JPH049861B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a low-temperature rope with excellent weld stiffness, particularly for liquefied natural gas (LNG) rope materials, etc.
This is an attempt to improve the properties of low-temperature steel, for which weld stiffness is an important factor when used at extremely low temperatures such as 160°C or lower. (Conventional technology) Steel materials used in LNG tanks and the like are required to have high toughness even at low temperatures. In this case, one particular problem is the low-temperature toughness of the weld. Various measures have been taken to solve this problem, for example, Japanese Patent Application Laid-Open No. 133312/1986.
The publication proposes a reduction in the amount of C, and ``Tetsu to Hagane (1982) 4, p. 168) proposes extremely low P and extremely low S. (Problems to be solved by the invention) However, these With this method, it is not possible to fully satisfy the required stiffness for welded joints, especially for heat-affected zones that are reheated to 700 to 900℃ or 1350℃ or higher. It was unavoidable that the value would be low due to the occurrence of pop-in.Moreover, in the former low C method, from the viewpoint of base material strength,
There is a limit to the reduction of Si and Mn, so
A major drawback was the low stiffness of the heat-affected zone heated to ℃. The present invention advantageously solves the above problems, and aims to propose a low-temperature steel that has excellent low-temperature stiffness in welded parts. (Means for Solving the Problems) The above objectives are advantageously achieved by the configurations listed in the following sections. (1) C: 0.04-0.12wt% (hereinafter simply expressed as %) Si: 0.02-0.30%, Mn: 0.05-0.27%, P: 0.01% or less, S: 0.005% or less, Ni: 6.5-12.0%, The composition contains Al: 0.01 to 0.10%, Ti: 0.004 to 0.015%, and the remainder is substantially Fe (hereinafter abbreviated as basic component). (2) 0.005 to 0.06% Nb and
A composition containing at least one component among 0.005 to 0.07% V. (3) A composition that further contains 0.02 to 0.40% Mo in the basic components. (4) 0.005 to 0.06% Nb and
At least one of 0.005 to 0.07% V;
Composition containing 0.02-0.40% Mo. (Function) The reason why the composition of the steel is limited to the above range in this invention is as follows. C: 0.04-0.12% C is a useful element for obtaining sufficient high tension, but if the content is less than 0.04%, as mentioned above, it will be necessary to increase Si and Mn, and the temperature will rise to 700-900℃. There is a problem that the toughness of the heated part is low,
On the other hand, even if it exceeds 0.12%, it will impair the chewiness, so 0.04 to 0.12
% range. Si: 0.02 to 0.30% Si is one of the features of the present invention, because reduction of Si has a remarkable effect on improving weld ductility. However, even if it is less than 0.02%, no gradual effect is observed, so the lower limit is set to 0.02%.
%. On the other hand, if it exceeds 0.3%, it will not only cause a deterioration of the stiffness but also an excessive increase in strength.
The upper limit was set at 0.3%. Mn: 0.05 to 0.27% Mn is also one of the features of this invention, like Si. The reduction of Mn, combined with the reduction of Si, has a significant effect on improving weld ductility. however
The lower limit was set at 0.05%, since a reduction below 0.05% does not show a progressive effect. If Mn is reduced within this range, it will gradually improve the weld ductility, especially
It is noticeable at 0.27% or less. However, on the other hand, if it exceeds 0.27%, it not only deteriorates the chewiness but also excessively increases the strength, so 0.27% is set as the upper limit. P≦0.01%, S≦0.005% Both P and S impair the stiffness of the base metal and the weld zone, so it is desirable to reduce them as much as possible, but they are permissible within the range of 0.01% or less and 0.005% or less, respectively. Ni: 6.5-12.0% Ni is an essential element for the low-temperature steel of this invention.
It has the effect of imparting high ductility at low temperatures, but
If it is less than 6.5%, the effect is poor, while if it is added in a large amount exceeding 12%, the effect reaches saturation and is also uneconomical, so it is limited to a range of 6.5 to 12.0%. Al: 0.01 to 0.10% Al is an element necessary for deoxidation, but if it is less than 0.01%, its effect is poor, and if it exceeds 0.10%, cleanliness is impaired, so it was set in the range of 0.01 to 0.10%. Ti: 0.004 to 0.015% Ti is one of the features of this invention, because the Ti component forms Ti nitrides in the steel, and this Ti nitride improves the weld joint stiffness, especially COD.
Since it has a positive effect on improving the value, it is added to improve the weld ductility. However, if the content is less than 0.004%, no improvement effect on the welded joint ductility will be observed, while if the content exceeds 0.015%, the burility will deteriorate, so the content should be within the range of 0.004 to 0.015. limited to. Also,
Ti is also an element that compensates for the decrease in strength of the base material due to the reduction of Si and Mn. The above-mentioned limited amounts of C, Si, Mn, P, S, Ni, Al, and Ti are the basic components of the low-temperature steel according to the present invention.
%, V: 0.005 to 0.07% or/and Mo: 0.02 to 0.40%. The reasons for these limitations will be explained next. Nb: 0.005-0.06%, V: 0.005-0.07% Both Nb and V contribute effectively to improving strength through precipitation strengthening, but both
If it is less than 0.005%, the addition effect will be small, so 0.005% is the lower limit.On the other hand, if Nb exceeds 0.06% and V exceeds 0.07%, the stiffness will be impaired, so the upper limit should be set for each.
Nb: 0.06%, V: 0.07%. Mo: 0.02 to 0.40% Mo is an element useful for improving strength through solid solution strengthening and improving hardenability, but if it is less than 0.02, the effect of adding it is poor, while if it exceeds 0.40%, it will actually increase the strength. It was limited to the range of 0.02 to 0.40%. As long as the components are within the above range, the base material may be manufactured by any method, such as direct quenching after rolling - two-phase region quenching - tempering (DQ-Q'-T), direct quenching after rolling - tempering ( DQ-T), reheating and quenching after rolling - two-phase region quenching - tempering (RQ-
Conventional methods such as rolling (Q'-T) and reheating and quenching and tempering after rolling (RQ-T) may be used. However, from the point of view of cost, DQ-T or RQ-T is preferable, and of course, even if manufactured by such a manufacturing method, this invention steel is -
Shows sufficient base material toughness even at temperatures below 160℃ -196℃. (Example) Steels with various chemical compositions shown in Table 1 were rolled to a thickness of 10 mm at a slab heating temperature of 1220°C and a finishing rolling temperature of 900°C, immediately cooled with water, and then
Tempering treatment was performed at 570℃ for 70min. Table 2 shows the results of examining the base metal strength and toughness (vE- ₁₉₆ ) at that time.

