JP7215518B2 - HOT PRESS MEMBER AND MANUFACTURING METHOD THEREOF - Google Patents

Description

The present invention is a high-strength pressed part mainly used in the automobile industry field, in which a heated steel plate is hot-pressed in a mold consisting of a die and a punch, and particularly has a tensile strength (TS) of 1300 MPa or more and 1760 MPa. The present invention relates to a method for manufacturing high-strength press parts with a steel plate diameter of less than

In recent years, automobile exhaust gas regulations have been strengthened from the viewpoint of global environmental conservation. Under these circumstances, improving the fuel efficiency of automobiles has become an important issue, and there is a demand for high-strength and thin-walled automobile parts. As means for increasing the strength and reducing the wall thickness of automobile parts, there is known a means of using a steel sheet as a raw material for the automobile part and forming the steel sheet into a desired part shape by hot press quenching. In hot press quenching, a blank (steel plate) heated to the austenite single phase region is hot press-formed into a desired shape using a die, and quenched by extracting heat in the die.
As described above, in hot press quenching, a steel sheet heated to a high temperature range, that is, a steel sheet in a state of being softened and easily worked, is press-formed, so that the steel sheet can be formed into a complicated part shape. In addition, since the steel sheet is quenched while being formed into a desired part shape, a hot pressed member having extremely high strength such that TS exceeds 1300 MPa after forming can be obtained. Furthermore, since quenching is performed in the mold, heat treatment strain can be suppressed, and a hot pressed member with excellent dimensional accuracy can be obtained.

Japanese Patent No. 5729213 Japanese Patent No. 5257062

However, as the strength of members increases, material properties such as ductility and delayed fracture resistance generally deteriorate. On the other hand, some structural members used in automobiles, such as door guard bars and side members, are required to have high ductility from the viewpoint of ensuring safety in the event of automobile collision. In addition, there is also a demand that delayed fracture (hydrogen embrittlement) does not occur due to hydrogen entering from the usage environment, that is, delayed fracture resistance is required.
In Patent Document 1, the heating temperature during hot pressing is controlled, and the member after hot pressing is heat-treated at a temperature range of 150 ° C. or higher and 200 ° C. or lower to produce a member having excellent delayed fracture resistance. Suggest a way to get it. However, in the method described in Patent Document 1, since the heating and cooling conditions during hot press forming are not appropriate, the prior austenite grain size is not sufficiently refined, and sufficient delayed fracture characteristics are not necessarily obtained. . Also, the control of the microstructure was not sufficient, and sufficient ductility was not obtained.
Patent Document 2 proposes a method of obtaining a member having excellent delayed fracture resistance by controlling the heating temperature during hot pressing to refine the grain size of prior austenite. However, in the method described in Patent Document 2, since the heating and cooling conditions during hot press forming are not appropriate, the prior austenite grain size is not sufficiently refined, and sufficient delayed fracture characteristics are not necessarily obtained. . Also, the control of the microstructure was not sufficient, and sufficient ductility was not obtained.

Accordingly, an object of the present invention is to provide a hot pressed member having a tensile strength of 1300 MPa or more and less than 1760 MPa in TS and excellent ductility and delayed fracture resistance, and a method for producing the same.

As a result of intensive studies, the inventors have found that the above objects can be achieved by adopting the following configuration, and completed the present invention.

(1) C: 0.15% or more and less than 0.26%, Si: 0.01% or more and 1.5% or less, Mn: 1.0% or more and 3.5% or less, P: 0.01% or more and 1.5% or less, Mn: 1.0% or more and 3.5% or less. 10% or less, S: 0.010% or less, Al: 0.01% or more and 1.5% or less, N: 0.010% or less, Ti: 0.01% or more and 0.20% or less, and B: a component composition containing 0.0005% or more and 0.020% or less, the balance being Fe and unavoidable impurities;
The total area ratio of martensite, tempered martensite, and bainite is 60% or more and less than 90%, the area ratio of ferrite is 10% or more and 40% or less, and the prior austenite average grain size is less than 3.0 μm. There is a hot press member.
(2) The above composition, in terms of % by mass, contains Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Sb: 0.10% or less, and Sn: 0.1% or less. The hot press member according to (1) above, containing at least one member selected from the group consisting of 10% or less.
(3) The above component composition further contains at least one selected from the group consisting of Nb: 0.20% or less, Mo: 1.0% or less, and V: 1.0% or less in mass %. , the hot press member according to the above (1) or (2).
(4) The above component composition is further composed of Bi: 0.10% or less, Ca: 0.10% or less, Co: 0.10% or less, Mg: 0.10% or less, and REM: 0.10% or less. Contains at least one selected from the group consisting of 10% or less, Ta: 0.10% or less, W: 0.10% or less, Zn: 0.10% or less, and Zr: 0.10% or less , the hot press member according to any one of the above (1) to (3).
(5) The hot press member according to any one of (1) to (4) above, having an Al-based plating layer or a Zn-based plating layer on the surface.
(6) A method for manufacturing a hot pressed member according to any one of (1) to (5) above, comprising:
A steel material having the chemical composition according to any one of the above (1) to (4) is heated at an average heating rate of 50 ° C./s or more to Ac ₃ temperature -50 ° C. or more and Ac ₃ temperature + 100 ° C. or less, and Ac After holding at a temperature of ₃ temperature -50 ° C. or higher and Ac ₃ temperature + 100 ° C. or lower for 0 seconds or more and 30 seconds or less, preheat treatment is performed to cool to Ms temperature of -100 ° C. or lower at an average cooling rate of 5 ° C./s or higher. a preheat treatment step;
After the preheating step, the preheated steel material is heated to Ac ₃ temperature -50 ° C. or more and Ac ₃ temperature at an average heating rate of 50 ° C./s or more, and Ac ₃ temperature -50 ° C. or more Ac After holding at a temperature of less than ₃ temperatures for 0 seconds or more and 30 seconds or less, cooling at an average cooling rate of 5 ° C./s or more, hot press molding at Ms temperature or more, Ms temperature -100 ° C. or less at 20 ° C./ A method for manufacturing a hot pressed member, comprising a hot pressing step of obtaining a hot pressed member by cooling at an average cooling rate of s or more.
(7) A method for manufacturing a hot pressed member according to any one of (1) to (5) above, comprising:
A steel material having the chemical composition according to any one of the above (1) to (4) is heated at an average heating rate of 50 ° C./s or more to Ac ₃ temperature -50 ° C. or more and Ac ₃ temperature + 100 ° C. or less, and Ac After holding at a temperature of ₃ temperature -50 ° C. or higher and Ac ₃ temperature + 100 ° C. or lower for 0 seconds or more and 30 seconds or less, preheating was performed by cooling to Ms temperature of -100 ° C. or lower at an average cooling rate of 5 ° C./s or higher. After that, a preheating step in which the preheating is repeated one or more times;
After the preheating step, the preheated steel material is heated to Ac ₃ temperature -50 ° C. or more and Ac ₃ temperature at an average heating rate of 50 ° C./s or more, and Ac ₃ temperature -50 ° C. or more Ac After holding at a temperature of less than ₃ temperatures for 0 seconds or more and 30 seconds or less, cooling at an average cooling rate of 5 ° C./s or more, hot press molding at Ms temperature or more, Ms temperature -100 ° C. or less at 20 ° C./ A method for manufacturing a hot pressed member, comprising a hot pressing step of obtaining a hot pressed member by cooling at an average cooling rate of s or more.

