JPH0841536A - Production of high tensile strength steel plate small in nonuniformity of hardness in plate thickness direction and excellent in dwtt property - Google Patents

Abstract

PURPOSE:To provide a method for producing a high strength steel plate for a thick pipeline in which the nonuniformity of the hardness in the plate thickness direction is small and the DWTT 85% fracture appearance transition temp. is regulated to <=60 deg.. CONSTITUTION:A steel slab contg. 0.01 to 0.18% C, 0.05 to 0.5% Si, 1.0 to 2.5% Mn and 0.01 to 0.1% Nb is heated to 900 to 1050 deg.C, is subjected to rolling reduction of >=65% in the unrecrystallized gamma region and that of 10 to 60% including water cooling passes for 1 to 6 times in the gamma+alpha two phase region and is subjected to accelerated cooling to <400 deg.C at 20 to 50 deg.C/s rate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low temperature DWTT (Drop weight tear tes) used for a material for a thick UOE steel pipe used in a pipeline for transporting crude oil, natural gas and the like.
t) The present invention relates to a method for producing a high-strength steel sheet having excellent properties and having little hardness unevenness in the sheet thickness direction.

[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 high strength and thick steel plate for UOE steel pipe is required. There is. In order to enhance the safety against brittle fracture, these pipelines need to improve the occurrence characteristics of brittle fracture as well as the ability to stop the generated brittle crack. The former is evaluated by the fracture surface transition temperature of the Charpy impact test and the CTOD test, while the latter is evaluated by the fracture surface transition temperature of DWTT. D
To decrease the WTT 85% fracture surface transition temperature (85% FATT), the conventional fracture surface transition temperature of the Charpy impact test and DWT
As it is well known that it is important to achieve grain refinement from the idea that there is a correlation with the fracture surface transition temperature of T, and for that reason, for example, the technique of grain refinement such as controlled rolling has been developed. Is.

However, when the plate thickness exceeds 20 mm, if the grain size is reduced, the Charpy fracture transition temperature shifts to the low temperature side, but the DWTT fracture transition temperature is not necessarily the low temperature side. It often happened that the required characteristics could not be satisfied. Until now, the minimum DWTT characteristics required for UOE steel pipes were -20 ℃ in the main line and -46 ℃ around the station, but in recent years, to develop gas and oil fields in the Arctic Ocean, etc.,
DWTT 85% UO for cryogenic temperatures with a fracture surface transition temperature of -60 ° C or less
E steel pipe is required.

Further, in order to increase the strength with a thick wall, for example, as disclosed in JP-A-54-68719, it is attempted to increase the strength by setting the cooling rate after controlled rolling to 15 ° C./s or more. However, due to the difference in cooling rate between the surface and the center, a difference in hardness occurs in the plate thickness direction, which causes distortion in the plate and material variation.

[0005]

DISCLOSURE OF THE INVENTION An object of the present invention is to provide a thick wall U having less hardness unevenness in the plate thickness direction and excellent DWTT characteristics.
An object of the present invention is to provide a method for manufacturing an OE steel pipe material.

[0006]

The present invention, in terms of weight ratio,
C: 0.01 to 0.18%, Si: 0.05 to 0.5%, Mn: 1.0 to 2.5
%, Al: 0.005-0.06%, Nb: 0.01-0.1%,
90 steel slabs with the balance being iron and inevitable impurities
After heating in the range of _{0 ~1050 ℃, (Ar 3 +} 70 ℃) above recrystallization γ region giving the reduction of 50% or more, and subsequently (Ar ₃ + 70 ℃)
~ 65% reduction in the unrecrystallized γ region of Ar ₃
In the (γ + α) two-phase region of Ar ₃ to (Ar ₃ -80 ° C), 10 to 60% reduction including 1 to 6 water cooling passes is applied before final finishing, and then cooling rate of 20 to 50 ° C / s. Is a method for producing a high-strength steel sheet having less hardness unevenness in the plate thickness direction and excellent low-temperature toughness, which is characterized by accelerating cooling to less than 400 ° C. at a temperature of Cu: 1.0% or less, N
i: 1.0% or less, Cr: 1.5% or less, Mo: 0.5% or less, V
: 0.01 to 0.1% or less, Ti: 0.005 to 0.1% or less, Ca:
0.001 to 0.01%, REM: 0.001 to 0.01% of 1 or 2
After heating a steel slab containing at least one species and the balance consisting of iron and unavoidable impurities in the range of 900 to 1050 ℃, (Ar ₃ +70
° C.) in the above recrystallization γ region giving the reduction of 50% or more, given subsequently (Ar ₃ + 70 ℃) ~Ar pressure of more than 65% non-recrystallized γ zone _3, then Ar ₃ ~ (Ar ₃ 10-60% including 1-6 water cooling passes before final finishing in the (γ + α) 2 phase region at -80 ° C)
At a cooling rate of 20 to 50 ° C / s at 400 ° C.
It is a method for producing a high-strength steel sheet having less hardness unevenness in the plate thickness direction and excellent low-temperature toughness, which is characterized in that accelerated cooling is performed to a temperature lower than that.

[0007]

[Working] As a result of diligent studies on the microstructure that improves the DWTT characteristics of thick-walled materials, the present inventors found that simply lowering the slab heating temperature and refining the ferrite structure would result in a DWTT 85% fracture surface transition temperature. Temperature does not fall below -60 ° C, while the fine acicular ferrite structure is mixed with island martensite,
It was newly found that the T characteristic is improved and the hardness unevenness in the plate thickness direction is reduced.

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 0.07% C-0.3% Si-1.9
% Mn-0.04% Nb steel, slabs from 850 ℃ to 1100
After heating ° C., after giving a 60% reduction in (Ar ₃ + 70 ° C.) or recrystallization gamma zone, the non-recrystallization to Ar ₃ from (Ar ₃ + 70 ℃) γ
The tensile strength and DWTT of 85% when accelerated cooling is applied after 70% reduction in the temperature range and 30% reduction in the (γ + α) two-phase range from Ar ₃ to (Ar ₃ -80 ° C). The relationship between the fracture surface transition temperatures is shown. Accelerated cooling fixes the cooling rate at 30 ° C / s,
Cooling stop temperature is 600 ℃ (□ mark) and 300 ℃ (△ mark and ○ mark)
It is a different one. The difference between the ○ and △ marks of the 300 ℃ stopped material is that in the (γ + α) two phase rolling before accelerated cooling, three water cooling passes were inserted between 1 pass and 6 passes before the final finish rolling (○ This is a comparison between the mark () and the one with no water cooling pass (marked with Δ). The plate thickness at this time was 24 mm.

When the cooling stop temperature is lowered from 600 ° C. to 300 ° C., the tensile strength increases, the DWTT characteristics shift to the low temperature side, and high strength and high DWTT characteristics can be achieved at the same time. Also
By introducing a water cooling path in the 300 ° C stop material,
It can be seen that the DWTT characteristic is further improved. Also DW
The TT characteristics strongly depend on the heating temperature, and by setting the heating temperature in the range of 900 ° C to 1050 ° C, the DWTT85% fracture surface transition temperature becomes -60 ° C or less.

FIG. 2 shows 0.07% C-0.3% Si-1.9% Mn.
Using -0.04% Nb steel, after heating the slab to 1000 ℃, (Ar ₃
After applying a 60% reduction in the recrystallization γ region above + 70 ° C, (A
70% reduction is applied in the unrecrystallized γ region from r ₃ + 70 ℃ to Ar ₃ and (γ + α) 2 from Ar ₃ to (Ar ₃ -80 ℃).
After applying a reduction of 30% in the phase region, 300 at a cooling rate of 30 ° C / s
The hardness distribution in the plate thickness direction when accelerated cooling is performed up to ℃ is shown.

In the figure, in the (γ + α) two-phase rolling before accelerated cooling, a water cooling pass was not introduced (marked Δ) and three water cooling passes were introduced between 1 pass and 6 passes before final finishing rolling. This is a comparison of the difference in hardness distribution with the one (marked with ○). It can be seen that the hardness difference is about 30 points when the water cooling path is not introduced, whereas the hardness difference becomes about 10 points when the water cooling path is introduced, and the hardness difference in the plate thickness direction becomes small.

That is, by introducing a water cooling path during the (γ + α) 2 phase rolling and setting the cooling stop temperature to less than 400 ° C., it is possible to produce a steel sheet having less hardness unevenness in the sheet thickness direction and excellent DWTT characteristics. You can see that Although the detailed mechanism of improving the DWTT characteristics by setting the cooling stop temperature to less than 400 ° C has not been clarified, in the DWTT test, since the plate thickness of the test piece is thicker than that of the Charpy test piece, the crack tip has a large internal Stress is generated. Due to this internal stress, the hard island martensite is peeled off and fine separation occurs. By finely dispersing this island-like martensite, a large number of fine separations occur, and the fracture surface transition temperature shifts to the low temperature side.

In order to finely disperse fine island martensite, it is important to form the matrix structure as an acicular ferrite structure and to form island martensite between acicular ferrites. Therefore, the cooling rate is 20
Untransformed γ is transformed into acicular ferrite by increasing the cooling rate to 400 ° C / s or more and the cooling rate to less than 400 ° C,
Island martensite is generated at the interface.

However, when island martensite is formed, there is a problem that hardness tends to increase. By changing the structure from polygonal ferrite to acicular ferrite, the untransformed γ is finely dispersed, and the concentration of C and the like in the finely dispersed untransformed γ is reduced, and the hardness increase of island martensite is small. Become. When the steel sheet is rolled while air-cooling the (γ + α) 2 phase region, ferrite coarsens, island martensite is formed, and hardness increases. But (γ + α) 2
When a water cooling pass is introduced during phase rolling, the steel plate is supercooled by the water cooling pass and many ferrite nuclei are generated, but grain growth of ferrite is blocked and ferrite grains are refined. Further, since a large amount of fine ferrite is generated, the concentration of C and the like in the untransformed γ is prevented, so that the hardness is lowered and the toughness is improved.

Conventionally, examples of accelerated cooling after (γ + α) two-phase rolling are disclosed in, for example, Japanese Patent Laid-Open No. 516152/1985 and Japanese Patent Laid-Open No. 57-1.
As can be seen in Japanese Patent No. 34518, in both cases, the second phase structure is transformed into acicular ferrite to finely disperse the island martensite as in the present invention, and the hardness unevenness in the plate thickness direction is small and the DWTT characteristics are improved. There is no disclosure of technology.

Next, the reasons for limiting the component composition of the steel sheet to obtain the above-described structure by such rolling and accelerated cooling treatment will be described. If the content of C is less than 0.01%, the strength of the steel sheet decreases and the weld heat affected zone (hereinafter abbreviated as HAZ) is large in softness, so the lower limit of the C content was made 0.01%. Further, when C exceeds 0.18%, the toughness of the base material deteriorates, and the hardening of the weld zone and the deterioration of crack resistance are remarkable, so the upper limit was made 0.18%.

Si is an element that is inevitably contained in deoxidation during steel refining, but if it is less than 0.05%, the base material toughness deteriorates, so the lower limit was made 0.05%. On the other hand, if the amount of Si is too large, the cleanliness of the steel deteriorates and the toughness decreases, so the upper limit is 0.5%.
And If the Mn content is less than 1.0%, the strength and toughness of the steel sheet will deteriorate, and the HAZ will soften significantly, so the lower limit was made 1.0%. On the other hand, if the Mn content is too high, the HAZ toughness deteriorates, so the upper limit was made 2.5%.

Since Al needs to be added so as to form a solid solution of at least 0.005% for deoxidation, the lower limit of Al was made 0.005%. On the other hand, if the solid solution exceeds 0.06%, HA
Not only the toughness of Z but also the toughness of the weld metal deteriorates remarkably.
Therefore, the upper limit of Al is set to 0.06%. Nb is an important element for obtaining acicular ferrite in the present invention, but is 0.1% to avoid deterioration of the toughness of the weld metal portion.
The upper limit was made 0.1% because it must be less than or equal to%.
On the other hand, if the Nb content is less than 0.01%, the effect of fine particles for improving the transition temperature cannot be obtained, and therefore the lower limit of the total Nb content is set to 0.
It was set to 01%.

The above are the basic components of the steel slab used in the present invention, and if necessary, Cu: 1.0% or less, Cr: 1.5% or less, N: N or less for increasing the strength or increasing the thickness.
i: 1.0% or less, Mo: 0.5% or less, V: 0.01 to 0.1% or less, one or more types, Ti: 0.005 to 0.1%, and MnS in order to reduce toughness unevenness or improve toughness.
C to improve morphology control, toughness and HIC resistance
One or two or more of a: 0.001 to 0.01% and REM: 0.001 to 0.01% can be contained.

Cu improves the toughness, but if it exceeds 1.0%, cracks are likely to occur during hot rolling and the surface properties of the steel sheet deteriorate, so the upper limit was made 1.0%. Ni
Has substantially the same effect as Cu, but if it is added and contained in excess of 1.0%, it causes an increase in the HAZ curability and production cost, and is not necessary for achieving the objects and effects of the present invention. The upper limit of Ni was set to 1.0%.

Since Cr forms bainite, Cr is added to improve the strength and toughness. However, if the addition amount is too large, the toughness of the base material and HAZ is significantly deteriorated, so the upper limit was made 1.5%. Mo improves the strength and toughness because it regulates the γ grains during rolling and produces fine bainite, but in order to achieve the object of the present invention,
It is not necessary to add more than 0.5%, and if it exceeds that amount, the manufacturing cost will rise, so the upper limit was made 0.5%.

V is added to improve the base metal strength and toughness of the steel sheet and to secure the joint strength. However, if the addition amount is too large, the toughness of the base material and HAZ is significantly deteriorated, so the upper limit was made 0.1%. Ti is added for the purpose of improving toughness by the effect of refining γ grains and increasing strength by Ti carbonitride. However, if the Ti content is less than 0.005%, the effect is not obtained, and if it exceeds 0.1%, the toughness deteriorates, so Ti
The lower limit of the added amount was 0.005% and the upper limit was 0.1%.

If the content of Ca is less than 0.001%, it is insufficient for controlling the morphology of MnS and there is no effect of improving the toughness in the T direction, so the lower limit of Ca was made 0.001%. On the other hand, if Ca exceeds 0.01%, the cleanliness of the steel deteriorates, causing internal defects.
The upper limit of a was 0.01%. If the REM content is less than 0.001%, it is insufficient for controlling the morphology of MnS and is not effective in improving the toughness in the T direction, so the lower limit of REM content was set to 0.001%. On the other hand, REM
If it exceeds 0.01%, the cleanliness of the steel will deteriorate and problems may occur during arc welding, so the upper limit of REM was set to 0.01%.

Next, heating, which is the second component of the present invention,
The reasons for limiting the rolling and cooling conditions will be described. The slab is heated first, but the reason for limiting the heating temperature to 900 ° C to 1050 ° C is to keep the γ grains during heating fine and to make the rolling structure finer. 1050 ° C. is an upper limit temperature at which the γ grains do not coarsen during heating, and if the heating temperature exceeds this temperature, the γ grains coarsen and remain as coarse bainite after rolling, degrading the toughness of the steel. On the other hand, if the heating temperature is too low, carbides and the like will not be sufficiently solutionized, the internal quality of the steel will deteriorate, and the temperature at the end stage of rolling will be too low, so a sufficient material improvement effect by accelerated cooling cannot be expected. Therefore, it is necessary to set the lower limit to 900 ° C.

However, even if the heating temperature is limited to a low value as described above, a good material cannot be obtained if the rolling conditions are inappropriate. For this reason, the condition is that the rolling reduction in the recrystallization γ region above (Ar ₃ + 70 ° C) is 50% or more. Recrystallization γ
In the region, rolling-recrystallization is repeated to reduce the grain size, but if the rolling reduction is less than 50%, the grain size of the γ grains becomes insufficient, and even if rolling is performed in the unrecrystallized γ region that follows. The toughness deteriorates. Therefore, the rolling reduction in the recrystallization γ region must be 50% or more.

Next rolling at (Ar ₃ + 70 ° C.) below Ar ₃ or more non-recrystallized γ region is carried out to introduce the variant in γ grains lengthening and γ the grains, (Ar ₃ +70 ℃) over temperature range or Ar
_In the temperature range of less than ₃ , the above object cannot be achieved. Furthermore, the rolling reduction in this temperature range needs to be 65% or more. If the rolling reduction is less than 65%, the elongation of γ grains and the introduction of the deformation zone become insufficient, and α during the subsequent (γ + α) two-phase rolling is coarsened, so that the toughness is significantly deteriorated.

Then Ar₃~ (Ar₃-80 ℃) (γ + α) 2
Rolling in the phase region introduces subgrains into transformed α
In addition, further refinement of untransformed γ and expansion of
Separation is generated on the fracture surface of the test piece, and the transition temperature is low.
Do it to get down. But Ar ₃Super temperature range or (A
r₃The above object is not achieved in the temperature range below -80 ° C.
Furthermore, if the rolling reduction in this temperature range is less than 10%, untransformed γ
Insufficient grain refinement and subsequent accelerated cooling
Large bainite is generated. If the rolling reduction exceeds 60%, it will change.
Since the number of subgrains introduced in the activated α increases,
Also significantly deteriorates toughness. Therefore, the reduction rate should be 10 to 60%.
Need to be

Before the final finishing of the above (γ + α) 2 phase zone rolling
Roll from 1 to 6 passes with water cooling.
This is done in order to reduce the hardness unevenness in the plate thickness direction by creating a supercooled state up to 3 mm from the surface of the steel plate and producing a large amount of fine ferrite, but the vicinity of the steel plate surface does not become supercooled unless a water cooling pass is performed. Therefore, hardness unevenness occurs. On the other hand, if it exceeds 6 passes, the supercooled area due to water cooling will exceed 3 mm, and the effect of preventing uneven hardness will be reduced, so
The range was 1 to 6 passes.

The purpose of water cooling is to supercool the steel sheet surface up to about 3 mm. For example, a shower for water cooling and temperature control in a descaling water cooling device installed in the front and rear surfaces of the rolling mill. Water cooling by means such as the above is conceivable, but the means of water cooling is not particularly limited. Accelerated cooling is performed after the (γ + α) two-phase rolling and the water-cooling pass are applied. In this cooling, untransformed γ is transformed into acicular ferrite and fine island martensite, and fine separation is generated to improve DWTT characteristics. To improve. If the cooling rate is less than 20 ° C / s, acicular ferrite will not be generated. On the other hand, if it exceeds 50 ° C / s, coarse bainite will be generated and the DWTT characteristics will be significantly deteriorated.
It was limited to the range of 20 ° C / s to 50 ° C / s. The accelerated cooling is 400
Cooling is continued to less than ℃, but if accelerated cooling is stopped at 400 ℃ or more, island martensite will not be formed, so DW
Since the TT characteristics deteriorate significantly, the cooling stop temperature was set to less than 400 ° C.

[0030]

[Examples] Table 2 shows the sample steels whose composition is shown in Table 1.
The changes in mechanical properties of the steel sheet treated under the heating-rolling-cooling conditions shown in Table 1 were investigated, and the results are summarized in Table 2.

[0031]

[Table 1]

[0032]

[Table 2]

In Table 2, tests No. 1 to 10 and No. 12 to
No. 16 is a product having a thickness of 24 mm, which is obtained by subjecting a slab of steel A having the composition within the range limited in the present invention to various heating-rolling-cooling conditions. First test No. 1
Since the slab heating temperature is as low as 850 ° C, the tensile strength is low and the DWTT characteristics are not sufficient.

In Test Nos. 2 and 3, the rolling reduction in the unrecrystallized γ region was
The DWTT characteristic is poor because it is as low as 50% or (γ + α) 2 phase rolling is not performed. Test Nos. 4 and 5 have a slow cooling rate of 15 ° C / s, or the cooling stop temperature is low.
Since it is as high as 500 ° C, the tensile strength is low and the DWTT characteristics are not sufficient. Test Nos. 6, 7, and 8 have slab heating temperature of 1100 ° C
The DWTT characteristics are poor because the rolling reduction is high in the recrystallization γ region and the rolling reduction is low at 40%, and the rolling reduction in the (γ + α) 2 phase region is too large at 70%.

Test No. 14 has a large hardness difference (ΔHv) between the surface and the center because the water cooling pass before final finishing in the (γ + α) 2 phase region is not performed. In the test No. 15, the cooling rate after rolling was too fast at 60 ° C./s, so the DWTT characteristics were poor.
On the other hand, since Test Nos. 9 and 10 were manufactured in accordance with the constitutional requirements of the present invention, high strength and excellent DWTT characteristics (85%
It can be seen that the fracture surface transition temperature is -60 ° C or less).

In Test No. 11, the manufacturing conditions satisfied the constitutional conditions of the present invention, but in the composition of the composition which is another important constitutional condition, since Nb was not contained, the strength and DW were
Poor TT characteristics. Next, Test Nos. 12 and 13 show the mechanical properties of the C and D steel slabs having the composition according to the present invention and the steel plate having a thickness of 30 mm manufactured by satisfying all the constituent requirements. It can be seen that all are steel sheets having high strength and excellent DWTT characteristics.

[0037]

EFFECTS OF THE INVENTION According to the present invention, the cooling unevenness in the plate thickness direction is small, the DWTT 85% fracture surface transition temperature is -60 ° C or less, and the DWTT characteristics are excellent. A steel plate can be easily obtained.

[Brief description of drawings]

FIG. 1 is a characteristic diagram showing the relationship between the heating temperature, the presence or absence of a water cooling pass during (γ + α) two-phase rolling, and the tensile strength and the DWTT 85% transition temperature (FATT) when the cooling stop temperature is changed.

FIG. 2 is a diagram showing a difference in hardness distribution in a plate thickness direction depending on the presence or absence of a water cooling pass during (γ + α) 2 phase rolling.

Claims (2)

[Claims] 1. A weight ratio of C: 0.01 to 0.18% and Si: 0.
05-0.5%, Mn: 1.0-2.5%, Al: 0.005-0.06%,
After heating a steel slab containing Nb: 0.01 to 0.1% and the balance of iron and unavoidable impurities in the range of 900 to 1050 ° C,
(Ar ₃ + 70 ℃) gave the reduction of 50% or more by recrystallization γ region above, the pre-recrystallization γ zone of subsequent _{(Ar 3 + 70 ℃) ~Ar} 3 65
% Or more, then Ar ₃ to (Ar ₃ -80 ° C) (γ
+ Α) In the two-phase region, 10 to 60% reduction including 1 to 6 water-cooling passes is applied before final finishing, and thereafter accelerated cooling is performed at a cooling rate of 20 to 50 ° C / s to less than 400 ° C. A method for manufacturing a high-strength steel sheet having less hardness unevenness in the plate thickness direction and excellent DWTT characteristics. 2. A weight ratio of C: 0.01 to 0.18% and Si: 0.
05-0.5%, Mn: 1.0-2.5%, Al: 0.005-0.06%,
Nb: 0.01 to 0.1%, Cu: 1.0% or less, N
i: 1.0% or less, Cr: 1.5% or less, Mo: 0.5% or less, V
: 0.01 to 0.1% or less, Ti: 0.005 to 0.1% or less, Ca:
0.001 to 0.01%, REM: 0.001 to 0.01% of 1 or 2
After heating a steel slab containing at least one species and the balance consisting of iron and unavoidable impurities in the range of 900 to 1050 ℃, (Ar ₃ + 70
° C.) in the above recrystallization γ region giving the reduction of 50% or more, given subsequently (Ar ₃ + 70 ℃) ~Ar pressure of more than 65% non-recrystallized γ zone _3, then Ar ₃ ~ (Ar ₃ 10-60% including 1-6 water cooling passes before final finishing in the (γ + α) 2 phase region at -80 ° C)
At a cooling rate of 20 to 50 ° C / s at 400 ° C.
A method for producing a high-strength steel sheet having less hardness unevenness in the plate thickness direction and having excellent DWTT characteristics, characterized by accelerating cooling to less than a certain level.