HOT PRESS MOLDING AND MANUFACTURING METHOD THEREFOR

Abstract

A hot press molding including: a first forming region exhibiting a metal structure, which contains 80-97 area % of martensite and 3-20 area % of retained austenite, respectively, and in which the residual structure is 5 area % or less; and a second forming region exhibiting a metal structure, which contains 70-97 area % of bainitic ferrite, 27 area % or less of martensite, and 3-20 area % of retained austenite, respectively, and in which the residual structure is 5 area % or less. As a result, hot press moldings, which have at least a region corresponding to a shock-resistant area and a region corresponding to an energy-absorbing area in a single molding and in which a high level of balancing of high strength with elongation according to the respective region can be achieved, are provided without using a welding method.

Description

TECHNICAL FIELD

The present invention relates to a hot press molded article which is used for a structural member of an automobile component and whose strength and ductility can be adjusted according to different regions in the molded article and to the method for producing such a hot press molded article. More specifically, the present invention relates to a hot press molded article exhibiting strength and ductility according to different regions in such a manner that when a pre-heated steel plate (blank) is molded into a predetermined shape, thermal treatment is applied at the same time as shaping, and also relates to the useful method for producing such a hot press molded article.

BACKGROUND ART

Development has been made to reduce the weight of vehicle bodies as one of the measures for improvement of the fuel efficiency of automobiles, such a measure stemming from global environmental issues. A highest possible strength has been required for steel plates used for the automobiles. However, with an increase in the strength of the steel plates for the purpose of reducing the weight of the automobiles, an elongation EL (elongation) and an r value (Lankford value) decrease, resulting in a lower press formability and a lower shape fixability.

In order to solve the above-described problem, the following hot press molding method has been employed for component production. After a steel plate is heated to a predetermined temperature (e.g., a temperature at which an austenite phase is exhibited) to lower strength (i.e., to facilitate molding), molding is, for shaping, performed using a die having a lower temperature (e.g., a room temperature) than that of the thin steel plate. At the same time, rapid-cooling thermal treatment (quenching) is performed using a temperature difference between the die and the steel plate, thereby ensuring strength after molding.

According to the above-described hot press molding method, since molding is performed in a low-strength state, springback is small (favorable shape fixability is obtained). In addition, since a material containing, e.g., alloy elements of Mn and B and exhibiting favorable hardenability is used, a 1500 MPa class strength in terms of tensile strength can be obtained by rapid cooling. Note that the above-described hot press molding method is, in addition to hot pressing, called as various names such as hot forming, hot stamping, a hot stamp method, and die quenching.

FIG. 1 is a schematic view illustrating a die structure for performing hot press molding (hereinafter sometimes represented by “hot stamping”) as described above. In FIG. 1, a reference numeral “1” denotes a punch, a reference numeral “2” denotes a die, a reference numeral “3” denotes a blank holder, a reference numeral “4” denotes a steel plate (blank), reference characters “BHF” denote wrinkle pressing force, reference characters “rp” denote a punch shoulder radius, reference characters “rd” denote a die shoulder radius, and reference characters “CL” denote a clearance between the punch and the die. Of these components, the punch 1 and the die 2 are formed respectively therein passages 1a, 2a through which a corresponding one of cooling media 5a, 6a (e.g., water) is able to pass. Configuration is made such that the cooling media 5a, 6a pass through these passages to cool the punch 1 and the die 2.

In hot stamping (e.g., hot deep drawing) using the above-described die, molding begins in such a state that the steel plate (blank) 4 is heated to a single-phase temperature range of an Acs transformation point or higher and then, is softened. That is, in the state in which the steel plate 4 in a high-temperature state is interposed between each pair of the die 2 and the blank holder 3, the punch 1 pushes the steel plate 4 (in the direction indicated by an arrow A) into a hole of the dies 2 (the space between the dies 2, 2 in FIG. 1). While the outer diameter of the steel plate 4 is being reduced, the steel plate 4 is molded into a shape corresponding to the outer shape of the punch 1. The punch 1 and the dies 2 are cooled in parallel with molding to draw heat from the steel plate 4 to the die (the punch 1 and the dies 2), and are further held and cooled at a lower dead point in molding (the point at which a punch tip end is positioned innermost: the state illustrated in FIG. 1) to quench the raw material (the steel plate 4). Such a molding method can be performed to obtain a 1500 MPa class molded article with favorable dimension accuracy. Moreover, since a forming load can be reduced as compared to the case of molding a component of the same strength class as the above-described molded article in a cold state, the capacity of a pressing machine can be smaller.

Steel plates containing 22MnB5 steel as a raw material have been known as widely-used current steel plates for hot stamping. These steel plates have a tensile strength of 1500 MPa and an elongation of about 6-8%, and have been applied to shock-resistant members (members least deforming and rupturing in collision). Moreover, further development has been made to increase a C content and increase strength (1500 MPa or higher, a 1800 MPa class) with 22MnB5 steel being used as a base material.

However, in current situation, steel grades other than 22MnB5 steel have been little applied, and little study has been made on steel grades and methods in order to control the strength and elongation of a component (e.g., strength reduction: a 980 MPa class, elongation enhancement: 20%) to expand the range of application beyond application to the shock-resistant members.

In passenger cars of a medium size or larger, components such as B pillars (center pillars), rear side members, and front side members may sometimes have both functions of a shock-resistant area and an energy-absorbing area, considering compatibility (the function of also protecting a collision partner in collision with a small-sized vehicle) in lateral collision or rear collision. In order to produce these members, the following method has been mainly employed: super-high tensile steel having a high strength of a 980 MPa class and high tensile steel having an elongation of a 440 MPa class are, for example, laser-welded (form a tailor welded blank (TWB)) to press-mold the TWB in a cold state. However, in recent years, development has been made on the technique of forming, by hot stamping, a component with portions different from each other in strength.

