KR20160042968A - Hot-pressing steel plate, press-molded article, and method for manufacturing press-molded article - Google Patents

Abstract

A Ti-containing precipitate having a predetermined chemical composition and having a circle equivalent diameter of 30 nm or less among the Ti-containing precipitates contained in the steel sheet has an average circle equivalent diameter of 6 nm or less and satisfies a predetermined relationship between the amount of precipitated Ti in the steel and the total amount of Ti, In addition, when the metal structure has a fraction of ferrite of 30% by area or more, it is possible to facilitate molding and processing before hot pressing, and when a uniform property is required in a molded article, balance of high strength and elongation is set to high level It is possible to achieve a high level of balance between the high strength and the elongation according to the respective regions when a region corresponding to the impact resistant region and the energy absorbing region is required in a single molded product, There is provided a hot-pressing steel sheet useful for obtaining a press-molded article having a good anti-softening property in HAZ .

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a hot-press steel sheet, a press-formed sheet, and a method of manufacturing a press-

TECHNICAL FIELD The present invention relates to a hot-press steel sheet used for manufacturing structural parts of automobiles and suitable for hot press forming, and press-formed articles obtained from such hot-press steel sheet, and a method for producing press-molded articles. And particularly to a hot-press-use steel sheet and press-molded article which are useful in application to a hot press forming method in which a predetermined strength is obtained by performing heat treatment at the same time as shaping of a preheated steel sheet (blank) into a predetermined shape, To a useful method for producing a press-molded article.

As one of the measures to improve the fuel efficiency of automobiles started from the global environment problem, weight reduction of the body is progressing, and it is necessary to make the steel sheet used for automobiles as strong as possible. On the other hand, if the steel sheet is made to have a high strength, the shape precision at the time of press forming is lowered.

For this reason, by heating the steel sheet at a predetermined temperature (for example, a temperature at which it is in the form of austenite) to lower the strength thereof and then molding the steel sheet at a lower temperature (for example, at room temperature) than the steel sheet, (Quenched) by using the temperature difference between them to secure the strength after molding is employed in the production of parts (press molded products). Such a hot press forming method may be carried out by various methods such as a hot forming method, a hot stamping method, a hot stamp method, a die quenching method, .

Fig. 1 is a schematic explanatory view showing a mold structure for carrying out the hot press forming as described above. 1 is a punch, 2 is a die, 3 is a blank holder, 4 is a steel plate (blank), BHF is a blank holder force, rp is a punch nose radius, rd is a die radius, CL is a punch / And a clearance between the dies. Of these components, the punch 1 and the die 2 are provided with passages 1a and 2a through which a cooling medium (for example, water) can pass, and the cooling medium So that these components are cooled.

When forming hot press using such a die (e.g., hot deep drawing), steel plate (blank) (4), 2 sangyeok temperature or Ac least ₃ transformation point phase inverse of (Ac ₁ transformation point to Ac ₃ transformation point) The molding is started in a softened state by heating at a temperature. 1) between the die 2 and the blank holder 3 with the punch 1 in a state where the steel plate 4 in a high temperature state is sandwiched between the die 2 and the blank holder 3, The steel plate 4 is press-fitted so as to be formed into a shape corresponding to the outer shape of the punch 1 while reducing the outer diameter of the steel plate 4. [ Heat is discharged from the steel plate 4 to the mold (the punch 1 and the die 2) by cooling the punch and the die in parallel with the molding, and at the same time, the molding bottom dead center (The state shown in Fig. 1), the material (steel plate) is quenched by additional cooling. By performing such a molding method, a molded article having a high dimensional accuracy of 1,500 MPa can be obtained. Further, compared with a case in which a part having the same strength class is formed in a cold state, the molding load can be reduced, so that the capacity of the press machine may be small.

BACKGROUND ART It is known that a 22MnB5 steel is used as a hot-press steel sheet widely used at present. Such a steel sheet has a tensile strength of 1,500 MPa and an elongation of about 6 to 8%, and is applied to an impact resistant member (a member which is not deformed extremely at the time of collision and is not broken). However, it is difficult to apply to parts requiring deformation such as an energy absorbing member because the elongation (ductility) is lowered.

As a steel sheet for hot pressing which exhibits a good elongation, for example, the techniques of Patent Documents 1 to 4 have been proposed. In these techniques, by setting the carbon content in the steel sheet in various ranges, it is possible to adjust the basic strength class of each steel sheet, to introduce ferrite having high deformability, and to reduce the average grain size of ferrite and martensite, . These techniques are effective for improving the elongation, but still insufficient from the viewpoint of improving the elongation according to the strength of the steel sheet. For example, a tensile strength (TS) of 1470 MPa or more and a elongation (EL) of 10.2% at the maximum are required to be further improved.

On the other hand, compared with hot stamped articles which have been studied so far, there is a problem in molding precision in cold presses, even in the case of a molded product having a low strength class, for example, a tensile strength TS of 980 MPa or 1180 MPa. There is a demand for a low-strength hot press. At this time, it is necessary to remarkably improve the energy absorption characteristic in the molded article.

Particularly in recent years, development of a technique of placing a difference in strength within a single component is underway. As a technique of this, there is proposed a technique in which a portion to be prevented from deformation is made to have a high strength (high strength side: impactor side) and a portion where energy absorption is required is low strength and high ductility (low strength side: energy absorbing side) . For example, considering the compatibility (a function of protecting the other side when a compact car collides) at the time of a side collision or a rear collision in a vehicle of a medium or more-sized automobile, There are cases in which both function and energy-absorbing functions are provided. In order to manufacture such a component, (a) a method of joining a steel sheet to a general hot-pressing steel sheet at a same temperature by heating or metal quenching (tailor welded blank: TWB) (b) a method in which the difference in strength is given to each region of the steel sheet with a difference in the cooling rate in the mold, and (c) a method in which the difference in strength is made at different heating temperatures for each region of the steel sheet.

In these techniques, a tensile strength of 1500 MPa is attained on the high strength side (impact side), but a maximum tensile strength is 700 MPa and an elongation (EL) is 17% on the low strength side In order to further increase the absorption characteristics, it is required to realize high ductility with higher strength.

Further, in order to realize a complicated shape by hot stamping, it is required to apply the hot stamping in a direction to perform hot stamping after making a certain shape by press molding at room temperature, or to cut out a steel sheet to be provided for hot stamping , It is simultaneously required that the strength of the steel sheet for hot stamping does not become too high.

However, it is known that the automobile parts are required to be joined by spot welding, but the strength of the welded joint is remarkably lowered in the weld heat affected zone (HAZ) and the strength of the welded joint is lowered (softened) Document 1).

Japanese Patent Application Laid-Open No. 2010-65292 Japanese Patent Application Laid-Open No. 2010-65293 Japanese Patent Application Laid-Open No. 2010-65294 Japanese Patent Application Laid-Open No. 2010-65295

Hirosue et al., "Nippon Steel Technical Report", No. 378, pp. 15-20 (2003)

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method of manufacturing a molded product, which can facilitate molding and processing before hot pressing, When a region corresponding to an impact-resistant region and an energy-absorbing region is required in a single molded product, a balance between high strength and elongation can be achieved at a high level in accordance with each region, and furthermore, Which is useful for obtaining a press-molded article having a good anti-softening property, and a press-molded article exhibiting such properties, and a useful method for producing such a press-molded article.

The steel sheet for hot-pressing according to the present invention,

C: 0.15-0.5% (meaning% by mass, hereinafter the same with respect to chemical composition)

Si: 0.2 to 3%

Mn: 0.5 to 3%

P: not more than 0.05% (not including 0%)

S: not more than 0.05% (not including 0%)

Al: 0.01 to 1%

B: 0.0002 to 0.01%

Ti: 3.4 [N] + 0.01% or more, 3.4 [N] + 0.1% or less (where [N] represents the content of N (mass%

N: 0.0010 to 0.01%

Respectively, the balance being iron and inevitable impurities,

A Ti-containing precipitate having a circle equivalent diameter of 30 nm or less and having an average circle equivalent diameter of 6 nm or less and a precipitated Ti amount in the steel and an overall Ti amount satisfying the following formula (1) , And the fraction of ferrite is 30% by area or more. The term "circle equivalent diameter" refers to the diameter (average diameter) of a Ti-containing precipitate (for example, TiC) to be.

[Equation 1]

(% By mass) - 3.4 [N] < 0.5 占 Total Ti amount (mass%) - 3.4 [N]

(In the formula (1), [N] represents the content (mass%) of N in the steel)

In the steel sheet for hot press forming of the present invention, if necessary, it is also useful to contain at least one of the following elements (a) to (c) as another element. The properties of the press-molded article are further improved, depending on the kind of element contained, if necessary.

(a) a total of at least 0.1% (excluding 0%) of at least one element selected from the group consisting of V, Nb and Zr;

(b) at least one selected from the group consisting of Cu, Ni, Cr, and Mo in a total amount of not more than 1% (not including 0%),

(c) a total of 0.01% or less (not including 0%) of at least one element selected from the group consisting of Mg, Ca and REM,

The method of producing a press-molded article according to the present invention, which can achieve the above object, is a method in which the steel sheet for hot pressing according to the present invention as described above is heated to a temperature of Ac ₁ transformation point + 20 ° C or higher and an Ac ₃ transformation point- The steel sheet is cooled to a temperature lower than the bainite transformation start temperature (Bs) by 100 占 폚 while ensuring an average cooling rate of 20 占 폚 / second or more in the mold during molding and after molding. do.

In the press-molded article of the present invention, the metal structure of the press-molded article is composed of 3 to 20 percent by area of retained austenite, 30 to 80 percent by area of ferrite, less than 30 percent by area (not including 0 percent by area) of bayonite ferrite, , Martensite: not more than 31% by area (not including 0 area%), and the Ti-containing precipitates contained in the press-molded article have an average circle equivalent diameter of not more than 10 nm, The amount of Ti and the total amount of Ti satisfy the relation of the following formula (1), and a balance between the high strength and the elongation in the molded article can be achieved as a uniform property at a high level.

[Equation 1]

(% By mass) - 3.4 [N] < 0.5 占 Total Ti amount (mass%) - 3.4 [N]

(In the formula (1), [N] represents the content (mass%) of N in the steel)

On the other hand, another method of manufacturing a press molded article of the present invention that could achieve the above object is, by using a hot press steel sheet as described above, dividing the heating zone of the steel plate to at least two regions, one region of the Ac ₃ At a temperature not lower than the transformation point and not higher than 950 占 폚, and at the same time, the other region is changed to an Ac ₁ transformation point + 20 ° C or higher, Ac ₃ After the molding is started at a temperature of -20 占 폚 or below, press molding is started for both regions, and an average cooling rate of 20 占 폚 / To a temperature not higher than the start temperature (Ms).

Another press-molded article of the present invention is a press-molded article of a steel sheet having the chemical composition as described above, wherein the press-molded article has a first region in which the metal structure has residual area of austenite of 3 to 20% and martensite of 80% , Ferritic: 30 to 80 area%, baynitic ferrite: less than 30 area% (excluding 0 area%), martensite: 31 area% or less (0 area% Of the Ti-containing precipitates contained in the steel in the second region is not more than 10 nm, and the average equivalent circle diameter of the Ti-containing precipitates having a circle equivalent diameter of 30 nm or less is 10 nm or less, Ti amount and total Ti amount satisfy the relation of the following formula (1). In such a press-molded article, a balance between high strength and elongation can be achieved at a high level in each region, and a region corresponding to an impact-resistant portion and an energy-absorbing portion exists in a single molded product. Further, The anti-softening property of the HAZ is improved.

[Equation 1]

(% By mass) - 3.4 [N] < 0.5 占 Total Ti amount (mass%) - 3.4 [N]

(In the formula (1), [N] represents the content (mass%) of N in the steel)

According to the present invention, it is possible to precisely define the chemical composition and to control the size of the Ti-containing precipitate and to control the precipitation rate of Ti not forming TiN and adjust the ratio of ferrite to metal structure Since the steel sheet is used, the strength-elongation balance of the press-molded article can be made high by hot pressing it under a predetermined condition. In addition, by hot pressing in a plurality of regions under different conditions, it is possible to form an impact-resistant region and an energy-absorbing region in a single molded product, and a balance between high strength and elongation can be achieved at a high level in each region, And the characteristics become good.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic explanatory view showing a mold structure for performing hot press forming; Fig.

The present inventors have found that when a steel sheet is heated to a predetermined temperature and then subjected to hot press forming to produce a press-molded article, a steel sheet for hot press that can obtain a press-molded article exhibiting good ductility (elongation) In order to realize, it was reviewed at various angles.

As a result, the chemical composition of the steel sheet for hot pressing is strictly defined, the size of the Ti-containing precipitate and the amount of precipitated Ti are controlled, and the steel sheet is subjected to hot press forming , And that a predetermined amount of retained austenite is retained after press forming to obtain a press molded article having enhanced inherent ductility (residual ductility), thereby completing the present invention.

In the steel sheet for hot press of the present invention, it is necessary to strictly specify the chemical composition, and the reason for limiting the range of each chemical component is as follows.

(C: 0.15-0.5%)

C is used in order to achieve a balance between high strength and elongation at a high level when uniform properties are required in a press-molded article, or when a region corresponding to an impact-resistant portion and an energy- · It is an important element in securing retained austenite at high ductility site. Further, by heating C in the hot press forming, C is concentrated to austenite, whereby retained austenite can be formed after quenching. It also contributes to an increase in the amount of martensite, thereby increasing the strength. In order to exhibit these effects, the C content needs to be 0.15% or more.

However, if the C content is excessive and exceeds 0.5%, the bimetallic heating zone becomes narrow, so that a balance between high strength and elongation can not be achieved at a high level when uniform properties are required in a molded article, (A structure obtained by securing a predetermined amount of ferrite, bainitic ferrite, or martensite) in a region where a region corresponding to the energy absorbing region is required, particularly in a low strength and high ductility region It becomes difficult. The preferable lower limit of the C content is 0.17% or more (more preferably 0.20% or more), and the more preferable upper limit is 0.45% or less (more preferably 0.40% or less).

