JPH02194122A - Manufacture of nickel steel plate for low temperature use excellent in toughness in weld zone - Google Patents

Abstract

PURPOSE:To easily manufacture a steel for low temp. use excellent in toughness at low temp. in a weld zone by hot-rolling a slab of Ni steel with a specific composition at a specific finishing temp., controlling cooling conditions, and further tempering the resulting steel plate at a specific temp. CONSTITUTION:A slab having a composition consisting of, by weight, 0.04-0.12% C, 0.02-0.30% Si, 0.05-0.4% 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 heated up to 1100-1300 deg.C. Subsequently, hot rolling is applied to the above slab at 1000-700 deg.C finishing temp. and cooling is carried out while controlling the average cooling rate in the range of 800 to 500 deg.C to >=2 deg.C/sec, followed by tempering treatment at a temp. between 450 deg.C and (Ac1+70 deg.C). It necessary, one or more kinds among 0.005-0.06% Nb, 0.005-0.07% V, and 0.05-0.50% Cu are incorporated to the above steel slab. By this method, superior toughness in a weld zone and strength can be obtained.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a method for manufacturing nickel steel sheets for low temperature use with excellent weld toughness, particularly for steel materials for use in liquefied natural gas (LNG), etc. at temperatures below -160°C. The present invention relates to a method for manufacturing nickel steel sheets for low temperature use, where weld toughness is an important factor when used at extremely low temperatures.

<Conventional technology> Steel materials used for LNG tanks, etc., such as 9% Mi
ll has already been reheated, quenched and tempered (RQ-T)
According to ASTM A353, direct quenching and tempering treatment (
DQ-T) is known as standard steel such as ASTM A344.

These steel materials are required to have high toughness at low temperatures, especially low-temperature toughness in welded parts.

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

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

As a means to solve the problem of the decrease in toughness of the heat-affected zone heated to 700 to 900° C. during welding, reduction of Sil has been proposed, for example, in Japanese Patent Application Laid-Open No. 53-112219. However, with this method, the strength of the base metal cannot satisfy the standard value, and a decrease in the toughness of the base metal cannot be avoided.

In addition, in order to solve the above problems, the applicant first
JP-A-63-290246 proposes a method of reducing the amount of i and Mn1i and adding TI. However, as a result of further investigation, we found that this method
In the Charby impact test at 196°C of the heat-affected zone heated to 200°C, it became clear that a brittle fracture surface appeared.

<Problems to be solved by the invention> This invention advantageously solves the above-mentioned problems.
The purpose of the present invention is to propose a method for manufacturing a nickel steel plate for low temperature use, which has excellent low-temperature toughness in a welded part heated to 0° C. or higher and has strength specified by ASTM A353 and ASTM A884.

<Means for Solving the Problems> The present invention has C: 0.04 to 0.12%, S: 0.04% to 0.12%, S
i: 0.02-0.30%, Mn: 0.05-0
．． 4%, P: 0.01% or less, S! 0.005
% or less, Ni: 6.5-12.0% 9M: 0, O 1-0
．． 10% and N: 0.0035% or less, optionally further Nb: 0,005 to 0.06%,
V: 0.005-0.07% and Cu:
110% of a slab containing at least one selected from 0.05 to 0.50%, with the remainder consisting essentially of Fe.
Heating to a temperature of 0 to 1300°C, then rolling finishing temperature 1
In the process of hot rolling at 000 to 700°C, and cooling the hot rolled steel sheet, the average cooling rate in the temperature range of 800 to 500″C was set to 2° (/so: or more), and then the steel plate was cooled to 45°C.
This is a method for producing a low-temperature nickel steel plate with excellent weld toughness, characterized by tempering in a temperature range of 0°C to (Ac1+70°C).

<Function> The basis of this invention is that among the low-temperature steel compositions containing N1 in the range of 665 to 12.0% (same below weight%), St is particularly 0.02 to 0.30%, Mn is 0.05~0.
It has been found that by reducing N to 0.0035% or less, high strength can be ensured through direct quenching and tempering, and high toughness of the welded joint can be ensured.

First, the reasons for limiting the composition of steel in this invention will be explained.

C: 0.04-0.12% C is an element that is necessary to obtain sufficient high tension, but
If the Si content is less than 0.04%, the strength will be lowered.

It became necessary to increase Mn, and as mentioned above, 700-9
There is a problem that the toughness of the part heated to 0.00℃ is low, and on the other hand, even if it exceeds 0.12%, the toughness will be reduced.
．． The range was 0.04 to 0.12%.

