WO2012020511A1 - 加工性および耐衝撃性に優れた高強度冷延鋼板およびその製造方法 - Google Patents
加工性および耐衝撃性に優れた高強度冷延鋼板およびその製造方法 Download PDFInfo
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- WO2012020511A1 WO2012020511A1 PCT/JP2010/063949 JP2010063949W WO2012020511A1 WO 2012020511 A1 WO2012020511 A1 WO 2012020511A1 JP 2010063949 W JP2010063949 W JP 2010063949W WO 2012020511 A1 WO2012020511 A1 WO 2012020511A1
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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
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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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
- 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")
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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
- 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
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- 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
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- 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
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- 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
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- 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
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- 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
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- 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
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- 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
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- 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
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- 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
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- 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
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- 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
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
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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
- 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
- the present invention relates to a high-strength cold-rolled steel sheet excellent in formability for skeletal members and suspension members mainly used in the automotive industry and a method for producing the same.
- Patent Document 1 discloses a method for producing a high-strength steel sheet excellent in workability that secures retained austenite and achieves high ductility by adding a large amount of Si.
- these DP steels and TRIP steels are excellent in elongation characteristics, there is a problem that the hole expandability is inferior.
- Hole expansibility is an index indicating workability when a processed hole is expanded to form a flange, and is an important characteristic required for high-strength steel sheets together with elongation characteristics.
- Patent Document 2 discloses a technique for improving the hole expandability by annealing and soaking, followed by quenching and tempering to obtain a composite structure of ferrite and tempered martensite. ing.
- Patent Document 2 discloses a technique for improving the hole expandability by annealing and soaking, followed by quenching and tempering to obtain a composite structure of ferrite and tempered martensite. ing.
- Such techniques can provide high hole expansibility, low elongation is a problem.
- the present invention has been made paying attention to the above-mentioned problems, and an object of the present invention is to provide a high-strength cold-rolled steel sheet excellent in ductility and stretch flangeability, and a method for producing the same.
- the present inventors have conducted earnest research from the viewpoint of the composition and microstructure of the steel sheet.
- the alloy elements are appropriately adjusted, and when cooling from the soaking temperature in the annealing process, the alloy element is strongly cooled to a temperature range of 150 to 350 ° C., and then reheated, whereby the ferrite is 20% or more in area ratio, It was found that a structure containing 10 to 60% tempered martensite and 3 to 15% residual austenite at a volume ratio was obtained, and high ductility and stretch flangeability became possible.
- both high ductility and high stretch flangeability can be achieved.
- the details of why high stretch flangeability is possible even in the presence of retained austenite are unclear, but the coexistence of retained austenite with tempered martensite reduces the adverse effect of retained austenite on stretch flangeability. it is conceivable that.
- the impact resistance characteristics can be improved in addition to high workability by making the steel sheet structure having an average crystal grain size of 3 ⁇ m or less of the low temperature transformation phase composed of martensite, tempered martensite and retained austenite. .
- the present invention has been made based on the above-described findings, and the gist thereof is as follows.
- 1st invention is the mass%, C: 0.05-0.3%, Si: 0.3-2.5%, Mn: 0.5-3.5%, P: 0.003-0 100%, S: 0.02% or less, Al: 0.010 to 0.5%, the balance is iron and inevitable impurities, and ferrite is 20% or more in area ratio, tempered martensite A high-strength cold-rolled steel sheet having a structure containing 10 to 60%, martensite 0 to 10%, and a volume ratio of 3 to 15% retained austenite and excellent workability and impact resistance.
- the second invention is a highly excellent workability and impact resistance according to the first invention having a structure in which the average crystal grain size of the low-temperature transformation phase comprising the martensite, tempered martensite, and retained austenite is 3 ⁇ m or less. It is a strength cold-rolled steel sheet.
- the third invention further includes, in mass%, Cr: 0.005 to 2.00%, Mo: 0.005 to 2.00%, V: 0.005 to 2.00%, Ni: 0.005.
- the processing according to the first invention or the second invention characterized by containing one or more elements selected from -2.00% and Cu: 0.005-2.00% It is a high-strength cold-rolled steel sheet excellent in heat resistance and impact resistance.
