JP2014520961A - Steel plate for hot press forming, formed member using the same, and method for producing them - Google Patents

Abstract

C: 0.3-1.0%, Mn: 0.01-4.0%, Si: 1.0-2.0%, Al: 0.01-2.0%, S: There is provided a steel sheet for hot press forming including 0.015% or less, N: 0.01% or less, and the balance, the balance being Fe and other inevitable impurities. In addition, the method includes a step of heating the steel slab having the above composition to 1100 to 1300 ° C., a step of finish hot rolling at an Ar ₃ transformation point to 950 ° C., and a step of winding at M _{S to} 720 ° C. A method for producing a hot-pressed steel sheet is provided. In addition, a hot press-molded member having the above composition and having a two-phase microstructure composed of bainite and retained austenite is provided. Furthermore, the step of heating the steel plate having the above composition at a temperature of Ac ₃ point or higher, the step of hot press forming the heated steel plate, and the temperature of M _{S to} 550 ° C. at a cooling rate of 20 ° C./sec or more. comprising the steps of cooling method of hot press forming member, characterized in that it comprises a step of heat treatment at M _S to 550 ° C. in a heating furnace is provided.

Description

  The present invention relates to a steel plate for hot press forming, a forming member using the same, and a method for producing the same, and more specifically, hot press forming that can be suitably used for not only an impact member of an automobile but also a collision member. The present invention relates to a steel plate for hot press forming excellent in the strength and ductility of the final product later, a formed member using the same, and a method for producing them.

  Recently, as safety laws and regulations for protecting automobile passengers and fuel efficiency regulations for environmental protection have been strengthened, societal demands for weight reduction have increased rapidly. In order to reduce the weight of automobile parts, high-strength steel sheets must be adopted to ensure rigidity and collision safety at the same time.

  However, increasing the strength of automobile steel sheets inevitably involves an increase in yield strength and a decrease in elongation, and thus there is a problem that formability is significantly reduced. In addition, the shape freezing property is also deteriorated, for example, the dimensional change of the part after molding occurs due to excessive spring back.

  In order to solve this problem, DP (dual phase) steel having a yield ratio characteristic in which a martensite structure is included in a ferrite base, or bainite and residual austenite (ferrite) in a ferrite base Advanced high strength steel (AHSS, Advanced high strength steel) such as TRIP steel (Transformation induced plasticity steel) that includes an austenite phase and has an excellent balance between strength and elongation has been developed and used. Yes.

  However, the tensile strength of these steel plates is in the range of 500 to 1000 MPa, and there is a problem that they are not suitable for satisfying the rigidity and the collision stability that meet the currently required weight reduction of automobiles.

Therefore, as a method for solving such problems and providing an ultra-high strength automobile part of 1 GPa class or higher, a molding method called hot press forming has become common. The blanking process is performed by blanking the steel sheet, heating it to an austenite region of Ac ₃ points or higher, continuously extracting and press forming, and then performing die quenching to finally obtain martensite. An ultra-high strength member of 1 GPa or more can be obtained by forming a microstructure in which sites or martensite and bainite are mixed, and since the molding is performed at a high temperature, the dimensional accuracy of the parts is extremely excellent.

  However, such a conventional hot press molding method can provide rigidity and crash stability suitable for reducing the weight of automobile parts, but because the elongation is less than 10%, the ductility of the final product is extremely low. There was a problem. That is, the parts manufactured by the conventional hot press molding method can be applied to an impact member of an automobile, but absorb energy directly at the time of a collision, so that it is not suitable for a collision member for protecting passengers. There was a problem.

  Therefore, in order to suitably apply the hot press-formed member to a collision member of an automobile, research on a formed member having excellent ductility after hot press forming and a steel sheet for hot press forming therefor is extremely necessary.

  As one aspect, the present invention provides a hot press-formed steel sheet capable of producing a hot press-formed member having excellent strength and ductility at the same time, a formed member using the same, and a method for producing the same.

  In the present invention, by weight, C: 0.3 to 1.0%, Mn: 0.01 to 4.0%, Si: 1.0 to 2.0%, Al: 0.01 to 2.0% , S: 0.015% or less, N: 0.01% or less, and the balance, wherein the balance is made of Fe and other inevitable impurities.

