KR102633542B1 - Hot stamp molded products and their manufacturing method - Google Patents

Abstract

This hot stamp molded product has a predetermined chemical composition. The hot stamped molded product has a metal structure, in terms of area%, of ferrite: more than 50.0%, tempered martensite: 5.0% or more, less than 50.0%, martensite: 0% or more, less than 10.0%, bainite: 0% or more, Contains less than 20.0%. The hot stamp molded product has a tensile strength of 440 MPa or more and less than 700 MPa, and ΔTS, which is the amount of decrease in the tensile strength when heat treated at 170°C for 20 minutes, is 100 MPa or less.

Description

Hot stamp molded products and their manufacturing method

The present invention relates to hot stamped products and methods for manufacturing the same.

This application claims priority based on Patent Application No. 2019-070212 filed in Japan on April 1, 2019, the contents of which are incorporated herein by reference.

Today, the industrial technology field is highly specialized, and materials used in each technology field are required to have special and high-level performance. For example, regarding steel sheets for automobiles, high strength is required out of consideration for the global environment and to improve fuel efficiency by reducing the weight of the vehicle body. When a high-strength steel plate is applied to the body of a car, the thickness of the steel plate can be thinned to lighten the car body and provide the desired strength to the car body.

However, in press forming, which is a process for forming automobile body members, the thinner the thickness of the steel plate used, the more likely it is that cracks and wrinkles will occur. Therefore, excellent press formability is also required for automotive steel sheets.

Since securing press formability and increasing the strength of the steel sheet are conflicting factors, it is difficult to satisfy these characteristics at the same time. Additionally, when a high-strength steel sheet is press-formed, the shape of the member changes significantly due to springback when the member is taken out of the mold, making it difficult to ensure dimensional accuracy of the member. In this way, it is not easy to manufacture high-strength car body members by press forming.

Until now, as a method of manufacturing ultra-high-strength vehicle body members, for example, as disclosed in Patent Document 1, a technique of press forming a heated steel sheet using a low-temperature press mold has been proposed. This technology is called hot stamping or hot pressing, and press-forms a steel sheet that has been made soft by heating to a high temperature, so that members of complex shapes can be manufactured with high dimensional accuracy. Additionally, since the steel sheet is rapidly cooled by contact with the mold, it becomes possible to significantly increase the strength simultaneously with press forming by quenching. For example, Patent Document 1 describes that a member with a tensile strength of 1,400 MPa or more can be obtained by hot stamping a steel sheet with a tensile strength of 500 to 600 MPa.

Among car body members, in skeletal structural parts such as center pillars and side members, hard and soft parts are often provided in the members in order to control the deformation state of the members when a car crashes.

As a method of manufacturing a member having a soft portion by hot stamping, Patent Document 2 discloses a method of partially changing the heating temperature of a steel sheet by induction heating or infrared heating and softening the portion heated to a low temperature. .

Patent Document 3 discloses a method of attaching an insulating material to a part of a steel sheet when heating the steel sheet in a furnace and partially lowering the heating temperature to soften the steel sheet.

Patent Document 4 and Patent Document 5 disclose a method of partially changing the cooling rate of a steel sheet by changing the contact area between the steel sheet and the mold during forming, and softening the portion with a low cooling rate.

Patent Document 6 discloses a hot stamping technique using a so-called tailored blank material in which two unprocessed plates are connected by welding.

In hot stamping, a steel sheet is usually heated to the austenite region and then cooled at a cooling rate higher than the critical cold speed to form a single martensite structure to increase strength. On the other hand, in the methods described in Patent Documents 2 to 5, as described above, the heating temperature or cooling rate of the steel sheet is partially lowered and a structure other than martensite is partially generated to soften the steel sheet. However, since the fraction of structures other than martensite changes sensitively in response to heating temperature and cooling rate, the methods of Patent Documents 2 to 5 have the problem that the strength of the soft portion is not stable.

In the technology described in Patent Document 6, a soft portion can be formed under certain heating and cooling conditions by using a steel plate with low hardening properties as one unprocessed plate. However, although the metal structure and strength characteristics of the soft part largely depend on the composition of the steel sheet, Patent Document 6 does not give any consideration to the composition of the steel sheet with low hardenability.

In response to this problem, Patent Documents 7 and 8 disclose a method of stabilizing the strength of the soft portion in a hot stamp member composed of a hard portion and a soft portion, or a hot stamp member that is soft as a whole.

Specifically, Patent Document 7 discloses a high-strength member for automobiles of the 600 to 1200 MPa class in which the C content is limited to a slightly low level, a certain amount of quenching elements are contained, and the formation of ferrite, pearlite, and martensite is suppressed during cooling. and its manufacturing method are disclosed.

In addition, Patent Document 8 discloses a hot stamp member with a tensile strength of 500 MPa or more and a method for manufacturing the same, in which the C content is limited to a slightly low level, Ti is contained, and the amount of martensite generated is controlled.

According to the techniques described in Patent Documents 7 and 8, it becomes possible to increase the uniformity of strength and elongation within the member. However, according to the present inventors' examination, in the technologies described in Patent Documents 7 and 8, the metal structure includes hard structures such as bainite and martensite, so the thermal stability is low, and when the member is subjected to a paint baking treatment, It was found that the strength sometimes decreased. Since paint baking treatment is often performed on automobile components, there is room for improvement in the techniques described in Patent Documents 7 and 8.

Japanese Patent Publication No. 2002-102980 Japanese Patent Publication No. 2005-193287 Japanese Patent Publication No. 2009-61473 Japanese Patent Publication No. 2003-328031 International Publication No. 2006/38868 Japanese Patent Publication No. 2004-58082 Japanese Patent Publication No. 2005-248320 International Publication No. 2008/132303

As described above, it is not easy to manufacture a soft member or a member including a soft portion by hot stamping. In particular, it was difficult in the prior art to manufacture a low-strength hot stamped member (molded product) with excellent thermal stability and partially or entirely containing a soft part by hot stamping.

The present invention solves the above problems and has excellent thermal stability, more specifically, a tensile strength of 440 MPa or more, 700 MPa, with small fluctuation in strength (tensile strength) before and after the paint baking process accompanying the paint baking process. The object is to provide a hot stamped product having a smaller portion and a method for manufacturing the same.

The present invention has been made to solve the above problems, and its main focus is the following hot stamp molded product and its manufacturing method.

(1) It is a hot stamp molded product, and all or part of the hot stamp molded product contains, in mass%, C: 0.001% or more, less than 0.090%, Si: less than 0.50%, Mn: 0.50% or more, less than 1.70%, P: 0.200% or less, S: 0.0200% or less, sol.Al: 0.001 to 2.500%, N: 0.0200% or less, B: 0.0002 to 0.0200%, Ti: 0 to 0.300%, Nb: 0 to 0.300%, V: 0 to 0 0.300%, Zr: 0 to 0.300%, Cr: 0 to 2.00%, Mo: 0 to 2.00%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100 %, REM: 0 to 0.1000%, Bi: 0 to 0.0500%, and the balance: Fe and impurities, and the metal structure, in area%, is ferrite: more than 50.0%, tempered martensite: 5.0% or more, Contains less than 50.0%, martensite: 0% or more and less than 10.0%, bainite: 0% or more and less than 20.0%, has a tensile strength of 440 MPa or more and less than 700 MPa, and heat treatment at 170 ° C. for 20 minutes. When implemented, ΔTS, which is the amount of decrease in the tensile strength, is 100 MPa or less.

(2) The hot stamp molded product described in (1) above has the chemical composition, in mass%, of Ti: 0.001 to 0.300%, Nb: 0.001 to 0.300%, V: 0.001 to 0.300%, and Zr: 0.001 to 0.300%. , Cr: 0.001 to 2.00%, Mo: 0.001 to 2.00%, Cu: 0.001 to 2.00%, Ni: 0.001 to 2.00%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, REM: 0.0001 to 0.1000%, and Bi: 0.0001 to 0.0500%. It may contain one or two or more types selected from the group consisting of.

(3) The hot stamp molded product described in (1) or (2) above may have a plating layer on the surface.

(4) A method of manufacturing a hot stamp molded product according to (1) or (2) above, wherein a steel sheet for hot stamping having the chemical composition described in (1) or (2) above is heated to a temperature exceeding the Ac ₃ point. a heating process, and for the steel sheet for hot stamping after the heating process, starting hot stamping at a temperature of (Ar ₃ point -200°C) or more but less than Ar ₃ point, and then cooling to a temperature below 90°C. It includes a hot stamping process and a reheating process in which the molded product after the hot stamping process is heated to a temperature of 100 to 140°C and maintained at that temperature for 3 to 120 minutes.

(5) A method of manufacturing a hot stamp molded product according to (1) or (2) above, wherein a hot stamping steel sheet having the chemical composition described in (1) or (2) above is joined to a bonding steel sheet to form a bonded steel sheet. A joining process, a heating process of heating the joined steel sheet after the joining process to a temperature exceeding the Ac ₃ point of the hot stamping steel sheet, and (Ar) of the hot stamping steel sheet for the joining steel sheet after the heating process. A hot stamping process in which hot stamping is started at a temperature of ₃ points -200°C or higher and less than Ar ₃ point and then cooled to a temperature below 90°C, and the molded article after the hot stamping process is cooled to a temperature of 100 to 140°C. A reheating process is provided in which the temperature is heated to and maintained at that temperature for 3 to 120 minutes.

