JP2019014938A - High-strength plating steel plate and method for producing the same - Google Patents

Abstract

To provide a high-strength plating steel plate having excellent corrosion resistance and a method of producing the same.SOLUTION: A high-strength plating steel plate has different steel plates and butt-welding parts thereof, where, a maximum tensile strength of at least one steel plate of the different steel plates is 780 MPa or more, and in the whole steel plate including the butt-welding part and a welding heat-affected zone, a plating layer is provided on the surface.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a high-strength plated steel sheet excellent in corrosion resistance and a method for producing the same.

  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.

  For example, Non-Patent Document 1 introduces “tailored blank welding” in which steel plates having different plate thicknesses, strengths, or plating types are welded to form a steel plate after welding. By adopting this method, high strength steel sheets with thin plate thickness are placed in areas where strength is required, and mild steel sheets are placed in areas where formability is required, resulting in high strength and complex shapes. It is possible to achieve both weight reduction and collision safety.

  There are parts in automobile bodies that require high corrosion resistance, and plated steel sheets are often applied to such parts. When tailored blank welding is applied to a plated steel sheet, the welded portion and the surrounding plating layer are evaporated, and therefore the welded portion and its surroundings are not protected by the plating layer, and there is a problem that the corrosion resistance is inferior to that of the base material. In response to this problem, for example, Patent Document 1 discloses that the peripheral base material of the peripheral base material even if the plating layer disappears by performing plating with sacrificial anticorrosive action on the steel plate surface containing Cu-P-Nb-Mo. It is said that the corrosion resistance can be enhanced by the sacrificial anticorrosive action by the plating layer. However, the corrosion resistance of the portion where the plating layer disappears remains inferior to that of the base material, and this problem cannot be solved.

  Moreover, in patent document 2, it is supposed that deterioration of welding part corrosion resistance can be suppressed by restrict | limiting the chemical composition of a steel plate. However, this technology can achieve a welded part with corrosion resistance equivalent to the base material without plating, and the corrosion resistance of the welded part is inferior to that of the base material with plating. Cannot be resolved.

  Furthermore, Patent Document 3 states that by using a Zn—Al—Mg alloy as the plating type, the sacrificial anticorrosive action can be enhanced as compared with the commonly used zinc plating, and the corrosion resistance of the welded portion can be improved. However, the corrosion resistance of the portion where the plating layer disappears remains inferior to that of the base material, and this problem cannot be solved.

  In addition, when tailored blank welding is applied to high-strength steel sheets, the welded part is heated to a high temperature and then hardened by quenching at the time of welding, so deterioration in formability in the welded part and impact resistance in the welded part are problems. It becomes. In addition, the periphery of the welded portion is a heat affected zone (HAZ) that is heated during welding. However, the HAZ is softened and the formability is deteriorated due to strain concentration on the HAZ, and the HAZ is hardened. Degradation of formability is also a problem.

  On the other hand, for example, Patent Document 4 proposes a means for avoiding the occurrence of cracks in the vicinity of the welded part by limiting the press forming method. Patent Document 5 proposes a technique for obtaining a welded portion having excellent formability in a high-strength steel sheet by limiting the welding conditions and controlling the range of the heat affected zone. However, in these techniques, since the forming method and the welding method are limited, applicable parts and materials are restricted, and an appropriate shape and material may not be selected. In addition, since special equipment is required to apply these techniques, the production cost also increases.

  In order to improve the formability of the welded part, for example, Patent Document 6 and Patent Document 7 propose a technique of suppressing the hardness of the welded part by applying a heat treatment around the welded part before or after welding. However, when heat is applied to the welded portion, the surrounding steel plates are also heated in the same manner, so that the characteristics are deteriorated, and there is a case where destruction during molding or shape failure occurs. Further, the formability and impact resistance of the HAZ by welding and / or post heat treatment may deteriorate.

  Patent Documents 8 and 9 propose high-strength steel sheets with small HAZ softening that limit the chemical composition of the steel sheets. However, no mention is made of the deterioration of the HAZ impact resistance, or the deterioration of the weld formability and impact resistance.

  Patent Documents 10 to 12 propose steel sheets that limit the chemical composition of the steel sheets and have excellent weldability and HAZ formability with controlled microstructure. However, there is no mention of impact resistance degradation in the weld zone and HAZ.

  As described above, it is possible to reduce the weight of the vehicle body by welding and using steel plates with different characteristics, but it is difficult to increase the corrosion resistance of the welded parts, and the use of such steel plates is limited. Was an issue. Moreover, it is required to further improve the impact resistance characteristics in the welded part without impairing the corrosion resistance.

JP-A-5-255806 JP 2013-53330 A International Publication No. 2014/156671 JP 2006-218501 A JP 2006-218500 A JP 2009-721 A JP-A-5-9561 JP 2000-87175 A JP 2000-178654 A JP 2000-290749 A JP 2003-231941 A JP 2007-277729 A

Book that understands iron and steel, ISBN 4-534-03835-6

  In view of the demand for improved corrosion resistance in addition to improving the formability-strength balance in high-strength steel sheets having a tensile strength of 780 MPa or more, the present invention provides a high-strength steel sheet (plated steel sheets) having a tensile strength of 780 MPa or more. It is an object of the present invention to provide a high-strength steel plate having a butt-welded joint that includes two or more types of steel plates having different yield strengths and / or different plate thicknesses, including butt weld joints, and a method for producing the same.

  The present inventors have intensively studied a method for solving the above problems. As a result, the surface of the welded portion and the heat of welding in a steel plate composed of two or more types of steel plates having different yield strength and / or thickness, including high strength steel plates (including plated steel plates) having a tensile strength of 780 MPa or more, and welds thereof. It has been found that the entire surface of the steel sheet including the surface of the affected part has a plated layer, so that both improvement in formability-strength balance and corrosion resistance can be achieved.

Furthermore, the hardness, plate thickness, and crystal grain size at the butt weld and its vicinity formed by butt welding the steel plate 1 and the steel plate 2 as shown in FIG. 1 contribute to the formability and impact resistance characteristics of the butt weld joint. I studied the effect of this. As a result, it has been found that the following requirements can improve the formability and impact resistance of the butt-welded joint in addition to the corrosion resistance.
(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.

  This invention was made | formed based on the said knowledge, and the summary is as follows.

但し、式（１）は、鋼板の温度が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式Ｆ_n(T_n, T^*, r, t_n, C^*, Si^*, Mn^*, Cr^*, Mo^*)の計算値を合計するものである。Ｔ_ｎ［℃］はｎステップ目における到達温度を、ｔ_ｎ［秒］はＴ^＊に到達してからｎステップ目までの総経過時間をそれぞれ表わす。Ｃ^＊、Ｓｉ^＊、Ｍｎ^＊、Ｃｒ^＊およびＭｏ^＊は、前記２種の鋼板の化学組成のＣ、Ｓｉ、Ｍｎ、Ｃｒ及びＭｏのそれぞれの含有量［質量％］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はそれぞれ定数項であり、それぞれ２．２５×１０^６、２．２０×１０^０、２．４１×１０^４とする。また、Ｔ^＊は下記の式（２）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％］、および板厚比ｒから求められる。但し、当該元素が含まれないときは、０を代入する。
（１０）前記鋼板の溶接後、めっき処理の前に、前記鋼板のうち少なくとも１つの鋼板の（Ａ_ｃ１＋４０）℃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（３）を満たすことを特徴とする、（７）〜（９）のいずれかに記載の高強度めっき鋼板の製造方法。

