JP2015175039A - Thick hot rolled steel sheet and production method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thick hot rolled steel sheet suitable as a raw material for a X80 class electric resistance welded steel tube or a raw material for X80 class spiral steel tube, excellent in strength and toughness and having low yield elongation, and to provide a production method of the thick hot rolled steel sheet.SOLUTION: There is provided the thick hot rolled steel sheet having a composition containing C, Nb, V and Ti and having structure in which a percentage of deposited Nb is 5% or more and less than 35% based on total added Nb amount, a volume fraction of bainitic ferrite in a sheet thickness center position is 95% or more and a volume fraction of tempered martensite and/or tempered bainite at sheet thickness surface layer 1 mm position is 95% or more. The above composition and structure provides the thick hot rolled steel sheet with excellent toughness and such low yield elongation that yield elongation in a tensile test in a hot rolled steel sheet longitudinal direction is 0.1% to 3.0%.

Description

  The present invention is a high strength, high toughness and low yield elongation suitable as a material of API standard X80 class ERW steel pipe or spiral steel pipe that shows excellent buckling resistance against compressive load caused by earthquakes, etc. The present invention relates to a suppressed thick hot-rolled steel sheet and a method for producing the same. The steel pipe manufactured from this thick hot-rolled steel sheet can be used for pipelines, oil well pipes, civil engineering and construction. In the present invention, the thick hot-rolled steel sheet means a hot-rolled steel sheet (hot-rolled steel strip) having a thickness of 12 mm to 30 mm.

  In response to increasing energy demand in recent years, demand for pipelines and oil well pipes has increased, and high-strength (especially API standard X80 class) and thick-walled steel pipes (plate thickness of 12 mm or more) are required for highly efficient transportation. It has come to be. Conventionally, UOE steel pipes made of thick plates are mainly used to meet this requirement. However, recently, due to the reduction of pipeline construction costs and the lack of supply capacity of UOE steel pipes, there is a strong demand for reducing the material cost of steel pipes, and hot rolled steel sheets that are more productive and cheaper than UOE steel pipes are being used. ERW steel pipes and spiral steel pipes have been used as raw materials.

Here, because pipelines are often laid in cold regions where natural gas reserves are abundant, for example, steel sheets for line pipe materials are not only high strength and thick, but also low temperature toughness. It is also required to be excellent.
Line pipes and the like are also required to have buckling resistance against deformation stress due to repeated freezing and thawing of the ground in cold regions and crustal movements such as earthquakes. Therefore, the steel sheet for a line pipe material also needs to have a small yield elongation in the rolling longitudinal direction from the viewpoint of improving the buckling resistance.

Under such circumstances, various techniques have recently been proposed for hot-rolling materials for line pipes.
For example, in Patent Document 1, the composition of a hot-rolled steel sheet is, in mass%, C: 0.02 to 0.08%, Si: 0.01 to 0.50%, Mn: 0.5 to 1.8%, P: 0.025% or less, S: 0.005% or less. , Al: 0.005 to 0.10%, Nb: 0.01 to 0.10%, Ti: 0.001 to 0.05%, and C, Ti, Nb ([% Ti] + ([% Nb] / 2)) / [% C The composition of the remaining Fe and unavoidable impurities is included so as to satisfy <4, and the structure of the hot-rolled steel sheet is the average crystal grain size of the ferrite phase that is the main phase at a position of 1 mm from the steel sheet surface in the sheet thickness direction. The difference between the average grain size of the ferrite phase, which is the main phase at the center of the plate thickness of the steel sheet, is less than 2μm, and the second phase structure fraction (volume%) at a position 1mm from the steel plate surface to the plate thickness direction. ) And the structure fraction (volume%) of the second phase at the center of the sheet thickness of the steel sheet is a structure where the ΔV is 2% or less, and the Vickers hardness H at a position of 1 mm from the steel sheet surface to the sheet thickness direction. There has been proposed a technology for producing a thick, high-tensile hot-rolled steel sheet with excellent low-temperature toughness by setting the difference ΔHV between V1mm and the Vickers hardness HV1 / 2t at the center position of the steel sheet thickness to 50 points or less.

  In Patent Document 2, in mass%, C: 0.05 to 0.12%, Si: 0.01 to 0.50%, Mn: 0.5 to 2.0%, Mo: 0.05 to 0.50%, Al: 0.01 to 0.08%, the balance Fe and inevitable A steel slab made of impurities is prepared, and after rolling at a rolling end temperature: (Ar3-10) to (Ar3 + 80) ° C., air-cooled to form a steel plate by cold forming, and then a pipe expansion ratio: 0.5 to 1.5 A technology has been proposed to produce steel pipes with excellent buckling resistance by cold working. In addition, in this patent document, when the yield elongation of the stress-strain curve of the steel sheet is 2.0% or less, the yield elongation easily disappears due to cold working by pipe forming and pipe expansion in the final pipe forming process. It is described that the buckling resistance is improved.

