JPH09263830A - Production of alloy steel tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To complete the isothermal transformation treatment, which is operated so far by off-line, instead by in-line by means of heating and cooling directly after hot working. SOLUTION: A steel billet, having a composition consisting of 0.05-0.35% C, 0.02-0.60% Si, 0.3-1.8% Mn, 0.2-10.0% Cr, 0.2-1.5% Mo, and the balance Fe with inevitable impurities, is used. This steel billet is heated to 1050-1280 deg.C, subjected, by the use of a piercer, a mandrel, and a sizer, to piercing and to hot rolling at >=750 deg.C finishing temp., reheated to 800-1050 deg.C, cooled down to an isothermal temp. at <=5 deg.C/sec cooling rate, subjected to isothermal treatment at 650-780 deg.C for 0.25-2.0hr, and air-cooled.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel pipe for a boiler having Cr-Mo steel as a basic component, which has a strength and ductility equivalent to those of conventional heat-treated steel by combining hot working and heat treatment. The present invention relates to a method for manufacturing an alloy steel pipe.

[0002]

2. Description of the Related Art Boilers for thermal power generation use various types of heat-resistant steel pipes such as water wall pipes, superheater pipes, reheater pipes, economizer pipes, feed water heater pipes, and main steam pipes. However, all of them require performance such as high temperature strength, oxidation resistance, and corrosion resistance. In order to satisfy these performances, the heat resistance is secured by Cr, and the performance is secured by utilizing solid solution strengthening by Mo, N, Nb and dispersion strengthening by carbide of each element.

As a conventional method for producing a heat-resistant steel pipe, after finishing to a predetermined shape by hot or cold working, heat treatment is performed to adjust the strength and the microstructure. As a heat treatment method for tempering, the boiler steel pipe added with Cr, Mo, N, Nb, etc. is heated to Ac ₃ point or higher to austenite, and then cooled to a certain temperature of Ae ₁ point or lower, and brought to that temperature. A constant temperature transformation process is applied to transform while keeping it. Generally, the room temperature strength after the isothermal transformation treatment is proof stress 250 to 350 N / mm ² , tensile strength 45.
It is required to be 0 to 550 N / mm ² .

In the production of the above boiler steel pipe, the finish processing timing of the steel pipe and the heat treatment for heat treatment are often inconsistent, the waiting time becomes long, and heating from room temperature is required during the heat treatment for heat treatment. Combined with the isothermal transformation treatment after heating, the treatment time is long and the efficiency is low.
There is a problem in that after the heat treatment for heat treatment, descaling treatment such as pickling or grinder is required. Especially,
The scale generation in the heat treatment can be solved by performing the heat treatment in the annealing furnace with controlled atmosphere gas for the cold finish processed material, but in the hot finish processed material, the scale generated by the hot finish exists. Therefore, there is a problem that even if the heat treatment is performed in an annealing furnace in which the atmosphere gas is controlled, the surface texture cannot be improved.

As a method for adjusting the strength and the microstructure by omitting the above-mentioned off-line heat treatment, C: 0.08
~ 0.13%, Si: 0.05-0.50%, Mn:
1.10 to 1.50%, Ni: 0.70% or less and /
Or Cu: 0.45% or less, and Cr: 0.20
% Or less, Mo: 0.05% or less, V: 0.02% or less and B: 0.0005% or less, at least one selected from the group: Al: 0.005 to 0.080%, Nb:
0.005-0.050%, N: 0.0010-0.0
090%, Ti: 0.005-0.035%, balance Fe
And a slab having a steel composition consisting of unavoidable impurities 1
A method (Japanese Patent Laid-Open No. 2-305919) in which hot working is performed after heating to a temperature of 000 ° C. or higher and normalization is further performed at a temperature of 880 to 950 ° C. has been proposed.

[0006]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
The method disclosed in Japanese Patent No. 919 is used as a steel material for welded structures or a steel pipe for line pipes used in cold regions, has a low carbon equivalent, has excellent weldability, and has excellent low-temperature impact characteristics. It is intended to obtain a normalizing type high toughness and high strength steel material, and is not a steel material for a boiler which requires high temperature strength, oxidation resistance and corrosion resistance.

An object of the present invention is to provide a method for manufacturing an alloy steel pipe which completes the above-described conventional isothermal transformation process, which has been performed offline, in-line by heating and cooling immediately after hot working.

