JPH09143557A - Production of thick nickel-containing steel plate excellent in toughness at low temperature and having high strength - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a thick Ni-containing steel plate excellent in toughness at low temp. and having high strength by subjecting a steel slab, containing specific amounts of C, Si, Mn, Ni, Al, Nb, N, P, S, Ca, and REM, to respectively specified rolling, hardening, and tempering. SOLUTION: A slab of a steel, which has a composition containing, by weight, 0.03-0.06% C, <=0.20% Si, 0.30-0.70% Mn, 7.5-12.0% Ni, 0.01-0.05% Al, 0.005-0.030% Nb, <=0.005% N, <=0.005% P, <=0.002% S, and one or more kinds among 0.0005-0.005% Ca and 0.001-0.03% each REM and further containing, if necessary, prescribed amounts of V, Cu, Mo, and Cr, is heated to 1100-1300 deg.C and hot-rolled into thick steel plate. At this time, the cumulative draft at 700 to 850 deg.C is regulated to 30-80%. Then, the steel plate is heated to a temp. between Ac3 and 850 deg.C and cooled to undergo primary hardening treatment and further heated to a temp. between the Ac1 and the Ac3 point and cooled to undergo secondary hardening treatment. After hardening, the steel plate is tempered at a temp. not higher than (Ac1 +50) deg.C and cooled at a rate of >=2 deg.C/sec.

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-containing steel sheet having a plate thickness of more than 30 mm and excellent in low-temperature toughness, and particularly in a tank for storing liquefied natural gas having a plate thickness of 40 mm or more. , An advantageous method for producing a high-strength Ni-containing thick steel sheet excellent in low-temperature toughness, which does not lose toughness even when used at extremely low temperatures of −160 ° C. or lower.

[0002]

BACKGROUND OF THE INVENTION Liquefied natural gas (hereinafter referred to as LNG), which is a clean energy source, against the backdrop of the recent increase in energy demand and fear of safety of nuclear power generation.
Demand) is rapidly increasing. Along with this, LNG
Construction of tanks for storing water is being promoted. A steel plate for a pressure vessel is naturally used for this LNG storage tank, but since the inside of the tank is exposed to an extremely low temperature atmosphere, a material particularly excellent in low temperature toughness is recommended. Therefore, Ni steel, especially 9 wt% Ni steel plate, is frequently used for the LNG storage tank.

In addition, there is a tendency to increase the capacity of the tank in order to increase the storage efficiency of LNG, and the thickness of the 9 wt% Ni steel sheet used for LNG storage tank material is 30 mm, which is the upper limit of the conventional steel sheet. There is a need for a material having a thickness exceeding 40 mm, particularly 40 mm or more. Many proposals have been made regarding a method for producing a 9 wt% Ni steel sheet having excellent low temperature toughness. For example, Japanese Patent Publication No. 47-23317 and Japanese Patent Publication No. 4
JP-A-40411 proposes a method of improving low temperature toughness in which the material is heated and quenched in the two-phase region between the Ac _{1 to} Ac ₃ transformation points and then tempered below the Ac ₁ transformation point.

However, these techniques have a plate thickness of 30 mm.
Although it is effective for the following steel plates, it is difficult to apply it similarly to thick steel plates having a plate thickness of more than 30 mm. That is, when the plate thickness is more than 30 mm, it is difficult to refine the crystal grains by rolling, and the cooling rate during quenching cooling is necessarily reduced, so that both strength and toughness decrease. There's a problem.

Further, in Japanese Patent Laid-Open No. 3-264617, the slab is heated to a relatively low temperature of 800 to 1000 ° C., hot-rolled, then quenched, and then subjected to intermediate quenching of two-phase region heating, and then Ac ₁ A method has been proposed for producing a 9 wt% Ni steel thick steel sheet having high yield strength by performing a tempering treatment at a temperature not higher than the point. Japanese Patent Publication No. 57-3739 discloses that S is reduced to 0.010% or less, treated with Ca or a Ca alloy, and the Ca / S ratio is 0.05 to
By setting the range to 1.50, a low temperature Ni steel having high low temperature toughness has been proposed.