  Patent Document 3 includes, by weight, C: 0.05 to 0.15%, Mn: 1.0 to 2.0%, and Cu: 0.05 to 0.30%, Ni: 0.05 to 0.30%, Cr: 0.05 to 0.30%, Mo: A steel slab containing one or more of 0.05 to 0.30%, Nb: 0.005 to 0.10%, V: 0.005 to 0.10%, Ti: 0.005 to 0.10% is heated to 1050 to 1180 ° C and hot rolled. After that, the steel sheet cooled at a cooling rate of 4 ° C / sec or more from the temperature range of (Ar3 + 40) to (Ar3-80) ° C determined by the components of the steel is cold-formed, and then the end is welded. Techniques have been proposed for producing line pipes with high strength, toughness, and earthquake resistance of X80 or higher.

JP 2010-196156 A Japanese Patent No. 3937998 JP-A-9-202922

  However, in the technique proposed in Patent Document 1, the buckling resistance performance of the steel pipe has not been studied. Further, in the technique proposed in Patent Document 1, as shown in the examples, a large amount of Cu, Ni, Mo, and Cr is added to obtain an X80 grade high-strength steel pipe. Since these elements are rare metals and have large price fluctuations, there is a problem that it is difficult to provide a stable supply in the future.

  On the other hand, in the technique proposed in Patent Document 2, the buckling resistance performance of a steel pipe is studied. However, with the technique proposed in Patent Document 2, the steel pipe strength does not reach X80. Moreover, addition of Mo, which is a rare element, is essential to achieve low yield elongation. Mo does not have an alternative element when it is difficult to secure it in the future, and it is not preferable as an essential additive element from the viewpoint of reducing rare metals. Furthermore, in order to obtain a steel pipe excellent in buckling resistance, it is said that a pipe expanding process is necessary, and there is a problem that the production efficiency is greatly reduced.

  Also in the technique proposed in Patent Document 3, the buckling resistance performance of the steel pipe is studied. However, the technique proposed in Patent Document 3 has a high C equivalent, and when used as an ERW steel pipe that does not use a weld metal, the toughness of the welded portion tends to deteriorate. Further, as shown in the examples, the Nb content is suppressed in order to achieve a low yield ratio. If the Nb content is low, the slab heating temperature needs to be lowered to 1180 ° C or less in order to prevent toughness reduction associated with the coarsening of the austenite grain size, which increases the rolling load, roll and equipment life. There is a problem that becomes shorter.

  The present invention solves the above-mentioned problems of the prior art, and is a thick-walled hot-rolled steel that combines the high strength, high toughness, and low yield elongation required as materials for X80 class ERW steel pipes and spiral steel pipes. An object is to provide a technique for stably producing a steel sheet without using complicated cooling techniques and equipment, rare elements, and without reducing the production efficiency.

  Based on the knowledge that the alloy elements and hot-rolling conditions of the thick-walled hot-rolled steel sheet used as the raw material have a significant effect on the strength, toughness, and yield elongation of the steel pipe, the present inventors have reduced the rare metal content of the X80 class. The effects of manufacturing conditions on the properties of thick hot rolled steel sheets for steel pipes were studied.

  First, steel ingots having various compositions were prepared, and hot-rolled steel sheets (thickness: 12 to 30 mm) in which the slab heating temperature, the rolling reduction, the cooling rate, the winding temperature, and the like were changed were prepared. A sample was taken from the produced hot-rolled coil and bent back, and then a full-thickness tensile specimen was taken and subjected to a tensile test to measure the yield elongation characteristics. And the multiple regression analysis was performed with respect to the steel composition, hot-rolling conditions, and the yield elongation characteristic of the obtained hot-rolled steel sheet. However, no significant correlation was obtained between the steel composition and hot rolling conditions and the yield elongation characteristics of the obtained hot rolled steel sheet.

  In response to this problem, the present inventors have developed a bauschinger effect due to the influence of strain at the time of coil bending back, and have changed the yield elongation characteristics of the hot-rolled steel sheet. The reason was that no significant correlation was obtained with the yield elongation characteristics.

  The fact that the contribution of manufacturing conditions to yield elongation cannot be accurately grasped is that high strength and high toughness hot rolling with low yield elongation is suitable as a material for ERW and spiral steel pipes with excellent buckling resistance. A steel plate cannot be obtained. Therefore, the present inventors have devised that a tensile test is performed by collecting a round bar tensile test piece from the center position of the plate thickness and the coil longitudinal direction from a hot-rolled steel plate that is not bent back and is not bent. The yield elongation length was measured from the stress-strain curve obtained from the round bar tensile test, and the correlation with the production conditions was investigated.

