JPH08337815A - Production of chromium-molybdenum steel excellent in strength and toughness - Google Patents

Abstract

PURPOSE: To produce a Cr-Mo steel excellent in strength, toughness and resistance to tempering embrittlement sensitivity by a direct hardening method by subjecting a Cr-Mo steel having a specified compsn. to heating and hot rolling, executing hardening and thereafter subjecting the same to cooling and tempering under specified conditions. CONSTITUTION: A slab contg., by weight, 0.05 to 0.20% C, <=0.30% Si, <=1.00% Mn, 1.00 to 3.50% Cr, 0.40 to 2.00% Mo and <=0.0200% N, and the balance Fe is prepd. This slab is heated at 950 to 1100 deg.C, is rolled under the conditions in which the cumulative draft in the unrecrystallized temp. region is regulated to >=20% and the rolling finishing temp. to the Ar3 point or above, is immediately hardened at <=500 deg.C, is thereafter cooled to a room temp. and is subsequently tempered at >=600 deg.C. If required, the same steel may furthermore be incorporated with one or more kinds among <=0.50% Cu, <=0.50% Ni, 0.0003 to 0.0030% B, 0.05 to 0.40% V, 0.003 to 0.050% Nb, 0.003 to 0.015% Ti, 0.005 to 0.050% Al, 0.0005 to 0.0100% Ca and 0.0005 to 0.0200% rare earth metals.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a steel for pressure vessels, and particularly to Cr- which is excellent in strength and toughness.
The present invention relates to a method for manufacturing Mo steel.

[0002]

2. Description of the Related Art Since Cr-Mo steel is excellent in hydrogen corrosion resistance and high temperature strength, it is widely used as a material for pressure vessels used under high temperature and high pressure hydrogen atmosphere in petroleum refining plants and chemical plants. There is. In recent years, these plants tend to be operated under conditions of higher temperature and higher pressure from the viewpoint of efficiency improvement. Therefore, Cr-Mo steel is required to have higher strength. Further, from the viewpoint of safe operation, higher toughness has been required to prevent brittle fracture of the pressure vessel. With respect to this toughness, since the pressure resistance test performed in the regular inspection of the pressure vessel is performed at room temperature, not only the high operating temperature but also the characteristics at room temperature are necessary. As described above, the Cr-Mo steel used for the pressure vessel has been required to have a material having higher strength and toughness than ever before due to changes in the use environment.

By the way, when various reheating treatments (quenching, annealing, normalizing) generally known in the past are applied to the production method of Cr-Mo steel, fine grain and good toughness are applied. However, there was a problem that sufficient strength could not be obtained. Therefore, some attempts have been made to improve the strength. For example, a so-called direct quenching method in which direct quenching is performed after hot rolling is disclosed in Japanese Patent Publication No. 1-29853 and Japanese Patent Publication No.
It is disclosed in Japanese Patent No. 9647. Since this direct quenching method is a heat treatment method that does not include a reheating step, it is a manufacturing technique having many advantages not only in terms of energy saving but also in terms of productivity and economy.

[0004]

However, in these conventional direct quenching methods, although the strength of the steel can be enhanced as compared with the above-mentioned reheating heat treatment method, since the crystal grains of the structure after rolling are coarse, There was a problem of low toughness.
Further, in this method, due to the coarse crystal grains, there is a high susceptibility to temper embrittlement, and there is a concern that embrittlement during use may occur. As described above, in the conventional technology for producing Cr-Mo steel by the reheating heat treatment method or the direct quenching method, the material of high strength and high toughness cannot be obtained, and the material of which temper temper embrittlement susceptibility is small There was a problem that could not be obtained.

