WO2015174530A1

WO2015174530A1 - Hot-rolled steel plate member

Info

Publication number: WO2015174530A1
Application number: PCT/JP2015/064101
Authority: WO
Inventors: 匹田　和夫; 啓達小嶋; 進一郎田畑
Original assignee: 新日鐵住金株式会社
Priority date: 2014-05-15
Filing date: 2015-05-15
Publication date: 2015-11-19
Also published as: EP3144405A1; EP3144405B1; KR20160147025A; PL3144405T3; JPWO2015174530A1; ES2753390T3; US20170073792A1; EP3144405A4; MX2016014884A; KR101908210B1; TWI563101B; JP6315087B2; CN106661685A; CN106661685B; TW201602360A

Abstract

　A hot-rolled steel plate member having a chemical composition, by mass%, of 0.08-0.16% of C, 0.19% or less of Si, 0.40-1.50% of Mn, 0.02% or less of P, 0.01% or less of S, 0.01-1.0% of sol. Al, 0.01% or less of N, 0.25-3.00% of Cr, 0.01-0.05% of Ti, 0.001-0.01% of B, 0-0.50% of Nb, 0-2.0% of Ni, 0-1.0% of Cu, 0-1.0% of Mo, 0-1.0% of V, and 0-0.005% of Ca, the balance being Fe and unavoidable impurities, in which the total volume fraction of martensite, tempered martensite, and bainite is 50% or higher, the volume fraction of ferrite is 3% or less, the average particle size of prior γ grains is 10 μm or less, and the number density of residual carbides that are present is 4 × 10³ per mm² or less.

Description

Hot forming steel plate

This specification relates to a hot-formed steel plate member formed by hot forming a steel plate.

In the field of steel sheets for automobiles, the application of high-strength steel sheets having high tensile strength has been expanded in order to achieve both weight reduction for improving fuel economy and improvement of impact resistance. However, since the press formability of the steel sheet decreases with increasing strength, it has become difficult to manufacture products having complicated shapes.

As a result, for example, with the increase in strength of the steel sheet, in addition to the problem that ductility decreases and breaks at a high degree of workability, the problem is that the dimensional accuracy deteriorates because the springback and wall warp become large. Has occurred. Therefore, it is not easy to press-mold a steel sheet having a high strength, particularly a tensile strength of 780 MPa or more, into a product having a complicated shape.

Therefore, in recent years, as disclosed in, for example, Japanese Patent Application Publication No. 2002-102980, a hot stamping technique has been adopted as a technique for press forming a difficult-to-form material such as a high-strength steel sheet. The hot stamping technique is a hot forming technique in which a material used for forming is heated to form. In this technique, since quenching is performed simultaneously with forming, the steel sheet is soft and has good formability during forming, and after forming, the formed member can obtain higher strength than the cold forming steel sheet.

Japanese Patent Application Publication No. 2006-213959 discloses a steel member having a tensile strength of 980 MPa.
Japanese Patent Application Publication No. 2007-314817 discloses that a hot pressed steel sheet member having excellent tensile strength and toughness can be obtained by reducing the cleanliness and the segregation degree of P and S. Yes.

Summary of disclosure

The metal material described in Japanese Patent Application Publication No. 2002-102980 has a problem that the hardenability at the time of hot pressing is insufficient, resulting in poor hardness stability. Although Japanese Patent Application Publication No. 2006-213959 and Japanese Patent Application Publication No. 2007-314817 disclose steel sheets having excellent tensile strength and toughness, there is still room for improvement in terms of local deformation characteristics. Has been.

An object of the embodiment of the present specification is to provide a hot-formed steel sheet member having excellent hardness stability and local deformability. In many cases, the hot-formed steel plate member is not a flat plate but a formed body. In this specification, the hot-formed steel plate member is also referred to as a “hot-formed steel plate member”.

According to one aspect of the present specification,
Chemical composition is mass%,
C: 0.08 to 0.16%,
Si: 0.19% or less,
Mn: 0.40 to 1.50%,
P: 0.02% or less,
S: 0.01% or less,
sol. Al: 0.01 to 1.0%
N: 0.01% or less,
Cr: 0.25 to 3.00%,
Ti: 0.01 to 0.05%,
B: 0.001 to 0.01%,
Nb: 0 to 0.50%,
Ni: 0 to 2.0%,
Cu: 0 to 1.0%,
Mo: 0 to 1.0%,
V: 0 to 1.0%,
Ca: 0 to 0.005%,
Balance: Fe and impurities,
The total volume ratio of martensite, tempered martensite and bainite is 50% or more, and the volume ratio of ferrite is 3% or less,
The average particle size of the former γ grains is 10 μm or less,
There is provided a hot-formed steel sheet member in which the number density of residual carbides present is 4 × 10 ³ pieces / mm ² or less.

It is a schematic diagram which shows the shape of the metal mold | die in the hat molding in an Example. It is a schematic diagram which shows the shape of the molded object obtained by hot forming in the Example. It is a schematic diagram which shows the shape of the notch tensile test piece in an Example.

As a result of intensive studies to provide a hot-formed steel sheet member having excellent hardness stability and local deformability, the present inventors have obtained the following knowledge.

(1) Since the old γ grains in the hot-formed steel sheet member are refined, the generation and connection of voids are delayed, so that local deformability is improved. Therefore, it is preferable to refine the old γ grains.
(2) If a large amount of residual carbide is present in the hot-formed steel sheet member, not only the hardenability after hot forming may be reduced and the hardness stability may be reduced, but the residual carbide may be a source of voids and become localized. Deteriorates deformability. Therefore, it is preferable to reduce the number density of residual carbides.

