JPS6339660B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to the creation of a non-heat-refined high-strength, high-toughness steel for pressure vessels, which can ensure excellent strength and toughness properties even through post-weld heat treatment. This is what we are trying to provide. Conventionally, C-0.5Mo steel has been widely used for containers used in high-temperature hydrogen atmospheres, such as ammonia converters and methanol converters, and for boilers that operate at high temperatures. That is, this C
-0.5Mo steel is standardized as A204 in ASTM standards, and in JIS as JIS.G3103, SB46M,
It is standardized as 49M, and its composition is basically relatively high C, 0.15 to 0.30wt%Si, 0.9wt
% Mn and 0.45 to 0.60 wt% Mo. By the way, since these materials are used at high temperatures, the focus of development has traditionally been on high-temperature strength and creep properties, and little attention has been paid to toughness, but recent accidents have led to the development of such materials. In addition, toughness is now required, and there is a strong desire for a material that has both strength and excellent toughness. In addition, such materials undergo several post-weld heat treatments during container manufacturing.
Hert Treatment (hereinafter referred to as PWHT)
It is necessary to have sufficient strength and toughness even after the conditions. However, the conventional C-
When the thickness of 0.5Mo steel exceeds 38 mm, it is generally normalized (Nor) or normalized-tempered (Nor-Temp), but due to the low Mn content and low hardenability, ferrite is used. It becomes a structure mainly composed of ferrite + bainite. Moreover, in order to ensure strength at high temperatures, it is generally a high C system, with a bainite structure with low toughness dispersed in a ferrite base, and such a composition system is used during the manufacturing of steel sheets. In addition to impairing toughness, the so-called SR (stress relief annealing) embrittlement phenomenon, which shows toughness deterioration due to increases in processing time and processing temperature during PWHT, becomes significant, making it increasingly difficult to maintain toughness after PWHT. Become. In addition, the SR embrittlement is a toughness deterioration phenomenon that is accompanied by a decrease in strength, and it is necessary to simultaneously suppress the decrease in strength and the deterioration in toughness as much as possible, but ensuring strength by increasing the amount of C reduces the toughness level and promotes the deterioration of toughness. Furthermore, this increase in the amount of C impairs weldability and workability, so in any case, the above-mentioned requirements cannot be met immediately. The present invention was devised after repeated studies in view of the above-mentioned circumstances, and is a non-heat-treated thick-walled C-
0.5Mo steel has sufficient strength even after the PWHT without impairing weldability and workability,
We also succeeded in obtaining a steel with higher toughness than conventional steel. That is, the component composition wt% (hereinafter simply referred to as %) of the product according to the present invention is 0.07%≦C≦
0.25%, 0.10%≦Si≦0.50%, 0.40%≦Mn≦0.90
%, P≦0.030%, S≦0.020%, 0.50%＜Mo≦
0.65%, 0.005%≦sol.Al≦0.04%, 0.05%≦Cr≦
Further, 0.005%≦Nb≦0.05% is added to 0.40%, and the remainder consists of Fe and unavoidable impurities, and further 0.05% is added to the basic invention.
It contains one or two of the following: %≦Ni≦0.40%, 0.0002%≦B≦0.0020%, and especially
This applies to steel with a plate thickness of 50 mm or more, which has severe PWHT conditions. However, in the case of the present invention, the strength
As a guideline for toughness, the tempering parameter [TP = T (Log t + 20)] after PWHT is within the range of 19.0 × 10 ³ or less, and in terms of strength, it is A204 or SB46,
Satisfy the standard value of 49M, toughness value vTs
The temperature shall be ≦0°C. In addition, in the above tempering parameter TP=T (Log t+20), T is
Processing temperature in PWHT (unit = °C + 273),
t is the processing time (unit: hr) in the relevant PWHT, and if tempering is performed after normalizing during steel plate manufacturing, T.
It goes without saying that the P value must also be considered. Furthermore, just to be sure, the above TP value is
19.0×10 ³ means that, for example, when no tempering treatment is performed and a PWHT temperature of 600 to 650° C. is employed, the treatment is performed at that temperature for approximately 3.5 to 58 hours. The features of the invention as described above are as follows:
-0.5Mo steel with a small amount of Cr of 0.05-0.4% and 0.005
The strength and toughness after PWHT were significantly improved by adding ~0.05% Nb. Figure 1 summarizes the changes in tensile strength (TS) due to changes in cooling rate during normalization and vTs after PWHT for the inventive steel and conventional steel shown in Table 1 as a row. .

