JP2003221640A - Steel plate showing excellent laser cuttability and its manufacturing process - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel plate which offers a fine cutting quality in laser cutting of a hot-rolled steel plate used for steel structures, and its manufacturing process. <P>SOLUTION: The steel plate showing an excellent laser cuttability generates scales at its surface, wherein the scales contain ≥85 vol.% Fe<SB>3</SB>O<SB>4</SB>(magnetite). In the manufacturing process, the steel plate is subjected to hot rolling at a finishing rolling temperature of 850-950°C while performing descaling before and during the rolling in every pass to form highly adhesive thin scales (≤6 μm). <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet having a good laser cut surface and a method for manufacturing the steel sheet.

[0002]

2. Description of the Related Art A large amount of thick steel plates are used for steel structures such as shipbuilding, construction, industrial machinery, and bridges. Occupied by cutting. Conventionally, as a method of cutting the thick steel plate, there are gas cutting, plasma cutting, laser cutting, and the like.

This laser cutting is a cutting method which has been put into practical use for about 20 years, and has been popular mainly in the thin plate processing industry where the plate thickness is 3.2 mm or less. Its use is expanding to the cutting of thicker steel plates.

This laser cutting is a method in which the material and oxide are melted by the heat energy of laser light and the melt is removed by an assist gas to form a cutting groove. And
The maximum plate thickness that can be cut is about 25 mm, but the cutting groove width is narrow and it is suitable for precision cutting, and the cutting strain is the smallest among thermal cutting. In addition, there are various kinds of materials that can be cut among the heat cutting, and it is possible to cut a complicated shape. Furthermore, since cutting work can be automated, the introduction of equipment is rapidly advancing as a rationalization measure for companies.

However, in the case of thick steel plates, the range of the appropriate laser cutting speed is limited, so that the expansion of the applicable range is hindered. That is, if the cutting speed is increased,
The energy efficiency per unit cutting length becomes small and the entire plate thickness cannot be melted, or even if it can be melted, the molten metal cannot be sufficiently removed. There is a problem of sticking to.

On the other hand, when the cutting speed is decreased, the cutting width is increased and the amount of molten metal is increased, so that the cutting width is large and uneven cutting portions called notches are formed on the cutting surface. Since the notch on the cut surface adversely affects welding and working of the steel plate, a great load is required along with the work of neatly finishing the cross section. Therefore, there is a demand for a thick steel plate having a stable laser cutting property.

For the purpose of improving the laser cutting property, many attempts have been made to improve the properties of the scale of the steel sheet. For example, as proposed in JP-A No. 11-61248, the scale thickness is set to 6 μm or less. With thinning,
There is a method for producing a steel sheet containing 60 vol% or less magnetite.

[0008]

However, when the amount of magnetite in the scale is 60 vol% or less, the adhesion of the scale is poor, and when the plate thickness is 19 to 25 mm, the effect of improving the laser cutting property is not sufficient. There was a case where the cut surface defective portion due to scale peeling, that is, the notch was generated. An object of the present invention is to provide a steel sheet excellent in laser cuttability in which a notch does not occur in a cut surface as described above and a manufacturing method thereof.

[0009]

Means for Solving the Problems The present invention has been made to solve the above-mentioned problems, and means 1 is, in mass%, C: 0.03 to 0.22%, Si: 0. 0
5 to 0.50%, Mn; 1.60% or less, P;
0.025% or less, S; 0.015% or less, Al
0.045% or less, N 2; 0.009% or less, and the balance being iron and unavoidable impurities, and the thickness of the scale formed on the surface is 6 μm or less. It is a steel sheet made of magnetite of 85 vol% or more and excellent in laser cutting property. Means 2
Furthermore, in mass%, Ni 2; 0.5% or less, Cu 2;
5% or less, Mo; 0.4% or less, Cr; 0.5% or less, Nb; 0.04% or less, V 1; 0.08% or less, or a laser cutting property containing two or more thereof. Excellent steel plate. Further, means 3 is a method in which a steel sheet containing the components described in the means 1 or 2 is heated in a heating furnace at 950 ° C to 1
After heating to 250 ° C., rolling is started, rolling is completed at 850 ° C. to 950 ° C. while descaling with a water pressure of 100 kg / mm 2 or more in all passes during the rolling, and thereafter,
It is a method for producing a steel sheet which is cooled by air and has excellent laser cutting properties.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have studied the mechanism of notch generation in the cut surface of a steel sheet when the steel sheet is cut by a laser. It was found that notches were generated due to the different melting widths of the steel, and the cause was the peeling of the scale on the steel plate surface due to the thermal shock of the laser. In order to prevent the scale from peeling, it is important to improve the adhesion of the scale. This is because if the adhesion of the scale is improved, the scale can be prevented from peeling during laser cutting, and the scale can be melted with a laser. The cutting width is the same, stable cutting quality can be obtained, and a wide range of cutting speed for obtaining good cutting quality can be adopted, resulting in good workability. In order to prevent scale peeling from the steel plate surface during laser cutting, the scale thickness should be 6 μm or less and the amount of magnetite in the scale should be 8 μm.
It is necessary to be 5 vol% or more, and for this purpose, the front and back surfaces of the steel sheet are descaled by water pressure of 100 kg / mm 2 or more in all rolling passes during rolling, and 850 ° C. to 95 ° C.
It was found necessary to finish rolling at 0 ° C.