The embodiment of the present specification is based on the above knowledge, and according to one aspect of the embodiment,
(1) The chemical composition is mass%,
C: 0.08 to 0.16%,
Si: 0.19% or less,
Mn: 0.40 to 1.50%,
P: 0.02% or less,
S: 0.01% or less,
sol. Al: 0.01 to 1.0%
N: 0.01% or less,
Cr: 0.25 to 3.00%,
Ti: 0.01 to 0.05%,
B: 0.001 to 0.01%,
Nb: 0 to 0.50%,
Ni: 0 to 2.0%,
Cu: 0 to 1.0%,
Mo: 0 to 1.0%,
V: 0 to 1.0%,
Ca: 0 to 0.005%,
Balance: Fe and impurities,
The total volume ratio of martensite, tempered martensite and bainite is 50% or more, and the volume ratio of ferrite is 3% or less,
The average particle size of the former γ grains is 10 μm or less,
There is provided a hot-formed steel sheet member in which the number density of residual carbides present is 4 × 10 ³ pieces / mm ² or less.

(2) A hot-formed steel plate member according to (1), preferably
The chemical composition is mass%,
Nb: 0.003 to 0.50%,
Ni: 0.01 to 2.0%,
Cu: 0.01 to 1.0%,
Mo: 0.01 to 1.0%,
V: 0.01 to 1.0%
And Ca: 0.001 to 0.005%
1 or more types selected from the group consisting of:

(3) The hot-formed steel plate member according to (1) or (2), preferably having a cleanliness value of steel defined by JIS G 0555 (2003) of 0.08% or less.

(4) The hot-formed steel sheet member according to any one of (1) to (3), preferably having a Mn segregation degree α represented by the following formula (i) of 1.6 or less.
α = [maximum Mn concentration (mass%) at the thickness center portion] / [average Mn concentration (mass%) at ¼ depth position of the thickness from the surface] (i)

(5) The hot-formed steel plate member according to any one of (1) to (4), preferably having a plating layer on the surface of the steel plate member.
(6) The hot-formed steel plate member according to any one of (1) to (5), preferably, the steel plate member has a tensile strength of 1.0 GPa or more.

Hereinafter, embodiments will be described in detail.

(A) Chemical composition The reason for limitation of each element is as follows. In the following description, “%” for the content means “% by mass”.

C: 0.08 to 0.16%
C is an important element for enhancing the hardenability of steel and ensuring the strength after quenching. Further, since C is an austenite-forming element, it has an effect of suppressing strain-induced ferrite transformation during high strain forming. Therefore, it is easy to obtain a stable hardness distribution in the steel sheet member after hot forming. When the C content is less than 0.08%, it becomes difficult to secure a tensile strength of 1.0 GPa or more after quenching and to obtain the above-described effects. Therefore, the C content is 0.08% or more. On the other hand, if the C content exceeds 0.16%, the strength after quenching excessively increases and local deformability deteriorates. Therefore, the C content is 0.16% or less. The C content is preferably 0.085% or more, and more preferably 0.9% or more. Moreover, it is preferable that C content is 0.15% or less, and it is more preferable that it is 0.14% or less.

Si: 0.19% or less Si is an element having an action of suppressing scale formation during high-temperature heating during hot forming. However, if the Si content exceeds 0.19%, the heating temperature necessary for austenite transformation during hot forming becomes extremely high. For this reason, the cost required for heat treatment increases, and quenching becomes insufficient due to insufficient heating. Moreover, since Si is a ferrite-forming element, if the Si content is too high, strain-induced ferrite transformation is likely to occur during high strain molding. For this reason, the hardness of the steel sheet member after hot forming is locally reduced, making it difficult to obtain a stable hardness distribution. Furthermore, when Si is contained in a large amount, non-plating may occur due to a decrease in wettability when a hot dipping process is performed. Therefore, the Si content is 0.19% or less. The Si content is preferably 0.15% or less. In order to obtain the above effect, the Si content is preferably 0.01% or more.

Mn: 0.40 to 1.50%
Mn is an element useful for enhancing the hardenability of the steel sheet and for ensuring the strength after hot forming stably. If the Mn content is less than 0.40%, it is difficult to obtain the above effects. Therefore, the Mn content is 0.40% or more. On the other hand, when the Mn content exceeds 1.50%, coarse MnS is generated, which causes deterioration of local deformability. Therefore, the Mn content is 1.50% or less. The Mn content is preferably 0.80% or more, and preferably 1.40% or less.

P: 0.02% or less P is an element contained as an impurity, but has the effect of improving the hardenability of steel and further ensuring the strength of steel after quenching stably. , You may make it contain positively. However, when the P content exceeds 0.02%, the local deformability is significantly deteriorated. Therefore, the P content is 0.02% or less. The P content is preferably 0.01% or less. The lower limit of the P content is not particularly limited, but excessive reduction of the P content causes a significant cost increase. For this reason, it is preferable that P content shall be 0.0002% or more.

S: 0.01% or less S is an element that is contained as an impurity and deteriorates local deformability. When the S content exceeds 0.01%, the local deformability is significantly deteriorated. Therefore, the S content is 0.01% or less. The lower limit of the S content is not particularly limited, but excessive reduction of the S content causes a significant cost increase, so the S content is preferably 0.0002% or more.

sol. Al: 0.01 to 1.0%
sol. Al is an element having an action of deoxidizing molten steel to make the steel sound. sol. If the Al content is less than 0.01%, deoxidation is not sufficient. Furthermore, sol. Al is an element that has the effect of enhancing the hardenability of the steel sheet and ensuring the strength after quenching stably, so it may be positively incorporated. Therefore, sol. Al content shall be 0.01% or more. However, even if it contains exceeding 1.0%, the effect acquired by the effect | action is small, and it causes a cost increase unnecessarily. For this reason, sol. Al content shall be 1.0% or less. sol. The Al content is preferably 0.02% or more, and preferably 0.2% or less.

