JP5703632B2 - Warm press molding material and panel manufacturing method - Google Patents

Description

  The present invention relates to a warm press-molding material used in applications such as automobiles and home appliances, and a method of manufacturing a panel member using this material.

  Conventionally, automobile outer panel that requires dent resistance such as hood, door, trunk lid, back door, and fender is TS: 340MPa class BH steel plate (bake hardening type steel plate, hereinafter simply referred to as 340BH). Has been applied. 340BH is a ferritic single-phase steel in which the amount of solid solution C is controlled by adding carbonitride-forming elements such as Nb and Ti in a very low carbon steel with C: less than 0.01% by mass, and solid solution strengthened with Mn and P. In recent years, the need for lighter vehicle bodies has further increased, and the outer panel to which these 340BH has been applied has been further strengthened to reduce the thickness of the steel sheet, or to reduce the R / F (reinforcing member) with the same thickness. Study is underway to reduce the baking process time and temperature.

  However, if a large amount of Mn and P is further added to the conventional 340BH to increase the strength, the surface strain resistance of the press-formed product is significantly deteriorated due to an increase in yield stress (YS). Here, 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 must have a low YS that is close to the current 340BH, while increasing the strength of the press-formed product while increasing the strength of the press-formed product. Is required.

  On the other hand, in order to increase the strength after press molding and baking coating while maintaining low YS, it is necessary to increase work hardening (WH) during press molding and baking hardening (BH) after press molding. The amount of strain at a position where dendability is required at a protruding portion such as a door is several percent, and WH requires such a low strain and a high value. Further, it is preferable to increase BH in order to stably ensure high dent resistance without depending on the amount of strain applied during press molding. However, increasing BH results in deterioration of aging resistance. In particular, due to the recent globalization of vehicle production bases, demand for panel steel sheets is increasing not only in North America and Northeast Asia, but also in Southeast Asia, South America, India, etc., and further improvement in aging resistance is required. ing. For example, when using steel sheets in the area near the equator, considering the transportation process and storage period in the local warehouse, the steel sheets are exposed to 40-50 ° C for 2-5 months. In (340BH), the aging resistance is not sufficient, and a wrinkled pattern is generated on the design surface of the outer plate after press molding. Thus, in recent years, it is required as a steel sheet characteristic that it has a higher aging resistance than conventional steel while maintaining a high BH.

From such a background, for example, Patent Document 1 discloses the 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% in weight ratio. A method for obtaining an alloyed galvanized steel sheet having both low yield stress (YS) and high bake hardening (BH) by optimizing and forming a composite structure mainly composed of ferrite and martensite is disclosed. Further, in Patent Document 2, 0.02 to 1.5% of Mo is added to steel containing, by weight%, C: more than 0.01% and less than 0.03%, Mn: 0.5 to 2.5%, B: 0.0025% or less, and further sol. Control the amount of Al, N, B, and Mn so that sol.Al ≧ 9.7 × N, B ≧ 1.5 × 10 ⁻⁴ × (Mn ² +1) to obtain a structure consisting of ferrite and low-temperature transformation generation phase Discloses a method for obtaining a hot-dip galvanized steel sheet excellent in both bake hardenability and room temperature aging resistance. Patent Document 3 discloses that, in the process of hot rolling a steel sheet containing C: 0.005% or more and less than 0.04% and Mn: 0.5 to 3.0% by mass%, cooling at 70 ° C./s or more within 2 seconds after the end of rolling. A method for obtaining a steel sheet having excellent aging resistance by cooling to 650 ° C. or lower at a speed is disclosed. In Patent Document 4, in steel containing, by mass%, 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, Cr / Al is 30 or more. Thus, a method for obtaining a steel sheet having a low yield ratio, high BH, and excellent normal temperature aging resistance is disclosed. In Patent Document 5, in steel containing C: 0.005 to 0.04%, Mn: 1.0 to 2.0%, Cr: 0.2 to 1.0%, Mn + 1.29Cr is controlled to 2.1 to 2.8 and Cr is added. A method of obtaining a hot-dip galvanized steel sheet having a low YS and a high BH by adding a relatively large amount is disclosed. Patent Document 6 discloses that, in mass%, C: 0.01% or more and less than 0.040%, Mn: 0.3 to 1.6%, Cr: 0.5% or less, and Mo: 0.5% or less, a temperature of 550 to 750 ° C. after annealing. A method of obtaining a steel plate having excellent bake hardenability is disclosed by cooling at a cooling rate of 3 to 20 ° C / s and cooling to a temperature of 200 ° C or less at a cooling rate of 100 ° C / s or more. Yes.

Japanese Patent Publication No.62-40405 JP 2005-8904 A JP 2005-29867 A JP 2008-19502 Gazette Japanese Unexamined Patent Publication No. 2007-211338 JP 2006-233294 A

  However, all of the steel sheets described in Patent Documents 1 to 6 are composite structure steels mainly composed of ferrite and martensite as the structure of the steel sheet, and sufficiently low YS and high compared to conventional solid solution strengthened steel sheets. Although it has BH, compared with 340BH currently used for the outer panel, etc., it has high YS, and its application has not progressed to a member that demands surface distortion resistance.

  The present invention solves such problems of the prior art by warmly pressing a panel material when manufacturing a panel member, and is made of a high-strength hot-dip galvanized steel sheet. It aims at providing the manufacturing method of the member for panels excellent in the raw material for warm press molding, and surface distortion resistance and dent resistance.

  The present inventors have conducted intensive studies on a method for providing a panel member having excellent surface strain resistance and dent resistance after press forming for a high-strength hot-dip galvanized steel sheet having a composite structure with low yield strength. The following conclusions were obtained.

