WO2013084477A1 - Tôle d'acier laminée à froid à haute résistance ayant d'excellentes caractéristiques de résistance au vieillissement et d'aptitude au durcissement après cuisson - Google Patents

Tôle d'acier laminée à froid à haute résistance ayant d'excellentes caractéristiques de résistance au vieillissement et d'aptitude au durcissement après cuisson Download PDF

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WO2013084477A1
WO2013084477A1 PCT/JP2012/007771 JP2012007771W WO2013084477A1 WO 2013084477 A1 WO2013084477 A1 WO 2013084477A1 JP 2012007771 W JP2012007771 W JP 2012007771W WO 2013084477 A1 WO2013084477 A1 WO 2013084477A1
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Prior art keywords
steel sheet
rolled steel
cold
less
mass
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PCT/JP2012/007771
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English (en)
Japanese (ja)
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金晴 奥田
英之 木村
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Ｊｆｅスチール株式会社
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Priority to CN201280060601.0A priority Critical patent/CN103975087B/zh
Publication of WO2013084477A1 publication Critical patent/WO2013084477A1/fr

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    • 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
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • 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
    • 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
    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
    • 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
    • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
    • 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
    • C22C38/12—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
    • C22C38/16—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
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum

Definitions

  • the present invention relates to a cold-rolled steel sheet suitable for automobile panel parts, and in particular, has a high strength with a tensile strength TS of 340 to 440 MPa class and excellent anti-aging property.
  • the present invention relates to a high-strength cold-rolled steel sheet having both bake-hardening property (hereinafter also referred to as BH property).
  • BH property bake-hardening property
  • the “steel plate” herein includes a steel plate and a steel strip.
  • the “cold rolled steel sheet” includes a cold rolled steel sheet and a cold rolled steel sheet obtained by subjecting the cold rolled steel sheet to a surface treatment such as electroplating.
  • the plating process includes a pure zinc plating process, a zinc-based alloy plating process in which an alloy element is added with zinc as a main component, or an Al-based alloy plating process in which an alloy element is added with Al or Al as a main component.
  • a BH steel sheet is generally a low-carbon steel sheet containing Nb in an atomic ratio equal to or less than that of carbon, and solid solution strengthened with Mn or P, and a small amount of solid steel after annealing. Due to the presence of molten C, dislocations introduced by processing such as pressing are fixed in the coating baking process, yield strength is increased, and dent resistance is improved.
  • BH steel plate As such a bake hardened steel plate (BH steel plate), for example, in Patent Document 1, C: 0.002 to 0.015%, Si: 1.2% or less, Mn: 0.04 to 0.8%, P: 0.03 to 0.10%, Al : High-tensile cold-rolled steel sheet having 0.02% or more and N% ⁇ 4 or more, Nb: C% ⁇ 3 to ⁇ C% ⁇ 8 + 0.020% ⁇ , and the balance being substantially made of Fe Has been.
  • the high-tensile cold-rolled steel sheet described in Patent Document 1 hot-rolls a steel slab having the above composition at a total rolling reduction of 90% or more and a rolling speed of 40 m / min or more, and is wound at a temperature of 600 ° C. or more.
  • This cold-rolled steel sheet is said to be a steel sheet having a tensile strength TS of 340 to 440 MPa, excellent anti-aging properties and excellent formability.
  • Patent Document 1 has a problem that yield elongation appears when the Nb content is reduced or the aging temperature is increased to increase the seizure curability. That is, there has been a problem that the conventional production methods that have been studied cannot have both aging resistance and seizure curability.
  • a high-strength cold-rolled steel sheet that solves the problems of the prior art, has a tensile strength TS of 340 to 440 MPa, and has both excellent aging resistance and excellent seizure hardenability (BH property).
  • the purpose is to provide.
  • excellent aging resistance here refers to a case where the yield elongation is as low as 0.5% or less after aging promotion treatment at room temperature (aging treatment at 70 ° C. ⁇ 60 days).
