JPS63303008A - Manufacture of 0.5%-mo steel plate for boiler and pressure vessel excellent in weldability - Google Patents

Abstract

PURPOSE:To manufacture a 0.5-Mo steel plate for boiler and pressure vessel excellent in weldability, by subjecting a rolling stock having a specific composition consisting of C, Si, Mn, Mo, B, Al, N, and Fe to hot rolling and normalizing at respectively specified temps. CONSTITUTION:A rolling stock which has a composition consisting of, by weight, 0.01-0.12% C, 0.05-1% Si, 0.1-2% Mn, 0.2-0.8% Mo, 0.0002-0.002% B, 0.005-0.1% Sol.Al, <=0.007% N, and the balance Fe with inevitable impurities and further containing, if necessary, one or more kinds among 0.05-0.5% Cu, 0.05-0.5% Ni, 0.05-0.8% Cr, 0.005-0.08% Ti, 0.005-0.08% Nb, and 0.0053-0.1% V and/or 0.0005-0.01% Ca and in which the value calculated from PCM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B is regulated to <=0.23% is used. This stock is hot-rolled at 900-1,150 deg.C heating temp., and the resulting steel plate is normalized at 910-1,000 deg.C. By this method, the steel plate for pressure vessel excellent in weldability can be obtained.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing 0.5% Mowi plates for boilers and pressure vessels, particularly for boilers and pressure vessels that have excellent weldability, high strength, and high toughness. The present invention relates to a method for manufacturing a 0.5% Mo steel plate for containers.

[Prior Art] Structural materials used for boilers and pressure vessels used in power generation plants, chemical plants, etc. are required to have heat resistance and oxidation resistance, as well as good weldability and workability.

As such structural materials, JIS S84 has traditionally been used.
6M, 3849M (ASTM A204 Gr, A
, B), etc. have been used.

The above-mentioned rolled steel sheet is mainly made of ferrite +
It has a bainite structure or a ferrite + pearlite structure, but in order to ensure strength at room temperature and high temperature (approximately 300 to 450 degrees Celsius), it is desirable to increase the proportion of bainite and pearlite, and this is achieved. Therefore, the amount of C contained is set relatively high at 0.15 to 0.3%.

However, as the amount of C increases, the weld cracking susceptibility composition Pcw (hereinafter referred to as PCM) naturally increases.

Therefore, when welding such rolled steel materials, high-temperature preheating of about 150 to 300 degrees Celsius is usually necessary to prevent low-temperature cracking, which leads to longer production times, large consumption of thermal energy, etc., and increases production costs. Leads to.

[Problems to be Solved by the Invention] On the other hand, it is already well known that reducing the C content in the steel to reduce the PCM is effective as a means to improve the weldability of copper steel sheets. . However, C in steel
If the content is reduced, there is a problem in that conventional steels cannot maintain a sufficient level of strength after normalization or stress relief annealing (hereinafter referred to as SR treatment).

Under these circumstances, a technique has recently been developed to obtain high strength by adding a small amount of B to a steel sheet having a low C and low pcM composition and normalizing the steel sheet. However, until now on the 0.5% Mail board, B
The reality is that no consideration has been given at all to manufacturing conditions (heating temperature during rolling and normalizing conditions) that can effectively utilize the hardenability improvement effect.

The present invention has been made in view of the above-mentioned circumstances, and its purpose is to improve the hardenability by B in a 0.5% Mofi plate with low C1 and low pew and a trace amount of B added. The objective is to provide a manufacturing method that can maximize the effects.

[Means for Solving the Problems] The method of the present invention that achieves the above object includes the following four inventions.

The first invention is as follows: C: 0.01-0.12% Si: 0.05-1% Mn: 0.1-2% Mo: 0.2-0.8% B: 0.0002 ~0.002% So1. A 1
: 0.005-0.1% as an essential component,
A rolled material in which N is limited to 0.007% or less, the remainder is Fe and unavoidable impurities, and the value calculated by ◆-■ (zero) ◆5B (zero) is 0.23% or less. A boiler with excellent weldability, characterized in that the rolled material is hot-rolled at a heating temperature of 900 to 1150°C to form a steel plate, and then the steel plate is normalized at a temperature range of 910 to 1000°C. - A method for manufacturing 0.5% Mo steel plate for pressure vessels.

In addition to the constituent features of the first invention, the second invention
: 0.05~0.5%, Ni: 0.05~
0.5%.

