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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575

Tetsu-to-Hagané Vol. 68 (1982), No. 9

  • Trends of High Strength Steel Sheets and Autobody Forming Techniques

    pp. 1129-1135

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  • Future Trend of Automobile and the High Strength Sheet Steel

    pp. 1136-1146

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  • Strength and Ductility Related with Structure of Dual Phase High Strength Steel Sheet

    pp. 1147-1158

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  • Aging Characteristics and Formability of Dual Phase Steel Sheet

    pp. 1159-1168

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  • Effects of Grain Size and Solid-solution Strengthening Elements on the Bake-hardenability of Low Carbon Aluminum-killed Steel

    pp. 1169-1176

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    Effect of metallurgical factors on the bake hardenability (BH) of low carbon Al-killed steel is studied. The yield stress change due to baking (ΔYS) is determined by grain size and dissolved (C+N) content. On the other hand, the ultimate tensile stress change due to baking (ΔTS) depends mostly on the dissolved (C+N) content. The locking mechanism of dislocation is discussed in relation to grain size dependence of these properties. The influence of alloying elements (Mn, Si, and P) on BH through their metallurgical factors is also studied. Mn decreases ΔYS through the reduction of dissolved C content. Si gives rise to the increase of dissolved C content and grain refining up to 0.5% Si content. Over 0.5% Si content, the grain becomes coarser in turn. Consequently, ΔYS increases up to 0.5% Si content and levels off after that. Though the addition of 0.06% P does not affect the dissolved (C+N) content, it gives rise to a little increase of ΔYS through grain refining. Larger grain size dependence of ΔYS is observed at5 ppm of dissolved (C+N) content for 0.06% P containing steel.
  • Microstructure, Work Hardening and Ductility of Continuously Annealed Dual Phase Steel

    pp. 1177-1184

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    Dual phase steel sheets offer potential for weight reduction in automotive components, because of combination of excellent formability and high strength.
    In this paper, a study is made on the effect of heating temperature on the properties of hot rolled dual phase steel, which is obtained by continuous annealing of a steel containing 0.06%C-2.6%Mn.
    The result can be summarized as follows.
    1) The steel heated up-to about Ac3 has very fine ferrite/martensite structure and also excellent strength-ductility combination. This superior ductility is caused by the improvement of local elongation related to its unique microstructure.
    2) Retained austenite is observed in dual phase steels and its volume fraction is 3-5%. However the retained austenite can be easily transformed to martensite by tensile strain and completely disappears after small strain of 0.05.
    3) Work hardening of dual phase steel consists of three stages up-to necking. The first stage of work hardening is related to volume fraction and mean diameter of second phase. The next one is related to volume fraction of retained austenite and the last one is only related to volume fraction of second phase.
  • Deformation Behavior and Mechanical Properties of Ferrite Plus Bainite Plus Martensite (Triphase) Steel

    pp. 1185-1194

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    The effect of microstructure on the mechanical properties of multi-phase steels has been studied. The amount and nature of low temperature transformation products of the steels are changed through heat treatment and controlled cooling after hot rolling. Ferrite plus bainite steel has a superior stretch-flangeability and fatigue strength, and has an inferior strength-uniform elongation relationship when compared to ferrite plus martensite ("dual phase") steel.
    Introducing a small amount of martensite to ferrite plus bainite steel gives rise to a decrease in yield ratio and increase in work hardening rate. The microscopic examination shows that pearlite and martensite particles nucleate cracks easily and that the replacement of pearlite and martensite particles by more ductile bainite particles results in improved ductility and fatigue strength. The stretchflangeability of the triphase steel is also improved by decreasing martensite particle size. It is concluded that ferrite-bainite-martensite (triphase) steel may be preferred to ferrite-martensite steel in automobile applications.
  • Effect of Second Phase on the r-value of Dual Phase Sheet Steel

    pp. 1195-1202

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    Study has been made with regard to second phase parameters: hardness ratio (c) of second phase martensite to ferrite phase, second phase volume fraction (f), and second phase spacing (d). The parameters were varied by means of combination of heat treatments; d was controlled by box-annealing, f, by quenching, and c, by tempering.
    The r-value was affected strongly by the character of the second phase; it decreased with increasing c and/or f. The effect of c on the r-value was relatively small for small f and marked for large f.
    The decrease in d enhanced the effect of c and f on the r-value.
    The above effect of the second phase can be interpreted in terms of the internal stress caused by the difference in the deformability of both constituent phases.
    Finally, condition is presented for the production of dual phase steel with high r-value; furthermore, this steel has high bake hardenability and shows no yield point elongation after room temperature aging.
    x

