Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/02—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for springs

Definitions

• This disclosure relates to a high-strength cold rolled steel sheet having excellent formability for use in structural parts and suspension parts mainly used in the automobile industry field, and to a method for manufacturing the same.
• DP steel ferrite-martensite dual phase steel
• TRIP steel that utilizes the transformation-induced plasticity of retained austenite
• Japanese Unexamined Patent Application Publication No. 2-101117 discloses a method for manufacturing a high-strength steel sheet having good formability, with which high ductility is achieved by adding large quantities of Si and thereby reliably obtaining retained austenite.
• the stretch flangeability is an indicator of formability during flange-forming through expanding holes already made, and is a property as important as the elongation property required for high-strength steel sheets.
• Japanese Unexamined Patent Application Publication No., 2004-256872 discloses a method for manufacturing a cold rolled steel sheet having good stretch flangeability with which the stretch flangeability is improved by forming a ferrite-tempered martensite multi-phase microstructure by conducting quenching and tempering after annealing and soaking.
• a ferrite-tempered martensite multi-phase microstructure by conducting quenching and tempering after annealing and soaking.
• the elongation is low.
• this steel sheet microstructure can exhibit high formability and improved crashworthiness.
• a high-strength cold rolled steel sheet having excellent formability and crashworthiness including, on a mass % basis, C: 0.05 to 0.3%, Si: 0.3 to 2.5%, Mn; 0.5 to 3.5%, P: 0.003 to 0.100%. S: 0.02% or less, Al: 0.010 to 0.5%, and balance being iron and unavoidable impurities, the high-strength cold rolled steel sheet having a microstructure including 20% or more of ferrite on an area fraction basis, 10 to 50% of tempered martensite on an area fraction basis, 0 to 10% of martensite on an area fraction basis, and 3 to 15% of retained austenite on a volume fraction basis.
• a high-strength cold rotted steel sheet having excellent formability and crashworthiness further including, on a mass % basis, at least one element selected from Cr: 0.005 to 2.00%, Mo: 0.005 to 2.00%, V: 0.005 to 2.00%, Ni: 0.005 to 2.00%, and Cu: 0.005 to 2.00%.
• a high-strength cold rolled steel sheet having excellent formability and crashworthiness, further including, on a mass % basis, one or both of Ti: 0.01 to 0.20% and Nb: 0.01 to 0.20%.
• a high-strength cold rolled steel sheet having excellent formability and crashworthiness further including, on a mass % basis, one or both of Ca: 0.001 to 0.005% and REM: 0.001 to 0.005%.
• a method for manufacturing a high-strength cold roiled steel sheet having excellent formability and crashworthiness including hot-rolling and cold-rolling a slab having a composition described in any one of the first to sixth aspects of the invention to manufacture a cold rolled steel sheet and continuously annealing the cold rolled sheet, in which, during the continuous annealing, the steel sheet is held at a temperature of 750° C. or more for 10 seconds or more, cooled front 750° C. to a temperature in a temperature range of 150 to 350° C. at a cooling rate of 10° C./s or more on average, heated to a temperature of 350 to 600′c, held thereat for 10 to 600 seconds, and cooled to room temperature.
• a high-strength cold rolled steel sheet having excellent formability is thus obtained.
• We achieve advantageous effects such as realizing both weight reduction and improved crash safety of automobiles and greatly contributing to improving performance of automobile bodies.
• Carbon (C) is an element that stabilizes austenite and promotes generation of phases other than ferrite. Thus, carbon is needed to increase the steel sheet strength, generate a multiphase structure, and improve the TS-EL balance. At a C content less than 0.05%, it is difficult to reliably obtain phases other than ferrite even when the production conditions are optimized and TS ⁇ EL decreases as a result. At a C content exceeding 0.3%, hardening of welded portions and heat-affected zones is significant, and mechanical properties of the welded portions are deteriorated. Thus, the C content is 0.05 to 0.3% and preferably 0.08 to 0.15%. Si: 0.3 to 2.5% 1002$ Silicon(Si) is an element, effective that strengthens the steel.
• Silicon is also a ferrite-generating element, suppresses C from becoming concentrated and forming carbides in the austenite, and thus accelerates generation of retained austenite.
• the Si content is less than 0.3%, the effects of addition are low. Thus, the lower limit is 0.3%. Excessive addition deteriorates the surface quality and weldability.
• the Si content is 2.5% or less.
• the Si content is preferably 0.7 to 2.0%.
• Manganese (Mn) is an element effective that strengthens the steel and accelerates generation of low-temperature transformation-forming phase such as tempered martensite. Such an effect is observed at a Mn content of 0.5% or more. However, when the Mn, content exceeds 3.5%, the second phase fraction increases excessively, the ductility deterioration of ferrite due to solid solution strengthening becomes significant and formability is degraded. Accordingly, the Mn content is 0.5 to 3.5% and preferably 1.5 to 3.0%.
• Phosphorus (P) is an element effective that strengthens the steel and this effect is achieved at a P content of 0.003% or more.
• P content is 0.003% to 0.100%.
• S Sulfur
• the S content is preferably as low as possible, but is limited to 0.02% or less from the production cost point of view.
• Aluminum (Al) acts as a deoxidizing agent and is an element effective for cleanliness of the steel. Aluminum is preferably added in the deoxidizing process. When the Al content is less than 0.01%, the effect of addition is little and thus the lower limit is 0.01%. However, addition of large quantities of Al increases the risk of slab cracking during continuous casting and decreases the productivity. Thus, the upper limit of the Al content is 0.5%.
• Our high-strength cold rolled steel sheet contains the above-described components as the basic components and the balance is iron and unavoidable impurities. However, the following components can be adequately contained according to the desired properties.
• Chromium (Cr), molybdenum (Mo), vanadium (V), nickel (Ni), and copper (Cu) suppress generation of pearlite during cooling from the annealing temperature, promote generation of low-temperature transformation-forming phases, and effectively serve to strengthen the steel.
• Cr, Mo, V. Ni, and Cu are contained.
• the content of each of Cr, Mo, V. Ni, and Cu exceeds 2.00%, the effect is saturated and the cost will rise.
• the Cr, Mo, V, Ni, and Cu contents are each 0.005 to 2.00%
• Ti 0.01 to 0.20% and Nb: OMI to 0.20%
• Titanium (Ti) and niobium (Nb) form carbon nitrides and have an effect of strengthening the steel by precipitation. Such effects are observed at a content of 0.01% or more for each element. In contrast, when Ti and Nb are contained in amounts exceeding 0.20%, excessive strengthening occurs and the ductility is decreased. Thus, the Ti and Nb contents are each 0.01 to 0.20%.
• B Boron
• Ca Ca
• REM rare earth element
• the area fraction of the ferrite is less than 20%. TS ⁇ EL decreases. Thus, the area fraction of ferrite is limited to 20% or more and preferably 50% or more.
• Tempered martensite is a ferrite-cementite multiphase having a high dislocation density and obtained by heating martensite to a temperature equal to or lower than Ac 1 transformation point and preferably to a temperature lower than Ac 1 transformation point. Tempered martensite effectively strengthens the steel.
• the microstructure obtained by heating martensite to a temperature exceeding Ac 1 transformation point is a microstructure that does not contain cementite in ferrite and is fundamentally different from our tempered martensite.
• the tempered martensite Compared to martensite, the tempered martensite has less adverse effects on stretch flangeability and is a phase effective to reliably obtain the strength without significantly decreasing stretch flangeability.
• the area fraction of the tempered martensite is less than 10%, it becomes difficult to reliably obtain the strength.
• the area fraction exceeds 60%. TS ⁇ EL is decreased.
• the area fraction of the martensite is limited to 10 to 60%.
• Manensite effectively increases the strength of the steel, but significantly decreases stretch flangeability once the area fraction of the martensite exceeds 10%.
• the area fraction of the martensite is 0 to 10%.
• volume fraction of retained austenite 3 to 15%
• Retained austenite not only contributes to strengthening of the steel, but also effectively improves TS ⁇ EL of the steel. Such effects are achieved at a volume fraction of 3% or more. When the volume fraction of the retained austenite exceeds 15%, stretch flangeability is decreased. Accordingly, the volume fraction of the retained austenite is 3 to 15%.
• Average crystal grain diameter of low-temperature transformation-forming phases constituted by martensite, tempered martensite, and retained austenite 3 ⁇ m or less
• Low-temperature transformation-forming phases constituted of martensite, tempered martensite, and retained austenite effectively improve crashworthiness, in particular, finely dispersing the low-temperature transformation-forming phases improves the crashworthiness, and this effect becomes notable when the average crystal grain diameter of the low-temperature transformation-forming phases is 3 ⁇ m or less. Accordingly, the average crystal grain diameter of the low-temperature transformation-forming phases is 3 ⁇ m or less.
• the phases other than ferrite, tempered martensite, martensite and retained austenite may include pearlite and bainite, but such phases do not present a problem as long as the above-described phase structure is satisfied.
• the amount of pearlite is preferably 3% or less from the view point of ductility and stretch flangeability
• a steel having a composition controlled as described above is melted in a converter or the like and formed into a slab by continuous casting or the like.
• This steel is hot-rolled, cold-rolled, and continuously annealed.
• the manufacturing methods regarding casting, hot-rolling, and cold-rolling are not particularly limited, but preferable manufacturing methods are described below.
