WO2012033210A1 - 伸びフランジ性に優れた高強度冷延鋼板およびその製造方法 - Google Patents
伸びフランジ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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- 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
- 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
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
-
- 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
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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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
- C21D2211/002—Bainite
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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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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P10/00—Technologies related to metal processing
- Y02P10/20—Recycling
Definitions
- the present invention relates to a high-strength cold-rolled steel sheet suitable for use in automobile parts and the like that are press-formed into a complicated shape, and particularly relates to improvement of stretch flangeability.
- high-strength steel plate refers to a steel plate having a high strength of tensile strength: 590 MPa or more.
- the “steel plate” here includes a steel plate and a steel strip.
- Patent Document 1 describes “a method for producing a high-strength cold-rolled steel sheet having excellent stretch flangeability”.
- the technique described in Patent Document 1 includes C: 0.04% or more and less than 0.20%, Si: 1.50% or less, Mn: 0.50 to 2.00%, P: 0.10% or less, S: 0.005% or less, Cr: 2.00% or less, or further including one or more of Ca, Ti, Nb, REM, and Ni, and a cold steel plate composed of the remaining Fe and inevitable impurities After rolling, annealing is performed in a two-phase region, and cooling is performed so as to stay at a temperature between 650 ° C.
- Patent Document 2 describes “a composite structure steel plate excellent in elongation and stretch flangeability”.
- the steel sheet described in Patent Document 2 contains, by mass%, C: 0.02 to 0.12%, Si + Al: 0.5 to 2.0%, and Mn: 1.0 to 2.0%.
- the second phase structure which is composed of 80% or more of polygonal ferrite, 1 to 7% of retained austenite, the balance of bainite and / or martensite, and martensite and retained austenite, has an aspect ratio Has a composite structure in which the number of aggregated second phase structures having an average particle size of 0.5 ⁇ m or more is 15 or less in 750 ⁇ m 2 .
- elongation at room temperature and stretch flangeability are improved by shape control of the second phase structure.
- Patent Document 1 a large amount of Cr that adversely affects chemical conversion treatment is essential, and the C content is also high, leaving problems in chemical conversion treatment and spot weldability.
- Patent Document 2 contains a large amount of Si and Al that reduce chemical conversion property and spot weldability, and has a problem that chemical conversion property and spot weldability are low.
- Patent Document 3 describes “a method for producing a high-strength steel sheet excellent in elongation and stretch flangeability”.
- the technique described in Patent Document 3 is as follows: C: 0.05 to 0.3%, Si: 0.01 to 3%, Mn: 0.5 to 3.0%, Al: 0.01 to 0.1 And a composition containing one or more selected from Ti, Nb, V, and Zr in a total of 0.01 to 1% and a total space factor of martensite and / or bainite is 90%
- a steel plate having a prior austenite grain diameter of 20 ⁇ m or less in equivalent circle diameter is used as a raw steel plate, and after heating and holding in a temperature range of (Ac 3 points ⁇ 100 ° C.) to Ac 3 points for 1 to 2400 seconds, It is cooled to an Ms point or less at an average cooling rate of 10 ° C./second or more, and subsequently reheated and maintained in a temperature range of 300 to 550 ° C.
- Patent Document 3 has a problem that it contains a large amount of Si, has a high C content, and has reduced chemical conversion properties and spot weldability.
- the technique described in Patent Document 3 requires a heating and reheating step after cooling, and there is a concern that the manufacturing cost will increase.
- increasing the strength of a steel plate often involves adding a large amount of alloy elements such as C and Si.
- it is also required to adjust the C amount and Si amount to appropriate ranges in order to ensure the chemical conversion processability and spot weldability required for automobile bodies. ing.
- Patent Document 4 describes “a method for producing a high-strength cold-rolled steel sheet having excellent chemical conversion properties and stretch flangeability”.
- a steel slab having a composition different in surface layer part and other interiors is hot-rolled, then cold-rolled and heated to 800 ° C. or higher in a continuous annealing line, and then 30 ° C.
- This is a method for producing a high-strength cold-rolled steel sheet that is cooled to 350 to 500 ° C. at a cooling rate of at least / sec and is maintained in the temperature range for at least 40 seconds.
- the components of the surface layer are: C: 0.20% or less, Si: 0.04% or less, Mn: 0.1 to 3.0%, P: 0.025% or less, S: 0.005% or less, Al : 0.01 to 0.1%, or further including one or more of Ca, REM, and Zr, the balance being Fe and inevitable impurities, the other internal components are C: 0.04 ⁇ 0.20%, Si: 0.5-2.0%, Mn: 0.5-3.0%, and C, Si, Mn satisfies a specific relational expression, P: 0.025% or less, S: 0.005% or less, Al: 0.01 to 0.1%, or one or more of Ca, REM, and Zr, and the balance Fe and unavoidable impurities.
- Patent Document 5 describes “a method for producing a high-strength steel sheet excellent in workability”.
- the technique described in Patent Document 5 is as follows: C: 0.03-0.13%, Si: 0.02-0.8%, Mn: 1.0-2.5%, Al: 0.01-0 0.1%, N: 0.01% or less, Ti: 0.004 to 0.1% and / or Nb: 0.004 to 0.07% in a cold rolled steel sheet having an average heating rate of 5 After heating to a temperature range above the Ac3 transformation point at °C / s and holding for 10 to 300 seconds in the temperature range, from the temperature range to a temperature range of 400 to 600 °C with an average cooling rate of 2 °C / s or more This is a method for producing a high-strength steel sheet, which is cooled, held in the temperature range for 40 to 400 seconds, and then subjected to an annealing step for cooling to obtain a high-strength steel sheet.
- the area% is ferrite: 50 to 86%, bainite: 10 to 30%, martensite: 4 to 20%, and the bainite area ratio is larger than the martensite area ratio.
- the average particle size of the ferrite as the parent phase is 2.0 to 5.0 ⁇ m
- the second phase has a structure having bainite and martensite, and is excellent in TS-El balance and TS- ⁇ balance. It is said that a high-strength steel sheet of 590 to 780 MPa class that is excellent in properties can be obtained.
- Patent Document 6 describes “a method for producing a high-strength cold-rolled steel sheet excellent in the balance between elongation and stretch flangeability”.
- C 0.05 to 0.30%, Si: 3.0% or less, Mn: 0.1 to 5.0%, Al: 0.001 to 0.10% Nb: 0.02 to 0.40%, Ti: 0.01 to 0.20%, V: 0.01 to 0.20%, or one or more of (Nb / 96 + Ti / 51 + V / 48) Finishing rolling end temperature: 900 ° C. or higher, cooling time to 550 ° C .: (finishing end temperature ⁇ 550 ° C.) / 20 s or less, coiling temperature: 500 ° C.
- cold rolling rate 20-80% cold rolling, 600 ° C. to Ac1 temperature range satisfying specific relationship Heating up to a temperature in the range of (8 ⁇ Ac1 + 2 ⁇ Ac3) / 10 to 1000 ° C. at a rate of temperature increase, 3600 s or less at that temperature
- After holding rapidly cool to a temperature below the Ms point at a cooling rate of 50 ° C./s or gradually cool to a temperature of up to 600 ° C. and then cool to a temperature below the Ms point at a cooling rate of 50 ° C./s or less.
- the soft phase contains ferrite in an area ratio of 10 to 80%, further includes residual austenite, martensite, and a mixed structure thereof in an area ratio of less than 5%, and the balance is tempered martensite and / or It has a structure composed of a hard phase composed of tempered bainite, and has an excellent balance between elongation and stretch flangeability by reducing the amount of strain in ferrite as much as possible and enhancing the deformability of the hard phase.
- a high-strength cold-rolled steel sheet having a tensile strength of 780 MPa or more is obtained.