    Readers Who Read This Article Also Read

    1. AlN Precipitation of Al-killed Steel for Cold-rolled Sheets in Continuous Casting and Direct Rolling Process Tetsu-to-Hagané Vol.74(1988), No.7
    2. 連鋳 Tetsu-to-Hagané Vol.60(1974), No.11
    3. 薄板 Tetsu-to-Hagané Vol.60(1974), No.4
  • Formability of High Strength Steel Sheets

    pp. 1203-1210

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  • Niobium Added Ferrite-Bainite High Strength Hot-rolled Sheet Steel with Improved Formability

    pp. 1211-1220

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    Dual phase steels are known not to have superior characteristics in such properties as hole expanding property, Charpy impact property and especially in flash butt welding property which has a great influence on wheel rim fabrication. Through the investigation of the quality and volume fraction of second phase and alloying elemets, it has been clarified that Nb-added steels composed of ferrite and 10 to 20% bainite eliminate these problems in dual phase steels and exhibit excellent characteristics.
    In this paper, the effects of the quality of second phase such as martensite, bainite, and pearlite which are produced by changing the coiling temperature from room temperature to 650°C and the volume fraction of bainite on the mechanical properties and so on are investigated.
    As far as flash butt welding property is concerned, Nb-added steel composed of ferrite plus bainite dose not show a remarkable softening and localized necking in the heat affected zone which usually occur in dual phase steel because of the tempering of martensite. The superiority of the hole expanding property of Nb added ferrite plus bainite steel is due to the superior ductility of bainite compared to that of martensite or pearlite.
  • High Strength Hot Rolled Sheet Steels for Automotive Wheel Use

    pp. 1221-1227

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    Overall applicability of various types of high strength sheet steels to automotive wheels was investigated by means of conventional production tooling as well as laboratory forming and welding tests. Dual phase sheet steels, especially those of as hot rolled type, showed most hopeful potential in wheel discs forming because of their high workhardenability, good elongation and uniform dispersion of second phase. Special improvements in flash butt welding and roll forming conditions were necessary to make high strength sheet steels usable for the production of wheel rims and HSLA steels which contained Nb revealed comparatively stable quality in joints and HAZ of flash butt welds. Centrifugal cornering fatigue tests suggested that high strength sheet steel of 60 kgf/mm2 tensile strength could be substituted for mild sheet steel in wheels with weight saving as high as 25%. Influences of alloying elements on various properties and performances of sheet steels for wheel were summarized and utility of dual phase sheet steels was also discussed.
  • Deep Drawability and Recrystallization Texture of Rephosphorized Al-killed High Strength Cold Rolled Steel

    pp. 1228-1235

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    The deep drawability and the recrystallization texture of rephosphorized Al-killed high strength steels (P:0.07%) were studied as compared with those of a conventional Al-killed steel (P:0.016%). The rephosphorized Al-killed steel shows a peak r-value at a certain heating rate of annealing as well as the conventional Al-killed steel. However, phosphorus weakens the retardation effect of AlN particles on the recrystallization, which results in a lower r-value and a lower grain elongation ratio. The preferred orientation of the rephosphorized Al-killed steels is {111} <112>, whereas that of the conventional Al-killed steel is {111} <110>. These results are discussed by taking into account the interaction between phosphorus and AlN precipitation.
  • Analysis of Side Wall Curl in Draw-bending of High Strength Steel Sheet

    pp. 1236-1243

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    Geometrical defects observed in applying high strength steel sheet to auto-body structural members with hat-shaped cross-section were studied using a simple experimental draw-bend die. One of the typical troubles in forming of these parts is the side wall curl which appears along side walls from a bottom to a top (or a flange) and deflects walls outward.
    It was found that the side wall curl was eliminated by choosing the die profile radius about twice as large as sheet thickness under the optimum die clearance.
    A rather small die profile radius was advantageous for producing the reverse bending against bending at the die profile.
    This reverse bending contributed to reducing the side wall curl by cancelling the residual bending moment in side walls which had been subjected to bending at the die profile. Such behaviour of material between a die and a punch was an important fact pointed out in this investigation.
    The other interesting observation was that the dual phase steel had a larger tendency to the outward side wall curl than the precipitation hardening steel with the similar tensile strength. This difference was also discussed in this paper.
  • Recent Developments in Production Technology of Automotive High Strength Steel Sheets