JP-B-62-1453 discloses that P: 0.010 wt.
% Or less, S: 0.005 wt% or less, or further Ca,
Disclosed is a method for producing a low-temperature steel containing Ni, which contains REM and the like, is heated to 1050 to 1200 ° C., is controlled-rolled, is then rapidly cooled, and is excellent in temper displacement of crack opening. Japanese Unexamined Patent Publication (Kokai) No. 58-73717 proposes a technique in which Ca and La are added to low Ni steel to enhance low temperature toughness by a two-step quenching / tempering treatment including quenching in the two-phase region.

In Japanese Patent Laid-Open No. 6-44597, a Ni-containing steel slab added with Ti is quenched immediately after hot rolling, and then heated to a two-phase region between Ac ₁ and Ac ₃ and quenched, and Ac ₁ point A method for improving the low temperature toughness of Ni steel, which is tempered below, has been proposed.
Japanese Unexamined Patent Publication (Kokai) No. 63-290246 proposes a low temperature steel to which Ti is added and which has excellent toughness at the welded portion.

In Japanese Unexamined Patent Publication No. 6-240348, a slab having a reduced amount of P and S is formed into a thick steel plate by hot rolling.
A method for producing Ni steel has been proposed in which after heating and quenching to 00 to 900 ° C., heating to Ac _{3 to} 800 ° C., quenching again, and tempering at Ac ₁ point or more. However, even with the technology described above,
When the plate thickness is large and high strength is required, the low temperature toughness is still insufficient and a problem remains where stable and high toughness cannot be obtained.

[0009]

DISCLOSURE OF THE INVENTION The present invention provides a Ni-containing sheet having a thickness of more than 30 mm, which has a high yield strength and excellent low temperature toughness.
The purpose is to propose a method for manufacturing thick steel plates.

[0010]

In order to advantageously solve the above problems, the present inventors have added various components to 9% Ni steel and investigated the effects on yield strength and low temperature toughness. By reducing the C and Si contents and adding Nb,
We have obtained a new finding that the strength and toughness are significantly improved from the conventional ones.

Further, by significantly reducing S and adding Ca and / or REM, and simultaneously performing high temperature heating of the slab, formation of MnS which adversely affects toughness is prevented and low temperature toughness is remarkably improved. I found out. The present invention is constructed based on such new knowledge. That is, in 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 to 12.0 wt
%, Al: 0.01 to 0.05 wt%, Nb: 0.005 to 0.030 wt%,
N: 0.005 wt% or less, P: 0.005 wt% or less, S: 0.002
wt% or less, and Ca: 0.0005 to 0.005 wt% and
REM: A steel slab containing 1 or 2 of 0.001 to 0.03 wt% and the balance of Fe and unavoidable impurities is heated to 1100 to 1300 ° C and the cumulative rolling reduction at 700 to 850 ° C is 30 to Hot-rolled to 80% to make a thick steel plate, then subjected to primary quenching treatment by heating and cooling in the temperature range of Ac ₃ points to 850 ° C, and further heating and cooling in the temperature range of Ac ₁ point to Ac ₃ points. After carrying out the secondary quenching treatment, Ac ₁ point + 50 ° C
A method for producing a high-strength Ni-containing thick steel sheet excellent in low-temperature toughness, characterized by performing the following tempering treatment.

The present invention is also a method for producing a high-strength Ni-containing thick steel sheet having excellent low-temperature toughness, which is characterized by performing a cooling treatment at a cooling rate of 2 ° C./sec or more after the tempering treatment. Further, the steel slab further has V: 0.005 to 0.030.
wt% may be contained, and further Cu: 0.05 to 0.30 wt
%, Mo: 0.02 to 0.20 wt% and Cr: 0.05 to 0.30 wt%, at least one kind may be contained.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION 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 material and HAZ will be deteriorated, so the range is 0.03 to 0.06 wt%.

Si: 0.20 wt% or less Si contributes to the increase in strength, so 0.01 wt% or more is preferable. However, since a large amount of content causes a decrease in toughness, it is 0.20 wt% or less. From the viewpoint of toughness, it is more preferably 0.02 to 0.15 wt%. Mn: 0.30 to 0.70 wt% Mn is also a component that contributes to the increase in strength, and it is necessary to contain 0.30 wt% or more from the viewpoint of securing strength, but if it exceeds 0.70 wt%, toughness decreases, so 0.30 The range is to 0.70wt%. More preferably, it is 0.50 to 0.60 wt%.

