JPH07310140A - Double-layered steel sheet excellent in surface property and weldability - Google Patents

Abstract

PURPOSE:To produce a double-layered steel sheet excellent in surface properties and weldability by preparing a multilayered steel sheet in which each component in the surface layer and internal layer is respectively specified and the thickness of a transition layer on the boundary part between the surface layer and internal layer is specified. CONSTITUTION:In a steel sheet having a multilayered structure in the order of A, B and A of two kinds of componental systems, time component of A positioned in the surface layer contains, by weight, 0.05 to 0.25% C, 0.5 to 1.5% Si, 0.3 to 2.0% Mn, <=0.025% P, <=0.02% S, <=0.06% Al, <=0.006% N, and the balance iron with inevitable impurities. Moreover, the component of B positioned in the internal layer contains 0.05 to 0.25% C, <=0.5% Si, 0.3 to 2.0% Mn, <=0.025% P, <=0.02% S, <=0.06% Al, <=0.006% N, and the balance iron with inevitable impurities, and a transition layer having thickness of 2 to 10% that of the steel sheet is provided on the boundary layer between the surface layer and internal layer. Thus, the steel sheet free from the peeling of scales at the time of cold working and good in beauty in appearance after coating can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-layer steel sheet having excellent surface properties and weldability, which is used in fields such as industrial machinery and building materials.

[0002]

2. Description of the Related Art A steel plate having a thickness of 40 to 50 kg, which is generally used for industrial machines and building materials, is manufactured as it is rolled in order to reduce the manufacturing cost. In this case, since there are no strict requirements regarding the material and no special requirements regarding the surface properties of the steel sheet,
The manufacturing conditions are significantly different. For this reason, the steel plate manufactured as-rolled has a scale thickness of 50 μm on the steel plate surface.
If the thickness exceeds m, or if the scale thickness is thin, but the red scale is attached to the steel sheet, the surface properties of the steel sheet are greatly different.

On the other hand, in recent years, laser cutting has been carried out from the viewpoint of efficiency and precision in cutting steel plates,
It has been clarified that the cuttability by the laser has a great influence on the roughness of the steel sheet surface. Another problem is that the scale that peels off during cold working of steel sheets deteriorates the aesthetic appearance after painting.Furthermore, concrete contamination due to peeling of red rust on a steel concrete frame for construction becomes a problem. There is.

That is, there is a need for a steel sheet which is thinner, has a good surface roughness and does not cause red rust without peeling, but there has been no steel sheet having such characteristics. Regarding the hot rolled steel sheet, as a method for producing a high strength hot rolled steel sheet having excellent surface properties, for example, JP-A-1-
Japanese Patent Publication No. 209374 discloses a method for producing a hot-rolled steel sheet having a good surface property by adding a considerable amount of Si and regulating the slab heating temperature, heating time, rolling temperature, and winding temperature after rolling.

However, in the production of hot-rolled steel sheet, the strain rate during rolling is about five times as large as that in the production of thick steel sheet, and further, the steel sheet surface is taken up by winding the steel sheet at a predetermined temperature after rolling. The composition of the scale changes greatly.
That is, thick steel plates are air-cooled in the plate state after rolling, and the steel plate surface is exposed to the atmosphere, whereas in the case of hot-rolled steel plates, the cooling rate of the steel plate is significantly reduced due to winding after rolling. However, since the steel sheet surface does not come into direct contact with the atmosphere on the inner surface of the coil, the scale composition and properties of the steel sheet surface are different between the thick steel sheet and the hot rolled steel sheet.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a thick steel sheet having excellent surface properties by strictly controlling the manufacturing method of the thick steel sheet.

[0007]

The gist of the present invention is that in a steel sheet having a multilayer structure in the order of two kinds of component systems A, B and A, the component of A located in the surface layer is weight%
And C: 0.05 to 0.25%, Si: 0.5 to 1.5
%, Mn: 0.3 to 2.0%, P: 0.025% or less,
S: 0.02% or less, Al: 0.06% or less, N: 0.
Content of 006% or less, the balance consisting of iron and unavoidable impurities, and the content of B component in the inner layer is% by weight, and C:
0.05-0.25%, Si: 0.5% or less, Mn:
0.3-2.0%, P: 0.025% or less, S: 0.0
2% or less, Al: 0.06% or less, N: 0.06% or less, and if necessary, Nb: 0.005-
0.05%, V: 0.005-0.06%, Ti: 0.
A transition layer containing 005 to 0.02% of 1 type or 2 types or more, the balance consisting of iron and unavoidable impurities, and having a thickness of 2 to 10% of the steel plate thickness at the boundary between the surface layer and the inner layer. It is a multi-layer steel sheet having excellent surface properties and weldability, which is characterized by having

[0008]

The present invention applies 0.5 to 1.50% S to the surface layer (both sides).
This is a so-called cast clad steel plate in which i-containing steel is used and the inner layer is Si-containing steel of 0.5% or less, and the properties and weldability of the steel plate surface are ensured. That is, in such a cast clad steel plate, surface properties are secured with 0.5 to 1.50% Si-containing steel (surface layer component A), and weldability is achieved with 0.5% or less Si-containing steel (inner layer component B). Can be secured.

