JP2019014935A - Steel sheet for hot press, manufacturing method therefor, hot press molded member and manufacturing method therefor - Google Patents

Abstract

To provide a steel sheet for hot press excellent in processability, and a manufacturing method therefor, a hot press molded member excellent in impact resistance, and a manufacturing method therefor.SOLUTION: There is provided a steel sheet for hot press having minimum value HTof distribution of products HT between hardness and sheet thickness of 0.80 times or more of a smaller value of average HTand average HT, maximum value HTof 1.50 times or more of a lager value of HTand HT, difference ΔH between the maximum value of hardness Hand a larger value of hardness Hand hardness Hof 100 Hv or less, Hof 400 Hv or less, a ratio between effective crystal grain diameter d of a larger value of averages of effective crystal grain diameters of different steel sheets, and a maximum value dof effective crystal grain diameters of 5.0 or less, and a ratio between a minor diameter r of a larger value of averages of minor diameter of carbide of the different steel sheets and a maximum value rof the minor diameter of carbide of 3.0 or less.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a steel sheet for hot pressing excellent in workability and a manufacturing method thereof, and a hot press-formed member excellent in impact resistance and a manufacturing method thereof.

  In recent years, automobiles have high strength in order to reduce the body weight and improve fuel efficiency, reduce carbon dioxide emissions, and to absorb collision energy and ensure passenger protection and safety in the event of a collision. Many steel plates are used. However, in general, increasing the strength of a steel sheet decreases formability (ductility, hole expandability, etc.) and makes it difficult to process complex shapes, so strength and formability (ductility, hole expandability, etc.) It is not easy to achieve both, and various techniques have been proposed so far.

On the other hand, as disclosed in Patent Document 1, according to a method called hot press for press-molding a heated steel plate, the steel plate is soft and highly ductile at high temperatures, so that it has dimensions in a complicated shape. It is possible to mold with high accuracy. Furthermore, according to the hot pressing method, the steel sheet is heated in the austenite region and rapidly cooled (quenched) in the mold, so that the strength of the steel sheet can be increased by martensitic transformation.

  In the hot press method, increasing the shape accuracy of the member is a problem. On the other hand, Patent Document 2 discloses that the strength of the steel sheet is improved and the shape accuracy of the member is improved at the same time by heating it to an austenite region after being formed into a predetermined shape at room temperature and then rapidly cooling it in the mold. A pre-press quench method is disclosed that achieves.

  In a member for automobiles, there is a case where characteristics are improved by connecting regions having different characteristics in one member. In the hot press method, for example, in Patent Documents 3 and 4, steel members having different characteristics are previously connected by “tailored blank welding” and formed by the hot press method, thereby integrally forming members having different materials depending on places. Technology is disclosed.

  However, in a hot press base material manufactured by tailored blank welding, workability may be extremely deteriorated in a welded part and a heat affected zone (HEAT Affected Zone: HAZ), which is a problem in the manufacturing process. Specifically, since the welded portion and / or HAZ becomes brittle, there is a risk that when the base material is cut for use in the hot pressing process, cracks propagate along the weld line and the base material breaks. is there. In addition, since the formability of the welded part and / or HAZ deteriorates, the pre-press quench method may not be applied because the base material is broken in pre-pressing performed before hot pressing.

  Patent Document 5 discloses a technique for improving the impact resistance of a member by controlling the chemical composition of the steel sheet, suppressing the growth of the microstructure during hot pressing, and refining the matrix austenite grain size. ing. However, in a base material that has been tailored blank welded, the microstructure becomes coarse before hot pressing in the weld and / or HAZ, making it difficult to refine the parent phase austenite grain size during hot pressing. The toughness in the part and / or HAZ may deteriorate.

  Alternatively, in Patent Documents 6 and 7, a member having a different material depending on the location by controlling the cooling rate of the member being pressed separately for each place in the hot pressing process and controlling the phase transformation of the microstructure. A technique for integrally molding the above is disclosed. However, with this technique, the plate thickness of the member cannot be optimized for each location, and the member may not be sufficiently lightened.

British Patent No. 1490535 Japanese Patent Laid-Open No. 10-96031 JP 2007-154257 A JP 2006-21216 A JP 2006-152427 A Japanese Patent No. 5890710 Japanese Patent No. 5890711

  The present invention is a butt-welded joint comprising two or more types of steel plates having different chemical compositions and / or thicknesses in view of the need for further weight reduction and impact resistance improvement in hot press-formed members. An object of the present invention is to provide a hot-press steel plate having excellent workability and a method for producing the same, a hot press-formed member having excellent impact resistance obtained by using the steel plate, and a method for producing the same.

The inventors of the present invention solve the above-mentioned problems by determining the hardness, plate thickness and crystal grain size in the butt weld and its vicinity formed by butt welding the steel plate 1 and the steel plate 2 as shown in FIG. We have intensively studied the effects of butt welded joints on the workability before hot pressing and the impact resistance after hot pressing. As a result, it was found that (1) improvement in workability before hot pressing of the butt weld joint and (2) improvement in impact resistance after hot pressing can be achieved by the following requirements.
(1) In the distribution of the product HT of hardness and plate thickness in the range consisting of the steel plate 1 and the steel plate 2 and the butt welds of these steel plates 1 and 2, the HT over the weld and HAZ and the HT in the steel plate 1 and the steel plate 2 And the ratio of the maximum hardness in the range and the hardness difference between the steel plate 1 and the harder side of the steel plate 2 are reduced;
(2) Further, in the distribution of the effective crystal grain size in the range consisting of the steel plate 1 and the steel plate 2 and the butt welds of these steel plates 1 and 2, the maximum value of the effective crystal grain size over the weld and HAZ, and the steel plate 1. To reduce the ratio of the average value of the effective crystal grain size of the steel plate 2 to the average value of the coarser effective crystal grain size.

  In addition, in obtaining the hot-press steel sheet, the hot-rolled steel sheet and / or the cold-rolled steel sheet, which are semi-finished products in the manufacturing process of the hot-press steel sheet, are butt-welded, and then the entire steel sheet including the welded part and HAZ is adequate. It was found that a steel sheet for hot pressing having excellent workability can be produced by heat treatment.

  Furthermore, it turned out that the hot press-molding member excellent in the impact resistance characteristic can be manufactured by heating the steel plate for hot press on appropriate conditions, and carrying out press molding.

但し、各元素記号は鋼板における含有量［質量％］を表し、当該元素が含まれないときは、０を代入する。
（２）質量％で、
Ｃ：０．０５０％〜０．８００％、
Ｓｉ：０．００１％〜３．００％、
Ｍｎ：０．０１％〜１３．０％、
Ｐ：０．１００％以下、
Ｓ：０．０１００％以下、
Ａｌ：０．００１％〜２．５００％、
Ｎ：０．０１５０％以下、
Ｏ：０．００５０％以下、
を含有し、残部が鉄および不可避不純物からなる１つの鋼板と、
前記鋼板とは化学組成および／または板厚の異なる１種以上の他の鋼板とを、溶接部における板厚比を３．０以下として突き合わせ溶接し、
溶接した全ての鋼板のうち少なくとも１つの鋼板の（Ａ_ｃ１−５０）℃を上回る温度まで加熱する熱処理を行い、
前記熱処理は、加熱開始から冷却開始までの温度履歴が式（２）を満たし、且つ
冷却開始から冷却完了までの温度履歴が式（４）を満たすことを特徴とする熱間プレス用鋼板の製造方法。

但し、式（２）は、鋼板の温度が５５０℃から温度Ｔ^＊に到達するまでの時間を１０ステップに等分に分割し、分割した各ステップにおける式Ｆ_n(T_n, T^*, r, t_n, C^*, Si^*, Mn^*, Cr^*, Mo^*)の計算値を合計し、前記温度Ｔ^＊に到達してから冷却を開始するまでの時間を１０ステップに等分に分割し、分割した各ステップにおけるＧ_n(T_n, T^*, r, t_n, C^*, Si^*, Mn^*, Cr^*, Mo^*)の計算値を合計し、これらの合計値を合算するものである。Ｔ_ｎ［℃］は各温度域におけるｎステップ目における到達温度を、ｔ_ｎ［秒］は各温度域におけるｎステップ目までの総経過時間をそれぞれ表わす。なお、最高加熱温度がＴ^＊に到達しない場合、第２項のＧ_nの計算値の合計は０とする。また、Ｃ^＊、Ｓｉ^＊、Ｍｎ^＊、Ｃｒ^＊およびＭｏ^＊［質量％］は、前記２種の鋼板の化学組成の単純平均を示し、当該元素が含まれないときは、０を代入する。ｒは溶接部を除く前記２種の鋼板の板厚比であり、板厚の薄い鋼板の板厚に対する板厚の厚い鋼板の比率であり、鋼板の板厚が等しい場合、ｒ＝１とする。α、β、γおよびδ、ε、θはそれぞれ定数項であり、それぞれ１．３３×１０^６、１．８０×１０^０、２．２５×１０^４および２．２５×１０^６、２．２０×１０^０、２．４１×１０^４とする。また、Ｔ^＊は炭化物の溶解が始まる目安となる温度であり、下記の式（３）によって得られる。

ここで、元素の右肩に記載のかっこ内の添え字１および２は前記２種の鋼板をそれぞれ表わし、Ｔ^＊は各鋼におけるＡ_ｃ１［℃］、各鋼板の化学組成におけるＳｉ、Ｍｎ、Ｃｒ及びＭｏのそれぞれの含有量［質量％］、および式（２）に示した板厚比ｒから求められる。