    pp. 1244-1255

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  • Production and Quality in Hot Rolled 60kgf/mm2 Class Dual Phase Steel

    pp. 1256-1262

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    Manufacturing factors and properties of hot rolled 60 kgf/mm2 class dual phase (DP) steels by two producing methods, which were suitable for mass production, were discussed.
    (1) As rolled extra low temperature coiling method developed using three step cooling pattern, called as "Inline controlled quenching (ICQ) method" was suitable for massproduction of DP steel with low alloying component (1.5%Mn and low Si content less than 0.15%). It was important for stabilization of mechanical properties to control the temperature and period of the second air cooling stage between water cooling stages according to variation of chemical composition.
    (2) The continuous annealing (CA) method as the heat-treated type process was fit for the case that low yield ratio was especially required.
    (3) As the result of investigation about formability, bake hardenability and weldability, it was proved that the hole expansion ratio of DP steel was remarkably improved with inclusion shape control by calcium-addition, bake hardenability of ICQ-DP steel was larger than the CA steel due to higher solute nitrogen content, and that weldability was excellent except flash-butt welding for auto wheel rim.
    (4) DP steels were successfully appliable to auto wheel disc and most parts made of hot rolled mild steels and their applications have been useful for weight reduction.
  • The Production Technology of High Strength Steel Sheets and Their Properties

    pp. 1263-1269

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    The paper reviews Nippon Steel's research and development systems and describes its approach to dealing with the requirements of the automobile industry for high strength steel sheets in terms of steel properties, such as, formability, weldability, paintability and corrosion resistance.
    The paper also explains the corporation's technologies for the production of hot-rolled, cold-rolled and surface treated high strength steel sheets and the properties of these products.
    For the trends of the expanding applications of high strength steel sheets and the growing demand for higher strength steel, the paper emphasizes the necessity for overall measures to promote technical improvements in both the manufacture and the fabrication of high strength steel sheets.
    x

    Readers Who Read This Article Also Read

    1. Manufacturing Technology and Properties of Ultra High Strength Cold Rolled Strip Produced by Water Quenching Method Tetsu-to-Hagané Vol.68(1982), No.9
    2. Production and Quality in Hot Rolled 60kgf/mm2 Class Dual Phase Steel Tetsu-to-Hagané Vol.68(1982), No.9
    3. recoated Steel Sheets for Automotive Use Tetsu-to-Hagané Vol.72(1986), No.8
  • Manufacturing Technology and Properties of Ultra High Strength Cold Rolled Strip Produced by Water Quenching Method

    pp. 1270-1275

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    A new technique to manufacture cold rolled ultra high strength steel sheets through the continuous annealing line with a water quenching device has been developed.
    Full martensite and ferrite plus martensite dual phase steels with 80 to 150 kgf/mm2 in tensile strength can be easily obtained by water quenching after annealing in the austenite and ferrite plus austenite regions, respectively.
    These products having good formability and weldability are used successfully for automobile parts to improve the safety of the passengers in case of an accident and to reduce the weight of the automobiles.
    Furthermore, these products are being utilized in the industry of construction, and are expected to be used for machinery, electric appliances, containers, and others.
  • Production of 3540kgf/mm2 Cold Rolled High Strength Steels with Excellent Press Formability by Continuous Annealing

    pp. 1276-1282

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    Dual phase (DP) steels with low yield to tensile strength ratio of 45% and extra deep drawing high strength (EDDH) steels with high r value above 2.0 have been successfully produced in a multipurpose continuous annealing line with cooling rate of 40°C/s by gas jet.
    Dent resistance of trunk lid panel made from 1.2Mn-0.5Cr DP steel sheets of 0.70 mm thickness is higher than that made from Al killed mild steels of 0.90 mm thickness. In order to obtain tensile strength between, 40 to 46kgf/mm2, it is effective to control C content within 0.0250.040%, and to adjust annealing temperature precisely just above 740 °C.
    Excellent deep drawabilities are attained by reducing C content down to 0.004% or less in 0.03Nb-0.06P-0.05Al EDDH steels. In actual production, the extremely low carbon content is attained by the combination of decarburizing with bottom blown vessel (down to 0.015% C) and subsequently with reformed RH degasser in about 15 min (less than 0.003%).
    The EDDH steels have lower yield strength than 20 kgf/mm2 and higher elongation than 42%.
    In the multipurpose continuos annealing line, the strip tensions in soaking section and rapid cooling section in the furnace are regulated in dependently to avoid heat buckling at 830°C and fluttering caused by gas jet, respectively.
  • Development and Application of Box-annealed Bake-hardenable Steel Sheets