For example, Non-Patent Document 1 proposes the method for laser-welding (forming a tailor welded blank (TWB)) 22MnB5 steel for hot stamping and a material whose strength is not increased by quenching in a die and then, performing hot stamping. Different portions are formed in a component such that a tensile strength is 1500 MPa (an elongation of 6-8%) on a high-strength side (a shock-resistant area side) and that the tensile strength is 440 MPa (an elongation of 12% or higher) on a low-strength side (an energy-absorbing area side). From a similar point of view, the technique as described in Non-Patent Document 2 has been also proposed.

In the techniques of Non-Patent Documents 1 and 2, on the energy-absorbing area side, the tensile strength is 600 MPa or lower, and the elongation is about 12-18%. It is required to perform laser-welding (form the tailor welded blank (TWB)) in advance, leading to an increase in the number of steps and an increase in a cost. Moreover, an energy-absorbing area which intrinsically does not need to be quenched is heated, and therefore, these techniques are not preferable considering heat consumption.

In addition, e.g., the techniques as described in Non-Patent Documents 3 and 4 have been also proposed as the technique of forming portions different from each other in strength in a component. In the technique described in Non-Patent Document 3, the portions different from each other in strength are formed in such a manner that a temperature difference (distribution) is given to a blank in a heating furnace. Since 22MnB5 steel is used as a base material, poor robustness of the post-quenching strength against heating in a two-phase temperature range is exhibited due to addition of boron. Moreover, it is difficult to control the strength on the energy-absorbing area side, and only an elongation of about 15% can be obtained.

On the other hand, in the technique described in Non-Patent Document 4, the portions different from each other in strength are formed in such a manner that a cooling rate is changed in a die (by heating part of the die with a heater or using materials different from each other in coefficient of thermal conductivity). Since 22MnB5 steel is used as a base material, it is not rational in terms of controlling (die cooling control) such that 22MnB5 steel intrinsically having favorable hardenability is not quenched.

CITATION LIST

Non-Patent Document

• Non-Patent Document 1: Klaus Lamprecht, Gunter Deinzer, Anton Stich, Jurgen Lechler, Thomas Stohr, Marion Merklein, “Thermo-Mechanical Properties of Tailor Welded Blanks in Hot Sheet Metal Forming Processes,” Proc. IDDRG2010, 2010.
• Non-Patent Document 2: Usibor1500P(22MnB5)/1500 MPa·8%-Ductibor500/550˜700 MPa·17% [searched on Apr. 27, 2011], Internet <http://www.arcelomittal.com/tailoredblanks/pre/seifware.pl>
• Non-Patent Document 3: 22MnB5/above AC3/1500 MPa·8%-below AC3/Hv190·Ferrite/Cementite Rudiger Erhardt and Johannes Boke, “Industrial application of hot forming process simulation,” Proc, of 1st Int. Conf. on Hot Sheet Metal Forming of High-Performance steel, ed. By Steinhoff, K., Oldenburg, M, Steinhoff, and Prakash, B., pp 83-88, 2008.
• Non-Patent Document 4: Begona Casas, David Latre, Noemi Rodriguez, and Isaac Valls, “Tailor made tool materials for the present and upcoming tooling solutions in hot sheet metal forming,” Proc, of 1st Int. Conf. on Hot Sheet Metal Forming of High-Performance steel, ed. By Steinhoff, K., Oldenburg, M, Steinhoff, and Prakash, B., pp 23-35, 2008.

SUMMARY OF THE INVENTION

Technical Problems

The present invention has been made in view of the foregoing situation, and is intended to provide hot press molded articles having, without application of welding, at least regions corresponding respectively to a shock-resistant area and an energy-absorbing area in a single molded article and exhibiting a high-level balance between high strength and elongation according to each region and to provide the useful method for producing such a hot press molded article.

Solution to Problems

The hot press molded article of the present invention capable of accomplishing the above-described objective is a hot press molded article formed by hot press molding of a thin steel plate, which includes a first molding region exhibiting a metal structure which contains 80-97 area % of martensite and 3-20 area % of retained austenite and which has a residual structure at 5 area % or less; and a second molding region exhibiting a metal structure which contains 70-97 area % of bainitic ferrite, 27 area % or less of martensite, and 3-20 area % of retained austenite and which has a residual structure at 5 area % or less.

In the hot press molded article of the present invention, the chemical component composition thereof is not limited. However, examples of the chemical component composition include a chemical component composition in which the first and second molding regions have an identical chemical component composition, and steel of each component region contains, in units of mass %, 0.15-0.3% of C, 0.5-3% of Si, 0.5-2% of Mn, 0.05% or less of P, 0.05% or less of S, 0.01-0.1% of Al, 0.01-1% of Cr, 0.0002-0.01% of B, [N]×4-0.1% of Ti, and 0.001-0.01% of N, where 0% is not inclusive for the P and the S and [N] denotes an N content in units of %, and the steel of each component region has a residual consisting of iron and an inevitable impurity.

In the hot press molded article of the present invention, it is useful that as necessary, the steel further contains, as other element, (a) one or more selected from a group consisting of Cu, Ni, and Mo in a total amount of 1% or less, where 0% is not inclusive, and that the steel further contains, as other element, (b) at least one of V or Nb in a total amount of 0.1% or less, where 0% is not inclusive. Depending on the types of elements to be contained, the characteristics of the hot press molded article are further improved.

The method of the present invention is the method for producing the above-described hot press molded article by forming a thin steel plate so as to divide the thin steel plate into a plurality of regions including at least first and second molding regions, which includes after the thin steel plate is heated to a temperature of an Acs transformation point or higher and 1000° C. or lower, starting cooling at an average cooling rate of 20° C./sec or higher and molding by pressing at least the first and second molding regions together with a die; and terminating, in the first molding region, the molding at equal to or lower than a temperature lower than a martensite transformation start temperature by 50° C., and performing, in the second molding region, the cooling to a temperature range of equal to or lower than a temperature lower than a bainite transformation start temperature by 100° C. and equal to or higher than the martensite transformation start temperature and terminating the molding after a lapse of a stay time of 10 seconds or longer within the temperature range.

Effects of the Invention

According to the present invention, in the hot press molding method, the conditions therefor are properly controlled according to each region of the molded article. This can adjust the metal structure of each region while a proper amount of retained austenite is present. Moreover, the hot press molded article can be achieved, which has a higher intrinsic ductility (residual ductility) as compared to a conventional case of using 22MnB5 steel. Further, combination of thermal treatment conditions and a pre-molded steel plate can properly control the strength and the elongation according to each region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating a die configuration for performing hot press molding.

FIG. 2 is a schematic view illustrating a die used in an example.

FIGS. 3(a) and 3(b) are schematic views illustrating the shape of a press molded article formed in the example.

DESCRIPTION OF EMBODIMENTS

The inventor(s) of the present invention has conducted study from various angles in order to realize a hot press molded article which when a thin steel plate is heated to a predetermined temperature to produce a molded article by hot press molding, exhibits strength corresponding to the demand characteristics of each of different regions and exhibits favorable ductility (elongation) after molding.

As a result, the present invention has been accomplished based on the following findings. When a thin steel plate is press-molded using a die for press molding to produce a hot press molded article, a heating temperature and the conditions for each molding region in molding are properly controlled, and the structure of each molding region is adjusted so as to contain 3-20 area % of retained austenite. This realizes a hot press molded article exhibiting a strength-elongation balance according to each molding region.

The reasons for setting the range of each structure (a basic structure) in each molding region of the hot press molded article of the present invention are as follows.

(Structures of First Molding Region)

High-strength martensite is used for a main structure of a first molding region, thereby ensuring a high strength in a particular region of the hot press molded article. From this point of view, it is necessary that the area fraction of the martensite is 80 area % or greater. However, when such a fraction exceeds 97 area %, the area fraction of retained austenite (a retained austenite fraction) is insufficient, and ductility (residual ductility) is lowered. The lower limit of the martensite fraction is preferably 83 area % or greater (more preferably 85 area % or greater), and the upper limit of the martensite fraction is preferably 95 area % or less (more preferably 93 area % or less).

The retained austenite has the effect of transforming into the martensite during plastic deformation to increase a work hardening rate (transformation-induced plasticity) and improving the ductility of the molded article. In order to produce such an effect, it is necessary that the retained austenite fraction is 3 area % or greater. A greater retained austenite fraction results in better ductility. However, in a composition used for steel plates for automobiles, the retained austenite which can be ensured is limited, and the upper limit thereof is about 20 area %. The lower limit of the retained austenite fraction is preferably 5 area % or greater (more preferably 7 area % or greater).

In addition to the above-described structures, ferrite, pearlite, bainite, etc. may be contained as a residual structure. These structures are softer than the martensite, and less contribute to the strength as compared to other structures. For this reason, these structures are preferably contained in the minimum possible amount. Note that these structures can be contained up to 5 area %. The residual structure is more preferably 3 area % or less, and much more preferably 0 area %.

Since the structures of the first molding region are formed as described above, a portion (e.g., a shock-resistant area of an automobile component) where the strength (a tensile strength TS) is 1500 MPa or greater and the elongation (a total elongation EL) is 10% or greater can be formed.

(Structures of Second Molding Region)

Since high-strength bainitic ferrite having sufficient ductility is used for a main structure of a second molding region, both of a high strength and a high ductility of the hot press molded article can be realized. From this point of view, it is necessary that the area fraction of the bainitic ferrite (a bainitic ferrite fraction) is 70 area % or greater. However, when such a fraction exceeds 97 area %, the retained austenite fraction is insufficient, and the ductility (the residual ductility) is lowered. The lower limit of the bainitic ferrite fraction is preferably 75 area % or greater (more preferably 80 area % or greater), and the upper limit of the bainitic ferrite fraction is preferably 95 area % or less (more preferably 90 area % or less).

The high-strength martensite can be partially contained to increase the strength of the hot press molded article. However, a greater amount of martensite results in a lower ductility (a lower residual ductility). From this point of view, it is necessary that the area fraction of the martensite (the martensite fraction) is 27 area % or less. The lower limit of the martensite fraction is preferably 5 area % or greater (more preferably 10 area % or greater), and the upper limit of the martensite fraction is preferably 20 area % or less (more preferably 15 area % or less).

Because of the reasons similar to those of the first molding region, the retained austenite fraction is 3 area % or greater and 20 area % or less. The preferable lower limit of the retained austenite fraction is similar to that of the first molding region.

In addition to the above-described structures, ferrite, pearlite, bainite, etc. may be contained as a residual structure. These structures are softer than the martensite, and less contribute to the strength as compared to other structures. For this reason, these structures are preferably contained in the minimum possible amount. Note that these structures can be contained up to 5 area %. The residual structure is more preferably 3 area % or less, and much more preferably 0 area %.

Since the structures of the second molding region are formed as described above, a portion (e.g., an energy-absorbing area of the automobile component) where the strength (the tensile strength TS) is 1100 MPa or greater and the elongation (the total elongation EL) is 15% or greater can be formed.

The molded article of the present invention includes at least the first and second molding regions, but does not necessarily include only two molding regions. The molded article of the present invention may further include a third or fourth molding region. These molding regions can be formed according to a production method described later.

In production of the hot press molded article of the present invention, a thin steel plate (having the same chemical component composition as that of the molded article) may be formed so as to be divided into a plurality of regions including at least the first and second molding regions. Specifically, the above-described thin steel plate may be heated to a temperature of an Acs transformation point or higher and 1000° C. or lower. Then, for at least the first and second molding regions, cooling at an average cooling rate of 20° C./sec or higher and molding may begin by pressing of the first and second molding regions together with the die. In the first molding region, molding may be terminated at equal to or lower than a temperature (hereinafter sometimes referred to as “Ms point—50° C.”) lower than a martensite transformation start temperature by 50° C. In the second molding region, cooling may be performed to a temperature range of equal to or lower than a temperature (hereinafter sometimes referred to as “Bs point—100° C.”) lower than a bainite transformation start temperature by 100° C. and equal to or higher than the martensite transformation start temperature (the Ms point), and molding may be terminated after a lapse of a stay time of 10 seconds or longer within the above-described temperature range. The reasons for setting each requirement in this method are as follows. Note that the phrase of “molding is terminated” basically means the state at a lower dead point in molding (the point at which a punch tip end is positioned innermost: the state illustrated in FIG. 1). However, when the die needs to be cooled to a predetermined temperature in such a state, the phrase of “molding is terminated” also means the state until the die is detached after the die is maintained cooled.