- the fourth invention further includes one or two elements selected from Ti: 0.01 to 0.20% and Nb: 0.01 to 0.20% by mass%.
- the fifth aspect of the invention further includes the workability and impact resistance according to any one of the first to fourth aspects of the invention, characterized by containing B: 0.0002 to 0.005% by mass%. It is an excellent high-strength cold-rolled steel sheet.
- the sixth invention further includes one or two elements selected from Ca: 0.001 to 0.005% and REM: 0.001 to 0.005% by mass%.
- a temperature of 750 ° C. or higher is applied. After holding at temperature for 10 seconds or more, after cooling from 750 ° C. to a temperature range of 150 to 350 ° C. at an average cooling rate of 10 ° C./s or more, heating to 350 to 600 ° C. and holding for 10 to 600 seconds, It is a method for producing a high-strength cold-rolled steel sheet excellent in workability and impact resistance, characterized by cooling to a low temperature.
- the eighth invention is a high strength excellent in workability and impact resistance according to the seventh invention, characterized in that the temperature is raised at an average heating rate from 500 ° C. to Ac 1 transformation point at 10 ° C./s or more. It is a manufacturing method of a cold-rolled steel plate.
- a high-strength cold-rolled steel sheet having excellent workability can be obtained, and it is possible to achieve both the weight reduction of the automobile and the improvement of the collision safety, and the excellent effect of greatly contributing to the enhancement of the performance of the automobile body. Play.
- C 0.05 to 0.3%
- C is an element that stabilizes austenite, and is an element necessary for increasing the steel sheet strength and improving the TS-EL balance by increasing the steel sheet strength in order to easily generate phases other than ferrite. If the amount of C is less than 0.05%, it is difficult to secure phases other than ferrite even if the production conditions are optimized, and TS ⁇ EL decreases. On the other hand, when the amount of C exceeds 0.3%, the welded portion and the heat affected zone are significantly hardened, and the mechanical properties of the welded portion are deteriorated. From this point of view, the C content is in the range of 0.05 to 0.3%. Preferably it is 0.08 to 0.15% of range.
- Si 0.3 to 2.5%
- Si is an element effective for strengthening steel. Further, it is a ferrite-forming element and has the function of promoting the formation of retained austenite because it promotes the concentration of C in austenite and suppresses the formation of carbides. If the amount of Si is less than 0.3%, the effect of addition becomes poor, so the lower limit was made 0.3%. However, excessive addition deteriorates the surface properties and weldability, so Si was contained at 2.5% or less. Preferably it is 0.7 to 2.0% of range.
- Mn 0.5 to 3.5%
- Mn is an element effective for strengthening steel and promotes the generation of low-temperature transformation phases such as tempered martensite. Such an effect is recognized when the Mn content is 0.5% or more.
- the Mn content is set to a range of 0.5 to 3.5%. Preferably, it is in the range of 1.5 to 3.0%.
- P 0.003 to 0.100%
- P is an element effective for strengthening steel, and this effect is obtained at 0.003% or more.
- excessive addition over 0.100% causes embrittlement due to grain boundary segregation and degrades impact resistance. Therefore, the P content is in the range of 0.003% to 0.100%.
- S 0.02% or less S is an inclusion such as MnS, which causes deterioration of impact resistance and cracks along the metal flow of the weld. 0.02% or less.
- Al acts as a deoxidizer and is an element effective for the cleanliness of steel, and is preferably added in the deoxidation step. If the amount of Al is less than 0.01%, the effect of addition becomes poor, so the lower limit was made 0.01%. However, if added in a large amount, the risk of steel piece cracking during continuous casting increases and the productivity decreases. Therefore, the upper limit of the amount of Al is 0.5%.
- the high-strength cold-rolled steel sheet in the present invention has the above-described component composition as a basic component and the balance is composed of iron and unavoidable impurities, but can appropriately contain the components described below according to desired characteristics.
