JPH07242937A - Production of non-heat treated high tensile strength steel plate small in dispersion of hardness in plate thickness direction - Google Patents

Abstract

PURPOSE:To produce a non-heat created high tensile strength thick steel plate small in the dispersion of hardness in the plate thickness direction by subjecting a steel slab having a specified compsn. to slight rolling reduction and air cooling under specified conditions in an unrecrystallized gamma region and incorporating somewhat coarse polygonal ferrite therein. CONSTITUTION:A steel slab contg., by weight, 0.005 to 0.06% C, 0.05 to 1.0% Si, 0.8 to 2.5% Mn, 0.005 to 0.08% Al and 0.01 to 0.1% Nb, contg., at need, one or more kinds among 0.01 to 0.10% V, respectively <=1.0% Cu, Ni and Cr, <=0.5% Mo, 0.005 to 0.1% Ti and respectively 0.001 to 0.01% rare earth metals, and the balance iron is heated to 1050 to 1250 deg.C. This steel slab is applied with rolling reduction of >=30% the unrecrystallized gamma region of (the Ar3+150 deg.C) or above and is moreover applied with rolling reduction of >=50% including slight rolling reduction of at least <=5%/pass in the unrecrystallmzed region of less than (the Ar3+150 deg.C) to the Ar3 by l to 3 passes. Next, this steel sheet is air cooled to (the Ar3-10 deg.C) to (the Ar3 to 80 deg.C) and is thereafter subjected to accelerated cooling to <400 deg.C at the cooling rate of 20 to 50 deg.C/sec.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a non-heat treated high-strength steel plate having excellent properties in the plate thickness direction, which is used for a thick UOE steel pipe used in a pipeline for transporting crude oil, natural gas and the like. Is.

[0002]

2. Description of the Related Art Recently, in pipelines for transporting crude oil, natural gas, etc., high-pressure operation is aimed at in order to improve transportation efficiency, and there is a demand for UOE steel pipe sheets having high strength and thick sheet thickness. In order to increase the strength with a thick wall, for example, as disclosed in JP-A-54-68719, an attempt is made to increase the strength by increasing the cooling rate after controlled rolling to 15 ° C / s or more. The difference in the cooling rate between the center and the center causes a difference in hardness in the plate thickness direction, which causes strain in the plate and SSCC.
(Sulfide stress corrosion cracking).

This is because, as is well known, when the cooling rate is high, bainite and martensite are generated in the surface layer portion, so that hardness unevenness occurs in the plate thickness direction. Therefore, there is a technique to reduce the hardness of the surface layer portion by generating ferrite in the surface layer portion to reduce the unevenness in hardness. However, when ferrite is generated, the alloying elements such as C are concentrated in the untransformed γ (austenite) and the island Since martensite is formed, it is not possible to eliminate the hardness unevenness by simply producing ferrite.

Therefore, the inventors of the present invention have disclosed in Japanese Patent Laid-Open No. 3-223419.
In the publication, when a microstructure for reducing the thickness unevenness of the thick-walled material in the plate thickness direction was examined, it was found that the hardness unevenness in the plate thickness direction becomes smaller when acicular ferrite is generated without forming polygonal ferrite. Headline,
An invention based on this is proposed. Further, the present inventors examined a microstructure for further reducing the hardness unevenness in the plate thickness direction of the thick material, and found that the invention was based on a principle different from the one found in the above-mentioned JP-A-3-223419. Arrived

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide an invention based on this new principle, that is, a method for producing a non-heat treated high-strength thick steel plate with less hardness unevenness in the plate thickness direction.

[0006]

The present invention, in terms of weight ratio,
C: 0.005-0.06%, Si: 0.05-1.0%, Mn: 0.8-2.
5%, Al: 0.005-0.08%, Nb: 0.01-0.1%,
If necessary, V: 0.01 to 0.10%, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 0.5% or less, Ti: 0.005 to 0.1%, Ca: 0.001 to 0.01%. , RE
M: 1050 to 1250 steel slabs containing 0.001 to 0.01% of 1 type or 2 or more types and the balance of iron and unavoidable impurities
After heating in the temperature range of ℃, recrystallization of (Ar ₃ + 150 ℃) or more γ
Giving pressure of 30% or more by-pass, further (Ar ₃ +150 ℃) than non-recrystallization γ zone to Ar _3, at least 5% / pass less reduction of 50% or more, including 1 to 3 passes the soft reduction Give, then
Air cool to (Ar ₃ -10 ° C) to (Ar ₃ -80 ° C), then 20 to 50
It is a method for producing a non-heat treated high-strength steel sheet with little hardness unevenness in the sheet thickness direction, which is characterized by accelerating cooling to less than 400 ° C at a cooling rate of ° C / s.

