KR20180063304A - Hot press member and manufacturing method thereof - Google Patents

Abstract

A first region having a tensile strength TS of 1500 MPa or more and a uniform elongation uE1 of 6.0% or more and a second region having a tensile strength TS of 780 MPa or more and a uniform elongation uEl of 15.0% or more A hot press member is provided. The hot press member of the present invention is a hot press member having a predetermined (particularly, a low C of not less than 0.090% and not more than 0.30% and a high Mn of not less than 3.5% and not more than 11.0%), a martensite phase of not less than 80.0% A first region having a dislocation density of 1.0 x 10 < ¹⁶ > / m < 2 > or more, a ferrite phase having a volume ratio of 30.0% or more and 60.0% And a second region having a structure including a residual austenite phase of not less than 10.0% and not more than 70.0% and a martensite phase of not more than 30.0% in volume percentage.

Description

Hot press member and manufacturing method thereof

The present invention relates to a member formed by hot pressing a thin steel sheet, that is, a hot press member and a manufacturing method thereof.

Recently, from the viewpoint of preserving the global environment, improvement of fuel efficiency of automobiles is strongly desired. Therefore, there is a strong demand for reduction in weight of the vehicle body. Therefore, it is required to increase the strength of the steel sheet as the material of the member so as not to impair the safety even when the automotive member is made thin. However, in general, since the moldability is lowered as the strength of the steel sheet becomes higher, there arises a problem such as difficulty in molding or deterioration of shape durability in the production of a member made of a high-strength steel sheet .

To solve this problem, a technique of manufacturing a high-strength automobile member by applying a hot press method to a steel sheet has been put to practical use. In the hot press method, the steel sheet is heated to the austenite side and then conveyed to the press machine, which is molded into a desired shape member in the press machine and quenched at the same time. In the cooling process (quenching) in the mold, the structure of the member is phase-changed from the austenite phase to the martensite phase, whereby a high-strength member having a desired shape is obtained.

Further, in recent years, from the viewpoint of ensuring the safety of the passenger, it is desired to improve the impact resistance property of the automotive member. In order to satisfy this demand, it is effective to increase the uniform elongation of the automotive member from the viewpoint of enhancing the ability to absorb energy at impact (impact energy absorbing ability). Therefore, there is a strong demand for hot press members having high strength and excellent uniform stretchability.

With respect to this demand, Patent Document 1 proposes a hot press formed article in which a thin steel sheet is formed by a hot press forming method. The hot press molded article described in Patent Document 1 contains 0.15 to 0.35% of C, 0.5 to 3% of Si, 0.5 to 2% of Mn, 0.05% or less of P, 0.05% or less of S, (The content of N) x 4 to 0.1% and N: 0.001 to 0.01%, and the balance of Fe and inevitable impurities And has a structure composed of 80 to 97% of martensite, 3 to 20% of retained austenite, and 5% or less of residual structure in terms of composition and area ratio. According to the technique described in Patent Document 1, it is described that a hot pressed component in which a metal structure in which a proper amount of retained austenite remains can be obtained and the ductility inherent in a molded product is further increased can be realized.

In addition, Patent Document 2 proposes a hot press member having excellent ductility. The hot press member described in Patent Document 2 is characterized in that it contains 0.20 to 0.40% of C, 0.05 to 3.0% of Si, 1.0 to 4.0% of Mn, 0.05% or less of P, 0.05% or less of S, To 0.1% of N, 0.01% or less of Fe, and the balance of Fe and inevitable impurities; a composition ratio of the ferrite phase occupying in the whole structure is 5 to 55% and an areal ratio of the martensite phase is 45 to 95% And a microstructure having a microstructure in which an average particle size of a ferrite phase and a martensite phase is not more than 7 m, and has a high tensile strength TS: 1470 to 1750 MPa and a high elongation El: 8% or more.

In recent years, a hot press member having two portions having different mechanical characteristics in the same member and a manufacturing method thereof have been developed and have attracted attention as a technique for further improving the performance of parts such as a B-pillar and a rear side member. Patent Document 3 discloses a ferritic stainless steel having compositional composition of 0.1 to 0.3% of C, 0.5 to 3% of Si, 0.5 to 2% of Mn, 0.05% or less of P, 0.05% 0.1 to 0.1% of N and 0.001 to 0.01% of N, the balance of Fe and inevitable impurities, and the metal structure contains martensite: 80 to 97 area% and retained austenite: 3 to 20 area% A first region comprising 5% by area or less of residual structure and 30% or less by area of ferrite, 30% by area or less of bainitic ferrite, 30% or less of martensite, And a second region composed of an area%.

Japanese Patent Application Laid-Open No. 2013-79441 Japanese Laid-Open Patent Publication No. 2010-65293 Japanese Laid-Open Patent Publication No. 2013-194248

However, in the techniques described in Patent Documents 1 and 2, the strengthening of the martensite phase by C is achieved by the strengthening of the tensile strength TS: 1,500 MPa or more. However, from the viewpoint of increasing the impact energy absorbing ability, the problem that the uniform stretching becomes insufficient .

Further, in the technique described in Patent Document 3, there is a problem that the robustness to the molding initiation temperature at the time of hot pressing for forming two portions having different mechanical properties is restricted.

The hot press member is generally baked after the member is manufactured, and the yield stress YS is increased by the heat treatment at the time of baking. Here, in order to enhance the impact resistance property, it is also important that not only the uniform stretching but also YS is high, a hot press member having excellent heat-setting curability such that YS increases as much as possible by heat treatment at baking is required . However, the techniques described in Patent Documents 1, 2 and 3 do not consider this heat treatment curability at all.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems and has an object to provide a steel sheet which has tensile strength TS of 1,500 MPa or more and tensile properties of uniformly elongated uEl of 6.0% or more and excellent yield stress YS of 150 MPa or more when heat treatment (baking coating) A hot press member having a first region having heat treatment curability and a second region having a tensile strength TS of 780 MPa or more and a tensile strength of uniformly elongated uEl: 15.0% or more. Further, It is another object of the present invention to provide a method of manufacturing a hot press member capable of producing a member with high robustness against the molding initiation temperature at the time of hot pressing. In the present specification, "excellent heat setting curability" means that when the hot press member is heat treated, the difference between the yield stress YS after heat treatment and the yield stress YS before heat treatment (hereinafter referred to as "ΔYS") is 150 MPa or more Lt; / RTI > In addition, a large amount of mobile dislocation is generated on martensite, so that YS has a low property. Therefore, it is considered that increasing the YS of the first region having martensite as a main phase is very effective in solving the above problems.

In order to achieve the above object, the inventors of the present invention have found that, in a hot press member having a first region having a high strength of at least 1500 MPa and a second region having a tensile strength TS of at least 780 MPa, And various factors affecting the YS in the first area were studied. As a result, the following perceptions were obtained.

(A) In order to increase the homogeneously stretched uEl of the first region to 6.0% or more and the homogeneously stretched uEl of the second region to 15.0% or more, a structure having an appropriate amount of retained austenite is required. In order to obtain a structure having a C content of less than 0.30 mass% and an appropriate amount of retained austenite, Mn of at least 3.5% is required to be contained. Mn also contributes to an increase in strength, and even if C is less than 0.30%, higher strength can be secured.