Cr: 0.05-0.8%, Ti: 0.0
05-0.08%.

Nb: 0.005-0.08%, V: 0.
tl or 2 selected from the group consisting of 0.005-0.1%
The key point of this manufacturing method is that it uses a rolled material containing more than just seeds.

A third invention provides, in addition to the constituent features of the first invention,
: This manufacturing method has the gist of using a rolled material containing 0.005 to 0.01% as an essential component.

In addition to the constituent features of the invention of '11, the fourth invention provides C.
a: 0.005-0.01% is also included as an essential requirement, Cu: 0.05-0.5%, Nt:
, 0.05-0.5%, Cr: 0.05-0.8
%, Ti: 0.005-0.08%, Nb
: 0.005-0.08%, V: 0.005
This manufacturing method is characterized in that it uses a rolled material containing one or more selected from the group consisting of ~0.1%.

[Function] The present invention is configured as described above, but the point is that high strength and high toughness can be achieved by regulating the content and PCV of each of the above components, as well as regulating the heating temperature and normalizing conditions during rolling. A method for producing a 0.5% Mo@ plate for boilers and pressure vessels which has excellent weldability and has excellent weldability has been realized.

0.5% M for boilers and pressure vessels targeted by the method of the present invention
In the O steel sheet, B is dissolved in the austenite structure,
Since it segregates at grain boundaries and suppresses the occurrence of ferrite transformation, it has the effect of improving the hardenability of steel. On the other hand, B easily combines with N in steel, and when a large amount of N exists in steel, B combines with N to produce BN, reducing the effective amount of B in steel and reducing the hardenability of the steel. do.

In the present invention, since it is necessary to ensure an effective amount of B in the steel in order not to reduce the hardenability of the steel, it was decided to reduce the amount of N itself in the steel and fix it with Sol and AI. However, when the amount of B added is too large, a B compound precipitates at the austenite grain boundaries, resulting in a situation where the hardenability deteriorates. Therefore, in the present invention, taking such points into consideration, B, Sol, and A
The amount of N added was limited to within a predetermined range, and the amount of N was regulated to be below a predetermined amount.

The reasons for limiting each component in the present invention are as follows.

C: 0.01 to 0.12% C needs to be added in an amount of 0.01% or more to ensure the strength of the steel plate. On the other hand, if the amount of C is increased too much, weldability and toughness will decrease, so the amount of C added is 0.12%.
Must be limited to the following.

Si: 0.05 to 1% St is effective for ensuring the strength of steel sheets and improving oxidation resistance, and at least 0.05% is required to exhibit this effect.
It is necessary to contain it. However, if it is contained excessively, the toughness of the steel plate will decrease, so the upper limit needs to be 1%.

Mn: 0.1-2% Mn is effective in improving the hardenability of steel, and must be added in an amount of 0.1% or more. However, if more than 2% is added, the toughness decreases. Therefore, the amount of Mn added in the present invention must be limited to 0.1 to 2%.

Mo: 0.2 to 0.8% MO is an essential element of the steel of the present invention because it improves the hardenability during normalizing, especially when it coexists with B. In the present invention, the amount added must be 0.2% or more so that it can withstand use at high temperatures of about 450°C. However, MO is an expensive element, and the upper limit of its addition amount is set at 0.8% from a practical standpoint.

B: 0.0002 to 0.002% As mentioned above, B is an essential element for improving hardenability during normalizing and obtaining the effect of increasing strength. In the steel of the present invention, in order to achieve the above effects due to B, 0.
It is necessary to add 0002% or more. However, 0
．． If B is added in excess of more than 0.002%, B compounds are generated during normalizing, which reduces hardenability and also deteriorates toughness.

SoL, AI:Q, QO5~0.1% So1. As mentioned above, AI has the effect of fixing N, refining the structure of the steel plate, and increasing toughness, and is an essential element in the steel of the present invention. And its content is o, oo
If the content is less than s%, the above effects will not be achieved, while if the content exceeds 0.1%, it will cause cracks on the surface of the steel ingot. Therefore, in the present invention, the content is 0.
It was limited to a range of 0.005 to 0.1%.

N: 0.007 or less If the N content exceeds 0.007%, BN is likely to be generated, which is effective in developing hardenability of steel, and fi decreases. Therefore, in the present invention, it is necessary to limit the N content to 0.007% or less.