- Patent Document 7 describes “a method for producing a cold-rolled steel sheet” having good elongation and bendability while having a high tensile strength of 780 MPa or more and a thick plate thickness of 2.0 mm or more.
- the technique described in Patent Document 7 includes C: 0.08 to 0.20%, Si: 1.0% or less, Mn: 1.8 to 3.0%, sol.
- a hot rolled steel sheet having a composition containing Al: 0.005 to 0.5%, N: 0.01% or less, and Ti: 0.02 to 0.2% is subjected to cold rolling with a reduction ratio of 30 to 60%.
- the cold-rolled steel sheet is retained within a temperature range of Ac3 to (Ac3 + 50 ° C.) for 240 seconds, cooled to a temperature range of 680 to 750 ° C. at an average cooling rate of 1 to 10 ° C./second, This is a method for producing a cold-rolled steel sheet that is cooled to 400 ° C. or lower at an average cooling rate of 20 to 50 ° C./second.
- the volume ratio was 10% or more of ferrite, 20 to 70% of bainite, 3 to 20% of retained austenite, and 0 to 20% of martensite, and the average particle diameter was 10 ⁇ m or less for ferrite, 10 ⁇ m or less for bainite, While having a structure of 3 ⁇ m or less at the site, a high tensile strength TS of 780 MPa or more and a thick plate thickness of 2.0 mm or more, TS ⁇ E1 is 14000 MPa ⁇ % or more and the minimum bending radius is 1. It is supposed to be a cold-rolled steel sheet having excellent bending characteristics of 5 t or less.
- Patent Document 4 it is required to use a steel slab whose composition is changed between the surface layer and the other inside, and a special cladding technique or the like is used to make such a steel slab. There is a problem that the manufacturing cost increases. Further, the technique described in Patent Document 5 has a problem that the bainite fraction is low and excellent bending characteristics cannot be secured stably. Moreover, since the temperature rising rate at the time of annealing is high, there is also a problem that the stability of the structure is lacking. Further, the technique described in Patent Document 6 is directed to a steel sheet having a high Si content, has a high C content, and has left problems in chemical conversion property and weldability.
- the present invention advantageously solves the problems of the prior art, does not use a special cladding technique, and does not contain a large amount of an alloying element such as C or Si. It aims at providing a rolled steel plate and its manufacturing method.
- it does not contain Si, Cr which adversely affects chemical conversion property, does not contain a large amount of C, Si, Al which adversely affects spot weldability, and is an expensive alloy element, Ni, It is a component system that does not contain Cu, Mo, or the like, and aims to improve stretch flangeability while maintaining high strength of tensile strength: 590 MPa or more.
- excellent in stretch flangeability means the product of tensile strength TS and elongation El, strength-elongation balance TS ⁇ El of 16000 MPa% or more, product of tensile strength TS and hole expansion ratio ⁇ , The case where the strength-hole expansion rate balance TS ⁇ ⁇ satisfies 40000 MPa% or more is assumed.
- the present inventors diligently studied the influence of the metal structure on stretch flangeability.
- the heating and cooling conditions during annealing of cold-rolled sheets are devised, and the content of alloy elements such as C and Si is low by strictly adjusting the structural fraction of ferrite, bainite, martensite, and retained austenite. It was found that even in the component system, a cold-rolled steel sheet having excellent stretch flangeability can be produced while maintaining a high strength of tensile strength: 590 MPa or more.
- heating should be performed in two stages and cooling should be performed in two stages, particularly in the latter half compared to the first half. It has been found that it is important that the cooling time is slow and the latter cooling time is 0.2 to 0.8 of the total cooling time.
- the present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
- a high-strength cold-rolled steel sheet having excellent stretch flangeability.
- the steel material is mass% and C: 0.050 to 0.00. 090%, Si: 0.05% or less, Mn: 1.5 to 2.0%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.1%, N : 0.01% or less, Ti: 0.005 to 0.050%, Nb: 0.020 to 0.080%, a steel material having a composition composed of the remaining Fe and unavoidable impurities, the annealing step, The maximum temperature reached: 800 to 900 ° C., a process having two stages of heating and two stages of cooling, and the two stages of heating are performed from 50 ° C.
- the first stage of heating is performed up to the first stage heating temperature in the temperature range of (maximum temperature -50 ° C) to (maximum temperature -10 ° C).
- Heat and a second stage heating in which the temperature rise time from the temperature range to the maximum temperature reached is 30 to 150 s.
- the two-stage cooling is performed from the maximum temperature to an average cooling rate: 10 to Cooling at the first stage cooling at the first stage cooling rate of 40 ° C./s, followed by 400 to 500 ° C. at the average cooling rate: (0.2 to 0.8) ⁇ first stage cooling rate.
- a method for producing a high-strength cold-rolled steel sheet excellent in stretch flangeability characterized by retaining for 100 to 1000 seconds in a temperature range of 400 ° C to 500 ° C after completion of cooling of the stage.
- the tensile strength TS high strength of 590 MPa or more, excellent elongation satisfying the strength-elongation balance TS ⁇ El of 16000 MPa% or more and the strength-hole expansion rate balance TS ⁇ ⁇ of 40000 MPa% or more.
- a high-strength cold-rolled steel sheet that has flangeability and is suitable for automobile parts that are press-molded into a complicated shape can be manufactured stably and inexpensively, and has a remarkable industrial effect.
- C 0.050 to 0.090%
- C is an element that increases the strength of the steel by solid solution or precipitation as carbide in the steel, and forms a bainite phase and a martensite phase that are low-temperature transformation phases through an increase in hardenability. This contributes to an increase in the strength of the steel sheet by strengthening the structure.
- it is necessary to contain 0.050% or more.
- the content exceeds 0.090%, the spot weldability is adversely affected, and the martensite phase is excessively hardened, so that stretch flangeability is deteriorated.
- C is limited to the range of 0.050 to 0.090%. It is preferably 0.060 to 0.080%.
- Si 0.05% or less
- Si When Si is contained in a large amount, it hardens and the workability decreases. Moreover, when Si is contained in a large amount, a Si oxide is generated particularly during annealing, which adversely affects chemical conversion treatment properties. For this reason, Si is desirably reduced as much as possible as an impurity in the present invention, and is limited to 0.05% or less.
- Mn 1.5 to 2.0%
- Mn is an element that contributes to increasing the strength of the steel through solid solution to increase the strength of the steel and improve the hardenability. Such an effect becomes remarkable when the content is 1.5% or more.
- excessive content exceeding 2.0% improves hardenability and increases the amount of low-temperature transformation phase, so that the steel sheet is excessively hardened to ensure a desired ferrite phase fraction. Becomes difficult, and press formability decreases. For this reason, Mn was limited to the range of 1.5 to 2.0%. Note that the content is preferably 1.6 to 1.9%.
- P 0.030% or less P segregates at the grain boundary and has an adverse effect of reducing ductility and toughness. Moreover, P reduces spot weldability. For this reason, it is desirable to reduce P as much as possible. However, excessive reduction increases the refining time for dephosphorization, lowers the production efficiency, and increases the manufacturing cost. Is preferred. Further, if the content exceeds 0.030%, the spot weldability is significantly reduced. For this reason, P was limited to 0.030% or less. In addition, Preferably it is 0.001% or more and less than 0.020%.
- S 0.0050% or less S is present in steel as inclusions and hardly contributes to the strength. It also forms coarse MnS and has ductility, in particular, the origin of cracking during stretch flange molding, and stretch flangeability. In order to reduce, it is preferable to reduce as much as possible. However, excessive reduction increases the desulfurization time in the steel making process, lowers the production efficiency, and causes an increase in production cost, so 0.0001% or more is preferable. If the content exceeds 0.0050%, the stretch flangeability is remarkably lowered, so S is limited to 0.0050% or less. Preferably, the content is 0.0001 to 0.0030%.