Moreover, this invention is weight%, C: 0.3-1.0%, Mn: 0.01-4.0%, Si: 1.0-2.0%, Al: 0.01-2. 0%, S: not more than 0.015%, N: not more than 0.01%, and the balance, the step of heating the steel slab made of Fe and other inevitable impurities to 1100-1300 ° C., Ar ₃ transformation Provided is a method for producing a hot press-formed steel sheet, comprising a step of hot rolling at a point to 950 ° C and a step of winding at M _{S to} 720 ° C.

  Moreover, this invention is weight%, C: 0.3-1.0%, Mn: 0.01-4.0%, Si: 1.0-2.0%, Al: 0.01-2. 0%, S: 0.015% or less, N: 0.01% or less, and the balance, which consists of Fe and other inevitable impurities, and has a dual phase microstructure consisting of bainite and residual austenite A hot press-molded member is provided.

Moreover, this invention is weight%, C: 0.3-1.0%, Mn: 0.01-4.0%, Si: 1.0-2.0%, Al: 0.01-2. 0%, S: 0.015% or less, N: 0.01% or less, and the balance, wherein the balance is a step of heating a steel plate made of Fe and other inevitable impurities at a temperature of Ac ₃ or higher, and the above heating A step of hot-pressing the formed steel plate, a step of cooling to a temperature of M _{S to} 550 ° C. at a cooling rate of 20 ° C./sec or more, and a step of heat-treating in a heating furnace at M _{S to} 550 ° C. The manufacturing method of the hot press-molding member characterized by these is provided.

  The present invention can provide a steel sheet for hot press forming excellent in strength and ductility, and using this, the microstructure is composed of a dual phase of bainite and retained austenite, and TS (MPa) * A molded member having an El (%) value of 25,000 MPa% or more can be provided. Therefore, since it has excellent ductility as well as strength, it can be suitably used as a collision member for automobiles.

The concept of the manufacturing process of the hot press molding member of this invention is shown with the graph of the temperature with respect to time. In the method for producing a hot press-formed member, the microstructure is shown by the cooling rate after molding. (C) is an enlarged photograph of (b).

  The present invention is for producing a molded member having excellent strength and ductility that can be used for a collision member of an automobile, and provides not only a molded member but also a steel plate having excellent ductility so as to be suitable for the production of the molded member. There is also a feature in doing. Therefore, this invention consists of four categories, the steel plate for hot presses excellent in ductility, its manufacturing method, and a hot press molding member, and its manufacturing method.

(Hot press forming steel plate)
Below, the hot press-forming steel sheet of the present invention will be described in detail.

  The steel sheet for hot press forming is a steel sheet excellent in strength and ductility so that good strength and ductility of the final member after hot press forming can be ensured, and by weight%, C: 0.3-1. 0%, Mn: 0.01 to 4.0%, Si: 1.0 to 2.0%, Al: 0.01 to 2.0%, S: 0.015% or less, N: 0.01% In the following, including the remainder, the remainder consists of Fe and other inevitable impurities.

  First, carbon (C) is a component added to ensure the strength of the steel sheet. In the present invention, carbon (C) is transformed into martensite by diffusing into residual austenite with Si or the like and stabilizing the residual austenite. It also plays a role in preventing. C is preferably contained in an amount of 0.3 to 1.0% by weight, and if it is less than 0.3%, the amount of retained austenite decreases after molding, and it is difficult to simultaneously ensure the strength and ductility of the part. If it exceeds 0.0%, the transformation of bainite is remarkably slow, the generation of pearlite is promoted, and on the contrary, the properties of steel are lowered.

  Manganese (Mn) is a component added to prevent red heat embrittlement by FeS in which Fe and S inevitably added during the manufacturing process of steel are combined, and is added in an amount of 0.01 to 4.0%. It is preferred that When the addition amount is less than 0.01%, red hot brittleness occurs due to FeS, and when the addition amount exceeds 4.0%, the transformation rate of bainite is slowed down during the hot press forming process. Since it takes a long time, it not only hinders productivity but also increases steelmaking costs.