The reasons for limiting each component in the steel of the present invention are as described above, but in the present invention, it is also necessary to limit the PCM to 0.23 or less. That is, PCM is an index indicating the susceptibility to cold cracking during welding, and it is necessary to keep PCM as low as possible in order to lower the preheating temperature during welding.

In order to prevent cracks from occurring even when the preheating temperature is lower than about 50° C., the PCM content in the steel of the present invention is limited to 0.23% or lower. Note that PCM is a value calculated by the following formula.

12V (zero) + 5B (zero) The basic components of the steel to be used in the method of the present invention are as described above, with the exception of Cu, Ni, and Or.

By adding Ti, Nb, V, Ca, etc., the effects of the method of the present invention can be achieved even more effectively. That is, Cu, Ni,
Cr, Ti, Nb, V, etc. are all effective elements for improving the strength of steel sheets, and Ca is an effective element for improving toughness. The detailed reasons for limiting these components will be explained below.

Cu: 0.05-0.5% Ni: 0.05-0.5% Cr: 0.05-0.8% Ti: 0
．． 005-0.08% Nb: 0.005-0.08% V: 0.005-0.01% As mentioned above, these six elements are effective elements for improving the strength of steel sheets, and in that sense as well. Since they are equivalent elements, they each have their own specific effects as described below, but from the perspective of achieving the purpose of the invention, if 1 fit or 221 or more selected from these are combined. , the same effect can be obtained regardless of the selection content.

Cu is effective for solid solution strengthening and precipitation strengthening.

In order to effectively exhibit this effect, the amount of Cu added must be at least 0.05%. However, if added in excess of more than 0.5%, weldability deteriorates. Therefore, Cu
The amount of addition must be in the range of 0.05 to 0.5%.

Ni is effective in increasing the hardenability of steel, and is also effective in preventing hexagonal cracking caused by 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 of Ni. However, since Ni is an expensive element, from a practical standpoint, the upper limit was set at 0.5%.

Cr is effective in ensuring corrosion resistance and strength at high temperatures. In order to effectively exhibit this effect, it is necessary to add at least 0.05%, but since adding too much will deteriorate the toughness, the upper limit was set at 0.8%.

Ti is effective in fixing N up to a high temperature range of 1200°C or higher, but when the amount added is less than 0.005%, the above effect is insufficient; on the other hand, when added in excess of 0.08%, Ti is effective in fixing N. When doing so, the toughness is significantly impaired. Therefore, the amount of Ti added must be in the range of 0.005 to 0.08%.

Both Nb and V are effective in refining crystal grains and improving strength. However, if the amount added is less than 0.005%, these effects are poor; on the other hand, for Nb, 0.08%
When Nb exceeds 0.1%, it not only deteriorates toughness and weldability, but is also unfavorable from an economic point of view. Therefore, the amount of Nb added is 0.1%.
005-0.08%, 0.005-0.1 for ■
% range.

Ca: 0.0005 to 0.01% Ca improves the toughness of the steel plate as described above, improves the toughness of the bond part of the welded joint, and further improves the properties in the thickness direction of the plate. In order to effectively exhibit the above effect, it is necessary to add at least 0.0005% of Ca, but if it is added in excess of Q,01%, the amount of nonmetallic inclusions increases and the ductility decreases. Therefore, in the steel of the present invention, the amount of Ca added must be in the range of o, oos to 0.01%, but the preferable range for maximizing the effect of Ca is 0.0 (15 to 0.0%). It is 007%.

The above-mentioned elements such as Cu, Ni, Cr, Ti, Nb, V, and Ca are added in combination depending on the performance required of the steel sheet, but even when these elements are added, weldability Considering this, it is necessary to suppress the PCM to 0.23% or less.

In the method of the present invention, 0, which has the above-mentioned components,
Using 5% Mo rolled material, 900~l! After hot rolling a steel plate at a heating temperature of 50°C, the steel plate is heated to 910-10
Normalizing is carried out in a temperature range of 00°C, and the reason for regulating the temperature range and normalizing conditions during rolling is as follows.

The present inventors used steel type A (see Table 1 below) that satisfies the composition range specified in the present invention, and investigated the effects of slab heating temperature and normalizing temperature on the mechanical properties of the steel sheet. investigated. The results are shown in FIGS. 1 and 2.

As is clear from the results in Figure 1, the slab heating temperature is 1.
When the temperature exceeds 150°C, the proof stress ps, tensile strength TS, and elongation Ell decrease and their dispersion increases, and at the same time, at 0°C in the 2a+m V Charbi test
The absorbed energy vEo at is also decreasing.

These causes can be considered as follows. That is, in the present invention, in order to effectively utilize the effect of B addition, Ai is added and N is Aj! It is fixed as N, but when the slab heating temperature exceeds 1150°C, part of AILN becomes solid solution, and part of the solid solution N becomes B.
It is considered that B combines with B to produce BN, which impairs the hardenability improvement effect of B addition. The heating temperature during rolling may generally be considered to be the slab heating temperature, but in the case of directly rolling a steel plate from something other than a slab, such as a steel ingot, it means the heating temperature at that time.

As mentioned above, in the present invention, the heating temperature during rolling is 11
Although it is necessary to limit the heating temperature to 50℃ or less,
If the temperature is too low, below 00°C, the hot deformation resistance will increase and the efficiency of rolling will be significantly impaired, so in the present invention, the heating temperature during rolling needs to be in the range of 900 to 1150°C.

-1 As is clear from the results shown in Figure 2, the effect of improving hardenability due to the addition of B can only be achieved by increasing the normalizing temperature to 910°C or higher. The reason for this is that when the normalizing temperature is lower than 910°C, the amount of B dissolved in austenite decreases and the amount of B that segregates to grain boundaries is insufficient, so the addition of Bm has an effect on suppressing ferrite transformation. This is because they are not fully demonstrated. On the other hand, if the normalizing temperature exceeds 1000°C, the crystal grains become coarse and the toughness of the steel sheet deteriorates, which is not preferable. Therefore, in the present invention, the normalizing temperature is limited to 910 to 1000°C.

[Example] Slabs of 11 types of tI4 type A- shown in Table 1 below (
Each f1 steel plate A was obtained using a rolled material) under the manufacturing conditions (slab heating temperature during rolling, normalizing temperature) shown in Table 2.
-1 to A-4, B-1, 8-2, C-, 1, C-2, D
-1, D-2, E-1゜E-2, F-1, F-2, G-
1, G-2, H-1, H-2, I-1, I-2, J-1
, J-2°-1, its mechanical properties and weldability were investigated. The results are also listed in Table 2 along with the plate thickness.

Here, 1lfiA to J satisfy all of the chemical composition ranges specified by the present invention, and are conventional steel types that do not satisfy the chemical composition ranges specified by the present invention. Also steel plate A
-1, A-2, B-1, C-1, D-1, E-1, F-
1,0-1,H-1゜I-1,J-1 is an example obtained under the manufacturing conditions specified in the present invention, and steel plate A-3
, A-4, B-2, C-2, D-2, E-2, F-2°G-2, H-2, 1-2, J=2 was obtained by a method that does not follow the method of the present invention. This is a comparative example (the m-plate -1 does not even satisfy the manufacturing conditions).

For reference, Table 2 also shows the mechanical properties of the samples subjected to SR treatment.

From the results in Tables 1 and 2, the following considerations can be made.

'U4 types A-J aim to achieve low C0 and low PcM, so the preheating temperature for preventing root cracking in the diagonal Y-shaped weld cracking test is as low as 25°C or less. In contrast,
Steel f! contains a large amount of C, so its PCV is high,
The preheating temperature is as high as 150°C.

On the other hand, even if the chemical components are within the range specified by the present invention, the mechanical properties obtained are largely influenced by the manufacturing conditions. That is, steel plates A-1 and A that satisfy all the requirements of the present invention
-2,B-1,C-1゜D-1,E-1,F-1,G-
1, H-1, I-1, and J-1, a tensile strength TS of 50 kgf/mm' or more was obtained even after SR treatment at 625°C , elongation E) and impact properties (absorbed energy vEo at 0°C in 2 mm V Charpy impact test, 50
% ductile fracture surface transition temperature v Trs) is also good. On the other hand, steel sheets A-3 and C-
2, D-2, J-2 and steel plates A-4, B-2゜E-2, F-2, G-2, H-2 that adopted high slab heating temperature
, I-2, 48 kgf/+m even after normalizing
A tensile strength TSL of less than 2 was not obtained.

[Effects of the Invention] As described above, according to the present invention, by adopting the above-described configuration, a method for manufacturing a vine that maximizes the hardenability improvement effect of B in a 0.5% MO steel sheet is realized. did it.

