JP3242418B2 - High-strength low-carbon Cr-Mo steel sheet with excellent low-temperature crack resistance and SR crack resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-carbon Cr-Mo steel sheet, and more particularly to a low-temperature resistant steel plate capable of preventing low-temperature cracking generated during welding and subsequent SR cracking during stress relief annealing (hereinafter referred to as SR). The present invention relates to a high-strength low-carbon Cr-Mo steel sheet having excellent cracking and SR cracking resistance.

[0002]

2. Description of the Related Art Conventionally, Cr-Mo steel sheets have been widely used as structural members of power generation turbines such as high-temperature pressure vessels and turbine diaphragm materials in chemical industry plants such as petroleum refining and power generation plants. I have.

[0003] However, conventional Cr-Mo steel sheets usually have a high C content of about 0.15%, so they are highly susceptible to welding, and usually require preheating at about 150 to 350 ° C to prevent low temperature cracking. This has contributed to an increase in manufacturing costs, such as a prolonged manufacturing period and large consumption of heat energy.

[0004] In order to improve the low-temperature crack resistance of a Cr-Mo steel sheet, the present inventors have already disclosed in JP-A-2-6103.
As described in No. 5, while the amount of C is significantly reduced, by adding a small amount of B, Cu, and Ni in a complex manner, it is equivalent to a conventional Cr-Mo steel sheet that is a low carbon steel but has a large amount of C. It has been found that a Cr-Mo steel sheet having the following characteristics can be obtained.

Further, in order to effectively use the effect of improving the hardenability of B, N is fixed with Ti, and the amount of Sol.
It has also been found that by setting the content to less than%, the susceptibility to SR is suppressed to the same level as a conventional steel sheet.

[0006]

However, recently, even in plants where these Cr-Mo steel sheets are used, there is a tendency that operating conditions are increased to high temperatures and high pressures in order to improve efficiency. The demands are very strong. At the same time, a high residual stress tends to be applied to the welded joint after welding due to the complicated structure of the equipment. In such a case, since the occurrence of SR cracking is concerned, the structural material is required to have high SR cracking resistance.

In order to increase the strength, it is effective to add a precipitation strengthening element, as disclosed in JP-A-51-69422 and JP-A-60-238451.
JP-A-60-238452 discloses the addition of V and / or Nb. However, V and Nb form carbonitrides and exhibit precipitation strengthening, thereby deteriorating weldability and toughness. Therefore, simply adding these elements causes problems in weldability and toughness. In JP-A-60-238451 and JP-A-60-238452, reduction of Si and solid solution
Although the reduction in S improves the SR cracking resistance and toughness, it does not mention the low temperature cracking resistance.

[0008] Furthermore, since the SR resistance of the Cr-Mo steel greatly depends on the type of steel, the effects of the conventional improvement methods differ depending on the type of steel. For this reason, it has been particularly difficult to stably secure excellent SR cracking resistance in 1-1.25% Cr-0.5% Mo steel, which is considered to have high SR cracking sensitivity.

The present invention has been made in order to solve the above-mentioned problems, and it has been proposed to add a small amount of V in addition to utilizing B while ensuring low-temperature resistance to low carbon. Achieved high strength and, in addition to Sol.Al and O content regulations,
By controlling the amount of Cu, Ni and V according to the Cr content, S
It is an object of the present invention to provide a high-strength low-carbon Cr-Mo steel sheet which is excellent in low-temperature cracking resistance and SR cracking resistance in which R cracking is prevented.

[0010]

Means for Solving the Problems The present inventors have developed a low carbon material having high strength and excellent resistance to low-temperature cracking and SR.
In order to obtain a Cr-Mo steel sheet, intensive research was conducted on the basis of a low carbon Cr-Mo steel sheet, and the following findings were obtained. Add a small amount of V
High strength is achieved by finely depositing carbides. In low-carbon component systems, Si is effective in securing strength. By suppressing the amount of Sol.Al to less than 0.003% and at the same time regulating the amounts of Cu, Ni and V according to the Cr content, SR resistance can be greatly improved. Furthermore, by keeping O low, it is possible to keep the amount of Si oxide-based inclusions low, and it is resistant to low temperature cracking and S
It is effective for improving the R characteristic and toughness. To obtain the above effects to the maximum, it is important to suppress the formation of ferrite during normalizing. For this purpose, solid solution B, which is effective for improving hardenability, is used.