- Al 0.005 to 0.1%
- Al is an element that acts as a deoxidizer, and in order to obtain this effect sufficiently, it needs to be contained in an amount of 0.005% or more.
- the content exceeds 0.1%, weldability such as flash butt welding is deteriorated, the effect of Al addition is saturated, and the production cost increases due to the addition of a large amount. For this reason, Al was limited to the range of 0.005 to 0.1%.
- the content is preferably 0.02 to 0.06%.
- N 0.01% or less N is an impurity in the present invention, but it may reduce aging resistance as solute N, and is preferably reduced as much as possible, but excessive reduction increases the refining time, In order to increase the manufacturing cost, it is preferable that the amount is about 0.0020% or more from the viewpoint of economy. On the other hand, if the content exceeds 0.01%, the tendency of occurrence of slab cracks, slab internal defects, etc. is increased, and surface defects may occur. For this reason, N was limited to 0.01% or less. In addition, Preferably it is 0.0050% or less.
- Ti 0.005 to 0.050%
- Ti is an element that forms carbonitride and has the effect of suppressing the coarsening of austenite grains during slab heating, etc., and contributes effectively to the refinement and homogenization of hot rolled sheet structure and steel sheet structure after annealing. To do. In order to acquire such an effect, 0.005% or more of content is required. On the other hand, if the content exceeds 0.050%, precipitates are excessively generated in the ferrite phase, and the ductility of the ferrite phase is lowered. Moreover, the further excessive content of Ti hardens a hot-rolled sheet too much, and increases the rolling load at the time of hot rolling or cold rolling. For this reason, Ti was limited to the range of 0.005 to 0.050%. Note that the content is preferably 0.010 to 0.0040%.
- Nb 0.020 to 0.080%
- Nb is an element that contributes to increasing the strength of the steel sheet by forming a solid solution in the steel and by solid solution strengthening or by forming a carbonitride and by precipitation strengthening. To obtain such an effect, Nb is 0.020% or more. Containing is required.
- excessive content exceeding 0.080% excessively produces precipitates in the ferrite phase, lowers the ductility of the ferrite phase, and excessively hardens the hot-rolled sheet, during hot rolling or cold. Increase the rolling load during rolling. For this reason, Nb was limited to a range of 0.020 to 0.08%. Note that the content is preferably 0.030 to 0.050%.
- Ti contributes to the refinement and homogenization of the hot-rolled sheet structure and the steel sheet structure after annealing by suppressing the coarsening of the austenite grains, while Nb is dissolved in the steel and solidified. It contributes to increasing the strength of the steel sheet by melt strengthening or by forming carbonitride and by precipitation strengthening.
- Ti and Nb having such an action are combined and contained.
- the Nb content is greater than the Ti content in the present invention.
- Nb / Ti is 1.5 or more.
- Nb / Ti is preferably 1.8 or more and 5.0 or less.
- Nb and Ti are partly redissolved in the heating stage of hot rolling, but are precipitated as Ti-based carbonitrides or Nb-based carbonitrides in the subsequent rough rolling, finish rolling, and further winding stages.
- Ti-based carbonitrides are deposited at a high temperature, while Nb-based carbonitrides are deposited at a lower temperature than Ti-based carbonitrides. For this reason, Ti carbonitrides have a long residence time at high temperatures, and tend to grow and become coarse.
- Nb-based carbonitrides are finer and have a relatively dense distribution because the precipitation temperature is lower than that of Ti-based carbonitrides.
- Fine carbonitride has a pinning effect on crystal grains, and during annealing, recovery of cold-rolled structure, recrystallization, and grain growth are delayed, resulting in a uniform fine structure in the steel sheet finally obtained. can do.
- Ti and Nb in combination, such a uniform fine structure can be obtained, and the bending characteristics of the steel sheet are remarkably improved.
- Ca 0.0001 to 0.0050%
- Ca is an element that contributes effectively to the shape control of inclusions.
- MnS that is expanded in the cold rolling process and becomes plate-like inclusions into CaS that is a spherical inclusion, before the annealing process.
- the form of inclusions is controlled to improve ductility and stretch flangeability.
- Such an effect is recognized when the content is 0.0001% or more.
- the content exceeds 0.0050%, the effect is saturated and an effect commensurate with the content cannot be expected.
- Ca is preferably limited to a range of 0.0001 to 0.0050%. More preferably, the content is 0.0005 to 0.0020%.
- the balance other than the components described above consists of Fe and inevitable impurities.
- the cold-rolled steel sheet of the present invention is composed of 50% to 77% ferrite phase, 20% to 50% bainite phase, 2% to 10% martensite phase, and 1% to 5% residual austenite phase.
- the ferrite phase is soft and contributes to the ductility (elongation) of the cold-rolled steel sheet.
- the volume fraction of the ferrite phase needs to be 50% or more.
- the desired high strength TS: 590 MPa or more
- the volume fraction of the ferrite phase is limited to a range of 50 to 77%.
- Preferably it is 50 to 65%, More preferably, it is 50 to 60%.
- the crystal grain size of the ferrite phase is too large, the low-temperature transformation phase is localized, causing non-uniform deformation, and it becomes difficult to ensure excellent formability.
- the average crystal grain size of the ferrite phase is preferably in the range of 1 to 10 ⁇ m.
- Bainite phase 20-50%
- the bainite phase is one of the low-temperature transformation phases, and in order to ensure a desired high strength, the present invention needs to contain 20% or more.
- the content exceeds 50%, the steel sheet becomes excessively hard and formability decreases.
- the volume fraction of the bainite phase was limited to a range of 20 to 50%.
- the content is preferably 30 to 50%, more preferably more than 30% and 50% or less, and still more preferably 35 to 45%.
- the average crystal grain size of the bainite phase exceeds 10 ⁇ m, the structure becomes a non-uniform structure, and non-uniform deformation occurs during molding, making it difficult to ensure excellent formability.
- the average crystal grain size of the bainite phase is preferably in the range of 1 to 10 ⁇ m.
- the ratio between the bainite phase and the martensite phase is also important.
- the bainite phase is softer than the martensite phase, the strength difference (hardness difference) from the ferrite phase is smaller than that of the martensite phase, and the entire steel sheet is uniformly deformed during forming, so in particular from the viewpoint of improving stretch flangeability.
- the site phase More advantageous than the site phase.
- the low-temperature transformation phase is mainly composed of a bainite phase, and the martensite phase is contained in a small amount. Thereby, it is possible to ensure excellent moldability such as stretch flangeability while ensuring desired high strength.
- the low temperature transformation phase in this invention means a bainite phase and a martensite phase.
- the bainite phase contributes effectively to the improvement of bending workability.
- the bending strain can be uniformly deformed without locally concentrating.
- strain concentrates on the interface of the phase and cracks occur. This is because when a predetermined amount of the bainite phase having intermediate hardness exists, the strain is not locally concentrated during bending and the strain is dispersed, so that uniform deformation can be performed.
- Martensite phase 2-10%
- the martensite phase is hard as a low-temperature transformation phase and greatly contributes to an increase in the strength of the steel sheet.
- many voids are generated at the interface between the martensite phase and the ferrite phase due to the hardness difference between the martensite phase and the ferrite phase.
- the crack extends and leads to cracking.
- the presence of a large amount of martensite phase reduces stretch flangeability.
- the volume fraction of the martensite phase exceeds 10%, the strength becomes too high, the ductility is remarkably reduced, and the interface between the martensite phase and the ferrite phase increases, ensuring excellent stretch flangeability. It becomes difficult.