(6) A method of manufacturing a hot stamp molded product according to (3) above, wherein a steel sheet for hot stamping having the chemical composition described in (1) or (2) above and having a plating layer on the surface is used to produce a hot stamping steel sheet having an Ac temperature exceeding ₃ points. A heating process of heating to a temperature, and for the steel sheet for hot stamping after the heating process, hot stamping is started at a temperature above (Ar ₃ point -200°C) and below Ar ₃ point, and then continued at a temperature below 90°C. A hot stamping process of cooling to , and a reheating process of heating the molded product after the hot stamping process to a temperature of 100 to 140°C and maintaining it at that temperature for 3 to 120 minutes.

(7) A method of manufacturing the hot stamp molded product described in (3) above, wherein a steel sheet for hot stamping having the chemical composition described in (1) or (2) above and having a plating layer on the surface is joined to a steel sheet for joining. A joining process for forming a laminated steel sheet, a heating process for heating the laminated steel sheet after the joining process to a temperature exceeding the Ac ₃ point of the hot stamping steel sheet, and a bonding process for the laminated steel sheet after the heating process. A hot stamping process in which hot stamping is started at a temperature of the steel sheet above (Ar ₃ point -200°C) but below Ar ₃ point and then cooled to a temperature below 90°C, and the molded product after the hot stamping process is subjected to a temperature of 100 to 100°C. A reheating process is provided by heating to a temperature of 140°C and maintaining the temperature at that temperature for 3 to 120 minutes.

According to the present invention, it is possible to obtain a hot stamped product that has little variation in strength due to paint baking treatment (excellent thermal stability) and has a portion with a tensile strength of 440 MPa or more and less than 700 MPa.

Figure 1 is a schematic diagram showing the shape of a hot stamp molded product manufactured in Example 1.
Figure 2 is a schematic diagram showing the shape of the hot stamp molded product manufactured in Example 2.

The present inventors have intensively studied methods for suppressing strength reduction during paint printing for hot stamped products with a tensile strength of 440 MPa or more and less than 700 MPa. As a result, the following knowledge was obtained.

(A) If the metal structure of a hot stamp molded product contains a large amount of hard structures such as martensite or bainite, the tensile strength of the molded product is greatly reduced by the paint baking treatment. This is thought to be because the hard tissue is tempered and made soft.

(B) On the other hand, even if the hard structure fraction is low and the hot stamped product has a metal structure mainly composed of soft structure containing ferrite, the tensile strength is greatly reduced by the paint baking treatment depending on the chemical composition or hot stamping conditions. There are cases where it happens.

(C) In the hot stamping process, hot stamping is started in a temperature range where ferrite and austenite coexist, and the hot stamped product after the hot stamping process is reheated in a predetermined temperature range, thereby performing a paint baking process. The accompanying decrease in tensile strength is suppressed.

Although the reason for this is not clear, the present inventors speculate that it is due to the following reason. (a) In a hot stamp molded product, carbon in a solid solution state contained in ferrite precipitates as coarse iron carbide during stamping, causing a decrease in the strength of ferrite. (b) In hot stamp molded products, fine iron carbides or fine iron carbon clusters are converted into coarse iron carbides during stamping, causing a decrease in the strength of ferrite. (c) When hot stamping is performed in the presence of ferrite, dislocations are introduced into the ferrite in the hot stamped product. (d) When the hot stamped product is reheated, the dissolved carbon in the ferrite precipitates on the dislocation, and the amount of dissolved carbon decreases, and fine iron carbide or fine iron carbon clusters coarsen.

(D) When B (boron) is included in the chemical composition, the decrease in tensile strength accompanying paint baking treatment is suppressed. The reason for this is not clear, but when B is contained, the amount of dislocations introduced into ferrite in the hot stamped product increases, precipitation of carbides by reheating is promoted, and the amount of dissolved carbon is further reduced and iron carbide or It is assumed that this is due to accelerated coarsening of iron-carbon clusters.

From the knowledge of (A) to (D) above, the present inventors used a steel sheet containing a desired amount of B (boron), started hot stamping in a temperature range where ferrite and austenite coexist, and further hot stamped By reheating the hot stamped product after the process, a hot stamped product has a metal structure mainly composed of ferrite, has at least a partial tensile strength of less than 700 MPa, has excellent thermal stability, and has little strength decrease due to paint baking treatment. It was discovered that it could be manufactured.

Hereinafter, each requirement of the hot stamp molded product according to one embodiment of the present invention (hot stamp molded product according to this embodiment) and its manufacturing method will be described in detail. However, the present invention is not limited to the configuration disclosed in the present embodiment, and various changes are possible without departing from the spirit of the present invention.

<Chemical composition of hot stamp molded products>

All or part of the hot stamp molded article according to this embodiment has the chemical composition shown below. The reasons for limitation of each element are as follows. In the following description, “%” regarding the content of the chemical composition all means “mass%.” When a hot stamp molded product has a portion having a tensile strength of less than 700 MPa and a portion having a tensile strength of 700 MPa or more, at least the portion having a tensile strength of less than 700 MPa may have the following chemical composition.

C: 0.001% or more, less than 0.090%

C is an element that has the effect of increasing the tensile strength of the steel sheet (steel sheet included in the hot stamped product) after hot stamping. If the C content is less than 0.001%, an increase in tensile strength by hot stamping cannot be expected. Therefore, the C content is set to 0.001% or more. A preferable C content is 0.020% or more, 0.030% or more, 0.040% or more, or 0.050% or more.

On the other hand, when the C content is 0.090% or more, the area ratio of tempered martensite, martensite and/or bainite increases in the metal structure after hot stamping, and the tensile strength of the hot stamped product becomes 700 MPa or more, and The thermal stability of the stamped product cannot be secured. Therefore, the C content is set to less than 0.090%. Preferred C content is less than 0.085%, less than 0.080%, less than 0.075% or less than 0.070%.

Si: less than 0.50%

Si is an element contained as an impurity in steel. If the Si content is 0.50% or more, it becomes difficult to ensure thermal stability of the hot stamp molded product even if the hot stamp molded product is subjected to reheating treatment as described later. Therefore, the Si content is set to less than 0.50%. Preferred Si contents are less than 0.40%, less than 0.20%, less than 0.10% or less than 0.05%. When using a plated steel sheet as a hot stamping steel sheet, the Si content is preferably less than 0.40%, and more preferably less than 0.30%, to ensure plating properties.

The lower limit of the Si content is not particularly limited, but excessively reducing the Si content causes an increase in steelmaking costs. Therefore, it is preferable that the Si content is 0.001% or more. Additionally, Si has the effect of increasing the tensile strength of the steel sheet after hot stamping, so it may be actively included. From the viewpoint of high strength, the Si content is preferably 0.10% or more or 0.20% or more.

Mn: 0.50% or more, less than 1.70%

Mn is an element that improves the hardenability of steel, and is contained in an amount of 0.50% or more to obtain a metal structure containing ferrite and tempered martensite. A preferable Mn content is 0.60% or more or 0.70% or more.

On the other hand, if the Mn content is 1.70% or more, it becomes difficult to ensure thermal stability of the hot stamp molded product even if the hot stamp molded product is subjected to reheating treatment as described later. Therefore, the Mn content is set to less than 1.70%. The Mn content is preferably less than 1.50%, less than 1.20%, less than 1.00%, or less than 0.80%.

P:0.200% or less

P is an element contained as an impurity in steel. If the P content exceeds 0.200%, weldability and toughness after hot stamping are significantly deteriorated, so the P content is set to 0.200% or less. A preferable P content is 0.100% or less, 0.050% or less, or 0.020% or less.

The lower limit of the P content is not particularly limited, but excessively reducing the P content causes an increase in steelmaking costs. Therefore, it is preferable that the P content is 0.001% or more. In addition, since P has the effect of increasing the tensile strength of the molded product after hot stamping, it may be actively included. From the viewpoint of high strength, the preferable P content is 0.010% or more, 0.020% or more, or 0.030% or more. When using a plated steel sheet as a hot stamping steel sheet, the P content is preferably 0.050% or less, and more preferably 0.040% or less, to ensure plating properties.

S:0.0200% or less

S is an element that is contained as an impurity in steel and embrittles the steel. Therefore, the lower the S content, the more desirable it is. However, if the S content exceeds 0.0200%, embrittlement of the steel becomes significant, so the S content is set to 0.0200% or less. The preferred S content is 0.0100% or less, 0.0050% or less, or 0.0030% or less.

The lower limit of the S content is not particularly limited, but excessively reducing the S content causes an increase in steelmaking costs. Therefore, it is preferable that the S content is 0.0001% or more.

sol.Al:0.001 to 2.500%

Al is an element that has the effect of deoxidizing molten steel. If the sol.Al content (acid-soluble Al content) is less than 0.001%, deoxidation becomes insufficient. Therefore, the sol.Al content is set to 0.001% or more. The sol.Al content is preferably 0.005% or more, 0.010% or more, or 0.020% or more.

On the other hand, if the sol.Al content is too high, the transformation point increases, making it difficult to heat the steel sheet to a temperature exceeding the Ac ₃ point in the hot stamping heating process. Therefore, the sol.Al content is set to 2.500% or less. The sol.Al content is preferably less than 0.500%, less than 0.100%, less than 0.050% or less than 0.040%.