[Table] It is clear that the steel A of the present invention has higher strength and toughness than the conventional steel B at the same cooling rate, that is, at the same plate thickness. As shown in the figure, in the case of the steel of the present invention, a tensile strength of 50 Kg/mm ² or more and a vTs of 0°C or less are obtained after normalizing treatment and PWHT, even for extremely thick steel plates such as 150 mm thick. It is. Fig. 2 shows the present invention steel A (98 mm plate thickness material) and conventional steel B (50 mm plate thickness material and 100 mm simulation material,
In other words, the results of a comparison of changes in strength and toughness due to tempering parameters with a material normalized at a cooling rate equivalent to the cooling rate at which a 100 mm material is cooled during normalization are shown. Comparing the strength and toughness of the 98 mm steel of the present invention A and the 100 mm simulation material of the comparison steel, it is found that the steel of the present invention has a strength of at least 5 kg/mm ² or higher and a high toughness of at least 20°C or higher. it is obvious. In addition, the toughness of the 98 mm steel material of the invention steel A is the same as that of the 50 mm thick material of the conventional steel B, but in terms of strength, the thicker steel of the invention is higher as described above, indicating the effectiveness of the composite addition. is shown, and in the steel of the present invention, for example,
It is clear that it has sufficient strength and toughness even under severe PWHT conditions of 19.5×10 ³ . The reason for limiting the range of each element in the present invention is as follows. C is ASTMA204, JISG3103 SB46M,
In order to satisfy the strength standard value of C-0.5Mo steel in 49M, it is necessary to add 0.07% or more.On the other hand, addition of more than 0.25% deteriorates low temperature toughness and impairs weldability, so 0.07~ The rate shall be 0.25%. Si is an element necessary as a deoxidizing agent, as well as for preventing high-temperature oxidation and ensuring high-temperature strength. If it is less than 0.10%, it has no effect, and if it is less than 0.5%
Exceeding this range will deteriorate the toughness of the base material.
It was set at 0.10-0.50%. Mn is an element necessary to improve hardenability, as well as strength and toughness, and if it is less than 0.40%, it will not be possible to satisfy the specified strength and will cause a significant decrease in toughness. However, if it exceeds 0.9%, it impairs weldability and significantly increases the hardness of the weld heat affected zone, so it is necessary to keep the content within the range of 0.4 to 0.9%. P is an element that is inevitably included as an impurity element, but if it exceeds 0.030%, it causes deterioration of low temperature toughness, so the upper limit is set to 0.030%. Like P, S is an element that is unavoidably contained, and it causes deterioration of toughness, promotion of material anisotropy, deterioration of properties in the thickness direction, and reduction of hot workability, so S is an element that is unavoidably contained, especially in thick walls. For the steel of the present invention, which is frequently applied to materials, it is necessary to set the upper limit to 0.020%. Mo is added to steel materials used at high temperatures to prevent graphitization, ensure high-temperature strength, and prevent hydrogen corrosion, but if it is less than 0.50%, the effect is poor, and the hardenability decreases, resulting in increased strength. , leading to deterioration of toughness. On the other hand, addition of more than 0.65% inhibits weldability and reduces the toughness of welded joints. accordingly
It is necessary to keep it within the range of over 0.50 to 0.65%. sol.Al fixes solid solution N and improves toughness by refining the structure, but if it is less than 0.005%, it causes grain coarsening, and if it is added more than 0.04%, it causes a decrease in ductility during hot working. Therefore, it should be in the range of less than 0.005 to 0.04%. As mentioned above, the upper limit of Cr is set to 0.40%, and the lower limit of Cr is required to be in the range of 0.05 to 0.40% since the effect of Cr is not recognized if it is less than 0.05%. Nb forms fine carbides (some carbonitrites),
It improves toughness by making the structure finer and improves strength at room temperature and high temperature by precipitation strengthening.
It is an essential element in this type of pressure vessel steel used under severe PWHT treatment conditions in order to suppress the coarsening of cementite and Mo ₂ C that accompanies PWHT treatment. In other words, less than 0.005% Nb has no effect;
On the other hand, adding more than 0.05% Nb impairs weldability.
Degrades toughness. Therefore, for Nb, 0.005~
The range was set at 0.05%. In the present invention, Ni and B can be further added to the basic invention as described above, and the reason for limiting the range thereof is as follows. Ni is an element that is effective in solid solution to improve strength and toughness, but its effect is not clear when it is less than 0.05%, and when considering the economic efficiency of this class of steel, addition of more than 0.40% is extremely uneconomical, so it is set at 0.05 to 0.40%. B, like Cr, is an element that improves hardenability, and using B in place of a part of Cr is effective in terms of ensuring strength and toughness. However
Less than 0.0002% has no effect on hardenability;
Addition of more than 0.002% will lead to deterioration of weldability.
The range was set to 0.0002 to 0.002%. The characteristics of the present invention will be explained below by showing specific manufacturing examples of the present invention together with comparative examples thereof. Table 2 below shows the chemical composition of the test steels used by the inventors. Steels 1 to 3 are comparative steels, and Steel 1
is a conventional steel, and Steels 2 and 3 are conventional steels to which Nb and Cr are respectively added. On the other hand, Steels 4 to 11 are steels of the present invention, Steels 4 to 6 are Cr-Nb series, and Steels 7 to 8 are steels of the present invention.
is Cr-Nb-Ni system, steel 9 is Cr-Nb-B system, steel 10
~11 is Cr-Nb-Ni-B system.

