JPS62263924A - Production of tough steel pipe - Google Patents

Abstract

PURPOSE:To produce a tough steel pipe which has high strength and toughness and excellent corrosion resistance and is considerably decreased in the anisotropy of the toughness by subjecting a specifically composed billet successively to heating, hot rolling, quick cooling, reheating and diametral reducing under specific conditions. CONSTITUTION:The billet which contains (hereafter wt%) 0.2-0.55% C, 0.01-0.6% Si, 0.3-1.8% Mn, 0.005-0.06% solAl, and consists of the balance Fe and inevitable impurities is heated in a 1,050-1,250 deg.C range. AFter the heated billet is hot rolled at >=800 deg.C finishing temp. with a Pilger mill and mandrel mill, the billet is quickly cooled down to <=350 deg.C under the conditions under which the billet is cooled at >=200 deg.C/min cooling rate in a 800-500 deg.C range to form the structure consisting of substantially >=50vol% martensite and the balance mainly bainite. The rolled steel is further reheated to the temp. range of the Ac1 transformation point - (AC1 transformation point -200 deg.C) at which the austenite is substantially not formed; thereafter the steel is subjected to diametral reducing at >=5% reduction in area by using a reducer and is air-cooled to obtain the rolled steel pipe essentially consisting of martensite.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention has high strength and high toughness equivalent to conventional tempered steel pipes, and also has significantly lower toughness anisotropy.

The present invention also relates to a method for producing a strong steel pipe which has superior corrosion resistance, superior strength and toughness, and significantly smaller anisotropy of toughness when compared with conventionally rolled steel pipes. It is something.

[Conventional technology]

In general, tempered steel pipes and rolled steel pipes are used as oil country tubular goods, oil pipes, and steel pipes for various mechanical structures.

Conventionally, there is one method for manufacturing tempered steel pipes, for example.

In weight % (relative % indicates weight %).

fllc: 0.2-0.3%.

Si: 0.2-0.6J Mn: 0.8~], 5%.

soA, A6: 0.05% or less.

Contains and further if necessary.

Cr: 1% or less, Mo: below 0.296’a.

V: 0.06% U lower.

Ti: 0.1% or less.

A steel piece containing one or more of the following, with the remainder consisting of Fe and unavoidable impurities.

(2) As shown in the process heat curve diagram in FIG.

Heat to a temperature within the range of 1200-1250°C.

(3) After hot rolling using a pilger mill, a mandrel mill, and a yaw reducer, it is air-cooled.

(4) Then, by performing tempering heat treatment under the following conditions: quenching temperature: 920°C, tempering temperature: 2, 600 to 700°C, a tempered steel pipe having a structure consisting essentially of tempered martensite can be manufactured ( The method is known from the basic steps of 1) to (4).

For example, there is one method for manufacturing rolled steel pipes.

(1γ　C: 0.2-0.55%.

Si: 0.2-0.6%.

Mn: 1-1.9%.

soA, M: 0.05% or less.

Contains and further if necessary.

V: 0.1% or less.

A steel piece having a composition containing one or more of Nb: 0.1% or less, and the remainder consisting of Fe and unavoidable impurities.

(2)' Similarly, as shown in the process heat curve diagram in FIG. 2, it was heated to a temperature within the range of 1200 to 1250°C.

(3)' Using a pilger mill and a mandrel mill (
, > 1 After hot rolling, it is usually left as is (in some cases, reducer rolling is performed at a temperature of 800°C or higher)
.

C4)' Air cooling as necessary to a temperature of 400° C. or higher to obtain a structure consisting essentially of ferrite, canine-rite or austenite, and (5)' Subsequently, at least AC1 transformation point 1 Usually after reheating to a temperature above the Ac 3 transformation point.

(6) A rolled steel pipe having a structure substantially consisting of ferrite and pearlite is manufactured by performing diameter reduction with a cross-section reduction rate of 20% or more using a 'reducer' and cooling with 'ai & air.

A method comprising the basic steps (1)' to (6)' above is known.

[Problem that the invention seeks to solve]

However, tempered steel pipes manufactured by the method consisting of the basic steps (1) to (4) above have high strength and high toughness.

Also, it has the feature of small anisotropy in toughness.

Corrosion resistance is not necessarily sufficient, and additional heat treatment is required, which not only increases costs.

It is subject to shape constraints in terms of dimensional accuracy.

In addition, rolled steel pipes manufactured by the method consisting of the basic steps 11)' to (6)' above do not have good corrosion resistance, have relatively low strength and toughness, and have a large anisotropy in cut toughness. . In this way, the conventional method achieves 1 strength and toughness.

At present, it is difficult to manufacture steel pipes that satisfy both corrosion resistance and anisotropy.

[Means for solving problems]

Therefore, the inventors of the present invention, from the above-mentioned viewpoint.

As rolled, without the need for refining heat treatment, strength,
This is the result of research aimed at producing rolled steel pipes that have good toughness and corrosion resistance, but also low corrosion resistance.

Ia) C: 0.2~015596° Si: 0.01-0.6%.

Mn: 0.3-1.8%.

sol. Al: '0.005~0.06%,
Contains additional information as required.

Cr: 0.2-1.5%.

Mo: 0.2~196° V: 0.01-0.15%, Nb: 0.01-0.1%.

Ti: 0.01-0.1%.

B: 0.0005-0.003%.

A steel piece containing one or more of the following, with the remainder consisting of Fe and unavoidable impurities.

(bl　As shown in the process heat curve diagram in Figure 3.

Heating to a temperature within the range of 1050-1250°C, lcl
Using a pilger mill and a mandrel mill.

After hot rolling at a finish temperature of 800°C or higher.

[dl The temperature range of 800 to 500°C is rapidly cooled to a temperature of 350°C or less at a cooling rate of 200°C/min. Assume that it has a structure made of bainite.

(el Then, AC1 transformation point ~ (Ac1 transformation point - 200℃
) after reheating to a temperature within the range.

(f) Using a reducer, reduce the diameter with a cross-section reduction rate of 596 or more, and cool it with air at 12ti and above (al
The rolled steel pipe manufactured in the basic process of (a) to (a) has a structure consisting essentially of martensite, with the remainder mainly consisting of bainite.

It also has high strength, high toughness, and excellent corrosion resistance.

Furthermore, they found that it has significantly less anisotropy.

This invention was made based on the above findings, and the reason why the component composition and manufacturing conditions were limited as described above will be explained below.

A. Ingredient composition al C The C component has the effect of improving the strength of steel pipes, but if its content is less than 0.2%, the yield stress will be 60 Kqf /
- If the content exceeds 0.55%, Mandre/L.

Since quench cracking occurs during rapid cooling after rolling, the content was determined to be 0.2 to 0.55%.

(The bl 5i Si component has a deoxidizing effect as well as an effect of improving hardenability, but if its content is less than 010196, the desired effect cannot be obtained; on the other hand, if its content is less than 0.6
%, the refinement of crystal grains is hindered, and carbides that were previously finely and uniformly dispersed become coarser, resulting in a decrease in toughness and corrosion resistance, as well as a further decrease in hardenability. Since no effect was observed in this direction, the content was determined to be 0.01 to 0.696.

(Cl Mn The Mn component has the effect of improving hardenability and thereby making it easier to precipitate martensite during cooling after mandrel rolling, but if its content is less than 0.3%, the desired hardenability cannot be achieved. Unable to secure.

On the other hand, if the content exceeds 1.8%, depending on the cooling rate, bainite will be more likely to be generated than martensite, and the toughness will be significantly deteriorated. It was set at 1.8%.

(The dl so7.A/M component has the effect of refining the prior austenite grain size in addition to its deoxidizing effect.
! If the content is less than 0.00596, the desired effect cannot be obtained in the above action, and even if the content exceeds 0.06% for the same <soA, A41, the effect of further improving the action will not appear, and the effect will be from becoming saturated.

Its content is 0.005 to 0.0 for SOA, A7
696.

le) 0r The Cr component has the effect of improving hardenability and corrosion resistance, so it is included as necessary, but if the content is 0
．． If the content is less than 2%, the desired effect cannot be obtained, while if the content exceeds 1.596.

