JPH0920922A - High toughness low temperature steel sheet manufacturing method - Google Patents

Abstract

PURPOSE: To improve toughness at low temp. and to enlarge hardening temp. range by reducing the component segregation of Ni, Mn, etc., in an Ni steel of specific composition by means of diffusion heat treatment and uniformizing the structure of the steel. CONSTITUTION: A slab, having a composition consisting of, by weight, 0.04-0.12% C, 0.02-0.50% 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 Fe, is prepared. This slab is held at 1200-1350 deg.C for >=24hr to undergo diffusion heat treatment, followed by cooling to room temp. Then, the slab is heated to 1100-1300'C and hot-rolled at 1000-700 deg.C rolling finishing temp. The resultant rolled plate is cooled while regulating the average cooling rate at 800-500 deg.C to <=2 deg.C/sec and then tempered at a temp. between 450 deg.C and (Ac1 transformation point + 70 deg.C). Further, one or more kinds among 0.005-0.06% Nb, 0.005-0.07% V, and 0.05-0.50% Cu can be incorporated into Nb.

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, particularly liquefied natural gas (LNG).
TECHNICAL FIELD The present invention relates to a method for producing a low-temperature Ni-containing steel sheet suitable for use in an extremely low temperature range of −160 ° C. or less for use in steel sheets.

[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. Among these treatment methods, the DQ-T treatment is a preferable production method from the viewpoint of process saving. By the way, even in the manufacturing method by the DQ-T treatment, efforts have been conventionally made to improve the toughness. For example, Japanese Patent Application Laid-Open No. 2-194122 proposes a method of improving the strength, toughness, and weld zone toughness of a base material by subjecting a steel sheet having a low Si-low Mn content to a DQ-T treatment.

[0003]

However, in recent years,
The demand for low temperature toughness is increasing year by year, and depending on the above-mentioned known techniques, the toughness that meets the demand (currently vE-1
Characteristics of 96 = 25 kgf ・ m class are required. ) Has become difficult to satisfy. Further, in order to obtain high toughness of, for example, 20 kgf · m or more by the conventional manufacturing method, a certain limited quenching temperature range, specifically Ac ₃ to
There is a problem that it cannot be achieved unless the temperature is controlled within an extremely narrow quenching temperature range such as (Ac ₃ + 50 ° C).

The object of the present invention is to propose a manufacturing technique for improving the low temperature toughness and permitting a wide quenching temperature range when manufacturing a 9% Ni steel by the DQ-T process.

[0005]

Means for Solving the Problems Now, the inventors first set forth
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 is limited to a narrow range. Therefore, as a result of diligent examination by the inventors of the manufacturing conditions for eliminating the above cause,
In particular, it was concluded that it is possible to improve the toughness and alleviate the quenching temperature conditions by reducing the segregation of Ni, Mn, etc. components by diffusion heat treatment and making the steel structure uniform. The invention has been completed. The summary structure is as follows.

(1) C: 0.04 to 0.12 wt%, Si: 0.02 to 0.
Contains 50 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 balance is a slab consisting essentially of Fe, 1200 ~
Diffusion heat treatment is performed at 1350 ° C for 24 hours or more, cooling to room temperature, then heating to a temperature of 1100 to 1300 ° C, hot rolling to a rolling finish temperature of 1000 to 700 ° C, and immediately
The average cooling rate in the temperature range of 800-500 ℃ is 2 ℃ /
A method for producing a high toughness low temperature steel sheet, which comprises cooling for at least sec and then tempering at a temperature of 450 ° C to (Ac ₁ transformation point + 70 ° C).

(2) C: 0.04 to 0.12 wt%, Si: 0.02 to 0.
50 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, Perform diffusion heat treatment at 1200 to 1350 ° C for 24 hours or more, cool to room temperature, then heat to 1100 to 1300 ° C, perform hot rolling at a rolling finish temperature of 1000 to 700 ° C, and immediately Cooling is performed at an average cooling rate of 2 ° C / sec or more in the temperature range of 500 ° C, and then 450 ° C.
A method for producing a high-toughness low-temperature steel sheet, characterized by tempering at a temperature of (Ac ₁ transformation point + 70 ° C).

(3) The steel slab described in (1) or (2) above is subjected to a diffusion heat treatment of keeping it at 1200 to 1350 ° C for 24 hours or longer, cooled to room temperature, and then heated to a temperature of 1100 to 1300 ° C. Then, hot rolling with a rolling finishing temperature of 1000 to 700 ° C is performed, and immediately, cooling is performed at an average cooling rate of 2 ° C / sec or more in a temperature range of 800 to 500 ° C, and further (Ac ₃
A method for producing a steel sheet for high toughness low temperature, which comprises heating to −100 ° C. to (Ac ₃ + 10 ° C.), water cooling, and then tempering at a temperature of 450 ° C. to (Ac ₁ transformation point + 70 ° C.).

[0009]

First, the reasons for limiting the chemical 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 there is a problem that the toughness of the part heated to 700 to 900 ℃ becomes low. If it exceeds the limit, the toughness decreases, so the range is 0.04 to 0.12 wt%. In addition, the preferable range of C content is 0.06 to 0.10 wt%
It is.

