JPH09256038A - Heat treatment before stress relieving annealing treatment for thick steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a heat treating method before stress relieving annealing treatment for a thick steel plate (having >=50mm plate thickness in particular) excellent in mechanical properties and used for structures requiring stress relieving annealing treatment such as pressure vessels. SOLUTION: A steel plate contg., by weight, 0.05 to 0.20% C, 0.02 to 0.5% Si, 0.2 to 2.0% Mn and 0.005 to 0.10% Al, furthermore contg., at need, one or >= two kinds among Cu, Ni, Cr, Mo, V, Nb, Ti, Ca and rare earth elements, and the balance iron with inevitable impurities is heated at the Ac1 to the Ac3 transformation point and is subsequently subjected to gradual cooling treatment before stress relieving annealing treatment for forming the plate into a structural member.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thick steel plate having excellent mechanical properties (in particular, a plate thickness of 5) used for a structure requiring stress relief annealing (hereinafter abbreviated as PWHT) such as a pressure vessel.
0 mm or more) prior to PWHT.

[0002]

2. Description of the Related Art When manufacturing a structure such as a reaction vessel, a steel plate is formed by hot working in the production of a mirror plate, etc.
Since WHT is often carried out, it is important to secure strength and toughness after treatment. Since the material properties after PWHT largely depend on its chemical composition and microstructure, steel sheets having excellent strength and toughness after PWHT by optimizing alloying elements and applying thermomechanical treatment technology during steel sheet manufacturing have been conventionally used. Have been proposed. The following are some examples.

Japanese Patent Laid-Open No. 59-232234 discloses that
After hot rolling a steel containing C: 0.03 to 0.30%, Mn: 0.2 to 2.0%, and further satisfying C + Mn / 9.11 ≧ 0.26%, Ar ₃ or more points From the temperature of 500 ℃
50 for stress relief annealing characterized by controlled cooling at a cooling rate of 3 to 30 ° C./s to a temperature of less than 250 ° C. or higher.
A method for manufacturing a kilo steel is disclosed, and JP-A-62-474 is also disclosed.
No. 30 publication, C: 0.02 to 0.20%, Mn:
A step of heating a steel slab containing 0.50 to 2.5% to a temperature of Ac ₃ transformation point to 1250 ° C., and after the heating,
A step of rolling at a rolling reduction of 30% or more at a temperature of Ar ₃ transformation point to (Ar ₃ transformation point + 100 ° C.), and after the rolling, (α
+ Γ) In the two-phase region, a finish rolling with a rolling reduction of 5 to 60% and a finishing temperature of Ar ₃ transformation point to (Ar ₃ transformation point −80 ° C.) and 1 ° C./s after the finishing rolling. Disclosed is a method for producing a high-strength steel for stress relief annealing, which comprises the step of cooling to 600 ° C. or lower at the above cooling rate.

Further, in JP-A-62-93312, C: 0.02 to 0.18%, Si: 0.03 to 0.
60%, Mn: 0.5 to 2.5%, soluble Al: 0.0
05-0.06%, Nb: 0.005-0.05%, C
u: 0.05 to 0.7%, Ni: 0.05 to 0.7%, a step of heating a steel piece containing Ac ₃ transformation point to 1250 ° C., and Ar ₃ transformation after the heating. Rolling with a rolling reduction of 30% or more in the temperature range of the point to (Ar ₃ transformation point + 100 ° C), and immediately after the rolling, up to an arbitrary temperature of 600 ° C or less at a cooling rate of 1 to 30 ° C / s. Disclosed is a method for producing a high-strength steel material for stress relief annealing excellent in weldability and low-temperature toughness, characterized by comprising a step of cooling,
Further, in JP-A-62-240713, C: 0.0
2 to 0.18%, Si: 0.03 to 0.60%, Mn:
0.5-2.5%, Sol. Al: 0.005-0.0
6%, Nb: 0.005-0.03%, B: 0.000
A high-strength steel containing 3 to 0.002%, Ti: 0.005 to 0.02%, and satisfying the C equivalent Ceq: C + Mn / 6 ≦ 0.38 was prepared in a temperature range of Ac ₃ temperature to 1050 ° C. After heating, the steel sheet is rolled at a reduction rate of 30% or more in the austenite non-recrystallization temperature range of Ar ₃ temperature to (Ar ₃ temperature + 100 ° C), and immediately 600 ° C at a cooling rate of 1 to 10 ° C / s.
Plate thickness 50 characterized by forced cooling to the following temperatures
Disclosed is a method for producing an extra-thick high-strength steel sheet containing bainite of mm or more and having excellent weldability and low-temperature toughness and suitable for stress relief annealing.