The present invention has been made based on these five findings, and the first invention is that C: 0.03 to 0.12%, Si:
0.15 to 1.00%, Mn: 0.20 to 1.00%, P: 0.020% or less, S:
0.009% or less, Cu: 0.40% or less, Ni: 0.40% or less , Cr:
0.40 to 3.50%, Mo: 0.20 to 1.25%, V: 0.005% or more 0.09
%, Sol.Al: less than 0.003%, O: 0.0035% or less, Ti: 0.0
10 to 0.030%, B: 0.0003 to 0.0020%, the balance being Fe and unavoidable impurities, and Ti and B satisfy the following equation (1), and Cu, Ni, and V satisfy the following equation (2). It is a high-strength low-carbon Cr-Mo steel sheet that is excellent in satisfactory low-temperature crack resistance and SR crack resistance. 0.23Ti + B -0.79N ≥0.0003 (%) ……………………………………………………………………………………… ····························································· 1 where N indicates N% in steel. [｛Cu + (Ni / 2) + V｝ / Cr] ≦ 0.30 ……………………… 2 formula

The second invention is characterized in that Nb: 0.01 to 0.10% or Ca:
The high-strength low-carbon Cr-Mo steel sheet according to claim 1, which contains 0.0005 to 0.0100% and has excellent low-temperature cracking resistance and SR cracking resistance.

[0013]

Hereinafter, the present invention will be described in detail. First, the reasons for limiting the chemical components of the present invention will be described.

In the present invention, C is required to be added in an amount of 0.03% or more in order to secure the strength and erosion resistance of the steel. , The upper limit of the amount added is
0.12%. Therefore, the addition amount of C is 0.03-0.12%
Range.

Since Si is effective in securing strength and improving oxidation resistance, it is added in an amount of 0.15% or more. However, when added excessively, the tempering embrittlement susceptibility is increased.
1.00%. Therefore, the addition amount of Si is 0.15 to 1.00%
Range.

[0016] Mn is effective in increasing the strength and ductility of the steel, but if added excessively, the weldability is reduced. Therefore, the amount of Mn is set in the range of 0.20 to 1.00%.

P is contained as an impurity in steel,
It is desirable to reduce not only the toughness and weldability but also the tempering embrittlement susceptibility and the SR susceptibility. Therefore, in the present invention, the P content is set to 0.020% or less.

S, like P, is contained as an impurity in steel, but it significantly reduces the toughness of the steel and also increases the susceptibility to SR. Therefore, it is desirable that S be reduced. Shall be 0.009% or less.

Cu is an element effective for increasing the strength by solid solution strengthening and precipitation strengthening, and is also effective for improving the erosion resistance, and may be added. Since the workability and SR resistance are deteriorated, the content is set to 0.40% or less even when added.

Ni is effective in increasing the hardenability of the steel, and also in improving the erosion resistance, and is also effective in preventing cracking due to precipitation of Cu at the austenite grain boundaries at high temperatures. It may be added, but if added in excess of 0.40%, the resistance to SR is degraded, so even if added, the content is set to 0.40% or less.

Cr enhances oxidation resistance and tempering softening resistance,
It is effective for securing the strength, but if added excessively, the weldability is deteriorated. Therefore, the added amount of Cr is 0.40 to 3.50
% Range.

Mo is an element indispensable for improving the hardenability of steel, especially when normalizing when coexisting with B. Further, Mo is effective in increasing the tempering softening resistance and improving the high-temperature strength. For this purpose, it is necessary to add 0.20% or more. However, since it is an expensive element, the amount of Mo is 0.20 to 1.25. % Range.

V must be added in an amount of at least 0.005% in order to increase the strength by precipitating fine carbides, but if added in excess of 0.09%, the weldability is reduced. 0.005% or more and less than 0.09%.