The reason will be described below. First, the magnetite (Fe3
As a result of investigating the relationship between the O4) ratio and the scale peeling ratio, it was found that the relationship shown in FIG. The scale used in FIG. 1 was composed of wustite, hematite, and magnetite, had a thickness of 6 μm, and had a peeling rate of 5% strain applied to a flat tensile test piece.
Parallel part of the tensile test piece shown in Fig. 4 (40 mm x 50 mm)
The amount of scale peeled off from the area was measured and calculated. As can be seen from Fig. 1, the amount of magnetite is 85
It was found that when the content was more than vol%, the peeling rate of the scale sharply decreased. It is assumed that this is because the scale is dense and has high strength, the adhesion between the base steel of the steel sheet and the scale is improved, and the peeling and scattering of the scale is suppressed by the thermal shock during laser cutting.

Further, it is necessary that the scale thickness is 6 μm or less. This is because as the scale thickness becomes thicker than 6 μm, the thermal shock applied to the scale during laser cutting becomes large, and the scale is cracked, and the scale surface is partially peeled (if the scale is peeled from the base metal, However, since the scale is peeled from the middle in the thickness direction and a notch is generated due to a difference in the scale thickness, the thickness is 6 μm or less, which is a thickness where partial peeling of the scale surface does not occur.

Next, the manufacturing conditions for producing a scale having a thickness of 6 μm or less and containing 85 vol% or more of magnetite will be described. The scale on the surface of the steel sheet occurs when the steel sheet is in a high temperature state of 850 ° C. to 950 ° C. or higher, and thereafter hardly occurs. Then, first, it is generated as a scale made of wustite (FeO), and thereafter, as the steel sheet is cooled, this wustite is transformed into magnetite. Therefore, in order to reduce the thickness of the scale, it is necessary to suppress the generation of scale on the front and back surfaces of the steel sheet in a high temperature state of 850 ° C to 950 ° C or higher.

In order to suppress this scale generation, FIG.
As shown in, descaling is required to inject cooling water to the entire front and back surfaces of the steel sheet on the entrance side and exit side of the rolling roll. Further, the descaling cools the front and back surfaces of the steel sheet to transform the wustite formed on the front and back surfaces of the steel sheet into magnetite. Then, as shown in FIG. 3, this descaling reduces the magnetite ratio in the scale to 8
Descaling water pressure is 1 to get more than 5vol%
It is necessary to carry out at least 100 kg / mm 2 and in all passes from the first pass of rolling to the final finishing pass. In addition, FIG.
A slab heated at a plate thickness of 200 mm and 1200 ° C. was rolled in 10 passes, and the rolling was completed at a plate thickness of 22 mm and a finishing temperature of 900 ° C. The reason why the water pressure for descaling needs to be 100 kg / mm 2 or more is supposed to be that water remaining on the surface of the steel sheet can be prevented and the front and back surfaces of the steel sheet can be uniformly cooled. Further, by performing the descaling in all passes, it is possible to prevent the temperature rise on the front and back surfaces of the steel sheet due to the recuperation, so that the scale can be reliably made magnetite.

The limits of the individual elements contained in the steel sheet will be described below. C is 0.03 because it is a strength element
% Or more, but if it exceeds 0.22%, the toughness of the steel sheet deteriorates, so 0.22% was made the upper limit. Although Si deteriorates the laser cutting property, it has the function of improving the scale adhesion, so that the balance between the two is 0.05 to 0.
The range was 50%. Mn is an element that improves the scale adhesion without significantly degrading the laser cutting property, but if it exceeds 1.6%, it deteriorates the weldability.
The upper limit was 6%.

