JP2582148B2 - Method of producing low yield ratio nickel steel sheet for low temperature with excellent weld toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for producing a low-yield-ratio nickel steel sheet for low temperature with excellent weld toughness, and particularly to liquefied natural gas (LN).
The present invention relates to a method for producing a low-temperature low-yield-ratio nickel steel sheet in which weld toughness is an important factor when used at extremely low temperatures such as -160 ° C or lower, such as steel materials for G).

<Prior art> Steel materials used for LNG tanks, for example, 9% Ni steel are already known as standard steels, and are ASTM A553 by reheating quenching and tempering, and ASTM A844 by direct quenching and tempering. ASTM 35 by tempering
There are three. Nickel steel sheet with normalizing and tempering twice is often used for the head plate of LNG tank and the annular part of the outer tank. This is due to its low yield ratio (YR), which is advantageous for safety design. .

These steel materials are required to have high toughness at low temperatures, especially at low temperatures in welds.

Various measures have been taken to ensure the toughness of the heat-affected zone, which is heated to 1350 ° C or higher, of the welded portion.

However, recently, in the heat-affected zone of the weld zone heated to 700 to 900 ° C., it has become clear that toughness is reduced due to the formation of island-like martensite, which poses a problem.

In order to solve the decrease in toughness of the heat-affected zone heated to 700 to 900 ° C. during welding, for example, Japanese Patent Application Laid-Open No.
-112219 proposes a reduction in the amount of Si. However, in this method, the base material strength cannot satisfy the standard value. In addition, a decrease in base metal toughness cannot be avoided.

In order to solve the above problems, the applicant of the present application
A method of reducing the amount of Si and Mn and adding Ti (Japanese Patent Laid-Open No.
No. 0246). However, as a result of further study, it became clear that a brittle fracture surface appears in this method in a Charpy impact test at -196 ° C of a heat-affected zone heated to 1000 to 1200 ° C.

<Problems to be Solved by the Invention> The present invention is to advantageously solve the above-described problems, and has excellent low-temperature toughness in a welded portion heated to 700 ° C. or more, and has a strength specified in ASTM A353. Another object of the present invention is to provide a method for producing a low-temperature nickel steel having a low yield ratio.

<Means for Solving the Problems> In the present invention, C: 0.04 to 0.12% and Si: 0.02 to 0.
19%, Mn: 0.05 to less than 0.30%, P: 0.01% or less, S: 0.005% or less, Ni: 7.65 to 12.0%, Al: 0.01 to 0.10%, and N: 0.0035%
Including the following, further Nb: 0.005 to 0.06%, V: 0.005 to 0.07
% And Cu: 0.05 to 0.50%, containing at least one selected from the group consisting essentially of Fe, with the balance being substantially hot-rolled to a predetermined thickness by ordinary hot rolling. _Three
A first normalizing treatment of heating to a temperature range from the transformation point to (Ac ₃ transformation point + 200 ° C.) and then cooling to room temperature is performed, and further, (Ac ₃ transformation point −50 ° C.) to (Ac ₃ transformation point + 150 ° C.) ) Is subjected to a second normalizing process of heating to the temperature range and then cooling to room temperature.
In addition, (Ac ₁ transformation point + 70 ° C) ~ (Ac ₁ transformation point-150
(C), a tempering treatment of heating to a temperature range after cooling to room temperature, and producing a low yield ratio nickel steel sheet for low temperature with excellent weld toughness.

<Operation> The basis of the present invention is 7.65 to 12.0% (the same applies to weight% or less)
Among the low-temperature steel compositions containing Ni in the range of Ni, in particular, the Si content is 0.02-0.19%, the Mn content is 0.05-0.30%, and the N content is 0.0035.
It has been found that by reducing the content to not more than%, high toughness of the welded portion is ensured.

First, the reasons for limiting the composition of steel in the present invention will be described.

