WO2016147549A1 - 高強度冷延鋼板およびその製造方法 - Google Patents
高強度冷延鋼板およびその製造方法 Download PDFInfo
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
- C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
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- C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
- C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
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- C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
- C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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- C21D8/0273—Final recrystallisation annealing
Definitions
- the present invention relates to a high-strength cold-rolled steel sheet that has a tensile strength (TS) of 1300 MPa or more and is excellent in chemical conversion treatment and workability, and a method for producing the same, useful for the use of automobile members.
- TS tensile strength
- automotive steel sheets are used after being coated, and chemical conversion treatment such as phosphate treatment is performed as a pretreatment for the coating. Since the chemical conversion treatment of this steel plate is one of the important treatments for ensuring the corrosion resistance after coating, the automotive steel plate is also required to have excellent chemical conversion treatment properties.
- Patent Document 1 the balance between strength and ductility is improved by adding a large amount of C.
- a large amount of C is added, stretch flangeability is deteriorated due to the hardness difference between the two phases.
- Patent Document 2 Si is utilized. However, when a large amount of Si is added, in the case of the production method described in Patent Document 2, it is speculated that Si oxide is formed on the surface of the steel sheet in the continuous annealing line, and the chemical conversion property is deteriorated. It is not preferable.
- Patent Document 3 by adding a large amount of Mn, the Si-Mn composite oxide is finely dispersed on the steel sheet surface and used as a nucleation site for zinc phosphate crystals, and the steel sheet surface SiO 2 is reduced as much as possible. To ensure chemical conversion. However, it is difficult to achieve a tensile strength of 1300 MPa and an elongation of 10% or more with the amounts of C and Si described in Patent Document 3.
- an object of the present invention is to provide a high-strength cold-rolled steel sheet having a tensile strength of 1300 MPa or more and excellent in chemical conversion property and workability, and a method for producing the same.
- the microstructure needs to be a martensite single phase structure or a ferrite-martensite composite structure.
- optimization of component design, structure control, etc. is important to achieve both high strength and workability.
- Mn is effective for increasing the strength of steel sheets.
- Mn is added more than necessary, it segregates during casting, and a steel structure in which ferrite and martensite are distributed in a band shape is formed. For this reason, anisotropy occurs in the mechanical characteristics, and the workability deteriorates.
- Component composition is mass%, C: 0.15% to 0.22%, Si: 1.0% to 2.0%, Mn: 1.7% to 2.5%, P: 0.05% or less, S: 0.02% or less, Al: 0.01% or more and 0.05% or less, N: 0.005% or less, satisfying the following formula (1), the balance is composed of iron and inevitable impurities, the structure is area ratio, tempered martensite is 60% More than 100%, untransformed austenite is 5% or less (including 0%), the balance is ferrite, and the average crystal grain size of the ferrite is less than 3.5 ⁇ m.
- [Si] represents the Si content (mass%)
- [Mn] represents the Mn content (mass%).
- the composition contains one or more of% by mass: V: 0.01% to 0.30%, Mo: 0.01% to 0.30%, Cr: 0.01% to 0.30% [1 ]
- Cu includes 0.01% or more and 0.30% or less, and Ni: 0.01% or more and 0.30% or less in mass%.
- Sn 0.001% to 0.100%
- Sb 0.001% to 0.100%
- Ca 0.0002% to 0.0100%
- W 0.01% to 0.10%
- the high-strength cold-rolled steel sheet according to any one of the above [1] to [6] which contains at least one of Co: 0.01% to 0.10% and REM: 0.0002% to 0.0050%.
- a steel material having the composition described in any one of [1] to [7] above is heated to a temperature of 1200 ° C. or higher, and then hot rolled to a finish rolling outlet temperature of 800 ° C. or higher. Winding at a temperature of 450 ° C to 700 ° C, cold rolling, then heating to an annealing temperature of Ac 1 point or more and Ac 3 point or less, and the residence time in the temperature range from Ac 1 point to Ac 3 point 30 seconds or more and 1200 seconds or less, the primary cooling from the annealing temperature to the primary cooling stop temperature of 600 ° C or higher at an average cooling rate of less than 100 ° C / s, and the secondary cooling stop temperature of 100 ° C or lower to the average cooling rate An annealing treatment is performed to perform secondary cooling at 100 ° C./s or more and 1000 ° C./s or less, and then heating is performed to a temperature of 100 ° C.
- the high-strength cold-rolled steel sheet is a cold-rolled steel sheet having a tensile strength (TS) of 1300 MPa or more.
- a high-strength cold-rolled steel sheet having a tensile strength of 1300 MPa or more and excellent in chemical conversion property and workability can be obtained.
- the high-strength cold-rolled steel sheet of the present invention has a tensile strength of 1300 MPa or more and is excellent in chemical conversion treatment and workability, so it can be suitably used for the use of automobile structural members, etc. The effect is remarkable, such as improving its reliability.
- C 0.15% to 0.22%
- C is an element effective for improving the balance between strength and ductility of the steel sheet. If the C content is less than 0.15%, it is difficult to ensure a tensile strength of 1300 MPa or more. On the other hand, when the C content exceeds 0.22%, coarse cementite precipitates and workability such as stretch flangeability deteriorates. Therefore, the C content is in the range of 0.15% to 0.22%. Preferably it is 0.16% or more. Preferably it is 0.20% or less.
- Si 1.0% or more and 2.0% or less Si is an element effective for ensuring strength without significantly reducing the ductility of the steel sheet.
- the Si content is less than 1.0%, a steel plate with high strength and high workability cannot be produced.
- the amount of Si exceeds 2.0%, even if a step of re-acid washing after pickling is performed, the Si oxide on the surface of the steel sheet cannot be removed, and the chemical conversion treatment performance is lowered. Therefore, the Si content is in the range of 1.0% to 2.0%. Preferably it is 1.0% or more. Preferably it is 1.5% or less.
- Mn 1.7% to 2.5%
- Mn is an element that increases the strength of the steel sheet. If the Mn content is less than 1.7%, it is difficult to ensure a tensile strength of 1300 MPa or more. On the other hand, when the amount of Mn exceeds 2.5%, a steel structure in which ferrite and martensite are distributed in a band shape is formed due to segregation during casting. As a result, anisotropy occurs in the mechanical characteristics, and workability deteriorates. Therefore, the Mn content is in the range of 1.7% to 2.5%.
- [Si] represents the Si content (mass%)
- [Mn] represents the Mn content (mass%)
- the amount of Si-based oxide and Si-Mn composite oxide is determined by the balance between Si and Mn.
- Si silicon-based oxide
- Si-Mn composite oxide an oxide mainly composed of Si-Mn (Si-Mn composite oxide) is generated excessively, and the present invention.
- the intended chemical conversion processability cannot be obtained. Therefore, [Si] / [Mn] ⁇ 0.5.
- P 0.05% or less
- P is an impurity element and must be reduced to degrade ductility. If it exceeds 0.05%, the local ductility deteriorates due to grain boundary embrittlement accompanying P segregation to the austenite grain boundaries during casting. As a result, the balance between strength and ductility deteriorates. Therefore, the P content is 0.05% or less. Preferably it is 0.02% or less.
- S 0.02% or less S is present as MnS in the steel sheet, and causes a reduction in impact resistance, strength, and stretch flangeability. Therefore, the upper limit is 0.02%. Preferably it is 0.002% or less.
- Al 0.01% or more and 0.05% or less Al has an effect of reducing ductility by reducing oxides such as Si by itself forming an oxide. However, a significant effect cannot be obtained at less than 0.01%.
