JPH02175814A - Manufacture of thick steel plate for uoe steel tube - Google Patents

Abstract

PURPOSE:To improve the characteristic of stopping the propagation of generated brittle cracks and to increase safety by subjecting a slab as a stock hot-rolled by means of light rolling reduction in the unrecrystallization region to air cooling down to a temp. in the prescribed region lower than the Ar3 transformation point and then to rapid cooling without delay. CONSTITUTION:A slab as a stock is heated and hot-rolled at a temp. between 900 deg.C and the Ar3 transformation point at >=50% draft (1-5% draft per rolling pass, including light pass up to three times) so as to be finished to the prescribed plate thickness. The resulting steel plate is air-cooled down to a temp. between (Ar3 transformation point-20 deg.C) and (Ar3 transformation point-70 deg.C) and then cooled rapidly without delay, or further, tempering is further applied to the above plate. By this method, coarse polygonal ferrite is incorporated to a microstructure and safety can be increased.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a method for producing thick-walled steel plates for use in forming UoE steel pipes suitable for pipelines for transporting crude oil, natural gas, etc. Drop We
This is intended to improve the characteristics of the light tear.

Recently, pipelines are being operated at high pressure in order to improve transportation efficiency, so there is an increasing demand for steel plates for UOE I pipes that have high strength and thick plate thickness.

In order to increase safety against brittle fractures, such steel plates for UOE steel pipes not only improve the characteristics of brittle fracture occurrence, but also improve the characteristics of stopping the propagation of brittle cracks that have occurred. There is a need.

The brittle fracture initiation characteristics of the former are evaluated using the fracture surface transition temperature and CTOD (crack tip opening displacement) test values of the Charpy impact test, and the brittle crack propagation arrest characteristics of the latter are evaluated using the fracture surface transition temperature and CTOD (crack tip opening displacement) test values of the Charpy impact test.
It is generally evaluated by the fracture surface transition temperature of T.

(Prior art) Based on the idea that there is a correlation between the fracture surface transition temperature of the Charpy impact test and the fracture surface transition temperature of DWTT, the fracture surface transition temperature (8525ATT) of DWTT is lowered and the brittle crack propagation arresting property is In order to improve this, it is important to achieve grain refinement which is advantageous for improving brittle fracture occurrence characteristics, and conventionally, grain refinement techniques such as controlled rolling have been developed.

However, in the case of thick steel plates with a thickness exceeding 20 mm, the fracture surface transition temperature in the Charpy impact test shifts to the lower temperature side by grain refinement, but the DWT
The fracture surface transition temperature of T does not necessarily move to the lower temperature side, and therefore, there have often been cases where required properties cannot be satisfied.

In addition, rolling at a low temperature, for example (α+
γ) A method is often used to improve the toughness in both the rolling direction and the thickness direction T by performing two-phase region rolling to develop a (100) texture in the steel sheet and creating separation. There is.

However, the above method was not preferable because it imposed an excessive load on the rolling mill and reduced rolling efficiency.

Conventionally, there are examples in which the cooling start temperature after rolling is set to be below the Ar3 transformation point, for example, in JP-A-55-41927 and JP-A-57-126916, but in both cases the ferrite content of the structure is simply set. It's just to control.

(Problems to be Solved by the Invention) This invention is capable of producing I
This advantageously solves the problem of improving the brittle crack propagation arresting properties of thick-walled steel plates for JOE steel pipes, and improves the DWTT properties, specifically achieving a DWT785% ductile fracture transition temperature lower than -20°C. The purpose of this study is to propose a method for manufacturing thick steel plates for UOE Q pipes that have excellent brittle crack propagation arresting properties.

(Means for Solving the Problems) As a result of intensive research to improve the DWTT characteristics of thick-walled steel plates for UOE steel pipes, the inventors found that rather than simply refining grains in the microstructure, coarse polygons It has been found that the DWTT characteristics are improved by mixing a predetermined amount of nalferite.

That is, by including 5 to 40 vol.
TT characteristics are improved, and Charpy impact test characteristics are not deteriorated.

Based on this, we further conducted research on manufacturing methods that are advantageous for obtaining the above-mentioned structure, and as a result, we applied light reduction in the non-recrystallized area during hot rolling, air-cooled it to a predetermined temperature range below the Ar3 transformation point, Immediately after that, he discovered that the cooling method was suitable.

This invention is based on the above knowledge.