ADVANTAGE OF THE INVENTION According to this invention, the hot press member which has the tensile strength whose TS is 1300 MPa or more and less than 1760 MPa, and which is excellent in ductility and delayed fracture resistance, and its manufacturing method can be provided.
By using the hot pressed member of the present invention for automobile structural members, frame members, underbody members such as suspensions, etc., it is possible to reduce the weight of the automobile body while ensuring the safety of the automobile. Therefore, it can contribute to the reduction of the environmental load.

The hot press member of the present invention and the method for manufacturing the same will be described below.
In this specification, the numerical range represented by "-" means a range including the numerical values before and after "-" as lower and upper limits.

[Hot press member]
The hot press member of the present invention is
% by mass, C: 0.15% or more and less than 0.26%, Si: 0.01% or more and 1.5% or less, Mn: 1.0% or more and 3.5% or less, P: 0.10% or less , S: 0.010% or less, Al: 0.01% or more and 1.5% or less, N: 0.010% or less, Ti: 0.01% or more and 0.20% or less, and B: 0.0005 % or more and 0.020% or less, the balance being Fe and unavoidable impurities;
The total area ratio of martensite, tempered martensite, and bainite is 60% or more and less than 90%, the area ratio of ferrite is 10% or more and 40% or less, and the prior austenite average grain size is less than 3.0 μm. It is a hot press member.

<Component composition>
First, the reasons for limiting the component composition of the hot press member will be described. Hereinafter, "%" in component compositions means "% by mass" unless otherwise specified.

(C: 0.15% or more and less than 0.26%)
C is an effective element for increasing the strength of steel, and is a very important element for strengthening martensite, tempered martensite and bainite after hot pressing to increase the strength of hot pressed parts. In order to ensure a TS of 1300 MPa or more after hot pressing, the content should be at least 0.15% or more. Therefore, the C content is 0.15% or more, preferably 0.17% or more, and more preferably 0.19% or more.
On the other hand, when the C content is 0.26% or more, the strength after hot pressing becomes too high, and the delayed fracture resistance deteriorates. Therefore, the C content is less than 0.26%, preferably 0.25% or less, more preferably 0.24%.

(Si: 0.01% or more and 1.5% or less)
Si contributes to solid-solution strengthening, suppresses precipitation of cementite, improves resistance to temper softening of martensite during hot pressing, and contributes to strength improvement of hot pressed parts. In order to obtain such effects, the Si content is 0.01% or more, preferably 0.1% or more, and more preferably 0.2% or more.
On the other hand, Si is a ferrite-forming element, and if the Si content exceeds 1.5%, the ferrite fraction of the hot-pressed member increases, and the TS of the hot-pressed member may not be ensured. Therefore, the Si content is 1.5% or less, preferably 1.2% or less, and more preferably 0.7% or less.

(Mn: 1.0% or more and 3.5% or less)
Mn contributes to solid-solution strengthening, and also promotes the formation of martensite, tempered martensite, and bainite during hot pressing by improving hardenability, thereby contributing to improving the strength of hot-pressed parts. In order to obtain such effects, the Mn content is 1.0% or more, preferably 1.2% or more, and more preferably 1.5% or more.
On the other hand, when the Mn content exceeds 3.5%, the effect is saturated and the delayed fracture resistance of the hot pressed member is deteriorated. Therefore, the Mn content is 3.5% or less, preferably 3.0% or less, and more preferably 2.5% or less.

(P: 0.10% or less (including 0%))
P contributes to improving the strength of the hot pressed member by solid-solution strengthening. However, P segregates at grain boundaries and reduces ductility. Therefore, it is preferable to keep the P content as low as possible, and a P content of up to 0.10% is permissible. Therefore, the P content is 0.10% or less, preferably 0.050% or less, and more preferably 0.020% or less.

(S: 0.010% or less (including 0%))
S combines with Ti and Mn to form coarse sulfides, which lower the ductility and delayed fracture resistance of the hot pressed member. Therefore, it is preferable to reduce the S content as much as possible, and an S content of up to 0.010% is permissible. Therefore, the S content is 0.010% or less, preferably 0.0050% or less, and more preferably 0.0030% or less.

(Al: 0.01% or more and 1.5% or less)
Al acts as a deoxidizing agent and is effective in improving the cleanliness of hot pressed parts. If Al is too small, the effect is not necessarily sufficient. Therefore, the Al content is 0.01% or more, preferably 0.015% or more, and more preferably 0.020% or more.
On the other hand, excessive addition of Al causes an increase in oxide-based inclusions, degrading ductility and delayed fracture resistance. Therefore, the Al content is 1.5% or less, preferably 1.2% or less, and more preferably 1.0% or less.