[Table] The tensile properties and impact properties of these test steels are as shown in Table 3 below.

【table】

[Table] That is, steels 4 to 11 of the present invention have higher strength than conventional steel or comparative steel in any combination of ingredients.
It exhibits high toughness, and the improvement in impact properties is particularly remarkable, with vEo (Sharpey absorbed energy at 0°C) being 10 Kg·m or more. Conventional steels are high C-based steels, so although they exhibit sufficiently high strength values, they have low toughness; Nb-only steels have low toughness due to lack of hardenability, although improvements in toughness are observed due to microstructural refinement; In a single system, the strength is similarly high, but the toughness is low because the toughness has not been improved by microstructural refinement. According to the present invention as explained above, Cr and Nb are added in combination to the basic component system of non-tempered C-0.5Mo steel, or if necessary, this material is further added.
By adding one or both of Ni and B
This invention is capable of ensuring sufficient strength and toughness after PWHT, making it high in strength, and providing a steel type preferable as a high-toughness steel for pressure vessels, and is industrially highly effective.

[Brief explanation of the drawing]

The drawings show the technical contents of the present invention, and Fig. 1 is a chart showing the cooling rate dependence of strength and toughness for the inventive steel and comparative steel, and Fig. 2 is a chart showing the cooling rate dependence of the strength and toughness of the inventive steel and comparative steel. 1 is a chart showing changes in strength and toughness of steel due to tempering parameters.

Claims (1)

[Claims] 1 C: 0.07 to 0.25 wt%, Si: 0.10 to 0.50 wt%, Mn: 0.40 to 0.90 wt%, Cr: 0.05 to 0.40 wt%, Mo: over 0.50 to 0.65 wt%, P: 0.030wt% or less, S: 0.020wt% or less, sol.Al: 0.005 to less than 0.04wt%, Nb: 0.005 to 0.05wt%, and the remainder consists of iron and inevitable impurities. High-quality, high-strength, high-toughness pressure vessel steel. 2 C: 0.07-0.25wt%, Si: 0.10-0.50wt%, Mn: 0.40-0.90wt%, Cr: 0.05-0.40wt%, Mo: More than 0.50 up to 0.65wt%, P: 0.030wt% or less, S : 0.020wt% or less, sol.Al: 0.005 to less than 0.04wt%, Nb: 0.005 to 0.05wt%, and any one of Ni: 0.05 to 0.40wt%, B: 0.0002 to 0.002wt%. or 2 types, with the remainder consisting of iron and inevitable impurities.