Si: 0.02-0.30% Si is one of the features of the present invention, because the reduction of Si has a significant effect on improving the toughness of the weld zone. However, no gradual effect is observed even if the content is less than 0.02%, so the lower limit was set at 0.02%. On the other hand, if it exceeds 0.3%, it not only causes deterioration of toughness but also excessively increases strength, so 0.3% is set as the upper limit.

Mn: 0.05-0.4% Like Si, Mn is also one of the features of this invention.

The reduction of Mn, combined with the reduction of Si, shows a remarkable effect on improving the weld toughness.However, a reduction below 0.05% does not show a gradual effect, so the lower limit was set at 0.05%.
If 0Mn (%) is reduced within this range, the weld toughness will be gradually improved, and this is particularly noticeable when it is 0.3% or less. However, if the content exceeds 0.4%, it not only deteriorates the toughness but also excessively increases the strength, so 0.4% was set as the upper limit.

P≦0.01%, S≦o, oos% Both P and S damage the toughness of the base metal and weld zone, so it is desirable to reduce them as much as possible, but 0.01% each.
Hereinafter, it is acceptable within the range of o, oos% or less.

N1j6.5-12.0% Ni is an essential element for the low-temperature steel of this invention, and has the effect of imparting high toughness at low temperatures, but if it is less than 6.5%, the effect is poor; Even if added in large amounts, the effect reaches saturation and is also uneconomical.
It was limited to a range of 6.5 to 12.0%.

/V: 0.01-0.10% AI is a necessary element for deoxidation, but if it is less than 0.01%, its effect is poor, while if it exceeds 0.01-0.10%, the cleanliness is impaired. Therefore, the range was set to 0.01 to 0.10%.

N + 0.0035% or less N is one of the characteristics of the present invention. Reducing N reduces mobile dislocations and reduces island martensite, thereby improving toughness. In particular, when N is set to 0.0035% or less, the reduction in St and Mn together with the reduction in
The toughness of the heat affected zone heated to 900°C can be significantly improved. Furthermore, the toughness of the heat-affected zone heated to 1350° C. or higher can also be improved to the same extent as by adding Ti.

The above C, Si, Mn, P, S, Nl, /V,
Each limited amount of N is the basic component of the low-temperature steel according to the present invention, but in this invention, in addition, Nb + 0.00
5-0.06%.

V: 0.005-0.07% and Cu: 0.
It is also possible to contain at least one of 05 to 0.50%.

The reasons for these limitations will be explained next.

Nb: 0.005-0.06%, V: 0.0
05-0.07% Nb and V both contribute effectively to improving strength through precipitation strengthening, but both o
If it is less than , oos%, the effect of addition is small, so o, oos%
is the lower limit, while Nb is 0.06%, and ■ is 0.07%.
%, the toughness would be adversely affected, so the upper limits were limited to Nb: o, o: 6%, and v: 0.07%, respectively.

Cu: 0.05-0.50% Cu is an effective element for improving strength by improving hardenability, but if it is less than 0.05%, its effect is poor, while if it exceeds 0.50% On the contrary, the toughness deteriorates, so it was limited to a range of 0.05 to 0.50%.

All steel materials with the above-mentioned composition range are rolled by heating to a temperature of 1100°C or higher and 1300°C or lower, but when the heating temperature is lower than 1100°C, coarse precipitates/VN occur when the steel ingot is cooled. In addition to not melting and deteriorating toughness, there is also the problem that a sufficient reduction ratio cannot be achieved.

On the other hand, since heating above 1300°C causes the austenite grains to become coarse and is also uneconomical, the slab heating temperature is limited to 1100-1300''C.

The finishing temperature of hot rolling is limited to 1000 to 700°C. At temperatures exceeding 1000°C, sufficient fine-grained fibers cannot be obtained.
This is inconvenient for manufacturing high-toughness steel sheets, which is the objective of this invention. On the other hand, below 700°C, strain accumulates in crystal grains, resulting in low toughness.

A feature of this invention is that it can be quenched immediately after rolling, but this process makes it possible to omit reheating and quenching, resulting in lower costs. The strength increases, so the St amount increases.

It becomes possible to reduce the amount of Mn1i and N.

As mentioned above, reductions in the SL content, Mn content, and Nff1 improve the base metal and weld toughness.

The average cooling rate of direct quenching is required to be 2°C/see or more in the temperature range of 800 to 500°C, and a cooling rate slower than this cannot satisfy the required strength.

Further, the cooling rate must be at least 2°C/at least in the temperature range of 800 to 500°C, and the cooling rate in the temperature range of 800°C or less than 500°C is not particularly limited.

After direct quenching, tempering treatment is performed, but the heating temperature is 4
The temperature is 50°C or higher (Act+70°C) or lower, and 450
Toughness cannot be ensured if the temperature is not determined, and (Ac, +70℃)
If the temperature is exceeded, the strength will decrease.

