JPS62146247A - Cr-mo steel plate for multilayer vessel - Google Patents

Abstract

PURPOSE:To obtain the titled Cr-Mo steel plate capable of holding sufficient strength and toughness even after subjected to SR at high temp. for long period in assembling a multilayer vessel, by providing a composition in which C content is remarkably reduced, B, Cu, and Ni are added in combination, the amounts of C, Si, and Mn are relatively controlled, and impurities such as P, S, etc., are reduced. CONSTITUTION:The steel plate for multilayer vessel has a composition consisting of, by weight, 0.03-0.10% C, 0.05.-0.90% Si, 0.30-0.90% Mn, <=0.020% P, <=0.010% S, 0.05-0.35% Cu, 0.05-0.35% Ni, 0.40-1.60% Cr, 0.20-0.70% Mo, 0.0003-0.0020% B, 0.005-0.100% solAl, <=0.010% Sn, <=0.010% Sb, <=0.010% As, and the balance Fe and satisfying the conditions in an equation as well as in an inequality. Said steel plate is the one of <=32mm thick having a bainite structure, capable of welding without preheating, and maintaining high strength and high toughness even after subjected to high-temp. and long-period stress relief annealing treatment, and further it has superior weld crack resistance as well.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a Cr-Mo steel plate for multilayer containers, and in detail,
It has excellent weld cracking resistance, can be welded without preheating during welding work, and is further referred to as SR due to stress relief annealing at high temperatures and long periods of time. ) A normalized and tempered Cr-Mo steel plate for multilayer containers having a thickness of 32 mm or less and retaining high strength and high toughness even after treatment.

(Prior art) In the recent chemical industry, there has been a remarkable trend toward higher pressure and larger capacity for pressure vessels, and if conventional single-layer containers were to meet these demands, extremely thick copper plates would have to be used. As a result, the cost of machining is high (and the material is non-uniform in the thickness direction, and in practice, there is a limit to the thickness of the steel plate used in terms of press capacity). On the other hand, multilayer containers manufactured by laminating steel plates with a thickness of 32 mm or less can avoid the above problems, and are currently being used as pressure vessels in chemical plants and power generation. Widely used in plants.

In general, a multilayer container is manufactured by tightening a semicircular or triangular outer layer plate 2 to an inner cylinder 1 and performing longitudinal welding 4 at a seam 3, as shown in FIG. The unit cylinder 5 is constituted by adding them. Next, as shown in Figure 4,
It is manufactured by assembling the required number of unit cylinders 5 coaxially, performing circumferential welding 6 along the circumference of the unit cylinders to join them, and further welding flanges 7 or end plates to both ends.

Since Cr-Mo steel sheets generally have poor strength and oxidation resistance at high temperatures, they have traditionally been used in medium- and high-temperature pressure vessels in chemical industry plants such as oil refining, feed water heaters in power generation plants, etc. It is widely used as However, conventional Cr-Mo steel sheets usually have C
Because it has a high content of about 0.15% and also contains large amounts of alloy components such as Cr and Mo, it has a high weld cracking susceptibility composition PCM, so it is necessary to prevent cold cracking during welding. Therefore, high-temperature preheating at about 150 to 250°C is usually required.

However, when producing a multilayer container by longitudinal welding the unit cylinder, when preheating is performed at a high temperature as described above, the accuracy of the groove part decreases significantly due to thermal expansion, and the front layer A gap is created between the two, making longitudinal welding extremely difficult. Further, even if welding is possible, welding defects are likely to occur and it is difficult to obtain a sound joint. For this reason, although Cr-Mo steel sheets have excellent properties as a high-temperature material, their application to multilayer containers has not been put to practical use.

On the other hand, it is already well known that reducing PCM, particularly CJI, is effective in reducing cold cracking susceptibility and improving weldability. Cr
-Although it is possible to improve the weldability of Mo steel sheets by such means, (reducing j3 causes problems such as a decrease in strength and toughness, and a decrease in high-temperature creep strength. Although the steel plates used for manufacturing are thin, with a thickness of 32I1m or less, the wall thickness of the unit cylinder made by laminating them is about 100 to 200 mm, so the SR after circumferential welding when assembling the pressure vessel is carried out based on this wall thickness. Therefore, each Cr-Mo steel plate used for manufacturing a multilayer container has a thickness of 32 mm.
Even if the thickness is less than +m, it is required to maintain a predetermined strength and toughness even after SR which corresponds to a plate thickness of 100 to 200 fl. Therefore, simply reducing the amount of C does not make it possible to obtain Cr-Mo that can be applied to the production of multilayer cylindrical containers.

