JPH11199984A - High strength steel sheet excellent in gas cutting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength steel sheet capable of obtaining a good cut face free from the sticking of dross at the time of gas cutting. SOLUTION: The viscosity of a molten iron alloy μFe at 1550 deg.C is regulated to <=6 cP, and the viscosity of molten oxide μOx produced at the time of oxidizing the molten iron alloy at the same temp. is regulated to <=150 cP. Furthermore, in the case elemental symbols in the following each formula denote each containing wt.%. of the same elements, the value of the following formula I is regulated to <=4.3, the value of the following formula II is regulated to <=4.6, and the value of the following formula III is regulated to 1 to 9: the formula I=5-0.5-0.2(Si+Mn)-3.4P-8.5Al+Ti+1.2Nb+0.3(Cr+Mo)+2.7 V, the formula II=1.8Al+1.6Si+1.7(Ti+Nb+V)+2.1Cr, and the formula III=Mn/Si.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel sheet excellent in fusing property.

[0002]

2. Description of the Related Art In recent years, high-strength steel sheets have been used in the automobile industry from the viewpoints of reducing the weight of a vehicle body and improving the safety of occupants in the event of a collision in order to improve fuel efficiency. The biggest problem in putting such a high-strength steel sheet to practical use is that cutting is extremely difficult. In particular, removal of the leading and trailing edges during steel plate manufacturing,
In addition, when cutting and drilling holes in molded products at the time of raw sheet production by the user, the application of the shearing method that uses the mechanical force that has been conventionally used is accompanied by the strengthening of the steel sheet. It tends to be difficult. Such a tendency is not limited to high-strength steel sheets for automobiles, but also applies to high-strength steel sheets for shipbuilding.

[0003] As an alternative to the shearing method, there is a fusing method in which a material is melted and removed in a groove shape by thermal energy (chemical reaction heat, arc heat, electron, plasma beam heat, laser beam heat, etc.) and cut. According to this method, a high-strength thick steel plate can be cut unlike the shearing method. In addition, it has many advantages such as easy cutting in a curved shape and reduction in initial equipment investment.

[0004]

However, in the fusing method, dross (a mixture of molten metal and molten oxide) is applied to the cutting edge.
Easily adheres to the product, significantly impairing the appearance of the molded product,
When the peeling is difficult, a great effort is required to remove the dross. Further, when the dross adheres remarkably, the cut pieces cannot be separated by re-welding at the time of cutting the closed curve.
Furthermore, even when the dross is slightly attached, a decrease in cutting accuracy cannot be avoided. For this reason, prevention of dross adhesion is indispensable when applying the fusing method.

[0005] To prevent the dross from adhering, conventionally, attempts have been made to improve the fusing conditions such as the type of gas used during fusing, the amount of heat input, and the cutting speed. Has not yet been established. For this reason, dross-free at the time of fusing is not achieved at present.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a high-strength steel sheet which is free from dross adhesion at the time of fusing and can obtain a good cut surface.

[0007]

According to observations made by the present inventors, dross adheres to the cutting edge of a steel sheet during fusing.
This is because dross is not discharged downward from the cutting front surface but flows backward or to the side, flows around the plate back surface, and solidifies as it is due to inappropriate cutting conditions. Therefore, even if the dross flows backward or to the side, if the detachability from the back surface of the plate is good, it is measured that the adhesion does not occur.

Therefore, the present inventors focused on the viscosity which is considered to have the greatest influence on the releasability from the back surface of the plate, and examined various iron alloys and various oxides which would be generated at the time of fusing at a high temperature. The viscosities were measured and the relationship between them and dross adhesion was investigated. As a result, it was found that dross adhesion did not occur at all when the viscosities of the two became below a specific value. Furthermore, as a result of analyzing in detail the relationship between the viscosities of the molten iron alloy and the molten oxide and each element, when the following expressions 1 to 3 are satisfied, the viscosity can be ensured, and dross adhesion can be completely prevented. It has been found that the following inventions have been completed.

That is, the invention according to claim 1 has the following features.
This is a high-strength steel sheet having a characteristic that the viscosity μFe of the molten iron alloy at 50 ° C. is 6 cP or less, and the viscosity μOx of the molten oxide generated when the molten iron alloy is oxidized at the same temperature is 150 cP or less.

Further, in the invention according to claim 2, when the element symbol in each of the following formulas indicates the content% by weight of the same element, the value of the following formula 1 is 4.3 or less, and the value of the following formula 2 is 4. 6. The high-strength steel sheet according to claim 1, wherein the value of Formula 3 below is 1 to 9. Formula 1 = 5-0.5C-0.2 (Si + Mn) -3.4P
−8.5Al + Ti + 1.2Nb + 0.3 (Cr + M
o) + 2.7V Formula 2 = 1.8Al + 1.6Si + 1.7 (Ti + Nb +
V) + 2.1Cr Formula 3 = Mn / Si

[0011] The invention according to claim 3 is characterized in that:
And C: 0.02-0.30%, Si: 0.01-3.
00%, Mn: 0.50 to 3.00%, P: 0.15%
Hereinafter, Al: 0.01 to 0.07% is contained, and the balance Fe
And a high-strength steel sheet having excellent fusing properties according to claim 1 or 2 comprising unavoidable impurities.

Further, the invention according to claim 4 is characterized in that Ti: 0.25% or less, Nb: 0.25% or less, B: 0.005% or less, Cu: 1.0% or less, N
i: 0.5% or less, Cr: 1.0% or less, Mo: 0.6
% Or less, and V: 0.4% or less.

[0013] Regarding the viscosity μFe of the molten iron alloy and the viscosity μOx of the molten oxide, if at least one of μFe and μOx exceeds the upper limit, the viscosity of the dross increases, and adhesion to the back surface of the plate occurs. Become like

A regression analysis was performed to determine the relationship between the content weights of various component elements and the amount of dross deposited. As a result, the viscosity of the molten iron alloy changed the viscosity of the molten iron among the various elements forming the steel sheet. When the value of Equation 1 exceeds 4.3, μFe at 1550 ° C. becomes 6 cP
And dross adhesion occurs, and the viscosity of the molten oxide is determined by a comprehensive combination of elements having high oxide-forming ability among various elements forming the steel sheet. If the value of equation 2 exceeds 4.6, 15
The viscosity μOx at 50 ° C. also exceeded 150 cP, and it became clear that dross was attached. Also, when the formula 3 showing the ratio of Mn to Si, which has a particularly high oxide-forming ability, is less than 1 or more than 9, a composite oxide having a high melting point is likely to be formed, so that dross adhesion is also likely to occur. Sick. It should be noted that the content of each element in the formulas 1 and 2 is not indispensable, but means that the content is considered when it is contained.

The high-strength steel sheet of the present invention has a tensile strength of 60 kg.
Although the effect is great at f / mm ² or more, especially at 80 kgf / mm ² or more, preferred components of the high-strength steel sheet having a strength of 80 kgf / mm ² or more are as described in claims 3 and 4, and The reasons for limiting the components will be described below.

C: 0.02 to 0.30% C is an important element that controls the tensile strength of a steel sheet. To obtain a martensitic structure, at least 0.02%
Need to be added. From the viewpoint of increasing the strength, the larger the better, the better. However, if it exceeds 0.30%, the toughness and weldability are significantly deteriorated, so the upper limit is made 0.30%.

Si: 0.01 to 3.00% Si is an essential element as a deoxidizing element in addition to securing the strength of the steel sheet, and at least 0.01% is necessary to obtain its effect. On the other hand, if the amount is too large, the scale is remarkably generated in the hot rolling step, and the surface properties of the steel sheet are deteriorated. Therefore, the upper limit is set to 3.00%.

Mn: 0.50 to 3.00% Mn is an essential element for securing the strength, and at least 0.50% is necessary to obtain the effect. 3.00
%, The toughness and weldability deteriorate significantly, so the upper limit is made 3.00%.

P: not more than 0.15% If P is added in excess of 0.15%, the weldability is impaired, so the upper limit is made 0.15%.

Al: 0.01 to 0.07% Al needs 0.01% as a deoxidizing element.
If it exceeds 07%, not only the deoxidizing effect is saturated, but also alumina clusters are formed to deteriorate the surface properties.

The preferred components of the high-strength steel sheet according to the present invention comprise, in addition to the above basic components, the balance of Fe and unavoidable impurities. In order to further improve the mechanical properties of the steel sheet, Ti: 0.25% or less, Nb: 0.
25% or less, B: 0.005% or less, Cu: 1.0% or less, Ni: 0.5% or less, Cr: 1.0% or less, Mo:
One or more of 0.6% or less and V: 0.4% or less can be contained. These elements have the effect of increasing the strength and toughness of the steel sheet. If the content exceeds the upper limit of each element content, the toughness and weldability of the steel sheet will be significantly impaired.

[0022]

EXAMPLE A slab having the composition shown in Table 1 was subjected to hot rolling, pickling, cold rolling and annealing to produce a steel sheet (sheet thickness: 2.0 mm) having the tensile strength shown in the same table. For each steel sheet, the viscosity of the molten iron alloy and the molten oxide were measured by the following measurement method, and plasma cutting and laser cutting were performed under the following conditions, and dross adhesion was investigated in the following manner. The results are shown in Table 1.

(1) Method of measuring viscosity Molten iron alloy A sample cut from each steel plate was placed in an inert gas for 1550 minutes.
℃, melted, and the viscosity μFe of the molten iron alloy was measured by a crucible rotation vibration method. The crucible rotation vibration method utilizes the fact that when a container filled with a melt sample is hung with a catenary and rotated and vibrated, the vibration gradually attenuates due to the viscous resistance of the sample in the container. This is a method of measuring the logarithmic decay rate and the oscillation period, and obtaining the viscosity of the sample from these measured values, and is mainly used for measurement of low viscosity.

Molten oxide A sample cut from a steel sheet is heated and melted at 1550 ° C. in the air or an oxygen-enriched atmosphere to sufficiently oxidize, and the oxide collected at this time is reheated to 1550 ° C. in an inert gas. , Melted, and the viscosity μOx of the molten oxide was measured by a rotating cylinder method. The rotating cylinder method is a method in which a cylinder is rotated at a constant angular velocity in a molten sample, a rotational moment due to viscosity acting on the rotating body is measured at that time, and the viscosity of the sample is obtained from the measured value.

(2) Cutting method Plasma cutting Plasma cutting is performed by an air plasma method using air as an operating gas, cutting current 10 A, cutting speed 0.1 m / mi.
Made with n.

Laser cutting Laser cutting was performed using a carbon dioxide laser as a laser source at a laser output of 900 W and a cutting speed of 4 m / min.

(3) Method of measuring the amount of dross adhering The dross adhering to both sides of the cut portion was polished and removed, and the amount of adhering per 1 m of cutting length was determined from the weight difference before and after the removal.

[0028]

[Table 1]

From Table 1, it can be seen that Samples Nos. 21 and 2
Comparing Sample No. 1 with Sample Nos. 1 to 11 as Examples, the dross adhesion can be remarkably suppressed only when the viscosity μFe of the molten iron alloy is within the range of the present invention.
Can be achieved when the value is 4.3 or less. In addition, even if the cutting method is different from Inventive Example No. 10 and other invention examples, dross does not adhere to the cut edge at the time of fusing as long as the present invention is satisfied, and excellent fusing property is obtained. You can see that it is.

Sample Nos. 23 to 26, which are comparative examples,
Comparing Nos. 1 to 11, the dross adhesion can be significantly suppressed only when the viscosity μOx of the molten oxide is within the range of the present invention. It can be seen that the value of 3 can be achieved only within the range of 1 to 9.

[0031]

According to the high-strength steel sheet of the present invention, a good cut surface without dross adhesion at the time of fusing can be obtained, so that the steel sheet can be easily manufactured or processed into a product shape, and the productivity can be greatly improved. Applications can be expanded.

Claims (4)

[Claims] 1. The viscosity μ of a molten iron alloy at 1550 ° C.
A high-strength steel sheet excellent in fusing properties, in which the Fe is 6 cP or less and the viscosity μOx of the molten oxide generated when the molten iron alloy is oxidized at the same temperature is 150 cP or less. 2. When an element symbol in each of the following formulas indicates the content percentage by weight of the same element, the value of the following formula 1 is 4.3 or less, and the value of the following formula 2 is
2. The high-strength steel sheet excellent in fusing property according to claim 1, wherein the value of Eq. Formula 1 = 5-0.5C-0.2 (Si + Mn) -3.4P
−8.5Al + Ti + 1.2Nb + 0.3 (Cr + M
o) + 2.7V Formula 2 = 1.8Al + 1.6Si + 1.7 (Ti + Nb +
V) + 2.1Cr Formula 3 = Mn / Si 3. C: 0.02 to 0.30% by weight
%, Si: 0.01 to 3.00%, Mn: 0.50 to 0.5%
3.00%, P: 0.15% or less, Al: 0.01 to
The high-strength steel sheet having excellent fusing property according to claim 1 or 2, comprising 0.07%, and the balance being Fe and unavoidable impurities. 4. Ti: 0.25% or less, Nb:
0.25% or less, B: 0.005% or less, Cu: 1.0
%, Ni: 0.5% or less, Cr: 1.0% or less, M
4. The high-strength steel sheet excellent in fusing property according to claim 3, containing one or more of o: 0.6% or less and V: 0.4% or less.