JPH0558052B2 - - Google Patents

Description

[Detailed description of the invention]

Industrial application field The present invention is a low carbon tempered Cr-Mo system 80Kgf/mm ²
Regarding grade steel plates, for details, please refer to the plate thickness for city gas tanks, LPG tanks, etc., which have significantly improved weldability.
Concerning heat-treated 80Kgf/mm class ² high tensile strength steel plate of 40mm or less. (Conventional technology) In recent years, along with the trend toward larger city gas tanks and LPG tanks, there has been a strong demand for lighter weight.
f/mm The use of grade ² high tensile strength steel plates is increasing. These steel plates are highly susceptible to cold cracking during welding, so they are usually preheated to about 100 to 150°C during welding. However, particularly in recent years, there has been an increasing demand for reductions in construction costs for such preheating, and there has been a strong demand for 80 kgf/mm class ² steel plates that do not cause cold cracking even without preheating. It is already well known that it is effective to reduce the amount of C in steel and suppress hardening of the weld zone in order to reduce cold cracking susceptibility and improve weldability. Utilizing the effect of improving hardenability, 60Kgf/mm class ² steel plates that do not require preheating have been put into practical use. However, reducing the amount of C in steel leads to a decrease in strength, so the guaranteed strength is high.
In 80Kgf/mm class ² steel plate, even if the hardenability improvement effect of B is utilized, simply reducing the amount of C will not simultaneously provide the required strength and excellent weldability that does not require preheating. It is difficult to force them to do so. In order to solve this problem, for example,
No. 51-103018, JP-A-53-28017, JP-A-53-
As described in No. 29218 and Japanese Unexamined Patent Publication No. 54-38216, the preheating temperature for preventing weld cracking has been reduced to around 50°C by variously controlling the chemical composition and manufacturing conditions of the steel plate. A lower 80Kgf/mm ^2nd class steel plate has been proposed. In all of these, the decrease in strength due to the decrease in the amount of C is compensated for by adding alloying elements such as Cu, Ni, Cr, and Mn. However, on the other hand, the above-mentioned steel plate is expensive when using Ni, for example, and is also highly susceptible to sulfide stress corrosion cracking, making it unsuitable as a tank material. Furthermore, when the amount of Mn is relatively large, there are problems such as large temper embrittlement and SR (stress relief annealing) embrittlement. Problems to be Solved by the Invention The inventors of the present invention have conducted intensive research to obtain an 80Kgf/mm class ² steel plate that is highly resistant to sulfide stress corrosion cracking and SR embrittlement. 80
Kgf/mm In class ² steel sheets, by setting the C content to less than 0.10% and the Mn/C ratio within the specified range, the hardenability is much greater than that predicted from conventional knowledge. It has been found that it is possible to obtain a low carbon type and
The present invention was achieved by discovering that it is possible to obtain a tempered Cr-Mo steel sheet having a strength of 80 Kgf/mm ² class. That is, the present invention provides a high-strength, high-toughness, low-carbon heat-treated Cr-Mo system of 80 kgf/mm ² that has excellent weld cracking resistance and can be welded without preheating during welding work.
The purpose is to provide grade steel plates. Means for Solving the Problems The first low carbon type Cr-Mo 80Kgf/mm grade ² steel sheet according to the present invention has, in weight percent, C 0.05% to less than 0.10%, Si 0.05 to 0.50%, Mn 0.75 to 1.40. %, Cr 0.50~1.40%, Mo 0.20~0.80%, V 0.01~0.10%, B 0.0003~0.0018%, the balance consists of iron and unavoidable impurities, satisfies Mn/C≧10, and P _CM = C + Si/ 30+Mn+Cu+Cr/20+Mo/15+V/10+5
It is characterized by being a low carbon tempered Cr-Mo 80Kgf/mm ^2nd class steel plate with a thickness of 40mm or less and a _PCM defined as B (%) of 0.20% or less. The second low carbon type Cr-Mo type 80Kgf/mm grade ² steel plate according to the present invention has a steel plate that contains Cu in addition to the above elements.
It is characterized by containing in the range of 0.05 to 0.50%. The present invention will be explained in detail below. In order to explore the possibility of reducing the amount of C in Cr-Mo 80Kgf/mm class ² steel sheets, as shown in Table 1,
0.75Cr−0.20Mo−0.040V−0.0010B steel is the basic steel, Ceq=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4
+V/14 (%) was set at 0.52% (constant), and steels with various C and Mn contents were melted and subjected to geominy edge quenching using test pieces taken from as-rolled materials. I conducted a test. As shown in FIG. 1, the Jiomini curves of these steels show that the lower the C content, the lower the hardness of the quenched end, but on the other hand, there is a tendency for the Jiomini curves to become more horizontal. In order to quantitatively evaluate this tendency, the ideal critical diameter D _{I (caL)} calculated from the chemical composition
and the actual ideal critical diameter D _{I (pbs)} obtained from the Giyomini curve in Figure 1. As shown in Table 2, for steel A with a C content of 0.14%, D _{I (pbs)} is almost the same as D _{I (caL)} , but the C content is
It is observed that in steels B to E where the content is 0.12% or less, D _{I (pbs)} is approximately 1.4 times greater than D _{I (caL)} . still,