但し、ｖはＡ_ｃ１からＡ_ｃ１＋４０℃の区間における平均加熱速度［℃／秒］であり、ｋは２つの鋼板の平均冷間圧延率［％］をそれぞれ示す。
（１１）前記熱処理のうち、加熱を開始してから冷却を開始するまでの加熱工程において、予熱バーナーに用いる空気と燃料ガスの混合ガスにおいて、単位体積の混合ガスに含まれる空気の体積と、単位体積の混合ガスに含まれる燃料ガスを完全燃焼させるために理論上必要となる空気の体積との比である空気比：０．７〜１．２とされた条件の酸化帯において加熱し、次いで、水蒸気（Ｈ_２Ｏ）と水素（Ｈ_２）との分圧比Ｐ（Ｈ_２Ｏ）／Ｐ（Ｈ_２）：０．０００１〜２．０とされた還元帯において最高加熱温度まで加熱することを特徴とする、（９）または（１０）に記載の高強度めっき鋼板の製造方法。
（１２）突き合わせ溶接後、めっき処理の前に、溶接部を研削することを特徴とする（７）〜（１１）のうちいずれかに記載の高強度めっき鋼板の製造方法。
（１３）前記熱処理前に、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Ａ_ｃ１温度以上に加熱する予備熱処理を１回以上施すことを特徴とする（９）〜（１２）のうちいずれかに記載の高強度めっき鋼板の製造方法。
（１４）前記１つの鋼板及び他の鋼板のうち１種以上が下記式（４）を満たす化学組成を有することを特徴とする（７）〜（１３）のうちいずれかに記載の高強度めっき鋼板の製造方法。

但し、式（４）中の元素記号は前記１つの鋼板及び他の鋼板における各元素の含有量［質量％］を示し、当該元素が含まれないときは、０を代入する。
（１５）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（７）〜（１４）のうちいずれかに記載の高強度めっき鋼板の製造方法。
（１６）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、Ａｃ_３以上の温度まで加熱した後に１．０℃／秒以上の速度で冷却する予備熱処理を施した鋼板であることを特徴とする（９）〜（１５）のうちのいずれかに記載の高強度めっき鋼板の製造方法。
（１７）めっき処理が溶融亜鉛めっき処理であることを特徴とする（７）〜（１６）のいずれかに記載の高強度めっき鋼板の製造方法。
（１８）めっき処理の後に合金化処理を施すことを特徴とする（１７）に記載の高強度めっき鋼板の製造方法。 (1) consisting of different steel plates and their butt welds,
The maximum tensile strength of at least one of the different steel plates is 780 MPa or more,
A high strength plated steel sheet having a plated layer on the entire surface of the steel sheet including the butt weld and the weld heat affected zone.
(2) 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 of the different steel plates. of not less than 0.80 times the smaller value among the average values HT ₂ in the other 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. The high-strength plated steel sheet according to (1), characterized in that
(3) In the distribution of the effective crystal grain size in the region including the butt weld and the weld heat affected zone, 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 of the larger effective crystal grain diameter d to the maximum value d _max of the effective crystal grain diameter is 5.0 or less, and the high-strength plating according to (1) or (2) steel sheet.
(4) Residual austenite volume ratio V in the steel sheet on the side with a large amount of retained austenite in the distribution of the volume ratio of retained austenite in the region including the butt weld and the heat affected zone, and the maximum residual from the one steel sheet to the other steel sheet high strength plated steel sheet according to any one of and a difference austenite volume fraction V _max is equal to or less than 5.0% (1) to (3).
(5) The high strength plated steel sheet according to any one of (1) to (4), wherein the entire surface of the steel sheet including the welded portion and the weld heat affected zone has a galvanized layer.
(6) The high strength plated steel sheet according to (5), wherein the galvanized layer is an alloyed galvanized layer.
(7) By mass%
C: 0.020% or more and 0.800% or less,
Si: 0.001% to 3.00%,
Mn: 0.01% or more and 25.00% or less,
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 and other steel plates having different chemical compositions and / or plate thicknesses are butt welded with a plate thickness ratio of 3.0 or less at the weld,
A method for producing a high-strength galvanized steel sheet, characterized by performing a plating treatment after welding.
(8) 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 0.0004-0.0100%
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 0.0010-0.0100%
Mg 0.0010-0.0100%
Zr 0.0010-0.0100%
La 0.0010-0.0100%
Hf 0.0010 to 0.0100%
REM 0.0010-0.0100%
Any one of these is included, The manufacturing method of the high strength plated steel plate as described in (7) characterized by the above-mentioned.
(9) After the welding of the steel plate, before the plating treatment, a heat treatment is performed to heat to a temperature exceeding the _Ac1 temperature of at least one of the steel plates,
The method for producing a high-strength plated steel sheet according to (7) or (8), wherein in the heat treatment, a temperature history from the start of heating to the start of cooling satisfies the formula (1).

However, Formula (1) divides time from the time when the temperature of the steel plate reaches T ^* [° C.] until the start of cooling into 10 steps equally, and Formula F _n (T _n in each divided step is _{^{, T *, r, t n}} , C *, Si *, Mn *, Cr *, is to sum the calculated values of Mo ^*). T _n [° C.] represents the temperature reached at the n-th step, and t _n [second] represents the total elapsed time from reaching T ^* to the n-th step. C ^* , Si ^* , Mn ^* , Cr ^*, and Mo ^* are simple averages of the contents [mass%] of C, Si, Mn, Cr, and Mo of the chemical compositions of the two types of steel plates, respectively. If no element is included, 0 is substituted. r is a plate thickness ratio of the two types of steel plates, and is a ratio of a steel plate having a large plate thickness to a plate thickness of a steel plate having a small plate thickness. When the plate thicknesses of the steel plates are equal, r = 1. α, β, and γ are constant terms, which are 2.25 × 10 ⁶ , 2.20 × 10 ⁰ , and 2.41 × 10 ⁴ , respectively. T ^* is obtained by the following equation (2).

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 determined from the respective contents [% by mass] of Cr and Mo and the plate thickness ratio r. However, 0 is substituted when the element is not included.
(10) After the welding of the steel plate, before the plating treatment, heat treatment is performed to heat the steel plate to a temperature exceeding (A _c1 +40) ° C. of at least one of the steel plates,
The method for producing a high-strength plated steel sheet according to any one of (7) to (9), wherein in the heat treatment, a temperature history from the start of heating to the start of cooling satisfies formula (3).

However, v is an average heating rate [° C./second] in a section from A _c1 to A _c1 + 40 ° C., and k indicates an average cold rolling rate [%] of the two steel plates.
(11) Among the heat treatments, in the heating process from the start of heating to the start of cooling, in the mixed gas of air and fuel gas used for the preheating burner, the volume of air contained in the unit gas mixture; Heating in an oxidation zone under the condition of air ratio: 0.7 to 1.2, which is a ratio to the volume of air theoretically required to completely burn the fuel gas contained in the unit volume of the mixed gas, Next, heating is performed up to the maximum heating temperature in the reduction zone where the partial pressure ratio P (H ₂ O) / P (H ₂ ) of water vapor (H ₂ O) and hydrogen (H ₂ ) is 0.0001 to 2.0. The method for producing a high-strength plated steel sheet according to (9) or (10), wherein
(12) The method for producing a high-strength plated steel sheet according to any one of (7) to (11), wherein the welded portion is ground after the butt welding and before the plating treatment.
(13) Before the heat treatment, a pre-heat treatment for heating at least one of the one steel plate and the other steel plate to at least one _Ac1 temperature or more is performed once or more (9) to (12) The manufacturing method of the high strength plated steel plate in any one.
(14) The high-strength plating according to any one of (7) to (13), wherein at least one of the one steel plate and the other steel plate has a chemical composition satisfying the following formula (4): A method of manufacturing a steel sheet.

