STEEL PLATE MEMBER AND METHOD OF PRODUCING THE STEEL PLATE MEMBER
A method of producing a steel plate member including: quenching a steel plate by heating a steel plate to a temperature which is higher than a first transformation temperature at which austenitization is completed, and then cooling the steel plate at a cooling rate higher than an upper critical cooling rate; reheating a second area of the steel plate, without reheating a first area of the steel plate, to a temperature which is higher than a second transformation temperature, at which austenitization starts, and is lower than the first transformation temperature; and cooling the steel plate at a cooling rate lower than a lower critical cooling rate.
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The disclosure of Japanese Patent Application No. 2018-085085 filed on Apr. 26, 2018 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
BACKGROUND 1. Technical FieldThe present disclosure relates to a steel plate member and a method of producing the steel plate member, and particularly, to a steel plate member including a hard area containing martensite and a soft area containing ferrite and pearlite and a method of producing the steel plate member.
2. Description of Related ArtIn recent years, for example, as a structural member for a vehicle, in order to improve impact resistance, a steel plate member including a hard area that is resistant to shock and a soft area that absorbs shock has been developed. In Japanese Unexamined Patent Application Publication No. 2012-144773 (JP2012-144773 A), a method in which only a partial area of a steel plate member is heated to a temperature higher than an austenite transformation completion temperature A3 and quenched, and thus a hard area and a soft area are formed on one steel plate member is disclosed.
SUMMARYIn the method of producing a steel plate member including a hard area and a soft area disclosed in JP2012-144773 A, when only a partial area of the steel plate member is heated to a temperature higher than the austenite transformation completion temperature A3, a microstructure of the area changes to an austenite single phase. Thus, after quenching, the area becomes a hard area composed of martensite. On the other hand, in an area heated only to a temperature lower than an austenite transformation start temperature A1, austenite does not appear. Thus, even after quenching, the area remains as a soft area composed of ferrite and pearlite as before quenching.
Here, a boundary area between the hard area and the soft area is heated at a temperature between the austenite transformation start temperature A1 and the austenite transformation completion temperature A3, and some of ferrite and pearlite change to austenite. Thus, after quenching, the boundary area has an unstable microstructure in which hard martensite and soft ferrite and pearlite coexist. As a result, cracks are likely to occur in the boundary area between the hard area and the soft area, and local ductility and bendability deteriorate.
The present disclosure provides a steel plate member in which the occurrence of cracks in a boundary area between a hard area and a soft area is restrained and a method of producing the steel plate member.
An aspect of the disclosure relates to a method of producing a steel plate member including: quenching a steel plate by heating the steel plate to a temperature which is higher than a first transformation temperature at which austenitization is completed and then cooling the steel plate at a cooling rate higher than an upper critical cooling rate; reheating a second area of the steel plate, without reheating a first area of the steel plate, to a temperature which is higher than a second transformation temperature, at which austenitization starts, and is lower than the first transformation temperature; and cooling the reheated steel plate at a cooling rate lower than a lower critical cooling rate to obtain the steel plate member in which a hard area composed of martensite is formed in the first area, a soft area containing tempered martensite in addition to ferrite and pearlite is formed in the second area, and an area composed of only tempered martensite is formed in a third area between the first area and the second area.
In the method of producing a steel plate member of the above aspect, in the tempering step, the first area of the steel plate is not reheated, and the second area of the steel plate is reheated to a temperature that is higher than a transformation temperature at which austenitization starts and is lower than a transformation temperature at which austenitization is completed. Then, the steel plate is cooled at a cooling rate lower than a lower critical cooling rate. Therefore, a hard area composed of martensite is formed in the first area, a soft area containing tempered martensite in addition to ferrite and pearlite is formed in the second area, and an area composed of only tempered martensite is formed in the third area between the first area and the second area. That is, according to the method of producing a steel plate member of the above aspect, since an unstable microstructure in which the hard martensite and the soft ferrite and pearlite coexist is not formed in the third area, it is possible to restrain the occurrence of cracks in the third area.
In the above aspect, the second area of the steel plate may be reheated by induction heating.
According to the method of producing a steel plate member of the above aspect, it is possible to quickly heat the second area of the steel plate and it is possible to accurately maintain the temperature that is higher than the transformation temperature at which austenitization starts and is lower than the transformation temperature at which austenitization is completed.
In the above aspect, after the steel plate is heated, a press processing may be performed on the steel plate before the steel plate is cooled during quenching.
According to the method of producing a steel plate member of the above aspect, it is possible to obtain a high strength steel plate member by cooling after press molding while spring back occurring in cold press forming is restrained.
In the above aspect, the third area may be heated to a temperature which is lower than the second transformation temperature by heat conduction from the second area being reheated; and a structure of the third area may change from martensite to tempered martensite.
Another aspect of the disclosure relates to a steel plate member including: a hard area composed of martensite; a soft area containing ferrite and pearlite; and a third area which is formed between the hard area and the soft area, wherein: the soft area further contains tempered martensite; and the third area includes an area composed of only tempered martensite.
