WO2020204027A1 - Hot-stamping molded article and method for manufacturing same - Google Patents

Abstract

This hot-stamping molded article has a predetermined chemical composition. The metal structure of the hot-stamping molded article includes, in area%, more than 50.0% of ferrite, not less than 5.0% but less than 50.0% of tempered martensite, 0% or more but less than 10.0% of martensite, and 0% or more but less than 20.0% of bainite. The hot-stamping molded article has a tensile strength not less than 440 MPa but less than 700 MPa, and has a ΔTS, which is the reduction level of the tensile strength, of 100 MPa or less when being subjected to a heating process for 20 minutes at 170°C.

Description

Hot stamp molded product and its manufacturing method

The present invention relates to a hot stamp molded product and a method for producing the same.
The present application claims priority based on Japanese 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 divided, and the materials used in each technology field are required to have special and high performance. For example, steel sheets for automobiles are required to have high strength in order to improve fuel efficiency by reducing the weight of the vehicle body in consideration of the global environment. When a high-strength steel plate is applied to the vehicle body of an automobile, the desired strength can be imparted to the vehicle body while reducing the thickness of the steel plate to reduce the weight of the vehicle body.

However, in press molding, which is the process of forming automobile body members, the thinner the steel sheet used, the more likely it is that cracks and wrinkles will occur. Therefore, steel sheets for automobiles are also required to have excellent press formability.

Ensuring press formability and increasing the strength of the steel sheet are contradictory factors, so it is difficult to satisfy these characteristics at the same time. Further, when the high-strength steel sheet is press-formed, the shape of the member is significantly changed by the springback when the member is taken out from the mold, so that it becomes difficult to secure the dimensional accuracy of the member. As described above, it is not easy to manufacture a high-strength vehicle body member by press molding.

So far, as a method for manufacturing an ultra-high strength vehicle body member, for example, as disclosed in Patent Document 1, a technique of press-forming a heated steel sheet using a low-temperature press die has been proposed. .. This technology is called hot stamping or hot pressing, and since steel sheets that have become soft when heated to a high temperature are press-formed, it is possible to manufacture members with complex shapes with high dimensional accuracy. it can. In addition, since the steel sheet is rapidly cooled by contact with the mold, it is possible to significantly increase the strength at the same time as press molding by quenching. For example, Patent Document 1 describes that a member having a tensile strength of 1400 MPa or more can be obtained by hot stamping a steel sheet having a tensile strength of 500 to 600 MPa.

Among body members, skeletal structural parts such as center pillars and side members are often provided with a hard part and a soft part in the member in order to control the deformation state of the member when an automobile collides.

As a method for manufacturing a member having a soft portion by hot stamping, Patent Document 2 discloses a method in which the heating temperature of a steel sheet is partially changed by induction heating or infrared heating to soften the portion heated to a low temperature. ing.
Patent Document 3 discloses a method in which a heat insulating material is attached to a part of a steel sheet when the steel sheet is heated in a furnace, and the heating temperature is partially lowered 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 molding to soften the portion having a low cooling rate. There is.
Patent Document 6 discloses a technique of hot stamping using a so-called tailored blank material in which two base plates are welded and connected.

In hot stamping, the strength of martensite is increased by forming a single structure of martensite by heating the steel sheet to the austenite region and then cooling it at a cooling rate higher than the critical cooling rate. 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 the tissue other than martensite changes in response to the heating temperature and the cooling rate, the methods of Patent Documents 2 to 5 have a problem that the strength of the soft portion is not stable.

In the technique described in Patent Document 6, a soft portion can be formed under certain heating and cooling conditions by using a steel plate having a low hardenability for one of the base plates. However, although the metallographic structure and strength characteristics of the soft portion largely depend on the composition of the steel sheet, Patent Document 6 does not give any consideration to the composition of the steel sheet having low hardenability.

In response to such problems, Patent Documents 7 and 8 disclose a method for stabilizing the strength of a soft portion in a hot stamp member composed of a hard portion and a soft portion, or an overall soft hot stamp member. Has been done.

Specifically, Patent Document 7 describes a height for 600 to 1200 MPa class automobiles in which the C content is limited to a low level and the quenching element is contained in a certain amount or more to suppress the formation of ferrite, pearlite and martensite during cooling. A strength member and a method for manufacturing the same are disclosed.
Further, Patent Document 8 discloses a hot stamping member having a tensile strength of 500 MPa or more and a method for producing the same, in which the C content is limited to a low level and Ti is contained to control the amount of martensite produced. ..

According to the techniques described in Patent Documents 7 and 8, it is possible to improve the uniformity of strength and elongation in the member. However, according to the study by the present inventors, in the techniques described in Patent Documents 7 and 8, since the metal structure contains a hard structure such as bainite and martensite, the thermal stability is low and the member is coated and baked. It was found that the strength may decrease when the treatment is applied. Since paint baking treatment is often performed on automobile parts, there is room for improvement in the techniques described in Patent Documents 7 and 8.

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

As mentioned above, it is not easy to manufacture a soft member or a member including a soft part by hot stamping. In particular, it has been difficult in the prior art to produce a low-strength hot stamping member (molded product) having excellent thermal stability, which includes a soft portion in part or in whole by hot stamping.

The present invention solves the above-mentioned problems and is excellent in thermal stability, more specifically, the variation in strength (tensile strength) before and after the coating baking treatment accompanying the coating baking treatment is small, and the tensile strength is 440 MPa. As mentioned above, it is an object of the present invention to provide a hot stamp molded product having a portion of less than 700 MPa and a method for producing the same.

