KR20140121877A

KR20140121877A - Steel sheet for hot pressing use, press-molded article, and method for producing press-molded article

Info

Publication number: KR20140121877A
Application number: KR1020147024781A
Authority: KR
Inventors: 도시오 무라카미; 준야 나이토; 게이스케 오키타; 슈시 이케다
Original assignee: 가부시키가이샤 고베 세이코쇼
Priority date: 2012-03-09
Filing date: 2013-03-01
Publication date: 2014-10-16
Also published as: CN104160052A; EP2824209A4; US20180312947A1; JP5756773B2; WO2013133166A1; EP2824209A1; CN104160052B; US20150027602A1; JP2013185242A; KR101609967B1

Abstract

The steel sheet for hot press according to the present invention has a predetermined chemical composition and has an average circle equivalent diameter of a Ti-containing precipitate contained in a steel sheet having a circle equivalent diameter of 30 nm or less, The total amount of Ti satisfies the relation of the following formula (1), and the total percentage of bainite and martensite is not less than 80% by area of the metal structure.

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

Description

TECHNICAL FIELD [0001] The present invention relates to a hot-rolled steel sheet, a press-formed article, and a method of manufacturing a press-formed article,

TECHNICAL FIELD [0001] The present invention relates to a hot press steel sheet which is used for manufacturing structural parts of automobiles and which is suitable for hot press forming, and a press-formed article obtained from such a hot press steel sheet and a method for producing press- A steel sheet and a press-formed product for hot-pressing which are useful for application to a hot press forming method in which a predetermined strength is obtained by performing heat treatment at the same time when a shape is formed (blank) in a predetermined shape, &Lt; / RTI >

As one of countermeasures to improve fuel efficiency of automobiles in global environmental problems, weight reduction of the vehicle body is progressing, and it is necessary to strengthen the steel sheet used for automobiles as much as possible. On the other hand, if the strength of the steel sheet is increased, the shape accuracy at the time of press forming is lowered.

As a result, the steel sheet is heated to a predetermined temperature (for example, a temperature at which it becomes an austenite phase) to lower its strength and then molded in a metal mold having a lower temperature (for example, room temperature) than the steel sheet. At the same time, a hot press forming method of performing quenching (quenching) using the temperature difference between the both to secure the strength after molding is employed in part production. Such a hot press forming method is called various names such as a hot forming method, a hot stamping method, a hot stamp method, and a die kenning method in addition to a hot press method.

Brief Description of Drawings Fig. 1 is a schematic view showing a mold construction for carrying out the hot press forming as described above. In the figure, reference numeral 1 denotes a punch, 2 denotes a die, 3 denotes a blank holder, 4 denotes a steel plate (blank) Rp is a die shoulder radius, and CL is a punch / die clearance, respectively. Of these components, the punch 1 and the die 2 are respectively provided with passages 1a and 2a through which a cooling medium (for example, water) can pass, and the cooling medium So that these members are cooled.

Using this metal mold for hot press forming when (e. G., Hot deep drawing), steel plate (blank) (4), two or more of A _c3 transformation point temperature or sangyeok reverse phase of (A _c1 transformation point ~A _c3 transformation point) The molding is started in a softened state by heating at a temperature. That is, the steel plate 4 is pushed into the hole of the die 2 by the punch 1 while the steel plate 4 in a high temperature state is sandwiched between the die 2 and the blank holder 3, 4 are formed in a shape corresponding to the outer shape of the punch 1 while reducing the outer diameter of the punch 1. The punch 1 and the die 2 are cooled in parallel with the molding to heat the mold from the steel plate 4 to the mold 1 (punch 1 and die 2) The material is quenched by further holding and cooling at a point (a state where the tip of the punch is located at the deepest portion: the state shown in Fig. 1). By performing such a molding method, a molded product having a high dimensional accuracy of 1,500 MPa can be obtained, and the molding load can be reduced as compared with the case of forming a part having the same strength class in the cold, so that the capacity of the press machine becomes small .

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

As a steel sheet for hot press that 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 a wide range, it is possible to adjust the basic strength class of each steel sheet, introduce ferrite with high deformability, and reduce the average grain size of ferrite and martensite to improve the elongation . These techniques are effective for improving the elongation, but still insufficient from the viewpoint of improvement of elongation according to the strength of the steel sheet. For example, a tensile strength TS of 1470 MPa or more and an elongation percentage EL of 10.2% at the maximum are required, and further improvements are required.

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

Particularly in recent years, development of techniques for imparting a difference in strength within a single component is underway. As such a technique, a technique in which a portion to be prevented from deformation is high strength (high strength side: impactor side), a portion where energy absorption is required is low strength and high flexibility (low strength side: energy absorbing side) . For example, in a vehicle of a medium-sized or larger-sized vehicle, taking into consideration thecompatibility (a function of protecting the other side when a small car collides) at the time of a side collision or a rear collision, And an energy-absorbing region. (A) a method of joining a steel sheet to a steel sheet for a conventional hot press at a same temperature by heating or die quenching (a tailored weld blank: TWB), (b) cooling in a mold (C) a method of applying a difference in heating temperature between regions of the steel sheet to give a difference in strength, and the like have been proposed.

In these techniques, a tensile strength of 1500 MPa is attained on the high-strength side (impact-resistant side), but a maximum tensile strength of 700 MPa and an elongation EL of 17% It is required to realize high ductility with a higher strength.

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

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 hot press molded article which can achieve a balance of high strength and elongation at a high level when uniform properties are required in a molded article, A steel sheet for hot press which is useful for obtaining a press molded article which can achieve a balance of high strength and elongation at a high level according to each region when a region corresponding to an impact region and an energy absorbing region is required, And a useful method for producing such a hot press molded article.

The steel sheet for hot pressing according to the present invention, which has achieved the above object,

C: 0.15 to 0.5% (meaning% by mass, the same applies 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: 0.0010 to 0.01%

Respectively, the balance being composed of iron and unavoidable impurities,

The steel sheet according to any one of claims 1 to 3, wherein the Ti-containing precipitates contained in the steel sheet have an average circle equivalent diameter of not more than 30 nm and a circle equivalent diameter of not less than 3 nm, And the total fraction of bainite and martensite is not less than 80% by area. The term " circle equivalent diameter " refers to a diameter when the size (area) of a Ti-containing precipitate (for example, TiC) Average value).

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

In the steel sheet for hot press forming according to the present invention, if necessary, (a) 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% (excluding 0%), (c) at least one selected from the group consisting of Mg, Ca and REM (Not including 0%) in the total amount of not more than 0.01% is also useful, and the characteristics of the hot press molded article are further improved depending on the kind of the element contained.

The term method of manufacturing a press molded article of the present invention that could achieve the above object, using a hot press steel sheet of the present invention as described above, A _c1 transformation point + more than 20 ℃, A _c3 transformation point temperature below -20 ℃ , And press molding is started. After molding and after the molding is completed, the mold is cooled to a temperature lower than the bainite transformation start temperature (Bs) by 100 占 폚 while securing an average cooling rate of 20 占 폚 / .

In the press-formed article obtained by this manufacturing method, the metal structure is composed of 3 to 20% by area of retained austenite, 30 to 87% by area of annealing martensite and / or annealing bainite, 10 to 67% , And the amount of carbon in the retained austenite is 0.60% or more, so that a balance of high strength and elongation can be achieved as a uniform property at a high level in a molded article. The area ratio of the annealed martensite and / or the annealed bainite is the total area ratio of both the structures when both the structures are included and the area ratio of the structure when the structure is formed of either one.

On the other hand, another manufacturing method of the press-molded article of the present invention, which has been able to achieve the above object, is a method of manufacturing a hot-press steel sheet according to the present invention as described above by dividing a heating region of a steel sheet into two regions, Is heated to a temperature of not less than the A _c3 transformation point and not more than 950 ° C and the other region is heated to a temperature not less than the A _c1 transformation point + 20 ° C and not more than the A _c3 transformation point -20 ° C, , And cooling the mold to a temperature not higher than the martensitic transformation starting temperature (Ms) while securing an average cooling rate of not lower than 20 deg. C / sec in the mold.

In the press-formed article obtained by this manufacturing method, the metal structure is composed of a first region where the retained austenite is 3 to 20% by area and a martensite is 80% by area or more and a second region where the metal structure is retained austenite by 3 to 20% Martensite and / or annealing bainite: 30 to 87 area%, quenched martensite: 10 to 67 area%, and the carbon content in the retained austenite is 0.60% or more. Therefore, a balance between the high strength and the elongation can be achieved at a high level, and a region corresponding to the impact-resistant portion and the energy-absorbing portion exists in a single molded product.

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 deposition rate thereof for Ti not forming TiN, and to provide a tempering hard phase Site, bainite, and the like) and a steel sheet in which the ratio of the hard phase (quenched state martensite phase) to the retained austenite phase is adjusted. Therefore, by hot pressing the steel sheet under predetermined conditions, the balance of strength- It can be at a high level. In addition, when hot pressing is performed under different conditions in a plurality of regions, it is possible to form an impact-resistant portion and an energy-absorbing portion in a single molded product, and balance of high strength and elongation at each portion can be achieved at a high level.

BRIEF DESCRIPTION OF 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 product, the steel sheet for hot press forming, which can realize a press molded product exhibiting good ductility (elongation) , It has been studied from various angles.

