JPS6328964B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a low yield ratio nickel steel sheet with excellent low-temperature toughness, and more particularly to an improvement in a double normalizing and tempering treatment method for a nickel steel sheet. Conventionally, Ni steel sheets including 9% Ni have been required to have high toughness at low temperatures as materials for liquefied natural gas, and their heat treatments include quenching and tempering treatment and double normalizing and tempering treatment (NNT treatment).
is used. Quenched and tempered steel sheets can produce steel sheets with extremely high toughness due to the reduction of P, S, etc. However, twice-normalized and tempered nickel steel sheets are often used for the head plates of liquefied natural gas storage tanks and the annular parts of outer tanks, but this is because it has a low yield ratio (YR) and is advantageous in terms of safety design. This is due to Therefore, twice-normalized and tempered Ni steel sheets are required not only to have high toughness but also to have a low yield ratio, but the conventional problem is that if the yield ratio is low, sufficient high toughness cannot be obtained. It was hot. An object of the present invention is to solve the problems of the prior art described above and to provide a double normalizing and tempering treatment method capable of obtaining a low yield ratio nickel steel sheet with excellent low-temperature toughness. As a result of intensive research into improving nickel steel sheets produced by two-time normalizing and tempering treatments, the present inventors found that the cooling rate in the second normalizing treatment of the two normalizing treatments It was discovered that high toughness steel sheets can be obtained by controlling the yield ratio without increasing the yield ratio. The present invention has been made based on this knowledge. The above objects of the present invention are achieved by the following two inventions. The gist of the first invention is as follows. That is, in weight ratio: C: 0.01-0.18%, Si: 0.02-0.80% Mn: 0.40-2.0%, Ni: 6-13.5% Al: 0.010-0.10%, N: 0.010% or less, P: 0.020% or less, S : 0.010% or less, the balance being Fe and unavoidable impurities. A step of rolling a nickel steel to a predetermined thickness by normal hot rolling, and then rolling the hot rolled steel sheet to an Ac ₃ transformation point ~ Ac ₃ transformation point.
A primary normalizing treatment step in which the steel sheet is heated to a temperature range of transformation point +150°C and then air cooled to room temperature, and the first normalized steel sheet is heated to a temperature range of Ac ₃ transformation point to Ac ₃ transformation point + 150°C and then heated to a temperature range of 450°C. A secondary normalizing treatment process in which the steel sheet is air-cooled to a temperature range of ~550°C and then cooled to room temperature at a cooling rate of 25°C/min or more, and the secondary normalized steel sheet is heated to an Ac ₁ transformation point ~ an Ac ₁ transformation point. a tempering process of heating to a temperature range of -150°C and then air cooling to room temperature;
This is a method for producing a low yield ratio nickel steel sheet having excellent low temperature toughness. The gist of the second invention is that in addition to the same basic components as the first invention, Mo: 0.05 to 0.30%