(B) Before subjecting a steel sheet containing Mn of not less than 3.5% to the above-mentioned hot press, the steel sheet is preheated to the temperature of the ferrite-austenite 2 phase in advance and heated at a predetermined temperature within the range of 1 hour to 48 hours And the Mn is concentrated in the austenite, whereby an appropriate amount of retained austenite can be produced.

(C) the dislocation density of the hot press member and? Ys. In order to realize ΔYS: 150 MPa or more, the dislocation density of the hot press member must be 1.0 × 10 ¹⁶ / m 2 or more.

(D) When the Mn content is 3.5% by mass or more, austenite produced in the heating step immediately before the hot press forming step for the material steel sheet is subjected to heat treatment at a cooling rate equal to or higher than the air- There is no case that causes transformation. This is because Mn has the effect of delaying the ferrite transformation of austenite in the cooling step. As a result, there is no restriction on the molding initiation temperature for obtaining the first region and the second region having desired characteristics, and the robustness is widened as compared with the conventional one.

The present invention has been completed by the above-described recognition, and its essential structure is as follows.

(1) in mass%

C: 0.090% or more and less than 0.30%

Mn: 3.5% or more and less than 11.0%

Si: 0.01 to 2.5%

P: not more than 0.05%

S: 0.05% or less,

Al: 0.005 to 0.1%

N: 0.01% or less,

The balance being Fe and inevitable impurities,

A structure including a martensite phase of not less than 80.0% by volume and a retained austenite phase of not less than 3.0% and not more than 20.0% by volume and a tensile strength TS of 1500 MPa or more and a tensile strength of not less than 6.0% A first region having a dislocation density of 1.0 x 10 < ¹⁶ > / m < 2 &

A structure including 30.0% or more and 60.0% or less of ferrite phase by volume, 10.0% or more and 70.0% or less of residual austenite phase by volume, and 30.0% or less martensite phase by volume, A second region having a tensile property of not less than 780 MPa and a uniform elongation uEl of not less than 15.0%

Wherein the hot press member comprises:

(2) In the structure of the second region, the average grain size of the ferrite phase is 10 占 퐉 or less, the average grain size of the second phase is 10 占 m or less, the Mn concentration in the second phase is Mns and the Mn concentration in the ferrite phase is Mn? (1), wherein Mns / Mn alpha is not less than 1.5.

(3) The hot-press member according to (1) or (2) above, wherein the composition further comprises one or two or more groups selected from the group consisting of the following groups A to E in mass%.

group

A: at least one or two or more selected from the group consisting of 0.01 to 5.0% of Ni, 0.01 to 5.0% of Cu, 0.01 to 5.0% of Cr and 0.01 to 3.0% of Mo

B group: 0.005 to 3.0% of Ti, 0.005 to 3.0% of Nb, 0.005 to 3.0% of V and 0.005 to 3.0% of W

Group C: at least one selected from the group consisting of REM: 0.0005 to 0.01%, Ca: 0.0005 to 0.01%, and Mg: 0.0005 to 0.01%

Group D: Sb: 0.002 to 0.03%

E group: B: 0.0005 to 0.05%

(4) The hot-press member according to any one of (1) to (3), wherein a plated layer is provided on the surface.

(5) The hot-press member according to (4), wherein the plating layer is a Zn-based plating layer or an Al-based plating layer.

(6) The hot-press member according to (5), wherein the Zn-based plating layer contains 10 to 25 mass% of Ni.

(7)

C: 0.090% or more and less than 0.30%

Mn: 3.5% or more and less than 11.0%

Si: 0.01 to 2.5%

P: not more than 0.05%

S: 0.05% or less,

Al: 0.005 to 0.1%

N: 0.01% or less,

The remainder being Fe and inevitable impurities is heated to a first temperature of Ac1 point or more and Ac3 point or less and is stored and held at the first temperature for 1 hour to 48 hours or less and then cooled, ,

The material steel sheet is divided into a first region which is heated to a second temperature of not less than Ac3 point and not more than 1000 ° C and a second region which is heated to a third temperature of not less than Ac1 point (Ac3 point -20 ° C) A heating step,

Thereafter, the above-mentioned material steel sheet is subjected to a press forming process and a casting process simultaneously using a forming mold, and a hot press forming process

Wherein the hot press member has a plurality of through holes.

(8) The process for producing a hot-press member according to the above (7), wherein the above-mentioned component composition further contains, in mass%, one or two or more groups selected from the following group A to E below.

group

A: at least one or two or more selected from the group consisting of 0.01 to 5.0% of Ni, 0.01 to 5.0% of Cu, 0.01 to 5.0% of Cr and 0.01 to 3.0% of Mo

B group: 0.005 to 3.0% of Ti, 0.005 to 3.0% of Nb, 0.005 to 3.0% of V and 0.005 to 3.0% of W

Group C: at least one selected from the group consisting of REM: 0.0005 to 0.01%, Ca: 0.0005 to 0.01%, and Mg: 0.0005 to 0.01%

Group D: Sb: 0.002 to 0.03%

E group: B: 0.0005 to 0.05%

(9) The method of producing a hot-press member according to the above (7) or (8), further comprising a step of forming a plating layer on the surface of the material steel sheet before the heating step.

(10) The process for producing a hot-press member according to the above (9), wherein the plating layer is a Zn-based plating layer or an Al-based plating layer.

(11) The process for producing a hot-press member according to (10), wherein the Zn-based plating layer contains 10-25 mass% of Ni.

(12) The production method of a hot-press member according to any one of (9) to (11), wherein the amount of the plating layer adhered is 10 to 90 g / m 2 per one side.

The hot press member of the present invention has a tensile strength TS of 1,500 MPa or more and a tensile property of not less than 6.0% of uniformly drawn uEl and an excellent heat treatment curability in which a yield stress YS is increased by 150 MPa or more when heat treatment (baking coating) And a second region having a tensile strength TS of 780 MPa or more and a tensile property of uniformly elongated uEl: 15.0% or more. Further, according to the manufacturing method of the hot press member of the present invention, the hot press member having the above characteristics can be manufactured under conditions of high robustness against the molding initiation temperature at the time of hot pressing.

The hot press member according to one embodiment of the present invention has a tensile strength TS: 1,500 MPa or more, preferably 2300 MPa or less, and has a uniform elongation uEl: 6.0% or more, substantially 20% 1 region, and a second region having a tensile strength TS: not less than 780 MPa, preferably not more than 1320 MPa, and a uniform elongation uE1: not less than 15.0% and substantially not more than 40%. The first region (high strength and high ductility region) is an impact resistance characteristic region that has a certain degree of impact energy absorbing ability at the time of impact but does not allow deformation, and the second region (low strength / ultra soft portion) Is an energy absorbing region that allows deformation but has a very high impact energy absorption capacity. Thus, by having two regions having different characteristics in the same hot press member, the hot press member can be suitably used for a structural member requiring high impact energy absorbing ability such as an impact beam, a center pillar, a bumper, .

The positional relationship between the first region and the second region in the hot press member is not particularly limited and may be determined according to the use of the member. For example, when the member is used for the center pillar, the upper portion may be used as the first region and the lower portion may be used as the second region.

(Composition of components)

The composition of the hot press member according to one embodiment of the present invention will be described. Hereinafter, " mass% " is simply expressed as "% " unless otherwise specified.

