JPH06240348A - Production of high toughness steel for low temperature use - Google Patents

Abstract

PURPOSE:To produce steel having excellent low temp. toughness without deteriorating its proof stress by subjecting a steel slab having a specified componental compsn. to hot rolling, executing hardening for plural times under specified conditions and subjecting it to tempering. CONSTITUTION:A slab contg., by weight, 0.02 to 0.08%. C, 0.02 to 0.50% Si, 0.40 to 0.80% Mn, <=0.003% P, <=0.002% S, 8.5 to 9.5% Ni and 0.01 to 0.05% Al, and the balance iron with inevitable impurities is subjected to hot rolling at about <=1250 deg.C. Next, it is reheated at 800 to 900 deg.C, is thereafter subjected to primary hardening, is furthermore reheated at the Ac3 point to <800 deg.C, is held at the same temp. for >=15min, is subsequently subjected to secondary hardening and is tempered at the Ac1 point or below. In this way, the 9% Ni steel having excellent low temp. toughness can be obtd. without deteriorating its proof stress.

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 toughness low temperature steel, and more particularly to a method for producing a high toughness low temperature steel suitable for a tank for transporting or storing low temperature liquefied gas such as LNG and LEG. It is about.

[0002]

2. Description of the Related Art Generally, 9% Ni steel is the most widely used low temperature steel for transportation tanks and storage tanks of low temperature liquefied gas such as LNG and LEG.

9% Ni steel has an excellent low temperature toughness, but recently, in order to further improve the safety of a low temperature tank, the 9% Ni steel further improves the low temperature toughness. Is being requested.

The heat treatment of this 9% Ni steel is as follows:
Quenching and tempering (QT) are most commonly performed. For example, as shown in JP-A-62-205227, after quenching, Ac ₃ is performed for the purpose of improving low temperature toughness. A method (QQ'T) in which quenching is performed again at a temperature between ₁ and Ac ₁ and then tempering is proposed. However, although QQ'T in this method can improve the low temperature toughness, it has a problem that the yield strength is lower than that of QT.

[0005]

DISCLOSURE OF THE INVENTION In order to solve the various problems of the conventional low temperature toughness steel described above, the present invention has been earnestly studied by the present inventors, and as a result of repeated studies,
We have developed a method for producing high-toughness low-temperature steel suitable for containers such as low-temperature liquefied gas that can improve low-temperature toughness without lowering yield strength.

[0006]

The method for producing a high toughness low temperature steel according to the present invention comprises: C 0.02 to 0.08 wt%; Si
0.02 to 0.50 wt%, Mn 0.40 to 0.80 wt
%, P ≤ 0.003 wt%, S ≤ 0.002 wt%
%, Ni 8.5 to 9.5 wt%, Al 0.01 to 0.0
After hot-rolling a slab containing 5 wt% of the balance iron and unavoidable impurities, the slab is reheated to a temperature of 800 ° C. or higher and 900 ° C. or lower, followed by the first quenching, and then Ac _{It is characterized} by performing reheating to a temperature of ₃ points or more and less than 800 ° C., holding at that temperature for 15 minutes or more, second quenching, and then tempering at a temperature of Ac ₁ point or less. The first invention is a method for producing a toughness low temperature steel, and C 0.02 to 0.08 wt%, Si
0.02 to 0.50 wt%, Mn 0.40 to 0.80 wt
%, P ≤ 0.003 wt%, S ≤ 0.002 wt%
%, Ni 8.5 to 9.5 wt%, Al 0.01 to 0.0
5 wt%, further Cu 0.05-0.50w
t%, Cr 0.05 to 0.50 wt%, Mo 0.05
~ 0.50 wt%, Nb 0.005-0.05 wt%,
V 0.005 to 0.05 wt%, containing one or more selected from Ti 0.005 to 0.05
After hot rolling the slab containing wt% and the balance iron and unavoidable impurities, after reheating to a temperature of 800 ° C. or higher and 900 ° C. or lower, the first quenching is performed,
Next, it is reheated to a temperature of Ac ₃ points or higher and lower than 800 ° C., held at that temperature for 15 minutes or longer, secondly quenched, and then tempered at a temperature of Ac ₁ point or lower. Second method for manufacturing high toughness low temperature steel
The invention consists of two inventions.

