JPH0368100B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a Cr-Mo steel mainly used for boilers and pressure vessels, and more particularly to a high-strength, high-toughness, low-carbon Cr-Mo steel sheet that has excellent weld cracking resistance, erosion resistance, and creep resistance. . Conventionally, 1%Cr-0.5%Mo steel plate and 1.25%Cr-
0.5% Mo steel sheets are widely used as medium- and high-temperature pressure vessel steel sheets in chemical industry plants, including oil refining, and power generation plants. but,
These conventional Cr-Mo steel sheets usually have a high C content of about 0.15% and also contain large amounts of alloy components such as Cr and Mo, so they have a high weld crack susceptibility structure P _CM . Welding usually requires high-temperature preheating at around 150 to 350℃ to prevent cold cracking, which increases manufacturing costs by prolonging the manufacturing period and consuming a large amount of thermal energy. This is the cause. On the other hand, it is already well known that in order to reduce cold cracking susceptibility and improve weldability, it is effective to reduce P _CM , especially to reduce the amount of C. However, reducing the C content causes problems such as a decrease in strength, a decrease in erosion resistance, and a decrease in high-temperature creep strength, so low C Cr-Mo steel sheets have not been put into practical use. On the other hand, although B has conventionally been widely used in tempered steel that undergoes quenching and tempering, it has generally been rarely added to normalized steel or normalized and tempered steel, and until recently Accordingly, a method has been put into practical use to increase the hardenability of steel and increase its strength by adding a small amount of B to normalized steel or normalized and tempered steel.
However, in the case of Cr-Mo steel, B addition has conventionally been used to impart SR embrittlement resistance and increase strength and toughness.
The C content in such Cr-Mo steel is limited to the purpose of improving hot workability, etc.
Since it is almost the same as conventional steel and is about 0.11% or more, weldability has not been significantly improved. As a result of intensive research to improve the weldability of Cr-Mo steel, the present inventors have significantly reduced the amount of C while adding B while adding an appropriate amount of Al and reducing the amount of N. By adding Ni and Ni, it is a low C steel with excellent strength, toughness, erosion resistance, and creep resistance comparable to conventional Cr-Mo steel with a high C content of about 0.11 to 0.21%. The present invention was achieved by discovering that it is possible to obtain a novel Cr-Mo steel having the following characteristics. That is, an object of the present invention is to provide a high-strength, high-toughness normalized and tempered low carbon Cr-Mo steel sheet that has excellent weld cracking resistance, erosion resistance, and creep resistance. The first of the Cr-Mo steels according to the present invention is
C 0.03~0.10%, Si 0.05~1.00%, Mn 0.30~0.95%, P 0.020% or less, S 0.015% or less, Cu 0.05~0.50%, Ni 0.05~0.50%, Cr 0.40~1.75%, Mo 0.20~ 0.75%, B 0.0003-0.0020%, sol Al 0.005-0.100%, N 0.005% or less, balance consisting of iron and inevitable impurities, P _CM = C + Si / 30 + Mn + Cu + Cr / 2
It is characterized by having a P _CM defined as 0+Ni/60+Mo/15+V/10+5B (%) of 0.27% or less, and that its main structure is bainite. The second Cr-Mo steel according to the present invention is characterized by containing, in addition to the above chemical components, at least one member selected from the group consisting of 0.005 to 0.07% Nb and 0.005 to 0.07% V. . The third Cr-Mo steel according to the present invention is characterized by containing 0.0005 to 0.0070% Ca in addition to the above-mentioned chemical components. Further, the fourth Cr-Mo steel according to the present invention contains at least one member selected from the group consisting of Nb and V within the above-mentioned range in addition to the above-mentioned chemical components.
It is characterized by containing Ca. The present invention will be explained in detail below. In the Cr-Mo steel according to the present invention, B dissolves in solid solution in austenite, segregates at grain boundaries, and suppresses ferrite transformation, thereby improving the hardenability of the steel. However, on the other hand, B tends to combine with N in steel. Therefore, when B forms BN, the effective amount of B in the steel decreases, reducing the hardenability. Therefore, in the present invention, in order to improve the hardenability of steel and ensure an effective amount of B, the amount of N in the steel is reduced and N is fixed with Al. However, when adding too much B, the B compound precipitates at the austenite grain boundaries, which actually reduces the hardenability. Therefore, in the present invention, it is necessary to add an appropriate amount of B. However, in low C steel like the steel of the present invention, simply utilizing the hardenability improvement effect of B is not sufficient.
It is difficult to obtain desired properties such as strength and toughness. Here, the inventors
As a result of repeated research, we found that by adding an appropriate amount of B to low C steel and utilizing its hardenability effect,
By adding Cu and Ni in combination, the hardenability of the steel is further improved, and at the same time, due to the synergistic effect with the hardenability improving effect of B, the main structure of the steel becomes bainite, thus increasing the strength. At the same time, they discovered that the erosion resistance and creep resistance properties can be improved to the same level or better than conventional steels. Next, the reason for limiting the chemical composition of the Cr-Mo steel according to the present invention will be explained. In order to ensure the strength and erosion resistance of the steel, it is necessary to add 0.03% or more of C to the steel of the present invention, but on the other hand, as the amount of C increases, weldability and toughness decrease. , the upper limit of the amount added is 0.10%. Si is effective for securing strength and improving oxidation resistance, and is contained in an amount of 0.05% or more. However, when it is contained in an excessive amount, it increases the susceptibility to tempering embrittlement, so the upper limit of the content is 1.00%. %. Mn is effective in increasing the strength and ductility of steel, but when added in excess, weldability decreases, so the amount added is in the range of 0.30% to 0.95%. P is contained as an impurity in steel, but it not only impairs toughness and weldability, but also increases susceptibility to temper embrittlement, so it is desirable to reduce it as much as possible. Therefore, in the present invention, the content of P is
