JPH07242991A - High toughness chromium-molybdenum steel sheet excellent in weldability - Google Patents

Abstract

PURPOSE:To produce a normalized-tempered type Cr-Mo steel sheet excellent in weldability and toughness at low temp. CONSTITUTION:This steel sheet is a high toughness Cr-Mo steel sheet excellent in weldability, having a composition which contains 0.03-<0.05% C, 0.50-0.80% Si, 0.40-0.65% Mn, 0.05-0.50% Cu, 0.05-0.50% Ni, 1.00-1.50% Cr, 0.45-0.65% Mo, 0.005-0.100% sol.Al, and 0.005-0.030% Ti and in which P and S in impurities are regulated to <=0.020% and 0.015%, respectively, having a main structure composed of bainite, and having <=200Hv hardness. This high toughness steel sheet excellent in weldability can be produced by subjecting a steel sheet as material having the above composition to normalizing at 910-930 deg.C, to accelerated cooling by water cooling, and then to tempering treatment to form the main structure into bainite and regulate hardness to <=200Hv. In this steel sheet, maximum hardness after welding is <=300Hv, and the necessity of SR treatment after welding can be obviated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a chromium molybdenum steel sheet mainly used for boilers and pressure vessels and a method for producing the same.

[0002]

2. Description of the Related Art 1.1 / 4Cr-1 / 2Mo steel sheets are widely used as steel sheets for medium / high temperature pressure vessels in chemical industrial plants such as oil refining and power plants. However, the conventional 1 / 4Cr-1 / 2Mo steel sheet usually has a high C content of about 0.15% and contains a relatively large amount of alloying elements such as Cr and Mo. Therefore, when performing welding,
From the viewpoint of preventing cold cracking, preheating at a high temperature of about 150 to 350 ° C is usually required, and advanced welding technology is required, leading to a long construction period and a large consumption of thermal energy, leading to cost reduction. This is one of the reasons for the rise.

Further, in the case of a 1 / 4Cr-1 / 2Mo steel sheet, it is necessary to make the Si content range relatively high, 0.50 to 0.80%, from the viewpoint of securing high temperature strength characteristics and erosion resistance. , Low temperature toughness decreases. Therefore, it is unavoidable that the low-temperature toughness of the parts that come into contact with the outside air of various equipment is 2.1 / 4Cr-
An example of using 1Mo steel sheet is recognized.

Many studies have been made in order to solve the above-mentioned problems of the 1 / 4Cr-1 / 2Mo steel sheet.

For example, the Cr-Mo steel disclosed in Japanese Unexamined Patent Publication No. 58-37157 has improved cracking resistance during stress relief annealing performed after welding by utilizing precipitation hardening of Nb.

The main components are Si: 0.10 to 0.80%, Cr:
0.40 to 6.00%, Mo: 0.40 to 1.50% and 1 / 4Cr-
Different from 1 / 2Mo steel. Moreover, according to the examples, only the case where the Si content is 0.29% or less is described.
It is presumed that the Mo steels can obtain the above-mentioned properties mainly on the low Si side.

Cr-Mo disclosed in Japanese Patent Publication No. 3-57361
Similarly, in the steel sheet as well, although excellent weldability is secured mainly by reducing the C content, the structure is ferrite-pearlite or ferrite-pearlite-bainite, and there is no point in improving the low temperature toughness. Not considered.

Cr-Mo disclosed in Japanese Patent Publication No. 3-68100
In steel sheets, a weld crack susceptibility composition P _CM is introduced to determine its upper limit, and the C content is reduced to improve weldability, and Al and B are added for strength compensation and hardenability improvement. Is going. However, the essential addition of B causes an increase in the maximum hardness, and there is a problem in that the hardness of the welded joint part particularly increases.

JP-A-62-89844 and JP-A-61-
The 104055 gazette, like the Japanese Patent Publication No. 3-68100, can improve the weldability and toughness of Cr-.
Mo steel sheet is shown. However, all of them are due to Al-B treatment, and the results are not sufficient, and there is room for improvement.

Japanese Patent Laid-Open No. 3-180442 discloses that Nb is added to secure weldability and high temperature (600 ° C.) strength, and at the same time, it is well known as a drawback of continuously cast slabs and slabs containing Nb. Low S as a countermeasure against cracking on the surface of slabs
The Cr-Mo-Nb steel for steel building structures is shown. However, the heat treatment method, the structure of the steel, the maximum hardness, etc. are not shown, and it is not clear whether this steel can be applied to a pressure vessel or the like.

[0011]

From the above points, it is considered an urgent need to develop a 1.1 / 4Cr-1 / 2Mo steel sheet excellent in weldability and low temperature toughness and a manufacturing method thereof. An object of the present invention is to provide a normalization-tempering type chromium molybdenum steel sheet having excellent weldability and low temperature toughness, and a method for producing the same.

[0012]

The summary of the present invention is as follows.
It is in the chromium molybdenum steel plate of (1) and its manufacturing method of (2).

(1)% by weight, C: 0.03 to less than 0.05%, S
i: 0.50 to 0.80%, Mn: 0.40 to 0.65%, Cu: 0.05 to 0.50
%, Ni: 0.05 to 0.50%, Cr: 1.00 to 1.50%, Mo: 0.45 to
0.65%, sol.Al: 0.005-0.100% and Ti: 0.005-
It contains 0.030%, the balance is Fe and unavoidable impurities, P in the impurities is 0.020% or less, S is 0.015% or less, the main structure is bainite, and the hardness is Hv.
High toughness chrome molybdenum steel sheet with excellent weldability characterized by being 200 or less.

(2) After the material steel plates having the above components are normalized at 910 to 930 ° C., they are subjected to accelerated cooling by water cooling and then tempered to obtain bainite for the main structure and Hv for hardness.
Is 200 or less, a method for producing a high toughness chromium molybdenum steel sheet having excellent weldability.

The "main structure" referred to here is 50% by volume.
% Means at least a bainite single phase, or the bainite has a volume fraction of 50% or more and the rest is ferrite.

The present inventors have earnestly studied to improve weldability and toughness in the production of a 1 / 4Cr-1 / 2Mo steel sheet, and as a result, have obtained the following new findings. Completed the invention.

In order to improve the weldability, it is effective to add a trace amount of Ti in order to reduce the amount of C remarkably while adding no B and further lowering the solid solution N.

In order to improve the toughness, it is necessary to carry out accelerated cooling (water cooling) treatment at the time of normalizing and performing bainite at the time of performing normalizing-tempering heat treatment.

At the time of accelerated cooling, it is necessary to add a trace amount of Cu and Ni in order to secure sufficient hardenability.

During normalizing, excessive heating causes coarsening of γ grains, which is disadvantageous in improving toughness, so that the heating temperature must be limited.

If the hardness is suppressed to 200 or less in Hv, the maximum hardness of the welded portion can be set to 300 or less in Hv, and the stress relief annealing after welding can be omitted.

[0022]

First, the reason why the chemical composition of the chromium-molybdenum steel and the material steel thereof according to the present invention is limited as described above will be explained. % Means% by weight.

C: 0.03 to less than 0.05% C must be contained in 0.03% or more in order to secure the strength and erosion resistance of the steel. However, since an increase in the C content causes a decrease in weldability and toughness, the C content is preferably 0.03% or more and is as low as possible. In order to obtain the desired properties, a C content of less than 0.05% is sufficient, and the upper limit was made less than 0.05%.

Si: 0.50 to 0.80% Si is an effective component for securing the strength of steel and improving the oxidation resistance, and is contained in 0.50% or more. On the other hand, Si content is 0.80
%, The weldability and toughness deteriorate, so the upper limit of the content was made 0.80% or less.

Mn: 0.40 to 0.65% Mn is an effective component for increasing the strength, toughness and ductility of steel. To obtain these effects, it is necessary to contain 0.40% or more. On the other hand, if the Mn content exceeds 0.65%, the weldability deteriorates. Therefore, the range of Mn content is 0.40 to 0.65%
And

Cu: 0.05 to 0.50% Cu is an effective component for precipitation strengthening, and also has an effect of improving erosion resistance. In order to effectively exhibit these effects, a Cu content of 0.05% or more is necessary. On the other hand, if it exceeds 0.50%, the hot workability deteriorates and cracks are likely to occur.

Therefore, the range of Cu content is set to 0.05 to 0.50%.

Ni: 0.05 to 0.50% Ni is a component necessary for suppressing the generation of cracks accompanying the precipitation of Cu at the austenite grain boundaries at high temperatures. If the Cu content is in the above range and the Ni content is 0.05% or less, the above crack suppressing effect cannot be obtained. On the other hand, if it exceeds 0.50%, the crack suppressing effect is saturated. Therefore, the range of Ni content is set to 0.05 to 0.50%.

Cr: 1.00 to 1.50% Cr is an important component in determining the desired properties. That is, Cr has a function of securing oxidation resistance and strength at high temperature together with Si. If the Cr content is less than 1.00%, these effects cannot be obtained. On the other hand, when it exceeds 1.50%, these effects are saturated. Therefore, the Cr content range is 1.00 to 1.
It was set to 50%.

Mo: 0.45-0.65% Mo is also an important ingredient in determining the desired properties. That is, Mo has the effect of increasing the temper softening resistance and increasing the high temperature strength. If the Mo content is less than 0.45%, these effects cannot be obtained. On the other hand, if it exceeds 0.65%, these effects are saturated. Therefore, the Mo content range is 0.45 to 0.65%.