- Al is excessively added exceeding 0.05%, Al and N are combined to form a nitride. Since this nitride precipitates on the austenite grain boundary during casting and embrittles the grain boundary, it deteriorates stretch flangeability. Therefore, the Al content is in the range of 0.01% to 0.05%.
- N forms nitrides with Al and Ti and deteriorates stretch flangeability as described above.
- the N content exceeds 0.005%, the stretch flangeability is remarkably deteriorated by Ti and Al nitrides, and the decrease in elongation due to the increase in solute N is also remarkable. Therefore, the N content is 0.005% or less. Preferably it is 0.002% or less.
- Ti 0.010% or more and 0.020% or less Ti has an effect of refining the structure, and may be added as necessary. If the amount of Ti is less than 0.010%, the effect of refining the structure is small. On the other hand, even if added over 0.020%, not only the effect of refining the structure is saturated, but also coarse Ti and Nb composite carbides may be formed to deteriorate the balance between strength and ductility and stretch flangeability. Further, the manufacturing cost increases. For this reason, when adding Ti, it is made into 0.010% or more and 0.020% or less. Preferably it is 0.012% or more. Preferably it is 0.018% or less.
- Nb 0.02% or more and 0.10% or less Nb has the effect of refining the structure in the same manner as Ti, and may be added as necessary. If the Nb content is less than 0.02%, the effect of refining the structure is small. On the other hand, adding over 0.10% not only saturates the effect of refining the structure but also forms coarse Ti and Nb composite carbides, which may deteriorate the balance between strength and ductility and stretch flangeability. Furthermore, the manufacturing cost increases. For this reason, when adding Nb, it is made into 0.02% or more and 0.10% or less. Preferably it is 0.04% or more. Preferably it is 0.08% or less.
- B 0.0002% or more and 0.0020% or less B segregates at the austenite grain boundaries during heating in continuous annealing, suppresses ferrite transformation and bainite transformation from austenite during cooling, and facilitates the formation of tempered martensite. As a result, the steel plate is strengthened. Therefore, you may add as needed. If the amount of B is less than 0.0002%, the above effect is small. On the other hand, if the amount of B exceeds 0.0020%, borocarbide Fe 23 (C, B) 6 is generated, which may cause deterioration of workability and strength. For this reason, the B content is 0.0002% or more and 0.0020% or less when added.
- V 0.01% to 0.30%
- Mo 0.01% to 0.30%
- Cr 0.01% to 0.30%
- V 0.01% or more and 0.30% or less Fine carbide formed by combining V and C is effective for precipitation strengthening of the steel sheet, and V may be added as necessary. The effect is small when the V content is less than 0.01%. On the other hand, if the amount of V exceeds 0.30%, carbides may precipitate excessively and the balance between strength and ductility may deteriorate. For this reason, the amount of V is 0.01% or more and 0.30% or less when added.
- Mo 0.01% or more and 0.30% or less Mo is effective for strengthening the quenching of the steel sheet, and has the effect of refining the steel structure, so it may be added as necessary. The effect is small when the Mo content is less than 0.01%. On the other hand, if the amount of Mo exceeds 0.30%, not only the effect is saturated, but also the formation of Mo oxide on the steel sheet surface is promoted during continuous annealing, and the chemical conversion treatment property of the steel sheet may be remarkably lowered. For this reason, the Mo amount is 0.01% or more and 0.30% or less when added.
- Cr 0.01% or more and 0.30% or less Cr is effective for strengthening the quenching of the steel sheet, and may be added as necessary. If the Cr content is less than 0.01%, the strengthening ability is small. On the other hand, if the Cr content exceeds 0.30%, the formation of Cr oxides on the steel sheet surface is promoted during continuous annealing, so the chemical conversion property of the steel sheet may be significantly reduced. For this reason, when adding Cr, it is 0.01% or more and 0.30% or less. In the present invention, when further improving the characteristics, it is preferable to contain one or more of Cu: 0.01% to 0.30% and Ni: 0.01% to 0.30%.
- Cu 0.01% or more and 0.30% or less
- Cu suppresses ferrite transformation and bainite transformation from austenite during cooling in continuous annealing, facilitates the formation of tempered martensite, and strengthens the steel sheet. Therefore, you may add as needed. If the amount of Cu is less than 0.01%, the above effect is small. On the other hand, if the amount of Cu exceeds 0.30%, ferrite transformation is excessively suppressed and ductility may be reduced. For this reason, when adding Cu, it is 0.01% or more and 0.30% or less.
- Ni 0.01% or more and 0.30% or less Ni suppresses ferrite transformation and bainite transformation from austenite during cooling in continuous annealing, facilitates the formation of tempered martensite, and strengthens the steel sheet. Therefore, you may add as needed.
- the amount of Ni is less than 0.01%, the above effect is small.
- the Ni content exceeds 0.30%, ferrite transformation is excessively suppressed and ductility may be reduced. For this reason, when Ni is added, it is set to 0.01% or more and 0.30% or less.
- Sn 0.001% to 0.100%
- Sb 0.001% to 0.100%
- Ca 0.0002% to 0.0100%
- W It is preferable to contain one or more of 0.01% to 0.10%, Co: 0.01% to 0.10%, REM: 0.0002% to 0.0050%.
- Sn 0.001% or more and 0.100% or less
- Sb 0.001% or more and 0.100% or less Since Sn and Sb have the effect of suppressing surface oxidation, decarburization, and nitriding, they can be contained as necessary. However, the above effects are small when the Sn content and the Sb content are each less than 0.001%. On the other hand, even if the added amount exceeds 0.100%, the effect is saturated. For this reason, when adding Sn and Sb, it is made 0.001% or more and 0.100% or less, respectively. Preferably it is 0.005% or more. Preferably it is 0.010% or less.
- Ca 0.0002% or more and 0.0100% or less
- Ca has an effect of improving ductility through the form control of sulfide, grain boundary strengthening, and solid solution strengthening, and can be contained as necessary.
- the effect is small when the Ca content is less than 0.0002%.
- ductility will deteriorate by grain boundary segregation. For this reason, when adding Ca, it is made into 0.0002% or more and 0.0100% or less.
- W 0.01% or more and 0.10% or less
- Co 0.01% or more and 0.10% or less
- W and Co have the effect of improving ductility through sulfide morphology control, grain boundary strengthening, and solid solution strengthening. It can be included. However, the effect is small when the W content and the Co content are each less than 0.01%. On the other hand, if added excessively, ductility deteriorates due to grain boundary segregation and the like. For this reason, when adding W and Co, they are 0.01% or more and 0.10% or less, respectively.
- REM 0.0002% or more and 0.0050% or less REM has an effect of improving ductility through sulfide morphology control, grain boundary strengthening, and solid solution strengthening, and can be contained as necessary.
- the REM amount is less than 0.0002%, the above effect is small.
- ductility deteriorates due to grain boundary segregation and the like. For this reason, when adding REM, it is made into 0.0002% or more and 0.0050% or less.
- the balance other than the above is Fe and inevitable impurities.
- Inevitable impurities include O (oxygen) and the like, and an O content of 0.01% or less is acceptable.
- tempered martensite contains 60% or more and less than 100%, untransformed austenite is 5% or less (including 0%), the balance is ferrite, and the average grain size of ferrite is less than 3.5 ⁇ m.