As a result, it is possible to precisely define the chemical composition of the steel sheet for hot press forming, to control the size of the Ti-containing precipitate and the amount of precipitated Ti, and if the metal structure is appropriate, (Residual ductility) is increased by securing a predetermined amount of retained austenite after press forming, 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, but 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 high level of balance between high strength and elongation when a uniform property is required in a molded product or when a region corresponding to an impact resistant portion and an energy absorbing portion is required in a single molded product, · It is an important element in securing retained austenite in high ductility site. Further, at the time of heating in hot press forming, C is concentrated in austenite, so that 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, when the C content is excessive and exceeds 0.5%, the bimetallic heating zone is narrowed, 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 in which a predetermined amount of annealing martensite and / or annealing bainite is secured) in a region where an impact region and an energy absorbing region are required, particularly in a low strength and high ductility region It becomes difficult. The lower limit of the C content is preferably 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 has the effect of forming residual austenite by suppressing the formation of cementite or decomposition of untransformed austenite by tempering martensite during cooling of the mold quenching. In order to exhibit such an effect, the Si content needs to be 0.2% or more. In addition, when the Si content exceeds 3% and exceeds 3%, ferrite tends to form, and it becomes difficult to form a single phase at the time of heating, and a necessary fraction of bainite and martensite in the hot press steel sheet can not be secured. 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 quenching property 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 only the characteristics are considered, it is preferable that the Mn content is large, but since the cost of adding the alloy increases, 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 preferable upper limit is 2.5% or less (more preferably 2.0% or less).

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

P is an element that is inevitably included in the steel but deteriorates ductility, so that it is desirable to reduce P as much as possible. However, extreme reductions lead to an increase in steelmaking costs, and it is difficult to make 0%, so it 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 which is inevitably included in the steel as in P, and the ductility is deteriorated. Therefore, it is preferable that S is reduced as much as possible. However, extreme reductions lead to an increase in steelmaking costs, and it is difficult to make 0%, so it 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 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, when the Al content is excessive and exceeds 1%, Al ₂ O ₃ is excessively generated, 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 is, because of the effect of suppressing ferrite transformation, pearlite transformation, and bainite transformation in the high intensity portion side, the cooling after heating to a temperature of 2 sangyeok (A _c1 transformation point ~A _c3 transformation point), the ferrite, pearlite, 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 exhibits an improvement effect of hardness by fixing N and keeping B in a solid state. In order to exhibit such an effect, it is important to contain more than 0.01% of Ti and N than the stoichiometric ratio [3.4 times the content of N]. However, if the Ti content is excessively increased to more than 3.4 [N] + 0.1%, the Ti-containing precipitate to be formed is finely dispersed to inhibit the growth of martensite during cooling after the bimetallic heating, Lath-shaped martensite) is formed, the discharge of carbon (C) to the retained austenite between the laths is slowed, and the amount of carbon in the retained austenite 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% [N] + 0.08% or less).

[N: 0.001 to 0.01%]

N is an element which is inevitably incorporated, but it is preferable to reduce it. However, since there is a limit to reduction in the actual process, the lower limit is 0.001%. If the N content is excessive, ductility is deteriorated due to deformation aging, or BN is precipitated to lower the effect of improving the quenching property by the solid solution B, so that the upper limit is set at 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 composition of the steel sheet for hot pressing according to the present invention is as described above, and the remaining amount is iron and unavoidable impurities (for example, O, H, etc.) other than P and S. (A) 0.1% or less in total (excluding 0%) of at least one selected from the group consisting of V, Nb and Zr, (b) Cu, At least one element selected from the group consisting of Ni, Cr and Mo in an amount of 1% or less in total (excluding 0%), (c) at least one element selected from the group consisting of Mg, Ca and REM (rare earth element) 0.01% or less in total (not including 0%), and the like are also useful, and the properties of the hot press steel sheet are further improved depending on the kind of element contained. The preferable range for containing these elements and the reason for limiting the range are as follows.

[0.1% or less (not including 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, a coarse carbide is formed and becomes a starting point of fracture, thereby deteriorating ductility. For these reasons, 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.

[1% or less in total (excluding 0%) of at least one selected from the group consisting of Cu, Ni, Cr and Mo]

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, 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, 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. Further, 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, the steel sheet is preferably 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.

[Not more than 0.01% (excluding 0%) of at least one selected from the group consisting of Mg, Ca and REM]

These elements make the inclusions finer and thus act effectively in improving the ductility. In order to exhibit such effects, it is preferable that the total amount is 0.0001% or more. Considering only the characteristics, the content is preferably as large as possible, but the effect is saturated, so that 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, it is preferable that (A) the Ti-containing precipitates contained in the steel sheet have a circle equivalent diameter of 30 nm or less and an average circle equivalent diameter of 3 nm or more, (B) The relationship of 3.4 [N] > 0.5 x [total Ti amount (mass%) - 3.4 [N]] And the total fraction of bainite and martensite is not less than 80% by area.

If excess Ti is present in the steel sheet prior to hot pressing and finely dispersed or mostly present in a solid state, a large amount of Ti is present in a fine state during heating of the hot press. Then, in the martensitic transformation occurring during quenching in the mold after heating, the growth of the martensitic grains in the longitudinal direction is inhibited, and the growth in the width direction is promoted, thereby reducing the aspect ratio. As a result, the carbon discharge from the martensitic lath to the surrounding retained austenite is delayed, the amount of carbon in the retained austenite is reduced, and the stability of the retained austenite is lowered, so that the effect of improving the elongation can not be sufficiently obtained.

From this point of view, it is necessary to disperse the Ti-containing precipitates to a large extent. For this purpose, the Ti-containing precipitates contained in the steel sheet must have an average circle equivalent diameter of 3 nm or more in a circle equivalent diameter of 30 nm or less The requirements of (A) above]. The reason why the circle-equivalent diameter of the target Ti-containing precipitate is specified to be 30 nm or less is that the Ti-containing precipitate except for TiN, which does not affect the texture change or characteristics, It is necessary to control. The size of the Ti-containing precipitate (the average circle-equivalent diameter of Ti-containing precipitates having a circle-equivalent diameter of 30 nm or less) is preferably 5 nm or more, and more preferably 10 nm or more. The Ti-containing precipitates to be subjected in the present invention include not only TiC and TiN but also precipitates containing Ti such as TiVC, TiNbC, TiVCN, and TiNbCN.

Further, in the hot-press steel sheet, it is necessary that most of Ti other than those used for precipitation-fixing N in Ti is present in a precipitated state. In order to do so, the amount of Ti present as a precipitate other than TiN (i.e., the amount of precipitated Ti (mass%) - 3.4 [N]) is larger than 0.5 times the remainder obtained by subtracting Ti forming TiN from the total Ti (Total Ti amount (mass%) - 3.4 (N)] (the requirement of (B) above). The amount of precipitated Ti (mass%) - 3.4 [N] is preferably 0.6 x [total Ti amount (mass%) - 3.4 [N]] or more, more preferably 0.7 x -3.4 [N]].

The metal structure is originally a control necessary for achieving a desired strength-elongation balance in a molded product. However, the metal structure can not be controlled only by the hot press condition, and the structure of the raw steel (hot press steel sheet) It is necessary to do. In the molded steel sheet, it is necessary to set the total fraction of bainite and martensite in the steel sheet to not less than 80% by area in order to obtain a proper amount of annealing martensite and annealing bainite which are fine and have a large contribution to ductility. When the total fraction of bainite and martensite is less than 80% by area, it becomes difficult to secure the aimed annealing martensite and / or annealing bainite fraction, and the amount of other structure (for example, ferrite) - It lowers ductility balance. The total fraction of bainite and martensite is preferably 90% by area or more, and more preferably 95% by area or more.

In the steel sheet for hot press according to the present invention, the remaining amount of the metal structure is not particularly limited, but may be, for example, at least one of ferrite, pearlite and 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 cast piece, in which the steel material having the above chemical composition is dissolved, is heated at a heating temperature of 1100 DEG C or higher (preferably 1150 DEG C or higher) The hot rolling is performed at 1300 占 폚 or lower (preferably 1250 占 폚 or lower), the finish rolling temperature is 750 占 폚 or higher (preferably 780 占 폚 or higher) and 850 占 폚 or lower (preferably 830 占 폚 or lower) After cooling (slow cooling: intermediate cooling) so as to keep the temperature within 700 to 750 占 폚 (preferably 720 to 740 占 폚) for at least 10 seconds (preferably at least 50 seconds) (Preferably at least 150 deg. C) and not more than 450 deg. C (preferably not more than 400 deg. C) to 20 deg. C / .

The method comprises the steps of: (1) finishing rolling in a temperature region where a potential introduced by a hot rolling in austenite remains, and (2) annealing immediately after that to form a Ti-containing precipitate such as TiC on the potential, , (3) re-quenched and rewound to control bainite transformation or martensitic transformation.

The steel sheet for hot pressing 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. When the reduction rate is 10 to 80% (preferably 20 to 70% The cold rolling may be performed. The steel sheet for hot press or the cold rolled steel sheet is heated to a temperature of not more than 450 ° C (preferably not more than 400 ° C) after being heated to a temperature range (1000 ° C or less; for example, 870 to 900 ° C) The substrate is quenched at a cooling rate of 20 deg. C / sec or more (preferably 30 deg. C / sec or more) and then held at 450 deg. C or less for 10 seconds or more, 1000 seconds or less, or tempered at 450 deg. Heat treatment may be performed. 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 (surface of the base steel sheet).