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

(1) A hot stamped product, all or part of the hot stamped product, in mass%, C: 0.001% or more, less than 0.090%, Si: less than 0.50%, Mn. : 0.50% or more and 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 balance: Fe and chemical composition of impurities. The metal structure is% by area, ferrite: more than 50.0%, tempered martensite: 5.0% or more and less than 50.0%, martensite: 0% or more and less than 10.0%, bainite: ΔTS, which includes 0% or more and less than 20.0%, has a tensile strength of 440 MPa or more and less than 700 MPa, and is the amount of decrease in the tensile strength when heat-treated at 170 ° C. for 20 minutes. It is 100 MPa or less.
(2) The hot stamped product according to (1) above has a chemical composition of 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: It may contain one or more selected from the group consisting of 0.0001 to 0.0500%.
(3) The hot stamp molded product according to (1) or (2) above may have a plating layer on its surface.
(4) Ac ₃ points of a hot stamping steel plate having the chemical composition according to the above (1) or (2), which is a method for producing the hot stamping molded product according to the above (1) or (2). The heating step of heating to a temperature exceeding the above temperature and the hot stamping of the hot stamping steel plate after the heating step are started at a temperature of (Ar ₃ points-200 ° C.) or more and less than Ar ₃ points, and then 90. It includes a hot stamping step of cooling to a temperature lower than ° C., and a reheating step of heating the molded product after the hot stamping step to a temperature of 100 to 140 ° C. and holding the molded product at that temperature for 3 to 120 minutes.
(5) A hot stamping steel plate having the chemical composition according to the above (1) or (2), which is a method for producing the hot stamping molded product according to the above (1) or (2), is used as a joining steel plate. A joining step of joining with and forming a joined steel plate, a heating step of heating the joined steel plate after the joining step to a temperature exceeding Ac ₃ points of the hot stamping steel plate, and a heating step of the joined steel plate after the heating step. the hot stamping of the steel plate (Ar ₃ point -200 ° C.) or higher, to start a hot stamp at a temperature of Ar less than ₃ points, and hot stamping step of subsequently cooled to a temperature below 90 ° C., after the hot stamping process A reheating step of heating the molded product to a temperature of 100 to 140 ° C. and holding the molded product at that temperature for 3 to 120 minutes is provided.
(6) A steel plate for hot stamping, which is a method for producing the hot stamped product according to (3) above, has the chemical composition according to (1) or (2) above, and has a plating layer on the surface. the starting a heating process of heating to a temperature above the Ac ₃ point, with respect to the hot stamping steel sheet after the heating step, a hot stamp (Ar ₃ point -200 ° C.) or higher, Ar less than ₃ points temperature Then, a hot stamping step of cooling to a temperature of less than 90 ° C., and a reheating step of heating the molded product after the hot stamping step to a temperature of 100 to 140 ° C. and holding at that temperature for 3 to 120 minutes. , Equipped with.
(7) A steel sheet for hot stamping, which is a method for producing the hot stamped product according to (3) above, has the chemical composition according to (1) or (2) above, and has a plating layer on the surface. A joining step of joining the steel sheet for joining to form a joined steel sheet, a heating step of heating the joined steel sheet after the joining step to a temperature exceeding Ac ₃ points of the hot stamping steel sheet, and a heating step after the heating step. with respect to the bonding steel plate, wherein (Ar ₃ point -200 ° C.) of the hot stamping steel plate or to initiate hot stamping at a temperature of Ar less than ₃ points, followed by hot stamping step of cooling to a temperature below 90 ° C. and A reheating step of heating the molded product after the hot stamping step to a temperature of 100 to 140 ° C. and holding the molded product at that temperature for 3 to 120 minutes is provided.

According to the present invention, it is possible to obtain a hot stamped product having a portion having a tensile strength of 440 MPa or more and less than 700 MPa, in which the fluctuation of the strength due to the coating baking treatment is small (excellent in thermal stability).

It is a schematic diagram which shows the shape of the hot stamp molded article manufactured in Example 1. FIG. It is a schematic diagram which shows the shape of the hot stamp molded article manufactured in Example 2. FIG.

The present inventors have diligently studied a method for suppressing a decrease in strength at the time of coating baking for a hot stamped product having a tensile strength of 440 MPa or more and less than 700 MPa. As a result, the following findings were obtained.

(A) If the metal structure of the hot stamped product contains a large amount of hard structure such as martensite or bainite, the tensile strength of the molded product is greatly reduced by the coating baking process. It is considered that this is because the hard structure is tempered and softened.

(B) On the other hand, even a hot stamped product having a low fraction of hard structure and a metal structure mainly composed of a soft structure containing ferrite has tensile strength due to coating baking treatment depending on the chemical composition and hot stamping conditions. May drop significantly.

(C) In the step of performing hot stamping, hot stamping is started in a temperature range in which ferrite and austenite coexist, and the hot stamped molded product after the hot stamping step is reheated in a predetermined temperature range. , The decrease in tensile strength due to the coating baking process is suppressed.

The reason for this is not clear, but the present inventors presume that it is due to the following reason. (A) In a hot stamped product, carbon in a solid solution state contained in ferrite is precipitated as coarse iron carbide during coating and baking, which causes a decrease in ferrite strength. (B) In the hot stamped molded product, fine iron carbides or fine iron carbon clusters change to coarse iron carbides during coating baking, which causes a decrease in ferrite strength. (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 solid solution carbon in ferrite is precipitated on the dislocations, the amount of solid solution carbon is reduced, and fine iron carbides or fine iron carbon clusters are coarsened.

(D) When B (boron) is contained in the chemical composition, the decrease in tensile strength due to the coating baking treatment is suppressed. The reason for this is not clear, but when B is contained, the amount of dislocations introduced into ferrite increases in the hot stamped product, the precipitation of carbides due to reheating is promoted, and the amount of solid solution carbon decreases further. It is presumed that this is due to the fact that the coarsening of iron carbides or iron carbon clusters is promoted.

Based on the above findings (A) to (D), the present inventors started hot stamping in a temperature range in which ferrite and austenite coexist using a steel sheet containing a desired amount of B (boron), and further. By reheating the hot stamped molded product after the hot stamping process, it has a metal structure mainly composed of ferrite, at least a part of which has a tensile strength of less than 700 MPa, is excellent in thermal stability, and is coated. It has been found that a hot stamped product with a small decrease in strength due to the baking process can be produced.

Hereinafter, each requirement of the hot stamped product (hot stamped product according to the present embodiment) and the manufacturing method thereof according to the embodiment of the present invention will be described in detail. However, the present invention is not limited to the configuration disclosed in the present embodiment, and various modifications can be made without departing from the spirit of the present invention.

<Chemical composition of hot stamped products>
All or part of the hot stamped article according to this embodiment has the following chemical composition. The reasons for limiting each element are as follows. In the following description, all "%" for the content of the chemical composition mean "mass%". When the hot stamped product includes 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 has the following chemical composition. You just have to do it.

C: 0.001% or more and less than 0.090% C is an element having an effect of increasing the tensile strength of the steel sheet after hot stamping (the steel sheet provided in the hot stamped product). If the C content is less than 0.001%, the tensile strength cannot be expected to increase due to hot stamping. Therefore, the C content is set to 0.001% or more. The preferred C content is 0.020% or higher, 0.030% or higher, 0.040% or higher, or 0.050% or higher.
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 increases. It becomes 700 MPa or more, and the thermal stability of the hot stamped molded product cannot be ensured. 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. When the Si content is 0.50% or more, it becomes difficult to secure the thermal stability of the hot stamped product even if the hot stamped product is reheated as described later. Therefore, the Si content is set to less than 0.50%. Preferred Si content is less than 0.40%, less than 0.20%, less than 0.10%, or less than 0.05%. When a plated steel sheet is used as the hot stamping steel sheet, the Si content is preferably less than 0.40% and more preferably less than 0.30% in order to ensure the plating property.
The lower limit of the Si content is not particularly limited, but an excessive decrease in the Si content causes an increase in steelmaking cost. Therefore, the Si content is preferably 0.001% or more. Further, since Si has an effect of increasing the tensile strength of the steel sheet after hot stamping, it may be positively contained. From the viewpoint of increasing the strength, the Si content is preferably 0.10% or more, or 0.20% or more.

Mn: 0.50% or more and 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 in order to obtain a metallographic structure containing ferrite and tempered martensite. The preferred Mn content is 0.60% or more, or 0.70% or more.
On the other hand, when the Mn content is 1.70% or more, it becomes difficult to secure the thermal stability of the hot stamped product even if the hot stamped product is reheated 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%, the weldability and toughness after hot stamping are significantly deteriorated, so the P content is set to 0.200% or less. The preferred 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 lowering the P content causes an increase in steelmaking cost. Therefore, the P content is preferably 0.001% or more. Further, since P has an effect of increasing the tensile strength of the molded product after hot stamping, it may be positively contained. From the viewpoint of increasing the strength, the preferable P content is 0.010% or more, 0.020% or more, or 0.030% or more. When a plated steel sheet is used as the hot stamping steel sheet, the P content is preferably 0.050% or less, and more preferably 0.040% or less in order to ensure the plating property.