  • excellent bake hardenability (BH property) here refers to pre-strain of yield stress after applying pre-strain: 2% and heat treatment (paint baking process) at 170 ° C for 20 min. The increase amount (BH amount) with respect to the arrival stress at 30 is 30 MPa or more.
  • the present inventors have conducted a detailed examination on the influence of various factors on BH properties and aging resistance. As a result, in order to combine excellent aging resistance and excellent seizure curability, it has been found that control of the amount of dissolved C is important. And it was conceived that by including two kinds of alloy elements having different affinity with C, it was possible to combine excellent aging resistance and excellent seizure curability by controlling the amount of dissolved C. . As a result of further study, Cr or Cu, which has a weaker affinity for C than Nb, is used in addition to Nb, which has a stronger affinity for C, as two types of alloy elements having a different affinity for C. I came up with it.
  • the present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows. [1] By mass% C: 0.0010 to 0.0080%, Si: 1.0% or less, Mn: 0.1 to 1.8%, P: 0.100% or less, S: 0.03% or less, sol.Al: 0.01 to 0.50%, N: 0.0050% or less, Nb: 0.005 to 0.050%, Cr: 0.010-0.10% C, Nb, Cr satisfying the following formula (1) and the following formula (2), and having a composition including the balance Fe and unavoidable impurities, and high strength cold rolling excellent in aging resistance and bake hardenability steel sheet.
  • C * C- (12 / 92.9) Nb Cr, Nb, C: Content of each element (mass%) [2]
  • the tensile strength TS suitable for a panel part of an automobile body is 340 MPa or more, generally high strength of 340 to 440 MPa class, low yield ratio, high elongation and excellent formability.
  • a high-strength cold-rolled steel sheet having both excellent aging resistance and excellent bake hardenability can be produced easily and inexpensively, and has a remarkable industrial effect.
  • the high-strength cold-rolled steel sheet according to the present invention can be applied to the inner and outer plates of an automobile body, and has the effect of sufficiently contributing to the reduction of the weight of the automobile body and the improvement of collision safety.
  • the high-strength cold-rolled steel sheet according to the present invention can be applied as a home appliance or a pipe material.
  • C is an important element in the present invention in order to combine excellent aging resistance and excellent BH properties.
  • the content In order to ensure the desired excellent BH property, the content of 0.0010% or more is required.
  • the content exceeds 0.0080%, it is necessary to keep the Nb content high from the viewpoint of securing the desired excellent aging resistance, which leads to an increase in material cost.
  • the Nb content when the Nb content is low, the large content of C as described above may cause aging at room temperature, which reduces aging resistance. For this reason, C is limited to the range of 0.0010 to 0.0080%.
  • Preferably it is less than 0.0060%, More preferably, it is 0.0040% or less.
  • Si 1.0% or less
  • Si is an element that has the effect of increasing the strength of the steel sheet by solid solution strengthening and improving the work hardening ability. In order to acquire such an effect, it is desirable to contain 0.1% or more.
  • a large content exceeding 1.0% tends to generate a red scale during hot rolling to deteriorate the surface appearance of the steel sheet, and further promotes non-plating during galvanization.
  • the above-described large amount of Si also reduces chemical conversion properties. For this reason, Si was limited to 1.0% or less. In addition, Preferably it is 0.5% or less.
  • Mn 0.1-1.8%
  • Mn has a function of increasing the strength of the steel sheet by solid solution and fixing S as MnS to prevent hot cracking due to S. In order to obtain such an effect, a content of 0.1% or more is required. On the other hand, an excessive content exceeding 1.8% lowers the ductility and the r value. Therefore, Mn is limited to the range of 0.1 to 1.8%. In addition, Preferably it is 1.2% or less.
  • P 0.100% or less
  • P has an effect of solid solution and strengthening the steel sheet, but segregates at the grain boundary to reduce secondary work embrittlement resistance and weldability. Such an adverse effect of P becomes prominent with an excessive content exceeding 0.100%. For this reason, P was limited to 0.100% or less. In addition, Preferably it is 0.080% or less.