- Tempered martens The tensile strength of steel having a structure including sites and ferrite increases as the area ratio of tempered martensite increases. In tempered martensite and ferrite, tempered martensite has higher hardness, and deformation resistance during tensile deformation is borne by tempered martensite, which is a hard phase. The larger the tempered martensite area ratio, the more tempered martensite. This is because it approaches the tensile strength of the single phase structure.
- the area ratio of tempered martensite is less than 40%, a tensile strength of 1300 MPa or more cannot be obtained.
- the area of the interface between tempered martensite and ferrite is large, that is, when the area ratio of tempered martensite is 40% or more and less than 60%, the frequency of void formation due to the hardness difference between the two phases increases and the voids are connected. It becomes easy to do, and the progress of a crack is accelerated, and stretch flangeability will deteriorate. From the above, in order to improve workability while ensuring tensile strength, the area ratio of tempered martensite needs to be 60% or more.
- the area ratio of tempered martensite is 100%, excellent workability cannot be obtained. In addition, 5% or less of untransformed austenite may be inevitably mixed. However, if it is 5% or less, there is no problem in obtaining the effect of the present invention and it is allowed. From the above, the area ratio of tempered martensite is less than 100%, untransformed austenite is 5% or less (including 0%), and the balance is ferrite.
- the lower limit of the area ratio of a suitable tempered martensite is 70%.
- a preferred upper limit is 90%.
- the average crystal grain size of ferrite is 3.5 ⁇ m or more, the predetermined strength cannot be obtained because the grain refinement strengthening is insufficient. In addition, since deformation is likely to occur between crystal grains during deformation, workability deteriorates. Therefore, the average crystal grain size of ferrite is less than 3.5 ⁇ m.
- the area ratio of tempered martensite, the area ratio of ferrite, and the average crystal grain size of ferrite can be measured by the methods of Examples described later.
- the number of Si-Mn composite oxides having an equivalent circle diameter of 5 ⁇ m or less is less than 10/100 ⁇ m 2 , the presence of Si-Mn composite oxides on the steel sheet surface significantly deteriorates the chemical conversion treatment property. Needless to say, if a coarse Si-Mn composite oxide is present on the surface of the steel sheet, the chemical conversion treatment performance deteriorates. Even in the case of a Si-Mn composite oxide having a circle equivalent diameter of 5 ⁇ m or less, when the distribution form exceeds a certain number density, deterioration of chemical conversion treatment becomes obvious. Therefore, the number of Si-Mn composite oxides having an equivalent circle diameter of 5 ⁇ m or less is defined as less than 10/100 ⁇ m 2 . If it is 10 pieces / 100 ⁇ m 2 or more, a region where zinc phosphate crystals are not formed becomes obvious, and the chemical conversion treatment performance deteriorates. The number is preferably 0/100 ⁇ m 2 .
- the number of Si-Mn composite oxides having an equivalent circle diameter of 5 ⁇ m or less can be measured by the method of Examples described later.
- the surface is a range from the surface layer to the position of 3% with respect to the plate thickness in the plate thickness direction.
- the steel surface coverage of the oxide mainly composed of Si is 1% or less. If an oxide mainly composed of Si is present on the surface of the steel sheet, the chemical conversion treatment performance is remarkably lowered. Therefore, the steel sheet surface coverage of oxide mainly composed of Si is set to 1% or less. Preferably it is 0%.
- the oxide mainly composed of Si is, for example, SiO 2 . Further, the oxide mainly composed of Si can be measured by the method of Examples described later.
- the structure, the number of Si-Mn composite oxides, and the steel sheet surface coverage of the oxide mainly composed of Si can be controlled by controlling pickling after annealing, particularly re-acid cleaning, in the manufacturing method described later. Obtainable.
- the high-strength cold-rolled steel sheet of the present invention heats the steel material (steel slab) having the above-described composition to a temperature of 1200 ° C or higher, and then performs hot rolling to a finish rolling temperature of 800 ° C or higher, and 450 ° C or higher. Winding at a temperature of 700 ° C or less and cold rolling. Next, heating is performed to an annealing temperature of Ac 1 point or more and Ac 3 point or less, and the residence time in the temperature range from Ac 1 point to Ac 3 point is 30 seconds or more and 1200 seconds or less, and the primary cooling stop temperature is 600 ° C. from the annealing temperature.
- the primary cooling is performed at an average cooling rate of less than 100 ° C./s until the above, and the secondary cooling is performed at an average cooling rate of 100 ° C./s to 1000 ° C./s to a secondary cooling stop temperature of 100 ° C. or lower.
- it is heated to a temperature of 100 ° C. or higher and 300 ° C. or lower, subjected to a tempering treatment in which the residence time in the temperature range from 100 ° C. to 300 ° C. is 120 seconds or longer and 1800 seconds or shorter, and further pickled and re- pickled.
- the high-strength cold-rolled steel sheet of the present invention can be manufactured.
- Ac 1 point and Ac 3 point are values (° C.) obtained from a transformation expansion curve obtained at an average heating rate of 3 ° C./s using a thermal expansion measuring device.
- the method for melting steel is not particularly limited, and a known melting method such as a converter or an electric furnace can be employed. Further, secondary refining may be performed in a vacuum degassing furnace. Thereafter, the slab (steel material) is preferably formed by a continuous casting method from the viewpoint of productivity and quality, but the slab may be formed by a known casting method such as an ingot-bundling rolling method or a thin slab continuous casting method. .
- the heating temperature of the steel material is 1200 ° C. or higher and the heating temperature is lower than 1200 ° C., the carbide does not re-dissolve and the workability deteriorates. Therefore, the heating temperature of the steel material is set to 1200 ° C or higher. If the heating temperature becomes too high, it leads to an increase in scale loss accompanying an increase in oxidation mass. Therefore, the heating temperature of the steel material is preferably 1300 ° C. or lower.
- the heating temperature of the steel material is preferably 1300 ° C. or lower.
- the rough rolling conditions are not particularly limited.
- Finishing rolling exit temperature 800 ° C. or more
- the finish rolling exit temperature is set to 800 ° C. or higher.
- the upper limit of the finish rolling exit temperature is not particularly limited, but 1000 ° C. or less is preferable because rolling at an excessively high temperature causes scale wrinkles and the like.
- Winding temperature 450 ° C or more and 700 ° C or less
- the coiling temperature after hot rolling is lower than 450 ° C, the processed structure generated by hot rolling remains, and the rolling load of the next cold rolling increases.
- the coiling temperature exceeds 700 ° C., coarse grains are formed, the steel sheet structure becomes non-uniform, and the ductility decreases. Therefore, the coiling temperature is set to 450 ° C. or more and 700 ° C. or less.
- the lower limit of the suitable winding temperature is 500 ° C.
- a preferred upper limit is 650 ° C.
- pickling and then cold rolling as necessary.
- the conditions for pickling are not particularly limited. Cold rolling needs to be performed to obtain a desired plate thickness. Although there is no restriction
- annealing temperature residence time 30 seconds or more of a temperature range of from Ac 1 point or more Ac 3 point below the annealing temperature to heating Ac 1 point to Ac 3 point is less than 1 point Ac, predetermined during annealing Austenite (transformed into martensite after quenching) necessary for securing the strength is not generated, and a predetermined strength cannot be obtained even if quenching is performed after annealing.
- the annealing temperature is more than Ac 3 point, it is possible to obtain martensite with an area ratio of 60% or more by controlling the area ratio of ferrite precipitated during cooling from the annealing temperature. When annealing at more than 3 points, it becomes difficult to obtain a desired metal structure.
