JPH0941088A - Production of high toughness steel plate for low temperature use - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce the steel plate improved in toughness at low temp. and having a wide permissible hardening temp. range. SOLUTION: A slab, having a composition consisting of, by weight, 0.04-0.12% C, 0.02-0.80% Si, 0.05-0.8% Mn, <=0.01% P, <=0.005% S, 6.5-12.0% Ni, 0.01-0.10% Al, <=0.0035% N, and the balance essentially Fe, is held at 1,200-1,350 deg.C for >=24hr to undergo diffusion heat treatment, followed by air cooling down to room temp. Subsequently, the slab is hot-rolled to the prescribed plate thickness, and the resultant plate is heated to a temp. between the Ac3 transformation point and (Ac3 transformation point +200 deg.C), cooled, and then tempered at a temp. between 450 deg.C and (Ac1 transformation point +70 deg.C).

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 low temperature steel sheet having excellent toughness, and particularly to liquefied natural gas (LN
The present invention relates to a method for producing a Ni-containing steel sheet for low temperature, which is suitable for use in an extremely low temperature range of −160 ° C. or less such as a steel sheet for G).

[0002]

2. Description of the Related Art Ni-containing steel sheets such as 9% Ni steel have long been known as low-temperature steel materials used in LNG tanks and the like. All low-temperature steel materials are required to have high toughness at low temperatures. For example, 9% Ni steel requires ASTM A5
It is generally manufactured by reheating quenching-tempering treatment (RQ-T) or direct quenching-tempering treatment (DQ-T) as standardized in No. 53 and A844. In addition to this, reheating and normalizing-tempering (R
N-T) is also a frequently used manufacturing method. Among these treatment methods, heat treatment including a reheating step such as RQ-T treatment and RN-T treatment is a preferable manufacturing method from the viewpoint of improving toughness. In addition to this, even in the manufacturing method including the above-mentioned reheating step, efforts have been conventionally made to improve the toughness. For example, in JP-A-58-73717, steel containing 0.5 to 10.0 wt% of Ni is subjected to quenching treatment (primary quenching) from a temperature range of Ac ₃ points or more, and then Ac ₁ point or more Ac ₃ Quench from the two-phase region below the point (secondary quench),
A method of performing tempering treatment is also disclosed in JP-A-2-19412.
Japanese Patent Laid-Open No. 1-1985 proposes a method of performing a normalizing process on a steel having a low Si-low Mn content, followed by a secondary normalizing process.

[0003]

However, in recent years,
The demand for low temperature toughness is increasing more and more, and it has become difficult to satisfy the toughness (currently required characteristic is vE-196 = 25 kgf · m class) that meets the requirement by the above-mentioned known techniques. Further, according to the conventional manufacturing method, for example,
In order to obtain a high toughness of 20 kgf · m or higher, a certain limited heat treatment temperature range, specifically, the quenching temperature is Ac ₃ to (A
There was a problem that it could not be achieved unless it was controlled within an extremely narrow temperature range such as c ₃ + 50 ° C.

The object of the present invention is to improve the low temperature toughness in the production of 9% Ni steel by a heat treatment including a reheating step, and to allow a wide range of heat treatment temperature ranges such as quenching and normalizing. Is proposed.

[0005]

[Means for Solving the Problems] First, the inventors
In the 9% Ni steel manufactured by the conventional method, it was difficult to obtain high low temperature toughness, and as a result of pursuing the reason that the quenching conditions for obtaining the desired low temperature toughness are limited to an extremely narrow temperature range, the components in the steel sheet We found that it was due to segregation. That is, a dendrite structure is formed during ingot formation, segregation of Ni, Mn, etc. occurs, and these segregations are rolled,
After the heat treatment, it remains as a layer in the thickness direction. With respect to the degree of this segregation, for example, in the case of steel containing 9.0 wt% of Ni, a difference in Ni amount of 8.0 to 10.8 wt% occurs in the thickness direction. Due to such a difference in Ni concentration, a difference in transformation point in the thickness direction occurs, and the final structure also becomes nonuniform. Therefore, in order to make the final structure of the steel sheet uniform and to obtain high toughness, the quenching temperature was limited to a narrow range. Therefore, as a result of diligent studies on the manufacturing conditions for eliminating the above causes, the inventors have reduced the segregation of components such as Ni and Mn by diffusion heat treatment, and attempted to make the steel structure uniform by reducing the toughness. We have reached the conclusion that it is possible to improve and relax the heat treatment (quenching or normalizing) temperature conditions, and have completed the present invention. The summary structure is as follows.

