JPH09184043A - Low alloy heat resistant steel excellent in high temperature strength and weldability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a low alloy heat resistant steel excellent in weldability as well as in high temp. strength and useful, e.g. for chemical reaction vessels, industrial non-heat-resistant members, and further various high temp. equipment parts while obviating the necessity of the addition of large amounts of alloying elements. SOLUTION: This steel has a composition which consists of, by weight, 0.05-0.25% C, 0.1-1.0% Si, 0.2-2.0% Mn, 0.01-0.5% Cu, 0.01-1.5% Ni, 0.01-0.7% Cr, 0.01-0.7% Mo, 0.001-0.07% V, 0.002-0.01% N, one or >=2 kinds among 0.001-0.025% Al, 0.001-0.05% Nb, and 0.001-0.025% Ti, and the balance Fe with inevitable impurities and in which the value of N*, defined by N*=N-(0.4Al +0.15Nb+0.3Ti), is regulated to 0.0005-0.005%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to pressure-resistant members such as various chemical reaction vessels used in petroleum refining and chemical industries, industrial non-pressure-resistant members such as converter iron shells and zinc kettles, and further The present invention relates to a low alloy heat resistant steel which is useful for various high temperature equipment parts and has excellent high temperature strength and good weldability.

[0002]

2. Description of the Related Art In chemical reaction vessels, industrial non-pressure resistant members, parts of various high-temperature equipment, etc., equipment and devices are becoming large in size from the viewpoint of improving reaction and work efficiency. For this reason, the plate thickness of the low alloy heat resistant steels used for them is increasing. In order to cope with such an increase in plate thickness, for example, JP-A-6-256893 and JP-A-61-52.
As disclosed in Japanese Patent Publication No. 354, Japanese Patent Application Laid-Open No. 57-140855, Japanese Patent Publication No. 56-31868, and the like,
The amount of addition of alloying elements such as Ni, Cr, Mo and V tends to increase more and more.

However, although such addition of a large amount of alloying elements has the effect of significantly improving high temperature strength,
Not only does the price of steel rise, but the weldability of steel is impaired.

[0004]

The present invention is a low alloy heat resistant steel having excellent high temperature strength and good weldability, which is useful for chemical reaction vessels, industrial non-pressure resistant members, and parts of various high temperature equipment. Is provided without depending on the large addition of alloying elements.

[0005]

[Means for Solving the Problems] The present inventors have conducted various studies on the influence of various elements on the high-temperature strength of low alloy heat-resistant steel, and have determined the appropriate amounts of N addition and Al, Ti, Nb addition. It has been found that by controlling the temperature, excellent high temperature strength can be obtained, and a low alloy heat resistant steel having good weldability can be obtained.

The present invention has been made on the basis of this finding. (1) C: 0.05 to 0.25% Si: 0.1 to 1.0% Mn: 0.2 2.0% Cu: 0.01-0.5% Ni: 0.01-1.5% Cr: 0.01-0.7% Mo: 0.01-0.7% V: 0.001- 0.07% N: including 0.002 to 0.01%, Al: 0.001 to 0.025% Nb: 0.001 to 0.05%, Ti: 0.001 to 0.025%, or comprises two or more, and wherein the result from the remaining Fe and inevitable ^{impurities, N * = N- (0.4Al +} 0.15Nb + 0.3Ti) N * as defined is 0.0005 to 0.005% A low alloy heat resistant steel with excellent high temperature strength and good weldability.

Hereinafter, the present invention will be described in more detail.

0.15% C-0.25% Si-0.75%
Mn-0.1Cu-0.05Ni-0.1% Cr-0.25
In a steel having a chemical composition of% Mo-0.01V, A
1: 0.001 to 0.025%, Nb: 0.001 to 0.0
5%, Ti: 0.001 to 0.025% and N: 0.0
Al, Nb, Ti and N in the range of 005 to 0.01%
After varying the amount and hot rolling to a thickness of 25 mm, the material was held at 930 ° C. for 1 hour and then normalized. Tensile test pieces were machined from these steel plates and then JIS Z2241 or JI
A tensile test was performed at room temperature and 400 ° C. according to SZ0567.