- the volume fraction of the martensite phase is less than 2%, the dispersion in the structure becomes coarse and the influence on the stretch flangeability is reduced, but a desired high strength cannot be secured stably.
- the volume fraction of the martensite phase was limited to a range of 2 to 10%. Preferably, it is 4 to 8%.
- the average crystal grain size of the martensite phase is preferably in the range of 0.5 to 5.0 ⁇ m.
- the average grain size of the martensite phase is less than 0.5 ⁇ m, the hard martensite phase is finely dispersed in the soft ferrite phase, resulting in uneven deformation due to a large hardness difference. It is difficult to ensure the moldability.
- the average grain size of the martensite phase is larger than 5.0 ⁇ m, the martensite phase is unevenly distributed and the structure becomes non-uniform, so that the deformation becomes non-uniform and excellent moldability can be secured. It becomes difficult.
- the average crystal grain size of the martensite phase is preferably limited to a range of 0.5 to 5.0 ⁇ m.
- Residual austenite phase 1-5%
- the residual austenite phase contributes to the improvement of ductility (uniform elongation) through strain-induced transformation during molding.
- C is concentrated and hard, and the hardness difference from the ferrite phase is large. For this reason, the presence of the retained austenite phase becomes a factor that reduces stretch flangeability.
- the residual austenite phase exceeds 5%, a large number of voids are generated at the interface between the residual austenite phase and the ferrite phase during punching shearing due to the hardness difference from the ferrite phase. The voids are connected to form cracks, which are further extended to crack.
- the volume fraction of the retained austenite phase is less than 1%, the dispersion in the structure becomes coarse, so that the influence on stretch flangeability is reduced, but the improvement in ductility is small.
- the volume fraction of the retained austenite phase was limited to a range of 1 to 5%.
- the content is preferably 1 to 3%.
- the remainder other than the above-mentioned phase is a cementite phase that is inevitably generated. If the cementite phase inevitably produced is less than 3% in volume fraction, the effect of the present invention is not affected.
- the average crystal grain size of ferrite phase, bainite phase, martensite phase, etc. is observed with 5 or more fields of view with an optical microscope (magnification: 200 to 1000 times), and after identifying the structure, a cutting method or image based on JIS method. What is necessary is just to calculate by analysis.
- the steel material having the above composition is sequentially subjected to a hot rolling process, a cold rolling process, an annealing process, or a temper rolling process to obtain a cold rolled steel sheet.
- the manufacturing method of the steel material is not particularly limited.
- 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 is slabed by a conventional casting method such as a continuous casting method. It is preferable to use a steel material such as
- the steel material casting method is desirably an intermittent casting method in order to prevent macro segregation of components, but there is no problem with the ingot casting method or the thin slab casting method.
- the obtained steel material is then subjected to a hot rolling process, but the heating for hot rolling is performed by cooling to room temperature and then reheating, without cooling to room temperature.
- Energy-saving processes such as direct feed rolling and direct rolling, in which the material is charged into the heating furnace as it is or after a slight heat retention is performed, can be applied without any problem.
- the hot rolling process the steel material having the above-described composition is heated or subjected to normal hot rolling consisting of rough rolling and finish rolling without heating, to obtain a hot-rolled sheet having a predetermined size and shape, and then wound up. It is preferable to set it as a process. In the present invention, it is only necessary to obtain a hot-rolled sheet having a predetermined size and shape, and it is not particularly necessary to limit the conditions for hot rolling, but the following conditions are preferable.
- the heating temperature of the steel material is preferably 1150 ° C. or higher. If heating temperature is less than 1150 degreeC, the rolling load of hot rolling will become large.
- the upper limit of the heating temperature is not particularly limited, but is preferably set to 1300 ° C. or less from the viewpoint of crystal grain coarsening, scale loss due to oxidation, and the like.
- the heated steel material is roughly rolled into a sheet bar having a predetermined size and shape. However, the condition of the rough rolling is not particularly limited as long as it can be a sheet bar having a predetermined size and shape.
- the finish rolling end temperature in finish rolling is preferably 880 ° C. or higher.
- the finish rolling finishing temperature in finish rolling shall be 880 degreeC or more.
- the upper limit of finishing rolling finish temperature is not particularly limited, but if it becomes too high, there is a problem that the crystal grains become coarse and the workability of the cold-rolled sheet is lowered. It is preferable.
- the obtained hot rolled sheet is then wound into a coil.
- the cooling rate until winding is not particularly limited, and a cooling rate higher than air cooling is sufficient.
- the coiling temperature is preferably 450 to 650 ° C.
- the hot-rolled sheet becomes hard, the cold rolling load increases, and it becomes difficult to ensure the cold rolling reduction ratio.
- it exceeds 650 ° C. the cooling rate after winding varies in the longitudinal direction and width direction in the coil, the structure becomes non-uniform, and shape defects after cold rolling tend to occur.
- the hot-rolled sheet is then pickled and then cold-rolled.
- the cold rolling step it is preferable to subject the hot rolled sheet to cold rolling at a predetermined cold rolling reduction ratio to obtain a cold rolled sheet for ordinary cold rolling.
- the conditions of the cold rolling process need not be particularly limited, but the cold rolling reduction ratio is preferably determined by the thickness of the hot rolled sheet and the product sheet. Usually, if the cold rolling reduction ratio is 30% or more, there is no particular problem in workability and sheet thickness accuracy. On the other hand, if the cold rolling reduction ratio exceeds 70%, the load on the cold rolling mill becomes too large, and operation becomes difficult.
- the cold-rolled sheet is then subjected to an annealing process.
- the annealing process in the present invention is a process having two-stage heating and two-stage cooling.
- the maximum temperature reached in heating is 800 to 900 ° C., and then two-stage cooling is performed.
- the maximum temperature is less than 800 ° C.
- the amount of ⁇ ⁇ ⁇ transformation during heating is small, and therefore, the structure when reaching the maximum temperature becomes a ferrite + austenite two-phase structure with a large amount of ferrite, and is finally obtained.
- the steel sheet has an excessively high structure fraction of the ferrite phase, and the desired high strength cannot be ensured.
- the highest temperature exceeds 900 ° C.
- the crystal grain size of the ferrite phase and low-temperature transformation phase that are generated tends to be coarse, and the stretch flangeability decreases. For this reason, the maximum temperature reached was limited to a temperature in the range of 800 to 900 ° C.
- the two-stage heating consists of a first stage heating followed by a second stage heating.
- the heating process is important in adjusting the structural fraction of the ferrite phase and the bainite phase.
- the average temperature of the cold-rolled sheet is increased from at least 50 ° C. to the first stage heating attainment temperature in the temperature range from (maximum reached temperature ⁇ 50 ° C.) to (maximum achieved temperature ⁇ 10 ° C.).
- Speed A heating treatment at 0.5 to 5.0 ° C./s.
- the heating conditions up to 50 ° C. are not particularly limited, and may be appropriately performed according to a conventional method.
- the heating rate in the first stage heating is less than 0.5 ° C./s, the heating rate is too slow and coarsening of the austenite grains proceeds. Therefore, ⁇ ⁇ ⁇ due to the coarsening of the austenite grains during cooling. The transformation is delayed, the structural fraction of the ferrite phase to be formed is reduced, and it is hardened to deteriorate the workability.
- the rate of temperature increase in the first stage heating exceeds 5.0 ° C./s, the austenite grains to be produced become finer, and the finally obtained ferrite phase has a high structural fraction. Ensuring strength is difficult. For this reason, the rate of temperature increase in the first stage heating is limited to the range of 0.5 to 5.0 ° C./s on average. It is preferably 1.5 to 3.5 ° C./s.
- the first stage heating temperature is less than (maximum temperature -50 ° C.)
