JP5812115B2 - High-tensile hot-rolled steel sheet and manufacturing method thereof - Google Patents

Description

The present invention is for welded steel pipes that require high strength and high toughness, especially for transport pipes (line pipes) for transporting crude oil, natural gas, etc., and for oil well pipes, especially for high strength ERW steel pipes and high strength spiral steel pipes. The present invention relates to a high-strength hot-rolled steel sheet suitable as a material and a method for producing the same, and particularly relates to a steel sheet having improved deformation characteristics after being pipe-formed (tube-formed).
The “high-tensile hot-rolled steel sheet” herein refers to a hot-rolled steel sheet having a high strength of API5L-X65 grade or more and X80 grade or less.

  In recent years, oil and natural gas extraction and pipeline laying have been actively carried out in extremely cold regions due to soaring crude oil since the oil crisis and diversification of energy supply sources. Furthermore, in the pipeline, in order to improve the transport efficiency of natural gas and oil, there is a tendency to perform high-pressure operation with a large diameter. In order to withstand the high-pressure operation of the pipeline, the transport pipe (line pipe) must be a thick-walled steel pipe, a steel pipe made of a thick steel plate such as UOE steel pipe is used, and the API5L standard X80 grade Strength grade steel pipes are used. Recently, however, there is a strong demand for further reduction in pipeline construction costs and a reduction in material costs for steel pipes. As a transport pipe, instead of UOE steel pipes made of thick steel plates, coil-shaped hot-rolled steel sheets are used. High-strength ERW steel pipes or high-strength spiral steel pipes made from (hot rolled steel strip) have come to be used.

  These high-strength steel pipes are required to have excellent low-temperature toughness as well as high strength from the viewpoint of preventing line pipe breakage. In order to produce a steel pipe having both high strength and high toughness, in steel sheet as a steel pipe material, transformation strengthening using accelerated cooling after hot rolling and alloying elements such as Nb, Ti, V, etc. Strengthening by precipitation strengthening using precipitates and toughness by microstructure refinement using controlled rolling have been attempted.

  For example, Patent Document 1 contains appropriate amounts of C, Si, Mn, and N, further contains Si and Mn in a range where Mn / Si satisfies 5 to 8, and further contains Nb: 0.01 to 0.1%. Rough rolling to adjust the rolling reduction for each rolling temperature, and once cooling the surface layer part to Ar1 point or less, the temperature of the surface layer part is (Ac3-40 ° C) to (Ac3 + 40 ° C) by reheating or forced heating. ), And finish rolling is performed at a total rolling reduction at 950 ° C. or lower: 60% or higher, rolling finish temperature: Ac3 point or higher, and at 10 ° C./s or higher after finishing rolling. A method for producing a hot rolled steel sheet for high-strength ERW steel pipe that is cooled to 600 ° C. or lower and wound in a temperature range of 600 to 350 ° C. is described. In the technique described in Patent Document 1, a high-strength electric resistance welded steel pipe excellent in low-temperature toughness can be refined without adding an expensive alloy element or heat treating the entire steel pipe Can be manufactured. However, in the technique described in Patent Document 1, in the case of a steel plate having a large thickness, there is a problem that the cooling capacity is insufficient, a desired cooling rate cannot be secured, and further improvement of the cooling capacity is required. there were.

  In addition, the environment in which pipelines are laid is diversifying year by year, and there are cases where bending deformation of pipelines cannot be ignored due to ground fluctuations, ocean currents at the seabed, etc. after pipeline laying. It has become.

  For example, Patent Document 2 includes C: 0.02 to 0.09%, Si: 0.001 to 0.8%, Mn: 0.5 to 2.5%, and Ti: 0.005 to 0.03%, Nb: 0.005. -0.3%, Al: 0.001-0.1%, N: 0.001-0.008%, and Ni: 0.1-1.0%, Cu: 0.1-1.0, Mo: 0.05-0.6%, (Ni + Cu)- Aging that is formed by welding a steel sheet having a mixed structure of ferrite with a grain size of 15 μm or less on average and a balance of martensite and / or bainite, including Mo> 0.5 so as to satisfy Mo> 0.5. A high-strength steel pipe for pipelines having excellent later deformation characteristics is described. This steel pipe is said to be an X-70 to X-100 grade high-strength steel pipe having a uniform elongation after heating at 200 to 300 ° C. of 5% or more and excellent deformation characteristics after aging. However, in the technique described in Patent Document 2, it is necessary to contain a relatively large amount of alloy elements such as Cu and expensive Ni that cause liquid phase embrittlement during hot rolling, leaving a problem in weldability. It was.

  Recently, the reel barge method has been frequently used for laying submarine line pipes. In the reel barge method, circumferential welding, inspection, painting, etc. are performed in advance on land, and the resulting long pipe is wound on the reel of a barge ship at sea and laid on the seabed while rewinding at the target sea. Is the method. However, in the reel barge method, tensile and compressive stress due to bending-bending back acts on a part of the pipe when winding the pipe and laying the pipe. For this reason, local buckling arises in a pipe, and there exists a problem that destruction of a pipe arises from it.