- the annealing temperature is set to Ac 1 point or more and Ac 3 point or less.
- the annealing temperature is preferably set to 780 ° C. or more from the viewpoint of stably securing an equilibrium area ratio of austenite of 60% or more.
- the residence time at the annealing temperature is too short, the microstructure is not sufficiently annealed and becomes a non-uniform structure in which a cold-rolled processed structure exists, and the ductility is lowered.
- the residence time is 30 to 1200 seconds.
- a preferred lower limit of residence time is 150 seconds.
- a preferred upper limit is 600 seconds.
- Secondary cooling stop temperature at an average cooling rate of less than 100 ° C / s from the primary cooling annealing temperature to less than 100 ° C / s with an average cooling rate of less than 100 ° C / s. ) (Slow cooling). It becomes possible to precipitate ferrite during the slow cooling from the annealing temperature, and to control the balance between strength and ductility.
- the slow cooling stop temperature primary cooling stop temperature
- the slow cooling stop temperature is less than 600 ° C.
- a large amount of pearlite is generated in the microstructure and the strength rapidly decreases, so that a tensile strength of 1300 MPa or more cannot be obtained.
- 680 ° C In order to obtain a predetermined strength more stably, 680 ° C.
- the average cooling rate is 100 ° C./s or more, a sufficient amount of ferrite does not precipitate during cooling, so that excellent ductility cannot be obtained.
- the ductility of the metal structure having tempered martensite and ferrite intended in the present invention is attributed to the high work-hardening ability that is manifested by the mixture of hard tempered martensite and soft ferrite.
- the average cooling rate is 100 ° C./s or more, carbon concentration in the austenite during cooling becomes insufficient, and hard martensite cannot be obtained during rapid cooling. As a result, the work hardening ability of the final structure is lowered and sufficient ductility cannot be obtained.
- the average cooling rate is less than 100 ° C / s. In order to sufficiently cause carbon concentration in austenite, an average cooling rate of 5 ° C./s or less is preferable.
- Secondary cooling down temperature to 100 ° C or less Secondary cooling at an average cooling rate of 100 ° C / s or more and 1000 ° C / s or less Following the above-mentioned slow cooling, 100 ° C or less at an average cooling rate of 100 ° C / s or more and 1000 ° C / s or less Cool down (rapidly cool) to the secondary cooling stop temperature. Rapid cooling after slow cooling is performed in order to transform austenite into martensite, but when the average cooling rate is less than 100 ° C / s, austenite transforms into ferrite, bainite or pearlite during cooling, so that a predetermined strength is obtained. I can't.
- the average cooling rate during rapid cooling is set to 100 ° C./s or more and 1000 ° C./s or less.
- the quenching is preferably quenching by water quenching.
- the secondary cooling stop temperature is 100 ° C or less. When the secondary cooling stop temperature is higher than 100 ° C, it will cause a decrease in the area ratio of martensite due to insufficient quenching of austenite during rapid cooling, and a decrease in material strength due to self-tempering of martensite generated by rapid cooling. It is not preferable.
- a tempering process in which the residence time in the temperature range from 100 ° C to 300 ° C is 120 to 1800 seconds, and the tempering treatment is performed at a temperature of 100 ° C for the tempering of martensite. Re-heating to a temperature of 300 ° C or lower and tempering for 120 to 1800 seconds in the temperature range of 100 to 300 ° C.
- This tempering softens martensite and improves workability.
- tempering is performed at less than 100 ° C., the softening of martensite is insufficient, the effect of improving workability cannot be expected, and the hardness difference from ferrite becomes large, so that the stretch flangeability deteriorates.
- the residence time is less than 120 seconds, the martensite is not sufficiently softened in the temperature range from 100 ° C. to 300 ° C., and therefore an effect of improving workability cannot be expected.
- the residence time exceeds 1800 seconds, the strength is remarkably lowered due to excessive softening of martensite, and the manufacturing cost is increased due to an increase in reheating time, which is not preferable.
- the Si oxide and Si-Mn oxide on the steel sheet surface are removed, and the chemical conversion processability is improved.
- a non-oxidizing acid as the pickling solution, unlike the pickling solution used in pickling.
- Pickling can be performed by a conventional method, and the conditions are not particularly limited. For example, any one of nitric acid, hydrochloric acid, hydrofluoric acid, sulfuric acid and an acid obtained by mixing two or more of them can be used.
- the tempered steel sheet is pickled with a strong acid such as nitric acid with a concentration of more than 50 g / L and less than 200 g / L, for example.
- non-oxidizing acid examples include hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, oxalic acid, and acids obtained by mixing two or more of these.
- hydrochloric acid having a concentration of 0.1 to 50 g / L sulfuric acid having a concentration of 0.1 to 150 g / L
- an acid in which 0.1 to 20 g / L hydrochloric acid and 0.1 to 60 g / L sulfuric acid are mixed can be suitably used.
- a high-strength cold-rolled steel sheet having a tensile strength (TS) of 1300 MPa or more and excellent in chemical conversion property and workability is produced. Since the high-strength cold-rolled steel sheet of the present invention is excellent in plate shape (flatness) after annealing, a process for correcting the shape of the steel sheet, such as rolling and leveler processing, is not necessarily required. From the viewpoint of adjusting the surface roughness, there is no problem even if the annealed steel sheet is rolled at an elongation of about several percent.
- the high-strength cold-rolled steel sheet of the present invention does not affect the material depending on the plating treatment or the composition of the plating bath, so as a plating treatment, a hot dip galvanizing treatment, an alloyed hot dip galvanizing treatment, an electrogalvanizing treatment Any of the treatments can be applied.
- Test steels A to R having the composition shown in Table 1 were vacuum-melted into slabs, and then hot rolled under the conditions shown in Table 2 to obtain hot-rolled steel sheets.
- the hot-rolled steel sheet was pickled to remove the surface scale, and then cold-rolled (rolling rate: 60%).
- continuous annealing and tempering treatment were performed under the conditions described in Table 2, and pickling and re- pickling were performed.
- Ac 1 point and Ac 3 point were obtained from a transformation expansion curve obtained at an average heating rate of 3 ° C./s using a thermal expansion measuring device.
- the two-phase volume ratio was determined by the point counting method based on the SEM image at a magnification of 1000 times, and the particle size of each phase was determined by the line segment method. The obtained volume ratio was defined as the area ratio.
- the tensile test was carried out at a strain rate of 3.3 ⁇ 10 ⁇ 3 s ⁇ 1 by cutting a JIS No. 5 test piece (distance between gauge points: 50 mm, width of parallel part: 25 mm) parallel to the rolling direction. Total elongation was measured by test piece butt after fracture.
- the hole expansion test was performed with a 100mm x 100mm size test piece, and after punching a circular hole of ⁇ 10mm (d 0 ), the apex angle was 60 ° with a 75mm inner diameter die pressed with a wrinkle holding force of 9tons.
- the conical punch was pushed up from below against the hole, and the hole diameter (d) was measured when a plate thickness through crack occurred at the hole edge.
- the hole expansion rate: (lambda) (%) defined by following Formula was calculated
- the test was performed so that the hole punching and the hole expansion were in the same direction with the surface where burrs were generated by punching as the upper side (JIS 2256 compliant).
- ⁇ (%) ⁇ (d-d0) / d0 ⁇
- d0 initial hole diameter
- d hole diameter at the time when the crack penetrates the plate thickness.