C: 0.090% or more and less than 0.30%

C is an element that increases the strength of the steel. Further, the yield stress is increased by the dislocation fixing of the solid solution C by the heat treatment for the hot press member. In order to obtain such an effect and ensure tensile strength TS: 1500 MPa or more, the C content is set to 0.090% or more. On the other hand, when the C content is 0.30% or more, since the solid solution strengthening amount by C becomes large, it becomes difficult to adjust the tensile strength TS of the hot press member to less than 2300 MPa.

Mn: 3.5% or more and less than 11.0%

Mn is an element that increases the strength of the steel and is concentrated in the austenite and improves the stability of the austenite, and is the most important element in the present invention. In order to obtain such effects, and to secure a tensile strength TS of 1500 MPa or more in the first region and a uniformly elongated uEl of 6.0% or more, the Mn content is set to 3.5% or more. On the other hand, when the Mn content is 11.0% or more, the solid solution strengthening amount by Mn becomes large, and it becomes difficult to adjust the tensile strength TS of the hot press member to less than 2300 MPa.

Within the range of the C content and the Mn content, the tensile strength TS in the first region is preferably in the range of 1,500 MPa or more, and more preferably 2300 MPa or less, and the tensile strength at which the uniform elongation is stably 6.0% or more A hot press member can be obtained. More specifically, in order to secure the strength at a tensile strength TS of 1500 MPa or more and less than 1700 MPa in the first region, it is preferable that C: 0.090% or more and less than 0.12%, Mn: 4.5% Or C: not less than 0.12% and not more than 0.18%, and Mn: not less than 3.5% and not more than 5.5%. In order to secure the strength in a tensile strength TS of 1700 MPa or more and less than 1900 MPa in the first region, it is preferable that the content of C is 0.090% or more and less than 0.12%, the content of Mn is 6.5% or more and less than 8.5% % Or more and less than 0.18%, and Mn: 5.5% or more and less than 7.5%. Further, in order to secure the strength in the first region with a tensile strength TS of 1800 MPa or more and less than 1980 MPa, it is preferable that C: 0.18% or more and less than 0.30%, and Mn: 3.5% or more and less than 4.5%. In order to secure the strength of the tensile strength TS in the first region of not less than 2000 MPa and less than 2300 MPa, it is preferable that C: not less than 0.090% and not more than 0.12%, Mn: not less than 8.5% and not more than 11.0% , More preferably not less than 0.18% and not more than 11.0% of Mn, or not less than 0.18% and not more than 0.30% of Mn and not less than 4.5% and not more than 6.5% of Mn.

Even in the second region, C and Mn affect the mechanical properties of the region. However, in the range of C: not less than 0.090% and not more than 0.30%, and Mn: not less than 3.5% and not more than 11.0% Or a heating step immediately before the hot pressing step, a desired tensile strength TS: 780 MPa or more and a uniform elongation uEl of 15.0% or more are secured. That is, the mechanical characteristics in the second region are strongly influenced by the heating temperature T1 of the Mn-enriching heat treatment to be described later, or the heating temperature T3 immediately before the hot pressing process.

Si: 0.01 to 2.5%

Si is an element that increases the strength of steel by solid solution strengthening. To obtain this effect, the Si content is set to 0.01% or more. On the other hand, when the Si content exceeds 2.5%, surface defects called red scales remarkably occur at the time of hot rolling and the rolling load increases. Therefore, the Si content is set to 0.01% or more and 2.5% or less. The Si content is preferably 0.02% or more. The Si content is preferably 1.5% or less.

P: not more than 0.05%

P exists as an inevitable impurity in steel, segregates in crystal grain boundaries and the like, and has an adverse influence such as lowering toughness of the member. It is preferable to reduce it as much as possible, but up to 0.05% is acceptable. Therefore, the P content is set to 0.05% or less, more preferably 0.02% or less. In addition, since the excessive de-P treatment causes a high degree of refining cost, the P content is preferably 0.0005% or more.

S: not more than 0.05%

S is inevitably contained and exists as a sulfide-based inclusion in the steel and lowers the ductility, toughness, etc. of the hot press member. For this reason, it is preferable to reduce S as much as possible, but up to 0.05% is acceptable. In this respect, the S content is set to 0.05% or less, more preferably 0.005% or less. In addition, since the excessive de-S treatment causes high refining costs, the S content is preferably 0.0005% or more.

Al: 0.005 to 0.1%

Al is an element acting as a deoxidizer. In order to exhibit this effect, the Al content is set to 0.005% or more. On the other hand, when the Al content exceeds 0.1%, a large amount of nitride is generated due to binding with nitrogen, thereby lowering the blanking workability and shattering of the steel sheet. Therefore, the Al content should be 0.005% or more and 0.1% or less. The Al content is preferably 0.02% or more. The Al content is preferably 0.05% or less.

N: not more than 0.01%

N is inevitably contained in the steel in general, but when the N content exceeds 0.01%, nitrides such as AlN are formed during hot rolling or hot press heating, and the blanking workability of the steel sheet as the material and? The quenching is deteriorated. Therefore, the N content should be 0.01% or less. The N content is more preferably 0.0030% or more. The N content is more preferably 0.0050% or less. In addition, when it is inevitably contained without special adjustment, the N content is less than 0.0025%. Further, since the refining cost increases, the N content is preferably 0.0025% or more.

Further, in addition to the basic composition described above, a composition containing any of the following optional components may be used.

A: at least one or two or more selected from the group consisting of 0.01 to 5.0% of Ni, 0.01 to 5.0% of Cu, 0.01 to 5.0% of Cr and 0.01 to 3.0% of Mo

Ni, Cu, Cr, and Mo all contribute to improving the strength of steel as well as improving the quenching, and may optionally contain one or more species. In order to obtain such an effect, the content of each element should be 0.01% or more. On the other hand, the content of Ni, Cu and Cr is 5.0% or less and the Mo content is 3.0% or less from the viewpoint of not increasing the material cost. The preferable content of each element is 0.01% or more and 1.0% or less.

B group: 0.005 to 3.0% of Ti, 0.005 to 3.0% of Nb, 0.005 to 3.0% of V and 0.005 to 3.0% of W

Ti, Nb, V, and W all contribute to the increase in the strength of the steel by precipitation strengthening and contribute to the improvement in toughness due to refinement of the crystal grains. If necessary, one or more of Ti, .

Ti has an effect of improving the toughness by the solid solution B by forming a nitride in preference to B in addition to the effect of increasing the strength and improving the toughness. In order to obtain such an effect, the Ti content should be 0.005% or more. On the other hand, when the Ti content exceeds 3.0%, the rolling load during hot rolling increases extremely and the toughness of the hot press member decreases. Therefore, in the case of containing Ti, the content thereof is 0.005% or more and 3.0% or less. It is preferably 0.01% or more. And preferably 1.0% or less.

In order to obtain the above effect by Nb, the Nb content should be 0.005% or more. On the other hand, when the Nb content exceeds 3.0%, the amount of carbonitride increases and the ductility and delayed fracture resistance deteriorate. Therefore, when Nb is contained, the content thereof is 0.005% or more and 3.0% or less. It is preferably 0.01% or more. It is preferably 0.05%.

