JP2021181617A - Hot press member and method for producing the same - Google Patents

Abstract

To provide a hot press member that has a tensile strength TS of 1300 MPa or more and less than 1760 MPa and also excels in ductility and delayed fracture resistance, and a method for producing the same.SOLUTION: A hot press member contains, in 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 with the balance being Fe and inevitable impurities. In the micro structure, 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. The former austenite average particle size is less than 3.0 μm.SELECTED DRAWING: None

Description

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

In recent years, automobile emission regulations have been tightened from the viewpoint of global environmental conservation. Under such circumstances, improving the fuel efficiency of automobiles has become an important issue, and high strength and thinning of automobile parts are required. As a means for increasing the strength and thinning of an automobile part, a means for forming a steel sheet into a desired part shape by hot press quenching using a steel sheet as a material for the automobile part is known. In hot press quenching, a blank (steel plate) heated to the austenite single phase region is hot press molded into a desired shape using a die, and heat is removed in the die to perform quenching.
As described above, in hot press quenching, a steel sheet heated to a high temperature region, that is, a steel sheet that is softened and easily processed is press-formed, so that the steel sheet can be formed into a complicated part shape. Further, since the steel sheet is hardened while being formed into a desired part shape, a hot pressed member having extremely high strength such that TS exceeds 1300 MPa can be obtained after forming. Further, since quenching is performed in the mold, heat treatment strain can be suppressed, and a hot press member having excellent dimensional accuracy can be obtained.

Japanese Patent No. 5729213 Japanese Patent No. 5257062

However, as the strength of the member increases, the material properties such as ductility and delayed fracture resistance generally deteriorate. On the other hand, some structural members used for 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 a collision of automobiles. In addition, there are some that require delayed fracture (hydrogen embrittlement) due to hydrogen invading from the usage environment, that is, delayed fracture resistance.
Patent Document 1 describes a member having excellent delayed fracture resistance by controlling the heating temperature during hot pressing and heat-treating the member after hot pressing in a temperature range of 150 ° C. or higher and 200 ° C. or lower. I'm proposing a way to get it. However, in the method described in Patent Document 1, since the heating / cooling conditions at the time of hot press molding are not appropriate, the former austenite particle size is not sufficiently miniaturized, and sufficient delayed fracture characteristics cannot always be obtained. .. In addition, the microstructure was not sufficiently controlled and sufficient ductility could not be obtained.
Patent Document 2 proposes a method of controlling the heating temperature during hot pressing to reduce the particle size of the old austenite to obtain a member having excellent delayed fracture resistance. However, in the method described in Patent Document 2, since the heating / cooling conditions at the time of hot press molding are not appropriate, the former austenite particle size is not sufficiently miniaturized, and sufficient delayed fracture characteristics cannot always be obtained. .. In addition, the microstructure was not sufficiently controlled and sufficient ductility could not be obtained.

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

As a result of diligent studies, the present inventor has found that the above object can be achieved by adopting the following configuration, and has completed the present invention.

(1) In terms of 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: A component composition containing 0.0005% or more and 0.020% or less, and the balance consisting of Fe and unavoidable impurities.
The microstructure is martensite, tempered martensite, and bainite in total area ratio of 60% or more and less than 90%, ferrite is 10% or more and 40% or less in area ratio, and the old austenite average particle size is less than 3.0 μm. There is a hot press member.
(2) The composition of the above components 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. The hot press member according to (1) above, which contains at least one selected from the group consisting of 10% or less.
(3) The above-mentioned 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 (1) or (2) above.
(4) The composition of the above components is, in terms of mass%, Bi: 0.10% or less, Ca: 0.10% or less, Co: 0.10% or less, Mg: 0.10% or less, REM: 0. 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 (1) to (3) above.
(5) The hot press member according to any one of (1) to (4) above, which has an Al-based plating layer or a Zn-based plating layer on the surface.
(6) A method for manufacturing a hot press member according to any one of (1) to (5) above, wherein the hot press member is manufactured.
Above (1) was heated to (4) at an average heating rate of the steel material having the component composition described above 50 ° C. / s according to any one of the following Ac ₃ temperature -50 ° C. or higher Ac ₃ temperature + 100 ° C., Ac ₃ temperature -50 ° C or higher Ac ₃ temperature + 100 ° C or lower for 0 seconds or more and 30 seconds or less, then preheat to cool to Ms temperature -100 ° C or lower at an average cooling rate of 5 ° C / s or higher. Preheat treatment process and
After the preheating step, the preheating process the average heating rate of more than 50 ° C. / s a steel material which has been subjected, Ac ₃ was heated to a temperature -50 ° C. or higher Ac less than ₃ temperature, Ac ₃ temperature -50 ° C. or higher Ac _{After holding at a temperature of less than 3} temperatures for 0 seconds or more and 30 seconds or less, cool at an average cooling rate of 5 ° C./s or more, hot press molding at Ms temperature or more, and 20 ° C./to Ms temperature of -100 ° C or less. A method for manufacturing a hot-pressed member, comprising a hot-pressed 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 press member according to any one of (1) to (5) above, wherein the hot press member is manufactured.
Above (1) was heated to (4) at an average heating rate of the steel material having the component composition described above 50 ° C. / s according to any one of the following Ac ₃ temperature -50 ° C. or higher Ac ₃ temperature + 100 ° C., Ac _{After holding at 3} temperature -50 ° C or higher and Ac ₃ temperature + 100 ° C or lower for 0 seconds or longer and 30 seconds or lower, preheat treatment was performed to cool the Ms temperature to -100 ° C or lower at an average cooling rate of 5 ° C / s or higher. After that, the preheat treatment step of repeatedly performing the above preheat treatment once or more,
After the preheating step, the preheating process the average heating rate of more than 50 ° C. / s a steel material which has been subjected, Ac ₃ was heated to a temperature -50 ° C. or higher Ac less than ₃ temperature, Ac ₃ temperature -50 ° C. or higher Ac _{After holding at a temperature of less than 3} temperatures for 0 seconds or more and 30 seconds or less, cool at an average cooling rate of 5 ° C./s or more, hot press molding at Ms temperature or more, and 20 ° C./to Ms temperature of -100 ° C or less. A method for manufacturing a hot-pressed member, comprising a hot-pressed step of obtaining a hot-pressed member by cooling at an average cooling rate of s or more.

According to the present invention, it is possible to provide a hot press member having a TS having a tensile strength of 1300 MPa or more and less than 1760 MPa, and having excellent ductility and delayed fracture resistance, and a method for manufacturing the same.
By using the hot press member of the present invention for a structural member, a skeleton member, an undercarriage member such as a suspension, etc. of an automobile, the weight of the automobile body can be reduced while ensuring the safety of the automobile. Therefore, it can contribute to the reduction of the environmental load.

Hereinafter, the hot press member of the present invention and a method for manufacturing the same will be described.
In addition, the numerical range represented by using "~" in this specification means the range including the numerical values before and after "~" as the lower limit value and the upper limit value.

[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 A component composition containing% or more and 0.020% or less, and the balance consisting of Fe and unavoidable impurities.
The microstructure is martensite, tempered martensite, and bainite in total area ratio of 60% or more and less than 90%, ferrite is 10% or more and 40% or less in area ratio, and the old austenite average particle size is less than 3.0 μm. There is a hot press member.

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

(C: 0.15% or more and less than 0.26%)
C is an element effective 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 the hot pressed member. In order to secure TS1300MPa or more after hot pressing, it is necessary to make it 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 strengthening the solid solution, suppresses the precipitation of cementite, improves the tempering and softening resistance of martensite during hot pressing, and contributes to improving the strength of the hot pressed member. In order to obtain such an effect, 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 TS of the hot-pressed member may not be secured. 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 strengthening the solid solution, and also contributes to improving the strength of the hot-pressed member by promoting the formation of martensite, tempered martensite, and bainite during hot-pressing by improving hardenability. In order to obtain such an effect, 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 press 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 the improvement of the strength of the hot press member by strengthening the solid solution. However, P segregates at the grain boundaries and reduces ductility. Therefore, the P content is preferably as low as possible, and the content of P up to 0.10% is acceptable. 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 a coarse sulfide, which reduces the ductility and delayed fracture resistance of the hot press member. Therefore, the S content is preferably as low as possible, and the content of S up to 0.010% is acceptable. 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 the hot press member. If the amount of Al is too small, the effect is not always 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 and deteriorates 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 binding to an element forming a nitride, and contributes to the miniaturization of crystal grains. However, N tends to combine with Ti at a high temperature to form a coarse nitride, and too much content deteriorates the ductility and delayed fracture resistance of the hot press 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 press member by precipitation strengthening or solid solution strengthening. Ti forms a nitride in the austenite phase high temperature region (high temperature region in the austenite phase and higher temperature region than the austenite phase (casting stage)). As a result, the precipitation of BN is suppressed and B becomes a solid solution state. In this way, the hardenability required for the formation of martensite, tempered martensite, and bainite during hot pressing is obtained, which contributes to the improvement of strength. In addition, Ti has the effect of forming carbonitrides, suppressing the coarsening of austenite grains when the steel sheet to be hot-pressed _{is heated to Ac 3} temperature or higher, and making the old austenite grain size finer. It is an important element. In order to exhibit these effects, the Ti content is 0.01% or more. The Ti content is preferably 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, which deteriorates the ductility and delayed fracture resistance of the hot press 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 into the old 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 the improvement of 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-mentioned effect is 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 content shown below.