但し、式（４）は、冷却過程において炭化物の生成が始まる６５０℃から１００℃に至るまでの温度範囲における滞在時間を１０ステップに区切り、それぞれのステップにおける炭化物の生成挙動を評価し、足し合わせたものである。ここで、Ａは、前記式（２）の左辺が１．００以上の場合は１．００、それ以外の場合は前記式（２）の左辺の値を用いる。また、ΔＴ［℃］は、式（３）で得られるＴ^＊から１００℃低い温度を起点とし、そこからｎステップ目までの区間における最低到達温度Ｔ_ｍｉnを引いた値である。なお、Ｔ_ｍｉｎがＴ^＊−１００℃よりも高い場合、ΔＴは０とする。Ｔｎ［℃］は、ｎステップ目の区間における平均温度である。また、ｔ_ｎ［秒］は６５０℃に到達してからｎステップ目が完了するまでの総経過時間である。μ、η、ζ、ρは定数項であり、それぞれ５．５３×１０^−３、２．５０×１０^−１、２．５０×１０^−２、３．０７×１０^３とする。
（３）前記１つの鋼板の化学組成が、
Ｆｅの一部に替えて、更に質量％で、
Ｃｒ：０．０３〜５．００％
Ｍｏ：０．０３〜５．００％
Ｎｉ：０．０３〜５．００％
Ｃｕ：０．０３〜５．００％
Ｗ：０．０３〜５．００％
Ｂ：０．０００４〜０．０１００％
Ｎｂ：０．００５〜０．２００％
Ｔｉ：０．０１０〜０．５００％
Ｖ：０．０５〜２．００％
Ｓｂ：０．００３〜１．０００％
Ｓｎ：０．００５〜１．０００％
Ｃａ：０．００１０〜０．０１００％
Ｃｅ：０．００１０〜０．０１００％
Ｍｇ：０．００１０〜０．０１００％
Ｚｒ：０．００１０〜０．０１００％
Ｌａ：０．００１０〜０．０１００％
Ｈｆ：０．００１０〜０．０１００％
ＲＥＭ：０．００１０〜０．０１００％
のいずれか１種以上を含むことを特徴とする（２）に記載の熱間プレス用鋼板の製造方法。
（４）突き合わせ溶接後に溶接部を研削することを特徴とする（２）または（３）に記載の熱間プレス用鋼板の製造方法。
（５）前記１つの鋼板及び他の鋼板のうち少なくともいずれかの鋼板が、熱延鋼板に０．０１〜８５％の冷間圧延を施した冷延鋼板であることを特徴とする（２）〜（４）のうちいずれかに記載の熱間プレス用鋼板の製造方法。
（６）異なる鋼板およびそれらの突き合せ溶接部を有し、
前記突き合わせ溶接部及び溶接熱影響部を含む領域の硬度と板厚の積ＨＴ^＊の分布における最小値ＨＴ^＊ _ｍｉｎが、前記異なる鋼板のうち１つの鋼板における平均値ＨＴ^＊ _１と前記異なる鋼板のうち他の鋼板における平均値ＨＴ^＊ _２のうち小さい方の値の０．８０倍以上であり、
前記ＨＴ^＊の分布における最大値ＨＴ^＊ _ｍａｘが前記ＨＴ^＊ _１とＨＴ^＊ _２のうち大きい方の値の１．２０倍以下であり、
前記突き合わせ溶接部及び溶接熱影響部を含む領域の硬度の最大値Ｈ^＊ _ｍａｘと前記１つの鋼板における硬度Ｈ^＊ _１と前記他の鋼板における硬度Ｈ^＊ _２のうち大きい方の値との差ΔＨ^＊が５０Ｈｖ以下であり、かつ、前記硬度Ｈ^＊ _１と前記硬度Ｈ^＊ _２のうち大きい方の値が３００Ｈｖ以上であり、
前記突き合わせ溶接部及び溶接熱影響部を含む領域の母相オーステナイト粒径の最大値Ｄ_ｍａｘと前記１つの鋼板における母相オーステナイト粒径Ｄ_１と前記他の鋼板における母相オーステナイト粒径Ｄ_２のうち大きい方の値との比が５．０倍以下であり、
前記突き合わせ溶接部及び溶接熱影響部における粒子径１．０μｍ以上の炭化物の平均密度が１．０×１．０×１０^１０ｍ^−２以下であることを特徴とする熱間プレス成形部材。
（７）前記（１）に記載の熱間プレス用鋼板を用い、
最高加熱温度が式（１）を満たす鋼板におけるＡ_ｃ３温度以上とし、
５５０℃から加熱終了までの温度履歴が式（５）を満たす熱処理を行うことを特徴とする、熱間プレス成形部材の製造方法。

但し、式（５）の左辺は、式（２）と同一の形式であり、各項の意味および値は等しい。
（８）熱間プレス後に焼戻処理を施すことを特徴とする、（７）に記載の熱間プレス成形部材の製造方法。 This invention was made | formed based on the said knowledge, and the summary is as follows.
(1) consisting of different steel plates and their butt welds,
The chemical composition of at least one steel plate among the different steel plates satisfies the formula (1),
The minimum value HT _min in the distribution of the product HT of the hardness and the plate thickness HT in the region including the butt weld and the heat affected zone is an average value HT _{1 in one} of the different steel plates and the other of the different steel plates. It is 0.80 times or more the smaller value of the average value HT _{2 in} the steel sheet,
The maximum value HT _max in the HT distribution is not more than 1.50 times the larger value of HT ₁ and HT ₂ ;
The difference ΔH between the maximum value H _max of the hardness of the region including the butt weld and the weld heat affected zone, the hardness H 1 of the one steel plate, and the hardness H ₂ of the other steel plate is ₁₀₀ Hv or less. And
And H _max is 400 Hv or less,
In the distribution of effective crystal grain size in the region including the butt weld and the weld heat affected zone, the larger of the average value of the effective crystal grain size of the one steel plate and the average value of the effective crystal grain size of the other steel plate The ratio (d _max / d) of the effective crystal grain size d of the above and the maximum value d _max of the effective crystal grain size is 5.0 or less,
Further, in the region including the butt weld and the heat affected zone, the shorter of the average value of the minor axis of carbide of the one steel plate and the average value of the minor axis of carbide of the different steel plate among the different steel plates. A steel sheet for hot pressing, wherein a ratio (r _max / r) of the diameter r to the maximum value r _max of the minor axis of the carbide is 3.0 or less.

However, each element symbol represents the content [% by mass] in the steel sheet, and 0 is substituted when the element is not included.
(2) In mass%,
C: 0.050% to 0.800%,
Si: 0.001% to 3.00%,
Mn: 0.01% to 13.0%,
P: 0.100% or less,
S: 0.0100% or less,
Al: 0.001% to 2.500%
N: 0.0150% or less,
O: 0.0050% or less,
One steel plate containing the balance of iron and inevitable impurities,
The steel plate is butt welded with one or more other steel plates having different chemical compositions and / or plate thicknesses with a plate thickness ratio of 3.0 or less at the welded portion,
Heat treatment is performed to heat at least one steel plate out of all the welded steel plates to a temperature exceeding (A _c1 -50) ° C.
In the heat treatment, a steel sheet for hot pressing is characterized in that the temperature history from the start of heating to the start of cooling satisfies Equation (2) and the temperature history from the start of cooling to the completion of cooling satisfies Equation (4). Method.

However, the expression (2) divides the time until the temperature of the steel plate reaches the temperature T ^* from 550 ° C. into 10 steps equally, and the expression F _n (T _n , T ^* , r in each divided step is , t _n , C ^* , Si ^* , Mn ^* , Cr ^* , Mo ^* ) are summed up, and the time from reaching the temperature T ^* until starting cooling is divided equally into 10 steps and, G _n in each step of dividing ^{_{(T n, T *, r}} , t n, C *, Si *, Mn *, Cr *, Mo *) sums the calculated values of the sums these totals Is. T _n [° C.] represents the reached temperature at the n-th step in each temperature range, and t _n [second] represents the total elapsed time up to the n-th step in each temperature range. When the maximum heating temperature does not reach T ^* , the total of the calculated values of G _{n in} the second term is 0. C ^* , Si ^* , Mn ^* , Cr ^*, and Mo ^* [mass%] indicate a simple average of the chemical compositions of the two types of steel plates, and 0 is substituted when the element is not included. r is the plate thickness ratio of the two types of steel plates excluding the welded portion, and is the ratio of the thick steel plate to the plate thickness of the thin steel plate, and r = 1 when the steel plate thickness is equal. . α, β, γ and δ, ε, θ are constant terms, respectively, 1.33 × 10 ⁶ , 1.80 × 10 ⁰ , 2.25 × 10 ⁴ and 2.25 × 10 ⁶ , 2.20, respectively. × 10 ⁰ , 2.41 × 10 ⁴ Further, T ^* is a temperature that is a standard for starting dissolution of carbide, and is obtained by the following equation (3).

Here, the subscripts 1 and 2 in the parentheses described on the right shoulder of the element represent the two types of steel plates, respectively, T ^* is A _c1 [° C.] in each steel, Si, Mn in the chemical composition of each steel plate, It is calculated | required from each content [mass%] of Cr and Mo, and plate | board thickness ratio r shown in Formula (2).

However, equation (4) divides the residence time in the temperature range from 650 ° C. to 100 ° C. where the formation of carbides begins in the cooling process into 10 steps, and evaluates the formation behavior of carbides in each step and adds them up It is a thing. Here, A is 1.00 when the left side of the formula (2) is 1.00 or more, and the value of the left side of the formula (2) is used otherwise. Further, ΔT [° C.] is a value obtained by subtracting the minimum temperature T _min that is obtained from the temperature that is 100 ° C. lower than T ^* obtained by the equation (3), and from that point to the n-th step. Note that ΔT is 0 when T _min is higher than T ^* −100 ° C. Tn [° C.] is an average temperature in the n-th section. Also, t _n [seconds] is the total elapsed time from reaching 650 ° C. until the nth step is completed. μ, η, ζ, and ρ are constant terms, which are 5.53 × 10 ⁻³ , 2.50 × 10 ⁻¹ , 2.50 × 10 ⁻² , and 3.07 × 10 ³ , respectively.
(3) The chemical composition of the one steel plate is
In place of a part of Fe, further in mass%,
Cr: 0.03-5.00%
Mo: 0.03-5.00%
Ni: 0.03-5.00%
Cu: 0.03-5.00%
W: 0.03-5.00%
B: 0.0004 to 0.0100%
Nb: 0.005 to 0.200%
Ti: 0.010-0.500%
V: 0.05-2.00%
Sb: 0.003 to 1.000%
Sn: 0.005 to 1.000%
Ca: 0.0010 to 0.0100%
Ce: 0.0010 to 0.0100%
Mg: 0.0010 to 0.0100%
Zr: 0.0010 to 0.0100%
La: 0.0010 to 0.0100%
Hf: 0.0010 to 0.0100%
REM: 0.0010 to 0.0100%
Any one of these is included, The manufacturing method of the steel plate for hot presses as described in (2) characterized by the above-mentioned.
(4) The method for producing a hot-press steel plate according to (2) or (3), wherein the welded portion is ground after butt welding.
(5) At least one of the one steel plate and the other steel plate is a cold-rolled steel plate obtained by subjecting a hot-rolled steel plate to 0.01 to 85% cold rolling (2) The manufacturing method of the steel plate for hot presses in any one of-(4).
(6) having different steel plates and their butt welds,
The minimum value HT ^* _min in the distribution of the product HT ^* of the hardness and the plate thickness of the region including the butt weld and the weld heat affected zone is an average value HT ^* _{1 of one} of the different steel plates and the difference between the different steel plates. Of these, the average value HT ^* ₂ of other steel plates is 0.80 or more times the smaller value,
The HT ^* maximum ^HT _{* max} in the distribution of not more than 1.20 times the greater of the ^HT _{* 1} and ^HT _{* 2,}
The difference ΔH between the maximum value H ^* _max of the hardness of the region including the butt weld and the heat affected zone, and the larger value of the hardness H ^* _{1 of the one} steel plate and the hardness H ^* ₂ of the other steel plate ^* Is 50 Hv or less, and the larger value of the hardness H ^* ₁ and the hardness H ^* ₂ is 300 Hv or more,
Of the mother phase austenite grain size D ₂ at the maximum value D _max and the one of the other steel sheet matrix phase austenite grain size D ₁ of the steel matrix phase austenite grain size of the region containing the butt weld and heat affected zone The ratio of the larger value is 5.0 times or less,
The hot press-molded member, wherein an average density of carbides having a particle diameter of 1.0 μm or more in the butt weld and the weld heat affected zone is 1.0 × 1.0 × 10 ¹⁰ m ⁻² or less.
(7) Using the steel sheet for hot pressing described in (1) above,
The maximum heating temperature is not _lower than the _Ac3 temperature in the steel sheet satisfying the formula (1),
A method for producing a hot press-molded member, comprising performing a heat treatment in which a temperature history from 550 ° C. to the end of heating satisfies Formula (5).

However, the left side of Expression (5) has the same format as Expression (2), and the meaning and value of each term are the same.
(8) The method for producing a hot press-formed member according to (7), wherein a tempering treatment is performed after the hot pressing.

  ADVANTAGE OF THE INVENTION According to this invention, the hot press forming member excellent in the impact resistance property obtained using the steel plate for hot press excellent in workability and the steel plate can be provided.

It is a graph which shows the example of the plate | board thickness and hardness distribution in a general butt welding part. It is a graph which shows the example of the plate | board thickness and hardness distribution in the welding part in the high strength steel plate of this invention. It is explanatory drawing which shows the measuring method of distribution of hardness in a welding part. It is a graph which shows distribution of the effective crystal grain size around the welding part in a general steel plate for hot press, and distribution of the parent phase austenite grain size in a hot press member. It is a graph which shows distribution of the effective crystal grain size around the welding part in the steel plate for hot presses of this invention, and distribution of the parent phase austenite grain size in a hot press-formed member. It is a schematic diagram of a test piece with a notch. It is a schematic diagram of a hot press molding member. It is a schematic perspective view which shows the structure of the steel plate for hot presses of Experimental examples 31-33.

但し、各元素記号は鋼板における含有量［質量％］を表し、当該元素が含まれないときは、０を代入する。 Hereinafter, the steel sheet for hot pressing of the present invention and the manufacturing method thereof will be described.
The steel sheet for hot pressing according to the present invention comprises two or more kinds of steel sheets having different chemical compositions and / or thicknesses and butt welds thereof, and the steel sheet for hot pressing exhibits sufficient strength by hot pressing. Must contain a sufficient amount of C and have sufficient hardenability. Specifically, at least one of steel plates (hereinafter also referred to as “base material steel plate”) as a base material constituting the steel sheet for hot pressing needs to satisfy the following formula (1).

However, each element symbol represents the content [% by mass] in the steel sheet, and 0 is substituted when the element is not included.

  In the base steel sheet not satisfying the formula (1), since the maximum strength and / or hardenability obtained by quenching is insufficient, it is difficult to obtain a hardness of 300 Hv or more by hot pressing. In order to further increase the strength after hot pressing, the left side of the formula (1) is preferably 1.15 or more, and more preferably 1.50 or more.

  However, in the hot-press steel plate of the present invention, the second and subsequent base metal plates do not have to satisfy the formula (1). In particular, it is preferable to use a base steel plate that does not satisfy the formula (1) for a portion that is desired to have a low strength with a hardness of less than 300 Hv in a hot press-formed member. More preferably, it is used.

(Chemical composition)
At least one base metal plate of the base steel plate constituting the hot press steel plate of the present invention has a chemical composition of the following in order to make the hardness of the hot press-formed member of the present invention 300 Hv or higher. It is preferable to use it. In the chemical composition,% represents mass%.

(C: 0.050-0.800%)
C is an element that contributes to improving the strength. If the C content is less than 0.050%, the hardness after hot pressing does not reach 300 Hv, so the content is preferably 0.050% or more. C is preferably contained in an amount of 0.090% or more, and more preferably 0.140% or more. On the other hand, if the C content exceeds 0.800%, the cast slab becomes brittle and easily breaks, so the content is preferably 0.800% or less. Moreover, since the weldability in butt welding deteriorates, the C content is preferably 0.550% or less. In order to ensure the weldability of the steel sheet for hot pressing, the C content is more preferably 0.400% or less.

(Si: 0.001 to 3.00%)
Si is an element that refines iron-based carbides and contributes to improvement in strength and formability, but is also an element that embrittles steel. If the Si content exceeds 3.00%, the cast slab becomes brittle and easily cracked, and the weldability is lowered. Therefore, the Si content is preferably 3.00% or less. In order to ensure impact resistance, it is preferably 2.20% or less, more preferably 1.70% or less. On the other hand, since special treatment is required to reduce the Si content to less than 0.001%, the Si content is preferably 0.001% or more. In order to strengthen steel, the content of Si is preferably 0.010% or more, and more preferably 0.030% or more.

(Mn: 0.01 to 13.0%)
Mn is an element that enhances hardenability and contributes to improvement in strength, but is also an element that embrittles steel. If the Mn content exceeds 13.0%, the cast slab becomes brittle and easily cracked, and weldability deteriorates. Therefore, Mn is preferably 13.0% or less. In order to prevent embrittlement of the cast slab, the Mn content is preferably 10.0% or less, and more preferably 7.00% or less. On the other hand, since special treatment is required to make the Mn content less than 0.01%, the Mn content is preferably 0.01% or more. In order to strengthen steel, Mn is preferably contained in an amount of 0.10% or more, and more preferably 0.50% or more.

(Al: 0.001 to 2.500%)
Al functions as a deoxidizing material, but is also an element that embrittles steel. If the Al content is less than 0.001%, a sufficient deoxidation effect cannot be obtained. Therefore, the Al content is preferably 0.001% or more. On the other hand, when the Al content exceeds 2.500%, a coarse oxide is generated and the cast slab is easily cracked. Therefore, the Al content is preferably 2.500% or less. In view of securing good spot weldability, the Al content is preferably 2.000% or less.

  In manufacturing the steel sheet of the present invention, the component composition of the base steel sheet may include the following elements in addition to the above elements in order to improve characteristics.

(Cr: 0.03 to 5.00% or less)
Cr is an element that enhances hardenability and contributes to an improvement in steel sheet strength, and is an element that can replace a part of C and / or Mn. If the Cr content exceeds 5.00%, the hot workability decreases and the productivity decreases, so the Cr content is preferably 5.00% or less. The lower limit includes 0%, but it is preferable to contain 0.03% or more in order to sufficiently obtain the effect of improving the strength of Cr.

(Mo: 0.03-5.00% or less)
Mo is an element that suppresses phase transformation at a high temperature in the hot pressing step and contributes to the improvement of the steel sheet strength, and is an element that can replace a part of C and / or Mn. If the Mo content exceeds 5.00%, the hot workability decreases and the productivity decreases, so the Mo content is preferably 5.00% or less. The lower limit includes 0%, but in order to sufficiently obtain the effect of improving the strength of Mo, the content is preferably 0.03% or more.

(Ni: 0.03-5.00%)
Ni is an element that suppresses phase transformation at a high temperature in the hot pressing process and contributes to an improvement in steel sheet strength, and is an element that can replace a part of C and / or Mn. If Ni exceeds 5.00%, the weldability is lowered, so the Ni content is preferably 5.00% or less. The lower limit includes 0%, but it is preferable to contain 0.03% or more in order to sufficiently obtain the Ni strength improvement effect.

(Cu: 0.03 to 5.00% or less)
Cu is an element that exists in steel as fine particles and contributes to the improvement of the strength of the steel sheet, and is an element that can replace a part of C and / or Mn. If Cu exceeds 5.00%, the weldability decreases, so the Cu content is preferably 5.00% or less. The lower limit includes 0%, but it is preferable to contain 0.03% or more in order to sufficiently obtain the effect of improving the strength of Cu.

(W: 0.03-5.00% or less)
W is an element that suppresses phase transformation at a high temperature in the hot pressing process and contributes to an improvement in the steel sheet strength, and is an element that can replace a part of C and / or Mn. If W exceeds 5.00%, the hot workability is lowered and the productivity is lowered. Therefore, the W content is preferably 5.00% or less. The lower limit includes 0%, but in order to sufficiently obtain the effect of improving the strength of W, the content is preferably 0.03% or more.

(B: 0.0004 to 0.0100% or less)
B is an element that suppresses phase transformation at a high temperature in the hot pressing process and contributes to the improvement of the steel sheet strength, and is an element that can replace a part of C and / or Mn. If the B content exceeds 0.0100%, the hot workability decreases and the productivity decreases, so the B content is preferably 0.0100% or less. The lower limit includes 0%, but in order to sufficiently obtain the effect of improving the strength of B, the content is preferably 0.0004% or more.

(Nb: 0.005 to 0.200% or less)
Nb is an element that contributes to improvement of toughness by suppressing the growth of parent phase austenite crystal grains in the hot pressing step, and may be contained up to 0.200%. If the content of Nb exceeds 0.200%, a large amount of carbonitride precipitates and the moldability deteriorates, which is not preferable. The lower limit includes 0%, but it is preferable to contain 0.005% or more in order to obtain the effect of refining effective crystal grains in HAZ.

(Ti: 0.010 to 0.500% or less)
Ti is an element that contributes to improvement of toughness by suppressing the growth of parent phase austenite crystal grains in the hot pressing step, and may be contained up to 0.500%. If the Ti content exceeds 0.500%, a large amount of carbonitride precipitates and the formability deteriorates, such being undesirable. The lower limit includes 0%, but in order to obtain the effect of refining effective crystal grains in HAZ, the content is preferably 0.010% or more.

(V: 0.05 to 2.00% or less)
V is an element that contributes to improvement of toughness by suppressing the growth of parent phase austenite crystal grains in the hot pressing step, and may be contained up to 2.00%. If the V content exceeds 2.00%, a large amount of carbonitride precipitates and the formability deteriorates, such being undesirable. The lower limit includes 0%, but it is preferably 0.05% or more in order to obtain the effect of refining effective crystal grains in HAZ.

(Sb: 0.003 to 1.000% or less)
Sb is an element that suppresses the coarsening of the parent phase austenite crystal grains in the hot pressing step and contributes to the improvement of the toughness of the hot press-formed member. On the other hand, if the Sb content exceeds 1.000%, the steel sheet becomes brittle and may break during rolling. Therefore, the Sb content is preferably 1.000% or less. The lower limit includes 0%, but it is preferable to contain 0.003% or more in order to sufficiently obtain the effect of adding Sb.

(Sn: 0.005 to 1.000% or less)
Sn is an element that suppresses the coarsening of the parent phase austenite crystal grains in the hot pressing step and contributes to the improvement of the toughness of the hot press-formed member. On the other hand, if the Sn content exceeds 1.000%, the steel sheet becomes brittle and may break during rolling. Therefore, the Sn content is preferably 1.000% or less. The lower limit includes 0.000%, but the Sn content is preferably 0.005% or more in order to sufficiently obtain the effect of adding Sn.

  The component composition of the steel sheet of the present invention may include one or more of Ca, Ce, Mg, Zr, La, Hf, and REM as necessary so that the total amount is 0.0100% or less. Ca, Ce, Mg, Zr, La, Hf, and REM are elements that contribute to improvement in impact resistance by reducing the size of inclusions. However, if one or more of Ca, Ce, Mg, Zr, La, Hf and / or REM is contained in total exceeding 0.0100%, the formation of inclusions is promoted on the contrary, and impact resistance is increased. Therefore, the total content of the above elements is preferably 0.0100% or less, and more preferably 0.0070% or less. The lower limit of the total of one or more of Ca, Ce, Mg, Zr, La, Hf, and REM includes 0.0000%, but in order to sufficiently obtain the impact resistance improvement effect, the total is 0.0010. % Or more is preferable.

  REM (Rare Earth Metal) means an element belonging to the lanthanoid series. In many cases, La and Ce are added in the form of misch metal, but in addition to La and Ce, lanthanoid series elements may be unavoidably contained.

(Inevitable impurities)
In the component composition of the steel sheet of the present invention, the balance excluding the above elements is Fe and inevitable impurities. Inevitable impurities are elements that are inevitably mixed from the steel raw materials and / or in the steel making process. In the present invention, among the inevitable impurities, the contents of P, S, N, and O are defined as follows.