In addition, alloying element B has conventionally been widely used in tempered steel that undergoes quenching and tempering, but in general, it has rarely been added to normalized steel or normalized and tempered steel, but recently As a result, a method has been put into practical use in which B is added to normalized steel or normalized and tempered steel to increase the hardenability and strength of the steel. However, regarding Cr-Mo steel,
Conventionally, B addition has been limited to the purpose of imparting SR embrittlement resistance, increasing strength and toughness, and improving hot workability.Moreover, the C content in such CrMo steel is , is almost the same as that of conventional steel, and is about 0.11% or more, so no significant improvement in weldability has been achieved.

(Object of the Invention) As a result of intensive research to improve the weldability of Cr-Mo steel, the present inventors have found that while significantly reducing the amount of C,
Combined addition of B, Cu and Nt, and further addition of C, Si and M
In addition to relatively controlling nN, impurities P, S, Sn,
By reducing the amounts of Sb and As, it is possible to perform longitudinal welding without preheating, especially in the production of the unit cylinder, and to maintain sufficient strength and strength even after long-term SR at high temperatures when assembling a multilayer container. Having discovered that it is possible to obtain a Cr-Mo steel plate for multilayer containers that maintains toughness,
This led to the present invention.

That is, the present invention has excellent weld cracking resistance, can be welded without preheating during welding work, and has a thickness that maintains high strength and high toughness even after high-temperature and long-term SR treatment. Cr- for normalized and tempered multilayer containers of 32 mm or less
The purpose is to provide Mo steel sheets.

(Structure of the Invention) The first Cr-Mo steel plate for a multilayer container according to the present invention has, in weight percent, C0.03 to 0.10%, Si 0.05 to 0.90%, and Mn 0.30 to 0.90. %, P 0.020% or less, SO, oto% or less, Cu 0.05-0.35%, Ni 0.05-0.35%, Cr 0.40-1.60%, Mo 0.20- 0.70%, B 0.0003-0.0020%, sol A6
0.005~0.100%, Sn 0.010% or less, SBO, 010% or less, As 0.010% or less, the balance being iron and unavoidable impurities, satisfying 44C-7Si+7Mn(χ) ≧0, and defined PCl4 is 0.26% or less, and
The structure is bainite, it can be welded without preheating, it maintains high strength and toughness even after high-temperature and long-term annealing treatment to remove horns, and the plate thickness is 32+U or less. .

Further, a second CrM for a multilayer container according to the present invention.

In addition to the above chemical components, the steel plate contains: Ti 0.005-0.07%, Nb 0.005-0.07%, V 0.005-0.07%, and Ca 0.0
It is characterized by containing at least one element selected from the group consisting of 0.005% to 0.0070%.

The present invention will be explained in detail below.

Cr Mow! according to the invention! In I, 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 easily combines with N in steel,
When B forms BN, the effective amount of B in the steel decreases, reducing hardenability. Therefore, in the present invention, in order to improve the hardenability of the steel and ensure an effective amount of B, the amount of N in the steel is reduced and N is fixed by A2. 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 a low C steel like the steel of the present invention, it is difficult to obtain desired properties such as strength and toughness just by utilizing the hardenability improving effect of B. Here, as a result of repeated research, the present inventors have found that, at low Cw4, by adding an appropriate amount of B to utilize its hardenability improvement effect, and by adding Cu and Ni in combination, At the same time, the hardenability is further improved, and due to the synergistic effect with the hardenability improving effect of B, the steel structure becomes bainite, and at the same time, fine carbonitrides precipitate within the crystal grains.
It has been found that strength and toughness are improved.

Furthermore, compared to ferrite, which is the main structure of conventional steel, bainite has a higher solid solubility limit for each alloying element, making it difficult to form precipitates at grain boundaries. It has also been found that the properties and creep rupture strength after SR are improved.

In order to reliably obtain the above effects, it is necessary to sufficiently austenitize during heat treatment and to sufficiently dissolve the alloying elements in solid solution. If the solid solution of alloying elements is insufficient, even if sufficient strength and toughness are obtained after heat treatment, after long-term SR at high temperatures, the toughness will be significantly reduced due to agglomeration and coarsening of precipitates. do. Furthermore, in the case of low C@, as in the steel of the present invention, the temperature for austenitization, that is, Ac3, decreases as Cl decreases.
Since the point increases, sufficient austenitization becomes difficult by heating to a temperature of 910 to 930° C., which is a normal austenitization temperature.

However, increasing the austenitizing temperature should be avoided because it causes coarse grains and deteriorates toughness. Furthermore, during actual operation of steel sheet manufacturing, raising the austenitizing temperature will severely damage the heat treatment furnace and limit simultaneous charging with other steel materials, so this should also be avoided. It is.

Therefore, as a result of repeated detailed research on the elements that affect Ac=transformation point, the present inventors limited the chemical composition of Cr-Mo steel, and within the composition range, 3i and M n 4] without the need to increase the austenitization temperature.
It has been found that austenitization can be sufficiently achieved at normal austenitization temperatures.

Furthermore, as a cause of embrittlement of steel due to SR, apart from the agglomeration and coarsening of precipitates mentioned above, it is also necessary to consider temper embrittlement that occurs due to the slow cooling rate after SR treatment. This tempering embrittlement occurs at temperatures between 550 and 350°C after SR treatment.
Because it is slowly cooled in the temperature range of
This occurs because impurities such as , Sb, and As segregate and embrittle grain boundaries. The present inventors have found that in order to prevent these problems, it is effective to restrict the amount of impurity elements described above.

We have also found that in order to ensure excellent toughness even after high-temperature, long-term SR, it is necessary to obtain as high a toughness value as possible in the state before SR.

The present invention was completed based on the above-mentioned new findings.

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 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, so the addition The upper limit of the amount is 0.10%.

Si is effective for ensuring strength and improving oxidation resistance, but since it increases susceptibility to temper embrittlement, the amount added is in the range of 0.05 to 0.90%.

Mn is effective in increasing the strength and ductility of steel, but if added in excess, weldability decreases, so the amount added is 0.
The range is 30% to 0.90%.

Furthermore, as mentioned above, these elements are 44C-73i
It is necessary to satisfy the relationship: +7Mn(%)≧0(1).

Although P is contained as an impurity in steel, it is desirable to reduce it as much as possible because it not only impairs toughness and weldability (but also increases susceptibility to temper embrittlement).

Therefore, in the present invention, the P content is 0.020%
The following shall apply.

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 0.010% or less.

Cu is an effective component for solid solution strengthening and precipitation strengthening, and in order to effectively exhibit such effects, Cu must be at least 0.
It is necessary to add 0.05%. On the other hand, if added in excess of 0.35%, hot workability and weldability will deteriorate, so the amount of Cu added is in the range of 0.05 to 0.35%.

Ni is an element useful for increasing the hardenability of steel and preventing hexagonal cracking due to precipitation of Cu at austenite grain boundaries at high temperatures. In order to effectively express this effect, it is necessary to add 0.05% or more,
However, since it is an expensive element, from a practical standpoint, the amount added is in the range of 0.05 to 0.35%.

Cr is effective in ensuring oxidation resistance and strength at high temperatures, and in order to effectively obtain such effects, it is necessary to add a small amount (both 0.40%). However, if added in excess, Since weldability deteriorates, the upper limit of the amount added is 1.
．． It shall be 60%.

Mo is an essential element for improving the hardenability of steel, especially when it coexists with B, and is also effective in increasing temper softening resistance and high-temperature strength.
It is necessary to add 0% or more, but since it is an expensive element, the amount added is in the range of 0.20 to 0.70%.

As mentioned above, since B is effective in improving hardenability and increasing strength, it is necessary to add 0°0003% or more in the steel of the present invention. However, 0.0020
When added in excess of more than %, compound B is generated,
It reduces hardenability and is accompanied by deterioration of toughness. Therefore, the upper limit of the amount added is 0.002'O%.

As mentioned above, 5OIAl has the effect of fixing N and refining the structure, but when its content is 0.005
If the content is less than 0.10%, the above effects cannot be expected. On the other hand, if the content exceeds 0.10%, it may cause surface cracking of the steel ingot, so the content should be 0.005 to 0.10%. %
The range shall be .