C: 0.04 to 0.12% C is a useful element for obtaining a sufficiently high tensile strength. However, if the content is less than 0.04%, it is necessary to increase Si and Mn in terms of strength. There is a problem that the toughness of a portion heated to 900 ° C. is low. On the other hand, if it exceeds 0.12%, the toughness is impaired. Therefore, the range is 0.04 to 0.12%.

Si: 0.02 to 0.19% Si is one of the features of the present invention, because the reduction of Si has a remarkable effect on the improvement of weld toughness. However, even if less than 0.02%, no gradual effect is observed, so the lower limit was set to 0.02%. On the other hand, when the content exceeds 0.19%, not only is the toughness deteriorated but also the strength is excessively increased, so the upper limit is 0.19%.

Mn: 0.05 to less than 0.30% Mn is one of the features of the present invention like Si. The reduction of Mn, combined with the reduction of Si, has a remarkable effect on the improvement of weld toughness. However, a reduction below 0.05% does not show a gradual effect, so the lower limit was made 0.05%. If Mn is reduced in this range, the weld toughness is gradually improved, and is particularly remarkable at less than 0.30%. Therefore, Mn is 0.05 to 0.30
%.

P ≦ 0.01%, S ≦ 0.005% Since both P and S impair the toughness of the base material and the welded portion, it is desirable to reduce them as much as possible. However, P and S are acceptable in the range of 0.01% or less and 0.005 or less, respectively.

Ni: 7.65 to 12.0% Ni is an essential element in the low-temperature steel of the present invention, and has an effect of giving high toughness at low temperatures. However, if it is less than 7.65%, the effect is poor, while on the other hand, more than 12% Even if added, the effect reaches saturation and is uneconomical, so 7.65
Limited to the range of ~ 12.0%.

Al: 0.01 to 0.10% Al is an element necessary for deoxidation. If it is less than 0.01%, its effect is poor. On the other hand, if it exceeds 0.10%, cleanliness is impaired.

N: 0.0035% or less N is one of the features of the present invention. When N is reduced, toughness is improved because mobile dislocations are reduced and island martensite is reduced. In particular, by setting the N content to 0.0035% or less, the reduction of Si and Mn is combined with 700 to 900%.
The toughness of the heat-affected zone heated to ℃ can be significantly improved. Further, the toughness of the heat-affected zone heated to 1350 ° C. or more can be improved as well as the case where Ti is added.

Nb: 0.005 to 0.06%, V: 0.005 to 0.07% Both Nb and V effectively contribute to improving the strength by precipitation strengthening, but if both are less than 0.005%, the effect of addition is small, so the lower limit is 0.005%. And Nb is 0.0
When the content exceeds 6% and V exceeds 0.07%, the toughness is impaired. Therefore, the upper limits are respectively limited to 0.06% for Nb and 0.07% for V.

Cu: 0.05 to 0.50% Cu is a useful element for improving strength by improving hardenability. However, if it is less than 0.05%, the effect of its addition is poor. On the other hand, if it exceeds 0.50%, the toughness is impaired. ~
The range was limited to 0.50%.

Next, the heat treatment conditions for the nickel steel having the above-described limited components will be described. After the nickel steel of the above-mentioned limited component is made to have a predetermined thickness by a normal hot rolling process, Ac ₃ to (Ac ₃ +200
Temperature) and then cooled to room temperature. However, if the heating temperature is lower than Ac ₃ , the austenite structure will not be completely formed. If the heating temperature is higher than Ac ₃ + 200 ° C, austenite will not be formed. Since it is coarse and impairs toughness,
The temperature of the next normalizing treatment was set in the range of Ac ₃ to (Ac ₃ + 200 ° C.).

Next, a secondary normalizing treatment is performed. If the heating temperature is lower than (Ac ₃ -50 ° C.), the transformation from ferrite to austenite is insufficient, and untransformed ferrite remains, and the two-phase region substantially remains. , The strength of the two-phase region where the strength is significantly lowered after heat treatment is strongly affected, and if (Ac ₃ + 150 ° C) is exceeded, austenite coarsens and the toughness is impaired.
The temperature of the next normalizing process is (Ac ₃ -50 ° C) to (Ac ₃ 150 ° C)
Limited to the range.