The content was set at 0.2 to 1.5% because hardenability increases too much and quench cracking occurs, and corrosion resistance and toughness decrease.

(fl Mo and ■) These components facilitate the production of martensite during the rapid cooling of mandrel barley.

ACl - (Ac1-200℃) It has the effect of minimizing the strength loss when reheating to a temperature within the range of (Ac1-200℃), enabling reheating at a higher temperature, and thereby facilitating reducer rolling. However, the content is
Mo: less than 0.296 and V: 0.01, respectively
If the content is less than 1%, the desired effect cannot be obtained. On the other hand, if the content exceeds MO: 1% and V: 0.15%, the strength will improve.

Since the toughness decreases, the content is set to Mo: 0.2 to 1% and V-0.01 to 0.1, respectively.
It was set at 5%.

(g) Nb and Ti These components have the effect of making the austenite grain size finer after mandrel rolling, improving toughness, and preventing quench cracking, so they are included as necessary.
The content is Nb: less than 0.01% and Ti: 0.
If the Nb content is less than 0.1%, the desired effect cannot be obtained; on the other hand, even if the Nb content exceeds 0.1%,
Since the action becomes saturated and the toughness decreases when Ti exceeds 0.1%, the content is adjusted to Nb: 0.01 to 0.196. Ti:
It was set at 0.01 to 0.1%.

(h) The B S component has the effect of suppressing the formation of ferrite during cooling after mandrel rolling and making it easier to obtain a martensitic structure, so it is included as necessary, but if its content is 0 If the content is less than 0.0005%, the desired effect cannot be obtained, while if the content exceeds 0.003%, the toughness will decrease, so the content should be reduced to 0.00%.
It was set at 0.05 to 0.003%.

fil Unavoidable impurities As unavoidable impurities, the inclusion of N, P, and S components is unavoidable, but the N component should be kept at 0.06% or less to prevent cracking of the steel ingot and ensure hardenability due to the S component. is desirable, and the S component is desirable from the viewpoint of preventing segregation bands and improving toughness.

The S component should be 0.025% or less, preferably 0.015% or less, and furthermore, the S component should be 0.01% or less, preferably 0.00596 or less, from the viewpoint of toughness in the Tananobe direction. It is better to do so.

B. Manufacturing conditions (al Heating temperature This heating is done by uniformly heating the steel piece to the center.

It is applied to perform the primary process of drilling and rolling with micro-segregation removed, and even if the temperature is below 1050°C, it is effective in improving toughness by refining the crystal grains. This is undesirable because the deformation resistance increases significantly.

On the other hand, if the temperature exceeds 1250°C, scale formation becomes significant, resulting in a decrease in yield and surface roughness, so the heating temperature was set at 1050 to 1250°C.

(b) Finishing temperature The heated steel billet is pierced with a piercer and then rolled with a pilger mill and a mandrel mill. In order to obtain a hardness of 8°C (10°C or higher), if the temperature is lower than 8°C, ferrite will begin to form during BE stretching or quenching, resulting in a significant deterioration of toughness. There is no upper limit for the finishing temperature, but if it exceeds 1100°C, the grains will become extremely coarse, so it is desirable to set it at 850 to 1050'C.Furthermore, even if the rolling rate with the mandrel is small, Although the crystal grains become fine in subsequent steps, from the viewpoint of improving the surface texture of the steel pipe and preventing quench cracking, it is desirable that the rolling reduction is 3096 or more in area reduction ratio.

(d) Rapid cooling conditions In order to ensure sufficient toughness, the formation of a ferrite + pearlite structure is avoided, and the structure is composed of martensite and bainite, and martensite accounts for 50% by volume or more, preferably 9096 or more. From 800 to
The temperature range of 500'C is rapidly cooled at a cooling rate of 200°C or more, and in order to completely transform into austenite and form the above-mentioned martensite-based structure, the temperature range is 350'C.
It is necessary to cool the sample to a temperature below 1 liter, and if any of these cooling conditions are not met, the desired structure will not be obtained.