(N: 0.010% or less (including 0%))
N precipitates as a nitride by combining with an element that forms a nitride, and contributes to refinement of crystal grains. However, N tends to combine with Ti at high temperatures to form coarse nitrides, and an excessive content deteriorates the ductility and delayed fracture resistance of the hot pressed member. Therefore, the N content is 0.010% or less, preferably 0.008% or less, and more preferably 0.006% or less.

(Ti: 0.01% or more and 0.20% or less)
Ti improves the strength of the hot pressed member by precipitation strengthening or solid solution strengthening. Ti forms nitrides in the austenite phase high temperature region (the high temperature region in the austenite phase and the region higher than the austenite phase (casting stage)). As a result, precipitation of BN is suppressed, and B becomes a solid solution. In this way, the hardenability necessary for the formation of martensite, tempered martensite, and bainite during hot pressing is obtained, contributing to strength improvement. In addition, Ti forms carbonitrides, suppresses coarsening of austenite grains when the steel sheet to be subjected to hot pressing is heated to Ac ₃ temperature or higher, and has the effect of refining the prior austenite grain size. is an important element for In order to express these effects, the Ti content is 0.01% or more. The Ti content is desirably 0.015% or more, more preferably 0.02% or more.
On the other hand, if the Ti content is too high, coarse carbonitrides are formed, degrading the ductility and delayed fracture resistance of the hot pressed member. Therefore, the Ti content is 0.20% or less, preferably 0.10% or less, and more preferably 0.060% or less.

(B: 0.0005% or more and 0.020% or less)
B segregates at prior austenite grain boundaries and suppresses the formation of ferrite, thereby promoting the formation of martensite, tempered martensite, and bainite during hot pressing, and contributes to improving the strength of the steel sheet. In order to exhibit these effects, the B content is 0.0005% or more, preferably 0.0010% or more, and more preferably 0.0015% or more.
On the other hand, if the B content is too high, the above effects will be saturated. Therefore, the B content is 0.020% or less, preferably 0.010% or less, and more preferably 0.0050% or less.

The component composition of the hot press member may further contain at least one selected from the group consisting of Cr, Cu, Ni, Sb and Sn in the following content.

(Cr: 1.0% or less)
Cr contributes to solid-solution strengthening, promotes formation of martensite, tempered martensite, and bainite during hot pressing through improvement of hardenability, and contributes to strength improvement.
On the other hand, Cr deteriorates the chemical conversion treatability and platability of the hot press material and the hot press member, and deteriorates the corrosion resistance, thereby lowering the delayed fracture resistance. Therefore, when Cr is contained, the Cr content is 1.0% or less, preferably 0.5% or less. In order to express these effects, when Cr is contained, the Cr content is preferably 0.01% or more, more preferably 0.10% or more.

(Cu: 1.0% or less)
Cu contributes to solid-solution strengthening, promotes the formation of martensite, tempered martensite, and bainite during hot pressing by improving hardenability, and contributes to improving the strength of hot-pressed parts. In addition, since it improves corrosion resistance, it can improve delayed fracture resistance, so it can be added as necessary.
On the other hand, if the Cu content is too high, the above effects are saturated, and surface defects due to Cu are likely to occur. Therefore, when Cu is contained, the Cu content is preferably 1.0% or less, more preferably 0.50% or less. In order to obtain these effects, when Cu is contained, the Cu content is preferably 0.01% or more, more preferably 0.05% or more.

(Ni: 1.0% or less)
Ni contributes to solid solution strengthening, promotes formation of martensite during hot pressing, tempered martensite, and bainite through improvement of hardenability, and contributes to strength improvement. Further, since it improves corrosion resistance in the same manner as Cu, it can improve delayed fracture resistance, so it can be added as necessary.
On the other hand, when the Ni content is too high, ductility deteriorates. Therefore, when Ni is contained, the Ni content is preferably 1.0% or less, more preferably 0.50% or less. In order to obtain these effects, when Ni is contained, the Ni content is preferably 0.01% or more, more preferably 0.05% or more.

(Sb: 0.10% or less)
Sb suppresses nitridation of the surface of the steel material such as a slab and suppresses precipitation of BN on the surface layer of the steel material at the stage of heating the steel material. In addition, the presence of B in solid solution provides hardenability necessary for forming martensite, tempered martensite, and bainite in the surface layer of the hot-pressed member, thereby improving the strength of the hot-pressed member.
On the other hand, if the Sb content is too high, it may lead to an increase in the rolling load during the production of the hot press material, resulting in a decrease in productivity. Therefore, when Sb is contained, the Sb content is preferably 0.10% or less, more preferably 0.050% or less, and even more preferably 0.020% or less. In order to exhibit such an effect, when Sb is contained, the Sb content is preferably 0.0002% or more, more preferably 0.0010% or more.

(Sn: 0.10% or less)
Like Cu and Ni, Sn improves corrosion resistance and thus can improve delayed fracture resistance, so it can be added as necessary.
On the other hand, if the Sn content is too high, the hot workability may deteriorate and cracks may occur during hot pressing. Therefore, when Sn is contained, the Sn content is 0.10% or less, preferably 0.050% or less, and more preferably 0.020% or less. In order to obtain these effects, when Sn is contained, the Sn content is desirably 0.0002% or more, more preferably 0.0010% or more.

The component composition of the hot press member may further contain at least one selected from the group consisting of Nb, Mo and V in the following content.

(Nb: 0.20% or less)
Nb improves the strength of hot pressed parts by precipitation strengthening or solid solution strengthening. In addition, Nb, like Ti, forms carbonitrides, suppresses coarsening of austenite grains when the steel sheet to be subjected to hot pressing is heated to Ac ₃ temperature or higher, and makes the prior austenite grain size finer. It is an element that has the effect of
On the other hand, if the Nb content is too high, coarse carbonitrides are formed, degrading the ductility and delayed fracture resistance of the hot pressed member. Therefore, when Nb is contained, the Nb content is 0.20% or less, preferably 0.10% or less, and more preferably 0.060% or less. In order to exhibit these effects, when Nb is contained, the Nb content is preferably 0.005% or more. The Nb content is desirably 0.01% or more, more preferably 0.02% or more.

(Mo: 1.0% or less)
Mo promotes the formation of martensite, tempered martensite, and bainite during hot pressing through improvement of hardenability, and contributes to improvement of strength of the steel sheet. In addition, Mo, like Ti, forms carbonitrides, suppresses coarsening of austenite grains when the steel sheet to be subjected to hot pressing is heated to Ac ₃ temperature or higher, and makes the prior austenite grain size finer. It is an element that has the effect of
On the other hand, if the Mo content is too high, like Cr, the chemical convertibility and plating properties of the hot-pressed material and hot-pressed members are deteriorated, and the corrosion resistance is deteriorated, thereby lowering the delayed fracture resistance. Therefore, when Mo is contained, the Mo content is 1.0% or less, preferably 0.50% or less. In order to obtain such an effect, when Mo is contained, the Mo content is preferably 0.01% or more, more preferably 0.05% or more.

(V: 1.0% or less)
V improves the strength of the hot pressed member by precipitation strengthening or solid solution strengthening. In addition, like Ti, V forms carbonitrides, suppresses coarsening of austenite grains when the steel sheet to be subjected to hot pressing is heated to Ac ₃ temperature or higher, and makes the prior austenite grain size finer. It is an element that has the effect of
On the other hand, if the V content is too high, coarse carbides are formed, degrading the ductility and delayed fracture resistance of the hot pressed member. Therefore, when V is contained, the V content is 1.0% or less, preferably 0.50% or less. In order to express these effects, when V is contained, the V content is preferably 0.01% or more, more preferably 0.10% or more.

The component composition of the hot press member can further contain at least one selected from the group consisting of Bi, Ca, Co, Mg, REM, Ta, W, Zn and Zr in the following content.
REM (Rare Earth Metal) is a general term for a total of 17 elements, 2 elements Sc (scandium) and Y (yttrium), and 15 elements (lanthanoids) from La (lanthanum) to Lu (lutetium).

(Bi: 0.10% or less)
Bi is an element that has the effect of homogenizing substitutional elements such as Mn in martensite, tempered martensite, and bainite of hot pressed parts and improving ductility.
On the other hand, if the Bi content is too high, coarse oxides are formed, degrading the ductility and delayed fracture resistance of the hot pressed member. Therefore, when Bi is contained, the Bi content is 0.10% or less, preferably 0.050% or less, and more preferably 0.020% or less. In order to obtain such an effect, when Bi is contained, the Bi content is preferably 0.0002% or more, more preferably 0.0010% or more.

(Ca: 0.10% or less, Mg: 0.10% or less, REM: 0.10% or less)
Ca, Mg, and REM control the shape of oxides and sulfides and suppress the formation of coarse inclusions, thereby improving the ductility and delayed fracture resistance of the hot pressed member.
On the other hand, too much content of Ca, Mg, and REM causes an increase in inclusions, degrading the ductility and delayed fracture properties of the hot pressed member. Therefore, when one or more of these elements are contained, the content of each is 0.10% or less, preferably 0.050% or less, and more preferably 0.020% or less. In order to exhibit these effects, when one or more of these elements are contained, the content of each is preferably 0.0002% or more, more preferably 0.0010% or more.

(Co: 0.10% or less, Zr: 0.10% or less)
Like Cu and Ni, Co and Zr improve corrosion resistance and thus can improve delayed fracture resistance, so they can be added as necessary.
On the other hand, if the contents of Co and Zr are too large, the ductility is deteriorated. Therefore, when one or two of these elements are contained, the content of each is 0.10% or less, preferably 0.05% or less, and more preferably 0.020% or less. In order to obtain such an effect, when one or two of these elements are contained, the content of each is preferably 0.0002% or more, more preferably 0.0010% or more.

(Ta: 0.10% or less, W: 0.10% or less)
Ta and W form alloy carbides, contribute to precipitation strengthening, and contribute to improving the strength of hot pressed parts.
On the other hand, when the content of Ta and W is too large, coarse carbides are formed, and the ductility and delayed fracture resistance of the hot pressed member are deteriorated. Therefore, when one or two of these elements are contained, the content of each is 0.10% or less, preferably 0.050% or less, and more preferably 0.020% or less. In order to obtain such an effect, when one or two of these elements are contained, the content of each is preferably 0.0002% or more, more preferably 0.0010% or more.

(Zn: 0.10% or less)
Zn promotes the formation of martensite, tempered martensite, and bainite during hot pressing by improving the hardenability during hot pressing, and contributes to improving the strength of the steel sheet.
On the other hand, if the Zn content is too high, the ductility is degraded. Therefore, when Zn is contained, the Zn content is 0.10% or less, preferably 0.05% or less, and more preferably 0.020% or less. In order to obtain such an effect, when Zn is contained, the Zn content is preferably 0.0002% or more, more preferably 0.0010% or more.

(remainder)
In the component composition of the hot pressed member, the balance other than the components (elements) described above consists of Fe and unavoidable impurities. Examples of unavoidable impurities include Ag, As, Ce, O, etc., and the total content of these elements is acceptable as long as it is 0.5% or less.

<Microstructure>
Next, the reason for limiting the microstructure of the hot press member will be explained.

(Martensite, tempered martensite, and bainite are 60% or more and less than 90% in total area ratio)
If the total area ratio of martensite, tempered martensite, and bainite is less than 60%, it may not be possible to achieve a TS of 1300 MPa or more. 70% or more is preferable, and a total of 75% is more preferable.
On the other hand, if the total area ratio of martensite, tempered martensite, and bainite is 90% or more, excellent ductility may not be achieved. , the total is preferably 85% or less, more preferably 80% or less.