(P: 0.100% or less)
P is an element that embrittles steel. If P exceeds 0.100%, the cast slab becomes brittle and easily cracked, so P is made 0.100% or less. The lower limit includes 0%, but if P is reduced to less than 0.0001%, the manufacturing cost increases significantly, so 0.0001% is a practical lower limit on a practical steel sheet.

(S: 0.0100% or less)
S is an element that forms MnS and impairs impact resistance. If the S content exceeds 0.0100%, the impact resistance of the weld zone and the HAZ is significantly reduced, so the S content is set to 0.0100% or less. The lower limit includes 0%, but if it is reduced to less than 0.0001%, the manufacturing cost increases significantly, so 0.0001% is a practical lower limit on the practical steel sheet.

(N: 0.0150% or less)
N is an element that forms nitrides and impairs impact resistance, and also causes blowholes during welding and is an element that impairs weldability. If the N content exceeds 0.0150%, impact resistance and weldability deteriorate, so the N content is set to 0.0150% or less. The N content is preferably 0.0100% or less, and more preferably 0.0075% or less. The lower limit of the N content includes 0%, but if it is reduced to less than 0.0001%, the manufacturing cost increases significantly, so 0.0001% is a practical lower limit on the practical steel sheet.

(O: 0.0050% or less)
O is an element that forms an oxide and hinders impact resistance. When the O content exceeds 0.0050%, the impact resistance is remarkably lowered, so the O content is set to 0.0050% or less. The lower limit includes 0%, but if O is reduced to less than 0.0001%, the manufacturing cost increases significantly, so 0.0001% is a practical lower limit on a practical steel sheet.

  Inevitable impurities include H, Na, Cl, Sc, Co, Zn, Ga, Ge, As, Se, Y, Zr, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te. , Cs, Ta, Re, Os, Ir, Pt, Au, and Pb may be included in a total amount of 0.0100% or less.

  Subsequently, in the hot-press steel sheet of the present invention, for the butt weld including the steel sheet satisfying the formula (1), the reason why the HAZ is limited in the steel sheet 1, the steel sheet 2, the welded joint, and the steel sheet 1 and the steel sheet 2 sandwiching the welded part. Will be described.

[Product of hardness and thickness HT]
Cracks during processing include cracks due to strain concentration and cracks due to insufficient toughness, and the ease of occurrence of cracks due to strain concentration in the weld zone and HAZ can be organized by the product HT of the hardness and the plate thickness at that location. Since HT corresponds to the load resistance at the location, when deformation is applied to the steel sheet, the deformation tends to concentrate at locations where the HT is low compared to the periphery, that is, locations where the load resistance is low. Therefore, when the HT in the welded part or HAZ is remarkably small as compared with the steel plate part not affected by the welding, the strain may concentrate on the part having a small HT during the press forming and may crack.

In order to avoid such strain concentration, the HT in the weld zone and the HAZ should not be excessively small with respect to the smaller HT side of the butt welded steel sheet. Specifically, in the case of butt welding shown in FIG. 1, in order to avoid strain concentration, the minimum value HT _min in the distribution of HT in the region including the weld and HAZ is the average value HT ₁ in the steel plate 1. it is required to be more than 0.80 times the smaller value among the average values HT ₂ in the steel plate 2. The relationship between the two is preferably 0.85 times or more, more preferably 0.90 times or more, and most preferably the two are equal. The average value HT ₂ in the average value HT ₁ and the steel plate 2 in the steel sheet 1 is the average value of the hardness in the steel sheet area not including the welds and HAZ.

On the other hand, in places where HT is extremely high compared to the surrounding area, even if a load is applied, the deformation does not easily occur. In order to avoid this, the HT in the weld zone and the HAZ should not be an excessively large value with respect to the steel plate having the larger HT among the butt welded steel plates. Specifically, in the case of butt welding shown in FIG. 1, in order to avoid strain concentration, the maximum value HT _max in the HT distribution in the steel plate 1 to the steel plate 2 in the region including the weld and HAZ is the steel plate 1. or less is required 1.50 times the larger of the average value HT ₂ in the average value HT ₁ and the steel plate 2 in. The relationship between the two is preferably 1.40 times or less, more preferably 1.30 times or less, and most preferably both are equal.

[Maximum hardness H _max ]
On the other hand, the ease of cracking during molding due to insufficient toughness can be organized by hardness. When the hardness in the welded part and the HAZ is extremely high as compared with the surrounding steel plates, there is a risk that the portion is greatly embrittled as compared with the steel plates, and may break during forming. Specifically, in the case of butt welding shown in FIG. 1, the maximum value H _max of the hardness from the steel plate 1 to the steel plate 2 in the region including the butt weld and HAZ, the hardness H ₁ in the steel plate _1, and the steel plate 2 If the difference ΔH from the larger value of the hardness H ₂ exceeds 100 [Hv], cracks may occur during press molding, so the upper limit of ΔH is set to 100 [Hv]. ΔH is preferably as small as possible, preferably 50 [Hv] or less, and more preferably 30 [Hv] or less.

A method for measuring the hardness of the steel plate and the welded portion will be described. The hardness is measured by performing a micro Vickers test described in JIS Z 2244 in a cross section perpendicular to the welded part and the plate surface. In the measurement, the hardness is measured on a straight line parallel to the plate surface passing through ¼ of the plate thickness of the thin steel plate among the butt welded steel plates. First, the hardness is measured at the center of the welded portion, and the hardness is measured every 0.1 to 0.2 mm from there to each steel plate. The measurement on each steel plate is continued until the variation of the hardness measurement values at 10 consecutive points falls within ± 10% of the average value at 10 points, and the average hardness H _{1 of} each steel plate and the average value are obtained. and H _2. The measurement load is adjusted and set so that the size of the indentation is 100 μm or less in the range of 10 to 100 gf.

[Maximum effective crystal grain size d _max ]
In order to increase the impact resistance of the member after hot pressing, it is necessary to make the crystal grain size of the parent phase austenite fine during hot pressing in order to suppress the propagation of fracture. The crystal grain size of the parent phase austenite is greatly influenced by the crystal grain size of the steel sheet for hot pressing before heating, that is, if the crystal grain size of the steel sheet for hot pressing is coarse, the austenite grain size also becomes coarse. To do. In hot-pressed steel sheets, particularly in HAZ, the microstructure becomes coarse during welding, and the effective crystal grain size may be significantly larger than that of the surrounding steel, and accordingly the parent phase austenite grain size after hot pressing. Becomes coarse and the impact resistance of the hot press-formed member tends to deteriorate.

Specifically, in the case of butt welding shown in FIG. 1, the maximum value d _max of the effective crystal grain size in the region including the butt weld and the HAZ, and the average value d ₁ of the effective crystal grain size in the steel plate 1 By setting the ratio of the average value d 2 of the effective crystal grain sizes in the steel sheet 2 to the larger value d ₂ to be 5.0 or less, the impact resistance characteristics are improved. This ratio is preferably 4.0 or less, more preferably 3.0 or less, and most preferably both are equal. The average value d ₂ of the effective crystal grain size in average d ₁ and the steel plate 2 of the effective crystal grain size in the steel sheet 1 is the average value of the effective crystal grain size in each of the steel sheet region not including the welds and HAZ . Hereinafter, the “average value of effective crystal grain size” is simply referred to as “average effective crystal grain size”.

A method for measuring the effective crystal grain size will be described. The effective crystal grain size is measured by analyzing the crystal orientation around the midpoint of the hardness measurement point on the same plane as the surface on which the hardness measurement was performed. The crystal orientation is measured by a field emission scanning electron microscope (FE-SEM: Field Emission Scanning Electron Microscope) by an EBSD method (Electron Backscattering Diffraction) for obtaining an electron beam backscattering diffraction pattern. The measurement area per point is 1.0 × 10 ⁻⁸ m ² or more, and the size of the measurement point is 0.1 to 0.3 μm.

  The effective crystal grain size is obtained by analyzing the crystal orientation information obtained by the EBSD method, mapping the boundary having an orientation difference of 10 ° or more, measuring the average interval of the boundary by the cutting method, and obtaining the measured value as the effective crystal grain size. Consider diameter.

  In addition, the average effective crystal grain size in each steel plate excluding HAZ is any two or more of 9 intermediate points consisting of 10 measurement points used for determining the average hardness of each steel plate in the measurement of hardness. The crystal orientation is measured and the average value of the obtained values is regarded as the average effective crystal grain size in each steel plate. The data obtained by the EBSD method is analyzed using “OIM Analysis 7.0” manufactured by TSL.

[Maximum minor axis r _max of carbide]
In order to increase the impact resistance of the hot press-formed member, it is effective to suppress the formation of coarse carbides that act as starting points for fracture. In particular, in HAZ, the microstructure around coarse carbides is coarsened, so that breakage starting from coarse carbides is likely to propagate. The dissolution behavior of carbides in the steel sheet in the hot pressing step is greatly affected by the size of carbides before heating, that is, in the hot pressing steel sheet. In particular, when the minor axis of the carbide having a major axis exceeding 1.0 μm is coarse, the carbide tends to be dissolved in the hot pressing process. This is because the degree of influence as a starting point of carbide destruction depends on the major axis of the carbide, and the dissolution behavior during heating also depends on the minor axis.

Specifically, in the case of the butt welding shown in FIG. 1, in the region from the steel plate 1 to the steel plate 2 including the butt weld and the HAZ, the maximum value r _max of the short diameter of the carbide whose major axis is 1.0 μm or more. When, by setting the formed to ratio between the value r of the larger of the minor axis r ₂ of the carbide in the minor axis r ₁ and the steel plate 2 of carbides in the steel plate 1 3.0, near the welded portion in the hot press step The dissolution rate of carbides in the steel is equivalent to the dissolution rate in the base steel sheet, and the impact resistance characteristics of the hot press-formed member are improved.

  A method for measuring the minor axis of carbide will be described. The minor axis of the carbide is measured by observing the microstructure around the middle point of the hardness measurement point on the same plane as the surface on which the hardness measurement was performed. The measurement method is, for example, observing the region with a field emission scanning electron microscope (FE-SEM), measuring the minor axis of 25 arbitrary carbides having a major axis of 1.0 μm or more, The simple average is taken as the minor axis of the carbide at the hardness measurement point.

  Moreover, the average value of the minor axis of carbide in each steel plate excluding HAZ is an arbitrary 2 of 9 intermediate points consisting of 10 measurement points used for obtaining the average hardness of each steel plate in the measurement of hardness. The microstructure is observed above the point, the minor axis is measured for 25 arbitrary carbides having a major axis of 1.0 μm or more, and the average value of the obtained values is regarded as the average value of the minor axis of the carbide in each steel plate.

In addition, even if the measurement area per point exceeds 5.0 × 10 ⁻⁸ m ² , if the number of carbides having a major axis of 1.0 μm or more is less than 25, 5.0 × 10 ⁻⁸ m ² The minor axis of the carbide of 1.0 μm or more observed in the range may be measured, and the simple average may be used as the minor axis of the carbide at the measurement point. Or you may observe exceeding 5.0 * 10 < ^-8 > m < ² > until the number of the carbide | carbonized_material whose major axis is 1.0 micrometer or more reaches 25 pieces.