- Cr 0.005 to 2.00%
- Mo 0.005 to 2.00%
- V 0.005 to 2.00%
- Ni 0.005 to 2.00%
- Cu 0.005 to
- One or more selected from 2.00% Cr, Mo, V, Ni, Cu suppresses the formation of pearlite during cooling from the annealing temperature, promotes the formation of low-temperature transformation phase, and strengthens the steel Works effectively. This effect is obtained by adding 0.005% or more of at least one of Cr, Mo, V, Ni, and Cu. However, when each component of Cr, Mo, V, Ni, and Cu exceeds 2.00%, the effect is saturated, resulting in a cost increase. Accordingly, the amounts of Cr, Mo, V, Ni, and Cu are each in the range of 0.005 to 2.00%.
- Ti and Nb form carbonitrides and strengthen steel by precipitation strengthening Have the effect of Such an effect is recognized at 0.01% or more.
- Ti and Nb are contained in amounts exceeding 0.20%, the strength is excessively increased and the ductility is lowered. For this reason, the amounts of Ti and Nb are each in the range of 0.01 to 0.20%.
- B 0.0002 to 0.005%
- B has the effect of suppressing the formation of ferrite from the austenite grain boundaries and increasing the strength. The effect is obtained at 0.0002% or more. However, if the amount of B exceeds 0.005%, the effect is saturated, which causes a cost increase. Therefore, the B content is in the range of 0.0002 to 0.005%.
- Ca and REM both have the effect of improving workability by controlling the form of sulfide. And can contain 0.001% or more of one or two of Ca and REM as required. However, excessive addition may adversely affect cleanliness, so each content is made 0.005% or less.
- Ferrite area ratio 20% or more If the area ratio of ferrite is less than 20%, TS ⁇ EL decreases, so it is 20% or more. Preferably it is 50% or more.
- Tempered martensite area ratio 10-60%
- Ac 1 transformation point martensite and tempered martensite preferably an complex structure of high ferrite and cementite dislocation density obtained by heating to a temperature lower than the Ac 1 transformation point, effectively strengthening the steel work.
- the structure obtained by heating martensite to a temperature exceeding the Ac 1 transformation point is a structure not containing cementite in the ferrite, and is basically different from the tempered martensite intended in the present invention.
- tempered martensite has a smaller adverse effect on hole expansibility than martensite, and is an effective phase for ensuring strength without a significant decrease in hole expansibility. If the area ratio of tempered martensite is less than 10%, it is difficult to ensure strength, and if it exceeds 60%, TS ⁇ EL decreases, so the area ratio of tempered martensite is 10 to 60%.
- Martensite area ratio 0-10% Martensite works effectively to increase the strength of steel, but if the area ratio exceeds 10%, stretch flangeability is significantly reduced. Therefore, the area ratio of martensite is set to 0 to 10%.
- volume ratio of retained austenite 3-15% Residual austenite not only contributes to the strengthening of the steel, but also works effectively to improve and improve the TS ⁇ EL of the steel. Such an effect is obtained when the volume ratio is 3% or more. On the other hand, when the retained austenite exceeds 15%, the hole expansibility decreases. Therefore, the volume ratio of retained austenite is 3 to 15%.
- Average crystal grain size of low-temperature transformation phase composed of martensite, tempered martensite, and retained austenite 3 ⁇ m or less
- the low-temperature transformation phase composed of martensite, tempered martensite, and retained austenite works effectively to improve impact resistance.
- the impact resistance is improved by finely dispersing the low-temperature transformation phase, and the effect becomes remarkable when the average crystal grain size of the low-temperature transformation phase is 3 ⁇ m or less. Therefore, the average crystal grain size of the low temperature transformation phase is set to 3 ⁇ m or less.
- phases other than ferrite, tempered martensite, martensite, and retained austenite may contain pearlite and bainite, but there is no problem as long as the above phase structure is satisfied.
- the pearlite content is desirably 3% or less.
- Manufacturing conditions Steel adjusted to the above component composition is melted in a converter or the like, and is made into a slab by a continuous casting method or the like. This steel material is subjected to continuous annealing after hot rolling and cold rolling.
- the manufacturing method is not particularly limited, but a suitable manufacturing method is described below. Casting conditions
- the steel slab used is continuously cast to prevent macro segregation of components. However, it may be produced by an ingot-making method or a thin slab casting method.
- Hot rolling conditions Slab heating temperature: 1100 ° C or higher
- low-temperature heating is preferable in terms of energy, but if the heating temperature is less than 1100 ° C, carbides cannot be sufficiently dissolved or heat due to an increase in rolling load. Problems such as an increased risk of trouble during hot rolling occur.