This is a method for producing a low yield ratio nickel steel sheet with excellent low-temperature toughness, in which a nickel steel containing Fe and unavoidable impurities is produced by the same production process as in the first invention. The reason for limiting the composition of nickel steel in the present invention will be explained. C: C must be contained in an amount of 0.01% or more in order to obtain a sufficiently high tensile strength, but if it exceeds 0.18%, toughness will be impaired, so the content is set in the range of 0.01 to 0.18%. Si: 0.02% or more of Si is required for deoxidation in steel manufacturing, but if it exceeds 0.80%, toughness and weldability will be impaired, so it is set in the range of 0.02 to 0.80%. Mn: Mn is effective in ensuring sufficient high tensile strength and toughness, and if it is less than 0.40%, its effect is insufficient.
The lower limit was set at 0.40%, and if it exceeded 2.0%, weldability would be impaired, so the upper limit was set at 2.0%. Ni: Ni is an essential element for the nickel steel of the present invention.
It has the effect of providing high toughness at low temperatures, but 6
If it is less than 13.5%, there is no effect, and if it exceeds 13.5%, the effect will be saturated and uneconomical, so it was limited to a range of 6 to 13.5%. Al: Al is an element necessary for deoxidation, and if it is less than 0.010%, it has no effect, and if it exceeds 0.10%, cleanliness is impaired, so it was set in the range of 0.010 to 0.10%. N, P, S: Too much N, P, and S will damage the toughness of the base metal and weld, so 0.010% each.
Limited to 0.020%, 0.010% or less. Although the above-mentioned limited amounts of C, Si, Mn, Ni, Al, N, P, and S are the basic components of the nickel steel according to the present invention, nickel steel containing 0.05 to 0.30% of Mo at the same time may also be used. The purpose of the present invention can be achieved more effectively. Mo is effective in improving strength and toughness, but if it is less than 0.05%, the effect is small, so the lower limit is set at 0.05%, and if it exceeds 0.30%, the toughness is impaired, so the upper limit is set at 0.30%.
It was limited to the range of 0.05-0.30%. Next, heat treatment conditions for nickel steel having the above-mentioned limited components will be explained. After the nickel steel with the above-mentioned limited components is made into a predetermined thickness through a normal hot rolling process, it is subjected to a primary normalizing treatment in which it is heated to a temperature range of Ac ₃ to _{Ac 3} +150°C and then air cooled to room temperature. ,
If this heating temperature is less than Ac ₃ , a complete austenite structure will not be obtained, and if it exceeds Ac ₃ +150°C, the austenite will become coarse and lose toughness, so the temperature of the primary normalizing treatment should be adjusted to between Ac ₃ and Ac ₃ +150°C. limited to the range of Next, a secondary normalizing treatment is performed, and for the same reason as the primary normalizing treatment, heating is performed to a temperature range of Ac ₃ to _{Ac 3} +150°C, which is also a feature of the present invention.
Air cooling is performed to a temperature range of 450 to 550°C, and then accelerated cooling is performed to room temperature at a cooling rate of 25°C/min or more.
If the accelerated cooling start temperature exceeds 550°C, the structure becomes martensite, and although high toughness can be obtained, the yield ratio becomes high and the original purpose cannot be achieved. Furthermore, even if accelerated cooling is performed from a temperature below 450°C, high toughness cannot be obtained. For these reasons, the starting temperature of accelerated cooling in the secondary normalizing treatment is set to 450~
The temperature was limited to 550℃. Next, basic experiments conducted by the present inventors regarding the cooling rate of accelerated cooling will be explained. Two types of test steels, A and B, having the chemical compositions and transformation points shown in Table 1 were melted in a normal process and hot rolled into 9% Ni steel plates with a thickness of 14 mm. After performing a primary normalizing treatment of heating this hot-rolled steel plate at 890°C for 40 minutes and cooling it in air,