The reasons for limiting the components of the surface layer component A of the present invention will be described. C needs to be added in an amount of 0.05% or more to secure the strength, and if it exceeds 0.25%, the weldability and low temperature toughness deteriorate, so it is limited to the range of 0.05 to 0.25%. . Si is an essential element for improving the scale thickness, roughness, and adhesion to the base iron on the surface of the steel sheet, which is the object of the present invention. If the Si content is less than 0.5%, Fe ₂ SiO ₄ (firelite) having good adhesiveness is hardly generated, and the effect is not obtained. Therefore, the lower limit is set to 0.5%. Also, Si is 1.5
%, The weldability and HAZ toughness deteriorate.
The upper limit was 1.5%. By adding Si, Si in the steel is concentrated on the surface of the slab when the slab is heated, and a firelight having good adhesion to the base iron is generated. After that, Fe ₃ O ₄
(Magnetite), Fe ₂ O ₃ (hematite) and other scales are generated.

[0010] Mn is an important element for ensuring strength and toughness, and it is necessary to add 0.3% or more, but if it is too much, the weldability deteriorates, so the upper limit was made 2.0%. . The reason that the impurities P and S in the steel of the present invention are 0.025% or less and 0.02% or less, respectively, is to further improve the low temperature toughness of the base material and the welded portion. Reduction of P prevents grain boundary destruction, and reduction of S prevents deterioration of toughness due to MnS.

Al is an element usually contained in steel as a deoxidizer, but if it exceeds 0.06%, Al-based nonmetallic inclusions increase and impair the cleanliness of steel, so the upper limit is 0.06%. And N forms TiN and improves the base material toughness and HAZ toughness through the effect of suppressing the coarsening of γ grains. However, too much N causes deterioration of HAZ toughness due to solid solution N, so it is necessary to suppress the content to 0.006% or less.

Next, the reasons for limiting the components of the inner layer component B of the present invention will be explained. C needs to be added in an amount of 0.05% or more to secure the strength, and if it exceeds 0.25%, the weldability and low temperature toughness deteriorate, so it is limited to the range of 0.05 to 0.25%. . Si is an element that strengthens steel,
If a large amount is added, the weldability and HAZ toughness deteriorate extremely, so the content was made 0.5% or less. Further, Ti alone is sufficient for deoxidizing steel, and Si does not necessarily have to be added.

Mn is an important element for ensuring strength and toughness, and it is necessary to add 0.3% or more, but if it is too much, the weldability deteriorates, so the upper limit was made 2.0%. . The reason that the impurities P and S in the steel of the present invention are 0.025% or less and 0.02% or less, respectively, is to further improve the low temperature toughness of the base material and the welded portion. Reduction of P prevents grain boundary destruction, and reduction of S prevents deterioration of toughness due to MnS.

Al is an element usually contained in steel as a deoxidizer, but if it exceeds 0.06%, Al-based nonmetallic inclusions increase and impair the cleanliness of steel, so the upper limit is 0.06%. And N forms TiN and improves the base material toughness and HAZ toughness through the effect of suppressing the coarsening of γ grains. However, too much N causes deterioration of HAZ toughness due to solid solution N, so it is necessary to suppress the content to 0.06% or less.

Next, the reason for adding Nb, V and Ti will be described. The main purpose of adding these elements in addition to the basic components is to improve the properties such as strength and low temperature toughness of the base metal without impairing the excellent characteristics of the steel of the present invention. Therefore, the amount of addition is limited by itself. Nb and V form carbonitrides and have the effect of increasing the strength, but if they are too large, the weldability is impaired, so the upper limits were made 0.05% and 0.06%, respectively.

