JP2004238722A - High-strength steel strip excellent in plasma weldability and formability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength steel strip which is used for automobiles, industrial machines, etc., can be stably welded by plasma welding without causing burn through or high-temperature cracking even at a high welding rate, and is excellent in formability, such as hole-enlarging property or ductility. <P>SOLUTION: The high-strength steel strip excellent in plasma weldability and formability contains 0.05-0.10 mass% C, 0.5-2.0 mass% Mn, at most 0.02 mass% P, 0.0015-0.004 mass% S, 0.005-0.1 mass% Al, at most 0.005 mass% N, X to (1.2-X) mass% Si [wherein X=1.75×(10,000×S amount)<SP>-1/2</SP>], and the balance being iron and unavoidable impurities. The steel strip has a structure comprising at least 90% polygonal ferrite and the balance being a rigid phase such as pearlite or bainitic ferrite and has a hole-enlarging ratio of 80% or higher. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

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【発明の効果】
以上のように、本発明の方法に従えば、プラズマ溶接を施した際に、溶接速度を上げても溶け落ちや高温割れがなく安定して溶接することが可能で、かつ、穴拡げ性や延性に代表される成形性に優れた高強度鋼板を提供することができる。また、プラズマ溶接以外の融接を施す場合にも同様に適用できる。この鋼板を使用すれば、自動車や産業用機械などの分野でそれぞれがもつべき機能を向上させながら軽量化を図ることができ、産業上、非常に大きな効果を有する。
【図面の簡単な説明】
【図１】Ｓｉ量とＳ量をかえた場合のプラズマ溶接の適正電流範囲の変化を示す図である。
【図２】Ｓ量とＳｉ量をかえた場合のプラズマ溶接での高温割れ発生率の変化を示す図である。
【図３】Ｓｉ量とＳ量の適正範囲を示す図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a high-strength steel sheet used for automobiles, industrial machines, and the like, and particularly for use in performing fusion welding such as plasma welding.
[0002]
[Prior art]
2. Description of the Related Art In recent years, tailored blanks have been frequently used as a means for manufacturing a vehicle body. This is a method in which the material is cut and then welded first and then pressed. By using the material in the right place at the right place, it is possible to achieve weight reduction, collision safety improvement and cost reduction of the automobile. Laser welding and mash seam welding are the mainstream welding methods for tailored blanks, but the former requires high precision in the mating surface and requires high equipment costs. There is a disadvantage that the mechanical properties of the material are inferior to the butt welding method.
[0003]
On the other hand, there is plasma welding as a welding method in which the equipment cost is significantly lower than that of laser welding and the shape of the welded portion is relatively good. This method is a non-consumable arc welding using a tungsten electrode like the TIG, but uses a high energy density plasma arc by flowing a plasma gas flow through a small hole in the center of the welding torch nozzle. It has the best high-speed weldability among consumable arc welding. However, since the welding speed is lower than that of laser welding, it is necessary to increase the number of welding machines in order to secure productivity equal to or higher than that of laser welding. In addition, to reduce the total cost of laser welding, the welding speed must be increased as much as possible. A welding speed of at least 80 cm / min or more is required.
[0004]
When the welding speed was increased in plasma welding, it was found that in conventional high-strength steel sheets, when C ≦ 0.1%, burn-through and high-temperature cracking were likely to occur. Although the burn-through hardly occurs due to the increase in C, the welding speed is easy to increase, but the hole expandability is particularly deteriorated, and cracks are generated at the time of pressing in a part having stretch flange processing.
[0005]
[Problems to be solved by the invention]
In view of the above situation, the present invention provides stable welding without burn-through or high-temperature cracking even when the welding speed is increased when plasma welding is performed. It is an object to provide a high-strength steel sheet excellent in typical formability. It should be noted that the present invention can be applied to a case where plasma welding is performed on a material other than a tailored blank.
[0006]
[Means for Solving the Problems]
The present inventors have investigated the plasma weldability and hole expandability using high-strength steel sheets of various components in order to solve the above problems, and have obtained the following findings.
Regarding plasma weldability, Si and S are most important in low carbon steels with C ≦ 0.1%. The appropriate current range of plasma welding was examined by changing the amount of Si at C: 0.07% and S: 25 ppm.
[0007]
FIG. 1 shows a change in the appropriate current range of plasma welding when the amount of Si and the amount of S are changed. In this case, C is 0.07%, the other components are within the scope of the present invention, and the welding conditions are the same as those in Table 3 described later. As shown in this figure, it was found that there was a peak when Si: about 0.6%. The appropriate current range here is a current range in which a back bead of 1.5 mm or more when butt-welding with a gap between steel plates of 0.2 mm can be secured and no burn-through occurs. Further, the more S is, the more the appropriate current range is expanded. However, an increase in S facilitates hot cracking, and Si suppresses the occurrence.
