JPH0525584A - High weather resistant steel plate excellent in weldability and its production - Google Patents

Abstract

PURPOSE:To obtain a high weather resistant steel plate excellent in weldability by subjecting a steel having a specific composition to hot rolling at a temp. in specific range at specific draft, continuously cooling the resulting plate at specific velocity, and finishing the above cooling at a temp. in specific range. CONSTITUTION:A steel having a composition consisting of, by weight, <=0.05% C, <=0.5% Si, <=2.0% Mn, 0.05-0.25% P, 0.003-0.2% Al, <=1.0% Cr, <=1.0% Cu, <=1.0% Ni, >0.005-0.020% S, <=0.004% N, and the balance Fe with inevitable impurities is heated up to 1000-1200 deg.C and hot-rolled at a temp. in the range not lower than the Ar3 point at >=50% draft. Cooling is continuously done from the above temp. at a cooling velocity not lower than air cooling velocity. Cooling is finished at 550-300 deg.C, and >=80% of surface area is formed into bainite surface layer structure.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high weather resistance steel sheet having excellent weldability suitable for use in bridges, buildings and other structures, and a method for producing the same.

[0002]

2. Description of the Related Art Generally, P, Cu, Cr and Ni are applied to low alloy steel.
Corrosion resistance in the atmosphere can be improved by adding alloy elements such as. This low alloy steel is called weather resistant steel and is widely used in bridges, constructions and other structures. This weather-resistant steel is characterized by the fact that it forms rust that protects against corrosion in the outdoors for several years (hereinafter referred to as weather-resistant rust), and is so-called maintenance-free, which eliminates the need for anticorrosion treatment such as painting. However, since it contains alloying elements such as P, Cr and Cu, it has a problem in weldability.

Therefore, weather resistant steel having improved weldability has been developed. For example, Japanese Examined Patent Publication No. 1-29859 proposes weather resistance with extremely low C, extremely low N and extremely low S. However, this weather resistant steel takes several years to form the above weather resistant rust that is protective against corrosion,
During the period until then, floating rust such as red rust and yellow rust and flow rust are generated, which is not only visually unfavorable but also causes pollution of the surrounding environment.

On the other hand, as a method for preventing the occurrence of such floating rust and flow rust, Japanese Patent Laid-Open No. 142088/1989 discloses a surface treatment method for weather-resistant steel in which a phosphate treatment is performed after treatment with an acidic aqueous solution. Although proposed, such a treatment is not preferable because it increases the cost.

For these reasons, there is a strong demand for the development of highly weather-resistant steel that is maintenance-free and excellent in weldability, even if it is not subjected to anticorrosion treatment such as surface treatment or painting.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to meet the above-mentioned demands, and it has excellent weldability and produces dense weather-resistant rust at an early stage and can be used without treatment. It is an object of the present invention to provide maintenance-free high weather resistance steel and a method capable of stably producing such high weather resistance steel.

[0007]

The weather resistance is considered to be improved by the addition of alloying elements such as P, Cr and Cu as mentioned above, but these alloying elements impair the weldability.
On the other hand, floating rust and flow rust, which significantly impair the appearance, are considered to be suppressed by the early formation of dense weather-resistant rust.However, conventional weather-resistant steel is used until such weather-resistant rust is formed. Takes a long time.

However, the present inventors found that weathering rust was formed early by adding S in an appropriate amount and that weldability was improved by reducing C and N as much as possible.
Further, by adopting accelerated cooling after hot rolling to make the steel sheet surface layer a structure in which 80% or more of its surface area is bainite, early formation of weather resistant rust is further promoted and weather resistance is improved, It was also found that the strength decrease due to the extremely low carbon content is also compensated by the accelerated cooling to increase the strength.

Here, the gist of the present invention lies in the following high weather-resistant steel plate and its manufacturing method.

C: 0.05% or less, Si: 0.5% or less,
Mn: 2.0% or less, P: 0.05 to 0.25%, Al: 0.003 to
0.2%, Cr 1.0% or less, Cu: 1.0% or less, Ni:
1.0% or less, S: more than 0.005% and 0.020% or less, N is limited to 0.004% or less, the balance is Fe and unavoidable impurities, and has a surface layer structure in which 80% or more of the surface area is bainite High weather resistance steel plate with excellent weldability.

A steel having the above composition is
° C. subjected to hot rolling 50% or more in a temperature range of more than Ar ₃ point after heating to 1200 ° C. inclusive, then continuously cooled from Ar ₃ point or more temperature air cooling rate higher than 550 ° C. or less A method for producing a highly weather-resistant steel sheet having excellent weldability, which terminates cooling in a temperature range of 300 ° C or higher.

[0012]

The present invention will be described in more detail below.

First, the action of the alloy components in the high weather-resistant steel sheet of the present invention, the content thereof, and the reason for limiting the surface layer structure as described above will be explained. The "%" of the content in the alloy components is "% by weight" unless otherwise specified.

C: C is an element that generally reduces weldability. In particular, when the contents of P and S are large, the weld crack resistance is greatly reduced due to the interaction between C and these elements.

However, according to the knowledge of the present inventors, if the C content is suppressed to 0.05% or less and the P and S contents are adjusted to 0.25% or less and 0.02% or less, respectively, the weld cracking susceptibility is improved. Not so high. Based on this finding, the C content was set to 0.05% or less.

Si: Si is added for deoxidizing molten steel, but it is an element which adversely affects weldability and weather resistance.
When the Si content exceeds 0.5%, the weather resistance is particularly deteriorated, so the content is set to 0.5% or less.

Mn: Mn does not affect the weather resistance, but is added to improve deoxidation, hot workability and weldability. Further, in the present invention, Mn is added in order to compensate for the insufficient strength due to the C content being suppressed to 0.05% or less. For these purposes, the more Mn, the better. However, even if the content of Mn exceeds 2.0%, the above effect is saturated and only the cost increases, so the content is set to 2.0% or less.

P: P is an element that greatly affects the weather resistance, but it is an element that extremely impairs weldability in ordinary 0.15% C steel. With ultra-low carbon steel having a C content of 0.05% or less, the weather resistance is greatly improved with the addition of P. In order to obtain this effect, it is desirable to contain 0.05% or more, but if the content exceeds 0.25%, the weldability will deteriorate even if the C content is 0.03% or less. 0.05
~ 0.25%.

Al: Al has the function of refining the structure and improving the corrosion resistance. It also has a function of further enhancing the effect of the weather resistance improving elements such as P, Cr and Cu. In order to exert these effects, it is necessary to contain 0.003% or more, but if the content exceeds 0.2%, the effect will be saturated,
Only the cost will increase, so its content is 0.
It was set to 003 to 0.2%.

Cr, Cu, Ni: Cr, Cu and Ni
Is an element having the effect of improving the corrosion resistance of weather-resistant rust. However, even if the content of each exceeds 1.0%, the effect of improving the weather resistance is saturated and only the cost increases, so the content was set to 1.0% or less in all cases.

S: S is an important element in the present invention.
When S is contained in excess of 0.005%, the amount of reduction in plate thickness upon exposure becomes small, and the rate of increase with time is S
Was smaller than that of 0.005% or less.
That is, by containing S in an amount of more than 0.005%, weather resistant rust having a high anticorrosive ability is formed at an early stage.

However, in order to eliminate the adverse effects of P on the weldability and the toughness of the welded joint and to lower the weld crack susceptibility by the interaction with C and N, it is preferable that the content of P is not more than 0.020%. Therefore, the S content should be
It was set to more than 0.005% and 0.020% or less.

N: N is contained as much as possible in order to eliminate adverse effects on the weldability of P and toughness of welded joints and to avoid interaction with C and S, which increase the susceptibility to weld cracking, as much as possible. Therefore, 0.004% or less is preferable.

The high weather-resistant steel sheet of the present invention comprises, in addition to the above components, the balance Fe and unavoidable impurities, and the surface layer of the steel sheet has a bainite structure of 80% or more of its surface area. 80% or more of the surface area of the surface layer has a bainite structure, because it was found that dense weathering rust was formed earlier and the formation of floating rust such as red rust and flow rust was suppressed. Is. The surface layer refers to the surface of the steel plate up to a depth of about 5 mm.
If 80% or more of the structure is bainite, the formation of rust is suppressed. Therefore, even if the internal structure is 80% or more of bainite, there is no influence on the suppression of rust. The structure of the surface layer other than bainite is mainly ferrite, and the inside is a mixed structure of ferrite, pearlite and bainite.

Next, a desirable method for producing the high weather resistance steel sheet of the present invention will be described.

After heating the steel having the above-mentioned component composition, it is hot-rolled to a predetermined plate thickness. It is desirable that the heating is performed at a temperature of 1000 ° C. or higher in order to form a solid solution of the alloying elements, and in a temperature range of 1200 ° C. or lower in order to avoid steel ingot cracking caused by the high P material. Further, hot rolling is performed in a temperature range of Ar ₃ point or higher in order to refine the structure by recrystallization of γ grains.
It is desirable to carry out a reduction rate of 50% or more.

After hot rolling, the transformation is completed by continuously cooling from a temperature of Ar ₃ point or higher at a cooling rate of air cooling or higher.
Cooling is completed within the temperature range of 50 ℃ or below and 300 ℃ or above. When the cooling deviates from the above conditions, not only does the surface layer structure in which 80% or more of the surface area is bainite is formed, but also the effect of accelerated cooling cannot be obtained and the steel sheet does not have high strength. By cooling from a temperature of Ar ₃ or higher to a cooling rate of air or higher, precipitation of polygonal ferrite is suppressed and bainite formation is promoted.

[0028]

EXAMPLE Steels having the chemical compositions shown in Table 1 were heated, hot rolled and cooled under the manufacturing conditions shown in Tables 2 (1) and (2) to form thick steel plates.

Then, test pieces were cut out from these thick steel plates and evaluated for mechanical properties and weather resistance. Table 3 (1)
The results of these evaluations are shown in (2).

[0030]

[Table 1]

[0031]

[Table 2 (1)]

[0032]

[Table 2 (2)]

[0033]

[Table 3 (1)]

[0034]

[Table 3 (2)]

As is apparent from Table 3, all of the examples of the present invention have good weldability, strength and toughness, and also have excellent weather resistance. On the other hand, Comparative Examples in which the component composition or manufacturing conditions deviate from the conditions specified in the present invention are inferior in mechanical properties, weldability and weather resistance. Good weldability was not obtained for sample No. 1 having a large amount of C, sample Nos. 4 and 5 having a large amount of P, and sample No. 9 having a large amount of S. Sample No.6 with a small amount of S, Sample No.10 with a large amount of Si and JIS
Sample No. 11, which has a composition specified by G3106 for SM50A (weather resistant hot rolled steel for welded structures), does not have good weather resistance.

Samples Nos. 12, 13, 14 and 15 having low cooling start temperatures, Samples Nos. 20, 21, 22 and 23 having high cooling end temperatures, and Samples Nos. 32, 33, 34 and 35 having low heating temperatures. Is
80% or more of the steel surface layer does not have a bainite structure and the precipitation of relatively coarse ferrite is accompanied by poor weather resistance.
Sufficient strength is not obtained. Note that Sample Nos. 28, 29, 30 and 31 having high heating temperatures were incapable of rolling because of ingot cracking immediately after the start of rolling.

[0037]

EFFECTS OF THE INVENTION According to the present invention, maintenance-free high weather-resistant steel which has excellent weldability, produces dense weather-resistant rust at an early stage and can be used without treatment, and stably manufactures it at low cost. A method is provided.

Claims (2)

[Claims] 1. C: 0.05% or less, Si: 0.5% or less, M
n: 2.0% or less, P: 0.05 to 0.25%, Al: 0.003 to
0.2%, Cr 1.0% or less, Cu: 1.0% or less, Ni: 1.
0% or less, S: more than 0.005% and 0.020% or less, N is limited to 0.004% or less, the balance is Fe and inevitable impurities, and has a surface layer structure in which 80% or more of the surface area is bainite High weatherability steel plate with excellent weldability, which is characterized by 2. C: 0.05% or less, Si: 0.5% or less, M
n: 2.0% or less, P: 0.05 to 0.25%, Al: 0.003 to
0.2%, Cr 1.0% or less, Cu: 1.0% or less, Ni: 1.
0% or less, S: more than 0.005% and 0.020% or less, N is limited to 0.004% or less, and the balance is Fe and unavoidable impurities. 50% or more hot rolling is performed in a temperature range of ₃ points or more, then Ar is continuously cooled from a temperature of ₃ points or more at a cooling rate of air cooling or more, and cooling is completed in a temperature range of 550 ° C or less and 300 ° C or more. A method for producing a high weather-resistant steel sheet having excellent weldability, which comprises: