JPH07316654A - Production of thick steel plate of high strength ni steel excellent in toughness at low temperature - Google Patents

Abstract

PURPOSE:To secure high strength and superior toughness at low temp. even in the case of steel plates with large plate thickness by using an Ni steel slab having a specific composition in which C and Si contents are reduced and to which Nb is added and by specifying rolling and heat treatment conditions. CONSTITUTION:A steel slab, containing, by weight, 0.03-0.06% C, <=0.20% Si, 0.30-0.70% Mn, 0.01-0.05% Al, 0.005-0.030% Nb, <=0.005% P, <=0.002% S, and <=0.005% N, is used. This slab is heated to 1100-1300 deg.C and finished into a thick steel plate by hot rolling at 30-80% cumulative rolling reduction at temps. between 700 and 850 deg.C. This plate is heated to a temp. between the Ac3 point and 850 deg.C, cooled, further heated to a temp. between the Ac1 and the Ac3 point, cooled, and tempered at <=(Ac1 point + 50 deg.C). Moreover, it is preferable that cooling treatment at >=2 deg.C/sec cooling rate is performed after that and V having an effect similar to that of Nb is further incorporated into this steel by 0.005-0.03%. By this method, high strength and superior toughness can be secured even for a thick steel plate of 9%Ni steel having a plate thickness exceeding 30mm.

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 high-strength Ni steel thick steel sheet excellent in low temperature toughness, and particularly, a liquefied natural gas (hereinafter referred to as LNG) storage tank steel sheet as a typical example. The present invention provides an advantageous method for producing a high-strength Ni steel sheet having excellent low-temperature toughness without losing toughness even when used at extremely low temperatures of −160 ° C. or less.

The demand for LNG, which is a clean energy source, is rapidly increasing against the backdrop of the recent increase in energy demand and fear of the safety of nuclear power generation.
Along with this, the construction of tanks for storing LNG is being promoted. Naturally, steel sheets for pressure vessels are used for this LNG storage tank, but since the inside of the tank is exposed to an extremely low temperature atmosphere, a material with excellent low temperature toughness is recommended. Therefore, the LNG storage tank contains Ni steel, especially 9 wt.
% Ni steel sheet is often used. In addition, there is a tendency to increase the capacity of the tank in order to improve the storage efficiency of LNG.
G 9wt% Ni steel sheet used for storage tank material has a thickness exceeding 30mm which is the upper limit of conventional steel sheet, especially 40mm.
Those having the above thickness are required.

[0003]

2. Description of the Related Art A number of proposals have been made regarding a method for producing a 9 wt% Ni steel sheet having excellent low temperature toughness, and among them, it is described in Japanese Patent Publication Nos. 47-23317 and 4-40411. The method that includes a treatment of heating and tempering between the Ac ₁ and Ac ₂ transformation points is advantageous for improving the low temperature toughness.

However, these techniques have a plate thickness of 30.
Although it is effective for steel plates with a thickness of less than 30 mm, it is difficult to apply it to steel plates with a plate thickness exceeding 30 mm. That is, when the plate thickness exceeds 30 mm, it becomes difficult to refine the crystal grains by rolling, and the cooling rate during quenching cooling is inevitably reduced, so both strength and toughness decrease. Tend to do. Furthermore, in welding, which is indispensable for the construction of LNG storage tanks, the heat input amount increases as the plate thickness increases, so the material of the weld heat affected zone (hereinafter HAZ) changes and the toughness is impaired.

Therefore, in Japanese Patent Laid-Open No. 3-264617, a 9 wt% Ni steel thick steel plate having a high yield strength is obtained by hot rolling after low temperature slab heating, followed by quenching, intermediate quenching and tempering. A method of manufacturing was proposed.

[0006]

According to the above method,
Although the yield strength of thick Ni steel plate certainly increases, the problem remains where low temperature toughness is still insufficient. Therefore, it is an object of the present invention to propose a technique for assuring high strength and excellent low temperature toughness even for a 9 wt% Ni steel thick steel plate having a plate thickness of more than 30 mm, especially 40 mm or more.

[0007] In order to solve the above problems, the present inventors have investigated the effects on the yield strength and low temperature toughness when various components are added to 9 wt% Ni steel. An attempt was made to improve the toughness by reducing it compared with steel, and to compensate for the strength decrease due to the decrease of these contents by Nb and V. Not only the strength but also the drastic improvement effect of the toughness can be obtained unexpectedly. Was newly found. Also,
When the rolling and heat treatment conditions were also examined, it was found that cooling at a relatively high speed after tempering was effective in improving toughness, and the present invention was derived.