Should be 0.020% or less. S is also contained as an impurity in steel, but since it significantly impairs the toughness of steel, it is desirable to reduce it as much as possible, and the content should be 0.015% or less. Cu is an effective component for solid solution strengthening and precipitation strengthening, and is also effective for improving erosion resistance. In order to effectively demonstrate this effect,
It is necessary to add at least 0.05%. on the other hand,
If added in excess of 0.50%, hot workability deteriorates, so the amount of Cu added should be in the range of 0.05 to 0.50%. Ni is effective in increasing the hardenability of steel and improving erosion resistance, and is also effective in preventing hexagonal cracking due to precipitation of Cu at austenite grain boundaries at high temperatures. In order to effectively exhibit this effect, it is necessary to add 0.05% or more, but since it is an expensive element, the amount added is in the range of 0.05 to 0.50% from a practical standpoint. Cr is effective in ensuring corrosion resistance and strength at high temperatures, so add at least 0.40%. However, if excessively added, weldability deteriorates, so the upper limit of its addition amount is set at 1.75%. Mo is an essential element for improving the hardenability of steel, especially when coexisting with B, the hardenability during normalizing. Further, since it is effective in increasing temper softening resistance and improving high-temperature strength, it is necessary to add 0.20% or more, but since it is an expensive element, the amount added is in the range of 0.20 to 0.75%. As mentioned above, B improves the hardenability during normalizing and is effective in increasing strength, so it is necessary to add 0.0003% or more to the steel of the present invention. However, when added in excess of more than 0.0020%, a B compound is generated during normalization, resulting in a decrease in hardenability and deterioration in toughness. Therefore, the upper limit of the amount added is 0.0020%. As mentioned above, sol Al has the effect of fixing N and refining the structure, but when its content is less than 0.005%, the above effects cannot be expected; If it exceeds 0.10%, it will cause cracks on the surface of the steel ingot, so its content should be
It should be in the range of 0.005 to 0.10%. When the content of N exceeds 0.005%,
It becomes easier to generate BN, and as a result, the amount of B effective for hardenability decreases, and the hardenability of the steel sheet decreases, so the content is set to 0.005% or less. As is well known, the P _CM is an index indicating the susceptibility to cold cracking during welding, and in order to lower the preheating temperature during welding, it is necessary to keep this value as low as possible. In order to prevent cracking even when the preheating temperature is about 100℃, the present invention
For Cr-Mo steel, P _CM shall be 0.27% or less.
Furthermore, in order to keep the preheating temperature below about 50℃,
It is preferable that P _CM is 0.23% or less. According to the present invention, the Cr-Mo steel according to the second invention contains, in addition to the above-mentioned elements, at least one element selected from the group consisting of Nb, V, and Ca. Both Nb and V are effective in refining crystal grains and improving strength, but if each is less than 0.005%, the effect cannot be expected;
When it exceeds 0.07%, it not only deteriorates toughness and weldability but is also unfavorable from the economic point of view. Therefore, the amount added is 0.005~
The range shall be 0.07%. Ca improves the toughness of welded joints and bond parts, and also improves the properties in the thickness direction. In order to effectively exhibit this effect, it is necessary to add at least 0.0005%. However, when added in excess of more than 0.0070%, the amount of nonmetallic inclusions increases, reducing ductility. Therefore, in the steel of the present invention, the amount added is 0.0005
The range shall be ~0.0070%. The above Nb, V and Ca are used in appropriate combinations as necessary, but when V is added,
It is necessary to suppress P _CM to 0.27% or less. In order to produce the Cr-Mo steel according to the present invention, a steel billet having the chemical composition according to the present invention may be hot rolled and subsequently heat treated according to a conventional method. However, even in the production by such a method, it is desirable to maximize the hardenability improvement effect of B. That is, when N is fixed with Al,
If the heating temperature of the steel billet exceeds 1150°C during rolling of a thick plate product, the solid solution of AlN is promoted and the amount of solid solution N increases. At this time, since BN is precipitated at the austenite grain boundaries, the hardenability improving effect of B is not fully exhibited during the next normalizing treatment.
Therefore, in order to make the most of the hardenability-improving effect of B, it is desirable that the heating temperature of the steel slab be 1200°C or less. Further, by performing accelerated cooling during normalization, the properties of the steel of the present invention are greatly improved. Examples According to the invention having the chemical composition shown in Table 1
Cr-Mo steel 1 (Hv=176) and conventional steel Cr-Mo steel 2
Figure 1 shows the erosion resistance of Comparative Steel (Hv=178) and Comparative Steel 3 (Hv=136). The conventional steel has a high C content, and the comparative steel has a high Cu and carbon content in the inventive steel.
Chemical components excluding Ni

