WO2010150919A1 - 高強度溶融亜鉛めっき鋼板およびその製造方法 - Google Patents
高強度溶融亜鉛めっき鋼板およびその製造方法 Download PDFInfo
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- WO2010150919A1 WO2010150919A1 PCT/JP2010/061296 JP2010061296W WO2010150919A1 WO 2010150919 A1 WO2010150919 A1 WO 2010150919A1 JP 2010061296 W JP2010061296 W JP 2010061296W WO 2010150919 A1 WO2010150919 A1 WO 2010150919A1
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Images
Classifications
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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
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
- C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
- C23C2/0224—Two or more thermal pretreatments
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
- C23C2/06—Zinc or cadmium or alloys based thereon
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/26—After-treatment
- C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/26—After-treatment
- C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
- C23C2/29—Cooling or quenching
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
- C23C2/36—Elongated material
- C23C2/40—Plates; Strips
Definitions
- the present invention relates to a press-forming high-strength hot-dip galvanized steel sheet used in automobiles, home appliances and the like through a press-forming process, and a method for producing the same.
- 340MPa class BH steel plate (baking hardening type steel plate, hereinafter referred to as 340BH).
- 340BH is a ferrite single-phase steel in which the amount of solid solution C is controlled by addition of carbonitride-forming elements such as Nb and Ti in an ultra-low carbon steel with C: less than 0.01%, and solid solution strengthened with Si, Mn, and P It is.
- the surface distortion resistance of the press-formed product is significantly deteriorated due to the increase in YP.
- the surface distortion is a fine wrinkle or wavy pattern on the press-molded surface that is likely to occur on the outer periphery of the knob portion of the door. Since surface distortion significantly impairs the appearance quality of automobiles, the steel sheet applied to the outer panel may have a low YP that is close to the current level of 340BH, while increasing the strength of the pressed product while the strength of the pressed product is increased. Required.
- the flange portion In press molding of the part, the flange portion is bent for joining with the inner part, but the ductility of the blank end face after shearing or punching, so-called stretch flange formability is insufficient. If it is, a crack will occur in the end face. For example, if the tensile strength is reduced from 340BH and the flangeability decreases, the outer edge of the back door and the flange of the window frame opening of the door are hemmed, or the flange end of the joint to the side panel of the fender is bent. When processed, cracks often occur on the flange end face. For this reason, the steel sheet used for such an application is required to have excellent stretch flangeability.
- steel plates for automobiles.
- steel plates are in close contact with the hem-processed parts and spot weld peripheral parts of parts such as doors, hoods, and trunk lids, and rust is likely to occur because it is difficult to form a chemical conversion film during electrodeposition coating.
- rust perforation often occurs at corners in front of the hood and corners at the bottom of the door where water tends to accumulate and exposed to a moist atmosphere for a long time.
- studies have been conducted by car body manufacturers to improve the anti-corrosion performance of the car body and extend the drilling life from the previous 10 years to 12 years, and the steel sheet has sufficient corrosion resistance. Is essential.
- Patent Document 1 the amount of Ti is controlled so that Ti (%) / C (%) ⁇ 4.0 in steel of C: 0.020% or less, and Si, A method for obtaining a high strength steel sheet of 340 to 490 MPa class by adding a large amount of Mn and P is disclosed.
- Patent Document 2 discloses a cooling rate after annealing of steel containing C: 0.005 to 0.15%, Mn: 0.3 to 2.0%, Cr: 0.023 to 0.8%.
- a method of obtaining an alloyed galvanized steel sheet having both low yield stress (YP) and high ductility (El) by optimizing the above and forming a composite structure mainly composed of ferrite and martensite is disclosed.
- Patent Document 3 includes C: 0.02 to 0.033%, Mn: 1.5 to 2.5%, Cr: 0.03 to 0.5%, Mo: 0 to 0.5%. Steel sheet with excellent ductility (El) and stretch flangeability (hole expansion ratio, ⁇ ) when the total amount of Mn, Cr, and Mo in the steel is 1.8 to 2.5% and YP is 300 MPa or less. Is disclosed.
- Patent Document 4 describes the total amount of Mn and Cr in steels containing C: 0.02 to 0.14%, Mn: 1.3 to 3.0%, Cr: 0.3 to 1.5%.
- 440 to 590 MPa by setting the metal structure of the steel sheet to 2.0 to 3.5% and the metal structure of the steel sheet to be a composite structure composed of a ferrite phase of 50% or more in area ratio, 3 to 15% bainite and 5 to 20% martensite.
- Patent Document 5 contains C: 0.02 to 0.08%, Mn: 1.0 to 2.5%, P: 0.05% or less, Cr: more than 0.2% and 1.5% or less A method of obtaining a steel sheet having a low yield ratio, high BH, and excellent normal temperature aging resistance by setting Cr / Al to 30 or more in the obtained steel is disclosed.
- Patent Document 6 includes a steel containing C: 0.01% or more and less than 0.040%, Mn: 0.3 to 1.6%, Cr: 0.5% or less, Mo: 0.5% or less.
- Low YR and high bake hardening by cooling to a temperature of 550 to 750 ° C. at a cooling rate of 3 to 20 ° C./s after annealing and cooling to a temperature of 200 ° C. or less at a cooling rate of 100 ° C./s or more.
- a method of obtaining a steel sheet having properties is disclosed.
- Patent Document 1 is an IF steel in which C is fixed with Ti and is a ferrite single-phase steel, it is necessary to utilize the solid solution strengthening of Si, Mn, and P as a strengthening mechanism. YP is increased by adding a large amount of these elements, and the plating appearance quality and powdering resistance are significantly deteriorated.
- Patent Documents 2 and 3 are steels in which an appropriate amount of a second phase mainly composed of martensite is dispersed in a ferrite structure, and YP is reduced compared to a conventional solid solution strengthened steel such as IF steel.
- a conventional solid solution strengthened steel such as IF steel.
- Patent Document 5 has a relatively low YP and a high hole expansibility because it actively uses Cr. However, it has been revealed that many of the steel sheets described in the examples have insufficient corrosion resistance. In addition, these steel plates contain a large amount of expensive elements such as Cr and Mo, and the cost of such steel plates increases.
- Patent Document 6 since the technique described in Patent Document 6 requires rapid cooling after annealing, it can be applied to a continuous annealing line (CAL) that is not subjected to plating, but is maintained at 450 to 500 ° C. during cooling after annealing. In principle, it is difficult to apply in a continuous hot dip galvanizing line (CGL) in which a plating process is performed by dipping in a galvanizing bath.
- CAL continuous annealing line
- the present invention has been made to solve such problems, and does not require the addition of a large amount of expensive elements such as Mo and Cr, and has high corrosion resistance, low YP, and high stretch flangeability.
- An object is to provide a hot-dip galvanized steel sheet and a method for producing the same.
- the present inventors have conducted intensive studies on a method for simultaneously securing low YP and excellent stretch flangeability without using an expensive element while improving corrosion resistance for a conventional composite steel sheet having low yield strength. The following conclusions were obtained.
- I to III set the Mn equivalent to be described later as high as 2.2 or more, suppress the addition amount of Mn, Mo, Cr and actively use P and B, and the heating rate during annealing. This can be achieved by controlling to less than 5.0 ° C./sec.
- the reduction of the heating rate in the annealing process also has the effect of uniformly dispersing the second phase.
- Mn and P have the effect of slightly improving the corrosion resistance. Therefore, by adding P and B while controlling the addition amount of Mn, Mo and Cr within a predetermined range and reducing the heating rate in the annealing process, all of good corrosion resistance, low YP, and high stretch flangeability are satisfied. Steel can be obtained. In addition, since it is not necessary to add a large amount of expensive elements such as Mo and Cr, it can be manufactured at low cost.
- the present invention has been made based on the above findings, and the gist thereof is as follows.
- the second phase contains 2 to 12% volume fraction, the second phase contains 1 to 10% volume fraction martensite and 0 to 5% volume fraction residual ⁇ , and further contains martensite in the second phase.
- the ratio of the volume fraction of residual ⁇ is 70% or more, and the ratio of the volume fraction of the second phase volume fraction that exists at the triple point of the grain boundary is 50% or more.
- [Mneq] [% Mn] +1.3 [% Cr] +8 [% P] + 150B * + 2 [% V] +3.3 [% Mo]
- B * [% B] + [% Ti] /48 ⁇ 10.8 ⁇ 0.9+[%Al]/27 ⁇ 10.8 ⁇ 0.025
- [% Mn], [% Cr], [% P], [% B], [% Ti], [% Al], [% V], and [% Mo] are Mn, Cr, P, B, Ti, sol.
- Each content of Al, V, and Mo is represented.
- a steel slab having the composition described in any one of [1] to [5] is hot-rolled and cold-rolled, and then in a continuous hot-dip galvanizing line (CGL) in a range of 680 to 750 ° C. Is heated at an average heating rate of less than 5.0 ° C./sec, and then annealed at an annealing temperature of 750 ° C. or more and 830 ° C. or less, and the average cooling rate from the annealing temperature to immersion in the galvanizing bath is 2 to 30 ° C. After cooling so that the holding time in the temperature range of 480 ° C. or less is 30 seconds or less, and then dipping in a galvanizing bath and galvanizing.