  • S 0.03% or less S causes hot cracking and exists in the steel as sulfide inclusions, and reduces the ductility and the like of the steel sheet. Therefore, in the present invention, it is desirable to reduce as much as possible, but 0.03% is acceptable. For these reasons, S is limited to 0.03% or less. In addition, Preferably it is 0.01% or less.
  • sol.Al 0.01-0.50% Al acts as a deoxidizer and forms nitrides to fix solute N and improve aging resistance. In order to acquire such an effect, 0.01% or more of content is required. On the other hand, a large content exceeding 0.50% raises the material cost (alloy cost) and causes frequent surface defects. Therefore, sol.Al is limited to the range of 0.01 to 0.50%. In addition, Preferably it is 0.30% or less.
  • N 0.0050% or less
  • N is an element that increases the strength of the steel by solid solution, but inclusion exceeding 0.0050% lowers the aging resistance. Therefore, in the present invention, N is limited to 0.0050% or less.
  • Nb 0.005 to 0.050%
  • Nb has a high ability to form carbides, combines with C to form carbides and fixes C, and has the effect of refining the hot-rolled structure and increasing the r value, thereby contributing to improved formability. In order to acquire such an effect, 0.005% or more of content is required. On the other hand, an excessive content exceeding 0.050% increases the hot deformation resistance and increases the rolling load during hot rolling. Therefore, Nb is limited to the range of 0.005 to 0.050%. In addition, Preferably it is 0.006% or more. In addition, Nb is contained in the above-mentioned range and adjusted to satisfy the following formula (1) from the meaning of fixing C.
  • C * is the amount of C that is not fixed by Nb (theoretical value) (mass%), and means the maximum value of the solid solution C amount when all Nb is precipitated as NbC.
  • the desired BH property cannot be secured if C * is 0 or less. For this reason, the Nb content is adjusted in relation to the C content so that C * exceeds 0.
  • Cr 0.010-0.10% Cr is not as strong as Nb, but is an element that interacts with C. Cr traps (captures) C at room temperature and delays its diffusion. On the other hand, Cr separates from C and contributes to the improvement of BH properties during the coating baking process. In order to acquire such an effect, Cr needs to contain 0.010% or more. On the other hand, if the content exceeds 0.10%, C is excessively fixed and the BH property is lowered. For this reason, Cr is limited to the range of 0.010 to 0.10%. In the present invention, Cr is contained within the above-described content range and adjusted according to the amount of C not fixed with Nb so as to satisfy the following formula (2), that is, according to C *. To do.
  • Cu 0.010 to 0.050%
  • Cu like Cr, is not as strong as Nb, but is an element that interacts with C. Like Cr, Cu traps (captures) C at room temperature and delays its diffusion. On the other hand, Cu dissociates from C and contributes to the improvement of BH properties during paint baking. In order to acquire such an effect, Cu needs to contain 0.010% or more. On the other hand, if the content exceeds 0.050%, C is excessively fixed and the BH property is lowered. Therefore, Cu is limited to the range of 0.010 to 0.050%.
  • Cu is contained within the above-described range and adjusted according to C * so as to satisfy the following formula (3).
  • B 0.0050% or less may be further contained.
  • B: 0.0050% or less B is an element that segregates at the grain boundaries and improves the secondary work embrittlement resistance.
  • it is desirable to contain 0.0003% or more, but even if it exceeds 0.0050%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous.
  • Become For this reason, when it contains, it is preferable to limit B to 0.0050% or less. More preferably, it is 0.0030% or less.
  • the balance other than the above components contains Fe and inevitable impurities.
  • unavoidable impurities Ca: 0.01% or less, REM: 0.01% or less, Sb: 0.01% or less, Sn: 0.1% or less, Zn: 0.01% or less are acceptable.
  • the cold rolled steel sheet of the present invention has a ferrite single phase structure.
  • the term “ferrite single phase structure” as used herein refers to a case where the ferrite phase contains 95% or more by volume. In addition, Preferably it is 98% or more.