[0008]

A method for manufacturing an alloy steel pipe according to claim 1 of the present invention is C: 0.05 to 0.35%, S:
i: 0.02 to 0.60%, Mn: 0.3 to 1.8%,
A steel slab containing Cr: 0.2 to 10.0%, Mo: 0.2 to 1.5% and the balance being Fe and inevitable impurities is heated to a temperature range of 1050 to 1280 ° C. After performing piercing and hot rolling at a finishing temperature of 750 ° C or higher using a piercer, mandrel and sizer,
Reheat to a temperature range of 800 to 1050 ° C and cool to a constant temperature at a cooling rate of 5 ° C / sec or less, 650 to 780 ° C.
After the isothermal transformation treatment in the temperature range of 0.25 to 2.0 hours, air cooling is performed. In this way, the steel strip having the above composition is hot-rolled, and immediately subjected to the isothermal transformation treatment in-line immediately, so that the strength and ductility equal to those of the steel pipe subjected to the isothermal transformation treatment in the off-line state can be secured by omitting the offline heat treatment. At the same time, the surface quality can be improved.

Further, according to a second aspect of the present invention, there is provided a method for producing an alloy steel tube, which comprises C: 0.05 to 0.35% and Si: 0.0
2 to 0.60%, Mn: 0.3 to 1.8%, Cr: 0.
2 to 10.0%, Mo: 0.2 to 1.5%, V: 0.01 to 0.15%, Nb: 0.01 to 0.
10%, Ti: A steel slab containing one or more of 0.01 to 0.10% and a balance of Fe and unavoidable impurities in a temperature range of 1050 to 1280 ° C. After heating, piercing and hot rolling at a finishing temperature of 750 ° C or higher using a piercer, mandrel and sizer, then reheating to a temperature range of 800 to 1050 ° C to a constant temperature of 5 ° C / sec or less. Cool at a cooling rate of 65
After constant temperature treatment in the temperature range of 0 to 780 ° C. for 0.25 to 2.0 hours, it is cooled by air. In this way, the steel strip having the above composition is hot-rolled, and immediately subjected to the isothermal transformation treatment in-line immediately, so that the strength and ductility equal to those of the steel pipe subjected to the isothermal transformation treatment in the off-line state can be secured by omitting the offline heat treatment. At the same time, the surface quality can be improved.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The reason why the chemical composition of the steel slab, the conditions of hot rolling and the conditions of heat treatment for tempering are limited in the present invention will be explained.

C is an element having the function of improving the strength of steel, but if it is less than 0.05%, the tensile strength is 450 N /
It is not possible to secure the strength of 2 mm2 or more, and the strength of 0.
If it exceeds 35%, the tensile strength will rise too much, so 0.0
It was set to 5 to 0.35%.

Si is an element which has an effect of improving the strength of the steel material in addition to the deoxidizing effect, but if it is less than 0.02%, its effect is not sufficient, and if it exceeds 0.60%, it becomes fine and uniform. Since the carbides dispersed in the steel become coarser and the effect of improving the strength cannot be seen, 0.02 to 0.02
It was set to 0.60%.

Mn is an element having the action of improving the strength of the steel material like C, but if it is less than 0.3%, the effect is insufficient and the desired strength cannot be obtained.
%, The yield point is not reduced, so 0.3-
It was set to 1.8%.

Cr is an element having an effect of improving the oxidation resistance and strength of the steel material, but if it is less than 0.2%, its effect is not sufficient, and if it exceeds 10.0%, it is microscopic by the isothermal transformation treatment. Since the effect of improving the balance adjustment between the structure and the yield point and the tensile strength is reduced, 0.2 to 1
0.0%.

Mo is an element having the effect of improving the high-temperature strength of steel, but if it is less than 0.2%, its effect is not sufficient, and if it exceeds 1.5%, the high-temperature strength of steel is improved. , The ductility decreases, so 0.2
To 1.5%.

V, Nb, and Ti are all elements that improve the strength. Among them, V is an element that has the effect of improving the high temperature strength of the steel material, but if it is less than 0.01%, its effect is sufficient. Further, if it exceeds 0.15%, the high temperature strength of the steel material improves, but the ductility decreases, so it was made 0.01 to 0.15%.

Nb is an element having a function of refining the austenite grain size of the steel material after sizer rolling to improve strength, toughness and ductility, and is added as necessary,
If it is less than 0.01%, the effect is not sufficient, and if it is 0.
If it exceeds 10%, the action will be saturated, so 0.01-
0.10%.

Like Nb, Ti is an element having a function of refining the austenite grain size of the steel material after sizer rolling to improve strength, toughness and ductility, and is added as necessary, but if less than 0.01%. The effect is not sufficient, and if it exceeds 0.10%, the ductility decreases, so the content was made 0.01 to 0.10%.

Inclusion of N, P and S as unavoidable impurities is unavoidable, but N is added to prevent cracking of the steel ingot.
The content of P is preferably 0.06% or less, and P is preferably 0.025% or less, more preferably 0.015% or less, from the viewpoint of preventing segregation bands and improving toughness.
Is 0.01% or less, preferably 0.005% or less from the viewpoint of improving the toughness in the rolling direction.

The heating of the steel slab is performed in order to carry out the piercing and rolling in the next step in a state where the steel slab is uniformly heated to the center and the segregation of the microstructure is removed. Is less than 1050 ° C, it is effective for improving toughness by refining the crystal grains, but deformation resistance remarkably increases, which hinders hot working. On the other hand, when it exceeds 1280 ° C, it is zero due to processing heat during drilling. Since it reaches the ductility zone, the heating temperature was set to 1050 to 1280 ° C.

The heated billet is perforated with a piercer,
Next, hot rolling is performed with a mandrel mill and sizer, but if the finishing temperature in hot rolling is less than 750 ° C., ferrite will be generated and the desired isothermal transformation treatment cannot be performed. Also, there is no upper limit for the finishing temperature, but 110
If the temperature exceeds 0 ° C, the particles will become extremely coarse.
The temperature is preferably 050 ° C. Further, the rolling rate in the mandrel mill and sizer is fine even if the rolling rate is small, but considering the surface texture of the steel pipe, it is desirable that the cross-section reduction rate be 30% or more.

Reheating after hot rolling is performed for austenite formation. If the temperature is less than 800 ° C., the ferrite produced after hot rolling cannot be austenitized, and if it exceeds 1050 ° C., the austenite grain size is increased. Since it coarsened and solid-solved carbides and desired strength could not be obtained, the reheating temperature was set to 800 to 1050 ° C. The reheating time is not limited, but since the austenitization is completed in a short time, if the reheating time is unnecessarily lengthened, the cost per unit energy will deteriorate and the cost will increase. Is desirable.

When cooling the steel pipe, a temperature distribution is always generated in the wall thickness direction, and the cooling rate increases toward the surface. If the cooling rate increases, bainite is generated, and the original isothermal transformation process cannot be performed.In addition, the speed difference with the most uncooled portion increases, and the isothermal transformation conditions (temperature, time) differ in the thickness direction. This causes microscopic strength variations. When the cooling rate exceeds 5 ° C / sec, the above phenomenon occurs, so the cooling rate was set to 5 ° C / sec or less.

The constant temperature transformation treatment is performed at a temperature of Ae ₁ point or lower. Ae ₁ point of the steel material of this invention is 78
Since 0 ° C is the maximum, this was set as the upper limit. Also, 6
At a temperature of less than 50 ° C., it is necessary to retain the material for a considerably long time to obtain the desired strength, but the effect is small. If the holding time of the isothermal transformation treatment is less than 0.25 hours, martensite and bainite are generated from untransformed austenite after air cooling, and the strength is not improved. Further, if it exceeds 2.0 hours, the strength will decrease. Therefore, the constant temperature is 650 to 780 ° C., and the constant temperature time is 0.25 to 2.0 hours.

In the method for producing an alloy steel pipe according to the present invention, a steel slab having the above-mentioned predetermined composition is treated with 1050 as shown in FIG.
~ 1280 ℃ temperature range, using a piercer, mandrel mill and sizer, hot rolling at a finishing temperature of 750 ℃ or more, then immediately reheated to 800 ~ 1050 ℃ temperature range, then 5 ℃ / sec or less By maintaining a constant temperature of 650 to 780 ° C. at a cooling rate of 1, and performing a constant temperature transformation treatment by holding for 0.25 to 2.0 hours, tensile properties equivalent to those of the heat-treated tempered material can be imparted, and the amount of scale formation can be increased. It can be significantly reduced compared to heat treatment.

[0026]

EXAMPLES Steel pieces A to L of the present invention having an outer diameter of 225 mm and a length of 2 m and having the composition of components shown in Table 1 by a conventional melting method and casting method, and containing the components indicated by * in Table 1 among the constituent components. Comparative steel billets M to R having a composition outside the range of the steel billets of the present invention were prepared, and these steel billets A to L were used as raw materials, and a hot seamless steel pipe consisting of a piercer, a mandrel mill and a sizer. Using the manufacturing equipment, under the respective manufacturing conditions shown in Table 2, a seamless steel pipe of the present invention method having an outer diameter of 177.8 mm and a wall thickness of 16 mm was manufactured. Further, the comparative steel slabs M to R and the steel slabs A and I of the present invention were used as raw materials, and a hot seamless steel pipe manufacturing facility consisting of a piercer, a mandrel mill and a sizer was used, and the outer diameter was obtained under the respective manufacturing conditions shown in Table 3. 177.8m
A seamless steel pipe having a thickness of 16 mm and a thickness of 16 mm was manufactured by a comparative method. Then, a test piece was taken from each of the obtained seamless steel pipes to measure the tensile properties and the scale adhesion thickness. The results are shown in Table 4 and Table 5, respectively. In addition, * mark in Table 3 indicates outside the scope of the present invention, and
The off-line heat treatment was performed by heating the hot-worked seamless steel pipe from room temperature to 920 ° C. and holding it for 0.17 hours, then
The sample was cooled to 725 ° C. at a cooling rate of ° C./sec, and kept at 725 ° C. for 0.75 hours for a constant temperature transformation treatment.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

[Table 3]

[0030]

[Table 4]

[0031]

[Table 5]

As shown in Tables 4 and 5, the test Nos. Manufactured by the method of the present invention. The seamless steel pipes 1 to 23 do not require the conventional heat treatment for refining, but are immediately reheated in-line after hot rolling, cooled to a constant temperature and subjected to a constant temperature transformation treatment. Test N
o. It has the same strength and ductility as the seamless steel pipes of 40 and 41, and can significantly reduce the scale of adhesion on the surface. In contrast, Test No. 24-29
Of the comparative steel slab and the test No. 3
The seamless steel pipe that does not satisfy the specified manufacturing conditions of the method of the present invention of 0 to 39 is lower or higher than the strength requirement such as yield strength 250 to 350 N / mm ² and tensile strength 450 to 550 N / mm ^2. Even if the yield strength and tensile strength are satisfied, the scale thickness is three times thicker or more.

[0033]

INDUSTRIAL APPLICABILITY The method for producing an alloy steel pipe according to the present invention produces a steel pipe which does not require heat treatment for heat treatment, has the same tensile properties as a heat-treated heat treatment material, and has an extremely small amount of scale formation. Therefore, the descaling process can be reduced.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing changes in temperature conditions in a method for manufacturing an alloy steel pipe according to the present invention.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display location C22C 38/38 C22C 38/38

Claims (2)

[Claims] 1. C: 0.05 to 0.35%, Si: 0.
02-0.60%, Mn: 0.3-1.8%, Cr:
A steel slab containing 0.2 to 10.0% and Mo: 0.2 to 1.5% and the balance being Fe and unavoidable impurities is heated to a temperature range of 1050 to 1280 ° C, Drill and 75 with piercer, mandrel and sizer
After hot rolling at a finishing temperature of 0 ° C or higher, 80
Reheat to a temperature range of 0 to 1050 ° C to reach a constant temperature of 5
A method for producing an alloy steel pipe, which comprises cooling at a cooling rate of not more than C / sec, subjecting to constant temperature treatment in a temperature range of 650 to 780 ° C. for 0.25 to 2.0 hours, and then air cooling. 2. C: 0.05 to 0.35%, Si: 0.
02-0.60%, Mn: 0.3-1.8%, Cr:
0.2 to 10.0%, including Mo: 0.2 to 1.5%,
Further, V: 0.01 to 0.15%, Nb: 0.01 to
0.10%, Ti: 0.01 to 0.10% of 1 type or 2 types or more, and the balance has the composition which consists of Fe and unavoidable impurities. After heating to the range, piercing using a piercer, mandrel and sizer and hot rolling at a finishing temperature of 750 ° C or higher, reheating to a temperature range of 800 to 1050 ° C to a constant temperature of 5 ° C / sec. Cool at the following cooling rate,
A method for producing an alloy steel pipe, which comprises subjecting to constant temperature treatment in a temperature range of 650 to 780 ° C. for 0.25 to 2.0 hours and then performing air cooling.