  Silicon (Si) is an essential component to ensure the ductility of the final product according to the present invention, promotes the transformation of ferrite, and seeks stabilization by increasing carbon in the retained austenite by diffusing C in the retained austenite. It plays a role in preventing transformation to martensite. The amount of Si added is preferably 1.0 to 2.0% by weight. If the amount is less than 1.0%, the effect of stabilizing the retained austenite is not sufficient. The upper limit was limited to 2.0% because there was a problem that the rollability deteriorated, such as the generation of cracks in the rolling process.

  Aluminum (Al) removes oxygen present in the steel to prevent the formation of non-metallic inclusions during solidification, and promotes diffusion of C into the retained austenite as in the case of Si, thereby reducing the retained austenite. It plays a role of stabilization. The amount of Al added is preferably 0.01 to 2.0%, and if it is less than 0.01%, there is a limit to the removal of oxygen in the steel, thereby preventing the formation of nonmetallic inclusions. However, if it exceeds 2.0%, there is a problem that the cost of steelmaking increases.

  Sulfur (S) is a component that is unavoidably included in the steel manufacturing process, and combines with Fe to form FeS and induce red hot brittleness problems. Therefore, the amount of S can be managed as low as possible. preferable. Therefore, the sulfur content is preferably limited to 0.015% or less.

  Nitrogen (N) is a component inevitably included in the steel manufacturing process, and is preferably managed as low as possible. Therefore, the nitrogen content is preferably limited to 0.01% or less.

  The hot-press forming steel sheet has not only the above composition but also Mo: 0.5% or less (excluding 0), Cr: 1.5% or less (excluding 0), Ni: 0.5% or less It is preferable to further include one or more selected from the group consisting of (excluding 0), Nb: 0.005 to 0.1%, and V: 0.005 to 0.1%.

  Molybdenum (Mo) is a component added to suppress the formation of pearlite and is expensive. Therefore, in consideration of manufacturing costs, it is preferable to add 0.5% by weight or less.

  Chromium (Cr) is a component added to suppress the formation of ferrite and expand the transformation of bainite, and when added in excess of 1.5% by weight, Cr carbide is formed and solid solution C Since there is a problem of reducing the amount, it is preferable to add 1.5% by weight or less.

  Nickel (Ni) is a component added to increase the fraction of austenite and improve hardenability, and is expensive. Therefore, in consideration of manufacturing costs, it may be added in an amount of 0.5% by weight or less. preferable.

  Niobium (Nb) is a component added to increase the strength of the steel sheet, improve the grain refinement and toughness, suppress the crystal grain growth during the reheating process, and transform austenite to ferrite during cooling. It plays a role of delaying. The Nb is preferably added in an amount of 0.005 to 0.1% by weight. If the amount added is less than 0.005%, it is difficult to expect a crystal grain refining effect. If added in such a manner, carbonitrides may be excessively precipitated, causing delayed fracture in the steel sheet, or causing a problem that workability is deteriorated.

  Vanadium (V) is a component that is added to increase the strength of the steel sheet, refine crystal grains, and improve hardenability, and is preferably added in an amount of 0.005 to 0.1% by weight. If the addition amount is less than 0.005%, the above effect cannot be achieved. If the addition amount exceeds 0.1%, carbonitrides are excessively precipitated and delayed fracture occurs in the steel sheet. There is a risk that the processability may be lowered.

  Moreover, it is preferable that the steel plate for hot press forming further includes B: 0.005% or less (excluding 0) and Ti: 0.06% or less (excluding 0).

  First, boron (B) is a component added to suppress the formation of ferrite. When the added amount exceeds 0.005% by weight, a compound is formed by combining with Fe and C to form ferrite. Therefore, it is preferable to add 0.005% by weight or less.

  Titanium (Ti) is a component added in order to maximize the effect of B. By combining with N present as an impurity in the steel to form a TiN compound, B is combined with N. It plays a role in preventing the situation where the ferrite generation suppression function cannot be exhibited. Such an effect can be achieved by adding 0.06% by weight or less of Ti.

  On the other hand, the steel sheet may be a hot-rolled steel sheet or a cold-rolled steel sheet. In particular, a plated steel sheet having a plated layer formed on the surface of the cold-rolled steel sheet is used in order to improve corrosion resistance and suppress the formation of surface oxides. May be.