According to the above-described method, the steel plate is divided into at least two molding regions (e.g., a high-strength region and a low-strength region), and the production conditions are controlled according to each region. Thus, the molded article exhibiting the strength-ductility balance according to each region can be obtained. The production conditions for forming each region will be described.

(Production Conditions for First Molding Region (High-Strength Region))

In order to properly adjust the structures of the hot press molded article, it is necessary to control the heating temperature within a predetermined range. By proper control of the heating temperature, a predetermined amount of retained austenite can be ensured at a subsequent cooling step. Meanwhile, the first molding region can transform into the structure mainly containing the martensite. As a result, the final hot press molded article can be formed with desired structures. If the heating temperature of the thin steel plate is lower than the Ac₃ transformation point, a sufficient amount of austenite cannot be obtained in heating, and a predetermined amount of retained austenite cannot be ensured at the final structure (the structures of the molded article). On the other hand, if the heating temperature of the thin steel plate exceeds 1000° C., the particle size of the austenite increases in heating, and the martensite transformation start temperature (the Ms point) and a martensite transformation end temperature (an Mf point) increase. Thus, the retained austenite cannot be ensured in quenching, and as a result, favorable moldability cannot be achieved. The heating temperature is preferably (Acs transformation point+50° C.) or higher and 950° C. or lower.

The cooling conditions in molding and a molding end temperature need to be properly controlled according to each region. First, in a steel plate region (hereinafter sometimes referred to as a “first steel plate region”) corresponding to the first molding region of the molded article, it is necessary that an average cooling rate of 20° C./sec or higher is ensured at the die in molding and that molding is terminated at a temperature of (Ms point—50° C.) or lower.

In order that the austenite formed at the above-described heating step has a desired structure (a structure mainly containing the martensite) with generation of the structures such as ferrite, pearlite, and bainite being blocked, the average cooling rate in molding and the molding end temperature need to be properly controlled. From this point of view, the average cooling rate in molding is 20° C./sec or higher, and the molding end temperature is (Ms point—50° C.) or lower. In particular, in the case where a steel plate with a high Si content is targeted, the mixed structure of the martensite and the retained austenite can be formed by cooling under the above-described conditions. The average cooling rate in molding is preferably 30° C./sec or higher (more preferably 40° C./sec or higher).

While the molding end temperature in the first steel plate region is cooled to a room temperature at the above-described average cooling rate, molding may be terminated. However, after the molding end temperature is cooled to (Ms point—50° C.) or lower (preferably to a temperature of Ms point—50° C.), cooling (two-step cooling) may be performed to 200° C. or lower at an average cooling rate of 20° C./sec or lower. Such addition of the cooling step allows thickening of carbon of the martensite in untransformed austenite, and therefore, the amount of retained austenite can be increased. In such two-step cooling, the average cooling rate in a second cooling step is preferably 10° C./sec or lower (more preferably 5° C./sec or lower).

(Production Conditions for Second Molding Region (Low-Strength Region))

On the other hand, in order to properly adjust the structures of the second molding region of the hot press molded article, it is necessary to control the heating temperature of a steel plate region (hereinafter sometimes referred to as a “second steel plate region”) corresponding to the second molding region within a predetermined range. By proper control of the heating temperature, a predetermined amount of retained austenite can be ensured at a subsequent cooling step. Meanwhile, the second molding region can transform into the structure mainly containing the bainitic ferrite. As a result, the final hot press molded article can be formed with desired structures. If the heating temperature of the thin steel plate is lower than the Acs transformation point, a sufficient amount of austenite cannot be obtained in heating, and a predetermined amount of retained austenite cannot be ensured at the final structure (the structures of the molded article). On the other hand, if the heating temperature of the thin steel plate exceeds 1000° C., the state similar to that of the first steel plate region is brought about (the preferable temperature range is also similar to that of the first steel plate region).

In order that the austenite formed at the above-described heating step has a desired structure (a structure mainly containing the bainitic ferrite) with generation of the structures such as ferrite and pearlite being blocked, the average cooling rate in molding and a cooling end temperature need to be properly controlled. From this point of view, the average cooling rate in molding needs to be 20° C./sec or higher, and the cooling end temperature needs to be (Bs point—100° C.) or lower and the martensite transformation start temperature (the Ms point) or higher (such a temperature range is hereinafter sometimes referred to as a “cooling rate change temperature”). The average cooling rate is preferably 30° C./sec or higher (more preferably 40° C./sec or higher).

Cooling is temporarily stopped within the above-described temperature range (the cooling rate change temperature), and such a state stays for 10 seconds or longer within the above-described temperature range (i.e., a temperature range of (Bs point—100° C.) or lower and the martensite transformation start temperature Ms point or higher). In this manner, bainite transformation proceeds in supercooled austenite so that the structure mainly containing the bainitic ferrite can be formed. A stay time in this state is preferably 50 seconds or longer (more preferably 100 seconds or longer). If the stay time is too long, the austenite begins decomposing, and the retained austenite fraction cannot be ensured. For this reason, the stay time is preferably 1000 seconds or shorter (more preferably 800 seconds or shorter).

As long as the stay step as described above is performed within the above-described temperature range, the stay step may be any of an isothermal holding step, a monotonic cooling step, or a re-heating step. Moreover, regarding the relationship between the above-described stay step and molding, the stay step as described above may be added after molding is terminated, or the holding step may be added within the above-described temperature range in the middle of termination of molding. After molding is terminated as described above, cooling may be performed to the room temperature by cold heat radiation at a proper cooling rate.

Control of the average cooling rate in molding can be achieved by a unit such as (a) a unit for controlling the temperature of the die (the cooling media illustrated in FIG. 1) and (b) a unit for controlling the coefficient of thermal conductivity of the die (the same applies to cooling in the later-described method). In the method of the present invention, the cooling conditions in molding vary according to each steel plate region. The control units such as the units (a) and (b) may be provided separately in a single die, and cooling control may be performed corresponding to each steel plate region in the single die.

The method for producing the hot press molded article according to the present invention is applicable not only to the case (the direct method) of producing a hot press molded article in a simple shape as illustrated in FIG. 1, but also to the case of producing a molded article in a relatively-complicated shape. Note that in the case of the complicated component shape, it might be difficult to form the final shape of the product by a single process of press molding. In this case, the method (called an “indirect method”) for performing cold press molding as a preceding process of hot press molding can be employed. In this method, a portion which is difficult to be molded is pre-molded into an approximate shape by cold working, and the other portion is hot-press-molded. According to such a method, e.g., when a recessed-raised portion (ridge portion) is formed at three sections of a molded article, the first and second recessed-raised portions are formed by cold press molding, and then, the third recessed-raised portion is formed by hot press molding.

The present invention is intended for a hot press molded article made of a high-strength steel plate. Although the steel grade of such a steel plate may include steel grades of high-strength steel plates with typical chemical component compositions, C, Si, Mn, P, S, Al, Cr, B, Ti, and N are preferably adjusted to suitable ranged. From this point of view, the preferable ranges of these chemical components and the reasons for limiting these ranges are as follows.

(C: 0.15-0.3%)

C is an essential element (the low-strength region) in strength improvement made by micronizing the bainitic ferrite generated at the cooling process and increasing a dislocation density in the bainitic ferrite. Moreover, C is also an essential element (the high-strength region) in control of the strength of the martensite structure. A less C content results in insufficient strength even in full martensite. C is an element heavily involved with hardenability. An increase in the C content produces the effect of reducing formation of other soft structures such as ferrite during cooling after heating. Further, C is also an element necessary for ensuring the retained austenite. If the C content is less than 0.15%, the bainite transformation start temperature Bs increases, and a high strength of the hot press molded article cannot be ensured. On the other hand, if the C content becomes excess and exceeds 0.3%, the strength becomes too high, and favorable ductility cannot be obtained. The lower limit of the C content is more preferably 0.18% or greater (much more preferably 0.20% or greater), and the upper limit of the C content is more preferably 0.27% or less (much more preferably 0.25% or less).

(Si: 0.5-3%)

Si produces the effect of forming the retained austenite in quenching. Moreover, Si also produces the effect of increasing, by solid solution strengthening, the strength without lowering the ductility much. If a Si content is less than 0.5%, a predetermined amount of retained autstenite cannot be ensured, and favorable ductility cannot be obtained. On the other hand, if the Si content becomes excess and exceeds 3%, the degree of solid solution strengthening becomes too high, and the ductility is significantly lowered. The lower limit of the Si content is more preferably 1.15% or greater (much more preferably 1.20% or greater), and the upper limit of the Si content is more preferably 2.7% or less (much more preferably 2.5% or less).

(Mn: 0.5-2%)

Mn is a useful element in reduction of formation of ferrite and pearlite during primary cooling. Moreover, Mn is also a useful element in micronizing the structure unit of the bainitic ferrite by lowering (Bs point—100° C.), or in enhancement of the strength of the bainitic ferrite by increasing a dislocation density in the bainitic ferrite. Further, Mn is also a useful element in stabilizing the austenite to increase the amount of retained austenite. In order to produce these effects, Mn is preferably contained at 0.5% or greater. Only considering characteristics, a great Mn content is preferable. However, since the cost for alloy addition increases, the Mn content is preferably 2% or less. Moreover, with significant improvement of the strength of the austenite, a hot rolling load increases, and it is difficult to produce a steel plate. For this reason, it is not preferable that the Mn content exceeds 2%, considering productivity. The lower limit of the Mn content is more preferably 0.7% or greater (much more preferably 0.9% or greater), and the upper limit of the Mn content is more preferably 1.8% or less (much more preferably 1.6% or less).

(P: 0.05% or Less (0% is not Inclusive))

Although P is an element inevitably contained in steel, P lowers the ductility. For this reason, P is preferably reduced as much as possible. However, significant reduction results in an increase in a steel production cost, and in production, it is difficult to make a P content 0%. Thus, the P content is preferably 0.05% or less (0% is not inclusive). The upper limit of the P content is more preferably 0.045% or less (much more preferably 0.040% or less).

(S: 0.05% or Less (0% is not Inclusive))

As in P, S is also an element inevitably contained in steel, and lowers the ductility. For this reason, S is preferably reduced as much as possible. However, significant reduction results in an increase in the steel production cost, and in production, it is difficult to make an S content 0%. Thus, the S content is preferably 0.05% or less (0% is not inclusive). The upper limit of the S content is more preferably 0.045% or less (much more preferably 0.040% or less).

(Al: 0.01-0.1%)

Al is useful as a deoxidizing element, and is also useful in ductility improvement because Al fixes, as AlN, solid liquid N present in steel. In order to effectively produce these effects, an Al content is preferably 0.01% or greater. However, if the Al content becomes excess and exceeds 0.1%, Al₂O₃ is excessively generated, and the ductility is lowered. Note that the lower limit of the Al content is more preferably 0.013% or greater (much more preferably 0.015% or greater), and the upper limit of the Al content is more preferably 0.08% or less (much more preferably 0.06% or less).

(Cr: 0.01-1%)

Cr has the effect of reducing ferrite transformation and pearlite transformation. Thus, Cr is an element preventing formation of ferrite and pearlite during cooling and contributing to ensuring the retained austenite. In order to produce these effects, Cr is preferably contained at 0.01% or greater. Even if Cr is excessively contained at greater than 1%, a cost increases. Moreover, since Cr significantly increases the strength of the austenite, a hot rolling load increases, and it is difficult to produce a steel plate. For this reason, it is not preferable that a Cr content exceeds 1%, considering the productivity. The lower limit of the Cr content is more preferably 0.02% or greater (much more preferably 0.05% or greater), and the upper limit of the Cr content is more preferably 0.8% or less (much more preferably 0.5% or less).

(B: 0.0002-0.01%)

B has the effect of increasing the hardenability and reducing ferrite transformation and pearlite transformation. Thus, B is an element preventing formation of ferrite and pearlite during primary cooling after heating and contributing to ensuring the bainitic ferrite and the retained austenite. In order to produce these effects, B is preferably contained at 0.0002% or greater. If B is excessively contained, at greater than 0.01%, the effect thereof is saturated. The lower limit of a B content is more preferably 0.0003% or greater (much more preferably 0.0005% or greater), and the upper limit of the B content is more preferably 0.008% or less (much more preferably 0.005% or less).

(Ti: [N]×4-0.1%)

Ti produces the effect of improving the hardenability by fixing N and maintaining B at a solid solution state. In order to produce such an effect, Ti is preferably contained at at least equal to or greater than four times as great as an N content [N]. However, if a Ti content becomes excessive and exceeds 0.1%, a great amount of TiC is formed. In addition, the strength increases due to precipitation strengthening, but the ductility is lowered. The lower limit of the Ti content is more preferably 0.05% or greater (much more preferably 0.06% or greater), and the upper limit of the Ti content is more preferably 0.09% or less (much more preferably 0.08% or less).

(N: 0.001-0.01%)

Since N is an element capable of reducing a hardenability improvement effect by fixing B as BN, N is preferably reduced as much as possible. However, reduction of N is limited in an actual process, and for this reason, the lower limit of the N content is preferably 0.001%. If the N content becomes excessive, coarse TiN particles are formed, and such TiN functions as a starting point for destruction to lower the ductility. For this reason, the upper limit of the N content is preferably 0.01%. The upper limit of the N content is more preferably 0.008% or less (much more preferably 0.006% or less).

The basic chemical components in the press molded article of the present invention are as described above. The residual substantially consists of iron. Note that the phrase of “substantially consists of iron” can means, in addition to iron, not only a slight amount of components (e.g., Mg, Ca, Sr, Ba, REM such as La, and carbide-forming elements such as Zr, Hf, Ta, W, and Mo) not inhibiting the characteristics of the steel material of the present invention, but also inevitable impurities (e.g., O and H) other than P and S.

It is useful for the press molded article of the present invention to further contain, as necessary, (a) one or more selected from the group consisting of Cu, Ni, and Mo in the total amount of 1% or less (0% is not inclusive) and (b) at least one of V or Nb in the total amount of 0.1% or less (0% is not inclusive), for example. Depending on the types of the elements to be contained, the characteristics of the hot press molded article are further improved. The preferable ranges of these contained elements and the reasons for limiting these ranges are as follows.

(One or More Selected from the Group Consisting of Cu, Ni, and Mo in the Total Amount of 1% or Less (0% is not Inclusive))

Since Cu, Ni, and Mo reduce ferrite transformation and pearlite transformation, Cu, Ni, and Mo effectively function to prevent formation of ferrite and pearlite during primary cooling and to ensure the retained austenite. In order to produce these effects, Cu, Ni, and Mo are preferably contained at the total amount of 0.01% or greater. Only considering characteristics, a great content is preferable. However, since the cost for alloy addition increases, the total amount is preferably 1% or less. Moreover, since Cu, Ni, and Mo have the effect of significantly increasing the strength of the austenite, a hot rolling load increases, and it is difficult to produce a steel plate. For this reason, it is preferable that the total amount is 1% or less, considering the productivity. The lower limit of the total content of these elements is more preferably 0.05% or greater (much more preferably 0.06% or greater), and the upper limit of the total content of these elements is more preferably 0.9% or less (much more preferably 0.8% or less).

(At Least One of V or Nb in the Total Amount of 0.1% or Less (0% is not Inclusive))

V and Nb have the effect of forming fine carbide particles and micronizing a structure by a pinning effect. In order to produce these effects, at least one of V or Nb is preferably contained at the total amount of 0.001% or greater. However, if the content of these elements becomes excess, coarse carbide particles are formed, this serves as a starting point for destruction to lower the ductility. For this reason, the total content is preferably 0.1% or less. The lower limit of the total content of these elements is more preferably 0.005% or greater (much more preferably 0.008% or greater), and the upper limit of the total content of these elements is more preferably 0.08% or less (much more preferably 0.06% or less).

According to the present invention, the press molding conditions (the heating temperature and the cooling rate according to each steel plate region) can be properly adjusted to control the characteristics, such as the strength and the elongation, of the molded article according to each molding region. In addition, the hot press molded article can be obtained with a high ductility (a high residual ductility). Thus, the present invention is also applicable to a portion (e.g., a member requiring both of shock resistance and energy absorption reduction) that has been difficult to apply in a conventional hot press molded article, and is significantly useful in enlargement of the scope of application of the hot press molded article. Further, the molded article of the present invention has a higher residual ductility as compared to that of a molded article whose structure is adjusted by typical annealing performed after cold press molding.

Advantages of the present invention will be more specifically described below with reference to an example, but the later-described example does not limit the scope of the present invention. In light of the description made above and later, any design changes may be made within the technical scope of the present invention.

This application claims the benefit of and priority to Japanese Patent Application No. 2013-032615 filed on Feb. 21, 2013, the disclosure of which is hereby incorporated by reference in its entirety in this application.

Example

A steel material having a chemical component composition shown in Table 1 below was vacuum-fused, thereby forming an experimental slab. Then, after hot rolling was performed, the slab was cooled and rolled up. Further, cold rolling was performed, thereby forming a thin steel plate. Note that an AC3 transformation point, an Ms point, and (Bs point—100° C.) as shown in Table 1 were obtained using Expressions (1)-(3) described below (e.g., see “The Physical Metallurgy of Steels,” Maruzen Co., Ltd., 1985).

AC₃ Transformation Point (° C.)=910−203×[C]^1/2+44.7×[Si]−30×[Mn]+700×[P]+400×[Al]+400×[Ti]+104×[V]−11×[Cr]+31.5×[Mo]−20×[Cu]−15.2×[Ni]  (1)

Ms point (° C.)=550−361×[C]−39×[Mn]−10×[Cu]−17×[Ni]−20×[Cr]−5×[Mo]+30×[Al]  (2)

Bs point (° C.)=830−270×[C]−90×[Mn]−37×[Ni]−70×[Cr]−83×[Mo]  (3)

Note that [C], [Si], [Mn], [P], [Al], [Ti], [V], [Cr], [Mo], [Cu], and [Ni] represent the contents (mass %) of C, Si, Mn, P, Al, Ti, V, Cr, Mo, Cu, and Ni, respectively. If an element(s) shown as each term in Expressions (1)-(3) is not contained, calculation is made without taking such an element(s) into consideration.

TABLE 1
		Ac3		Bs Point
Steel	Chemical Component Composition* (mass %)	Transformation	Ms Point	−100° C.
Grade	C	Si	Mn	P	S	Cr	Al	Ti	B	N	Point (° C.)	(° C.)	(° C.)
A	0.232	1.19	1.41	0.014	0.0021	0.21	0.053	0.027	0.0033	0.0047	863	409	526
B	0.232	0.18	1.41	0.014	0.0021	0.21	0.053	0.027	0.0033	0.0047	817	409	526
*Residual: iron and inevitable impurities other than P and S

While the heating temperature was changed in each steel plate region of the obtained steel plate, molding and cooling treatment were performed. Specifically, press molding was performed using a bending die having a hat channel shape (a HAT shape) illustrated in FIG. 2. Note that in FIG. 2, a reference numeral “10” denotes an upper die (equivalent to the punch 1 illustrated in FIG. 1), and a reference numeral “11” denotes a lower die (equivalent to the die 2 illustrated in FIG. 1). Moreover, in this die, a pad 12 is provided, and is configured such that press molding is performed with a steel plate 4 being interposed between the pad 12 and the upper 11 while pressure (pad pressure) is being applied (at a pad pressure of 9800 N).

The heating temperature and the average cooling rate in each steel plate region are shown in Table 2 below (the molding end temperature (a die-detaching temperature) was 200° C. in any of the regions). The steel plate size in molding and cooling was 220 mm×500 mm (a plate thickness of 1.4 mm) (the area ratio between the first steel plate region and the second steel plate region was 1:1). The shape of a molded press molded article is illustrated in FIGS. 3(a) and 3(b) (FIG. 3(a) is a perspective view, and FIG. 3(b) is a view schematically illustrating the cross section). In FIG. 3(a), a reference numeral “15” denotes the first steel plate region (corresponding to the first molding region of the molded article), and a reference numeral “16” denotes the second steel plate region (corresponding to the second molding region of the molded article). Note that “Average Cooling Rate 1” of the first steel plate region as shown in Table 2 is an average cooling rate from the heating temperature to (Ms point—50° C.) or lower (the molding end temperature), and “Average Cooling Rate 2” of the first steel plate region is an average cooling rate from the molding end temperature to 200° C. or lower.

	TABLE 2
	Production Conditions
	Second Steel Plate Region
		Average
	First Steel Plate Region	Cooling		Average
		Steel Plate	Average		Average	Rate	Cooling Rate		Cooling Rate	Stay Time at
		Heating	Cooling	Molding End	Cooling	(° C./sec)	Change		(° C./sec)	[Bs −100° C.
Test	Steel	Temperature	Rate 1	Temperature	Rate 2	in Primary	Temperature	Retention	in Secondary	to Ms Point]
No.	Grade	(° C.)	(° C./sec)	(° C.)	(° C./sec)	Cooling	(° C.)	Time (sec)	Cooling	(sec)
1	A	930	40	200	15	40	N/A	N/A	N/A	3
2	A	930	40	200	15	40	480	0	5	15
3	A	930	40	200	15	40	420	0	20	5
4	A	930	40	200	15	40	420	10	50	15
5	A	930	40	200	15	40	600	10	5	23.4
6	B	930	40	200	15	40	480	N/A	5	15

For each steel plate subjected to the foregoing processes (heating, molding, and cooling), a tensile strength (TS) and an elongation (a total elongation EL) were measured in the following manner, and metal structures (the fraction of each structure) were observed in the following manner.

(Tensile Strength (TS) and Elongation (Total Elongation EL)) A tension test was conducted using a JIS 5 test piece, thereby measuring the tensile strength (TS) and the elongation (the total elongation EL). At this point, a strain rate in the tension test was 10 mm/sec. In the present invention, evaluation was made as “successful” when (a) in the first region, the tensile strength (TS) satisfies 1500 MPa or greater and the elongation (the total elongation EL) satisfies 10% or greater and (b) in the second region, the tensile strength (TS) satisfies 1100 MPa or greater and the elongation (the total elongation EL) satisfies 15% or greater.

(Observation of Metal Structures (Fraction of Each Structure))

(1) For the structures of martensite, ferrite, and bainitic ferrite in the steel plate, the steel plate was corroded with nital, and then, the fraction (the area ratio) of each structure was obtained by scanning electron microscope (SEM) observation (a magnification of 1000-power or 2000-power) with the ferrite and the bainitic ferrite being discriminated from each other.

(2) A retained austenite fraction (an area ratio) in the steel plate was measured in such a manner that after the steel plate was ground to the quarter of the thickness of the steel plate, chemical polishing was performed, and then, X-ray diffractometry was performed (e.g., ISJJ Int., Vol. 33, 1933, No. 7, P. 776).