  (1) By using warm forming as the press forming, the YS is reduced and the surface distortion resistance is improved. The temperature is 250 to 500 ° C., which has little influence on the properties of plating.

  (2) Although the cost is increased by warm forming, the dent resistance can be improved without passing through the paint baking process by prescribing the structure and mechanical properties of the material steel plate as it is.

  (3) Although a relatively large amount of Cr has been added to the conventional composite steel sheet in order to ensure hardenability while maintaining low YS, the corrosion resistance of the hem-processed portion is significantly degraded by the addition of Cr. For this reason, in order to ensure the corrosion resistance equivalent to or higher than that of 340BH, it is necessary to reduce the Cr content to less than 0.5% by mass.

  (4) To keep YS or the yield ratio (YR) low and ensure good aging resistance, the composite structure of ferrite and the second phase, which is mainly martensite, is increased by increasing the Mn equivalent to suppress the formation of pearlite. It is necessary to secure an area ratio of the martensite phase of 3% or more while controlling it.

  (5) In order to ensure sufficient Mn equivalent while reducing Cr from the viewpoint of ensuring corrosion resistance, it is necessary to utilize Mn, but when Mn is added in a large amount, ferrite grains expand and uneven particle size distribution At the same time, martensite is remarkably refined and YS increases. On the other hand, B (boron) and P (phosphorus) have a remarkable effect of improving hardenability, and also have the effect of polygonizing the ferrite grains uniformly and coarsely. There is an effect of evenly distributing the emphasis. Specifically, B has a strong action of uniformly and coarsening ferrite grains, and P has a strong action of uniformly dispersing martensite. For this reason, by adding P and B in a predetermined range and further suppressing the addition amount of Mn to a predetermined range, uniform and coarse ferrite grains and uniformly dispersed martensite grains can be obtained at the same time, Cr and Mo Low YS can be obtained even in steels with reduced slag.

  (6) Addition of a large amount of Mn significantly degrades BH due to a decrease in solute C and non-uniform dispersion of the second phase. On the other hand, P and B themselves have the effect of increasing BH when added. Therefore, BH increases remarkably by adding more than a predetermined amount of P and B to reduce the amount of Mn added. For this reason, low YS and high BH can be obtained simultaneously by controlling P, B, and Mn to a specific range in addition to controlling Mn equivalent.

  (7) In the steel of the present invention in which the Mn equivalent is increased by utilizing P and B, the ferrite transformation in the cooling process after hot rolling is delayed, so that the winding process is performed at a predetermined temperature without performing special rapid cooling. By applying the above, the hot rolled structure becomes fine ferrite and fine pearlite, or bainite, and the structure after cold rolling and annealing becomes uniform, and BH is further improved.

  Thus, while reducing Cr to less than 0.5% by mass and increasing the Mn equivalent, P and B were added in a predetermined amount and a hot-dip galvanized steel sheet in which the added amount of Mn was controlled within a predetermined range was 250 to 500. By performing warm forming at 0 ° C., it is possible to provide a panel member that has all of surface distortion resistance, dent resistance, corrosion resistance, and aging resistance while suppressing the manufacturing cost as compared with the conventional case.

  The present invention has been made based on the above findings and provides the following inventions.

  (1) It is composed of a high strength hot-dip galvanized steel sheet having a composite structure with a martensite phase area ratio of 3-15%, BH content of 45 MPa or more, YS of 280 MPa or less, and YR of 60% or less. A material for warm press molding is provided.

  (2) As a component composition of steel, in mass%, C: more than 0.015% and less than 0.1%, Si: 0.2% or less, Mn: 1.0% or more and less than 1.9%, P: 0.015% or more and 0.05% or less, S: 0.02% Sol.Al: 0.01% or more and 0.5% or less, N: 0.005% or less, Cr: less than 0.5%, B: 0.0003% or more and 0.005% or less, Mo: 0.10% or less, Ti: containing less than 0.014%, and The warm press-molding material described in (1) is provided, which satisfies 2.2 ≦ [Mneq] ≦ 3.1 and has a composition composed of the balance iron and inevitable impurities.

Where [Mneq] = [% Mn] +1.3 [% Cr] +8 [% P] +3.3 [% Mo] + 150B ^* , B ^* = [% B] + [% Ti] /48×10.8× 0.9 + [% Al] /27×10.8×0.025, [% Mn], [% Cr], [% P], [% Mo], [% B], [% Ti], [% Al] Represents each content of Mn, Cr, P, Mo, B, Ti, sol.Al. When [% B] = 0, B ^* = 0 and when [% B] + [% Ti] /48×10.8×0.9 + [% Al] /27×10.8×0.025≧0.0022, B ^* = 0.0022 To do.

  (3) Furthermore, it is characterized by containing at least one of V: 0.4% or less, Nb: 0.015% or less, Zr: 0.1% or less, Cu: 0.5% or less, Ni: 0.5% or less in mass%. The material for warm press molding described in (2) is provided.

  (4) Excellent surface distortion resistance and dent resistance, characterized in that the warm press molding material according to any one of (1) to (3) is warm press molded at 250 ° C to 500 ° C. A method for manufacturing a panel member is provided.

  When a warm press-molding material comprising the high-strength hot-dip galvanized steel sheet of the present invention is used and the warm press-molding is performed by the method of the present invention, a panel member having excellent surface distortion resistance and dent resistance can be obtained. . In addition, since the dent resistance can be ensured even if the coating baking process is omitted or simplified, it is possible to manufacture a panel member having excellent dent resistance without a significant cost increase.

  Details of the present invention will be described below.

  First, the structure and mechanical properties of a high-strength hot-dip galvanized steel sheet, which is a warm press-forming material, will be described.