- the steel sheet surface coverage of the oxide mainly composed of Si is mainly composed of Si in the same manner as above by observing 5 fields of view at 1000 times on the steel sheet surface using SEM and analyzing the same field of view by EDX.
- the oxide was identified, and the coverage was determined by a point counting method (a method in which 15 straight lines were drawn in each of the vertical and horizontal directions of the SEM image to determine the probability that an Si-based oxide was present at the intersection (225 points)).
- chemical conversion treatment was performed using a commercially available chemical treatment chemical (Nippon Parkerizing Co., Ltd., Palbond PB-L3065 (registered trademark)) at a bath temperature of 35 ° C and a treatment time of 120 seconds.
- the processed steel sheet surface is observed with 5 fields using SEM at a magnification of 500 times, and when all the 5 fields have a uniform conversion crystal with an area ratio of 95% or more, the chemical conversion processability is good.
- the chemical conversion treatment ability was evaluated as inferior “ ⁇ ” when a defect with an area ratio exceeding 5% was observed even in the visual field.
- the examples that meet the conditions of the present invention have excellent strength and high tensile strength (TS) of 1300 MPa or more, elongation (EL) of 10% or more, and hole expansion ratio ( ⁇ ) of 30% or more. Workability is obtained. Moreover, it is excellent in chemical conversion treatment property.
- No. 12 is a comparative example in which the C content is higher than the range of the present invention. Since the C content is high, the strength of martensite is increased and the balance between strength and ductility is excellent, but it can be seen that the stretch flangeability is extremely low due to the hardness difference between ferrite and martensite.
- No. 13 and 14 are comparative examples in which the Si content is outside the scope of the present invention.
- No. 13 does not satisfy the chemical conversion treatment property because Si oxide exists on the steel plate surface even after the two-step pickling treatment.
- No. 14 does not have a predetermined elongation.
- No. 15 and 16 are comparative examples in which the Mn content is outside the scope of the present invention. Since Mn is an element that greatly fluctuates the martensite fraction, No. 15 having a high content does not achieve a predetermined elongation. In No. 16 having a low content, the martensite fraction is small, so that a predetermined strength is not obtained.
- Nos. 18 to 23 are comparative examples in which the manufacturing conditions are outside the scope of the present invention.
- No. 18 is a comparative example in which the component composition and production conditions are outside the scope of the present invention. In addition to the fact that the predetermined elongation cannot be obtained, the stretch flangeability and chemical conversion treatment properties are inferior.
- No. 19 has a high annealing temperature, so the prescribed strength and elongation cannot be obtained.
- No. 20 to 22 do not have a sufficient martensite fraction, and the prescribed strength is not obtained.
- No. 23 does not have a sufficient martensite fraction and has poor stretch flangeability.
- No. 24 is an example in which the pickling treatment after annealing is omitted.
- Si oxide is present on the surface of the steel sheet and does not satisfy the chemical conversion processability.
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Abstract
Description
[Si]/[Mn] ≧ 0.5 … (1)
ただし、式中[Si]はSi含有量(質量%)を示し、[Mn]はMn含有量(質量%)を示す。