N: 0.01% or less N is an element that is contained as an impurity and deteriorates toughness. If the N content exceeds 0.01%, coarse nitrides are formed in the steel, and the local deformability and toughness are significantly deteriorated. Therefore, the N content is 0.01% or less. The N content is preferably 0.008% or less. The lower limit of the N content is not particularly limited, but excessive reduction of the N content causes a significant cost increase. For this reason, the N content is preferably 0.0002% or more, and more preferably 0.0008% or more.

Cr: 0.25 to 3.00%
Cr is an element having an effect of enhancing the hardenability of steel. Therefore, it is an especially important element in the embodiment in which the Mn content is limited to 1.50% or less. Cr is an austenite-forming element and has an action of suppressing strain-induced ferrite transformation during high strain forming. Therefore, by containing Cr, it becomes easy to obtain a stable hardness distribution in the steel sheet member after hot forming. If the Cr content is less than 0.25%, the above effects cannot be obtained sufficiently. Therefore, the Cr content is 0.25% or more. On the other hand, if the Cr content exceeds 3.00%, Cr concentrates in carbides in the steel, delays the solid solution of carbides in the heating process when subjected to hot forming, and decreases hardenability. Therefore, the Cr content is 3.00% or less. The Cr content is preferably 0.30% or more, and more preferably 0.40% or more. Moreover, it is preferable that Cr content is 2.50% or less, and it is preferable that it is 2.00% or less.

Ti: 0.01 to 0.05%
Ti is an element having an action of suppressing recrystallization of austenite grains when the hot forming steel sheet is heated to Ac ₃ point or more and used for hot forming. Furthermore, it has the effect | action which forms a fine carbide | carbonized_material and suppresses the grain growth of an austenite grain and makes it a fine grain. For this reason, it has the effect | action which improves the local deformability of a hot-formed steel plate member greatly. Moreover, since Ti preferentially bonds with N in the steel, the consumption of B due to the precipitation of BN is suppressed, and as a result, it has the effect of improving the hardenability by B. Therefore, the Ti content is 0.01% or more. However, if the content exceeds 0.05%, the amount of TiC deposited increases, C is consumed, and the strength after quenching decreases. For this reason, Ti content shall be 0.05% or less. The Ti content is preferably 0.015% or more. Moreover, it is preferable that Ti content is 0.04% or less, and it is preferable that it is 0.03% or less.

B: 0.001 to 0.01%
B is an element having an effect of enhancing the hardenability of steel and ensuring the strength after quenching stably. Therefore, it is an especially important element in the embodiment in which the Mn content is limited to 1.50% or less. If the B content is less than 0.001%, the above effects cannot be obtained sufficiently. Therefore, the B content is 0.001% or more. On the other hand, when the B content exceeds 0.01%, the above effect is saturated, and further, local deformability deterioration of the quenched portion is caused. Therefore, the B content is 0.01% or less. The B content is preferably 0.005% or less.

The hot-formed steel plate member of the embodiment has a chemical composition composed of the above elements C to B, the remaining Fe and impurities.

Here, "impurities" are components mixed in due to various factors in the manufacturing process, such as ores and scraps, when manufacturing steel sheets industrially, and are allowed within a range that does not adversely affect the embodiment. Means what will be done.

In addition to the above elements, the hot-formed steel sheet member of the embodiment further contains one or more elements selected from the group consisting of Nb, Ni, Cu, Mo, V, and Ca in the amounts shown below. You may let them.

Nb: 0 to 0.50%
Nb suppresses recrystallization and forms fine carbides to suppress grain growth when heating a hot forming steel sheet to Ac ₃ point or higher to be used for hot forming. Fine austenite grains It is an element which has the effect | action which makes it. For this reason, it has the effect | action which improves the local deformability of a hot-formed steel plate member greatly. Therefore, you may contain Nb as needed. However, if the content exceeds 0.50%, the amount of NbC deposited increases, C is consumed, and the strength after quenching decreases. For this reason, when it contains Nb, the content shall be 0.50% or less. The Nb content is preferably 0.45% or less. To obtain the above effect, the Nb content is preferably 0.003% or more, and more preferably 0.005% or more.

Ni: 0 to 2.0%
Ni is an element effective for enhancing the hardenability of the steel sheet and stably securing the strength after quenching, and therefore Ni may be contained as necessary. However, even if Ni is contained in excess of 2.0%, the effect is small, and the cost is unnecessarily increased. For this reason, when it contains Ni, the content shall be 2.0% or less. The Ni content is preferably 1.5% or less. In order to obtain the above effect, the Ni content is preferably 0.01% or more, and more preferably 0.05% or more.

Cu: 0 to 1.0%
Cu is an element effective for enhancing the hardenability of the steel sheet and stably securing the strength after quenching, and may be contained as necessary. However, even if Cu is contained exceeding 1.0%, the effect is small, and the cost is unnecessarily increased. For this reason, when it contains Cu, the content shall be 1.0% or less. The Cu content is preferably 0.5% or less. In order to obtain the above effect, the Cu content is preferably 0.01% or more, and more preferably 0.03% or more.

Mo: 0 to 1.0%
Mo is an element having an action of forming fine carbides to suppress grain growth and making austenite grains fine when heating a hot-forming steel sheet to Ac ₃ or more to be used for hot forming. is there. It also has the effect of greatly improving the local deformability of the hot-formed steel sheet member. For this reason, you may contain Mo as needed. However, when the Mo content exceeds 1.0%, the effect is saturated and the cost is increased unnecessarily. Therefore, when it contains Mo, the content shall be 1.0% or less. The Mo content is preferably 0.7% or less. To obtain the above effect, the Mo content is preferably 0.01% or more, and more preferably 0.04% or more.

V: 0 to 1.0%
V is an element effective for enhancing the hardenability of the steel sheet and stably securing the strength after quenching, and may be contained as necessary. However, even if V is contained exceeding 1.0%, the effect is small, and the cost is unnecessarily increased. For this reason, when it contains V, the content shall be 1.0% or less. The V content is preferably 0.08% or less. In order to obtain the above effect, the V content is preferably 0.01% or more, and more preferably 0.02% or more.