[Table] Heated at 800℃ for 40 minutes, air cooled to 500℃, then cooled by changing the cooling rate in the range of 15℃/min to about 1800℃/min, performed secondary normalizing treatment, and finally
Tempering treatment was performed by heating at 585°C for 35 minutes and cooling in air. The obtained 9% Ni steel plate was subjected to a Charpy impact test and a tensile test at -196°C, and the results are shown in FIGS. 1 and 2. In addition, as a comparative example, the same 9% Ni steel plate shown in Table 1 was subjected to normalizing, quenching, and tempering treatments.
Similarly, a Charpy impact test and a tensile test were conducted and the results are shown in Table 2. The conditions for normalizing, hardening, and tempering are as follows. In other words, the normalizing treatment involves heating at 890℃ for 40 minutes, followed by air cooling.
Quenching treatment involves heating at 800°C for 40 minutes followed by water cooling, and tempering treatment involves heating at 585°C for 35 minutes followed by air cooling.

[Table] As is clear from Figures 1 and 2, the cooling rate after 500℃ in the secondary normalizing treatment is 25℃/min.
In the above manner, a Ni steel sheet with excellent low-temperature toughness can be obtained without increasing the yield ratio too much. On the other hand, in the comparative examples, high absorbed energy is obtained as shown in Table 2, but the yield ratios are as high as 97 and 96, so that the intended purpose cannot be achieved. Based on the results of the above basic experiments, in the present invention, the cooling rate after air cooling to 450 to 500°C during the secondary normalizing treatment was limited to 25°C/min or more. Next, after the secondary normalizing treatment, a tempering treatment is performed in which the material is heated to a temperature range of Ac ₁ to Ac ₁ -150°C and then air cooled to room temperature, but heating at temperatures exceeding Ac ₁ produces austenite and deteriorates toughness. and also Ac ₁ −150
Since heating at temperatures below 0.degree. C. has no effect on tempering, the heating temperature for tempering treatment was limited to a temperature range of Ac ₁ to _{Ac 1} -150°C. In the present invention, the components of the nickel steel sheet are limited, and the hot-rolled steel sheet is heated twice to a temperature range of Ac ₃ to _{Ac 3} +150°C and then cooled. Tempering treatment in which the material is air cooled to a temperature range of 450 to 550°C, then cooled to room temperature at a cooling rate of 25°C/min or more, and then heated to a temperature range of Ac ₁ to Ac ₁ -150°C and then air cooled to room temperature. By doing this, we were able to produce nickel steel sheets with excellent low-temperature toughness and low yield ratio.

[Brief explanation of the drawing]

The attached drawings are all diagrams showing the influence of the cooling rate from 500℃ to room temperature in the secondary normalizing treatment of 9% Ni steel sheets, and Figure 1 shows the relationship with the Charpy absorbed energy at -196℃, Figure 2 shows the relationship between lower yield point, tensile strength, and yield ratio.

Claims (1)

[Claims] 1. C: 0.01-0.18%, Si: 0.02-0.80% Mn: 0.40-2.0%, Ni: 6-13.5% Al: 0.010-0.10%, N: 0.010% or less P: 0.020% or less, S: 0.010% or less, and the remainder consists of Fe and unavoidable impurities. A process of rolling nickel steel to a predetermined thickness by normal hot rolling, and rolling the hot-rolled steel plate to an Ac ₃ transformation point. ~
A primary annealing and normalizing process of heating to a temperature range of Ac ₃ transformation point + 150°C and then air cooling to room temperature, and heating the primary annealing and normalizing treated steel sheet to a temperature range of Ac ₃ transformation point to Ac ₃ transformation point + 150°C A secondary normalizing treatment step in which the steel sheet is air cooled to a temperature range of 450 to 550°C and then cooled to room temperature at a cooling rate of 25°C/min or more, and the secondary normalizing steel sheet is cooled to Ac ₁ transformation point ~ Ac ₁ A method for producing a low yield ratio nickel steel sheet with excellent low-temperature toughness, comprising the steps of heating to a temperature range of -150°C, which is a transformation point, and then air-cooling to room temperature. 2 Weight ratio C: 0.01-0.18%, Si: 0.02-0.80% Mn: 0.40-2.0%, Ni: 6-13.5% Al: 0.010-0.10%, N: 0.010% or less P: 0.020% or less, S: In addition to 0.010% or less, it also contains Mo: 0.05 to 0.30%, and the balance is Fe.
and unavoidable impurities, to a predetermined thickness by normal hot rolling, and the hot-rolled steel sheet is heated to a temperature range of Ac ₃ transformation point to Ac ₃ transformation point + 150°C, and then air cooled to room temperature. 450°C to 450°C after heating the primary normalized steel sheet to a temperature range of Ac ₃ transformation point to Ac ₃ transformation point + 150°C.
Air cooled to a temperature range of 550℃ and then cooled to room temperature by 25℃/
A secondary annealing and normalizing process in which the steel sheet is cooled at a cooling rate of min or more, and a tempering process in which the secondary annealing-normalized steel sheet is heated to a temperature range of Ac ₁ transformation point to Ac ₁ transformation point - 150°C and then air cooled to room temperature. A method for producing a low yield ratio nickel steel sheet with excellent low-temperature toughness, comprising the steps of: