KR20140099321A - Hot rolled high tensile strength steel sheet and method for manufacturing same - Google Patents

Abstract

An object of the present invention is to provide a high-strength hot-rolled steel sheet which does not generate local buckling when subjected to bending deformation as a steel pipe, exhibits excellent deformation characteristics after pipe forming, and is suitable for use as a line pipe and further as a well pipe. The composition of the present invention comprises, by mass%, 0.04 to 0.08% of C, 0.50% or less of Si, 0.8 to 2.2% of Mn, 0.02% or less of P, 0.006% or less of S, 0.008% or less and Cr: 0.05 to 0.8%, and further containing 0.01 to 0.08% of Nb, 0.001 to 0.12% of V and 0.005 to 0.04% of Ti, with the balance being Fe and inevitable impurities Wherein the surface layer contains bainite as a main phase, martensite having a volume ratio of 0.5 to 4% as a second phase, and a ferrite phase having a total volume ratio of 10% or less, pearlite, cementite , And a structure containing at least a species as a third phase.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a hot rolled steel sheet,

The present invention relates to a welded steel pipe requiring high strength and high toughness for use as a transportation pipe (line pipe) for transporting crude oil, natural gas or the like, and particularly, a high strength steel pipe for a high strength spiral steel pipe To a high tensile strength hot-rolled steel sheet and a method of manufacturing the same, and particularly to an improved deformation characteristic after tube forming (tube forming).

The term "high-tensile hot-rolled steel sheet" as used herein refers to a hot-rolled steel sheet having a strength as high as API5L-X65 or higher and X80 or lower.

In recent years, oil and natural gas mining and pipelines have been actively installed in extreme cold regions due to high oil prices after the oil crisis and demands for diversification of energy supply sources. In addition, in pipelines, high-pressure operation is performed with a large diameter in order to improve the transportation efficiency of natural gas and oil. In order to withstand the high pressure operation of the pipeline, the piping of the pipeline (pipeline) needs to be a steel pipe of the aftertreatment, and a steel pipe made of a steel sheet like the UOE steel pipe is used. In addition, Of the steel pipe is used. In recent years, however, there has been a strong demand for further reduction in piping construction costs and a reduction in the material cost of steel pipes. In addition, as a transport pipe, instead of a UOE steel pipe made of a steel plate, High strength steel pipe or high strength spiral steel pipe is used.

From the viewpoint of preventing breakage of the line pipe, these high strength steel pipes are required to have high strength and good low temperature toughness. In order to manufacture a steel pipe having such high strength and high toughness, it is desired that the steel sheet, which is a steel pipe material, has a high strength and a high strength due to transformation strengthening using accelerated cooling after hot rolling and precipitation strengthening using precipitates of alloying elements such as Nb, Ti and V , And controllability of the structure by means of control rolling and the like.

For example, Japanese Patent Application Laid-Open No. H07-338994 discloses a method of producing a steel sheet which contains an appropriate amount of C, Si, Mn, and N and contains Si and Mn in a range satisfying Mn / Si of 5 to 8, And the surface layer portion is cooled to the Ar1 point or less and the temperature of the surface layer portion is increased from (Ac3 - 40 deg. C) to (Ac3 + 40 占 폚), finish rolling is started and finish rolling is carried out at a rolling reduction end temperature of Ac3 point or more at a total rolling reduction of not less than 60% at not more than 950 占 폚. lt; 0 > C or less and cooling to 600 [deg.] C or lower and winding at a temperature in the range of 600 to 350 [deg.] C. According to the technique described in Patent Document 1, it is possible to fabricate a high-strength electro-plated steel pipe which can finely structure the surface layer of a steel sheet without adding expensive alloying elements and heat the entire steel pipe, and is excellent in low temperature toughness . However, in the technique described in Patent Document 1, there is a problem that, in the case of a steel sheet having a large sheet thickness, the cooling ability is insufficient, the desired cooling rate can not be secured, and further cooling capability is required.

In addition, the environment in which the pipeline is laid is diversified year by year. After the pipeline is laid, the bending deformation of the pipeline can not be ignored due to the influence of the ground fluctuation and the ocean current at the seabed. It is becoming a problem.

For example, in Patent Document 2, there is disclosed a steel sheet comprising 0.02 to 0.09% of C, 0.001 to 0.8% of Si, 0.5 to 2.5% of Mn, 0.005 to 0.03% of Ti, 0.001 to 0.3% of Al, 0.001 to 0.1% of Al and 0.001 to 0.008% of N, and further 0.1 to 1.0% of Ni, 0.1 to 1.0% of Cu and 0.05 to 0.6% of Mo Ni + Cu) -Mo > 0.5, and a ferrite having an average grain size of not more than 15 mu m as an area ratio of not more than 50% and a steel sheet having a mixed structure of martensite and / or bainite as the remainder A high-strength steel pipe for a pipeline excellent in deformation characteristics after aging is disclosed. This steel pipe is said to be a high-strength steel pipe of X-70 to X-100 grade 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 alloying elements such as Cu and expensive Ni, which cause liquid phase embrittlement during hot rolling, and this has left a problem in weldability.

In recent years, the reel pile method has been widely used for laying down the submarine line pipe. The reel punching method is a method of preliminary welding, inspection, painting, etc. in the land form, winding the completed long pipe on a reel of a barge in the sea, laying it on the seabed while rewinding it at the desired sea to be. However, in the reel pile method, stresses of tensile and compression due to bending-bending recoil are applied to portions of the pipe at the time of pipe winding and at the time of pipe laying. Therefore, local buckling (local buckling) occurs in the pipe, and there is a problem that the pipe is broken starting from that point.

For example, Japanese Patent Application Laid-Open Publication No. 2003-229904 discloses a method of manufacturing a multilayer ceramic substrate which comprises 0.03 to 0.20% of C, 0.05 to 0.50% of Si, 0.50 to 1.5% of Mn, 0.005 to 0.060% of Al, Of not more than 0.04%, a yield ratio of not more than 85% with a composition having a carbon equivalent Ceq of 0.20 to 0.35%, a weld cracking susceptibility index Pcm of not more than 0.25%, and a welded softened portion A steel pipe is described. According to the technique described in Patent Document 3, it is possible to prevent local buckling occurring in the pipe when the pipeline is laid by using the reel punching method.

Patent Document 4 discloses that a steel strip having a composition containing not more than 0.1% of C and not more than 2.3% of Mn is subjected to a deformation imparting step of imparting deformation of not more than 15% A manufacturing method of a steel pipe is described. This makes it possible to prevent local buckling when pipes are installed.

Japanese Laid-Open Patent Publication No. 2001-207220 Japanese Laid-Open Patent Publication No. 2006-144037 Japanese Patent Application Laid-Open No. 03-211255 Japanese Laid-Open Patent Publication No. 2006-122932

However, in the technique described in Patent Document 3, in order to secure a high strength of X65 or higher stably, it is necessary to increase C, leaving a problem that toughness can not be secured. In addition, in the technique described in Patent Document 4, it is necessary to impart a deformation imparting step to a steel strip, and there is a problem that a large-scale deformation introducing facility is required.

Also, in the line pipe, painting is usually performed on the surface of the pipe for corrosion prevention. Then, in order to bake the coating, a coating baking treatment for heating at a temperature in the range of 200 to 300 占 폚 is carried out. Therefore, the steel pipe into which deformation has been introduced at the time of casting is hardened by the deformation aging, and the yield strength is increased to exhibit deformation characteristics indicating yield elongation. In the steel pipe exhibiting such deformation characteristics, local buckling occurs at the time of bending deformation with respect to the subsequent steel pipe, thereby causing a problem of breaking the pipe.

It is an object of the present invention to solve the problems of the prior art described above and to provide a high impact strength steel sheet having a high strength of API5L-X65 or more and X80 or less and a fracture surface transition temperature vTrs of Charpy impact test of -80 deg. It is an object of the present invention to provide a high-tensile hot-rolled steel sheet having excellent deformation characteristics in which the uniform elongation at the surface layer and the central portion in the thickness direction is 10% or more, and is excellent in deformation characteristics after tube forming.

The term " excellent deformation characteristics after tube forming " as used herein means that the surface layer portion exhibits a uniform elongation of 10% or more in a tensile test using a JIS No. 5 test piece (GL: 50 mm) in accordance with JIS Z 2241, Baking and curing properties in which the coating baking hardening amount? Y after the coating baking hardening treatment of heating at 250 占 폚 for 60 minutes after imparting 2% tensile deformation as a deformation has a low coating baking hardening property, It means that the occurrence of elongation is suppressed and the tube has deformation characteristics capable of suppressing occurrence of local buckling when subjected to bending deformation as a tube.

In order to achieve the above object, the inventors of the present invention have extensively studied the deformation characteristics after coating baking treatment as a tube, in particular, various factors affecting occurrence of yield elongation.

As a result, it was confirmed that Cr, and also Nb, Ti and V were essentially contained, the total amount of Nb, Ti and V was adjusted to an appropriate range, bainite was used as a main phase, (Coating baking treatment) of 250 占 폚 占 60 min after giving a pre-strain of 2% and having a high deformation rate of 10% or more in the uniform stretching of the surface layer portion by using a structure containing martensite, (Yield stress after coating baking treatment) - (deformation stress after application of preliminary deformation)) is as low as 40 MPa or less and yield elongation can be suppressed after the coating baking treatment.

The present invention has been completed based on these findings by further study. That is, the gist of the present invention is as follows.

(1) A ferritic stainless steel comprising: 0.04 to 0.08% of C, 0.50% or less of Si, 0.8 to 2.2% of Mn, 0.02% or less of P, 0.006% or less of S, (V): 0.001 to 0.12%, Ti: 0.005 to 0.04%, and the following formula (1): ????????