Ca: 0 to 0.005%
Ca is an element that has the effect of reducing the inclusions in the steel and improving the local deformability after quenching, and thus may be included as necessary. However, when the Ca content exceeds 0.005%, the effect is saturated and the cost is unnecessarily increased. Therefore, when it contains Ca, the content shall be 0.005% or less. The Ca content is preferably 0.004% or less. When it is desired to obtain the above effect, the Ca content is preferably 0.001% or more, and more preferably 0.002% or more.

(B) Metal structure In the embodiment, in order to improve local deformability, it is preferable to suppress variation in hardness in the metal structure after hot forming. When the difference in hardness in the structure becomes large, it becomes a starting point of voids. Therefore, it is preferable to suppress the mixture of low-temperature transformation structure such as hard martensite and bainite and soft ferrite structure as much as possible. Therefore, the hot-formed steel sheet member of the embodiment preferably has a metal structure mainly composed of a low-temperature transformation structure and a volume ratio of ferrite of 3% or less.

The metal structure mainly composed of a low temperature transformation structure means a metal structure in which the total volume ratio of martensite, tempered martensite and bainite is 50% or more. The tempered martensite here means martensite in which martensite transformed during quenching is tempered by automatic tempering, and martensite that has undergone low temperature tempering such as a paint baking process after quenching. The low temperature transformation structure in the metal structure is preferably 80% or more and more preferably 90% or more in volume ratio.

Moreover, since retained austenite improves ductility by the TRIP effect, there is no problem even if it is included. However, since martensite transformed from austenite is hard, it can be a starting point for voids. Therefore, the retained austenite contained in the metal structure is preferably 10% or less by volume ratio.

Mn segregation degree α: 1.6 or less α = [maximum Mn concentration (mass%) at the center of the plate thickness] / [average Mn concentration (mass%) at the 1/4 depth position of the plate thickness from the surface]・・ (I)
In the center part of the thickness cross section of the hot-formed steel sheet member, Mn is concentrated due to center segregation. For this reason, MnS concentrates in the center as inclusions, and it becomes easy to form hard martensite, resulting in a difference in hardness from the surroundings and a deterioration in local deformability. Particularly, when the value of the segregation degree α of Mn represented by the above formula (i) exceeds 1.6, the local deformability is remarkably deteriorated. Therefore, in order to improve local deformability, the value of α of the hot-formed steel sheet member is preferably set to 1.6 or less. In order to further improve the local deformability, the value of α is more preferably 1.2 or less.

The segregation of Mn in the steel sheet is controlled mainly by the steel sheet composition, particularly the impurity content, and the conditions for continuous casting, and does not substantially change before and after hot rolling and hot forming. Therefore, the inclusions and segregation status of the hot-forming steel sheet are almost the same as the inclusions and segregation status of the hot-formed steel sheet member produced by hot forming. Since the value of α does not change greatly by hot forming, the value of α of the hot-formed steel sheet member is also reduced to 1.6 or less by setting the value of α of the hot-forming steel sheet to 1.6 or less. When the value of α is 1.2 or less, the value of α of the hot-formed steel sheet member can also be 1.2 or less.

The maximum Mn concentration at the center of the plate thickness is determined by the following method. Using an electronic probe microanalyzer (EPMA), line analysis is performed at the center of the plate thickness of the steel sheet, three measured values are selected in descending order from the analysis result, and the average value is calculated. In addition, the average Mn concentration at the 1/4 depth position of the plate thickness from the surface is determined by the following method. Similarly, using EPMA, analysis is performed at 10 positions at the 1/4 depth position of the steel sheet, and the average value is calculated.

Cleanliness: 0.08% or less When a large amount of inclusions of A, B, and C inclusions described in JIS G 0555 (2003) are present in the steel sheet member, the inclusions are likely to be the starting point of destruction. If the inclusions increase, crack propagation easily occurs and local deformability deteriorates. In particular, in the case of a hot-formed steel sheet member having a tensile strength of 1.0 GPa or more, it is preferable to suppress the presence ratio of inclusions. If the value of the cleanliness of the steel specified in JIS G 0555 (2003) exceeds 0.08%, it is difficult to ensure a practically sufficient local deformability because the amount of inclusions is large. Therefore, the cleanliness value of the hot forming steel plate is preferably 0.08% or less. In order to further improve the local deformability, the cleanliness value is more preferably 0.04% or less. In addition, the value of the cleanliness of steel is obtained by calculating the area percentage occupied by the above-mentioned A-type, B-type and C-type inclusions.

Since the cleanness value does not change greatly by hot forming, the cleanness value of the hot-formed steel sheet member is also 0 by setting the cleanness value of the hot-formed steel sheet to 0.08% or less. 0.08% or less, and by setting it to 0.04% or less, the cleanliness value of the hot-formed steel sheet member can also be made 0.04% or less.

In the embodiment, the cleanliness value of the hot-formed steel plate or hot-formed steel plate member is obtained by the following method. For the hot forming steel plate or hot forming steel plate member, test materials are cut out from five locations. And when the plate thickness of the hot-formed steel plate or hot-formed steel plate member is t, 1 / 8t, 1 / 4t, 1 / 2t, 3 / 4t, 7 / 8t in the plate thickness direction of each test material For each position, the cleanliness is investigated by point calculation. The numerical value having the largest cleanliness value (lowest cleanliness) at each plate thickness is taken as the cleanliness value of the specimen.

