JPH09256039A - Production of high yield strength and high toughness nickel-containing thick steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing an Ni-contg. thick steel plate with >=50mm plate thickness having high yield strength and excellent low temp. toughness. SOLUTION: An Ni-contg. thick steel plate contg. 7.5 to 12.0wt.% Ni is subjected to primary quenching treatment of heating to the Ac3 point or above and executing cooling, is then subjected to secondary quenching treatment of holding under heating in such a manner that the temp. range is regulated to (Ac1 +10) to (Ac3 -10) deg.C and the amt. of austenite (γ) formed in the same temp. range is regulated to 20 to 65% or 85 to 95% by area ratio and is furthermore subjected to tempering treatment at 500 to (Ac1 -10) deg.C.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a sheet having a thickness of 50 mm or more.
Regarding the manufacturing method of Ni thick steel plate, especially the high yield strength,
The present invention also relates to a method for producing a high yield strength, high toughness Ni-containing thick steel sheet which does not lose its toughness even when used at an extremely low temperature 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 sheet, is often 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 9 wt% Ni steel sheet used for LNG storage tank material has a thickness exceeding the upper limit of 30 mm of the conventional steel sheet. Especially, those having a thickness of 50 mm or more are required. Excellent low temperature toughness 9
Many proposals have been made regarding the manufacturing method of the wt% Ni steel sheet. For example, Japanese Patent Publication No. 47-23317 and Japanese Patent Publication No. 4-4041.
Japanese Patent Laid-Open No. 1-1993 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 at the Ac ₁ transformation point or lower.

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 to thick steel plates having a plate thickness of more than 50 mm. That is, when the plate thickness is more than 50 mm, it becomes 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.

In Japanese Patent Laid-Open No. 3-264617, a slab is heated to a relatively low temperature of 800 to 1000 ° C., hot rolled, then quenched and then an intermediate quench of two-phase region heating is performed. A method for producing a 9 wt% Ni thick steel plate having a high yield strength by performing a tempering treatment at ₁ point or less has been proposed. In Japanese Patent Laid-Open No. 4-371520, after heating a slab, hot rolling with a cumulative rolling reduction of 30 to 80% at 700 to 850 ° C. was performed, followed by quenching and intermediate quenching by heating in a two-phase region. After that, tempering at the transformation point from 550 to Ac ₁ and excellent CTOD characteristics of the base metal and the weld heat affected zone 9% thick plate thickness of 40 mm or more
A method for manufacturing Ni steel has been proposed.

Japanese Unexamined Patent Publication (Kokai) No. 58-73717 discloses that low Ni steel contains Ca,
A technique has been proposed in which La is added and the low temperature toughness is enhanced by a two-step quenching / tempering treatment including quenching in the two-phase region. In Japanese Unexamined Patent Publication (Kokai) No. 6-240348, a slab with a reduced amount of P and S is formed into a thick steel plate by hot rolling, then heated and quenched at 800 to 900 ° C, further heated to Ac _{3 to} 800 ° C, and then quenched again. However, there has been proposed a method for manufacturing a Ni steel that is tempered at an Ac of ₁ point or more.

However, even the above-mentioned technique has a problem that high strength and high toughness cannot be stably obtained when the plate thickness is large.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to propose a method for producing a Ni-containing thick steel plate having a plate thickness of 50 mm or more, which has high yield strength and excellent low temperature toughness.

[0009]

First, experimental results on which the present invention is based will be described. With a thick 9% Ni steel having a plate thickness of 50 mm or more, it becomes difficult to maintain high yield strength as well as low temperature toughness as shown in FIG. 3, for example. The present inventors investigated the structure of 9% Ni steel in detail and examined the mechanical properties, especially the relationship between the yield strength and the microstructure.

In order to improve low temperature toughness, 9% Ni steel has conventionally been subjected to a primary quenching (Q) which is heated and cooled in the austenite (γ) region and a secondary quenching treatment which is heated and cooled in the γ + α two-phase region and Ac ₁ A heat treatment consisting of a tempering process for tempering below the point is performed. The microstructure of the 9% Ni steel thus obtained is composed of a soft matrix that is ferrite (α) during the secondary quenching heating, martensite that occurs during secondary quenching, bainite that occurs during secondary quenching, and austenite that precipitates during tempering. The strength depends on the structure fraction and hardness of martensite and bainite, the structure fraction of precipitated austenite, the existence form and stability.