1) Structure The steel sheet structure mainly includes ferrite and martensite, and may further include residual γ, pearlite, bainite, and a small amount of carbide. In particular, in the present invention, the presence of the martensite phase is important. First, a method for measuring the area ratio of the martensite phase will be described.

  The area ratio of the martensite phase is that the L section (vertical section parallel to the rolling direction) of the steel sheet was corroded with nital after being polished, and 10 fields of view were observed with a SEM (scanning electron microscope) at a magnification of 4000 times. Obtained by image analysis of photographs. In the tissue photograph, particles with white contrast were martensite. Fine dot-like particles with a diameter of 0.4 μm or less recognized on the SEM photograph are mainly carbides by TEM (transmission electron microscope) observation, and since these area ratios are very small, almost no material is used. In this case, particles having a particle size of 0.4 μm or less were excluded from the evaluation of the area ratio. In addition, ferrite is a region with a slightly black contrast, and the region in which carbides are generated in a lamellar or dotted pattern is pearlite or bainite, and particles with white contrast are not observed in the lath structure on the substrate. Residual γ.

Martensite phase area ratio: 3-15%
In order to obtain low YS while ensuring excellent aging resistance, the area ratio of the martensite phase needs to be 3% or more. When the area ratio of the martensite phase is less than 3%, aging resistance deteriorates and YS increases. On the other hand, when the area ratio of the martensite phase exceeds 15%, YS increases and BH decreases. Therefore, in order to secure excellent aging resistance and obtain low YS and high BH, the area ratio of the martensite phase is set in the range of 3 to 15%. In order to obtain low YS while obtaining higher BH, the area ratio of the martensite phase is preferably 10% or less, and more preferably 7% or less.

  Ferrite, residual γ, pearlite, and bainite are not particularly specified, but ferrite is preferably 70% or more from the viewpoint of formability, and pearlite and bainite are each preferably 10% or less. Since the ductility is improved by the presence of the residual γ, its presence is effective from the viewpoint of formability, but the residual γ is preferably 10% or less in order to increase the yield strength.

2) Mechanical properties
BH amount: 45MPa or more
The amount of BH was determined by subtracting the ultimate stress at the time of pre-strain from the upper yield stress when the specimen subjected to 2% pre-strain in the tensile test was heat-treated at 170 ° C for 20 minutes and the tensile test was performed again. (Difference between upper yield stress during re-tension test and ultimate stress during pre-strain). A general BH steel has a BH amount of 30 MPa or more. However, in the present invention, the BH amount is set to 45 MPa or more because it is intended for a member that requires more dent resistance.

YS: 280MPa or less If YS at room temperature exceeds 280MPa, surface distortion occurs during warm pressing and it is difficult to eliminate even by adjusting the mold, so YS should be 280MPa or less.

YR: 60% or less As described above, low YS is an important requirement, but at the same time, from the viewpoint of dent resistance, work hardening and a high BH amount are required. From the viewpoint of work hardening, it is preferable that YR is 60% or less and TS is higher than YS.

3) Steel component composition Next, a preferred steel component composition of the high-strength hot-dip galvanized steel sheet will be described. Note that% representing the amount of a component means mass% unless otherwise specified.

Cr: Less than 0.5%
Cr is an important element that needs to be strictly controlled in the present invention. In other words, Cr has been actively used for the purpose of reducing YS and improving BH. However, Cr is not only an expensive element, but when it is added in a large amount, the corrosion resistance of the hem-processed part is significantly deteriorated. It was revealed that That is, when evaluating the corrosion resistance of wet outer parts of door outer and hood outer parts made of conventional composite steel with low YS, the pierced life of the hem-processed part is reduced by 1 to 4 years compared to the conventional steel. Was recognized. Furthermore, it has been clarified that such deterioration of corrosion resistance occurs when the Cr content is 0.5% or more. Therefore, in order to ensure sufficient corrosion resistance, the Cr content needs to be less than 0.5%.

[Mneq]: 2.2 or more and 3.1 or less In order to secure high BH while simultaneously ensuring low YS and excellent aging resistance, it is necessary to make the steel structure a composite structure mainly composed of ferrite and second phase. . In conventional steels, many steel sheets where YS or YR is not sufficiently reduced and steel sheets with insufficient aging resistance are found, and as a result of investigating the cause, such steel sheets have martensite and a small amount of residual as the second phase. In addition to γ, pearlite and bainite were generated. Since this pearlite is as fine as 1 to 2 μm and is formed adjacent to martensite, it is difficult to distinguish it from martensite with an optical microscope, and it is identified by observing at a magnification of 3000 times or more using SEM. it can. For example, when the structure of the conventional 0.03% C-1.5% Mn-0.5% Cr steel is investigated in detail, only coarse pearlite is identified by observation with an optical microscope or SEM at a magnification of about 1000 times. The area ratio of pearlite or bainite in the area ratio of the second phase is measured to be about 10%, but when investigated in detail by SEM observation 4000 times, the ratio of the area ratio of pearlite or bainite in the second phase Account for 30-40%. By suppressing such pearlite or bainite, low YS can be obtained while ensuring high BH.

  In order to sufficiently reduce the CGL thermal history in which such fine pearlite or bainite is slowly cooled after annealing, the hardenability of various elements was investigated. As a result, it has been clarified that, in addition to Mn, Cr, B, and Mo, which are well known as hardenable elements, P has a great effect of improving hardenability. In addition, when B is added in combination with Ti or Al, the effect of improving hardenability is remarkably increased. However, the effect of improving hardenability is saturated even if it is added in a predetermined amount or more. It was found that it is expressed as an Mn equivalent formula.