However, the element symbol in Formula (4) shows content [mass%] of each element in the said 1 steel plate and another steel plate, and substitutes 0 when the said element is not contained.
(15) 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 (7) The manufacturing method of the high strength plated steel plate in any one of-(14).
(16) At least one of the one steel plate and the other steel plate is a steel plate that has been subjected to preliminary heat treatment that is heated to a temperature of Ac ₃ or higher and then cooled at a rate of 1.0 ° C./second or higher. (9) The manufacturing method of the high strength plated steel plate according to any one of (9) to (15).
(17) The method for producing a high-strength plated steel sheet according to any one of (7) to (16), wherein the plating treatment is a hot dip galvanizing treatment.
(18) The method for producing a high-strength plated steel sheet according to (17), wherein an alloying treatment is performed after the plating treatment.

  ADVANTAGE OF THE INVENTION According to this invention, the high strength plated steel plate excellent in corrosion resistance and its manufacturing method 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 diameter around the welding part in the high strength steel plate of this invention. It is a schematic diagram of a test piece with a notch. It is a schematic perspective view which shows the structure of Experimental Examples 20-22.

Hereinafter, the steel sheet of the present invention and the manufacturing method thereof will be described.
The steel sheet of the present invention comprises two or more kinds of steel sheets having different yield strengths and / or thicknesses and butt welds thereof. At least one of the steel sheets has a maximum tensile strength of 780 MPa or more. The entire steel plate including the affected part is characterized in that the surface has a plating layer.

  Furthermore, in order to improve the formability and impact resistance, the HAZ 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 is as follows for the butt weld including a steel plate having a maximum tensile strength of 780 MPa or more. It is preferable to satisfy the requirements.

[Product of hardness and thickness HT]
Cracks at the time of forming 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 hardness and plate thickness at the relevant 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 steel plate having the smaller 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 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. is preferably at least 0.80 times the smaller value among the average values HT ₂ in the steel plate 2. The relationship between the two is more preferably 0.85 times or more, further preferably 0.90 times or more, and most preferably both 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. it is preferably not more than 1.50 times the value of 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 more preferably 1.40 times or less, still 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 preferably set to 100 [Hv]. ΔH is preferably as small as possible, more preferably 50 [Hv] or less, and even more preferably 30 [Hv] or less. Since the surface which does not have plating is exposed in the site | part cracked at the time of shaping | molding and corrosion resistance after shaping | molding deteriorates remarkably, it is preferable to satisfy | fill the said also from a corrosion-resistant viewpoint.

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 molded part, it is necessary to control the hardness that contributes to the occurrence of fracture as described above, and to reduce the crystal grain size in order to suppress the propagation of the fracture. In particular, 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 the impact resistance is likely 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, the average value d ₁ of the effective crystal grain size in the steel plate _1, and the steel plate By making the ratio of the average value d 2 of the effective crystal grain diameters of _{2 and} the larger value d ₂ 5.0 or less, the impact resistance is 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”.

[Maximum retained austenite volume fraction V _max ]
Residual austenite contained in the microstructure may be contained in the steel sheet, HAZ and welds in order to improve formability. However, the retained austenite becomes hard martensite by molding and acts as a base point of fracture at the time of impact, and therefore, when there is a large amount of retained austenite in the HAZ and the welded portion in comparison with the steel plate, the impact resistance is significantly deteriorated.

Specifically, in the case such as the butt weld shown in Figure 1, butt welds and in the distribution of the volume fraction of residual austenite region including the HAZ, the maximum residual austenite volume fraction V _max and high residual austenite side steel If the difference from the retained austenite volume fraction V exceeds 5.0%, the impact resistance may deteriorate. Therefore, the difference between the two is preferably 5.0% or less, more preferably 3.5% or less, and even more preferably 2.0% or less. The smaller the difference between the two, the better, and it is most preferable that the two are equal.

A method for measuring the effective crystal grain size and the retained austenite fraction will be described. Both are 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. On the other hand, the crystal orientation information obtained by the EBSD method is analyzed, the area ratio occupied by the point where the crystal structure is FCC is obtained, and is regarded as the volume ratio of residual austenite in the region.

  Moreover, the average effective crystal grain size and the retained austenite fraction in each steel plate excluding HAZ are 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 at two or more arbitrary points, and the average value of the obtained values is regarded as the average effective crystal grain size and the volume fraction of retained austenite in each steel plate. The data obtained by the EBSD method is analyzed using “OIM Analysis 7.0” manufactured by TSL.

(Chemical composition)
Since at least one base steel plate of the steel plate (hereinafter also referred to as “base steel plate”) as the base material constituting the plated steel plate of the present invention has a strength of the steel plate of the present invention of 780 MPa or more, It is preferable to use a steel plate having a chemical composition of In the chemical composition,% represents mass%.

(C: 0.020-0.800%)
C is an element that contributes to improving the strength. If the C content is less than 0.020%, the effect of addition cannot be sufficiently obtained, so the content is preferably 0.020% or more. C is preferably contained in an amount of 0.050% or more, and more preferably 0.100% 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 0.600% or less. In order to ensure the weldability of the member, the C content is more preferably 0.300% 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. From the viewpoint of ensuring impact resistance, it is preferably 2.50% or less, and more preferably 2.00% 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 25.00%)
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 25.00%, the cast slab becomes brittle and easily cracked, and weldability deteriorates, so Mn is preferably 25.00% or less. In order to prevent embrittlement of the cast slab, the Mn content is preferably 12.00% 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. From the viewpoint of ensuring good spot weldability, the Al content is preferably 2.000% or less.

  In producing the plated steel sheet according to 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 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 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 and contributes to an improvement in the strength of the steel sheet, 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 and contributes to an improvement in the strength of the steel sheet, 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 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 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 the improvement of toughness by strengthening by precipitates and suppressing the growth of crystal grains, 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 strengthening by precipitates and suppressing the growth of crystal grains, 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 strengthening by precipitates and suppressing the growth of crystal grains, 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 crystal grains and contributes to the improvement of steel plate strength. 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 crystal grains and contributes to the improvement of steel plate strength. 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 of moldability 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 the moldability is improved. Since there is a possibility of deterioration, 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%, but 0.0010% or more in total is necessary to sufficiently obtain the formability improvement effect. 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 formability such as ductility, hole expansibility, stretch flangeability, and bendability. If the S content exceeds 0.0100%, the formability 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 inhibits formability such as ductility, hole expansibility, stretch flangeability, and bendability, and also causes blowholes during welding, resulting in poor weldability. It is an element that inhibits. If the N content exceeds 0.0150%, formability 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 inhibits formability such as ductility, hole expansibility, stretch flangeability, and bendability. If the O content exceeds 0.0050%, the moldability 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.

(Production method)
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 treatment within 5.0 seconds after the 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 plate thicknesses are used, and one or more of them use steel plates having the above chemical composition in order to make the strength of the steel plates be 780 MPa 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, and the structure after heat treatment, which will be described later, is particularly homogenized and the characteristics are improved.

In addition, the maximum heating temperature in the preliminary heat treatment is set to Ac ₃ 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. The structure can be a homogeneous fine structure. Particularly, when the heat treatment described later is performed, the structure after the heat treatment is homogenized and refined, and the characteristics 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 may be used. If the plate thickness of the two steel plates sandwiching the butt weld and HAZ is excessively different, unplating due to a step may occur in the plating process, and corrosion resistance may be impaired. Therefore, it is preferable 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.