[0007]

[Operation] The present inventors examined a microstructure that reduces the hardness in the vicinity of the surface of a thick-walled material, and found that a polygonal
It has been found that the mixing of ferrite reduces the hardness unevenness in the plate thickness direction. That is, 10-25 μm in the tissue
It was found that the difference in surface hardness and the hardness difference (ΔHv) between the center of the plate thickness and the surface hardness was significantly reduced by including 5 to 40% of the polygonal ferrite of (1).

The present invention relates to a manufacturing method for industrially realizing the above structure. First, the experiment that was the basis of the present invention will be described. Figure 1 shows 0.04% C-1.4% Mn-0.03%
Using Nb steel, after heating to 1150 ° C., giving a 50% reduction in (Ar ₃ + 150 ℃) or recrystallization γ region, (Ar ₃ +150 ℃) less than ~ A
changing the rolling reduction / path in conferring 70% reduction in non-recrystallized γ region of r _3, air-cooled to thereafter _{(Ar 3 -10 ℃) ~ (} Ar 3 -80 ℃), cooling of 30 ° C. / s The hardness distribution (○, Δ marks) in the plate thickness direction when accelerated cooling to 300 ° C at a speed is shown. The finished thickness at this time was 25 mm. Also, in the figure, as a comparison, 1150 ℃
After heating, at (Ar ₃ +150 ℃) or recrystallization γ region giving a 50% reduction in non-recrystallized γ region of (Ar ₃ +150 ℃) less than ~ Ar ₃
Immediately apply 70% reduction and 300 ℃ at a cooling rate of 30 ℃ / s.
Also shown is the hardness distribution in the plate thickness direction (□ mark) when accelerated cooling is performed. □ indicates the cooling start temperature before accelerated cooling is Ar ₃
Because of the above, coarse polygonal ferrite was not generated, and ΔHv (difference between surface hardness and center of thickness) was about 60.
More than points. The △ mark indicates that the cooling start temperature is (A) because the light reduction pass in the unrecrystallized γ region is 15% / pass (more than 5 passes).
When air-cooled in the range of (r ₃ −10 ° C.) to (Ar ₃ −80 ° C.), fine ferrite is generated, so ΔHv becomes as high as about 30 points. On the other hand, the ○ mark indicates that the light reduction pass in the unrecrystallized γ region is 3% / pass (5 passes or less), so (Ar ₃ −10 ° C) to (A
When air-cooled to the range of (r ₃ −80 ° C.), coarse polygonal ferrite is generated, so ΔHv becomes about 10 points or less.

That is, it is found that even if ferrite is simply generated, the unevenness of hardness in the plate thickness direction cannot be eliminated, but by mixing coarse polygonal ferrite, the unevenness of hardness in the plate thickness direction is reduced. When a reduction pressure of less than 5% / pass is applied in the unrecrystallized γ region, strain-induced grain boundary migration occurs in the γ grain boundary, resulting in an appropriate mixed grain state. When this mixed grain γ is air-cooled in the range of (Ar ₃ −10 ° C.) to (Ar ₃ −80 ° C.), coarse polygonal ferrite with a small strain of 10 to 25 μm is generated. When such coarse ferrite with less strain is generated in fine acicular ferrite with an average grain size of about 6 μm, the unevenness of hardness is reduced. Further, when such coarse ferrite is generated, C or the like is appropriately concentrated in the untransformed γ. It is considered that the accelerated transformation of this moderately concentrated untransformed γ transforms it into acicular ferrite, which suppresses the formation of island martensite and reduces unevenness in hardness.

Conventionally, the cooling start temperature after rolling is set to A
Examples of r ₃ or less are found, for example, in JP-A-52-123921 and JP-A-58-77529, both of which use ferrite by lightly reducing in the unrecrystallized γ region as in the present invention. There is no disclosure about the appropriate grain mixing and the reduction of hardness unevenness. Next, the reasons for limiting the component composition of the material used in the present invention will be described.

C: 0.005 to 0.06% If C is less than 0.005%, the steel sheet strength is insufficient, and the weld heat affected zone (hereinafter referred to as HAZ) is softened. While 0.06
If it exceeds 0.1%, martensite is generated in the surface layer portion and uneven hardness in the plate thickness direction occurs. Therefore, it is necessary to set C in the range of 0.005 to 0.06%.

Si: 0.05 to 1.0% Si is an element which is inevitably contained in deoxidizing steel during steel refining, but if it is less than 0.05%, the base material toughness is insufficient. On the other hand, 1.0%
If it exceeds 1.0%, the cleanliness of the steel deteriorates and the toughness decreases, so Si must be in the range of 0.05 to 1.0%. Mn: 0.8-2.5% If Mn is less than 0.8%, the strength and toughness of the steel sheet will be insufficient, and further HAZ softening will be severe, while if it exceeds 2.5%, the HAZ toughness will deteriorate, so Mn is 0.8-2.5%. Must be in the range.

Al: 0.005 to 0.08% It is necessary to contain at least 0.005% Al so as to form a solid solution in deoxidizing the steel. On the other hand, if it exceeds 0.08%, not only the toughness of the HAZ but also the toughness of the weld metal are required. As it deteriorates significantly, Al
Should be within the range of 0.005 to 0.08%. Nb: 0.01 to 0.1% Nb is effective in reducing the grain size of ferrite, but if it is less than 0.01%, it does not exhibit the effect.On the other hand, if it exceeds 0.1%, it diffuses into the weld metal during welding, and Nb is limited to the range of 0.01 to 0.10% because it lowers the toughness.

In the above component composition, the intended effect by the method of the present invention is exhibited, but even when the following components are included for the purpose of adding them,
It does not prevent the achievement of the effects of the present invention. Ni: 1.0% or less Ni is useful for improving the strength and toughness of the base metal without adversely affecting the hardenability and toughness of the HAZ, but containing more than 1.0% increases the manufacturing cost. Cause 1.0% or less.

Cu: 1.0% or less Cu not only has almost the same effect as Ni, but also contributes to the improvement of corrosion resistance, but if it exceeds 1.0%, cracks tend to occur during hot rolling, and Since the surface quality deteriorates, it should be 1.0% or less. Mo: 0.5% or less Mo is effective in improving strength and toughness because it forms γ grains during rolling and forms fine bainite.
It is not necessary to exceed 0.5%, but rather there is a disadvantage that the manufacturing cost rises, so Mo is limited to 0.5% or less.

V: 0.01 to 0.10% V may be contained in an amount of 0.01% or more to improve the strength and toughness of the base material of the steel sheet and to secure the strength of the joint portion, but if it exceeds 0.10%, V of the base material and HAZ V is restricted to the range of 0.01 to 0.10% because it significantly deteriorates the toughness. Cr: 1.0% or less Cr can be contained in order to secure the base metal strength and joint strength of the steel sheet, but if it exceeds 1.0%, not only the toughness of the base metal but also the toughness of the welded part will be adversely affected, so it should be 1.0% or less. There is a need.

Ti: 0.005 to 0.1% Ti is added for the purpose of improving toughness due to the effect of refining γ grains and increasing strength due to Ti carbonitride. However, the Ti content is 0.0
If it is less than 05%, it has no effect, and if it exceeds 0.10%, the toughness deteriorates, so the range of Ti content is made 0.005 to 0.1%. Ca: 0.001 to 0.01% With a trace amount of 0.001%, Ca has the effect of controlling the morphology of MnS and is effective in improving the toughness in the rolling and right-angle direction of the steel sheet, but if it exceeds 0.01%, the cleanliness of the steel is poor. Since it causes internal defects, it was limited to the range of 0.001 to 0.01%.

REM: 0.001 to 0.01% REM (La, Ce, Pr, Nd, Il, Sm, Eu, Gd, Tb, Dy, Ho,
Er, Tu, Yb, Lu, Sc, and Yt (rare earth elements) show a morphological control effect of MnS even in a small amount of about 0.001%, and are effective for rolling steel sheets and improving toughness in the perpendicular direction, but 0.01%
If it exceeds 1.0, the cleanliness of the steel deteriorates and there is a disadvantage in terms of arc welding, so the range was limited to 0.001 to 0.01%.