The method of manufacturing the high toughness low temperature steel according to the present invention will be described in detail below. First, the components contained and the ratio of components of the steel used in the method for producing a high toughness low temperature steel according to the present invention will be described.

C is an element necessary to secure the strength, and if the content is less than 0.02 wt%, such an effect is small, and if it exceeds 0.08 wt%, the toughness and weldability are improved. Deteriorate. Therefore, the C content is 0.
It is set to 02 to 0.08 wt%.

Si is necessary as a deoxidizing element and for ensuring strength. If the content is less than 0.02 wt%, such an effect is small, and if it exceeds 0.50 wt%, Si is contained. Deteriorates toughness. Therefore, the Si content is 0.02 to 0.50 wt%.

Mn is an element necessary to secure the strength, and if the content is less than 0.40 wt%, such an effect is small, and if it exceeds 0.80 wt%, the toughness deteriorates. Therefore, the Mn content is 0.40 to 0.
80 wt%.

P and S are elements that are harmful to toughness, and in order to obtain an extremely high level of toughness in the manufacturing method of high toughness low temperature steel, the P content is 0.003 wt% and the S content is If each of them exceeds 0.002 wt%, sufficient toughness cannot be obtained even if special heat treatment is performed. Therefore, the P content is ≤ 0.003 wt% and the S content is ≤
It is indispensable to set it to 0.002 wt%.

Ni is an element that enhances both strength and toughness at the same time, and is indispensable for ensuring the performance as a low temperature steel. When the content is less than 8.5 wt%, sufficient strength and toughness are obtained. Can't get
If the content exceeds 9.5 wt%, not only the toughness improving effect is saturated, but also the manufacturing cost is greatly increased. Therefore, N
The i content is 8.5 to 9.5 wt%.

Al is an element effective for suppressing coarsening of crystal grains and preventing deterioration of toughness, and its content is 0.01.
If it is less than wt%, a sufficient effect cannot be expected, and it is 0.0
If the content exceeds 5 wt%, the effect will be saturated.
Therefore, the Al content is set to 0.01 to 0.05 wt%.

Cu, Cr and Mo increase the strength, and
It is an element effective for fine adjustment of strength, and when the content of each element is less than 0.05 wt%, such an effect is small,
Further, if the content exceeds 0.50 wt%, the toughness deteriorates. Therefore, the Cu content is 0.05 to 0.50 wt.
%, Cr content is 0.05 to 0.50 wt%, and Mo content is 0.05 to 0.50 wt%.

Nb and V are effective elements for fine adjustment of the strength, and if the content is less than 0.005 wt%, this effect is not sufficient, and if the content exceeds 0.05 wt%, toughness is obtained. Deteriorate. Therefore, the Nb content is 0.005 to 0.05 wt% and the V content is 0.005.
˜0.05 wt%.

Ti is an element having an effect of improving the toughness of the heat-affected zone of welding, and if the content is less than 0.005, such an effect is not sufficient, and if it exceeds 0.05 wt%, it is contained. Deteriorates toughness. Therefore, the Ti content is set to 0.005 to 0.05 wt%.

Next, the manufacturing conditions in the method of manufacturing the high toughness low temperature steel according to the present invention will be described.

In the method for producing a high toughness low temperature steel according to the present invention, the steel (slab) having the above-described components and component ratios is hot-rolled at a temperature of 1250 ° C. or lower. In this hot rolling, if the heating temperature is extremely high, the γ grains are coarsened, and even if the subsequent heat treatment cannot sufficiently reduce the size, the temperature is set to 1250 ° C. or lower. After that, when the first quenching is performed to generate a mixed texture of martensite and lower bainite and the subsequent second heating before quenching is performed, the temperature is Ac as described below. _This is to enable uniform austenitization even at a relatively low temperature such as _{3 to} 800 ° C.