In particular, the inventors of the present invention have found that in 9% Ni steel having a plate thickness of 50 mm or more, the decrease in yield strength has a large effect on the amount of γ formed during heating in the two-phase region, as shown in FIG. I found out that it is being done. If the amount of γ is more than 65% and less than 85%, the yield strength will be significantly reduced. That is, in order to obtain a yield strength of 590 MPa or more, the amount of γ formed during heating in the two-phase region is 65
% Or less, preferably 20 to 65% or 85% or more, preferably 85 to 95%.

For example, after the amount of austenite formed during the secondary quenching is set to 40%, the quenching cooling rate is changed to cool, and the yield strength (Y) after tempering at 570 ° C.
The relationship between S), tensile strength (TS) and the secondary quenching cooling rate is shown in FIG. 2 as a solid line. For comparison, an example in which the amount of generated austenite at the time of secondary quenching is set to 75% and then quenching and tempering is shown by a broken line. In this way, by setting the amount of austenite produced during the secondary quenching to 40%, it is possible to reduce the decrease in yield strength even for thick steel plates that cool slowly.

The present invention is constructed based on the above findings. That is, the present invention relates to 7.5 to 12.0 wt% Ni
A nickel-containing thick steel plate containing Al is subjected to a primary quenching treatment in which it is heated to a temperature of Ac ₃ points or higher and cooled, and then Ac ₁ + 10 ° C. or higher, A
c ₃ −10 ° C. or less, and the amount of austenite (γ) produced in the temperature range is maintained at 20 to 65% in area ratio by heating and holding, followed by secondary quenching treatment of cooling, and then 500 A high yield strength, high toughness Ni-containing thick steel sheet having a plate thickness of 50 mm or more, which is characterized by performing a tempering treatment at a temperature of ℃ or more and (Ac ₁ -10 ℃) or less, and the present invention provides: 7.5 ~ 1
Ni-containing thick steel plate containing 2.0 wt% Ni was subjected to a primary quenching treatment of heating and cooling to a temperature of Ac ₃ points or higher, and then Ac ₁ +10
After carrying out a secondary quenching treatment in which the austenite (γ) produced in the temperature range of ℃ or more and Ac ₃ -10 ℃ or less and the area ratio is 85 to 95% by heating and holding, and cooling is performed. A method for producing a high yield strength, high toughness Ni-containing steel plate having a plate thickness of 50 mm or more, which is characterized by performing a tempering treatment at 500 ° C. or more and (Ac _1-10 ° C.) or less.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. The present invention is intended for a thick steel plate having a plate thickness of 50 mm or more. The steel sheet that has undergone hot rolling is cooled once and then Ac ₃
A primary quenching treatment of heating to a temperature above the point and cooling is performed.

Here, the primary quenching treatment is performed in order to obtain a high yield strength and excellent low temperature toughness by forming a fine martensite (+ bainite) structure and combining it with the heat treatment of the next step. Therefore, it is necessary to cool from a uniform austenite (γ) structure. for that reason,
Ac It is necessary to heat to ₃ points or more. However, if the heating temperature is too high, the austenite grains become coarse and the toughness decreases, so the temperature is preferably 850 ° C. or less. In addition,
The cooling rate after heating is preferably 1 ° C./sec or more.

The subsequent secondary quenching treatment is carried out in order to make the austenite that precipitates after the tempering treatment have a precipitation amount, a morphology, and stability that are advantageous for strength and toughness. That is, the steel sheet is in the two-phase region of the temperature range between Ac ₁ + 10 ° C. and Ac ₃ −10 ° C., and the amount of austenite produced in this temperature range is 65% or less, preferably 20 to 65 or 85% in area ratio. As described above, heating and holding is preferably performed so as to be in the range of 85 to 95%. If the amount of austenite produced is less than 20%, the amount of austenite precipitated during the tempering treatment is small and the low temperature toughness deteriorates. Further, when the amount of austenite produced is more than 65% and less than 85%, the yield strength does not satisfy 590 MPa or more. In this range, since the amount of austenite precipitated during tempering increases and lacks stability, it easily deforms,
Yield strength decreases. Also, if it exceeds 95%, the amount of austenite that precipitates stably during tempering decreases,
Low temperature toughness deteriorates.