As mentioned above, Sn, Sb, and As segregate at grain boundaries during the cooling process during SR treatment and impair toughness, so the content of each needs to be 0.010% or less.

Said P. As is well known, M is an index indicating the sensitivity to cold cracking during welding, and in order to enable welding without preheating and without cracking during welding, M is used in the Cr-Mo1 plate of the present invention. The PCM shall be 0.26% or less.

According to the invention, Cr-M according to the second invention.

In addition to the above-mentioned elements, the 1211 plate contains Ti, Nb, and V.
and at least one element selected from the group consisting of Ca.

Ti has the effect of stably fixing N even at high temperatures and refining the structure. In order to effectively exhibit this effect, it is necessary to add at least 0.005%, but if added in excess of 0.07%, the toughness deteriorates, so the amount added is 0.005%. The range is 0.07%.

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, and -, when each exceeds 0.07%, , which not only deteriorates toughness and weldability but also is unfavorable from an economic point of view. Therefore, the amount added is
Each is in the range of 0.005 to 0.07%.

Ca improves toughness, improves the toughness of welded joints and bond parts, and further improves properties in the thickness direction. In order to effectively exhibit this effect, at least 0.00
It is necessary to add 0.05%. However, 0.00
When added in excess of more than 70%, the amount of nonmetallic inclusions increases, reducing ductility. Therefore, in the steel of the present invention, the amount added is 0.0005 to 0.0. The range is OO70%.

The above Ti, Nb, (2) and Ca are used in appropriate combinations as necessary, but even when (2) is added, it is necessary to suppress Poo to below the above-mentioned value.

In addition, when the amount of N contained as an impurity is too large, as mentioned above, it becomes easy to generate BN, and as a result, the amount of N that is effective for hardenability is reduced, and the hardenability of the steel is reduced. Therefore, in the present invention, the amount of N is 0.
It is preferable to set it to 0.007% or less.

The Cr-Mo steel plate according to the present invention may be produced by hot rolling a steel ingot or slab having the above-mentioned predetermined chemical composition according to a conventional method, and then subsequently heat-treating the steel ingot or slab.

(Effects of the Invention) As described above, according to the present invention, not only is it possible to significantly reduce the amount of C, lower the photosensitivity to cold cracking, and weld without preheating, but also it is possible to weld together with B. Add a complex of
Furthermore, while relatively controlling C, Si and Mnl,
By reducing the impurities P, S, Sn, sb, and As, 77, it maintains excellent strength and toughness even after high-temperature and long-term SR treatment, and therefore has a thickness that can be particularly suitably applied to the production of multilayer containers. A Cr-Mo steel plate with a diameter of 32 mm or less can be obtained.

(Examples) 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 Table 1 shows the chemical composition and PC of Cr-Mo steel sheet A-F according to the present invention and Cr-Mo steel sheet G- as a comparative example.
Table 2 shows the thicknesses, heat treatment conditions, tensile properties, impact properties, and weldability of these steel plates.

All of the Cr-Mo steel sheets according to the present invention have a significantly reduced C content compared to conventional high-C content Cr-Mo steel sheets, resulting in low PCM and, as a result, diagonal Y-shaped weld cracking. The root crack prevention preheating temperature in the test was room temperature, and welding was possible without preheating.

Furthermore, in the steel sheet of the present invention, not only the as-normalized and as-tempered steel sheet, but also the above (1) cost is set to 0% or more, so that 67
Even after high-temperature and long-term SR treatment at 0°C for 23 hours, a tensile strength of 53 kgf/mm or more can be obtained, and in terms of impact properties, the absorbed energy vEo at 0°C is 10 kgf-m. That's all.

Comparative steels G and H are conventional Cr-Mo steel plates and have a large amount of C, so the root crack prevention preheating temperature in the diagonal Y-shaped weld cracking test is 100°C or higher. Therefore, when welding these steel plates, it is not necessary to preheat them in order to prevent the occurrence of cold cracking, so for the reasons mentioned above, these steel plates cannot be applied to the manufacture of multilayer containers. I can't.

In addition, since comparative steels ①, J and K have a reduced C content, their weldability satisfies the requirements for steel plates for multilayer containers. However, since Comparison @I is a steel plate without B additives, and Comparative Steel J is a steel plate without Cu and Ni additives, both have a tensile strength of 50 after SR treatment at 670°C for 23 hours.
kgf/mm" or less, and the strength is insufficient for a steel plate for multilayer containers.