Next, the heating which is the second constitutional requirement of the present invention,
The reasons for limiting the rolling and cooling conditions will be described. First, the slab is heated, but if the heating temperature is less than 1050 ° C, the solid solution amount of Nb will be less than 0.01%, so high strength cannot be achieved. When the steel is heated above 1250 ° C, the subsequent recrystallization γ
Even if rolled in the zone, the austenite grains are not sufficiently refined and the toughness deteriorates. Therefore, the heating temperature is 1050-1250
Must be in the range of ° C.

The slab heated under the above conditions is (Ar ₃ +
Rolling is performed in the recrystallization γ region of 150 ° C or higher. The rolling-recrystallization is repeated to refine the γ grains, but if the rolling reduction in the recrystallization γ region is less than 30%, the γ grains will not be refined sufficiently.
The toughness deteriorates even if the rolling is continued in the unrecrystallized γ region.
Therefore, the rolling reduction in the recrystallization γ region must be 30% or more.

Next, rolling in an unrecrystallized γ region of less than (Ar ₃ + 150 ° C.) and Ar ₃ or more causes the γ grains to coarsen due to strain-induced grain boundary migration, and the γ grains to be elongated or transformed into γ grains. This is an important step in which both grain refinement by introducing the band is performed at the same time. It does not occur unless the γ grains are coarsened due to strain-induced grain boundary movement and the light reduction per pass is not more than 5% / pass. If the number of passes is more than 3, the γ grains grow too much.
The final α grain size becomes 25 μm or more and the toughness deteriorates. Therefore, the light rolling reduction / pass rolling reduction should be 5% or less, and the number of passes should be 3 times or less. The temperature range for giving this light reduction / pass may be within the range of the unrecrystallized γ region of less than (Ar ₃ +150 ℃) and Ar ₃ or more, but it is preferable to perform rolling from the initial stage of rolling in the unrecrystallized γ region. It is better to give it on the way or near the end of rolling.

Further, in the non-recrystallized γ region of less than (Ar ₃ + 150 ° C.) and Ar ₃ or more, elongation of γ grains and introduction of a deformation zone into γ grains are carried out in order to make the grains finer. (Ar ₃ +150
The above-mentioned object cannot be achieved in a temperature range above (° C) or below a temperature range below Ar ₃ . Next, the rolling reduction in this temperature range must be 50% or more. If the rolling reduction is less than 50%, the elongation of γ grains and the introduction of the deformation zone become insufficient and the toughness deteriorates significantly. Therefore, the lower limit of the rolling reduction is 50%.

Subsequently, air cooling to a temperature range of (Ar ₃ −10 ° C.) to (Ar ₃ −80 ° C.) is carried out. This is because the main object of the present invention is to produce a steel plate of low hardness, so that coarse polygonal ferrite is used. To generate. The end temperature of air cooling is (Ar ₃ −10
℃), the coarse polygonal ferrite grain size is
It does not exceed 10 μm and its volume ratio does not exceed 5%. On the other hand, when the end temperature of air cooling is lower than (Ar ₃ -80 ° C), the coarse polygonal ferrite grain size is 25 μm or more, and the volume ratio thereof is 40% or more, and the toughness deteriorates. Therefore, the temperature range for air cooling is (Ar ₃ −10 ° C) to (Ar ₃ −80 ° C).
C) range.

Although accelerated cooling is performed after air cooling to the above range, if the cooling rate is less than 20 ° C./s, there is no effect of increasing the strength by accelerated cooling, while if the cooling rate exceeds 50 ° C./s, high strength is obtained. The cooling rate is limited to the range of 20 to 50 ° C./s because the chemical reaction is saturated. The accelerated cooling continues to be accelerated to less than 400 ° C, but if the cooling is stopped at 400 ° C or higher, the effect of accelerated cooling becomes insufficient, so the cooling stop temperature was limited to less than 400 ° C.

[0025]

EXAMPLES The sample steels whose composition is shown in Table 1 were treated under the heating-rolling-cooling conditions shown in Table 2. Investigating changes in mechanical properties of steel sheets and changes in ferrite structure,
The results are summarized in Table 2. Test N in Table 2
o. Nos. 1 to 13 are slabs of steel B shown in Table 1 having a composition within the range limited by the present invention, subjected to various heating-rolling-cooling conditions, and were all products having a plate thickness of 24 mm.