N:0.0200% or less

N is an element contained as an impurity in steel and forming nitrides during continuous casting of steel. Since this nitride deteriorates the toughness after hot stamping, a lower N content is preferable. If the N content exceeds 0.0200%, the deterioration of toughness becomes significant. Therefore, the N content is set to 0.0200% or less. The N content is preferably less than 0.0100%, less than 0.0080% or less than 0.0050%.

The lower limit of the N content is not particularly limited, but excessively reducing the N content causes an increase in steelmaking costs, so it is desirable to set the N content to 0.0010% or more.

B:0.0002 to 0.0200%

B is an element that has the effect of improving the thermal stability of hot stamped products with a metal structure containing ferrite and tempered martensite. When the B content is less than 0.0002%, the effects due to the above actions are not sufficiently obtained. Therefore, the B content is set to 0.0002% or more. A preferable B content is 0.0006% or more, 0.0010% or more, or 0.0015% or more.

On the other hand, if the B content exceeds 0.0200%, the tensile strength of the steel sheet after hot stamping becomes too high, and the thermal stability of the hot stamped product deteriorates. Therefore, the B content is set to 0.0200% or less. The preferred B content is less than 0.0050%, less than 0.0030%, or less than 0.0020%.

Ti:0 to 0.300%

Nb:0 to 0.300%

V:0 to 0.300%

Zr:0 to 0.300%

Ti, Nb, V, and Zr are elements that increase the tensile strength of hot stamped products by refining the metal structure. In order to obtain this effect, one or more types selected from Ti, Nb, V, and Zr may be contained as needed. Since these elements do not need to be contained, the lower limit of the content of these elements is 0%.

In order to obtain the above effect, it is preferable to contain 0.001% or more of at least one selected from Ti, Nb, V, and Zr. Furthermore, it is more preferable to contain at least one of 0.005% or more Ti, 0.005% or more Nb, 0.010% or more V, and 0.005% or more Zr.

When containing Ti, it is more preferable that the Ti content is 0.020% or more, and it is especially preferable that it is 0.030% or more.

When containing Nb, it is more preferable that the Nb content is 0.020% or more, and it is especially preferable that it is 0.030% or more.

When containing V, it is more preferable that the V content is 0.020% or more.

When containing Zr, it is more preferable that the Zr content is 0.010% or more.

On the other hand, when the contents of Ti, Nb, V, and Zr each exceed 0.300%, not only the effect is saturated, but also the manufacturing cost of the steel sheet increases. Therefore, even when these elements are included, the contents of Ti, Nb, V, and Zr are each set to 0.300% or less.

Additionally, when the content of Ti, Nb, V, and Zr is high, a large amount of carbides of these elements may precipitate, which may impair the toughness after hot stamping.

Therefore, the Ti content is preferably less than 0.060%, more preferably less than 0.040%.

The Nb content is preferably less than 0.060%, more preferably less than 0.040%.

The V content is preferably less than 0.200%, more preferably less than 0.100%.

The Zr content is preferably less than 0.200%, more preferably less than 0.100%.

Cr:0 to 2.00%

Mo:0 to 2.00%

Cu:0 to 2.00%

Ni:0 to 2.00%

Cr, Mo, Cu, and Ni have the effect of increasing the tensile strength of hot stamped products (steel sheets after hot stamping). Therefore, one or more types selected from Cr, Mo, Cu, and Ni may be contained as needed. Since these elements do not need to be contained, the lower limit of the content of these elements is 0%.

In order to obtain the above effect, it is preferable to contain 0.001% or more of one or more types selected from Cr, Mo, Cu, and Ni. The preferable Cr content is 0.05% or more, the preferable Mo content is 0.05% or more, the preferable Cu content is 0.10% or more, and the preferable Ni content is 0.10% or more.

On the other hand, if the contents of Cr, Mo, Cu, and Ni each exceed 2.00%, the tensile strength of the steel sheet after hot stamping becomes too high, and the thermal stability of the hot stamped product deteriorates.

Therefore, even when the above elements are contained, the contents of Cr, Mo, Cu, and Ni are each set to 2.00% or less. The preferred Cr content is less than 0.50% or less than 0.20%, the preferred Mo content is less than 0.50% or less than 0.20%, the preferred Cu content is less than 1.00%, and the preferred Ni content is less than 1.00%.

Ca:0 to 0.0100%

Mg:0 to 0.0100%

REM:0 to 0.1000%

Ca, Mg, and REM are elements that have the effect of improving toughness after hot stamping by adjusting the shape of inclusions. Therefore, one or more types selected from Ca, Mg, and REM may be contained as needed. Since these elements do not need to be contained, the lower limit of the content of these elements is 0%.

In order to obtain the above effect, it is preferable to contain 0.0001% or more of one or more types selected from Ca, Mg, and REM.

On the other hand, when the Ca or Mg content exceeds 0.0100%, or when the REM content exceeds 0.1000%, the above effect is saturated and the manufacturing cost of the steel sheet increases. Therefore, even when containing the above elements, the Ca and Mg contents are each set to 0.0100% or less, and the REM content is set to 0.1000% or less.

In this embodiment, REM refers to a total of 17 elements including Sc, Y, and lanthanoid, and REM content means the total content of these elements. Lanthanoids are industrially added in the form of misch metal.

Bi:0 to 0.0500%

Bi is an element that has the effect of improving toughness after hot stamping by refining the solidified structure. Therefore, Bi may be contained as needed. Since Bi does not need to be contained, the lower limit of Bi content is 0%.

In order to obtain the above effect, the Bi content is preferably 0.0001% or more. The Bi content is more preferably 0.0003% or more, and even more preferably 0.0005% or more.

On the other hand, when the Bi content exceeds 0.0500%, the above effect is saturated, and the manufacturing cost of the steel sheet increases. Therefore, even when Bi is included, the Bi content is set to 0.0500% or less. The Bi content is preferably 0.0100% or less, and more preferably 0.0050% or less.

In the above chemical composition, the balance is Fe and impurities. Here, “impurities” are components that are mixed in by raw materials such as ore, scrap, and various factors in the manufacturing process when industrially manufacturing steel sheets, and are allowed as long as they do not adversely affect the hot stamp molded product according to this embodiment. It means becoming.

The chemical composition of the above-mentioned hot stamp molded product can be measured by a general analysis method. For example, it can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). In addition, sol.Al can be measured by ICP-AES using the filtrate after heating and decomposing the sample with acid. C and S can be measured using the combustion-infrared absorption method, and N can be measured using the inert gas fusion-thermal conductivity method.

<Metal structure of hot stamped products>

The metal structure of the hot stamp molded product according to this embodiment will be described. All or part of the hot stamp molded product according to the present embodiment has a metal structure containing ferrite, tempered martensite, martensite, and bainite as shown below. In the following description of metal structure, “%” means “area %.”

Ferrite: over 50.0%

If the area ratio of ferrite is 50.0% or less, the tensile strength of the molded product after hot stamping becomes 700 MPa or more, and thermal stability cannot be secured. Therefore, the area ratio of ferrite is set to exceed 50.0%. The area ratio of ferrite is preferably greater than 60.0%, more preferably greater than 70.0%, and even more preferably greater than 80.0%.

There is no need to specifically determine the upper limit of the area ratio of ferrite, but in order to increase the strength of the hot stamped product, it is preferably less than 95.0%, more preferably less than 90.0%, and even more preferably less than 85.0%. do.

In this embodiment, the ferrite includes, in addition to polygonal ferrite, pseudo-polygonal ferrite and granular bainitic ferrite, which have a higher dislocation density than polygonal ferrite, and acicular ferrite having serrated grain boundaries. From the viewpoint of thermal stability, it is preferable that the ratio of polygonal ferrite to the entire ferrite is 5.0% or more in terms of area ratio.

Tempering martensite: 5.0% or more, less than 50.0%

Tempered martensite is a structure that increases the strength of a hot stamped product while maintaining its thermal stability. If the area ratio of tempered martensite is less than 5.0%, the effect of the above action cannot be sufficiently obtained, making it difficult to ensure thermal stability and/or strength of the hot stamped molded product. Therefore, the area ratio of tempered martensite is set to 5.0% or more. The area ratio of tempered martensite is preferably 8.0% or more, more preferably 10.0% or more, and even more preferably 12.0% or more.

On the other hand, if the area ratio of tempered martensite is 50.0% or more, the tensile strength of the steel sheet after hot stamping becomes too high, and the thermal stability of the hot stamped product deteriorates. Therefore, the area ratio of tempered martensite is set to less than 50.0%. The area ratio of tempered martensite is preferably less than 40.0%, more preferably less than 30.0%, and even more preferably less than 20.0%.

Martensite: 0% or more, less than 10.0%

Bainite: 0% or more, less than 20.0%

If the metal structure (microstructure) contains a large amount of martensite (refers to untempered martensite, also called fresh martensite) and bainite, the thermal stability of the hot stamped product deteriorates. Therefore, the area ratio of martensite is set to less than 10.0%, and the area ratio of bainite is set to less than 20.0%. The area ratio of martensite is preferably less than 5.0%, more preferably less than 2.0%, and even more preferably less than 1.0%. The area ratio of bainite is preferably less than 10.0%, more preferably less than 5.0%, and even more preferably less than 2.0%.