The present invention was developed in view of the above-mentioned problems in the material of the conventional Cr-Mo steel, and an object of the present invention is to propose a method for producing Cr-Mo steel in which strength and toughness are advantageously improved. And

[0006]

Means for Solving the Problems In order to achieve the above object, the inventors have conducted extensive studies on the effects of the composition of components, the conditions of hot rolling and direct quenching on the strength and toughness. Came to obtain the knowledge of. By imparting a cumulative rolling reduction of 20% or more in the unrecrystallized region and directly quenching, transformation strengthening and microstructure refinement are achieved, and both strength and toughness are improved. Since the γ grain size obtained by rolling in the non-recrystallization region should not be as fine as that of the reheating heat treatment, it was feared that the tempering embrittlement susceptibility would decrease, but this tempering embrittlement resulted in a Si content of 0.30 wt%. It can be suppressed by controlling the following. The heating temperature is the same as that of the conventional direct quenching method (110
By setting the temperature lower than 0 ° C. or higher), the crystal grains are made finer, and the toughness and the tempering embrittlement susceptibility are improved. By making the heating temperature higher than that of the reheating heat treatment method (reheating temperature is generally 900 to 950 ° C.), Cr,
The solid solution and diffusion of alloy elements such as Mo, V, Nb, and Ti are promoted, the temper softening resistance is increased, and high strength is obtained.

The present invention is based on the above findings, and its gist is as follows. 1) C: 0.05 to 0.20 wt%, Si: 0.30 wt% or less, Mn:
1.00wt% or less, Cr: 1.00 to 3.50wt%, Mo: 0.40 to
2.00wt%, N: 0.0200wt% or less, balance is Fe
And a slab consisting of the composition of unavoidable impurities.
When heated to 00 ℃, the cumulative rolling reduction in the unrecrystallized temperature range is 2
A strength characterized by rolling under 0% or more and a rolling end temperature of Ar ₃ points or more, immediately quenching at a temperature of 500 ° C or less, cooling to room temperature, and then tempering at a temperature of 600 ° C or more. And a method for producing Cr-Mo steel excellent in toughness.

2) C: 0.05 to 0.20 wt%, Si: 0.30 wt%
% Or less, Mn: 1.00 wt% or less, Cr: 1.00 to 3.50 wt
%, Mo: 0.40 to 2.00 wt%, N: 0.0200 wt% or less, Cu: 0.50 wt% or less, Ni: 0.50 wt% or less,
B: 0.0003 to 0.0030 wt%, V: 0.05 to 0.40 wt%, Nb: 0.
003 to 0.050 wt%, Ti: 0.003 to 0.015 wt%, Al: 0.00
5 to 0.050 wt%, Ca: 0.0005 to 0.0100 wt%, REM: 0.00
A steel slab containing one or more selected from 05 to 0.0200 wt% and the balance Fe and inevitable impurities is heated to 950 to 1100 ℃ and accumulated in the non-recrystallization temperature range. Characterized by rolling under a rolling reduction of 20% or more and a rolling end temperature of Ar ₃ points or more, immediately quenching at a temperature of 500 ° C or less, cooling to room temperature, and then tempering at a temperature of 600 ° C or more Cr-M with excellent strength and toughness
o Steel manufacturing method.

[0009]

In the following, the reason why the present invention is limited to the above-mentioned essential constitution will be explained. C: 0.05 to 0.20 wt% C is an element effective for securing strength, and at least 0.0
A content of 5 wt% is necessary. On the other hand, when the content exceeds 0.20 wt% and is excessive, the weldability is greatly deteriorated, so 0.05 to 0.20 wt%, preferably 0.10 to
0.16 wt%.

Si: 0.30 wt% or less Si is effective for increasing strength, but if it is excessively contained in excess of 0.30 wt%, the temper embrittlement susceptibility becomes high, so 0.30 wt% or less, preferably 0 15 wt% or less.

Mn: 1.00 wt% or less Mn is an element effective in increasing the strength, but if too much is added, the temper embrittlement susceptibility is increased and the weldability is lowered, so 1.00 wt% or less , Preferably 0.80 wt%
Below.

Cr: 1.00 to 3.50 wt% Cr is an element effective in improving high temperature strength, oxidation resistance and hydrogen corrosion resistance, and it is necessary to add at least 1.00 wt% or more. On the other hand, if added in excess of 3.50 wt%, the weldability and creep strength decrease, so 1.00
To 3.50 wt%.

Mo: 0.40 to 2.00 wt% Mo is an element effective in improving hydrogen corrosion resistance and high temperature strength, especially creep strength, and in order to exert its effect, 0.40 wt% or more must be added. is there. On the other hand, 2.0
If added in excess of 0 wt%, the effect is saturated, which is uneconomical and reduces weldability.
Limit to the range of 2.00 wt%. The preferable addition range is 0.50 to 1.50 wt%.

N: 0.0200 wt% or less N is an element that adversely affects toughness, and especially 0.0
If the content exceeds 200 wt%, the toughness is drastically reduced.
Further, when the amount of N increases, in the steel containing B, BN is formed to reduce the hardenability of B, and in the steel containing Ti, Ti is added.
The amount of N is 0.0200 wt% because it forms N and hinders solid solution of Ti (precipitation strengthening by Ti) at low temperature heating.
% Or less, preferably 0.0060 wt% or less.

Cu: 0.50 wt% or less Cu is an element effective for improving hardenability and toughness, but if added in excess of 0.50 wt%, it increases susceptibility to temper embrittlement, so 0.50 wt% % Or less, preferably 0.30 wt% or less.

Ni: 0.50 wt% or less Ni is an element effective in improving hardenability and toughness, but if added in excess of 0.50 wt%, it increases susceptibility to temper embrittlement, so 0.50 wt% % Or less, preferably 0.30 wt% or less.

B: 0.0003 to 0.0030 wt% B is an element useful for improving hardenability, strength and toughness. In order to exert these addition effects, it is necessary to add 0.0003 wt% or more. However, even if added over 0.0030 wt%, the hardenability rather deteriorates, especially the toughness decreases, so 0 .00
03-0.0030, preferably 0.0005-0.0
The range is 015 wt%.

V: 0.05 to 0.40 wt% V is an element that precipitates stable carbides during tempering and heat treatment after welding, improves the strength at room temperature and high temperature, and enhances the resistance to water line erosion. In order to exert these effects, the addition amount of at least 0.05 wt% is necessary. On the other hand, even if added in excess of 0.40 wt%, the effect tends to be saturated, and further toughness and weldability are deteriorated, so 0.05 to 0.40 wt%, preferably 0.10 to 0. 0.35 wt%.

Nb: 0.003 to 0.050 wt% Nb is an element that precipitates stable carbides during tempering and during post-weld heat treatment to improve strength at room temperature and high temperature and enhance water line erosion resistance. In order to exert these effects, the added amount of at least 0.003 wt% is necessary. On the other hand, even if added in excess of 0.050 wt%, the effect tends to be saturated, and further toughness and weldability are deteriorated, so 0.003 to 0.0
50 wt%, preferably 0.005-0.025 wt%.

Ti: 0.003 to 0.015 wt% Ti is an element that precipitates stable carbides at the time of tempering and at the time of heat treatment after welding to improve the strength at room temperature and high temperature and to enhance the resistance to water line erosion. In order to exert these effects, the added amount of at least 0.003 wt% is necessary. On the other hand, even if added in excess of 0.015 wt%, the effect tends to be saturated, and further toughness and weldability are deteriorated, so 0.003 to 0.0
15 wt%, preferably 0.008 to 0.012%.

Al: 0.005 to 0.050 wt% Al is an element effective for deoxidizing steel and for suppressing coarsening in heating at 1100 ° C. or lower, and in order to exert these effects, 0 It is necessary to add more than 0.005% by weight. On the other hand, if added in excess of 0.050 wt%, the creep strength will be reduced. Therefore, the addition amount of Al is 0.005 to 0.050 wt%, preferably 0.0
The range is 10 to 0.025 wt%.

Ca: 0.0005 to 0.0100 wt% Ca is an element useful for enhancing stress relief annealing cracking resistance and toughness, and in order to exert these effects, addition of 0.0005 wt% or more is necessary. Is. on the other hand,
If these elements are added excessively, the cleanliness of steel is deteriorated and the toughness is rather lowered. Therefore, the addition amount of Ca is 0.0005 to 0.0100 wt%, preferably 0.00
It is set to 10 to 0.0050 wt%.