In the steel plate member of the above aspect, an area composed of only tempered martensite is formed in the third area, and an unstable microstructure in which the hard martensite and the soft ferrite and pearlite coexist is not formed in the third area. Therefore, according to the steel plate member of the above aspect, it is possible to restrain the occurrence of cracks in the third area.
According to the present disclosure, it is possible to provide a steel plate member in which the occurrence of cracks in a boundary area between a hard area and a soft area is restrained and a method of producing the steel plate member.
Features, advantages, and technical and industrial significance of exemplary embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:
Specific embodiments in which the present disclosure is applied will be described below in detail with reference to the drawings. However, the present disclosure is not limited to the following embodiments. In addition, in order to clarify the description, the following description and drawings are appropriately simplified.
1. First Embodiment 1-1. Method of Producing a Steel Plate MemberFirst, a method of producing a steel plate member according to a first embodiment will be described with reference to
First, in the quenching step, the entire steel plate member is heated at a temperature that is higher than a transformation temperature A3 at which austenitization is completed. The transformation temperature A3 at which austenitization is completed is, for example, 910° C. In this case, the microstructure of the entire steel plate member changes from ferrite and pearlite to an austenite single phase. Then, the steel plate member is cooled at a cooling rate that is higher than an upper critical cooling rate. Thereby, the steel plate member undergoes martensitic transformation, and the microstructure of the entire steel plate member changes to hard martensite. The upper critical cooling rate is a minimum cooling rate at which the microstructure transforms only to martensite.
Here, after the steel plate member is heated, before the steel plate member is cooled, the steel plate member is preferably press-molded. Since hot press forming is performed, it is possible to obtain a high strength steel plate member by quenching after press molding while spring back occurring in cold press forming is restrained. Here, such hot pressing is generally called a hot stamp. Although not particularly limited, as a steel plate for a hot stamp, for example, a steel plate made of manganese and boron steel with a thickness of about 1 mm to 4 mm is used.
Next, in the partial tempering step, only a partial area of the steel plate member is reheated and softened. Specifically, as shown in
Then, the steel plate member is cooled at a cooling rate lower than a lower critical cooling rate so that the second area 12 does not undergo martensitic transformation. In this case, as indicated by a dashed arrow in
Here,
Here,
Next, the microstructure of the steel plate member 10 during heating in the partial tempering step shown in the center in
Thus, as shown in the center in
On the other hand, the first area 11 of the steel plate member 10 is an area that is not reheated and is not thermally affected. Thus, the microstructure of the first area 11 does not change from the martensite M. Here, the boundary area 13 between the first area 11 and the second area 12 is heated at a temperature that is lower than the transformation temperature A1 at which austenitization starts by heat conduction from the second area 12 and is thermally affected. Thus, the microstructure of the boundary area 13 changes from the martensite M to the tempered martensite TM.
More specifically, in the boundary area 13, when a distance from the second area 12 is shorter, it is heated at a temperature closer to the transformation temperature A1 at which austenitization starts. Thus, the microstructure on the side of the second area 12 which is most of the boundary area 13 is composed of only the tempered martensite TM. Here, in the vicinity of the first area 11 in the boundary area 13, the tempered martensite TM and the martensite M coexist, and toward the first area 11 composed of only the martensite M, an amount of the tempered martensite TM decreases and an amount of the martensite M increases. Note that, this part of the boundary area 13 in which the tempered martensite TM and the martensite M coexist is not specifically shown in
Next, a microstructure of the steel plate member 10 after the partial tempering step shown on the right side in
Here, when the second area 12 is reheated to a temperature that is higher than the transformation temperature A3 at which austenitization is completed, the second area 12 after cooling becomes a microstructure composed of only ferrite/pearlite FP and sufficient strength is not obtained. Thus, a temperature of the second area 12 before cooling is set to be lower than the transformation temperature A3 at which austenitization is completed during reheating so that an area composed of only the ferrite/pearlite FP does not occur in the second area 12.
1-2. Configuration of Steel Plate MemberNext, a steel plate member according to the first embodiment will be described with reference to
As shown in
As shown in
Here, a steel plate member according to a comparative example of the first embodiment and a method of producing the same will be described with reference to
In the partial quenching step, only a partial area of the steel plate member is heated to a temperature that is higher than the transformation completion temperature A3 at which austenitization is completed. Specifically, as shown in
Then, a boundary area 23 between the first area 21 and the second area 22 is heated to a temperature that is higher than the transformation temperature A1 at which austenitization starts and is lower than the transformation temperature A3 at which austenitization is completed by heat conduction from the second area 22. Thus, in the microstructure of the boundary area 23, a part of ferrite and pearlite changes to austenite. That is, the microstructure of the boundary area 23 is a mixed structure of ferrite/pearlite and austenite.
Then, the steel plate member is cooled at a cooling rate higher than the upper critical cooling rate. Therefore, the entire austenite undergoes martensitic transformation, and the microstructure of the first area 21 changes to hard martensite. In addition, the microstructure of the boundary area 23 is a mixed structure of soft ferrite and pearlite and hard martensite. Here, the microstructure of the second area 22 does not change from ferrite and pearlite.
Next, the steel plate member according to the comparative example of the first embodiment will be described with reference to
As shown in
In this manner, in the steel plate member 20 according to the comparative example, the boundary area 23 has an unstable microstructure in which hard martensite M and soft ferrite/pearlite FP coexist. Thus, cracks are likely to occur in a boundary area between the hard area and the soft area.
1-4. Effects According to Steel Plate Member According to First Embodiment and Method of Producing the SameNext, effects according to a steel plate member according to the first embodiment and a method of producing the same will be described. As described above, in the steel plate member 20 according to the comparative example shown in
In addition, in the boundary area 13 of the steel plate member 10 according to the first embodiment, when a distance from the second area 12 heated in the partial tempering step is shorter, since it is heated at a higher temperature, the area becomes soft. That is, the boundary area 13 of the steel plate member 10 according to the first embodiment gradually becomes soft from the side of the hard first area 11 toward the side of the soft second area 12. Therefore, it is possible to more effectively restrain the occurrence of cracks in the boundary area 13 between the hard first area 11 and the soft second area 12.
Here, in the steel plate member 10 according to the first embodiment, the second area 12 which is a soft area has a microstructure in which ferrite/pearlite FP and tempered martensite TM coexist. However, since the ferrite/pearlite FP and the tempered martensite TM are also soft, cracks are unlikely to occur in the second area 12.
2. Second EmbodimentNext, a method of producing a steel plate member according to a second embodiment will be described with reference to
As shown in
Here, in high frequency induction heating, since the heating efficiency sharply decreases at a Curie point at which the steel plate member 10 loses its magnetism, it is difficult to increase the heating temperature near the Curie point. Since austenite is nonmagnetic, and martensite, ferrite and pearlite are ferromagnetic, the Curie point is between the transformation temperature A1 at which austenitization starts and the transformation temperature A3 at which austenitization is completed.
Therefore, when high frequency induction heating is used, it is possible to quickly heat only the second area 12 of the steel plate member 10 and it is possible to easily and accurately maintain the temperature that is higher than the transformation temperature A1 at which austenitization starts and is lower than the transformation temperature A3 at which austenitization is completed. Since it is possible to quickly heat only the second area 12 of the steel plate member 10, it is possible to narrow the boundary area 13 that is thermally affected by heat conduction from the second area 12.
An example of the second embodiment in which a steel plate member is induction-heated in partial tempering will be described below. As a steel plate member, a quenched component of a steel plate for a hot stamp made of manganese and boron steel (22MnB5 steel) and with a thickness of 2.0 mm, a width of 100 mm, and a length of 300 mm is used.
As will be described in detail below, as a result of the partial tempering performed according to the example, a hard area composed of the martensite M is formed in the first area 11 shown in
As shown in
In addition, as shown in
On the other hand, as shown in
Here, the present disclosure is not limited to the above embodiments, and can be appropriately changed without departing from the spirit and scope the disclosure.
Claims
1. A method of producing a steel plate member comprising:
- quenching a steel plate by heating the steel plate to a temperature which is higher than a first transformation temperature at which austenitization is completed and then cooling the steel plate at a cooling rate higher than an upper critical cooling rate;
- reheating a second area of the steel plate, without reheating a first area of the steel plate, to a temperature which is higher than a second transformation temperature, at which austenitization starts, and is lower than the first transformation temperature; and
- cooling the reheated steel plate at a cooling rate lower than a lower critical cooling rate to obtain the steel plate member in which a hard area composed of martensite is formed in the first area, a soft area containing tempered martensite in addition to ferrite and pearlite is formed in the second area, and an area composed of only tempered martensite is formed in a third area between the first area and the second area.
2. The method of producing the steel plate member according to claim 1, wherein the second area of the steel plate is reheated by induction heating.
3. The method of producing the steel plate member according to claim 1, wherein, after the steel plate is heated, a press processing is performed on the steel plate before the steel plate is cooled during quenching.
4. The method of producing the steel plate member according to claim 1, wherein:
- the third area is heated to a temperature which is lower than the second transformation temperature by heat conduction from the second area being reheated; and
- a structure of the third area changes from martensite to tempered martensite.
5. A steel plate member comprising:
- a hard area composed of martensite;
- a soft area containing ferrite and pearlite; and
- a third area which is formed between the hard area and the soft area, wherein: the soft area further contains tempered martensite; and
- the third area includes an area composed of only tempered martensite.
Type: Application
Filed: Aug 31, 2018
Publication Date: Apr 11, 2019
Applicant: TOYOTA JIDOSHA KABUSHIKI KAISHA (Toyota-shi)
Inventors: Tomoaki IHARA (Toyota-shi), Satoshi YAMAZAKI (Nagoya-shi), Shinobu OKUMA (Toyota-shi)
Application Number: 16/119,096