That is, this invention hot-rolls a raw material steel billet for a tlOE steel pipe, finishes the rolling at a temperature equal to or higher than the Ary transformation point, and cools it in air.
In a method for producing thick-walled steel plates for UOE steel pipes, which involves rapid cooling from a temperature below Ar+transformation point or further tempering, 1 to 5% per rolling pass is applied in the temperature range of 900°C-Ar, transformation point. Finish to the specified thickness by hot rolling at a reduction rate of 50% or more, including up to 3 light reduction passes, up to a temperature range of Ar3 transformation point -20° (knee Ar, transformation point -70°C) This is a method for producing a thick flat plate for UOE steel pipes, which is characterized by air cooling and then immediately starting rapid cooling.

The experiments that formed the basis of this invention will be explained below.

C: 0.06wt%, St: 0.20wL%, M
n: 1.15wt%, Nb: 0.03wt%,
V: 0.05wt%, Cu: 0.2wt%,
A raw material slab for a UOE steel pipe was prepared which contained 0.2 wt% of Ni and the remainder had a composition of substantially Fe excluding unavoidable impurities.

This raw material slab was heated to 1150°C, and
When hot rolling is carried out at a rolling reduction rate of 70% in a temperature range of 0°C with a reduction schedule of 10% reduction per rolling pass, and when rolling is carried out at a rolling reduction ratio of 3% per rolling pass at 8.
The rolling schedule was roughly the same except that rolling was carried out once at 75°C.
After finishing to 5.4 mm, it was air cooled to various temperatures below the Arz transformation point (815° C. for this steel type), immediately quenched (water cooled) from this temperature, and then tempered at 550° C. for an hour.

The steel plate thus obtained was subjected to DWTT, and the results are shown in FIG. 1 in relation to the cooling start temperature.

As is clear from the figure, when rolling is performed at a reduction rate of 10% per rolling pass in the temperature range of 900 to 820°C, the DWTT characteristics deteriorate as the quenching start temperature decreases. , when a rolling reduction of 3% is applied once per rolling pass at 875°C, Ar3 transformation point -20°C x A
The DWTT characteristics were significantly improved in the temperature range of −70° C., the transformation point.

This is because by applying a low rolling reduction per rolling pass in the non-recrystallized region, the γ grain boundaries undergo strain-induced movement, the crystal structure becomes a moderately mixed grain state, and this γ structure is brought to the quenching start temperature ( Ar3 transformation point -20'C to Ar3 transformation point -70'C
), the α grains became mixed grains.

(Function) In this invention, the reason why the reduction rate and the number of times of the light reduction pass in the temperature range of 900° (knee Ar3 transformation point), the reduction rate, and the quenching start temperature are limited will be explained.

If the rolling reduction rate per rolling pass of the light rolling pass is less than 1%, strain-induced movement will not occur, while if it exceeds 5%, the strain-induced movement will not occur as it remains in an unrecrystallized state. The content was limited to 1 to 5% because the desired mixed grain structure could not be obtained.

It is necessary to perform the light reduction pass at least once in order to strain-induced movement of one grain boundary, but if it exceeds 3 times, the γ grains will grow too much and the coarse polygonal ferrite grains that are finally obtained will be reduced. 1. Because the diameter becomes 25 μm or more, not only the DWTT characteristics but also the Charpy test characteristics deteriorate.
It was limited to 3 times.

If the rolling reduction in the transformation point range is less than 50%, the average ferrite grain size excluding coarse polygonal ferrite grains will become coarse, the degree of refinement of the structure will be small, and the toughness will decrease. Since there is no improvement, it is set at 50% or more.

If the quenching start temperature is higher than the ^r3 transformation point -20°C, the coarse polygonal ferrite grain size will not exceed 10 μm, and the volume fraction will not exceed 5%; on the other hand, if the quenching start temperature is higher than the Ar3 transformation point -70 If the temperature is lower than ℃, the coarse polygonal ferrite grain size will be 25 μm or more, and the volume fraction will be 40 μm or more.
% or more, neither of them satisfy the condition that the coarse polygonal ferrite grain size includes 5 to 40 vol. ^r3
The transformation was limited to a range from -20°C to 70°C.

In this invention, the rolling reduction in the range of 900°C-Ar transformation point is 9
It is calculated from the plate thickness at 00°C and the plate thickness at the end of rolling.

In this invention, light rolling may be carried out at any rolling time within the range of 900°C to ^r transformation point, preferably 900 to 850°C.