[0008]
FIG. 2 shows a change in the hot crack occurrence rate in plasma welding when the S amount and the Si amount are changed. In this case, C is 0.07%, the other components are within the scope of the present invention, and the welding conditions are the same as those in Table 3 described later. That is, it shows the effect of the component on the hot crack occurrence rate. Based on the results of FIG. 1 or FIG. 2, the range of Si and S that can be welded without any problem at a welding speed of 80 cm / min or more in actual welding is shown in FIG. That is, FIG. 3 is a diagram illustrating an appropriate range of the Si amount and the S amount. The other components in this case are within the scope of the present invention, and the welding conditions are the same as in Table 3 described later.
[0009]
As shown in FIG. 3, in the range of S: 15 to 40 ppm (0.0015 to 0.004%), the range of the amount of Si depends on the amount of S. When the range is formulated, when X = 1.75 × (10000 × S amount) ^−1/2 (where the S amount is mass%), Si: X〜 (1.2−X) %. Regarding S, the hole expansion rate is also affected, but the lower the better, the better the hole expansion rate is in this component range.
[0010]
The addition of Mg or the addition of Ti / REM (Ce or La) makes the solidified structure finer, so that hot cracking is less likely to occur. Therefore, the upper limit of S can be reduced to 60 ppm. In order to exert the effect, Mg: 0.0005 to 0.02%, Ti: 0.01 to 0.2%, and REM: 0.0005 to 0.02% are required. In addition, since the inclusions become finer due to the addition of these elements, the hole expansion rate is also improved, which is very preferable.
[0011]
Other elements were limited for the following reasons.
C improves the plasma weldability as described above, but deteriorates the hole expansion rate. In order to exert the effect of Si and S on the plasma weldability, 0.05% or more is necessary, and in order to suppress the deterioration of the hole expansion rate, it is necessary to be 0.1% or less.
Mn does not significantly affect the plasma weldability and the hole expansion rate, and its amount is adjusted mainly to obtain the required strength. However, since excessive addition causes deterioration of ductility, the upper limit is 2%. On the other hand, in order to improve hot brittleness by S and secure the strength, it is necessary to add 0.5% or more.
[0012]
P segregates at the time of solidification similarly to S and induces hot cracking. From the viewpoint of preventing hot cracking, the upper limit is made 0.02%.
Al is required as a deoxidizing element. In order to exhibit the effect, 0.005% or more is required. Since excessive addition remains in the steel as an oxide and lowers ductility, the upper limit is set to 0.1%.
[0013]
N is preferably small in order to cause deterioration of ductility, but an excessive decrease greatly increases steelmaking costs, so the upper limit is 0.005%.
Nb, V, Mo, Cr and Ti contribute to securing the strength, but excessive addition causes a decrease in ductility and easily causes hot cracking. From the viewpoint of suppressing hot cracking, the respective amounts are set to 0.2% or less.
[0014]
B increases the hardenability and the strength. To achieve the effect, 0.0005% or more is required. However, excessive addition leads to an increase in the hard phase and lowers the hole expansion rate. Therefore, the upper limit is made 0.005%.
Ca is added for controlling the form of MnS formed in the steel. Although the hole expansion rate is improved, 0.0005% or more is required to exhibit the effect. Excessive addition causes inclusions to remain in the steel, so the upper limit is 0.005%.
[0015]
It should be noted that the inclusion of trace amounts of Cu, Ni, Sn, etc. due to the use of scrap does not impair the effects of the present invention. Use of scrap in the steelmaking process is preferable from the viewpoint of recycling.
Regarding the structure, a single-phase structure of polygonal ferrite is ideal for improving the hole expansion ratio. However, in order to secure the strength, it is necessary to allow a hard phase such as pearlite and bainitic ferrite to be mixed. The goal was to secure a hole expansion rate of 80% or more based on actual automotive parts pressing results and the like, and found that it is necessary to secure 90% or more of polygonal ferrite for that purpose.
[0016]
The present invention has been completed based on the above findings. The gist is:
In mass%, C: 0.05 to 0.10%, Mn: 0.5 to 2.0%, P: 0.02% or less, S: 0.0015 to 0.004%, Al: 0.005 ０．１0.1%, N: 0.005% or less, X = 1.75 × (10000 × S amount) ^−1/2
Si: contains X to (1.2-X)%, the balance consists of unavoidable impurities and iron, and the structure consists of 90% or more of polygonal ferrite and the balance consists of hard phases such as pearlite and bainitic ferrite. A high-strength steel sheet excellent in plasma weldability and formability, having a hole expansion ratio of 80% or more.
(2) A high-strength steel sheet excellent in plasma weldability and formability further containing 0.2% or less of Ti with respect to the high-strength steel sheet according to (1).
[0017]
(3) In mass%, C: 0.05 to 0.10%, Mn: 0.5 to 2.0%, P: 0.02% or less, S: 0.0015 to 0.006%, Al: When 0.005 to 0.1%, N: 0.005% or less, and X = 1.75 × (10000 × S amount) ^−1/2 , Si: X to (1.2−X)% is contained. Further, one or both of the following (1) and (2) are satisfied, and (1) Mg: 0.0005 to 0.02% is contained; (2) Ti: 0.01 to 0.2 % And one or two of Ce and La in a total amount of 0.0005 to 0.02%. The balance is composed of unavoidable impurities and iron, and the structure is 90% or more of polygonal ferrite and the balance. Characterized by a hard phase such as pearlite or bainitic ferrite having a hole expansion ratio of 80% or more. High-strength steel sheet having excellent formability and contact resistance.
[0018]
(4) Plasma weldability of the high-strength steel sheet according to any one of (1) to (3), further containing 0.2% or less of one or more of Nb, V, Mo, and Cr, respectively. High strength steel sheet with excellent formability.
(5) A high-strength steel sheet excellent in plasma weldability and formability further containing 0.0005 to 0.005% of B with respect to the high-strength steel sheet according to any of (1) to (4).
[0019]
(6) A high-strength steel sheet excellent in plasma weldability and formability, further containing Ca: 0.0005 to 0.005% with respect to the high-strength steel sheet according to any of (1) to (5).
(7) A high-strength steel sheet excellent in plasma weldability and formability, characterized by having a plating layer on the surface of the high-strength steel sheet according to any of (1) to (6).
[0020]
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention is basically carried out by a hot rolled steel sheet manufacturing process in a normal continuous hot rolling line or a cold rolled steel sheet manufacturing process through pickling, cold rolling, continuous annealing, and skin pass. As long as the requirements of the present invention are satisfied, either a hot-rolled steel sheet or a cold-rolled steel sheet may be used. The molten steel may be produced by a usual blast furnace method, or may be one using a large amount of scrap as in the electric furnace method. The slab may be manufactured by a normal continuous casting process or may be manufactured by thin slab casting.
[0021]
In addition, the continuous hot rolling line is a normal line, other than the use of a coil box for winding and holding the coarse bar after the rough rolling and further rewinding of the coarse bar that has been rolled and joined with the preceding coarse bar for rolling, The so-called hot-rolling continuous process is preferable from the viewpoint of reducing material variation. Further, the pickling, cold rolling, continuous annealing, and skin pass may be a line in which two or more processes are continuously performed, or may be a line in which each is processed independently.
[0022]
Even if plating is performed on a hot-rolled steel sheet or a cold-rolled steel sheet, the effect of the present invention is not impaired. The plating may be Zn, Al, Sn, Ni alone or a composite component thereof, and any plating method such as hot-dip plating or electroplating may be used. Further, the surface may be covered with a bond coat, a resin, or the like. Regarding welding, it is particularly intended for plasma welding. However, in addition to plasma welding, the invention is also effective when the welding speed is increased to the limit in so-called fusion welding such as ordinary arc welding, laser welding, electron beam welding, and gas welding in which a steel sheet is welded while melting.
[0023]
【Example】
A hot-rolled steel sheet or a cold-rolled steel sheet as shown in Table 4 was manufactured using the steel slabs having the components shown in Table 1. Table 2 shows the range of the manufacturing conditions, and Table 3 shows the plasma welding test conditions. Table 4 also shows the results of the material inspection and the results of the evaluation in the plasma welding test. The hot-rolled steel sheet is hot-dip galvanized, and the cold-rolled steel sheet is electrogalvanized.
The hole expansion ratio shown in Table 4 is measured by a hole expansion test. A hole with a diameter of 10 mm is punched out at the center of a 150 mm square steel plate with a clearance of 12%, and pushed out with a conical punch with a vertical angle of 60 degrees so that the burrs are on the outside. Measure. That is, the hole expansion rate is calculated by (hole diameter after fracture−initial hole diameter) / initial hole diameter × 100.
[0024]
Table 1 shows Comparative Example Nos. Produced using steel grades L to S out of the component range of the present invention. Comparative Examples Nos. 12 to 19, in which a steel type having a component range defined by the present invention was used, but the structure was out of the requirement. In No. 20, any one of elongation, hole expansion rate, appropriate welding current range evaluation, and hot crack evaluation was poor.
On the other hand, in the example of the present invention, no. In Nos. 1 to 11, the hole expansion ratio was as good as 80% or more and the elongation was good.
[0025]
[Table 1]