FIG. 1 is a diagram showing the relationship between the ratio R of tensile strength TS (RT) at room temperature to tensile strength TS (400) at 400 ° C. and N ^* . Here, R = TS (400) / TS (RT) N ^* = N- (0.4Al + 0.15Nb + 0.3Ti).

As shown in FIG. 1, when N ^* is less than 0.0005%, R is as small as less than 0.7. N ^* is 0.
R increases at 0005% or more, and R becomes 1 or more when N ^* exceeds 0.0005%. N ^* is R by higher but further increased, increase of R is saturated when N ^* is more than 0.005%, even if the N ^* is increased beyond this improvement in R is no longer observed. Thus, N ^* is 0.0005% or more,
In the range of 0.005% or less, the high temperature tensile strength is remarkably improved with respect to the room temperature strength.

The reason for this is considered as follows. Generally, the precipitation of carbonitrides tends to coarsen when the amount of the metal element constituting the carbonitride is atomic% and exceeds the amount of carbon or nitrogen. Therefore, if N ^* is too small with respect to Al, Nb, and Ti, Nb and Ti nitrides, which are expected to precipitate during a high temperature tensile test and increase the high temperature strength of the steel material, become coarse, and the strengthening effect Is lost. N ^* is 0.000
In the range of 5% to 0.005%, the larger the N ^* is, the more effectively the coarsening of the nitride is suppressed. However, when N ^* is 0.005%, the effect is saturated, and the N * of more than this is saturated. Increasing does not improve R.

Generally, the weldability of steel materials is arranged by the carbon equivalent (Ceq) calculated by Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14, The larger this value, the lower the weldability. On the other hand, Al, N
Control of N ^* by b and Ti does not affect Ceq. That is, the steel of the present invention can improve high-temperature strength without impairing weldability, and can provide a steel material having excellent high-temperature strength and good weldability.

The reasons for limiting the other constituent elements will be described below.

C is an element effective for increasing the strength of the steel material, and in the case of the steel material of the present invention, addition of 0.05% or more is preferable. However, if the addition amount is too large, the weldability is impaired, so the upper limit of the addition amount is set to 0.25%.

Si is added in an amount of 0.1% or more for deoxidation, but if the addition amount is large, the weldability and toughness deteriorate, so the upper limit is made 1.0%.

Mn is added in an amount of 0.2% or more in order to increase the strength of the steel material, but if it exceeds 2.0%, the striped structure of ferrite and pearlite (or bainite) becomes strong and the anisotropy of toughness increases. , 0.2 to 2.0%.

Cu is an element having the effect of increasing the strength without impairing the toughness of the steel material. Therefore, 0.01
% Or more is added. However, if added in a large amount, the hot workability is impaired, so the upper limit of the added amount is 0.5%.

Ni is an element that improves the toughness of steel materials, and is added in an amount of 0.01% or more. However, if it exceeds 1.5%, this effect tends to be saturated, and the upper limit of addition is 1.5%.
And

Cr has the effect of increasing strength. Therefore, 0.01% or more is added. However, if over 0.7% is added, the weldability and toughness deteriorate, so the upper limit is made 0.7%.

Like Cr, Mo is an element whose strength, especially high temperature strength, increases with addition, and is added in an amount of 0.01% or more. However, the addition of more than 0.7% increases the cost, so the upper limit is made 0.7%.

V, like Cr and Mo, also has the effect of improving the strength of the steel material. Therefore, 0.001% or more is added. However, if it exceeds 0.07%, the toughness is adversely affected, so the upper limit is made 0.07%.

N becomes a nitride in the steel and is effective for refining the structure and strengthening the steel, and is added in an amount of 0.002% or more. However, if the addition amount exceeds 0.01%, the weldability deteriorates, so the addition amount is set to 0.002% to 0.01%.

Al is an element useful for deoxidizing and refining the structure, and is related to the improvement of strength by the addition of Nb and Ti. Therefore, 0.001% or more is added. However, if the addition amount exceeds 0.025%, the effect is saturated and the risk of impairing the toughness is increased, so the addition amount is set to 0.001% to
The amount is 0.025%.