V is precipitated as a precipitate or a precipitate in addition to the effect of increasing strength and toughness, and has an effect of improving hydrogen embrittlement as a trap site of hydrogen. In order to obtain such an effect, the V content should be 0.005% or more. On the other hand, when the V content exceeds 3.0%, the amount of carbonitride is remarkably increased and the ductility is lowered. Therefore, when V is contained, the content thereof is 0.005% or more and 3.0% or less. It is preferably 0.01% or more. Preferably 2.0% or less.

W has an effect of improving the hydrogen embrittlement resistance in addition to the effect of increasing the strength and improving the toughness. In order to obtain such an effect, the W content is set to 0.005% or more. On the other hand, when the W content exceeds 3.0%, ductility decreases. Therefore, when W is contained, the content thereof is 0.005% or more and 3.0% or less. It is preferably 0.01% or more. Preferably 2.0% or less.

Group C: at least one selected from the group consisting of REM: 0.0005 to 0.01%, Ca: 0.0005 to 0.01%, and Mg: 0.0005 to 0.01%

REM, Ca, and Mg are elements that improve ductility and hydrogen embrittlement by shape control of inclusions, and may contain one or two or more species depending on necessity. In order to obtain this effect, the content of each element should be 0.0005% or more. On the other hand, from the viewpoint of not lowering the hot workability, the REM content and the Ca content are all set to 0.01% or less. From the viewpoint of preventing the ductility from being lowered by the formation of coarse oxides or sulfides, the Mg content is set to 0.01% or less. The preferable content of each element is 0.0006 to 0.01%.

Group D: Sb: 0.002 to 0.03%

Sb may be contained as necessary in order to suppress the formation of decarburized layer in the surface layer of the steel sheet during heating and cooling of the steel sheet. In order to obtain this effect, the Sb content should be 0.002% or more. On the other hand, when the Sb content exceeds 0.03%, the rolling load is increased and the productivity is lowered. Therefore, when Sb is contained, the content thereof is 0.002% or more and 0.03% or less, preferably 0.002% or more and 0.02% or less.

E group: B: 0.0005 to 0.05%

B may be contained as needed in order to contribute to improvement in toughness during hot pressing or toughness after hot pressing. In order to obtain this effect, the B content should be 0.0005% or more. On the other hand, when the B content exceeds 0.05%, martensite phase or bainite phase occurs after the increase of the rolling load at the time of hot rolling and the hot rolling, and cracks of the steel sheet may occur. Therefore, when B is contained, the content thereof is 0.0005% or more and 0.05% or less, preferably 0.0005% or more and 0.01% or less.

The balance other than the above-mentioned components is composed of Fe and inevitable impurities. As the inevitable impurities, O (oxygen): 0.0100% or less can be allowed.

(group)

The structure of the hot press member according to one embodiment of the present invention will be described.

Martensite phase in the first region: not less than 80.0% by volume

In order to secure a tensile strength TS of 1500 MPa or more in the first region, a martensite phase of 80.0% or more by volume is required to be a columnar phase. Further, the martensite phase is preferably at most 97% in order to contain a desired amount of the retained austenite phase.

The retained austenite phase in the first region: 3.0 to 20.0% by volume,

The retained austenite phase is the most important structure in the present invention, which enhances uniform elongation by the TRIP effect (transformation organic calcination) at the time of deformation. In the present embodiment, in order to realize a homogeneously stretched uEl of 6.0% or more in the first region, 3.0% or more of the retained austenite phase is contained in the volume ratio. On the other hand, if the volume percentage of the retained austenite phase exceeds 20.0%, the hard martensite phase which has been transformed after the TRIP effect is manifested becomes too large and the toughness lowers. Therefore, the volume percentage of the retained austenite phase is set to 3.0% or more and 20.0% or less. The volume percentage of the retained austenite phase is preferably at least 5.0. The volume percentage of the retained austenite phase is preferably 18.0% or less.

In addition, in the first region, the remainder other than the martensite phase and the retained austenite phase allow a bainite phase, a ferrite phase, a cementite, and a pearlite in a total volume ratio of 10% or less (including 0% .

Ferritic phase in the second region: not less than 30.0% and not more than 60.0% by volume

The ferrite phase is soft and acts to increase ductility of the hot press member. If the volume percentage of the ferrite phase is less than 30.0%, uniform elongation of 15.0% or more can not be ensured. On the other hand, if the volume percentage of the ferrite phase exceeds 60.0%, the tensile strength TS can not be 780 MPa or more. Therefore, the volume percentage of the ferrite phase is set to 30.0% or more and 60.0% or less, preferably 35.0% or more and 55.0% or less.

The retained austenite phase in the second region: not less than 10.0% and not more than 70.0%

The retained austenite phase is the most important structure in the second region, which enhances uniform stretching due to the TRIP effect (transformation organic firing) at the time of deformation. If the volume percentage of retained austenite is less than 10.0%, a uniformly elongated uEl of 15.0% or more can not be secured. On the other hand, when the volume percentage of the retained austenite phase exceeds 70.0%, the hard martensite phase which has been transformed after the TRIP effect is exhibited becomes excessively large, and the toughness is lowered. Therefore, the volume percentage of the retained austenite phase is 10.0% or more and 70.0% or less. The volume percentage of the retained austenite phase is preferably 15.0% or more. The volume percentage of the retained austenite phase is preferably 65.0% or less.

Martensite phase in the second region: not more than 30.0% by volume

The martensite phase is hard and has an effect of increasing the strength. A martensite phase of 30.0% or less (inclusive of 0%) by volume is contained from the viewpoint that the tensile strength TS is 780 MPa or more. However, when the volume ratio of martensite exceeds 30.0%, uniformly stretched uEl: 15.0% or more can not be secured. Therefore, the volume ratio of the martensite phase is 30.0% or less (including 0%).

The ferrite phase, the retained austenite phase and the remainder other than the martensite phase in the second region are allowed to contain bainite phase, cementite, and pearlite in a total volume ratio of 10% or less (including 0%) can do.

In order to generate the above-mentioned residual austenite phase common to the first and second regions, a steel sheet containing an appropriate amount of Mn is used. The steel sheet is subjected to a predetermined heat treatment before hot pressing to obtain Mn It becomes important to concentrate them in the austenite, and furthermore to appropriately heat the heating process at the time of hot pressing.

In the present invention, the volume ratios of the respective phases are determined as follows.

First, the volume percentage of retained austenite is obtained by the following method. The X-ray diffraction test piece is cut out from the first region or the second region of the hot press member, and subjected to mechanical polishing and chemical polishing so that the 1/4 thickness is the measurement surface, and then X-ray diffraction is performed. CoK? Lines are used for the incident X-rays and the integrated intensities of the peaks of the {200} plane, {220} plane and {311} plane of the retained austenite and the integrated intensity of the {200} plane of the ferrite } Plane is measured. 200} -α {200} -γ {200}, α {200} -γ {220}, α {200} -γ {311}, α {211} the residual gamma volume ratio obtained from the integral intensity ratio is calculated for each of six kinds of the total of? {211} -γ {311}. The average value of these is referred to as " volume percentage of retained austenite phase ".