- the slab heating temperature is desirably 1300 ° C. or less because of an increase in scale loss accompanying an increase in oxidized weight.
- a sheet bar heater which heats a sheet bar from a viewpoint of preventing the trouble at the time of hot rolling even if slab heating temperature is made low.
- Finish rolling end temperature Ar 3 transformation point or higher If the finish rolling end temperature is less than the Ar 3 transformation point, ferrite and austenite are generated during rolling, and a band-like structure is easily generated in the steel sheet. It may remain even after hot rolling or after annealing, causing anisotropy in material properties or reducing workability. For this reason, it is desirable that the finish rolling temperature is not less than the Ar 3 transformation point.
- Winding temperature 450 ⁇ 700 °C
- the coiling temperature is less than 450 ° C.
- problems such as a decrease in cold rolling property may occur.
- the coiling temperature exceeds 700 ° C.
- problems such as decarburization may occur in the surface layer of the railway.
- the coiling temperature is preferably in the range of 450 to 700 ° C.
- part or all of the finish rolling may be lubricated rolling in order to reduce the rolling load during hot rolling.
- Performing lubrication rolling is also effective from the viewpoint of uniform steel plate shape and uniform material.
- the friction coefficient during the lubricating rolling is preferably in the range of 0.25 to 0.10.
- the application of the continuous rolling process is also desirable from the viewpoint of the operational stability of hot rolling.
- pickling conditions and cold rolling conditions are not particularly limited, and may be in accordance with conventional methods.
- the rolling reduction of cold rolling is preferably 40% or more.
- Average heating rate at 500 ° C. to Ac 1 transformation point 10 ° C./s or more
- average heating rate at the Ac 1 transformation point from 500 ° C. which is the recrystallization temperature range in the steel of the present invention to 10 ° C./s or more
- Recrystallization at the time of heating and heating is suppressed, and it effectively works for refining austenite generated at the Ac 1 transformation point or higher, and thus for refining the structure after annealing and cooling, and the average grain size of the low temperature transformation phase is 3 ⁇ m or less. It becomes possible.
- the average heating rate at 500 ° C. to Ac 1 transformation point is set to 10 ° C./s or more.
- a preferable range of the average heating rate is 20 ° C./s or more.
- the heating temperature is lower than 750 ° C. or holding time is shorter than 10 seconds, austenite is not sufficiently generated during annealing, and a sufficient amount of low-temperature transformation phase cannot be secured after annealing cooling. .
- the upper limit of the holding temperature and holding time is not particularly specified, but if the holding temperature is 900 ° C. or more and the holding time is 600 seconds or more, the effect is saturated and the cost is increased. Therefore, the holding temperature is less than 900 ° C. and the holding time is 600 Less than a second is preferred.
- the cooling rate from 750 ° C. is set to 10 ° C./s or more.
- Cooling temperature condition is one of the most important conditions in this technology. When the cooling is stopped, a part of austenite is transformed into martensite, and the rest becomes untransformed austenite. By reheating from there and cooling to room temperature after plating and alloying treatment, martensite becomes tempered martensite and untransformed austenite becomes retained austenite or martensite.
- the final martensite and residual austenite and tempered martensite are controlled by controlling the cooling temperature. Is determined.
- the temperature reached by cooling is higher than 350 ° C.
- the martensite transformation at the time of cooling stop is insufficient and the amount of untransformed austenite increases, resulting in excessive formation of final martensite or residual austenite, which decreases hole expandability.
- the temperature reached by cooling is lower than 150 ° C.
- most of the austenite is transformed into martensite during cooling and the amount of untransformed austenite is reduced, and 3% or more of retained austenite cannot be obtained. Therefore, the temperature at which the cooling is achieved is in the range of 150 to 350 ° C.
- any cooling method such as gas jet cooling, mist cooling, water cooling, or metal quenching may be used as long as the target cooling rate and cooling stop temperature can be achieved.
- the martensite generated during the cooling is tempered
- the hole expandability is improved, and untransformed austenite that has not been transformed into martensite at the time of cooling is stabilized, and finally 3% or more of retained austenite is obtained, and ductility is improved. improves.