(Si: 0.2 to 3%)

Si exhibits an effect of forming retained austenite by suppressing formation of cementite or decomposition of untransformed austenite by tempering martensite during cooling of the quenching mold. In order to exhibit such an effect, the Si content needs to be 0.2% or more. When the Si content exceeds 3% and the Si content exceeds 3%, the ferrite transformation is promoted during cooling after hot rolling, so that TiC in the ferrite formed at this time is formed easily, and HAZ softening prevention characteristics can not be obtained . The lower limit of the Si content is preferably 0.5% or more (more preferably 1.0% or more), and the preferable upper limit is 2.5% or less (more preferably 2.0% or less).

(Mn: 0.5 to 3%)

Mn is an element effective for increasing the hardenability and suppressing formation of a structure (ferrite, pearlite, bainite, etc.) other than martensite and retained austenite during cooling of the mold quenching. It is also an element that stabilizes austenite and contributes to an increase in the amount of retained austenite. In order to exhibit such an effect, Mn should be contained in an amount of 0.5% or more. When considering only the characteristics, it is preferable that the Mn content is large, but since the cost of adding the alloy increases, the upper limit is set to 3% or less. The lower limit of the Mn content is preferably 0.7% or more (more preferably 1.0% or more), and the upper limit is preferably 2.5% or less (more preferably 2.0% or less).

(P: not more than 0.05% (not including 0%))

P is an element inevitably included in the steel, but deteriorates ductility, so that it is preferable that P is extremely reduced. However, extreme reductions lead to an increase in steelmaking costs, and it is difficult to manufacture at 0%, so the upper limit is 0.05% or less (0% is not included). The preferable upper limit of the P content is 0.045% or less (more preferably 0.040% or less).

(S: not more than 0.05% (not including 0%))

S is an element inevitably included in the steel as in P and deteriorates ductility, so that it is preferable that S is extremely reduced. However, extreme reductions lead to an increase in steelmaking costs, and it is difficult to manufacture at 0%, so the upper limit is 0.05% or less (0% is not included). The preferable upper limit of the S content is 0.045% or less (more preferably 0.040% or less).

(Al: 0.01 to 1%)

Al is useful as a deoxidizing element and is also useful for improving ductility by fixing solid solution N present in the steel as AlN. In order to effectively exhibit such an effect, the Al content needs to be 0.01% or more. However, if the Al content is excessive and exceeds 1%, Al ₂ O ₃ is excessively produced, and the ductility is deteriorated. The lower limit of the Al content is preferably 0.02% or more (more preferably 0.03% or more), and the preferable upper limit is 0.8% or less (more preferably 0.6% or less).

(B: 0.0002 to 0.01%)

B has a function of suppressing ferrite transformation, pearlite transformation and bainite transformation at the high strength site side. Therefore, during cooling after heating at a bimetallic temperature (Ac ₁ transformation point to Ac ₃ transformation point) of ferrite, pearlite and bainite And thus contributes to securing retained austenite. In order to exhibit such an effect, it is necessary to contain B in an amount of 0.0002% or more, but if the amount of B exceeds 0.01%, the effect is saturated. The lower limit of the B content is preferably 0.0003% or more (more preferably 0.0005% or more), and the preferable upper limit is 0.008% or less (more preferably 0.005% or less).

(Ti: 3.4 [N] + 0.01% or more, 3.4 [N] + 0.1%

Ti has an effect of improving the hardenability by fixing N and keeping B in a solid state. In order to exhibit such an effect, it is important to contain at least 0.01% more than the stoichiometric ratio of Ti and N (3.4 times the content of N). In addition, since Ti added in excess to N is present in the hot stamped molded article in a solid state, and the precipitated compound is finely dispersed, the solidified Ti is formed as TiC when the hot stamped product is welded It is possible to suppress the decrease in strength in the HAZ by effects such as precipitation strengthening and delay in increasing the dislocation density due to the effect of preventing the dislocation of the dislocations by TiC. However, if the Ti content becomes excessive and exceeds 3.4 [N] + 0.1%, the formed Ti-containing precipitate (for example, TiN) is coarsened and the ductility of the steel sheet is lowered. A more preferable lower limit of the Ti content is 3.4 [N] + 0.02% or more (more preferably 3.4 [N] + 0.05% or more), and a more preferable upper limit is 3.4 [N] + 0.09% Is 3.4 [N] + 0.08% or less).

(N: 0.001 to 0.01%)

N is an element that is inevitably incorporated and is preferably reduced as much as possible. However, since there is a limit to reduction in an actual process, the lower limit of 0.001% is set. Further, when the N content is excessive, the formed Ti-containing precipitate (for example, TiN) is coarsened and this precipitate acts as a starting point of fracture to lower the ductility of the steel sheet, so that the upper limit is set to 0.01% . A more preferable upper limit of the N content is 0.008% or less (more preferably 0.006% or less).

The basic chemical components of the steel sheet for hot pressing according to the present invention are as described above and the balance is iron and unavoidable impurities (for example, O, H, etc.) other than P, S and N. It is also useful to add at least one of the following (a) to (c) to the steel sheet for hot press of the present invention, if necessary. The characteristics of the steel sheet for hot press (i.e., press-molded article) are further improved, depending on the type of element contained as needed. The preferable range for containing these elements and the reason for limiting the range are as follows.

(a) a total of at least 0.1% (excluding 0%) of at least one element selected from the group consisting of V, Nb and Zr;

(b) at least one selected from the group consisting of Cu, Ni, Cr, and Mo in a total amount of not more than 1% (not including 0%),

(c) a total of 0.01% or less (not including 0%) of at least one element selected from the group consisting of Mg, Ca and REM,

(0.1% or less (excluding 0%) in total of at least one selected from the group consisting of V, Nb and Zr)

V, Nb and Zr form fine carbides and have the effect of making the structure finer by the pinning effect. In order to exhibit such effects, it is preferable that the total content is 0.001% or more. However, when the content of these elements is excessive, coarse carbides are formed, which is a starting point of fracture, which in turn deteriorates ductility. Therefore, it is preferable that the total of these elements is 0.1% or less. A more preferable lower limit of the content of these elements is 0.005% or more (more preferably 0.008% or more) in total, and a more preferable upper limit is 0.08% or less (more preferably 0.06% or less) in total.

(At least one selected from the group consisting of Cu, Ni, Cr and Mo: not more than 1% in total (not including 0%))

Since Cu, Ni, Cr and Mo inhibit ferrite transformation, pearlite transformation and bainite transformation, formation of ferrite, pearlite and bainite is prevented during cooling after heating, and effectively acts to secure retained austenite . In order to exhibit such effects, it is preferable that the total amount is 0.01% or more. Considering the characteristics alone, it is preferable that the content is large, but since the cost of adding the alloy increases, it is preferable that the total content is 1% or less. In addition, since the steel has an effect of greatly increasing the strength of austenite, the load of hot rolling becomes large, and it becomes difficult to manufacture a steel sheet. Therefore, it is preferable to be 1% or less from the viewpoint of productivity. A more preferable lower limit of the content of these elements is 0.05% or more (more preferably 0.06% or more) in total, and a more preferable upper limit is 0.5% or less (more preferably 0.3% or less) in total.