- the second stage heating up to the maximum temperature becomes rapid heating, and a desired tissue fraction can be stably secured. It becomes difficult.
- the temperature reached in the first stage exceeds the maximum temperature ( ⁇ 10 ° C.)
- the second stage heating up to the maximum temperature is gradually heated, and the residence time in the high temperature range becomes longer. The crystal grains become too coarse and workability is reduced. For this reason, the first stage heating ultimate temperature was limited to a temperature range of (maximum ultimate temperature ⁇ 50 ° C.) to (maximum ultimate temperature ⁇ 10 ° C.).
- the second stage heating is a process of heating so that the temperature rise time from the first stage heating reaching temperature to the maximum reaching temperature is 30 to 150 s. If the heating time from the first stage heating temperature to the highest temperature was less than 30 s, the heating to the highest temperature became too rapid, the ⁇ ⁇ ⁇ transformation was delayed, and finally the highest temperature was reached. At this time, the structure fraction of the ferrite phase becomes high, and a desired high strength cannot be secured. In addition, the diffusion of alloy elements such as C and Mn becomes insufficient, resulting in a non-uniform structure and workability is reduced. On the other hand, if the length is longer than 150 s, the crystal grain size becomes coarse and the workability tends to be lowered. For this reason, the temperature raising time for the second stage heating is adjusted to a range of 30 to 150 s.
- Cooling is performed immediately after the second stage heating is completed.
- the cooling after heating is a two-stage cooling. Cooling is important in order to adjust the structural fraction of the soft ferrite phase and the hard bainite phase, and to combine high strength with a tensile strength of TS: 590 MPa or more and excellent workability. For this reason, it is necessary to strictly adjust the cooling pattern, that is, the cooling rate and the cooling time so that the desired metal structure can be secured.
- the two-stage cooling consists of a first-stage cooling followed by a second-stage cooling that is slower than the first-stage cooling. The first stage cooling and the second stage cooling are important for adjusting the structure fraction of the ferrite phase and the bainite phase.
- the first-stage cooling is performed by cooling from the highest temperature at an average cooling rate of 10 to 40 ° C./s (first-stage cooling rate).
- first-stage cooling rate When the first stage cooling rate is less than 10 ° C./s, the soft ferrite phase has a high structural fraction, and it becomes difficult to ensure a desired high strength.
- the first stage cooling rate is rapid cooling exceeding 40 ° C./s, the amount of ferrite phase generated is reduced, the steel sheet becomes hard and workability is lowered.
- the second stage cooling immediately follows the first stage cooling, and immediately depends on the first stage cooling rate (0.2 to 0.8) ⁇ (first stage cooling rate) second stage cooling. It is set as the process which cools to the 2nd stage cooling stop temperature of 400-500 degreeC with a cooling rate. If the second stage cooling rate is less than 0.2 ⁇ (first stage cooling rate), the cooling is too slow and the formation of a soft ferrite phase is promoted, the bainite phase structure fraction is lowered, and the desired high strength is achieved. It cannot be secured.
- the second stage cooling rate was limited to the range of 0.2 to 0.8 ⁇ (first stage cooling rate).
- the cooling time of the first stage cooling and the second stage cooling is distributed.
- the cooling time for the second stage cooling is set to a cooling time of 0.2 to 0.8 of the total cooling time, which is the sum of the cooling times for the first stage cooling and the second stage cooling. That is, the second stage cooling time is (0.2 to 0.8) ⁇ total cooling time. If the cooling time of the second stage cooling is less than 0.2 of the total cooling time, the cooling time at the first stage cooling rate becomes longer, the amount of ferrite phase generated decreases, and the structure fraction of the bainite phase increases. Thus, the steel plate becomes hard and the desired stretch flangeability cannot be secured.
- the cooling time of the second stage cooling is limited to 0.2 to 0.8 of the total cooling time.
- the cooling stop temperature in the second stage cooling is less than 400 ° C.
- the structure is mainly composed of a hard martensite phase
- the steel sheet becomes excessively hard, and the stretch flangeability deteriorates.
- the cooling stop temperature of the second stage cooling exceeds 500 ° C.
- the structure fraction of the ferrite phase decreases
- the steel plate becomes hard, and a pearlite phase is generated, and excellent workability. It will be difficult to ensure.
- the cooling stop temperature of the second stage cooling is limited to the range of 400 to 500 ° C.
- the residence time after stopping the cooling is important for adjusting the structure fraction of the bainite phase. If the residence time is less than 100 s, the transformation from austenite to bainite is insufficient, and the untransformed austenite is transformed into the martensite phase. Therefore, the structural fraction of the martensite phase is increased, and the steel sheet is hardened and has workability. descend. On the other hand, when the residence time is longer than 1000 s, the structure fraction of the bainite phase increases, and it becomes difficult to ensure desired excellent workability. For this reason, the residence time after stopping the cooling is limited to 100 to 1000 s. Although cooling is continued after the above-mentioned residence, the conditions are not particularly limited and may be appropriately determined according to the production equipment.
- the cold-rolled annealed plate may be further subjected to a temper rolling step for the purpose of shape correction and surface roughness adjustment.
- a temper rolling step for the purpose of shape correction and surface roughness adjustment.
- Excessive temper rolling rolls crystal grains to form a rolled structure, so that the ductility is lowered and the workability is lowered. Therefore, the temper rolling process has an elongation of 0.05 to 0.5. % Temper rolling is preferable.
- Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab (steel material) by a continuous casting method. These steel materials (slabs) were used as starting materials, heated to 1200 ° C, and then subjected to hot rolling at a finish rolling finish temperature of 900 ° C and a coiling temperature of 600 ° C to form hot rolled sheets. . Next, the hot-rolled sheet is subjected to hydrochloric acid pickling and then cold-rolled to form a cold-rolled sheet, followed by two-stage heating and two-stage cooling under the conditions shown in Table 2.
- An annealing process for annealing was performed to obtain a cold-rolled annealed sheet having a thickness of 1.4 mm. From the obtained cold-rolled steel plate (cold-rolled annealed plate), test pieces were collected and subjected to a structure observation test, a tensile test, a hole expansion test, and a bending test.
- the test method was as follows.
- Microstructure observation test Samples for microstructural observation were collected from the obtained cold-rolled steel sheet, the cross section in the rolling direction was polished, corroded (Nital solution), and the optical microscope ( The number of fields: 5 or more fields were observed and imaged with a scanning electron microscope (magnification: 3000 times). While identifying the structure
- the average crystal grain size of the ferrite phase was determined by a cutting method in accordance with the method defined in JIS G 0552. Moreover, it carried out similarly about the bainite phase and the martensite phase.
- All of the examples of the present invention have excellent tensile strength TS: high strength of 590 MPa or more, strength-elongation balance TS ⁇ El of 16000 MPa% or more, and strength-hole expansion ratio balance TS ⁇ ⁇ of 40000 MPa% or more. It is a high-strength cold-rolled steel sheet having excellent bendability that can withstand severe bending as well as stretch flangeability.
- the strength is insufficient, the elongation El is low, or TS ⁇ El is less than 16000 MPa%, and the stretch flangeability is deteriorated.
- the hole expansion rate is low, and TS ⁇ ⁇ is less than 40000 MPa%.
- the comparative examples (steel plates No. 8 and No. 9) whose composition falls outside the scope of the present invention have few ferrite phases, cannot secure a desired structure, have low elongation El, and have low stretch flangeability and bending workability. .
- Comparative example (steel plate No. 10) where the rate of temperature rise in the annealing process is slow and deviates from the scope of the present invention comparative example (steel plate No. 13) having a high maximum temperature and deviates from the scope of the present invention, temperature rise time of the second stage heating
- a comparative example (steel plate No. 15) that deviates from the scope of the present invention for a long time a comparative example (steel plate No. 17) that has a fast cooling rate for the first stage cooling, and a large cooling rate for cooling the second stage.