[1]成分組成は、質量%で、C:0.15%以上0.22%以下、Si:1.0%以上2.0%以下、Mn:1.7%以上2.5%以下、P:0.05%以下、S:0.02%以下、Al:0.01%以上0.05%以下、N:0.005%以下を含有し、かつ下記式(1)を満たし、残部は鉄及び不可避的不純物からなり、組織は、面積率で、焼戻しマルテンサイトが60%以上100%未満、未変態オーステナイトが5%以下(0%含む)、残部がフェライトであり、該フェライトの平均結晶粒径が3.5μm未満であり、鋼板表面において、円相当直径5μm以下のSi-Mn複合酸化物の個数が10個/100μm2未満であり、Siを主体とする酸化物の鋼板表面被覆率が1%以下であり、引張強度が1300MPa以上である高強度冷延鋼板。
[Si]/[Mn] ≧ 0.5 … (1)
ただし、式中[Si]はSi含有量(質量%)を示し、[Mn]はMn含有量(質量%)を示す。
[2]前記成分組成に加えて、質量%で、Ti:0.010%以上0.020%以下を含有する上記[1]に記載の高強度冷延鋼板。
[3]前記成分組成に加えて、質量%で、Nb:0.02%以上0.10%以下を含有する上記[1]または[2]に記載の高強度冷延鋼板。
[4]前記成分組成に加えて、質量%で、B:0.0002%以上0.0020%以下を含有する上記[1]~[3]のいずれかに記載の高強度冷延鋼板。
[5]前記成分組成に加えて、質量%で、V:0.01%以上0.30%以下、Mo:0.01%以上0.30%以下、Cr:0.01%以上0.30%以下の1種以上を含有する上記[1]~[4]のいずれかに記載の高強度冷延鋼板。
[6]前記成分組成に加えて、質量%で、Cu:0.01%以上0.30%以下、Ni:0.01%以上0.30%以下の1種以上を含有する上記[1]~[5]のいずれかに記載の高強度冷延鋼板。
[7]前記成分組成に加えて、質量%で、Sn:0.001%以上0.100%以下、Sb:0.001%以上0.100%以下、Ca:0.0002%以上0.0100%以下、W:0.01%以上0.10%以下、Co:0.01%以上0.10%以下、REM:0.0002%以上0.0050%以下の1種以上を含有する上記[1]~[6]のいずれかに記載の高強度冷延鋼板。
[8]上記[1]~[7]のいずれかに記載の成分組成を有する鋼素材を、1200℃以上の温度に加熱し、次いで、仕上げ圧延出側温度800℃以上とする熱間圧延を施し、450℃以上700℃以下の温度で巻取り、冷間圧延し、次いで、Ac1点以上Ac3点以下の焼鈍温度まで加熱しAc1点からAc3点までの温度域の滞留時間が30秒以上1200秒以下であり、前記焼鈍温度から一次冷却停止温度600℃以上の温度まで平均冷却速度100℃/s未満で一次冷却し、二次冷却停止温度100℃以下の温度まで平均冷却速度100℃/s以上1000℃/s以下で二次冷却する焼鈍処理を施し、次いで、100℃以上300℃以下の温度まで加熱し、100℃から300℃までの温度域の滞留時間が120秒以上1800秒以下である焼戻し処理を施し、さらに、酸洗、再酸洗を施す高強度冷延鋼板の製造方法。
[9]前記再酸洗では、前記酸洗で用いる酸洗液とは異なり、かつ、非酸化性の酸を酸洗液として用いる上記[8]に記載の高強度冷延鋼板の製造方法。
なお、本発明において、高強度冷延鋼板とは、引張強さ(TS)が1300MPa以上の冷延鋼板である。
Cは鋼板の強度と延性のバランスを改善するのに有効な元素である。C量が0.15%未満では、引張強度1300MPa以上を確保するのが困難である。一方、C量が0.22%を超えると粗大なセメンタイトが析出し、伸びフランジ性等の加工性が劣化する。よって、C量は0.15%以上0.22%以下の範囲とする。好ましくは0.16%以上である。好ましくは0.20%以下である。
Siは鋼板の延性をさほど低下させることなく強度を確保するために有効な元素である。Si量が1.0%未満の場合、高強度かつ高加工性の鋼板が製造できない。一方、Si量が2.0%を超えると、酸洗後に再酸洗する工程を経たとしても鋼板表面のSi酸化物が除去しきれず、化成処理性が低下する。よって、Si量は1.0%以上2.0%以下の範囲とする。好ましくは1.0%以上である。好ましくは1.5%以下である。
Mnは鋼板の強度を高める元素である。Mn量が1.7%未満の場合、引張強度1300MPa以上を確保するのが困難である。一方、Mn量が2.5%を超えると、鋳造時の偏析によりフェライトとマルテンサイトが帯状に分布した鋼組織を形成する。その結果、機械的特性に異方性が生じ、加工性が劣化する。よって、Mn量は1.7%以上2.5%以下の範囲とする。
ただし、式中[Si]はSi含有量(質量%)を示し、[Mn]はMn含有量(質量%)を示す。
Pは不純物元素であり、延性を劣化させるため低減しなければならない。0.05%を超えると、鋳造時のオーステナイト粒界へのP偏析に伴う粒界脆化により局部延性が劣化する。その結果、強度と延性のバランスが劣化する。よって、P量は0.05%以下とする。好ましくは0.02%以下である。
Sは鋼板中にMnSとして存在し、耐衝撃特性、強度および伸びフランジ性の低下を招くため、極力低減させることが好ましい。そのため、上限は0.02%とする。好ましくは0.002%以下である。
Alは自身が酸化物を形成することによってSiなどの酸化物を低減させ、延性を改善する効果がある。しかしながら、0.01%未満では有意な効果は得られない。一方、0.05%を超えてAlを過度に添加すると、AlとNとが結合して窒化物が形成される。この窒化物は鋳造時にオーステナイト粒界上に析出して粒界脆化させるため、伸びフランジ性を劣化させる。よって、Al量は0.01%以上0.05%以下の範囲とする。
NはAlおよびTiと窒化物を形成し、上記のように伸びフランジ性を劣化させる。N量が0.005%を超えるとTi、Al窒化物により伸びフランジ性が顕著に劣化し、また、固溶Nの増加による伸びの低下も著しい。このためN量は0.005%以下とする。好ましくは0.002%以下とする。
Tiは組織を微細化する効果を有するため、必要に応じて添加しても良い。Ti量が0.010%未満では組織を微細化する効果は小さい。一方、0.020%を超えて添加しても組織微細化の効果が飽和するばかりか、粗大なTi、Nb複合炭化物を形成して強度と延性のバランスおよび伸びフランジ性を劣化させる場合がある。さらに製造コストが増大する。このため、Tiを添加する場合には、0.010%以上0.020%以下とする。好ましくは0.012%以上である。好ましくは0.018%以下である。
NbはTiと同様に組織を微細化する効果を有するため、必要に応じて添加してもよい。Nb量が0.02%未満では組織を微細化する効果は小さい。一方、0.10%を超えて添加しても組織微細化の効果は飽和するばかりか、粗大なTi、Nb複合炭化物を形成して強度と延性のバランスおよび伸びフランジ性を劣化させる場合がある。さらに製造コストも増大する。このため、Nbを添加する場合には、0.02%以上0.10%以下とする。好ましくは0.04%以上である。好ましくは0.08%以下である。
Bは連続焼鈍における加熱時にオーステナイト粒界に偏析し、冷却時のオーステナイトからのフェライト変態およびベイナイト変態を抑制して、焼もどしマルテンサイトの形成を容易化する。その結果、鋼板を強化する。そのため、必要に応じて添加してもよい。B量が0.0002%未満では、上記効果は小さい。一方、B量が0.0020%を超えると、ホウ炭化物Fe23(C、B)6が生じて加工性の劣化と強度の低下が起きる場合がある。このため、B量は添加する場合には、0.0002%以上0.0020%以下とする。
VとCとが結合して形成される微細炭化物は鋼板の析出強化に有効であり、Vを必要に応じて添加してもよい。V量が0.01%未満では上記効果が小さい。一方、V量が0.30%を超えると、炭化物が過剰に析出して強度と延性のバランスが劣化する場合がある。このため、V量は添加する場合には0.01%以上0.30%以下とする。
Moは鋼板の焼入強化に有効であり、鋼組織の微細化効果も有するので必要に応じて添加してもよい。Mo量が0.01%未満では上記効果は小さい。一方、Mo量が0.30%を超えると、効果が飽和するばかりか、連続焼鈍時に鋼板表面にMo酸化物の形成が促進され、鋼板の化成処理性が著しく低下する場合がある。このため、Mo量は添加する場合には、0.01%以上0.30%以下とする。
Crは鋼板の焼入強化に有効であり、必要に応じて添加してもよい。Cr量が0.01%未満では強化能が小さい。一方、Cr量が0.30%を超えると、連続焼鈍時に鋼板表面にCr酸化物の生成が促進されるため、鋼板の化成処理性が著しく低下する場合がある。このため、Cr量は添加する場合には、0.01%以上0.30%以下とする。