- CGL continuous hot-dip galvanizing line
- the average cooling rate is 5 to 100 ° C./sec.
- High strength characterized by cooling to below °C, or after galvanizing, further alloying treatment of plating, and cooling to 300 °C or less at an average cooling rate of 5-100 ° C./sec after alloying treatment Of manufacturing hot-dip galvanized steel sheet.
- a high-strength hot-dip galvanized steel sheet having excellent corrosion resistance, low YP and excellent stretch flangeability can be produced at low cost. Since the high-strength hot-dip galvanized steel sheet according to the present invention has excellent corrosion resistance, excellent surface strain resistance, and excellent stretch flangeability, the strength and thickness of automobile parts can be increased.
- the figure which shows the relationship between YP and P content The figure which shows the relationship between hole expansion rate (lambda) and P content.
- % showing the quantity of a component means the mass%.
- Component composition of steel Cr less than 0.40% Cr is an important element that needs to be strictly controlled in the present invention. That is, conventionally, Cr has been actively used for the purpose of reducing YP and improving stretch flange formability, but Cr is not only an expensive element, but when added in a large amount, It has been found that the corrosion resistance is significantly degraded. That is, when door outer and hood outer parts are made of conventional composite steel with low YP and corrosion resistance is evaluated in a wet environment, the drilling life of the hem-processed part is reduced by 1 to 4 years compared to conventional steel. A steel plate was observed.
- Cr is an element that can be arbitrarily added from the viewpoint of optimizing [Mneq] shown below, and the lower limit is not specified (including Cr: 0%), but from the viewpoint of low YP, Cr is 0. It is preferable to add 02% or more, and more preferably 0.05% or more.
- This pearlite is likely to be formed adjacent to hard martensite and tends to be a starting point of cracks at the shear end face. Therefore, even if a small amount of steel containing martensite is present, stretch flangeability is significantly deteriorated. Moreover, bainite is a hard phase and raises YP notably.
- this pearlite and bainite are as fine as about 1 to 2 ⁇ m and are formed adjacent to martensite, it is difficult to distinguish them from martensite with an optical microscope, and they are observed using a SEM at a magnification of 3000 times or more. Can be identified. For example, when the structure of conventional 0.03% C-1.5% Mn-0.5% Cr steel is examined in detail, it is coarse in observation with an optical microscope or SEM at a magnification of about 1000 times.
- [% Mn], [% Cr], [% P], [% B], [% V], [% Mo], [% Ti], and [% Al] are Mn, Cr, P, B, V, Mo, Ti + sol. Each content of Al is represented.
- [Mneq] When this [Mneq] is set to 2.2 or more, pearlite and bainite are sufficiently suppressed even in the CGL thermal history in which slow cooling is performed after annealing. Therefore, in order to secure excellent stretch flange formability while reducing YP, [Mneq] needs to be 2.2 or more. Furthermore, from the viewpoint of lowering YP and improving stretch flange formability, [Mneq] is preferably 2.3 or more, and more preferably 2.4 or more. When [Mneq] exceeds 3.1, the amount of Mn, Mo, Cr, and P added is too large, and it becomes difficult to ensure a sufficiently low YP and excellent corrosion resistance at the same time. Therefore, [Mneq] is set to 3.1 or less.
- Mn 1.0% or more and 1.9% or less
- at least [Mneq] is necessary to improve stretch flange formability while reducing YP, but that alone is insufficient. It is necessary to control the amount and the contents of Mo, P, and B described later within a predetermined range. That is, Mn is added to increase the hardenability and increase the ratio of martensite in the second phase. However, if the content is too high, the ⁇ ⁇ ⁇ transformation temperature in the annealing process becomes low, and ⁇ grains are formed at the fine grain boundary immediately after recrystallization or at the interface of the recovery grains during recrystallization. , And becomes non-uniform and the second phase is refined to increase YP.
- the amount of Mn is set to 1.9% or less.
- the amount of Mn is set to 1.9% or less.
- the amount of Mn is too small, it becomes difficult to ensure sufficient hardenability even if a large amount of other elements are added.
- the Mn content is 1.0% or more and 1.9% or less.
- Mn is desirably 1.2% or more, and from the viewpoint of lowering YP, the Mn content is desirably 1.8% or less.
- Mo Less than 0.15% Mo can be added from the viewpoint of improving hardenability, suppressing the formation of pearlite, and improving stretch flange formability.
- Mo like Mn, has a strong effect of refining the second phase, and also has a strong effect of refining ferrite grains. Therefore, when Mo is added excessively, YP is remarkably increased. Further, Mo is an extremely expensive element, and a large amount of addition leads to a significant cost increase. Therefore, the amount of Mo added is limited to less than 0.15% (including 0%) from the viewpoint of reducing YP and reducing costs. From the viewpoint of further reducing the YP, Mo is desirably 0.05% or less, and more desirably 0.02% or less. Most preferably, it does not contain Mo.
- P 0.015% or more and 0.050% or less P is an important element for achieving low YP and improvement in stretch flangeability in the present invention. That is, P is contained in a predetermined range in combination with Cr and B described later, so that low YP and excellent stretch flange formability can be simultaneously obtained at a low production cost, and excellent corrosion resistance can be secured. .
- P has been conventionally used as a solid solution strengthening element, and it was considered desirable to reduce it from the viewpoint of low YP.
- P has a large effect of improving hardenability even when added in a small amount.
- P has an effect of uniformly and coarsely dispersing the second phase at the triple point of the ferrite grain boundary. Therefore, it has been clarified that YP is lower when P is used than when Mn or Mo is used even with the same Mn equivalent. Further, it has been clarified that it has an effect of improving the balance between strength and stretch flangeability and an effect of improving corrosion resistance. Therefore, by using P as a quenching element and reducing the addition amount of Mn and Mo, low YP and high stretch flangeability can be obtained at the same time, and by using P and reducing Cr, corrosion resistance is improved. Remarkably improved.
- the test piece was prepared by the following method. That is, a slab having a thickness of 27 mm was heated to 1200 ° C., hot-rolled to 2.8 mm at a finish rolling temperature of 850 ° C., and immediately after rolling, water spray cooling was performed and a winding process was performed at 570 ° C. for 1 hr. Further, after cold rolling at a rolling rate of 73% to 0.75 mm, heating was performed so that the average heating rate in the range of 680 to 750 ° C. was 2 ° C./sec, and the soaking was maintained at 780 ° C. for 40 sec. Cooling so that the holding time in the temperature range of 480 ° C.
- a JIS No. 5 tensile test piece was collected from the obtained steel plate and subjected to a tensile test (based on JIS Z2241). The stretch flange formability was evaluated by a hole expansion test in accordance with the provisions of JFST1001.
- the hardenability is improved by adding P in a steel with a relatively low Mn content of 1.6%, resulting in a structure mainly composed of ferrite and martensite or residual ⁇ . Since the two phases are uniformly dispersed, YP is significantly reduced and the hole expansion ratio ⁇ is remarkably increased.
- the addition amount of P is 0.015% or more and 0.050% or less, YP is suppressed to 220 MPa or less, and a high ⁇ of TS ⁇ ⁇ ⁇ 38000 (MPa ⁇ %) and ⁇ ⁇ 90% is obtained. Since both TS and ⁇ are increased by the addition of P, TS ⁇ ⁇ is significantly increased by the addition of P.
- the steel added with a large amount of Mn and Mo has high ⁇ but high YP.
- steel added with a large amount of Cr has a low YP and a high ⁇ , but the corrosion resistance is remarkably inferior due to a large amount of Cr.
- B 0.005% or less B has an effect of uniformly and coarsening ferrite grains and martensite and an effect of suppressing pearlite by improving hardenability. For this reason, by replacing Mn with B while securing a predetermined amount of [Mneq], low YP can be achieved while ensuring high stretch flange formability. However, when B is added in excess of 0.005%, the castability and rollability are remarkably lowered. Therefore, it is desirable to add B in the range of 0.005% or less. In order to further exhibit the effect of lowering YP due to the addition of B, B is preferably added in an amount of 0.0002% or more, and more preferably more than 0.0010%.
- FIG. 4 a steel plate of YP ⁇ 215 MPa and TS ⁇ ⁇ ⁇ 40000 (MPa ⁇ %) is indicated by ⁇ , and a steel plate of 215 MPa ⁇ YP ⁇ 220 MPa and TS ⁇ ⁇ ⁇ 40000 (MPa ⁇ %) is indicated by ⁇ , and 215 MPa ⁇ YP A steel sheet satisfying ⁇ 220 MPa and 38000 (MPa ⁇ %) ⁇ TS ⁇ ⁇ ⁇ 40000 (MPa ⁇ %) is indicated by ⁇ . Further, a steel plate of YP> 220 MPa or TS ⁇ ⁇ ⁇ 38000 (MPa ⁇ %) that does not satisfy the above characteristics is indicated by ⁇ .