P and S have almost no effect on the laser cutting property, but the lower the material is, the more preferable it is, and P is 0.02.
The upper limit was set to 5% or less and S to 0.015% or less. A
1 is necessary for deoxidation and was set to 0.045%. N is
If it is 0.0090% or less, it does not adversely affect the weldability and the slab quality, so the upper limit was made it. It is possible to produce a steel sheet excellent in laser cuttability within the range of the steel sheet components described above, but it is effective to add the following elements in order to obtain higher strength without impairing the properties. .

Ni, Cu, Mo and Cr are solid solution strengthening elements and can enhance the strength of the steel sheet. However, addition of a large amount impairs weldability economically and in terms of quality, so the upper limit values are Ni ≦ 0.5% and Cu ≦ 0.5, respectively.
0.5%, Mo ≦ 0.4%, Cr ≦ 5%. Nb,
V is a precipitation strengthening element and has the effect of increasing the strength of the steel sheet. However, the addition of a large amount deteriorates the toughness of the welded portion economically and in terms of quality, so the upper limit values were set to Nb ≦ 0.04% and V ≦ 0.08%, respectively.

[0018]

EXAMPLES Tables 1 and 2 are examples of the present invention. Table 2 shows that the steel having the composition shown in Table 1 is melted and then continuously cast into a slab, which is immediately or reheated. It is rolled.

[0019]

[Table 1]

[0020]

[Table 2]

The scale thickness shown in Table 2 is a value obtained by rolling a steel sheet and then cooling it to room temperature, taking a sample, and measuring it with an optical microscope or a scanning microscope. The composition of the scale is a value measured by X-ray diffraction of the sample. Furthermore, the scale adhesion is 5% for steel plates.
Is the amount of scale peeled off per 40 mm x 50 mm area, and the amount of scale is the value measured by an electronic balance. As for the cutting result, the steel plate output is 6 kW.
When the laser was used to perform 1 m cutting at a cutting speed of 750 to 1200 mm / min, the cut surface was visually observed, and ⊚ has 1 or less notch in the cut surface. △
Has two or more notches on the cut surface. In the case of ×, burning occurred during cutting and it became impossible to cut.

As can be seen from Table 2, Examples 1 to 4 of the present invention obtained good laser cutting results. On the other hand, in Comparative Example 1, there is a path that is not descaled,
In Comparative Example 2, the descaling pressure is 100 kg / mm2.
Because of the following, the results of cutting with a laser were all poor.

[0023]

As described above, according to the present invention,
Since the scale adhesion of the steel sheet is improved and the scale can be prevented from peeling due to thermal shock during laser cutting, a stable and good cut surface can be obtained, and a remarkable effect industrially useful is achieved.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a magnetite ratio and a scale exfoliation ratio.

FIG. 2 is a diagram showing a descaling method.

FIG. 3 is a diagram showing a relationship between a descaling pressure and a magnetite ratio.

FIG. 4 is a view showing a 5% tensile peel test piece.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI Theme Coat (reference) C22C 38/58 C22C 38/58 (72) Inventor Akira Shibori Oita City Oita City Nishinosu No. 1 Nippon Steel Oita Steel Works F-term (reference) 4K032 AA01 AA04 AA05 AA11 AA14 AA16 AA19 AA21 AA22 AA23 AA27 AA29 AA31 AA36 BA01 CA01 CA02 CA03 CC04 CD05

Claims (3)

[Claims] 1. In mass%, C: 0.03 to 0.22%, Si: 0.05 to 0.50%, Mn; 1.60% or less, P: 0.025% or less, S; 0 A steel plate containing 0.15% or less, Al: 0.045% or less, N: 0.009% or less, and the balance of iron and inevitable impurities, and the thickness of the scale formed on the surface was 6 μm. A steel sheet having excellent laser cuttability, characterized in that the scale is composed of 85 vol% or more of magnetite. 2. Further, in mass%, Ni: 0.5% or less, Cu: 0.5% or less, Mo: 0.4% or less, Cr: 0.5% or less, Nb: 0.04% or less. The steel sheet excellent in laser cuttability according to claim 1, containing one or more of V 2 and 0.08% or less. 3. A steel sheet having the components according to claim 1 or 2 is heated to 950 to 1250 ° C. in a heating furnace, then rolling is started, and 100 kg / in all passes during the rolling.
Descaling with water pressure of mm2 or more, 850 ℃
A method for producing a steel sheet having excellent laser cuttability, which comprises rolling at 950 ° C. and then air cooling.