【table】

【table】

[Table] Next, for steels with various chemical compositions shown in Table 1, welding reproduction thermal cycle in which the maximum heating temperature is 1350℃, cooling immediately after reaching the maximum heating temperature, and then cooling from 800 to 500℃ in 30 seconds and at that time
Charpy impact test and vTrs and δc respectively
Obtained by COD test. The results are shown in Figure 1a,
b - Figure 3 a and b show the Mn content, Si content and
Shown in relation to Ti content. Furthermore, regarding invention steel No. 2 and comparison steel No. 15,
The fourth section examines the results of the Charpy impact test (vE- ₁₉₆ ) when the maximum heating temperature is changed from 500 to 1000℃ and a thermal cycle corresponding to the weld heat affected zone, which is cooled immediately after reaching the maximum heating temperature, is applied. As shown in the figure. Conventional steel becomes brittle when heated to 700-1000℃ (such thermal history always appears during welding,
(which may cause pop-in, etc.), but the invented steel does not become brittle. Furthermore, Fig. 5 a, b, and c show the results of the Charpy test when heated at 800°C in relation to the amounts of Mn, Si, and Ti, which show excellent toughness within the appropriate range of this invention. . (Effects of the Invention) Thus, according to the present invention, it is possible to obtain a low-temperature steel that is excellent in low-temperature stiffness, particularly in welded joint stiffness, which is advantageous.

[Brief explanation of drawings]

Figures 1 a, b to 3 a, b are, respectively,
A graph showing the relationship between Mn, Si and Ti contents and vTrs and δc, Figure 4 shows the maximum heating temperature and vE
- Graph showing the relationship with ₁₉₆ , Figure 5 a, b,
c is the Mn, Si and Ti content and vE−, respectively.
This is a graph showing the relationship with ₁₉₆ .

Claims (1)

[Claims] 1 C: 0.04 to 0.12wt%, Si: 0.02 to 0.30wt%, Mn: 0.05 to 0.27wt%, P: 0.01wt% or less, S: 0.005wt% or less, Ni: 6.5 to 12.0 A low-temperature steel with excellent weld stiffness, containing Al: 0.01 to 0.10 wt% and Ti: 0.004 to 0.015 wt%, with the remainder being substantially Fe. 2 C: 0.04-0.12wt%, Si: 0.02-0.30wt%, Mn: 0.05-0.27wt%, P: 0.01wt% or less, S: 0.005wt% or less, Ni: 6.5-12.0wt%, Al: 0.01 -0.10wt% and Ti: 0.004-0.015wt%, and contains at least one selected from Nb: 0.005-0.06wt%, V: 0.005-0.07wt%, and the remainder is substantially Fe. A low-temperature rope with excellent weld strength. 3 C: 0.04-0.12wt%, Si: 0.02-0.30wt%, Mn: 0.05-0.27wt%, P: 0.01wt% or less, S: 0.005wt% or less, Ni: 6.5-12.0wt%, Al: 0.01 ~0.10wt%, Ti: 0.004~0.015wt%, and Mo: 0.02~0.40wt%, with the remainder being essentially Fe.A low-temperature steel with excellent weld stiffness. 4 C: 0.04-0.12wt%, Si: 0.02-0.30wt%, Mn: 0.05-0.27wt%, P: 0.01wt% or less, S: 0.005wt% or less, Ni: 6.5-12.0wt%, Al: 0.01 -0.10wt% and Ti: 0.004-0.015wt%, and contains at least one selected from Nb: 0.005-0.06wt% and V: 0.005-0.07wt% together with Mo: 0.02-0.40wt%. A low-temperature steel with excellent weld stiffness, with the remainder essentially consisting of Fe.