REM: 0.0005 to 0.0200 wt% REM (rare earth element) is an element useful for enhancing stress relief annealing cracking resistance and toughness, and in order to exert these effects, 0.0005 wt% or more Is required. On the other hand, if these elements are excessively added, the cleanliness of steel deteriorates and the toughness deteriorates. Therefore, R
The amount of EM added is 0.0005 to 0.0200 wt%, preferably 0.0010 to 0.0080 wt%.

The lower the content of P and S as impurity elements, the better. However, P is 0.015 wt% or less from the viewpoint of suppressing temper embrittlement, and S is 0.1% from the viewpoint of ensuring good toughness. It is desirable to reduce it to 008 wt% or less.

After the steel having the above chemical composition is melted in a converter or an electric furnace, ladle refining or vacuum degassing treatment is applied if necessary, and the ingot-separating method or continuous casting is performed. Method to make cast steel slab. After heating this steel slab, it is manufactured by hot rolling and direct quenching. The manufacturing method according to the present invention will be described below. -Heating temperature: 950 to 1100 ° C If the heating temperature is lower than 950 ° C, solid solution of Cr, Mo, V,
Since the diffusion is not performed sufficiently, good hardenability and high temper softening resistance cannot be obtained. Also, below 950 ° C,
The precipitation strengthening effect of Nb and Ti is not exhibited. For this reason,
In order to secure sufficient strength, heating at 950 ° C. or higher, which is higher than in the reheating heat treatment method, is necessary. On the other hand, 1100
If the heating is performed at a temperature higher than 0 ° C, the γ-grains become too coarse, and they cannot be sufficiently refined in the rolling process, resulting in a decrease in toughness. Therefore, the heating temperature is 950 to 1100 ° C, preferably 1030 to 1100 ° C.

Cumulative rolling reduction in the non-recrystallization temperature range: 2
The slab heated to 0% or more is hot-rolled by reduction of multiple passes. In this hot rolling, in order to achieve transformation strengthening, microstructure refinement, and strength and toughness improvement by quenching of working γ, at least a cumulative rolling reduction of at least 20% is required in the non-recrystallization temperature range. is there. Therefore, the cumulative reduction rate in the non-recrystallized region is set to 20% or more, preferably 25% or more. In the present invention, the negative side in strength due to the lowering of the heating temperature is sufficiently eliminated by adopting the above-mentioned means that the cumulative rolling reduction in the non-recrystallization temperature region is 20% or more.

Rolling end temperature: Ar ₃ points or higher If the rolling temperature is lower than Ar ₃ points, ferrite is formed and good quenchability cannot be obtained even after rapid cooling.
Strength and toughness decrease. Therefore, the rolling end temperature is A
r ₃ points or more.

Cooling condition: Quenching to a temperature of 500 ° C. or lower, and cooling to room temperature. After quenching, strong quenching with a fountain is desirable, and efficient quenching with an accelerated cooling facility is preferable. When the cooling stop temperature is low, a fine quenching structure can be obtained, while
If it exceeds 500 ° C., a coarse structure is formed and strength and toughness are reduced. Therefore, the cooling stop temperature is set to 500 ° C. or lower, preferably 450 ° C. or lower. Also, after quenching,
The reason for cooling to room temperature is to obtain good toughness by performing tempering after completely completing the transformation.

Tempering temperature: 600 ° C. or higher In order to obtain uniform and excellent toughness, tempering treatment is performed after quenching. In order to achieve this purpose, the tempering temperature must be 600 ° C or higher. The preferable tempering temperature is preferably 625 to 725 ° C.

[0030]

【Example】

Example 1 A slab having the chemical composition shown in Table 1 was heated, hot rolled, and directly quenched (water cooled) under the conditions shown in Table 2, cooled to room temperature, and then tempered. In addition, for comparison, some of them were subjected to air-cooling-reheating quenching after rolling instead of the above direct quenching. After these quenching treatments, a tempering treatment was performed under the condition of holding at 690 ° C. for 24 hours.