  Thus, since the hot press-forming steel sheet of the present invention has excellent strength and ductility at the same time due to the above composition, it is used in the production of the following hot press-formed members, and is a molded member having excellent strength and ductility. To play an important role.

(Method for producing hot press-formed steel sheet)
Below, although the manufacturing method of the hot press forming steel plate of this invention is demonstrated in detail, this is only an example of the preferable manufacturing method of the steel plate for obtaining the hot press forming member excellent in ductility.

First, the manufacturing method of the steel plate for hot press forming of this invention is weight%, C: 0.3-1.0%, Mn: 0.01-4.0%, Si: 1.0-2. 0%, Al: 0.01-2.0%, S: 0.015% or less, N: 0.01% or less, and the balance, including 1100 steel slab made of Fe and other inevitable impurities Heating to 1300 ° C., finishing hot rolling at Ar ₃ transformation point to 950 ° C., and winding at M _{S to} 720 ° C.

When the steel slab heating temperature is less than 1100 ° C., there is a problem that homogenization of the continuous cast structure is not sufficient, and there is a problem that it is difficult to ensure the temperature during finish rolling. When the temperature exceeds 1300 ° C., the crystal grain size increases. The steel slab heating temperature is preferably 1100 to 1300 ° C., because there is a risk that surface oxidation may occur and the strength may decrease or the surface characteristics may be poor. Further, when the finish rolling temperature is lower than the Ar ₃ transformation point, rolling is performed in a two-phase region and mixed grains are generated. When the finishing rolling temperature is higher than 950 ° C., the crystal grains are coarsened and surface oxidation may occur during rolling. Therefore, the finish rolling temperature is preferably Ar ₃ transformation point to 950 ° C. Further, below the coiling temperature M _S, austenite very poor hot winding workability by deteriorating the ductility of the steel sheet was transformed into martensite, it exceeds 720 ° C., thick on the surface of the steel sheet Since the oxide film is generated and internal oxidation occurs, the winding temperature is preferably M _{S to} 720 ° C.

Moreover, the manufacturing method of the steel plate for hot press forming of this invention is weight%, C: 0.3-1.0%, Mn: 0.01-4.0%, Si: 1.0-2. 0%, Al: 0.01-2.0%, S: 0.015% or less, N: 0.01% or less, and the balance, including 1100 steel slab made of Fe and other inevitable impurities Heating to 1300 ° C., finishing hot rolling at Ar ₃ transformation point to 950 ° C., winding up at M _{S to} 720 ° C., pickling treatment, cold rolling, A step of continuous annealing at 750 to 900 ° C. and a step of performing an overaging heat treatment at M _{S to} 550 ° C. may be included.

The pickling treatment is to remove the surface oxide generated in the heating and hot rolling steps. Then, cold rolling is performed. When the annealing temperature of the cold-rolled steel sheet is less than 750 ° C., recrystallization is not sufficiently performed, and there is a limit in securing workability, and when it exceeds 900 ° C., heating cannot be performed due to the limit of equipment. Further, the overage heat treatment temperature below M _S, since an adverse effect becomes too high strength of the steel sheet martensite obtained in the ductility, deteriorate the operability during blank operation before hot press forming, 550 ° C. If it exceeds 1, the roll surface of the annealing furnace deteriorates, adversely affecting the operability, and there is a problem that the function of the overaging zone for carbide precipitation and bainite transformation is not met.

Moreover, the manufacturing method of the steel plate for hot press forming of this invention is weight%, C: 0.3-1.0%, Mn: 0.01-4.0%, Si: 1.0-2. 0%, Al: 0.01-2.0%, S: 0.015% or less, N: 0.01% or less, and the balance, including 1100 steel slab made of Fe and other inevitable impurities Heating to 1300 ° C., finishing hot rolling at Ar ₃ transformation point to 950 ° C., winding up at M _{S to} 720 ° C., pickling treatment, cold rolling, A step of continuous annealing at 750 to 900 ° C., a step of performing an overaging heat treatment at M _{S to} 550 ° C., and a hot dip galvanizing treatment, an alloyed galvanizing treatment, an electrogalvanizing treatment or the like Plating stage that performs any one of hot-dip aluminum plating treatment And it may include.