When Ti is added, fine TiN is formed, and when the slab is reheated, the coarseness of γ grains in the welded portion is suppressed and the base metal toughness,
It is effective in improving the weld heat affected zone (HAZ) toughness. However, if the amount of Ti is too large, coarsening of TiN and precipitation hardening due to TiC occur and the low temperature toughness deteriorates, so the upper limit must be limited to 0.02%. The steel of the present invention has a multi-layer structure in which two kinds of components A, B, and A are formed in this order, but it may have a transition layer having a thickness of 2 to 10% of the steel plate thickness at the boundary between A and B. is necessary. This is because peeling or cracking does not occur at the boundary between A and B. When A and B are in direct contact with each other in the casting method, elements such as Si segregate at the interface between A and B, causing peeling or cracking. The reason why the thickness of the transition layer is 2% or more of the thickness of the steel plate is that 2% is the limit value for preventing the occurrence of peeling or cracking, and the thickness of the transition layer exceeds 10% of the thickness of the steel plate. It becomes difficult to secure the necessary weldability of the base metal. Therefore, the thickness of the transition layer is limited to 2 to 10% of the steel plate thickness.

This transition layer is obtained by the casting method in which A first starts solidification from the surface and B subsequently starts solidification at the site where the solidification ends. Therefore, it is necessary that the A and B layers sandwiching the transition layer are solidified continuously without being mixed with each other and are macroscopically separated from the viewpoint of component analysis. The transition layer formed during casting is retained even after heating, rolling and heat treatment.

[0018]

EXAMPLES Tables 1 and 2 (continued from Table 1-1), Table 3
The steel sheet surface properties, strength, low temperature toughness (Charpy impact test) and weldability (HAZ toughness) of the various component clad steel sheets shown in ((continuation-2 of Table 1)) were investigated.

[0019]

[Table 1]

[0020]

[Table 2]

[0021]

[Table 3]

In Tables 1 to 3, steels 1 to 10 are steels of the present invention and 11 to 18 are comparative steels. The steels 1 to 10 of the present invention have excellent strength, low temperature toughness, and HAZ toughness, and the surface properties of the steel sheets are all good with a thin scale thickness of 20 μm or less and a surface roughness of 10 μm or less. In addition, the adhesion of the scale by bending evaluation was good, and the color of the scale was light blue.

On the other hand, the comparative steels 11 to 18 are inferior in properties. Since the comparative steel 11 has a small amount of Si in the surface layer, it has a large scale thickness, a rough surface texture, and poor adhesion. In Comparative Steel 12, the HAZ toughness deteriorated because the amount of Si in the surface layer was too large. In Comparative Steel 13, the HAZ toughness was deteriorated because the amount of Si in the inner layer was too large.

Comparative Steel 17 had cracks in the transition layer because the proportion of the transition layer was too small. Since the ratio of the transition layer of Comparative Steel 18 was too large, the strength and toughness were deteriorated.

[0025]

According to the present invention, a thick steel plate having excellent surface properties and weldability has become possible. As a result, the scale peeling that occurs during cold working was eliminated, and the aesthetic appearance after coating was improved. In addition, the processing accuracy and efficiency during laser cutting were improved, and the weldability was also improved. In this way, the improvements in various properties have improved the safety and aesthetics of steel structures.

Claims (2)

[Claims] 1. In a steel sheet having a multilayer structure in the order of two kinds of component systems A, B and A, the component of A located in the surface layer is% by weight, and C: 0.05 to 0.25. %, Si: 0.5-
1.5%, Mn: 0.3 to 2.0%, P: 0.025%
Hereinafter, S: 0.02% or less, Al: 0.06% or less,
N: 0.006% or less is contained, the balance is composed of iron and inevitable impurities, and the B component located in the inner layer is wt%.
And C: 0.05 to 0.25%, Si: 0.5% or less,
Mn: 0.3-2.0%, P: 0.025% or less, S:
0.02% or less, Al: 0.06% or less, N: 0.06
% Or less, the balance consisting of iron and unavoidable impurities, and having a transition layer having a thickness of 2 to 10% of the thickness of the steel plate at the boundary between the surface layer and the inner layer, and welding. Multi-layer steel sheet with excellent properties. 2. In a steel sheet having a multilayer structure in the order of two kinds of component systems A, B and A, the component of A located in the surface layer is% by weight and C: 0.05 to 0.25. %, Si: 0.5-
1.5%, Mn: 0.3 to 2.0%, P: 0.025%
Hereinafter, S: 0.02% or less, Al: 0.06% or less,
N: 0.006% or less is contained, the balance is composed of iron and inevitable impurities, and the B component located in the inner layer is wt%.
And C: 0.05 to 0.25%, Si: 0.5% or less,
Mn: 0.3-2.0%, P: 0.025% or less, S:
0.02% or less, Al: 0.06% or less, N: 0.06
% Or less, and Nb: 0.005 to 0.05
%, V: 0.005-0.06%, Ti: 0.005-
0.02% of 1 type or 2 types or more is contained, the balance consists of iron and unavoidable impurities, and a transition layer having a thickness of 2 to 10% of the steel plate thickness is provided at the boundary between the surface layer and the inner layer. A multi-layer steel sheet with excellent surface properties and weldability, which is characterized by