(The area ratio of ferrite is 10% or more and 40% or less)
If the area ratio of ferrite is less than 10%, excellent ductility may not be achieved. If the area ratio of ferrite exceeds 40%, it may not be possible to achieve a TS of 1300 MPa or more.
Pearlite, retained austenite, and the like can be considered as the residual structure of the hot-pressed member, but these are permissible if the total area ratio is 10% or less.

(Previous austenite average grain size is less than 3.0 μm)
The finer the prior austenite average grain size, the better the delayed fracture resistance. If the prior austenite average grain size is 3.0 μm or more, excellent delayed fracture resistance cannot be achieved. The average grain size of prior austenite is less than 3.0 μm, preferably 2.5 μm or less, more preferably 2.0 μm or less, even more preferably 1.5 μm or less.

<Plating layer>
The hot press member of the present invention may have a plating layer on its surface.
The hot pressed member of the present invention preferably has an Al-based plating layer or a Zn-based plating layer on its surface.
When a steel sheet for hot pressing to which an Al-based plating layer or a Zn-based plating layer is applied is heated and then hot-pressed, part of the plating layer components contained in the Al-based plating layer or the Zn-based plating layer Alternatively, all diffuse into the base steel sheet to form solid solution phases and intermetallic compounds. Form intermetallic compounds. An oxide film containing Al is formed on the surface of the Al-based plating layer, and an oxide film containing Zn is formed on the surface of the Zn-based plating layer.
For example, when an Al—Si plating layer is heated, the plating layer changes into a plating layer mainly composed of an Fe—Al intermetallic compound containing Si. When hot-dip Zn-plated layers, alloyed hot-dipped Zn-plated layers, electro-Zn-plated layers, etc. are heated, FeZn solid solution phases in which Zn is dissolved in Fe, ZnFe intermetallic compounds, surface ZnO layers, and the like are formed. Furthermore, when the electric Zn—Ni alloy plating layer is heated, a solid solution phase containing Ni in which plating layer components are dissolved in Fe, an intermetallic compound mainly composed of ZnNi, a ZnO layer on the surface, and the like are formed. be.
The Al-based plating layer or the Zn-based plating layer is not limited to the above-mentioned plating layers. The plating layer may contain one or more of B, P, S, Ti, V, W, Mo, Sb, Cd, Nb, Cr, Sr, and the like. Although a plating layer is generally applied to the steel sheet before hot pressing, the plating layer may be applied after hot pressing. The method of applying the Al-based plating layer or the Zn-based plating layer is also not limited at all, and any known hot-dip plating method, electroplating method, vapor deposition plating method, or the like can be applied. Moreover, it may be a plated layer that is alloyed after being plated.
The plating layer may be applied to any steel sheet such as a hot-rolled steel sheet, a cold-rolled steel sheet, or an annealed steel sheet after cold-rolling. The surface of the steel sheet may be pickled with hydrochloric acid or the like before the plating layer is provided.
Before or after any of the hot rolling process, the pickling process, the cold rolling process, the annealing process, and the plating layer applying process, temper rolling with an elongation rate of 0.01% or more and 5% or less may be performed.
The adhesion amount of the plating layer is not particularly limited as long as it is a general one. For example, it is preferable to have a plating layer with a plating amount of 5 to 150 g/m ² per side. If the coating amount is less than 5 g/m ² , it may become difficult to ensure corrosion resistance, while if it exceeds 150 g/m ² , the coating peeling resistance may be deteriorated.
After hot pressing, the surface of the member may be coated by a conventional method. For example, any of spray coating, electrodeposition coating, and the like can be applied. If necessary, a conventional baking treatment may be applied after coating. For example, it is preferable to perform a baking treatment at 100° C. or higher and 300° C. or lower for 1 minute or longer and 60 minutes or shorter. Painting and baking can be repeated without any problem. As a coating base, chemical conversion treatment may be applied by a conventional method. For example, zinc phosphate treatment, iron phosphate treatment, zirconium treatment, and the like are all applicable.

[Method for manufacturing hot press member]
Next, a method for manufacturing the hot press member of the present invention will be described.
The method of manufacturing the hot pressed member of the present invention is not particularly limited, but a method comprising the following preheating step and the following hot pressing step is preferred.
(1) Preheating step A steel material having the above-described chemical composition is heated at an average heating rate of 50 ° C./s or more to Ac ₃ temperature -50 ° C. or more and Ac ₃ temperature + 100 ° C. or less, and Ac ₃ temperature -50 ° C. or more. Step (2) Hot pressing of performing preheating by holding at a temperature of Ac ₃ temperature + 100 ° C. or less for 0 seconds or more and 30 seconds or less, and then cooling to Ms temperature of -100 ° C. or less at an average cooling rate of 5 ° C./s or more. Step After the preheating step, the preheated steel material is heated at an average heating rate of 50 ° C./s or more to Ac ₃ temperature -50 ° C. or more and Ac ₃ temperature, and Ac ₃ temperature -50 ° C. or more. After holding at a temperature of less than Ac ₃ temperature for 0 seconds or more and 30 seconds or less, cooling at an average cooling rate of 5 ° C./s or more, hot press molding at Ms temperature or more, Ms temperature -100 ° C. or less at 20 ° C. A step of obtaining a hot pressed member by cooling at an average cooling rate of /s or more

<Preheat treatment step>
In the preheating step, the steel material having the above-described chemical composition is heated at an average heating rate of 50 ° C./s or more to Ac ₃ temperature -50 ° C. or higher and Ac ₃ temperature + 100 ° C. or lower, and Ac ₃ temperature -50 ° C. or higher. ₃ After holding at a temperature of +100° C. or less for 0 to 30 seconds, a preheating step is performed to cool to an Ms temperature of −100° C. or less at an average cooling rate of 5° C./s or more.
The preheating step is a step for making a fine structure before hot pressing, and is a very important step because the prior austenite average grain size after hot pressing can be less than 3.0 μm.

(average heating rate)
If the average heating rate of the steel material is less than 50° C./s (seconds), recrystallization and grain growth of ferrite proceed, and the grain size of austenite when reversely transformed to austenite becomes coarse, resulting in hot pressing. The later prior austenite average grain size of less than 3.0 μm cannot be obtained. Therefore, the average heating rate was set to 50° C./s or higher. It is preferably 60° C./s or higher, more preferably 70° C./s or higher. There is no particular upper limit to the heating rate, but if it exceeds 1000° C./s, it becomes difficult to control the heating temperature.

(Heating temperature)
If the heating temperature is less than the Ac ₃ temperature −50 ° C., the reverse transformation to austenite will be insufficient, and a coarse ferrite structure will remain before hot pressing, and the former austenite average grain size after hot pressing will be 3. less than 0.0 μm cannot be achieved. Therefore, the lower limit of the heating temperature was set to Ac ₃ temperature -50°C. Preferably, the lower limit of the heating temperature is Ac ₃ temperature -20°C or higher, more preferably Ac ₃ temperature or higher.
On the other hand, when the heating temperature exceeds the Ac ₃ temperature +100°C, recrystallization and grain growth of austenite proceed, and the prior austenite average grain size after hot pressing of less than 3.0 µm cannot be achieved. Therefore, the upper limit of the heating temperature was set to be the Ac ₃ temperature + 100°C or less. Preferably, the upper limit of the heating temperature is Ac ₃ temperature + 80°C or less, more preferably Ac ₃ temperature + 50°C or less. More preferably less than Ac ₃ temperature + 10°C.

(holding time)
When the holding time at the temperature of Ac ₃ temperature -50 ° C. or higher and Ac ₃ temperature + 100 ° C. or lower exceeds 30 seconds, the austenite grain growth proceeds, and the prior austenite average grain size after hot pressing is less than 3.0 μm. be unable to do so. Therefore, the holding time at the temperature of Ac ₃ temperature -50°C or higher and Ac ₃ temperature + 100°C or lower was set to 30 seconds or less. It is preferably 20 seconds or less, more preferably 10 seconds or less. The lower limit of the retention time is not particularly limited and is 0 second (no retention time).

(Average cooling rate up to Ms temperature -100 ° C or less)
If the average cooling rate to the Ms temperature of −100° C. or lower is less than 5° C./s, coarse ferrite may occur, and the prior austenite average grain size after hot pressing of less than 3.0 μm cannot be achieved. Gone. Therefore, the average cooling rate to the Ms temperature of -100°C or lower was set to 5°C/s or higher. It is preferably 10° C./s or higher, more preferably 20° C./s or higher, and still more preferably 50° C./s or higher.
The steel plate may be press-formed during cooling.

(preferred embodiment)
The preheating described above can be performed repeatedly. By repeating the process, the prior austenite grain size becomes even finer. Although there is no particular limitation on the number of repetitions, 10 times or less is preferable from the viewpoint of manufacturing costs.

(Ac ₃ temperature)
where Ac ₃ temperature can be determined by the following equation:
Ac ₃ temperature (°C) = 881 - 206C + 53Si - 15Mn - 20Ni - 1Cr - 27Cu + 41Mo
However, the element symbol in the formula represents the content (% by mass) of each element, and is calculated as 0 when the element is not contained.

(Ms temperature)
Here, the Ms temperature can be obtained by the following equation.
Ms temperature (°C) = 561-474C-33Mn-18Ni-17Cr-21Mo
However, the element symbol in the formula represents the content (% by mass) of each element, and is calculated as 0 when the element is not contained.

<Hot press process>
In the hot press step, after the preheating step described above, the steel material subjected to the preheating described above is heated at an average heating rate of 50 ° C./s or more to Ac ₃ temperature -50 ° C. or more and Ac ₃ temperature, After holding at a temperature of Ac ₃ temperature -50 ° C. or more and less than Ac ₃ temperature for 0 seconds or more and 30 seconds or less, cooling at an average cooling rate of 5 ° C./s or more, hot press molding at Ms temperature or more, Ms temperature In this step, a hot pressed member is obtained by cooling to −100° C. or lower at an average cooling rate of 20° C./s or higher.

(average heating rate)
If the average heating rate of the steel material is less than 50°C/s, recrystallization and grain growth of ferrite proceed, and the grain size of austenite becomes coarse when reversely transformed into austenite. An average austenite grain size of less than 3.0 μm cannot be obtained. Therefore, the average heating rate was set to 50° C./s or higher. It is preferably 70° C./s or higher, more preferably 100° C./s or higher. There is no particular upper limit to the heating rate, but if it exceeds 1000° C./s, it becomes difficult to control the heating temperature, so 1000° C./s or less is preferable.

(Heating temperature)
If the heating temperature is less than Ac ₃ temperature -50 ° C., the area ratio of ferrite will become too high, and the total area ratio of martensite, tempered martensite, and bainite after hot pressing will not be able to achieve 60% or more. . Therefore, the lower limit of the heating temperature was set to Ac ₃ temperature -50°C or more. Preferably, the lower limit of the heating temperature is Ac ₃ temperature -40°C or higher, more preferably Ac ₃ temperature -30°C or higher.
On the other hand, if the heating temperature is the Ac ₃ temperature or higher, the austenite single phase region is formed, and the ferrite area ratio of 10% or higher after hot pressing cannot be achieved. Therefore, the upper limit of the heating temperature was set so that the heating temperature is less than the Ac ₃ temperature. Preferably, the upper limit of the heating temperature is Ac ₃ temperature -10°C or less.

(holding time)
When the holding time at the temperature of Ac ₃ temperature -50 ° C. or more and less than Ac ₃ temperature exceeds 30 seconds, the austenite grain growth proceeds and the prior austenite average grain size after hot pressing is less than 3.0 μm. I can't do it. Therefore, the holding time at the temperature above Ac ₃ temperature -50°C and below Ac ₃ temperature was set to 30 seconds or less. It is preferably 20 seconds or less, more preferably 10 seconds or less. The lower limit of the retention time is not particularly limited and is 0 second (no retention time).