  The hot-dip galvanized steel sheet may be manufactured by depositing the cold-rolled steel sheet in a galvanizing bath. The electrogalvanized steel sheet may be manufactured by performing zinc electroplating or Zn-Fe electroplating on a continuous electroplating line using the cold-rolled steel sheet. The hot-dip aluminum-plated steel sheet is manufactured by heating the cold-rolled steel sheet to 750 to 900 ° C. and then depositing it in an aluminum plating bath and then cooling to room temperature at a cooling rate of 5 to 15 ° C./sec. can do.

  The steel slab has Mo: 0.5% or less (excluding 0), Cr: 1.5% or less (excluding 0), Ni: 0.5% or less (excluding 0), Nb: 0.00%. It is preferable to further include one or more selected from the group consisting of 005 to 0.1% and V: 0.005 to 0.1%. B: 0.005% or less (excluding 0) and Ti : More preferably 0.06% or less (excluding 0).

(Hot press-molded member)
Hereinafter, the hot press-formed member of the present invention will be described in detail.

  The above hot press-formed member is intended to be an ultra-high strength product excellent in ductility, so that by weight%, C: 0.3-1.0%, Mn: 0.01-4.0% , Si: 1.0-2.0%, Al: 0.01-2.0%, S: 0.015% or less, N: 0.01% or less, and the remainder, Fe and other inevitable It is preferable that it has a fine structure consisting of bainite and retained austenite.

  The molded member has Mo: 0.5% or less (excluding 0), Cr: 1.5% or less (excluding 0), Ni: 0.5% or less (excluding 0), Nb: 0.0. It is preferable to further include one or more selected from the group consisting of 005 to 0.1% and V: 0.005 to 0.1%. B: 0.005% or less (excluding 0) and Ti : It may further contain 0.06% or less (excluding 0).

  The conventional hot press-formed member is intended to have ultra-high strength, and therefore always includes martensite. However, there is a problem that ductility is lowered and it is not suitable for an automobile collision member. Therefore, in the present invention, in the microstructure of the molded member, the formation of martensite is suppressed and the amount of retained austenite is increased, so that it consists of a dual phase of bainite and retained austenite.

  A hot press-molded member having the above composition and a fine structure is extremely excellent in balance between strength and ductility, so that TS (tensile strength, MPa) * El (elongation rate,%) value is 25,000 or more, It can be suitably applied not only to automobile impact members but also to collision members.

(Method for producing hot press-formed member)
Below, the manufacturing method of the hot press molding member of this invention is demonstrated in detail.

The manufacturing method of the hot press-molded member provides an ultra-high-strength automobile part having excellent ductility after forming by hot press-molding the steel sheet, and in weight%, C: 0.3 to 1.0 %, Mn: 0.01 to 4.0%, Si: 1.0 to 2.0%, Al: 0.01 to 2.0%, S: 0.015% or less, N: 0.01% or less , And the balance, wherein the balance is a steel plate made of Fe and other inevitable impurities, a step of heating to a temperature of Ac ₃ point or higher, a step of hot press forming, and a cooling rate of 20 ° C./sec or more at a rate of M _S. Cooling to 550 ° C. and heat treating in a furnace heated to a temperature of M _S to 550 ° C.

  Further, the steel sheet has Mo: 0.5% or less (excluding 0), Cr: 1.5% or less (excluding 0), Ni: 0.5% or less (excluding 0), Nb: 0.005 It is preferable to further include one or more selected from the group consisting of 0.1% and V: 0.005 to 0.1%, B: 0.005% or less (excluding 0) and Ti: 0 0.06% or less (excluding 0) may be further included, and the steel sheet may be one of a hot-rolled steel sheet, a cold-rolled steel sheet, or a plated steel sheet in which a plating layer is formed on the surface of the cold-rolled steel sheet.

  The method for producing the hot press-formed member of the present invention is to obtain a molded part having a microstructure different from the conventional one by controlling the heat treatment stage after the hot press forming to be different from the conventional method, The purpose is to improve the ductility of the product. That is, in the past, since the ultimate goal was to obtain an ultra-high-strength part, heat treatment conditions were included to include martensite as the main structure. This technology has a problem that excellent ductility cannot be obtained and it cannot be applied to a collision member of an automobile, and therefore, it has led to the application of heat treatment conditions for eliminating martensite and forming a structure composed only of bainite and residual austenite. It was.