S: 0.0200% or less S is an element contained in steel as an impurity and embrittles the steel. Therefore, the smaller the S content is, the more preferable it is, but if the S content exceeds 0.0200%, the embrittlement of the steel becomes remarkable, 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 an excessive decrease in the S content causes an increase in steelmaking cost. Therefore, the S content is preferably 0.0001% or more.

sol. Al: 0.001 to 2.500%
Al is an element having an action of deoxidizing molten steel. sol. If the Al content (acid-soluble Al content) is less than 0.001%, deoxidation becomes insufficient. Therefore, sol. The Al content is 0.001% or more. sol. The Al content is preferably 0.005% or more, 0.010% or more, or 0.020% or more.
On the other hand, sol. If the Al content is too high, the transformation point rises and it becomes difficult to heat the steel sheet to a temperature exceeding ₃ points in the hot stamp heating step. Therefore, sol. The Al content is 2.500% or less. sol. The 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 a nitride during continuous casting of steel. Since this nitride deteriorates the toughness after hot stamping, it is preferable that the N content is low. When the N content is more than 0.0200%, the deterioration of toughness becomes remarkable. 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 it is preferable that the N content is 0.0010% or more because excessively lowering the N content causes an increase in steelmaking cost.

B: 0.0002 to 0.0200%
B is an element having an action of improving the thermal stability of a hot stamped product having a metal structure containing ferrite and tempered martensite. If the B content is less than 0.0002%, the effect of the above action cannot be sufficiently obtained. Therefore, the B content is 0.0002% or more. The preferred B content is 0.0006% or higher, 0.0010% or higher, or 0.0015% or higher.
On the other hand, when 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 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 have the effect of increasing the tensile strength of the hot stamped product by refining the metal structure. In order to obtain this effect, one or more selected from Ti, Nb, V and Zr may be contained as required. Since these elements do not have to be contained, the lower limit of the content of these elements is 0%.

When the above effect is desired, it is preferable to contain 0.001% or more of each of one or more selected from Ti, Nb, V and Zr. Further, it is more preferable to contain any one or more of 0.005% or more Ti, 0.005% or more Nb, 0.010% or more V, and 0.005% or more Zr.

When Ti is contained, the Ti content is more preferably 0.020% or more, and particularly preferably 0.030% or more.
When Nb is contained, the Nb content is more preferably 0.020% or more, and particularly preferably 0.030% or more.
When V is contained, it is more preferable that the V content is 0.020% or more.
When Zr is contained, 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%, the effect is saturated and the manufacturing cost of the steel sheet increases. Therefore, even when these elements are contained, the contents of Ti, Nb, V and Zr are set to 0.300% or less, respectively.

Further, when the contents of Ti, Nb, V and Zr are large, a large amount of carbides of these elements may be precipitated to 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 the hot stamped product (steel plate after hot stamping). Therefore, one or more selected from Cr, Mo, Cu and Ni may be contained as required. Since these elements do not have to be contained, the lower limit of the content of these elements is 0%.

When the above effect is desired, it is preferable to contain 0.001% or more of each of one or more selected from Cr, Mo, Cu and Ni. The preferred Cr content is 0.05% or higher, the preferred Mo content is 0.05% or higher, the preferred Cu content is 0.10% or higher, and the preferred Ni content is 0.10% or higher. is there.

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. To do.
Therefore, even when the above elements are contained, the contents of Cr, Mo, Cu and Ni are set to 2.00% or less, respectively. 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%, and the preferred Cu content is less than 1.00%. 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 selected from Ca, Mg and REM may be contained as required. Since these elements do not have to be contained, the lower limit of the content of these elements is 0%.

When the above effect is desired, it is preferable to contain 0.0001% or more of each of one or more selected from Ca, Mg and REM.
On the other hand, when the content of Ca or Mg is more than 0.0100%, or when the content of REM is more than 0.1000%, the above effect is saturated and the manufacturing cost of the steel sheet is increased. Therefore, even when the above elements are contained, the Ca and Mg contents are 0.0100% or less, and the REM content is 0.1000% or less.

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

Bi: 0-0.0500%
Bi is an element having an action of improving toughness after hot stamping by refining the solidified structure. Therefore, Bi may be contained as needed. Since Bi does not have to be contained, the lower limit of the Bi content is 0%.

When the above effect is desired, the Bi content is preferably 0.0001% or more. The Bi content is more preferably 0.0003% or more, still 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 contained, the Bi content is 0.0500% or less. The Bi content is preferably 0.0100% or less, more preferably 0.0050% or less.

In the above chemical composition, the balance is Fe and impurities. Here, the "impurity" is a component mixed by various factors of raw materials such as ore and scrap, and various factors in the manufacturing process when the steel sheet is industrially manufactured, and has an adverse effect on the hot stamped product according to the present embodiment. Means what is allowed within the range that does not give.

The chemical composition of the hot stamped product described above may be measured by a general analysis method. For example, ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrum) may be used for measurement. In addition, sol. Al may be measured by ICP-AES using a filtrate obtained by heat-decomposing the sample with an acid. C and S may be measured by using the combustion-infrared absorption method, and N may be measured by using the inert gas melting-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 stamped article according to this embodiment has a metallographic structure containing the following amounts of ferrite, tempered martensite, martensite, and bainite. In the following description of the metallographic structure, "%" means "area%".

Ferrite: More than 50.0% When 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 ensured. Therefore, the area ratio of ferrite is set to more than 50.0%. The area ratio of ferrite is preferably more than 60.0%, more preferably more than 70.0%, and even more preferably more than 80.0%.
The upper limit of the area ratio of ferrite does not need to be set in particular, but it is preferably less than 95.0%, more preferably less than 90.0%, in order to increase the strength of the hot stamped product, 85. More preferably, it is less than 0.0%.

In the present embodiment, the ferrite includes, in addition to the polygonal ferrite, a pseudo-polygonal ferrite having a higher dislocation density than the polygonal ferrite, a granular bainitic ferrite, and an acidic ferrite having a serrated grain boundary. From the viewpoint of thermal stability, the ratio of polygonal ferrite to the entire ferrite is preferably 5.0% or more in terms of area ratio.

Tempering martensite: 5.0% or more and less than 50.0% Tempering martensite is a structure having an action of increasing the strength of a hot stamped product while maintaining the thermal stability of the hot stamped product. If the area ratio of the tempered martensite is less than 5.0%, the effect of the above action cannot be sufficiently obtained, and the thermal stability of the hot stamped product and / or the strength of the hot stamped product should be ensured. Becomes difficult. 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, still 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%, still more preferably less than 20.0%.

Martensite: 0% or more and less than 10.0% Bainite: 0% or more and less than 20.0% Metallographic (microstructure) is martensite (refers to untempered martensite, also called fresh martensite) And when a large amount of bainite is contained, the thermal stability of the hot stamped product deteriorates. Therefore, the area ratio of martensite is less than 10.0%, and the area ratio of bainite is 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 have to be contained, the lower limit of the area ratio of martensite and bainite is 0%.
However, since martensite and bainite have an effect of increasing the strength of the hot stamped product, they may be contained 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 of 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 rest of the metallographic structure may contain pearlite or retained austenite, and may further contain precipitates such as cementite. Since it is not necessary to positively contain pearlite, retained austenite and precipitates, the lower limit of the area ratio of pearlite, retained austenite and precipitates is 0%.