(Including 0% or less) of at least one element selected from the group consisting of Mg, Ca and REM (rare earth element) in a total amount of not more than 0.01%

Since these elements make the inclusions finer, they act effectively for improving the ductility. In order to exhibit such effects, it is preferable that the total content is 0.0001% or more. Considering the characteristics alone, it is preferable that the content is large, but since the effect is saturated, the total content is preferably 0.01% or less. A more preferable lower limit of the content of these elements is 0.0002% or more (more preferably 0.0005% or more) in total, and a more preferable upper limit is 0.005% or less (more preferably 0.003% or less) in total.

In the steel sheet for hot press according to the present invention, the average circle equivalent diameter of the Ti-containing precipitates (A) having a circle equivalent diameter of 30 nm or less in the steel sheet is 6 nm or less, (B) (C) the ratio of ferrite in the metal structure is not less than 30% by area, and (3) the ratio of the total content of Ti It is also an important requirement.

The Ti-containing precipitates and the control of the formula (1) are intended to prevent the softening of the HAZ. Essentially, the control is necessary for the molded article, but the change of these values is small before and after the hot press forming. Therefore, it is necessary to control in advance at the stage before molding (steel sheet for hot press). By allowing excess Ti to be present in a solid state or a fine state in excess of N in a steel sheet before molding, the Ti-containing precipitate can be maintained in a solid state or a fine state during heating of the hot press. Thereby, the amount of precipitated Ti in the press-molded article can be controlled to a predetermined amount or less, and the softening in the HAZ can be prevented, thereby improving seam characteristics.

From this point of view, it is necessary to finely disperse the Ti-containing precipitate. For this purpose, the average circle-equivalent diameter of the Ti-containing precipitates contained in the steel sheet having a circle equivalent diameter of 30 nm or less needs to be 6 nm or less The requirement of the above (A)). The reason why the circle equivalent diameter of the target Ti-containing precipitate is specified to be 30 nm or less is that it is formed in the solvent stage coarsely, and then the Ti-containing precipitate except for TiN, It is necessary to control the operation of the apparatus. The size (average circle equivalent diameter) of the Ti-containing precipitate is preferably 5 nm or less, and more preferably 3 nm or less. The Ti-containing precipitates to be subjected in the present invention include precipitates containing Ti such as TiVC, TiNbC, TiVCN and TiNbCN in addition to TiC and TiN.

As described later, the average circle-equivalent diameter of the Ti-containing precipitates in the press-molded article is specified to be 10 nm or less, whereas the shape before molding (hot press steel sheet) is specified to be 6 nm or less. The reason for this is that although the steel sheet contains fine precipitates or Ti in a solid state, the Ti-containing precipitates are slightly coarse when heated at about 800 DEG C for 15 minutes or more, so that the molded article has a larger size of the precipitate than the steel sheet . In order to ensure the properties as a molded article, it is necessary that the average circle-equivalent diameter of the Ti-containing precipitates is 10 nm or less. To realize the precipitation state as a hot stamped product, the average number of fine precipitates of 30 nm or less It is necessary that the equivalent diameter is 6 nm or less and most of Ti is present in a solid state.

Further, in the hot press steel sheet, most of the Ti other than that used for precipitation fixing of N in Ti needs to be present in a solid state or a fine state. For this purpose, the amount of Ti existing as a precipitate other than TiN (that is, the amount of precipitation Ti (mass%) - 3.4 [N]) is 0.5 times the remainder obtained by subtracting Ti forming TiN from the total Ti The total amount of Ti (mass%) - 3.4 [N]] must be less than the amount of Ti (the requirement of (B) above). The amount of precipitated Ti (mass%) - 3.4 [N] is preferably 0.4 x [total Ti amount (mass%) - 3.4 [N]] or less, more preferably 0.3 x [total Ti amount 3.4 [N]] or less.

In addition, it is necessary to process the steel material before the hot stamping and to perform the press forming. In this case, it is necessary to secure a predetermined amount of ferrite that is a soft tissue. From this point of view, it is necessary to set the fraction of ferrite in the hot press steel sheet to 30% by area or more (the requirement (C) above). The fraction of ferrite is preferably at least 50% by area, more preferably at least 70% by area.

In the steel sheet for hot pressing, the remainder of the metal structure is not particularly limited, but may include, for example, at least one of pearlite, bainite, martensite, or retained austenite.

In order to produce the steel sheet (steel sheet for hot pressing) of the present invention as described above, it is preferable that the slab prepared by dissolving the steel material having the above chemical composition is heated at a heating temperature of 1100 DEG C or higher (preferably 1150 DEG C or higher) , The hot rolling is performed at a finish rolling temperature of 850 DEG C or higher (preferably 900 DEG C or higher) and 1050 DEG C or lower (preferably 1000 DEG C or lower) (Preferably at least 625 DEG C) at an average cooling rate of 20 DEG C / second or more (preferably 30 DEG C / second or more), and cooling (quenching) from 620 DEG C to 580 DEG C at 10 DEG C / (Preferably 5 ° C / sec or less), and then cooled to an average cooling rate of 10 ° C / sec or more, and thereafter cooled to 350 ° C or higher (preferably 380 ° C or higher) and 450 ° C or lower 430 ° C or less).

The method comprises (1) finishing rolling at a temperature range where a potential introduced into austenite by hot rolling remains, and (2) quenching immediately thereafter to finely form a Ti-containing precipitate such as TiC in the entire phase , And (3) the steel sheet is further cooled in two steps and wound, thereby controlling ferrite transformation while securing the Ti-containing precipitate amount.

The hot press steel sheet having the chemical composition, the metal structure and the Ti precipitation state as described above may be provided as it is in the production of the hot press as it is. The hot rolled steel sheet may be provided with a cold rolling rate of 60% or less (preferably 40% or less) Rolled and then provided for the production of a hot press. The hot-rolled steel sheet of the present invention may be formed by heat-treating the hot rolled sheet by a continuous annealing furnace or a continuous hot-dip galvanizing line. In short, it is included in the steel sheet for hot pressing of the present invention as long as it satisfies the required properties such as the metal structure and the Ti precipitation state.

The steel sheet for hot press was heated to a temperature not lower than the Ac ₁ transformation point + 20 ° C (Ac ₁ + 20 ° C) and not higher than the Ac ₃ transformation point -20 ° C (Ac ₃ -20 ° C) (Bs-100 deg. C) lower than the bainite transformation start temperature (Bs) while ensuring an average cooling rate of 20 deg. C / sec or higher in the mold during and after the molding, In a press-molded product (hereinafter sometimes referred to as a single-region molded product), it is possible to obtain an optimal structure with low strength and high ductility. The reason for defining each of the requirements in this molding method is as follows.