  Moreover, when performing the heat processing mentioned later, if both plate | board thickness differs too much, the temperature fluctuation of a steel plate and a welding part and the nonuniformity of a plating process may arise, and the stable characteristic may not be acquired. Also from this viewpoint, it is necessary to select the plate set of the two steel plates sandwiching the butt weld and the HAZ 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.

  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 can be made homogeneous and fine. And the properties of the steel sheet are improved.

  After the butt welding, it is necessary to remove oxides present on the surface of the welded portion and the weld heat affected zone in order to improve the wettability in the plating process and improve the appearance quality and the corrosion resistance. The method for removing the oxide is not particularly limited. For example, pickling treatment can be performed. Alternatively, shot peening may be performed.

  Or it is preferable to grind the surface in a welding part and a welding heat affected zone after butt welding. Grinding removes oxides and reduces the level difference at the welded portion, making the weld bead of the product less noticeable and improving the appearance. This treatment may be performed before and after the pickling treatment.

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

[Plating treatment]
In producing the high-strength plated steel sheet of the present invention, the entire surface of the steel sheet including the butt weld and HAZ is plated after welding to obtain a plated steel sheet. By performing electroplating after welding, an electroplated steel sheet in which either Zn or an alloy mainly composed of Zn is adhered to the steel sheet surface is obtained.

  Or, after welding, it is reheated to a temperature equivalent to a molten metal bath, and immersed in the molten metal bath, so that either Zn, Al, an alloy mainly composed of Zn or an alloy mainly composed of Al adheres to the steel sheet surface. A hot-dip galvanized steel sheet is obtained.

  Or you may give the heat processing mentioned later between welding and plating processes. When heat treatment and plating treatment are performed continuously, either Zn, Al, an alloy mainly composed of Zn or an alloy mainly composed of Al is steel sheet by being immersed in a molten metal bath during cooling from the maximum heating temperature. A hot dipped steel sheet adhered to the surface is obtained. Alternatively, after cooling to room temperature once after the heat treatment, reheating to a temperature equivalent to a molten metal bath and immersing in a molten metal bath, Zn, Al, an alloy mainly composed of Zn or an alloy mainly composed of Al A hot-dip galvanized steel sheet in which either one is adhered to the steel sheet surface is obtained.

  Since all of these plating treatments are performed after butt welding, the plating layer of the welded portion evaporates at the time of welding in a normal tailored blank material, but in the developed steel, a steel plate having a plated layer in the welded portion is obtained.

  After being immersed in the molten metal bath, it may be subjected to an alloying treatment in which the boundary between the plating layer and the ground iron is alloyed by continuous or once cooling and then reheating.

  Zinc plating layer and galvannealed layer are Al, Ag, B, Be, Bi, Ca, Cd, Co, Cr, Cs, Cu, Fe, Ge, Hf, Zr, I, K, La, Li, Mg , Mn, Mo, Na, Nb, Ni, Pb, Rb, Sb, Si, Sn, Sr, Ta, Ti, V, W, Zr, REM, one or more types, do not inhibit corrosion resistance and moldability In a range, you may contain. In particular, Ni, Al, and Mg are effective in improving corrosion resistance, and may be positively added in a range where Zn is the maximum in the mass ratio of the constituent elements. The chemical composition of the plating layer is not limited to the above. For example, Al, Ni, an alloy mainly containing Al, or an alloy mainly containing Ni may be used as the plating layer.

[Heat treatment]
After the welding, prior to the plating treatment, it is preferable to perform a heat treatment to produce the steel sheet, the HAZ and the microstructure of the welded portion, thereby manufacturing the steel sheet of the present invention. 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.

  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.

Upon heat treatment, for maximum tensile strength of at least 780 MPa, a maximum heating temperature of the base material steel plate having a chemical composition described above is preferably set to A _c1 temperature or more at the base steel sheet. This is because the carbide is dissolved and the steel sheet is strengthened with carbon. In order to sufficiently dissolve the carbide, the maximum heating temperature for the base steel plate is more preferably (A _c1 +30) ° C. or higher, and further preferably (A _c1 +45) ° C. or higher. The upper limit of the maximum heating temperature is not particularly set, but if the heating temperature exceeds 1000 ° C., the effect of improving the characteristics is not seen. Therefore, the maximum heating temperature is preferably set to 1000 ° C. or less from the viewpoint of manufacturing cost.

  Moreover, it is preferable that the temperature history in the base material steel plate in the heating process from the start of heating to the start of cooling among the heat treatments satisfies the following formula (1). Equation (1) is an index representing the degree of carbide dissolution in the HAZ and welds around the base steel plate. If equation (1) is not satisfied, low strength parts are produced in the HAZ and welds, and distortion during molding Makes it easier to concentrate.

式（１）は、炭化物の溶け始める目安となる温度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炭化物の溶け具合をＴ_ｎ、Ｔ^＊、鋼１と鋼２の化学組成のＣ、Ｓｉ、Ｍｎ、Ｃｒ及びＭｏのそれぞれの含有量の単純平均及び鋼１の板厚tk₁に対する鋼２の板厚tk₂の比率（tk₂/tk₁；但し、tk₂≧tk₁）をパラメータとして含む関数Ｆ_n(T_n, T^*, r, t_n, C^*, Si^*, Mn^*, Cr^*, Mo^*)にて計算し、合計するものである。但し、当該元素が含まれないときは、０を代入する。Ｔ_ｎ［℃］はｎステップ目における到達温度を、ｔ_ｎ［秒］はＴ^＊に到達してからｎステップ目までの総経過時間をそれぞれ表わす。Ｃ^＊、Ｓｉ^＊、Ｍｎ^＊、Ｃｒ^＊およびＭｏ^＊［質量％］は、溶接される鋼１と鋼２の化学組成のＣ、Ｓｉ、Ｍｎ、Ｃｒ及びＭｏのそれぞれの含有量の単純平均を示す。
ｒは前記２種の鋼板のうち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、板厚の薄い鋼板を鋼１とし、板厚の厚い鋼板を鋼２とした場合、鋼２の板厚を鋼１の板厚で除した値とする。α、β、γはそれぞれ定数項であり、それぞれ２．２５×１０^６、２．２０×１０^０、２．４１×１０^４とする。なお、加熱温度がＴ^＊よりも低い場合、左辺の値を０とし、式（１）は満たされないものとする。

Formula (1) divides the time from reaching the temperature T ^* [° C.], which is a guideline for starting the dissolution of carbide, to the start of cooling into 10 steps, and the melting degree of the carbide in each divided step. _Tn , T ^* , the simple average of the contents of C, Si, Mn, Cr and Mo in the chemical composition of steel 1 and steel 2 and the thickness tk ₂ of steel 2 relative to the thickness tk ₁ of steel ₁ Function F _n (T _n , T ^* , r, t _n , C ^* , Si ^* , Mn ^* , Cr ^* , Mo ^* ) including the ratio (tk ₂ / tk ₁ ; tk ₂ ≧ tk ₁ ) as a parameter It is calculated by and summed up. However, 0 is substituted when the element is not included. T _n [° C.] represents the temperature reached at the n-th step, and t _n [second] represents the total elapsed time from reaching T ^* to the n-th step. C ^* , Si ^* , Mn ^* , Cr ^*, and Mo ^* [mass%] are the simple average of the respective contents of C, Si, Mn, Cr, and Mo of the chemical compositions of Steel 1 and Steel 2 to be welded. Show.
r is the ratio of the thick steel plate to the thin steel plate thickness of the two types of steel plates, where the thin steel plate is steel 1 and the thick steel plate is steel 2. The thickness of steel 2 is divided by the thickness of steel 1. α, β, and γ are constant terms, which are 2.25 × 10 ⁶ , 2.20 × 10 ⁰ , and 2.41 × 10 ⁴ , respectively. In addition, when heating temperature is lower than T ^* , the value of a left side shall be 0 and Formula (1) shall not be satisfy | filled.