Next, the volume ratio of the ferrite phase and the residual structure is obtained by the following method. From the first region or the second region of the hot press member, a test piece for tissue observation is sampled so that the surface parallel to the rolling direction and perpendicular to the rolled surface is the observation surface. The observation surface is polished, and the tissue is corroded with 3 vol.% Or the leaching solution. The tissue at the position where the plate thickness becomes 1/4 is observed with a scanning electron microscope (magnification: 1500 times) and picked up. From the obtained tissue photographs, identification of tissues and tissue fractions are determined by image analysis. The phase observed in black on a relatively smooth surface is a ferrite phase, the phase observed in a film form or a lump form in a crystal grain is cementite, the phase in which a ferrite phase and cementite are formed in a layer form is pearlite, And the phase composed of bainitic ferrite having no carbide in the lip is identified as a bainite phase. The occupied area ratio of each phase in the tissue photographs was determined, and the tissue was considered to be homogeneous three-dimensionally, and the area ratio was defined as the volume ratio.

The " volume ratio of martensite phase " was obtained by subtracting the volume percentage of the above residual structure and the volume percentage of retained austenite from 100%.

Average particle size of the ferrite phase in the second region: 10 mu m or less

The fine grain size of ferrite in the second region contributes to the improvement of TS. In order to secure a desired TS, the average particle size of the ferrite phase is preferably 10 占 퐉 or less, more preferably 5 占 퐉 or less. The lower limit value of the average particle size of the ferrite is not particularly limited, but is preferably about 0.2 占 퐉 on the industrial scale.

In the second region, the average particle size of the second phase: 10 mu m or less

The coarsening of the second phase in the second region causes a decrease in ductility. Therefore, the average particle diameter of the second phase is preferably 10 탆 or less, more preferably 5 탆 or less. The lower limit value of the average particle diameter of the second phase is not particularly limited, but it is preferably about 0.2 탆 for industrial use. In the second region, the " second phase " is a residual structure other than ferrite, mainly including residual austenite and martensite, but also includes martensite, pearlite and bainite.

The "average particle diameter of ferrite" and the "average particle diameter of the second phase" were obtained by the following methods. From the tissue photograph of the second region obtained by the above-described method, the structure was identified by the method described previously, and the average particle size of the ferrite and the second phase was determined by the line segment method described in JIS G 0551 (2005).

Mn < / RTI > in the second region is not less than 1.5

It is preferable that Mns / Mn alpha is 1.5 or more when the Mn concentration in the second phase is Mns and the Mn concentration in the ferrite phase is Mn alpha. The second phase in the second region is mainly retained austenite, and the state in which the Mn concentration is high, that is, the state in which Mn is concentrated indicates that the stability of the retained austenite is high. The retained austenite having high stability has a high TRIP effect (transformation organic distortion) at the time of deformation, thereby increasing the uniform elongation. That is, in order to secure a good ductility, it is necessary that Mns / Mn alpha, which is a state in which the Mn concentration of the second phase is high, is 1.5 or more. Preferably 1.6 or more. The upper limit value is not particularly limited, but is practically 10.0 or less.

Mns / Mn? In the second phase was determined by the following method. The specimen for tissue observation was taken, the observation surface was polished, and the tissue was corroded with 3vol.% Or a leaching solution. The tissue at a position where the plate thickness was 1/4 was analyzed with an Electron Probe Micro Analyzer (Electron Probe Micro Analyzer ), And quantitative analysis of Mn was performed on 30 particles of each of the ferrite and the second phase. As a result of quantitative analysis of Mn, a value obtained by dividing the average value of ferrite by Mn ?, the average value of second phase by Mns and the average value Mns of second phase by the average value Mn? Of ferrite was Mns / Mn ?.

(Dislocation density)

Dislocation density in the first region: 1.0 x 10 ¹⁶ / m 2 or more

The dislocation density of the hot press member is the most important index in the present invention which affects? Y. When the hot press member is subjected to a heat treatment (baking coating), it is considered that the solid solution C adheres to the movable potential and the yield stress YS increases. ? YS: To realize 150 MPa or more, the dislocation density of the hot press member needs to be 1.0 x 10 ¹⁶ / m 2 or more. The upper limit of dislocation density is substantially 5.0 x 10 ¹⁶ / m 2. The dislocation density of the hot press member is preferably 1.2 x 10 ¹⁶ / m 2 or more. The dislocation density of the hot press member is preferably 4.5 x 10 ¹⁶ / m 2 or less. Particularly, since martensite in the first region generates a movable potential, generally YS is low. Therefore, it is considered that improving the YS of the first region effectively works as an effect of the component characteristics.

In the present invention, the dislocation density is obtained by the following method. A test piece for X-ray diffraction is cut out from the first area of the hot press member, and subjected to mechanical polishing and chemical polishing so that the 1/4 thickness is the measurement surface, and then X-ray diffraction is performed. CoKα ₁ line is used for the incident X-ray and the half width of the peaks of α {110}, α {211}, and α {220} is measured. The half-width of the actually measured peaks of? {110},? {211}, and? {220} was corrected to a true half-width using a standard test piece Si having no strain, , A strain (?) Is obtained. The dislocation density p is obtained by the following equation using the strain epsilon and the Burgess vector (b = 0.286 nm).

ρ = 14.4 × ε ² / b ²

(Plating layer)

The hot press member according to an embodiment of the present invention preferably has a plating layer.

When the steel sheet used as the material of the hot press member is a plated steel sheet, the plated layer remains on the surface layer of the obtained hot press member. In this case, scale generation is suppressed during heating of the hot press. Therefore, the hot press member can be provided for use without scaling off the surface, and the productivity is improved.

The plating layer is preferably a Zn-based plating layer or an Al-based plating layer. When the corrosion resistance is required, the Zn-based plating layer is superior to the Al-based plating layer. This is because the sacrificial protection action of zinc can lower the corrosion rate of the steel strip. Further, in the case of hot-pressing the plated steel sheet, a zinc oxide film is formed at the initial stage of heating in the hot pressing step, and evaporation of Zn in the subsequent processing of the hot press member can be prevented.

As Zn-based plating, general hot dip galvanizing (GI), galvannealed hot dip galvanizing (GA), Zn-Ni based plating and the like can be mentioned. Among them, Zn-Ni based plating is preferable. The Zn-Ni-based plating layer not only inhibits scale generation at the time of hot press heating, but also prevents liquid metal brittle cracking. From the viewpoint of obtaining this effect, it is preferable that the Zn-Ni based plating layer contains 10 to 25 mass% of Ni. Even if Ni is contained in an amount exceeding 25%, this effect saturates.

As the Al-based plating layer, Al-10% by mass Si plating can be mentioned.

(Manufacturing method)

A method of manufacturing a hot press member in an embodiment of the present invention will be described. First, the slab having the above composition is heated and hot-rolled to obtain a hot-rolled steel sheet. Thereafter, this hot-rolled steel sheet is subjected to a predetermined heat treatment (Mn enrichment heat treatment) to be described later to obtain a first material steel sheet. Thereafter, optionally, the first material steel sheet is cold-rolled to obtain a cold-rolled steel sheet, and then the cold-rolled steel sheet is subjected to predetermined annealing to obtain a second material steel sheet.

The first material steel sheet or the second material steel sheet thus obtained is subjected to a predetermined heating step and a hot press forming step to obtain a hot press member. Hereinafter, each step will be described in detail.

≪ Step of obtaining steel sheet &

The step of obtaining the steel sheet is not particularly limited, and it is good according to the regular method. It is preferable that the molten steel having the above-mentioned composition is used as a slab in a continuous casting process in order to prevent macro segregation by solvent in a converter or the like. Instead of the continuous casting method, a roughing method or a thin slab continuous casting method may be used.