    pp. 1283-1289

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    The effects of chemical compositions and processing factors on the bake-hardenability have been studied on the box-annealed rephosphorized steels and the steels strengthened by silicon and manganese. The effects of bake-hardening on the dent and crush resistance have been also studied.
    Lowering carbon content, increasing hot coiling temperature, and increasing annealing temperature are shown to be effective for the improvement of bake-hardenability. Bake-hardenability also increnses with increassing silicon and phosphorus contents and decreasing manganese content. The influence of these elements is diminished by lowering carbon contents.
    In order to meet the demands of automobile manufacturers, bake-hardenable rephosphorized steel sheets are successfully produced on an industrial scale in Unitized Annealing Department.
    Application of bake hardening is effective to improve dent and crush resistance. Dent resistance is more improved than crush resistance due to bake-hardening.
  • Production Technology and Mechanical Properties of High Strength Hot Rolled Steel Sheets

    pp. 1290-1296

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    The various types of high strength hot rolled steel sheets for automotive use have been developed. The effects of hot rolling conditions, especially cooling condition and coiling temperature, on the mechanical properties and the flatness of hot strip are discussed. It was found that the cooling pattern during traveling on runout table as well as coiling temperature was important factor to obtain as-hotrolled dual phase steel sheet having the uniform properties.
    The dual phase steel sheets have a lower yield ratio, high work hardening rate and good ductility, compared with conventional high strength steel sheets. Some examples of the application of these high strength steel sheets are also introduced.
  • New Method for the Production of As-hot Rolled Dual Phase Sheet Steel

    pp. 1297-1305

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    For the purpose of developing the as-hot-rolled dual phase sheet steel, hot rolling conditions and subsequent cooling pattern were investigated by using 1%Si-1.5%Mn-1.0%Cr steels with carbon contents of 0.05%C and 0.08% and 0.05%C-1.0%Si-1.5%Mn steel.
    In order to study the exact transformation behavior immediately after finish-rolling, hot-rolled strips of these steels were stopped on the runout table and water-cooled and/or air-cooled to room temperature.
    This unique experiment exhibited that the optimum cooling pattern consisted of three stages;
    1) rapid cooling to a suitable temperature (700°C) to increase ferrite nucleation sites,
    2) holding at the temperature (700°C) to enhance ferrite transformation, thereby stabilizing untransformed austenite due to C and N concentration, and
    3) rapid cooling to a coiling temperature to avoid the formation of pearlite and bainite.
    Experimental rolling was performed at the production mill. The steels obtained showed dual phase structure with excellent tensile properties Lin terms of low yield to tensile strength ratio and high ductility.
  • The Manufacture of Si-Mn As-hot-rolled Dual Phase Steel Sheets

    pp. 1306-1312

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    The Si-Mn steels have been hot-rolled and coiled at the very low temperature after rapid cooling in runout table of hot strip mills to produce as-hot-rolled dual phase (AHR DP) steel sheets which prove to be both economical and excellent in quality. The results obtained are as follows:
    (1) The strengthening effect of carbon is very large but ductility does not decrease so much by increasing carbon content as to deteriorate the good strength-ductility combination.
    (2) The Si addition of a higher content raises the critical coiling temperature under which DP steel can be obtained and it improves much the strength-ductility combination.
    (3) The coiling temperature is the most important factor affecting strength and elongation of AHR DP steel and its appropriate range depends on the controlling capacity of cooling and the quality level of materials.
    (4) Variations of mechanicl properties within a coil as well as throughout coils can be minimized by the control of chemical composition and rolling condition, and especially by the use of continuously-casted steel.
    (5) When compared to conventional high strength low alloy steels, the hot-rolled DP steel shows a smaller planar anisotropy of properties and larger work- and bake-hardenability; the Si-Mn AHR DP steel, on the other hand, has a better combination of strength and ductility than the heat treated DP steel.
    (6) Performances of Si-Mn AHR DP steel sheets are good especially in hole expansion property and stretchabilty.
  • Manufacturing Process of As Hot Rolled Dual Phase Steel