After the steel sheet for hot pressing as described above is heated to a temperature of A _c1 transformation point + 20 ° C or higher and an A _c3 transformation point -20 ° C or lower, press molding is started, and 20 (Hereinafter sometimes referred to as a single-zone molded article) having a single characteristic can be obtained by cooling to a temperature lower than the bainite transformation start temperature (Bs) by 100 占 폚 while securing an average cooling rate of not less than 占 폚 / Also, it can be made into an optimal structure by making it highly ductile. The reason for specifying each of the requirements in this molding method is as follows.

In order to form annealed martensite or annealed bainite having excellent ductility by forming austenite between martensite or lath of bainite in a steel sheet and annealing martensite or bainite, the heating temperature is in a predetermined range . &Lt; / RTI > When the heating temperature of the steel sheet is less than the A _c1 transformation point + 20 캜, a sufficient amount of austenite is not 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 A _c3 transformation point of -20 캜, the amount of transformation to austenite during heating is excessively increased to secure a predetermined amount of annealing martensite or annealing bainite in the final structure Can not.

It is necessary to appropriately control the average cooling rate and the cooling termination temperature during and after the 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 changing the cooling end temperature to the bainite transformation start temperature (Bs) or lower, bainite or martensite can be obtained by transforming the austenite present at the time of heating into bainite or martensite while inhibiting formation of a structure such as ferrite or pearlite While maintaining fine austenite between the bainite and the lath of martensite to secure a predetermined amount of retained austenite.

When the cooling end temperature is higher than the temperature lower than the bainite transformation start temperature (Bs) by 100 占 폚 or the average cooling rate is lower 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 lowered.

Control of the average cooling rate is basically unnecessary at a 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 completion of molding can be performed by controlling the temperature of the molding die (the cooling medium shown in Fig. 1) or (b) controlling the thermal conductivity of the mold Can be achieved.

In the press-formed article produced by the hot press as described above, the metal structure is composed of 3 to 20% by weight of retained austenite, 30 to 87% by area of annealing martensite and / or annealing bainite, To 67% by area, and the amount of carbon in the retained austenite is 0.60% 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 requirements (basic structure and amount of carbon in retained austenite) in such hot press formed products is as follows.

The retained austenite is transformed into martensite during the plastic deformation, thereby increasing the work hardening rate (transformational organic calcination) and improving the ductility of the press-molded article. 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 ensured 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).

The ductility (elongation percentage) of the press-molded article can be increased while securing a predetermined strength by making the main structure into an annealing martensite and / or an annealing bainite which is fine and has a low dislocation density. From this viewpoint, the fraction of the annealed martensite and / or the annealed bainite is 30% or more by area. However, if this fraction exceeds 87% by area, the fraction of the retained austenite is insufficient and the ductility (residual ductility) is lowered. The preferable lower limit of the percentage of the annealed martensite or annealed bainite is 40% or more (more preferably 50% or more) and the preferable upper limit is less than 80% (more preferably, less than 70%).

Quenching state Since martensite is a structure with insufficient ductility, if it exists in a large amount, the elongation rate is lowered. However, in order to realize a high strength of more than 100 kg in a matrix having a low strength such as annealing martensite, It is necessary to secure a predetermined amount. From this viewpoint, the fraction of the quenched state martensite is 10 percent by area or more. However, if the fraction of the quenched state martensite becomes too large, the strength becomes too high and the elongation becomes insufficient. Therefore, the fraction should be 67% or less by area. The preferable lower limit of the fraction of the quenched state martensite is at least 20% by area (more preferably at least 30% by area), and the preferable upper limit is at most 60% by area (more preferably at most 50% by area).

Apart from the above-mentioned structure, ferrite, pearlite, bainite and the like can be included as the residual amount structure. However, it is preferable that these tissues are basically not contained, because their contribution to strength and contribution to ductility are lower than those of other tissues 0 area%). However, it is acceptable if it is up to 20% by area. The residual portion structure is more preferably 10 percent by area or less, and more preferably 5 percent by area or less.

The amount of carbon in the retained austenite affects the timing at which the retained austenite undergoes transformational organic transformation into martensite at the time of the deformation such as the tensile test and the more the carbon amount is, the more the transformation organic transformation (TRIP) . In the process of the present invention, during cooling, carbon is discharged from the formed martensitic rust into the surrounding austenite. At this time, if Ti carbide or carbonitride dispersed in the steel is dispersed in a large amount, the growth in the longitudinal direction of the martensitic phase progresses without being inhibited, resulting in a martensiticite having a narrow width and a long aspect ratio. As a result, carbon is liable to be discharged in the width direction from the martensitic lath, so that the amount of carbon in the retained austenite increases, and ductility is improved. From this viewpoint, in the press-molded article of the present invention, the amount of carbon in the retained austenite in the steel is specified to be 0.60% or more. The amount of carbon in the retained austenite can be increased to about 0.70%, but is limited to about 1.0%.