Nb forms carbonitrides and improves the strength of steel. Therefore, 0.0001% or more is added as needed, but if the content exceeds 0.05%, the effect is saturated, so the addition amount is suppressed to 0.05% or less.

Ti also forms carbonitrides and improves the strength of steel. Therefore, 0.0001% or more is added as needed, but the effect is saturated if it exceeds 0.025% as with Nb, so the addition amount is suppressed to 0.025% or less.

The effect of the present invention can be maintained even by adding elements such as Ca, REM, Y, and Mg, which have the function of suppressing the morphology of inclusions. There is no.

Next, the manufacturing conditions of the material will be described.

The steel having the above-mentioned chemical composition is obtained by smelting in a converter or an electric furnace, then subjecting it to ladle refining or vacuum degassing treatment, and is made into a slab by continuous casting. Casting may be performed by a conventional mold or a unidirectionally solidifying mold, and in this case, a slab is formed by slabbing. Even if it is a continuous casting slab, lumping may be performed as needed. The soaking in the mass may be any. That is, the steel ingot may be cooled and then soaked, or the hot ingot may be charged into the soaking furnace. The soaking temperature is preferably 1000-1320 ° C.

Any amount of shape deformation such as reduction in rolling may be used, and the effect of the steel of the present invention is not impaired. After rolling, it is heated to the Ac ₃ temperature or higher and cooled by cooling. The effect of the present invention can be recognized even in the structure after air-cooling after rolling, and the normalizing can be omitted. The form of the steel material may be any of a thick steel plate, a steel strip (hot rolling or cold rolling), a steel pipe and the like.

[0030]

BEST MODE FOR CARRYING OUT THE INVENTION

(Example) Using steel having the chemical composition shown in Table 1, Table 2
It hot-rolled to the plate thickness shown inside. Normalized as-rolled or at the temperature in Table 2.

Round bar tensile test pieces having diameters and lengths of parallel portions cut out from these steel materials of 6 mm and 30 mm were used, and the tensile strength at room temperature and the high temperature tensile strength at the temperatures shown in Table 2 were measured in accordance with JISZ2241 or JISZ0567. A test was conducted to determine the tensile strength.

Table 2 shows these values and R values (normal temperature tensile strength / high temperature tensile strength). Steel materials 1A-8A
In the A series steel, the chemical composition is within the range of the present invention, the R value is 1.0 or more, and the high temperature strength is excellent.

On the other hand, in the B series steel materials of 1B to 8B, N ^* is outside the scope of the present invention, the R value is smaller than 0.7, and the high temperature strength is poor.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

The steel material according to the present invention has a carbon equivalent (Ce
The high temperature strength can be improved without increasing q), the weldability is good, and the high temperature strength is excellent. For this reason, it can be used for pressure resistant materials such as various chemical reaction vessels used in petroleum refining and chemical industries, industrial non-pressure resistant materials such as converter iron shells and zinc kettles, and parts for various high temperature equipment. It is suitable and has great industrial value.

[Brief description of the drawings]

1] N ^* = N- (0.4Al + 0.15Nb + 0.3T)
i) and the ratio of tensile strength TS (RT) at room temperature to tensile strength TS (400) at 400 ° C. R = RS (400) / TS
It is a figure which shows the relationship with (RT).

Claims (1)

[Claims] 1. By weight%, C: 0.05-0.25% Si: 0.1-1.0% Mn: 0.2-2.0% Cu: 0.01-0.5% Ni: 0.01 to 1.5% Cr: 0.01 to 0.7% Mo: 0.01 to 0.7% V: 0.001 to 0.07% N: 0.002 to 0.01% , Al: 0.001 to 0.025%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.025%, containing one or more kinds, and consisting of residual Fe and unavoidable impurities. , N ^* = N- (0.4Al + 0.15Nb + 0.3Ti), N ^* is 0.0005 to 0.005%, low alloy heat resistance with excellent high temperature strength and good weldability. steel