    pp. 1313-1322

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    The effects of processing variables and chemistry on the microstructure and the mechanical properties of as-hot rolled dual phase steel were investigated in relation to transformation behavior in order to optimize the processing condition. As-hot rolled dual phase structures with good strength-ductility balance and low yield ratio below 0.60 were obtained, when C-Si-Mn steels with such plain compositions as 0.040.10%C-0.51.5%Si-1.21.7%Mn were rolled at finishing temperature above and near Ar3 point and coiled at temperatures below 200°C. Si was so effective for strengthening without sacrifice of ductility, and for broadening the suitable range of the finishing rolling temperature.
    Deformation of austenite accelerated the transformation into ferrite and raised the Ar3 point. The rise of Ar3 point was remarkably influenced by deformation temperature, holding time in austenitic region and Si content. The optimum thermomechanical process for producing the dual phase steel was discussed on the basis of transformation characteristics of deformed austenite.
  • Effects of Steel Chemistry and Processing Factors on the Mechanical Properties of As-hot-rolled Dual Phase Steels

    pp. 1323-1332

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    As-hot-rolled, ferrite-martensite dual-phese steels of rather simple chemistry can be produced by "DPR Process" which uses a low finish rolling temperature and a very low coiling temperature. Laboratory DPR experiments have been carried out using C-Mn steels and these with Cr or Si additions, to examine the effects of alloying and processing factors on the structure and mechanical properties of the processed steels. Major results obtained are as follows:
    (1) To attain a sufficiently low yield-to-tensile strength ratio, the final finish pass temperature should be at about Ar3 point which varies depending on the chemistry, so as to bring about early phase separation of alpha from gamma before cooling starts. The coiling after a rapid cooling should be done at a temperature lower than about 200°C, almost regardless of the steel composition, to suppress auto-tempering of the transformed martensite and aging of the ferrite.
    (2) Both Cr and Si additions enhance the hardenability of partitioned austenite, allowing a slower cooling rate to obtain martensite phases. However, Cr addition is prone to hinder the early phase separation, making the gamma-to-alpha transformation sluggish. Si addition accelerates the phase separation so that a wide range of finishing temperature is available.
  • The Development of Ti-bearing 70 and 80 kgf/mm2 Class Hot Rolled Steel Sheets with Bainitic Structure

    pp. 1333-1339

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    The fundamental researches and the mill trials have been carried out in order to determine the suitable manufacturing conditions for Ti-containing bainitic 70 and 80 kgf/mm2 class steel sheets with excellent bending and stretch-flanging formability.
    The results obtained are as follows:
    (1) Addition of Ti to low C-2.0Mn steel enhances acicularity, and the higher the acicularity, the better is the tensile strength-notch elongation balance without decrease of tensile strength-elongation balance.
    (2) The tensile strength-notch elongation balance of 0.03C-Ti bearing steel is better than that of 0.08C-Ti bearing steel.
    (3) The excellent stretch flanging formability of 70 kgf/mm2 class hot rolled steel sheets is obtained in 0.03C-0.5Si-2.0Mn-0.3Cr-0.08Ti steel coiled at about 600°C.
    (4) The excellent bending formability of 80 kgf/mm2 class hot rolled steel sheets is obtained in 0.08C-0.5Si-2.0Mn-0.3Cr-0.15Ti steel coiled at about 550°C.
  • Effect of Chemical Composition on the Formability of Flash Butt Welded Joints in Hot-rolled High-strength Steel Sheets

    pp. 1340-1347

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    A study has been made of the formability of flash-butt welded joints in three types of hot-rolled high-strength sheet steels, solid solution hardened steel, precipitation hardened steel and dual phase steel, to clarify the factors which are required to high strength steels for wheel rim use.
    Formability in stretch-flanging depends strongly on the hardness and microstructure at the weld. It is impaired by localized fracture due to the softening at heat affected zone or weld interface in dual phase steels and precipitation hardened steels having low alloy content. To avoid the softening in precipitation hardened steel, the carbon equivalent should be raised in proportion to the tensile strength in newly proposed equation; Ceq [FBW].
    Formability in bending is deteriorated by two types of defects, penetorator crack and hook crack. The former is caused by oxides generated at weld interface during welding. It is suppressed by controlling Si and Mn content to maintain Mn/Si in an adequate range, 423. The hook crack can be suppressed by lowering the amount of sulfide inclusions.
  • Ultra High Strength Cold Rolled Steel Sheet

    pp. 1348-1354

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  • Development of High Strength Cold-rolled-phosphorus Steel Produced by Continuous Annealing

    pp. 1355-1361

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    A high strength cold-rolled phosphorus-containing steel has been developed by continuous annealing process. The study is focussed on the improvement of Lankford value and aging characteristics of phosphorus-containing steel.
    The effects of coiling temperature after hot rolling and cold reduction on Lankford value have been investigated for a low C-low Mn-B-P steel. Investigations have also made clear the effects of cooling rate after continuous annealing on the precipitation rate and the size of precipitates of carbides. Experimental results show that (1) the combination of proper coiling temperature and large cold reduction results in high Lankford value over 1.5 and (2) the combination of high paint baking hardenability over 40 MPa with retarded aging property at room temperature can be obtained in the range of cooling rate from 10 to 100 deg C/s. The factors affecting such results are also discussed.
  • Development of Cold Rolled High Strength Steel Sheet with Bake Hardenability and Excellent Deep Drawability

    pp. 1362-1368

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    Effects of carbon and niobium contents, and continuous annealing conditions on mechanical properties, structures and embrittlement of cold rolled steel sheets containing extra low carbon, low niobium and phosphorus have been investigated. The following results are obtained:
    (1) The steel sheets with Nb/C (atomic ratio) ≤0.5 show high bake hardenability but poor drawability. The steel sheets with 0.7≤Nb/C≤1.2 exhibit excellent deep drawability and high bake hardenability originated in solution of niobium carbides by annealing at over 850°C. (2) For maintaining high bake hardenability, rapid cooling is effective especially at near 750°C and at near 300°C. (3) Cold-work embrittlement due to adding phosphorus can be prevented by means of leaving a small amount of solute carbon in steel sheets through the rapid cooling at the above temperature ranges. Critical content of phosphorus (Per) causing cold-work embrittlement can be expressed in terms of the ageing index as follows.
    Per(wt%)=0.04·AI+0.045
    (AI: Ageing index (kgf/mm2), Cooling rate:≥1°C/s)
    Based on the above results, the cold rolled high strength steel sheet with bake hardenability and excellent deep drawability (r=2.3) has been produced_in a continuous annealing line.
  • Bake-hardenable Al-killed Steel Sheet by Box-annealing Process

    pp. 1369-1377

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    The bake-hardenable cold-rolled steel sheets for the outer panels of auto-bodies exhibiting a sufficient formability and an excellent resistance to denting have been developed in box-annealing process. Examinations in the effects of chemical composition and box-annealing condition on the bakehardenability and on Snoek peak height of Al-killed steels revealed that lowering total carbon content to less than 0.02% for tight coil annealing and raising annealing temperature to α/γ intercritical range for open coil annealing, besides, adding silicon or phosphorus and reducing manganese content in steel can increase the bake-hardenability to 40 MPa, retaining 10ppm carbon in solution. The mill produced bake-hardenable Al-killed steel sheets showed practically non-aging quality and high r-value. Discussions were made on the compatibility of retarded aging property and bake-hardenability of the steels by using Hundy's equation. A retarded aging and bake-hardenable rimmed steel was also produced by controlling nitrogen to 8 ppm in the open coil annealing furnace.
  • Effect of Alloying Elements and Annealing Cycles on Strengthening of Continuously Annealed Cold Rolled Sheet

    pp. 1378-1387

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    Effects of alloying elements (C, Si, Mn, and P) and continuous annealing conditions (soaking temperature, cooling rate, and over-aging temperature) on strengthening of steels, have been investigated so as to develop a desirable process to produce commercial quality high strength cold rolled sheets for automobiles by continuous annealing.
    In subcritical annealing, cooling rate after soaking has no influence on strengthening effects of alloying elements. Degree of strengthening effects is in the order cf P>C>Si>Mn. In case of intercritical annealing, strengthening effect of C is largely influenced by cooling rate after soaking.
    Continuous annealing just above recrystallization temperature, which accompanies ferrite grain refining due to subcritical annealing is the most desirable strengthening procedure on the viewpoint of economy. In this case, moreover, the similar strength-ductility balance and the superior bake hardenability to those by other processes can be obtained even after subcritical annealing without over-aging.
  • Effects of Continuous Annealing Conditions on Mechanical Properties of Drawing Quality High Strength Steel Sheet