(average cooling rate)
If the average cooling rate is less than 5° C./s, ferrite may occur, and the total area ratio of martensite, tempered martensite, and bainite after hot pressing may not reach 60% or more. Therefore, the average cooling rate was set to 5°C/s or more.

(Hot press temperature)
If hot press forming is performed at a temperature lower than the Ms temperature, part of the steel sheet undergoes martensite transformation, and the formability may deteriorate, and cracks may occur during forming. Therefore, hot press molding was performed at Ms temperature or higher.

(Average cooling rate up to Ms temperature -100 ° C or less)
If the average cooling rate to the Ms temperature of −100° C. or lower is less than 20° C./s, the tempering of transformed martensite becomes significant, and TS may not be able to achieve 1300 MPa or higher. Therefore, the average cooling rate to the Ms temperature of −100° C. or less was set to 20° C./s or more. It is preferably 30° C./s or higher, more preferably 40° C./s or higher, and still more preferably 50° C./s or higher.

<Heating method>
The heating method described above is not particularly limited, and if a desired heating rate can be achieved, heating can be performed by furnace heating, high-frequency heating, electric heating, etc., but electric heating is preferable because the effect of the present invention is more excellent. .

<Cooling method>
The cooling method described above is not particularly limited, and if the desired cooling rate can be achieved, cooling can be performed by air cooling, forced air cooling, water cooling, mist cooling, gas cooling, mold cooling, etc., but the effect of the present invention is more excellent. , mold cooling is preferred.

EXAMPLES The present invention will be specifically described below with reference to examples. However, the present invention is not limited to the examples described below. Appropriate modifications may be made within the scope of the present invention, and they are all included in the technical scope of the present invention.

[Manufacturing of hot pressed parts]
A hot pressed member was manufactured as follows.

<Preparation of steel material>
Steel materials (steel sheets) having chemical compositions shown in Table 1 were prepared.
Here, the Ac ₃ temperature, Ms temperature and plate thickness columns represent the Ac ₃ temperature, Ms temperature and plate thickness of each steel material. In addition, in the plating column, HR is a hot-rolled steel sheet after hot rolling, HGI is a hot-rolled steel sheet to which hot-dip Zn plating has been applied, HGA is a hot-rolled steel sheet that has been further alloyed with hot-dip Zn plating, and CR is Cold-rolled steel sheet after cold rolling, AC is cold-rolled steel sheet annealed after cold-rolling, GI is cold-rolled steel sheet to which hot-dip Zn plating is applied, and GA is cold-rolled to which hot-dip Zn plating is alloyed. Steel sheet, AI is cold-rolled steel sheet with hot-dip Al plating, AA is cold-rolled steel sheet with further alloying treatment of hot-dip Al plating, EG is cold-rolled steel sheet with electro-Zn plating, EGN is electro-Zn plating represents a cold-rolled steel sheet alloyed with Ni.

<Preheating step and hot pressing step>
The obtained steel material was subjected to a preheating step under the conditions described in the preheating step shown in Table 2 below, and then subjected to a hot pressing step under the conditions described in the hot pressing step shown in Table 2 below.
For example, No. If it is 1, the steel material of A in Table 1 is heated to 830 ° C. at an average heating rate of 50 ° C./s, and the average cooling rate is 30 ° C./s to Ms temperature (417 ° C.) −100 ° C. or less without holding time. cooled at a rate (preheating step). After that, it is heated to 820°C at an average heating rate of 50°C/s, cooled at an average cooling rate of 10°C/s without holding time, hot press-molded at 800°C, and Ms temperature (417°C) -100. C. or less at an average cooling rate of 30.degree. C./s (hot pressing step). In this way, no. No. 1 hot pressed member was obtained.

In addition, No. For Nos. 20 to 22, preheating was repeatedly performed as shown in Table 2 below. Also, N. For Nos. 29 and 38, only the hot pressing process was performed without preheating.
Also, No. The heating method in Nos. 1 to 37 is electric heating. The heating mode in 38 was furnace heating. Moreover, the cooling method was die cooling in any of the examples.
The mold used for hot pressing had a punch width of 70 mm, a punch shoulder radius of 4 mm, a die shoulder radius of 4 mm, and a forming depth of 30 mm.

[Evaluation of hot press member]
A test piece was taken from the hat bottom portion of the hot pressed member thus obtained, and the test and evaluation described below were performed. In addition, No. No. 28 was not evaluated because cracks occurred during hot press forming.

<Observation of Microstructure>

(Measurement of area ratio of martensite, tempered martensite, bainite, and ferrite)
The sampled test piece was polished to expose a cross section (a cross section parallel to the rolling direction) at a position of 1/4 of the plate thickness. The exposed cross section was corroded using an etchant (3 mass % nital solution), and then observed and photographed for 10 fields of view at a magnification of 5000 using a scanning electron microscope (SEM). The area ratio [%] of martensite, tempered martensite, bainite, and ferrite is determined by measuring and quantifying the microstructure photographs of 10 fields of view taken by the point counting method (according to ASTM E562-83 (1988)). turned into The lath-like structure was martensite, tempered martensite and bainite, and the lamellar structure was pearlite. It was possible to distinguish between ferrite and retained austenite from the contrast of the remaining structure, and it was assumed that the dark area was ferrite and the bright area was retained austenite. Table 3 shows the results.

(Measurement of prior austenite average grain size)
The prior austenite grain size of the sampled test piece was measured according to JIS G0551:2013.
Specifically, the sampled test piece was polished to expose a cross section (a cross section parallel to the rolling direction) at the 1/4 thickness position. The exposed cross section is exposed to a prior austenite structure with a corrosive solution (an aqueous solution containing picric acid, a surfactant, and oxalic acid), and an optical microscope is used at a position of 1/4 of the plate thickness at a magnification of 1000 times. Ten fields of view were photographed, and the average equivalent circle diameter of the prior austenite grains was measured. Table 3 shows the results.