0.05? Nb + V + Ti? 0.20 ... (One)

(Where Nb, V, Ti represent the content (mass%) in the steel)

And the remainder being Fe and inevitable impurities, the surface layer having bainite as a main phase, the martensite having a volume ratio of 0.5 to 4% as a second phase, and a total of And a structure containing at least one selected from ferrite phase, pearlite and cementite having a volume fraction of 10% or less as a third phase.

(2) The steel sheet according to (1), wherein the central portion of the steel sheet has bainite as a main phase, martensite having a volume ratio of 0.5 to 4% as a second phase, and a total volume percentage of not more than 20% Of at least one member selected from the group consisting of ferrite phase, pearlite and cementite as a third phase, and the uniform elongation is 10% or more.

(3) The steel sheet according to item (1) or (2), further comprising, in addition to the above composition, at least one selected from the group consisting of 0.3 mass% or less of Mo, 0.5 mass% or less of Cu, 0.5 mass% or less of Ni, Or two or more of them.

(4) The steel sheet according to any one of (1) to (3), further comprising one or two selected from the group consisting of Zr: 0.04% or less and Ta: 0.07% High-strength hot-rolled steel sheet.

(5) The steel sheet according to any one of (1) to (4), further comprising one or two selected from the group consisting of Ca in an amount of 0.005% or less and REM in an amount of 0.005% High-strength hot-rolled steel sheet.

(6) A method for producing a hot-rolled steel sheet, which is obtained by hot-rolling the steel material to obtain a hot-rolled sheet, accelerating the hot-rolled sheet immediately after the hot-rolling, Wherein said steel material comprises, by mass%, 0.04 to 0.08% of C, 0.50% or less of Si, 0.8 to 2.2% of Mn, 0.02% or less of P, 0.006% or less of S, % Of Cr, 0.05 to 0.8% of Cr, 0.01 to 0.08% of Nb, 0.001 to 0.12% of V and 0.005 to 0.04% of Ti,

0.05? Nb + V + Ti? 0.20 ... (One)

(Where Nb, V, Ti represent the content (mass%) in the steel)

And the balance of Fe and inevitable impurities, wherein the heating of the steel material is carried out at a temperature in the range of 1100 to 1250 ° C, and a temperature of 930 ° C or lower of the finish rolling in the hot rolling , The cooling down is started at an average cooling rate CR of 7 to 50 占 폚 / s immediately after completion of the finish rolling, , 550 占 폚 or more, and the following expression (2):

SCT (℃) = 750 - 270C - 90Mn + 4Si (25 - CR) - 80Mo - 30 (Cu + Ni) ... (2)

(Mass%) in the steel, and CR denotes the average cooling rate (° C / s) in the accelerated cooling), where C, Mn, Si, Mo, Cu,

(SCT - 20 ° C) to (SCT + 20 ° C) after stopping the cooling at the cooling stop temperature, which is a temperature within the range of the temperature SCT + (SCT - 50 deg. C) or less by performing a cold treatment or a sintering treatment with a retention time of 10 to 60 seconds at a temperature range of 30 to 30 DEG C (Method for manufacturing high - strength high - strength hot - rolled steel sheet).

(7) The steel sheet according to item (6), further comprising, in mass%, one or two selected from the group consisting of Mo: 0.3% or less, Cu: 0.5% or less, Ni: 0.5% By weight or more based on the total weight of the high-strength hot-rolled steel sheet.

(8) The steel sheet according to (6) or (7), further comprising one or two selected from the group consisting of Zr: 0.04% or less and Ta: 0.07% A method of manufacturing a hot - rolled steel sheet.

(9) The honeycomb structured body according to any one of (6) to (8), further comprising one or two selected from the group consisting of Ca in an amount of not more than 0.005% and REM in an amount of not more than 0.005% Of the hot-rolled steel sheet.

According to the present invention, it is possible to produce a high-strength hot-rolled steel sheet which is excellent in deformation characteristics after forming a pipe without causing local buckling when subjected to bending deformation and is suitable for use in a line pipe, It has a remarkable effect.

The high-strength hot-rolled steel sheet of the present invention is a hot-rolled steel sheet having a strength as high as API5L-X65 or more and X80 or less and capable of producing a steel pipe excellent in deformation characteristics after pipe forming (tube forming) .

First, the reason for limiting the composition of the high-strength hot-rolled steel sheet of the present invention will be described. Hereinafter, unless otherwise stated, mass% is simply expressed in%.

C: 0.04 to 0.08%

C is an element having an effect of increasing the strength of steel, and in the present invention, it is required to contain 0.04% or more to ensure a desired strength. On the other hand, if it is contained in a large amount exceeding 0.08%, the toughness of the base material and the toughness of the welded heat affected portion are lowered. For this reason, C is limited to a range of 0.04 to 0.08%. Further, it is preferably 0.05 to 0.07%.