(3) The area ratio of the martensite (as-quenched martensite) was measured as follows. The steel plate was subjected to Repera corrosion. Then, the area ratio of a white contrast as the mixed structure of the as-quenched martensite and the retained austenite was measured by SEM observation. The retained austenite fraction obtained by X-ray diffractometry was subtracted from the area ratio of the white contrast, thereby measuring the as-quenched martensite fraction.

The measurement results of the metal structures in each region of the molded article are shown in Table 3 below, and the mechanical characteristics in each region of the molded article are shown in Table 4 below.

	TABLE 3
	Structures of Molded Article (area %)
	First Molding Region	Second Molding Region
Test	Steel		Retained	Other	Bainitic	As-Quenched	Retained	Other
No.	Grade	Martensite	Austenite	Structure	Ferrite	Martensite	Austenite	Structure
1	A	95	5	0	0	95	5	0
2	A	95	5	0	85	7	8	0
3	A	95	5	0	90	8	2	0
4	A	95	5	0	87	4	9	0
5	A	95	5	0	40	30	10	20 (Ferrite)
6	B	100	0	0	95	5	0	0

	TABLE 4
	Mechanical Characteristics
	First Molding Region	Second Molding Region
		Tensile		Tensile
Test	Steel	Strength	Elongation	Strength	Elongation
No.	Grade	TS (MPa)	EL (%)	TS (MPa)	EL (%)
1	A	1550	10	1515	10
2	A	1550	10	1203	17
3	A	1550	10	1220	13
4	A	1550	10	1198	18
5	A	1550	10	980	10
6	B	1545	7	1098	13

The following consideration was made based on these results. Test Nos. 2 and 4 were examples satisfying the requirements defined in the present invention, and the results show that in each of Test Nos. 2 and 4, the molded article exhibiting high performance, i.e., a high strength-ductility balance, in each region was obtained.

On the other hand, Test Nos. 1, 3, 5, and 6 were comparative examples not satisfying any of the requirements defined in the present invention, and any of the characteristics was lowered. That is, in Test No. 1, the stay time at (Bs—100° C.) to the Ms point was short in the second steel plate region, the fraction of the bainitic ferrite in the structure of the second region of the molded article was low, the fraction of the martensite in the structure of the second region of the molded article was high, and only a low elongation (a low total elongation EL) was obtained in the second region.

In Test No. 3, the cooling rate change temperature was proper in the second steel plate region, but the stay time at (Bs—100° C.) to the Ms point was short. Although a proper fraction of the bainitic ferrite in the structure of the second region of the molded article was ensured, the amount of retained austenite was small. Thus, only a low elongation (a low total elongation EL) was obtained in the second region.

In Test No. 5, the cooling rate change temperature was high in the second steel plate region. The ferrite was formed, and the amount of bainitic ferrite was not ensured. Thus, only a low strength and a low elongation (a low total elongation EL) were obtained in the second region. In Test No. 6, a Si content was small in a steel component. Thus, even if the cooling conditions were proper, the amount of retained austenite was not formed in any of the regions of the molded article, and only a low elongation (a low total elongation EL) was obtained (the strength in the second region was also low).

INDUSTRIAL APPLICABILITY

The press molded article of the present invention includes the first molding region exhibiting the metal structure which contains 80-97 area % of the martensite and 3-20 area % of the retained austenite and which has the residual structure at 5 area % or less; and the second molding region exhibiting the metal structure which contains 70-97 area % of the bainitic ferrite, 27 area % or less of the martensite, and 3-20 area % of the retained austenite and which has the residual structure at 5 area % or less. As a result, at least regions corresponding respectively to a shock-resistant area and an energy-absorbing area can be, without application of welding, formed in a single molded article, and a high-level balance between high strength and elongation can be achieved according to each region.

EXPLANATION OF REFERENCE NUMERALS

• 1 Punch
• 2 Die
• 3 Blank Holder
• 4 Steel Plate (Blank)

Claims

1. A hot press molded article formed by hot press molding of a thin steel plate, comprising:

a first molding region exhibiting a metal structure which contains 80-97 area % of martensite and 3-20 area % of retained austenite and which has a residual structure at 5 area % or less; and

a second molding region exhibiting a metal structure which contains 70-97 area % of bainitic ferrite, 27 area % or less of martensite, and 3-20 area % of retained austenite and which has a residual structure at 5 area % or less.

2. The hot press molded article according to claim 1, wherein

the first and second molding regions have an identical chemical component composition, and

steel of each component region contains, in units of mass %, 0.15-0.3% of C, 0.5-3% of Si, 0.5-2% of Mn, 0.05% or less of P, 0.05% or less of S, 0.01-0.1% of Al, 0.01-1% of Cr, 0.0002-0.01% of B, [N]×4-0.1% of Ti, and 0.001-0.01% of N, where 0% is not inclusive for the P and the S, and [N] denotes an N content in units of %, and

the steel of each component region has a residual consisting of iron and an inevitable impurity.

3. The hot press molded article according to claim 2, wherein

the steel further contains, as other element, one or more selected from a group consisting of Cu, Ni, and Mo in a total amount of 1% or less, where 0% is not inclusive.

4. The hot press molded article according to claim 2, wherein

the steel further contains, as other element, at least one of V or Nb in a total amount of 0.1% or less, where 0% is not inclusive.

5. A method for producing the hot press molded article according to claim 1 by forming a thin steel plate so as to divide the thin steel plate into a plurality of regions including at least first and second molding regions, the method comprising:

after the thin steel plate is heated to a temperature of an Ac3 transformation point or higher and 1000° C. or lower, starting cooling at an average cooling rate of 20° C./sec or higher and molding by pressing at least the first and second molding regions together with a die; and

terminating, in the first molding region, the molding at equal to or lower than a temperature lower than a martensite transformation start temperature by 50° C., and performing, in the second molding region, the cooling to a temperature range of equal to or lower than a temperature lower than a bainite transformation start temperature by 100° C. and equal to or higher than the martensite transformation start temperature and terminating the molding after a lapse of a stay time of 10 seconds or longer within the temperature range.