From the above, the secondary quenching treatment is
In the temperature range of Ac ₁ + 10 ° C to Ac ₃ -10 ° C and the amount of austenite produced is 65% or less, preferably 20 to 65% or
The heating and holding condition was 85% or more, preferably 85 to 95%. The amount of austenite to be generated is determined by the combination of heating temperature, holding time and chemical composition.
For 0.05% C-0.6% Mn-9.4% Ni steel, hold time is 20
When min is kept constant, it is preferably in the range of 620 to 660 ° C, or 680 to 690 ° C. Holding at low temperature for a long time is also suitable from the viewpoint of stability of precipitated austenite as long as the amount of austenite is generated during secondary quenching in the above range. Rather, it has a strong time dependency in the early stage of transformation, and when the retention time is short, a slight difference in retention time causes a large difference in the structure. In thick-walled materials, the time for holding at the heat treatment temperature is different between the surface part and the central part, so if the holding time is short, the structure of the surface and the central part will be different. If the target heat treatment temperature is held for 10 minutes or longer in the center of the plate thickness, a substantially uniform structure is obtained. Therefore, the holding time is preferably 10 minutes or longer.

By making the holding time sufficient, alloying elements such as Ni are sufficiently concentrated in the austenite formed during heating in the two-phase region. As a result, hard martensite can be generated during cooling, and fine and stable γ can be precipitated during tempering, and a structure excellent in strength and toughness can be obtained. Next, a tempering treatment is performed at 500 ° C. or higher and Ac _1-10 ° C. or lower.

The tempering treatment is carried out in order to reduce the dislocation density of the above-mentioned quenched structure and at the same time to produce stable precipitated austenite. However, as the tempering temperature approaches the Ac ₁ transformation point, the amount of precipitated austenite is increased. To reduce the yield strength and to obtain fine austenite precipitation effective for low temperature toughness, Ac _1-10
It needs to be performed in a temperature range of ℃ or less. The lower limit of the tempering temperature is preferably 500 ° C. or higher in order to secure the tempering effect. Cooling after tempering is preferably set to 2 ° C./sec or more because the toughness can be further improved. If cooling is slow, grain boundary segregation of P or formation of Fe phosphide is caused, which adversely affects toughness.

The slab used for the thick steel plate of the present invention can be suitably manufactured by either ingot ingot or continuous casting. The slab manufactured by the above method is hot-rolled and has a plate thickness of
Finished with thick steel plate of 50 mm or more. Here, the hot rolling preferably heats the slab to 1100-1300 ° C.

This temperature range is for melting MnS in the slab and for refining the crystal grains in the subsequent rolling step, which requires heating at 1100 ° C. or higher, while 1300 ° C. If it exceeds, the γ grains are remarkably coarsened and the crystal grains after rolling are not refined. In order to sufficiently form a solid solution of MnS, heating is preferably performed at 1200 to 1300 ° C. Hot rolling must be performed at 700 to 900 ° C with a cumulative reduction of 20 to 90% in order to refine the crystal grains.

In rolling in a temperature range of less than 700 ° C., a texture that inhibits toughness develops, and in rolling in a temperature range of more than 900 ° C., γ grains recrystallize instantly,
The grain refinement by rolling cannot be achieved. The finishing temperature is preferably 750 to 900 ° C from the viewpoint of rolling productivity. Cumulative rolling reduction in the range of 700 to 900 ℃ is 20
If it is less than%, the grain refinement cannot be achieved. If the cumulative rolling reduction exceeds 90%, the anisotropy becomes too strong.

The Ni-containing thick steel plate of the present invention preferably has the following component composition. 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 an increase in strength, so 0.01 wt% or more is preferable, but if a large amount of Si 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 an 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 may decrease. Therefore, the range is 0.30 to 0.70 wt%. More preferably, it is 0.50 to 0.60 wt%. Ni: 7.5 to 12.0 wt% Ni is a component that is added in order to impart low temperature toughness to steel and at the same time obtain a structure mainly composed of martensite by quenching treatment, and it is necessary to contain 7.5 wt% or more.
Even if the content exceeds 12.0wt%, its effect will be saturated,
The upper limit is 12.0wt%.

Al: 0.01 to 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%. 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, but since it can be up to 0.005 wt% or less, the P content is 0.005 wt% or less.

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.

N: 0.005 wt% or less N reduces the toughness in the solid solution state, particularly the toughness of the heat-affected zone of the weld, 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.

In the present invention, in addition to the composition described above, the following elements can be added if necessary. Cu: 0.05-0.30wt%, Mo: 0.02-0.20wt%, Cr: 0.05-
One or more selected from 0.30 wt% Cu, Mo and Cr are effective components for increasing 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.

Nb, V: 0.005 to 0.030 wt% Nb, V is a component effective in increasing the strength due to precipitation hardening or grain refinement, and is also effective in improving the toughness. V is effective at 0.005 wt% or more, but 0.030 wt
%, The toughness of the weld will decrease, so 0.030 wt%
Is the upper limit.

[0030]

EXAMPLES Examples will be described below. The continuously cast steel slab having the chemical composition shown in Table 1 was formed into a thick steel plate having a plate thickness of 50 to 100 mm under the hot rolling conditions shown in Table 2. Then, under the heat treatment conditions shown in Table 2, primary quenching, secondary quenching and tempering were performed. Cooling rate of secondary quenching is 2.2-8.8 ℃ /
It was in the range of sec.

The tensile properties (yield strength, tensile strength, elongation) and the absorbed energy at -196 ° C. of the Charpy impact test of the thick steel sheet treated under the above conditions were determined, and the results are also shown in Table 2.

[0032]

[Table 1]

[0033]

[Table 2]

As shown in Table 2, steel sheets Nos. 1 to 10 to which the present invention is applied have a yield strength of 590 MPa or more and E _-196 30 kgf.
-It has excellent strength and toughness even for thick steel plates with a thickness of m or more and a thickness of 50 mm or more. On the other hand, steel plate Nos. 11, 12, 13, 14
Is 70-80 in the amount of austenite produced during secondary quenching and heating.
The yield strength is reduced due to the range of%. Steel plate No. 15
The tempering temperature is too high and the yield strength is low, and the tempering temperature of steel sheet No. 16 is too low, so the low temperature toughness is deteriorated.

[0035]

According to the present invention, it is possible to stably obtain a Ni-containing thick steel sheet having excellent yield strength and low temperature toughness even in a thick steel sheet having a thickness of 50 mm or more, and to supply a thick steel sheet that can withstand practical use. it can.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the amount of austenite (γ) generated during secondary quenching heating, absorbed energy ( _v E ₋₁₉₆ ) at −196 ° C., and yield strength (YS).

FIG. 2 is a graph showing the relationship between the secondary quenching cooling rate and the yield strength (YS) and tensile strength (TS).

FIG. 3 Secondary quenching cooling rate and yield strength (YS), tensile strength (TS), absorbed energy at −196 ° C. ( _v E
_-196 ) is a graph showing the relationship.

Claims (2)

[Claims] 1. A primary quenching treatment in which a Ni-containing thick steel sheet containing 7.5 to 12.0 wt% Ni is heated to a temperature of Ac ₃ points or higher and cooled, and then Ac ₁ + 10 ° C. or higher and Ac ₃ -10 ° C. or lower. In the temperature range of and the amount of austenite (γ) produced in the temperature range is 20 to 65% in area ratio, and is held by heating and cooled.
After the next quenching treatment, further (500 ° C or more, (Ac ₁
A method for producing a high yield strength, high toughness Ni-containing steel plate having a plate thickness of 50 mm or more, characterized by performing a tempering treatment at -10 ° C or less. 2. A Ni-containing thick steel sheet containing 7.5 to 12.0 wt% Ni is subjected to a primary quenching treatment of heating to a temperature of Ac ₃ points or higher and cooling, and then Ac ₁ + 10 ° C. or higher and Ac ₃ -10 ° C. or lower. In the temperature range of and the amount of austenite (γ) generated in the temperature range is 85 to 95% in area ratio, and heating and holding are performed. 2
After the next quenching treatment, further (500 ° C or more, (Ac ₁
A method for producing a high yield strength, high toughness Ni-containing steel plate having a plate thickness of 50 mm or more, characterized by performing a tempering treatment at -10 ° C or less.