  Here, it should be noted that the absolute value of yield elongation depends on the shape of the test piece. As a result of the investigation, it is clear that the full-thickness plate tensile test specimen collected from the unrolled hot rolled coil has a lower yield elongation value than the round bar tensile test specimen collected from the non-bent hot rolled coil. became.

  Based on these investigation results, the present inventors conducted a tensile test using a round bar tensile specimen taken from a hot-rolled steel sheet that has not been bent back as described above. By regression analysis, the manufacturing conditions (composition, hot-rolling conditions) on the properties of hot-rolled steel sheets and the degree of influence of the structure are evaluated, and it is significant in making coiled hot-rolled steel sheets have high strength, high toughness and low yield elongation. The production conditions were determined.

  Subsequently, a hot-rolled steel sheet (sheet thickness: 12 to 30 mm) was produced according to the production conditions obtained above, and steel pipes (an electric-welded steel pipe and a spiral steel pipe) were produced using the hot-rolled steel sheet as a raw material. And when the buckling-proof test was done using the produced steel pipe, the steel pipe which has desired buckling-proof performance was obtained.

  As described above, the present inventors conducted a tensile test using a tensile test piece taken from a hot-rolled coil that is not bent back, and based on the test results, the composition of the hot-rolled steel sheet and the hot-rolling conditions By specifying the microstructure, it is possible to obtain hot-rolled steel sheets in which the yield elongation in the longitudinal direction of the hot-rolled steel sheet is suppressed to a low level while maintaining high strength and high toughness. It was found that the buckling performance was also excellent.

  And as a result of further investigation, without adding rare metals such as Cu, Ni, Mo, Cr, using a steel material added with a predetermined amount of precipitation strengthening elements Nb, V, Ti, at the time of finish rolling By specifying the reduction ratio in the non-recrystallization temperature range, the finish rolling finish temperature, the subsequent cooling rate and the coiling temperature, it is suitable as a material for X80 grade thick-walled steel pipes with high toughness and low yield elongation. It has been found that a thick hot-rolled steel sheet can be obtained.

This invention is made | formed based on the above knowledge, The summary is as follows.
[1] By mass%, C: 0.04% to 0.15%, Si: 0.01% to 0.55%, Mn: 1.0% to 3.0%, P: 0.03% or less, S: 0.01% or less, Al: 0.003% Contains 0.1% or less, N: 0.006% or less, Nb: 0.035% or more and 0.1% or less, V: 0.001% or more and 0.1% or less, Ti: 0.001% or more and 0.1% or less, and the balance is Fe and inevitable impurities It has a composition, the ratio of precipitated Nb to the total amount of added Nb is 5% or more and less than 35%, the volume fraction of bainitic ferrite at the center of the plate thickness is 95% or more, and the plate thickness surface layer is 1 mm. It has a structure in which the volume fraction of tempered martensite and / or tempered bainite at the position is 95% or more, and the yield elongation in the tensile test in the longitudinal direction of the hot-rolled steel sheet is 0.1% or more and 3.0% or less. Thick hot rolled steel sheet.

[2] A thick hot-rolled steel sheet according to [1], wherein the composition satisfies the following formulas (1) and (2).
Record
Pcm = [% C] + [% Si] / 30 + ([% Mn] + [% Cu] + [% Cr]) / 20
+ [% Ni] / 60 + [% V] / 10 + [% Mo] / 7 + 5 × [% B] ≦ 0.25 (1)
Px = 701 × [% C] + 85 × [% Mn] ≧ 181 (2)
Here, in the formulas (1) and (2), [% C], [% Si], [% Mn], [% Cu], [% Cr], [% Ni], [% V], [% % Mo] and [% B] are the contents of each element (% by mass).

[3] The thick hot-rolled steel sheet according to [1] or [2], further containing Ca: 0.0001% to 0.005% by mass% in addition to the composition.

[4] In any one of the above [1] to [3], in addition to the above composition, Cu: 0.001% to 0.5%, Ni: 0.001% to 0.5%, Mo: 0.001% or more in mass% A thick-walled hot-rolled steel sheet containing one or more selected from 0.5% or less, Cr: 0.001% or more and 0.5% or less, and B: 0.0001% or more and 0.004% or less.