- the heating temperature is 350 to 600 ° C.
- the reheating temperature is in the range of 350 to 600 ° C., and the holding time in that temperature range is 10 to 600 seconds.
- the annealed steel sheet may be subjected to temper rolling for adjustment of shape correction, surface roughness, and the like.
- the cross-sectional microstructure of the steel sheet is revealed with a 3% nital solution (3% nitric acid + ethanol), the depth direction plate thickness 1/4 position is observed with a scanning electron microscope, and the photographed structure photograph is used.
- Image analysis processing was performed to quantify the ferrite phase fraction (in addition, commercially available image processing software can be used for image analysis processing).
- the martensite area ratio and tempered martensite area ratio were quantified with image processing software by taking SEM photographs at an appropriate magnification of 1000 to 3000 times depending on the fineness of the structure.
- the average particle size of the low-temperature transformation phase was obtained by dividing the area of the low-temperature transformation phase in the observed visual field by the number of low-temperature transformation phases to obtain an average area, and the 1/2 power was taken as the average particle size.
- the volume ratio of retained austenite was determined by polishing the steel plate to a 1 ⁇ 4 surface in the plate thickness direction and diffracting X-ray intensity of the 1 ⁇ 4 surface thickness. MoK ⁇ rays are used as incident X-rays, and the peaks of ⁇ 111 ⁇ , ⁇ 200 ⁇ , ⁇ 220 ⁇ , ⁇ 311 ⁇ planes of retained austenite and ⁇ 110 ⁇ , ⁇ 200 ⁇ , ⁇ 211 ⁇ planes of ferrite phases are used. Intensity ratios were determined for all combinations of integrated intensities, and the average value of these ratios was taken as the volume fraction of retained austenite.
- the tensile properties were determined by performing a tensile test in accordance with JISZ2241, using a JIS No. 5 test piece sampled so that the tensile direction was perpendicular to the rolling direction of the steel sheet, and TS (tensile strength) and EL (elongation).
- TS tensile strength
- EL elongation
- the strength-elongation balance value represented by the product of strength and elongation (TS ⁇ EL) was determined.
- the hole expansion rate ⁇ was measured as an index for evaluating stretch flangeability.
- the hole expansion rate ⁇ is determined by performing a hole expansion test in accordance with the Japan Iron and Steel Federation Standard JFST1001, and the hole diameter at the time of punching and punching through the hole thickness penetrates the plate thickness. It was determined from the ratio with the diameter of the hole at the time of the test.
- the shock absorption characteristic is obtained by distorting a stress-true strain curve obtained by performing a tensile test at a strain rate of 2000 / s using a test piece having a width of 5 mm and a length of 7 mm taken from a direction perpendicular to the rolling direction of the steel sheet. Absorption energy was calculated and evaluated by integrating in the range of 0 to 10% (see Iron and Steel, 83 (1997) p748).
- the steel plate of the present invention shows excellent strength, ductility and stretch flangeability with TS ⁇ EL of 22000 MPa ⁇ % or more and ⁇ of 70% or more.
- the steel plate of the comparative example which deviates from the scope of the present invention has TS ⁇ EL of less than 22000 MPa ⁇ % and / or ⁇ of less than 70%, and has excellent strength, ductility and stretch flangeability like the steel plate of the present invention. Cannot be obtained. Furthermore, by setting the average particle size of the low-temperature transformation phase to 3 ⁇ m or less, excellent impact resistance characteristics with a ratio of absorbed energy to TS (AE / TS) of 0.063 or more can be obtained.
- the present invention can contribute to reducing the weight and fuel consumption of automobiles as a high-strength cold-rolled steel sheet excellent in workability and impact resistance.