Sol.Al: 0.005 to 0.100% sol.Al is an effective component for fixing N and removing oxygen in steel. These effects have a sol.Al content of 0.00
You can't expect it to be more than 5%. On the other hand, if the sol.Al content is
If it exceeds 0.100%, it may cause surface cracking of the slab, so the upper limit of its content was made 0.100%.

Ti: 0.005 to 0.030% Ti is an extremely effective component for fixing Free (solid solution) N in steel during welding and reducing hardening of the heat affected zone.
This effect cannot be obtained unless the Ti content is 0.005% or more. On the other hand, if the Ti content exceeds 0.030%, the toughness of the welded joint is decreased. Therefore, the range of Ti content is
It was set to 0.005 to 0.030%.

P: 0.020% or less P is contained in steel as an impurity, but not only impairs toughness and weldability but also increases temper embrittlement susceptibility,
It is desirable to reduce it as much as possible. However, a significant reduction will lead to an increase in cost, so the allowable upper limit of P content is 0.020
%.

S: 0.015% or less S, like P, is contained as an impurity in the steel, but it significantly reduces the toughness of the steel, so it is desirable to reduce it as much as possible. However, a significant reduction causes an increase in cost, so the allowable upper limit of S content was set to 0.015%.

The main structure of the chromium molybdenum steel sheet of the present invention having the above chemical composition is bainite, and the hardness is Hv of 200 or less for the following reason.

The "main structure" referred to here is 50 in volume ratio.
% Means at least a bainite single phase, or the bainite has a volume fraction of 50% or more and the rest is ferrite.

If the above structure is deviated, desired mechanical properties, hardness, low temperature toughness and weldability cannot be obtained.

Hardness means hardness at the cross-section, and when the hardness of the steel sheet after tempering exceeds 200 in Hv, the maximum hardness of the welded portion after welding according to JIS Z 3101 is 300 in Hv. Beyond
The stress relief annealing (hereinafter referred to as SR) cannot be omitted.

When assembling a structure by welding,
Residual stress is generated inside. Conventional 1 / 4Cr-1 / 2Mo
In order to reduce this internal stress, it is essential that the steel sheet be subjected to SR treatment. Since this residual stress concentrates on the hardened part of the heat-affected zone of the welding, if the residual stress of the hardened part can be reduced or eliminated, the SR process that requires a large-scale equipment and time can be omitted. . If the hardness of the steel sheet after tempering is 200 or less in Hv at the cross section, JIS Z
The hardened part of the weld heat affected zone by the maximum hardness test according to 3101 can also be maintained at 300 or less in Hv, and no significant stress concentration occurs, so the SR treatment after welding can be omitted.

Next, the reason why the manufacturing method of the present invention is limited to the above manufacturing steps and conditions will be described.

In order to produce a chrome molybdenum steel according to the present invention, a steel plate (slab) having the above chemical composition is produced according to a conventional method, and a hot rolled steel sheet is obtained.
After normalizing at 10 to 930 ° C., accelerated cooling by water cooling may be performed, and then tempering treatment may be performed.

In normalizing, the upper limit of the heating temperature must be 930 ° C. in order to prevent the γ grains from coarsening. When the heating temperature is lower than 910 ° C, it is insufficient to form austenite. The desired holding time at this time is the plate thickness
At least 1 hour per 25 mm.

Next, accelerated cooling is performed using water cooling for the purpose of securing strength and improving toughness. The desirable cooling rate at this time is 400 ° C / min or more.

After completion of cooling, tempering is performed for the purpose of stabilizing the structure and improving toughness. The preferred temperature range is 650-730
C., holding time is 1 hour per 25 mm plate thickness. Cooling after tempering is preferably air cooling.

The effect of quenching by the above accelerated cooling (water cooling) decreases as the plate thickness increases.

Therefore, in order to make the main structure a stable and high toughness bainite structure, it is desirable that the upper limit of the applicable plate thickness is 100 mm in the method of the present invention.

[0047]

EXAMPLE Steels having the chemical compositions shown in Table 1 were melted in an electric furnace,
A 24 ton steel ingot was used.

[0048]

[Table 1]

Among the examples of the present invention, steel types 1 and 2 are steels having the most typical composition. Steel type 3 is an example in which the Cr content is the lower limit, and steel type 4 is an example in which the Mo content is the lower limit. Steel type 5 is an example in which the C content is the upper limit, and steel type 6 is an example in which the C content is also the lower limit.

Among the comparative examples, steel types 7 to 12 are steels having compositions known in the past. Grades 7 and 13 are Al-B treated low C type Cr-Mo steels, grade 8 is a normal C level JIS SC.
Cr-Mo steel corresponding to MV3-1. Grade 9 is a Cr-Mo steel corresponding to JIS SCMV3-2 with Cu and Ni added, and grade 10 is
It is a Cr-Mo steel corresponding to JIS SCMV3-2 that has been subjected to Al-B treatment. Steel type 11 has a C content below the lower limit, and steel type 12 has a C content above the upper limit.

The obtained steel ingots were heated and slab-rolled according to a conventional method to produce slabs, and then these slabs were hot-rolled to obtain raw steel plates having a thickness of 50 mm.

After heat-treating these steel sheets under the same constant conditions shown in Table 2 to make product steel sheets, JIS No. 10 test pieces and JIS No. 4 test pieces were sampled from the steel sheets to investigate the mechanical properties and toughness. .

The accelerated cooling rate during the heat treatment is 700 ° C./mi
n, holding time for tempering was 2.0 hours, and cooling after tempering was air cooling.

Further, the tempered steel sheet was investigated for its metallographic structure and hardness, a weldability test was carried out, and the maximum hardness after welding according to JIS Z 3101 was measured.
It was judged whether or not the processing could be omitted.

In carrying out these various tests, the tensile test characteristics were aimed at the value of SCMV3-1 specified in JIS G 4109 (1987). The toughness is evaluated by the Charpy impact test, and 0 ° C is used so that sufficient absorbed energy can be obtained even at low temperature.
The target energy absorption value was 200 J or more.
Hardness is hardness Hv that makes it possible to omit SR after welding.
The target was 200 or less. Weldability was evaluated by this hardness and the preheating temperature required to prevent cracking in the diagonal y-groove weld cracking test (JIS Z3158), and the tempered steel sheet had a Hv of 200 or less,
After welding, Hv of 300 or less and preheating temperature of 50 ° C or less were considered good. Tables 2 and 3 show the above test results.

[0056]

[Table 2]

[0057]

[Table 3]

Next, a test for investigating the influence of the plate thickness was conducted on the steel type 2. Table 4 shows the plate thickness, the heat treatment conditions, the mechanical properties, and the metal structure when the product plate thickness is changed and the heat treatment conditions are the same.

[0059]

[Table 4]

From the above results, it is clear that all of the examples of the present invention satisfy the target performance.

In the comparative examples, the one having a high C content (steel types 8, 9, 10, 12) and the one having a low C type and subjected to Al-B treatment (steel types 7, 13) all had a high maximum hardness. Too much.

When the C content is less than the lower limit (steel type 11), precipitation of carbides is insufficient and a large amount of ferrite is formed, so that the toughness is low.

Further, a test for investigating the effect of accelerated cooling conditions was conducted on steel types 1 to 3. Table 5 shows the heat treatment conditions, mechanical properties, and metallographic structure when the cooling was air cooling.

[0064]

[Table 5]

Since the cooling was done by air cooling and not accelerated cooling, pearlite and coarse ferrite were produced and strength,
It can be seen that the toughness is greatly reduced.

[0066]

EFFECTS OF THE INVENTION According to the present invention, it has a high toughness and a high weldability, and the SC shown in JIS G 4109 (1987).
It is possible to obtain a normalization-tempering type Cr-Mo steel sheet that sufficiently satisfies the mechanical properties equivalent to MV3-1. This steel sheet has a maximum hardness Hv of 300 or less after welding as specified in JIS Z 3101, so that SR treatment after welding is unnecessary.

Claims (2)

[Claims] 1. By weight%, C: 0.03 to less than 0.05%, Si: 0.
50 to 0.80%, Mn: 0.40 to 0.65%, Cu: 0.05 to 0.50%, N
i: 0.05 to 0.50%, Cr: 1.00 to 1.50%, Mo: 0.45 to 0.65
%, Sol.Al: 0.005 to 0.100% and Ti: 0.005 to 0.03
The content is 0%, the balance is Fe and inevitable impurities, P in the impurities is 0.020% or less, S is 0.015% or less, the main structure is bainite, and the hardness is 20 at Hv.
A high toughness chromium molybdenum steel sheet having excellent weldability, which is 0 or less. 2. By weight%, C: 0.03 to less than 0.05%, Si: 0.
50 to 0.80%, Mn: 0.40 to 0.65%, Cu: 0.05 to 0.50%, N
i: 0.05 to 0.50%, Cr: 1.00 to 1.50%, Mo: 0.45 to 0.65
%, Sol.Al: 0.005 to 0.100% and Ti: 0.005 to 0.03
Containing 0%, the balance consisting of Fe and unavoidable impurities, P in the impurities is 0.020% or less, S is 0.015% or less, and after normalizing it at 910 to 930 ℃, it is accelerated by water cooling. A method for producing a high toughness chromium molybdenum steel sheet having excellent weldability, which comprises cooling, followed by tempering to obtain a bainite main structure and a hardness of 200 or less in Hv.