First, Ac is heated to _three or more points because of transformation to austenite, and then undergoes a process of hot press forming a steel sheet heated to the austenite region.

The heat treatment conditions after the molding is a very important factor in determining the structure of the product microstructure, conventionally, by die quenching at a temperature immediately M _S products that are press-formed, final It is common to increase the strength by including martensite as the main structure in the microstructure of the product.

However, an object of the present invention is to eliminate martensite from the microstructure in order to obtain a final product excellent in ductility while maintaining a strength suitable for weight reduction. Thus, instead of being cooled to below room temperature immediately M _S a formed product, after first cooling in the M _S to 550 ° C., by heat treatment at M _S to 550 ° C. in a heating furnace, the process of transformation to bainite Go through. When cooled below M _S, martensite is generated there is a problem that ductility is lowered, if it exceeds 550 ° C., pearlite phase occurs, the physical properties of the steel is lowered. Therefore, by controlling the cooling rate to M _S ˜550 ° C., a dual phase microstructure composed of bainite and retained austenite is obtained.

Fe ₃ C carbides are not formed in the bainite produced in the above process. This is because Si or the like is sufficiently added to the composition of the steel used for the hot press forming and C is diffused into the retained austenite. Ie, C can not be used to form carbides in bainite, to stabilize the residual austenite is dissolved in the retained austenite, lowering the M _S, in the cooling process after to prevent transformation to martensite To play a role. Thereby, since the retained austenite not transformed into martensite remains in the final product, excellent ductility can be ensured.

At this time, the cooling rate is preferably 20 ° C./sec or more, and when cooled at a low rate of less than 20 ° C./sec, since it easily transforms into a pearlite structure, there may be a problem that the physical properties of the final product are deteriorated. There is. That is, as shown in FIG. 2A, when cooled at 30 ° C./sec, a bainite structure appears, but as shown in FIGS. 2B and 2C, 5 ° C. / It can be seen that a pearlite structure in which ferrite and Fe ₃ C are layered appears when cooled in sec.

An example of the manufacturing process of the hot press-formed member of the present invention described above will be briefly described with reference to FIG. 1. First, a steel plate is poured into a heating furnace, and a temperature of Ac ₃ or higher is used for forming austenite. And then hot press forming. After molding, cooling is performed at a cooling rate of 20 ° C./sec or more so as not to generate pearlite. The cooling temperature is set to M _{S to} 550 ° C., and similarly, heat treatment is performed at M _{S to} 550 ° C. in a heating furnace. This is for the transformation to bainite, M _S is lowered by the diffusion of the C in the austenite in the process. Even if the molded product produced by the above process is cooled to room temperature without artificial control, it does not transform into martensite, and a two-phase microstructure of bainite and retained austenite can be obtained.

  Hereinafter, the present invention will be described in detail through examples. However, this is merely for explaining the present invention more completely, and the scope of rights of the present invention is not limited by the following examples.

(Example)
A steel ingot having the composition shown in Table 1 is manufactured to a thickness of 90 mm and a width of 175 mm by vacuum induction melting, reheated at 1200 ° C. for 1 hour, and then hot rolled to a thickness of 3 mm. did. The hot rolling finish temperature is set to the Ar ₃ transformation point or higher, and after cooling, it is charged into a heating furnace preheated to 600 ° C., held for 1 hour, and then cooled in the furnace to simulate hot rolling and heat The cold-rolled sheet was further cold-rolled at a reduction rate of 60% to 1.2 mm, and then annealed at 900 ° C. Subsequently, bainite transformation was performed at 400 degreeC. The unit of the following components is wt%, and the units of S and N are ppm.

  Using 1.2 mm steel manufactured by the above method, heat to 900 ° C for simulation of heat treatment in the hot press forming process, hold for 30 seconds, and then cool at 30 ° C / second Then, it was put into a heating furnace, heat-treated at a temperature similar to the cooling temperature for 400 to 10800 seconds, and air-cooled to obtain a hot press-formed part. The process conditions and the mechanical properties of the final product are shown in Table 2.