Abstract
Description
しかし、これらDP鋼やTRIP鋼は伸び特性には優れるものの穴拡げ性が劣るという問題がある。穴拡げ性は加工穴部を拡張してフランジ成形させるときの加工性を示す指標で、伸び特性と共に高強度鋼板に要求される重要な特性である。
本発明は、上記した知見に基づいてなされたもので、その要旨は以下の通りである。
まず、本発明において鋼の成分組成を上記の範囲に限定した理由について説明する。なお、成分に関する%表示は特に断らない限り質量%を意味するものとする。
Cはオーステナイトを安定化させる元素であり、フェライト以外の相を生成しやすくするため、鋼板強度を上昇させるとともに、組織を複合化してTS−ELバランスを向上させるために必要な元素である。C量が0.05%未満では製造条件の最適化を図ったとしてもフェライト以外の相の確保が難しく、TS×ELが低下する。一方、C量が0.3%を超えると、溶接部および熱影響部の硬化が著しく、溶接部の機械的特性が劣化する。こうした観点からC量は、0.05~0.3%の範囲とする。好ましくは0.08~0.15%の範囲である。
Siは鋼の強化に有効な元素である。また、フェライト生成元素であり、オーステナイト中へのCの濃化促進および炭化物の生成を抑制することから、残留オーステナイトの生成を促進する働きを有する。ここに、Si量が0.3%に満たないとその添加効果に乏しくなるので、下限を0.3%とした。ただし過剰な添加は、表面性状、溶接性を劣化させるので、Siは2.5%以下で含有させるものとした。好ましくは0.7~2.0%の範囲である。
Mnは鋼の強化に有効な元素であり、焼戻しマルテンサイト等の低温変態相の生成を促進する。このような作用は、Mn含有量が0.5%以上で認められる。ただし、Mnを3.5%を超えて過剰に添加すると、第二相分率の過剰な増加や固溶強化によるフェライトの延性劣化が著しくなり成形性が低下する。従って、Mn量を0.5~3.5%の範囲とする。好ましくは1.5~3.0%の範囲である。
Pは鋼の強化に有効な元素であり、この効果は0.003%以上で得られる。しかし、0.100%を超えて過剰に添加すると粒界偏析により脆化を引き起こし、耐衝撃性を劣化させる。従って、P量は0.003%~0.100%の範囲とする。
SはMnSなどの介在物となって、耐衝撃特性の劣化や溶接部のメタルフローに沿った割れの原因になるので極力低い方が良いが、製造コストの面から0.02%以下とする。
Alは脱酸剤として作用し、鋼の清浄度に有効な元素であり、脱酸工程で添加することが好ましい。ここに、Al量が0.01%に満たないとその添加効果に乏しくなるので、下限を0.01%とした。しかし多量に添加すると連続鋳造時の鋼片割れ発生の危険性が高まり製造性を低下させる。従ってAlの添加量の上限は0.5%とする。
Cr、Mo、V、Ni、Cuは焼鈍温度からの冷却時にパーライトの生成を抑制し、低温変態相の生成を促進し鋼の強化に有効に働く。この効果は、Cr、Mo、V、Ni、Cuの少なくとも1種を0.005%以上含有させることで得られる。しかし、Cr、Mo、V、Ni、Cuのそれぞれの成分が2.00%を超えるとその効果は飽和し、コストアップの要因となる。従ってCr、Mo、V、Ni、Cuの量はそれぞれ、0.005~2.00%の範囲とする。
Ti、Nbは炭窒化物を形成し、鋼を析出強化により高強度化する作用を有する。このような効果はそれぞれ0.01%以上で認められる。一方、Ti、Nbはそれぞれ0.20%を超えて含有しても、過度に高強度化し、延性が低下する。このため、Ti、Nbの量はそれぞれ0.01~0.20%の範囲とする。
Bはオーステナイト粒界からのフェライトの生成を抑制し強度を上昇させる作用を有する。その効果は0.0002%以上で得られる。しかし、B量が0.005%を超えるとその効果は飽和し、コストアップの要因となる。従って、B量は0.0002~0.005%の範囲とする。
Ca、REMはいずれも硫化物の形態制御により加工性を改善する効果を有しており、必要に応じてCa、REMの1種または2種を0.001%以上含有させることができる。しかしながら過剰な添加は清浄度に悪影響を及ぼす恐れがあるため、それぞれ0.005%以下とする。
次に鋼の組織について説明する。