【table】

【table】

【table】

[Table] It has a composition. Erosion resistance was determined by impinging high-pressure water at 150°C onto the center of a 10mm diameter specimen with a polished surface at a speed of 5m/sec on the grain side for 500 hours, then measuring the weight loss due to corrosion of the specimen. evaluated. The comparison steel has lower erosion resistance than the conventional steel, but the steel of the present invention with a composite addition of Cu and Ni has better erosion resistance than the conventional steel. This is because the corrosion resistance of the steel itself has improved due to the combined addition of Cu and Ni, the overall hardness has increased due to improved hardenability, and the lower C has reduced the precipitation of carbides. This is because local corrosion is less likely to occur because the difference in strength between grain boundaries and within grains is small. Figure 2 shows the Cr-Mo steel 1 according to the present invention and the conventional steel.
Shows the high temperature creep rupture strength of Cr-Mo steel. The horizontal axis [P] shows the Larson-Miller parameter, T is the test temperature (K), and t is the rupture time (h). The breaking strength of the steel of the present invention is equal to or higher than that of conventional steel. This is because, according to the steel of the present invention, in the creep temperature range, there is less precipitation of carbides on the grain boundaries, which become the core of void generation, and the difference in strength between the grain boundaries and the grain interior is small, so the grain boundaries are strained. This is due to the fact that it is difficult to concentrate. Next, the heat treatment conditions, mechanical properties, and weldability of steel plates of the present invention steels A to G and comparative steels H to K having the chemical components shown in Table 1 are shown. Since the steel of the present invention has a significantly reduced C content compared to the comparative steel, the P _CM is 0.27% or less, which is lower than the comparative steel. Therefore, the preheating temperature for preventing root cracking in the diagonal Y-shaped weld cracking test for the steel of the present invention is 100°C or higher for the comparative steel.
According to the steel of the present invention, the temperature is 100°C or less. At the same time, the absorbed energy vEo of the steel of the present invention at 0°C
is 10.0 Kgf·m or less for the comparative steel, whereas it is 10.0 Kgf·m or more for the steel of the present invention. Furthermore, the fracture surface transition temperature vTrs is also 0℃ for the comparison steel.
However, according to the steel of the present invention, the temperature is 0° C. or lower. As described above, the steel of the present invention has excellent weld cracking resistance, erosion resistance, and creep resistance, and has high strength and high toughness.