The steel sheet targeted by this invention is required not only to have DWTT properties but also to have high strength, excellent Charpy toughness, weldability, etc., so C: 0.2 wt% or less, and 0.2 wt% or less.
02 to 0.2 wt%, Si: 0.1 to 1.0 wt% or less. Owt%, Mn: 0.5 to 2.5wt% within 2.5wt%, P: 0.02wt%
Below, S: 0.01wt% or less, and Al: 0.0
The basic component is 0.005 to 0.03 wt%, even if it is 3 wt% or less, and Nb: 0.0, even if it is 0.2 wt% or less.
05-0.2wt%, V: 0 among 0.5wL% or less
．． 01-0.5-1%, Ti: 0.3 wt% 0.005-0.3 wt%, Mo: 0.5w
0.02 to 0.5 wt%, Cr:0.
0.02 to 0.5 wt%, Cu
: 0.05 to 1.5 wt%, even below 1.5 wt%
, Ni: 0.05 to 1.0 within 1.0 wt%
Round t%, B: 0.0002 among 0.01wt% or less
~0,01 wt%, Ca: 0.01 or less, 0.0005 to 0.01 wt% Oyobi REM: 0
．． 0.02 wt% or less, preferably 0.002 to 0.02 wt% as a selected component.

(Example) Steels having various compositions shown in Table 1 were heated to 1180'C, and the rolling reduction rate was 1 to 5% per rolling pass in the temperature range of 900 to Ar3 transformation point shown in Table 2. After hot rolling at a rolling reduction rate of up to 3 light reduction passes and a rolling end temperature, the material was air cooled, quenched from the quenching start temperature shown in Table 2, and then tempered at 550° C. for 11 hours.

Average ferrite grain size excluding coarse grains, ferrite coarse grain size of 7 μm or more, area ratio of ferrite coarse grains, DWTT 85% ductile fracture transition temperature, Charpy impact test 50% brittle fracture surface of the steel sheet thus obtained The transition temperatures are also listed in Table 2.

As is clear from Table 2, the test Na 1 using steel type A
Regarding Nα2, test Nα2, in which the reduction rate in the temperature range of 900 to Ar and the transformation point was outside the range of the present invention, was inferior to test Nα1 in both Charpy fracture transition temperature and DWTT characteristics. Also, test N which deviates from the manufacturing conditions of this invention
α3. Na5. Nα8 and Nα9 are the test Nα4. which is the scope of this invention. When compared with No. 7, it is Nα3. N
α5 has inferior DWTT characteristics compared to Nα4, and No.
, 8. Nα9 had inferior DWTT characteristics compared to No.7. This is because unless the manufacturing conditions of this invention are met, the microstructure will not contain 5 to 40 vol% of coarse polygonal ferrite with a size of 10 to 25 μm, and DWT
This is because the conditions for improving the T characteristics are not satisfied.

On the other hand, test No. that satisfies the manufacturing conditions of this invention,
4, 8 (16, No. 7 and No. 10 are DWT
It was possible to produce a steel sheet that had excellent T properties and Charpy impact test properties, in other words, had both excellent brittle crack propagation arresting properties and brittle fracture initiation properties.

Although only the DWTT properties have been described above as brittle crack propagation arresting properties, it has been confirmed that other brittle crack propagation arresting properties (for example, the ESSO test) exhibit excellent properties.

(Effects of the Invention) According to the present invention, regarding the method for manufacturing thick-walled steel plates for UOE steel pipes, it is possible to obtain a microstructure in which predetermined coarse polygonal ferrite is mixed (Ar, hot rolling that ends at a transformation point or higher). This is especially desirable when the steel plate is thick, without causing problems such as overloading the rolling mill or reducing rolling efficiency, by applying light reduction and then air cooling to a predetermined temperature range, followed by immediate rapid cooling. Accordingly, it is possible to provide a steel sheet with excellent brittle fracture occurrence characteristics and brittle crack propagation arresting characteristics.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the effect of quenching start temperature on DWT 785% ductile fracture transition temperature. Figure 1

Claims (1)

[Claims] 1. Hot rolling a raw steel billet for UOE steel pipes, finishing the rolling at a temperature of Ar_3 transformation point or higher, air cooling, and quenching from a temperature of Ar_3 transformation point or lower, or further tempering thereafter. , U
In the manufacturing method of thick steel plate for OE steel pipes, rolling pass 1 is carried out at a temperature range of 900°C to Ar_3 transformation point.
Finish to the specified thickness by hot rolling at a reduction rate of 50% or more, including up to 3 light reduction passes at a reduction rate of 1 to 5% per pass, Ar_3 transformation point -20℃ - Ar_3 transformation point -70 A method for producing a thick steel plate for UOE steel pipes, which is characterized by air cooling to a temperature range of °C and then immediately starting rapid cooling.