フェライトの面積率が20%未満だとTS×ELが低下するため20%以上とする。好ましくは50%以上である。
焼戻しマルテンサイトとはマルテンサイトをAc1変態点以下、好ましくはAc1変態点よりも低い温度に加熱して得られる転位密度の高いフェライトとセメンタイトとの複合組織を示し、鋼の強化に有効に働く。また、マルテンサイトをAc1変態点を超える温度に加熱して得られる組織は、フェライト中にセメンタイトを含まない組織であり、本願発明で意図する焼戻しマルテンサイトとは基本的に異なるものである。
マルテンサイトは鋼の高強度化に有効に働くが、その面積率が10%を超えると伸びフランジ性が顕著に低下する。従って、マルテンサイトの面積率は0~10%とする。
残留オーステナイトは鋼の強化に寄与するだけでなく、鋼のTS×ELの向上およびに有効に働く。このような効果は体積率が3%以上で得られる。また、残留オーステナイトが15%を超えると穴拡げ性が低下する。従って、残留オーステナイトの体積率は3~15%とする。
マルテンサイト、焼戻しマルテンサイト、残留オーステナイトからなる低温変態相は耐衝撃特性の向上に有効に働く。特に低温変態相を微細に分散させることにより耐衝撃特性が向上し、低温変態相の平均結晶粒径が3μm以下でその効果が顕著となる。従って低温変態相の平均結晶粒径を3μm以下とする。
上記の成分組成に調整した鋼を転炉などで溶製し、連続鋳造法等でスラブとする。この鋼素材を熱間圧延および冷間圧延を施した後、連続焼鈍を行う。鋳造、熱延圧延、冷間圧延については特に製造方法を限定するものではないが、以下に好適な製造方法について説明する
鋳造条件
使用する鋼スラブは、成分のマクロ偏析を防止するために連続鋳造法で製造するのが好ましいが、造塊法、薄スラブ鋳造法で製造してもよい。また、鋼スラブを製造したのち、いったん室温まで冷却し、その後再度加熱する従来法に加え、室温まで冷却しないで、温片のままで加熱炉に挿入する、あるいはわずかの保熱をおこなった後に直ちに圧延する直送圧延・直接圧延などの省エネルギープロセスも問題なく適用できる。
スラブ加熱温度:1100℃以上
スラブ加熱温度は、低温加熱がエネルギー的には好ましいが、加熱温度が1100℃未満では、炭化物が十分に固溶できなかったり、圧延荷重の増大による熱間圧延時のトラブル発生の危険が増大するなどの問題が生じる。なお、酸化重量の増加にともなうスケールロスの増大などから、スラブ加熱温度は1300℃以下とすることが望ましい。
仕上げ圧延終了温度がAr3変態点未満では、圧延中にフェライトとオーステナイトが生成して、鋼板にバンド状組織が生成し易くなり、かかるバンド状組織は冷間圧延後や焼鈍後にも残留し、材料特性に異方性を生じさせたり、加工性を低下させる原因となる場合がある。このため、仕上げ圧延温度はAr3変態点以上とすることが望ましい。
巻取り温度が450℃未満だと巻取り温度の制御が難しく温度ムラが生じやすくなり、その結果冷間圧延性が低下するなどの問題が生じることがある。また巻取り温度が700℃を超えると地鉄表層で脱炭が生じるなどの問題が起こることがある。このため、巻取り温度は450~700℃の範囲とするのが望ましい。
本発明の鋼における再結晶温度域である500℃からAc1変態点における平均加熱速度を10℃/s以上とすることで、加熱昇温時の再結晶が抑制され、Ac1変態点以上で生成するオーステナイトの微細化、ひいては焼鈍冷却後の組織の微細化に有効に働き、低温変態相の平均粒経を3μm以下とすることが可能となる。
加熱温度が750℃未満あるいは保持時間が10秒未満では、焼鈍時のオーステナイトの生成が不十分となり、焼鈍冷却後に十分な量の低温変態相が確保できなくなる。保持温度および保持時間の上限は特に規定しないが、保持温度が900℃以上および保持時間が600秒以上では効果が飽和する上、コストアップにつながるので、保持温度は900℃未満および保持時間は600秒未満が好ましい。
750℃からの冷却速度が10℃/s未満ではパーライトが生成し、TS×ELおよび穴拡げ性が低下する。従って、750℃からの冷却速度は10℃/s以上とする。冷却到達温度条件は本技術で最も重要な条件の一つである。冷却停止時にはオーステナイトの一部がマルテンサイトに変態し、残りは未変態のオーステナイトとなる。そこから再加熱し、めっき・合金化処理後、室温まで冷却することで、マルテンサイトは焼戻しマルテンサイトとなり、未変態オーステナイトは残留オーステナイトまたはマルテンサイトとなる。焼鈍からの冷却到達温度が低いほど、冷却中に生成するマルテンサイト量が増加し、未変態オーステナイト量が減少するため、冷却到達温度の制御により、最終的なマルテンサイトおよび残留オーステナイトと焼戻しマルテンサイトの面積率が決定される。