ここで、かっこ内の添え字は溶接される鋼１、鋼２をそれぞれ表わす。すなわち、式（２）は各鋼におけるＡ_ｃ１［℃］、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％］（但し、当該元素が含まれないときは、０を代入する）、および式（１）に示した板厚比ｒからなる式である。なお、鋼２がＡ_ｃ１を持たない場合、Ｔ^＊は鋼１のＡ_ｃ１と等しいとする。 T ^* is obtained by the following equation (2).

Here, the subscripts in parentheses represent steel 1 and steel 2 to be welded, respectively. That is, the formula (2) is A _c1 [° C.] in each steel, and each content [mass%] of Si, Mn, Cr and Mo in the chemical composition (however, when the element is not included, 0 is substituted. And the plate thickness ratio r shown in the equation (1). When steel 2 does not have A _c1 , T ^* is assumed to be equal to A _c1 of steel 1.

  When the temperature history in the heating step satisfies the formula (1), the carbides in the HAZ and the welded portion are sufficiently dissolved, so that a welded portion having a strength close to that of the base steel plate is obtained. From this viewpoint, the left side of the formula (1) is more preferably 1.10 or more, and further preferably 1.20 or more.

The points A _c1 and A _{c3 of the} steel sheet are the starting point and the completion point of reverse transformation in the heating process, respectively. Specifically, a small piece is cut out from the steel sheet after hot rolling prior to the heat treatment at 10 ° C./second. It is obtained by heating to 1200 ° C. and measuring the volume expansion during that time.

ここでｖはＡ_ｃ１からＡ_ｃ１＋４０℃の区間における平均加熱速度［℃／秒］であり、ｋは２つの鋼板の平均冷間圧延率［％］をそれぞれ示す。式（３）の値が０．１を下回ると、Ａ_ｃ１近傍で生成したオーステナイト粒の一部が極端に成長し、ｄ_ｍａｘ／ｄが小さくならない場合がある。また、式（３）の値が１０．０を超えると、Ａ_ｃ１近傍で隣接して生成するオーステナイト粒の結晶方位が揃いやすくなり、これらオーステナイト粒が合体して粗大な組織となるため、ｄ_ｍａｘ／ｄが小さくならない場合がある。なお、最高加熱温度がＡ_ｃ１＋４０℃に達しない場合、式（３）の値は０とする。 In the heat treatment step, most of the steel sheet structure is made to be austenite, and then cooled and transformed, thereby eliminating the coarse structure caused by welding and reducing the variation in effective crystal grain size d _max / d. An excellent steel sheet is obtained. Specifically, d _max / d can be made sufficiently small by satisfying the following formula (3).

Here, v is an average heating rate [° C./second] in a section from A _c1 to A _c1 + 40 ° C., and k is an average cold rolling rate [%] of the two steel plates. When the value of the formula (3) is less than 0.1, a part of austenite grains generated in the vicinity of A _c1 grows extremely, and d _max / d may not become small. Further, since the value of the equation (3) exceeds 10.0, easily aligned crystal orientations of the austenite grains produced by adjacent A _c1 vicinity become coarse tissue these austenite grains coalesce, d _max / d may not be reduced. When the maximum heating temperature does not reach A _c1 + 40 ° C., the value of equation (3) is 0.

In order to reduce d _max / d and improve the impact resistance, the value of formula (3) is preferably 0.5 or more and 5.0 or less.

  In the high-strength plated steel sheet of the present invention, it is preferable to control the atmosphere in the heat treatment in order to improve the wettability of the steel sheet, HAZ and welds in the plating process, and to improve the appearance of the product and the adhesion between the plating and the steel sheet. For example, by controlling the dew point in the heat treatment to −35 ° C. or lower, the generation of oxide on the steel sheet surface is suppressed, and the wettability can be improved by cleaning the steel sheet surface before plating.

  Alternatively, it is preferable that the heat treatment is divided into a preheating process in an oxidizing atmosphere and a main heating process in which heating is performed to a maximum heating temperature in a reducing atmosphere, and heating is performed while controlling the atmosphere.

  The preheating process in the oxidizing atmosphere is performed by generating an oxide on the steel sheet surface layer in a preheating furnace controlled to an air ratio of 0.7 to 1.2. The “air ratio” is the ratio between the volume of air contained in a unit volume of mixed gas and the volume of air theoretically required for complete combustion of the fuel gas contained in the unit volume of mixed gas. It is. When the completion temperature of the preheating process is less than 400 ° C., oxide formation at the steel sheet surface layer becomes insufficient. On the other hand, when the completion temperature of the preheating process exceeds 800 ° C., decarburization proceeds excessively in the steel sheet surface layer portion, and the steel sheet strength deteriorates. The completion temperature of the preheating process can be any temperature below the maximum heating temperature in the range of 400-800 ° C. When the air ratio is less than 0.7, oxide formation at the surface layer of the steel sheet becomes insufficient. On the other hand, when the air ratio exceeds 1.2, decarburization proceeds excessively in the surface layer portion of the steel sheet, and the strength of the steel sheet and / or the welded portion deteriorates. The air ratio is preferably controlled in the range of 0.7 to 1.2, and more preferably in the range of 0.8 to 1.1.

Subsequently, in the main heating process, heating is performed up to the maximum heating temperature in the main heating furnace in which the partial pressure ratio of H ₂ O and H ₂ is P (H ₂ O) / P (H ₂ ): 0.0001 to 2.00. Thus, the wettability can be greatly improved by reducing the oxide generated in the preheating process and cooling it after obtaining a clean surface. If the partial pressure ratio is less than 0.001, oxides are generated on the surface of the steel sheet, and a clean surface cannot be obtained. On the other hand, when the partial pressure ratio exceeds 2.0, decarburization proceeds excessively in the steel sheet surface layer portion, and the strength of the steel plate and / or the welded portion deteriorates. The partial pressure ratio is preferably controlled in the range of 0.001 to 2.00, more preferably in the range of 0.005 to 1.50.

  In the high-strength plated steel sheet of the present invention, in order to increase the strength of the portion made of the base steel sheet, the average cooling rate from the maximum heating temperature to 600 ° C. is preferably 1 ° C./second or more. This is to suppress the formation of soft tissues and coarse carbides during cooling, and from this viewpoint, the average cooling rate is more preferably 5 ° C./second or more.

In order to improve the formability by retained austenite, it is preferable to set the residence time in the temperature range of 450 to 300 ° C. to 10 seconds or more in order to promote the concentration of carbon to austenite. In order to increase the volume ratio of austenite and further improve the moldability, the residence time in the temperature range is more preferably 30 seconds or more.
The residence time indicates the total time spent in the temperature range, and cooling and / or heating may be performed as appropriate within the temperature range. In addition, if the cooling end point temperature is 100 ° C. or higher or the heating end point temperature is 600 ° C. or lower, the temperature once deviates from the temperature range of 450 to 300 ° C. during the staying and then returns to the temperature range and stays again. Absent.