[Mneq] = [% Mn] +1.3 [% Cr] +8 [% P] +3.3 [% Mo] + 150B ^*
B ^* = [% B] + [% Ti] /48×10.8×0.9 + [% Al] /27×10.8×0.025
However, when [% B] = 0, B ^* = 0, and when [% B] + [% Ti] /48×10.8×0.9 + [% Al] /27×10.8×0.025≧0.0022, B ^* = 0.0022.
Where [% Mn], [% Cr], [% P], [% Mo], [% B], [% Ti], [% Al] are Mn, Cr, P, Mo, B, Ti , Each content of sol.Al.

B ^* is an index that represents the effect of improving the hardenability by remaining solid solution B by adding B, Ti, Al, and B ^* = 0. When B ^* is 0.0022 or more, the effect of improving hardenability by B is saturated, so B ^* is set to 0.0022.

  By setting this [Mneq] to 2.2 or more, pearlite or bainite is sufficiently suppressed even in the CGL thermal history in which slow cooling is performed after annealing. Therefore, in order to obtain excellent aging resistance while reducing YS, [Mneq] needs to be 2.2 or more. Further, from the viewpoint of lowering YS, [Mneq] is preferably 2.3 or more, and more preferably 2.4 or more.

  If [Mneq] is too large, the quenching effect will be saturated and the alloy addition cost will be increased.

Mn: 1.0% or more and less than 1.9% As mentioned above, at least [Mneq] is necessary to achieve high BH while reducing YS, but that alone is not sufficient, and the amount of Mn and P and B described later are insufficient. It is necessary to control the content of to a predetermined range. That is, Mn is added to increase the hardenability and increase the ratio of martensite in the second phase. Therefore, 1.0% or more is necessary. However, if the content is too high, the α → γ transformation temperature in the annealing process becomes low, and γ grains are formed at the fine grain boundary immediately after recrystallization or at the interface of the recovery grains during recrystallization. Expands and becomes non-uniform, and the second phase becomes finer and YS increases. Also, the addition of Mn has the effect of reducing the solid solution C in the ferrite by shifting the A ₁ line of the Fe-C phase diagram to low temperature and low C side, and also disperse the second phase non-uniformly. , Significantly reduce BH. Therefore, in order to obtain low YS and high BH simultaneously, it is necessary to make it less than 1.9%.

P: 0.015% to 0.05%
P is an important element for achieving low YS and high BH in the present invention. In other words, P can be used in combination with B, which will be described later, in a predetermined range, so that low YS, high BH, and good aging resistance can be simultaneously obtained at a low manufacturing cost, and excellent corrosion resistance can be secured. Become.

  Conventionally, P has been used as a solid solution strengthening element, and it was considered desirable to reduce it from the viewpoint of lowering YS. However, as described above, it has been clarified that P has a great effect of improving hardenability even when added in a small amount. Furthermore, it has been clarified that P has an effect of uniformly and coarsely dispersing the second phase at the triple point of the ferrite grain boundary and an effect of increasing BH. Therefore, we have intensively studied methods for reducing YS and increasing BH by utilizing the effect of improving the hardenability of P. As a result, by replacing Mn with P while maintaining a predetermined [Mneq], it was found that the second phase can be dispersed very uniformly, and YS decreases and BH increases significantly. It was.

  Moreover, since P is also an element that improves corrosion resistance, replacing Cr with P can improve corrosion resistance while maintaining a good material. In order to obtain such effects by addition of P, it is necessary to add at least 0.015% or more, and it is preferable to add 0.020% or more.

  However, if P is added in an amount exceeding 0.05%, the effect of improving hardenability, the homogenization of the structure, and the effect of coarsening are saturated, and the solid solution strengthening amount becomes too large to obtain a low YS. Also, the effect of increasing BH is reduced. Moreover, when P is added in excess of 0.05%, the alloying reaction between the base iron and the plating layer is remarkably delayed, and the powdering resistance is deteriorated. Moreover, weldability also deteriorates. Therefore, the P content is 0.05% or less.

B: 0.0003% to 0.005%
B has the effect of making the ferrite grains uniform and coarse, the effect of improving hardenability, and the effect of increasing BH. For this reason, low YS and high BH can be achieved by replacing Mn with B while securing a predetermined amount of [Mneq]. Steel with uniformly coarse ferrite grains and martensite evenly dispersed at the triple point of the grain boundary by using P, which has the effect of forming martensite at grain boundaries, and B, which has the effect of uniformly coarsening ferrite grains A structure is obtained, and YS reduction and BH improvement are remarkably achieved. In order to obtain such an effect of addition of B, B needs to be 0.0003% or more. In order to further exhibit the effect of lowering YS due to the addition of B, B is preferably added in an amount of 0.0005% or more, more preferably more than 0.0010%. However, when B is added in excess of 0.005%, castability and rollability are remarkably lowered. For this reason, B is made 0.005% or less. From the viewpoint of securing castability and rollability, B is preferably 0.004% or less.

C: More than 0.015% and less than 0.1%
C is an element necessary for ensuring a predetermined area ratio of the martensite phase. If the amount of C is too small, a sufficient area ratio of the martensite phase cannot be secured, and sufficient aging resistance and low YS cannot be obtained. In order to obtain aging resistance equivalent to or higher than that of conventional steel, C needs to be more than 0.015%. From the viewpoint of further improving the aging resistance and further reducing YS, C is preferably 0.020% or more. On the other hand, when the C content is 0.1% or more, the area ratio of the martensite phase becomes too large, and YS increases and BH also decreases. Moreover, weldability also deteriorates. Therefore, the C content is less than 0.1%. In order to obtain high BH while obtaining lower YS, the C content is preferably less than 0.06%, and more preferably less than 0.04%.