Si: 0.50% or less

Si is an element which acts as a deoxidizer, and the effect is observed to be 0.01% or more. Further, Si forms an oxide containing Si at the time of the complete welding, deteriorating the quality of the welded part and lowering the toughness of the welded heat affected part. From this point of view, Si is preferably reduced as much as possible, but up to 0.50% is acceptable. In view of this, Si was limited to 0.50% or less. Further, it is preferably 0.40% or less.

Mn: 0.8 to 2.2%

Mn is an element that improves the quenching property and contributes to increase the strength of the steel sheet by improving the quenching property. Further, Mn forms MnS and fixes S, thereby preventing grain boundary segregation of S and suppressing slab cracking. In order to obtain such an effect, a content of 0.8% or more is required. On the other hand, an excessive content exceeding 2.2% promotes segregation during solidification and causes the Mn-enriched portion to remain in the steel sheet, thereby increasing occurrence of separation. For this reason, Mn is limited to the range of 0.8 to 2.2%. Further, it is preferably 0.9 to 2.1%.

P: not more than 0.02%

P has an effect of increasing the strength of steel, but tends to segregate and toughness is deteriorated. Therefore, in the present invention, P is preferably reduced as much as possible, but up to 0.02% is acceptable. In this respect, P was limited to 0.02% or less. Further, it is preferably 0.016% or less.

S: not more than 0.006%

S exists mainly as an inclusion (sulfide) in the steel, and adversely affects the ductility and toughness of the steel. For this reason, S is preferably reduced as much as possible, but up to 0.006% is acceptable. In this respect, S was limited to 0.006% or less. Further, it is preferably not more than 0.004%.

Al: 0.1% or less

Al is an element acting as a deoxidizer, and it is preferable that Al is contained in an amount of 0.001% or more to obtain such effect. On the other hand, the content exceeding 0.1% significantly deteriorates the cleanliness of the welded portion during welding. Therefore, the content of Al is limited to 0.1% or less.

N: not more than 0.008%

N is an element inevitably contained, but excessive content causes cracking at the time of slab casting. In addition, since the solid solution N causes aging and increases the yield strength (coating baking hardening) in the coating baking treatment, it is desirable to reduce the solid content as much as possible. In this respect, N is limited to 0.008% or less.

Cr: 0.05 to 0.8%

Cr is an element having an effect of improving the quenching property and increasing the steel sheet strength, and Cr has an action of suppressing the occurrence of yield elongation after the coating baking treatment. In order to obtain such an effect, a content of 0.05% or more is required. On the other hand, an excessive content exceeding 0.8% causes an excessively high strength and low ductility and toughness. For this reason, Cr is limited to a range of 0.05 to 0.8%. Further, it is preferably 0.3 to 0.5%.

Nb: 0.01 to 0.08%

Nb is an element having an effect of suppressing grain boundary migration of austenite and suppressing coarsening and recrystallization of austenite grains. Further, Nb has a function of increasing the strength of the hot-rolled steel sheet with a small content without deteriorating the weldability by fine precipitation as carbonitride. Nb fixes C and N to reduce the curing amount in the coating baking process. In order to obtain such an effect, it is required to contain 0.01% or more. On the other hand, if it exceeds 0.08%, the strength becomes too high, and the ductility and toughness are lowered. For this reason, Nb is limited to a range of 0.01 to 0.08%. Further, it is preferably 0.02 to 0.07%.

V: 0.001 to 0.12%

V has an action of increasing the strength of the steel sheet by fine precipitation as carbonitride. In addition, V fixes C and N, suppresses the yield elongation after the coating baking treatment, and improves the deformation characteristics after the tube formation. In order to obtain such an effect, the content is required to be 0.001% or more. On the other hand, if it exceeds 0.12%, the strength becomes excessively high, and the ductility and toughness are lowered. Therefore, V is limited to the range of 0.001 to 0.12%. Further, it is preferably 0.001 to 0.08%.

Ti: 0.005 to 0.04%

Ti has an action of increasing the strength of the steel sheet by fine precipitation as carbonitride. Further, Ti fixes C and N, suppresses occurrence of yield elongation after coating baking treatment, and improves deformation characteristics after tube forming. In order to obtain such an effect, the content is required to be 0.005% or more. On the other hand, a content exceeding 0.04% deteriorates the weldability. For this reason, Ti is limited to a range of 0.005 to 0.04%.

The above composition is adjusted so that Nb, V, and Ti satisfy the above-mentioned formula (1) in the above-mentioned range.

0.05? Nb + V + Ti? 0.20 (1)

(Where Nb, V, Ti: content (mass%))

When the total content of Nb, V and Ti is less than 0.05%, desired high strength can not be ensured and occurrence of yield elongation after the coating baking treatment can not be suppressed. On the other hand, in the case of exceeding 0.20%, deterioration of ductility and toughness becomes remarkable. In view of this, Nb, V and Ti are adjusted so as to satisfy the expression (1).