- [Mneq] is 2.2 or more, [% Mn] +3.3 [% Mo] is 1.9 or less, and ([% Mn] +3.3 [% Mo]) / (1.3 [% When [Cr] +8 [% P] + 150B * ) ⁇ 3.5 is satisfied, it can be seen that a low YP and a high TS ⁇ ⁇ can be obtained simultaneously.
- Such a steel sheet has a structure mainly composed of martensite with ferrite, and the generation amount of pearlite and bainite is reduced. Further, the ferrite grains are uniform and coarse, and the martensite is uniformly dispersed mainly at the triple points of the ferrite grains.
- [% Mn] +3.3 [% Mo] is set to 1.9 or less. Further, ([% Mn] +3.3 [% Mo]) / (1.3 [% Cr] +8 [% P] + 150B * ) is less than 3.5, more preferably less than 2.8.
- C more than 0.015% and less than 0.10% C is an element necessary for ensuring a predetermined volume ratio of the second phase.
- C is preferably 0.02% or more.
- the amount of C is 0.10% or more, the volume fraction of the second phase is excessively increased, YP is increased, and stretch flangeability is also deteriorated. Moreover, weldability also deteriorates. Therefore, the C content is less than 0.10%. In order to ensure excellent stretch flangeability while obtaining a lower YP, the C content is preferably less than 0.060%, and more preferably less than 0.040%.
- Si 0.5% or less
- Si is desirably 0.3% or less, and more desirably less than 0.2%.
- Si is an element that can be added arbitrarily, and the lower limit is not specified (including Si: 0%), but from the above viewpoint, Si is preferably added in an amount of 0.01% or more, and 0.02% or more is added. Further preferred.
- S 0.03% or less S can be contained because it has an effect of improving the primary scale peelability of the steel sheet and improving the appearance quality of the plating by containing an appropriate amount of S.
- MnS precipitated in the steel becomes too much, and the elongation of the steel sheet and the stretch flangeability are deteriorated.
- hot-rolling a slab hot ductility is reduced and surface defects are easily generated.
- the corrosion resistance is slightly reduced. For this reason, the amount of S is made into 0.03% or less.
- S is preferably 0.02% or less, more preferably 0.01% or less, and further preferably 0.002% or less.
- sol. Al 0.01% or more and 0.5% or less
- Al is the purpose of fixing N to promote the effect of improving the hardenability of B, the purpose of improving aging resistance, and improving the surface quality by reducing inclusions. Add for purpose.
- sol. Al is 0.01% or more.
- sol. Al is preferably contained in an amount of 0.015% or more, and more preferably 0.04% or more.
- sol. Al Even if Al is added in excess of 0.5%, the effect of remaining solid solution B and the effect of improving the aging resistance are saturated, resulting in an increase in cost. In addition, the castability is deteriorated and the surface quality is deteriorated. For this reason, sol. Al is 0.5% or less. From the viewpoint of ensuring excellent surface quality, sol. Al is preferably less than 0.2%.
- N 0.005% or less
- N is an element that forms nitrides such as BN, AlN, TiN, etc. in steel, and has the adverse effect of eliminating the effect of improving the stretch flangeability while reducing YP through the formation of BN.
- fine AlN is formed to lower the grain growth property and increase YP.
- N must be strictly controlled.
- the N content exceeds 0.005%, YP increases and the aging resistance deteriorates, and the applicability to the outer panel becomes insufficient. From the above, the N content is set to 0.005% or less. From the viewpoint of further reducing YP by reducing the amount of precipitated AlN, N is preferably 0.004% or less.
- Ti Less than 0.020% Ti has the effect of fixing N and improving the hardenability of B, the effect of improving aging resistance, and the effect of improving castability. It is an element that can be optionally added to obtain. However, when the content increases, fine precipitates such as TiC and Ti (C, N) are formed in the steel to significantly increase YP, and TiC is generated during cooling after annealing to reduce BH. Since there exists an effect
- the content of Ti is preferably 0.002% or more, and the Ti content is preferably less than 0.010% in order to suppress the precipitation of TiC and obtain a low YP.
- V 0.4% or less
- V is an element that improves hardenability, has little effect on YP and stretch flangeability, and has little effect on deterioration of plating quality and corrosion resistance. Use as an alternative to Mn and Cr. can do. From the above viewpoint, V is preferably added in an amount of 0.002% or more, and more preferably 0.01% or more. However, if added over 0.4%, the cost increases significantly. Therefore, it is desirable to add V at 0.4% or less.
- the balance is iron and inevitable impurities, but the following elements can also be contained in predetermined amounts.
- Nb less than 0.02%
- Nb has the effect of refining the structure and precipitating NbC and Nb (C, N) to strengthen the steel sheet, so it can be added from the viewpoint of increasing the strength.
- Nb is preferably added in an amount of 0.002% or more, and more preferably 0.005% or more.
- Nb is preferably added at less than 0.02%.
- W 0.15% or less W can be used as a hardenable element and a precipitation strengthening element. From the above viewpoint, W is preferably added in an amount of 0.002% or more, and more preferably 0.005% or more. However, if the amount added is too large, YP is increased, so it is desirable to add W at 0.15% or less.
- Zr 0.1% or less Zr can also be used as a hardenable element and a precipitation strengthening element. Zr is preferably added in an amount of 0.002% or more, more preferably 0.005% or more from the above viewpoint. However, if the amount added is too large, YP will increase, so it is desirable to add Zr at 0.1% or less.
- Ni 0.5% or less
- Ni is also an element having an action of improving corrosion resistance. Moreover, Ni has the effect
- Ca 0.01% or less Ca has an effect of fixing S in steel as CaS, further increasing the pH in the corrosion product, and improving the corrosion resistance around the hem-processed portion and the spot welded portion. Moreover, the production
- Ce 0.01% or less Ce can also be added for the purpose of fixing S in steel and improving stretch flangeability and corrosion resistance. Ce is preferably added in an amount of 0.0005% or more from the above viewpoint. However, since it is an expensive element, adding a large amount increases the cost. Therefore, it is desirable to add Ce at 0.01% or less.
- La 0.01% or less
- La can also be added for the purpose of fixing S in steel and improving stretch flangeability and corrosion resistance. From the above viewpoint, La is preferably added in an amount of 0.0005% or more. However, since it is an expensive element, adding a large amount increases the cost. Therefore, it is desirable to add La at 0.01% or less.
- Mg 0.01% or less Mg can be added from the viewpoint of finely dispersing the oxide and homogenizing the structure. From the above viewpoint, Mg is preferably added in an amount of 0.0005% or more. However, since the surface quality deteriorates when the content is large, it is desirable to add Mg at 0.01% or less.
- At least one of the following Sn and Sb: Sn: 0.2% or less Sn is preferably added from the viewpoint of suppressing decarburization and de-B in the several tens of microns region of the steel sheet surface layer caused by nitriding, oxidation, or oxidation of the steel sheet surface. This improves fatigue properties, aging resistance, surface quality, and the like. From the viewpoint of suppressing nitriding and oxidation, it is preferable to add 0.002% or more of Sn, and more preferably 0.005% or more. However, if it exceeds 0.2%, YP increases and toughness deteriorates. Therefore, it is desirable to contain Sn at 0.2% or less.
- Sb 0.2% or less Sb is also preferably added from the viewpoint of suppressing decarburization and de-B of the several tens of micron region of the steel sheet surface layer caused by nitriding, oxidation, or oxidation of the steel sheet surface, similarly to Sn.
- the steel sheet surface layer can be prevented from reducing the amount of martensite generated, and the decrease in B can prevent the hardenability from decreasing, thereby improving fatigue properties and aging resistance.
- the wettability of hot dip galvanization can be improved and plating external appearance quality can be improved.
- Sb is preferably added in an amount of 0.002% or more, more preferably 0.005% or more. However, if it exceeds 0.2%, YP increases and toughness deteriorates. Therefore, it is desirable to contain Sb at 0.2% or less.
- the steel sheet structure of the present invention is mainly composed of ferrite, martensite, a trace amount of residual ⁇ , pearlite, and bainite, and also contains a trace amount of carbide.
- the volume ratio of the second phase was determined by corroding the L cross section (vertical cross section parallel to the rolling direction) of the steel sheet with Nital after polishing, observing 10 fields of view at a magnification of 4000 times with a SEM at the 1/4 thickness position of the steel sheet, and photographing.
- the obtained tissue photograph was subjected to image analysis to determine the area ratio of the second phase. That is, the steel sheet according to the present invention has a small difference in structure between the rolling direction and the direction perpendicular to the rolling direction, and the area ratio of the second phase measured in either direction showed almost the same value.
- the area ratio of the second phase thus obtained was defined as the volume ratio of the second phase.
- ferrite is a slightly black contrast region
- the region where carbides are generated in a lamellar or dot-like shape is pearlite or bainite, and particles with white contrast are martensite or residual ⁇ .
- the volume ratio of martensite and residual ⁇ was obtained by measuring the area ratio of this white contrast region.
- the fine dot-like particles having a diameter of 0.4 ⁇ m or less recognized on the SEM photograph are mainly carbides by TEM observation, and since these area ratios are very small, it is considered that the material is hardly affected.
- volume ratio was calculated
- the volume fraction of the second phase indicates the total amount of these tissues.
- particles in contact with three or more ferrite grain boundaries were defined as second phase particles existing at the triple point of the ferrite grain boundary, and the volume ratio was determined.
- the volume fraction of residual ⁇ is determined by using a K ⁇ X-ray source with Co as a target, and the ⁇ 200 ⁇ ⁇ 211 ⁇ ⁇ 220 ⁇ plane of ⁇ and ⁇ 200 ⁇ ⁇ 220 of ⁇ by X-ray diffraction at the 1/4 thickness position of the steel plate. ⁇ It was determined from the integrated intensity ratio of the ⁇ 311 ⁇ plane.
- the volume ratio of martensite was determined by subtracting the volume ratio of residual ⁇ determined by X-ray diffraction from the volume ratio of martensite and residual ⁇ determined by the above SEM observation.
- volume ratio of the second phase 2-12%
- the volume ratio of the second phase needs to be 2% or more.
- the volume ratio of the second phase is in the range of 2 to 12%.
- the volume ratio of the second phase is preferably 10% or less, more preferably 8% or less, and even more preferably 6% or less. preferable.
- Martensite volume ratio 1-10%
- the volume ratio of martensite needs to be 1% or more.
- the volume ratio of martensite is in the range of 1 to 10%.
- the volume ratio of martensite is preferably 8% or less, and more preferably 6% or less.
- the residual ⁇ can be contained in an amount of 0 to 5%. That is, in the present invention, the component composition of steel is optimized, and the heating rate and cooling rate in CGL are optimized, and the holding time at 480 ° C. or less is optimized. The three key points are generated. Residual ⁇ is softer than martensite and bainite, and does not have quenching strain formed around martensite. For this reason, it became clear that the residual ⁇ formed in this steel hardly contributes to the increase in YP when the volume ratio is 5% or less. However, when the volume ratio of the residual ⁇ exceeds 5%, YP slightly increases and stretch flangeability deteriorates. Therefore, the volume ratio of residual ⁇ is set to a range of 0 to 5%. From the viewpoint of improving stretch flange formability, the volume ratio of the residual ⁇ is preferably 4% or less, and more preferably 3% or less.
- Ratio of volume ratio of martensite and residual ⁇ to second phase volume ratio 70% or more If [Mneq] is not optimized in the thermal history of CGL subjected to slow cooling after annealing, it is adjacent to martensite. Fine pearlite is generated and stretch flangeability is remarkably deteriorated, and bainite is generated to increase YP. In order to sufficiently suppress pearlite and bainite and ensure low YP and excellent stretch flangeability at the same time, the ratio of the volume ratio of martensite and residual ⁇ to the second phase volume ratio must be 70% or more. .
- Ratio of volume fraction of second phase volume fraction present at grain boundary triple point 50% or more
- type of second phase and volume fraction it is necessary to optimize the position of the second phase. That is, even if the steel plate has the same second phase volume ratio, the ratio of the martensite to the same second phase and the volume ratio of residual ⁇ , the steel plate in which the second phase is fine and the second phase is generated non-uniformly. YP is high. In addition, if the second phase is generated non-uniformly, stretch flangeability deteriorates.
- the ratio of the volume fraction of the second phase volume fraction that exists at the triple point of the grain boundary may be controlled to 50% or more. did.
- the existence position of the second phase may be either within the ferrite grain or the ferrite grain boundary, but the second phase usually generates and selects an energy stable ferrite grain boundary.
- 80% or more of the second phase precipitates at the ferrite grain boundaries.
- the second phase is easily generated by being connected on the ferrite grain boundary, and is easily dispersed unevenly.
- the steel composition and annealing conditions it is possible to disperse the second phase at the triple point of the grain boundary among the ferrite grain boundaries. In this case, the second phase is uniformly dispersed.
- the ratio of the volume fraction of the second phase volume fraction that exists at the grain boundary triple point is 50% or more.
- Such a structural form can be obtained by optimizing the composition range of Mn, Mo, Cr, P, B, etc., and optimizing the heating rate during annealing.
- the steel sheet of the present invention after hot rolling and cold rolling the steel slab having the component composition limited as described above, in a continuous hot dip galvanizing line (CGL),
- the temperature range from 680 to 750 ° C. is heated at an average heating rate of less than 5.0 ° C./sec, further annealed at an annealing temperature of 750 ° C. or more and 830 ° C. or less, and the average from the annealing temperature to immersion in the galvanizing bath After cooling so that the cooling rate is 2 to 30 ° C./sec and the holding time in the temperature range of 480 ° C.
- Hot rolling In order to hot-roll steel slabs, a method of rolling the slab after heating, a method of directly rolling the slab after continuous casting without heating, a method of rolling the slab after continuous casting by performing a short heat treatment, etc. You can do it.
- the hot rolling may be performed according to a conventional method.
- the slab heating temperature is 1100 to 1300 ° C.
- the finish rolling temperature is Ar 3 transformation point to Ar 3 transformation point + 150 ° C.
- the winding temperature is 400 to 720 ° C. do it.
- the cooling rate after hot rolling is desirably 20 ° C./sec or more
- the coiling temperature is 600 ° C. or less. desirable.
- the slab heating temperature is set to 1250 ° C. or lower, and descaling is sufficiently performed to remove the primary and secondary scales generated on the steel plate surface, and the finish rolling temperature is set to 900. It is desirable that the temperature is not higher than ° C.
- the rolling rate may be 50 to 85%. From the viewpoint of improving the r value and improving the deep drawability, the rolling rate is preferably 65 to 73%, and from the viewpoint of reducing the r value and the in-plane anisotropy of YP, the rolling rate is 70 to 73%. 85% is preferable.
- CGL The steel sheet after cold rolling is annealed and plated by CGL, or further alloyed after plating.
- the heating rate during annealing is an important production condition that must be controlled in order to obtain a desired structure form to achieve both low YP and excellent stretch flangeability.
- FIG. 5 shows C: 0.028%, Si: 0.01%, Mn: 1.73%, P: 0.030%, Cr: 0.15%, sol.
- the relationship between the average heating rate of 680 to 750 ° C., YP, and hole expansion rate during annealing in steel containing Al: 0.06% and B: 0.0013% is shown.
- the sample preparation conditions other than the heating rate were the same as the previous conditions (in the case of FIGS.
- the heating rate during annealing is less than 5.0 ° C./sec, the second phase is uniformly and coarsely dispersed, and YP is significantly reduced. At this time, the hole expansion rate maintains a high value. That is, it is possible to achieve both low YP and high stretch flange formability by optimizing the heating rate.
- the reason why the heating rate at 680 to 750 ° C during annealing has a significant effect on YP is that recrystallization and ⁇ ⁇ ⁇ transformation proceed simultaneously in this temperature range. This is because the ⁇ ⁇ ⁇ transformation proceeds without being performed, and a large number of ⁇ is generated at the interface of the non-recrystallized grains, and the second phase is finely dispersed after cooling. From the above, the average heating rate at 680 to 750 ° C. during annealing is less than 5.0 ° C./sec.
- the annealing temperature is 750 ° C. or higher and 830 ° C. or lower. If it is less than 750 ° C., the solid solution of carbide becomes insufficient, and the volume fraction of the second phase cannot be secured stably. If it exceeds 830 ° C., pearlite and bainite are likely to be generated, or the amount of residual ⁇ is excessively increased, so that a sufficiently low YP cannot be obtained.
- the soaking time may be 20 sec or more and 200 sec or less, more preferably 40 sec or more and 200 sec or less, in a temperature range of 750 ° C. or higher, which is performed by normal continuous annealing.
- the average cooling rate from the annealing temperature until dipping in a galvanizing bath normally maintained at 450 to 500 ° C. is 2 to 30 ° C./sec, and in the temperature range of 480 ° C. or lower in the cooling process. Cooling is performed so that the holding time is 30 sec or less.
- YP can be kept low. Further, by setting the holding time in the temperature range of 480 ° C. or less to 30 sec or less, fine bainite, fine residual ⁇ , and fine martensite are suppressed from being generated at positions other than the grain boundary triple point, and YP Can be kept low.
- the alloying treatment is followed by cooling to 300 ° C. or lower at an average cooling rate of 5 to 100 ° C./sec.
- the cooling rate is lower than 5 ° C./sec, pearlite is generated around 550 ° C., and bainite is generated in the temperature range of 400 ° C. to 450 ° C., thereby increasing YP.
- the cooling end temperature exceeds 300 ° C., tempering of martensite proceeds significantly and YP increases.
- the cooling rate in the temperature range below 300 ° C is not specified, but it should be cooled at a cooling rate in the normal range of 0.1 to 1000 ° C / s that can be used in the cooling line length and cooling method of existing annealing equipment. Desired characteristics can be obtained. If there is equipment that can be tempered and tempered, it is possible to perform an overaging treatment at a temperature of 300 ° C. or lower for 30 sec to 10 min from the viewpoint of low YP.
- the obtained galvanized steel sheet can be subjected to skin pass rolling from the viewpoint of stabilizing the press formability such as adjusting the surface roughness and flattening the plate shape.
- the skin pass elongation rate is preferably 0.1 to 0.6% from the viewpoint of low YP and high El.
- the slab was heated to 1180 to 1250 ° C. and then hot-rolled at a finish rolling temperature in the range of 820 to 900 ° C. Thereafter, the film was cooled to 640 ° C. or lower at an average cooling rate of 15 to 35 ° C./sec, and wound at a winding temperature CT: 400 to 640 ° C.
- the obtained hot-rolled sheet was cold-rolled at a rolling rate of 70 to 77% to obtain a cold-rolled sheet having a thickness of 0.8 mm.
- the obtained cold-rolled sheet was heated so that the heating rate (average heating rate) in the temperature range of 680 to 750 ° C. was 0.8 to 18 ° C./sec. Then, annealing was performed at an annealing temperature AT for 40 seconds, and the average cooling rate from the annealing temperature AT to the plating bath temperature was cooled at the primary cooling rate shown in Tables 3 and 4. Moreover, at this time, the time until it is immersed in the plating bath after being cooled to 480 ° C. or lower is shown in Tables 3 and 4 as the holding time of 480 ° C. or lower.
- the average cooling rate from the plating bath temperature to 300 ° C. after galvanization is in the case of cooling to 300 ° C. or less so as to obtain the secondary cooling rate shown in Table 3 and Table 4 and alloying treatment after galvanization, the average cooling rate from the alloying temperature to 300 ° C. is shown after alloying treatment. 3 and Table 4 were cooled to 300 ° C. or less so as to achieve the secondary cooling rate.
- Zinc plating is performed at a bath temperature of 460 ° C. and Al in the bath of 0.13%, and the alloying treatment is performed by plating from 480 to 540 ° C.
- the obtained hot-dip galvanized steel sheet was subjected to temper rolling with an elongation of 0.1%, and a sample was collected.
- the volume ratio of the second phase and the ratio of the volume ratio of martensite and residual ⁇ to the second phase volume ratio (ratio of martensite and residual ⁇ in the second phase) by the method described above.
- the ratio of the volume ratio of the second phase existing at the grain boundary triple point (the ratio of the second phase existing at the grain boundary triple point in the second phase) was investigated.
- the type of steel structure was separated by SEM observation.
- a JIS No. 5 test piece was collected from the direction perpendicular to the rolling direction and subjected to a tensile test (based on JIS Z2241) to evaluate YP and TS. Further, the hole expansion rate ⁇ was evaluated by the method described above.
- each steel plate was evaluated with a structure simulating the periphery of the hem-processed part and spot welded part.
- two steel plates obtained were spot welded to form a state where the steel plates were in close contact with each other, and further subjected to a corrosion test under SAEJ2334 corrosion cycle conditions after applying chemical conversion treatment and electrodeposition coating simulating the painting process in an actual vehicle. Went.
- the electrodeposition coating film thickness was 20 ⁇ m. Corrosion products were removed from the corrosion samples after 90 cycles, and the reduction amount of the plate thickness from the original plate thickness measured in advance was determined as the corrosion loss.
- the steel sheet of the present invention has a significantly reduced corrosion weight loss compared to conventional steels whose Cr, Mn, and P contents are not optimized, and has a low Mn equivalent, steel with a large amount of Mn added, Mo Compared with steel to which steel is added or steel whose heating rate is not optimized during annealing, the steel with the same TS level has a low YP, that is, a low YR and a high hole expansion ratio ⁇ at the same time.
- this steel (conventional 340BH)
- the chemical components of this steel are C: 0.002%, Si: 0.01%, Mn: 0.4%, P: 0.05%, S: 0.008%, Cr : 0.04%, sol. Al: 0.06%, Nb: 0.01%, N: 0.0018%, B: 0.0008%.
- the steel of the present invention has substantially the same corrosion resistance as the conventional steel.
- component steels with a Cr content of less than 0.30% steels G, H, I, J, K added with a large amount of P while further reducing the Cr content, and further Cr reduction, P Steel M, R, and S in which Ce, Ca, and La are added in combination are also good in corrosion resistance, and steel N in which Cu and Ni are added in combination are particularly good in corrosion resistance.
- steel A having a heating rate of less than 5.0 ° C./sec during annealing is TS: 440 MPa class, low YP of 220 MPa or less, low YR of 49% or less, and high TS ⁇ ⁇ of 38000 MPa ⁇ % or more (hole expansion) Rate).
- Steels B and C increase the P and B contents while decreasing the Mn contents ([% Mn] +3.3 [% Mo]) / (1.3 [% Cr] +8 [% P] +150 B * ).
- the ratio of the second phase existing at the grain boundary triple point increases in the order of steels A, B, and C, and YP is reduced. Yes.
- the component steels within the scope of the present invention have a predetermined microstructure if the annealing temperature, the heating rate during annealing, the primary cooling rate, the holding time in the temperature range of 480 ° C. or less, and the secondary cooling rate are within the predetermined range. And a good material is obtained.
- the heating rate during annealing and reducing the holding time in the temperature range of 480 ° C. or lower, the second existing in the martensite ratio in the second phase and the grain boundary triple point in the second phase.
- the phase ratio increases, resulting in a much lower YP and a higher hole expansion ratio ⁇ .
- steels T and Y in which [Mneq] is not optimized have a high YP and a low hole expansion rate ⁇ . Even if [Mneq] is optimized ([% Mn] +3.3 [% Mo]) / (1.3 [% Cr] +8 [% P] + 150B * ) is not optimized, steel U is YP. Is expensive. Steel AC to which P is added excessively has high YP. Steel AD to which a large amount of Mo is added has a high YP. Steels AE, AF and AG in which Ti, C and N are not optimized have high YP.
- a high-strength hot-dip galvanized steel sheet having excellent corrosion resistance, low YP and high hole expansion rate can be manufactured at low cost. Since the high-strength hot-dip galvanized steel sheet according to the present invention has excellent corrosion resistance, excellent surface strain resistance, and excellent stretch flange formability, it is possible to increase the strength and thickness of automobile parts.