Next, tempering treatment is performed. If the heating temperature is lower than (Ac ₁ -150 ° C.), toughness cannot be ensured, and (Ac ₁
When the temperature exceeds (+ 70 ° C.), the strength is reduced. Therefore, the temperature of the tempering treatment is limited to a range of (Ac ₁ −150 ° C.) to (Ac ₁ + 70 ° C.).

<Example> Steel having the chemical composition shown in Table 1 was subjected to a normal hot rolling process.
Rolled to a thickness of 15 mm, then subjected to a first normalizing process at 900 ° C, then a second normalizing process at 800 ° C,
A tempering treatment (N-NT) was performed at 70 ° C. For comparison, the steels shown in Table 1 were rolled to a thickness of 15 mm in a normal hot rolling step, then heated again at 780 ° C., then immediately re-quenched with water, and then tempered at 570 ° C. (PQ-T). gave. Table 2 shows the results obtained by examining the base metal strength at that time. It can be seen that a low yield ratio was obtained in the N-N-T process of the present invention.

Next, steel plates with various chemical compositions shown in Table 3 (thickness 10m
m) was subjected to NNT processing in the same manner as described above, and submerged arc welding was performed using an austenitic wire under the conditions of a heat input of 19 kJ / cm and three passes. And Bond part,
Charpy impact test pieces were sampled from HAZ 3, 5, 7 mm and used for the experiment. Table 4 shows the results together with the strength of the base material.

Table 4 shows that the steels satisfying the composition of the present invention were subjected to N-N-T treatment and the test steels Nos. 1 to 11 exhibited low yield ratios and sufficient weld toughness compared to the comparative steels. ing. In particular, the improvement of the weld toughness of HAZ7mm where the heating temperature reaches 700 to 900 ° C is remarkable. On the other hand, even if C, P, S, Ni, and Al are satisfied among the basic components of the steel of the present invention, such as test steel Nos. 12 to 17 (comparative steel), the remaining Si, Mn, and N If at least one of them is not satisfactory, sufficient weld toughness cannot be secured, and if Ti is added as in test steel 18 (comparative steel), the temperature will rise to 1000-1200 ° C.
It can be seen that sufficient weld toughness cannot be secured with HAZ of 3 mm.

<Effect of the Invention> Thus, according to the present invention, a low-temperature steel having a low yield ratio and excellent weld toughness can be easily obtained, which is advantageous.

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-194122 (JP, A) JP-A-53-97917 (JP, A) JP-B-46-13478 (JP, B1) Iron and steel 68 [ 12] (1982) Fukushige 5 P. 419

Claims (1)

(57) [Claims] (1) C: 0.04 to 0.12% and Si: 0.02 to 0.
19%, Mn: 0.05 to less than 0.30%, P: 0.01% or less, S: 0.005% or less, Ni: 7.65 to 12.0%, Al: 0.01 to 0.10%, and N: 0.0035%
Including the following, further Nb: 0.005 to 0.06%, V: 0.005 to 0.07
% And Cu: 0.05 to 0.50%, containing at least one selected from the group consisting essentially of Fe, with the balance being substantially hot-rolled to a predetermined thickness by ordinary hot rolling. _Three
A first normalizing treatment of heating to a temperature range from the transformation point to (Ac ₃ transformation point + 200 ° C.) and then cooling to room temperature is performed, and further, (Ac ₃ transformation point −50 ° C.) to (Ac ₃ transformation point + 150 ° C.) ) Is subjected to a second normalizing process of heating to the temperature range and then cooling to room temperature.
In addition, (Ac ₁ transformation point + 70 ° C) ~ (Ac ₁ transformation point-150
A method for producing a low-yield-ratio nickel steel sheet for low-temperature use having excellent weld toughness, characterized by performing a tempering treatment of heating to a temperature range of (° C) and then cooling to room temperature.