(Cr: 1.0% or less)
Cr contributes to strengthening the solid solution, and promotes the formation of martensite, tempered martensite, and bainite during hot pressing through the improvement of hardenability, and contributes to the improvement of strength.
On the other hand, Cr deteriorates the chemical conversion treatment property and the plating property of the hot press material and the hot press member, and deteriorates the corrosion resistance, thus 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 exhibit 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 strengthening the solid solution, and promotes the formation of martensite, tempered martensite, and bainite during hot pressing through the improvement of hardenability, and contributes to the improvement of the strength of the hot pressed member. Further, since the corrosion resistance can be improved and the delayed fracture resistance can be improved, it can be added as needed.
On the other hand, if the Cu content is too high, the above-mentioned effects are saturated, and surface defects caused by 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 the strengthening of solid solution, and promotes the formation of martensite, tempered martensite, and bainite during hot pressing through the improvement of hardenability, and contributes to the improvement of strength. Further, since the corrosion resistance is improved like Cu, the delayed fracture resistance can be improved, so that it can be added as needed.
On the other hand, if the Ni content is too high, the 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 nitriding of the surface of the steel material at the stage of heating the steel material such as a slab, and suppresses the precipitation of BN on the surface layer portion of the steel material. Further, the presence of the solid solution B provides the hardenability required for the formation of martensite, tempered martensite, and bainite in the surface layer portion of the hot press member, and improves the strength of the hot press member.
On the other hand, if the Sb content is too large, the rolling load during the production of the hot pressed material may increase, which may reduce the productivity. Therefore, when Sb is contained, the Sb content is preferably 0.10% or less, more preferably 0.050% or less, still 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)
Similar to Cu and Ni, Sn can improve the corrosion resistance and thus the delayed fracture resistance, and can be added as needed.
On the other hand, if the Sn content is too large, the hot workability may be deteriorated 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 preferably 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 content shown below.

(Nb: 0.20% or less)
Nb improves the strength of the hot press member by precipitation strengthening or solid solution strengthening. Further, like Ti, Nb forms a carbonitride _{to suppress coarsening of austenite grains when the steel plate to be hot-pressed is heated to Ac 3} temperature or higher, and the old austenite particle size is made 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, which deteriorates the ductility and delayed fracture resistance of the hot press 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 preferably 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 the improvement of hardenability, and contributes to the improvement of the strength of the steel sheet. Further, like Ti, Mo forms carbonitride _{to suppress coarsening of austenite grains when the steel sheet to be hot-pressed is heated to Ac 3} temperature or higher, and the old austenite particle size is made finer. It is an element that has the effect of
On the other hand, if the Mo content is too large, the chemical conversion treatment property and the plating property of the hot press material and the hot press member are deteriorated like Cr, and the corrosion resistance is deteriorated, so that the delayed fracture resistance is deteriorated. 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 press member by precipitation strengthening or solid solution strengthening. _{Further, V suppresses the coarsening of austenite grains when the steel sheet to be hot-pressed is heated to Ac 3} temperature or higher by forming carbonitride, as in Ti, and the old austenite particle size is made 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, which deteriorates the ductility and delayed fracture resistance of the hot press member. Therefore, when V is contained, the V content is 1.0% or less, preferably 0.50% or less. In order to exhibit 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 content shown below.
REM (Rare Earth Metal) is a general term for a total of 17 elements, including two 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 martensite, tempered martensite, and Mn in bainite of hot press members and improving ductility.
On the other hand, if the Bi content is too high, a coarse oxide is formed, which deteriorates the ductility and delayed fracture resistance of the hot press 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, so that the ductility and delayed fracture resistance of the hot press member are improved.
On the other hand, if the content of Ca, Mg and REM is too large, it causes an increase in inclusions and deteriorates the ductility and delayed fracture characteristics of the hot press 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)
Similar to Cu and Ni, Co and Zr can improve the corrosion resistance and thus the delayed fracture resistance, and can be added as needed.
On the other hand, if the content of Co and Zr is too large, the ductility is deteriorated. Therefore, when these one or two 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 these one or two 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 generate alloy carbides, contribute to precipitation strengthening, and contribute to improving the strength of the hot press member.
On the other hand, if the contents of Ta and W are too large, coarse carbides are formed, and the ductility and delayed fracture resistance of the hot press member are deteriorated. Therefore, when these one or two 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 these one or two 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 through the improvement of hardenability during hot pressing, and contributes to the improvement of the strength of the steel sheet.
On the other hand, if the Zn content is too high, the ductility is deteriorated. 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.