Conventionally, Sol. Al has been added to B-added steel for the purpose of fixing N in order to make effective use of B. However, this method produces sulfide finely connected to grain boundaries. Therefore, the resistance to SR is reduced.
For this reason, the addition amount of Sol.Al is less than 0.003%.

In the steel of the present invention in which O is added to suppress the amount of Sol. Al to a low level, O forms Si oxide-based inclusions,
The resistance to low-temperature cracking and the resistance to SR cracking are reduced, and the toughness is also impaired. Therefore, in order to prevent these, the O content is set to 0.0035% or less.

B enhances the hardenability during normalizing and is effective in increasing the strength.
% Must be added. But B is 0.0020
%, An excessive amount of B compound is generated at the time of normalizing to reduce the hardenability and at the same time to reduce the SR resistance.
With deterioration of brittleness and toughness. Therefore, the addition amount of B is set in the range of 0.0003 to 0.0020%.

Ti, as a strong nitride-forming element, fixes N, prevents B from reacting with N, and makes B a solid solution B, thereby exhibiting the effect of improving the hardenability of B. In the present invention, in order to secure high strength, it is necessary to suppress the formation of ferrite during normalizing, and Ti is added for the purpose of securing a solid solution B amount that enhances hardenability.

Further, Ti combines with S to form a stable sulfide, thereby reducing the amount of solute S at the crystal grain boundaries and increasing the SR resistance. To obtain such an effect, it is necessary to add Ti in an amount of 0.010% or more.
Since the effect is saturated even if it exceeds 0%, the addition amount of Ti is set in the range of 0.010 to 0.030%.

The amounts of Ti and B added must satisfy the following formula. 0.23Ti + B -0.79N ≥0.0003 (%) where N indicates N% in steel. In the formula, Ti is used as N in steel in order to secure 0.0003% or more of solid solution B from added B
Indicates the amount of Ti required to fix the.

Further, in the present invention, the amounts of Cu, Ni and V are regulated in accordance with the following formula according to the amount of Cr for the purpose of ensuring stable resistance to SR. [{Cu + (Ni / 2) + V} / Cr] ≦ 0.30 where Cu, Ni, V, and Cr are the contents (%) in steel.

That is, the reasons for regulating Cu, Ni, V and Cr are as follows:
As shown in FIG. 1, [{Cu + (Ni / 2) + V} / Cr] is 0.3
Is exceeded, the SR rate increases sharply. Fig. 1 shows a C: 0.06-0.09% low carbon Cr-Mo steel sheet (1.25% Cr-0.
5% Mo, 2.25% Cr-1% Mo) were obtained by examining the SR resistance characteristics by changing the amounts of Cu, Ni and V. S resistance
For the R characteristic, a 25 mm-thick oblique Y-shaped welded test piece was used, and the cross-sectional deformation rate was defined as the SR crack rate. As is clear from FIG. 1, in order to obtain a stable and good SR resistance regardless of the type of Cr-Mo steel, [｛Cu + (Ni / 2) +
[V 肝 / Cr] is set to 0.30 or less. In addition,
The B of the Cr-Mo steel used in the survey was 0.0006-0.0009%, and Sol.
Al is less than 0.003%.

The Cr—Mo steel sheet according to the present invention can contain Nb or Ca in addition to the above elements. Nb effectively acts to increase the strength by precipitating a stable carbonitride. To achieve this effect, Nb should be 0.01%
Although it is necessary to add Nb as described above, the effect is saturated even if it exceeds 0.10%, so that the amount of Nb added is in the range of 0.01 to 0.10%.

Ca has the effect of refining the crystal grains and generates sulfide to reduce the amount of solid solution S at the crystal grain boundaries, so that the resistance to SR is further enhanced. In order to effectively obtain this effect, Ca must be added in an amount of 0.0005% or more. However, when excessively added, the amount of nonmetallic inclusions increases and ductility decreases, so the upper limit is 0.01%.
00%. Therefore, the amount of Ca added is 0.0005 to 0.0100
% Range.

It is known that impurity elements such as Sb, Sn, As and the like deteriorate SR resistance and deteriorate characteristics.
Therefore, in the present invention, it is preferable that each of these elements is 0.01% or less. Cr according to the invention
In order to manufacture a -Mo steel sheet, a steel slab containing a chemical component defined by the present invention may be manufactured according to a conventional method, hot-rolled, and subsequently heat-treated.

[0035]

EXAMPLES The present invention will be described below with reference to examples.
The present invention is not limited by these examples. Table 1 shows the chemical components of the steel of the present invention and the comparative steel, and Table 2 shows the mechanical properties, low-temperature cracking resistance and SR cracking resistance of the steel of the present invention and comparative steel. Low temperature crack resistance and S resistance
The R characteristic is obtained by using an oblique Y-shaped welded test piece having a thickness of 25 mm.

[0036]

[Table 1]

[0037]

[Table 2]

The steels A to G are comparisons between the steel according to the present invention and a comparative steel relating to 1.25% Cr-0.5% Mo steel. The steels A to D according to the present invention have a high strength of 612 N / mm ² or more, and are excellent in low temperature crack resistance and SR crack resistance.

On the other hand, the comparative steel E is a conventionally known steel sheet having a C content of 0.14%, and has a preheating temperature of 150 ° C. for preventing rooting in the oblique Y-shaped welding test.
Requires a higher preheating temperature than low carbon steel sheets. Comparative steel F is an example in the case where the amount of Sol.Al is large. However, since the amount of Sol.Al is large, the SR resistance is inferior. Comparative steel G is
Because [{Cu + (Ni / 2) + V} / Cr] exceeds 0.30,
Low resistance to SR.

The steels HK compare the steel according to the invention with a comparative steel relating to 2.25% Cr-1% Mo steel. The steels H and I according to the present invention are superior to the comparative steels J and K in strength, low-temperature cracking resistance and SR cracking resistance. On the other hand, comparative steel J
Is a conventionally known steel sheet with a C content of 0.14%.
175 ° C, which requires a higher preheating temperature than low carbon steel sheets. Comparative steel K is [{Cu + (Ni / 2) + V} / Cr]
Is more than 0.30, the SR resistance is low.

[0041]

As described above, the present invention provides a low carbon
In Cr-Mo steel sheets, high strength is ensured by adding a small amount of V, and the amount of Sol.Al is reduced to less than 0.003%.
By controlling the amounts to 0.0035% or less and simultaneously controlling the amounts of Cu, Ni, and V in accordance with the Cr content, excellent SR resistance is stably secured. It is possible to obtain a high-strength low-carbon Cr-Mo steel sheet having strength and having excellent weld resistance and SR resistance, which can prevent low-temperature cracking and SR cracking that occur during welding.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between [{Cu + (Ni / 2) + V} / Cr] and SR cracking rate.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (2)

(57) [Claims] [Claim 1] C: 0.03-0.12%, Si: 0.15-1.00%, M
n: 0.20 to 1.00%, P: 0.020% or less, S: 0.009% or less, C
u: 0.40% or less, Ni: 0.40% or less , Cr: 0.40 to 3.50%,
Mo: 0.20-1.25%, V: 0.005% or more and less than 0.09%, Sol.Al:
Less than 0.003%, O: 0.0035% or less, Ti: 0.010-0.030%,
B: contains 0.0003 to 0.0020%, the balance consists of Fe and unavoidable impurities, and Ti and B satisfy the following equation (1), and Cu, Ni, and V satisfy the following equation (2). High-strength low-carbon Cr-Mo steel sheet with excellent low-temperature cracking and SR cracking resistance. 0.23Ti + B -0.79N ≧ 0.0003 (%) ……………………………………………………………………………………………………………… ············································································································ 1 [｛Cu + (Ni / 2) + V｝ / Cr] ≦ 0.30 ……………………… 2 2. Nb: 0.01-0.10% or Ca: 0.0005-0.
The high-strength, low-carbon Cr— excellent in low-temperature cracking resistance and SR cracking resistance according to claim 1 containing 0100%.
Mo steel plate.