Since pearlite has an effect of increasing the strength of the hot stamped molded product, when increasing the strength, the area ratio of pearlite is preferably 1.0% or more, more preferably 2.0% or more. It is preferably 5.0% or more, and more preferably 5.0% or more.
On the other hand, when pearlite is excessively contained, the toughness after hot stamping deteriorates. Therefore, the area ratio of pearlite is preferably 20.0% or less, and more preferably 10.0% or less.

Residual austenite has the effect of improving the shock absorption of hot stamped articles. Therefore, when this effect is obtained, the area ratio of retained austenite is preferably 0.5% or more, and more preferably 1.0% or more.
On the other hand, if the retained austenite is excessively contained, the toughness after hot stamping decreases. Therefore, the area ratio of retained austenite is preferably less than 3.0%, more preferably less than 2.0%.

In the present embodiment, the area ratio of each metal structure is calculated as follows.
First, a test piece is collected from a hot stamped product, and the sheet thickness cross section (vertical cross section of the steel sheet) is polished. Then, in the case of a non-plated steel sheet, the depth position is 1/4 of the sheet thickness of the steel sheet (steel plate). From the surface to the depth of 1/8 of the plate thickness to the region of the steel plate surface to the depth of 3/8 of the plate thickness), in the case of a plated steel plate, the plate of the steel plate that is the base material from the boundary between the steel plate of the base material and the plating layer Structure observation at 1/4 depth position of thickness (1/8 depth from the boundary to 1/8 depth of the base steel plate thickness to 3/8 depth from the boundary to the base steel plate thickness) To do. When the hot stamped 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, a test piece is collected from the portion where the tensile strength is less than 700 MPa and observed. I do.

Specifically, after the polished plate thickness cross section is subjected to nital corrosion or electrolytic polishing, the structure is observed using an optical microscope and a scanning electron microscope (SEM), and the obtained tissue photograph is image-analyzed. , Ferrite, pearlite, baynite, and tempered martensite, respectively. Then, after repeller corrosion at the same observation position, tissue observation is performed using an optical microscope and a scanning electron microscope (SEM), and image analysis is performed on the obtained tissue photograph to obtain retained austenite and Calculate the total area ratio of martensite.

Also, at the same observation position, after electropolishing the thick cross section, the area ratio of retained austenite is measured using an SEM equipped with an electron backscatter pattern analyzer (EBSP).

Based on these results, the area ratios of ferrite, pearlite, bainite, tempered martensite, martensite, and retained austenite are obtained.
Tempering martensite can be distinguished from martensite by the presence of iron carbide inside, and can be distinguished from bainite by the fact that iron carbide existing inside extends in multiple directions. Can be done.

<Strength of hot stamped products>
All or part of the hot stamped article 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 sheet of the hot stamped product according to the present embodiment is 440 MPa or more and less than 700 MPa. If the tensile strength is 700 MPa or more, the thermal stability of the hot stamped product cannot be ensured. 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 article is less than 650 MPa or less than 600 MPa. On the other hand, in order to improve the impact 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, the tensile strength of all or part of the hot stamped product is 460 MPa or more, 490 MPa or more, or 540 MPa or more.

In the hot stamped product according to the present embodiment, a soft portion having a tensile strength of 440 MPa or more and less than 700 MPa and a hard portion having a tensile strength of 700 MPa or more may coexist in the hot stamp molded product. .. By providing the portions having different strengths, it is possible to control the deformed state of the hot stamped molded product at the time of collision, and it is possible to improve the impact absorption of the hot stamped molded product. As will be described later, a hot stamped product having parts having different strengths can be produced by joining two or more types of steel sheets having different chemical compositions and then hot stamping.

<Thermal stability of hot stamped products>
The hot stamp molded product according to the present embodiment has a decrease in tensile strength (ΔTS) of 100 MPa or less with respect to the tensile strength before the heat treatment when the heat treatment is performed at 170 ° C. for 20 minutes. ΔTS is preferably 60 MPa or less, and more preferably 30 MPa or less. The lower limit of ΔTS is not particularly limited, but is preferably 1 MPa or more, 5 MPa or more, or 10 MPa or more from the viewpoint of steel sheet manufacturability.

In a hot stamped product having a structure mainly composed of ferrite (over 50.0% in area ratio), the reason why the strength decreases during paint baking is that the solid-dissolved carbon present in the ferrite is removed by the paint baking process. It is considered that this is due to the precipitation as coarse iron charcoal and the change of fine iron charcoal or fine iron carbon clusters present in the ferrite to coarse iron charcoal by the heat treatment at the time of coating baking. Although it is not easy to directly and quantitatively evaluate the existence state of this solid solution carbon and fine iron carbides or fine iron carbon clusters, the tensile strength when heat-treated at 170 ° C. for 20 minutes. It can be evaluated indirectly by the amount of decrease in (ΔTS). When ΔTS is 100 MPa or less, the amount of solid solution carbon in ferrite and the amount of fine iron carbides or fine iron carbon clusters produced are low, and it is judged that the thermal stability is excellent.

Tensile strength is obtained by collecting 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 the present embodiment may have a plating layer on the surface. By providing a plating layer on the surface, it is possible to prevent scale formation during hot stamping and further 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 stamped product having a plated layer can be obtained by hot stamping using a plated steel sheet, as will be described later. Examples of hot stamped products having a plating layer include zinc-based galvanized steel sheets or aluminum-based plated steel sheets, specifically, for example, hot-dip galvanized steel sheets, alloyed hot-dip galvanized steel sheets, hot-dip aluminum-plated steel sheets, and hot-dip Zn-Al alloy plating. A galvanized layer or a galvanized layer hot-stamped using a steel sheet, a hot-dip Zn-Al-Mg alloy plated steel sheet, a hot-dip Zn-Al-Mg-Si alloy plated steel sheet, an electrogalvanized steel sheet, an electric Ni-Zn alloy plated steel sheet, or the like. An example is a hot stamped product having an aluminum-based plating layer. The plating layer may be formed on one side or both sides.

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

<Chemical composition of steel sheet for hot stamping>
Since the chemical composition is not substantially changed by hot stamping, the chemical composition of the hot stamping steel sheet has the same chemical composition as the above-mentioned hot stamping molded product.

<Metal structure of steel plate for hot stamping>
It is preferable that the metal structure of the steel plate for hot stamping according to the present embodiment contains iron carbide, and the chemical composition of the iron carbide (Mn content and Cr content in the iron carbide) satisfies the following formula (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%.

If the chemical composition of the iron carbide contained in the metal structure of the steel sheet for hot stamping satisfies the above formula (i), the thermal stability of the steel sheet after hot stamping can be further improved. The lvalue of the above equation (i) is preferably more than 3.0, more preferably more 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 step described later, which impairs the manufacturability of the steel sheet. Therefore, the lvalue of the above equation (i) 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 taken from an arbitrary position of the steel plate, and a plate thickness cross section (longitudinal cross section) parallel to the rolling direction of the steel plate is polished, and then a 1/4 depth position from the steel plate surface (from the steel plate surface to the plate). Precipitates are extracted by the replica method at a depth of 1/8 of the thickness to a region of 3/8 of the thickness from the surface of the steel sheet). This precipitate is observed using a transmission electron microscope (TEM), and the precipitate is identified and composed by electron diffraction and energy dispersive X-ray analysis (EDS).

