KR20160042070A - Method for manufacturing press-molded article, and press-molded article - Google Patents

Abstract

A Ti-containing precipitate having a prescribed chemical composition and having a circle equivalent diameter of 30 nm or less and an average circle equivalent diameter of 6 nm or less among the Ti containing precipitates contained in the steel sheet and having a precipitation Ti amount and a total Ti amount satisfying a predetermined relationship The steel sheet for press is heated to a temperature of 900 DEG C or higher and 1100 DEG C or lower and then press molding is started and after bending of the bainite is started while maintaining an average cooling rate of 20 DEG C / Cooling to 200 ° C or lower at an average cooling rate of less than 20 ° C / sec after cooling to a temperature of 100 ° C lower than the temperature (Bs) or lower than the martensitic transformation start temperature (Ms) Can be obtained at a high level, and further, there is provided a useful method for obtaining a press-molded article having good anti-softening property in HAZ The.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a method of manufacturing a press-

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a press-molded article used for manufacturing a structural part of an automobile, and a method of manufacturing such a press-molded article. Particularly, the present invention relates to a press-formed article which is produced by applying to a press-molding method for obtaining a predetermined strength by performing heat treatment at the same time as the shape-imparting when a preheated steel sheet (blank) is formed into a predetermined shape, and a press- ≪ / RTI >

As one of measures to improve the fuel efficiency of automobiles started from global environmental problems, weight reduction of the vehicle 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, And a hot press forming method in which quenching heat treatment (quenching) using the temperature difference between them is performed to secure the strength after molding is employed in part production. 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, And CL represents a punch / die clearance, respectively. 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 a cooling medium So that these members 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. [ In addition, heat is discharged from the steel plate 4 to the mold (punch and die) by cooling the punch and the die in parallel with the molding, and at the same time the molding bottom dead center (at the time when the tip of the punch is located at the deepest portion: To thereby perform quenching of the workpiece. 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 a elongation of about 6 to 8%, and is applied to an impact resistant member (a member that is not extremely deformed 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, a technique similar to that of Patent Documents 1 to 4 has 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, the tensile strength (TS) is 1270 MPa or more and the elongation (EL) is 12.7% at maximum, and further improvement is demanded.

On the other hand, automobile parts need to be bonded by spot welding. However, in a hot stamped product having a structure mainly composed of martensite, the strength of the welded heat affected zone (HAZ) Softening) (see, for example, Non-Patent 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 press molded article capable of achieving a balance between high strength and elongation at a high level, and further, a method useful for obtaining a press molded article having good anti- And a press molded product exhibiting the above characteristics.

The production method of the press-molded article of 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: 0.001 to 0.01%

Respectively, the balance being iron and inevitable impurities,

A steel sheet for hot press having a circle equivalent diameter of not more than 30 nm and an average circle equivalent diameter of not more than 6 nm among the Ti containing precipitates contained in the steel sheet and satisfying the relation of the following formula , Press molding is started after heating to a temperature of 900 DEG C or more and 1100 DEG C or less and the bainite transformation start temperature (Bs) is maintained while maintaining an average cooling rate of 20 DEG C / Cooling to a temperature not higher than 100 占 폚 lower than the martensitic transformation start temperature (Ms), and then cooled to 200 占 폚 or lower at an average cooling rate lower than 20 占 폚 / sec. 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)

The steel sheet for hot press used in the production method of the present invention may contain at least one of the following (a) to (c) as another element, if necessary. 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,

In the press-formed article obtained by this manufacturing method, the metal structure is composed of 60 to 97% by area of bainitic ferrite, 37% by area or less of martensite, 3 to 20% by area of retained austenite, Of the Ti-containing precipitates contained in the press-molded article is not more than 5% by area 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, satisfies the relationship of the above- And the elongation can be achieved as a uniform characteristic at a high level.

According to the present invention, since the chemical composition is strictly regulated, and the Ti-containing precipitate is controlled in size, and the precipitation rate of Ti not forming TiN is controlled, a steel sheet is used. By hot pressing, the strength-elongation balance of the molded article can be made high, and further, the anti-softening property in the HAZ can be improved.