That is, according to the present invention, C: 0.03 to 0.06 wt.
%, Si: 0.20 wt% or less, Mn: 0.30 to 0.70 wt%, Ni: 7.5
~ 12.0wt%, Al: 0.01-0.05wt%, Nb: 0.005-0.030
1100 to 1300 steel slabs containing wt%, P: 0.005 wt% or less, S: 0.002 wt% or less, and N: 0.005 wt% or less.
After heating to ℃, finish rolling into a thick steel plate by hot rolling with a cumulative reduction of 30 to 80% at 700 to 850 ℃, then heating to a temperature range of Ac ₃ points to 850 ℃, then cooling, and further Ac ₁ This is a method for producing a high-strength Ni steel thick steel sheet having excellent low-temperature toughness, which is characterized by heating to a temperature range of from 1 to ₃ points of Ac, then cooling, and then tempering at ₁ point of Ac + 50 ° C or less.

The present invention also provides the above method,
High-strength Ni with excellent low-temperature toughness, characterized by being subjected to a cooling treatment at a cooling rate of 2 ° C / s or more after the tempering treatment.
It is a method for manufacturing a steel thick steel plate. The toughness can be further improved by adding V: 0.005 to 0.03 wt% to the steel slab.

[0010]

The method of the present invention will be described in detail below. First, the reason for limiting the component composition of the slab in the present invention will be described. C: 0.03 to 0.06 wt% C requires 0.03 wt% or more to secure the strength,
If it exceeds 0.06 wt%, the toughness of the base metal and HAZ will deteriorate, so the range is 0.03 to 0.06 wt%.

Si: 0.20 wt% or less Si contributes to an increase in strength, so 0.01 wt% or more is preferable. However, a large amount of Si causes a decrease in toughness, so Si is made 0.20 wt% or less.

Mn: 0.30 to 0.70 wt% Mn is also a component that contributes to an increase in strength, and it is necessary to contain 0.30 wt% or more. However, if it exceeds 0.70 wt%, the toughness is deteriorated, so 0.30 wt% is added. The range is to 0.70wt%.

Ni: 7.5 to 12.0 wt% Ni is a component that imparts low temperature toughness to steel and at the same time improves hardenability and obtains a martensite structure by quenching treatment. It is necessary, but even if the content exceeds 12.0 wt%, the effect is saturated, so the upper limit is 12.0 wt%.

Al: 0.01 to 0.05 wt% Al is contained as a deoxidizing agent and has the effect of becoming AlN in the steel to refine the crystal grains. It is necessary to contain 0.01 wt% or more. When the content exceeds wt%, the toughness is deteriorated, so the range is 0.01 to 0.05 wt%.

Nb: 0.005 to 0.030 wt% Nb is the most important component and has the effect of increasing both strength and low temperature toughness, and particularly contributes to the improvement of HAZ toughness. Here, as shown in FIG. 1, the relationship between the toughness of HAZ and the Nb content is 0.
It can be seen that the toughness of HAZ is significantly improved in the range of 005 to 0.030 wt%. Therefore, the Nb content is 0.005 to 0.03.
It was limited to the range of 0 wt%. In addition, the experiment shown in FIG.
C: 0.05 wt%, Si: 0.08 wt%, Mn: 0.60 wt%, Ni: 9.4w
After heating a steel slab containing t% and Al: 0.027 wt% to 1240 ° C and rolling it into a steel plate with a thickness of 50 mm at a finishing temperature of 800 ° C,
In the thick steel plate produced by heating and quenching at ℃, further heating and quenching at 670 ° C, and then tempering at 550 ° C,
The toughness of HAZ by welding with a heat input of 3.0 kJ / mm is evaluated by the absorbed energy at -196 ° C.

P: 0.005 wt% or less, S: 0.002 wt% or less and N: 0.005 wt% or less P and S are components that reduce toughness.
Limit to 0.005 wt% and 0.002 wt% or less. Similarly,
Since N also lowers the toughness, it is limited to 0.0050 wt% or less.

In the present invention, in addition to the above component composition, V: 0.005 to 0.03 wt% can be further added. That is, V is a component effective in improving strength and toughness like Nb, and is effective at 0.005 wt% or more. However, if it exceeds 0.03 wt%, the toughness of the welded portion decreases, so 0.03 wt% % Is the upper limit.