<Tensile test>
A JIS No. 5 test piece (gauge length GL: 50 mm) was taken from the position of the bottom of the hat of the hot pressed member thus obtained, and tensile strength (TS) and total elongation (El) were determined.
Specifically, the sampled test piece was subjected to a tensile test in accordance with JIS Z 2241:2011 to determine tensile strength (TS) [MPa] and total elongation (El) [%]. The tensile test was performed twice for each hot-pressed member, and the average value of the two times was taken as the TS and El of the hot-pressed member. Samples with a TS of 1300 MPa or more and less than 1760 MPa and an El of 8% or more from the viewpoint of ductility were passed. Table 3 shows the results.

<Evaluation of delayed fracture resistance>
A 4-point bending test piece was taken from the position of the hat bottom portion of the hot pressed member, and a 4-point bending test was performed in accordance with ASTM G39-99 (2016). Bending stress was applied while being immersed in a solution of hydrochloric acid (pH=1.0) at room temperature, and the presence or absence of breakage was evaluated. When the bending stress was TS, the delayed fracture resistance was evaluated as good (○) when the fracture did not occur for 100 hours or longer, and the delayed fracture resistance was evaluated as poor (x) when the fracture occurred in less than 100 hours. The n number of test pieces was 2 and the test was performed. It was rated as good (o) when no two wires were broken, and poor (x) when even one wire was broken. Table 3 shows the results.

As can be seen from Tables 1-3, no. The hot pressed parts of 1 to 22 and 30 to 32 had a tensile strength with a TS of 1300 MPa or more and less than 1760 MPa, and exhibited excellent ductility and delayed fracture resistance.
On the other hand, no. No. 33 to 37 hot press members and No. 3 with microstructures outside the specified range. 23-27, 29, 34-35 and 38 were insufficient in at least one of tensile strength, ductility and delayed fracture resistance.

Claims (7)

% by mass, C: 0.15% or more and less than 0.26%, Si: 0.01% or more and 1.5% or less, Mn: 1.0% or more and 3.5% or less, P: 0.10% or less , S: 0.010% or less, Al: 0.01% or more and 1.5% or less, N: 0.010% or less, Ti: 0.01% or more and 0.20% or less, and B: 0.0005 % or more and 0.020% or less, the balance being Fe and unavoidable impurities;
The total area ratio of martensite, tempered martensite, and bainite is 60% or more and less than 90%, the area ratio of ferrite is 10% or more and 40% or less, and the prior austenite average grain size is less than 3.0 μm. There is a hot press member. Further, the above component composition is, in mass %, Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Sb: 0.10% or less, and Sn: 0.10% or less. The hot press member according to claim 1, containing at least one selected from the group consisting of: The component composition further contains at least one selected from the group consisting of Nb: 0.20% or less, Mo: 1.0% or less, and V: 1.0% or less in mass %. 3. The hot press member according to 1 or 2. Further, the component composition is, in mass %, Bi: 0.10% or less, Ca: 0.10% or less, Co: 0.10% or less, Mg: 0.10% or less, REM: 0.10% or less , Ta: 0.10% or less, W: 0.10% or less, Zn: 0.10% or less, and Zr: 0.10% or less, containing at least one selected from the group consisting of The hot press member according to any one of 1 to 3. The hot press member according to any one of claims 1 to 4, which has an Al-based plating layer or a Zn-based plating layer on its surface. A hot pressed member manufacturing method for manufacturing the hot pressed member according to any one of claims 1 to 5,
A steel material having the chemical composition according to any one of claims 1 to 4 is heated at an average heating rate of 50 ° C./s or more to Ac ₃ temperature -50 ° C. or more and Ac ₃ temperature + 100 ° C. or less, and Ac ₃ After holding at a temperature of -50 ° C. or higher and Ac ₃ temperature + 100 ° C. or lower for 0 seconds or more and 30 seconds or less, preheat treatment is performed to cool to Ms temperature of -100 ° C. or lower at an average cooling rate of 5 ° C./s or higher. a heat treatment step;
After the preheating step, the preheated steel material is heated at an average heating rate of 50 ° C./s or more to Ac ₃ temperature -50 ° C. or more and Ac ₃ temperature, and Ac ₃ temperature -50 ° C. or more Ac After holding at a temperature of less than ₃ temperatures for 0 seconds or more and 30 seconds or less, cooling at an average cooling rate of 5 ° C./s or more, hot press molding at Ms temperature or more, Ms temperature -100 ° C. or less at 20 ° C./ A method for manufacturing a hot pressed member, comprising a hot pressing step of obtaining a hot pressed member by cooling at an average cooling rate of s or more. A hot pressed member manufacturing method for manufacturing the hot pressed member according to any one of claims 1 to 5,
A steel material having the chemical composition according to any one of claims 1 to 4 is heated at an average heating rate of 50 ° C./s or more to Ac ₃ temperature -50 ° C. or more and Ac ₃ temperature + 100 ° C. or less, and Ac ₃ After holding at a temperature of −50° C. or more and Ac ₃ temperature + 100° C. or less for a time of 0 to 30 seconds, and then cooling to Ms temperature of −100° C. or less at an average cooling rate of 5° C./s or more. , a preheating step of repeating the preheating one or more times;
After the preheating step, the preheated steel material is heated at an average heating rate of 50 ° C./s or more to Ac ₃ temperature -50 ° C. or more and Ac ₃ temperature, and Ac ₃ temperature -50 ° C. or more Ac After holding at a temperature of less than ₃ temperatures for 0 seconds or more and 30 seconds or less, cooling at an average cooling rate of 5 ° C./s or more, hot press molding at Ms temperature or more, Ms temperature -100 ° C. or less at 20 ° C./ A method for manufacturing a hot pressed member, comprising a hot pressing step of obtaining a hot pressed member by cooling at an average cooling rate of s or more.