The heating temperature before the first quenching is set to 800 ° C. or higher and 900 ° C. or lower when the heating temperature is 80 ° C.
If it is less than 0 ° C, uniform austenite cannot be achieved, and if the heating temperature exceeds 900 ° C, the austenite grains become coarse. Therefore, the heating temperature before the first quenching is set to 800 to 900 ° C.

Next, the second quenching is for refining the austenite grains to improve the toughness, and the heating temperature before quenching is set to the Ac ₃ point or more and less than 800 ° C. If the heating temperature is below the Ac ₃ point, uniform austenite cannot be achieved, and 800
If the temperature exceeds ℃, austenite grains become coarse.

By setting the heating and holding time before the second quenching to 15 minutes or more, the heating temperature approaches the Ac ₃ point or more and less than 800 ° C., so that uniform austenitization cannot be achieved in less than 15 minutes. . Therefore, the heating temperature before the second quenching is set to Ac ₃ point to 800 ° C., and the heating and holding time at this temperature is set to 15 minutes or more.

It is stated below that the low temperature toughness of the steel obtained by the method for producing a high toughness low temperature steel according to the present invention is improved by heat treatment under the production conditions in the method for producing a high toughness low temperature steel according to the present invention. Explained. In addition, as steel, 0.0
5C-0.23Si-0.56Mn-0.002P-0.0
01S-9.12Ni-0.035Al-Fe (hereinafter simply referred to as 9% Ni steel) was used.

The quenching conditions for the above 9% Ni steel were examined. That is, FIG. 1 shows the result of investigation on the influence of the heating temperature before quenching on the Charpy absorbed energy (vE _-196 ) at a temperature of -196 ° C.

As can be seen from FIG. 1, (vE _-196 ) shows a high value when the heating temperature before quenching is in the temperature range of 800 to 900 ° C, and when the heating temperature is less than 800 ° C, , The value of (vE _-196 ) is lower in both cases. The heating and holding time is 15 minutes, and the tempering temperature is 580 ° C.

When the heating temperature is lower than 800 ° C., the value of (vE ₋₁₉₆ ) decreases because the difference ΔT between the heating temperature and the Ac ₃ point is small, so that the part not austenitized It exists and a uniform quenching structure cannot be obtained. Incidentally, as well known in the art, this phenomenon specifies that the heating temperature before quenching of 9% Ni steel is 800 ° C. or higher in ASTM. When the heating temperature is higher than 900 ° C, (v
The value of E ₋₁₉₆ ) is decreased because the heated γ grains are coarsened.

FIG. 2 shows the result of investigation on the influence of the time of holding the heating temperature at 800 ° C. before quenching on the value of (vE ₋₁₉₆ ). As is clear from FIG. 2, (vE
_-196 ) is not observed. This is 8
This is because when heated to a temperature of 00 ° C., austenitization is sufficiently achieved even when the holding time is short such as about 1 minute. The tempering temperature is 580 ° C.

As described above, in the first quenching, the best toughness can be obtained in the case of quenching from near the temperature specified in the standard, and the value is the heating holding time before quenching. Even if a process such as changing is added, it cannot be improved any more.

Next, FIG. 3 shows the result of investigation on the influence of the number of times of quenching for improving the value of (vE ₋₁₉₆ ). In addition, the heating temperature before quenching is 800 ° C, and the heating holding time is 15
The tempering temperature was 580 ° C. for minutes. As is clear from FIG. 3, when the number of quenching increases, the (v
The value of E ₋₁₉₆ ) tends to be slightly improved, but it cannot be said to have a sufficient improvement effect practically.