但し、元素記号は各元素の母材鋼板における含有量［質量％］を示し、当該元素が含まれないときは、０を代入する。 In particular, when the steel sheet has a microstructure containing residual austenite and improves formability, the chemical composition of the steel sheet preferably satisfies the following (formula (4)). Equation (4) is an index representing the ease with which austenite remains in the steel sheet. The larger the value of equation (4), the more austenite remains after heat treatment, and the more likely retained austenite is obtained.

However, the element symbol indicates the content [% by mass] of the base material steel plate of each element, and 0 is substituted when the element is not included.

  The steel plate after heat treatment may be tempered to further improve the properties. When the tempering temperature exceeds 600 ° C, the maximum tensile strength of the portion made of the high-strength steel sheet may be lower than 780 MPa, and the tempering temperature is preferably 600 ° C or lower. Further, if the tempering temperature is lower than 150 ° C., a sufficient effect cannot be obtained, and therefore the tempering temperature is preferably 150 ° 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.

  When the plating treatment is performed after the heat treatment, it is preferable to subject the steel plate after the heat treatment to a pickling treatment. Moreover, when performing a plating process after heat processing, in order to remove | exclude the oxide of a surface, you may grind only the welding part or the whole steel plate surface containing a welding part after heat processing.

  In order to improve the corrosion resistance and / or formability, the steel plate of the present invention may have a film made of a phosphorus oxide and / or a composite oxide containing phosphorus on the entire surface of the steel plate including the butt weld and HAZ. I do not care.

Next, examples of the present invention will be described.
Evaluation of corrosion resistance after coating is performed on the test piece after bending and bending back. A test piece of 150 x 60 mm with the welded portion placed in the center is cut out, bent at a bending angle of 60 degrees in accordance with JIS Z 2248 so that the welded portion is perpendicular to the bending ridgeline, and is bent back to flat by pressing. After the chemical conversion treatment with a conversion treatment liquid (PB-SX35) manufactured by Nippon Parkerizing Co., Ltd., the electrodeposition paint (Powernics 110) manufactured by Nippon Paint Co., Ltd. (Powernics 110) becomes 20 μm in thickness as a test piece after reversion. And then baked at 170 ° C. to obtain a corrosion resistance test material after coating. In addition, the inner radius in the said bending process shall be 3.0 to 5.0 times the plate | board thickness of the thicker steel plate which comprises a welding part.

  The corrosion resistance after coating was evaluated by a corrosion test method specified in JASO M609 established by the Automotive Engineers Association. A crosscut is made in advance with a cutter centering on the intersection of the weld line and the bending ridgeline on the coating of the welded part and the base steel plate, and the width of the film bulge from the crosscut after the corrosion test 180 cycles (60 days) ( The maximum value on one side) is measured, and the case where it exceeds 7 mm is regarded as rejected (x). In addition, when the bulge width is 7 mm or less and pitting corrosion is not observed on the bending ridge line and the weld line, ◎ is indicated, and when pitting corrosion is observed, both are acceptable. .

  Casting a slab having the chemical composition of A, E, AA, AB shown in Table 1-1 and Table 1-2, and matching the base steel plate manufactured in accordance with a conventional method with the plate assembly described in Table 2 Plating is performed after the laser welding process. Table 2 shows the characteristics of the obtained steel sheet.

  Experimental Example A3 is a comparative example in which a plated steel plate is used as a base material and butt welding is performed. Since the plating layer of the welded portion and the weld heat affected zone evaporates, a steel plate with inferior corrosion resistance is obtained.

  Experimental Example A4 is an example in which a cold-rolled steel plate and a hot-rolled steel plate are used as a base material, butt welding is performed, the steel is heated to 461 ° C., and immersed in a molten zinc bath. Since the thickness ratio of the base steel plate is large and deviates from the scope of the present invention, plating failure occurs in the vicinity of the weld line, and the corrosion resistance of the steel plate becomes inferior.

  Experimental examples A1, A2, A5, and A6 are examples in which a plated steel sheet having excellent corrosion resistance, in which the entire steel sheet surface including the welded portion has a plating layer, is produced by the manufacturing method of the present invention.

  Experimental Example A1 is an example in which a cold-rolled steel sheet is welded, then heated to 462 ° C., immersed in a molten zinc bath, and further heated for alloying treatment. The entire steel sheet surface including the welded portion has a plating layer. An alloyed hot-dip galvanized steel sheet having excellent corrosion resistance is obtained.

  Experimental Example A2 is an example in which a hot-rolled steel sheet is welded, heated to 459 ° C., immersed in a molten zinc alloy bath containing Al and Mg, and further heated to be alloyed. An alloyed hot dip galvanized steel sheet having excellent corrosion resistance, having a plated layer on the entire surface, is obtained.

  Experimental Example A5 is an example in which a cold-rolled steel plate is welded and then heated to 458 ° C. and immersed in a hot-dip zinc bath. The hot-dip galvanized steel plate having excellent corrosion resistance, in which the entire steel plate surface including the welded portion has a plating layer. Is obtained.

  Experimental example A6 is an example in which electroplating is performed after welding a cold-rolled steel sheet, and a galvanized steel sheet having excellent corrosion resistance in which the entire steel sheet surface including the welded portion has a plating layer is obtained.

  Subsequently, an example in which heat treatment is performed prior to plating after welding will be described. Slabs having chemical compositions A to AE shown in Table 1-1 and Table 1-2 were cast, heated to slab heating temperatures shown in Tables 3-1 to 3-3, and Tables 3-1 to 3- Hot rolling is performed in a temperature range from the rolling start temperature shown in 3 to the rolling completion temperature. Then, it cools to the cooling start time shown to Table 3-1 to Table 3-3, cools to cooling stop temperature with the average cooling rate shown to Table 3-1 to Table 3-3, and winds up as a coil.

  Thereafter, the hot-rolled steel sheet is pickled and cold-rolled to obtain the total reduction shown in Tables 3-1 to 3-3 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 is subjected to plastic deformation by applying a tension for shape correction.

  Next, the steel plates are welded in combinations shown in Tables 3-1 to 3-3. Prior to welding, the butt portion is cut to obtain an end portion having excellent linearity. In particular, Experimental Examples 2 to 10 are examples in which an end portion after cutting is subjected to taper processing.

  Experimental example 27 is a comparative example in which laser welding is performed after heat treatment and plating treatment described later. Experimental Example 28 is an example in which laser welding is performed after the heat treatment and plating treatment described later, and the weld is irradiated with laser again to perform post-heat treatment on the weld.

  Experimental examples 19 and 24 are examples of welding by the mash seam welding method. Other experimental examples are examples of welding by laser welding.

  After welding, prior to heat treatment, the surface of the welded portion is ground in Experimental Examples 3, 7, 9-16, and 21-24.

  Experimental Example 18 is an example in which the steel plate after welding is subjected to pickling again before heat treatment.

  Experimental Examples 5 and 9 are examples in which a pickling treatment is performed after a preliminary heat treatment in which the sample is heated to 900 ° C. and rapidly cooled to room temperature before the heat treatment.

  Experimental Examples 2 and 14 are examples in which Ni plating treatment is performed on a steel plate after welding before heat treatment.

  Experimental Examples 20 to 22 are examples in which three steel plates are connected by welding to obtain one hot pressing steel plate. As shown in FIG. 6, in Experimental Examples 20 to 22, the steel plate 1 and the steel plate at the end of the steel plate 1 on the opposite side of the steel plate 2a and the welded portion a obtained by welding the steel plate 2a to the end of the steel plate 1. It is the structure which has the welding part b obtained by welding 2b.