In a steel sheet containing a predetermined amount of ferrite, since the ferrite is partially transformed into austenite while partially remaining, the heating temperature needs to be controlled within a predetermined range. When the heating temperature of the steel sheet is lower than the Ac ₁ transformation point + 20 ° C, a sufficient amount of austenite can not be obtained at the time of heating, and a predetermined amount of retained austenite can not be secured in the final structure (the structure of the molded article). When the heating temperature of the steel sheet exceeds the Ac ₃ transformation point -20 ° C, the transformation amount to the austenite at the time of heating is excessively increased, and a predetermined amount of ferrite can not be secured in the final structure (the structure of the molded article).

It is necessary to appropriately control the average cooling rate and the cooling end temperature during molding and after molding in order to make the austenite formed in the heating step a desired structure while preventing the formation of the structure such as ferrite or pearlite. From this point of view, it is necessary to set the average cooling rate during molding to not less than 20 占 폚 / second and the cooling end temperature to be not more than 100 占 폚 lower than the bainite transformation start temperature (Bs). The average cooling rate during molding is preferably 30 DEG C / second or more (more preferably 40 DEG C / second or more). By making the cooling end temperature lower than the bainite transformation starting temperature (Bs) by 100 占 폚 or lower, formation of a structure such as ferrite or pearlite is prevented, and the austenite present at the time of heating is transformed into bainite or martensite Fine austenite is retained between laths of bainite and martensite to secure a predetermined amount of retained austenite while securing bainite or martensite.

If the cooling end temperature is higher than the bainite transformation start temperature (Bs) by 100 占 폚 or the average cooling rate is less than 20 占 폚 / sec, a structure such as ferrite or pearlite is formed, and a predetermined amount of retained austenite And the elongation (ductility) of the molded article is deteriorated. The cooling end temperature is not particularly limited as long as it is 100 ° C lower than Bs, and may be, for example, lower than the martensitic transformation start temperature (Ms).

The control of the average cooling rate is basically unnecessary at the stage where the temperature is lower than the bainite transformation start temperature (Bs) by 100 占 폚. However, for example, at an average cooling rate of 1 占 폚 / It may be cooled to room temperature. The control of the average cooling rate during molding and after the completion of molding can be performed by controlling the temperature of the molding die (the cooling medium shown in Fig. 1), (b) the means of controlling the thermal conductivity of the mold Can be achieved.

In the press-formed product (single-domain molded product) produced by the hot press as described above, the metal structure is preferably composed of 3 to 20 percent by area of retained austenite, 30 to 80 percent by area of ferrite, and less than 30 percent by area of bainite ferrite 0 area%) and martensite: not more than 31 area% (not including 0 area%), so that a balance between high strength and elongation in a molded article can be achieved as a uniform property at a high level. The reason for setting the range of each requirement (basic organization) in such a hot press molded article is as follows.

The retained austenite is transformed into martensite during plastic deformation, thereby increasing the work hardening rate (transformational organic plasticity) and improving the ductility of the molded product. In order to exhibit such an effect, it is necessary to set the residual austenite fraction to 3% by area or more. Regarding ductility, the greater the retained austenite fraction, the better. In the composition used for the steel sheet for automobiles, the retained austenite that can be secured is limited, and the upper limit is about 20% by area. The preferable lower limit of the retained austenite is 5% by area or more (more preferably 7% by area or more).

By making the main structure a fine and highly ductile ferrite, ductility (elongation) of the press-molded article can be increased. From this viewpoint, the fraction of ferrite is 30% or more by area. However, when such a fraction exceeds 80% by area, the strength of the molded article can not be secured. The lower limit of the ferrite content is preferably at least 35% by area (more preferably at least 40% by area), and the upper limit is preferably at most 75% by area (more preferably at most 70% by area).

Bainitic ferrite is an effective structure for improving the strength of a molded product, but since it is a structure lacking softness, it is deteriorated when it is present in a large amount. From this viewpoint, the fraction of the baynitic ferrite is less than 30% by area. The preferable upper limit of the fraction of the baynitic ferrite is 25% or less (more preferably 20% or less).

Although martensite (martensite as quenched) is an effective structure for improving the strength of a molded article, it is a structure lacking ductility, so that it is deteriorated when it is present in a large amount. From this viewpoint, the fraction of martensite is 31% or less by area. The preferable upper limit of the fraction of martensite is 25% or less (more preferably 20% or less).

Other than the above-mentioned tissue, there is no particular limitation, and pearlite or the like may be included as the remaining tissue. However, these tissues are preferably basically not included because their contribution to strength and contribution to ductility are lower than those of other tissues % May be used).

In the press-molded article (single-domain molded article), the average circle-equivalent diameter of the Ti-containing precipitates contained in the press-molded articles (that is, the steel plates constituting the press-molded article) of 30 nm or less in circle equivalent is 10 nm or less. By satisfying these requirements, it is possible to obtain a press-molded article capable of achieving a balance between high strength and elongation at a high level. The average circle-equivalent diameter of the Ti-containing precipitates is preferably 8 nm or less, and more preferably 6 nm or less.

In addition, in the press-molded article (single-domain molded product), the amount of Ti (precipitated Ti amount: 3.4 [N]) existing as a precipitate other than TiN is less than 0.5 times of the remaining amount of Ti minus Ti forming TiN That is, less than 0.5 x [total Ti amount (%) - 3.4 [N]]). By satisfying these requirements, the Ti dissolved in the welding is finely precipitated in the HAZ, or the existing fine Ti-containing precipitates inhibit the recovery of the dislocation and the like, thereby preventing the softening in the HAZ and improving the weldability. The amount of precipitated Ti of -3.4 [N] is preferably 0.4 x [total Ti amount (mass%) - 3.4 [N]] or less, more preferably 0.3 x [total Ti amount (mass% ] Or less.

By using the steel sheet for hot press according to the present invention, it is possible to control properties such as strength and elongation of a press-molded article by appropriately adjusting press molding conditions (heating temperature and cooling rate), and furthermore, (For example, an energy absorbing member), which is difficult to apply in the conventional press-molded articles, and is extremely useful in expanding the application range of the press-molded article. In addition to the above-mentioned single-domain molded product, when a steel sheet is press-formed by using a press-formed metal mold, the heating temperature and the conditions of each region at the time of molding are appropriately controlled, (Hereinafter may be referred to as a multi-region molded product) exhibiting an intensity-ductility balance according to each region can be obtained.

When manufacturing the multi-region molded product using the steel sheet for hot-pressing according to the present invention, the heating region of the steel sheet is divided into at least two regions, one region (hereinafter referred to as the first region) ₃ (Hereinafter referred to as the second region) is heated to a temperature of Ac ₁ transformation point + 20 ° C or higher and an Ac ₃ transformation point -20 ° C or lower, and then the first region is heated to a temperature not lower than the transformation point and not higher than 950 ° C, And the first and second regions in both the first and second regions are maintained at an average cooling rate of 20 deg. C / sec or more in the mold and the martensitic transformation start temperature (Ms ) Or less.

In this method, the heating area of the steel sheet is divided into at least two areas (high-strength side area and low-strength side area), and manufacturing conditions are controlled according to each area, Can be obtained. The second region of the two regions corresponds to the low-strength side region, and the manufacturing conditions, organization and characteristics in this region are basically the same as those of the single-region molded article described above. Hereinafter, manufacturing conditions for forming the other first region (corresponding to the high-strength-side region) will be described. Further, when this manufacturing method is carried out, it is necessary to form a region having a different heating temperature in a single steel sheet. However, by using a conventional heating furnace (for example, far infrared ray, electric furnace + shield) It is possible to control the boundary portion to be 50 mm or less.

(Manufacturing conditions of the first area and the high strength side area)

In order to properly adjust the structure of the press-molded article, it is necessary to control the heating temperature to a predetermined range. By appropriately controlling such a heating temperature, it is transformed into a structure mainly composed of martensite while securing a predetermined amount of retained austenite in the subsequent cooling process, and can be made into a desired structure in the region of the final hot press molded article have. When the steel sheet heating temperature in this region is Ac ₃ If it is less than the transformation point, a sufficient amount of austenite can not be obtained at the time of heating, and a predetermined amount of retained austenite can not be secured in the final structure (the structure of the molded article). When the heating temperature of the steel sheet exceeds 950 DEG C, the grain size of austenite increases during heating, and the martensitic transformation starting temperature (Ms point) and the martensitic transformation end temperature (Mf point) increase, Austenite can not be ensured and good moldability is not achieved. The heating temperature of the steel sheet is preferably Ac ₃ transformation point + 50 ° C or higher and 930 ° C or lower.

It is necessary to appropriately control the average cooling rate and the cooling end temperature during molding and after molding in order to make the austenite formed in the heating step a desired structure while preventing the formation of the structure such as ferrite or pearlite. From this point of view, it is necessary to set the average cooling rate during molding to 20 deg. C / sec or more and to set the cooling end temperature to the martensitic transformation start temperature (Ms point) or less. The average cooling rate during molding is preferably 30 DEG C / second or more (more preferably 40 DEG C / second or more). By making the cooling end temperature lower than the martensitic transformation starting temperature (Ms point), martensite is secured by transforming austenite present at the time of heating into martensite while inhibiting the formation of a structure such as ferrite or pearlite . The cooling end temperature is concretely not higher than 400 캜, preferably not higher than 300 캜.

In the press-molded article obtained by this method, the first region and the second region have different metal structures, precipitates, and the like. In the first region, the metal structure has a residual austenite of 3 to 20% by area (the effect of the retained austenite is the same as described above) and a martensite of 80% by area or more. In the second region, the same metal structure, Ti state (average circle equivalent diameter of Ti-containing precipitates, precipitated Ti amount (mass%) - 3.4 [N], etc.) is satisfied.

By making the main structure of the first region into a high-strength martensite containing a predetermined amount of retained austenite, ductility and high strength of a specific region in a press-molded article can be ensured. From this viewpoint, the area fraction of the martensite needs to be not less than 80% by area. The fraction of martensite is preferably at least 85% by area (more preferably at least 90% by area). In addition, ferrite, pearlite, bainite, or the like may be included as a part of the residual structure in the first region.

Hereinafter, the effects of the present invention will be described in more detail with reference to Examples. However, the present invention is not limited to the following Examples, and all of the design changes in light of the above and the following will be included in the technical scope of the present invention will be.

[Example]

[Example 1]

Steel materials (steel Nos. 1 to 16, 18 to 32) having the chemical composition shown in Tables 1 and 2 below were vacuum-melted and turned into experimental slabs, followed by hot rolling to form steel sheets, (Plate thickness: 1.6 mm or 3.0 mm). In the method of the wrapping simulation, a sample was put in a furnace heated to a coiling temperature after cooling to a coiling temperature, held for 30 minutes, and then cooled. The steel sheet production conditions at this time are shown in Tables 3 and 4 below. In Table 1 and Table 2, Ac ₁ The transformation point, the Ac ₃ transformation point, the Ms point and the Bs point are obtained using the following equations (2) to (5) (see, for example, "Leslie Steel Materials" Maruzen, (1985)). The treatments (1) and (2) shown in the remarks column of Table 3 are performed by the following treatments (rolling, cooling and alloying).

&Quot; (2) "

Ac ₁ Cr = -16.9 [Ni] + 16.9 x [Cr] -16.9 [Ni]

&Quot; (3) "

Ac ₃ transformation point (℃) = 910-203 × [C ] 1/2 + 44.7 × [Si] -30 × [Mn] + 700 × [P] + 400 × [Al] + 400 × [Ti] + 104 × [V] -11 x [Cr] + 31.5 x [Mo] -20 x [Cu] -15.2 x [Ni]

&Quot; (4) "

Ms point (° C) = 550-361 × C -39 × [Mn] -10 × [Cu] -17 × [Ni] -20 × [Cr] -5 × [

&Quot; (5) "

-70 占 폚 -70 占 [Cr] -83 占 [Mo] -70 占 [Ni]

[C], [Si], [Mn], [P], [Al], [Ti], [V], [Cr], [Mo], [Cu] and [Ni] (Mass%) of Mn, P, Al, Ti, V, Cr, Mo, Cu and Ni. In the case where the elements shown in the respective items of the equations (2) to (5) are not included, it is calculated that there is no such term.

Process (1): The hot-rolled steel sheet was cold-rolled (plate thickness: 1.6 mm), simulated by continuous annealing with a heat treatment simulator, heated at 800 ° C, held for 90 seconds, cooled at an average cooling rate of 20 ° C / And kept for 300 seconds.

(2): After hot-rolled steel sheet was cold-rolled (plate thickness: 1.6 mm), it was heated to 860 ° C to simulate a continuous hot-dip galvanizing line with a heat treatment simulator and then cooled to 400 ° C at an average cooling rate of 30 ° C / After cooling and holding, the substrate was further held at 500 ° C for 10 seconds in order to simulate an immersion-alloying treatment into a plating bath, and then cooled to room temperature at an average cooling rate of 20 ° C / sec.

[Table 1]

[Table 2]

[Table 3]

[Table 4]

For the obtained steel sheet (steel sheet for press forming), analysis of the precipitation state of Ti and observation of metal structure (fraction of each structure) were carried out in the following manner. The tensile strength (TS) of each steel sheet was measured by a method described later. The results are shown in the following Tables 5 and 6 together with the calculated value of 0.5 x [total Ti amount (mass%) - 3.4 [N]] [0.5 x

(Analysis of Ti precipitation state of steel sheet)

An extraction model sample was prepared, and a transmission electron microscope (magnification: 100,000 times) of the Ti-containing precipitate was photographed by a transmission electron microscope (TEM). At this time, a Ti-containing precipitate (having a circle-equivalent diameter of 30 nm or less) was specified by analyzing the composition of the precipitate with an energy dispersive X-ray spectroscope (EDX). The area of at least 100 Ti-containing precipitates was measured by image analysis, and the circle-equivalent diameter was determined from the area. The average value was determined as the precipitate size (average circle-equivalent diameter of Ti-containing precipitates). The amount of precipitated Ti (mass%) - 3.4 [N] (amount of Ti present as a precipitate) was subjected to extraction residue analysis using a mesh having a mesh diameter of 0.1 탆 (during the extraction treatment, (Mass%) - 3.4 [N] (indicated in Table 5 and in Table 6, the amount of precipitated Ti is -3.4 [N]) was obtained. When the Ti-containing precipitate partially contains V or Nb, the content of these precipitates was also measured.

(Observation of metal structure (fraction of each tissue))

(1) For the structure of ferrite, bay nitride ferrite, pearlite and martensite in the steel sheet, the steel sheet was corroded with nital, and SEM (magnification: 1,000 or 2000) Tear ferrite, pearlite, and martensite were identified to obtain respective fractions (area ratio).

(2) The residual austenite fraction in the steel sheet was measured by X-ray diffractometry after grinding to a thickness of 1/4 of the steel sheet, followed by chemical polishing (for example, ISJJ Int. Vol. 33 (1933) , No. 7, p. 766).

[Table 5]

[Table 6]

The thickness (t) was adjusted to 1.6 mm by hot rolling for each steel sheet (1.6 mm ^t x 150 mm x 200 mm) except for the above treatments (1) and (2) , And then subjected to press molding and cooling treatment with a hat-shaped mold (Fig. 1) to prepare a molded product. Table 7 shows press molding conditions (heating temperature during press molding, average cooling rate, and rapid cooling termination temperature).

[Table 7]

The tensile strength (TS), the elongation (total elongation (EL)), the observation of the metal structure (the fraction of each structure) and the degree of decrease in hardness after heat treatment were measured for the obtained press-molded article by the following method, Analysis of the state was measured by the method described above.

(Measurement of tensile strength (TS) and elongation (total elongation (EL)))

The tensile strength (TS) and elongation (EL) were measured using a JIS No. 5 test piece. At this time, the strain rate of the tensile test was 10 mm / sec. In the present invention, when the tensile strength (TS) is 980 MPa or more and the elongation (EL) is 16% or more and the strength-elongation balance (TS x EL) is 16000 (MPa.

(Observation of metal structure (fraction of each tissue))

(1) For the structure of ferrite, baynitic ferrite, and pearlite in the steel sheet, the steel sheet was corroded into and detached from the steel sheet to distinguish ferrite, baynitic ferrite, and pearlite from each other by SEM (magnification: 1000 or 2000) (Including the distinction of ferrite and acicular ferrite), and the respective fractions (area ratio) were obtained.

(2) The retained austenite fraction in the steel sheet was measured by X-ray diffractometry after grinding to a thickness of 1/4 of the steel sheet, followed by chemical polishing (see, for example, ISJJ Int. Vol. 33 (1933) , No. 7, p. 766).

(3) With respect to the martensite (quenched martensite) fraction, the area ratio was measured as a mixed structure of martensite and retained austenite as the steel plate was subjected to LePera erosion and the white contrast was quenched, The residual austenite fraction obtained by X-ray diffraction was subtracted therefrom, and the martensite fraction was calculated.

(Reduction in hardness after heat treatment)

The sample was heated to 700 deg. C at an average heating rate of 50 deg. C / second by a heat treatment simulator and then cooled to an average cooling rate of 50 deg. C / sec to obtain a thermal degradation amount corresponding to the spot hardness (? Hv ) Were measured. When the degree of hardness decrease (? Hv) was 50 Hv or less, it was judged that the anti-softening property in HAZ was good.

The results of observation of the metal structure (Ti precipitation state, fraction of each structure, amount of precipitation Ti -3.4 [N]) are shown in Tables 8 and 9 below. Table 10 shows the mechanical properties (tensile strength TS, elongation (EL), (TS EL), and hardness lowering amount (Hv)) of the molded article. The value of the amount of precipitated Ti of -3.4 [N] in the molded product is slightly different from the value of precipitated Ti of -3.4 [N] in the steel sheet for press forming, which is a measurement error.

[Table 8]

[Table 9]

[Table 10]

From these results, it can be considered as follows. River No. 1, 2, 4 to 6, 10, 11, 15, 16, 19 to 21, and 23 to 32 satisfy the requirements specified in the present invention and are excellent in strength- It can be seen that this good component is obtained.

On the contrary, 3, 7 to 9, 12 to 14, 18 and 22 are comparative examples which do not satisfy any one of the requirements specified in the present invention, and any one of the characteristics is deteriorated. That is, 3 is a steel sheet used for press forming with a small Si content. The retained austenite fraction in the press-formed article is not ensured, the elongation is not achieved, and the strength-elongation balance is deteriorated. River No. 7 is that the finish rolling temperature at the time of steel sheet production is low and the Ti containing precipitate in the steel sheet for press forming is coarsened and the relationship of Equation 1 is not satisfied at any stage of the press forming steel sheet and the press molded article So that the anti-softening property is deteriorating.

River No. 8 is that the cooling rate of 620 ° C to 580 ° C at the time of steel sheet production is high and the ferrite transformation is not sufficiently progressed and the ferrite fraction in the steel sheet for press forming can not be ensured so that the strength is increased, It is expected that molding or processing becomes difficult. River No. 9 is that the coiling temperature is high at the time of steel sheet production and the Ti-containing precipitate in the steel sheet for press forming is coarse, and when the steel sheet is press molded using such steel sheet, the molding conditions are appropriate, The anti-softening property is deteriorated.

River No. 12 is that the heating temperature at the time of press forming is high and martensite is produced and ferrite is not produced and the elongation is decreased and the strength-elongation balance (TS x EL) is deteriorated. River No. 13, the cooling rate at the time of press forming becomes slow, and the ferrite fraction increases at the stage of the press-molded article, and the strength decreases.

River No. 14 is that the cooling end temperature at the time of press forming becomes high and pearlite is generated and the retained austenite can not be secured and the strength and elongation are lowered and the strength-elongation balance (TS x EL) is deteriorated. River No. 18 is a steel sheet for press forming in which the content of C is excessive and the ferrite fraction of the steel sheet can not be ensured and the ferrite fraction of the press-molded article can not be ensured and only a low elongation (EL) (TS EL) is also deteriorated. River No. 22 is based on a steel sheet for press forming having an excessive Ti content and does not satisfy the relationship of the formula (1) at any stage of the steel sheet for press forming and the press-molded article, so that the Ti-containing precipitate is coarsened, Deterioration characteristics are deteriorating.

[Example 2]

(Steel Nos. 33 to 37) having the chemical composition shown in the following Table 11 were vacuum-melted and turned into experimental slabs, followed by hot rolling, cooling and winding (plate thickness: 3.0 mm). The steel sheet production conditions at this time are shown in Table 12 below.