Si: 0.02 to 0.50 wt% Si is preferable to reduce since it adversely affects the toughness of the welded portion, but 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 0.50 wt%, not only the toughness is deteriorated but also the strength is excessively increased, so 0.50 wt% is set as the upper limit. Note that Si
The preferable range of the content is 0.2 to 0.4 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. However, even if it is reduced to less than 0.05 wt%, it does not show a gradual effect. 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 ensuring the 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
If it exceeds 0.0035 wt%, the toughness of the heat-affected zone heated to 700 to 900 ℃ will be reduced, so the upper limit is 0.0035 wt%.
And

The above C, Si, Mn, P, S, Ni, Al, and N are the basic components of the steel in the present invention, and further, Nb, V
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: 0.005-0.07 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 addition effect 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 DQ-T treatment after the diffusion heat treatment. These manufacturing conditions will be described below. -Diffusion heat treatment An important point in the method of the present invention is that the diffusion heat treatment is performed prior to the DQ-T treatment. The diffusion heat treatment conditions are heating to a temperature of 1200 to 1350 ° C., holding for 24 hours or more, and then cooling to room temperature. If the heating temperature is less than 1200 ℃, Ni
It becomes difficult to make the components uniform because the diffusion rate of the is rapidly reduced. 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. Further, when the retention time is less than 24 hr, the diffusion of Ni is not sufficient and the components are not homogenized, so the retention time is 24 hr or more, preferably 36 to 48.
hr. 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.

Hot rolling and cooling After the above diffusion heat treatment, heating is performed at a temperature of 1100 to 1300 ° C., and hot rolling is performed at a finishing temperature of 1000 to 700 ° C.
If the heating temperature is less than 1100 ° C., coarsely precipitated AlN does not dissolve in the cooled steel ingot, the toughness deteriorates, and a sufficient reduction ratio cannot be obtained. On the other hand, if heated above 1300 ° C, the austenite grains coarsen and it is economically disadvantageous, so the slab heating temperature is 1100 to 1300 ° C.
The temperature is preferably 1100 to 1200 ° C. Further, if the finishing temperature of hot rolling exceeds 1000 ° C., a sufficient fine grain structure cannot be obtained, which is inconvenient for the production of the high toughness steel sheet aimed at by the present invention. On the other hand, below 700 ° C, strain accumulates in the crystal grains,
The toughness becomes low. Therefore, the finishing temperature for hot rolling is
The temperature is 1000 to 700 ° C, preferably 900 to 800 ° C. Immediately after the hot rolling, a direct quenching treatment is carried out by cooling in the temperature range of 800 ° C to 500 ° C with an average cooling rate of 2 ° C / sec or more. By this treatment, the strength is increased as compared with reheating and quenching, so that it becomes possible to reduce the amounts of Si, Mn and N. At this time, if the average cooling rate is less than 2 ° C / sec, the quenching becomes insufficient and the strength decreases, so the average cooling rate of direct quenching is 2 ° C / sec.
Or higher, preferably 5 ° C./sec or higher.

Tempering After direct quenching, tempering is performed under the conditions of 450 ° C or higher (Ac ₁ point transformation point + 70 ° C) or lower. Tempering treatment temperature is 45
If the temperature is less than 0 ° C, sufficient toughness cannot be secured, while if it exceeds (Ac ₁ point transformation point + 70 ° C), the strength decreases.

In order to further improve the toughness of the base material, between the above direct quenching treatment and tempering treatment,
It is effective to carry out a quenching treatment of heating to (Ac ₃ -100 ° C) to (Ac ₃ + 10 ° C) and then water cooling.

[0024]

EXAMPLE 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, rolled, and immediately quenched and tempered. In addition,
Cooling during quenching is water cooling (average cooling rate of 800 to 500 ℃ 50 ℃
/ S), holding time during tempering was 70 min, and cooling was air cooling.

[0025]

[Table 1]

[0026]

[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 24 kgf · m. In contrast, No.1
The vE-196 of the comparative method represented by 0 to 13 and the conventional method are all 19 kgf · m to 20 kgf · m. The comparison method is
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.

[0028]

As described above, according to the manufacturing method of the present invention, it is possible to manufacture a steel sheet having excellent low temperature toughness by the direct quenching method. Moreover, since the quenching temperature range can be allowed in a wide range, the quenching process is easy and the manufacturability is improved, which greatly contributes to the industry.

Claims (2)

[Claims] 1. C: 0.04-0.12 wt%, Si: 0.02-0.50 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 ℃
Perform a diffusion heat treatment for 24 hours or more at room temperature to cool to room temperature, then heat to a temperature of 1100 to 1300 ° C, perform hot rolling to a rolling finish temperature of 1000 to 700 ° C, and immediately
Average cooling rate in the temperature range of ~ 500 ℃ 2 ℃ / sec
The above cooling is performed, and then 450 ° C to (Ac ₁ transformation point +
A method for producing a high toughness low temperature steel sheet, which comprises tempering at a temperature of 70 ° C. 2. C: 0.04 to 0.12 wt%, Si: 0.02 to 0.50 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, Perform diffusion heat treatment at 1200 to 1350 ° C for 24 hours or more, cool to room temperature, then heat to 1100 to 1300 ° C, perform hot rolling at a rolling finish temperature of 1000 to 700 ° C, and immediately Cooling is performed at an average cooling rate of 2 ° C / sec or more in the temperature range of 500 ° C, and then 450 ° C.
A method for producing a high-toughness low-temperature steel sheet, characterized by tempering at a temperature of (Ac ₁ transformation point + 70 ° C).