Quantitative analysis of iron carbide by EDS was carried out for three elements, Fe, Mn and Cr, and the Mn content (atomic%) and Cr content (atomic%) were determined when the total content thereof was 100 atomic%. , Obtained as [Mn] _θ and [Cr] _θ , respectively. This quantitative analysis is performed on a plurality of iron carbides, and the average value thereof is taken as the Mn content and Cr content in the iron carbides of the steel sheet. The number of iron carbides to be measured is 10 or more, and the larger the number, the more preferable. The iron carbide includes cementite that exists isolated in the metal structure in addition to cementite that constitutes pearlite.

In the present embodiment, in the case of a hot-rolled tempered steel plate, a cold-rolled steel plate, or a tempered steel plate, the depth position is 1/4 of the plate thickness from the steel plate surface (1/8 depth from the steel plate surface to the plate thickness from the steel plate surface). (3/8 depth region), in the case of plated steel sheet, from the boundary between the base steel plate and the plating layer to the 1/4 depth position of the base steel plate thickness (from the above boundary to the base steel plate) The above-mentioned metal structure is defined in the region from 1/8 depth of the plate thickness to 3/4 depth of the plate thickness of the steel plate as the base material from the boundary.

The area ratio of iron carbide does not need to be specified, but the area ratio of iron carbide is preferably 1% or more in order to finely granulate the metal structure after hot stamping and increase the tensile strength. Is more preferable.
On the other hand, if the area ratio of the iron carbide becomes excessive, the tensile strength of the steel sheet after hot stamping becomes too high and the 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 plate for hot stamping according to the present embodiment is preferably mainly ferrite, but may contain martensite, tempered martensite, bainite and retained austenite as the balance, and other than iron carbides. May contain a precipitate of. However, martensite, tempered martensite, bainite and retained austenite deteriorate the toughness after hot stamping, so a smaller area ratio of these tissues is preferable. The area ratios of martensite, tempered martensite, bainite and retained austenite are all preferably less than 1.0%, more preferably less than 0.5%.

The area ratio of the hot stamped steel sheet in the metal structure can be obtained by the same method as in the case of the hot stamped product.
The tensile strength of the steel sheet for hot stamping is not particularly limited, but it is preferably 300 MPa or more or 340 MPa or more from the viewpoint of steel sheet manufacturability, and 650 MPa or less or less than 590 MPa from the viewpoint of steel sheet cutability. preferable.

<Manufacturing method>
A preferred method for producing a hot stamped product according to the present embodiment and a steel sheet for hot stamping according to the present embodiment will be described.

[Manufacturing method of hot stamped products]
The method for producing a hot stamped product according to the present embodiment includes a heating step of heating a hot stamping steel sheet having the above-mentioned chemical composition, hot stamping the heated hot stamping steel sheet, and then cooling. It includes a hot stamping step and a reheating step of reheating the molded product after the hot stamping step. In the hot stamping step, molding and cooling are performed by a mold to obtain a hot stamped product.

In the heating step of heating the hot stamping steel sheet, the heating temperature is set to more than ₃ points of Ac. The Ac ₃ points are the temperatures at which ferrite disappears in the metal structure when the material steel sheet is heated, and can be obtained from the thermal expansion change of the steel sheet in the heating step. When the heating temperature is Ac ₃ points or less, the dissolution of carbides during heating becomes insufficient, and the strength of the hot stamped molded product decreases. The heating temperature is preferably (Ac ₃ points + 20 ° C.) or higher, and more preferably (Ac ₃ points + 40 ° C.) or higher.
Further, the steel sheet for hot stamping to be heated preferably has the above-mentioned structure.

The upper limit of the heating temperature is not particularly limited, but if the heating temperature is too high, the austenite becomes coarse 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.
The holding time of the preferable hot stamp at the heating temperature is 1 to 5 minutes.

In the hot stamping step of performing hot stamping on the heated steel sheet for hot stamping, the starting temperature of hot stamping is (Ar ₃ points-200 ° C.) or more and less than Ar ₃ points. The Ar ₃ points are the temperatures at which ferrite begins to be formed in the metal structure when the material steel sheet is cooled from a temperature exceeding Ac ₃ points. Ar ₃ points are obtained from the change in thermal expansion when the steel sheet is cooled after the heating step. When the hot stamping start temperature is Ar ₃ points or more, 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 points −200 ° C.), the area ratio of tempered martensite in the metal structure of the hot stamped product decreases, and the strength of the hot stamped product becomes insufficient. The preferred upper limit of the hot stamping start temperature is less than (Ar ₃ points -20 ° C), less than (Ar ₃ points -40 ° C), or less than (Ar ₃ points -60 ° C). The preferable lower limit of the hot stamp start temperature is (Ar ₃ points-170 ° C) or higher, (Ar ₃ points-140 ° C) or higher, or (Ar ₃ points-110 ° C) or higher.

After molding by hot stamping, the molded product is held in the mold and / or the molded product is taken out from the mold and cooled by an arbitrary method to bring the molded product to a temperature of less than 90 ° C. Cooling. When the cooling stop temperature is 90 ° C. or higher, the area ratio of tempered martensite decreases in the metal structure of the hot stamped product, and the strength of the hot stamped product becomes insufficient. The cooling stop temperature is preferably less than 50 ° C., more preferably room temperature. In order to increase productivity, it is preferable to keep the temperature in the mold below 90 ° C.

In the reheating step of reheating the hot stamped molded product, 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 formed, and the thermal stability of the hot stamped product deteriorates. 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 solute carbon remains in the 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, and when the reheating temperature is 100 ° C. or higher and lower than 120 ° C., the holding time is preferably more than 60 minutes, more than 70 minutes, or more than 80 minutes. Also, the retention time is preferably less than 110 minutes, less than 100 minutes, or less than 90 minutes. When the reheating temperature is 120-140 minutes, the retention time is preferably greater than 5 minutes, greater than 7 minutes, or greater than 9 minutes, and the retention time is preferably less than 30 minutes, less than 20 minutes, or less than 15 minutes. is there. From the viewpoint of productivity, the reheating temperature is preferably 120 to 140 ° C.

Further, another method for producing the hot stamped product according to the present embodiment includes a joining step of joining a steel plate having the above-mentioned chemical composition (a steel plate for hot stamping) to a steel plate for joining to form a joined steel plate, and the above-mentioned joining step. A step of heating the joined steel sheet, a step of performing hot stamping on the above-mentioned heated joined steel sheet and subsequently cooling, and a step of reheating the hot stamped molded product after the hot stamping step. Including. Examples of the joining method include a method in which a hot stamping steel plate and a joining steel plate are butted or overlapped and joined by welding.

The above-mentioned bonded steel sheet is heated to a temperature exceeding Ac ₃ points of the hot stamping steel sheet, and hot stamping is started at a temperature of (Ar ₃ points-200 ° C.) or more and less than Ar ₃ points of the hot stamping steel sheet. Cool to a temperature below 90 ° C. Then, the hot stamped product is reheated to a temperature of 100 to 140 ° C. and held at that temperature for 3 to 120 minutes. A preferable heating temperature in the step of heating the bonded steel plate, a preferable hot stamping start temperature and a preferable cooling stop temperature in the step of hot stamping the bonded steel plate, and a preferable reheating temperature and a preferable holding time in the step of reheating the hot stamped product. Is the same as the method for manufacturing a hot stamped product that does not include a joining step, and the reasons for these are the same as those that do not include a joined steel plate.

The chemical composition and mechanical properties of the steel sheet for joining are not particularly limited. However, in order to increase the impact absorption energy of the hot stamped product, the steel sheet for joining preferably 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 secure the tensile strength of the joining steel sheet after hot stamping, the C content of the joining steel sheet 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, the Mn content of the joining steel sheet is preferably 0.50% or more. More preferably, it is 0.80% or more, 1.00% or more, or 1.20% or more.

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

In order to improve the corrosion resistance of the hot stamped molded product, a plated steel plate having a plated surface may be used as the hot stamping steel plate and the joining steel plate. The type of the plated steel sheet is not particularly limited, but is a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet, a hot-dip aluminum-plated steel sheet, a hot-dip Zn-Al alloy plated steel sheet, a hot-dip Zn-Al-Mg alloy-plated steel sheet, and a hot-dip Zn-Al-. Examples thereof include Mg—Si alloy plated steel sheets, electrogalvanized steel sheets, and electric Ni—Zn alloy plated steel sheets.

[Manufacturing method of steel sheet for hot stamping]
The hot stamping steel sheet according to the present embodiment is subjected to a hot rolling step of hot rolling a slab having the above-mentioned chemical composition and then winding it in a temperature range of 800 ° C. or lower to obtain a hot rolled steel sheet. It is preferable that the hot-rolled steel sheet is manufactured by a manufacturing method including a hot-rolled sheet annealing step of subjecting the hot-rolled steel sheet to a temperature range exceeding 650 ° C. to obtain a hot-rolled annealed steel sheet.

In the hot rolling step, it is preferable that the winding temperature after hot rolling is 800 ° C. or lower. When the winding 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 take-up temperatures are less than 650 ° C, less than 600 ° C, or less than 550 ° C. Further, if the winding temperature is too low, the hot-rolled steel sheet becomes hard and cold rolling becomes difficult. Therefore, the winding temperature is preferably 400 ° C. or higher.

The slab manufacturing method used in the hot stamping steel sheet manufacturing method according to the present embodiment is not particularly limited. In the preferred method for producing the slab exemplified, the steel having the above-mentioned composition (chemical composition) is melted by a known means and then made into an ingot by a continuous casting method, or an arbitrary casting method. It is made into a steel piece by a method such as ingot rolling after making it into a steel ingot. In the continuous casting step, it is preferable to generate an external additional flow such as electromagnetic agitation in the molten steel in the mold in order to suppress the occurrence of surface defects caused by inclusions. The ingot or piece of steel may be reheated once cooled and subjected to hot rolling, and the ingot in a high temperature state after continuous casting or the piece of steel in a high temperature state after ingot rolling may be used as it is. Alternatively, it may be kept warm or supplementarily heated for hot rolling. In the present embodiment, such 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 less than 1250 ° C., more preferably less than 1200 ° C. in order to prevent coarsening of austenite. The hot rolling is preferably completed in a temperature range of Ar ₃ points or more in order to miniaturize the metal structure of the hot-rolled steel sheet by transforming austenite after the rolling is completed.

When the 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 the 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 at the start of finish rolling to 140 ° C. or lower by heating the rear end of the rough-rolled material so that the temperature is higher than that of the tip. As a result, the uniformity of product characteristics in the coil after the winding process is improved.

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

It is preferable that the hot-rolled and wound steel sheet is annealed after being degreased or the like according to a known method, if necessary. The annealing applied to the hot-rolled steel sheet is called hot-rolled sheet annealing, and the steel sheet after hot-rolled sheet annealing is called hot-rolled annealed steel sheet. Before annealing the hot-rolled plate, descaling may be performed by pickling or the like.

The heating temperature in the hot-rolled plate annealing step is preferably over 650 ° C. This is to increase the Mn content and Cr content in the iron carbide in the metal structure of the hot-rolled annealed steel sheet. The heating temperature in the hot-rolled plate annealing step is more preferably more than 680 ° C, and even more preferably more than 700 ° C. On the other hand, if the heating temperature in the hot-rolled sheet annealing step 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 plate annealing step is more preferably less than 750 ° C., and even more preferably less than 720 ° C.

In order to obtain the sufficient effect of hot-rolled sheet annealing, it is preferable to keep it at 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 plate annealing step 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 step described above, it is preferable that the hot-rolled annealed steel sheet is cold-rolled to obtain a cold-rolled steel sheet having a plate thickness of 2.8 mm or less. In order to reduce the weight of the hot stamped product, the thickness of the cold-rolled steel sheet is more preferably 2.3 mm or less, further preferably 2.0 mm or less, and particularly preferably 1.8 mm or less. It is even more preferably 1.6 mm or less. Further, from the viewpoint of steel sheet manufacturability, the thickness of the cold-rolled steel sheet is preferably 0.6 mm or more.

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

The cold-rolled steel sheet may be continuously annealed to obtain an annealed steel sheet. The continuous annealing may be carried out according to a conventional method, and a treatment such as degreasing may be performed by a known method before the continuous annealing. In order to refine the metal structure of the annealed steel sheet by recrystallization, the soaking temperature in continuous annealing is preferably 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 increases. descend. Therefore, the average heating rate to the soaking temperature in annealing is preferably 1 ° C./sec or more, the soaking temperature is preferably 800 ° C. or lower, or 760 ° C. or lower, and the soaking time is less than 300 seconds. Alternatively, it is preferably less than 120 seconds.

The hot-rolled annealed steel sheet, cold-rolled steel sheet, and annealed steel sheet thus obtained may be temper-rolled according to a conventional method.

The steel sheet for hot stamping according to the present embodiment may be provided with a plating layer on the surface layer for the purpose of preventing scale formation during hot stamping and improving the corrosion resistance of the steel sheet after hot stamping. The type of plating is not particularly limited as long as it meets the above-mentioned purpose, but is not particularly limited, but is limited to 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-. Examples thereof include Mg alloy plated steel sheets, molten Zn—Al—Mg—Si alloy plated steel sheets, electrogalvanized steel sheets, and electric Ni—Zn alloy plated steel sheets.

When the hot-rolled plated steel sheet is manufactured, the hot-rolled annealed steel sheet, the cold-rolled steel sheet or the annealed steel sheet manufactured by the above method may be used as the material steel sheet and plated according to a conventional method. When a cold-rolled steel sheet is used as the material steel sheet, the soaking temperature in the annealing process of continuous hot-dip galvanizing is set to 600 ° C. or higher, 650 ° C. or higher, or 700 ° C. or higher in order to refine the metal structure of the plated steel sheet by recrystallization. Is preferable.