[0026]
[Table 2]

[0027]
[Table 3]

[0028]
[Table 4]

[0029]
【The invention's effect】
As described above, according to the method of the present invention, when plasma welding is performed, it is possible to perform stable welding without burn-through or high-temperature cracking even when the welding speed is increased, and to increase the hole expandability. A high-strength steel sheet excellent in formability represented by ductility can be provided. Further, the present invention can be similarly applied to a case where fusion welding other than plasma welding is performed. If this steel sheet is used, it is possible to reduce the weight while improving the functions that each should have in the field of automobiles, industrial machines, and the like, which has a great industrial effect.
[Brief description of the drawings]
FIG. 1 is a diagram showing a change in an appropriate current range for plasma welding when the amount of Si and the amount of S are changed.
FIG. 2 is a diagram showing a change in a hot crack occurrence rate in plasma welding when the amount of S and the amount of Si are changed.
FIG. 3 is a diagram showing an appropriate range of Si amount and S amount.

Claims (7)

In mass%,
C: 0.05 to 0.10%,
Mn: 0.5 to 2.0%,
P: 0.02% or less,
S: 0.0015 to 0.004%,
Al: 0.005 to 0.1%,
N: 0.005% or less,
X = 1.75 × (10000 × S amount) ^−1/2
Si: containing X to (1.2-X)%,
The plasma is characterized in that the balance is composed of unavoidable impurities and iron, the structure is composed of polygonal ferrite with 90% or more and the hard phase such as pearlite and bainitic ferrite, and the hole expansion ratio is 80% or more. High strength steel sheet with excellent weldability and formability. 2. A high-strength steel sheet according to claim 1, which further contains 0.2% or less of Ti and has excellent plasma weldability and formability. In mass%,
C: 0.05 to 0.10%,
Mn: 0.5 to 2.0%,
P: 0.02% or less,
S: 0.0015 to 0.006%,
Al: 0.005 to 0.1%,
N: 0.005% or less,
X = 1.75 × (10000 × S amount) ^−1/2
Si: containing X to (1.2-X)%,
Furthermore, one or both of the following (1) and (2) are satisfied,
(1) Mg: 0.0005 to 0.02% is contained (2) Ti: 0.01 to 0.2%
And further contains one or two of Ce and La in a total amount of 0.0005 to 0.02%. The balance is made of unavoidable impurities and iron, and the structure is 90% or more of polygonal ferrite and the remainder is iron. A high-strength steel sheet made of a hard phase such as pearlite or bainitic ferrite and having a hole expansion ratio of 80% or more and excellent in plasma weldability and formability. The high-strength steel sheet according to any one of claims 1 to 3, which further includes one or more of Nb, V, Mo, and Cr, each containing 0.2% or less, and is excellent in plasma weldability and formability. High strength steel plate. The high-strength steel sheet according to any one of claims 1 to 4, further containing B by 0.0005 to 0.005% and having excellent plasma weldability and formability. A high-strength steel sheet excellent in plasma weldability and formability further containing Ca: 0.0005 to 0.005% with respect to the high-strength steel sheet according to any one of claims 1 to 5. A high-strength steel sheet excellent in plasma weldability and formability, comprising a plating layer on the surface of the high-strength steel sheet according to any one of claims 1 to 6.

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