The obtained slab is once cooled to room temperature, and then charged into a heating furnace for reheating. However, an energy saving process such as a process of charging the slab into a heating furnace with a warm slab or a process of immediately hot rolling after maintaining the slab for a short time can be applied without cooling the slab to room temperature.

The obtained slab is heated to a predetermined heating temperature and then hot-rolled to obtain a hot-rolled steel sheet. The heating temperature is, for example, 1000 to 1300 占 폚. The heated slab is usually hot rolled under the condition that the finishing rolling inlet side temperature is not higher than 1100 占 폚 and the finishing rolling out side temperature is 800 to 950 占 폚 and the average cooling rate is not lower than 5 占 폚 / And wound in a coil shape at a coiling temperature of 300 to 750 캜 to obtain a hot-rolled steel sheet.

Thereafter, the hot-rolled steel sheet is preferably cold-rolled to form a cold-rolled steel sheet. The cold-rolled steel sheet is susceptible to thinning, and the sheet thickness accuracy is good. The reduction rate at the time of cold rolling is preferably 30% or more, more preferably 50% or more, in order to prevent abnormal grain growth during subsequent annealing or heating step immediately before hot pressing do. In addition, since the rolling load is increased and the productivity is lowered, the reduction rate is preferably 85% or less. When the rolling load becomes remarkably high, the hot-rolled steel sheet may be soft annealed before cold rolling. The softening annealing is preferably performed in a batch annealing furnace, a continuous annealing furnace or the like.

≪ Mn enrichment heat treatment >

Next, the hot-rolled steel sheet or preferably the cold-rolled steel sheet is heated to a first temperature of Ac1 point or more and Ac3 point or less, and is stored for at least one hour but not longer than 48 hours at the first temperature and then cooled to obtain a material steel sheet. This treatment is to concentrate Mn in the austenite and realize a homogeneous stretched uEl of 6.0% or more with an appropriate amount of retained austenite in the first region, and to achieve a dislocation density of 1.0 x 10 < ¹⁶ > : This is the most important process for manufacturing a hot press member that realizes 150 MPa or more.

Heating temperature (first temperature T1): Ac 1 point or more Ac 3 point or less

The hot-rolled steel sheet, or preferably the cold-rolled steel sheet, is heated to the temperature of the ferrite-austenite 2 phase and the Mn is concentrated in the austenite. In the austenite in which Mn is concentrated, the martensitic transformation end temperature is lower than the room temperature, and residual austenite is likely to be formed. When the heating temperature is lower than the Ac1 point, austenite is not generated and Mn can not be concentrated into austenite. On the other hand, if the heating temperature exceeds the Ac3 point, the austenite single phase temperature state is obtained, and the Mn concentration to the austenite is not carried out. Further, both of the case where the heating temperature is lower than the Ac1 point and the case where the heating temperature exceeds the Ac3 point, the dislocation density in the first region of the hot press member can not be 1.0 x 10 ¹⁶ / m 2 or more. Therefore, the heating temperature should be Ac1 point or more and Ac3 point or less. The heating temperature is preferably set to (Ac1 point + 20 deg. C) or higher. The heating temperature is preferably set to (Ac3 point -20 DEG C) or lower.

The values Ac1 point (占 폚) and Ac3 point (占 폚) are calculated by using the following equations.

Ac1 point (占 폚) = 751-16C + 11Si-28Mn-5.5Cu-16Ni + 13Cr + 3.4Mo

Ac3 point (占 폚) = 910-203C ^1/2 + 44.7Si-4Mn + 11Cr

Here, C, Si, Mn, Ni, Cu, Cr, and Mo in the formula are the content (mass%) of each element, and when the element is not contained, the content of the element is taken as zero .

Heat retention time: 1 hour to 48 hours

The concentration of Mn in the austenite proceeds with the passage of the heating and holding time. When the heat storage and holding time is less than 1 hour, the concentration of Mn into austenite is insufficient, and the desired uniform stretching in the first region can not be obtained. Further, when the heat storage and holding time is less than 1 hour, the Mn concentration is insufficient, the Ms point in the hot press step is not lowered, and the dislocation density in the first region of the hot press member is 1.0 x 10 ¹⁶ / Can not. On the other hand, if the heat storage and holding time exceeds 48 hours, pearlite is generated and the desired uniform stretching in the first region is not obtained. Further, the dislocation density in the first region of the hot press member can not be 1.0 x 10 ¹⁶ / m 2 or more. Therefore, the heating preservation holding time should be 1 hour or more and 48 hours or less. The heating and holding time is preferably 1.5 hours or more. The heat storage and holding time is preferably 24 hours or less.

The Ms point (° C) is to be a value calculated using the following formula. Ms point (占 폚) = 539-423C-30.4Mn-17.7Ni-12.1Cr-7.5Mo

Here, C, Mn, Ni, Cr, and Mo in the formula are the content (mass%) of each element, and when the element is not contained, the content of the element is calculated as zero.

The cooling after the heating preservation and holding is not particularly limited, and it is preferable to cool (cool) or control cooling appropriately according to the heating furnace or the like to be used.

The Mn concentration heat treatment is preferably performed in a batch annealing furnace or a continuous annealing furnace. The processing conditions in the batch annealing furnace are not particularly limited except for the above-mentioned conditions. For example, the heating rate is 40 ° C / hr or more and the cooling rate after heating and holding is 40 ° C / hr or more , And Mn concentration. The conditions for the treatment in the continuous annealing furnace are not particularly limited except for the above. For example, after the above-mentioned heating and preservation is performed, the hot-rolled steel sheet or the cold-rolled steel sheet is cooled at a cooling rate of 350 From the viewpoint of productivity, it is preferable to cool to a cooling stop temperature in a temperature range of about 600 DEG C to 600 DEG C, and then to stay for 10 to 300 seconds in the temperature range, and thereafter cooled and wound.

The material steel sheet thus produced can be used as a hot-pressing steel sheet.

<Plating Process>

In the case where a plating layer is not formed on the surface of the material steel sheet, it is necessary to perform a scale peeling treatment such as shot blasting on the hot press member after the hot pressing step. On the other hand, in the case of forming the plating layer on the surface of the material steel sheet, since scale formation is suppressed at the time of heating the hot press, the scale peeling process after the hot press process is not required, and productivity is improved.

The coating amount of the plating layer is preferably 10 to 90 g / m 2 per one surface, more preferably 30 to 70 g / m 2. When the deposition amount is 10 g / m 2 or more, the effect of suppressing scale formation at the time of heating is sufficiently obtained, and if the deposition amount is 90 g / m 2 or less, the productivity is not impaired. The components of the plating layer are as described above.

<Heating process>

Subsequently, the material steel sheet is divided into a first region which is heated to a second temperature of not less than Ac3 point and not more than 1000 ° C and a second region which is heated to a third temperature of not less than Ac1 point (Ac3 point -20 ° C) Is performed.