That is, the toughness cannot be remarkably improved only by increasing the number of times of quenching. The value of (vE _-196 ) is slightly improved,
It is considered that the austenite grains are refined by repeating the transformation and the reverse transformation.

Next, in the case of quenching twice (v
FIG. 4 shows the effect of the heating temperature before the second quenching on the value of (E ₋₁₉₆ ). In addition, the heating temperature before the first quenching is 800 ° C., and the heating holding time is 15 minutes.
For comparison, FIG. 4 also shows the effect of the heating temperature before quenching on the value of (vE ₋₁₉₆ ) in the case of the single quenching shown in FIG. 1. By the way, the tempering temperature is 580 ° C.

As is apparent from FIG. 4, when the heating temperature before the second quenching is 800 ° C. or higher, the effect of improving the (vE ₋₁₉₆ ) value by the second quenching is slight, and
In the case of less than 00 ° C., the value of (vE ₋₁₉₆ ) is significantly improved as compared with the case of single quenching. This is
This is an important finding as it is very different from the case of single quenching. The reason will be described below.

That is, in the case of one-time quenching, since heating is performed for quenching after hot rolling, the structure before heating is a structure mainly of upper bainite containing relatively large carbides, and therefore lower than 800 ° C. At the heating temperature, it takes time to form a solid solution of carbide, and uniform austenitization hardly occurs.

Therefore, when the heating temperature is low, the toughness deteriorates, and the effect of refining the austenite grains by low temperature heating can be utilized. On the other hand, in the case of the two-time quenching, the structure before heating for the second-time quenching is a martensite with a small amount of carbides generated by the first-time quenching and a relatively small size. Since it has a mixed structure of lower bainite, uniform austenitization can be achieved even at a heating temperature lower than 800 ° C., and excellent toughness can be obtained.

The first quenching in the second quenching is to obtain a mixed structure of martensite and lower bainite in order to facilitate uniform austenitization during heating before the second quenching.

In order to obtain such a mixed structure, it is indispensable to achieve uniform austenitization at the time of heating before the first quenching, and from the viewpoint of fine austenite grains, uniform austenitization is required. Needless to say, it is better to heat at a temperature as low as possible within the range where

In order to confirm this, FIG. 5 shows the result of investigation on the influence of the heating temperature before the first quenching on the value of (vE _-196 ) of the double quenching material. As is clear from FIG. 5, the temperature at which the uniform austenitization is achieved is 8
An excellent value of (vE _-196 ) is obtained at 00 to 900 ° C.

However, the value of (vE _-196 ) decreases at a temperature below 800 ° C. at which uniform austenite formation is difficult, or at a temperature above 900 ° C. at which austenite grains become coarse. In FIG. 5, the temperature before the second quenching was 750 ° C., the heating and holding time was 15 minutes, and the tempering temperature was 5
It is 80 ° C.

Next, FIG. 6 shows the result of investigation on the effect of the heating holding time before the second quenching on the value of (vE _-196 ). As is clear from FIG. 6, (vE
The value of _-196 ) improves as the holding time becomes longer, but this effect tends to be saturated when the holding time exceeds 15 minutes. This indicates that if the holding time is short, the austenitization is insufficient even if the quenching is performed twice.

As described above, in the method for producing a high toughness low temperature steel according to the present invention, the steel containing the specific contained components in the specific component ratio is firstly heated at the specific heating temperature. By quenching, then heating to a specific heating temperature and holding for a specific time, then performing a second quenching, and then tempering at a specific temperature, excellent low temperature toughness without lowering the yield strength. It is possible to produce a 9% Ni steel having

[0040]

[Examples] Examples of the method for producing a high toughness low temperature steel according to the present invention will be described together with comparative examples.

A slab made of a steel having the contained components and component ratios shown in Table 1 by an ordinary melting method was hot-rolled, and then heat-treated under the production conditions shown in Table 2. Table 2 shows the Charpy impact absorbed energy (vE _-196 ).