Abstract
Description
ここで、[Mneq]=[%Mn]+1.3[%Cr]+8[%P]+150B*+2[%V]+3.3[%Mo]、B*=[%B]+[%Ti]/48×10.8×0.9+[%Al]/27×10.8×0.025で表され、[%Mn]、[%Cr]、[%P]、[%B]、[%Ti]、[%Al]、[%V]、[%Mo]はMn、Cr、P、B、Ti、sol.Al、V、Moのそれぞれの含有量を表す。[%B]=0のときはB*=0、B*≧0.0022のときはB*=0.0022とする。
Cr:0.40%未満
Crは本発明において厳密に制御される必要のある重要な元素である。すなわち、従来、CrはYPを低減し、伸びフランジ成形性を向上させる目的で積極的に活用されてきたが、Crは高価な元素であるばかりでなく、多量に添加されるとヘム加工部の耐食性を著しく劣化させることが明らかになった。すなわち、従来のYPの低い複合組織鋼でドアアウタやフードアウタの部品を作製し、湿潤環境下での耐食性を評価したところ、ヘム加工部の穴明き寿命が従来鋼より1~4年も減少する鋼板が認められた。例えば、Crを0.42%添加した鋼では穴明き寿命が1年低下し、Crを0.60%添加した鋼では穴明き寿命が2.5年低下する。このような穴明き寿命の低下は、Crが0.40%未満では小さく0.30%未満ではほとんど生じないことが明らかになった。したがって、良好な耐食性を確保するためには、Crの含有量は0.40%未満とする必要がある。さらに優れた耐食性を付与するためには、Crは0.30%未満とすることが望ましい。Crは以下に示す[Mneq]を適正化する観点から任意に添加することができる元素であり、下限は規定しないが(Cr:0%を含む)、低YP化の観点からはCrは0.02%以上添加するのが好ましく、0.05%以上添加するのがさらに好ましい。
優れた伸びフランジ性を確保しつつ低いYPを確保するためには、少なくとも鋼組織としてフェライトと主としてマルテンサイトからなる複合組織とする必要がある。従来鋼では、伸びフランジ性に優れていない、あるいはYPあるいはYRが十分低減されていない鋼板が多く見られ、その原因を調査した結果、伸びフランジ成形性に劣る鋼板では第2相としてマルテンサイトと少量の残留γに加えてパーライトが生成しており、YPの高い鋼板ではマルテンサイトと少量の残留γに加えてパーライトあるいはベイナイトが生成していることが明らかになった。このパーライトは、硬質なマルテンサイトに隣接して生成しやすく、せん断端面におけるクラックの起点になりやすいので、マルテンサイトを含む鋼では微量に存在していても伸びフランジ成形性を著しく劣化させる。また、ベイナイトは硬質相でありYPを顕著に上昇させる。
[Mneq]=[%Mn]+1.3[%Cr]+8[%P]+150B*+2[%V]+3.3[%Mo]
B*=[%B]+[%Ti]/48×10.8×0.9+[%Al]/27×10.8×0.025
但し、[%B]=0の場合はB*=0、B*≧0.0022のときはB*=0.0022とする。
ここで、[%Mn]、[%Cr]、[%P]、[%B]、[%V]、[%Mo]、[%Ti]、[%Al]は、Mn、Cr、P、B、V、Mo、Ti+sol.Alのそれぞれの含有量を表す。
上述のとおり、低YP化しつつ伸びフランジ成形性を向上させるには少なくとも[Mneq]の適正化が必要であるが、それだけでは不十分であり、Mn量や後述するMo、P、Bの含有量を所定範囲に制御する必要がある。すなわち、Mnは焼入性を高め、第2相中のマルテンサイトの比率を増加させるために添加される。しかしながら、その含有量が多すぎると、焼鈍過程におけるα→γ変態温度が低くなり、再結晶直後の微細なフェライト粒界あるいは再結晶途中の回復粒の界面にγ粒が生成するので、フェライト粒が展伸して不均一になるとともに第2相が微細化してYPが上昇するので、Mn量は1.9%以下とする。一方、Mn量が少なすぎると他の元素を多量に添加しても十分な焼入性を確保することは困難になる。また、MnSが微細に多数分散して耐食性が劣化する。十分な焼入性ならびに耐食性を確保するためにMnは少なくとも1.0%以上添加する必要がある。
したがって、Mn量は1.0%以上1.9%以下とする。さらに耐食性を向上させる観点からはMnは1.2%以上とすることが望ましく、さらに低YP化する観点からはMn量は1.8%以下とすることが望ましい。
Moは焼入性を向上させてパーライトの生成を抑制し、伸びフランジ成形性を向上させる観点から添加することができる。しかしながら、MoはMnと同様に第2相を微細化する作用が強く、さらにフェライト粒を微細化する作用も強い。したがって、Moは過剰に添加するとYPを著しく増加させる。また、Moは極めて高価な元素であり添加量が多いと著しいコストアップにつながる。したがって、YPの低減ならびに低コスト化の観点からMoの添加量は0.15%未満に限定する(0%を含む)。より一層低YP化する観点からはMoは0.05%以下とすることが望ましく、0.02%以下とすることがより望ましい。Moを含まないことが最も好ましい。
低YP化するには、Mn、Moのそれぞれの含有量に加え、これらの含有量を所定範囲に制限する必要がある。これらの含有量の重み付け当量式である[%Mn]+3.3[%Mo]が1.9を超えるとYPが上昇するので[%Mn]+3.3[%Mo]は1.9以下にする必要がある。
Pは本発明において低YP化と伸びフランジ成形性の向上を達成する重要な元素である。つまり、Pは後述するCrやBと併用して所定範囲で含有させることで、低い製造コストで低YP化、優れた伸びフランジ成形性が同時に得られるとともに、優れた耐食性の確保も可能になる。
しかしながら、Pは0.050%を超えて添加されると焼入性向上効果や組織の均一化、粗大化効果が飽和するとともに、固溶強化量が大きくなり過ぎて低いYPが得られなくなる。また、Pは0.050%を超えて添加されると地鉄とめっき層の合金化反応が著しく遅延して耐パウダリング性が劣化する。また、溶接性も劣化する。したがって、P量は0.050%以下とする。
Bはフェライト粒やマルテンサイトを均一、粗大化する作用、焼入性を向上させてパーライトを抑制する作用がある。このため、所定量の[Mneq]を確保しつつMnをBで置換することで高い伸びフランジ成形性を確保しつつ、低YP化が図られる。しかしながら、Bは0.005%を超えて添加すると鋳造性や圧延性が著しく低下する。したがって、Bは0.005%以下の範囲で添加することが望ましい。B添加による低YP化の効果をさらに発揮させるにはBは0.0002%以上添加するのがよく、さらには0.0010%超添加するのがよい。
極めて低いYPと高い伸びフランジ成形性を両立するには、Mn当量の適正化やMn、Moの添加量の適正化に加えて、Mn、Moといった第2相やフェライト粒を微細化する作用のある元素とCr、P、Bといった第2相を均一粗大に分散させる作用のある元素の組成比を所定範囲に制御する必要がある。これにより、第2相が粒界の3重点に分散した組織となり、高い伸びフランジ成形性を維持しつつ低いYPを得ることができる。
以上より、[%Mn]+3.3[%Mo]は1.9以下とする。また、([%Mn]+3.3[%Mo])/(1.3[%Cr]+8[%P]+150B*)は3.5未満とし、さらに好ましくは2.8未満とする。
Cは所定量の第2相の体積率を確保するために必要な元素である。C量が少ないと第2相が形成されなくなり、穴拡げ性は増加するが、YPが著しく増加する。所定量の第2相の体積率を確保し十分低いYPを得るためには、Cは0.015%超とする必要がある。耐時効性を向上させ、YPをさらに低減する観点からはCは0.02%以上とすることが望ましい。一方、C量が0.10%以上となると第2相の体積率が多くなりすぎてYPが増加し、伸びフランジ成形性も低下する。また、溶接性も劣化する。したがって、C量は0.10%未満とする。より低いYPを得つつ優れた伸びフランジ成形性を確保するためにはC量は0.060%未満とすることが好ましく、0.040%未満とすることがさらに好ましい。
Siは微量添加することで熱間圧延でのスケール生成を遅延させて表面品質を改善する効果、めっき浴中あるいは合金化処理中の地鉄と亜鉛の合金化反応を適度に遅延させる効果、鋼板のミクロ組織をより均一、粗大化する効果等があるので、このような観点から添加することができる。しかしながら、Siを0.5%超えで添加するとめっき外観品質が劣化して外板パネルへの適用が難しくなるとともにYPの上昇を招くので、Si量は0.5%以下とする。さらに表面品質を向上させ、YPを低減する観点からSiは0.3%以下とするのが望ましく、0.2%未満とするのがさらに望ましい。Siは任意に添加できる元素であり、下限は規定しないが(Si:0%を含む)、上記の観点からSiは0.01%以上添加するのが好ましく、0.02%以上添加するのがさらに好ましい。
Sは適量含有させることで鋼板の一次スケールの剥離性を向上させ、めっき外観品質を向上させる作用があるので、含有させることが出来る。しかしながら、Sはその含有量が多いと鋼中に析出するMnSが多くなりすぎ鋼板の伸びや伸びフランジ成形性を低下させる。また、スラブを熱間圧延する際に熱間延性を低下させ、表面欠陥を発生させやすくする。さらには耐食性を僅かに低下させる。このため、S量は0.03%以下とする。伸びフランジ成形性や耐食性を向上させる観点から、Sは0.02%以下とすることが望ましく、0.01%以下とすることがより望ましく、0.002%以下とすることはさらに望ましい。
AlはNを固定してBの焼入性向上効果を促進する目的、耐時効性を向上させる目的、介在物を低減して表面品質を向上させる目的で添加される。Bの焼入性向上効果や耐時効性を向上させる観点からsol.Alの含有量は0.01%以上とする。このような効果をより発揮させるためには、sol.Alは0.015%以上含有させることが望ましく、0.04%以上とすることがさらに望ましい。一方、sol.Alを0.5%を超えて添加しても固溶Bを残存させる効果や耐時効性を向上させる効果は飽和し、徒にコストアップを招く。また、鋳造性を劣化させて表面品質を劣化させる。このためsol.Alは0.5%以下とする。優れた表面品質を確保する観点からはsol.Alは0.2%未満とするのが望ましい。