[0005]

SUMMARY OF THE INVENTION These prior arts are:
By applying the alloying element to be added and forced cooling after hot rolling to optimize the microstructure, the mechanical properties after PWHT are improved. Therefore, as mentioned above, after the steel material is shipped, if the hot working or warm working is secondarily performed again such as spinning to make the structure at the customer's side, the effect is burned out. There was a risk of doing it.

[0006]

According to the present invention, a PWHT is subjected to a simple heat treatment after hot working before PWHT.
The purpose was to prevent the deterioration of the mechanical properties of the steel sheet after HT, and the gist thereof is as follows. (1) C: 0.05 to 0.20% by weight, Si:
0.02-0.5%, Mn: 0.2-2.0%, Al:
A steel sheet containing 0.005 to 0.10% and the balance being iron and unavoidable impurities is heated between the Ac ₁ to Ac ₃ transformation points before being subjected to stress relieving annealing for forming a structural member, and then gradually cooled. A heat treatment method before stress-relief annealing treatment of a thick steel sheet, which is characterized by performing a treatment.

(2) C, which is a group of strength improving elements, in weight%
u: 0.1 to 1.5%, Ni: 0.1 to 2.0%, C
r: 0.1 to 1.0%, Mo: 0.05 to 0.50%,
V: 0.005 to 0.10%, Nb: 0.005 to 0.
A heat treatment method before stress relieving annealing treatment of a thick steel sheet, characterized by using the steel sheet according to the above (1), which contains one or more of 0.05% and Ti: 0.005 to 0.04%.

(3) Ca: 0.001 to 0.010%, which is a group of inclusion controlling elements, and rare earth element: 0.0, by weight.
A heat treatment method before stress relieving annealing of a thick steel sheet, which comprises using the steel sheet according to the above (1) or (2) containing one or two of 1 to 0.10%. (4) Any of the above items (1) to (3), characterized in that hot working or warm working for forming a structural member is performed before heating and cooling after heating between Ac ₁ to Ac ₃ transformation points. The heat treatment method before the stress relief annealing treatment of the thick steel sheet according to Item 1.

[0009]

DETAILED DESCRIPTION OF THE INVENTION Generally, when a low alloy steel is PWHT, its mechanical properties deteriorate. This is because the heat treatment for a long time softens the ferrite material to lower the strength and also causes the coarsening of the cohesive carbides to lower the toughness. Therefore, in order to prevent deterioration of mechanical properties after PWHT, it is necessary to refine the structure and strengthen the ferritic matrix before PWHT.

The inventors of the present invention prevent the deterioration of mechanical properties after PWHT carried out after secondary hot working (a warm working at a relatively low temperature is acceptable) for forming a structure. In addition, from the viewpoint of optimizing the structure, a simple heat treatment
We have found a heat treatment method that can prevent the deterioration of mechanical properties after PWHT by adding before HT. 1 and 2
Is 0.13% C-0.44% Si-1.46% Mn-
0.18% Cu-0.42% Ni-0.02% V-0.
5% for 02% Nb-0.008% Ti steel (plate thickness 60 mm)
% Hot bending, then heat at the temperature shown on the horizontal axis and hold for 1 hour, then PWH for 10 hours at 625 ° C.
The tensile strength and the Charpy absorbed energy in -45 degreeC when T processing is implemented are shown. As is clear from FIGS. 1 and 2, it is clear that when PWHT is performed after heating between the Ac ₁ to Ac ₃ transformation points, the tensile strength and toughness are obviously improved.

FIG. 3 shows the main microstructure after heat treatment. When the heat treatment temperature is 900 ° C. above the Ac ₃ transformation point, a typical ferrite-pearlite structure is exhibited,
When the heat treatment temperature is below the Ac ₃ transformation point (800 ° C),
The ferrite becomes finer and the carbides are finely dispersed. That is, due to the effects of both of them, the deterioration of mechanical properties after PWHT as shown in FIG. 1 can be prevented.

Hereinafter, the present invention will be described in detail. In the present invention, the starting material is melted in an electric furnace, a converter, etc.,
Basically, through continuous casting or ingot making / separating process,
C: 0.05 to 0.20%, Si: 0.02 to 0.5
%, Mn: 0.2 to 2.0%, Al: 0.005 to 0.
It is a slab containing 10%. The reasons for limiting the chemical composition will be described below.

C: C is an element necessary for obtaining the strength of the member itself (mother steel plate) obtained by processing a steel material, and is a steel plate having a plate thickness of 50 mm or more intended by the present invention and having a tensile strength of 4
In order to achieve 0 Mpa or more, it is necessary to add 0.05% or more. On the other hand, if C is added in an amount of more than 0.20%, the carbide becomes coarse, and the effect of fine dispersion of the carbide according to the present invention cannot be obtained.