- Comparative example (steel plate No. 19) that deviates from the scope of the invention comparative example (steel plate No.
- the comparative examples (steel plate No. 23) and the comparative examples (steel plates No. 24 and No. 25) whose residence time deviates from the scope of the present invention have low ferrite phase structure fractions, and stretch flangeability is reduced. .
- the comparative example (steel plate No. 25) in which the residence time is out of the range of the present invention has a structure fraction of the bainite phase that is out of the range of the present invention, and the stretch flangeability is deteriorated.
- the comparative example (steel plate No. 11) where the temperature rising rate in the annealing process is fast and deviates from the range of the present invention the comparative example (steel plate No. 12) whose maximum ultimate temperature is low and deviates from the range of the present invention, Comparative example (steel plate No. 14) that is short in time and out of the scope of the present invention, comparative example (steel plate No. 16) in which the cooling rate of the first stage cooling is slow and out of the scope of the present invention, and cooling speed in the second stage is slow.
Abstract
Description
しかし、使用する鋼板の高強度化にともない、プレス成形性が低下する。とくに伸びフランジ性が大きく低下する傾向にある。このため、プレス成形性、とくに伸びフランジ性に優れた高強度鋼板が要求されている。
このように、鋼板の高強度化には、C,Si等の合金元素の多量添加を伴う場合が多く、このような場合にはプレス成形性の低下とともに、化成処理性やスポット溶接性の低下を伴う。そこで、伸びフランジ性などのプレス成形性の向上とともに、自動車車体用として要求される化成処理性、スポット溶接性を確保するために、とくにC量およびSi量を適正範囲に調整することも要求されている。
また、特許文献5に記載された技術では、ベイナイト分率が低く優れた曲げ特性を安定して確保できないという問題を残していた。また、焼鈍時の昇温速度が速いため、組織の安定性に欠けるという問題もある。
また、特許文献6に記載された技術では、Si含有量が高い組成の鋼板を指向しており、しかもC含有量が高く、化成処理性、溶接性に問題を残していた。さらに特許文献6に記載された技術では、昇温再加熱工程を必要とし、製造工程が複雑になり、製造コストが高騰するという懸念がある。
また、特許文献7に記載された技術では、C,Mn,Ti含有量が高く、溶接性が低下するという問題がある。また、Mn含有量が高いため、伸びフランジ性に悪影響を及ぼすMnバンドが残存し、さらに介在物の球状化が不十分であるため、伸びフランジ性が低下するという問題を残している。
なお、ここでいう「伸びフランジ性に優れた」とは、引張強さTSと伸びElの積、強度−伸びバランスTS×Elが16000MPa%以上、引張強さTSと穴拡げ率λの積、強度−穴拡げ率バランスTS×λが40000MPa%以上を満足する場合をいうものとする。
本発明は、かかる知見に基づき、さらに検討を加えて完成されたものである。すなわち、本発明の要旨は次のとおりである。
Cは、鋼中に固溶してあるいは炭化物として析出して、鋼の強度を増加させる元素であり、また、焼入れ性の増加を介して、低温変態相であるベイナイト相やマルテンサイト相を形成しやすくし、組織強化により、鋼板の強度増加に寄与する。このような作用を利用して、引張強さTS590MPa以上を確保するためには、0.050%以上の含有を必要とする。一方、0.090%を超える含有は、スポット溶接性に悪影響を及ぼすとともに、マルテンサイト相が過度に硬質化するため、伸びフランジ性を低下させる。このようなことから、Cは0.050~0.090%の範囲に限定した。なお好ましくは0.060~0.080%である。
Siは、多量に含有すると硬質化し、加工性が低下する。また、Siを多量に含有すると、とくに焼鈍時にSi酸化物を生成し、化成処理性を阻害するなどの悪影響を及ぼす。このようなことから、Siは、本発明では不純物として、できるだけ低減することが望ましく、0.05%以下に限定した。
Mnは、固溶して鋼の強度を増加させるとともに、焼入れ性の向上を通じて鋼の強度増加に寄与する元素である。このような作用は、1.5%以上の含有で顕著となる。一方、2.0%を超える過度の含有は、焼入れ性が向上して低温変態相の生成量が増加しすぎるため、鋼板の過度の硬質化が進み、所望のフェライト相分率を確保することが難しくなり、プレス成形性が低下する。このため、Mnは1.5~2.0%の範囲に限定した。なお、好ましくは1.6~1.9%である。
Pは、粒界に偏析して、延性や靭性を低下させる悪影響を及ぼす。また、Pは、スポット溶接性を低下させる。このため、Pはできるだけ低減することが望ましいが、過度の低減は脱リンのための精錬時間が長くなり、生産能率が低下し、製造コストの高騰を招くため、0.001%以上とすることが好ましい。また、0.030%を超える含有は、スポット溶接性の著しい低下を招く。このため、Pは0.030%以下に限定した。なお、好ましくは0.001%以上0.020%未満である。
Sは、鋼中ではほとんどが介在物として存在し強度にほとんど寄与しないばかりか、粗大なMnSを形成し、延性、とくに伸びフランジ成形時に割れの起点となり伸びフランジ性を低下させるため、できるだけ低減することが好ましい。しかし、過度の低減は製鋼工程での脱硫時間が長くなり、生産能率が低下し、製造コストの高騰を招くため、0.0001%以上とすることが好ましい。0.0050%を超えて含有すると、伸びフランジ性が顕著に低下するため、Sは0.0050%以下に限定した。なお、好ましくは、0.0001~0.0030%である。
Alは、脱酸剤として作用する元素であり、この効果を十分に得るためには0.005%以上の含有を必要とする。一方、0.1%を超えて含有すると、フラッシュバット溶接などの溶接性を低下させるとともに、Al添加効果が飽和し、多量添加のため製造コストが高騰する。このため、Alは0.005~0.1%の範囲に限定した。なお、好ましくは0.02~0.06%である。
Nは、本発明では不純物であるが、固溶Nとして耐時効性を低下させることもあり、できるだけ低減することが好ましいが、過度の低減は精錬時間が長くなり、製造コストの高騰を招くため、経済性の観点からは0.0020%程度以上とすることが好ましい。一方、0.01%を超える含有は、スラブ割れ、スラブ内部欠陥等の発生傾向が強まり、表面疵が発生する恐れがある。このため、Nは0.01%以下に限定した。なお、好ましくは0.0050%以下である。
Tiは、炭窒化物を形成し、スラブ加熱時等のオーステナイト粒の粗大化を抑制する作用を有する元素であり、熱延板組織、焼鈍後の鋼板組織の微細化、均一化に有効に寄与する。このような効果を得るためには、0.005%以上の含有を必要とする。一方、0.050%を超える含有は、析出物がフェライト相中に過度に生成し、フェライト相の延性を低下させる。またTiの更なる過度の含有は、熱延板を過度に硬化させ、熱間圧延時や冷間圧延時の圧延負荷を増大させる。このため、Tiは0.005~0.050%の範囲に限定した。なお、好ましくは0.010~0.0040%である。
Nbは、鋼中に固溶し固溶強化により、あるいは炭窒化物を形成し析出強化により鋼板の強度増加に寄与する元素であり、このような効果を得るためには0.020%以上の含有を必要とする。一方、0.080%を超える過度の含有は、析出物がフェライト相中に過度に生成し、フェライト相の延性を低下させるとともに、熱延板を過度に硬化させ、熱間圧延時や冷間圧延時の圧延負荷を増大させる。このため、Nbは0.020~0.08%の範囲に限定した。なお、好ましくは0.030~0.050%である。
TiとNbを複合して含有するに際し、Nb含有量をTi含有量より多くすることにより、Ti単独、あるいはTiとNbの複合含有ではあるがNb含有量をTi含有量より少なくした場合に比べ、結晶粒が均一、微細な組織が得られる。このため、曲げ特性が向上する。このような効果は、(Nb含有量)と(Ti含有量)との比、Nb/Ti、を1.5以上とすることにより顕著となる。なお、Nb/Tiは、好ましくは1.8以上、5.0以下である。
Ca:0.0001~0.0050%
Caは、介在物の形態制御に有効に寄与する元素であり、例えば冷間圧延工程にて展伸し板状介在物となるMnSを球状介在物であるCaSへと、焼鈍工程の前までに介在物の形態を制御して、延性、伸びフランジ性を向上させる。このような効果は0.0001%以上の含有で認められるが、0.0050%を超えて含有しても、効果が飽和し含有量に見合う効果が期待できなくなる。このため、含有する場合には、Caは0.0001~0.0050%の範囲に限定することが好ましい。なお、より好ましくは0.0005~0.0020%である。
上記した成分以外の残部は、Feおよび不可避的不純物からなる。
本発明冷延鋼板は、体積%で、50~77%のフェライト相と、20~50%のベイナイト相と、2~10%のマルテンサイト相と、1~5%の残留オーステナイト相からなる組織を有する。
フェライト相は、軟質であり冷延鋼板の延性(伸び)に寄与する。このような効果を得るためには、フェライト相の体積分率を50%以上とする必要がある。一方、77%を超える多量な含有は、所望の高強度(TS:590MPa以上)を確保できなくなる。このため、フェライト相の体積分率は50~77%の範囲に限定した。なお、好ましくは50~65%であり、より好ましくは50~60%である。また、フェライト相の結晶粒径が大きすぎると、低温変態相が局在し、不均一変形の原因となり、優れた成形性を確保することが困難となる。一方、フェライト相の結晶粒径が細かくなると、低温変態相とフェライトとが隣接し、フェライト相の変形が阻害され、優れた成形性を確保できにくくなる。そのため、フェライト相の平均結晶粒径は1~10μmの範囲とすることが好ましい。
ベイナイト相は、低温変態相の一つであり、所望の高強度を確保するために、本発明では20%以上の含有を必要とする。一方、50%を超える過度の含有は、鋼板が過度に硬質化し成形性が低下する。このため、ベイナイト相の体積分率は20~50%の範囲に限定した。なお、好ましくは30~50%、より好ましくは30%超50%以下、さらに好ましくは35~45%である。また、ベイナイト相の平均結晶粒径が10μmを超えて大きくなると、組織が不均一組織となり、成形時に不均一な変形を生じ、優れた成形性を確保することが困難となる。一方、ベイナイト相の平均結晶粒径が1μm未満と細かくなると、加工時の変形能に及ぼすベイナイト相の寄与が大きくなり、フェライト相の変形が阻害され、優れた成形性を確保できにくくなる。そのため、ベイナイト相の平均結晶粒径は1~10μmの範囲とすることが好ましい。
マルテンサイト相は、低温変態相として硬質であり、鋼板の強度増加に大きく寄与する。しかし、打抜剪断加工時に、マルテンサイト相とフェライト相の硬度差に起因してマルテンサイト相とフェライト相との界面でボイドが多数発生し、プレス成形過程においてそれらのボイドが連結し、亀裂になりさらにその亀裂が伸展し割れに至る。このため、多量のマルテンサイト相の存在は、伸びフランジ性を低下させることになる。マルテンサイト相の体積分率が10%を超えて大きくなると、強度が高くなりすぎ、延性が著しく低下するとともに、マルテンサイト相とフェライト相との界面が増加し、優れた伸びフランジ性の確保が難しくなる。一方、マルテンサイト相の体積分率が2%未満と少なくなると、組織中の分散が粗くなるため伸びフランジ性への影響は少なくなるが、所望の高強度を安定して確保できなくなる。このようなことから、マルテンサイト相の体積分率は、2~10%の範囲に限定した。なお、好ましくは4~8%である。
残留オーステナイト相は、成形加工時に歪誘起変態を介し延性(均一伸び)の向上に寄与する。しかし、残留オーステナイト相には、Cが濃化し硬質となっており、フェライト相との硬度差が大きくなっている。このため、残留オーステナイト相の存在は伸びフランジ性を低下させる要因となる。残留オーステナイト相が5%を超えて多くなると、フェライト相との硬度差に起因して、打抜剪断加工時に、残留オーステナイト相とフェライト相との界面でボイドが多数発生し、プレス成形過程においてそれらのボイドが連結し、亀裂となりさらにその亀裂が伸展し割れに至る。一方、残留オーステナイト相の体積分率が1%未満と少なくなると、組織中の分散が粗くなるため、伸びフランジ性への影響は少なくなるが、延性の向上が少ない。このようなことから、残留オーステナイト相の体積分率は1~5%の範囲に限定した。なお、好ましくは1~3%である。
なお、フェライト相、ベイナイト相、マルテンサイト相等の平均結晶粒径は、光学顕微鏡(倍率:200~1000倍)で5視野以上観察し、組織を同定したのち、JIS法に準拠した切断法や画像解析により算出すればよい。
上記した組成の鋼素材に、熱延工程と、冷延工程と、焼鈍工程と、あるいはさらに調質圧延工程と、を順次施して、冷延鋼板とする。
鋼素材の製造方法はとくに限定する必要はなく、上記した組成の溶鋼を、転炉法、電炉法等の常用の溶製方法で溶製し、連続鋳造法等の、常用の鋳造方法でスラブ等の鋼素材とすることが好ましい。鋼素材の鋳造方法は、成分のマクロな偏析を防止すべく違続鋳造法とすることが望ましいが、造塊法、薄スラブ鋳造法によってもなんら問題はない。
熱延工程は、上記した組成の鋼素材を、加熱しあるいは加熱することなく、粗圧延,仕上圧延からなる、常用の熱間圧延を施し、所定の寸法形状の熱延板とし、ついで巻き取る工程とすることが好ましい。本発明では、所定の寸法形状の熱延板とすることができればよく、とくに熱間圧延の条件を限定する必要はないが、下記の条件とすることが好ましい。
加熱後の冷却は、二段階の冷却とする。冷却は、軟質なフェライト相と硬質なベイナイト相の組織分率を調整し、引張強さTS:590MPa以上の高強度と優れた加工性を兼備させるために重要である。このため、冷却は、所望の金属組織を確保することができるように、冷却パターン、すなわち冷却速度、冷却時間を厳密に調整する必要がある。二段階の冷却は、第一段の冷却とそれに続く、第一段の冷却より緩冷の第二段の冷却とからなる。第一段の冷却と第二段の冷却はフェライト相とベイナイト相の組織分率を調整するために重要となる。
第二段冷却速度が0.2×(第一段冷却速度)未満では、冷却が遅すぎて軟質なフェライト相の生成が促進され、ベイナイト相の組織分率が低くなり、所望の高強度を確保できなくなる。一方、0.8×(第一段冷却速度)を超えると、冷却が速すぎてベイナイト変態開始から終了までに滞留する時間が短くなり、ベイナイト相の組織分率が低くなり、所望の高強度を確保できなくなる。このため、第二段冷却速度を0.2~0.8×(第一段冷却速度)の範囲に限定した。本発明では、所望のフェライト相とベイナイト相の分率を確保するため、第一段の冷却と第二段の冷却の冷却時間を配分する。
表1に示す組成の溶鋼を、転炉で溶製し、連続鋳造法でスラブ(鋼素材)とした。これら鋼素材(スラブ)を出発素材とし、1200℃に加熱したのち、仕上圧延終了温度:900℃、巻取温度:600℃とする熱間圧延を施し熱延板とする熱延工程を施した。ついで、該熱延板に塩酸酸洗を施したのち、冷間圧延を施し冷延板とする冷延工程と、それに引続き、表2に示す条件の二段階の加熱、二段階の冷却を有する焼鈍処理を施す焼鈍工程を施し、板厚:1.4mmの冷延焼鈍板を得た。
得られた冷延鋼板(冷延焼鈍板)から、試験片を採取し、組織観察試験、引張試験、穴拡げ試験、曲げ試験を実施した。試験方法は次のとおりとした。
得られた冷延鋼板から組織観察用試験片を採取し、圧延方向断面を研磨し、腐食(ナイタール液)して、板厚の1/4の位置について、光学顕微鏡(倍率:1000倍)または走査型電子顕微鏡(倍率:3000倍)で視野数:5視野以上を観察し、撮像した。得られた組織写真から、組織の同定を行うとともに、各相の粒径、組織分率(体積%)を求めた。
フェライト相の平均結晶粒径は、JIS G 0552に規定された方法に準拠して切断法で求めた。また、ベイナイト相、マルテンサイト相についても同様に行った。
また、倍率:1000倍の組織写真を用いて、画像解析装置で任意に設定した組織写真上の100×100mmの正方形領域内に存在する各相の占有面積を求め、各相の組織分率(体積%)に換算した。オーステナイト相からの低温変態相であるベイナイト相、マルテンサイト相の区別は、倍率:3000倍の組織写真を用いて、フェライト相以外の低温変態相において、炭化物が観察される相をベイナイト相とし、炭化物が観察されず平滑な相として観察されたものをマルテンサイト相あるいは残留オーステナイト相とした。なお、残留オーステナイト量はX線回折により求めた。そして、フェライト相、ベイナイト相、残留オーステナイト相以外の残りをマルテンサイト相の組織分率とした。
得られた冷延鋼板から、圧延方向と直角方向が引張方向となるように、JIS Z 2201の規定に準拠してJIS 5号引張試験片を採取し、JIS Z 2241の規定に準拠して引張試験を行い、引張特性(降伏強さYS、引張強さTS、伸びEl)を求めた。
得られた冷延鋼板から試験片(大きさ:100×100mm)を採取し、日本鉄鋼連盟規格 JFST1001の規定に基づき、穴拡げ試験を実施した。試験片に初期直径d0:10mmφの穴を打抜き、該穴に頂角:60°の円錐ポンチを挿入し上昇させて、該穴を押し広げ、亀裂が板厚を貫通したところで、円錐ポンチの上昇を停止し、亀裂貫通後の打抜き穴の径dを測定し、穴拡げ率λ(%)を求めた。穴拡げ率λは、次式で算出した。
λ(%)={(d−d0)/d0}×100
なお、同一鋼板について、試験は3回行い、その平均値を該鋼板の穴拡げ率λとした。
得られた冷延鋼板から曲げ試験片(大きさ:40×50mm)を採取し、先端曲げ半径R=1.0mmで90°V曲げを実施し、曲げ頂点での割れの有無を目視観察し、曲げ性を評価した。
得られた結果を表3に示す。
Claims (4)
- mass%で、
C:0.050~0.090%、 Si:0.05%以下、
Mn:1.5~2.0%、 P:0.030%以下、
S:0.0050%以下、 Al:0.005~0.1%、
N:0.01%以下、 Ti:0.005~0.050%、
Nb:0.020~0.080%
を含み、残部Feおよび不可避的不純物からなる組成と、体積%で、50~77%のフェライト相と、20~50%のベイナイト相と、2~10%のマルテンサイト相と、1~5%の残留オーステナイト相からなる組織と、を有することを特徴とする伸びフランジ性に優れた高強度冷延鋼板。 - 前記組成に加えてさらに、mass%で、Ca:0.0001~0.0050%を含有することを特徴とする請求項1に記載の高強度冷延鋼板。
- 鋼素材に、熱延工程と、冷延工程と、焼鈍工程と、を順次施して、冷延鋼板とするにあたり、前記鋼素材を、mass%で、
C:0.050~0.090%、 Si:0.05%以下、
Mn:1.5~2.0%、 P:0.030%以下、
S:0.0050%以下、 Al:0.005~0.1%、
N:0.01%以下、 Ti:0.005~0.050%、
Nb:0.020~0.080%
を含み、残部Feおよび不可避的不純物からなる組成の鋼素材とし、
前記焼鈍工程を、最高到達温度:800~900℃とし二段階の加熱と二段階の冷却とを有する工程とし、前記二段階の加熱が、50℃から平均昇温速度:0.5~5.0℃/sで、(最高到達温度−50℃)~(最高到達温度−10℃)の温度域の第一段の加熱到達温度まで加熱する第一段の加熱と、該温度域から前記最高到達温度までの昇温時間を30~150sとする第二段の加熱とからなり、前記二段階の冷却が、前記最高到達温度から、平均冷却速度:10~40℃/sの第一段冷却速度で冷却する第一段の冷却と、引続き、平均冷却速度:(0.2~0.8)×第一段冷却速度の冷却速度で、400~500℃の温度域の冷却停止温度まで、第一段の冷却と第二段の冷却の総冷却時間の0.2~0.8の冷却時間で冷却する第二段の冷却とからなり、前記第二段の冷却終了後、400℃~500℃の温度域で100~1000s滞留させること、を特徴とする伸びフランジ性に優れた高強度冷延鋼板の製造方法。 - 前記鋼素材が、前記組成に加えてさらに、mass%で、Ca:0.0001~0.0050%を含有することを特徴とする請求項3に記載の高強度冷延鋼板の製造方法。
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WO2017169869A1 (ja) * | 2016-03-31 | 2017-10-05 | Jfeスチール株式会社 | 薄鋼板およびめっき鋼板、並びに熱延鋼板の製造方法、冷延フルハード鋼板の製造方法、薄鋼板の製造方法およびめっき鋼板の製造方法 |
JP6278162B1 (ja) * | 2016-03-31 | 2018-02-14 | Jfeスチール株式会社 | 薄鋼板およびめっき鋼板、並びに熱延鋼板の製造方法、冷延フルハード鋼板の製造方法、薄鋼板の製造方法およびめっき鋼板の製造方法 |
US11254995B2 (en) | 2016-03-31 | 2022-02-22 | Jfe Steel Corporation | Steel sheet, coated steel sheet, method for producing hot-rolled steel sheet, method for producing full hard cold-rolled steel sheet, method for producing steel sheet, and method for producing coated steel sheet |
CN115698365A (zh) * | 2020-07-20 | 2023-02-03 | 安赛乐米塔尔公司 | 经热处理的冷轧钢板及其制造方法 |
CN115698365B (zh) * | 2020-07-20 | 2024-03-26 | 安赛乐米塔尔公司 | 经热处理的冷轧钢板及其制造方法 |
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Publication number | Publication date |
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JP2012077377A (ja) | 2012-04-19 |
KR20130058044A (ko) | 2013-06-03 |
CN103080357B (zh) | 2015-03-25 |
JP5126399B2 (ja) | 2013-01-23 |
EP2615191A4 (en) | 2014-05-21 |
US20130160907A1 (en) | 2013-06-27 |
CN103080357A (zh) | 2013-05-01 |
KR101515730B1 (ko) | 2015-04-27 |
TWI429761B (zh) | 2014-03-11 |
EP2615191A1 (en) | 2013-07-17 |
TW201219579A (en) | 2012-05-16 |
EP2615191B1 (en) | 2016-04-13 |
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