[Brief explanation of drawings]

FIG. 1 is a graph showing the effect of slab heating temperature on the mechanical properties of a steel plate, and FIG. 2 is a graph showing the effect of normalizing temperature on the mechanical properties of a steel plate.

Claims (4)

[Claims] (1) C: 0.01 to 0.12% (weight%, same below) Si:
0.05-1% Mn: 0.1-2% Mo: 0.2-0.8% B: 0.0002-0.002% Sol. Al: 0.005-0.1% is an essential component, N is limited to 0.007% or less, the remainder is Fe and unavoidable impurities, and P_C_M=C(%)+[1/ 30] Si (%) + [1
/20]Mn (%) + [1/20]Cu (%) + [1/
10] Using a rolled material with a value calculated by V (%) + 5B (%) of 0.23% or less, hot rolling the rolled material at a heating temperature of 900 to 1150 ° C. to make a steel plate, A method for producing a 0.5% Mo steel plate for boilers and pressure vessels with excellent weldability, which comprises normalizing the steel plate at a temperature in the range of 910 to 1000°C. (2) C: 0.01-0.12% Si: 0.05-1% Mn: 0.1-2% Mo: 0.2-0.8% B: 0.0002-0.002% Sol ．． Al: 0.005-0.1% is an essential component, Cu: 0.05-0.5% Ni: 0.05-0.5% Cr: 0.05-0.8% Ti: 0 .005~0.08% Nb: 0.005~0.08% V: 0.0053~0.1% Contains one or more types selected from the group consisting of N
is limited to 0.007% or less, the remainder is Fe and unavoidable impurities, and P_C_M=C(%)+[1/30]Si(%)+[1
/20]Mn (%) + [1/20]Cu (%) + [1/
60] Ni (%) + [1/20] Cr (%) + [1/1
5] Using a rolled material whose value calculated by Mo (%) + [1/10] V (%) + 5B (%) is 0.23% or less, the rolled material is heated at a heating temperature of 900 to 1150 ° C. A method for producing a 0.5% Mo steel plate for boilers and pressure vessels with excellent weldability, which comprises hot rolling the steel plate and then normalizing the steel plate at a temperature range of 900 to 1000°C. (3) C: 0.01-0.12% Si: 0.05-1% Mn: 0.1-2% Mo: 0.2-0.8% B: 0.0002-0.002% Sol ．． Al: 0.005 to 0.1% and Ca: 0.0005 to 0.01% are essential components, and N is limited to 0.007% or less, the remainder being Fe and inevitable impurities, And P_C_M=C(%)+[1/30]Si(%)[1/
20] Mn (%) + [1/20] Cu (%) + [1/6
0] Ni (%) + [1/20] Cr (%) + [1/15
] Mo (%) + [1/10] V (%) + 5B (%) Using a rolled material whose calculated value is 0.23% or less, heat the rolled material at a heating temperature of 900 to 1150 ° C. A method for producing a 0.5% Mo steel plate for boilers and pressure vessels with excellent weldability, which comprises rolling the steel plate into a steel plate and then normalizing the steel plate at a temperature range of 910 to 1000°C. (4) C: 0.01-0.12% Si: 0.05-1% Mn: 0.1-2% Mo: 0.2-0.8% B: 0.0002-0.002% Sol ．． Al: 0.005-0.1% and Ca: 0.0005-0.01% are essential components, Cu: 0.05-0.5% Ni: 0.05-0.5% Cr: 0.05-0.8% Ti: 0.005-0.08% Nb: 0.005-0.08% V: 0.005-0.1% One or two selected from the group consisting of: Including the above, N
is limited to 0.007% or less, the remainder is Fe and unavoidable impurities, and P_C_M=C(%)+[1/30]Si(%)+[1
/20]Mn (%) + [1/20]Cu (%) + [1/
60] Ni (%) + [1/20] Cr (%) + [1/1
5] Using a rolled material with a value calculated by Mo (%) + [1/10] V (%) + 5B (%) of 0.23% or less, heat the rolled material at a heating temperature of 900 to 1150 ° C. A method for producing a 0.5% Mo steel plate for boilers and pressure vessels with excellent weldability, which comprises rolling the steel plate into a steel plate and then normalizing the steel plate at a temperature range of 910 to 1000°C.