  Next, heat treatment under the conditions shown in Table 4-1 and Table 4-2 is performed on the steel plate after welding. A steel plate is heated on the heating conditions represented by Formula (1) to the heating temperature shown to Table 4-1 and Table 4-2. Then, it cools to 600 degreeC with the average cooling rate shown to Table 4-1, Table 4-2, and it is made to stay only for the residence time shown to Table 4-1, Table 4-2 at 450-300 degreeC, and the temperature below 100 degreeC Cool to area. Thereafter, a tempering process and / or a skin pass rolling process is performed on some steel plates.

  Further, during or after the heat treatment, the types of plating shown in Table 4-1 and Table 4-2, that is, hot dip galvanizing (GI), alloyed hot dip galvanizing (GA), hot dip zinc alloy plating (Zn alloy), molten aluminum Each plating treatment of plating (Al) and zinc plating (EG) is performed.

  In Experimental Examples 2, 6, 10, 13, 16, 28, 29, 31, 32, and 34, heat treatment is performed in a heating furnace with a controlled dew point. In other experimental examples, heat treatment is performed using a preheating furnace in an oxidizing atmosphere and a main heating furnace in a reducing atmosphere.

  In particular, Experimental Examples 8, 9, 13, 21, and 26 are examples in which a hot dip galvanized steel sheet is obtained by cooling the steel sheet to 600 ° C. and then immersing it in a hot dip zinc bath and then cooling to 450 ° C. or lower. is there.

  Experimental Example 14 is an example of obtaining a hot dip galvanized steel sheet by reheating the steel sheet to 460 ° C. after dwelling at 450 to 300 ° C., immersing it in a hot dip galvanizing bath, and then cooling to room temperature.

  Furthermore, in Experimental Example 6, after the heat treatment, that is, the steel sheet was cooled to 100 ° C. or lower, and then subjected to a tempering treatment that was reheated to 457 ° C., immersed in a molten zinc bath after heating, and cooled to room temperature. This is an example of obtaining a hot-dip galvanized steel sheet.

  Experimental Examples 1, 2, 4, 10, 17, 19, 20, 22, 25, 27 to 38 are alloys in which a steel sheet is cooled to 600 ° C., immersed in a molten zinc bath, and reheated to 470 to 560 ° C. This is an example of obtaining an alloyed hot-dip galvanized steel sheet by performing a heat treatment and cooling to 450 ° C. or lower.

  In Experimental Examples 11 and 18, the steel sheet was reheated to 460 ° C after dwelling at 450 to 300 ° C, immersed in a molten zinc bath, and subjected to an alloying treatment to be reheated to 470 to 560 ° C. This is an example of obtaining a galvannealed steel sheet by cooling.

  Further, in Experimental Example 3, after the heat treatment, that is, the steel sheet was cooled to 100 ° C. or lower, reheated to 460 ° C., immersed in a molten zinc bath, and reheated to 488 ° C. to be tempered and alloyed. This is an example of obtaining an alloyed hot-dip galvanized steel sheet by simultaneously performing the treatment and cooling to room temperature.

  Experimental examples 12 and 15 are examples in which a hot-dip galvanized steel sheet is obtained by cooling the steel sheet to 600 ° C., then immersing it in a hot-dip zinc alloy bath containing Al and Mg, and cooling to 450 ° C. or lower.

  Experimental Examples 5 and 16 are examples of obtaining an aluminum-plated steel sheet by dipping in a molten aluminum bath in the process of cooling the steel sheet to 600 ° C. after heating.

  Experimental Example 22 is an example of obtaining an aluminum-plated steel sheet by immersing it in a molten aluminum alloy bath containing Mg in the process of cooling the steel sheet to 600 ° C. after heating.

  Experimental Examples 7 and 24 are examples in which a galvanized steel sheet is obtained by pickling the steel sheet after heat treatment and performing electroplating treatment.

  In the high-strength plated steel sheet obtained as described above, evaluation of hardness and microstructure distribution, evaluation of mechanical properties, and evaluation of corrosion resistance after coating were performed. Evaluation of corrosion resistance after painting was carried out in the same manner as in the examples in Table 2, using the corrosion test method defined in JASO M609 established by the Automotive Engineers Association.

  FIG. 1 is an example of sheet thickness and hardness distribution in a general butt weld, and is generally tailored for steel sheets of chemical composition A and chemical composition AC shown in Table 1-1 and Table 1-2. It is obtained from the welded part obtained by the blank method and its periphery. FIG. 2 is an example of the thickness and hardness distribution in the welded portion of the high-strength steel sheet of the present invention, and can be obtained by subjecting the same steel sheet to welding and heat treatment by the method of the present invention. FIG. 4 is a distribution of effective crystal grain size around the welded portion in the high-strength steel sheet of the present invention and a general butt weld, and is obtained from Experimental Examples 1 and 2 by the measuring method shown in FIG.

The mechanical properties of the steel sheet are evaluated by a tensile test. The characteristics of the base metal part are evaluated by using a JIS No. 5 test piece described in JIS Z 2201 whose tensile axis is a direction perpendicular to the weld line. Other conditions conform to the tensile test method described in JIS Z 2241. These evaluation results are shown in Tables 5-1 to 5-3. In addition, the item “ratio (1)” in Tables 5-1 and 5-2 is the average value HT ₁ of the product distribution of the hardness and the plate thickness of the steel plate ₁ and the product distribution of the hardness and the plate thickness of the steel plate 2. The ratio of the maximum value HT _max in the distribution of the product HT of the hardness and thickness of the region including the butt weld and the weld heat affected zone to the average value HT ₂ , that is, HT _max / HT ₁ and HT _max / HT ₂ Of these, the larger value. In addition, the item “ratio (2)” is the smaller of the ratio of the minimum value HT _min in the distribution of the HT to the average value HT ₁ and the HT ₂ , that is, HT _min / HT ₁ and HT _min / HT _2. Is the value of “Ratio (3)” includes the larger effective crystal grain size d of the average values of the effective crystal grain sizes of the welded steel plate 1 and steel plate 2, the butt weld and the weld heat affected zone. This is the ratio (d _max / d) to the maximum value d _max in the effective crystal grain size distribution in the region.

  The properties of the weld were 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. This test result is shown in the column of item “Direct welding line” in Table 5-3. 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 times or more 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. Considerable moldability can be expected. The item “ratio (4)” in Table 5-3 indicates the ratio of the load in the column “Welding line direct” to the maximum load of the base metal part in this tensile test.

  The second is a test piece with a notch shown in FIG. 5. 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 is 25 mm. The test results are shown in the column “Notch Test” in Table 5-3. The maximum load in this tensile test is an index that represents the fracture strength around the welded part due to dynamic deformation at the time of impact. 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 the welded part is difficult to brittle fracture at the time of impact, and the steel sheet is expected to have impact resistance equivalent to the base material part. it can. The item “ratio (5)” in Table 5-3 indicates the ratio of the load in the column of “notch test” with respect to the maximum load of the base material portion in this tensile test.

  Experimental Example 27 is an example in which a high-strength plated steel sheet is obtained by a normal tailored blank method. Since the plating around the welded portion disappears during welding, the corrosion resistance is extremely inferior.

  Experimental example 28 is an example in which the welded portion is post-heat treated after welding the plated steel sheet, and since the plating around the welded portion disappears during welding, the corrosion resistance is extremely inferior.