The above-mentioned components are basic components, and in addition to these basic compositions, at least one selected from among at least one selected from the group consisting of Mo: not more than 0.3%, Cu: not more than 0.5%, Ni: not more than 0.5%, and B: , And / or one or two of Zr: 0.04% or less and Ta: 0.07% or less, and / or Ca: 0.005% or less and REM: 0.005% ≪ / RTI >

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 an effect of increasing the steel sheet strength. In the present invention, one or more of Mo, Cu, Ni and B may be selected and contained as required.

Mo has an effect of increasing the strength of the steel sheet through the improvement of the quenching property and finely precipitates as carbonitride and contributes to the increase of the strength of the steel sheet. Mo has an effect of suppressing occurrence of yield elongation after the coating baking treatment. In order to obtain such an effect, it is preferable that the content is 0.05% or more. On the other hand, a content exceeding 0.3% deteriorates the weldability. Therefore, when contained, it is preferable that Mo is limited to 0.3% or less.

Further, Cu is dissolved or precipitated to increase the strength of the steel sheet. In order to obtain such an effect, it is preferable that the content is 0.05% or more. On the other hand, if it exceeds 0.5%, the surface quality of the steel sheet may be deteriorated. Therefore, when contained, it is preferable that the content of Cu is limited to 0.5% or less.

Further, Ni is solved to increase the strength of the steel sheet and contribute to the improvement in toughness of the steel sheet. In order to obtain such an effect, it is preferable to contain 0.05% or more. On the other hand, a content exceeding 0.5% leads to a high production cost. In view of this, in the case of containing, it is preferable that Ni is limited to 0.5% or less.

In addition, B significantly contributes to an increase in steel sheet strength by significantly improving the quenching property due to the presence of a small amount. Such an effect becomes remarkable with the content of 0.0003% or more, but the effect is saturated even when the content exceeds 0.001%. In view of this, when it is contained, it is preferably limited to 0.001% or less.

Zr: 0.04% or less, and Ta: 0.07% or less

Zr and Ta are elements having an action of increasing the steel sheet by fine precipitation as carbonitride, and may be selected and contained as necessary. In order to obtain such an effect, it is preferable to contain 0.005% or more of Zr and 0.01% or more of Ta, but if it contains more than 0.04% of Zr and 0.07% of Ta, the weldability is lowered. Therefore, when contained, it is preferable that Zr is 0.04% or less and Ta is 0.07% or less.

Ca: 0.005% or less, and REM: 0.005% or less

Ca and REM are elements contributing to the control of the shape of sulphides in which sulphides of spherical forms are coarse-grained sulphides, and can be selected and contained if necessary. In order to obtain such an effect, it is preferable that Ca is contained in an amount of 0.001% or more and REM is contained in an amount of 0.001% or more, but if it is contained in an amount exceeding 0.005% of Ca and 0.005% of REM, the cleanliness of the steel sheet is lowered. For this reason, Ca is preferably limited to 0.005% or less, and REM should be limited to 0.005% or less.

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

The high-tensile hot-rolled steel sheet of the present invention is characterized in that, in addition to having the above composition, the surface layer of the steel sheet contains bainite as a main phase, martensite having a volume percentage of 0.5 to 4% as a second phase, , And a structure containing at least one of ferrite, pearlite and cementite at a total volume ratio of 10% or less.

As used herein, the term " columnar phase " refers to an image occupying 50% or more, preferably 80% or more by volume. Here, the " surface layer " used herein refers to a region up to 2 mm in the plate thickness direction from the surface of the steel sheet.

By making the structure of the surface layer 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%, uniform stretching preferably has an excellent deformation property of 10% or more . Further, even if the coating baking treatment is performed after the tube forming, the curing amount is small and the yield elongation can be suppressed after the coating baking treatment, so that buckling does not occur even if the tube is bent, and the steel tube becomes excellent in bending workability . The term " bainite " as used herein includes bainite and bainitic ferrite.

Further, by containing the martensitic phase as the second phase, the yield ratio is lowered, the deformation characteristics after the tube formation are improved, the curing amount during the coating baking treatment can be reduced, and the occurrence of yield elongation after tube formation is suppressed can do. The third phase other than bainite and martensite may contain at least one selected from ferrite phase, pearlite and cementite. These are preferably as small as possible because they lower the uniform stretching, but it is permissible if the total volume percentage is 10% or less.

The central portion of the plate thickness of the steel sheet is preferably composed of bainite as a main phase, a martensite having a volume percentage of 0.5 to 4% as a second phase, and a ferrite phase having a total volume ratio of not more than 20% , Pearlite, and cementite as a third phase.

By making the structure of the central portion of the plate thickness of the steel sheet into a structure in which bainite is a columnar phase and martensite having a volume ratio of 0.5 to 4% as a second phase as a second phase, high strength and high toughness can be combined . Specifically, 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. The third phase other than bainite and martensite may contain at least one selected from ferrite phase, pearlite and cementite. These are preferably as small as possible because they lower the strength and toughness, but they are preferably 20% or less in terms of the total volume ratio.