Heating temperature of the first region (second temperature T2): Ac3 point or more and 1000 占 폚 or less

In the first region, the material steel sheet is heated to the Ac3 point or more, which is the austenite single phase region. If the heating temperature is lower than the Ac3 point, the austenitization becomes insufficient, the desired amount of martensite can not be secured in the first region of the hot press member, and the desired tensile strength can not be obtained. Further, the dislocation density in the first region of the hot press member can not be 1.0 x 10 ¹⁶ / m 2 or more, and thus ΔYS: 150 MPa or more can not be realized. On the other hand, if the heating temperature exceeds 1000 ° C, the Mn concentrated in the austenite is homogenized, the desired amount of retained austenite can not be secured in the first region, and the desired uniform stretching can not be obtained. Further, by homogenization of Mn, the Ms point can not be lowered, and the dislocation density of the hot press member can not be 1.0 x 10 ¹⁶ / m 2 or more, so that ΔYS: 150 MPa or more can not be realized. Therefore, the heating temperature is set to Ac 3 point or more and 1000 ° C or less. The heating temperature is preferably set to (Ac 3 point + 20 ° C) or higher. The heating temperature is preferably 950 占 폚 or lower.

Heating temperature of the second region (third temperature T3): Ac1 point or more (Ac3 point -20 占 폚) or less

If the heating temperature of the second region exceeds (Ac3 point -20 占 폚), a desired amount of the ferrite phase and the retained austenite phase can not be obtained, and the uniformly drawn uEl of 15.0% or more can not be realized. Further, when the heating temperature is less than the heating temperature Ac1 of the second region, the ferrite volume ratio increases and the strength decreases. Therefore, the heating temperature of the second region is set to Ac 1 point or higher (Ac 3 point -20 ° C) or lower. The heating temperature in the second region is preferably set to (Ac1 point + 10 ° C) or higher. The heating temperature in the second region is preferably set to (Ac3 point -30 占 폚) or lower.

The mechanical properties of the second region are summarized as follows by the heating temperature T3 of the heating step and the heating temperature T1 of the Mn-enriching heat treatment described above. When T3 is more than T1 Ac3 point -20 or less, the mechanical characteristics of the second region are strongly influenced by T3. When T3 is T1 or less, the mechanical characteristics of the second region are strongly influenced by T1. This is because the volume ratio of the second phase does not change when T3 is equal to or less than T1 and the volume ratio of the second phase increases when T3 exceeds T1 with respect to the structure formed by the Mn enrichment heat treatment.

The rate of temperature rise to the heating temperature (second temperature and third temperature) is not particularly limited, but is preferably 1 to 400 占 폚 / s, more preferably 10 to 150 占 폚 / s. If the heating rate is 1 deg. C / s or more, the productivity is not deteriorated and if the heating rate is 400 deg. C / s or less, the temperature control will not become unstable.

Hold time: 900 seconds or less (including 0 seconds)

As the storage and holding time at the heating temperatures (the second temperature and the third temperature) is elapsed, the concentrated Mn diffuses around and becomes uniform. Therefore, if the holding and holding time exceeds 900 seconds, the desired amount of retained austenite can not be secured and the desired uniform stretching can not be obtained. Further, by homogenization of Mn, the Ms point can not be lowered, and the dislocation density in the first region of the hot press member can not be 1.0 x 10 ¹⁶ / m 2 or more, so that ΔYS: 150 MPa or more can be realized none. Therefore, the holding and holding time is 900 seconds or less. The holding time may be 0 seconds, that is, the heating may be terminated immediately after reaching the second temperature.

The heating method is not particularly limited, and any of the general heating methods such as an electric furnace, a gas furnace, an infrared heating, a high frequency heating, and a direct heating heating can be applied. The atmosphere is not particularly limited, and can be applied to both the atmosphere and the inert gas atmosphere.

There is no particular limitation on the method of heating and dividing into the first and second regions, and a method of covering a part of the material steel sheet, a method of partially spraying a cooling medium such as gas, a method of removing a part of the steel sheet from the heating furnace (For example, a method of discharging a part of the material steel sheet from the high-frequency coil to the outside or a method of adjusting the clamp position of the electrode for direct energization heating).

&Lt; Hot press forming step &

In the hot press forming step, a hot press member having a predetermined shape is obtained by simultaneously performing press forming and finishing on a material steel sheet subjected to a heating step using a molding die. The "hot press forming" is a method of press molding a heated thin steel sheet with a die and quenching it at the same time, which is also called "hot forming", "hot stamping", "die nicking" and the like.

The molding initiation temperature in the press machine is not particularly limited. When the Mn content is 3.5% by mass or more, the austenite generated in the heating step just before the hot press forming step for the material steel causes the ferrite transformation to occur newly at the cooling rate higher than the cooling rate in the subsequent cooling step after the heating step none. This is because Mn has the effect of delaying the ferrite transformation of austenite in the cooling step. As a result, there is no restriction on the molding initiation temperature for obtaining the first region and the second region having desired characteristics, and the robustness is widened as compared with the conventional one. However, from the viewpoint of the molding load, for example, it is preferable that the press molding start temperature of the first region is 500 캜 or higher. During the transportation of the material steel sheet to the start of molding, the material is generally cooled by air. Therefore, the upper limit of the molding initiation temperature is the heating temperature in the heating step immediately before the production step. It is preferable to reduce the cooling rate by means of a heat-insulating jig such as a heat-insulating box when it is transported under an environment in which the cooling rate is accelerated by a refrigerant such as gas or liquid.

The cooling rate in the mold is not particularly limited, but from the viewpoint of productivity, the average cooling rate up to 200 占 폚 is preferably 20 占 폚 / s or more, and more preferably 40 占 폚 / s or more.

The time taken out from the mold and the cooling rate after taking out are not particularly limited. As a cooling method, for example, a punch die is held and held at bottom dead center for 1 to 60 seconds, and the hot press member is cooled by using a die die and a punch die. Thereafter, the hot press member is taken out from the mold and cooled. The cooling after taking out from the mold in the mold and the mold can be combined with a cooling method using a refrigerant such as gas or liquid, thereby improving the productivity.

Example

Molten steel having the composition shown in Tables 1 and 4 (the balance being Fe and inevitable impurities) was dissolved in a small vacuum melting furnace to obtain a slab. The slab was heated to 1250 占 폚 and further subjected to hot rolling including rough rolling and finish rolling to obtain a hot-rolled steel sheet. The temperature at the finish rolling-in side temperature was 1100 占 폚 and the temperature at the finish rolling-out side temperature was 850 占 폚. The cooling rate after completion of the hot rolling was set at 15 占 폚 / s as an average of 800 to 600 占 폚, and the coiling temperature was set at 650 占 폚. The obtained hot-rolled steel sheet was pickled and cold-rolled at a reduction ratio of 54% to obtain a cold-rolled steel sheet (plate thickness: 1.6 mm).

The obtained cold-rolled steel sheet was heated to a heating temperature T1 (first temperature) shown in Tables 2 and 5, kept at that temperature for the time shown in Tables 2 and 5, and then cooled to obtain a material steel sheet.

As shown in Table 2 and Table 5, in some test examples, the material steel sheet was plated. In Table 2 and Table 5, "GI" is a hot-dip galvanized layer, "GA" is an alloying hot-dip galvanized layer, "Zn-Ni" is a Zn-12 mass% Ni plating layer, "Al- , And the adhesion amount of all the plated layers was 60 g / m &lt; 2 &gt;

The thus obtained material steel sheet was subjected to a heating step and a hot press forming step under the conditions shown in Tables 3 and 6 to obtain a hot press member having a hat shape. The hot press was performed at a forming depth of 30 mm using a punch mold having a width of 70 mm, a shoulder radius R of 6 mm, and a die having a shoulder radius R of 7.6 mm.