(1) C: 0.04 to 0.12 wt%, Si: 0.02 to 0.
80 wt%, Mn: 0.05 to 0.8 wt%, P: 0.01 wt% or less,
S: 0.005 wt% or less, Ni: 6.5 to 12.0 wt%, Al: 0.01
~ 0.10 wt% and N: 0.0035 wt% or less, the balance consisting essentially of Fe is subjected to a diffusion heat treatment of holding at 1200 to 1350 ° C for 24 hours or more, cooled to room temperature, and then to a predetermined plate thickness. Hot rolling, heating to a temperature range from Ac ₃ transformation point to (Ac ₃ transformation point + 200 ° C), then cooling, and then 450 ° C
A method for producing a high-toughness low-temperature steel sheet, which comprises tempering in a temperature range of to (Ac ₁ transformation point + 70 ° C).

(2) C: 0.04 to 0.12 wt%, Si: 0.02 to 0.
80 wt%, Mn: 0.05 to 0.8 wt%, P: 0.01 wt% or less,
S: 0.005 wt% or less, Ni: 6.5 to 12.0 wt%, Al: 0.01
Up to 0.10 wt% and N: 0.0035 wt% or less, and further N
b: 0.005-0.06wt%, V: 0.005-0.07wt% and C
u: a slab containing one or more selected from 0.05 to 0.50 wt% and the balance consisting essentially of Fe, 12
Diffusion heat treatment is carried out at 00 to 1350 ° C for 24 hours or more, cooling is performed to room temperature, and then hot rolling is performed to a predetermined plate thickness.
Steel sheet for high toughness low temperature, characterized by heating to a temperature range of c ₃ transformation point to (Ac ₃ transformation point + 200 ° C), cooling, and then tempering in a temperature range of 450 ° C to (Ac ₁ transformation point + 70 ° C). Manufacturing method.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION First, the reasons for limiting the composition of steel in the present invention will be explained. C: 0.04 to 0.12 wt% C is an element useful for obtaining a sufficiently high tension. C
If the content is less than 0.04 wt%, it is necessary to increase Si and Mn in order to secure the strength, and as described above, there is a problem that the toughness of the part heated to 700 to 900 ° C becomes low. Even if it exceeds wt%, toughness decreases, so 0.04 to 0.
The range is 12wt%. The preferred range of C content is
It is 0.05 to 0.09 wt%.

Si: 0.02 to 0.80 wt% Si has an adverse effect on the toughness of the welded portion, so it is preferable to reduce it. However, even if it is less than 0.02 wt%, no gradual effect is observed, so the lower limit was made 0.02 wt%. . On the other hand, if it exceeds 0.80 wt%, not only the toughness is deteriorated but also the strength is excessively increased, so 0.80 wt% is made the upper limit. In addition,
The preferable range of Si content is 0.1 to 0.5 wt%.

Mn: 0.05-0.8 wt% Mn, like Si, adversely affects the toughness of the weld zone, so it is preferable to reduce it, but even if it is reduced to less than 0.05 wt% it does not show a gradual effect, so the lower limit is Was set to 0.05 wt%. on the other hand,
The addition of more than 0.8 wt% not only deteriorates the toughness but also increases the strength excessively, so 0.8 wt% was made the upper limit. If Mn is reduced in this range, the toughness of the welded portion is gradually improved, and the effect is remarkable especially in the range of 0.3 wt% or less.

P: 0.01 wt% or less, S: 0.005 wt% or less P and S both impair the toughness of the base material and the welded portion, so it is desirable to reduce them as much as possible, but 0.01 wt% or less and 0.005 wt% respectively. The following range is acceptable. Preferably,
It is recommended to limit the content to 0.005 wt% or less and 0.002 wt% or less, respectively.

Ni: 6.5 to 12.0 wt% Ni is an essential element for the steel for low temperature use of the present invention, and has a remarkable effect in securing toughness at low temperatures, but if it is less than 6.5 wt%, its effect is poor. Even if added in excess of 12.0wt%, its effect is saturated and it is uneconomical, so 6.5-12.0wt%
%. The preferred range of Ni content is
7.5 to 10.0 wt%.

Al: 0.01 to 0.10 wt% Al is an element necessary for deoxidizing steel. The amount of Al added is 0.
If it is less than 01 wt%, the effect is poor, while if it exceeds 0.10 wt%, the cleanliness is impaired, so the range is 0.01 to 0.10 wt%.

N: 0.0035 wt% or less N is an element that increases mobile dislocations and increases island martensite to deteriorate toughness. N content is
Above 0.0035 wt%, especially in the temperature range of 700 to 900 ° C
Since it lowers the toughness of the heat-affected zone that is heated to
The amount is 0.0035wt%.

The above C, Si, Mn, P, S, Ni, Al, and N are the basic components of the steel in the present invention.
It is also possible to contain at least one of Cu and Cu. The reasons for these limitations will be described below.

Nb: 0.0055 to 0.06 wt% Nb effectively contributes to improving the strength by precipitation strengthening, but if it is less than 0.005 wt%, the addition effect is small, while
If it exceeds 06wt%, the toughness will be impaired, so 0.0055〜
The amount is 0.06 wt%, preferably 0.008 to 0.03 wt%.

V: 0.005 to 0.07 wt% V is an element effective for improving the strength by precipitation strengthening. If the addition amount of V is less than 0.005 wt%, its effect is small, while if it exceeds 0.07 wt%, the toughness is impaired, so 0.005 to 0.07 wt%, preferably 0.008 to 0.03 wt% is set.

Cu: 0.05 to 0.50 wt% Cu is an element effective for improving the strength by improving the hardenability, but if it is less than 0.05 wt%, its effect of addition is poor.
On the other hand, if it exceeds 0.50 wt%, the toughness is rather deteriorated.
It is set to 05 to 0.50 wt%, preferably 0.05 to 0.10 wt%.

The intended purpose can be achieved by subjecting the steel having the above-mentioned compositional range to the diffusion heat treatment and then the reheating heat treatment. These manufacturing conditions will be described below. Diffusion heat treatment An important point in this invention method is that the diffusion heat treatment is performed prior to the reheating heat treatment. The diffusion heat treatment conditions are heating within a temperature range of 1200 to 1350 ° C., holding for 24 hours or more, and then air cooling to room temperature. If the heating temperature is less than 1200 ° C., it becomes difficult to make the components uniform because the diffusion rate of Ni sharply decreases. On the other hand, since the steel ingot itself may be melted by heating for a long time when heated above 1350 ° C, the diffusion annealing treatment is performed in the temperature range of 1200 to 1350 ° C, preferably 1300 to 1350. The temperature range is ° C. Also,
When the holding time is less than 24 hours, the diffusion of Ni is not sufficient and the components are not made uniform. Therefore, the holding time is set to 24 hours or more, preferably 36 to 48 hours. Note that the reason for temporarily cooling to room temperature after the diffusion annealing is to make the crystal grains coarsened after the annealing finer by reheating before rolling.

Reheating heat treatment after hot rolling After the above diffusion heat treatment, ordinary hot rolling is subsequently performed to obtain a steel sheet. This steel sheet is first subjected to the Ac ₃ transformation point ~ (Ac ₃
Reheat heat treatment is performed by heating (water cooling, air cooling, etc.) after heating in the temperature range of transformation point + 200 ° C. If the heating temperature is below the Ac ₃ transformation point, the austenitization of the steel will be incomplete, and the structure will eventually contain a high temperature tempered martensite containing coarse carbides, adversely affecting toughness and strength. Further, if heating is performed above (Ac ₃ transformation point + 200 ° C.), the austenite grains are coarsened and the low temperature toughness is adversely affected. Therefore, the heating temperature range is from the Ac ₃ transformation point to (A
c ₃ transformation point + 200 ° C.), preferably Ac ₃ transformation point to (Ac ₃ transformation point + 100 ° C.).