  • the second phase other than the ferrite phase include martensite, bainite, and pearlite.
  • a steel material having the above-described composition is used as a starting material, the steel material is heated and hot-rolled to form a hot-rolled sheet, and the hot-rolled sheet is cold-rolled and cold-rolled sheet A cold rolling process, an annealing process for subjecting the cold-rolled sheet to a cold-rolled annealed sheet, and a pressure-adjusting process for subjecting the cold-rolled annealed sheet to temper rolling, in order. It is a rolled steel sheet.
  • the manufacturing method of the steel material is not particularly limited, but the molten steel having the above composition is melted by a conventional melting method such as a converter method or an electric furnace method, and a conventional method such as a continuous casting method is used. It is preferable to use a steel material such as a slab by the casting method.
  • the casting method of the steel material (slab) is preferably a continuous casting method in order to prevent macro segregation of components, but there is no problem even with the ingot casting method or the thin slab casting method.
  • the obtained steel material (slab) is then subjected to a hot rolling process.
  • Heating for hot rolling is preferably a method in which the steel material (slab) is once cooled to room temperature and then reheated, but is not cooled to room temperature, but is charged in a heating furnace as it is.
  • energy-saving processes such as direct feed rolling and direct rolling, in which rolling is performed immediately after performing a slight heat retention, can be applied without any problem.
  • the hot rolling step is preferably a step of heating the steel material to a predetermined temperature, subjecting the steel material to hot rolling consisting of rough rolling and finish rolling to form a hot rolled plate, and then winding it.
  • the heating temperature of the steel material is preferably 1000 to 1300 ° C. When the heating temperature is less than 1000 ° C., the deformation resistance is high and the rolling load increases, and the risk of trouble occurring during hot rolling increases. On the other hand, at a high temperature exceeding 1300 ° C., the scale loss increases as the oxidized weight increases. For this reason, the heating temperature of the steel material is preferably limited to a range of 1000 to 1300 ° C.
  • the heated steel material is made into a sheet bar by rough rolling.
  • the conditions for rough rolling need not be particularly limited, and can be performed according to a conventional method. From the viewpoint of lowering the steel material heating temperature and preventing problems during hot rolling, it is an effective method to use a so-called sheet bar heater that heats the sheet bar.
  • the finish rolling finish temperature is preferably 860 ° C. or higher. This temperature is the surface temperature of the steel sheet. This is to obtain a fine hot-rolled sheet structure so that excellent deep drawability can be secured after cold rolling and recrystallization annealing.
  • the finish rolling finish temperature is less than 860 ° C, the surface layer is below the transformation point, coarse grains are formed, the transformation texture from unrecrystallized austenite ( ⁇ ) develops strongly, and the texture develops after cold rolling annealing. Furthermore, the rolling load at the time of hot rolling becomes high. On the other hand, when the finish rolling finish temperature exceeds 980 ° C.
  • the finish rolling finish temperature is limited to 860 ° C. or higher and 980 ° C. or lower. More preferably, the temperature is 880 to 940 ° C.
  • lubrication rolling may be performed between some or all passes of finish rolling. Performing the lubrication rolling is effective from the viewpoint of homogenizing the shape of the steel plate and homogenizing the material.
  • the friction coefficient during the lubrication rolling is preferably in the range of 0.10 to 0.25.
  • the coiling temperature is preferably in the range of 550 to 720 ° C. This temperature is the surface temperature of the steel sheet.
  • the coiling temperature is less than 550 ° C., the precipitation of NbC becomes insufficient.
  • the coiling temperature is higher than 720 ° C., the r value after cold rolling annealing may be hindered.
  • the winding temperature is preferably limited to a range of 550 to 720 ° C. The temperature is more preferably 660 ° C. or lower.
  • the hot-rolled sheet that has finished the hot-rolling process is then subjected to a cold-rolling process.
  • pickling suitably before a cold rolling process.
  • the pickling method any conventional method can be applied.
  • the rolling reduction of the cold rolling is preferably 50% or more.
  • the rolling reduction ratio is less than 50%, the ⁇ 111 ⁇ recrystallization texture does not develop, and the deep drawability may decrease. is there.
  • the r value increases.
  • the cold rolling reduction ratio is preferably 50% or more and 85% or less.
  • the cold-rolled sheet that has finished the cold-rolling process is subjected to an annealing process that is subjected to an annealing process to form a cold-rolled annealed sheet.
  • the annealing treatment is preferably a continuous annealing line, and the cold-rolled sheet is soaked at a temperature in the range of 760 to 900 ° C., and then the average cooling rate up to 300 ° C. is 10 ° C./s or more. As a cooling process, it is preferable.
  • the annealing temperature is less than 760 ° C.
  • an unrecrystallized structure remains and ductility decreases.
  • the annealing temperature is preferably limited to a temperature in the range of 760 to 900 ° C.
  • the temperature is more preferably 800 ° C. or higher.
  • the cooling rate after soaking is less than 10 ° C./s at an average cooling rate up to 300 ° C., the cooling is too slow, so that solid solution C is reprecipitated and the BH property is lowered.
  • the cooling rate after soaking is limited to 10 ° C./s or more at an average cooling rate up to 300 ° C. More preferably, it is 40 ° C./s or less.
  • the structure of the cold-rolled annealed sheet after the annealing process is a ferrite single-phase structure.
  • a second phase other than the ferrite phase such as a martensite phase
  • the aging resistance changes, and the subsequent production conditions for temper rolling and low-temperature heat treatment deviate from the optimum conditions in the present invention.
  • the cold-rolled annealed plate after the annealing process may be subjected to a plating process such as electroplating.
  • a plating process such as electroplating.
  • the plating treatment include pure zinc plating treatment, zinc-based alloy plating treatment with zinc as the main component and addition of alloy elements, or Al-based alloy plating treatment with addition of alloy elements as the main component of Al or Al.
  • the cold-rolled annealed plate that has been subjected to the annealing step or the cold-rolled annealed plate (plated plate) that has been subjected to a plating treatment is subjected to a temper rolling step.
  • temper rolling is performed on the cold-rolled annealed plate or plated plate for the purpose of shape correction, adjustment of surface roughness, and the like.
  • the elongation in temper rolling is preferably in the range of 0.5 to 2.0%. If the elongation rate is less than 0.5%, the intended purpose of shape correction and surface roughness adjustment cannot be achieved.
  • the elongation ratio in temper rolling is preferably 0.5 to 2.0%. In addition, More preferably, it is 1.0% or less.
  • Molten steel having the composition shown in Table 1 was melted in a converter, which is a conventional melting furnace, and made into a slab (steel material: wall thickness 250 mm) by a conventional continuous casting method. These slabs were heated to 1250 ° C. and roughly rolled into sheet bars. Then, these sheet bars were subjected to finish rolling under the conditions shown in Table 2 to form a hot rolled sheet and wound in a coil shape. Next, the hot-rolled sheet wound in a coil shape was subjected to a pickling treatment to remove the surface scale, and then a cold-rolling step of performing cold rolling with a cold reduction ratio of 70% was performed to obtain a cold-rolled sheet.
  • These cold-rolled sheets were further subjected to an annealing process using a continuous annealing line to obtain cold-rolled annealed sheets under the conditions shown in Table 2. Furthermore, these cold-rolled annealed sheets were subjected to a temper rolling step for temper rolling under the conditions shown in Table 2.
  • Specimens were collected from the obtained cold-rolled annealed plates and subjected to a structure observation, a tensile test, a bake hardenability test, and an aging test to investigate the microstructure, tensile properties, seizure hardenability, and aging resistance.
  • the test method was as follows.