Average particle size of old γ grains: 10 μm or less Local deformability is improved by reducing the old γ particle size in the hot-formed steel sheet member. In steel sheets mainly composed of martensite, voids are generated at the boundaries of the old γ grain boundaries and the substructure within the grains. However, the refinement of the old γ grains suppresses the generation of voids and delays the connection. Deformability can be improved. If the average particle size of the former γ exceeds 10 μm, this effect cannot be exhibited. Therefore, the average particle diameter of the old γ grains in the hot-formed steel sheet member is 10 μm or less. In order to refine the old γ grains, it is effective to lower the heating temperature, delay the dissolution of carbide during heating, and suppress the grain growth.

The average particle diameter of old γ grains can be measured using a method defined by ISO643. That is, the number of crystal grains in the measurement visual field is measured, the average area of the crystal grains is obtained by dividing the area of the measurement visual field by the number of crystal grains, and the crystal grain diameter in the equivalent circle diameter is calculated. At that time, it is preferable to measure the number of grains at the boundary of the visual field as 1/2 and adjust the magnification so that the number of crystal grains is 200 or more. In order to improve accuracy, it is preferable to measure a plurality of visual fields.

Residual carbide: 4 × 10 ³ pieces / mm ² or less In the case of hot forming, sufficient hardenability can be secured by re-dissolution of carbide generally present in steel. However, a part of the carbide may remain without being re-dissolved. Residual carbide has an effect of suppressing γ grain growth during heating and holding during hot forming by pinning. Therefore, it is desirable for residual carbides to be present during heating and holding. The smaller the residual carbide after hot forming, the better the hardenability and the higher the strength. Therefore, it is preferable that the number density of residual carbides can be reduced upon completion of heating and holding.

If there is a large amount of residual carbide, not only the hardenability after hot forming may be lowered, but the residual carbide becomes a source of voids and deteriorates local deformability. In particular, if the number density of residual carbides exceeds 4 × 10 ³ pieces / mm ² , the hardenability after hot forming may be deteriorated. Therefore, the number density of residual carbides present in the hot-formed steel plate member is preferably 4 × 10 ³ pieces / mm ² or less.

(C) Plating layer The high-strength hot-formed steel plate member according to the embodiment may have a plating layer on the surface for the purpose of improving corrosion resistance. The plating layer may be an electroplating layer or a hot dipping layer. Examples of the electroplating layer include electrogalvanizing, electro-Zn—Ni alloy plating, electro-Zn—Fe alloy plating and the like. The hot dip galvanizing layer includes hot dip galvanizing, alloyed hot dip galvanizing, hot dip aluminum plating, hot dip Zn-Al alloy plating, hot dip Zn-Al-Mg alloy plating, hot dip Zn-Al-Mg-Si alloy plating, etc. Illustrated. The amount of plating adhesion is not particularly limited, and may be adjusted within a general range.

(D) Manufacturing method of hot forming steel plate There are no particular restrictions on the manufacturing conditions of the hot forming steel plate used for manufacturing the hot forming steel plate member according to the embodiment, but by using the manufacturing method shown below. , Can be preferably produced.

鋼 After melting the steel having the above chemical composition in a furnace, a slab is produced by casting. In order to reduce the cleanliness of the steel sheet to 0.08% or less, when continuously casting the molten steel, the heating temperature of the molten steel is set to 5 ° C. higher than the liquidus temperature of the steel, and the molten steel per unit time. It is desirable to limit the casting amount to 6 t / min or less.

When the casting amount of molten steel per unit time exceeds 6 t / min during continuous casting, the molten steel flow in the mold is fast, so that inclusions are easily trapped in the solidified shell and inclusions in the slab increase. In addition, when the molten steel heating temperature is less than 5 ° C higher than the liquidus temperature, the viscosity of the molten steel increases, and inclusions hardly float in the continuous casting machine, resulting in an increase in inclusions in the slab. Cleanliness is likely to deteriorate.

On the other hand, when the molten steel heating temperature from the liquidus temperature of the molten steel is 5 ° C. or more and the molten steel casting amount per unit time is 6 t / min or less, inclusions are hardly brought into the slab. As a result, the amount of inclusions at the stage of producing the slab can be effectively reduced, and the steel plate cleanliness of 0.08% or less can be easily achieved.

When continuously casting molten steel, the molten steel heating temperature is more preferably 8 ° C. or higher than the liquidus temperature, and the molten steel casting amount per unit time is more preferably 5 t / min or less. By making the molten steel heating temperature 8 ° C. or more higher than the liquidus temperature and setting the molten steel casting amount per unit time to 5 t / min or less, it becomes easy to make the cleanliness 0.04% or less. Therefore it is desirable.

In order to suppress the concentration of MnS that causes deterioration of local deformability, it is desirable to perform a center segregation reduction process that reduces the center segregation of Mn. Examples of the center segregation reduction treatment include a method of discharging molten steel enriched in Mn in an unsolidified layer before the slab is completely solidified.

Specifically, by performing treatment such as electromagnetic stirring and unsolidified layer pressure, molten steel enriched with Mn before complete solidification can be discharged. The electromagnetic stirring treatment can be performed, for example, by applying a flow to the unsolidified molten steel at 250 to 1000 gauss, and the unsolidified layer reduction treatment, for example, reduces the final solidified portion with a gradient of about 1 mm / m. Can be done.

ソー Soaking (soaking) treatment may be performed on the slab obtained by the above method as necessary. By performing the soaking process, segregated Mn can be diffused and the degree of segregation can be reduced. In the soaking process, a preferable soaking temperature is 1200 to 1300 ° C., and a soaking time is 20 to 50 hours.

After that, hot rolling is performed on the slab. In terms of hot rolling conditions, it is preferable that the hot rolling start temperature is 1000 to 1300 ° C. and the hot rolling completion temperature is 850 ° C. or higher from the viewpoint of more uniformly generating carbides. The coiling temperature is preferably higher from the viewpoint of workability, but if it is too high, the yield decreases due to scale formation, so it is preferably 500 to 650 ° C. The hot-rolled steel sheet obtained by hot rolling is descaled by pickling or the like.

In the embodiment, in order to refine the old γ grain size after hot forming and reduce the number density of residual carbide, the hot-rolled steel sheet subjected to descaling treatment is annealed to obtain a hot-rolled annealed steel sheet and It is preferable to do.

In order to make the old γ grain size after hot forming fine, it is preferable to suppress the growth of γ grains by the carbide being dissolved. However, in the hot-formed steel sheet member, it is preferable to reduce the number density of residual carbides in order to improve hardenability, ensure high strength, and suppress the generation of voids.

In order to reduce the old γ grain size in the hot-formed steel sheet member and reduce the number density of residual carbides, the form of carbides existing in the steel sheet before hot forming and the concentration of elements in the carbides Degree is important. Although it is desirable that the carbide is finely dispersed, in that case, since the dissolution of the carbide is accelerated, the effect of suppressing grain growth cannot be expected. If elements such as Mn and Cr are concentrated in the carbide, the carbide is difficult to dissolve. For this reason, it is desirable that the form of carbide in the steel sheet before hot forming is finely dispersed and the concentration of elements in the carbide is high.

The form of carbide can be controlled by adjusting the annealing conditions after hot rolling. Specifically, it is preferable to perform annealing for 5 hours or less by setting the annealing temperature to Ac1 point or lower and Ac1 point to −100 ° C. or higher.

When the coiling temperature after hot rolling is set to 550 ° C. or less, the carbide is easily finely dispersed. However, since the concentration level of the element in the carbide also decreases, the concentration of the element is advanced by annealing.

When the coiling temperature is 550 ° C. or higher, pearlite is generated, and element concentration in the carbide in pearlite is progressing. In this case, annealing is performed in order to divide the pearlite and disperse the carbide.

As the steel sheet for the hot-formed steel sheet member in the embodiment, the hot-rolled annealed steel sheet, the cold-rolled steel sheet that has been cold-rolled to the hot-rolled annealed steel sheet, or the cold-rolled annealed steel that has been annealed A steel plate can be used. What is necessary is just to select a process process suitably according to the plate | board thickness precision required level etc. of a product. In addition, since carbide is hard, even when cold rolling is performed, the form does not change, and the form before cold rolling is maintained even after cold rolling.

Cold rolling may be performed using a normal method. From the viewpoint of ensuring good flatness, the rolling reduction in cold rolling is preferably 30% or more. On the other hand, in order to avoid an excessive load, the rolling reduction in cold rolling is preferably 80% or less.

When the cold-rolled steel sheet is annealed, it is desirable to perform a degreasing process in advance. For the purpose of cold-rolling distortion removal, annealing is preferably performed at an Ac1 point or less and for a time or less, preferably for 3 hours or less.

(E) Method for forming plating layer As described above, the hot-formed steel sheet member according to the embodiment may have a plating layer on the surface for the purpose of improving corrosion resistance or the like. The plating layer is preferably formed on the steel plate before hot forming. When zinc-based plating is applied to the surface of the steel sheet, it is preferable to perform hot-dip galvanizing in a continuous hot dip galvanizing line from the viewpoint of productivity. In that case, annealing may be performed prior to the plating process in the continuous hot dip galvanizing line, or only the plating process may be performed without performing annealing at a low heating holding temperature. Further, an alloying heat treatment may be performed after hot dip galvanization to form an alloyed hot dip galvanized steel sheet. Zinc-based plating can also be applied by electroplating. The zinc-based plating can be applied to at least a part of the surface of the steel material, but in the case of a steel plate, it is generally applied to the entire surface of one side or both sides.

(F) Manufacturing method of hot-formed steel plate member A high-strength hot-formed steel plate member can be obtained by performing hot forming on the hot-formed steel plate. The heating rate of the steel sheet during hot forming is preferably 20 ° C./sec or more from the viewpoint of suppressing grain growth. More preferably, it is 50 ° C./sec or more. It is desirable that the heating temperature of the steel sheet during hot forming exceeds Ac ₃ point and is 1050 ° C. or lower. When the heating temperature is ₃ points or less of Ac, the austenite single phase state is not obtained before hot forming, and ferrite, pearlite, or bainite remains in the steel sheet. As a result, the metal structure is not mainly composed of martensite after hot forming, and the desired hardness may not be obtained. Further, not only the hardness variation of the hot-formed steel sheet member is increased, but also the local deformability is deteriorated.

On the other hand, when the heating temperature exceeds 1050 ° C., austenite becomes coarse, and the local deformability of the steel sheet member may deteriorate. Therefore, the heating temperature of the steel sheet during hot forming is preferably 1050 ° C. or lower. Further, if the heating time is less than 1 min, the austenite single phase may be insufficient even when heated, and further, the carbide is not sufficiently dissolved. The number density of carbides increases. When it exceeds 10 minutes, austenite will coarsen and the local deformability of a hot-formed steel plate member may deteriorate. Accordingly, the heating time of the steel sheet during hot forming is desirably 1 to 10 minutes.

If the hot forming start temperature is lower than the Ar ₃ point, ferrite transformation starts, so that even if forced cooling is performed thereafter, a structure mainly composed of martensite may not be formed. Therefore, it is desirable that the hot forming start temperature is Ar ₃ points or more. After hot forming, it is desirable to rapidly cool at a cooling rate of 10 ° C./sec or more, and it is more desirable to quench at a rate of 20 ° C./sec or more. There is no particular upper limit on the cooling rate.

In order to obtain a hot-formed steel sheet member having a martensite-based metal structure with little hardness variation, it is desirable to rapidly cool the steel sheet until the surface temperature of the steel sheet is 350 ° C. or lower after hot forming. The cooling end temperature is preferably 100 ° C. or lower, more preferably room temperature.

Hereinafter, the embodiments will be described more specifically by way of examples. However, the present invention is not limited to these examples.

Steel having the chemical components shown in Table 1 was melted in a test converter, and continuous casting was performed with a continuous casting tester to produce a slab having a width of 1000 mm and a thickness of 250 mm. In Table 1, * means out of the composition range of the embodiment. Under the conditions shown in Table 2, the heating temperature of the molten steel and the amount of molten steel cast per unit time were adjusted. The cooling rate of the slab was controlled by changing the amount of water in the secondary cooling spray zone. The center segregation reduction treatment was performed by performing light reduction at a gradient of 1 mm / m using a roll in the final solidification portion and discharging the concentrated molten steel in the final solidification portion. Some slabs were then soaked at 1250 ° C. for 24 hours.

The obtained slab was hot rolled by a hot rolling tester to obtain a hot rolled steel sheet having a thickness of 3.0 mm. After winding, the hot rolled steel sheet was pickled and further annealed. Some of the steel sheets were further cold-rolled with a cold rolling tester to obtain cold-rolled steel sheets having a thickness of 1.5 mm. Further, some of the cold-rolled steel sheets were annealed at 600 ° C. for 2 hours to obtain cold-rolled annealed steel sheets.

Thereafter, as shown in FIG. 1 and FIG. 2, hot pressing (hat forming) is performed on the steel sheet for hot forming 1 with a die (punch 11, die 12) using a hot press test apparatus. The hot-formed steel plate member 2 was obtained. The steel sheet is heated in a heating furnace with changing the surface temperature between 820 ° C. and 1100 ° C., held at that temperature for 90 seconds, then removed from the heating furnace and immediately heated in a mold with a cooling device. An intermediate press was performed, and a quenching treatment was performed simultaneously with molding. The following evaluation was performed about the said hot-formed steel plate member. The evaluation results are shown in Table 2. In Table 2, “hot rolled” means a hot-rolled steel sheet having a thickness of 3.0 mm that has been hot-rolled, and “cold-rolled” means a thickness that has been further cold-rolled to the hot-rolled steel sheet. This means a cold-rolled steel sheet having a thickness of 1.5 mm. * Means outside the scope of the embodiment.

<Evaluation of mechanical properties of hot-formed steel sheet member>
About the hot-formed steel plate member, a JIS No. 5 tensile test was taken from the direction perpendicular to the rolling direction, a tensile test was performed according to JIS Z 2241 (2011), and a tensile strength (TS) was measured.

<Identification of metal structure>
The hot-formed steel plate member was cut out so that the central portion of the plate thickness became the observation surface in the cross section parallel to the rolling direction, and then mirror polished. Thereafter, the metal structure was observed with respect to 5 fields of view of each sample using a scanning electron microscope (magnification 2000 times). The obtained micrograph was subjected to image processing to obtain the area ratio of ferrite, which was defined as the volume ratio of ferrite. Moreover, about the volume ratio of the retained austenite in a metal structure, it calculated | required using X-ray diffraction (XRD). The remainder was calculated as the volume fraction of the low temperature transformation structure. Residual γ volume fraction is obtained by chemically polishing an inner layer of 1/8 of the plate thickness from the surface of the steel sheet, and then by X-ray diffraction using a Mo tube. The diffraction strength Iα (200) of ferrite (200) and ferrite (211) It was obtained from the intensity ratio of the diffraction intensity Iα (211) and the diffraction intensity Iγ (220) of (220) of austenite and the diffraction intensity Iγ (311) of (311).
Vγ (volume%) = 0.25 × {Iγ (220) / (1.35 × Iα (200) + Iγ (220)) + Iγ (220) / (0.69 × Iα (211) + Iγ (220)) + Iγ (311) / (1.5 × Iα (200) + Iγ (311)) + Iγ (311) / (0.69 × Iα (211) + Iγ (311))}

<Evaluation of cleanliness>
With respect to the hot-formed steel plate member, test materials were cut out from five locations. The cleanliness of each position of 1 / 8t, 1 / 4t, 1 / 2t, 3 / 4t, and 7 / 8t with respect to the thickness t of each test material was investigated by a point calculation method. And the numerical value with the largest cleanliness value (lowest cleanliness) at each plate thickness was taken as the cleanliness value of the specimen.

<Measurement of Mn segregation degree α>
In the central part of the thickness of the hot-formed steel sheet member, line analysis using EPMA is performed, and after selecting three measured values in order from the analysis result, the average value is calculated, and the maximum Mn at the central part of the thickness is calculated. The concentration was determined. In addition, at the 1/4 depth position of the sheet thickness from the surface of the hot-formed steel plate member, analysis is performed at 10 locations using EPMA, the average value is calculated, and the 1/4 depth position of the sheet thickness from the surface is calculated. The average Mn concentration was determined. And Mn segregation degree (alpha) was calculated | required by dividing the maximum Mn density | concentration in said board thickness center part by the average Mn density | concentration in the 1/4 depth position of board thickness from the surface.

<Measurement of average particle diameter of old γ grains>
The average grain size of old γ grains in hot-formed steel sheet members is the equivalent of a circle by measuring the number of crystal grains in the measurement field of view and dividing the area of the measurement field by the number of crystal grains to determine the average area of the crystal grains It calculated | required by calculating the crystal grain size by a diameter. At that time, the number of grains at the boundary of the visual field was measured as ½, and the observation magnification was appropriately adjusted so that the number of crystal grains was 200 or more.

<Number density of residual carbides>
The surface of the hot-formed steel sheet member was corroded using a picral solution, magnified 2000 times with a scanning electron microscope, and observed in multiple fields. At this time, the number per 1 mm ² was calculated by counting the number of fields of view in which carbides exist.