150~350℃の温度範囲までの冷却後、350~600℃の温度範囲で10秒以上保持することで、前記冷却時に生成したマルテンサイトが焼戻され焼戻しマルテンサイトとなることで、穴拡げ性が向上し、さらに前記冷却時にマルテンサイトに変態しなかった未変態オーステナイトが安定化され、最終的に3%以上の残留オーステナイトが得られ、延性が向上する。
得られた鋼板の断面ミクロ組織、引張特性および穴拡げ性について調査を行い、その結果を表3に示した。
Claims (8)
- 質量%で、C:0.05~0.3%、Si:0.3~2.5%、Mn:0.5~3.5%、P:0.003~0.100%、S:0.02%以下、Al:0.010~0.5%を含み、残部が鉄および不可避的不純物からなり、かつ、面積率でフェライトを20%以上、焼戻しマルテンサイトを10~60%、マルテンサイトを0~10%、体積率で残留オーステナイトを3~15%含む組織を有する加工性および耐衝撃性に優れた高強度冷延鋼板。
- 前記マルテンサイト、焼戻しマルテンサイト、残留オーステナイトからなる低温変態相の平均結晶粒径が3μm以下の組織を有する請求項1記載の加工性および耐衝撃性に優れた高強度冷延鋼板。
- 更に、質量%で、Cr:0.005~2.00%、Mo:0.005~2.00%、V:0.005~2.00%、Ni:0.005~2.00%、Cu:0.005~2.00%の中から選ばれる1種または2種以上の元素を含有することを特徴とする請求項1または2に記載の加工性および耐衝撃性に優れた高強度冷延鋼板。
- 更に、質量%で、Ti:0.01~0.20%、Nb:0.01~0.20%の中から選ばれる1種または2種の元素を含有することを特徴とする請求項1~3のいずれかに記載の加工性および耐衝撃性に優れた高強度冷延鋼板。
- 更に、質量%で、B:0.0002~0.005%を含有することを特徴とする請求項1~4のいずれかに記載の加工性および耐衝撃性に優れた高強度冷延鋼板。
- 更に、質量%で、Ca:0.001~0.005%、REM:0.001~0.005%の中から選ばれる1種または2種の元素を含有することを特徴とする請求項1~5のいずれかに記載の加工性および耐衝撃性に優れた高強度冷延鋼板。
- 請求項1~6のいずれかに記載の成分を有するスラブに熱間圧延および冷間圧延を施して製造した冷延鋼板に連続焼鈍を施すに際し、750℃以上の温度で10秒以上保持した後、750℃から平均10℃/s以上の冷却速度で150~350℃の温度域まで冷却した後、350~600℃まで加熱し10~600秒保持した後、室温まで冷却することを特徴とする加工性および耐衝撃性に優れた高強度冷延鋼板の製造方法。
- 500℃~Ac1変態点における平均加熱速度を10℃/s以上で昇温することを特徴とする請求項7に記載の加工性および耐衝撃性に優れた高強度冷延鋼板の製造方法。
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Also Published As
Publication number | Publication date |
---|---|
EP2604715A1 (en) | 2013-06-19 |
CA2805834C (en) | 2016-06-07 |
US20130133792A1 (en) | 2013-05-30 |
MX2013001456A (es) | 2013-04-29 |
KR20130036763A (ko) | 2013-04-12 |
CA2805834A1 (en) | 2012-02-16 |
EP2604715B1 (en) | 2019-12-11 |
CN103069040A (zh) | 2013-04-24 |
EP2604715A4 (en) | 2017-12-13 |
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