本発明において、更に特性を向上させる場合、Cu:0.01%以上0.30%以下、Ni:0.01%以上0.30%以下の1種以上を含有することが好ましい。
Cuは連続焼鈍における冷却時のオーステナイトからのフェライト変態およびベイナイト変態を抑制して、焼もどしマルテンサイトの形成を容易化し、鋼板を強化する。そのため、必要に応じて添加してもよい。Cu量が0.01%未満では、上記効果は小さい。一方、Cu量が0.30%を超えると、フェライト変態が過度に抑制されて延性が低下する場合がある。このため、Cu量は添加する場合には、0.01%以上0.30%以下とする。
Niは連続焼鈍における冷却時のオーステナイトからのフェライト変態およびベイナイト変態を抑制して、焼もどしマルテンサイトの形成を容易化し、鋼板を強化する。そのため、必要に応じて添加してもよい。Ni量が0.01%未満では、上記効果は小さい。また、Ni量が0.30%を超えると、フェライト変態が過度に抑制されて延性が低下する場合がある。このため、Ni量は添加する場合には、0.01%以上0.30%以下とする。
Sn、Sbはいずれも表面酸化や脱炭、窒化を抑制する効果を有するため、必要に応じて含有させることができる。しかしながら、Sn量、Sb量がそれぞれ0.001%未満では上記効果は小さい。一方、添加量がそれぞれ0.100%を超えてもその効果は飽和する。このため、Sn、Sbを添加する場合には、それぞれ0.001%以上0.100%以下とする。好ましくは0.005%以上である。好ましくは0.010%以下である。
Caは、硫化物の形態制御や粒界強化、固溶強化を通じて延性を向上する効果を有するため、必要に応じて含有させることができる。しかしながら、Ca量が0.0002%未満では上記効果は小さい。また、過度に添加すると粒界偏析などにより延性が劣化する。このため、Caを添加する場合には、0.0002%以上0.0100%以下とする。
W、Coはいずれも硫化物の形態制御や粒界強化、固溶強化を通じて延性を向上する効果を有するため、必要に応じて含有させることができる。しかしながら、W量、Co量がそれぞれ0.01%未満では上記効果は小さい。一方、過度に添加すると粒界偏析などにより延性が劣化する。このため、W、Coを添加する場合には、それぞれ0.01%以上0.10%以下とする。
REMは、硫化物の形態制御や粒界強化、固溶強化を通じて延性を向上する効果を有するため、必要に応じて含有させることができる。しかしながら、REM量が0.0002%未満では上記効果は小さい。一方、過度に添加すると粒界偏析などにより延性が劣化する。このため、REMを添加する場合には、0.0002%以上0.0050%以下とする。
焼戻しマルテンサイトとフェライトを有する組織の鋼の引張強度は、焼戻しマルテンサイトの面積率の増加に伴って上昇する。これは焼戻しマルテンサイトとフェライトでは、焼戻しマルテンサイトの方が硬度が高く、引張変形時における変形抵抗は硬質相である焼戻しマルテンサイトが担っており、焼戻しマルテンサイトの面積率が大きいほど焼戻しマルテンサイト単相組織の引張強度に漸近するためである。本発明の鋼成分範囲においては、焼戻しマルテンサイトの面積率が40%未満では引張強度1300MPa以上は得られない。また、焼戻しマルテンサイトとフェライトの界面の面積が大きい、つまり焼戻しマルテンサイトの面積率が40%以上60%未満の時、2相間の硬度差に起因したボイドの生成頻度が増加し、ボイドが連結し易くなり亀裂の進展を早めるため、伸びフランジ性が劣化してしまう。以上より、引張強度を確保しつつ加工性を向上させるには、焼戻しマルテンサイトの面積率は60%以上必要である。一方、焼戻しマルテンサイトの面積率が100%の場合には優れた加工性が得られない。また、5%以下の未変態オーステナイトが不可避的に混在する場合がある。しかし、5%以下であれば、本発明の効果を得る上で問題はなく、許容される。以上より、焼戻しマルテンサイトの面積率は100%未満とし、未変態オーステナイトが5%以下(0%含む)であり、残部はフェライトとする。好適な焼戻しマルテンサイトの面積率の下限は70%である。好適な上限は90%である。
Si-Mn複合酸化物が鋼板表面に存在すると、化成処理性が著しく劣化する。粗大なSi-Mn複合酸化物が鋼板表面に存在すると、化成処理性を劣化させることは言うまでも無い。円相当直径5μm以下のSi-Mn複合酸化物であっても、ある一定の個数密度を超えた分布形態になったときには、化成処理性の劣化が顕在化する。そこで、円相当直径5μm以下のSi-Mn複合酸化物の個数を10個/100μm2未満と規定する。10個/100μm2以上ではリン酸亜鉛結晶の生成していない領域が顕在化し、化成処理性が劣化する。好ましくは0個/100μm2である。
Siを主体とする酸化物が鋼板表面に存在すると、化成処理性が著しく低下する。そこで、Siを主体とする酸化物の鋼板表面被覆率は1%以下とする。好ましくは0%である。なお、Siを主体とする酸化物とは、例えばSiO2である。また、Siを主体とする酸化物は後述する実施例の方法にて測定することができる。
本発明の高強度冷延鋼板は、上記した成分組成の鋼素材(鋼スラブ)を1200℃以上の温度に加熱し、次いで、仕上げ圧延温度800℃以上とする熱間圧延を施し、450℃以上700℃以下の温度で巻取り、冷間圧延する。次いで、Ac1点以上Ac3点以下の焼鈍温度まで加熱しAc1点からAc3点までの温度域の滞留時間が30秒以上1200秒以下であり、前記焼鈍温度から一次冷却停止温度600℃以上まで平均冷却速度100℃/s未満で一次冷却し、二次冷却停止温度100℃以下まで平均冷却速度100℃/s以上1000℃/s以下で二次冷却する焼鈍処理を施す。次いで、100℃以上300℃以下の温度まで加熱し、100℃から300℃までの温度域の滞留時間が120秒以上1800秒以下である焼戻し処理を施し、さらに、酸洗、再酸洗を施すことで本発明の高強度冷延鋼板を製造することができる。再酸洗では、酸洗で用いる酸洗液とは異なり、かつ、非酸化性の酸を酸洗液として用いることが好ましい。
なお、Ac1点およびAc3点は熱膨張測定装置を用いて平均加熱速度3℃/sで得られた変態膨張曲線から求められる値(℃)である。
加熱温度が1200℃未満では、炭化物が再溶解せず、加工性が劣化する。したがって、鋼素材の加熱温度は1200℃以上とする。加熱温度があまりに高くなると酸化質量の増加に伴うスケールロスの増大につながるため、鋼素材の加熱温度は1300℃以下とすることが好ましい。但し、鋼素材に熱間圧延を施すに際し、鋳造後の鋼素材が1200℃以上の温度域にある場合、或いは鋼素材の炭化物が溶解している場合には、鋼素材を加熱することなく直送圧延してもよい。なお、粗圧延条件については特に限定されない。
仕上げ圧延出側温度を800℃以上とすることで、均一な熱延母相組織を得ることができる。仕上げ圧延出側温度が800℃を下回ると、鋼板の組織が不均一となり、延性が低下するとともに成形時に種々の不具合を発生する危険性が増大する。したがって、仕上げ圧延出側温度は800℃以上とする。なお、仕上げ圧延出側温度の上限は特に規制されないが、過度に高い温度で圧延するとスケール疵などの原因となるため、1000℃以下が好ましい。
熱間圧延後の巻取り温度が450℃を下回ると、熱間圧延によって生じた加工組織が残留し、次工程である冷間圧延の圧延荷重が大きくなる。巻取り温度が700℃を超えると粗大粒が生成し、鋼板組織が不均一となり、延性が低下する。そのため、巻取り温度は450℃以上700℃以下とする。好適な巻取り温度の下限は500℃である。好適な上限は650℃である。
焼鈍温度がAc1点未満になると、焼鈍中に所定の強度確保に必要なオーステナイト(焼入れ後にマルテンサイトに変態)が生成せず、焼鈍後焼入れを実施しても所定強度が得られない。焼鈍温度がAc3点超であっても、焼鈍温度からの冷却中に析出するフェライトの面積率を制御することにより、面積率で60%以上のマルテンサイトを得ることが可能であるが、Ac3点超で焼鈍した場合、所望の金属組織が得られにくくなる。そのため、焼鈍温度はAc1点以上Ac3点以下とする。この温度範囲においてオーステナイトの平衡面積率が60%以上を安定して確保する観点から、焼鈍温度は780℃以上とするのが好ましい。また、焼鈍温度での滞留時間が短すぎるとミクロ組織が十分に焼鈍されずに冷間圧延による加工組織が存在した不均一な組織となり延性が低下する。一方、滞留時間が長すぎると製造時間の増加を招き製造コスト上好ましくない。このため、滞留時間は30~1200秒とする。好適な滞留時間の下限は150秒である。好適な上限は600秒である。