  • Microstructure observation A specimen for microstructural observation is collected from the obtained cold-rolled annealed plate, a cross section (L cross section) parallel to the rolling direction is polished, Nital corrosion is performed, and an optical microscope (magnification: 400 times) is used. The structure was observed, imaged, and the area ratio of the ferrite phase was determined by image analysis, and this was defined as the volume ratio. In the photomicrograph of the optical microscope, the inside of the ferrite phase grains is not corroded and is white. When there was a second phase that corroded in black other than the ferrite phase, the structure was observed with a scanning electron microscope (magnification: 3000 times), imaged, and the volume fraction of the second phase was determined by image analysis. .
  • the ferrite phase exhibits a slightly black contrast, and the martensite phase as the second phase is observed as particles exhibiting a white contrast.
  • generated to the lamellar form or the point sequence form was made into the pearlite and bainite phase.
  • a fine second phase having a diameter of 0.2 ⁇ m or less was excluded.
  • Each of the inventive examples has a ferrite single phase structure, a tensile strength TS of 340 to 440 MPa class, an elongation El of 40% or more, excellent workability, and an excellent BH amount of 30 MPa or more. It is a high-strength cold-rolled steel sheet that exhibits hardenability, has a low yield elongation of 0.5% or less after aging treatment, and has excellent aging resistance. On the other hand, in comparative examples that are out of the scope of the present invention, the seizure curability is lowered when the BH amount is less than 30 MPa, or the yield elongation exceeding 0.5% occurs after the aging treatment, and the aging resistance is lowered.
  • Steel plate No. 12 has a high Mn content outside the scope of the present invention, and has a bainite-like structure, a low elongation El, and a low formability.

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Abstract

La présente invention concerne une tôle d'acier laminée à froid à haute résistance qui combine résistance au vieillissement et aptitude au durcissement après cuisson. Ladite tôle d'acier contient, en masse, de 0,0010 à 0,0080 % de carbone, jusqu'à 1,0 % de silicium, de 0,1 à 1,8 % de manganèse, jusqu'à 0,100 % de phosphore, de 0,01 à 0,5 % d'aluminium soluble, jusqu'à 0,0050 % d'azote, de 0,005 à 0,050 % de niobium en une relation telle que C* = C - (12/92,9)Nb > 0, et de 0,01 à 0,10 % de chrome en une relation telle que Cr > 60C* − 0,03 ou de 0,010 à 0,050 % de cuivre en une relation telle que Cu > 42,8C* + 0,018. Une telle composition permet d'obtenir une tôle d'acier laminée à froid à haute résistance qui présente les caractéristiques suivantes : une haute résistance, une résistance à la traction se situant dans la plage de 340 à 440 MPa ; une excellente aptitude au durcissement après cuisson, le durcissement après cuisson étant d'au moins 30 MPa après l'application d'une précontrainte de 2 % suivie d'un processus de durcissement après peinture de 20 minutes à 170 °C ; et une excellente résistance au vieillissement, un allongement à la limite d'élasticité étant inférieur ou égal à 0,5 % au cours d'un essai de traction après 60 jours de vieillissement naturel à 70 °C.
PCT/JP2012/007771 2011-12-08 2012-12-04 Tôle d'acier laminée à froid à haute résistance ayant d'excellentes caractéristiques de résistance au vieillissement et d'aptitude au durcissement après cuisson WO2013084477A1 (fr)

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JPH07278654A (ja) * 1994-04-08 1995-10-24 Nippon Steel Corp 成形加工性に優れ、塗装焼付け硬化性を有し、かつ幅方向の塗装焼付け硬化性の変動の少ない自動車用高強度冷延鋼板の製造方法
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JPH07278654A (ja) * 1994-04-08 1995-10-24 Nippon Steel Corp 成形加工性に優れ、塗装焼付け硬化性を有し、かつ幅方向の塗装焼付け硬化性の変動の少ない自動車用高強度冷延鋼板の製造方法
JP2004043884A (ja) * 2002-07-11 2004-02-12 Jfe Steel Kk 低温焼付硬化性および耐時効性に優れる加工用薄鋼板
JP2009102673A (ja) * 2007-10-22 2009-05-14 Jfe Steel Corp 高張力冷延鋼板、高張力亜鉛めっき鋼板およびそれらの製造方法

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