Next, a method of manufacturing the hot-rolled steel sheet of the present invention will be described.

First, the starting material is a steel material having the above-mentioned composition.

The production method of the steel material is not particularly limited, and any conventionally known solvent method such as a converter can be applied. As the molten steel to be molten, all commonly known casting methods such as the continuous casting method and the like can be applied, and casting can be performed using a steel material such as a slab.

The obtained steel material is then reheated.

Reheating of steel material is carried out at a temperature in the range of 1100 ~ 1250 ℃. When the reheating temperature is less than 1100 DEG C, the amount of increase in strength due to the solidification of Nb and the precipitation after the rolling is reduced, making it difficult to secure a desired high strength. On the other hand, at a high temperature exceeding 1250 占 폚, the crystal grains coarser and low-temperature toughness is lowered, and the scale production amount is increased, the surface property is lowered and the yield is lowered. Therefore, the heating temperature of the steel material is preferably limited to a range of 1100 to 1250 ° C. When the steel material has heat capable of ensuring the temperature within the above range, it may be subjected to hot rolling through reheating without heating or by heating for a short period of time.

The heated steel material is then subjected to hot rolling consisting of rough rolling and finish rolling.

The rough rolling needs only to be capable of straightening a sheet having a predetermined dimension, and the condition is not particularly required to be specified. On the other hand, the finish rolling is a rolling in which the cumulative rolling reduction at a temperature range of 930 캜 or lower is 50% or more and the finishing rolling finish temperature is 760 캜 or higher. If the cumulative rolling reduction at a temperature range of 930 DEG C or lower (in the non-recrystallized temperature range) is less than 50%, it is impossible to achieve finer crystal grains and desired desired toughness can not be secured. Further, it is preferably 85% or less. When the cumulative rolling reduction exceeds 85%, the amount of rolling is excessively increased, and the crystal grains exhibit an extremely flat shape in the rolling direction, and are peeled in the thickness direction at the time of fracture, causing separation. For this reason, the cumulative reduction ratio at the non-recrystallized temperature region is 50% or more, preferably 85% or less.

If the finish rolling finish temperature is less than 760 占 폚, the austenite-to-ferrite transformation progresses particularly in the surface layer, and it becomes impossible to make the structure of the surface layer a bainite phase as a main phase, Can not. Also, it is preferably 870 占 폚 or less. If the finishing rolling finish temperature exceeds 870 占 폚, it is impossible to achieve a finer structure and the toughness is lowered. In this respect, the finishing rolling finishing temperature is limited to 760 占 폚 or higher, preferably 870 占 폚 or lower.

After completion of finish rolling, accelerated cooling is started immediately, preferably within 15 seconds, more preferably within 10 seconds.

Accelerated cooling is performed at an average cooling rate of 7 to 50 ° C / s to the cooling stop temperature to stop the cooling. As a result, generation of ferrite phase and pearlite is suppressed, and coarsening of crystal grains can be prevented. When the average cooling rate is less than 7 占 폚 / s, the ferrite phase is excessively generated, making it difficult to secure desired high strength and high toughness. When a large amount of ferrite produced at high temperature is formed, it is difficult to form a fine bainite phase. On the other hand, at a cooling rate exceeding 50 DEG C / s, a martensitic phase tends to be easily formed, and a desired structure having a bainite phase as a main phase is hardly formed. In this respect, the cooling rate of accelerated cooling is limited to a range of 7 to 50 DEG C / s on average. Further, it is preferably 20 DEG C / s or less.

The cooling stop temperature for accelerated cooling is set to a temperature in the range of 550 ° C or higher (SCT + 30 ° C) or lower.

When the cooling stop temperature of accelerated cooling is less than 550 캜, a martensite phase is easily formed as a columnar phase, and it becomes difficult to form a desired structure having a bainite phase as a main phase. On the other hand, at a temperature exceeding (SCT + 30 占 폚), a large amount of ferrite and pearlite are produced, making it difficult to stably secure desired characteristics.

Here, SCT is expressed by the following equation (2)

SCT (℃) = 750 - 270C - 90Mn + 4Si (25 - CR) - 80Mo - 30 (Cu + Ni) ... (2)

(Mass%) in the steel, and CR denotes the average cooling rate (° C / s) in the accelerated cooling), where C, Mn, Si, Mo, Cu,

And is a value indicating the degree of easiness of formation of a bainite phase containing martensite therein, and is a value depending on the amount of the alloy element contained and the degree of accelerated cooling.

After the accelerated cooling is stopped, in the present invention, a cold treatment or a cold treatment is carried out until the retention time becomes 10 to 60 s at the temperature range of (SCT - 20 캜) - (SCT + 30 캜) do. As a result, the surface of the steel sheet is reheated, the temperature distribution in the plate thickness direction becomes uniform, and a bainite phase containing martensite is easily produced while suppressing the formation of ferrite. When the residence time at the above-mentioned temperature range is less than 10 s, the double heat is insufficient and the amount of martensite in the surface layer is insufficient. On the other hand, if it exceeds 60 s, the bainite is ingrown and the toughness is lowered, and the productivity of the steel sheet is lowered. For this reason, the period from the stop of accelerated cooling to the winding up was a cold treatment or a cold treatment in which the residence time at a temperature range of (SCT - 20 ° C) to (SCT + 30 ° C) was 10 to 60 s.

Then, it is coiled into a coil. Coiling temperature should be 430 ℃ or higher (SCT - 50 ℃) or lower. When the coiling temperature is lower than 430 占 폚, diffusion of C is suppressed, and no martensite phase is formed in the main phase bainite. On the other hand, when (SCT - 50 DEG C) is exceeded, pearlite is generated, and a desired structure can not be formed.

Example

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

Molten steel having the composition shown in Table 1 was dissolved in a converter, and a slab (thickness: 220 mm) was formed by a continuous casting method. These slabs were heated to 1200 DEG C and 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 cold annealing treatment under the cooling conditions shown in Table 2 after finishing rolling , And rolled in a coiled state under the conditions shown in Table 2 and cooled to obtain a hot-rolled steel sheet (hot-rolled steel strip) having a thickness of 12 to 16 mm.

A test piece was taken from the obtained hot-rolled steel sheet (hot-rolled steel strip) and subjected to a texture observation, a tensile test, an impact test, and a tensile test after the coating baking treatment to evaluate the structure, tensile properties, toughness and tensile properties after the baking treatment. The evaluation method is as follows.

(1) Tissue observation

A specimen for observing the structure was taken from the obtained hot-rolled steel sheet and subjected to abrasion and corrosion in the rolling direction end face to obtain a surface layer (having a thickness of 1 mm from the surface) with an optical microscope (magnification: 1,000 times) or a scanning electron microscope Position) and the central position of the plate thickness. The type of tissue and tissue fraction were measured with an image analyzer for the obtained tissue photographs. The obtained results are shown in Table 3.

(2) Tensile test

JIS No. 5 tensile test specimen (GL: 50 mm) was sampled from the surface layer (area from surface to 2 mm in the sheet thickness direction) of the obtained hot-rolled steel sheet and from the plate thickness center position in the direction parallel to the rolling direction, The tensile properties (yield strength, tensile strength, total elongation, and uniform elongation) were measured in accordance with the provisions of Z 2241. The tensile test specimen in the surface layer (the area from the surface to the sheet thickness direction of 2 mm) was sampled so that the position of 1 mm from the surface was located at the central position in the thickness direction, and the thickness of the specimen was 1.6 mm. The tensile test piece at the central position of the plate thickness was prepared by cutting the surface layer (the area from the surface to 2 mm in the plate thickness direction) by cutting so that the center position of the plate thickness was the center position in the thickness direction. The obtained results are shown in Table 4.

(3) Impact test

A V-notch test piece (10 mm in width) was taken from the central portion of the obtained hot-rolled steel sheet so that the direction orthogonal to the rolling direction was the longitudinal direction, and Charpy impact test was performed in accordance with JIS Z 2242, (占 폚), and the toughness was evaluated. The obtained results are shown in Table 4.

(4) Tensile test after coating baking treatment

JIS No. 5 tensile test specimen (GL: 50 mm) was sampled such that the direction parallel to the rolling direction was the tensile direction from the surface layer (region up to 2 mm in the sheet thickness direction) of the obtained hot-rolled steel sheet and the central position of the sheet thickness. The tensile test specimen was subjected to a preliminary deformation of 2% at room temperature and then subjected to a heat treatment (250 DEG C x 60 min) equivalent to the coating baking treatment and subjected to a tensile test in the same manner as in (2) Stress and yield elongation were measured, and the amount of baking hardened by baking was determined. The obtained results are shown in Table 4.

It can be seen from the results of Tables 1 to 4 that all of the examples of the present invention have a structure having a bainite phase as a main phase, a high strength of X65 class having a yield strength YS of 450 MPa or more and a high toughness of vTrs of- And the uniform elongation at the central portion in the plate thickness direction was 10% or more. Further, even after the coating baking treatment, no occurrence of yield elongation was observed, and the amount of baking baking was also 40 MPa or less. And the hardenability was also low.

On the other hand, the comparative example deviating from the scope of the present invention has a problem that the strength is insufficient, the toughness is lowered, the stretching property is lowered, the yield elongation occurs, and a desired property is secured as a high strength hot- not.