[0031]

[Table 1]

[0032]

[Table 2]

With respect to the obtained test materials, the tensile properties at room temperature, the Charpy impact properties, and the embrittlement amount ΔvTrs after GE type step cooling treatment (accelerated heat treatment for measuring temper embrittlement properties) were investigated. The results of these tests are also shown in Table 2. Further, FIG. 1 shows that the tensile strength (TS) and the fracture surface transition temperature (v
Trs) is plotted as a relation. In FIG. 1, the ● marks indicate the characteristics of the materials produced by the invention method and the ○ marks indicate the characteristics of the materials produced by the comparative method.

From Table 2 or FIG. 1, the Cr-Mo steel produced according to the method of the present invention is superior in toughness to the comparative method in which direct quenching conditions are inappropriate and has higher strength than the comparative method by reheating and quenching. I understand. Furthermore, it can be seen that the Cr-Mo steel produced according to the method of the present invention has a smaller ΔvTrs and a lower susceptibility to temper embrittlement than those obtained by the comparative method in which direct quenching conditions are inappropriate. In addition,
Steel (F) with an excessively low Cr and Mo content shows almost no improvement in mechanical properties even if the direct quenching conditions are appropriate, and steel with an excessive Mn content (G). Even if the direct quenching condition is appropriate, ΔvTr
It can be seen that S is large and temper embrittlement susceptibility is large.

[0035]

As described above, according to the present invention, a Cr-Mo steel having excellent strength and toughness and a low susceptibility to temper embrittlement can be obtained.
It can be manufactured by the direct quenching method. Therefore, according to the present invention, a material for a pressure vessel used under a higher temperature and high pressure hydrogen atmosphere can be efficiently produced in an energy saving process, which brings about an extremely large industrial contribution.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between tensile strength (TS) and fracture surface transition temperature (vTrs) at room temperature.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akihiro Matsuzaki 1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama Prefecture (no address) Inside the Mizushima Works, Kawasaki Steel Co., Ltd. (72) Shinichi Amano, Mizushima-kawasaki-dori, Kurashiki-shi, Okayama Prefecture 1 chome (without street number) Kawasaki Steel Works Mizushima Steel Works

Claims (2)

[Claims] 1. C: 0.05 to 0.20 wt%, Si: 0.30 wt% or less, Mn: 1.00 wt% or less, Cr: 1.00 to 3.50 wt%, Mo: 0.40 to 2.00 wt%, N: 0.0200 wt% or less A steel slab containing Fe and unavoidable impurities in the balance is heated to 950 to 1100 ° C., the cumulative rolling reduction in the unrecrystallized temperature range is 20% or more, and the rolling end temperature is Ar ₃ points or more. A method for producing a Cr-Mo steel excellent in strength and toughness, which comprises rolling under conditions, immediately quenching at a temperature of 500 ° C or lower, cooling to room temperature, and then tempering at a temperature of 600 ° C or higher. 2. C: 0.05 to 0.20 wt%, Si: 0.30 wt% or less, Mn: 1.00 wt% or less, Cr: 1.00 to 3.50 wt%, Mo: 0.40 to 2.00 wt%, N: 0.0200 wt% or less Including,
And Cu: 0.50 wt% or less, Ni: 0.50 wt% or less, B: 0.0003 to 0.0030 wt%, V: 0.05 to 0.40 wt%, Nb: 0.003 to 0.050 wt%, Ti: 0.003 to 0.015 wt%, Al: 0.005 ~ 0.050 wt%, Ca: 0.0005-0.0100 wt%, REM: 0.0005-0.0200 wt%, containing one or more selected from the rest, the balance is a steel piece composed of Fe and inevitable impurities, Roll at a temperature of 950 to 1100 ° C, a cumulative rolling reduction in the non-recrystallization temperature range of 20% or more, and a rolling end temperature of Ar ₃ points or more, immediately quench to a temperature of 500 ° C or less, and then reach room temperature. A method for producing a Cr—Mo steel having excellent strength and toughness, which comprises cooling and then tempering at a temperature of 600 ° C. or higher.