    pp. 1388-1396

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    Effects of continuous annealing conditions on mechanical properties of drawing quality high strength steel sheet were studied using the continuous annealing heat cycle simulator which had been developed by the authors.
    Continuous annealing conditions which were studied are as follows:
    (1) Cooling rate before over aging. (2) Starting temperature of cooling in the cases of AcC (accelerated cooling) method and W. Q (water quenching) method. (3) End temperature of cooling in the case of AcC method. At the same time, changes of the amount of carbon in solution and carbide precipitation during heat treatments were studied.
    As a result of this study, it was made clear that the most desirable heat cycle for drawing quality and non aging high strength steel sheet consisted of slow cooling of about 7°C/s from soaking temperature to 675°C, accelerated cooling of about 100°C/s from 675°C to 400°C and over aging at 400°C. In this annealing condition, most of the fine carbides precipitated on grain boundaries and the amount of carbon in solution decreased in short over aging time.
  • Development of High Strength Galvannealed Steel Sheets with Extra Deep Drawability

    pp. 1397-1403

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    The effects of silicon, manganese and phosphorus addition on the mechanical properties of extra low carbon titanium-stabilized steel sheets have been examined in order to develop high-strength galvannealed steel sheets with extra deep drawability. Titanium-stabilized steels containing silicon, manganese and phosphorus have excellent r-value, good strength/elongation balance and low yield ratio (yield stress/tensile strength) as in base steel without these substitutional alloying elements. In particular, the r-value of silicon and phosphorus-containing steel sheets are higher than that of base steel at a given grain size after annealing.
    The effects of phosphorus addition in steels on phosphate coating, galvannealing behavior after galvanizing, and the properties of galvannealing coating have been investigated. Titanium-stabilized sheet steels with phosphorus show excellent coating properties after phosphating or galvannealing.
    The spot weldability of galvannealed sheet steels manufactured in accordance with these experimental results are sufficient to meet the requirements of automobile manufacturers.
  • Influence of Alloying Elements in Hot Dip Galvanized High Tensile Strength Sheet Steels on the Adhesion and Iron-zinc Alloying Rate

    pp. 1404-1410

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    The influences of 11 elements in sheet steel, temperature and Al content in zinc bath on the adhesion were investigated by use of continuous galvanizing line and laboratory galvanizing simulator. The adhesion was remarkably affected by Si in the steels, the temperature and Al content.
    The influences of Ti, Si and P in the steels on Fe-Zn alloying rate were also investigated. Ti accelerated the alloying rate, while Si and P slowed down it in the zinc bath containing Al. In the case of pure zinc bath, addition of these elements accelerated the rate. Based on the SEM observation of interface between alloyed layer and steel substrate, a model illustrating the adhesion and alloying rate has been proposed.
  • Spot Weldability of High Strength Steel Sheets

    pp. 1411-1420

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  • Hot-rolled Steel Sheet with Excellent Flash Weldability for Automobile Wheel Rim Use

    pp. 1421-1429

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    As for the steels for wheel rim, CC killed steel will be more prevalent instead of rimmed steel in accordance with the increase of CC ratio in steel production. In addition, from the point of weight reduction of cars, high strength steels are tried to put into practice. As far as wheel rim is concerned, flash weldability is very important as well as formability of steels.
    In this report, flash weldability of CC killed steel, HSLA (High Strength Low Alloy) and DP (Dual Phase) steels are investigated. In flash welding of CC killed steel, control of Al content, in addition to Si and Mn contents, is found to be required in order to avoid the formation of Si-Mn-Al oxide. On the other hand, coiling temperature is important in high strength steels, especially DP steels, in order to prevent softening of weld heat affected zone.
  • Fatigue Properties of High Strength Steel Sheets