The comparative steel has the above-mentioned (1) price of -0.16%,
It is not within the range specified in the present invention, and has high strength and toughness as normalized and tempered, but S
After the R treatment, vEo was 3.2 kgf-m, and the toughness was significantly deteriorated.

As described above, according to the Cr-Mo steel sheet of the present invention, it is possible to weld without preheating, and welding can be performed at high temperatures and for long periods of time such as 23 hours at 670°C, which is the SR condition required for plate thickness 200 mm. Even after SR treatment, it maintains excellent strength and toughness.

FIG. 1 shows the relationship between the above-mentioned (1) price and impact properties for Cr-Mo steel plates having a thickness of 9 to 32 mm and having chemical components within the range specified by the present invention. In the normalized and tempered state, all have good toughness, but after SR treatment at 670'C for 23 hours, vEo sharply decreases when the cost (1) above is less than 0%. At the same time, the fracture surface transition temperature vTrs also increased significantly. On the other hand, when the cost (1) is 0% or more, νEo is 10 kgf-m or more and vTrs is 0'c or less even after the SR treatment.

Further, when the multilayer container is used in a creep region of 450° C. or higher, the steel plate used must have a sufficient leap rupture strength.

The creep rupture strength of the steel sheets A and B of the present invention is shown in FIG. 2 in comparison with a conventional steel sheet (C content 0.11 to 0.16%). In FIG. 2, the horizontal axis (P) is Larson-Old! 1
er parameters, T is the test temperature (K), and t is the test time (h). The breaking strength of the steel of the present invention is equal to or higher than that of the 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.

[Brief explanation of drawings]

Figure 1 shows (44C-73i+7Mn) (%) and 0
Absorbed energy νEo and fracture surface transition temperature vT in °C
FIG. 2 is a graph showing the high temperature creep rupture strength of the steel sheets of the present invention and comparative steel sheets. FIG. 3 is a perspective view showing a unit container used for manufacturing a multilayer container. FIG. 4 FIG. 1 is a sectional view showing an example of a multilayer container. Figure 1 (44C-7St' +7 MJ r%) 8
Figure 2

Claims (2)

[Claims] (1) C 0.03 to 0.10%, Si 0.05 to 0.90%, Mn 0.30 to 0.90%, P 0.020% or less, S 0.010% or less, in weight%. Cu 0.05-0.35%, Ni 0.05-0.35%, Cr 0.40-1.60%, Mo 0.20-0.70%, B 0.0003-0.0020%, Consists of solAl 0.005-0.100%, Sn 0.010% or less, Sb 0.010% or less, As 0.010% or less, balance iron and unavoidable impurities, and satisfies 44C-7Si+7Mn(%)≧0 At the same time, P_C_M=C+Si/30+(Mn+Cu+Cr)/
20+Ni/60+Mo/15+V/10+5B(%)
P_C_M defined as 0.26% or less, the structure is bainite, it can be welded without preheating, and it has high strength and high toughness even after high temperature and long stress relief annealing treatment. Cr for multilayer containers with a holding plate thickness of 32 mm or less
-Mo steel plate. (2) In weight% (a) C 0.03-0.10%, Si 0.05-0.90%, Mn 0.30-0.90%, P 0.020% or less, S 0.010 % or less, Cu 0.05-0.35%, Ni 0.05-0.35%, Cr 0.40-1.60%, Mo 0.20-0.70%, B 0.0003-0. 0020%, solAl 0.005-0.100%, Sn 0.010% or less, Sb 0.010% or less, and As 0.010% or less, and (b) Ti 0.005-0.010%. 0.07%, Nb 0.005-0.07%, V 0.005-0.07%, and Ca 0.0005-0.0070%, the balance being iron. and unavoidable impurities, satisfying 44C-7Si+7Mn(%)≧0, and P_C_M=C+Si/30+(Mn+Cu+Cr)/
20+Ni/60+Mo/15+V/10+5B(%)
P_C_M defined as 0.26% or less, the structure is bainite, it can be welded without preheating, and it has high strength and high toughness even after high temperature and long stress relief annealing treatment. Cr for multilayer containers with a holding plate thickness of 32 mm or less
-Mo steel plate.