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

DISCLOSURE OF THE INVENTION The present inventors have studied at various angles in order to realize a press molded article exhibiting good ductility (elongation) while ensuring high strength after press forming when a steel sheet is heated at a predetermined temperature and then subjected to hot press forming to produce a press molded article did.

As a result, if the chemical composition of the steel sheet for hot press is strictly regulated and the size of the Ti-containing precipitate and the amount of precipitated Ti are controlled, the steel sheet is subjected to hot press forming under predetermined conditions, The inventors of the present invention have found that a retained austenite is secured to enhance the inherent ductility (residual ductility), and furthermore, a press molded article having a good anti-softening property in HAZ is obtained.

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

(C: 0.15-0.5%)

C is an important element for improving the strength by decreasing the bainite transformation start temperature (Bs), making the bainitic ferrite produced in the cooling process fine, and increasing the dislocation density in the bainitic ferrite. In addition, by increasing the amount of fine retained austenite formed between the laths of the bainitic ferrite, it is possible to secure a balance of high strength and elongation at a high level. When the C content is less than 0.15%, the bainite transformation initiation temperature (Bs) rises and the bainite ferrite becomes coarse and low dislocation density, and the strength of the hot press molded product can not be secured. Further, if the C content is excessive and exceeds 0.5%, the strength becomes too high, and good ductility can not be obtained. The lower limit of the C content is preferably 0.18% or more (more preferably 0.20% or more), and the preferred upper limit is 0.45% or less (more preferably 0.40% or less).

(Si: 0.2 to 3%)

Si suppresses the formation of cementite by decomposing the retained austenite formed between the laths of the bainite ferrite during cooling of the quenching mold, thereby exhibiting the effect of forming the retained austenite. In order to exhibit such an effect, the Si content needs to be 0.2% or more. When the Si content exceeds 3% and exceeds 3%, ferrite tends to be formed, which makes it difficult to single-phase the austenite at the time of heating, and the steel sheet for hot press has a structure fraction other than the bainite ferrite and retained austenite 5 area%. 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 the formation of a soft texture such as ferrite or pearlite during cooling of the mold quenching. Further, by lowering the bainite transformation start temperature (Bs), it is an important element for improving the strength by making the bainitic ferrite produced in the cooling process finer and increasing the dislocation density in the bainitic ferrite. It is also an element that stabilizes austenite and contributes to an increase in the amount of retained austenite. In order to exhibit these effects, Mn should be contained in an amount of 0.5% or more. When only the characteristics are taken into account, it is preferable that the Mn content is large, but the cost of adding the alloy increases, so that it is 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 which is 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 less than 0.05% (0% is not included) because 0% is difficult to manufacture. 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 less than 0.05% (0% is not included) because 0% is difficult to manufacture. 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 an effect of suppressing ferrite transformation and pearlite transformation, it is possible to prevent the formation of ferrite and pearlite during cooling after heating at a bimodal temperature (from Ac ₁ transformation point to Ac ₃ transformation point) to secure a retained austenite It is a contributing element. 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 fixes N and fixes B in a solid state, thereby exhibiting an effect of improving the hardenability. 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). Further, Ti added in excess to N is allowed to exist in a solid state in the hot stamped molded article, and the precipitated compound is finely dispersed, whereby Ti solidified when the hot stamped product is welded is formed as TiC 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. The 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 the preferable upper limit is 3.4 [N] + 0.09% [N] + 0.08% or less).

(N: 0.001 to 0.01%)

N is preferably reduced as much as possible because the effect of improving the hardenability is deteriorated by fixing B as BN. However, since there is a limit to reduction in the actual process, N is set to a lower limit of 0.001%. Further, when the N content is excessive, Ti-containing precipitates (for example, TiN) to be formed are coarsened and this precipitate acts as a starting point of fracture and lowers the ductility of the steel sheet, so that the upper limit is set to 0.01% . The preferable upper limit of the N content is 0.008% or less (more preferably 0.006% or less).

The basic chemical components in the hot-press steel sheet used in 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 elements (a) to (c) as another element, if necessary, to the hot-press steel sheet used in the present invention. The properties of the press-molded article are further improved, depending on the kind of element contained, if necessary. 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. For this reason, 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 and pearlite transformation, formation of ferrite and pearlite is prevented during cooling after heating, thereby effectively retaining retained austenite. In order to exhibit such effects, it is preferable that the total amount is 0.01% or more. Considering the characteristics alone, the content is preferably as large as possible, 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.

(0.01% or less (excluding 0%) in total of at least one selected from the group consisting of Mg, Ca and REM)

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 used in 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) It is also an important requirement to satisfy the relationship of -3.4 [N] < 0.5 x [total Ti amount (mass%) - 3.4 [N]]

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 in the steel sheet before molding to exist in a solid state or in a fine state, Ti-containing precipitates can be maintained in a solid state or a fine state at the time of heating 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 to improve the seam characteristics.

From this point of view, it is necessary to finely disperse the Ti-containing precipitates. To do so, the Ti-containing precipitates contained in the steel sheet must have an average circle equivalent diameter of 6 nm or less in the circle equivalent diameter of 30 nm or less (The requirement of (A) above). 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.

In the steel sheet for hot pressing, 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. In order to do so, the amount of Ti present as a precipitate other than TiN (i.e., the amount of precipitated Ti of -3.4 [N]) is less than 0.5 times the remainder obtained by subtracting Ti forming TiN from the whole Ti (%) - 3.4 [N]]) (the requirement of (B) above). The precipitation Ti amount -3.4 [N] is preferably 0.4 x [total Ti amount (%) - 3.4 [N]] or less, more preferably 0.3 x [total Ti amount to be.

In order to produce such a steel sheet (steel sheet for hot press), a slab made of a steel material having the chemical composition as described above is heated at a heating temperature of 1100 DEG C or higher (preferably 1150 DEG C or higher) and 1300 DEG C or lower 1250 占 폚 or less), the hot rolling is performed at a finish rolling temperature of 850 占 폚 or higher (preferably 900 占 폚 or higher) and 1000 占 폚 or lower (preferably 950 占 폚 or lower) (Preferably at least 450 ° C) at an average cooling rate of at least 20 ° C / second (preferably at least 30 ° C / second) Preferably 450 DEG 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 , (3) further quenched and then wound up to control bainite transformation or martensitic transformation.

The hot-rolled steel sheet having the chemical composition and the Ti precipitation state as described above may be provided as it is in the production of a hot press as it is, and a reduction ratio of 10 to 80% (preferably 20 to 70%) after pickling (pickling) And then subjected to cold rolling to provide a hot press. The steel sheet for hot press or the cold rolled steel sheet is heated at a temperature of 830 DEG C or higher (preferably 850 DEG C or higher and 900 DEG C or lower) and then heated to 500 DEG C or lower (preferably 450 DEG C or lower) at 20 DEG C / Preferably 30 ° C / second or more), and then subjected to heat treatment at 500 ° C or less for 10 seconds or more, 1000 seconds or less, or at a temperature of 500 ° C or less. The steel sheet for hot press according to the present invention may be plated with at least one of Al, Zn, Mg, and Si on its surface (base steel sheet surface).

After the above-mentioned hot press steel sheet is heated to a temperature of 900 DEG C or higher and 1100 DEG C or lower, press molding is started, and an average cooling rate of 20 DEG C / sec or more (Bs-100 deg. C) lower than the bainite transformation initiation temperature (Bs) to a temperature equal to or higher than the martensitic transformation starting temperature (Ms), and then cooled at an average cooling rate of less than 20 deg. C / , It is possible to obtain a structure which is a press-molded article having a single characteristic and which is optimal in that it has a predetermined strength and high cohesion (a structure mainly composed of bainitic ferrite). The reason for defining each of the requirements in this molding method is as follows.

When the heating temperature of the steel sheet is lower than 900 캜, sufficient austenite is not obtained at the time of heating, and the martensite fraction becomes excessive in the final structure (the structure of the molded article). When the heating temperature of the steel sheet exceeds 1100 占 폚, the grain size of austenite becomes large at the time of heating, the martensite transformation start temperature (Ms) and the martensite finish temperature (Mf) rise and the retained austenite It can not be ensured and good moldability is not achieved. The heating temperature is preferably 950 DEG C or more and 1050 DEG C or less. If the heating time at this time is too long, the Ti-containing precipitates in the steel sheet are hardly miniaturized, and even if the Ti-containing precipitates are small, the Ti-containing precipitates are formed and coarsened during heating, desirable. The preferable range of the heating time is 3600 seconds or less, more preferably 20 seconds or less.

In order to convert austenite formed in the heating step into a desired structure (a structure mainly composed of bainitic ferrite) while preventing the formation of a structure such as ferrite or pearlite, the average cooling rate during molding and after molding and the cooling end It is necessary to appropriately control the temperature. 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 and not more than the martensitic transformation start temperature (Ms) have. 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, the austenite existing at the time of heating is transformed into bainite while inhibiting formation of a structure such as ferrite or pearlite, Minute austenite is retained between the laths of the bayite ferrite while securing the amount of the niacitic ferrite to secure a predetermined amount of the retained austenite.

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. Further, when the temperature is lowered to a temperature lower than the martensitic transformation starting temperature (Ms), the amount of martensite produced increases and the elongation (ductility) of the molded article is deteriorated.

Rapid cooling is stopped at a temperature lower than the bainite transformation start temperature (Bs) by 100 占 폚 or lower and a temperature higher than the martensitic transformation start temperature (Ms), and thereafter an average cooling rate of less than 20 占 폚 / Is performed. By adding such a cooling step, the transformation of the bainitic ferrite is accelerated. When the average cooling rate at this time is 20 DEG C / second or more, martensite is formed and the strength is increased, but a good elongation is not obtained. The average cooling rate at this time is preferably 15 占 폚 / second or less, and more preferably 10 占 폚 / second or less. The reason for this cooling by cooling to 200 ° C or less is that carbon is distributed from non-ferrite austenite to ferrite to increase the amount of retained austenite remaining at room temperature.

After performing the above-described two-stage cooling, the control of the average cooling rate is basically unnecessary. However, it may be cooled to room temperature at an average cooling rate of 1 deg. C / sec or more and 100 deg. The control of the average cooling rate during press molding and after completion of molding is carried out by controlling the temperature of the molding die (the cooling medium shown in Fig. 1), (b) controlling the thermal conductivity of the mold Can be achieved by means.

In the press-formed article obtained by this manufacturing method, the metal structure is composed of 60 to 97% by area of bainitic ferrite, 37% by area or less of martensite, 3 to 20% by area of retained austenite, , And the amount of carbon in the retained austenite is 0.50% or more. Thus, a balance between high strength and elongation can be achieved at a high level as a uniform property in a molded article. The reason for setting the ranges of the respective requirements (basic structure and amount of carbon in retained austenite) in such hot press formed products is as follows.

By making the main structure of the press-molded article to be a high-strength and ductile baynitic ferrite, high strength and high ductility of the press-molded article can be achieved. From this point of view, the area fraction of the baynitic ferrite needs to be not less than 60% by area. However, when the content exceeds 97% by area, the fraction of the retained austenite is insufficient and the ductility (residual ductility) is lowered. The preferable lower limit of the proportion of the baynitic ferrite is 65% by area or more (more preferably 70% by area or more), and the preferable upper limit is 95% by area or less (more preferably 90% by area or less).

By including a part of the high-strength martensite, the strength of the hot-press molded article can be increased. However, when the amount of martensite is increased, the ductility (residual ductility) is lowered. From this viewpoint, it is necessary that the area fraction of martensite is 37% by area or less. The preferable lower limit of the martensite fraction is 5% by area or more (more preferably 10% by area or more), and the preferable upper limit is 30% by area or less (more preferably 25% by area or less).

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 steel sheets 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).

In addition to the above-described structure, ferrite, pearlite, and the like can be included as the residual structure. However, these tissues are preferably basically not included because their contribution to strength and contribution to ductility are lower than those of other tissues good). However, it is acceptable if it is up to 5 area%. The residual structure is more preferably 4% by area or less, and more preferably 3% by area or less.

In the above press-molded article, among the Ti-containing precipitates contained in the press-molded articles (that is, the steel plates constituting the press-molded article), the average circle equivalent diameter of those having a circle equivalent diameter of 30 nm or less 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.

Further, in the press-molded article, the amount of Ti (precipitated Ti amount -3.4 [N]) existing as a precipitate other than TiN is smaller than 0.5 times of the remaining amount of Ti minus Ti forming TiN among the whole Ti The total Ti content (%) - 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 precipitation Ti amount -3.4 [N] is preferably 0.4 x [total Ti amount (%) - 3.4 [N]] or less, more preferably 0.3 x [total Ti amount to be.

According to the method of the present invention, it is possible to control properties such as strength and elongation of a molded article by appropriately adjusting press molding conditions (heating temperature and cooling rate), and furthermore, to obtain a press molded article having high ductility Therefore, it can be applied to a region (for example, an energy absorbing member) which has been difficult to apply in the conventional hot press molded products, and is extremely useful in expanding the application range of hot press formed products.

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]

Steel materials (steel Nos. 1 to 31) having the chemical composition shown in the following Table 1 were vacuum-melted and turned into experimental slabs, followed by hot rolling to obtain steel sheets, which were then cooled and subjected to simulated winding (Plate thickness: 3.0 mm). In the method of the wrapping simulation, after cooling to the winding temperature, the sample was placed in a furnace heated to the winding temperature, held for 30 minutes, and then cooled. The steel sheet production conditions at this time are shown in Table 2 below. The Ac ₃ transformation point, Ms point and Bs point in Table 1 are obtained by using the following equations (2) to (4) (see, for example, "Leslie Steel Materials" Maruzen, (1985)). The treatments (1) and (2) shown in the remarks column of Table 2 are performed by the following treatments (rolling, cooling and alloying).

&Quot; (2) "

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; (3) "

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

&Quot; (4) "

-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 Si, Mn, P, Al, Ti, V, Cr, Mo, Cu and Ni. Further, in the case where the elements shown in the respective items of the equations (2) to (4) 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), and the continuous annealing was simulated using 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 wafer was held at 500 ° C for 10 seconds to simulate an immersion-alloying treatment into a plating bath, and then cooled to room temperature at an average cooling rate of 20 ° C / second.

[Table 1]

[Table 2]

Analysis of the precipitation state of Ti (amount of precipitated Ti: -3.4 [N], average circle equivalent diameter of Ti-containing precipitates) was performed on the obtained steel sheet (press forming steel sheet) in the following manner. The results are shown in Table 3 together with the calculated values of 0.5 占 total Ti amount -3.4 [N].

(Analysis of the precipitation state of Ti in the 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, the composition of the precipitate was analyzed by an energy dispersive X-ray spectroscope (EDX) to determine a Ti-containing precipitate (having a circle equivalent diameter of 30 nm or less). An area of at least 100 Ti-containing precipitates was measured by image analysis, and a 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). Further, the amount of precipitated Ti of -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 (in the extraction treatment, Precipitate can also be measured), and the amount of precipitated Ti of -3.4 [N] was obtained. When the Ti-containing precipitate partially contains V or Nb, the content of these precipitates was also measured.

[Table 3]

For each of the above steel plates (1.6 mm ^t x 150 mm x 200 mm) (except for the above treatments (1) and (2), the thickness t was adjusted to 1.6 mm by hot rolling) After heating to a temperature, press molding and cooling treatment were performed with a hat-shaped mold (Fig. 1) to obtain a molded article. Table 4 shows press molding conditions (heating temperature, press time, heating time, average cooling rate, and rapid cooling termination temperature).

[Table 4]

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, The state was measured by the above-mentioned method.

(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 1180 MPa or more and the elongation (EL) is 12.0% 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 the bainite ferrite, martensite and ferrite in the molded product, the steel sheet was corroded with nital and subjected to SEM (magnification: 1000 or 2000) Martensite, and ferrite, and the respective fractions (area ratio) were obtained.

(2) The retained austenite fraction in the molded product 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. 7, p. 766).

(Reduction in hardness after heat treatment)

The sample was heated to 700 ° C at an average heating rate of 50 ° C / second with a heat treatment simulator, followed by cooling at an average cooling rate of 50 ° C / sec to obtain a thermal degradation amount corresponding to the spot hardness (Vickers hardness) Hv) was 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 (the fraction of each structure, the precipitation state of Ti, and the amount of precipitated Ti -3.4 [N]) are shown in Table 5 below. The mechanical properties (tensile strength TS, elongation (ELTS x EL) and hardness decrease amount (Hv)) of the press-molded article are shown in Table 6 below. The value of the precipitated Ti amount -3.4 [N] in the press-molded article is slightly different from the value of the precipitated Ti amount -3.4 [N] in the steel sheet for press forming, which is a measurement error.

[Table 5]

[Table 6]

From these results, it can be considered as follows. River No. 1, 2, 4 to 6, 8 to 10, 15, 16, 18 to 20, and 22 to 31 are examples satisfying the requirements defined in the present invention, and are excellent in strength-ductility balance, It can be seen that this good molded article is obtained.

On the contrary, 3, 7, 11 to 14, 17 and 21 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 uses a steel sheet having a small Si content and does not secure the retained austenite fraction in the press-molded article, and only a low elongation (EL) is obtained, and the strength-elongation balance (TS x EL) also deteriorates. River No. 7 is that the finishing rolling temperature at the time of steel sheet production is low and does not satisfy the relation of the formula (1), and the Ti-containing precipitates are coarse to lower the strength-elongation balance (TS x EL) The characteristics are deteriorating.

River No. 11 is that the cooling rate after rapid cooling at the time of press forming is high and the production of martensite becomes excessive and the strength becomes too high to obtain only a low elongation (EL) and the strength-elongation balance (TS EL) Is also deteriorating. River No. 12 is that the rapid cooling termination temperature at the time of press forming is low and martensite is excessively produced and the strength is too high to obtain only a low elongation (EL), and the strength-elongation balance (TS x EL) Is deteriorating.

River No. 13, the average cooling rate at the time of press forming is slow, the area ratio of the bayonetic ferrite can not be ensured, the strength becomes too low, and the strength-elongation balance (TS x EL) also deteriorates. River No. 14 is that the rapid cooling termination temperature at the time of press forming becomes high and ferrite is generated and the area ratio of the bayonetic ferrite can not be secured and the strength becomes too low and the strength-elongation balance (TS x EL) also deteriorates have.

River No. 17 uses a steel sheet having an excess C content, and the strength of the molded product is increased, and only a low elongation (EL) is obtained. River No. 21 is a steel sheet in which the Ti content is excessive, the press-molded article does not satisfy the relationship of the formula (1), the Ti-containing precipitate in the molded article is coarsened, and the anti-softening property is deteriorated.

[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 Is heated to a temperature of 900 占 폚 or more and 1100 占 폚 or lower and then press molding is started and the average cooling rate of 20 占 폚 / After cooling to a temperature not higher than the bainite transformation start temperature (Bs) by 100 占 폚 or lower and a temperature higher than the martensitic transformation start temperature (Ms), and then cooled to 200 占 폚 or lower at an average cooling rate of less than 20 占 폚 / sec, Can be obtained at a high level of balance, and a press-molded article having good anti-softening property in HAZ can be realized.

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

Claims (3)

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: 0.001 to 0.01%
Respectively, the balance being iron and inevitable impurities,
A steel sheet for hot press having a circle equivalent diameter of not more than 30 nm and an average circle equivalent diameter of not more than 6 nm among the Ti containing precipitates contained in the steel sheet and satisfying the relation of the following formula , Press molding is started after heating to a temperature of 900 DEG C or more and 1100 DEG C or less and the bainite transformation start temperature (Bs) is maintained while maintaining an average cooling rate of 20 DEG C / Cooling to a temperature not higher than 100 占 폚 lower than the martensitic transformation start temperature (Ms), and then cooled to 200 占 폚 or lower at an average cooling rate lower than 20 占 폚 / sec.
A method of manufacturing a press molded article.
[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,
The steel sheet for hot press comprises, as another element, at least one of the following (a) to (c):
A method of manufacturing a press molded article.
(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 press-molded article of a steel sheet having the chemical composition according to any one of claims 1 to 3,
Wherein the metal structure of the press-molded article comprises 60 to 97% by area of baynitic ferrite, 37% by area of martensite, 3 to 20% by area of retained austenite and 5% Of the Ti-containing precipitates having a circle equivalent diameter of 30 nm or less and having an average circle equivalent diameter of 10 nm or less and satisfying the relationship of the formula
Pressed products.

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