<Measurement of local deformability>
The local deformability was measured by a notch tensile test. The tensile test piece had a parallel part width of 16.5 mm and a parallel part length of 60 mm, and the rolling direction was taken as the longitudinal direction. Further, a V-notch having a depth of 2 mm was processed at the center of the length of the tensile test piece to obtain a notched tensile test piece. The thickness of the notch test piece was 1.4 mm. The shape of the notch tensile test piece is shown in FIG. A tensile test was performed using the above-described notched tensile test piece, and the notch elongation at the time of breaking at the V notch portion was measured to evaluate the local deformability. The gauge distance was 5 mm, and the tensile speed (crosshead speed) during the tensile test was 0.5 mm / min.

<Hardness variation>
The following tests were performed as evaluation of hardness stability. The steel sheet for hot forming was heated to 900 ° C. at 10 ° C./sec with a heat treatment simulator and then held for 150 sec. Then, it cooled to room temperature with each cooling rate of about 80 degreeC / sec and 10 degreeC / sec. About each sample, the Vickers hardness test was implemented in the 1/4 position of the plate | board thickness of a cross section. The hardness was measured in accordance with JIS Z 2244 (2009), the test force was 9.8 N, five points were measured, and the average was obtained. The average values of the respective hardnesses when the cooling rate was about 80 ° C./sec and 10 ° C./sec were HS ₈₀ and HS ₁₀ , and the difference ΔHv was used as an index of hardness stability.

In the evaluation of hardness stability and local deformability, those having ΔHv of 50 or less and notch elongation of 6% or more were judged to be good.

As can be seen from Table 2, the test number 2 indicates that the composition of the steel satisfies the range of the embodiment, but the casting amount of molten steel per unit time is large. The deformability was inferior.
In Test No. 3, since the center segregation reduction treatment and the soaking treatment were not carried out, the Mn segregation degree exceeded 1.6 and the local deformability was inferior.
Test No. 5 resulted in poor local deformability due to the cleanliness value exceeding 0.08% because the molten steel heating temperature was low.
In Test No. 6, since the hot forming temperature is low, the ferrite volume fraction exceeds 3% after hot forming, resulting in poor hardness stability, and the number density of residual carbides is also 8.0 × 10 ³ pieces / for higher and mm ^2, resulted in local deformability is poor.
In Test No. 9, since the heating temperature at the time of hot forming was high, the old γ particle size was increased, resulting in poor local deformability.
Test No. 11 had a high coiling temperature after hot rolling, resulting in a high residual carbide density and poor local deformability.
In Test No. 14, since the annealing temperature after hot rolling was high and the annealing time was long, the ferrite volume fraction exceeded 3% after hot forming, resulting in poor hardness stability. Further, the dissolution of the carbide was insufficient, the residual carbide density was increased, and the local deformability was inferior.
In test number 16, since the S content exceeded the upper limit of the range of the embodiment, the cleanliness value exceeded 0.08%, resulting in poor local deformability.
In Test No. 17, since the Mn content exceeded the upper limit of the range of the embodiment, the Mn segregation degree exceeded 1.6 and the local deformability was inferior.
For Test No. 18 where the Si content exceeds the upper limit of the range of the embodiments, A _3-point increases, the volume ratio of ferrite is more than 3% after hot forming, hardness less stable results It became.
Test No. 19 resulted in inferior local deformability because the C content exceeded the upper limit of the range of the embodiment.
Test No. 20 resulted in inferior hardness stability because the Cr content was lower than the range of the embodiment.

On the other hand, Test Nos. 1, 4, 7, 8, 10, 12, 13 and 15 satisfying the range of the embodiment resulted in excellent hardness stability and local deformability.

The disclosures of Japanese Patent Application No. 2014-101443 filed on May 15, 2014 and Japanese Patent Application No. 2014-101444 filed on May 15, 2014 are hereby incorporated by reference in their entirety. It is captured.
All documents, patent applications, and technical standards mentioned in this specification are to the same extent as if each individual document, patent application, and technical standard were specifically and individually described to be incorporated by reference, Incorporated herein by reference.

Although various typical embodiments have been described above, the present invention is not limited to these embodiments. The scope of the present invention is limited only by the following claims.

Claims

Chemical composition is mass%,
C: 0.08 to 0.16%,
Si: 0.19% or less,
Mn: 0.40 to 1.50%,
P: 0.02% or less,
S: 0.01% or less,
sol. Al: 0.01 to 1.0%
N: 0.01% or less,
Cr: 0.25 to 3.00%,
Ti: 0.01 to 0.05%,
B: 0.001 to 0.01%,
Nb: 0 to 0.50%,
Ni: 0 to 2.0%,
Cu: 0 to 1.0%,
Mo: 0 to 1.0%,
V: 0 to 1.0%,
Ca: 0 to 0.005%,
Balance: Fe and impurities,
The total volume ratio of martensite, tempered martensite and bainite is 50% or more, and the volume ratio of ferrite is 3% or less,
The average particle size of the former γ grains is 10 μm or less,
A hot-formed steel plate member in which the number density of residual carbides present is 4 × 10 3 pieces / mm 2 or less.
The chemical composition is mass%,
Nb: 0.003 to 0.50%,
Ni: 0.01 to 2.0%,
Cu: 0.01 to 1.0%,
Mo: 0.01 to 1.0%,
V: 0.01 to 1.0%
And Ca: 0.001 to 0.005%
The hot-formed steel plate member according to claim 1, comprising at least one selected from the group consisting of:
The hot-formed steel sheet member according to claim 1 or 2, wherein a steel cleanliness value specified in JIS G 0555 (2003) is 0.08% or less.
The hot-formed steel plate member according to any one of claims 1 to 3, wherein a Mn segregation degree α represented by the following formula (i) is 1.6 or less.
α = [maximum Mn concentration (mass%) at the thickness center portion] / [average Mn concentration (mass%) at ¼ depth position of the thickness from the surface] (i)
The hot-formed steel plate member according to any one of claims 1 to 4, further comprising a plating layer on a surface of the steel plate member.
The hot-formed steel plate member according to any one of claims 1 to 5, wherein the steel plate member has a tensile strength of 1.0 GPa or more.