By using the steel sheet for hot press according to the present invention, properties such as strength and elongation of a press-formed article can be controlled by suitably adjusting press molding conditions (heating temperature and cooling rate) (For example, an energy absorbing member), which is difficult to apply in the conventional press-molded articles, and is extremely useful in broadening the application range of the press-molded article. In addition to the above-mentioned single-domain molded product, when a press-formed product is manufactured by press-forming a steel sheet 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 is obtained.

In the production of the multi-region molded product as described above using the steel sheet for hot press 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) the a _c3 transformation point or more, heating to with also heated to a temperature not higher than 950 ℃, the other area (hereinafter, the term second region), the a _c1 transformation point + more than 20 ℃, a _c3 transformation point temperature below -20 ℃ , Press molding is started with respect to both the first and second regions, and during the molding and after the molding is completed, martensitic transformation starts to be carried out in both the first and second areas while securing an average cooling rate of 20 deg. C / It may be cooled to a temperature not higher than the temperature (Ms).

In the above method, a press-molded article exhibiting a strength-ductility balance according to each region is obtained by dividing the heating region of the steel sheet into two regions (high strength side region and low strength side region) . The second region of the two regions corresponds to the low-strength side region, and the manufacturing conditions, structure 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 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 the 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 a desired structure can be obtained in the region of the final hot press molded article have. When the steel sheet heating temperature in this region is less than the A _c3 transformation point, a sufficient amount of austenite is not 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 the austenite increases at the time of heating, the martensitic transformation start temperature (Ms point) and the martensitic transformation end temperature (Mf point) rise, Austenite can not be ensured and good formability is not achieved. The heating temperature of the steel sheet is preferably A _c3 transformation point + 50 ° C or higher, and 900 ° C or lower.

It is necessary to appropriately control the average cooling rate and the cooling termination temperature during and after the 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 setting the cooling end temperature to the martensitic transformation starting temperature (Ms point) or less, martensite is secured by transforming the austenite present at the time of heating into martensite while inhibiting formation of a structure such as ferrite or pearlite. The cooling termination 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 is composed of residual austenite: 3 to 20% by area (the effect of the retained austenite is the same as described above) and martensite: 80% by area or more. In the second region, the carbon content in the same metal structure and retained austenite as the single-domain molded product satisfies 0.60% or more.

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 the hot press-molded product 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). As a structure in the first region, a part of the structure may contain ferrite, pearlite, bainite, or the like.

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 the light of the above and below are included in the technical scope of the present invention .

Example

[Example 1]

Steel materials (steel Nos. 1 to 32) having the chemical composition shown in the following Table 1 were vacuum-melted and turned into experimental slabs, followed by hot rolling to form steel sheets, followed by cooling and simulating winding Plate thickness: 3.0 mm). In the method of the wrapping simulation, a sample was put into a furnace heated to a coiling temperature after cooling to a coiling temperature, maintained for 30 minutes, and then cooled. The steel sheet manufacturing conditions at this time are shown in Table 2 below. The A _c1 transformation point, the A _c3 transformation point, the Ms point and the Bs point in Table 1 are obtained by using the following expressions (2) to (5) (see, for example, "Leslie Steel Materials" Maruzen ), (1985)). The treatments (1) to (3) shown in the remarks column of Table 2 were obtained by the following treatments (rolling, cooling and alloying).

[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. In the case where the elements shown in the respective terms of the above-mentioned formulas (2) to (5) are not included, the calculation is made without the term.

Process (1): After the finish rolling, the steel sheet was cooled to an average cooling rate of 50 deg. C / second up to 650 deg. C and then cooled for 10 seconds at an average cooling rate of 5 deg. C / second from 650 deg. / Sec < / RTI > Then, in order to match the plate thickness with the treatments (2) and (3), the front and back surfaces were polished and reduced in thickness to 1.6 mm.

Process (2): The hot-rolled steel sheet was cold-rolled and then subjected to continuous annealing. After heating to 860 캜, the steel sheet was cooled to 400 캜 at an average cooling rate of 30 캜 / sec and held.

(3): After cold-rolling the hot-rolled steel sheet, the hot-rolled steel sheet was heated to 860 ° C to simulate the continuous hot-dip galvanizing line, cooled to 400 ° C at an average cooling rate of 30 ° C / After heating for 10 seconds, it was cooled.

For the obtained steel sheet, analysis of the precipitation state of Ti and observation of metal structure (fraction of each structure) were carried out in the following manner. The results are shown in Table 3 together with the calculated value of 0.5 占 total Ti amount (mass%) - 3.4 [N] (0.5 占 total Ti amount -3.4 [N]).

[Analysis of the precipitation state of Ti in the steel sheet]

An extract replica sample was prepared and a transmission electron microscope image (magnification: 100,000 times) of the Ti-containing precipitate was photographed with a transmission electron microscope (TEM). At this time, the composition of the precipitate was analyzed by an energy dispersive X-ray spectroscope (EDX) to identify the i-containing precipitate. An area of at least 100 Ti-containing precipitates was measured by image analysis, and a sample having a circle-equivalent diameter of 30 nm or less was extracted, and the average value was determined as the precipitate size. In the table, "average circle equivalent diameter of Ti-containing precipitates" is shown. 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 (in the extraction treatment, (Mass%) - 3.4 [N] (in Table 3, the amount of precipitated Ti is indicated by 3.4 [N]). In the case where the Ti-containing precipitate partially contains V or Nb, the content thereof is also measured.

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

(1) For the structure of martensite and bainite in the steel sheet, martensite and bainite were identified by SEM (magnification: 1,000 or 2,000 magnifications) and the respective fractions (Area ratio).

(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).

For each of the above steel plates (1.6 mm ^t x 150 mm x 200 mm) [with the exception of the above treatments (1) to (3), the thickness was adjusted to 1.6 mm by hot rolling) After heating, press molding and cooling treatment were carried out in a hat-shaped mold (Fig. 1) to obtain a press-molded product. Table 4 shows press molding conditions (heating temperature at the time of press molding, average cooling rate, and rapid cooling termination temperature).

Tensile strength (TS), elongation (elongation elongation EL), and observation of metal structure (fraction of each structure) of the obtained molded article were measured by the following methods.

[Measurement of tensile strength (TS) and elongation (EL elongation)] [

The tensile strength (TS) and the 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 to 1179 MPa and the elongation (EL) is 20% or more and the strength-elongation balance (TS x EL) is 24000 (MPa.

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

(1) Annealing in steel sheets For the structure of martensite, bainite and annealed bainite, the steel sheet was corroded by separation and SEM (magnification: 1,000 or 2,000 times) was observed, and annealed martensite, bainite, The annealing bainite was discriminated to obtain the respective fractions (area ratios).

(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). At this time, the amount of carbon in the retained austenite was also measured.

(3) Quenched state With regard to the martensite fraction of the quenched state, the steel sheet was subjected to Repera erosion and the white contrast was determined to be a mixed structure of quenched martensite and retained austenite, and the area ratio was measured. The knot fraction was subtracted to calculate the quenched state martensite fraction.

Observation results (fractions of the respective tissues) of the metal tissues are shown in Table 5 below. The mechanical properties (tensile strength TS, elongation EL and TS EL) of the molded article are shown in Table 6 below.

From these results, it can be considered as follows. The steel Nos. 1, 2, 4, 5, 11 to 13, 15 to 17, 19 to 21 and 23 to 32 satisfy the requirements specified in the present invention. It can be seen that it is obtained.

On the other hand, the steel Nos. 3, 6 to 10, 14, 18, and 22 are comparative examples that do not satisfy any one of the requirements specified in the present invention, and one of the properties is deteriorated. That is, the steel No. 3 is made of a steel sheet having a small Si content, so that the retained austenite fraction in the molded product is not ensured and the amount of carbon in the retained austenite is reduced, so that the elongation is not obtained. In the case of steel No. 6, the heating temperature at the time of molding was high, only a low elongation EL was obtained, and the strength-elongation balance (TS 占 EL) also deteriorated.

In the case of steel No. 7, the average cooling rate at the time of press forming was slow, pearlite and ferrite were generated and the quenched martensite fraction could not be secured, and the strength-elongation balance (TS x EL) was deteriorated. (TS EL) of the steel No. 8 can not obtain only the low elongation EL because the rapid cooling termination temperature is high and pearlite and ferrite are generated and the quenched martensite fraction can not be secured. Is also deteriorated.

Steel Nos. 9 and 10 are not suitable for the steel material production, and the amount of precipitated Ti is insufficient (Nos. 9 and 10), Ti-containing precipitates are small (Steel No. 10) , The strength-elongation balance (TS x EL) is deteriorated even if the molding conditions are appropriate.

Steel No. 14 had a steel sheet having a metal structure of ferrite and pearlite of 100% by area due to the coiling temperature, and the annealed martensite and / or annealed bainite fraction in the molded product could not be secured, (TS EL) is deteriorated. Steel No. 18 was made of a steel sheet having an excess of C content and had only a low elongation EL because of its high strength. Steel No. 22 was a steel sheet having an excessive Ti content, and the strength-elongation balance (TS 占 EL) was deteriorated.

[Example 2]

Steel materials (steel Nos. 33 to 37) having the chemical composition shown in the following Table 7 were vacuum-melted and turned into experimental slabs, then subjected to hot rolling, and then cooled and wound (sheet thickness: 3.0 mm). The steel sheet manufacturing conditions at this time are shown in Table 8 below.

Analysis of the precipitation state of the Ti-containing precipitates and observation of the metal structure (fraction of each structure) were performed on the obtained steel sheet in the same manner as in Example 1. The results are shown in Table 9 below.

Each of the steel plates (3.0 mm ^t x 150 mm x 200 mm) was heated to a predetermined temperature in a heating furnace, and then press molded and cooled in a hat-shaped mold (Fig. 1) . At this time, the steel sheet is placed in the infrared ray path so that the portion to be intensified (the portion of the steel sheet corresponding to the first region) is brought into direct contact with the infrared ray so as to be heated at a high temperature, A corresponding steel plate portion) was covered with a lid so as to block a part of infrared rays so as to be able to be heated at a low temperature. Therefore, the molded article has regions having different intensities in a single component. Table 10 shows press molding conditions (heating temperature, average cooling rate, and rapid cooling termination temperature in each region at the time of press molding).

The tensile strength (TS), elongation (elongation percentage EL), observation of metal structure (fraction of each structure), and amount of carbon in retained austenite in each region of the obtained molded article were determined in the same manner as in Example 1.

Observation results (fractions of the respective tissues) of the metal tissues are shown in Table 11 below. The mechanical properties (tensile strength TS, elongation EL and TS EL) of the molded product are shown in Table 12 below. 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 占 EL) was 14000 (MPa 占%) or more Evaluation criteria on the strength side are the same as in Example 1).

From this result, it can be considered as follows. Steel Nos. 33, 35 and 37 are examples satisfying the requirements specified in the present invention, and it can be seen that a component having good strength-ductility balance in each region is obtained.

On the other hand, the steels Nos. 34 and 36 are comparative examples that do not satisfy any one of the requirements specified in the present invention, and any one of the characteristics is deteriorated. That is, in the case of steel No. 34, the heating temperature at the time of press forming is low, and the strength at the high strength side is lowered. Steel No.36 was made of a steel sheet having a small Ti-containing precipitate. Only a low strength was obtained on the high strength side, and the strength-elongation balance (TS 占 EL) on the low strength side was deteriorated.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to those skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the invention.

The present application is based on Japanese Patent Application (Japanese Patent Application No. 2012-053844) filed on March 9, 2012, the content of which is incorporated herein by reference.

INDUSTRIAL APPLICABILITY The present invention is suitable for a hot-press steel sheet used for manufacturing a structural component of an automobile.

1: punch
2: Die
3: Blank holder
4: Steel plate (blank)

Claims

C: 0.15 to 0.5% (meaning% by mass, the same applies 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 composed of iron and unavoidable impurities,
The steel sheet according to any one of claims 1 to 3, wherein the Ti-containing precipitates contained in the steel sheet have an average circle equivalent diameter of not more than 30 nm and a circle equivalent diameter of not less than 3 nm, Wherein the total content of bainite and martensite is 80% by area or more.

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

The steel sheet for hot pressing according to claim 1, further comprising at least one of the following (a) to (c) as another element.
(a) 0.1% or less in total (excluding 0%) of at least one 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 (excluding 0%) of at least one element selected from the group consisting of Mg, Ca and REM;

Claim 1 or by using for hot press steel sheet according to 2, A _c1 transformation point + more than 20 ℃, A _c3 transformation point and then heated to a temperature not higher than -20 ℃, discloses a press forming, and of and forming end formed after Is cooled to a temperature lower than the bainite transformation start temperature (Bs) by 100 占 폚 while securing an average cooling rate of 20 占 폚 / second or more in the mold.

A press-molded article obtained by the manufacturing method according to claim 3, wherein the metal structure is composed of 3 to 20% by area of retained austenite, 30 to 87% by area of annealing martensite and / or annealing bainite, To 67% by area, and the carbon content in the retained austenite is 0.60% or more.

A steel sheet for hot pressing as claimed in any one of claims 1 to 3, wherein the heating region of the steel sheet is divided into two regions and one region thereof is heated to a temperature not lower than the Ac ₃ transformation point and not higher than 950 ° C, Is heated to a temperature of A _c1 transformation point + 20 ° C or higher and an A _c3 transformation point -20 ° C or lower, press molding is started, and an average cooling rate of 20 ° C / (Ms) of the martensite transformation starting temperature while securing the martensite transformation starting temperature (Ms).

A press-formed article obtained by the manufacturing method according to claim 5, wherein the metal structure is composed of a first region where the residual austenite is 3 to 20% by area and the martensite is not less than 80% by area and the first region where the metal structure is retained austenite: 20% by area, annealed martensite and / or annealed bainite: 30 to 87% by area, quenched martensite: 10 to 67% by area, and a carbon content in the retained austenite of 0.60% Wherein the press-molded article is a press-molded article.