Si: 0.2% or less
Addition of a small amount of Si has the effect of improving the surface quality by delaying the scale formation in hot rolling, the effect of moderately delaying the alloying reaction between the iron and zinc in the plating bath or during alloying, From this point of view, it can be added because of the effect of making the microstructure of the layer more uniform and coarse. However, if Si is added in excess of 0.2%, the appearance quality of the plating deteriorates, making it difficult to apply to the outer panel and causing an increase in YS. Therefore, Si is made 0.2% or less from the viewpoint of improving the surface quality and reducing YS.

S: 0.02% or less
S can be contained because it has the effect of improving the peelability of the primary scale of the steel sheet and improving the appearance quality of the plating by containing an appropriate amount of S. However, if the content of S is large, too much MnS precipitates in the steel, and ductility such as elongation and stretch flangeability of the steel sheet is lowered, and press formability is lowered. Moreover, when hot-rolling a slab, hot ductility is reduced and surface defects are easily generated. Furthermore, corrosion resistance is reduced. Therefore, the S content is 0.02% or less. From the viewpoint of improving ductility and corrosion resistance, S is preferably 0.01% or less, more preferably 0.002% or less.

sol.Al: 0.01% to 0.5%
Al is added for the purpose of fixing N and promoting the hardenability improvement effect of B, the purpose of improving aging resistance, and the purpose of improving the surface quality by reducing inclusions. The effect of improving the hardenability of Al is small in B-free steel and is about 0.1 to 0.2 times Mn. However, in steel with B added, N is fixed as AlN and the effect of leaving solid solution B is small. The amount of sol.Al added is also large. Conversely, if the content of sol.Al is not optimized, the effect of improving the hardenability of B cannot be obtained, and solid solution N remains and the aging resistance deteriorates. From the viewpoint of improving the hardenability and aging resistance of B, the content of sol.Al is 0.01% or more. In order to exert such effects more, sol.Al is preferably contained in an amount of 0.015% or more, more preferably 0.04% or more. On the other hand, even if sol.Al is added in excess of 0.5%, the effect of leaving the solid solution B and the effect of improving the aging resistance are saturated, resulting in a cost increase. 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, and TiN in steel, and has a detrimental effect of eliminating the effect of B through the formation of BN. In addition, fine AlN is formed to lower the grain growth property and increase YS. Furthermore, when solid solution N remains, the aging resistance deteriorates. From this point of view, N must be strictly controlled. If the N content exceeds 0.005%, the effect of improving the hardenability of B cannot be obtained sufficiently and YS increases. Moreover, with such component steels, the aging resistance deteriorates, and the applicability to the outer panel becomes insufficient. From the above, the N content is 0.005% or less. From the viewpoint of effectively utilizing B and further reducing YS by reducing the precipitation amount of AlN, N is preferably 0.004% or less.

Mo: 0.10% or less
Mo can be added from the viewpoint of improving hardenability to suppress the formation of pearlite, lowering YR, or improving BH while maintaining good aging resistance. However, since Mo is an extremely expensive element, a large amount of addition leads to a significant cost increase. In addition, Y increases as the amount of Mo increases. Therefore, the amount of Mo added is limited to 0.10% or less from the viewpoint of YS reduction and cost reduction. From the viewpoint of further reducing YS, it is preferably 0.05% or less, and more preferably no addition (0.02% or less).

Ti: Less than 0.014%
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. However, if the content increases, fine precipitates such as TiC and Ti (C, N) are formed in the steel to significantly increase YS, and TiC is generated during cooling after annealing to reduce BH. Since there exists an effect | action, it is necessary to control content of Ti to an appropriate range. When the Ti content exceeds 0.014%, YS increases remarkably and BH decreases. Therefore, the Ti content is less than 0.014%. In order to fix N by precipitation of TiN and exert the effect of improving the hardenability of B, the Ti content is preferably 0.002% or more, and the TiC precipitation is suppressed and low YS and high BH are obtained. Therefore, the Ti content is preferably less than 0.010%.

  The balance is iron and inevitable impurities, but further contains at least one of V: 0.4% or less, Nb: 0.015% or less, Zr: 0.1% or less, Cu: 0.5% or less, Ni: 0.5% or less You can also

V: 0.4% or less
V is an element that improves hardenability and has a small effect on the deterioration of plating quality and corrosion resistance, so it can be used as an alternative to Mn and Cr. However, if added over 0.4%, the cost increases significantly, so V is added at 0.4% or less.

Nb: 0.015% or less
Nb refines the structure and precipitates NbC and Nb (C, N), strengthens the steel sheet, and has the effect of increasing BH by grain refinement, so it is added from the viewpoint of increasing strength and increasing BH. can do. However, if adding over 0.015%, YS increases remarkably, so Nb is added at 0.015% or less.

Zr: 0.1% or less
Zr can also be used as a hardenable element and precipitation strengthening element. However, if the amount added is too large, YS increases, so Zr is added at 0.1% or less.

Cu: 0.5% or less
Since Cu improves the corrosion resistance, it is preferably added from the viewpoint of improving the corrosion resistance. Moreover, it is an element mixed when scrap is used as a raw material. By allowing Cu to be mixed, recycled materials can be used as raw materials, and manufacturing costs can be reduced. From the viewpoint of improving the corrosion resistance, Cu is preferably added in an amount of 0.03% or more. However, if the content is too large, it causes surface defects, so Cu is 0.5% or less.

Ni: 0.5% or less
Ni is also an element having an action of improving the corrosion resistance. Ni also has the effect of reducing surface defects that are likely to occur when Cu is contained. Therefore, Ni is preferably added in an amount of 0.02% or more from the viewpoint of improving the surface quality while improving the corrosion resistance. However, if the amount of Ni added is too large, scale generation in the heating furnace becomes non-uniform, causing surface defects and a significant cost increase. Therefore, Ni is 0.5% or less.

  Furthermore, 1 or more types of the following components can also be added.

W: 0.15% or less
W can be used as a hardenable element and a precipitation strengthening element. However, if the amount added is too large, YS increases, so W is preferably added at 0.15% or less.

Sn: 0.2% or less
Sn is preferably added from the viewpoint of suppressing decarburization and de-B in the tens of microns region of the steel sheet surface layer caused by nitridation, oxidation, or oxidation of the steel sheet surface. From the viewpoint of suppressing nitriding and oxidation, it is desirable to add 0.005% or more of Sn, but if it exceeds 0.2%, YS increases and toughness deteriorates, so Sn is preferably contained at 0.2% or less.

Sb: 0.2% or less
Sb is also preferably added from the viewpoint of suppressing decarburization and de-B in the tens of microns region of the steel sheet surface layer caused by nitriding, oxidation, or oxidation of the steel sheet surface, as with Sn. By suppressing such nitriding and oxidation, the amount of martensite generated on the steel sheet surface layer can be prevented from decreasing, hardenability can be prevented from decreasing due to the decrease in B, and the wettability of hot dip galvanizing. Can improve the plating appearance quality. From the viewpoint of suppressing nitridation and oxidation, Sb is preferably added in an amount of 0.005% or more. However, if it exceeds 0.2%, YS increases and toughness deteriorates, so Sb is preferably contained in an amount of 0.2% or less.

Ca: 0.01% or less
Ca fixes S in steel as CaS, and further increases the pH in corrosive organisms, and has the effect of improving the corrosion resistance around heme-processed parts and spot welds. Moreover, it has the effect | action which suppresses the production | generation of MnS which reduces stretch flangeability by the production | generation of CaS, and improves stretch flangeability. From such a viewpoint, Ca is preferably added in an amount of 0.0005% or more. However, Ca easily floats and separates as an oxide in molten steel, and it is difficult to leave a large amount in Ca. Therefore, the Ca content is 0.01% or less.

Ce: 0.01% or less
Ce can also be added for the purpose of fixing S in the steel. However, since it is an expensive element, adding a large amount increases the cost. Therefore, Ce is preferably added at 0.01% or less.

La: 0.01% or less
La can also be added for the purpose of fixing S in the steel. However, since it is an expensive element, adding a large amount increases the cost. Therefore, La is preferably added at 0.01% or less.

  The high-strength hot-dip galvanized steel sheet has YS and TS higher than those of the current 340BH, and has excellent aging resistance. When this high-strength hot-dip galvanized steel sheet is warm-press formed, excellent surface strain resistance can be obtained because YS can be reduced. In addition, work hardening during warm press molding (WH) and bake hardening after warm press molding (BH) can be obtained even with a low amount of strain. Excellent dent resistance without passing through.

4) Method for producing high-strength hot-dip galvanized steel sheet The method for producing the above-described high-strength hot-dip galvanized steel sheet will be described.

  As described above, the steel sheet of the present invention, after hot rolling and cold rolling the steel slab having the limited component composition as described above, is in a continuous hot dip galvanizing line (CGL) exceeding 740 ° C. After annealing at an annealing temperature of less than ℃, cooled from the annealing temperature at an average cooling rate of 2 to 30 ℃ / sec, dipped in a galvanizing bath and galvanized, after galvanization, an average of 5 to 100 ℃ / sec It can be manufactured by a method of cooling to 100 ° C. or lower at a cooling rate, or further subjecting it to an alloying treatment after galvanization, and cooling to 100 ° C. or lower at an average cooling rate of 5 to 100 ° C./sec after the alloying treatment. This will be described in more detail below.

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. In the hot rolling, for example, the slab heating temperature may be 1100 to 1300 ° C., the finishing rolling temperature may be Ar ₃ transformation point to Ar ₃ transformation point + 150 ° C., and the winding temperature may be 400 to 720 ° C. After finish rolling, it is preferable to cool to 640 ° C. or less at an average cooling rate of 20 ° C./s or more, and then to wind at 400 to 620 ° C.

  When the average cooling rate is less than 20 ° C./s, the ferrite of the hot-rolled sheet becomes coarse and pearlite is coarsely formed. This is not preferable because it causes a problem that the martensite dispersion is not made uniform after cold rolling and annealing and ductility is lowered. For the same reason, the cooling stop temperature is preferably 640 ° C. or lower. For the coiling temperature, the upper limit of 620 ° C. is a preferable requirement for the above reason. However, if the coiling temperature is less than 400 ° C., the hot-rolled sheet becomes hard and the cold rolling load increases, which is not preferable.

  In order to obtain a beautiful plating surface quality for the outer panel, the slab heating temperature is preferably 1250 ° C or less, and sufficient descaling is performed to remove the primary and secondary scales generated on the steel sheet surface, and finish rolling. The temperature is preferably 900 ° C. or lower.

Cold rolling:
In cold rolling, 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 in-plane anisotropy of YS, the rolling rate is 70 to 73%. 85% is preferable.

Continuous hot dip galvanizing:
The steel sheet after cold rolling is annealed and galvanized with CGL, or further alloyed after galvanizing. The annealing temperature is more than 740 ° C and less than 840 ° C. Below 740 ° C, the solid solution of carbide becomes insufficient, and the area ratio of the martensite phase cannot be secured stably. Above 840 ° C, a sufficiently low YS cannot be obtained. The soaking time may be 20 seconds or more in a temperature range exceeding 740 ° C., which is carried out by normal continuous annealing, and more preferably 40 seconds or more. The soaking time is preferably 10 min or less from the viewpoint of deterioration of surface quality and ductility due to coarsening of crystal grains.

  After soaking, cooling is performed at an average cooling rate of 2 to 30 ° C./sec from the annealing temperature to the temperature of the galvanizing bath normally maintained at 450 to 500 ° C. When the average cooling rate is slower than 2 ° C / sec, a large amount of pearlite is generated in the temperature range of 500 to 650 ° C, and a sufficiently low YS cannot be obtained. On the other hand, when the average cooling rate is higher than 30 ° C / sec, the γ → α transformation proceeds remarkably around 500 ° C before and after being immersed in the galvanizing bath, and the martensite phase becomes finer and exists at the triple point of grain boundaries. The area ratio of the martensite phase decreases and YS increases.

  Then, it is immersed in a galvanizing bath and galvanized. The galvanizing method is not particularly limited, but it is preferable that the bath temperature is 460 ° C. and the Al in the bath is about 0.13%. After galvanization, cool to 100 ° C or less at an average cooling rate of 5-100 ° C / sec. If necessary, alloying treatment can also be performed by holding within 30 seconds within a temperature range of 470 to 650 ° C. after galvanization. The alloying treatment is preferably held for 10 to 25 seconds after immersion in the plating bath and heated to 480 to 540 ° C. so that the Fe% during plating is in the range of 9 to 12%. In conventional steel sheets that have not been optimized for [Mneq], the material has deteriorated significantly due to such alloying treatment. However, in the steel sheet of the present invention, the increase in YS is small and a good material can be obtained. it can.

  When alloying after galvanization, the alloy is cooled to 100 ° C. or less at an average cooling rate of 5 to 100 ° C./sec after the alloying treatment. When the average cooling rate is less than 5 ° C / sec, pearlite is generated around 550 ° C, and bainite is generated in the temperature range of 400 ° C to 450 ° C, increasing YS. On the other hand, if the average cooling rate is higher than 100 ° C./sec, the self-tempering of martensite that occurs during continuous cooling becomes insufficient, the martensite becomes too hard, YS increases, and ductility decreases. 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 lowering YS.

  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. In that case, the skin pass elongation rate is preferably 0.2 to 0.6% from the viewpoint of low YS and high EL.

5) Method for producing panel member In the present invention, the above-described high-strength hot-dip galvanized steel sheet is used, and a panel member is produced by hot press forming at a forming temperature of 250 ° C to 500 ° C.

  The forming temperature refers to the steel sheet heating temperature before press forming. In steel sheets with low solid solution C such as IF steel, YS decreases with increasing temperature, but in steel sheets with BH properties that are the subject of the present invention, dynamic strain aging occurs at around 200 ° C, and in the second phase. Since tempering of certain martensite occurs, if the heating temperature of the steel sheet before press forming is less than 250 ° C., there is a risk that YS will rise and yield elongation will occur. When the molding temperature is 250 ° C. or higher, YS decreases and surface distortion resistance is improved. From the viewpoint of surface strain resistance, the molding temperature is 250 ° C or higher. When the molding temperature exceeds 500 ° C., the degree of alloying of hot dip galvanizing increases and powdering is worsened. Therefore, the molding temperature is limited to a range of 250 ° C to 500 ° C. A more preferable molding temperature is 400 ° C. or less.

  When the above-mentioned high-strength hot-dip galvanized steel sheet is used for warm press forming on panel members at the above temperature, work hardening (WH) during warm press forming, even after low strain, baking after warm press forming Since curing (BH) can be obtained, excellent dent resistance can be obtained without going through a paint baking process.

  Next, examples of the present invention will be described.

Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab by a continuous casting method. These steel slabs are heated to 1220 ° C and roughly rolled into sheet bars, then hot rolled by a hot rolling process in which a finish rolling temperature of 850 ° C, an average cooling rate after finish rolling of 20 ° C / sec, and a winding temperature of 550 ° C are applied. A board was used. These hot-rolled sheets were formed into a cold-rolled sheet having a thickness of 0.7 mm by pickling and a cold rolling process with a rolling rate of 65%. The obtained cold-rolled sheet was subjected to hot dip galvanization (plating bath temperature: 480 ° C) after annealing at the annealing temperature shown in Table 2 with a soaking time of 40 seconds in a continuous hot dip galvanizing line (CGL), and after hot dip galvanizing Alloying treatment was performed to produce a hot dip galvanized steel sheet with a plating film made of an Fe-Zn alloy. Some hot-dip galvanized steel sheets that were not subjected to alloying after hot-dip galvanizing were also produced. The hot-dip galvanized steel sheet produced above was subjected to temper rolling with an elongation of 0.5%. The zinc plating amount was adjusted to 45 g / m ² per side, and the alloying treatment was performed so that Fe% of the plating film was 9.5%.

  Each obtained hot-dip galvanized steel sheet was examined for steel sheet structure, tensile properties, BH properties, corrosion resistance, and warm forming properties. The survey method is as follows.

(1) Steel plate structure The area ratio of the martensite phase was determined by corroding the L cross-section (vertical cross-section parallel to the rolling direction) of the steel plate with Nital and observing 10 fields of view with a SEM (scanning electron microscope) at a magnification of 4000 times. The obtained tissue photographs were obtained by image analysis. In addition, ferrite is a region with a slightly black contrast, and the region in which carbides are generated in a lamellar or dotted pattern is pearlite or bainite, and particles with white contrast are not observed in the lath structure on the substrate. Residual γ.

(2) Tensile properties JIS No. 5 tensile test specimens were taken from each of the obtained hot-dip galvanized steel sheets in the 90 ° direction (C direction) with respect to the rolling direction, and the crosshead speed was 10 mm / in accordance with the provisions of JIS Z 2241. A tensile test was performed at min, and yield stress (YS) and tensile strength (TS) were determined. Furthermore, YR was calculated from the ratio of YS to TS, YS / TS.