(Remaining)
In the component composition of the hot press member, the balance other than the above-mentioned components (elements) is composed of Fe and unavoidable impurities. Examples of the unavoidable impurities include Ag, As, Ce, O and the like, and a total content of these impurities of 0.5% or less is acceptable.

<Micro organization>
Next, the reason for limiting the microstructure of the hot press member will be described.

(Martensite, tempered martensite, and bainite total 60% or more and less than 90% in area ratio)
If the total area ratio of martensite, tempered martensite, and bainite is less than 60%, TS may not reach 1300 MPa or more. Therefore, the total area ratio of martensite, tempered martensite, and bainite should be 60% 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. Therefore, the total area ratio of martensite, tempered martensite, and bainite is less than 90%. A total of 85% or less is preferable, and 80% or less is more preferable.

(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. Therefore, the area ratio of ferrite is preferably 10% or more, preferably 15% or more, and more preferably 20% or more. If the area ratio of ferrite exceeds 40%, TS may not be able to achieve 1300 MPa or more. Therefore, the area ratio of ferrite is 40% or less, preferably 35% or less, and more preferably 30% or less.
It should be noted that pearlite, retained austenite, and the like can be considered as the residual structure of the hot pressed member, but these are acceptable as long as the total area ratio is 10% or less.

(Old austenite average particle size is less than 3.0 μm)
The finer the average austenite particle size, the better the delayed fracture resistance. If the old austenite average particle size is 3.0 μm or more, excellent delayed fracture resistance cannot be achieved. The average austenite particle size of the old austenite is less than 3.0 μm, preferably 2.5 μm or less, more preferably 2.0 μm or less, still more preferably 1.5 μm or less.

<Plating layer>
The hot press member of the present invention may have a plating layer on the surface.
The hot press member of the present invention preferably has an Al-based plating layer or a Zn-based plating layer on the surface.
When a hot-pressed steel plate to which an Al-based plating layer or a Zn-based plating layer is attached is heated and then hot-pressed, a part of the plating layer components contained in the Al-based plating layer or the Zn-based plating layer is performed. Alternatively, all of them diffuse into the base steel plate to form a solid-soluble phase or an intermetallic compound, and at the same time, Fe, which is a component of the base steel plate, diffuses into the Al-based plating layer or the Zn-plated layer to form a solid-soluble phase or an intermetallic compound. Form an intermetallic compound. Further, an oxide film containing Al is formed on the surface of the Al-based plating layer, and a Zn-containing oxide film layer is formed on the surface of the Zn-based plating layer.
As an example, when the Al—Si plating layer is heated, the plating layer changes to a plating layer mainly composed of a Si-containing Fe-Al intermetallic compound. Further, when the molten Zn plating layer, the alloyed fused Zn plating layer, the electric Zn plating layer and the like are heated, a FeZn solid-soluble phase in which Zn is solid-dissolved in Fe, a ZnFe intermetallic compound, a ZnO layer on the surface and the like are formed. Further, when the electric Zn—Ni alloy plating layer is heated, a solid solution phase containing Ni in which the plating layer component is solid-dissolved in Fe, an intermetallic compound mainly composed of ZnNi, a ZnO layer on the surface, and the like are formed. NS.
The Al-based plating layer or Zn-based plating layer is not limited to the above-mentioned plating layer, and in addition to the main components Al or Zn, Si, Mg, Ni, Fe, Co, Mn, Sn, Pb, Be, It may be a plating layer containing one or more of B, P, S, Ti, V, W, Mo, Sb, Cd, Nb, Cr, Sr and the like. Generally, a plating layer is applied to the steel sheet before hot pressing, but a plating layer may be applied after hot pressing. The method for applying the Al-based plating layer or the Zn-based plating layer is not limited, and any known hot-dip plating method, electroplating method, vapor deposition plating method, or the like can be applied. Further, it may be a plating layer that has been alloyed after being plated.
The plating layer may be applied to any steel sheet such as a steel sheet after hot rolling, a steel sheet after cold rolling, and a steel sheet annealed after cold rolling. The surface of the steel sheet may be pickled with hydrochloric acid or the like before the plating layer is applied.
Temporary rolling with an elongation rate of 0.01% or more and 5% or less may be performed before and after any of the hot rolling step, pickling step, cold rolling step, baking step, and plating layer applying step.
The amount of adhesion of the plating layer is not particularly limited, and may be a general one. For example, it is preferable to have a plating layer having a plating adhesion amount of 5 to 150 g / m ^{2 per side.} If the amount of plating adhesion is ^{less than 5 g / m 2} , it may be difficult to secure corrosion resistance, while if it ^{exceeds 150 g / m 2} , the plating peel resistance may deteriorate.
The surface of the member after hot pressing 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 process may be applied after painting. 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. There is no problem even if the painting and baking process is repeated. Chemical conversion treatment may be applied as a coating base by a conventional method. For example, any of zinc phosphate treatment, iron phosphate treatment, zirconium treatment and the like can be applied.