(Manufacturing method of steel sheet for hot pressing)
The manufacturing method of the base steel sheet is not particularly defined, but from the viewpoint of production cost, it is preferable that the cast slab is hot-rolled and cold-rolled as necessary. As the slab used for hot rolling, a slab produced by a continuous casting slab, a thin slab caster or the like can be used. Although the slab after casting may be once cooled to room temperature, it is more preferable to subject it to hot rolling directly at a high temperature because energy required for heating can be reduced.

  In the hot rolling step, the heating temperature of the slab is preferably 1150 ° C. or higher. This is to dissolve coarse carbides produced during casting. On the other hand, there is no effect of improving the characteristics even if the heating temperature is higher than 1300 ° C. From the viewpoint of production cost, the heating temperature is preferably 1300 ° C. or lower.

  When the hot rolling start temperature is lowered, the strength of the slab is increased and the predetermined sheet thickness accuracy may not be obtained. Therefore, the hot rolling start temperature is preferably 1030 ° C. or higher. On the other hand, when the completion temperature of hot rolling exceeds 1000 ° C., the structure becomes excessively coarse and the structure of the final product may also become coarse, and the completion temperature of hot rolling is preferably 1000 ° C. or less. . On the other hand, when the completion temperature of hot rolling is less than 830 ° C., the load during rolling is excessively increased, and a predetermined sheet thickness accuracy may not be obtained. Therefore, the completion temperature of hot rolling is 830 ° C. or more. It is preferable to do.

  In order to prevent coarsening of the structure after completion of hot rolling, it is preferable to start the cooling process within 10.0 seconds after completion of rolling. Moreover, in order to prevent the coarsening of a structure | tissue, it is preferable that the average cooling rate in a cooling process shall be 10 degreeC / second or more, and it is preferable that a cooling stop temperature shall be 680 degrees C or less.

  The obtained hot-rolled steel sheet is preferably subjected to pickling treatment. For example, the hot-rolled steel plate manufactured as described above can be used as a base steel plate for manufacturing the high-strength steel plate of the present invention. As the base steel plate, steel plates having different chemical compositions and / or thicknesses are used, and one or more of them use steel plates having the above chemical composition in order to make the hardness after hot pressing 300 Hv or more.

  The base steel plate may be subjected to a shape correction treatment in order to make the steel plate flat and facilitate butt welding. In order to increase the flatness, the amount of plastic deformation applied to the steel sheet is preferably 0.01% or more, and more preferably 0.05% or more. In addition to shape correction, the base steel plate may be cold-rolled in order to easily obtain the plate thickness required for the product. However, if the cold rolling rate exceeds 85%, the steel sheet may break during rolling, so the cold rolling rate is preferably 85% or less, and more preferably 75% or less.

  The cold rolling may be performed under individual conditions in a plurality of base steel plates. For example, a steel plate that is subjected to cold rolling and a steel plate that is not subjected to cold rolling may be mixed as a base steel plate.

Furthermore, prior to the welding process described later, a preliminary heat treatment may be performed on the base steel sheet. By setting the maximum heating temperature in the preliminary heat treatment to the _Ac1 temperature or higher, coarse carbides in the base steel plate can be reduced, the structure after heat treatment described later is homogenized, and the characteristics are improved.

In addition, the maximum heating temperature in the preliminary heat treatment is set to the _Ac3 temperature or higher, and the average cooling rate from the highest heating temperature to 400 ° C in the cooling step after heating is set to 1.0 ° C / second or higher, so that The structure can be made into a homogeneous fine structure, and the structure after heat treatment described later is homogenized and refined, and the characteristics after hot pressing are improved. The preliminary heat treatment may be performed on each of a plurality of base material steel plates under individual conditions. For example, a steel plate that is subjected to preliminary heat treatment and a steel plate that is not subjected to the heat treatment may be mixed as a base material steel plate.

  Two or more types of steel plates having different chemical compositions and / or plate thicknesses are subjected to a butt welding process to form a single plate. Prior to welding, it is preferable that the butt portion is cut and tapered as necessary so that stable welding can be performed.

  The steel sheet may be subjected to a butt welding process in the longitudinal direction of the steel strip coil to produce a welded steel strip coil, and may be subjected to a heat treatment described later. Alternatively, a steel plate cut to an appropriate size may be welded and subjected to heat treatment described later.

  The butt welding is not limited as long as a welded portion with little welding abnormality can be obtained. For example, in addition to laser welding, mash seam welding or plasma welding may be used. In a plate set of two steel plates including a butt weld and HAZ, if the thicknesses of both are excessively different, in the heat treatment described later, temperature fluctuations of the steel plate and the weld may occur, and stable characteristics may not be obtained. is there. Therefore, it is necessary to select the plate set of the two steel plates so that the thickness ratio of the base steel plate is 3.0 or less. In order to stabilize the temperature of the entire steel plate and obtain excellent impact characteristics, the thickness ratio of the base steel plate is preferably 2.6 or less.

  In order to improve the appearance quality of the product after heat treatment after butt welding, the surface may be ground at the welded part and the peripheral part. By grinding the surface, the weld bead is less noticeable and the appearance is improved. Any grinding means may be used, but it is preferable to continuously grind in the length direction of the weld bead using, for example, a brush roll. Alternatively, grinding may be performed using a grinder or the like.

After butt welding, preliminary heat treatment may be performed before heat treatment. In particular, by setting the maximum heating temperature of the preliminary heat treatment to an _Ac3 temperature or higher in one or more types of base steel plates, the base steel plate, the HAZ composed of the base steel plates, and the microstructure of the weld are made uniform and fine. And the properties of the steel sheet are improved.

  After butt welding, in order to stabilize the emissivity of the welded part and the peripheral part and improve the temperature control accuracy in the heat treatment, a pickling treatment may be performed before the heat treatment.

  After the butt welding, in order to stabilize the emissivity of the welded part and the peripheral part and improve the temperature control accuracy in the heat treatment, a shot peening treatment may be performed before the heat treatment.

  In order to improve the appearance quality of the product after butt welding and heat treatment, surface treatment may be performed before heat treatment. For example, a pre-plating process mainly composed of Fe or Ni may be performed.

[Heat treatment]
Then, it heat-processes, the steel plate, HAZ, and the microstructure of a welding part are made, and the steel plate for hot presses of this invention is manufactured. What is necessary is just to perform heat processing in the arbitrary heat processing apparatuses which can achieve the conditions mentioned later. For example, a heat treatment may be performed by inserting a steel plate into a sufficiently heated reducing atmosphere furnace. Alternatively, heat treatment may be performed by an induction heating method or an electric heating method. Alternatively, the atmosphere may be controlled and oxidized in the pretropical zone and then heated in a reduction furnace.

  In particular, when a welded steel strip coil subjected to butt welding is manufactured, the steel sheet of the present invention can be manufactured at low cost by processing the coil with a continuous heat treatment furnace. Or you may process the said coil with a box-type annealing furnace.

Upon heat treatment, to enhance the welds and HAZ of the workability, and (A _{c1 -50)} or more at the base steel sheet of the maximum heating temperature of the base material steel plate having a chemical composition described above. This is because in the microstructure in the weld zone and HAZ, carbides are generated and / or retransformed into a softer microstructure. When the maximum heating temperature is increased, the difference in microstructure between the weld zone and HAZ and the surrounding base material is reduced, and the workability of the steel sheet for hot pressing is improved. On the other hand, when heated at an excessively high temperature, In some cases, the strength of the entire steel plate increases, and the workability deteriorates. In order to avoid this, the maximum heating temperature is preferably 1000 ° C. or lower.

Of the heat treatment, the temperature history of the base steel sheet from the start of heating to the start of cooling needs to satisfy the following formula (2). Formula (2) is an index representing the growth and dissolution degree of carbides in the HAZ and welds around the base steel sheet. When Formula (2) is not satisfied, a large number of coarse carbides are generated in the HAZ and welds. It remains undissolved in the hot pressing process, and the strength and impact resistance of the hot press-formed member deteriorate.

However, the equation (2) indicates that the time from when the temperature of the steel sheet reaches 550 ° C. at which the carbide growth starts until the cooling is started until the temperature T ^* , which is a standard for starting the dissolution of the carbide, is reached. Each of the sections from reaching the temperature T ^* to reaching the maximum heating temperature is equally divided into 10 steps, and the growth and dissolution degree of carbide in each divided step is calculated and totaled. .

Equation (2) is obtained by dividing the time until the temperature of the steel sheet reaches the temperature T ^* from 550 ° C. into 10 steps equally, and the equation F _n (T _n , T ^* , r, t in each divided step is _n , C ^* , Si ^* , Mn ^* , Cr ^* , Mo ^* ) are summed, and the time from when the temperature T ^* is reached to when cooling is started is divided equally into 10 steps, G _n in each step of dividing ^{_{(T n, T *, r}} , t n, C *, Si *, Mn *, Cr *, Mo *) in which the sum of the calculated values of the sums these totals is there.

T _n [° C.] represents the reached temperature at the n-th step in each temperature range, and t _n [second] represents the total elapsed time up to the n-th step in each temperature range. When the maximum heating temperature does not reach T ^* , the value of the second term (the sum of the calculated values of the _Gn term) is set to zero. C ^* , Si ^* , Mn ^* , Cr ^*, and Mo ^* [mass%] indicate a simple average of chemical compositions of the two types of base steel sheets, and 0 is substituted when the element is not included. To do. r is the thickness ratio of the two types of base steel plates excluding the welded portion, and is the ratio of the base steel plate having a large thickness to the thickness of the base steel plate having a small thickness, and the thickness of the base steel plate is If equal, r = 1. α, β, γ and δ, ε, θ are constant terms, respectively, 1.33 × 10 ⁶ , 1.80 × 10 ⁰ , 2.25 × 10 ⁴ and 2.25 × 10 ⁶ , 2.20, respectively. × 10 ⁰ , 2.41 × 10 ⁴ Further, T ^* is a temperature that is a standard for starting dissolution of carbide, and is obtained by the following equation (3).

Formula (3) is a formula consisting of A _c1 [° C.], chemical composition [mass%], and the plate thickness ratio r shown in Formula (2) in each steel. Here, the subscripts 1 and 2 in the parentheses on the right shoulder of the element represent the above-mentioned two types of base steel plates, the steel plate having the thin plate thickness being steel 1, and the steel plate having the thick plate thickness being steel. 2. When steel 2 does not have A _c1 , T ^* is assumed to be equal to A _c1 of steel 1.

The points A _c1 and A _{c3 of the} steel sheet are the starting point and the completion point of the reverse transformation to austenite in the heating step, respectively. Specifically, prior to heat treatment, a small piece is cut out from the steel sheet after hot rolling. It is obtained by heating up to 1200 ° C./sec and measuring the volume expansion during that time.

  When the temperature history in the heat treatment satisfies the formula (2), the carbide in the HAZ and the welded portion becomes sufficiently fine, so that the strength and impact resistance of the hot press-formed member are improved. From this viewpoint, the left side of the formula (2) is preferably −0.70 or more, and more preferably −0.40 or more.

In the steel sheet for hot pressing according to the present invention, the temperature history during cooling needs to satisfy the formula (4) in order to reduce the strength of the steel sheet and improve the workability. Equation (4) is an equation representing the dissolution behavior of carbides and the formation behavior of fine carbides. The smaller the value of equation (4), the smaller the contribution of carbon to strengthening the steel sheet. In order to reduce the strength of the steel sheet for hot pressing and improve the workability, the left side of the formula (4) is preferably −0.20 or less, and more preferably −0.40 or less. On the other hand, if the left side of the formula (4) is excessively small, coarse carbides are formed and dissolved until hot pressing may impair the characteristics. Therefore, the left side of the formula (4) is −40.00 or more. It is preferable to set it to -30.00 or more.