Since martensite and bainite do not necessarily need to be contained, the lower limits of the area ratios of martensite and bainite are both 0%.

However, since martensite and bainite have the effect of increasing the strength of hot stamped products, they may be included in the metal structure as long as they are within the above range. If the area ratios of martensite and bainite are both less than 0.1%, the effect due to the above action cannot be sufficiently obtained. Therefore, when increasing the strength, the lower limit of the area ratio of martensite and bainite is preferably 0.1% or more, and more preferably 0.5% or more.

The remainder of the metal structure may contain pearlite or retained austenite, and may also contain precipitates such as cementite. Since there is no need to actively contain pearlite, retained austenite, and precipitates, the lower limit of the area ratio of pearlite, retained austenite, and precipitates is all 0%.

Since pearlite has the effect of increasing the strength of hot stamped products, when increasing the strength, the area ratio of pearlite is preferably 1.0% or more, more preferably 2.0% or more, and 5.0% or more. It is more desirable to do so.

On the other hand, when pearlite is contained excessively, the toughness after hot stamping deteriorates. Therefore, it is preferable that the area ratio of pearlite is 20.0% or less, and more preferably 10.0% or less.

Retained austenite has the effect of improving the shock absorption of hot stamped products. Therefore, when trying to obtain this effect, it is preferable that the area ratio of retained austenite is 0.5% or more, and more preferably 1.0% or more.

On the other hand, if retained austenite is excessively contained, the toughness after hot stamping decreases. Therefore, it is preferable that the area ratio of retained austenite is less than 3.0%, and more preferably less than 2.0%.

In this embodiment, the area ratio of each metal structure is calculated as follows.

First, a test piece is taken from the hot stamped product, the plate thickness cross section (longitudinal cross section of the steel plate) is polished, and then, in the case of an uncoated steel plate, a test piece is taken at a depth of 1/4 of the plate thickness from the steel plate surface (the plate thickness from the steel plate surface). 1/8 depth to 3/8 depth of the sheet thickness from the surface of the steel sheet), in the case of plated steel sheets, a depth position of 1/4 of the sheet thickness of the base steel sheet from the boundary between the base steel sheet and the plating layer (from the above boundary) The structure is observed in an area (from a depth of 1/8 of the thickness of the base steel sheet to a depth of 3/8 of the thickness of the base steel sheet from the boundary). When a hot stamp molded product has a portion with a tensile strength of less than 700 MPa and a portion with a tensile strength of 700 MPa or more, a test piece is taken from the portion with a tensile strength of less than 700 MPa and observed.

Specifically, after nital corrosion or electrolytic polishing of the polished plate thickness cross section, the structure is observed using an optical microscope and a scanning electron microscope (SEM), and the obtained structure photo is analyzed to determine ferrite and pearlite. , obtain the respective area ratios of bainite and tempered martensite. Afterwards, Repera corrosion was performed on the same observation position, tissue was observed using an optical microscope and a scanning electron microscope (SEM), and image analysis was performed on the obtained tissue photo to determine retained austenite and marten. Calculate the total area ratio of the site.

Additionally, at the same observation position, after electrolytic polishing of the plate thickness cross section, the area ratio of retained austenite is measured using an SEM equipped with an electron beam backscattering pattern analyzer (EBSP).

Based on these results, the respective area ratios of ferrite, pearlite, bainite, tempered martensite, martensite and retained austenite are obtained.

In addition, tempered martensite can be distinguished from martensite in that iron carbide exists inside, and it can also be distinguished from bainite in that iron carbide present inside extends in multiple directions.

<Strength of hot stamped products>

All or part of the hot stamp molded product according to this embodiment has a tensile strength of 440 MPa or more and less than 700 MPa. For this purpose, it is necessary that the tensile strength of all or part of the base steel plate of the hot stamped product according to this embodiment is 440 MPa or more and less than 700 MPa. If the tensile strength is more than 700 MPa, the thermal stability of the hot stamp molded product cannot be secured. Therefore, the tensile strength of all or part of the hot stamped product is set to less than 700 MPa. Preferably, the tensile strength of all or part of the hot stamped product is less than 650 MPa or less than 600 MPa. Meanwhile, in order to improve the shock absorption of the hot stamped product, the tensile strength of all or part of the hot stamped product is set to 440 MPa or more. Preferably, all or part of the hot stamp molded product has a tensile strength of 460 MPa or more, 490 MPa or more, or 540 MPa or more.

In the hot stamp molded product according to the present embodiment, soft parts with a tensile strength of 440 MPa or more and less than 700 MPa and hard parts with a tensile strength of 700 MPa or more may be mixed. By providing portions with different strengths, it becomes possible to control the deformation state of the hot stamped molded product during a collision, thereby improving the shock absorbency of the hot stamped molded product. Hot stamped products having sections with different strengths can be manufactured by joining two or more types of steel sheets with different chemical compositions and then hot stamping them, as described later.

<Thermal stability of hot stamped products>

In the hot stamp molded product according to this embodiment, when heat treatment is performed at 170°C for 20 minutes, the decrease in tensile strength (ΔTS) relative to the tensile strength before heat treatment is 100 MPa or less. ΔTS is preferably 60 MPa or less, and more preferably 30 MPa or less. The lower limit of ΔTS is not particularly limited, but from the viewpoint of manufacturability of steel sheets, it is preferably 1 MPa or more, 5 MPa or more, or 10 MPa or more.

In a hot stamped product having a structure mainly composed of ferrite (exceeding 50.0% in area ratio), the reason why the strength decreases during painting is that the dissolved carbon present in the ferrite becomes coarse due to the painting baking process. This is thought to be because the fine iron carbide or fine iron carbon clusters that precipitate as iron carbide and exist in ferrite are changed into coarse iron carbide by heat treatment during painting and printing. It is not easy to directly quantitatively evaluate the presence of this dissolved carbon and fine iron carbide or fine iron carbon clusters, but the amount of decrease in tensile strength (ΔTS) when heat treatment at 170°C for 20 minutes is not easy. It can be evaluated indirectly. When ΔTS is 100 MPa or less, the amount of dissolved carbon in the ferrite and the amount of fine iron carbide or fine iron carbon clusters generated are low, and thermal stability is judged to be excellent.

The tensile strength is obtained by taking a JIS13B tensile test piece and performing a tensile test at a tensile speed of 10 mm/min.

<Plating layer>

The hot stamp molded product according to this embodiment may have a plating layer on the surface. By providing a plating layer on the surface, it becomes possible to prevent scale formation during hot stamping and improve the corrosion resistance of the hot stamped molded product. The type of plating is not particularly limited as long as it is suitable for the above purpose. A hot stamp molded product with a plating layer can be obtained by hot stamping using a plated steel sheet, as described later. A hot stamped product having a plating layer, which is a zinc-based plated steel sheet or an aluminum-based plated steel sheet, specifically, for example, hot-dip galvanized steel sheet, alloyed hot-dip galvanized steel sheet, hot-dip aluminum-plated steel sheet, hot-dip Zn-Al alloy-plated steel sheet, hot-dip galvanized steel sheet, Hot stamped using Zn-Al-Mg alloy-plated steel sheet, hot-dip Zn-Al-Mg-Si alloy-plated steel sheet, electro-galvanized steel sheet, and electro-Ni-Zn alloy-plated steel sheet, etc., having a zinc-based plating layer or an aluminum-based plating layer. Hot stamp molded articles are exemplified. The plating layer may be formed on one side or on both sides.

Next, a steel sheet for hot stamping suitable for manufacturing the above hot stamped molded product will be described.

<Chemical composition of steel sheet for hot stamping>

Since the chemical composition does not change substantially due to hot stamping, the chemical composition of the steel sheet for hot stamping has the same chemical composition as that of the hot stamping molded article described above.

<Metal structure of steel sheet for hot stamping>

The metal structure of the steel sheet for hot stamping according to the present embodiment preferably includes iron carbide, and the chemical composition of the iron carbide (Mn content and Cr content in the iron carbide) satisfies the following equation (i).

[Mn] _θ +[Cr] _θ >1.7 … (i)

However, the meaning of each symbol in the above formula is as follows.

[Mn] _θ : Mn content (atomic %) in iron carbide when the total content of Fe, Mn, and Cr contained in iron carbide is 100 atomic %

[Cr] _θ : Cr content (atomic %) in iron carbide when the total content of Fe, Mn, and Cr contained in iron carbide is 100 atomic %.

When the chemical composition of iron carbide contained in the metal structure of the steel sheet for hot stamping satisfies the above equation (i), it becomes possible to further improve the thermal stability of the steel sheet after hot stamping. The value of the left side of equation (i) above is preferably greater than 3.0, and more preferably greater than 4.0.

On the other hand, in order to increase the Mn content and Cr content in the iron carbide, it is necessary to anneal the hot-rolled steel sheet at a high temperature in the hot-rolled sheet annealing process described later, so the manufacturability of the steel sheet is impaired. Therefore, the value of the left side of equation (i) above is preferably less than 20.0, and more preferably less than 10.0.

In this embodiment, the chemical composition of iron carbide is measured by the following procedure.