(3) BH characteristics JIS5 tensile test specimens were taken from each obtained galvanized steel sheet in the 90 ° direction (C direction) with respect to the rolling direction, and a tensile test was conducted at a crosshead speed of 10 mm / min. After pre-straining, the test piece was once removed from the tensile tester, subjected to aging treatment at 170 ° C. for 20 minutes in a high-temperature bath, and a tensile test was performed again at a crosshead speed of 10 mm / min. The amount of BH was obtained by subtracting the ultimate stress during pre-strain from the upper yield stress during re-tensioning.

(4) Warm forming characteristics Warm forming characteristics were evaluated by high-temperature tension. The test piece was 10 mm wide and the distance between grades was 50 mm. After holding at a predetermined temperature for 10 minutes with a high-temperature tensile tester, the YS was pulled at a crosshead speed of 0.15 mm / min and after YS at 4.5 mm / min, and 2% pre-strained. Thereafter, those that were not subjected to aging treatment at 170 ° C. for 20 minutes were re-tensioned at room temperature, and the upper yield point (YP ′) at that time was used as an index of dent resistance. The surface strain resistance was evaluated by YS at the time of pre-strain.
Based on YP ', the dent resistance was evaluated to be good at dent resistance of 320 MPa or more and poor at dent resistance below 320 MPa.
Based on YS, the surface strain resistance was evaluated to be good at a surface strain resistance of 250 MPa or less, and to be poor at a surface strain of over 250 MPa.

(5) Corrosion resistance The corrosion resistance of each hot-dip galvanized steel sheet was evaluated using a structure that simulated the periphery of the hem-processed part and spot welded part. In other words, two steel sheets obtained were spot welded together to bring them into close contact with each other, and after applying chemical conversion treatment and electrodeposition coating to simulate the painting process in an actual vehicle, corrosion was performed under SAE J2334 corrosion cycle conditions. A test was conducted. The electrodeposition coating film thickness was 20 μm. Corrosion products were removed from the corrosion samples after 60 cycles, and the reduction in plate thickness from the original plate thickness measured in advance was determined as the corrosion loss. Corrosion weight loss was judged to be good when it was less than 0.5mm, and corrosion resistance was poor when it was 0.5mm or more.

  Table 2 shows the annealing conditions for CGL, the presence or absence of alloying treatment, and the survey results. The primary cooling rate in Table 2 is the average cooling rate from the annealing temperature to immersion in the plating bath, and the secondary cooling rate is the average cooling rate from the alloying treatment temperature to 100 ° C (no alloying treatment is applied) The average cooling rate until cooling to 100 ° C. after hot dip galvanization.

  As is clear from Table 2, using the hot-dip galvanized steel sheet whose steel sheet structure and mechanical properties satisfy the scope of the present invention, all of the present invention examples subjected to warm forming under the conditions specified by the present invention method are surface resistant. Strain and dent resistance are good evaluations, and corrosion resistance is also good.

  On the other hand, comparative examples (No. 8, 10, 20 to 20) in which hot-dip galvanized steel sheets in which at least one of the steel sheet structure and mechanical properties do not satisfy the scope of the present invention were used and warm forming was performed under the conditions specified in the present invention. 23), even if a hot-dip galvanized steel sheet whose steel sheet structure and mechanical properties satisfy the scope of the present invention was used, comparative examples (No. Either distortion, dent resistance, or corrosion resistance is poor.

  When warm press-molding is performed by the method of the present invention using the high-strength hot-dip galvanized steel sheet of the present invention, a panel member having excellent surface strain resistance and dent resistance can be obtained. In addition, since the dent resistance can be ensured even if the coating baking process is omitted or simplified, it is possible to manufacture a panel member having excellent dent resistance without a significant cost increase.

Claims (3)

As composition of steel, in mass%, C: more than 0.015% and less than 0.06%, Si: 0.2% or less, Mn: 1.0% or more and less than 1.9%, P: 0.015% or more and 0.05% or less, S: 0.02% or less, sol Al: 0.01% or more and 0.5% or less, N: 0.005% or less, Cr: Less than 0.5%, B: 0.0003% or more and 0.005% or less, Mo: 0.10% or less, Ti: Less than 0.014%, and 2.2 ≦ [ Mneq] ≦ 3.1, having a composition composed of the balance iron and inevitable impurities, having a composite structure with a martensite phase area ratio of 5 to 15%, BH amount of 45 MPa or more, YS of 280 MPa or less, YR A material for warm press forming with a forming temperature of 250 ° C to 500 ° C, characterized by comprising a high-strength hot-dip galvanized steel sheet having a H of 60% or less.
Where [Mneq] = [% Mn] +1.3 [% Cr] +8 [% P] +3.3 [% Mo] + 150B ^* , B ^* = [% B] + [% Ti] /48×10.8× 0.9 + [% Al] /27×10.8×0.025, [% Mn], [% Cr], [% P], [% Mo], [% B], [% Ti], [% Al] Represents each content of Mn, Cr, P, Mo, B, Ti, sol.Al. When [% B] = 0, B ^* = 0 and when [% B] + [% Ti] /48×10.8×0.9 + [% Al] /27×10.8×0.025≧0.0022, B ^* = 0.0022 To do. Further, it is characterized by containing at least one of V: 0.4% or less, Nb: 0.015% or less, Zr: 0.1% or less, Cu: 0.5% or less, Ni: 0.5% or less in mass%. Item 2. The warm press-molding material according to Item 1 . A method for producing a panel member having excellent surface strain resistance and dent resistance, wherein the warm press molding material according to claim 1 or 2 is warm press molded at 250 ° C to 500 ° C.