Si: Si is an element necessary as a deoxidizing element for steelmaking, and it is necessary to add 0.02% or more to the steel, but if it exceeds 0.5%, the mother steel plate after PWHT and the welding heat effect are affected. Reduces the toughness of the part. Mn: Mn is an element necessary to secure the strength and toughness, but if it exceeds 2.0%, the toughness is significantly impaired, and conversely, it is 0.
If it is less than 2%, it is difficult to secure the strength of the mother steel sheet after PWHT, so the range is made 0.2 to 2.0%.

Al: Al is an element which is added as a deoxidizer and at the same time has an effect of reducing the crystal grain size.
It is necessary to add 0.005% or more. On the other hand, 0.10
If Al is added in excess of%, coarse alumina is produced,
Inhibits toughness. It should be noted that although not particularly limited, if P is added in a large amount as a grain boundary segregation element, it impairs the toughness of the base steel plate and the weld heat affected zone. Therefore, the lower the addition amount, the better, but generally it is preferably 0.04% or less.

In the present invention, as elements for improving strength and toughness, Cu, Ni, Cr, Mo, V,
One or more elements of Nb and Ti can be added. Cu: Cu is an element effective in increasing the strength without lowering the toughness, but if it is less than 0.1%, it has no effect, and if it exceeds 1.5%, it heats during billet heating or welding. Makes it easier for cracks to occur. Therefore, the Cu content is set to 0.1 to 1.5%.

Ni: Ni is an element effective in improving toughness and strength, and it is necessary to add 0.1% or more to obtain the effect, but if it exceeds 2.0%, weldability is improved. Therefore, the range is set to 0.1 to 2.0%. Cr: Cr is an element effective for improving the strength of steel by precipitation strengthening, and in order to obtain the effect, 0.1% or more is required to be added. On the other hand, if a large amount of Cr is added, the hardenability is increased and a bainite structure is generated to reduce the toughness, so the upper limit is made 1.0%.

Mo: Mo is an element that improves hardenability and at the same time forms carbonitrides to improve strength. To obtain this effect, addition of 0.05% or more is necessary. Addition of a large amount brings about not only strengthening more than necessary but also a remarkable decrease in toughness, so the range is set to 0.05 to 0.5.
0%. V: V is an element that forms carbides and nitrides and is effective in improving strength. Addition of less than 0.005% has no effect, and addition of more than 0.10% adversely affects toughness. Since it causes a decrease, the range is made 0.005 to 0.10%.

Nb: Nb is also an element that forms carbonitrides and is effective in improving strength. However, addition of less than 0.005% has no effect, and addition of more than 0.05% has the opposite effect. Since the toughness is reduced, the range is 0.005
0.05%. Ti: Ti is an element that can be expected to be effective in forming nitrides and making crystal grains finer. However, addition of a large amount causes a significant decrease in toughness due to the formation of carbides, so its upper limit is set to 0.
It needs to be 04%. In order to obtain the desired effect, 0.005% or more must be added, so the range of Ti is 0.005 to 0.04%.

Further, in the present invention, Ca and rare earth elements can be added for the purpose of controlling inclusions. Ca and rare earth elements (REM): Ca and REM
(For example, Ce and the like) is effective in improving the toughness in the direction perpendicular to the rolling direction by fixing S in the steel as sulfide such as CaS and suppressing the generation of MnS that inhibits the toughness. Ca is 0.001% or more, REM is 0.0
It is necessary to add 1% or more, but excessive addition causes inclusions in the steel to increase and lowers cleanliness. Therefore, the respective upper limits are 0.010% for Ca and 0.10% for REM. And

P and S: In the present invention, although not particularly specified, both are elements that affect the toughness of the steel, and when P is added in excess of 0.04% and S in excess of 0.03%, the toughness is increased. Since it remarkably inhibits, it is preferable to set this to the upper limit. The method for producing the steel sheet is not particularly limited, and for example, a steel slab having the above chemical composition is formed into a steel sheet having a thickness of 50 mm or more by ordinary heating and hot rolling. These steel sheets may be subjected to quenching and tempering treatment or normalizing treatment depending on the application. When these steel plates are processed as structural members, hot working such as hot spinning is performed again, but thereafter, Ac _{1 to} Ac ₃
A heat treatment for heating between transformation points and gradually cooling (any means such as air cooling or steam cooling as long as it does not generate martensite or bainite structure) is added.

This heat treatment is the main part of the present invention, and as described above, it aims to refine the ferrite grains and finely disperse the carbides, and to minimize the deterioration of strength and toughness during the subsequent PWHT. It is to suppress.