First, in the comparative steel 1, when the cooling rate is 400 ° C., it can be seen that the TS * El value is as low as 16785 MPa%, which is not suitable as an automobile collision member. This can be analyzed because the composition does not contain enough C and the retained austenite has not been stabilized well, and when the cooling rate is 250 ° C., it is cooled to below M 2 _S and martensite. By transformation to many sites, the tensile strength is quite high, but the elongation is extremely low and the TS * El value is 9066 MPa%, which makes it less suitable as a collision member for automobiles.

Also in Comparative Steel 2, because C is not sufficient, not enough even Si, not performed well the stabilization of the residual austenite, the cooling temperature or less M _S, elongation percentage being made transformation to martensite Is very low and the TS * El value is 10150 MPa%, which is not good. Also in comparative steels 3, C is not sufficient, the cooling temperature or less _{M S,} since TS * El value is 8940MPa%, it can be seen that unsuitable as a collision member of the motor vehicle.

  In Comparative Steel 4, C was sufficiently added, but Si was insufficient, and C was not sufficiently diffused into retained austenite. Therefore, the TS * El value was 19216 MPa%, which was higher than that of other comparative steels. Although it is high, it still does not exceed 25000 MPa%, indicating that it is not suitable as a collision member for automobiles.

  In addition, the inventive steel 7 whose steel composition is within the scope of the present invention was tested separately when cooled at 30 ° C./sec and when cooled at 5 ° C./sec. When cooled, the TS * El value is as high as 46923 MPa% and suitable for automobile collision members, but when cooled at 5 ° C./sec, the TS * El value is as low as 12480 MPa% and suitable for automobile collision members. There wasn't. As shown in FIG. 2, it can be analyzed that a pearlite structure is generated at a low cooling rate and the physical properties of the product are lowered.

Claims (22)