First, a test piece is collected from an arbitrary position on the steel sheet, a cross-section (longitudinal cross-section) of the sheet thickness parallel to the rolling direction of the steel sheet is ground, and then a test piece is taken at a depth of 1/4 of the sheet thickness from the surface of the steel sheet (1/4 of the sheet thickness from the surface of the steel sheet). Precipitates are extracted by the replica method in an area (/8 depth or 3/8 depth of the plate thickness from the surface of the steel plate). This precipitate is observed using a transmission electron microscope (TEM), and the precipitate is identified and composition analyzed by electron beam diffraction and energy dispersive X-ray analysis (EDS).

Quantitative analysis of iron carbide by EDS was performed on the three elements Fe, Mn, and Cr, and the Mn content (atomic %) and Cr content (atomic %) when their total content was set to 100 atomic % were respectively [ It is obtained as [Mn] _θ and [Cr] _θ . This quantitative analysis is performed on a plurality of iron carbides, and the average value is taken as the Mn content and Cr content in the iron carbide in the steel sheet. The number of iron carbides to be measured should be 10 or more, and the greater the number of measurements, the more preferable. Iron carbide includes cementite that exists in isolation in the metal structure in addition to the cementite that constitutes pearlite.

In this embodiment, in the case of hot rolled annealed steel sheet, cold rolled steel sheet, or annealed steel sheet, the position is at a depth of 1/4 of the sheet thickness from the steel sheet surface (1/8 of the sheet thickness from the steel sheet surface to a depth of 3/8 of the sheet thickness from the steel sheet surface). area), in the case of plated steel sheets, a depth position of 1/4 of the sheet thickness of the base steel sheet from the boundary between the base steel sheet and the plating layer (1/8 depth of the sheet thickness of the base steel sheet from the boundary to the base point from the boundary) In the area (3/8 depth of the steel plate thickness), the above-mentioned metal structure is defined.

There is no need to specifically determine the area ratio of iron carbide, but in order to refine the metal structure after hot stamping and increase tensile strength, the area ratio of iron carbide is preferably 1% or more, and more preferably 3% or more. do.

On the other hand, if the area ratio of iron carbide is excessive, the tensile strength of the steel sheet after hot stamping becomes too high and thermal stability is impaired. Therefore, the area ratio of iron carbide is preferably 20% or less, and more preferably 15% or less.

The metal structure of the steel sheet for hot stamping according to this embodiment is preferably composed mainly of ferrite, but may also contain martensite, tempered martensite, bainite, and retained austenite as the remainder, and may also contain other elements other than iron carbide. It may contain precipitates. However, martensite, tempered martensite, bainite, and retained austenite deteriorate the toughness after hot stamping, so a smaller area ratio of these structures is preferable. It is preferable that the area ratios of martensite, tempered martensite, bainite, and retained austenite are all less than 1.0%, and more preferably less than 0.5%.

The area ratio in the metal structure of a hot stamping steel sheet can be obtained by the same method as in the case of a hot stamping molded product.

The tensile strength of the steel sheet for hot stamping is not particularly limited, but from the viewpoint of the manufacturability of the steel sheet, it is preferably 300 MPa or more or 340 MPa or more, and from the viewpoint of the cutability of the steel sheet, it is preferably 650 MPa or less or less than 590 MPa. .

<Manufacturing method>

A preferred method of manufacturing the hot stamped product according to this embodiment and the steel sheet for hot stamping according to this embodiment will be described.

[Manufacturing method of hot stamp molded product]

The method of manufacturing a hot stamp molded product according to the present embodiment includes a heating process of heating a steel sheet for hot stamping having the above-described chemical composition, a hot stamping process of performing hot stamping on the heated steel sheet for hot stamping, and then cooling it. , including a reheating process to reheat the molded product after the hot stamping process. In the hot stamping process, molding and cooling are performed using a mold, and a hot stamped product is obtained.

In the heating process of heating the steel sheet for hot stamping, the heating temperature is set to exceed Ac ₃ point. The Ac ₃ point is the temperature at which ferrite disappears from the metal structure when the raw steel sheet is heated, and can be obtained from the change in thermal expansion of the steel sheet during the heating process. If the heating temperature is below the Ac ₃ point, dissolution of the carbide during heating becomes insufficient, and the strength of the hot stamped product decreases. The heating temperature is preferably (Ac ₃ point + 20°C) or higher, and more preferably (Ac ₃ point + 40°C) or higher.

Additionally, the steel sheet for hot stamping used for heating preferably has the structure described above.

The upper limit of the heating temperature is not particularly limited, but if the heating temperature is too high, austenite coarsens and the strength of the hot stamped product decreases. Therefore, the heating temperature is preferably 1000°C or lower, more preferably 950°C or lower, and even more preferably 900°C or lower.

Additionally, the preferred holding time at the hot stamp heating temperature is 1 to 5 minutes.

In the hot stamping process in which hot stamping is performed on a heated steel sheet for hot stamping, the starting temperature of hot stamping is set to be higher than (Ar ₃ point -200°C) or higher and lower than Ar ₃ point. The Ar ₃ point is the temperature at which ferrite begins to form in the metal structure when the raw steel sheet is cooled from a temperature exceeding the Ac ₃ point. The Ar ₃ point is determined from the change in thermal expansion when cooling the steel sheet after the heating process. If the hot stamping start temperature is above the Ar ₃ point, the amount of dislocations introduced into the ferrite is insufficient, and the thermal stability of the hot stamped product is impaired. If the hot stamping start temperature is less than (Ar ₃ point -200°C), the area ratio of tempered martensite in the metal structure of the hot stamped product is reduced, and the strength of the hot stamped product is insufficient. The upper limit of the preferred hot stamping start temperature is less than (Ar ₃ point -20°C), less than (Ar ₃ point -40°C) or less than (Ar ₃ point -60°C). The lower limit of the preferred hot stamping start temperature is (Ar ₃ point -170°C) or more, (Ar ₃ point -140°C) or more, or (Ar ₃ point -110°C) or more.

After performing molding by hot stamping, the molded article is cooled to a temperature of less than 90°C by holding the molded article in the mold and/or taking the molded article out of the mold and cooling it by any method. If the cooling stop temperature is 90°C or higher, the area ratio of tempered martensite is reduced in the metal structure of the hot stamped molded product, and the strength of the hot stamped molded product is insufficient. The cooling stop temperature is preferably less than 50°C, and is more preferably room temperature. In order to increase productivity, it is desirable to maintain the temperature in the mold below 90°C.

In the reheating process of reheating the hot stamp molded product described above, the reheating temperature is set to 100 to 140°C, and the holding time at the reheating temperature is set to 3 to 120 minutes. If the reheating temperature is less than 100°C, fine iron carbides or fine iron carbon clusters are generated, and the thermal stability of the hot stamped product is deteriorated. On the other hand, if the reheating temperature exceeds 140°C, the strength of the hot stamped product decreases.

If the holding time is less than 3 minutes, a large amount of dissolved carbon remains in ferrite in the metal structure of the hot stamped product, and the thermal stability of the hot stamped product deteriorates. On the other hand, if the holding time exceeds 120 minutes, the strength of the hot stamped product decreases. The holding time is preferably adjusted according to the reheating temperature. When the reheating temperature is 100°C or more and less than 120°C, the holding time is preferably longer than 60 minutes, longer than 70 minutes, or longer than 80 minutes, and the holding time is Preferably it is less than 110 minutes, less than 100 minutes, or less than 90 minutes. When the reheating temperature is 120 to 140 minutes, the holding time is preferably greater than 5 minutes, greater than 7 minutes, or greater than 9 minutes, and the holding time is preferably less than 30 minutes, less than 20 minutes, or less than 15 minutes. From the viewpoint of productivity, it is preferable that the reheating temperature is 120 to 140°C.

In addition, another method of manufacturing a hot stamp molded product according to the present embodiment includes a joining process of joining a steel sheet (steel sheet for hot stamping) having the above-described chemical composition with a steel sheet for bonding to form a bonded steel sheet, and heating the bonded steel sheet. A process includes a process of hot stamping the heated bonded steel sheet and subsequent cooling, and a process of reheating the hot stamped product after the hot stamping process. Examples of the joining method include a method of butting or overlapping a hot stamping steel sheet and a joining steel sheet and joining them by welding.

The above-mentioned bonded steel sheet is heated to a temperature exceeding the Ac ₃ point of the hot stamping steel sheet, and hot stamping is started at a temperature above (Ar ₃ point -200°C) but below the Ar ₃ point of the hot stamping steel sheet, Continue cooling to a temperature below 90°C. Thereafter, the hot stamp molded product is reheated to a temperature of 100 to 140° C. and maintained at that temperature for 3 to 120 minutes. A preferable heating temperature in the process of heating the laminated steel sheet, a preferable hot stamping start temperature and a preferable cooling stop temperature in the process of hot stamping the laminated steel sheet, and a preferable reheating temperature in the process of reheating the hot stamped molded product, and The preferable holding time is the same as the method for manufacturing a hot stamped molded product that does not include a joining process, and the reasons are the same as in the case that does not include a joined steel sheet.

There are no particular limitations on the chemical composition and mechanical properties of the steel sheets for joining. However, in order to increase the shock absorption energy of hot stamped products, it is preferable that the steel sheet for joining has a tensile strength of 700 MPa or more after reheating. A more preferable tensile strength of the steel sheet for joining after reheating is more than 1000 MPa, more than 1200 MPa, or more than 1500 MPa.