[0023]

Next, an embodiment of the present invention will be described. A steel sheet having the chemical composition shown in Table 1 and Table 2 (continued from Table 1) is used as a mother steel sheet, and after the hot working (hot rolling) shown in Table 3 and Table 4 (continued from Table 3) on the steel sheet, before PWHT. Was subjected to a predetermined heat treatment and then subjected to PWHT to determine the tensile strength using the tensile strength in the tensile test as an index, and the toughness using the transition temperature in the Charpy test as an index. The results are shown in Tables 3 and 4 as the difference from the mother steel plate before PWHT.

Reference numerals 1, 3, 4, 5, 5, 6, 8, 9, 10,
Reference numerals 11 and 12 represent examples of the present invention. As is clear from Tables 3 and 4, even after PWHT, no reduction in strength was observed from the mother steel sheet, and at the same time, no reduction in toughness was observed. On the other hand, reference numerals 2, 7, 13,
14, 15, 16, and 17 show comparative examples that deviate from the present invention.

That is, reference numeral 2 indicates that the chemical composition is within the range of the present invention, but the heat treatment conditions are 940 ° C., which exceeds the Ac ₃ transformation point. Therefore, the tensile strength and toughness after PWHT are lower than those of the mother steel sheet. Reference numeral 7 indicates that the heat treatment was performed at a temperature lower than the Ac ₁ transformation point.
In this case as well, the tensile strength and toughness are significantly lower than those of the mother steel sheet before PWHT.

Further, reference numerals 13 to 17 are examples in which the chemical composition deviates from the scope of the present invention. Code 13 (Steel J)
Indicates that C is added by 0.25% and exceeds the upper limit of the range of the present invention. Therefore, the tensile strength is not reduced, but the toughness is reduced.

Reference numeral 14 (Steel K) is an example in which Mn was added in excess of the upper limit of the range of the present invention, and although the tensile strength after PWHT was not reduced, the toughness was significantly reduced.
Reference numeral 15 (Steel L) is V added in an amount exceeding the upper limit of the range of the present invention. In this case, the toughness after PWHT is significantly reduced. Reference numeral 16 (Steel M) is an example in which Nb was added exceeding the upper limit of the range of the present invention. In this case, P
The tensile strength and toughness after WHT are reduced.

Reference numeral 17 (Steel N) is an example in which Al was added in excess of the range of the present invention. In this case, the toughness after PWHT is reduced.

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

[Table 3]

[0032]

[Table 4]

[0033]

EFFECTS OF THE INVENTION PW of steel materials limited to the chemical composition of the present invention
By applying the heat treatment of the present invention before HT, the plate thickness 5
It became possible to prevent the decrease in strength and toughness after PWHT, which was observed in a steel plate having a thickness of 0 mm or more, and the reliability of the structure using the steel plate member thus obtained could be significantly improved.

[Brief description of drawings]

1 is a heat treatment before PWHT in the present invention is PWHT
It is a figure which shows the influence which acts on the tensile test strength after that.

FIG. 2 shows that the heat treatment before PWHT in the present invention is PWHT.
It is a figure which shows the influence which acts on the toughness after that.

FIG. 3 is a diagram showing the influence of the heat treatment temperature before PWHT on the microstructure of a steel sheet in the present invention.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location C22C 38/06 C22C 38/06 38/58 38/58

Claims (4)

[Claims] 1. By weight%, C: 0.05 to 0.20%, Si: 0.02 to 0.5%, Mn: 0.2 to 2.0%, Al: 0.005 to 0. A steel sheet containing 10% and the balance being iron and unavoidable impurities is characterized by being subjected to a slow cooling treatment after heating between the Ac ₁ to Ac ₃ transformation points before the stress relief annealing treatment for forming a structural member. Heat treatment method before stress relief annealing of thick steel plate. 2. Cu, which is a group of strength improving elements, in% by weight:
0.1 to 1.5%, Ni: 0.1 to 2.0%, Cr: 0.1 to 1.0%, Mo: 0.05 to 0.50%, V: 0.005 to 0. 10%, Nb: 0.005-0.05%, Ti: 0.005-0.04%, The steel plate of Claim 1 containing 1 type, or 2 or more types, The thick steel plate characterized by the above-mentioned. Treatment method before stress relieving annealing of. 3. C, which is a group of inclusion controlling elements, in% by weight.
a: 0.001 to 0.010%, rare earth element: 0.01 to 0.10%, one or two
A heat treatment method before stress relieving annealing of a thick steel plate, characterized in that the steel plate according to claim 1 or 2 containing a seed is used. 4. The hot working or warm working for forming a structural member is performed after heating between the Ac ₁ to Ac ₃ transformation points and before cooling treatment. The heat treatment method before stress-relief annealing of the thick steel plate according to Item 1.