  C: 0.3-1.0%, Mn: 0.01-4.0%, Si: 1.0-2.0%, Al: 0.01-2.0%, S: A steel plate for hot press forming characterized by including 0.015% or less, N: 0.01% or less, and the balance, the balance being made of Fe and other inevitable impurities.   Mo: 0.5% or less (excluding 0), Cr: 1.5% or less (excluding 0), Ni: 0.5% or less (excluding 0), Nb: 0.005 to 0.1% and The steel sheet for hot press forming according to claim 1, further comprising one or more selected from the group consisting of V: 0.005 to 0.1%.   The steel sheet for hot press forming according to claim 1 or 2, further comprising B: 0.005% or less (excluding 0) and Ti: 0.06% or less (excluding 0).   3. The hot press forming according to claim 1, wherein the steel sheet is any one of a hot-rolled steel sheet, a cold-rolled steel sheet, or a plated steel sheet having a plating layer formed on a surface of the cold-rolled steel sheet. Steel plate. C: 0.3-1.0%, Mn: 0.01-4.0%, Si: 1.0-2.0%, Al: 0.01-2.0%, S: 0.015% or less, N: 0.01% or less, and the balance including a balance, the remainder being a steel slab composed of Fe and other inevitable impurities, heated to 1100-1300 ° C., and at an Ar ₃ transformation point of 950 ° C. A method for producing a hot press-formed steel sheet, comprising: a step of finish hot rolling and a step of winding at M _{S to} 720 ° C.   The steel slab is Mo: 0.5% or less (excluding 0), Cr: 1.5% or less (excluding 0), Ni: 0.5% or less (excluding 0), Nb: 0.005-0 The method for producing a steel sheet for hot press forming according to claim 5, further comprising one or more selected from the group consisting of 0.1% and V: 0.005 to 0.1%. .   The hot press forming according to claim 5 or 6, wherein the steel slab further includes B: 0.005% or less (excluding 0) and Ti: 0.06% or less (excluding 0). Steel plate manufacturing method. C: 0.3-1.0%, Mn: 0.01-4.0%, Si: 1.0-2.0%, Al: 0.01-2.0%, S: 0.015% or less, N: 0.01% or less, and the balance including a balance, the remainder being a steel slab composed of Fe and other inevitable impurities, heated to 1100-1300 ° C., and at an Ar ₃ transformation point of 950 ° C. Finish hot rolling, winding at M _{S to} 720 ° C, pickling, cold rolling, continuous annealing at 750 to 900 ° C, M _{S to} 550 ° C And a step of performing an overaging heat treatment at the step of manufacturing a steel sheet for hot press forming.   The steel slab is Mo: 0.5% or less (excluding 0), Cr: 1.5% or less (excluding 0), Ni: 0.5% or less (excluding 0), Nb: 0.005-0 The method for producing a steel sheet for hot press forming according to claim 8, further comprising one or more selected from the group consisting of 0.1% and V: 0.005 to 0.1%. .   The hot press forming according to claim 8 or 9, wherein the steel slab further includes B: 0.005% or less (excluding 0) and Ti: 0.06% or less (excluding 0). Steel plate manufacturing method. C: 0.3-1.0%, Mn: 0.01-4.0%, Si: 1.0-2.0%, Al: 0.01-2.0%, S: 0.015% or less, N: 0.01% or less, and the balance including a balance, the remainder being a steel slab composed of Fe and other inevitable impurities, heated to 1100-1300 ° C., and at an Ar ₃ transformation point of 950 ° C. Finish hot rolling, winding at M _{S to} 720 ° C, pickling, cold rolling, continuous annealing at 750 to 900 ° C, M _{S to} 550 ° C And a plating step of performing any one of hot dip galvanizing treatment, alloying galvanizing treatment, electrogalvanizing treatment or hot dip aluminum plating treatment on the surface of the steel sheet subjected to the over aging heat treatment. Of hot-press forming steel sheet, characterized in that Production method.   The steel slab is Mo: 0.5% or less (excluding 0), Cr: 1.5% or less (excluding 0), Ni: 0.5% or less (excluding 0), Nb: 0.005-0 The method for producing a steel sheet for hot press forming according to claim 11, further comprising one or more selected from the group consisting of 0.1% and V: 0.005 to 0.1%. .   The hot press forming according to claim 11 or 12, wherein the steel slab further includes B: 0.005% or less (excluding 0) and Ti: 0.06% or less (excluding 0). Steel plate manufacturing method.   C: 0.3-1.0%, Mn: 0.01-4.0%, Si: 1.0-2.0%, Al: 0.01-2.0%, S: 0.015% or less, N: 0.01% or less, and the balance, Fe is composed of Fe and other inevitable impurities, and has a dual phase microstructure composed of bainite and retained austenite A hot press-formed member.   Mo: 0.5% or less (excluding 0), Cr: 1.5% or less (excluding 0), Ni: 0.5% or less (excluding 0), Nb: 0.005 to 0.1% and The hot press-formed member according to claim 14, further comprising one or more selected from the group consisting of V: 0.005 to 0.1%.   The hot press-formed member according to claim 14 or 15, further comprising B: 0.005% or less (excluding 0) and Ti: 0.06% or less (excluding 0).   The hot press-molded member according to claim 14 or 15, wherein a TS (MPa) * El (%) value is 25,000 MPa% or more.   The hot press-molded member according to claim 16, wherein a TS (MPa) * El (%) value is 25,000 MPa% or more. C: 0.3-1.0%, Mn: 0.01-4.0%, Si: 1.0-2.0%, Al: 0.01-2.0%, S: 0.015% or less, N: 0.01% or less, and the balance including a balance, wherein the balance is a step of heating a steel plate made of Fe and other inevitable impurities at a temperature of Ac ₃ or higher, and the heated steel plate is hot A step of press molding, a step of cooling to a temperature of M _{S to} 550 ° C. at a cooling rate of 20 ° C./sec or more, and a step of heat treatment at M _{S to} 550 ° C. in a heating furnace, Manufacturing method of hot press-formed member.   The steel sheet has Mo: 0.5% or less (excluding 0), Cr: 1.5% or less (excluding 0), Ni: 0.5% or less (excluding 0), Nb: 0.005-0. The method for producing a hot press-formed member according to claim 19, further comprising one or more selected from the group consisting of 1% and V: 0.005 to 0.1%.   The hot-press formed member according to claim 19 or 20, wherein the steel sheet further includes B: 0.005% or less (excluding 0) and Ti: 0.06% or less (excluding 0). Manufacturing method.   The hot press forming according to claim 19 or 20, wherein the steel sheet is any one of a hot-rolled steel sheet, a cold-rolled steel sheet, or a plated steel sheet having a plating layer formed on a surface of the cold-rolled steel sheet. Manufacturing method of member.

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