In order to ensure the tensile strength of the steel sheet for joining after hot stamping, the C content of the steel sheet for joining is preferably 0.090% or more. More preferably, it is 0.100% or more, 0.120% or more, or 0.200% or more. For the same reason, it is preferable that the Mn content of the steel sheet for joining is 0.50% or more. More preferably, it is 0.80% or more, 1.00% or more, or 1.20% or more.

The steel sheet (steel sheet for hot stamping) used as the above material is preferably subjected to hot-rolled sheet annealing as described later. After annealing the hot rolled sheet, cold rolling or cold rolling and continuous annealing may be performed. On the other hand, as the steel sheet for joining, any of a hot rolled steel sheet, a cold rolled steel sheet obtained by cold rolling a hot rolled steel sheet, a hot rolled annealed steel sheet obtained by annealing a hot rolled steel sheet, and a cold rolled annealed steel sheet obtained by annealing a cold rolled steel sheet may be used.

In order to improve the corrosion resistance of hot stamped products, plated steel sheets with plating on the surface may be used as steel sheets for hot stamping and steel sheets for joining. The type of galvanized steel sheet is not particularly limited, but includes hot-dip galvanized steel sheet, alloyed hot-dip galvanized steel sheet, hot-dip aluminum-coated steel sheet, hot-dip Zn-Al alloy-coated steel sheet, hot-dip Zn-Al-Mg alloy-coated steel sheet, hot-dip Zn-Al-Mg -Si alloy-plated steel sheets, electro-galvanized steel sheets, and electro-Ni-Zn alloy-coated steel sheets, etc. are examples.

[Method of manufacturing steel plate for hot stamping]

The steel sheet for hot stamping according to this embodiment includes a hot rolling process of performing hot rolling on a slab having the above-described chemical composition and then coiling it in a temperature range of 800°C or lower to obtain a hot-rolled steel sheet, and forming the hot-rolled steel sheet at 650°C. It is preferable that it is manufactured by a manufacturing method including a hot-rolled sheet annealing process of heating a hot-rolled sheet to a temperature range exceeding the temperature range to obtain a hot-rolled annealed steel sheet.

In the hot rolling process, it is preferable that the coiling temperature after hot rolling is 800°C or lower. If the coiling temperature exceeds 800°C, the metal structure of the hot-rolled steel sheet becomes excessively coarse, and the tensile strength of the steel sheet after hot stamping decreases. More preferred coiling temperatures are below 650°C, below 600°C or below 550°C. Additionally, if the coiling temperature is too low, the hot-rolled steel sheet becomes hard and cold rolling becomes difficult, so the coiling temperature is preferably 400°C or higher.

In addition, the method for manufacturing the slab provided in the method for manufacturing a steel sheet for hot stamping according to the present embodiment is not particularly limited. In the preferred manufacturing method of the exemplified slab, the steel having the above-mentioned component composition (chemical composition) is melted by a known means and then formed into a steel ingot by a continuous casting method, or after being formed into a steel ingot by an arbitrary casting method. It is made into steel pieces by crushing and rolling methods. In the continuous casting process, in order to suppress the occurrence of surface defects caused by inclusions, it is desirable to generate external additional flow, such as electromagnetic stirring, in the molten steel within the mold. The steel ingot or steel piece may be reheated once cooled and then submitted to hot rolling, or the steel ingot in a high temperature state after continuous casting or the steel piece in a high temperature state after crush rolling may be hot rolled as is, by insulating the steel, or by performing auxiliary heating. You may provide it to . In this embodiment, such steel ingots and steel pieces are collectively referred to as “slabs” as materials for hot rolling.

The temperature of the slab to be subjected to hot rolling is preferably lower than 1250°C, and more preferably lower than 1200°C, in order to prevent coarsening of austenite. Hot rolling is preferably completed in a temperature range of Ar ₃ point or higher in order to refine the metal structure of the hot rolled steel sheet by transforming austenite after completion of rolling.

When hot rolling consists of rough rolling and finish rolling, the rough rolled material may be heated between rough rolling and finish rolling in order to complete finish rolling at the above temperature. At this time, it is desirable to suppress the temperature fluctuation over the entire length of the rough rolled material to 140°C or lower at the start of finish rolling by heating the rear end of the rough rolled material to a higher temperature than the leading end. This improves the uniformity of product characteristics within the coil after the winding process.

The method of heating the rough rolled material may be performed using a known means. For example, a solenoid-type induction heating device may be provided between the rough rolling mill and the finishing mill, and the amount of heating temperature rise may be controlled based on the temperature distribution in the longitudinal direction of the rough rolled material on the upstream side of the induction heating device.

It is preferable that the hot-rolled and coiled steel sheet is subjected to treatment such as degreasing according to a known method as needed and then annealed. Annealing performed on a hot-rolled steel sheet is called hot-rolled sheet annealing, and the steel sheet after hot-rolled annealing is called a hot-rolled annealed steel sheet. Before annealing the hot-rolled sheet, descaling may be performed by pickling or the like.

It is preferable that the heating temperature in the hot-rolled sheet annealing process exceeds 650°C. This is to increase the Mn content and Cr content in iron carbide in the metal structure of the hot-rolled annealed steel sheet. The heating temperature in the hot-rolled sheet annealing process is more preferably over 680°C, and even more preferably over 700°C. On the other hand, if the heating temperature in the hot-rolled sheet annealing process becomes too high, the metal structure of the hot-rolled annealed steel sheet becomes coarse, and the tensile strength after hot stamping decreases. Therefore, the upper limit of the heating temperature in the hot-rolled sheet annealing process is more preferably less than 750°C, and even more preferably less than 720°C.

In order to fully obtain the effect of hot-rolled sheet annealing, it is preferable to maintain the heating temperature for 30 minutes or more. On the other hand, if the holding time is too long, the metal structure of the hot rolled annealed steel sheet becomes coarse, and the tensile strength after hot stamping decreases. Therefore, the holding time at the heating temperature in the hot-rolled sheet annealing process is preferably less than 10 hours, more preferably less than 5 hours, and even more preferably less than 2 hours.

After the hot-rolled sheet annealing process described above, it is preferable to perform cold rolling on the hot-rolled annealed steel sheet to obtain a cold-rolled steel sheet with a sheet thickness of 2.8 mm or less. In order to reduce the weight of hot stamped products, the thickness of the cold rolled steel sheet is more preferably 2.3 mm or less, further preferably 2.0 mm or less, particularly preferably 1.8 mm or less, and even more preferably 1.6 mm or less. Additionally, from the viewpoint of manufacturability of the steel sheet, it is preferable that the sheet thickness of the cold rolled steel sheet is 0.6 mm or more.

Cold rolling may be performed according to a normal method, and descaling may be performed by pickling or the like before cold rolling. In cold rolling, in order to refine the metal structure after hot stamping and increase tensile strength, the cold rolling rate (cumulative rolling reduction during cold rolling) is preferably 30% or more, and more preferably 40% or more. If the cold pressing rate is too high, the toughness after hot stamping deteriorates. Therefore, it is preferable to set the cold pressure ratio to 65% or less, and more preferably to 60% or less. As described later, when continuous annealing is performed after cold rolling, the cold rolling ratio is preferably 60% or more, and more preferably 70% or more, in order to refine the metal structure of the annealed steel sheet.

Continuous annealing may be performed on a cold rolled steel sheet to obtain an annealed steel sheet. Continuous annealing may be performed according to a normal method, and before performing continuous annealing, treatment such as degreasing may be performed by a known method. In order to refine the metal structure of an annealed steel sheet by recrystallization, it is desirable to set the cracking temperature in continuous annealing to 600°C or higher, 650°C or higher, or 700°C or higher.

On the other hand, if the heating rate during continuous annealing is too slow, the soaking temperature is too high, or the soaking time is too long, the metal structure of the annealed steel sheet becomes coarse due to grain growth, and the tensile strength after hot stamping decreases. Therefore, it is preferable that the average heating rate up to the quenching temperature in annealing is 1°C/sec or more, the quenching temperature is preferably 800°C or lower or 760°C or lower, and the quenching time is less than 300 seconds or 120°C or less. Preferably less than a second.

The hot rolled annealed steel sheet, cold rolled steel sheet, and annealed steel sheet obtained in this way may be subjected to temper rolling according to a normal method.

The steel sheet for hot stamping according to this embodiment may be provided with a plating layer on the surface layer for the purpose of preventing the formation of scale during hot stamping and improving the corrosion resistance of the steel sheet after hot stamping. The type of plating may be any suitable for the above purpose and is not particularly limited, but includes hot-dip galvanized steel sheet, alloyed hot-dip galvanized steel sheet, hot-dip aluminum-plated steel sheet, hot-dip Zn-Al alloy-plated steel sheet, and hot-dip Zn-Al-Mg alloy plating. Examples include steel sheets, hot-dip Zn-Al-Mg-Si alloy-plated steel sheets, electro-galvanized steel sheets, and electro-Ni-Zn alloy-coated steel sheets.

When manufacturing a hot-dip galvanized steel sheet, the hot-rolled annealed steel sheet, cold-rolled steel sheet, or annealed steel sheet manufactured by the method described above can be used as a raw steel sheet, and plating can be performed according to a conventional method. When using a cold rolled steel sheet as a raw steel sheet, in order to refine the metal structure of the plated steel sheet by recrystallization, the cracking temperature during the annealing process of continuous hot dip plating is set to 600°C or higher, 650°C or higher, or 700°C or higher. desirable.