Ni: 7.5 to 12.0 wt% Ni is a component contained in order to impart low temperature toughness to steel and at the same time to obtain a structure mainly composed of martensite by quenching, and it is necessary to contain 7.5 wt% or more. But
Even if the content exceeds 12.0wt%, its effect will be saturated,
The upper limit is 12.0wt%.

Al: 0.01-0.05 wt% Al is contained as a deoxidizing agent and has the effect of becoming AlN in steel to refine the crystal grains. To obtain this effect,
The content is required to be 0.01 wt% or more, but if it exceeds 0.05 wt%, toughness may be deteriorated, so the content is made 0.01 to 0.05 wt%. Nb: 0.005 to 0.030 wt% Nb is the most important component in the present invention and has the effect of increasing both strength and low temperature toughness, and particularly contributes to the improvement of the toughness of the heat affected zone (HAZ). Is. HAZ in the Nb content range of 0.005 to 0.030 wt%
Nb content is 0.005〜
It was limited to the range of 0.030 wt%. Furthermore, more preferably
It is in the range of 0.01 to 0.025 wt%.

P: 0.005 wt% or less P reduces the toughness as much as possible because it is a component that lowers the toughness of the weld heat affected zone. However, since P is 0.005 wt% or less, the P content is 0.005 wt% or less. And S: 0.002 wt% or less S is a component that lowers the toughness due to the formation of MnS, especially the toughness of the steel sheet base material, and therefore reduces it as much as possible. In the steel sheet of the present invention, ductile fracture becomes dominant even in an impact test at -196 ° C. It is known that in order to increase the ductile fracture energy, it is necessary to reduce the amount of inclusions and control the morphology. In the present invention, the amount of MnS is reduced by reducing S, and the low temperature toughness is improved. Therefore, the S content is 0.00
2 wt% or less.

As will be described later, the slab heating temperature is set to a high temperature to accelerate the dissolution of MnS, and
The addition of REM prevents the formation of MnS. N: 0.005 wt% or less N reduces the toughness in the solid solution state, particularly the toughness of the weld heat affected zone, but also acts as AlN to refine the crystal grains. Therefore, N is reduced as much as possible within the range where the crystal grains are not coarse. 0.005 wt% or less, preferably 0.0
If it is 05 to 0.003 wt%, grain coarsening does not occur, so N
The content was 0.005 wt% or less.

Ca: 0.0005 to 0.005 wt%, REM: 0.001 to
One or two selected from 0.03 wt% These elements are one of the features of the present invention. In the present invention, the S content is reduced to 0.002 wt% or less, but even with such a small amount of S content, MnS-based inclusions are formed, which extend in the rolling direction of the steel sheet and cause a decrease in low temperature toughness. Becomes In the present invention, Ca and / or REM are added to improve the low temperature toughness. REM may be added as any one kind or two or more kinds of Ca, La and Y. Ca and REM form high melting point sulfides in the steel and become fine, equiaxed or spherical inclusions, which do not elongate during rolling. REM is often used to control the morphology of sulfide inclusions, and Ca is often used for oxide inclusions rather than sulfides. In the low S material as in the present invention, the amount of sulfide inclusions is small, so that the degree of influence of oxide inclusions such as alumina increases. Therefore, when S is extremely reduced as in the present invention, the effect of Ca addition is great.

To obtain such an effect, Ca is 0.00
It is necessary to add more than 05wt% and 0.001wt% of REM. However, when Ca exceeds 0.005 wt%, REM is 0.03
Since the number of inclusions increases when wt% is exceeded, Ca is 0.005
The upper limit of wt% is 0.03 wt% for REM. The preferable ranges are: Ca: 0.0005 to 0.003 wt%, REM: 0.002 to 0.01
wt%.

In the present invention, the following elements can be added, if necessary, in addition to the above composition. V: 0.005 to 0.030 wt% V is a component effective in increasing the strength by precipitation hardening, and also has an effect in improving the toughness. V is 0.005 wt%
The above is effective, but if the content exceeds 0.030 wt%, the toughness of the welded part deteriorates, so 0.030 wt% is the upper limit.

Cu: 0.05 to 0.30 wt%, Mo: 0.02 to 0.20 wt
%, Cr: One or more selected from 0.05 to 0.30 wt% Cu, Mo and Cr are effective components for increasing the strength by solid solution hardening. Cu is 0.05 wt% and Mo is 0.
The effect is recognized when the content of 02 wt% and Cr is 0.05 wt% or more. However, 0.30 wt% for Cu and 0.20 wt% for Mo
%, And Cr, if over 0.30 wt%, the toughness decreases, so these values are made upper limits.