From Table 2, the values of the Charpy impact absorbed energy (vE _-196 ) of Steels 1 to 4 produced by the method for producing a high toughness low temperature steel according to the present invention show a high value of 273 J or more. .

However, the values of the Charpy impact absorbed energy (vE _-196 ) in Comparative Examples 5 to 10 are low for the reasons explained below. That is, Comparative Example 5 is the second
The second quenching is not done. In Comparative Example 6, the heating temperature before the second quenching is too high. In Comparative Example 7, the heating and holding time before the second quenching is insufficient. In Comparative Example 8, the heating temperature before the first quenching is too high. Comparative Example 9 has too much P content. Comparative Example 10 has too much S content.

[0044]

[Table 1]

[0045]

[Table 2]

[0046]

As described above, since the method for producing a high toughness low temperature steel according to the present invention has the constitution as described above, it is possible to obtain excellent low temperature toughness without lowering the yield strength. There is an excellent effect that the 9% Ni steel that it has can be manufactured.

[Brief description of drawings]

FIG. 1 is a diagram showing an influence of a heating temperature before quenching on a value of Charpy impact absorbed energy (vE ₋₁₉₆ ).

FIG. 2 is a diagram showing the effect of heating and holding time before quenching on the value of Charpy impact absorbed energy (vE ₋₁₉₆ ).

FIG. 3 is a diagram showing the influence of the number of times of quenching on the value of Charpy impact absorbed energy (vE ₋₁₉₆ ).

[Fig. 4] Charpy impact absorption energy of single-quenched material
Effect of heating temperature before quenching on the value of (vE _-196 ), and Charpy impact absorption energy (vE _-196 )
It is a figure which shows the influence of the heating temperature before the 2nd hardening on the value of.

FIG. 5 Charpy impact absorbed energy of double-hardened material
shows the effect of the first quenching prior to the heating temperature on the value of (vE _-196).

FIG. 6 Charpy impact absorbed energy of double-hardened material
shows the effect of a second quenching prior to the heating holding time on the value of (vE _-196).

Claims (2)

[Claims] 1. C 0.02 to 0.08 wt%, Si 0.04.
02 to 0.50 wt%, Mn 0.40 to 0.80 wt%, P ≤ 0.003w
t%, S ≤ 0.002 wt%, Ni 8.5-9.5 wt
%, Al 0.01-0.05 wt%, and after hot rolling to a slab consisting of balance iron and unavoidable impurities, 8
After reheating to a temperature of 00 ° C or more and 900 ° C or less, the first quenching is performed, and then Ac ₃ points or more and 800 ° C or more.
Of the high toughness low temperature use steel characterized by performing reheating to a temperature of less than 15 minutes, holding at that temperature for 15 minutes or more, second quenching, and then tempering at a temperature of Ac ₁ point or less. Manufacturing method. 2. C 0.02 to 0.08 wt%, Si 0.04.
02 to 0.50 wt%, Mn 0.40 to 0.80 wt%, P ≤ 0.003w
t%, S ≤ 0.002 wt%, Ni 8.5-9.5 wt
%, Al 0.01 to 0.05 wt%, and further, Cu 0.05 to 0.50 wt%, Cr 0.05 to 0.5
0 wt%, Mo 0.05 to 0.50 wt%, Nb 0.005 to 0.5.
Slab containing one or more selected from 0.05 wt% and V 0.005 to 0.05 wt% and containing Ti 0.005 to 0.05 wt% and the balance iron and inevitable impurities. After hot rolling to
After reheating to a temperature above 800 ° C and below 900 ° C,
First quenching, then Ac ₃ points or more 800
A high-toughness low-temperature steel, characterized by being reheated to a temperature lower than ℃, kept at that temperature for 15 minutes or more, secondly quenched, and then tempered at a temperature of Ac ₁ point or lower. Manufacturing method.