Nは鋼中でBN、AlN、TiN等の窒化物を形成する元素であり、Bの低YP化しつつ伸びフランジ成形性を向上させる効果をBNの形成を通じて消失させる弊害がある。また、微細なAlNを形成して粒成長性を低下させ、YPの上昇をもたらす。さらには、固溶Nが残存すると耐時効性が劣化する。このような観点からNは厳密に制御されなければならない。N含有量が0.005%を超えるとYPが上昇するとともに耐時効性が劣化し、外板パネルへの適用性が不十分となる。以上より、Nの含有量は0.005%以下とする。AlNの析出量を軽減してより一層YPを低減する観点からはNは0.004%以下にすることが望ましい。
Tiは、Nを固定してBの焼入性を向上させる効果、耐時効性を向上させる効果や鋳造性を向上させる効果があり、このような効果を補助的に得るために任意に添加できる元素である。しかし、その含有量が多くなると鋼中でTiCやTi(C,N)といった微細な析出物を形成して著しくYPを上昇させるとともに、焼鈍後の冷却中にTiCを生成してBHを減少させる作用があるので、添加する場合はTiの含有量は適正範囲に制御する必要がある。Tiの含有量が0.020%以上になると著しくYPが増加する。したがって、Tiの含有量は0.020%未満とする。Tiは任意に添加できる元素であり、下限は規定しないが(Ti:0%を含む)、TiNの析出によりNを固定してBの焼入性の向上効果を発揮させるためにはTiの含有量は0.002%以上とするのが好ましく、TiCの析出を抑えて低いYPを得るためにはTiの含有量は0.010%未満とするのが好ましい。
Vは焼入性を向上させる元素であり、YPや伸びフランジ成形性に及ぼす影響は小さく、めっき品質や耐食性を劣化させる作用も小さいので、MnやCrの代替として活用することができる。Vは上記の観点から0.002%以上添加するのが好ましく、0.01%以上添加するのがさらに好ましい。しかしながら、0.4%を超えて添加すると著しいコスト増になるので、Vは0.4%以下で添加することが望ましい。
Nb:0.02%未満
Nbは組織を細粒化するとともにNbC、Nb(C,N)を析出させ鋼板を強化する作用があるので、高強度化の観点から添加することができる。Nbは上記の観点から0.002%以上添加するのが好ましく、0.005%以上添加するのがさらに好ましい。しかしながら、0.02%以上添加するとYPが著しく上昇するので、Nbは0.02%未満で添加することが望ましい。
Wは焼入性元素、析出強化元素として活用できる。Wは上記の観点から0.002%以上添加するのが好ましく、0.005%以上添加するのがさらに好ましい。しかしながら、その添加量が多すぎるとYPの上昇を招くのでWは0.15%以下で添加することが望ましい。
Zrも同様に焼入性元素、析出強化元素として活用できる。Zrは上記の観点から0.002%以上添加するのが好ましく、0.005%以上添加するのがさらに好ましい。しかしながら、その添加量が多すぎるとYPの上昇を招くのでZrは0.1%以下で添加することが望ましい。
Cu:0.5%以下
Cuは耐食性を僅かに向上させるので、耐食性向上の観点から添加することが望ましい。また、スクラップを原料として活用するときに混入する元素であり、Cuの混入を許容することでリサイクル資材を原料資材として活用でき、製造コストを削減することができる。耐食性向上の観点からCuは0.01%以上添加するのが好ましく、0.03%以上添加するのがさらに望ましい。しかしながら、その含有量が多くなりすぎると表面欠陥の原因となるので、Cuは0.5%以下とするのが望ましい。
Niも耐食性を向上する作用のある元素である。また、NiはCuを含有させる場合に生じやすい表面欠陥を低減する作用がある。したがって、耐食性を向上させつつ表面品質を改善する観点からNiは0.01%以上添加するのが好ましく、0.02%以上添加するのがさらに望ましい。しかし、Niの添加量が多くなりすぎると加熱炉内でのスケール生成が不均一になり表面欠陥の原因になるとともに、著しいコスト増となる。したがって、Niは0.5%以下とする。
Caは鋼中のSをCaSとして固定し、さらには腐食生成物中のpHを増加させ、ヘム加工部やスポット溶接部周辺の耐食性を向上させる作用がある。また、CaSの生成により伸びフランジ成形性を低下させるMnSの生成を抑制し、伸びフランジ成形性を向上させる作用がある。このような観点からCaは0.0005%以上添加することが望ましい。しかしながら、Caは溶鋼中で酸化物として浮上分離しやすく、鋼中に多量に残存させることは難しい。したがって、Caの含有量は0.01%以下とする。
Ceも鋼中のSを固定し、伸びフランジ成形性ならびに耐食性を向上させる目的で添加することができる。Ceは上記の観点から0.0005%以上添加するのが好ましい。しかし、高価な元素であるので多量添加するとコストアップになる。したがって、Ceは0.01%以下で添加するのが望ましい。
Laも鋼中のSを固定し、伸びフランジ成形性ならびに耐食性を向上させる目的で添加することができる。Laは上記の観点から0.0005%以上添加するのが好ましい。しかし、高価な元素であるので多量添加するとコストアップになる。したがって、Laは0.01%以下で添加するのが望ましい。
Mgは酸化物を微細分散させ、組織を均一化する観点から添加することが出来る。Mgは上記の観点から0.0005%以上添加するのが好ましい。しかしながら、その含有量が多いと表面品質が劣化するので、Mgは0.01%以下で添加することが望ましい。
Sn:0.2%以下
Snは鋼板表面の窒化、酸化、あるいは酸化により生じる鋼板表層の数十ミクロン領域の脱炭や脱Bを抑制する観点から添加するのが望ましい。これにより、疲労特性、耐時効性、表面品質などが改善される。窒化や酸化を抑制する観点からSnは0.002%以上添加するのが好ましく、0.005%以上添加するのがさらに望ましいが、0.2%を超えるとYPの上昇や靱性の劣化を招くのでSnは0.2%以下で含有させるのが望ましい。
SbもSnと同様に鋼板表面の窒化、酸化、あるいは酸化により生じる鋼板表層の数十ミクロン領域の脱炭や脱Bを抑制する観点から添加するのが望ましい。このような窒化や酸化を抑制することで鋼板表層においてマルテンサイトの生成量が減少するのを防止したり、Bの減少により焼入性が低下するのを防止し、疲労特性や耐時効性を改善する。また、溶融亜鉛めっきの濡れ性を向上させてめっき外観品質を向上させることが出来る。窒化や酸化を抑制する観点からSbは0.002%以上添加するのが好ましく、0.005%以上添加するのがさらに望ましいが、0.2%を超えるとYPの上昇や靱性の劣化を招くのでSbは0.2%以下で含有させるのが望ましい。
本発明の鋼板組織は、主としてフェライト、マルテンサイト、微量の残留γ、パーライト、ベイナイトからなり、この他に微量の炭化物を含む。最初にこれらの組織形態の測定方法を説明する。
マルテンサイトの体積率は、上記のSEM観察により求められたマルテンサイトおよび残留γの体積率からX線回折により求められた残留γの体積率を差し引くことにより求めた。
低いYPを得るためには、第2相の体積率を2%以上とする必要がある。しかしながら、第2相の体積率が12%を超えるとYPが上昇するとともに伸びフランジ成形性が劣化する。したがって、第2相の体積率は2~12%の範囲とする。さらに低いYPと優れた伸びフランジ成形性を得るためには第2相の体積率は10%以下とするのが好ましく、8%以下とすることが更に好ましく、6%以下とすることがより一層好ましい。
低いYPを得るためには、マルテンサイトの体積率を1%以上とする必要がある。しかしながら、マルテンサイトの体積率が10%を超えるとYPが上昇するとともに伸びフランジ成形性が劣化する。したがって、マルテンサイトの体積率は1~10%の範囲とする。さらに低いYPと優れた伸びフランジ成形性を得るためにはマルテンサイトの体積率は8%以下とするのが好ましく6%以下とすることが更に好ましい。
本発明において残留γは0~5%含有させることができる。すなわち、本発明においては、鋼の成分組成が適正化されており、なおかつCGLにおける加熱速度、冷却速度、480℃以下における保持時間が適正化されているので、残留γは粗大に主に粒界の3重点に生成している。また、残留γはマルテンサイトやベイナイトと比べると軟質であり、マルテンサイトの周囲に形成される焼入歪も有していない。このため、本鋼において形成される残留γは体積率が5%以下ではYPの上昇に殆ど寄与しないことが明らかになった。しかしながら、残留γの体積率が5%を超えるとYPがわずかに上昇するとともに伸びフランジ成形性が劣化する。したがって、残留γの体積率は0~5%の範囲とする。伸びフランジ成形性を向上させる観点からは残留γの体積率は4%以下とすることが望ましく、3%以下とすることがさらに望ましい。
焼鈍後に緩冷却が施されるCGLの熱履歴では[Mneq]が適正化されていなければ、マルテンサイトに隣接して微細なパーライトが生成し伸びフランジ成形性が著しく劣化するとともに、ベイナイトが生成してYPを上昇させる。パーライトならびにベイナイトを十分抑制して低いYPと優れた伸びフランジ成形性を同時に確保するためには、第2相体積率に対するマルテンサイトおよび残留γの体積率の比率を70%以上とする必要がある。
優れた伸びフランジ成形性を確保しつつYPを十分低減するためには、第2相の種類や体積率の制御に加えて、第2相の存在位置も適正化する必要がある。つまり、同一の第2相体積率、同一の第2相に対するマルテンサイトおよび残留γの体積率の比率の鋼板であっても、第2相が微細で第2相が不均一に生成した鋼板ではYPが高い。また、第2相が不均一に生成すると伸びフランジ成形性が低下する。これに対して第2相が主に粒界3重点に均一、粗大に分散した鋼板では高い伸びフランジ成形性を維持しつつYPが低減されることを知見した。また、このような低いYPと高い伸びフランジ成形性を得るためには、第2相体積率のうち粒界3重点に存在するものの体積率の比率を50%以上に制御すればよいことを知見した。つまり、第2相の存在位置としては、フェライト粒内、フェライト粒界のいずれかが考えられるが、第2相はエネルギー的に安定なフェライト粒界を通常は選択して生成する。通常、第2相のうち80%以上はフェライト粒界に析出する。このため、第2相はフェライト粒界上に連結して生成しやすく、不均一に分散しやすい。一方、鋼組成や焼鈍条件を適正化することで、フェライト粒界の中でも粒界の3重点に第2相を分散させることができる。この場合、第2相は均一に分散する。組織形態をこのように制御することで、第2相を粗大に分散させつつ第2相同士の連結した箇所を低減することが出来、YPを低減しつつ高い伸びフランジ成形性を維持することが出来る。YPが低減される理由については必ずしも明らかではないが、第2相が均一、粗大に分散してマルテンサイト粒同士の間隔が十分確保されることで、初期のマルテンサイト周囲からの変形が容易に生じるようになると考えられる。したがって、第2相体積率のうち粒界3重点に存在するものの体積率の比率は50%以上とする。
本発明の鋼板は、上述したように、上記のように限定された成分組成を有する鋼スラブを、熱間圧延および冷間圧延した後、連続溶融亜鉛めっきライン(CGL)において、680~750℃の温度範囲を5.0℃/sec未満の平均加熱速度で加熱し、さらに750℃以上830℃以下の焼鈍温度で焼鈍し、前記焼鈍温度から亜鉛めっき浴に浸漬するまでの平均冷却速度が2~30℃/secでなおかつ480℃以下の温度域の保持時間が30sec以下となるように冷却した後、亜鉛めっき浴に浸漬して亜鉛めっきし、亜鉛めっき後5~100℃/secの平均冷却速度で300℃以下まで冷却し、あるいは亜鉛めっき後さらにめっきの合金化処理を施し、合金化処理後5~100℃/secの平均冷却速度で300℃以下まで冷却する方法により製造できる。
鋼スラブを熱間圧延するには、スラブを加熱後圧延する方法、連続鋳造後のスラブを加熱することなく直接圧延する方法、連続鋳造後のスラブに短時間加熱処理を施して圧延する方法などで行える。熱間圧延は、常法にしたがって実施すればよく、例えば、スラブ加熱温度は1100~1300℃、仕上圧延温度はAr3変態点~Ar3変態点+150℃、巻取温度は400~720℃とすればよい。r値の面内異方性を低減する観点、BHを向上させる観点からは、熱延後の冷却速度は20℃/sec以上とすることが望ましく、巻取温度は600℃以下とするのが望ましい。
冷間圧延では、圧延率を50~85%とすればよい。r値を向上させて深絞り性を向上させる観点からは圧延率は65~73%とするのが好ましく、r値やYPの面内異方性を低減する観点からは、圧延率は70~85%にすることが好ましい。
冷間圧延後の鋼板には、CGLで焼鈍とめっき処理、又はめっき処理後さらに合金化処理が施される。焼鈍時の加熱速度は低YPと優れた伸びフランジ成形性を両立するための所望の組織形態を得るために制御しなければならない重要な製造条件である。図5にC:0.028%、Si:0.01%、Mn:1.73%、P:0.030%、Cr:0.15%、sol.Al:0.06%、B:0.0013%を含有する鋼における焼鈍時の680~750℃の平均加熱速度とYP、穴拡げ率の関係を示す。なお、加熱速度以外のサンプル作製条件については先(図1、2の場合)と同じ条件とした。焼鈍時の加熱速度が5.0℃/sec未満になると第2相が均一、粗大に分散し、YPが顕著に低下する。また、このとき、穴拡げ率は高い値を維持する。つまり、加熱速度を適正化することで低いYPと高い伸びフランジ成形性を両立することが出来る。焼鈍時の680~750℃における加熱速度がYPに顕著な影響を及ぼすのは、この温度域で再結晶とα→γ変態が同時に進行するためであり、加熱速度が速いと再結晶が十分完了しないままα→γ変態が進行し、γが未再結晶粒の界面に多数生成して冷却後に第2相が微細分散するためである。以上より、焼鈍時の680~750℃の平均加熱速度は5.0℃/sec未満とする。
Claims (6)
- 鋼の成分組成として、質量%で、C:0.015%超0.10%未満、Si:0.5%以下、Mn:1.0%以上1.9%以下、P:0.015%以上0.050%以下、S:0.03%以下、sol.Al:0.01%以上0.5%以下、N:0.005%以下、Cr:0.40%未満、B:0.005%以下、Mo:0.15%未満、V:0.4%以下、Ti:0.020%未満を含有し、更に2.2≦[Mneq]≦3.1および[%Mn]+3.3[%Mo]≦1.9、([%Mn]+3.3[%Mo])/(1.3[%Cr]+8[%P]+150B*)<3.5を満足し、残部鉄および不可避不純物からなり、鋼の組織として、フェライトと第2相を有し、第2相の体積率が2~12%、第2相として1~10%の体積率のマルテンサイトと0~5%の体積率の残留γを含み、さらに第2相におけるマルテンサイトおよび残留γの体積率の比率が70%以上、第2相体積率のうち粒界3重点に存在するものの体積率の比率が50%以上であることを特徴とする高強度溶融亜鉛めっき鋼板。
ここで、[Mneq]=[%Mn]+1.3[%Cr]+8[%P]+150B*+2[%V]+3.3[%Mo]、B*=[%B]+[%Ti]/48×10.8×0.9+[%Al]/27×10.8×0.025で表され、[%Mn]、[%Cr]、[%P]、[%B]、[%Ti]、[%Al]、[%V]、[%Mo]はMn、Cr、P、B、Ti、sol.Al、V、Moのそれぞれの含有量を表す。[%B]=0のときはB*=0、B*≧0.0022のときはB*=0.0022とする。 - ([%Mn]+3.3[%Mo])/(1.3[%Cr]+8[%P]+150B*)<2.8を満足することを特徴とする請求項1に記載の高強度溶融亜鉛めっき鋼板。
- 更に、質量%で、Nb:0.02%未満、W:0.15%以下およびZr:0.1%以下のうちの少なくとも1種を含有することを特徴とする請求項1または2に記載の高強度溶融亜鉛めっき鋼板。
- 更に、質量%で、Cu:0.5%以下、Ni:0.5%以下、Ca:0.01%以下、Ce:0.01%以下、La:0.01%以下およびMg:0.01%以下のうちの少なくとも1種を含有することを特徴とする請求項1乃至3のいずれかに記載の高強度溶融亜鉛めっき鋼板。
- 更に、質量%で、Sn:0.2%以下およびSb:0.2%以下のうちの少なくとも1種を含有することを特徴とする請求項1乃至4のいずれかに記載の高強度溶融亜鉛めっき鋼板。
- 請求項1乃至5のいずれかに記載の成分組成を有する鋼スラブを、熱間圧延および冷間圧延した後、連続溶融亜鉛めっきライン(CGL)において、680~750℃の範囲を5.0℃/sec未満の平均加熱速度で加熱し、その後750℃以上830℃以下の焼鈍温度で焼鈍し、前記焼鈍温度から亜鉛めっき浴に浸漬するまでの平均冷却速度が2~30℃/secでなおかつ480℃以下の温度域の保持時間が30sec以下となるように冷却した後、亜鉛めっき浴に浸漬して亜鉛めっきし、亜鉛めっき後5~100℃/secの平均冷却速度で300℃以下まで冷却する、または亜鉛めっき後さらにめっきの合金化処理を施し、合金化処理後5~100℃/secの平均冷却速度で300℃以下まで冷却することを特徴とする高強度溶融亜鉛めっき鋼板の製造方法。
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2010
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EP2623629A4 (en) * | 2010-09-30 | 2016-08-31 | Jfe Steel Corp | HIGH-STRENGTH HOT-GALVANIZED STEEL SHEET WHICH HAS EXCELLENT FATIGUE FEATURES AND METHOD FOR MANUFACTURING THE SAME |
US20150034215A1 (en) * | 2012-03-20 | 2015-02-05 | Salzgitter Flachstahl Gmbh | High strength multi-phase steel, and method for producing a strip from said steel |
Also Published As
Publication number | Publication date |
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AU2010263547B2 (en) | 2013-12-05 |
JP5740847B2 (ja) | 2015-07-01 |
JP2011026699A (ja) | 2011-02-10 |
EP2447390A1 (en) | 2012-05-02 |
MX2011013823A (es) | 2012-01-30 |
CN102803543B (zh) | 2015-01-28 |
US9255318B2 (en) | 2016-02-09 |
CA2764663C (en) | 2013-11-12 |
KR20120025591A (ko) | 2012-03-15 |
BRPI1013802A2 (pt) | 2016-04-12 |
KR101375413B1 (ko) | 2014-03-17 |
BRPI1013802B1 (pt) | 2019-10-29 |
CN102803543A (zh) | 2012-11-28 |
AU2010263547B8 (en) | 2013-12-19 |
CA2764663A1 (en) | 2010-12-29 |
US20120118439A1 (en) | 2012-05-17 |
EP2447390A4 (en) | 2016-03-30 |
AU2010263547A1 (en) | 2012-01-12 |
EP2447390B1 (en) | 2018-11-21 |
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