Next, the reasons for limiting each of the hot rolling and heat treatment conditions will be described. The steel slab adjusted to the above composition is 1100
It is heated to ~ 1300 ℃, but the reason for limiting this temperature range is to refine the crystal grains in the rolling process of the next process, and to sufficiently dissolve AlN necessary for expressing the refinement effect. Requires heating above 1100 ° C, while 1300 ° C
If it exceeds, the γ grains are remarkably coarsened and the crystal grains after rolling are not refined.

In the next hot rolling, the slab has a plate thickness of 30.
Finished into thick steel plate that exceeds mm. Here, the hot rolling needs to be performed at a cumulative rolling reduction of 30 to 80% at 700 to 850 ° C. This is because rolling is mainly performed in the temperature range of 700 to 850 ° C, and texture rolling that inhibits toughness develops in rolling in the temperature range of less than 700 ° C, and in rolling in the temperature range of more than 850 ° C. This is because the recrystallization of the γ-grains occurs instantly and the grain refinement by rolling cannot be achieved. The finishing temperature is 700 from the viewpoint of rolling productivity.
It is preferable to set the temperature to 800 ° C.

The steel sheet that has undergone the above-mentioned rolling is once cooled, then heated to a temperature range of Ac ₃ point to 850 ° C. and then cooled, and then subjected to a primary quenching treatment, and further heated to a temperature range of Ac ₁ point to Ac ₃ point. After that, each of them is subjected to a secondary quenching treatment of cooling and then Ac ₁
Tempering is performed at + 50 ° C or below.

Here, the primary quenching treatment is carried out in order to obtain fine strength and toughness by forming a fine martensite structure and combining it with a heat treatment in a subsequent step. Therefore, it is necessary to cool from a uniform austenite structure, and it is necessary to heat to the Ac ₃ point or higher, but 850 ℃
Above this temperature, coarsening of austenite grains occurs and toughness decreases, so the heating temperature is limited to the Ac ₃ point to 850 ° C. range. The cooling rate after heating is preferably 1.0 ° C./s or more.

The subsequent secondary hardening treatment is carried out in order to produce a large amount of stable precipitated austenite after the tempering treatment. That is, by heating the steel sheet in the two-phase region, a two-phase structure of ferrite and austenite is generated, and by rapidly cooling this, a martensite structure containing ferrite and a high alloy is generated. Therefore, the heating temperature needs to be in the temperature range of Ac ₁ point to Ac ₃ point. The cooling rate after heating is 1.0 ° C /
It is preferably s or more.

The subsequent tempering treatment is carried out in order to reduce the dislocation density of the martensitic structure described above and at the same time to form stable precipitated austenite. ₁ + 50 ° C
It needs to be performed in the following temperature range. The lower limit is preferably Ac ₁ -45 ° C or higher in order to secure the tempering effect.

Further, if the temperature is cooled at a rate of 2 ° C./s or more after the above-mentioned tempering treatment, the toughness can be further improved. This is because if the cooling rate is slow, segregation of P grain boundaries or the formation of Fe phosphide is generated, which adversely affects toughness.

As shown in FIG. 2, the relationship between the toughness of the base material of the steel sheet and the cooling rate after tempering shows that the toughness improves as the cooling rate increases. That is, the improvement in toughness is recognized from the point where the cooling rate exceeds 1 ° C./s, and the improvement in toughness is remarkable at 2 ° C./s or more. In addition, in the experiment of FIG. 2, C: 0.05 wt
%, Si: wt%, Mn: 0.60 wt% and Ni: 9.4 wt% are heated to 1240 ° C, rolled at a finishing temperature of 800 ° C, then heat-quenched at 800 ° C, and then at 670 ° C. The absorbed energy at -196 ° C was investigated by the test piece taken from the position of 1/2 of the thickness of the steel sheet obtained by variously changing the cooling rate after heating and quenching and then heating to 550 ° C. It is a thing.

The effect when the cooling rate after tempering is set to 2 ° C./s or more is particularly remarkable when a steel plate having a plate thickness of 40 mm or more is targeted, which is for a steel plate of less than 30 mm. In the air cooling process after tempering that is usually performed, the cooling rate is 0.
This is due to the extremely small value of 1 ° C / s or less, and the improvement of toughness cannot be expected for thick plates of 40 mm or more.