On the other hand, if the cracking temperature is too high, the metal structure of the annealed steel sheet becomes coarse due to grain growth, so regardless of the type of raw steel sheet, the cracking temperature during the annealing process of continuous hot dip plating should be 800°C or lower or 760°C or lower. It is desirable to do so. After hot dip plating, the steel sheet may be reheated to perform alloying treatment.

When manufacturing an electroplated steel sheet, the hot rolled annealed steel sheet, cold rolled steel sheet, or annealed steel sheet manufactured by the above-mentioned method is used as a raw steel sheet, and after performing known pretreatment for surface cleaning and adjustment as necessary, the usual method is used. Electroplating may be performed according to . The plated steel sheet obtained in this way may be subjected to temper rolling according to a normal method.

Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited to these examples.

Example

(Example 1)

Molten steel was cast using a vacuum melting furnace to produce steels A to N having the chemical compositions shown in Table 1. The Ac ₁ point and Ac ₃ point in Table 1 were obtained from the change in thermal expansion when cold-rolled steel sheets of steels A to N were heated at 2°C/sec. In addition, the Ar ₃ point in Table 1 was obtained from the change in thermal expansion when cold-rolled steel sheets of steels A to N were heated to 920°C and then cooled at 10°C/sec. Steels A to N were heated to 1200°C and held for 60 minutes, and then hot rolled under the hot rolling conditions shown in Table 2.

Specifically, 10 passes of rolling were performed on steels A to N in a temperature range of Ar ₃ or higher to obtain a hot rolled steel sheet with a thickness of 3.6 mm. After hot rolling, the hot-rolled steel sheet is cooled to 540 to 580°C with water spray, the cooling end temperature is set as the coiling temperature, the hot-rolled steel sheet is charged into an electric heating furnace maintained at this coiling temperature, and maintained for 60 minutes. , the hot-rolled steel sheet was furnace cooled to room temperature at an average cooling rate of 20°C/hour to simulate slow cooling after coiling.

After slow cooling, hot rolled annealing was performed on some of the hot rolled steel sheets. Specifically, a hot-rolled steel sheet is heated to 710°C at an average heating rate of 50°C/hour using an electric heating furnace, held for 1 hour, and then cooled at an average cooling rate of 20°C/hour to perform hot-rolled annealing. It was made of steel plate.

A hot rolled steel sheet and a hot rolled annealed steel sheet were pickled to serve as a base material for cold rolling, and cold rolled at a cold rolling rate of 61% to obtain a cold rolled steel sheet with a thickness of 1.4 mm. Some cold rolled steel sheets were heated to the annealing cracking temperature shown in Table 2 at an average heating rate of 10°C/sec using a continuous annealing simulator and cracked for 60 seconds. It was then cooled to 400°C and held for 180 seconds, and then cooled to room temperature to obtain an annealed steel sheet. For the obtained annealed steel sheet, “ACR” was written in the “steel grade” column in Table 3, and “-” was written in the “plated type” column. In addition, for cold-rolled steel sheets, “CR” was written in the “steel grade” column in Table 3, and “-” was written in the “plated type” column.

Additionally, some cold rolled steel sheets were heated to the annealing cracking temperature shown in Table 2 at an average heating rate of 10°C/sec using a hot dip plating simulator and cracked for 60 seconds. It was then cooled and immersed in a hot-dip galvanizing bath or a hot-dip aluminum plating bath to perform hot-dip galvanizing or hot-dip aluminum plating to obtain a hot-dip galvanized steel sheet or a hot-dip aluminum-plated steel sheet. After hot-dip galvanizing, some of the steel sheets were subjected to alloying treatment by heating to 520°C to obtain alloyed hot-dip galvanized steel sheets. For the obtained plated steel sheet, "ACR" was written in the "steel grade" column in Table 3, and "GI", "GA", or "AL" was written in the "plated type" column.

From the cold-rolled steel sheet, annealed steel sheet, hot-dip galvanized steel sheet, alloyed hot-dip galvanized steel sheet, and hot-dip aluminum-plated steel sheet (these steel sheets are collectively referred to as hot stamping steel sheets) obtained in this way, a test piece for structure observation was collected, and the structure was analyzed. Observation was made.

Specifically, in the case of non-coated steel sheets (cold rolled steel sheets and annealed steel sheets), after grinding the sheet thickness cross section parallel to the rolling direction, a position at a depth of 1/4 of the sheet thickness of the steel sheet from the steel sheet surface (plate thickness from the steel sheet surface) 1/8 depth to 3/8 depth of the sheet thickness from the surface of the steel sheet), in the case of plated steel sheets, a depth position of 1/4 of the sheet thickness of the base steel sheet from the boundary between the base steel sheet and the plating layer (from the above boundary) Precipitates were extracted by a replica method from a region (from a depth of 1/8 of the thickness of the base steel sheet to a depth of 3/8 of the thickness of the steel sheet as a base from the above boundary), and iron carbides were identified using TEM. For 10 iron carbides, quantitative analysis was performed for the three elements Fe, Mn, and Cr using EDS. When the total content of Fe, Mn, and Cr is set to 100 atomic%, the Mn content (atomic %) and Cr content (atomic %) in iron carbide are set as [Mn] _θ and [Cr] _θ , respectively, and [Mn ] The average value of the sum of _θ and [Cr] _θ was obtained.

Additionally, a JIS13B tensile test piece was taken from the hot stamping steel sheet along a direction straight to the rolling direction, and a tensile test was performed at a tensile speed of 10 mm/min to determine the tensile strength. Table 3 shows the results of observing the metal structure of the hot stamping steel sheet and the mechanical properties of the hot stamping steel sheet.

From the steel sheet for hot stamping, a raw plate for hot stamping with a width of 240 mm and a length of 170 mm was taken, and a hat member of the shape shown in FIG. 1 was manufactured by hot stamping. In the hot stamping process, a gas heating furnace is used, and the raw plate is heated for 4 minutes at the heating temperature shown in Table 4, then taken out from the heating furnace, left to cool, and placed in a mold equipped with a cooling device at the starting temperature shown in Table 4. The hat was molded by fitting, and then cooled in the mold to the cooling stop temperature shown in Table 4. Additionally, some of the hat members were reheated using an electric heating furnace under the conditions shown in Table 4. RT of the hot stamp conditions in Table 4 indicates room temperature, and “-” indicates that no reheating process was performed.

Some hat members (hot stamp molded products) were subjected to heat treatment at 170°C for 20 minutes using an electric heating furnace.

A test piece for SEM observation is taken from the vertical wall portion of the hat member before heat treatment, and a section through the thickness of the test piece parallel to the rolling direction of the steel sheet is polished, and then subjected to Nital corrosion and Repera corrosion on the section through the thickness of the plate, In the case of non-plated steel sheets, a depth location of 1/4 of the sheet thickness of the steel sheet from the surface of the steel sheet (area from a depth of 1/8 of the sheet thickness from the surface of the steel sheet to a depth of 3/8 of the sheet thickness from the surface of the steel sheet), in the case of plated steel sheets. is a depth position of 1/4 of the plate thickness of the base steel sheet from the boundary between the base steel sheet and the plating layer (1/8 depth of the plate thickness of the base steel sheet from the boundary to 3/3 of the plate thickness of the base steel sheet from the boundary). The metal structure in an area of 8 depth was observed. Using the method described above, the area ratios of ferrite, pearlite, retained austenite, tempered martensite, martensite, and bainite were measured by image processing. The results are shown in Table 4. The remainder of the structure shown in Table 4 was pearlite, retained austenite, and/or precipitates. In addition, in the table, in the test numbers satisfying the provisions of the present invention, the proportion of polygonal ferrite in ferrite in the metal structure of the hot stamped product was 5.0% or more.

Additionally, a JIS13B tensile test piece was taken from the vertical wall portion of the hat member before and after heat treatment along the longitudinal direction of the member, and a tensile test was performed at a tensile speed of 10 mm/min to determine the tensile strength. The difference (ΔTS) between the tensile strength of the hat member that was not heat treated and the tensile strength of the hat member that was heat treated was determined, and if ΔTS was 100 MPa or less, it was judged that the thermal stability of the hat member was good.

When the tensile strength before heat treatment was 440 MPa or more and less than 700 MPa, and ΔTS was 100 MPa or less, the test was judged as passing as satisfying the provisions of the present invention. On the other hand, cases where the tensile strength before heat treatment was less than 440 MPa or more than 700 MPa, or ΔTS was more than 100 MPa were judged to be disqualified as they did not satisfy the provisions of the present invention.

Table 4 shows the results of observing the metal structure of the hat member and the results of evaluating the mechanical properties of the hat member. In Tables 1 to 4, the underlined values mean outside the scope of the present invention or outside the preferred manufacturing conditions.

Test numbers 1 to 3, 9 to 11, 14 to 17, and 25 to 33 that satisfy the provisions of the present invention all show good strength characteristics, with the tensile strength of the hot stamped molded product being 440 MPa or more and less than 700 MPa. , and ΔTS is 100 MPa or less, showing good thermal stability.

In addition, in the manufacturing process of hot stamping steel sheets, test numbers 2, 10, 16, 25, and 27 in which hot-rolled sheet annealing was performed had ΔTS of hot stamping molded products of 30 MPa or less, showing particularly good thermal stability.