• the steel slab is preferably manufactured by continuous casting to prevent macrosegregation of the components, but an ingot casting technique or a thin slab casting technique may be employed.
• an energy-saving process such as hot direct rolling or direct roiling which involves sending the hot slab to a heating furnace without cooling the slab to room temperature or which involves rolling the slab immediately after a short period of heat retention may be employed without any difficulty.
• Hot rolling conditions Slab heating temperature: 1100° C. or more
• the slab heaving temperature is preferably low from the viewpoint of energy consumption. At a heating temperature less than 1100T, carbides cannot be sufficiently dissolved or the risks of troubles during hot-rolling increases our to an increased rolling load.
• the slab heating temperature is preferably 1300° C. or less to prevent the increase in scale loss attributable to oxidation weight gain, the slab heating temperature is preferably 1300° C. or less.
• a sheet bar heater that heats the sheet bar may be employed to avoid troubles during hot-rolling despite the decreased slab heating temperature.
• Finishing temperature Ar 3 transformation point or more.
• the finishing temperature is less than the Ar 3 transformation point, ferrite and austenite are generated during rolling and a band-like microstructure readily occurs in the steel sheet. Such a band-like microstructure remains after cold rolling and annealing, may generate anisotropy in the material properties, and may decrease formability. Accordingly, the finishing temperature is preferably equal to or higher than Ar 3 transformation point.
• Coiling temperature 450 to 700° C.
• the coiling temperature is preferably 450 to 700° C.
• pan or all of the finish rolling may be conducted by lubrication rolling.
• Lubrication rolling is effective from the viewpoint of uniform steel sheet shape and material homogeneity.
• the coefficient of friction during lubrication rolling is preferably 0.25 to 0.10.
• Preferable is a continuous rolling process of joining sheet bars next to each other and continuously finish-rolling the sheet bars.
• the continuous rolling process is also preferable from the viewpoint of operation stability of hot rolling.
• the oxidized scales on the surface of the hot-rolled steel sheet are preferably removed by pickling and the steel sheet is cold-rolled to form a cold-rolled steel sheet having a particular thickness.
• the pickling conditions and the cold rolling conditions are not particularly limited and typical conditions may be used.
• the reduction of cold rolling is preferably 40% or more
• Average heating rate from 500° C. to Ac 1 transformation point 10° C./s or more
• the average heating rate in the recrystallization temperature zone 500° C. to Ac 1 transformation point is 10° C./s or more, recrystallization during heating is suppressed, austenite generated at Ac 1 transformation temperature or higher becomes finer, and the microstructure after annealing and cooling becomes finer. As a result, the average grain diameter of the low temperature transformation-forming phase can be reduced to 3 ⁇ m or less.
• the average heating rate front 500° C. to Ac 1 transformation point is limited to 10° C./s or more and more preferably 20° C./s or more.
• the heating temperature is loss than 750° C. or the holding time is less than 10 seconds, generation of austenite during annealing is insufficient and a sufficient amount of low-temperature transformation-forming phases cannot be reliably obtained after annealing and cooling.
• the upper limits of the holding temperature and the holding time are not particularly defined, the effects saturate and the cost will increase when the holding temperature is 900° C. or more and the holding time is 600 seconds or more, Accordingly, the holding temperature is preferably less than 900° C. and the holding time is preferably less than 600 seconds.
• the cooling rate from 750° C. is less than PVC's, pearlite is generated and TS EL and stretch flangeability are degraded.
• the cooling rate from 750° C. is limited to 10° C./s or more.
• the temperature condition of ending the cooling is one of the most crucial conditions of this method. At the time cooling is stopped, part of austenite transforms into martensite and the rest forms untransformed austenite. When reheated, plated and alloyed, and cooled to room temperature, martensite turns into tempered martensite and untransformed austenite transforms into retained austenite or martensite.
• controlling the temperature of ending the cooling determines the final area fractions of the martensite, the retained austenite, and the tempered martensite.
• the temperature of ending the cooling is higher than 350° C.
• martensite transformation at the time cooling is stopped is insufficient and the amount of untransformed austenite is large, thereby ultimately generating excessive amounts of martensite or retained austenite and degrading stretch flangeability.
• the temperature of ending the cooling is lower than 150° C.
• most of austenite transforms into martensite during cooling the amount of untransformed austenite decreases, and 3% or more of retained austenite is not obtained. Accordingly, the temperature of ending the cooling is 150 to 350° C.
• any cooling method such as gas jet cooling, mist cooling, water cooling, or metal quenching, may be employed as long as the target cooling rate and cooling end temperature are achieved. Heating to 350 to 600° C. and holding thereat for 10 to 600 seconds
• the martensite generated during cooling is tempered and forms tempered martensite.