  For such problems, for example, Patent Document 3 includes C: 0.03 to 0.20%, Si: 0.05 to 0.50%, Mn: 0.50 to 1.5%, Al: 0.005 to 0.060%, and Nb + V + Ti to 0.04% or less. ESR with excellent reel barge laying property with a yield ratio of 85% or less and less weld softening, with a limited composition, carbon equivalent Ceq of 0.20 to 0.35% and weld crack sensitivity index Pcm of 0.25% or less Steel pipes are described. According to the technique described in Patent Document 3, local buckling that occurs in a pipe when a pipeline is laid using a reel barge method can be prevented.

  Patent Document 4 discloses that a strain having a composition containing C: 0.1% or less and Mn: 2.3% or less is given a strain of 15% or less on the average in the thickness direction before the pipe forming step. A method for producing an electric resistance welded steel pipe, which is subjected to an applying step, is described. Thereby, local buckling at the time of pipe laying can be prevented.

Japanese Patent Laid-Open No. 2001-207220 JP 2006-144037 Japanese Patent Laid-Open No. 03-211255 JP 2006-122932 A

However, in the technique described in Patent Document 3, it is necessary to increase the amount of C in order to stably secure a high strength of X65 grade or higher, and there remains a problem that toughness cannot be ensured. Further, the technique described in Patent Document 4 has a problem in that it is necessary to apply a strain applying process to the steel strip, and a large-scale strain introducing facility is required.
Also, line pipes are usually painted on the pipe surface for corrosion protection. And in order to bake the coating, the coating baking process heated to the temperature of the range of 200-300 degreeC is given. Therefore, a steel pipe in which strain is introduced during pipe making is hardened by strain aging, yield strength increases, and may exhibit deformation characteristics indicating yield elongation. In the steel pipe showing such deformation characteristics, there is a problem that local buckling occurs during bending deformation of the steel pipe and the pipe is broken.

The present invention solves the above-described problems of the prior art, has high strength of API5L-X65 grade or more and X80 grade or less, and has high toughness with a fracture surface transition temperature vTrs of -80 ° C. or less in the Charpy impact test. Another object of the present invention is to provide a high-tensile hot-rolled steel sheet having excellent deformation characteristics such that the uniform elongation at the center in the sheet thickness direction is 10% or more and excellent deformation characteristics after tube forming.
Note that “excellent deformation characteristics after tube forming” as used herein means that the surface layer portion has a uniform elongation of 10 in a tensile test using a JIS No. 5 test piece (GL: 50 mm) in accordance with the provisions of JIS Z 2241. %, And after applying 2% tensile strain as a pre-strain, the paint bake-hardening amount ΔYS after coating bake-hardening treatment heated at 250 ° C x 60 min is 40MPa or less. Further, it means that the occurrence of yield elongation is suppressed after pipe forming-paint baking treatment, and it has a deformation characteristic that can suppress the occurrence of local buckling when subjected to bending deformation as a pipe.

In order to achieve the above-described object, the present inventors diligently studied various factors that affect the deformation characteristics, particularly the occurrence of yield elongation, after being subjected to paint baking as a pipe.
As a result, Cr and Nb, Ti, V are essential, the total amount of Nb, Ti, V is adjusted to an appropriate range, and the structure of the surface layer part is bainite as the main phase and a small amount as the second phase. By having a structure containing martensite, the surface layer has a uniform deformation at a high elongation of 10% or more, and after applying 2% pre-strain, heat treatment (paint baking treatment) is performed at 250 ° C x 60 min. The coating bake hardening amount ΔYS (= {(yield stress after coating baking) − (deformation stress after applying pre-strain))} is as low as 40 MPa or less, and the yield elongation can be suppressed after coating baking. I found it.

The present invention has been completed based on these findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.04 to 0.08%, Si: 0.50% or less, Mn: 0.8 to 2.2%, P: 0.02% or less, S: 0.006% or less, Al: 0.1% or less, N: 0.008% or less Cr: 0.05 to 0.8%, Nb: 0.01 to 0.08%, V: 0.001 to 0.12%, Ti: 0.005 to 0.04%, the following formula (1):
0.05 ≦ Nb + V + Ti ≦ 0.20 (1)
(Here, Nb, V, and Ti indicate the content (mass%) in the steel.)
Contains adjusted so as to satisfy a composition balance being Fe and unavoidable impurities, the surface layer is a region from the steel plate surface to 2mm in thickness direction, the bainite and the main phase, the volume ratio 0.5 to 4% martensite as the second phase, further having a structure containing as a third phase one or more selected from ferrite phase, pearlite, and cementite in a total volume ratio of 10% or less, The central portion of the thickness, which is a region other than the surface layer of the steel plate, has bainite as the main phase, 0.5 to 4% martensite by volume ratio as the second phase, and a ferrite phase having a total volume ratio of 20% or less, A high-tensile hot-rolled steel sheet having a structure containing at least one selected from pearlite and cementite as a third phase and having a uniform elongation of 10% or more.

(2) In (1) , in addition to the above-mentioned composition, by mass%, Mo: 0.3% or less, Cu: 0.5% or less, Ni: 0.5% or less, B: 0.001% or less A high-tensile hot-rolled steel sheet containing two or more kinds.

(3) In (1) or (2) , in addition to the above-mentioned composition, the composition further contains one or two selected from the group consisting of Zr: 0.04% or less and Ta: 0.07% or less in mass%. High tensile hot-rolled steel sheet.

(4) In any one of (1) to (3) , in addition to the above composition, the composition further contains one or two kinds selected from Ca: 0.005% or less and REM: 0.005% or less in mass%. A high-tensile hot-rolled steel sheet characterized by that.

(5) After heating, the steel material is hot-rolled to form a hot-rolled sheet. Immediately after the hot-rolling, the hot-rolled sheet is subjected to accelerated cooling, and then coiled at a coiling temperature. A method for producing a rolled steel sheet, wherein the steel material is, in mass%, C: 0.04 to 0.08%, Si: 0.50% or less, Mn: 0.8 to 2.2%, P: 0.02% or less, S: 0.006% or less, Al: 0.1% or less, N: 0.008% or less, Cr: 0.05 to 0.8%, Nb: 0.01 to 0.08%, V: 0.001 to 0.12%, Ti: 0.005 to 0.04%, the following formula (1):
0.05 ≦ Nb + V + Ti ≦ 0.20 (1)
(Here, Nb, V, and Ti indicate the content (mass%) in the steel.)
Is adjusted to satisfy the content, the balance is composed of Fe and inevitable impurities, the heating of the steel material is a temperature in the range of 1100-1250 ° C., in the finish rolling in the hot rolling, The cumulative rolling reduction in the temperature range of 930 ° C or lower is set to 50% or higher, the rolling end temperature is set to 760 ° C or higher, and the accelerated cooling is performed immediately after the finish rolling is completed at an average cooling rate CR of 7 to 50 ° C / s. Start, over 550 ° C, the following formula (2):
SCT (° C) = 750−270C−90Mn + 4Si (25−CR) −80Mo−30 (Cu + Ni) (2)
(Here, C, Mn, Si, Mo, Cu, and Ni indicate the content (% by mass) in the steel, and CR indicates the average cooling rate (° C./s) in accelerated cooling.)
The temperature is defined as SCT + 30 ° C. or less, and the cooling is stopped at a cooling stop temperature, and after the accelerated cooling is stopped, the time until the winding is (SCT−20 ° C.) to (SCT + 30 ℃) in the temperature range is subjected to a cooling treatment or slow cooling treatment in which the residence time is 10 to 60 s, and the winding temperature is set to 430 ° C or higher (SCT-50 ° C) or lower ,
In the obtained high-tensile hot-rolled steel sheet, the surface layer that is a region from the steel sheet surface to 2 mm in the sheet thickness direction has bainite as the main phase, volume ratio of 0.5 to 4% martensite as the second phase, One or more selected from ferrite phase, pearlite, and cementite with a total volume ratio of 10% or less, and having a structure containing as a third phase, the plate thickness central portion that is a region other than the surface layer of the steel plate, One or more selected from ferrite phase, pearlite, and cementite having a volume ratio of 0.5 to 4% martensite as the second phase and a total volume ratio of 20% or less as the second phase. A method for producing a high-tensile hot-rolled steel sheet having excellent deformability after tube forming , having a structure including a phase and exhibiting a uniform elongation of 10% or more .

(6) In (5) , in addition to the above-mentioned composition, by mass%, Mo: 0.3% or less, Cu: 0.5% or less, Ni: 0.5% or less, B: 0.001% or less A method for producing a high-tensile hot-rolled steel sheet, comprising two or more kinds.

(7) In (5) or (6) , in addition to the above-described composition, the composition further contains one or two kinds selected from Zr: 0.04% or less and Ta: 0.07% or less in mass%. A method for producing a high-tensile hot-rolled steel sheet.

(8) In any one of (5) to (7) , in addition to the above composition, the composition further contains one or two selected from Ca: 0.005% or less and REM: 0.005% or less by mass%. A method for producing a high-tensile hot-rolled steel sheet.

  According to the present invention, a high-tensile hot-rolled steel sheet that does not cause local buckling when subjected to bending deformation as a steel pipe, has excellent deformation characteristics after pipe forming, and is suitable for line pipes and oil well pipes. Can be manufactured at a low cost, and has a remarkable industrial effect.

  The high-tensile hot-rolled steel sheet of the present invention has a high strength of API5L-X65 grade or more and X80 grade or less, and is suitable for line pipes and oil well pipes, and has excellent deformation characteristics after pipe forming (piping). It is a hot-rolled steel sheet that can be manufactured.

  First, the reasons for limiting the composition of the high-tensile hot-rolled steel sheet of the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply expressed as%.

C: 0.04-0.08%
C is an element having an effect of increasing the strength of steel, and in the present invention, it is necessary to contain 0.04% or more in order to ensure a desired strength. On the other hand, when it contains more than 0.08%, the toughness of the base metal and the weld heat affected zone toughness are lowered. For this reason, C was limited to 0.04 to 0.08% of range. In addition, Preferably it is 0.05 to 0.07%.

Si: 0.50% or less
Si is an element that acts as a deoxidizer, and such an effect is recognized with a content of 0.01% or more. Moreover, Si forms an oxide containing Si during ERW welding, lowers the weld zone quality, and lowers the weld heat affected zone toughness. From this point of view, it is desirable to reduce Si as much as possible, but it is acceptable up to 0.50%. For these reasons, Si is limited to 0.50% or less. In addition, Preferably it is 0.40% or less.

Mn: 0.8-2.2%
Mn is an element that improves the hardenability and contributes to an increase in the strength of the steel sheet through the improvement of the hardenability. Further, Mn forms MnS and fixes S, thereby preventing S grain boundary segregation and suppressing slab cracking. In order to obtain such an effect, the content of 0.8% or more is required. On the other hand, an excessive content exceeding 2.2% promotes segregation during solidification, leaves Mn-concentrated portions in the steel sheet, and increases the occurrence of separation. For this reason, Mn was limited to the range of 0.8 to 2.2%. In addition, Preferably it is 0.9 to 2.1%.

P: 0.02% or less P has an effect of increasing the strength of steel, but has a strong segregation tendency and lowers toughness. Therefore, in the present invention, it is desirable to reduce P as much as possible, but it is acceptable up to 0.02%. Therefore, P is limited to 0.02% or less. In addition, Preferably it is 0.016% or less.

S: 0.006% or less S is present as an inclusion (sulfide) in steel, and has an adverse effect on reducing ductility and toughness. For this reason, it is desirable to reduce S as much as possible, but it is acceptable up to 0.006%. For these reasons, S is limited to 0.006% or less. In addition, Preferably it is 0.004% or less.

Al: 0.1% or less
Al is an element that acts as a deoxidizer, and in order to obtain such an effect, it is desirable to contain 0.001% or more. On the other hand, a content exceeding 0.1% significantly impairs the cleanliness of the welded part during ERW welding. For this reason, Al was limited to 0.1% or less.

N: 0.008% or less N is an element inevitably contained, but excessive inclusion frequently causes cracks during slab casting. In addition, solute N causes aging and increases yield strength (paint bake hardening) during paint baking, so it is desirable to reduce it as much as possible. For these reasons, N is limited to 0.008% or less.

Cr: 0.05-0.8%
Cr is an element that has the effect of improving the hardenability and increasing the strength of the steel sheet, and Cr has the effect of suppressing the occurrence of yield elongation after the coating baking process. In order to acquire such an effect, 0.05% or more of content is required. On the other hand, if it exceeds 0.8%, the strength becomes too high and the ductility and toughness are lowered. For this reason, Cr was limited to the range of 0.05 to 0.8%. In addition, Preferably it is 0.3 to 0.5%.

Nb: 0.01-0.08%
Nb is an element that has the action of suppressing the austenite grain boundary movement and suppressing the austenite grain coarsening and recrystallization. Moreover, Nb has the effect | action which makes a hot-rolled steel plate high intensity | strength with little content, without impairing weldability by carrying out fine precipitation as a carbonitride. Moreover, Nb fixes C and N, and reduces the amount of curing during the coating baking process. In order to acquire such an effect, 0.01% or more of content is required. On the other hand, if the content exceeds 0.08%, the strength becomes too high and the ductility and toughness are lowered. For this reason, Nb was limited to the range of 0.01 to 0.08%. In addition, Preferably it is 0.02 to 0.07%.

V: 0.001 to 0.12%
V has the effect | action which increases the intensity | strength of a steel plate by carrying out fine precipitation as a carbonitride. Moreover, V fixes C and N, suppresses the occurrence of yield elongation after paint baking, and improves the deformation characteristics after tube forming. In order to obtain such an effect, a content of 0.001% or more is required. On the other hand, if the content exceeds 0.12%, the strength becomes too high and the ductility and toughness are lowered. For this reason, V was limited to the range of 0.001 to 0.12%. In addition, Preferably it is 0.001 to 0.08%.

Ti: 0.005-0.04%
Ti has the effect of increasing the strength of the steel sheet by fine precipitation as carbonitride. Ti also fixes C and N, suppresses the occurrence of yield elongation after paint baking, and improves the deformation characteristics after tube forming. In order to acquire such an effect, 0.005% or more of content is required. On the other hand, if the content exceeds 0.04%, weldability decreases. For this reason, Ti was limited to 0.005 to 0.04% of range.

In the composition, Nb, V, and Ti are contained within the above-described range and adjusted to satisfy the following formula (1).
0.05 ≦ Nb + V + Ti ≦ 0.20 (1)
(Nb, V, Ti: content (mass%))
If the total content of Nb, V, and Ti is less than 0.05%, the desired high strength cannot be ensured, and the occurrence of yield elongation after paint baking cannot be suppressed. On the other hand, when the content exceeds 0.20%, the ductility and toughness deteriorate significantly. For these reasons, Nb, V, and Ti are adjusted and contained so as to satisfy the expression (1).

  The above components are the basic components, and in addition to these basic compositions, the elements selected from Mo: 0.3% or less, Cu: 0.5% or less, Ni: 0.5% or less, B: 0.001% or less are selected. One or two or more selected from Zr: 0.04% or less, Ta: 0.07% or less, and / or Ca: 0.005% or less, REM: 0.005% or less 1 type or 2 types chosen from can be selected and contained as needed.

One or more selected from Mo: 0.3% or less, Cu: 0.5% or less, Ni: 0.5% or less, B: 0.001% or less
Mo, Cu, Ni, and B all have the effect of increasing the strength of the steel sheet, and in the present invention, one of Mo, Cu, Ni, and B is selected as required, and one or more are selected as necessary. Can be contained.
Mo has an effect of increasing the strength of the steel sheet through the improvement of hardenability, and contributes to an increase in the strength of the steel sheet by being finely precipitated as carbonitride. Moreover, Mo has the effect | action which suppresses generation | occurrence | production of the yield elongation after a paint baking process. In order to acquire such an effect, it is desirable to contain 0.05% or more. On the other hand, the content exceeding 0.3% lowers the weldability. For this reason, when it contains, it is preferable to limit Mo to 0.3% or less.
Further, Cu dissolves or precipitates to increase the steel sheet strength. In order to acquire such an effect, it is desirable to contain 0.05% or more. On the other hand, if the content exceeds 0.5%, the surface quality of the steel sheet may be deteriorated. For this reason, when it contains, it is preferable to limit Cu to 0.5% or less.
Ni also dissolves to increase the strength of the steel sheet and contributes to improving the toughness of the steel sheet. In order to acquire such an effect, it is desirable to contain 0.05% or more. On the other hand, if the content exceeds 0.5%, the production cost increases. Therefore, when Ni is contained, Ni is preferably limited to 0.5% or less.
Further, B, when contained in a small amount, significantly improves the hardenability and contributes to an increase in steel sheet strength. Such an effect becomes remarkable when the content is 0.0003% or more, but even if the content exceeds 0.001%, the effect is saturated. For these reasons, when it is contained, it is preferably limited to 0.001% or less.

One or two selected from Zr: 0.04% or less, Ta: 0.07% or less
Zr and Ta are elements having an action of increasing the steel sheet by fine precipitation as carbonitride, and can be selected and contained as necessary. In order to obtain such an effect, it is desirable to contain Zr: 0.005% or more and Ta: 0.01% or more, but if it contains more than Zr: 0.04%, Ta: 0.07%, the weldability is lowered. For this reason, when it contains, it is preferable to limit to Zr: 0.04% or less and Ta: 0.07% or less.

Ca: One or two selected from 0.005% or less, REM: 0.005% or less
Both Ca and REM are elements that contribute to the control of the morphology of sulfides in which the expanded coarse sulfide is a spherical sulfide, and can be selected and contained as necessary. In order to obtain such an effect, it is desirable to contain Ca: 0.001% or more, REM: 0.001% or more, but if it contains a large amount exceeding Ca: 0.005% and REM: 0.005%, respectively, the cleanliness of the steel sheet Reduce. For this reason, it is preferable to limit Ca to 0.005% or less and REM to 0.005% or less.

  The balance other than the above-described composition components is composed of Fe and inevitable impurities.

In addition to having the above-described composition, the high-tensile hot-rolled steel sheet of the present invention includes a surface layer of the steel sheet that includes bainite as a main phase, and a volume ratio of 0.5 to 4% martensite as a second phase. As a three-phase, it has a structure containing at least one of ferrite, pearlite, and cementite in a total volume ratio of 10% or less.
Here, the “main phase” means a phase occupying 50% or more, preferably 80% or more by volume ratio. The “surface layer” here refers to a region from the steel plate surface to 2 mm in the plate thickness direction.

By making the structure of the surface layer of the steel sheet into a structure containing bainite as the main phase and martensite at a volume ratio of 0.5 to 4% as the second phase, excellent deformation characteristics with a uniform elongation of preferably 10% or more are preferable. You will be able to have Furthermore, even if paint baking is performed after pipe forming, the amount of curing is small, and after paint baking, yield elongation can be suppressed, and buckling does not occur even if the pipe is bent. It becomes a steel pipe with excellent properties. The “bainite” here includes bainite and bainitic ferrite.
In addition, by including a martensite phase as the second phase, the yield ratio is reduced, the deformation characteristics after pipe forming are improved, and the amount of hardening during the coating baking process can be reduced. The occurrence of yield elongation can be suppressed. In addition, the third phase other than bainite and martensite may include one or more selected from ferrite phase, pearlite, and cementite. These are preferably as small as possible in order to reduce the uniform elongation, but are acceptable if the total volume ratio is 10% or less.

Further, the plate thickness center part of the steel sheet, a bainite and a main phase, 0.5 to 4% of the martensite and the second phase by volume, further, the volume ratio of the total of 20% or less of the ferrite phase, pearlite, cementite It has the structure | tissue which contains 1 or more types selected from among them as a 3rd phase.
By making the structure of the steel plate thickness central part of the steel sheet into a structure containing bainite as a main phase and martensite as a second phase in a volume ratio of 0.5 to 4% as a second phase, it has both high strength and high toughness. Can be made. Specifically, a uniform elongation of 10% or more can be realized while having high strength. Here, the “plate thickness central portion” refers to a portion other than the surface layer. In addition, the third phase other than bainite and martensite may include one or more selected from ferrite phase, pearlite, and cementite. These are preferably as small as possible in order to reduce the strength and toughness, but the total volume ratio is 20% or less .

Below, the manufacturing method of this invention hot rolled sheet steel is demonstrated.
First, a steel material having the above-described composition is used as a starting material.
The method for producing the steel material is not particularly limited, and any generally known melting method such as a converter can be applied. The melted steel can be applied to any generally known casting method such as a continuous casting method, and can be cast on a steel material such as a slab.

The obtained steel material is then reheated.
The reheating of the steel material is performed at a temperature in the range of 1100 to 1250 ° C. When the reheating temperature is less than 1100 ° C., the amount of increase in strength due to the solid solution of Nb and precipitation after rolling is reduced, and it becomes difficult to secure a desired high strength. On the other hand, at a high temperature exceeding 1250 ° C., the crystal grains become coarse and the low temperature toughness decreases, the amount of scale generation increases, the surface properties decrease, and the yield decreases. For this reason, it is preferable to limit the heating temperature of a steel raw material to the range of 1100-1250 degreeC. In addition, when the steel raw material has the heat which can ensure the temperature of the above-mentioned range, you may perform hot rolling, without reheating or through a short time heating furnace holding | maintenance.

The heated steel material is then subjected to hot rolling comprising rough rolling and finish rolling.
Rough rolling only needs to be a sheet bar having a predetermined size and shape, and there is no need to particularly define the conditions. On the other hand, finish rolling is rolling in which the cumulative rolling reduction in a temperature range of 930 ° C. or lower is 50% or higher and the finish rolling finish temperature is 760 ° C. or higher. If the cumulative rolling reduction in a temperature range of 930 ° C. or lower (non-recrystallization temperature range) is less than 50%, crystal grain refinement cannot be achieved, and desired high toughness cannot be ensured. In addition, Preferably it is 85% or less. If the cumulative reduction ratio exceeds 85%, the reduction amount becomes too large, and the crystal grains form a shape that is extremely flat in the rolling direction, and peels in the plate thickness direction at the time of breakage, causing separation. For this reason, the cumulative rolling reduction in the non-recrystallization temperature range is 50% or more, preferably 85% or less.

Furthermore, when the finish rolling finish temperature is less than 760 ° C., the austenite → ferrite transformation proceeds particularly in the surface layer, and the surface layer structure cannot be made a structure having a desired bainite phase as the main phase, and the desired high toughness is achieved. It cannot be secured. In addition, Preferably it is 870 degrees C or less. When the finish rolling finish temperature exceeds 870 ° C., the structure cannot be refined and the toughness is lowered. Therefore, the finish rolling finish temperature is limited to 760 ° C. or higher, preferably 870 ° C. or lower.
Immediately after finishing rolling, accelerated cooling is preferably started within 15 s, more preferably within 10 s.

  Accelerated cooling is performed by cooling to a cooling stop temperature at an average cooling rate of 7 to 50 ° C./s, and cooling is stopped. Thereby, the generation of ferrite phase and pearlite is suppressed, and coarsening of crystal grains can be prevented. When the cooling rate is less than 7 ° C./s on average, the ferrite phase is excessively generated, and it becomes difficult to secure desired high strength and high toughness. When a large amount of ferrite formed at high temperature is formed, it becomes difficult to form a fine bainite phase. On the other hand, at a cooling rate exceeding 50 ° C./s, a martensite phase is easily formed, and a desired structure having a bainite phase as a main phase is hardly formed. For this reason, the cooling rate of accelerated cooling was limited to a range of 7 to 50 ° C./s on average. In addition, Preferably it is 20 degrees C / s or less.

The cooling stop temperature for accelerated cooling shall be in the range of 550 ° C or higher (SCT + 30 ° C) or lower.
When the cooling stop temperature for accelerated cooling is less than 550 ° C., a martensite phase is easily formed as a main phase, and a desired structure having a bainite phase as a main phase is hardly formed. On the other hand, when the temperature exceeds (SCT + 30 ° C.), a large amount of ferrite and pearlite are generated, making it difficult to stably secure desired characteristics.
Here, SCT is the following equation (2)
SCT (℃) ＝ 750−270C−90Mn + 4Si (25−CR) −80Mo−30 (Cu + Ni) (2)
(Here, C, Mn, Si, Mo, Cu, and Ni indicate the content (% by mass) in the steel, and CR indicates the average cooling rate (° C./s) in accelerated cooling.)
Is a value that represents the degree of ease of forming a bainite phase containing martensite inside, and is a value that depends on the amount of alloy elements contained and the degree of accelerated cooling.

  In the present invention, after accelerating cooling is stopped, during the period until winding, a cooling process or a slow cooling process in which the residence time in the temperature range of (SCT−20 ° C.) to (SCT + 30 ° C.) is 10 to 60 s. Apply. As a result, the surface of the steel plate is reheated, the temperature distribution in the plate thickness direction is made uniform, and the bainite phase containing martensite is easily generated while suppressing the generation of ferrite. When the residence time in the above temperature range is less than 10 s, recuperation is insufficient and the surface martensite amount is insufficient. On the other hand, when it exceeds 60 s, bainite grows and the toughness decreases, and the productivity of the steel sheet decreases. For this reason, the period from the accelerated cooling stop to the winding is set as a cooling treatment or a slow cooling treatment in which the residence time in the temperature range of (SCT−20 ° C.) to (SCT + 30 ° C.) is 10 to 60 seconds.

  Then, it is wound in a coil shape. The coiling temperature should be 430 ° C or higher (SCT-50 ° C) or lower. When the coiling temperature is less than 430 ° C., the diffusion of C is suppressed, and the martensite phase is not generated in the bainite that is the main phase. On the other hand, if it exceeds (SCT-50 ° C.), pearlite is produced, and a desired tissue cannot be produced.

  Hereinafter, the present invention will be described in detail based on examples.

Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab (wall thickness: 220 mm) by a continuous casting method. These slabs are heated to 1200 ° C., subjected to hot rolling consisting of rough rolling and finish rolling under the conditions shown in Table 2, and then subjected to accelerated cooling and cooling treatment under the cooling conditions shown in Table 2 after finishing rolling. These were wound into a coil shape under the conditions shown in Table 2 and allowed to cool to obtain a hot rolled steel sheet (hot rolled steel strip) having a thickness of 12 to 16 mm.
Samples are taken from the obtained hot-rolled steel sheet (hot-rolled steel strip) and subjected to structure observation, tensile test, impact test, and tensile test after paint baking treatment, and the structure, tensile properties, toughness, paint baking treatment Later tensile properties were evaluated. The evaluation method is as follows.

(1) Microstructure observation A specimen for microstructural observation is collected from the obtained hot-rolled steel sheet, the cross section in the rolling direction is polished and corroded, and an optical microscope (magnification: 1000 times) or a scanning electron microscope (magnification: 1000 times). Then, at least 5 fields of view were observed at the surface layer (position 1 mm from the surface) and the center position of the plate thickness and imaged. About the obtained structure | tissue photograph, the kind of structure | tissue and the tissue fraction were measured with the image-analysis apparatus. The obtained results are shown in Table 3.

(2) Tensile test JIS No. 5 tensile test so that the direction parallel to the rolling direction is the tensile direction from the surface layer of the obtained hot-rolled steel sheet (area from the surface to 2 mm in the sheet thickness direction) and the sheet thickness center position. A piece (GL: 50 mm) was collected and subjected to a tensile test in accordance with the provisions of JIS Z 2241 to measure tensile properties (yield strength, tensile strength, total elongation, uniform elongation). In addition, the tensile test piece in the surface layer (region from the surface to 2 mm in the plate thickness direction) was sampled so that the position of 1 mm from the surface was the central position in the thickness direction, and the thickness of the test piece was set to 1.6 mm. . Further, the tensile test piece at the center position of the plate thickness was prepared by removing the surface layer (region from the surface to 2 mm in the plate thickness direction) by cutting so that the center position of the plate thickness becomes the center position in the thickness direction. Table 4 shows the obtained results.

(3) Impact test V-notch test piece (width 10mm) was sampled from the center of the thickness of the obtained hot-rolled steel sheet so that the direction perpendicular to the rolling direction was the longitudinal direction, and conformed to the provisions of JIS Z 2242 Then, a Charpy impact test was performed to determine the fracture surface transition temperature vTrs (° C.) and toughness was evaluated. Table 4 shows the obtained results.

(4) Tensile test after paint baking treatment From the surface layer of the obtained hot-rolled steel sheet (region from the surface to 2 mm in the sheet thickness direction) and the center position of the sheet thickness, the direction parallel to the rolling direction is the tensile direction. A JIS No. 5 tensile test piece (GL: 50 mm) was collected. And after giving 2% pre-strain to the tensile test piece at room temperature, it was subjected to the same heat treatment (250 ° C. × 60 min) as the coating baking process, and the tensile test was carried out in the same manner as (2). Yield strength (deformation stress) and yield elongation were measured to determine the amount of paint bake hardening. Table 4 shows the obtained results.

From the results of Tables 1 to 4, all of the examples of the present invention have a structure having a bainite phase as a main phase, a yield strength YS: high strength of X65 grade of 450 MPa or more, and vTrs of −80 ° C. or less. It has high toughness, exhibits excellent deformation characteristics with a uniform elongation of 10% or more at the surface layer and in the center in the plate thickness direction, and yield elongation has been observed even after paint baking. The paint bake hardening amount was 40MPa or less, and the steel was also low in paint bake hardenability.
On the other hand, the comparative example out of the scope of the present invention is insufficient in strength, has reduced toughness, has deteriorated elongation characteristics, or has yield elongation, as a high-strength hot-rolled steel sheet for line pipes. Desired characteristics are not secured.

  Further, the hot rolled steel sheet of the present invention example was made into an electric resistance steel pipe by cold forming using a roll, and further subjected to diameter reduction rolling to obtain a steel pipe having an outer diameter of 406 mmφ. In the diameter reduction rolling, a tensile strain (tube making strain) of 3.5% or more was applied in the tube axis direction. The obtained ERW steel pipe was further subjected to heat treatment at 250 ° C. for 60 minutes. From the obtained steel pipe, an arc-shaped tensile test piece with the pipe axis direction as the tensile direction was collected, and a tensile test was carried out according to the API 5L standard. It was confirmed that the ERW steel pipe showed elongation and had excellent deformation characteristics. These steel pipes are steel pipes in which the occurrence of buckling is suppressed even when subjected to bending.