The heating process before the hot press forming process was conducted in the air using an electric heating furnace, and the heating rate was 7.5 占 폚 / s as an average from room temperature to 750 占 폚 in the first region. The second area was covered with a heat-resistant cover having a thickness of 10 mm. As a result, the heating temperature T2 of the first region and the heating temperature T3 of the second region were as shown in Tables 3 and 6. Table 3 and Table 6 also show the holding and holding times in the heating process. The molding initiation temperatures in the first region are shown in Tables 3 and 6. Cooling was carried out by maintaining the punch mold at the bottom dead center for 15 seconds and cooling it to 150 占 폚 or less by a combination of clamping using a die mold and a punch mold and air cooling on an open die, did. The average cooling rate from the molding initiation temperature to 200 占 폚 was 100 占 폚 / s.

JIS No. 5 tensile test piece (parallel portion: 25 mm width, parallel portion length: 60 mm, GL = 50 mm) was collected from the position of the hat top plate of each of the first region and the second region of the obtained hot press member, 2241, tensile strength TS, total elongation tEl and uniform elongation uE1 were determined. The yield stress YS was also obtained for the first region. The results are shown in Tables 3 and 6.

The volume percentage of the martensite phase, the volume percentage of the retained austenite phase, the volume percentage of the residual structure and the volume fraction of the ferrite phase in the second region in the first region of the obtained hot press member, The volume ratio of the martensite phase, the volume ratio of the residual structure, the average particle size of the ferrite phase, the average particle size of the second phase and the Mns / Mn alpha were measured in the same manner as described above and the results are shown in Tables 3 and 6.

Further, the obtained hot press member was subjected to a heat treatment (low temperature heat treatment) at 170 캜 for 20 minutes. This corresponds to the baking coating conditions in the manufacturing process of a typical automobile member. JIS No. 5 tensile test piece (parallel portion: 25 mm width, parallel portion length: 60 mm, GL = 50 mm) was sampled from the position of the first region of the hat top plate portion in accordance with JIS Z 2241 And subjected to a tensile test to obtain yield stress YS, tensile strength TS, total elongation tEl, and uniform elongation uEl. The results are shown in Tables 3 and 6.

[Table 1]

[Table 2]

[Table 3]

Continuation of [Table 3]

[Table 4]

[Table 5]

[Table 6]

Continuation of [Table 6]

In the first region, the tensile strength TS is 1,500 MPa or more, the uniform elongation uEl is 6.0% or more,? YS is 150 MPa or more, the tensile strength TS is 780 MPa or more in the second region, 15.0% or more could be realized. On the other hand, the comparative example did not satisfy at least one of the characteristics.

(Industrial availability)

The hot press member of the present invention can be suitably used as a structural member that requires a high impact energy absorbing ability such as an impact beam of a vehicle, a center pillar, a bumper, and the like.

Claims (12)

In terms of% by mass,
C: 0.090% or more and less than 0.30%
Mn: 3.5% or more and less than 11.0%
Si: 0.01 to 2.5%
P: not more than 0.05%
S: 0.05% or less,
Al: 0.005 to 0.1%
N: 0.01% or less,
The balance being Fe and inevitable impurities,
A structure including a martensite phase of not less than 80.0% by volume and a retained austenite phase of not less than 3.0% and not more than 20.0% by volume and a tensile strength TS of 1500 MPa or more and a tensile strength of not less than 6.0% A first region having a dislocation density of 1.0 x 10 &lt; ¹⁶ &gt; / m &lt; 2 &
A structure including 30.0% or more and 60.0% or less of ferrite phase by volume, 10.0% or more and 70.0% or less of residual austenite phase by volume, and 30.0% or less martensite phase by volume, A second region having a tensile property of not less than 780 MPa and a uniform elongation uEl of not less than 15.0%
Wherein the hot press member comprises: The method according to claim 1,
When the average grain size of the ferrite phase is 10 mu m or less and the average grain size of the second phase is 10 mu m or less and the Mn concentration in the second phase is Mns and the Mn concentration in the ferrite phase is Mn alpha, / Mn &amp;alpha; is at least 1.5. 3. The method according to claim 1 or 2,
The hot-press member according to any one of claims 1 to 3, further comprising one or two or more groups selected from the following groups A to E, in terms of mass%.
group
A: at least one or two or more selected from the group consisting of 0.01 to 5.0% of Ni, 0.01 to 5.0% of Cu, 0.01 to 5.0% of Cr and 0.01 to 3.0% of Mo
B group: 0.005 to 3.0% of Ti, 0.005 to 3.0% of Nb, 0.005 to 3.0% of V and 0.005 to 3.0% of W
Group C: at least one selected from the group consisting of REM: 0.0005 to 0.01%, Ca: 0.0005 to 0.01%, and Mg: 0.0005 to 0.01%
Group D: Sb: 0.002 to 0.03%
E group: B: 0.0005 to 0.05% 4. The method according to any one of claims 1 to 3,
A hot press member having a plated layer on its surface. 5. The method of claim 4,
Wherein the plating layer is a Zn-based plating layer or an Al-based plating layer. 6. The method of claim 5,
Wherein the Zn-based plating layer contains 10 to 25% by mass of Ni. In terms of% by mass,
C: 0.090% or more and less than 0.30%
Mn: 3.5% or more and less than 11.0%
Si: 0.01 to 2.5%
P: not more than 0.05%
S: 0.05% or less,
Al: 0.005 to 0.1%
N: 0.01% or less,
The remainder being Fe and inevitable impurities is heated to a first temperature of Ac1 point or more and Ac3 point or less and is stored and held at the first temperature for 1 hour to 48 hours or less and then cooled, ,
The material steel sheet is divided into a first region which is heated to a second temperature of not less than Ac3 point and not more than 1000 ° C and a second region which is heated to a third temperature of not less than Ac1 point (Ac3 point -20 ° C) A heating step,
Thereafter, the above-mentioned material steel sheet is subjected to a press forming process and a casting process simultaneously using a forming mold, and a hot press forming process
Wherein the hot press member has a plurality of through holes. 8. The method of claim 7,
Further comprising one or two or more groups selected from the group consisting of the following groups A to E, in terms of mass%.
group
A: at least one or two or more selected from the group consisting of 0.01 to 5.0% of Ni, 0.01 to 5.0% of Cu, 0.01 to 5.0% of Cr and 0.01 to 3.0% of Mo
B group: 0.005 to 3.0% of Ti, 0.005 to 3.0% of Nb, 0.005 to 3.0% of V and 0.005 to 3.0% of W
Group C: at least one selected from the group consisting of REM: 0.0005 to 0.01%, Ca: 0.0005 to 0.01%, and Mg: 0.0005 to 0.01%
Group D: Sb: 0.002 to 0.03%
E group: B: 0.0005 to 0.05% 9. The method according to claim 7 or 8,
Further comprising a step of forming a plating layer on the surface of the workpiece steel before the heating step. 10. The method of claim 9,
Wherein the plating layer is a Zn-based plating layer or an Al-based plating layer. 11. The method of claim 10,
Wherein the Zn-based plating layer contains 10 to 25% by mass of Ni. 12. The method according to any one of claims 9 to 11,
Wherein the amount of the plating layer adhered is 10 to 90 g / m &lt; 2 &gt; per one side.