[Brief explanation of drawings]

FIG. 1 is a graph showing the erosion resistance of the inventive steel, comparative steel, and conventional steel, and FIG. 2 is a graph showing the creep rupture strength of the inventive steel and the conventional steel.

Claims (1)

[Claims] 1% by weight: C 0.03-0.10%, Si 0.05-1.00%, Mn 0.30-0.95%, P 0.020% or less, S 0.015% or less, Cu 0.05-0.50%, Ni 0.05-0.50%, Cr 0.40~1.75%, Mo 0.20~0.75%, B 0.0003~0.0020%, sol Al 0.005~0.100%, N 0.005% or less, the balance consisting of iron and inevitable impurities, P _CM = C + Si / 30 + Mn + Cu + Cr / 2
0+Ni/60+Mo/15+V/10+5B (%) _PCM defined as 0+Ni/60+Mo/15+V/10+5B (%) is 0.27% or less, and the main structure is bainite, which has excellent weld cracking resistance, erosion resistance, and creep resistance. Excellent high strength, high toughness normalized and tempered low carbon Cr-Mo steel sheet. 2 Weight% (a) C 0.03-0.10%, Si 0.05-1.00%, Mn 0.30-0.95%, P 0.020% or less, S 0.015% or less, Cu 0.05-0.50%, Ni 0.05-0.50%, Cr 0.40- 1.75%, Mo 0.20-0.75%, B 0.0003-0.0020%, sol Al 0.005-0.100%, N 0.005% or less, (b) Nb 0.005-0.07%, and V 0.005-0.07%. more than species,
The balance consists of iron and unavoidable impurities, P _CM = C + Si / 30 + Mn + Cu + Cr / 2
0+Ni/60+Mo/15+V/10+5B (%) _PCM defined as 0+Ni/60+Mo/15+V/10+5B (%) is 0.27% or less, and the main structure is bainite, which has excellent resistance to melt cracking, erosion resistance, and creep resistance. Excellent high strength, high toughness normalized and tempered low carbon Cr-Mo steel sheet. 3 In weight% (a) C 0.03-0.10%, Si 0.05-1.00%, Mn 0.30-0.95%, P 0.020% or less, S 0.015% or less, Cu 0.05-0.50%, Ni 0.05-0.50%, Cr 0.40- 1.75%, Mo 0.20-0.75%, B 0.0003-0.0020%, sol Al 0.005-0.100%, N 0.005% or less, (b) Ca 0.005-0.0070% The balance consists of iron and inevitable impurities, P _CM = C + Si / 30 + Mn + Cu + Cr /2
0+Ni/60+Mo/15+V/10+5B (%) _PCM defined as 0+Ni/60+Mo/15+V/10+5B (%) is 0.27% or less, and the main structure is bainite, which has excellent weld cracking resistance, erosion resistance, and creep resistance. Excellent high strength, high toughness normalized and tempered low carbon Cr-Mo steel sheet. 4 In weight% (a) C 0.03-0.10% Si 0.05-1.00%, Mn 0.30-0.95%, P 0.020% or less, S 0.015% or less, Cu 0.05-0.50%, Ni 0.05-0.50%, Cr 0.40-1.75 %, Mo 0.20-0.75%, B 0.0003-0.0020%, sol Al 0.005-0.100%, N 0.005% or less, (b) Nb 0.005-0.07%, and V 0.005-0.07%. Above, (c) Ca 0.0005-0.0070%, balance iron and unavoidable impurities, P _CM = C + Si / 30 + Mn + Cu + Cr / 2
0+Ni/60+Mo/15+V/10+5B (%) _PCM defined as 0+Ni/60+Mo/15+V/10+5B (%) is 0.27% or less, and the main structure is bainite, which has excellent resistance to melt cracking, erosion resistance, and creep resistance. Excellent high strength, high toughness normalized and tempered low carbon Cr-Mo steel sheet.