First, the test No. 1 has a slab heating temperature of 1000
Since it is as low as 0 ° C (limited range 1050 to 1250 ° C, the limited range is shown in parentheses below), TS is low. Test No. No. 2 has a high slab heating temperature of 1300 ° C, and Test No. 3 has a low rolling reduction in the recrystallization γ region of 10% (30% or more).
In No. 5, the rolling reduction in the unrecrystallized γ region was as low as 40% (50% or more), or the number of light rolling passes of 5% / pass or less in the unrecrystallized γ region was as many as 4 passes ( 1 ~ 3 pass), Charpy characteristics are poor. Test No. 6 and 7 have low TS due to the slow cooling rate of 10 ° C / s (20 to 50 ° C) or the high cooling stop temperature of 500 ° C (less than 400 ° C) under accelerated cooling conditions.

For these, the test No. Since Nos. 8 to 12 were manufactured in accordance with the constitutional requirements of the present invention, it is understood that the steel sheets have high strength and sufficient low temperature toughness, have low surface hardness, and have little hardness unevenness in the sheet thickness direction. Test No. 1
3 satisfies the limiting condition of the present invention in the production conditions, but since the C content of the component composition which is another important constituent condition is as high as 0.09% (0.005 to 0.06%), the surface hardness is high, It can be seen that the steel plate has large hardness unevenness in the plate thickness direction.

Test No. Nos. 14, 15 and 16 are slabs of C-steel, D-steel and E-steel having the composition according to the present invention, and 30 mm manufactured satisfying the constitutional requirements of the present invention.
And the characteristics of 15 mm thick steel plate are shown. It can be seen that the steel sheet has sufficient tensile properties and low temperature toughness, has low surface hardness, and has little unevenness in hardness in the plate thickness direction.

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

EFFECTS OF THE INVENTION According to the present invention, a non-heat treated high-strength thick steel plate having small hardness unevenness in the plate thickness direction and excellent in low temperature toughness can be easily manufactured by a method different from the conventional manufacturing method. As a result, it has great significance to be able to satisfy the demand for higher quality line pipes.

[Brief description of drawings]

FIG. 1 is a graph showing a hardness distribution in a plate thickness direction.

Claims (2)

[Claims] 1. A weight ratio of C: 0.005 to 0.06%, Si:
0.05 to 1.0%, Mn: 0.8 to 2.5%, Al: 0.005 to 0.08
%, Nb: 0.01-0.1%, with the balance being steel and unavoidable impurities, the steel slab is heated to a temperature range of 1050 to 1250 ° C and then recrystallized in the γ region above (Ar ₃ + 150 ° C) to 30%. giving more pressure, further non-recrystallized γ zone (Ar ₃ +150 ° C.) below to Ar _3, giving the reduction of 50% or more, including 1 to 3 passes at least 5% / pass less soft reduction, then ( Ar ₃ −10 ° C) ~ (A
r ₃ −80 ° C.), and then accelerated cooling to less than 400 ° C. at a cooling rate of 20 to 50 ° C./s, and a method for producing a non-heat treated high strength steel plate with little hardness unevenness in the plate thickness direction. . 2. A weight ratio of C: 0.005 to 0.06%, Si:
0.05 to 1.0%, Mn: 0.8 to 2.5%, Al: 0.005 to 0.08
%, Nb: 0.01 to 0.1%, V: 0.01 to 0.10.
%, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 0.5% or less, Ti: 0.005 to 0.1%, Ca: 0.001
~ 0.01%, REM: 0.001 ~ 0.01% of one or more kinds of steel slabs containing iron and inevitable impurities in the balance, and after heating to a temperature range of 1050 to 1250 ℃, (Ar ₃ + 150 ℃)
In the above recrystallization γ region, a reduction of 30% or more is applied, and (Ar ₃
Below + 150 ° C) to the unrecrystallized γ region of Ar ₃ of at least 5
% / Pass or less, 1 to 3 passes including 50% or more reduction, then air-cooling from (Ar ₃ -10 ℃) to (Ar ₃ -80 ℃), then cooling at 20-50 ℃ / s A method for producing a non-heat treated high-strength steel sheet with little hardness unevenness in the sheet thickness direction, which comprises accelerating cooling to less than 400 ° C at a speed.