  However, equation (4) divides the residence time in the temperature range from 650 ° C. to 100 ° C. where the formation of carbides begins in the cooling process into 10 steps, and evaluates the formation behavior of carbides in each step and adds them up It is a thing.

Here, A is a parameter representing the dissolved state of carbide, and when the left side of the formula (2) is 1.00 or more, 1.00 is used, and otherwise, the value of the left side of the formula (2) is used. . ΔT [° C.] is a value corresponding to the degree of supercooling in the formation of carbide, starting from a temperature 100 ° C. lower than the T ^* obtained by the equation (3), and reaching the lowest in the section from that point to the nth step. It is a value obtained by subtracting the temperature Tmin.

Note that ΔT is 0 when T _min is higher than T ^* −100 ° C. Tn [° C.] is an average temperature in the n-th section. Also, t _n [seconds] is the total elapsed time from reaching 650 ° C. until the nth step is completed. μ, η, ζ, and ρ are constant terms, which are 5.53 × 10 ⁻³ , 2.50 × 10 ⁻¹ , 2.50 × 10 ⁻² , and 3.07 × 10 ³ , respectively.

  The steel sheet for hot pressing after the heat treatment may be tempered to further improve the workability. If the tempering temperature exceeds 600 ° C., the carbides become excessively coarse and the characteristics of the hot press-formed member are deteriorated. Therefore, the tempering temperature is preferably 600 ° C. or lower. Further, if the tempering temperature is lower than 100 ° C., a sufficient effect cannot be obtained, and therefore the tempering temperature is preferably 100 ° C. or higher. The tempering time is not particularly specified, and may be set as appropriate according to the processing temperature and target characteristics.

  The steel plate after the heat treatment may be subjected to skin pass rolling with a maximum reduction ratio of 2.00% for the purpose of correcting the shape.

(Hot press material)
Subsequently, in the hot press-formed member of the present invention, for the butt weld including the steel plate satisfying the formula (1), the reason why the HAZ is limited in the steel plate 1, the steel plate 2, the welded joint, and the steel plate 1 and the steel plate 2 sandwiching the weld portion. Will be described.

[Product of hardness and thickness HT]
Cracks at the time of collision include cracks due to strain concentration and cracks due to insufficient toughness, and the ease of occurrence of cracks due to strain concentration in the weld zone and HAZ can be arranged by the product HT of the hardness and plate thickness at that location. Since HT corresponds to the load resistance at the location, when deformation is applied to the hot press-formed member, the deformation tends to concentrate at a location where the HT is lower than the periphery, that is, a location where the load resistance is low. Therefore, when the HT in the welded part or HAZ is remarkably small compared with the steel plate part not affected by the welding, when the member collides, strain may concentrate on the part where the HT is small and crack.

In order to avoid such concentration of strain, the HT in the weld zone and the HAZ should not be excessively small with respect to the smaller HT side of the butt welded steel plate. Specifically, in the case of butt welding shown in FIG. 1, in order to avoid strain concentration, the minimum value HT _min in the distribution of HT in the region including the weld and HAZ is the average value HT ₁ in the steel plate 1. it is required to be more than 0.80 times the smaller of the average value HT ₂ in the steel plate 2. The relationship between the two is preferably 0.85 times or more, more preferably 0.90 times or more, and most preferably the two are equal. The average value HT ₂ in the average value HT ₁ and the steel plate 2 in the steel sheet 1 is the average value of the hardness in the steel sheet area not including the welds and HAZ.

On the other hand, in places where HT is extremely high compared to the surrounding area, even if a load is applied, the deformation does not easily occur. In order to avoid this, the HT in the weld zone and the HAZ should not be an excessively large value with respect to the steel plate having the larger HT among the butt welded steel plates. Specifically, in the case of butt welding shown in FIG. 1, in order to avoid strain concentration, the maximum value HT _max in the HT distribution in the steel plate 1 to the steel plate 2 in the region including the weld and HAZ is the steel plate 1. or less is required 1.20 times the larger of the average value HT ₂ in the average value HT ₁ and the steel plate 2 in. The relationship between the two is preferably 1.15 times or less, more preferably 1.10 times or less, and most preferably the two are equal.

[Maximum hardness H _max ]
On the other hand, the ease of cracking at the time of collision due to insufficient toughness can be organized by hardness. When the hardness in the welded part and the HAZ is extremely high as compared with the surrounding steel plates, there is a risk that the part is greatly embrittled as compared with the steel plates, and may break at the time of collision. Specifically, in the case of butt welding shown in FIG. 1, the maximum value H _max of the hardness from the steel plate 1 to the steel plate 2 in the region including the butt weld and HAZ, the hardness H ₁ in the steel plate _1, and the steel plate 2 If the difference ΔH from the larger value of the hardness H ₂ exceeds 50 [Hv], cracks may occur during press molding, so the upper limit of ΔH is set to 50 [Hv]. ΔH is preferably as small as possible, and more preferably 35 [Hv] or less.

  In addition, the measuring method of the hardness of the steel plate and welding part in a hot press-forming member is the same as the measuring method in the steel plate for hot press.

[Maximum value D _max of parent phase austenite grain size]
In the impact-resistant welded part and HAZ of the member after hot pressing, it is necessary to reduce the crystal grain size of the parent phase austenite during hot pressing in order to suppress propagation of fracture at the time of collision. The crystal grain size of the parent phase austenite is greatly influenced by the crystal grain size in the steel sheet for hot press, that is, when the crystal grain size of the steel sheet for hot press is coarse, the austenite grain size is also coarsened. The impact resistance of the hot press-formed member tends to deteriorate.

Specifically, in the case of butt welding shown in FIG. 1, the maximum value D _max of the parent phase austenite grain size and the parent phase in the steel plate 1 in the region from the steel plate 1 to the steel plate 2 including the butt weld and HAZ. by formed to ratio between the value D of the larger of the parent phase austenite grain size D ₂ in the austenite grain size D ₁ and the steel plate 2 and 5.0 or less, impact resistance is improved. This ratio is preferably 4.0 or less, more preferably 3.0 or less, and most preferably both are equal.

  A method for measuring the matrix austenite particle size will be described. First, corrosion is performed using saturated picric acid after mirror polishing in the same plane as the surface on which the hardness was measured. Next, the microstructure is observed centering on the intermediate point of the hardness measurement points, one or more straight lines having a total length of 100 μm or more are drawn in the thickness direction, and the average interval between the grain boundaries is measured by a cutting method. The measured value of the average grain boundary interval is regarded as the parent phase austenite grain size.

[Coarse carbides]
In the welded part and the weld heat affected zone, coarse carbides act as a starting point for brittle fracture and / or a propagation path. Therefore, it is necessary to reduce the coarse carbides in order to improve the impact resistance characteristics of the hot press-formed member. Since carbide having a particle diameter of 1.0 μm or more works as a starting point of fracture and / or a propagation path, the average density of the carbide in the welded portion and the weld heat affected zone is set to 1.0 × 10 ¹⁰ m ⁻² or less. In order to improve the impact resistance characteristics of the hot press-formed member, the average density of the carbide is preferably 5.0 × 10 ⁹ m ⁻² or less.

A method for measuring the average density of carbide will be described. In the same plane as the surface on which the matrix austenite grain size is measured and the hardness is measured, it is mirror-polished and then corroded with nital, and the microstructure is observed around the midpoint of the hardness measurement point. In the measurement, an area of 5.0 × 10 ⁻¹⁰ m ² or more per field of view is taken and 3 fields or more are taken, and a simple average of the density of the three fields of view is taken as the average carbide density in the weld zone and the weld heat affected zone.

(Method for producing hot press-formed member)
Then, the manufacturing method of the hot press molding member of this invention is demonstrated.
The hot press-formed member of the present invention can be obtained by subjecting the hot press steel sheet of the present invention to hot pressing under appropriate conditions. When the chemical composition of at least one of the two or more steel plates constituting the hot-press steel plate satisfies the formula (1), the hardness of at least a part of the hot-press formed member is 300 Hv or more.

  In addition, the maximum effective crystal grain size in the weld zone and HAZ in the steel sheet for hot press is 5.0 times or less of the effective crystal grain size in the base steel plate, so that the base in the weld zone and HAZ after hot press The maximum crystal grain size of the phase austenite is not more than 5.0 times the matrix phase austenite grain size in the base steel sheet.

  Prior to hot pressing, the steel sheet for hot pressing of the present invention may be preformed. As a pre-forming process, cutting, punching, pressing, bending, drawing, roll forming, and burring may be performed as long as the steel sheet for hot pressing does not break. The preforming process is not limited to the above example, and any process may be performed as long as it is within the forming limit of the steel sheet for hot pressing.

In performing the hot press forming by heating the hot press steel plate of the present invention, the maximum heating temperature is set to _Ac3 or higher in the base steel plate satisfying the formula (1). When the maximum heating temperature is less than _{Ac3 of the} base material steel plate, soft ferrite remains in the microstructure before hot press forming, so that the hardness and / or toughness of the part deteriorates. In order to increase the strength of the hot press-molded member, the maximum heating temperature is preferably A _c3 + 10 ° C. or higher. On the other hand, even if the maximum heating temperature exceeds A _c3 + 120 ° C, the effect of increasing the strength of the part is not recognized, and the matrix austenite is coarsened and the toughness is impaired, so the maximum heating temperature is set to A _c3 + 120 ° C or less. It is preferable.

Further, in the heating before hot pressing, the temperature from 550 ° C. to the end of heating needs to satisfy the formula (5). Equation (5) is an equation for evaluating the growth and dissolution behavior of carbides in the weld zone and HAZ. The smaller the left side of equation (5), the coarser the carbide is formed in the hot press-formed member, and the impact resistance is higher. to degrade. In order to improve impact resistance, the left side of the formula (5) is preferably 2.00 or more, and more preferably 3.00 or more. In addition, the left side of Formula (5) is the same format as Formula (2) which evaluates the growth / dissolution behavior of carbide during heating in the manufacturing process of the steel sheet for hot pressing, and the meaning and value of each term are the same. It is.

  Moreover, when the short diameter of the carbide | carbonized_material in the weld part and HAZ in a hot press base material is excessively large compared with the short diameter of the carbide | carbonized_material in a base material steel plate, even if heating conditions satisfy | fill Formula (5), it is coarse. Carbide may form and impact resistance may deteriorate.

  The hot press forming start temperature is set to 600 ° C. or higher in a hot press steel plate in a portion made of a base steel plate whose chemical composition satisfies the formula (1). When the temperature of the part is lower than 600 ° C., soft ferrite starts to be formed, and there is a concern that strength and / or impact resistance deteriorate. The starting temperature of hot press molding is preferably 650 ° C. or higher.

  After starting the hot press molding, the average cooling rate in the temperature range of 600 ° C to 300 ° C is set to 10 ° C / second or more. When the cooling rate in the temperature range is insufficient, soft and low toughness bainite is generated, and the impact resistance of the steel sheet deteriorates. The average cooling rate in the temperature range is preferably 20 ° C./second or more.