【table】

[Table] How to calculate D _{I (caL)} and D _{I (pbs)} is shown in the footnotes of Table 2. Next, the horizontal axis is the Mn/C ratio of each steel, and the vertical axis is
As shown in the graph of D _{I (pbs)} / D _{I (caL)} in Figure 2, especially when the Mn/C ratio is 10 or more,
It is understood that D _I(pbs) is larger than D _I(caL) . In most cases, the thickness of steel plates for tanks is 40 mm or less, but the position of the Jyomini curve, which cools at the same cooling rate as the center of a 40 mm thick plate, is approximately 13 mm from the quenched edge. . The hardness at this position is Hv300 or higher for all of the steels shown in Table 1, which is almost sufficient for the as-quenched hardness of an 80Kgf/mm ² class steel plate. Therefore, the amount of C and alloying elements can be further reduced than in the component system shown in Table 1. Based on this knowledge, various steels with P _CM of 0.20% or less were produced and subjected to the Jiomini test. An example of the results is shown in Figure 3, 0.07C−0.96Mn
−0.73Cr−0.35Mo−0.040V−0.0009B−0.48%
In the composition system of Ceq−0.19%P _CM , 13 from the quenched end
At the mm position, it is possible to obtain as-quenched hardness of about Hv315, and the plate thickness is 80Kg without preheating of 40mm.
It is understood that f/mm ² grade steel plate can be obtained. Next, the reason for limiting the chemical composition in the Cr-Mo steel sheet according to the present invention will be explained. In general, the smaller the amount of C added, the better the weldability. To eliminate the need for preheating,
It is necessary to keep it below 0.10%. However, if it is too small, it will not be possible to obtain the required strength as an 80Kgf/mm ^2nd class steel plate, so the lower limit of the amount of C added is set at 0.05%. Si is necessary for deoxidizing steel and ensuring strength, but when added in excess, it deteriorates weldability, so the amount added is in the range of 0.05 to 0.50%. Mn is necessary to improve the hardenability of steel, and in the present invention it is necessary to add 0.75% or more, but if added in excess, it impairs weldability and may cause tempering embrittlement. The upper limit of the amount added is 1.40% as it increases sensitivity and SR embrittlement susceptibility.
shall be. Furthermore, in the present invention, in order for the steel to have sufficient hardenability, Mn and C must satisfy the relationship of Mn/C≧10. Cr needs to be added in an amount of at least 0.50% in order to improve the hardenability of the steel and increase its corrosion resistance. However, if excessively added, weldability deteriorates, so the upper limit of the amount added is set at 1.40%. Mo is also effective in increasing the hardenability of steel, increasing its resistance to temper softening, and reducing its susceptibility to temper embrittlement. In order to effectively obtain these effects, it is necessary to add 0.20% or more, but
When added in excess, it impairs weldability, and
Since it is an expensive element, the upper limit of its addition amount is
It shall be 0.80%. V also has the effect of increasing the hardenability of steel and increasing the resistance to temper softening. In order to effectively obtain these effects, it is necessary to add 0.01% or more, but when adding too much, weldability and toughness deteriorate, so the upper limit of the amount added is 0.10% or more.
%. When added in a small amount, B is effective in improving hardenability and increasing strength. In order to effectively obtain this effect, it is necessary to add 0.0003% or more. However, when added in excess of more than 0.0018%, a B compound is produced, which lowers hardenability and at the same time causes deterioration of toughness. In addition to the above-described elements, the Cr-Mo steel sheet according to the present invention may contain Cu in a range of 0.05 to 0.50%. Although Cu has a relatively small negative effect on cold weld cracking, it is effective in increasing strength through solid solution strengthening and precipitation strengthening, and in order to effectively obtain this effect, it is necessary to add at least 0.05%. However, when added in excess, hot workability deteriorates, so the amount added should be 0.50% or less. Furthermore, in the present invention, P _CM =C+Si/30+Mn+Cu+Cr/20+Mo/15+V/10+5
It is necessary that P _CM defined as B (%) is 0.20% or less. As is well known, P _CM is an index indicating the susceptibility to cold cracking during welding, and in order to enable welding without cracking without preheating during welding, we developed the Cr-Mo steel sheet of the present invention. In this case, P _CM shall be 0.20% or less. The Cr-Mo steel sheet according to the present invention is suitable for a sheet thickness of 40 mm or less. The thickness of most steel plates for tanks is 40 mm or less, and in the case of steel plates with a thickness exceeding 40 mm, the amount of C and alloying elements must be increased to avoid incomplete quenching in the center due to the mass effect. This is because, as a result, preheating at a high temperature is required. The Cr-Mo steel sheet according to the present invention can be produced by hot rolling a steel ingot or billet having the above-mentioned predetermined chemical composition according to a conventional method, and then directly quenching and tempering, or quenching and tempering after reheating. Therefore, it can be manufactured. Effects of the Invention As described above, according to the present invention, Cr-Mo 80
Kgf/mm In class ² steel sheets, by setting the C content to less than 0.10% and the Mn/C ratio to 10 or more, the hardenability is much greater than that predicted from conventional knowledge. Thus, it was possible to obtain a low carbon type material with a P _CM of 0.20% or less without using a large amount of alloying elements.
Example that can obtain a 80Kgf/mm ^2nd class tempered Cr-Mo steel sheet The present invention will be described below with reference to Examples, but the present invention is not limited to these Examples in any way. Example A Cr-Mo steel sheet having the chemical components shown in Table 3 was manufactured by a quenching and tempering method or a direct quenching and tempering method. Steel plates F to K are steel plates of the present invention, and steel plates L to O are comparison steel plates. Table 3 also shows the P _CM and Mn/Si ratios of these steel plates. Table 4 shows the base material properties of these steel plates and the results of the diagonal Y-shaped weld cracking test.