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 dissolve MnS in the slab and to refine the crystal grains in the rolling process of the next process.
The above heating is required, while if it exceeds 1300 ° C., the γ grains are remarkably coarsened and the crystal grains after rolling are not refined. MnS
In order to sufficiently perform the solid solution of (1), it is preferably heated to 1200 to 1300 ° C.

The solid solution temperature of MnS decreases as the S content decreases. For example, when Mn: 0.6% and S: 0.001%, the solid solution temperature is about 1200 ° C. Therefore, if heated to 1200 ° C. or higher, the MnS content is almost completely within the S content range of the present invention.
Dissolves and eliminates harmful coarse MnS. In addition, when the amount of alloying elements such as Ni increases, segregation bands of alloying elements such as Ni and Mn due to solidification segregation are present, and they are elongated by rolling to form a non-uniform structure in which they are stacked in layers. In particular, since the rolling reduction decreases as the thickness increases, the width of the layers and the spacing between the layers increase, and the adverse effect due to the nonuniformity of the structure is likely to appear. Increasing the slab heating temperature also has the effect of homogenizing the segregation zone due to diffusion and reducing the adverse effect on toughness.

Next, the slab is subjected to hot rolling to reduce the plate thickness.
It is finished in thick steel plate over 30 mm. Here, the hot rolling needs to be carried out at a cumulative rolling reduction of 30 to 80% at 700 to 850 ° C. Rolling in the temperature range below 700 ° C develops a texture that hinders toughness, and in the temperature range above 850 ° C, recrystallization of γ grains occurs instantly, so grain refinement by rolling cannot be achieved. . The finishing temperature is preferably 700 to 800 ° C from the viewpoint of rolling productivity.

Cumulative rolling reduction in the range of 700-850 ° C is 30%
If the amount is less than 1, the grain size cannot be reduced. If the cumulative rolling reduction exceeds 80%, the anisotropy becomes too strong. From this, the cumulative rolling reduction of 700 to 850 ℃ is 30 to 80
%. The steel sheet that has undergone the above rolling is once cooled, then heated to a temperature range of Ac ₃ point to 850 ° C. and then cooled 1.
After the secondary quenching treatment is performed, the secondary quenching treatment is further performed in which the material is heated in the temperature range of Ac ₁ point to Ac ₃ point and then cooled, and then the tempering treatment at Ac ₁ + 50 ° C. or less is performed.

Here, the primary quenching treatment is carried out in order to form a fine martensite structure and to obtain excellent strength and toughness by 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 it to the Ac ₃ point or higher. However, at temperatures above 850 ° C, austenite grains become coarse and toughness decreases, so the heating temperature is set to Ac ₃ Limit the temperature to 850 ℃. The cooling rate after heating is preferably 1.0 ° C./sec or more.

The subsequent secondary hardening treatment is carried out in order to generate 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, it is necessary to set the heating temperature in the temperature range of Ac ₁ point to Ac ₃ point. The cooling rate after heating is 1.0 ° C /
It is preferably set to sec or more.

The subsequent tempering treatment is carried out in order to reduce the dislocation density of the above-mentioned martensite structure and at the same time to produce stable precipitated austenite. In order to obtain fine austenite precipitation, Ac ₁ +50
It needs to be performed in a temperature range of ℃ or less. 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./sec or more after the above tempering treatment, the toughness can be further improved. If the cooling rate is slow, segregation of P grain boundaries or formation of Fe phosphide is caused, which adversely affects toughness. The effect when the cooling rate after tempering is 2 ℃ / sec or more, especially when the plate thickness is 40 mm
This is remarkable when using the above steel plates, which is 30 mm.
In the air-cooling process after tempering that is usually performed for steel sheets of less than 0.1%, the cooling rate is extremely low at 0.1 ° C / sec or less,
This is because improvement of toughness cannot be expected for thick plates of 40 mm or more.

The slab of the present invention can be suitably manufactured by either ingot-segmentation or continuous casting. As described above, the present invention (i) significantly reduces the S content,
(B) Control the inclusion morphology by adding Ca and REM,
(C) High toughness is achieved by using three means for heating the slab at a high temperature, and a more remarkable effect is obtained by the combined effect of these three means.

[0032]

EXAMPLES Examples will be described below. The continuous cast steel slabs having the chemical composition shown in Table 1 were subjected to hot rolling conditions (slab heating temperature, cumulative rolling reduction, finishing temperature) shown in Table 2 under the plate thickness.
A thick steel plate of 30 to 60 mm was used. Then, under the heat treatment conditions shown in Table 2, primary quenching, secondary quenching, and tempering were performed. The cooling rate after tempering was changed to 15 to 1 ° C / sec for cooling.

The tensile properties (yield strength, tensile strength, elongation) and the absorbed energy at -196 ° C. of the Charpy impact test of the thick steel plate treated under the above conditions were determined, and the welding heat cycle shown in FIG. A reproduced welding heat affected zone (HAZ) was produced and the Charpy absorbed energy at -196 ° C was determined. Table 2 also shows these results.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

[Table 3]

As shown in Table 2, the steel sheets Nos. 1 to 14 have excellent low temperature toughness in both the base metal and the HAZ because both the chemical composition and the rolling and heat treatment conditions satisfy the present invention. Above all, by setting the cooling rate after the tempering treatment to 2 ° C./sec or more, more excellent low temperature toughness was obtained. In addition, steel plate No. 8 containing V, Cu, Mo, Cr
11 has increased strength.

On the other hand, Steel plate No. 15 lacks strength because it has a small amount of C, and Steel plates No. 16 and 17 have poor toughness because they have a large amount of C or Si. Steel plates No. 18, 21, and 22 have Nb. Since the amount is small, the strength and toughness are low even if the cooling rate after tempering is changed, and the steel plates No. 19 and 20 have a large amount of Nb or V, so that the HAZ has low toughness. Steel sheets No. 26 to 28 are Cu, Mo or Cr
However, the toughness decreased. Steel plate No. 23 has low toughness because it does not contain Ca or REM, and steel plates No. 24 and 25 have low toughness because they contain much Ca or REM. Steel plate No. 33 has a high S and therefore has low toughness. Steel plate No. 31 has low toughness due to low slab heating temperature, and steel plate No. 32 has low toughness due to high slab heating temperature. Steel plates No. 34 and 35 have low toughness because the quenching temperature is too high. Steel plate No. 36 had low toughness due to low cumulative rolling reduction. Steels Nos. 29 and 30 contained Ti, so the HAZ toughness decreased.

[0039]

The Ni-containing thick steel plate produced according to the present invention not only has excellent base metal strength and low temperature toughness, but also
Since HAZ is also excellent in low-temperature toughness, strength and low-temperature toughness that can be practically used can be guaranteed even in a steel plate having a plate thickness of more than 30 mm, particularly 40 mm or more.

[Brief description of the drawings]

FIG. 1 is a diagram showing a welding heat cycle.

Claims (4)

[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%, N: 0.005 wt
% Or less, P: 0.005 wt% or less, S: 0.002 wt% or less, and Ca: 0.0005 to 0.005 wt% and REM: 0.001
Steel slabs containing 1 or 2 of 0.03 wt% and the balance of Fe and inevitable impurities at 1100-1300 ° C.
After heating to 70 ° C, the cumulative rolling reduction at 700 to 850 ℃ is 30
~ 80% hot-rolled into thick steel plate, then primary quenching treatment by heating and cooling to a temperature range of Ac ₃ points to 850 ° C, and further heating to a temperature range of Ac ₁ point to Ac ₃ points A method for producing a high-strength Ni-containing steel sheet having excellent low-temperature toughness, which comprises performing a secondary quenching treatment for cooling and then performing a tempering treatment at Ac ₁ point + 50 ° C or lower. 2. A cooling rate of 2 ° C./sec after the tempering treatment.
The method for producing a high-strength Ni-containing thick steel sheet excellent in low-temperature toughness according to claim 1, wherein the above cooling treatment is performed. 3. The steel slab further comprises V: 0.005-0.
030 wt% is contained, Claim 1 or 2 characterized by the above-mentioned.
A method for producing a high-strength Ni-containing steel sheet having excellent low-temperature toughness as described above. 4. The steel slab further comprises Cu: 0.05 to 0.30.
%, Mo: 0.02 to 0.20 wt%, Cr: 0.05 to 0.30 wt%, at least one or more of them being contained. Manufacturing method of thick steel plate.