Further, the hot-rolled steel sheet of the present invention example was formed into a steel pipe by cold forming using a roll and subjected to a reduction in diameter by shrinkage to obtain a steel pipe having an outer diameter of 406 mmφ. In shaper-milled rolling, tensile strain (tubular deformation) of 3.5% or more was given in the tube axis direction. The obtained steel pipe was further subjected to heat treatment at 250 캜 for 60 min. An arc-shaped tensile test specimen was taken from the obtained steel pipe in the direction of the tube axis in the tensile direction and subjected to a tensile test in accordance with the API 5L standard. However, no yield elongation occurred and a uniform elongation of not less than 4% It is confirmed that it is made of a steel pipe with characteristics. These steel pipes are steel pipes in which occurrence of buckling is suppressed even if bending is performed.

Claims (9)

In terms of% by mass,
0.04 to 0.08% of C, 0.50% or less of Si,
Mn: 0.8 to 2.2%, P: 0.02% or less,
S: not more than 0.006%, Al: not more than 0.1%
0.008% or less of N, 0.05 to 0.8% of Cr,
And a balance of Fe and inevitable impurities, wherein the composition contains 0.01 to 0.08% of Nb, 0.001 to 0.12% of V, and 0.005 to 0.04% of Ti so as to satisfy the following expression (1) ,
Wherein the surface layer comprises bainite as a main phase, martensite having a volume percentage of 0.5 to 4% as a second phase, and at least one selected from ferrite phase, perlite and cementite having a total volume ratio of not more than 10% High-strength hot-rolled steel sheet having a structure contained as a third phase,
0.05? Nb + V + Ti? 0.20 ... (One)
Here, Nb, V and Ti represent the content (mass%) in the steel. The method according to claim 1,
The central portion of the plate thickness of the steel sheet is selected from a ferrite phase, a pearlite and a cementite having a bainite as a main phase, a martensite having a volume percentage of 0.5 to 4% as a second phase and a total volume fraction of not more than 20% As a third phase, and at least one kind selected from the group consisting of
And a uniform elongation of 10% or more. 3. The method according to claim 1 or 2,
Further comprising, in mass%, at least one selected from the group consisting of Mo: not more than 0.3%, Cu: not more than 0.5%, Ni: not more than 0.5%, and B: not more than 0.001% Hot rolled steel sheet. 4. The method according to any one of claims 1 to 3,
Further comprises one or two selected from the group consisting of Zr: 0.04% or less and Ta: 0.07% or less, in addition to the above composition. 5. The method according to any one of claims 1 to 4,
Further comprising, in mass%, at least one selected from the group consisting of Ca: 0.005% or less and REM: 0.005% or less. A method of producing a high-strength hot-rolled steel sheet, comprising the steps of: hot-rolling a steel material to obtain a hot-rolled steel sheet; subjecting the hot-rolled steel sheet to accelerated cooling immediately after the hot-
The steel material, in mass%
0.04 to 0.08% of C, 0.50% or less of Si,
Mn: 0.8 to 2.2%, P: 0.02% or less,
S: not more than 0.006%, Al: not more than 0.1%
0.008% or less of N, 0.05 to 0.8% of Cr,
And a balance of Fe and inevitable impurities, wherein the composition contains 0.01 to 0.08% of Nb, 0.001 to 0.12% of V, and 0.005 to 0.04% of Ti so as to satisfy the following expression (1) ,
The heating of the steel material is carried out at a temperature in the range of 1100 to 1250 DEG C,
The cumulative rolling reduction in the temperature range of 930 DEG C or lower of the finish rolling in the hot rolling is 50% or more and the rolling finish temperature is 760 DEG C or more,
Immediately after completion of the finish rolling, the cooling is started at an average cooling rate CR of 7 to 50 占 폚 / s and the cooling is stopped at a temperature of 550 占 폚 or more and a temperature SCT of 30 占 폚 or less defined by the following formula (2) The cooling is stopped at the temperature,
After the accelerated cooling is stopped, a cold treatment or a slackening treatment is carried out with a retention time in the temperature range of (SCT - 20 ° C) to (SCT + 30 ° C)
Wherein the coiling temperature is set to 430 DEG C or higher (SCT - 50 DEG C) or lower.
0.05? Nb + V + Ti? 0.20 ... (One)
Here, Nb, V and Ti represent the content (mass%) in the steel.
SCT (℃) = 750 - 270C - 90Mn + 4Si (25 - CR) - 80Mo - 30 (Cu + Ni) ... (2)
Here, C, Mn, Si, Mo, Cu, and Ni represent the content (mass%) in the steel,
CR represents an average cooling rate (° C / s) in accelerated cooling. The method according to claim 6,
Further comprising, in mass%, at least one selected from the group consisting of Mo: not more than 0.3%, Cu: not more than 0.5%, Ni: not more than 0.5%, and B: not more than 0.001% A method of manufacturing a hot - rolled steel sheet. 8. The method according to claim 6 or 7,
Further comprising one or two selected from the group consisting of Zr: 0.04% or less and Ta: 0.07% or less, in addition to the above composition. 9. The method according to any one of claims 6 to 8,
Further comprising one or two selected from the group consisting of Ca in an amount of not more than 0.005% and REM in an amount of 0.005% or less, in addition to the above composition.