It becomes impossible to ensure high toughness.

(dl Reheating temperature As mentioned above, in the traditional rolled steel pipe manufacturing method.

At least the reheating temperature is above the Ac1 transformation point, usually Ac
Although austenite was formed at a temperature of 3 or more transformation points, it is assumed that no austenite is formed as the reheating temperature is set to a lower AC1 transformation point or lower and within a temperature range of Ac1 transformation point - 200° C. or more.

In combination with the structure obtained by rapid cooling (martensis: 50% or more), the toughness is significantly improved, and the toughness in the direction perpendicular to the rolling direction is also significantly improved. If the temperature exceeds the Ac1 transformation point -200°C, austenite will begin to form and the desired toughness cannot be secured.
If the temperature is below the Ac1 transformation point, the decomposition of martensite will be insufficient and the deformation resistance due to the reducer in the primary process will increase significantly, and cracks will occur during rolling due to deterioration of deformability. ~(Ac1 metamorphosis point-
200℃). For products where cold workability is important, it is desirable to set the reheating time to 0.5 to 2 hours, so that martensite can be sufficiently decomposed.

(Reduction rate of area by el reducer) If diameter reduction processing is not performed by the reducer, the structure, strength, and toughness are similar to that of tempered steel pipes.

However, when the reducer is used to reduce the diameter by 5% L/J and 7, the deformation and recrystallization accompanying the processing lead to finer grains and promote the decomposition of martensite. , and the finer dispersion of carbides.

Corrosion resistance is improved, and since the desired effect of improving corrosion resistance cannot be obtained by diameter reduction processing with a reduction in area of less than 596, the reduction in area by the reducer is set to 5% or more. Note that while the desired characteristics can be obtained by air cooling after the diameter reduction process using the reducer, forced cooling may also be performed.

Even if the characteristics are slightly improved by this, the characteristics are not impaired.

〔Example〕

Next, the method of the present invention will be explained in detail using embodiments.

With the usual melting method and casting method, each has the component composition shown in Table 1, and diameter = 180 - x length = 2 m
Steel billets A-M of the present invention having dimensions of .

Comparative steel billet N-3& having a component composition (those marked with * in Table 1) outside the scope of this invention was prepared, and then these steel billets were prepared. Methods 1 to 19 of the present invention were used under the conditions shown in Table 2, respectively. Comparative Methods 1 to 11 and Conventional Methods 1 and 2 were carried out to produce steel pipes with an outer diameter of 160 mm and a wall thickness near.

In addition, comparative methods 1 to 6 show cases in which the above-mentioned comparative steel pieces N to S are used as steel pieces, although the manufacturing conditions are within the scope of the present invention, and comparative methods 7 to 11 show cases in which one of the steel pieces of the present invention is used. A is used to indicate the case where any of the manufacturing conditions (marked with ζ2* in Table 2) is outside the scope of this invention, and conventional method 1 is A case is shown in which a rolled steel pipe is manufactured according to the curve diagram, and furthermore, Conventional Method 2 is shown in a case in which a tempered steel pipe is manufactured in the same manner as shown in the process heat curve diagram in FIG.

Next, we observed the microstructure of the various steel pipes obtained as a result, and measured the tensile properties and fracture surface transition temperature in the direction perpendicular to the rolling direction. A test piece with dimensions of x length = 75 mm and two vertical stress concentration drill holes in the center was used, and a change of 120 kg/- or less was applied to the center of the test piece. in a 0.5% acetic acid aqueous solution saturated with H2S.
A corrosion test was conducted under the condition of time immersion, and the critical stress value was determined from the cracking resistance. These results are shown in Table 2.

〔Effect of the invention〕

From the results shown in Table 2, the rolled steel pipes manufactured by Methods 1 to 1'tJ of the present invention all have excellent corrosion resistance equivalent to the rolled steel pipes manufactured by Conventional Method 1.

It also has far superior strength and toughness, and has significantly less anisotropy in toughness.

In addition, in comparison with heat-treated steel pipe manufactured by conventional method 2, it has high strength, high toughness, and low anisotropy of Iy) and J:, which are equivalent to this, while corrosion resistance is even better than this. It has become something like this. In this way, methods 1 to 1 of the present invention
It is clear that the rolled steel pipes manufactured in Example No. 9 all have high strength and high toughness, excellent corrosion resistance, and also extremely low anisotropy.

On the other hand, as seen in the rolled steel pipes manufactured by Comparative Methods 1 to 11, if at least one of the composition and manufacturing conditions of the steel slab deviates from the scope of the present invention, the strength, toughness, corrosion resistance, and It is clear that at least one of the properties of the orientation becomes inferior, and a rolled steel pipe having all of these properties cannot be obtained.

As described above, according to the method of the present invention, the as-rolled product has high strength and toughness, excellent corrosion resistance, and excellent properties in the direction perpendicular to the rolling direction without requiring refining heat treatment. This makes it possible to produce rolled steel pipes with extremely low anisotropy.

[Brief explanation of drawings]

Figure 1 is a thermal curve diagram showing the manufacturing process of conventional rolled steel pipes;
The figure is a thermal curve diagram showing the manufacturing process of a conventional tempered steel pipe, and FIG. 3 is a thermal curve diagram showing the manufacturing process of a rolled steel pipe of the present invention.

Claims (2)

[Claims] (1) (a) C: 0.2-0.55%, Si: 0.01-0.6%, Mn: 0.3-1.8%, sol. (b) A steel billet having a composition (weight%) containing Al: 0.005 to 0.06% and the remainder consisting of Fe and unavoidable impurities to a temperature within the range of 1050 to 1250 ° C. ,
(c) Using a pilger mill and a mandrel mill, 80
After hot rolling at a finishing temperature of 0°C or higher, (d)8
The product is rapidly cooled to a temperature of 350°C or less in the temperature range of 00°C to 500°C at a cooling rate of 200°C/min or more, and is made of substantially 50% by volume or more of martensite and the remainder mainly of bainite. (e) Then, A with substantially no austenite formation.
After reheating to a temperature within the range of c_1 transformation point to (Ac_1 transformation point - 200°C), (f) using a reducer, perform diameter reduction processing with a cross-section reduction rate of 5% or more, and then air cooling, A method for manufacturing rolled steel pipes having a structure consisting essentially of martensite and the rest mainly consisting of bainite, which is characterized by comprising the basic processes (a) to (f) above, and having high strength and high strength as rolled. A method for producing a strong steel pipe that has high toughness, excellent corrosion resistance, and significantly less anisotropy in toughness. (2) (a) C: 0.2-0.55%, Si: 0.01-0.6%, Mn: 0.3-1.8%, sol. Contains Al: 0.005-0.06%, furthermore, Cr: 0.2-1.5%, Mo: 0.2-1%, V: 0.01-0.15%, Nb: Contains one or more of the following: 0.01-0.1%, Ti: 0.01-0.1%, B: 0.0005-0.003%, and the remainder is Fe and unavoidable impurities. (b) heating a steel billet having a composition (the above weight %) to a temperature within the range of 1050 to 1250°C,
(c) Using a pilger mill and a mandrel mill, 80
After hot rolling at a finishing temperature of 0°C or higher, (d)8
Rapidly cooling the temperature range from 00 to 500 °C to a temperature of 350 °C or less under cooling conditions with a cooling rate of 200 °C / min or more,
A structure consisting of substantially 50% by volume or more of martensite and the remainder mainly of bainite; (e) A with substantially no austenite formation;
After reheating to a temperature within the range of c_1 transformation point to (Ac_1 transformation point - 200°C), (f) using a reducer, perform diameter reduction processing with a cross-section reduction rate of 5% or more, and then air cooling, A method for manufacturing rolled steel pipes having a structure consisting essentially of martensite and the rest mainly consisting of bainite, which is characterized by comprising the basic processes (a) to (f) above, and having high strength and high strength as rolled. A method for producing a strong steel pipe that has high toughness, excellent corrosion resistance, and significantly less anisotropy in toughness.