焼鈍温度から100℃/s未満の平均冷却速度で600℃以上の一次冷却停止温度(徐冷停止温度)まで冷却(徐冷)する。焼鈍温度からの徐冷中にフェライトを析出させ、強度と延性のバランスを制御することが可能となる。徐冷停止温度(一次冷却停止温度)が600℃未満の場合、ミクロ組織中にパーライトが多量に生成し強度が急激に低下するために、1300MPa以上の引張強度を得ることができない。また、より安定的に所定の強度を得るためには、680℃以上が好ましい。
また、平均冷却速度が100℃/s以上の場合、冷却中に十分な量のフェライトの析出が生じないため優れた延性を得ることができない。本発明で意図する焼戻しマルテンサイトとフェライトを有する金属組織の延性は硬質な焼戻しマルテンサイトと軟質なフェライトが混在することによって発現する高い加工硬化能に起因する。しかしながら、平均冷却速度が100℃/s以上の場合、冷却中のオーステナイト中への炭素濃化が不十分となり、急冷時に硬質なマルテンサイトが得られない。その結果、最終組織の加工硬化能が低下し十分な延性が得られない。以上のことから平均冷却速度は100℃/s未満とする。オーステナイト中への炭素濃化を十分に生じさせるためには、5℃/s以下の平均冷却速度とすることが好ましい。
上記徐冷に引き続き、100℃/s以上1000℃/s以下の平均冷却速度で100℃以下の二次冷却停止温度まで冷却(急冷)する。徐冷後の急冷はオーステナイトをマルテンサイトに変態させるために行うが、その平均冷却速度が100℃/s未満では、冷却中にオーステナイトがフェライト、ベイナイトまたはパーライトに変態するため、所定の強度を得ることができない。一方、平均冷却速度が1000℃/sを超えると、冷却による鋼板の収縮割れが生じる可能性がある。このため、急冷時の平均冷却速度は100℃/s以上1000℃/s以下とする。なお、急冷は、水焼入れによる急冷が好ましい。
二次冷却停止温度は100℃以下とする。二次冷却停止温度が100℃超では急冷時にオーステナイトの焼入れが十分に生じないことによるマルテンサイトの面積率低下、および急冷により生成したマルテンサイトの自己焼戻しによる材料強度の低下を誘引するため製造上好ましくない。
上記急冷に引き続き、マルテンサイトの焼戻しのために、100℃以上300℃以下の温度まで再加熱して100~300℃の温度域で120~1800秒間滞留する焼戻し処理を行う。この焼戻しはマルテンサイトを軟質化させ、加工性を向上させる。焼戻しを100℃未満で行った場合、マルテンサイトの軟質化が不十分であり、加工性の向上効果が期待できず、フェライトとの硬度差が大きくなるため伸びフランジ性が劣化する。また、焼戻しを300℃超で行うことは、再加熱のための製造費用を高めるだけでなく、著しい強度の低下を招き、有用な効果を得ることができない。好ましくは150~250℃の範囲である。滞留時間を120秒未満とした場合、100℃から300℃までの温度域でのマルテンサイトの軟質化が十分には生じないため、加工性の向上効果が期待できない。また、滞留時間が1800秒を超える場合、マルテンサイトの軟質化が過度に進行することにより強度が著しく低下することに加え、再加熱時間の増加により製造費用を高めるため好ましくない。
酸洗、再酸洗を施すことで、鋼板表面のSi酸化物、Si-Mn酸化物を除去し、化成処理性を向上させる。再酸洗では、酸洗で用いる酸洗液とは異なり、かつ、非酸化性の酸を酸洗液として用いることが好ましい。
酸洗は常法にて行うことができ、条件は特に限定されない。例えば、硝酸、塩酸、弗酸、硫酸およびそれらを2種以上混合した酸のいずれかを用いることができる。
焼戻し処理後の鋼板に例えば濃度:50g/L超え200g/L以下の硝酸等の強酸を用いて酸洗することで、化成処理性を劣化させる鋼板表面のSiを主体とする酸化物やSi-Mn複合酸化物を除去することが可能である。しかし、この強酸洗によって鋼板表面から溶解したFeが鉄系酸化物を生成し、鋼板表面に沈殿析出して鋼板表面を覆うことにより化成処理性が劣化してしまう。そのため、化成処理性改善のためには、上記強酸洗後にさらに適正な条件で再酸洗し、鋼板表面に析出した鉄系酸化物を溶解・除去することが必要である。以上の理由により、再酸洗では、酸洗で用いる酸洗液とは異なり、かつ、非酸化性の酸を酸洗液として用いることが好ましい。上記非酸化性の酸とは、例えば、塩酸、硫酸、リン酸、ピロリン酸、ギ酸、酢酸、クエン酸、弗酸、シュウ酸およびこれらの2種以上を混合した酸のいずれかが挙げられる。例えば、濃度が0.1~50g/Lの塩酸、0.1~150g/Lの硫酸、0.1~20g/Lの塩酸と0.1~60g/Lの硫酸を混合した酸などが好適に利用できる。
Ac1点およびAc3点は熱膨張測定装置を用いて平均加熱速度3℃/sで得られた変態膨張曲線から得た。
λ(%)={(d-d0)/d0}
ここで、d0:初期穴径、d:クラックが板厚を貫通した時点の穴径である。
Claims (9)
- 成分組成は、質量%で、C:0.15%以上0.22%以下、Si:1.0%以上2.0%以下、Mn:1.7%以上2.5%以下、P:0.05%以下、S:0.02%以下、Al:0.01%以上0.05%以下、N:0.005%以下を含有し、かつ下記式(1)を満たし、残部は鉄及び不可避的不純物からなり、
組織は、面積率で、焼戻しマルテンサイトが60%以上100%未満、未変態オーステナイトが5%以下(0%含む)、残部がフェライトであり、該フェライトの平均結晶粒径が3.5μm未満であり、
鋼板表面において、円相当直径5μm以下のSi-Mn複合酸化物の個数が10個/100μm2未満であり、Siを主体とする酸化物の鋼板表面被覆率が1%以下であり、
引張強度が1300MPa以上である高強度冷延鋼板。
[Si]/[Mn] ≧ 0.5 … (1)
ただし、式中[Si]はSi含有量(質量%)を示し、[Mn]はMn含有量(質量%)を示す。 - 前記成分組成に加えて、質量%で、Ti:0.010%以上0.020%以下を含有する請求項1に記載の高強度冷延鋼板。
- 前記成分組成に加えて、質量%で、Nb:0.02%以上0.10%以下を含有する請求項1または2に記載の高強度冷延鋼板。
- 前記成分組成に加えて、質量%で、B:0.0002%以上0.0020%以下を含有する請求項1~3のいずれか一項に記載の高強度冷延鋼板。
- 前記成分組成に加えて、質量%で、V:0.01%以上0.30%以下、Mo:0.01%以上0.30%以下、Cr:0.01%以上0.30%以下の1種以上を含有する請求項1~4のいずれか一項に記載の高強度冷延鋼板。
- 前記成分組成に加えて、質量%で、Cu:0.01%以上0.30%以下、Ni:0.01%以上0.30%以下の1種以上を含有する請求項1~5のいずれか一項に記載の高強度冷延鋼板。
- 前記成分組成に加えて、質量%で、Sn:0.001%以上0.100%以下、Sb:0.001%以上0.100%以下、Ca:0.0002%以上0.0100%以下、W:0.01%以上0.10%以下、Co:0.01%以上0.10%以下、REM:0.0002%以上0.0050%以下の1種以上を含有する請求項1~6のいずれか一項に記載の高強度冷延鋼板。
- 請求項1~7のいずれか一項に記載の成分組成を有する鋼素材を、1200℃以上の温度に加熱し、次いで、仕上げ圧延出側温度800℃以上とする熱間圧延を施し、450℃以上700℃以下の温度で巻取り、冷間圧延し、
次いで、Ac1点以上Ac3点以下の焼鈍温度まで加熱しAc1点からAc3点までの温度域の滞留時間が30秒以上1200秒以下であり、前記焼鈍温度から一次冷却停止温度600℃以上の温度まで平均冷却速度100℃/s未満で一次冷却し、二次冷却停止温度100℃以下の温度まで平均冷却速度100℃/s以上1000℃/s以下で二次冷却する焼鈍処理を施し、
次いで、100℃以上300℃以下の温度まで加熱し、100℃から300℃までの温度域の滞留時間が120秒以上1800秒以下である焼戻し処理を施し、
さらに、酸洗、再酸洗を施す高強度冷延鋼板の製造方法。 - 前記再酸洗では、前記酸洗で用いる酸洗液とは異なり、かつ、非酸化性の酸を酸洗液として用いる請求項8に記載の高強度冷延鋼板の製造方法。
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