【table】

【table】

【table】

[Table] The steel plates of the present invention all have a yield strength of 70Kgf/mm ²
Above, tensile strength is above 80Kgf/ ^mm2 , vTrs is -70
℃ or less, and P _CM is 0.20% or less.
In addition, no cracking occurs at room temperature in the diagonal Y-shaped weld cracking test. On the other hand, according to the comparative steel sheet, P _CM is 0.20
% or less, but because the Mn/Si ratio is less than 10, the tensile strength is less than 80 Kgf/mm ² , or even if the tensile strength is 80 Kgf/mm ² or more,
vTrs has a temperature of -10°C or higher, making it unsuitable as a tank material.

[Brief explanation of drawings]

Figure 1 shows 0.75Cr−0.20Mo−0.040V−0.0010B
Figure 2 shows Giyomini curves for test specimens taken from as-rolled steels of various composition systems in which the base steel is the base steel, Ceq is 0.52% (constant), and the C content and Mn content are varied. For a steel plate having the chemical composition shown in Table 1, the actual ideal critical diameter D _{I (pbs)} obtained from the Giyomini curve in Figure 1 and the ideal critical diameter D _{I (caL)} calculated from the chemical composition are A graph showing the influence of the Mn/C ratio on the ratio of
0.07C−0.96Mn−0.73Cr−0.35Mo−0.040V−
It is a graph showing the results of the Jiomini test for steel with a composition of 0.0009B-0.48%Ceq-0.19%P _CM .

Claims (1)

[Claims] 1% by weight: C 0.05% to less than 0.10%, Si 0.05 to 0.50%, Mn 0.75 to 1.40%, Cr 0.50 to 1.40%, Mo 0.20 to 0.80%, V 0.01 to 0.10%, B 0.0003 ~0.0018%, the balance consists of iron and unavoidable impurities, satisfies Mn/C≧10, and P _CM = C + Si / 30 + Mn + Cu + Cr / 20 + Mo / 15 + V / 10 + 5
A low carbon tempered Cr-Mo 80Kgf/mm grade ² steel plate with a thickness of 40mm or less and a _PCM defined by B (%) of 0.20% or less. 2% by weight: C 0.05% to less than 0.10%, Si 0.05% to 0.50%, Mn 0.75% to 1.40%, Cr 0.50% to 1.40%, Mo 0.20% to 0.80%, V 0.01% to 0.10%, Cu 0.05% to 0.50%, B 0.0003 ~0.0018%, the balance consists of iron and unavoidable impurities, satisfies Mn/C≧10, and P _CM = C + Si / 30 + Mn + Cu + Cr / 20 + Mo / 15 + V / 10 + 5
A low carbon tempered Cr-Mo 80Kgf/mm grade ² steel plate with a thickness of 40mm or less and a _PCM defined by B (%) of 0.20% or less.