· Tempering After the above reheating heat treatment, 450 ℃ or more (Ac ₁ point transformation point +70
Tempering treatment is performed under the following conditions. If the tempering temperature is less than 450 ° C, sufficient toughness cannot be secured, while
If it exceeds (Ac ₁ point transformation point + 70 ° C), the strength decreases.

[0022]

[Examples] Steels having the chemical compositions shown in Table 1 were subjected to diffusion annealing heat treatment under the conditions shown in Table 2, cooled to room temperature, heated and rolled into steel sheets, subjected to reheat heat treatment, and then tempered. gave. The holding time during the heat treatment was 70 min, the cooling was water cooling, and the holding time during the tempering was 70 min, and the cooling was air cooling.

[0023]

[Table 1]

[0024]

[Table 2]

The tensile properties and Charpy impact properties of the base metal of each of the obtained steel sheets were investigated. The results are shown in Table 2. In Table 2, in the case of Nos. 1 to 9 in which the invention method is applied and the diffusion annealing heat treatment is applied, vE-196 is all
The value exceeds 25 kgf ・ m. Further, since the structure is made uniform by diffusion annealing, the temperature of heat treatment is allowed in a wide range as shown. In contrast, No.10
The vE-196 of the comparative method typified by ~ 13, and vE-196 of the conventional method all have 18 to 20 kgf · m. Also, the allowable heat treatment temperature range is narrow. In the comparison method, the heating temperature during the diffusion annealing treatment was too low or the holding time was too short. The conventional method is one in which no diffusion annealing treatment is applied.

[0026]

As described above, according to the manufacturing method of the present invention, a steel sheet having excellent low temperature toughness can be manufactured by the reheating heat treatment method. In addition, since the heat treatment (quenching or normalizing) temperature range can be allowed in a wide range, reheating heat treatment such as quenching treatment is easy and the manufacturability is improved, so that the industrial contribution is extremely large.

Claims (2)

[Claims] 1. C: 0.04 to 0.12 wt%, Si: 0.02 to 0.80 wt%
%, Mn: 0.05 to 0.8 wt%, P: 0.01 wt% or less, S: 0.005 wt% or less, Ni: 6.5 to 12.0 wt%, Al: 0.01 to 0.10 wt% and N: 0.0035 wt% or less, The rest is a slab consisting essentially of Fe, 1200-1350 ℃
Diffusion heat treatment for 24 hours or more, cooling to room temperature, hot rolling to a predetermined plate thickness, heating to a temperature range from Ac ₃ transformation point to (Ac ₃ transformation point + 200 ° C), and then cooling.
Next, a method for producing a high-toughness low-temperature steel sheet, which comprises tempering in a temperature range of 450 ° C to (Ac ₁ transformation point + 70 ° C). 2. C: 0.04 to 0.12 wt%, Si: 0.02 to 0.80 wt
%, Mn: 0.05 to 0.8 wt%, P: 0.01 wt% or less, S: 0.005 wt% or less, Ni: 6.5 to 12.0 wt%, Al: 0.01 to 0.10 wt% and N: 0.0035 wt% or less,
Furthermore, a slab containing one or more selected from Nb: 0.005 to 0.06 wt%, V: 0.005 to 0.07 wt% and Cu: 0.05 to 0.50 wt%, and the balance substantially consisting of Fe, Diffusion heat treatment at 1200 to 1350 ° C for 24 hours or more, cooling to room temperature, then hot rolling to a prescribed plate thickness, and then heating and cooling to a temperature range of Ac ₃ transformation point to (Ac ₃ transformation point + 200 ° C). And then tempering in the temperature range of 450 ° C to (Ac ₁ transformation point + 70 ° C).