[Table 11]

[Table 12]

The obtained steel sheet (steel sheet for press forming) was analyzed in the same manner as in Example 1, by analyzing the precipitation state of Ti-containing precipitates, observing the metal structure (fraction of each structure), and measuring the tensile strength. The results are shown in Table 13 below.

[Table 13]

Each of the above steel plates (3.0 mm ^t x 150 mm x 200 mm) was heated to a predetermined temperature in a heating furnace and subjected to press molding and cooling treatment with a hat-shaped mold (Fig. 1) did. At this time, the steel sheet is put in the infrared ray path so that the infrared ray directly touches the portion (the steel sheet portion corresponding to the first region) which is desired to be intensified by the high temperature, , A heating temperature difference was set by covering a part of the infrared ray so as to be able to be heated at a low temperature. Therefore, the molded article has regions having different strengths in a single component. Table 14 shows press molding conditions (heating temperature, average cooling rate, and rapid cooling termination temperature in each region at the time of press forming).

[Table 14]

The tensile strength (TS), the elongation (total elongation (EL)), the observation of the metal structure (the fraction of each structure) and the degree of hardness decrease (? Hv) in each area of the obtained press- Respectively.

The results of the observation of the metal structure (the fraction of each structure) and the precipitation state of Ti are shown in Table 15 below. Table 16 shows the mechanical properties (tensile strength TS, elongation EL, TS EL and hardness decrease DELTA Hv) of the press-molded article. It was evaluated as acceptable when the tensile strength TS on the high strength side was 1470 MPa or more and the elongation percentage EL was 8% or more and the strength-elongation balance (TS x EL) was 14000 (MPa% Evaluation criteria on the strength side are the same as in Example 1).

[Table 15]

[Table 16]

From these results, it can be considered as follows. River No. 33 to 36 are examples satisfying the requirements specified in the present invention, and it can be seen that a press-molded article having excellent strength-ductility balance in each region is obtained. On the contrary, 37, the heating temperature at the time of press forming is low, and the martensite fraction at the high strength side is insufficient and the strength at the high strength side is lowered (the difference in strength from the low strength side is less than 300 MPa).

[Industrial Availability]

A Ti-containing precipitate having a prescribed chemical composition and having a circle equivalent diameter of 30 nm or less among the Ti-containing precipitates contained in a steel sheet has an average circle equivalent diameter of 6 nm or less and a precipitated Ti amount and a total Ti amount in the steel have a predetermined relationship And the metal structure has a fraction of ferrite of not less than 30% by area, it is possible to facilitate molding and processing before hot pressing, and when uniform properties are required in a molded article, the strength and elongation It is possible to obtain a press-molded article capable of achieving a high level of balance. When a region corresponding to an impact-resistant region and an energy-absorbing region is required in a single molded product, balance between high strength and elongation can be achieved at a high level In addition, a hot press useful for obtaining a press molded article having good softening prevention characteristics in HAZ It is possible to realize a steel sheet.

1: punch 2: die
3: blank holder 4: steel plate (blank)

Claims (6)

C: 0.15-0.5% (meaning% by mass, hereinafter the same with respect to chemical composition)
Si: 0.2 to 3%
Mn: 0.5 to 3%
P: not more than 0.05% (not including 0%),
S: not more than 0.05% (not including 0%),
Al: 0.01 to 1%
B: 0.0002 to 0.01%
Ti: 3.4 [N] + 0.01% or more, 3.4 [N] + 0.1% or less [where N represents the content (mass%
N: 0.001 to 0.01%
Respectively, the balance being iron and inevitable impurities,
A Ti-containing precipitate having a circle equivalent diameter of 30 nm or less and an average circle equivalent diameter of 6 nm or less and a precipitated Ti amount and a total Ti amount satisfying the relationship of the following formula (1) , And the fraction of ferrite is 30% by area or more
Steel sheet for hot press.
[Equation 1]
(% By mass) - 3.4 [N] < 0.5 占 Total Ti amount (mass%) - 3.4 [N]
(In the formula (1), [N] represents the content (mass%) of N in the steel) The method according to claim 1,
As another element, at least one of the following (a) to (c)
Steel sheet for hot press.
(a) a total of at least 0.1% (excluding 0%) of at least one element selected from the group consisting of V, Nb and Zr;
(b) at least one selected from the group consisting of Cu, Ni, Cr, and Mo in a total amount of not more than 1% (not including 0%),
(c) a total of 0.01% or less (not including 0%) of at least one element selected from the group consisting of Mg, Ca and REM, A steel sheet for hot pressing according to any one of claims ₁ to 3, which has an Ac ₁ transformation point + 20 ° C or higher, Ac ₃ The bainite transformation start temperature (Bs) is increased while maintaining an average cooling rate of 20 deg. C / sec or more in the mold during molding and after the molding is finished, And the temperature is lower than a temperature lower than 100 deg.
A method of manufacturing a press molded article. A press-molded article of a steel sheet having the chemical composition according to claim 1 or 2,
Wherein the metal structure in the press-molded article is at least one selected from the group consisting of 3 to 20 percent by area of retained austenite, 30 to 80 percent by area of ferrite, 30 percent by area of baynitic ferrite, 0 percent by area of martensite, % Of the Ti-containing precipitates contained in the press-formed article, the average equivalent circle diameter of the Ti-containing precipitates having a circle equivalent diameter of 30 nm or less is 10 nm or less, and the amount of precipitated Ti in the steel and the total amount of Ti And satisfies the following expression (1)
Pressed products.
[Equation 1]
(% By mass) - 3.4 [N] < 0.5 占 Total Ti amount (mass%) - 3.4 [N]
(In the formula (1), [N] represents the content (mass%) of N in the steel) First by using a steel sheet for hot press according to any of the preceding claims, dividing the heating zone of the steel plate to at least two regions, the one region at the same time, heated to a temperature not higher than Ac over ₃ transformation point, 950 ℃, other One area is Ac ₁ After the molding is started at a temperature of the transformation point + 20 deg. C or higher and the Ac ₃ transformation point -20 deg. C or lower, press molding is started for both regions and cooling is performed in the mold at 20 deg. C / (Ms) of the martensite transformation start temperature while securing the temperature of the martensite transformation starting temperature
A method of manufacturing a press molded article. A press-molded article of a steel sheet having the chemical composition according to claim 1 or 2,
The press-molded article of the present invention is characterized in that the metal structure has a first region where the retained austenite is 3 to 20% by area and martensite is 80% by area or more and a second region where the metal structure is retained austenite: 3 to 20% (Not including 0 area%), martensite: not more than 31 area% (not including 0 area%), and the second region Wherein the average circle equivalent diameter of the Ti-containing precipitates contained in the steel of the steel sheet having a circle equivalent diameter of 30 nm or less is 10 nm or less and the amount of precipitated Ti in the steel and the total amount of Ti satisfy the following formula
Pressed products.
[Equation 1]
(% By mass) - 3.4 [N] < 0.5 占 Total Ti amount (mass%) - 3.4 [N]
(In the formula (1), [N] represents the content (mass%) of N in the steel)

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