<Example> Example 1 Steel having the chemical composition shown in Table 1 was heated to a slab heating temperature of 1200 ml.
℃, rolled to a thickness of 15cm at a finishing temperature of 900℃, immediately cooled with water, set the average cooling rate in the temperature range of 800 to 500℃ to 50℃/shape, and then subjected to a 70-mm tempering treatment (DQ) at 570℃. -T) was applied. For comparison, the steels shown in Table 1 were heated at a slab heating temperature of 1200°C and a rolling finishing temperature of 9.
Rolled to a thickness of 15 teeth at 00°C and air cooled to room temperature (8°C).
Average cooling rate of 00~soo'c 0.8℃/5et)
After that, 60 teeth were heated at 780°C and immediately cooled in water for reheating and quenching, and then 70 teeth were tempered at 570°C (
Submerged arc welding was performed using an austenitic wire under the conditions of a heat input of 20 kJ/cm and a number of passes of 4.

Table 2 shows the results of examining the base material strength and toughness (vE-+*i”C) at that time.
The Charpy impact test results of Z3, 5.7 tone part are shown in Table 3. In the DQ-T treatment of the present invention, extremely high strength,
A base material with toughness is obtained, and the toughness of the weld zone is also improved.

Table 1 Can (%) Example 2 Steel having the chemical composition shown in Table 4 was heated to a slab heating temperature of 120
Rolled to IOM thickness under conditions of 0°C and finishing temperature of 900°C, immediately cooled with water, set average cooling rate in the temperature range of 800 to 500°C to 50°C/hoe, and then rolled to 70°C at 570°C.
A tempering treatment (DQ-T) was performed.

Next, submerged arc welding was performed using an austenitic wire under the conditions of a heat input of 119 kJ/c+a and a number of buses of 3. And Bond Department, HAZ 3, 5, 7
Charpy impact test pieces were taken from m and used for experiments.

The results are shown in Table 5 along with the strength of the base material.

Table 5 shows that the test steel N11l-13, which is a steel that satisfies the composition of the present invention and has been subjected to the DQ-T treatment, secures the strength specified in the ASTM standard and has sufficient welded areas compared to the comparative steels NcL14 to 20. Toughness has been obtained. In particular, the improvement in the toughness of the welded part of HAZ7wm where the heating temperature reaches 700-900°C is remarkable. ,
S, N1. Even if /u is satisfied, the remaining Si,
If either Mn or N is not satisfied, sufficient weld toughness cannot be ensured, and if TI is added as in test steel 20 (comparative steel), too
It can be seen that sufficient weld toughness cannot be ensured in HAZ 3m where the temperature rises to ℃.

Table <Effects of the Invention> Thus, according to the manufacturing method of the present invention, it is possible to easily obtain a low-temperature steel having excellent low-temperature toughness, particularly weld zone toughness.

Claims (1)

[Claims] 1. In weight%, C: 0.04 to 0.12%, Si: 0.
02-0.30%, Mn: 0.05-0.4%, P: 0
．． 01% or less, S: 0.005% or less, Ni: 6.5~
12.0%, Al: 0.01-0.10% and N: 0.
A slab containing 0.035% or less and the remainder substantially consisting of Fe is heated to a temperature of 1100 to 1300°C, then hot rolled at a finishing temperature of 1000 to 700°C, and the hot rolled steel plate is cooled. A low-temperature product with excellent weld toughness characterized by cooling at an average cooling rate of 2°C/sec or more in the temperature range of 800 to 500°C, and then tempering in the temperature range of 450°C to (Ac_1+70°C). Method of manufacturing nickel steel plate. 2. In weight%, C: 0.04-0.12%, Si: 0.
02-0.30%, Mn: 0.05-0.4%, P: 0
．． 01% or less, S: 0.005% or less, Ni: 6.5~
12.0%, Al: 0.01-0.10% and N: 0.
0.0035% or less, and further contains Nb: 0.005 to 0.0035%.
06%, V: 0.005-0.07% and Cu: 0.0
The slab contains at least one selected from 5 to 0.50%, and the remainder is substantially Fe.
Heated to a temperature of 300°C, then rolled to a finishing temperature of 1000°C.
Hot rolling is performed at ~700°C, and in the process of cooling the steel plate after hot rolling, the average cooling rate in the temperature range of 800 ~ 500°C is 2°C/s.
Cool to ec or higher, then 450℃~(Ac_1+
A method for producing a low-temperature nickel steel plate with excellent weld toughness, characterized by tempering in a temperature range of 70°C.