[Manufacturing method of hot press member]
Next, a method for manufacturing the hot press member of the present invention will be described.
The method for producing the hot press member of the present invention is not particularly limited, but a method including the following preheat treatment step and the following hot press step is preferable.
(1) in the preheating step described above a steel material having a chemical composition 50 ° C. / s or more an average heating rate, Ac ₃ temperature -50 ° C. or higher Ac ₃ was heated to a temperature + 100 ° C. or less, Ac ₃ temperature -50 ° C. or higher Ac ₃ Temperature + 100 ° C or lower for 0 seconds or more and 30 seconds or less, then preheat to cool to Ms temperature -100 ° C or less at an average cooling rate of 5 ° C / s or more (2) Hot press after step the preheating step, the steel material of the preheater treatment is performed at 50 ° C. / s or more an average heating rate, Ac ₃ temperature -50 ° C. or higher Ac ₃ was heated to a temperature below, Ac ₃ temperature -50 ° C. or higher After holding at a temperature of less than Ac ₃ temperature for 0 seconds or more and 30 seconds or less, cool at an average cooling rate of 5 ° C./s or more, hot press mold at Ms temperature or more, and 20 ° C. to Ms temperature of -100 ° C or less. A process of obtaining a hot pressed member by cooling at an average cooling rate of / s or more.

<Preheat treatment process>
Preheating step, at an average heating rate of the steel material having the component composition described above 50 ° C. / s or higher, Ac ₃ temperature -50 ° C. or higher Ac ₃ was heated to a temperature + 100 ° C. or less, Ac ₃ temperature -50 ° C. or higher Ac _This is a step of performing a preheat treatment for holding at a temperature of 3 temperature + 100 ° C. or lower for 0 seconds or more and 30 seconds or less, and then cooling to an Ms temperature of −100 ° C. or lower at an average cooling rate of 5 ° C./s or more.
The preheat treatment step is a step of forming a fine structure before hot pressing, and is a very important step because the average particle size of the old austenite 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 (sec), ferrite recrystallization and grain growth will proceed, and the grain size of austenite when reverse-transformed to austenite will become coarse, resulting in hot pressing. The later old austenite average particle size 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, and 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. Therefore, 1000 ° C./s or less is preferable, and 100 ° C./s or less is more preferable.

(Heating temperature)
If the heating temperature is _{less than the Ac 3} temperature of -50 ° C, the reverse transformation to austenite becomes insufficient, a coarse ferrite structure remains before the hot press, and the old austenite average particle size 3 after the hot press. It becomes impossible to achieve less than 0.0 μm. 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, and more preferably Ac ₃ temperature or higher.
On the other hand, when the heating temperature exceeds Ac ₃ temperature + 100 ° C., recrystallization and grain growth of austenite proceed, and it becomes impossible to achieve the old austenite average particle size of less than 3.0 μm after hot pressing. Therefore, the upper limit of the heating temperature is set so that the heating temperature is Ac ₃ temperature + 100 ° C. or less. The upper limit of the heating temperature is preferably Ac ₃ temperature + 80 ° C. or lower, and more preferably Ac ₃ temperature + 50 ° C. or lower. More preferably, the Ac ₃ temperature is less than + 10 ° C.

(Retention time)
When _{the holding time at the Ac 3} temperature -50 ° C or higher and the Ac ₃ temperature + 100 ° C or lower exceeds 30 seconds, the grain growth of austenite progresses and the old austenite average particle size after hot pressing is less than 3.0 μm. You will not be able to. 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, and more preferably 10 seconds or less. The lower limit of the holding time is not particularly limited and is 0 seconds (no holding time).

(Average cooling rate up to Ms temperature -100 ° C)
If the average cooling rate to Ms temperature of -100 ° C or lower is less than 5 ° C / s, coarse ferrite may occur, and the old austenite average particle size of less than 3.0 μm after hot pressing can be achieved. It disappears. Therefore, the average cooling rate up 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 even more preferably 50 ° C./s or higher.
The steel sheet may be press-formed during cooling.

(Preferable aspect)
The above-mentioned preheat treatment can be repeated. By repeating the process, the particle size of the old austenite becomes finer. The number of repeated operations is not particularly limited, but 10 times or less is preferable from the viewpoint of manufacturing cost.

(Ac ₃ temperature)
Here, the Ac ₃ temperature can be obtained 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 (mass%) of each element, and when the element is not contained, it is calculated as 0.

(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 (mass%) of each element, and when the element is not contained, it is calculated as 0.

<Hot press process>
In the hot pressing step, after the above-mentioned preheat treatment step, the steel material subjected to the above-mentioned preheat treatment is heated at an average heating rate of _{50 ° C./s or more to an Ac 3} temperature of −50 ° C. or more and _{less than an Ac 3} temperature. Ac ₃ temperature -50 ° C or higher and Ac ₃ temperature or lower for 0 seconds or longer and 30 seconds or shorter, then cooled at an average cooling rate of 5 ° C / s or higher, hot press molded at Ms temperature or higher, and Ms temperature. This is a step of obtaining a hot pressed member 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, ferrite recrystallization and grain growth will proceed, and the grain size of austenite when it is reverse-transformed to austenite will become coarse, and it will be old after hot pressing. Austenite average particle size 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, and 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 the Ac 3} temperature of -50 ° C, the area ratio of ferrite becomes too high, and the total area ratio of martensite, tempered martensite, and bainite after hot pressing cannot be achieved at 60% or more. .. Therefore, the lower limit of the heating temperature was set to Ac ₃ temperature −50 ° C. or higher. Preferably, the lower limit of the heating temperature is Ac ₃ temperature −40 ° C. or higher, and more preferably Ac ₃ temperature −30 ° C. or higher.
On the other hand, if the heating temperature is Ac ₃ or higher, the austenite single-phase region is reached, and it becomes impossible to achieve an area ratio of ferrite of 10% or more after hot pressing. Therefore, the upper limit of the heating temperature is 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 lower.

(Retention time)
Ac ₃ temperature When the holding time at a temperature of -50 ° C or higher and lower than Ac ₃ temperature exceeds 30 seconds, the grain growth of austenite progresses and the average particle size of the old austenite after hot pressing is less than 3.0 μm. Can't be done. Therefore _{, the holding time at the temperature of Ac 3} temperature −50 ° C. or higher _{and lower than Ac 3} temperature was set to 30 seconds or less. It is preferably 20 seconds or less, and more preferably 10 seconds or less. The lower limit of the holding time is not particularly limited and is 0 seconds (no holding 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 be achieved at 60% or more. Therefore, the average cooling rate was set to 5 ° C./s or higher.

(Hot press temperature)
When hot press forming is performed at a temperature lower than the Ms temperature, a part of the steel sheet undergoes martensitic transformation, so that the formability deteriorates and cracks may occur during forming. Therefore, the hot press molding was set to Ms temperature or higher.

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

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

<Cooling method>
The above-mentioned cooling method is not particularly limited, and if a 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 for the reason that the effect of the present invention is more excellent. , Mold cooling is preferred.

Hereinafter, the present invention will be specifically described with reference to examples. However, the present invention is not limited to the examples described below. It is also possible to make appropriate modifications to the extent that it is compatible with the gist of the present invention, and all of them are included in the technical scope of the present invention.

[Manufacturing of hot press members]
The hot press member was manufactured as follows.

<Preparation of steel material>
A steel material (steel plate) having the composition shown in Table 1 was prepared.
Here, Ac ₃ temperature, Ms temperature and thickness of the column represents the Ac ₃ temperature, Ms temperature and the thickness of each steel material. 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 is applied, HGA is a hot-rolled steel sheet further alloyed with hot-dip Zn plating, and CR is Cold-rolled steel sheet after cold rolling, AC is cold-rolled steel sheet that has been annealed after cold-rolling, GI is cold-rolled steel sheet with hot-dip Zn plating, and GA is cold-rolled with hot-dip Zn-plated alloying. Steel sheet, AI is cold-rolled steel sheet with hot-dip Al plating, AA is cold-rolled steel sheet with alloying treatment of hot-dip Al plating, EG is cold-rolled steel sheet with electric Zn plating, and EGN is electric Zn plating. Represents a cold-rolled steel sheet in which Ni is alloyed with.

<Preheat treatment process and hot press process>
The obtained steel material was subjected to a preheat treatment step under the conditions described in the preheat treatment step shown in Table 2 below, and a hot press step was carried out under the conditions described in the hot press step shown in Table 2 below.
For example, No. If 1, the steel material of Table 1 A is heated to 830 ° C. at an average heating rate of 50 ° C./s, and is cooled to an Ms temperature (417 ° C.) -100 ° C. or lower with an average cooling rate of 30 ° C./s without holding time. Cooled at a rate (preheat treatment step). Then, 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. It was cooled to a temperature of 30 ° C. or lower at an average cooling rate of 30 ° C./s (hot pressing step). In this way, No. 1 hot press member was obtained.

In addition, No. For 20 to 22, preheat treatment was repeatedly performed as shown in Table 2 below. In addition, N. For 29 and 38, only the hot pressing step was performed without preheat treatment.
In addition, No. The heating method in 1-37 is energization heating, and No. The heating method in 38 was furnace heating. Further, in each example, the cooling method was mold cooling.
The dies used in the hot press had a punch width of 70 mm, a punch shoulder R of 4 mm, a die shoulder R of 4 mm, and a molding depth of 30 mm.

[Evaluation of hot press members]
A test piece was collected from the bottom of the hat of the hot press 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 molding.

<Observation of microstructure>

(Measurement of area ratio of martensite, tempered martensite, bainite, ferrite)
The collected test piece was polished to expose a cross section (cross section parallel to the rolling direction) at the position of 1/4 of the plate thickness. The exposed cross section was corroded with a corrosive solution (3 mass% nital solution), and then 10 visual fields were observed and photographed at a magnification of 5000 times using a scanning electron microscope (SEM). The area ratio [%] of martensite, tempered martensite, bainite, and ferrite is quantified by measuring the area ratio of 10-field microstructure photographs taken by the point counting method (based on ASTM E562-83 (1988)). It became. The lath-like structure was martensite, tempered martensite and bainite, and the lamellar structure was pearlite. Ferrite and retained austenite can be distinguished from the light-dark contrast of the residual structure. The dark part is ferrite and the bright part is retained austenite. The results are shown in Table 3.

(Measurement of old austenite average particle size)
For the collected test pieces, the particle size of the old austenite was measured according to JIS G0551: 2013.
Specifically, the collected test piece was polished to expose a cross section (a cross section parallel to the rolling direction) at a plate thickness of 1/4. The exposed cross section is exposed to the old austenite structure with a corrosive solution (an aqueous solution containing picrinic acid, a surfactant, and oxalic acid), and the old austenite structure is exposed at a plate thickness of 1/4 using an optical microscope at a magnification of 1000 times. 10-field photography was performed to measure the average circle-equivalent diameter of the old austenite grains. The results are shown in Table 3.

<Tensile test>
From the position of the hat bottom of the hot press member thus obtained, a JIS No. 5 test piece (distance between gauge points GL: 50 mm) was collected, and the tensile strength (TS) and total elongation (El) were determined.
Specifically, the collected test pieces were subjected to a tensile test in accordance with the provisions of JIS Z 2241: 2011, and the tensile strength (TS) [MPa] and the total elongation (El) [%] were determined. Tensile tests were performed twice for each hot press member, and the average value of the two tests was taken as the TS and El of the hot press member. TS was 1300 MPa or more and less than 1760 MPa, and El was 8% or more from the viewpoint of ductility. The results are shown in Table 3.

<Evaluation of delayed fracture resistance>
A 4-point bending test piece was collected from the position of the bottom of the hat of the hot press member, and a 4-point bending test was performed in accordance with ASTM G39-99 (2016). Bending stress was applied while immersed in a solution of hydrochloric acid (pH = 1.0) at room temperature, and the presence or absence of fracture was evaluated. When the bending stress was TS, the delayed fracture resistance was good (◯) when the fracture did not occur for 100 hours or more, and the delayed fracture resistance was inferior (×) when the fracture occurred in less than 100 hours. The test was carried out with the n number of the test pieces being 2. The case where both of them did not break was regarded as good (◯), and the case where even one of them broke was regarded as inferior (×). The results are shown in Table 3.

As can be seen from Tables 1 to 3, No. 1 having a specific composition and a specific microstructure. In the hot press members 1 to 22 and 30 to 32, the TS had a tensile strength of 1300 MPa or more and less than 1760 MPa, and exhibited excellent ductility and delayed fracture resistance.
On the other hand, No. 1 whose component composition deviates from a specific range. No. 33 to 37 hot press members and microstructures outside the specific range. 23-27, 29, 34-35 and 38 lacked 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 A component composition containing% or more and 0.020% or less, and the balance consisting of Fe and unavoidable impurities.
The microstructure is martensite, tempered martensite, and bainite in total area ratio of 60% or more and less than 90%, ferrite is 10% or more and 40% or less in area ratio, and the old austenite average particle size is less than 3.0 μm. There is a hot press member. Further, the component composition is 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 in mass%. The hot press member according to claim 1, which contains 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%. The hot press member according to 1 or 2. Further, the component composition is, in terms of 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, which comprises at least one selected from the group. 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 the surface. A method for manufacturing a hot press member according to any one of claims 1 to 5, wherein the hot press member is manufactured.
A steel material having the component composition according to any one of claims 1 to 4 is heated at an average heating rate of _{50 ° C./s or more to an Ac 3} temperature of -50 ° C. or higher and an Ac ₃ temperature of + 100 ° C. or lower, and Ac ₃ After holding for a time of 0 seconds or more and 30 seconds or less at a temperature of -50 ° C or higher and Ac ₃ temperature + 100 ° C or lower, a preheat treatment is performed to cool the Ms temperature to -100 ° C or lower at an average cooling rate of 5 ° C / s or higher. Heat treatment process and
After the preheating step, the steel material preheating treatment is performed at an average heating rate of more than 50 ° C. / s, Ac ₃ temperature -50 ° C. or higher Ac ₃ was heated to a temperature below, Ac ₃ temperature -50 ° C. or higher Ac _{After holding for a time of 0 seconds or more and 30 seconds or less at a temperature of less than 3} temperatures, cool at an average cooling rate of 5 ° C./s or more, hot press molding at Ms temperature or more, and 20 ° C./to Ms temperature of -100 ° C or less. A method for manufacturing a hot-pressed member, comprising a hot-pressed step of obtaining a hot-pressed member by cooling at an average cooling rate of s or more. A method for manufacturing a hot press member according to any one of claims 1 to 5, wherein the hot press member is manufactured.
A steel material having the component composition according to any one of claims 1 to 4 is heated at an average heating rate of _{50 ° C./s or more to an Ac 3} temperature of -50 ° C. or higher and an Ac ₃ temperature of + 100 ° C. or lower, and Ac ₃ After holding for a time of 0 seconds or more and 30 seconds or less at a temperature of -50 ° C or higher and Ac ₃ temperature + 100 ° C or lower, and then performing a preheat treatment to cool the Ms temperature to -100 ° C or lower at an average cooling rate of 5 ° C / s or higher. , The preheat treatment step of repeatedly performing the preheat treatment once or more,
After the preheating step, the steel material preheating treatment is performed at an average heating rate of more than 50 ° C. / s, Ac ₃ temperature -50 ° C. or higher Ac ₃ was heated to a temperature below, Ac ₃ temperature -50 ° C. or higher Ac _{After holding for a time of 0 seconds or more and 30 seconds or less at a temperature of less than 3} temperatures, cool at an average cooling rate of 5 ° C./s or more, hot press molding at Ms temperature or more, and 20 ° C./to Ms temperature of -100 ° C or less. A method for manufacturing a hot-pressed member, comprising a hot-pressed step of obtaining a hot-pressed member by cooling at an average cooling rate of s or more.