In comparison with these, in Test Nos. 20 to 24 of Comparative Examples using steel sheets whose chemical compositions deviate from the scope of the present invention, the tensile strength of the hot stamped product was less than 440 MPa and the strength characteristics were poor, or ΔTS was 100 MPa or more. As a result, thermal stability decreased.

Specifically, in Test No. 21 using steel E, the Mn content of the steel was too low, so the tempered martensite area ratio was insufficient in the metal structure of the hot stamped product, and the tensile strength of the hot stamped product was low.

In test number 20 using steel D, the Mn content of the steel was too high, so the tensile strength of the hot stamped product was more than 700 MPa and ΔTS was large.

In test number 22 using steel F, the C content of the steel was too high, so the ferrite area ratio was insufficient in the metal structure of the hot stamped product, the tensile strength of the hot stamped product was 700 MPa or more, and ΔTS was large.

In test number 23 using steel G, ΔTS was large because the Si content of the steel was too high.

In test number 24 using steel H, ΔTS was large because the B content of the steel was too low.

Test numbers 4 to 8, 12, 13, 18 and 19 of comparative examples in which the chemical composition is within the scope of the present invention but the manufacturing conditions of the hot stamp molded product are outside the scope of the present invention show that the tensile strength of the hot stamp molded product is less than 440 MPa. , strength characteristics were poor, or ΔTS was 100 MPa or more, resulting in poor thermal stability.

Specifically, in Test Nos. 4 and 5 using steel A, the holding time in the reheating process was too long or the reheating temperature was too high, so the tensile strength of the hot stamped product was low.

Test No. 6 using steel A had low tensile strength because the heating temperature in the heating process was too low.

Test numbers 7 and 8 using steel A had low tensile strength because the forming start temperature in the hot stamping process was too low or the cooling stop temperature was too high.

In tests numbers 12 and 13 using steel B, the holding time in the reheating process was too short or the reheating temperature was too low, so the tempered martensite area ratio was insufficient in the metal structure of the hot stamped product, and ΔTS was large. .

In test number 18 using steel C, ΔTS was large because the forming start temperature in the hot stamping process was too high. In test number 19 using steel C, the reheating process was not performed, so the tempered martensite area ratio was insufficient and ΔTS was large.

(Example 2)

Molten steel was cast using a vacuum melting furnace to produce steels A to C having the chemical compositions shown in Table 1 in Example 1. Using steels A to C, hot rolling, hot-rolled sheet annealing, cold rolling, and annealing were performed under the conditions shown in Table 5, as in Example 1, and then plating was performed to produce hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets. Steel sheets and hot-dip aluminum-plated steel sheets (steel sheets for hot stamping) were manufactured.

The metal structure and mechanical properties of these hot stamping steel sheets were investigated in the same manner as Example 1. Table 6 shows the results of observing the metal structure of the hot stamping steel sheet and the mechanical properties of the hot stamping steel sheet.

From these hot stamping steel plates, unprocessed hot stamping plates with a thickness of 1.4 mm, a width of 240 mm, and a length of 170 mm were taken. This unprocessed plate was joined to a bonding steel plate of the same size by laser welding to produce a bonded steel plate with a thickness of 1.4 mm, a width of 240 mm, and a length of 340 mm. The steel sheet for joining is cold-rolled with a chemical composition in mass% of 0.21%C-0.13%Si-1.31%Mn-0.012%P-0.0018%S-0.043%sol.Al-0.0030%N-0.21%Cr-0.0018%B. A steel plate was used.

As in Example 1, the bonded steel sheet was hot stamped under the conditions shown in Table 7, and a hat member of the shape shown in FIG. 2 was manufactured. After that, heat treatment was performed on some of the hat members at 170°C for 20 minutes using an electric heating furnace.

Then, in the hat member before and after heat treatment, the metal structure and mechanical properties of the parts made of steel A to C were investigated in the same manner as in Example 1. Table 7 shows the results of observing the metal structure of the hat member (hot stamp molded product) and the results of evaluating the mechanical properties of the hat member.

The test results of any of Test Numbers 34 to 36 show that the tensile strength of the hot stamped product is 440 MPa or more and less than 700 MPa, and ΔTS is 100 MPa or less, showing good strength characteristics and thermal stability. The tensile strength of the steel plate portion for joining the hat member was 1545 MPa, 1540 MPa, and 1536 MPa for test numbers 34 to 36, respectively.

According to the present invention, it is possible to obtain a hot stamped product with excellent thermal stability, which has a portion with a tensile strength of 440 MPa or more and less than 700 MPa, with little variation in strength due to paint baking treatment.

Claims (7)

It is a hot stamp molded product,
All or part of the hot stamp molded product,
By mass%,
C: 0.001% or more, less than 0.090%,
Si: less than 0.50%,
Mn: 0.50% or more, less than 1.70%,
P:0.200% or less,
S:0.0200% or less,
sol.Al:0.001 to 2.500%,
N:0.0200% or less,
B:0.0002 to 0.0200%,
Ti:0 to 0.300%,
Nb:0 to 0.300%,
V:0 to 0.300%,
Zr:0 to 0.300%,
Cr:0 to 2.00%,
Mo:0 to 2.00%,
Cu:0 to 2.00%,
Ni:0 to 2.00%,
Ca:0 to 0.0100%,
Mg:0 to 0.0100%,
REM:0 to 0.1000%,
Bi:0 to 0.0500%, and
The remainder: has a chemical composition of Fe and impurities,
Metal structure, in area%,
Ferrite: Exceeding 50.0%,
Tempering martensite: 5.0% or more, less than 50.0%,
Martensite: 0% or more, less than 10.0%,
Bainite: Contains 0% or more and less than 20.0%,
The tensile strength is 440 MPa or more and less than 700 MPa,
When heat treatment is performed at 170°C for 20 minutes, ΔTS, which is the amount of decrease in tensile strength, is 100 MPa or less,
Hot stamp molding. According to paragraph 1,
The chemical composition is expressed in mass%,
Ti:0.001 to 0.300%,
Nb:0.001 to 0.300%,
V:0.001 to 0.300%,
Zr:0.001 to 0.300%,
Cr:0.001 to 2.00%,
Mo:0.001 to 2.00%,
Cu: 0.001 to 2.00%,
Ni:0.001 to 2.00%,
Ca: 0.0001 to 0.0100%,
Mg: 0.0001 to 0.0100%,
REM:0.0001 to 0.1000%, and
Bi:0.0001 to 0.0500%
A hot stamp molded article containing one or two or more types selected from the group consisting of. According to claim 1 or 2,
A hot stamped product with a plating layer on the surface. A method of manufacturing the hot stamp molded product according to claim 1 or 2,
A heating process of heating a steel sheet for hot stamping having the chemical composition according to claim 1 or 2 to a temperature exceeding the Ac ₃ point;
A hot stamping process in which the steel sheet for hot stamping after the heating process is started at a temperature of -200°C or more at the Ar ₃ point and less than the Ar ₃ point, and then cooled to a temperature of less than 90°C;
A reheating process is provided in which the molded product after the hot stamping process is heated to a temperature of 100 to 140° C. and maintained at that temperature for 3 to 120 minutes.
Method for manufacturing hot stamped molded parts. A method of manufacturing the hot stamp molded product according to claim 1 or 2,
A joining process of joining a hot stamping steel sheet having the chemical composition according to claim 1 or 2 with a joining steel sheet to form a joined steel sheet;
A heating process of heating the joined steel sheet after the joining process to a temperature exceeding the Ac ₃ point of the hot stamping steel sheet;
A hot stamping process in which the bonded steel sheet after the heating process is started at a temperature of -200°C or more at the Ar ₃ point and less than the Ar ₃ point of the hot stamping steel sheet, and then cooled to a temperature of less than 90°C. class,
A reheating process is provided in which the molded product after the hot stamping process is heated to a temperature of 100 to 140° C. and maintained at that temperature for 3 to 120 minutes.
Method for manufacturing hot stamped molded parts. A method of manufacturing the hot stamp molded product according to claim 3,
A heating process of heating a steel sheet for hot stamping having the chemical composition according to claim 1 or 2 and having a plating layer on the surface to a temperature exceeding the Ac ₃ point;
A hot stamping process in which the steel sheet for hot stamping after the heating process is started at a temperature of -200°C or more at the Ar ₃ point and less than the Ar ₃ point, and then cooled to a temperature of less than 90°C;
A reheating process is provided in which the molded product after the hot stamping process is heated to a temperature of 100 to 140° C. and maintained at that temperature for 3 to 120 minutes.
Method for manufacturing hot stamped molded parts. A method of manufacturing the hot stamp molded product according to claim 3,
A joining process of bonding a hot stamping steel sheet having the chemical composition according to claim 1 or 2 and having a plating layer on the surface with a joining steel sheet to form a joined steel sheet;
A heating process of heating the bonded steel sheet after the joining process to a temperature exceeding the Ac ₃ point of the hot stamping steel sheet;
A hot stamping process in which the bonded steel sheet after the heating process is started at a temperature of -200°C or more at the Ar ₃ point and less than the Ar ₃ point of the hot stamping steel sheet, and then cooled to a temperature of less than 90°C. class,
A reheating process is provided in which the molded product after the hot stamping process is heated to a temperature of 100 to 140° C. and maintained at that temperature for 3 to 120 minutes.
Method for manufacturing hot stamped molded parts.