[0027]

EXAMPLES Steel slabs having the chemical composition shown in Table 1 were
After heating to 1140 ° C, hot rolling at a cumulative rolling reduction of 50% at 700 to 850 ° C is completed at a finishing temperature of 750 ° C, heated to 800 ° C, water-cooled, and further heated to 670 ° C and water-cooled. Then, after tempering treatment at 550 ° C., cooling was performed according to the conditions shown in Table 2 to obtain 9 wt% Ni steel steel plates (base metal) having plate thicknesses of 40, 60 and 100 mm. The thus-obtained base material was subjected to a tensile test and a Charpy impact test to obtain mechanical properties and −
The shock absorption energy at 196 ℃ was investigated. Furthermore, the welding heat cycle shown in Fig. 3 was applied to the base metal.
HAZ was created and the impact absorption energy of HAZ at -196 ° C was also investigated. The results of these investigations are also shown in Table 2.

[0028]

[Table 1]

[0029]

[Table 2]

As shown in Table 2, the steel sheets Nos. 1 to 6 satisfy the present invention both in the chemical composition of the slab and in the rolling and heat treatment conditions, so that excellent low temperature toughness can be obtained for both the base metal and HAZ. It was Among them, Steel Sheet Nos. 1 to 5 were able to obtain more excellent low temperature toughness by setting the cooling rate after the tempering treatment to 2 ° C./s or more.

On the other hand, Steel plate No. 7 lacked strength due to the small amount of C in the slab, and Steel plates No. 8 and 9 had poor toughness due to the large amount of C or Si in the slab.
No. 10 was low in strength and toughness due to a small amount of Nb, and steel plates No. 11 and 12 were low in toughness of the HAZ due to a large amount of Nb or V in the slab.

[0032]

INDUSTRIAL APPLICABILITY 9 wt% Ni manufactured according to the present invention
Steel thick steel plate not only has excellent base metal strength and low temperature toughness, but also has excellent low temperature toughness of HAZ, so it can be used practically even for thick steel plates with a thickness over 30 mm, especially 40 mm or more. It can assure strength and low temperature toughness.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the HAZ toughness of steel sheets and the Nb content.

FIG. 2 is a diagram showing the relationship between the toughness of a base material of a steel sheet and the cooling rate after tempering.

FIG. 3 is a diagram showing a welding heat cycle.

Front page continuation (72) Inventor Yoshihiro Kataoka 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Technical Research Division, Kawasaki Steel Co., Ltd. (72) Inventor Hiroaki Ishii 1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama ) Kawasaki Steel Co., Ltd. Mizushima Steel Works (72) Inventor Osamu Tanigawa 1-chome, Mizushima Kawasaki-dori, Kurashiki City, Okayama Prefecture (no house number) Kawasaki Steel Co., Ltd. Mizushima Steel Works (72) Inventor Yoshifumi Nakano Kawasaki, Chuo-ku, Chiba City, Chiba Prefecture Town No. 1 Kawasaki Steel Co., Ltd. Technical Research Division (72) Inventor Yuji Kusuhara 2-2-3 Uchisaiwaicho, Chiyoda-ku, Tokyo Kawasaki Steel Co., Ltd. Tokyo headquarters (72) Inventor Naoshi Uemura Mizushima Kawasaki, Kurashiki City, Okayama Prefecture Tsudori 1-chome (No house number) Kawasaki Steel Co., Ltd. Mizushima Steel Works

Claims (3)

[Claims] 1. C: 0.03 to 0.06 wt%, Si: 0.20 wt% or less, Mn: 0.30 to 0.70 wt%, Ni: 7.5 to 12.0 wt%, Al: 0.01 to 0.05 wt%, Nb: 0.005 to 0.030 wt. %, P: 0.005 wt% or less, S: 0.002 wt% or less, and N: 0.
A steel slab containing 005 wt% or less is heated to 1100 to 1300 ° C, then hot rolled at a cumulative reduction of 30 to 80% at 700 to 850 ° C to finish a thick steel plate, and then Ac ₃ points to 850 Low temperature toughness characterized by heating in the temperature range of ℃, then cooling, heating in the temperature range of Ac _{1 to} Ac ₃ and then cooling, and then tempering at Ac ₁ point + 50 ℃ or less Excellent high strength
Ni steel thick steel plate manufacturing method. 2. A high strength Ni steel thick steel sheet excellent in low temperature toughness, which is characterized by performing a cooling treatment at a cooling rate of 2 ° C./s or more after the tempering treatment in the method according to claim 1. Production method. 3. The steel slab further comprises V: 0.005 to 0.03 wt.
%, The high-strength Ni according to claim 1 or 2
Steel thick steel plate manufacturing method.