  Experimental examples 29 and 30 are examples in which the thickness ratio of the base material constituting the high-strength plated steel sheet is large, in which poor plating occurs in the vicinity of the weld and the corrosion resistance is inferior. In addition, due to temperature unevenness that occurs around the weld during heat treatment, the hardness deviation around the weld becomes large, resulting in inferior formability and impact resistance.

  Experimental example 31 is an example in which the heating temperature in the heat treatment is low and the strength of the steel sheet is lower than 700 MPa.

  Experimental examples 32 and 33 are examples in which the heating conditions in the heat treatment do not satisfy the formula (1), and the dissolution of the carbide around the welded portion does not proceed sufficiently, resulting in inferior formability.

  Experimental Example 34 is an example in which the dew point of the heating furnace is excessively high and the corrosion resistance is inferior.

  Experimental Example 35 is an example in which the air ratio of the preheating furnace is excessively small and the corrosion resistance is inferior.

  Experimental Example 36 is an example in which the air ratio of the preheating furnace is excessively large, the strength around the welded portion is lowered, and the formability is inferior.

  Experimental Example 37 is an example in which the atmosphere of the heating furnace deviates from the scope of the present invention and the corrosion resistance is inferior.

  Experimental Example 38 is an example in which the atmosphere of the heating furnace departs from the scope of the present invention, the strength around the welded portion is lowered, and the formability is inferior.

  Experimental Examples 1 to 26 are examples of producing a high-strength plated steel sheet according to the present invention, and high strength excellent in formability, impact resistance and corrosion resistance by performing an appropriate heat treatment prior to plating after welding. A plated steel sheet 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.

  The high-strength plated steel sheet of the present invention is excellent in corrosion resistance and is suitable for application to the body of an automobile.

Claims (18)

Consisting of different steel plates and their butt welds,
The maximum tensile strength of at least one of the different steel plates is 780 MPa or more,
A high strength plated steel sheet having a plated layer on the entire surface of the steel sheet including the butt weld and the weld heat affected zone. 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. The high-strength plated steel sheet according to claim 1, wherein 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 high-strength plated steel sheet according to claim 1 or 2, wherein the ratio of the effective crystal grain size d to the maximum value d _max of the effective crystal grain size is 5.0 or less. In the distribution of the volume fraction of retained austenite in the region including the butt weld and the weld heat affected zone, the retained austenite volume ratio V in the steel sheet on the side with much retained austenite and the maximum retained austenite volume from the one steel sheet to the other steel sheet high strength plated steel sheet according to any one of claims 1 to 3, the difference between the rate V _max is equal to or less than 5.0%.   The high-strength galvanized steel sheet according to any one of claims 1 to 4, further comprising a galvanized layer on the entire surface of the steel sheet including the welded portion and the weld heat affected zone.   6. The high strength plated steel sheet according to claim 5, wherein the galvanized layer is an alloyed galvanized layer. % By mass
C: 0.020% or more and 0.800% or less,
Si: 0.001% to 3.00%,
Mn: 0.01% or more and 25.00% or less,
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 and other steel plates having different chemical compositions and / or plate thicknesses are butt welded with a plate thickness ratio of 3.0 or less at the weld,
A method for producing a high-strength galvanized steel sheet, characterized by performing a plating treatment after welding. 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 0.0004-0.0100%
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 0.0010-0.0100%
Mg 0.0010-0.0100%
Zr 0.0010-0.0100%
La 0.0010-0.0100%
Hf 0.0010 to 0.0100%
REM 0.0010-0.0100%
Any one or more of these are included, The manufacturing method of the high strength plated steel plate of Claim 7 characterized by the above-mentioned. After the steel plate is welded and before the plating treatment, heat treatment is performed to heat the steel plate to a temperature exceeding the _Ac1 temperature of at least one of the steel plates,
The method for producing a high-strength plated steel sheet according to claim 7 or 8, wherein the heat history satisfies a formula (1) in a temperature history from a heating start to a cooling start.

However, Formula (1) divides time from the time when the temperature of the steel plate reaches T ^* [° C.] until the start of cooling into 10 steps equally, and Formula F _n (T _n in each divided step is _{^{, T *, r, t n}} , C *, Si *, Mn *, Cr *, is to sum the calculated values of Mo ^*). T _n [° C.] represents the temperature reached at the n-th step, and t _n [second] represents the total elapsed time from reaching T ^* to the n-th step. C ^* , Si ^* , Mn ^* , Cr ^*, and Mo ^* are simple averages of the contents [mass%] of C, Si, Mn, Cr, and Mo of the chemical compositions of the two types of steel plates, respectively. If no element is included, 0 is substituted. r is a plate thickness ratio of the two types of steel plates, and is a ratio of a steel plate having a large plate thickness to a plate thickness of a steel plate having a small plate thickness. When the plate thicknesses of the steel plates are equal, r = 1. α, β, and γ are constant terms, which are 2.25 × 10 ⁶ , 2.20 × 10 ⁰ , and 2.41 × 10 ⁴ , respectively. T ^* is obtained by the following equation (2).

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 determined from the respective contents [% by mass] of Cr and Mo and the plate thickness ratio r. However, 0 is substituted when the element is not included. After the welding of the steel plate, prior to the plating treatment, heat treatment is performed to heat at least one of the steel plates to a temperature exceeding (A _c1 +40) ° C.,
The method for producing a high-strength plated steel sheet according to any one of claims 7 to 9, wherein in the heat treatment, a temperature history from the start of heating to the start of cooling satisfies formula (3).

However, v is an average heating rate [° C./second] in a section from A _c1 to A _c1 + 40 ° C., and k indicates an average cold rolling rate [%] of the two steel plates. The heat treatment is theoretically necessary to completely burn the volume of air contained in the unit volume of mixed gas and the fuel gas contained in the unit volume of mixed gas in the mixed gas of air and fuel gas used for the preheating burner. comprising air ratio is the ratio of the volume of air: heating in the oxidation zone of 0.7 to 1.2 and conditions, then, water vapor _(H 2 O) and the partial pressure ratio P of hydrogen _{(H 2)} ( _{H 2 O) / P (H} 2): characterized by heating to a maximum heating temperature in has been reducing zone and 0.0001 to 2.0, the production of high-strength plated steel sheet according to claim 9 or 10 Method.   The method for producing a high-strength plated steel sheet according to any one of claims 7 to 11, wherein the welded portion is ground after the butt welding and before the plating treatment. In any one of Claims 9-12, before the said heat processing, the preliminary heat processing heated to more than _Ac1 temperature more than at least any one among said one steel plate and another steel plate is given once or more. The manufacturing method of the high strength plated steel plate of description. One or more types among said one steel plate and another steel plate have a chemical composition which satisfy | fills following formula (4), The manufacture of the high strength plated steel plate of any one of Claims 7-13 characterized by the above-mentioned. Method.

However, the element symbol in Formula (4) shows content [mass%] in the said one steel plate and another steel plate, and substitutes 0 when the said element is not contained.   The steel sheet according to claim 7, 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 high strength plated steel plate of any one of them. At least one of the one steel plate and the other steel plate is a steel plate that has been subjected to a preheat treatment that is heated to a temperature of Ac ₃ or higher and then cooled at a rate of 1.0 ° C./second or higher. The manufacturing method of the high strength plated steel plate of any one of Claims 9-15.   The method for producing a high-strength plated steel sheet according to any one of claims 7 to 16, wherein the plating process is a hot dip galvanizing process.   18. The method for producing a high-strength plated steel sheet according to claim 17, wherein an alloying treatment is performed after the plating treatment.

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