  Moreover, when the plate | board thickness ratio of the base material steel plate in the steel plate for hot press exceeds 3.0, the cooling rate in the said temperature range may fall locally or increase. The local fluctuation of the cooling rate in the temperature range causes a local strength decrease and / or strength increase in the hot press-formed member, thereby impairing the impact resistance of the hot press-formed member.

  The hot press-formed member may be tempered in the range of 100 to 500 ° C. in order to improve impact resistance.

  The hot press-formed member may be subjected to shot blasting in order to improve the appearance.

Next, examples of the present invention will be described.
Slabs having chemical compositions A to BM shown in Table 1-1 and Table 1-2 were cast. Using these slabs, Experimental Examples 1 to 49 having combinations shown in Tables 2-1 to 2-4 were manufactured. First, the slabs for the steel plates 1 and 2 constituting Experimental Examples 1 to 49 are heated to the slab heating temperatures shown in Tables 2-1 to 2-4, and in the temperature range from the rolling start temperature to the rolling completion temperature. Hot rolled. Then, it cooled to the cooling start time shown to Table 2-1-Table 2-4, cooled to the cooling stop temperature with the average cooling rate, and wound up as a coil.

In addition, the A _c1 temperature and the A _c3 temperature in Table 1-1 and Table 1-2 are volume changes when a small piece is cut out from a hot-rolled steel sheet having each chemical composition and heated to 1050 ° C. at a heating rate of 10 ° C./sec. Was measured and read from the inflection point of the volume expansion curve.

  Then, the hot-rolled steel sheet was pickled and cold-rolled to obtain the total rolling reduction shown in Tables 2-1 to 2-4 to obtain a cold-rolled steel sheet to be used for welding. Note that the hot-rolled steel sheet is subjected to welding under the condition where the cold rolling rate is 0%. In addition, a part of the hot-rolled steel sheet used for welding was plastically deformed by applying tension to correct the shape.

  Subsequently, the steel plates 1 and 2 were welded in combinations shown in Tables 2-1 to 2-4. However, Experimental Examples 31 to 33 are examples in which three base materials are welded to form one hot pressing steel plate. In Experimental Examples 31 to 33, the steel plate 1 has a structure welded to the steel plate a2 and the steel plate 2b, and the steel plate a2 and the steel plate 2b correspond to the steel plate 2 of another experimental example.

  Prior to welding, the butt portion was cut to obtain an end portion having excellent linearity. In particular, Experimental Examples 1 to 21 are examples in which an end portion after cutting is tapered.

  Experimental Examples 34 and 35 are comparative examples in which steel sheets are welded in combinations shown in Table 2-3 after the steel sheet is subjected to heat treatment described in Table 3-2, which will be described later. In particular, Experimental Example 35 is an example in which post-heat treatment is performed by applying a laser to the welded portion after welding and heating.

  In other experimental examples, the steel plates were welded prior to the heat treatment. Experimental examples 17 and 18 are examples in which welding is performed by a mash seam welding method. In other experimental examples, welding was performed by a laser welding method.

  Experimental Examples 2-18 and 30-33 are examples in which the surface of the welded portion is ground before the heat treatment.

  Experimental example 28 is an example in which the steel plate after welding is subjected to pickling again before heat treatment.

  Experimental Examples 3 to 5 are examples in which a pre-plating treatment of Ni is performed on the welded steel plate before the heat treatment.

  Experimental Examples 31 to 33 are examples in which three steel plates are connected by welding to obtain one hot pressing steel plate. As shown in FIG. 8, in each of Experimental Examples 31 to 33, a steel plate 1 is formed at a welded portion a obtained by welding a steel plate 2a to an end portion of the steel plate 1, and an end portion of the steel plate 1 on the opposite side of the steel plate 2a. And a steel plate for hot pressing having a welded portion b obtained by welding the steel plate 2b.

Subsequently, the steel plate after welding was subjected to heat treatment under the conditions shown in Tables 3-1 and 3-2. Heat treatment was performed by heating the steel sheet to the heating temperatures shown in Tables 3-1 and 3-2 under the heating conditions represented by the formula (2). As each _{A c1} temperature of Example 1 to 49 were adopted _{A c1} temperature of the steel sheet 1.
Then, it cooled to the temperature range below 100 degreeC on the cooling conditions represented by Formula (4). Thereafter, some steel plates were subjected to tempering treatment and / or skin pass rolling treatment.

  Subsequently, a small piece was cut out from the steel plate for hot press after the heat treatment, and the hardness, microstructure and workability were confirmed. The workability was evaluated by measuring the maximum load by a tensile test. The maximum load of the base metal part was evaluated using a JIS No. 5 test piece described in JIS Z 2201 with the direction perpendicular to the weld line as the tensile axis. Other conditions were in accordance with the tensile test method described in JIS Z 2241.

These measurement results are shown in Tables 4-1 to 4-4. In Tables 4-1 and 4-2, H ₁ and HT ₁ are the average hardness of the steel plate 1 and the average value of HT, respectively. H ₂ and HT ₂ are the average hardness of the steel plate 2 and the average value of HT, respectively. It is. Further, [Delta] H is the difference between the value of the larger of Experimental Example 1 to 49 and the maximum value of the hardness of the region containing the respective butt welds and heat affected zone, the H ₁ and H _2.

  The workability of the weld was evaluated using two types of tensile test pieces. The first was a JIS No. 5 test piece, and a test piece was prepared and evaluated by placing the weld line in the center of the test piece with the direction perpendicular to the weld line as the tensile axis. The maximum load in this tensile test is an index of the likelihood of strain concentration around the weld due to static deformation. If the maximum load is 0.80 or more times the maximum load in the tensile test of the base metal part, it can be judged that strain concentration around the welded part due to static deformation is difficult to occur, and Can be expected to have moldability equivalent to the base material.

  On the other hand, if the maximum load of the tensile test piece including the weld is greatly inferior to the maximum load indicated by the base metal part, the strain concentrates around the weld during deformation, and the predetermined shape cannot be obtained, and the steel plate breaks. There is also a case. Specifically, there is a concern that the periphery of the welded portion may be cracked by pre-pressing or bending prior to hot pressing.

  The second is a test piece with a notch shown in FIG. 6, and a test piece is prepared by arranging the weld line at the center of the test piece with the direction perpendicular to the weld line as the tensile axis and aligning the center of the weld line with the notch bottom. ,evaluated. The notch bottom radius is 1.5 mm. The interval between the notch bottoms was 25 mm. The maximum load in this tensile test serves as an index representing the fracture strength around the welded part due to dynamic deformation. When the maximum load is 0.80 times or more the maximum load in the tensile test of the base material part, it can be determined that the welded part is difficult to brittle fracture, and the steel sheet can be expected to have fracture resistance equivalent to the base material part.

  On the other hand, when the maximum load of the notched tensile test piece is greatly inferior to the maximum load indicated by the base material, brittle fracture is likely to occur around the weld. Specifically, there is a concern that the periphery of the welded portion may be broken by cutting or punching prior to hot pressing.

Subsequently, the hot press-formed steel plates of Experimental Examples 1 to 49 were subjected to hot press molding under the conditions shown in Tables 5-1 and 5-2 to manufacture hot press-formed members of Experimental Examples 1 to 49. The shape of the member was the hat shape shown in FIG. Heat treatment was applied to the heating temperatures shown in Tables 5-1 and 5-2 under the heating conditions indicated by the value of the formula (5). The maximum heating temperature in the heat treatment is shown in the item “heating temperature” in Tables 5-1 and 5-2. Further, as _{"A c3} temperature" in the heat treatment, it was adopted _{A c3} temperature of the steel sheet 1.

  After the completion of heating, the product was air-cooled to the press start temperatures shown in Tables 5-1 and 5-2, and cooled with a mold by press molding. The cooling rates between 600 and 300 ° C. during cooling were as shown in Tables 5-1 and 5-2. After cooling to room temperature, some members were tempered.

  Next, a small piece is cut out from the hot press-formed member, and the hardness, microstructure and impact resistance are confirmed. The cutout position of the small piece is as shown in FIG. The evaluation of impact resistance is the same as the method for evaluating workability in a steel sheet for hot pressing, and the evaluation criteria are also the same.

The measurement results of the microstructure and impact resistance are shown in Tables 6-1 to 6-4. The items “H ^* ₁ ” in Table 6-1 and Table 6-2 are the average hardness of the steel plate 1 of the hot press-formed member, and “HT ^* ₁ ” is the hardness of the steel plate 1 of the hot press-formed member. And the average value of the product HT of the plate thickness. “HT ^* ₂ ” is the average value of the product HT of the hardness and thickness of the steel plate 2 of the hot press-formed member. In addition, the item “D” of “matrix austenite grain size” is the matrix phase austenite grain size D ₁ in the steel plate 1 of each hot press-formed member of Experimental Examples 1 to 49 and the matrix phase austenite grain size D in the steel plate 2. _{2 is} the larger value D, and the item “D _max ” is the maximum value of the matrix austenite grain size in the hot-press formed members of Experimental Examples 1 to 49.

  Experimental Examples 22 to 26 are examples in which all the base materials used in the hot-press steel sheet do not satisfy the formula (1), and the hardness of the hot-press formed member is less than 300 Hv, and sufficient strength is obtained. This is not an example.

  Experimental Example 34 is an example of a hot press steel plate manufactured by a tailored blank method and a hot press-formed member obtained using the steel plate. In the steel sheet for hot pressing obtained in this experimental example, a part of the welded portion is cured, so that workability is inferior. Furthermore, in a hot press-formed member obtained using this hot-press steel plate, the crystal grain size of the parent phase austenite is partially increased, so that the impact resistance characteristics of the member are inferior.

  Experimental Example 35 is an example in which a steel sheet for hot pressing is manufactured by a tailored blank method, a weld is post-heat treated with a laser, and a member is obtained by a hot pressing method. In the steel sheet for hot pressing obtained in this experimental example, the local hardening of the welded portion is eliminated by the effect of the post heat treatment. However, the post-heat treatment cannot suppress local softening or coarsening of the crystal grain size in the weld zone, and the workability of the hot-press steel sheet is inferior. Furthermore, in a hot press-formed member obtained using this hot-press steel plate, the crystal grain size of the parent phase austenite is partially increased, so that the impact resistance characteristics of the member are inferior.

  Experimental examples 36 and 37 are examples in which the thickness ratio of the base material of the hot-press steel sheet exceeds 3.00. In the heat treatment after welding, the temperature around the welded part becomes extremely heterogeneous, This is an example in which the workability of the steel sheet for hot pressing is inferior due to partial hardening. Further, the hot-pressed steel sheet obtained by cutting the hot-press steel sheet into a predetermined size by laser trim and obtained under the conditions shown in Tables 5-1 and 5-2 is a welded part during heating and pressing. The temperature at the periphery becomes extremely inhomogeneous, and the periphery of the weld is locally hardened or softened, so that the impact resistance of the member becomes inferior.

  Experimental examples 38 and 39 are examples in which the heating temperature in the heat treatment after welding is low, and in the steel sheet for hot pressing, coarse carbide exists in the vicinity of the welded portion. The impact resistance characteristics of the member are inferior.

  Experimental examples 40 and 41 are examples in which the heating condition in the heat treatment after welding does not satisfy the formula (2), a softened region is formed around the welded portion, and the workability of the steel sheet for hot pressing is inferior. is there.

  Experimental example 42 is an example in which the cooling condition in the heat treatment after welding does not satisfy the formula (4), and excessive carbides grow during cooling, so there is coarse carbide around the welded part in the hot-press steel sheet. Therefore, the density of coarse carbides in the hot press-formed member is high, and the impact resistance characteristics of the member are inferior.

  Experimental Example 43 is an example in which the cooling condition in the heat treatment after welding does not satisfy the formula (4), and carbides are not sufficiently generated during cooling, so that the strength of the steel sheet for hot pressing becomes excessive, and for hot pressing. The workability of the steel sheet is inferior.

  Experimental Examples 44 to 49 are examples in which a steel sheet for hot pressing excellent in workability is obtained according to the present invention. However, in these examples, the hot press conditions shown in Table 5-1 and Table 5-2 are out of the scope of the present invention, and therefore, a hot press molded member having excellent impact resistance according to the present invention cannot be obtained.

  Experimental Examples 44 to 46 are examples in which the heating temperature in the hot press is low, and the hardness in the hot press-formed member is less than 300 Hv, and sufficient strength cannot be obtained.

  Experimental Examples 47 to 49 are examples in which the heating condition in the hot press does not satisfy the formula (5), and a large number of coarse carbides exist in the hot press-formed member, so that the impact resistance of the member is inferior. .

  Experimental Examples 1-21 and Experimental Examples 27-33 produce hot-press steel sheets with excellent workability according to the conditions of the present invention, and the obtained hot-press steel sheets are hot-pressed according to the conditions of the present invention. This is an example of obtaining a hot press-formed member having excellent impact resistance.

  Experimental examples 9, 12, 15, and 29 are examples in which a steel sheet for hot pressing is obtained by performing a tempering treatment after the heat treatment in order to improve workability.

  Experimental Example 3 is an example in which, in order to improve workability and corrosion resistance, an alloying treatment is performed by reheating after heat treatment, dipping in a molten zinc bath, and further reheating to 488 ° C., and having an alloyed hot dip galvanized layer. Furthermore, a hot-press steel sheet having improved workability due to the effect of tempering by reheating is obtained.

  Further, Experimental Examples 3, 7, 9, and 15 are examples in which a tempering process is performed on a hot press-formed member after hot pressing, and a hot press-formed member having excellent impact resistance is obtained.

  As mentioned above, although each embodiment of this invention was described in detail, all the said embodiment showed only the example of actualization in implementing this invention. The technical scope of the present invention should not be limitedly interpreted by these embodiments. That is, the present invention can be implemented in various forms without departing from the technical idea or the main features thereof.

In particular, the shape of the hot press-formed member of the present invention is not limited to the hat shape shown in FIG. If the heating and cooling conditions shown as the method for producing a hot press-formed member of the present invention are satisfied, the shape of the hot press-formed member may be an arbitrary shape.
Further, four or more steel plates may be welded to obtain a hot press steel plate.

  The steel sheet for hot pressing according to the present invention is excellent in workability and contributes to the reduction of production cost in the hot pressing process and the improvement in dimensional accuracy of the hot press forming member, and is obtained using the steel sheet for hot pressing. The hot press-formed member of the present invention having excellent impact resistance is suitable for application to the body of an automobile.

Claims (8)

Consisting of different steel plates and their butt welds,
The chemical composition of at least one steel plate among the different steel plates satisfies the formula (1),
The minimum value HT _min in the distribution of the product HT of the hardness and the plate thickness HT in the region including the butt weld and the heat affected zone is an average value HT _{1 in one} of the different steel plates and the other of the different steel plates. It is 0.80 times or more the smaller value of the average value HT _{2 in} the steel sheet,
The maximum value HT _max in the HT distribution is not more than 1.50 times the larger value of HT ₁ and HT ₂ ;
The difference ΔH between the maximum value H _max of the hardness of the region including the butt weld and the weld heat affected zone, the hardness H 1 of the one steel plate, and the hardness H ₂ of the other steel plate is ₁₀₀ Hv or less. And
And H _max is 400 Hv or less,
In the distribution of effective crystal grain size in the region including the butt weld and the weld heat affected zone, the larger of the average value of the effective crystal grain size of the one steel plate and the average value of the effective crystal grain size of the other steel plate The ratio (d _max / d) of the effective crystal grain size d of the above and the maximum value d _max of the effective crystal grain size is 5.0 or less,
Further, in the region including the butt weld and the heat affected zone, the shorter of the average value of the minor axis of carbide of the one steel plate and the average value of the minor axis of carbide of the different steel plate among the different steel plates. A steel sheet for hot pressing, wherein a ratio (r _max / r) of the diameter r to the maximum value r _max of the minor axis of the carbide is 3.0 or less.

However, each element symbol represents the content [% by mass] in the steel sheet, and 0 is substituted when the element is not included. % By mass
C: 0.050% to 0.800%,
Si: 0.001% to 3.00%,
Mn: 0.01% to 13.0%,
P: 0.100% or less,
S: 0.0100% or less,
Al: 0.001% to 2.500%
N: 0.0150% or less,
O: 0.0050% or less,
One steel plate containing the balance of iron and inevitable impurities,
The steel plate is butt welded with one or more other steel plates having different chemical compositions and / or plate thicknesses with a plate thickness ratio of 3.0 or less at the welded portion,
Heat treatment is performed to heat at least one steel plate out of all the welded steel plates to a temperature exceeding (A _c1 -50) ° C.
In the heat treatment, a steel sheet for hot pressing is characterized in that the temperature history from the start of heating to the start of cooling satisfies Equation (2) and the temperature history from the start of cooling to the completion of cooling satisfies Equation (4). Method.

However, the expression (2) divides the time until the temperature of the steel plate reaches the temperature T ^* from 550 ° C. into 10 steps equally, and the expression F _n (T _n , T ^* , r in each divided step is , t _n , C ^* , Si ^* , Mn ^* , Cr ^* , Mo ^* ) are summed up, and the time from reaching the temperature T ^* until starting cooling is divided equally into 10 steps and, G _n in each step of dividing ^{_{(T n, T *, r}} , t n, C *, Si *, Mn *, Cr *, Mo *) sums the calculated values of the sums these totals Is. T _n [° C.] represents the reached temperature at the n-th step in each temperature range, and t _n [second] represents the total elapsed time up to the n-th step in each temperature range. When the maximum heating temperature does not reach T ^* , the total of the calculated values of G _{n in} the second term is 0. C ^* , Si ^* , Mn ^* , Cr ^*, and Mo ^* [mass%] indicate a simple average of the chemical compositions of the two types of steel plates, and 0 is substituted when the element is not included. r is the plate thickness ratio of the two types of steel plates excluding the welded portion, and is the ratio of the thick steel plate to the plate thickness of the thin steel plate, and r = 1 when the steel plate thickness is equal. . α, β, γ and δ, ε, θ are constant terms, respectively, 1.33 × 10 ⁶ , 1.80 × 10 ⁰ , 2.25 × 10 ⁴ and 2.25 × 10 ⁶ , 2.20, respectively. × 10 ⁰ , 2.41 × 10 ⁴ Further, T ^* is a temperature that is a standard for starting dissolution of carbide, and is obtained by the following equation (3).

Here, the subscripts 1 and 2 in the parentheses described on the right shoulder of the element represent the two types of steel plates, respectively, T ^* is A _c1 [° C.] in each steel, Si, Mn in the chemical composition of each steel plate, It is calculated | required from each content [mass%] of Cr and Mo, and plate | board thickness ratio r shown in Formula (2).

However, equation (4) divides the residence time in the temperature range from 650 ° C. to 100 ° C. where the formation of carbides begins in the cooling process into 10 steps, and evaluates the formation behavior of carbides in each step and adds them up It is a thing. Here, A is 1.00 when the left side of the formula (2) is 1.00 or more, and the value of the left side of the formula (2) is used otherwise. Further, ΔT [° C.] is a value obtained by subtracting the minimum temperature T _min that is obtained from the temperature that is 100 ° C. lower than T ^* obtained by the equation (3), and from that point to the n-th step. Note that ΔT is 0 when T _min is higher than T ^* −100 ° C. Tn [° C.] is an average temperature in the n-th section. Also, t _n [seconds] is the total elapsed time from reaching 650 ° C. until the nth step is completed. μ, η, ζ, and ρ are constant terms, which are 5.53 × 10 ⁻³ , 2.50 × 10 ⁻¹ , 2.50 × 10 ⁻² , and 3.07 × 10 ³ , respectively. The chemical composition of the one steel plate is
In place of a part of Fe, further in mass%,
Cr: 0.03-5.00%
Mo: 0.03-5.00%
Ni: 0.03-5.00%
Cu: 0.03-5.00%
W: 0.03-5.00%
B: 0.0004 to 0.0100%
Nb: 0.005 to 0.200%
Ti: 0.010-0.500%
V: 0.05-2.00%
Sb: 0.003 to 1.000%
Sn: 0.005 to 1.000%
Ca: 0.0010 to 0.0100%
Ce: 0.0010 to 0.0100%
Mg: 0.0010 to 0.0100%
Zr: 0.0010 to 0.0100%
La: 0.0010 to 0.0100%
Hf: 0.0010 to 0.0100%
REM: 0.0010 to 0.0100%
One or more types of these are included, The manufacturing method of the steel plate for hot presses of Claim 2 characterized by the above-mentioned.   The method for manufacturing a steel sheet for hot pressing according to claim 2 or 3, wherein the welded portion is ground after butt welding.   The steel sheet according to claim 2, wherein at least one of the one steel sheet and the other steel sheet is a cold-rolled steel sheet obtained by subjecting a hot-rolled steel sheet to 0.01 to 85% cold rolling. The manufacturing method of the steel plate for hot press of any one of them. Having different steel plates and their butt welds,
The minimum value HT ^* _min in the distribution of the product HT ^* of the hardness and the plate thickness of the region including the butt weld and the weld heat affected zone is an average value HT ^* _{1 of one} of the different steel plates and the difference between the different steel plates. Of these, the average value HT ^* ₂ of other steel plates is 0.80 or more times the smaller value,
The HT ^* maximum ^HT _{* max} in the distribution of not more than 1.20 times the greater of the ^HT _{* 1} and ^HT _{* 2,}
The difference ΔH between the maximum value H ^* _max of the hardness of the region including the butt weld and the heat affected zone, and the larger value of the hardness H ^* _{1 of the one} steel plate and the hardness H ^* ₂ of the other steel plate * Is 50 Hv or less, and the larger value of the hardness H ^* ₁ and the hardness H ^* ₂ is 300 Hv or more,
Of the mother phase austenite grain size D ₂ at the maximum value D _max and the one of the other steel sheet matrix phase austenite grain size D ₁ of the steel matrix phase austenite grain size of the region containing the butt weld and heat affected zone The ratio of the larger value is 5.0 times or less,
The hot press-molded member, wherein an average density of carbides having a particle diameter of 1.0 μm or more in the butt weld and the weld heat affected zone is 1.0 × 1.0 × 10 ¹⁰ m ⁻² or less. Using the steel sheet for hot pressing according to claim 1,
The maximum heating temperature is not _lower than the _Ac3 temperature in the steel sheet satisfying the formula (1),
A method for producing a hot press-molded member, comprising performing a heat treatment in which a temperature history from 550 ° C. to the end of heating satisfies Formula (5).

However, the left side of Expression (5) has the same format as Expression (2), and the meaning and value of each term are the same.   The method for producing a hot press-formed member according to claim 7, wherein a tempering treatment is performed after the hot pressing.

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