Claims (8)

% By mass
C: 0.04 to 0.08%, Si: 0.50% or less,
Mn: 0.8-2.2%, P: 0.02% or less,
S: 0.006% or less, Al: 0.1% or less,
N: 0.008% or less, Cr: 0.05-0.8%
In addition, Nb: 0.01 to 0.08%, V: 0.001 to 0.12%, Ti: 0.005 to 0.04% are contained so as to satisfy the following formula (1), and the balance is composed of Fe and inevitable impurities. Yes,
The surface layer, which is the area from the steel sheet surface to 2 mm in the thickness direction, has bainite as the main phase, 0.5 to 4% martensite in the volume ratio as the second phase, and ferrite with a total volume ratio of 10% or less. Having a structure containing at least one selected from a phase, pearlite, and cementite as a third phase,
The central portion of the thickness, which is a region other than the surface layer of the steel plate, has bainite as the main phase, 0.5 to 4% martensite by volume ratio as the second phase, and a ferrite phase having a total volume ratio of 20% or less, Having a structure containing at least one selected from pearlite and cementite as a third phase,
A high-tensile hot-rolled steel sheet characterized by a uniform elongation of 10% or more.
Record
0.05 ≦ Nb + V + Ti ≦ 0.20 (1)
Here, Nb, V, and Ti show content (mass%) in steel.   In addition to the above composition, the composition further contains one or more selected from Mo: 0.3% or less, Cu: 0.5% or less, Ni: 0.5% or less, and B: 0.001% or less. The high-tensile hot-rolled steel sheet according to claim 1.   The high tension according to claim 1 or 2, further comprising one or two kinds selected from Zr: 0.04% or less and Ta: 0.07% or less in mass% in addition to the composition. Hot rolled steel sheet.   4. In addition to the composition, the composition further contains one or two kinds selected from Ca: 0.005% or less and REM: 0.005% or less by mass%. High tensile hot rolled steel sheet. The steel material is heated and then hot-rolled to form a hot-rolled sheet. Immediately after the hot-rolling, the hot-rolled sheet is subjected to accelerated cooling and then wound into a coil at a coiling temperature. A method of manufacturing a steel sheet,
The steel material is mass%,
C: 0.04 to 0.08%, Si: 0.50% or less,
Mn: 0.8-2.2%, P: 0.02% or less,
S: 0.006% or less, Al: 0.1% or less,
N: 0.008% or less, Cr: 0.05-0.8%
And Nb: 0.01 to 0.08%, V: 0.001 to 0.12%, Ti: 0.005 to 0.04%, adjusted so as to satisfy the following formula (1), and the balance from Fe and inevitable impurities And the composition
The heating of the steel material is a temperature in the range of 1100-1250 ° C,
In the finish rolling in the hot rolling, the cumulative rolling reduction in the temperature range of 930 ° C or lower is 50% or more, the rolling end temperature is 760 ° C or higher,
Immediately after completion of the finish rolling, the accelerated cooling starts at an average cooling rate CR of 7 to 50 ° C./s, and is cooled in the range of 550 ° C. or higher and temperature SCT + 30 ° C. or lower defined by the following formula (2). Processing to stop cooling at the stop temperature,
After stopping the accelerated cooling, until the winding, a cooling treatment or a slow cooling treatment in which the residence time in the temperature range of (SCT-20 ° C.) to (SCT + 30 ° C.) is 10 to 60 seconds,
The winding temperature is set to 430 ° C or higher (SCT-50 ° C) or lower ,
In the obtained high-tensile hot-rolled steel sheet, the surface layer that is a region from the steel sheet surface to 2 mm in the sheet thickness direction has bainite as the main phase, volume ratio of 0.5 to 4% martensite as the second phase, One or more selected from ferrite phase, pearlite, and cementite with a total volume ratio of 10% or less, and having a structure containing as a third phase, the plate thickness central portion that is a region other than the surface layer of the steel plate, One or more selected from ferrite phase, pearlite, and cementite having a volume ratio of 0.5 to 4% martensite as the second phase and a total volume ratio of 20% or less as the second phase. A method for producing a high-tensile hot-rolled steel sheet, characterized by having a structure included as a phase and a uniform elongation of 10% or more .
Record
0.05 ≦ Nb + V + Ti ≦ 0.20 (1)
Here, Nb, V, and Ti show content (mass%) in steel.
SCT (℃) ＝ 750−270C−90Mn + 4Si (25−CR) −80Mo−30 (Cu + Ni) (2)
Here, C, Mn, Si, Mo, Cu, and Ni indicate the content (% by mass) in the steel,
CR indicates an average cooling rate (° C./s) in accelerated cooling.   In addition to the above composition, the composition further contains one or more selected from Mo: 0.3% or less, Cu: 0.5% or less, Ni: 0.5% or less, and B: 0.001% or less. The method for producing a high-tensile hot-rolled steel sheet according to claim 5,   The high tension according to claim 5 or 6, further comprising one or two kinds selected from Zr: 0.04% or less and Ta: 0.07% or less in mass% in addition to the composition. A method for producing a hot-rolled steel sheet.   In addition to the composition, the composition further contains one or two kinds selected from Ca: 0.005% or less and REM: 0.005% or less by mass%. Manufacturing method of high-tensile hot-rolled steel sheet.