On the other hand, if the soaking temperature is too high, the metal structure of the annealed steel sheet becomes coarse due to grain growth. Therefore, regardless of the type of the material steel sheet, the soaking temperature in the annealing process of continuous hot dip galvanizing is 800 ° C. or lower or 760 ° C. The following is preferable. After hot dip galvanizing, the steel sheet may be reheated for alloying treatment.

When an electroplated steel sheet is manufactured, a hot-rolled annealed steel sheet, a cold-rolled steel sheet or an annealed steel sheet manufactured by the above method is used as a material steel sheet, and if necessary, a well-known pretreatment for surface cleaning and adjustment is performed. After the application, electroplating may be performed according to a conventional method. The plated steel sheet thus obtained may be temper-rolled according to a conventional method.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

(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 points Ac ₁ and Ac _{3 in} Table 1 were determined from the changes in thermal expansion when the cold-rolled steel sheets of steels A to N were heated at 2 ° C./sec. The Ar ₃ points in Table 1 were obtained from the change in thermal expansion when the 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, steels A to N were rolled for 10 passes in a temperature range of ₃ points or more in Ar to obtain a hot-rolled steel sheet having a thickness of 3.6 mm. After hot rolling, the hot-rolled steel sheet is cooled to 540 to 580 ° C. with a water spray, the cooling end temperature is set as the winding temperature, and the hot-rolled steel sheet is charged into an electric heating furnace maintained at this winding temperature. After holding 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 winding.

After slow cooling, some hot-rolled steel sheets were annealed. Specifically, the 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. It was made into a hot-spread annealed steel sheet.

The hot-rolled steel sheet and the hot-rolled annealed steel sheet were pickled to form a base material for cold rolling, and cold-rolled at a cold pressure ratio of 61% to obtain a cold-rolled steel sheet having a thickness of 1.4 mm. Some cold-rolled steel sheets were heated to the annealing temperature of table 2 at an average heating rate of 10 ° C./sec using a continuous annealing simulator, and the heat was equalized for 60 seconds. Subsequently, it was cooled to 400 ° C. and held for 180 seconds, and then cooled to room temperature to obtain an annealed steel sheet. The obtained annealed steel sheet was described as "ACR" in the "Steel type" column and "-" in the "Plating type" column in Table 3. For cold-rolled steel sheets, "CR" was described in the "Steel type" column and "-" was described in the "Plating type" column in Table 3.

In addition, some cold-rolled steel sheets were heated to the annealing temperature equalizing temperature shown in Table 2 at an average heating rate of 10 ° C./sec using a hot-dip galvanizing simulator, and the heat was equalized for 60 seconds. Subsequently, it was cooled and immersed in a hot-dip galvanized bath or a hot-dip aluminum plating bath to be subjected to 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 steel sheets were heated to 520 ° C. and alloyed to obtain alloyed hot-dip galvanized steel sheets. The obtained plated steel sheet was described as "ACR" in the "Steel type" column and "GI", "GA" or "AL" in the "Plating type" column in Table 3.

The structure is obtained from the cold-rolled steel sheet, the tempered steel sheet, the hot-dip galvanized steel sheet, the alloyed hot-dip galvanized steel sheet, and the hot-dip aluminum-plated steel sheet (these steel sheets are collectively called the hot stamping steel sheet) obtained in this manner. A test piece for observation was collected and the tissue was observed.

Specifically, in the case of non-plated steel sheets (cold-rolled steel sheets and tempered steel sheets), after polishing the sheet thickness section parallel to the rolling direction, the depth position from the steel sheet surface to 1/4 of the sheet thickness of the steel sheet (steel surface surface) From 1/8 depth of the plate thickness to 3/8 depth of the plate thickness from the surface of the steel plate), in the case of plated steel plate, from the boundary between the steel plate of the base material and the plating layer, the plate thickness of the steel plate which is the base material From the 1/4 depth position (1/8 depth from the boundary to the thickness of the base steel plate as the base material to 3/8 depth from the boundary to the thickness of the steel plate as the base material) by the replica method. The precipitate was extracted and the iron carbide was identified using TEM. Quantitative analysis of three elements, Fe, Mn and Cr, was performed on 10 iron carbides using EDS. When the total content of Fe, Mn and Cr is 100 atomic%, the Mn content (atomic%) and Cr content (atomic%) in the iron carbide are [Mn] _θ and [Cr] _θ , respectively. , The average value of the sum of [Mn] _θ and [Cr] _θ was calculated.

Further, from the above-mentioned steel sheet for hot stamping, a JIS13B tensile test piece was taken along the direction orthogonal 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 metallographic structure of the hot stamping steel sheet and the results of investigating the mechanical properties of the hot stamping steel sheet.

From the above steel plate for hot stamping, a base plate for hot stamping having a width of 240 mm and a length of 170 mm was collected, and a hat member having a shape shown in FIG. 1 was manufactured by hot stamping. In the hot stamping step, the base plate was heated at the heating temperature shown in Table 4 for 4 minutes using a gas heating furnace, then taken out from the heating furnace and allowed to cool, and a cooling device was provided at the starting temperature shown in Table 4. It was sandwiched between molds to form a hat, and then cooled in the mold to the cooling stop temperature shown in Table 4. In addition, some of the hat members were reheated using an electric heating furnace under the conditions shown in Table 4. The RT of the hot stamping condition in Table 4 indicates that the temperature is room temperature, and “-” indicates that the reheating step was not performed.

Some hat members (hot stamped products) were heat-treated at 170 ° C for 20 minutes using an electric heating furnace.

A test piece for SEM observation is taken from the vertical wall of the hat member before heat treatment, and a sheet thickness section parallel to the rolling direction of the steel plate of this test piece is polished, and then nightal corrosion and repeller corrosion are applied to this sheet thickness cross section. In the case of non-plated steel sheet, the depth position from the surface of the steel sheet to the depth of 1/4 of the thickness of the steel sheet (the region from the surface of the steel plate to the depth of 1/8 to the surface of the steel plate to the depth of 3/8 of the thickness). In the case of a plated steel sheet, the depth position is 1/4 of the thickness of the base steel plate from the boundary between the base steel plate and the plating layer (1/8 depth from the boundary to the thickness of the base steel plate). -The metallographic structure in the region of 3/8 depth of the thickness of the steel plate as the base material was observed from the above boundary. The area ratios of ferrite, pearlite, retained austenite, tempered martensite, martensite and bainite were measured by image processing using the method described above. The results are shown in Table 4. The rest of the tissue shown in Table 4 was pearlite, retained austenite and / or precipitates. Further, in the table, in the test number satisfying the provisions of the present invention, the ratio of polygonal ferrite in the ferrite was 5.0% or more in the metal structure of the hot stamped molded product.

In addition, JIS13B tensile test pieces were collected from the vertical wall of the hat member before and after the 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 has not been heat-treated and the tensile strength of the hat member that has been heat-treated is determined. If ΔTS is 100 MPa or less, the thermal stability of the hat member is good. It was judged.

When the tensile strength before the heat treatment was 440 MPa or more and less than 700 MPa and the ΔTS was 100 MPa or less, it was judged to be acceptable as satisfying the provisions of the present invention. On the other hand, when the tensile strength before the heat treatment was less than 440 MPa, 700 MPa or more, or the ΔTS was more than 100 MPa, it was determined that the product did not satisfy the provisions of the present invention and was rejected.

Table 4 shows the results of observing the metallographic 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 that they are outside the scope of the present invention or that they are outside the preferred manufacturing conditions.

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

Further, in the test numbers 2, 10, 16, 25 and 27 in which the hot-rolled sheet was annealed in the manufacturing process of the hot-stamped steel sheet, the ΔTS of the hot-stamped product was 30 MPa or less, and the thermal stability was particularly good. Met.

On the other hand, in the test numbers 20 to 24 of the comparative example using the steel sheet whose chemical composition is out of the range of the present invention, the tensile strength of the hot stamped product was less than 440 MPa, and the strength characteristics were inferior or , ΔTS was 100 MPa or more, and the thermal stability was inferior.

Specifically, in Test No. 21 using steel E, since the Mn content of the steel was too low, 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 insufficient. 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 700 MPa or more, and ΔTS was large.

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

Test No. 23 using steel G had a large ΔTS because the Si content of the steel was too high.

Test number 24 using steel H had a large ΔTS because the B content of 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 production conditions of the hot stamped article are outside the scope of the present invention are the tensile strength of the hot stamped article. The strength was less than 440 MPa and the strength characteristics were inferior, or the ΔTS was 100 MPa or more and the thermal stability was inferior.

Specifically, in test numbers 4 and 5 using steel A, the tensile strength of the hot stamped product was low because the holding time in the reheating step was too long or the reheating temperature was too high.

Test No. 6 using steel A had a low tensile strength because the heating temperature in the heating step 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 test numbers 12 and 13 using steel B, the tempered martensite area ratio was insufficient in the metallographic structure of the hot stamped product because the holding time in the reheating step was too short or the reheating temperature was too low. ΔTS was large.

Test number 18 using steel C had a large ΔTS because the molding start temperature in the hot stamping process was too high. In Test No. 19 using steel C, the tempered martensite area ratio was insufficient and ΔTS was large because the reheating step was not performed.

(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 rolling plate annealing, cold rolling, and annealing are performed under the conditions shown in Table 5 in the same manner as in Example 1, and then plating treatment is performed to obtain a hot-dip zinc-plated steel sheet. , An alloyed hot-dip zinc-plated steel sheet and a hot-stamped steel sheet (steel for hot stamping) were manufactured.

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

From these hot stamping steel plates, hot stamping base plates having a thickness of 1.4 mm, a width of 240 mm, and a length of 170 mm were collected. This base plate was joined to a steel plate for joining having the same dimensions by laser welding to prepare a joined steel plate having a thickness of 1.4 mm, a width of 240 mm, and a length of 340 mm. The steel sheet for joining has a chemical composition of 0.21% C-0.13% Si-1.31% Mn-0.012% P-0.0018% S-0.043% sol. A cold-rolled steel sheet having Al-0.0030% N-0.21% Cr-0.0018% B was used.

The joined steel plate was hot stamped under the conditions shown in Table 7 in the same manner as in Example 1, and a hat member having the shape shown in FIG. 2 was manufactured. Then, some of the hat members were heat-treated at 170 ° C. for 20 minutes using an electric heating furnace.

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

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

According to the present invention, it is possible to obtain a hot stamped product having excellent thermal stability, having a portion having a tensile strength of 440 MPa or more and less than 700 MPa, in which the fluctuation of the strength due to the coating baking treatment is small. ..

Claims (7)

1. 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 and 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: 0-2.00%,
   Cu: 0-2.00%,
   Ni: 0 to 2.00%,
   Ca: 0-0.0100%,
   Mg: 0 to 0.0100%,
   REM: 0 to 0.1000%,
   Bi: 0-0.0500%, and balance: Fe and impurities with a chemical composition.
   The metal structure is% of the area
   Ferrite: over 50.0%,
   Tempering martensite: 5.0% or more and less than 50.0%,
   Martensite: 0% or more and less than 10.0%,
   Bainite: Includes 0% or more and less than 20.0%
   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 molded product.
2. When the chemical composition is 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: 0.001-2.00%,
   Cu: 0.001-2.00%,
   Ni: 0.001-2.00%,
   Ca: 0.0001-0.0100%,
   Mg: 0.0001-0.0100%,
   REM: 0.0001 to 0.1000%, and Bi: 0.0001 to 0.0500%
   Containing one or more selected from the group consisting of
   The hot stamp molded product according to claim 1.
3. Has a plating layer on the surface,
   The hot stamp molded product according to claim 1 or 2.
4. The method for producing a hot stamped article according to claim 1 or 2.
   A heating step of heating a steel sheet for hot stamping having the chemical composition according to claim 1 or 2 to a temperature exceeding ₃ points of Ac.
   A hot stamping step of starting hot stamping on the hot stamping steel sheet after the heating step at a temperature of (Ar ₃ points-200 ° C.) or more and less than Ar ₃ points, and then cooling to a temperature of less than 90 ° C. When,
   A reheating step of heating the molded product after the hot stamping step to a temperature of 100 to 140 ° C. and holding the molded product at that temperature for 3 to 120 minutes is provided.
   A method for manufacturing a hot stamped product.
5. The method for producing a hot stamped article according to claim 1 or 2.
   A joining step of joining a hot stamping steel sheet having the chemical composition according to claim 1 or 2 to a joining steel sheet to form a joining steel sheet.
   A heating step of heating the joined steel sheet after the joining step to a temperature exceeding Ac ₃ points of the hot stamping steel sheet, and a heating step.
   To the joining steel sheet after the heating step, the (Ar ₃ point -200 ° C.) of the hot stamping steel plate or to initiate hot stamping at a temperature of Ar less than ₃ points, followed by cooling to a temperature below 90 ° C. Hot stamping process and
   A reheating step of heating the molded product after the hot stamping step to a temperature of 100 to 140 ° C. and holding the molded product at that temperature for 3 to 120 minutes is provided.
   A method for manufacturing a hot stamped product.
6. The method for producing a hot stamped molded product according to claim 3.
   A heating step 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 ₃ points of Ac.
   A hot stamping step of starting hot stamping on the hot stamping steel sheet after the heating step at a temperature of (Ar ₃ points-200 ° C.) or more and less than Ar ₃ points, and then cooling to a temperature of less than 90 ° C. When,
   A method for producing a hot stamped molded product, comprising a reheating step of heating the molded product after the hot stamping step to a temperature of 100 to 140 ° C. and holding the molded product at that temperature for 3 to 120 minutes.
7. The method for producing a hot stamped molded product according to claim 3.
   A joining step of joining a hot stamping steel sheet having the chemical composition according to claim 1 or 2 and having a plating layer on the surface to a joining steel sheet to form a joining steel sheet.
   A heating step of heating the joined steel sheet after the joining step to a temperature exceeding Ac ₃ points of the hot stamping steel sheet, and
   To the joining steel sheet after the heating step, the (Ar ₃ point -200 ° C.) of the hot stamping steel plate or to initiate hot stamping at a temperature of Ar less than ₃ points, followed by cooling to a temperature below 90 ° C. Hot stamping process and
   A reheating step of heating the molded product after the hot stamping step to a temperature of 100 to 140 ° C. and holding the molded product at that temperature for 3 to 120 minutes is provided.
   A method for manufacturing a hot stamped product.

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