• stretch flangeability is improved, the untransformed austenite that did not transform into martensite during cooling is stabilized, and 3% or more of retained austenite is obtained at the final stage, thereby improving ductility.
• the reheating temperature is less than 350° C., the martensite is not sufficiently tempered and the austenite is not sufficiently stabilized, thereby degrading stretch flangeability and ductility. If the reheating temperature exceeds 600° C., untransformed austenite at the time cooling is stopped transforms into pearlite and 3% or more of retained austenite cannot be obtained at the final stage. Accordingly, the heating temperature is 350 to 600° C.
• the reheating temperature is 350 to 600° C. and the holding time within that temperature range is 10 to 600 seconds.
• the annealed steel sheet may be subjected to temper roiling to correct shape, adjust surface roughness or the like. Moreover, treatment such as resin or oil/fat coating and various other coating may be performed.
• a steel having the composition shown in Table 1 and balance being Fe and unavoidable impurities was melted in a converter and continuously casted into a slab.
• the slab was hot-rolled to a thickness of 3.0 mm.
• the hot rolling conditions were as follows: finishing temperature: 900° C., cooling rate after rolling: 10° C./s, and coiling temperature: 600° C. Then the hot-rolled steel sheet was pickled and cold-rolled to a thickness of 1.2 mm to manufacture a cold roiled steel sheet.
• the cold rolled steel sheet was annealed under the conditions described in Table 2 by using a continuous annealing line.
• the cross-sectional microstructure of the steel sheet was observed by exposing the microstructure by using a 3% nitAl solution (3% nitric acid+ethanol), observing the position 1 ⁇ 4 of the thickness in the depth direction by using a scanning electron microscope, and conducting an image processing of a picture of the microstructure taken to determine the fraction of the ferrite phase (the image processing can be performed by using commercially available image processing software).
• the area fractions of the martensite and tempered martensite were determined by taking SEM photographs of adequate magnification, e.g., about 1000 to 3000 magnification, depending on the fineness of the microstructure and then determining the quantity by using image processing software.
• the average grain diameter of the low-temperature transformation-forming phase was determined by dividing the area of the low-temperature transformation-forming phases in the observed area by the number of the low-temperature transformation-forming phases, determining the average area therefrom, and raising the average to the power of 1 ⁇ 2.
• the volume ratio of the retained austenite was determined by polishing the steel sheet to a surface 1 ⁇ 4 in the thickness direction and measuring X-ray diffraction intensity of the 1 ⁇ 4 thickness surface.
• a MoKa, line was used as the incident X ray, the intensity ratios were determined for all combinations of the integrated intensities of peaks of ⁇ 111 ⁇ , ⁇ 200 ⁇ , 201, and ⁇ 311 ⁇ faces of the retained austenite phase and the ⁇ 110 ⁇ , ⁇ 200 ⁇ , and ⁇ 211 ⁇ faces of the ferrite phase, and the average value was assumed to be the volume fraction of the retained austenite.
• the tensile property was determined by using a HS No. 5 specimen sampled from the steel sheet in such a manner that the tensile direction was orthogonal to the roiling direction, conducting a tensile test according to JIS Z2241 to measure TS (tensile strength) and EL (elongation), and determining the strength-elongation balance value represented by the product of the strength and elongation (TS ⁇ EL).
• the hole expanding ratio was measured as an indicator of stretch flangeability.
• the hole expanding ratio ⁇ was determined by conducting a hole expanding test according to the Japan Iron and Steel Federation standard JFST1001 and determining the ratio from the initial diameter (10 mm ⁇ ) of the hole upon punching and the diameter of hole at the time the crack at the hole edge penetrated the sheet upon hole expanding.
• the shock absorption property was determined by using a specimen 5 mm in width and 7 mm in length sampled from the steel sheet in a direction orthogonal to the rolling direction, conducting a tensile test at a strain rate of 2000/s, and integrating the stress-true strain curve obtained by the tensile test within the range of 0 to 10% to calculate the absorption energy (refer to Tetsu-to-Hagane, 83 (1997) p, 748).
• the steel sheets of OUT examples have excellent strength, ductility, and stretch flangeability, i.e., TS ⁇ EL of 22000 MPa ⁇ % or more and ⁇ of 70% or more.
• the steel sheets of comparative examples outside our range did not achieve excellent strength, ductility, and stretch flangeability unlike the steel sheets of our examples since TS ⁇ EL was less than 22000 MPa. % and/or ⁇ was less than 70%.
• the ratio of the absorption energy to TS is 0.063 or more, thereby achieving excellent crashworthiness.
• Our steel sheets can contribute to weight reduction and decreasing the fuel consumption of automobiles by providing a high-strength cold rolled steel sheet having excellent formability and crashworthiness.

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