    pp. 1430-1436

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    Fatigue properties of high strength steel sheets were studied in connection with the effects of tensile strength, sheet thickness, yield ratio and tensile prestrain.
    Materials examined in the present study were cold rolled and hot rolled steel sheets of conventional and low yield ratio types. Fatigue tests were carried out under the conditions of pulsative axial tension and alternative out-of-plane bending.
    Principal results are as follows:
    (1) Fatigue strength increased in proportion to the increase in tensile strength up to 70kgf/mm2, then it gradually leveled off above 70kgf/mm2 up to 140kgf/mm2.
    (2) The thinner steel sheets had the higher fatigue strength. This tendency was more significant under out-of-plane bending condition than under axial tension condition.
    (3) Low yield ratio type steel sheets in virgin state had lower fatigue strength compared with conventionl steels of the same tensile strength. However, this difference in fatigue strength almost disappered after prestraining of sheets.
    (4) Empirical formulas to estimate fatigue endurance limit were proposed on the basis of a multiple regression analysis in connection with tensile strength, sheet thickness, yield ratio and elongation of steel sheet.
  • Spot Weldability of Cold Rolled High Strength Steel Sheets

    pp. 1437-1443

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    The acceptable welding current range for nugget formation and mechanical strength of welded joint have been studied in relation to tensile strength and electrical resistivity of steel sheet, and electrode force in welding.
    The lower limit, Ilow(kA), of welding current for nugget formation and the upper limit, Iupp(kA), for avoiding expulsion decrease with the increase in tensile strength of steel sheet, in which the upper limit decreases more remarkably than the lower one. The decreasing tendency is well explained in terms of tensile strength, TS(kgf/mm2), and electrical resistivity, ρ(μ-Ωcm), of steel sheet as in the following:
    Ilow=9.50-0.030TS-0.060ρ
    Iupp=11.27-0.032TS-0.081ρ
    Tensile shear strength and cross tension strength of welded joint increase with the increase in tensile strength of steel sheet. The increment of cross tension strength is smaller than that in tensile shear strength. Cross tension strength appears to be largely affected by hardness distribution across the heat affected zone, which is influenced by chemical composition of steel sheet.
  • Improvement in Fatigue Strength of Spot Welded High Strength Sheet Steel Joints

    pp. 1444-1451

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    Fatigue strength of spot welded hot rolled high-strength sheet steel joints in long-life testing does not vary with tensile strength of the sheets. By extensive experiments using solid solution hardened steel, precipitation hardened steel and dual phase steel, two effective methods for improving the fatigue strength have been found.
    One method is to generate moderate expulsion by welding with a current just above the upper lobe curve; endurance limits at 107 cycles increase by about twice as compared with those of the normal welding method. In this case, the decrease in tensile shear strength is less than 15%. This improvement in fatigue strength is due to the increase in effective area of corona bond, and due to the increase in residual compressive stress of nugget.
    The other is the controlled tempering treatmet during the welding schedule; endurance limits at 107 cycles increase by about twice as compared with those of the normal welding method. This improvement is due to the residual compressive stress at the weld generated by the tempering treatment.
  • Factor Analysis of Explosion in Spot Welds of Hot Dip Galvannealed High Strength Steel Sheets and the Countermeasures

    pp. 1452-1459

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    A serious problem in the spot welding of hot dip galvannealed high strength steel sheets is the occurence of violent spattering of molten metal in the nugget and formation of explosion.
    A study was carried out to develop hot dip galvannealed high strength steel sheets having excellent resistance to explosion and also to develop welding methods for supressing the formation of cavities by explosion of the nugget.
    Results proved that the occurence of explosion depends upon the sort of iron-zinc compounds in the coating, and that when the coating has Γ phase of thickness more than 0.3 micrometers, explosion occurs, often exceeding a rate of 10%. As the Γ phase increases the electric resistance of the coating, it accelerates the formation of nugget in the initial period of welding and melts rapidly into the nugget. Melting of the Γ phase increases the zinc concentration in the nugget and causes the formation of cavities, because the vapour pressure of zinc in the molten nugget is extremely high.
    Based upon the results of study, countermeasures were taken with regard to galvannealing and welding conditions.
  • Automotive Application and Forming Problems for New Sheet Materials/Problems of New Sheets for Automobile Parts in Europe

    pp. 1460-1474

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  • 製管の歴史(4)-鍛接管方式-/冬のヨーロッパを訪れて/世界の石炭生産高

    pp. 1477-1478

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