[5] By mass%, C: 0.04% to 0.15%, Si: 0.01% to 0.55%, Mn: 1.0% to 3.0%, P: 0.03% or less, S: 0.01% or less, Al: 0.003% Contains 0.1% or less, N: 0.006% or less, Nb: 0.035% or more and 0.1% or less, V: 0.001% or more and 0.1% or less, Ti: 0.001% or more and 0.1% or less, with the balance being Fe and inevitable impurities The steel slab having the composition is reheated to a temperature range of 1000 ° C. or more and 1250 ° C. or less, and after the rough rolling and the rough rolling, the reduction rate in the non-recrystallization temperature range is 20% or more and 85% or less, and the finish rolling finish temperature is (Ar ₃ −50 ° C.) to (Ar ₃ + 100 ° C.) in a temperature range of less than or equal to (Ar ₃ + 100 ° C.). After the finish rolling, the average cooling rate in the temperature range of 750 ° C. Cool at an average cooling rate of 50 ° C / s or more and 300 ° C / s or less at a temperature range of 750 ° C or less and 650 ° C or more at a position of 5mm / s or more and less than 50 ° C / s, 1mm position of the plate thickness, 200 ° C or more and 550 ° C Less than Method for producing a hot rolled thick steel plate, wherein a wound in the coiling temperature.

[6] The thick hot rolled steel sheet according to [5], wherein the composition satisfies the following formulas (1) and (2), and the winding temperature satisfies the following formula (3): Production method.
Record
Pcm = [% C] + [% Si] / 30 + ([% Mn] + [% Cu] + [% Cr]) / 20
+ [% Ni] / 60 + [% V] / 10 + [% Mo] / 7 + 5 × [% B] ≦ 0.25 (1)
Px = 701 × [% C] + 85 × [% Mn] ≧ 181 (2)
0.0 ≦ 7.09 × [% C] −1.31 × [% Mn] + 0.00910 × CT ≦ 3.3 (3)
Here, in the formulas (1), (2) and (3), [% C], [% Si], [% Mn], [% Cu], [% Cr], [% Ni], [% % V], [% Mo], and [% B] are the contents (% by mass) of each element.
In equation (3), CT is the coiling temperature (° C).

[7] The method for producing a thick hot-rolled steel sheet according to [5] or [6], further comprising Ca: 0.0001% to 0.005% by mass% in addition to the composition.

[8] In any one of the above [5] to [7], in addition to the above composition, Cu: 0.001% to 0.5%, Ni: 0.001% to 0.5%, Mo: 0.001% or more in mass% A method for producing a thick hot-rolled steel sheet, comprising 0.5% or less, Cr: 0.001% or more and 0.5% or less, and B: 0.0001% or more and 0.004% or less selected from one or more.

  According to the present invention, it has the high strength, high toughness, and low yield elongation characteristics required for X80 grade electric resistance welded steel pipe material and X80 grade spiral steel pipe material applied to line pipe, oil well pipe, steel pipe for civil engineering and construction. The thick-walled hot-rolled steel sheet can be manufactured stably without using rare elements and without reducing the manufacturing efficiency, and is extremely useful industrially.

Details of the present invention will be described below.
First, the reason for limitation of the component composition of the thick hot-rolled steel sheet in the present invention will be described. In addition,% showing the following component composition shall mean the mass% unless there is particular notice.

C: 0.04% to 0.15%
C is an important element for securing the strength of the hot-rolled steel sheet by forming carbides with Nb, V, Ti, etc. In order to satisfy the desired strength, the C content is set to 0.04% or more. There is a need. On the other hand, when the C content exceeds 0.15%, the carbon equivalent becomes high, and when such a hot-rolled steel sheet is piped and welded, the toughness of the welded portion deteriorates. In addition, the yield ratio increases with the excessive production of precipitates, and the yield elongation in the longitudinal direction of the hot-rolled steel sheet increases. Therefore, the C content is 0.04% or more and 0.15% or less.

Si: 0.01% or more and 0.55% or less
When the Si content is increased, Mn-Si-based non-metallic inclusions are formed and the weld toughness is deteriorated. Therefore, the upper limit of Si content is 0.55%. On the other hand, the lower limit of the Si content is set to 0.01% from the deoxidation effect and the steelmaking technology limit.

Mn: 1.0% to 3.0%
Mn is an element that can secure strength and toughness by suppressing the formation of polygonal ferrite and pearlite, which cause strength reduction and toughness reduction, and has the effect of suppressing the yield elongation in the longitudinal direction of the hot-rolled steel sheet to a low level. It is also an element. In order to express these effects, the Mn content needs to be 1.0% or more. On the other hand, if the Mn content exceeds 3.0%, the weld zone toughness may deteriorate with an increase in carbon equivalent. Therefore, the Mn content is 1.0% or more and 3.0% or less.

P: 0.03% or less, S: 0.01% or less, N: 0.006% or less
P exists as an impurity in the steel, but is an element that easily segregates and causes toughness deterioration of the steel. Therefore, the upper limit of the P content is 0.03%.
S and N, like P, deteriorate the toughness of steel, so the upper limit of the S content is 0.01% and the upper limit of the N content is 0.006%.
In addition, since P, S, and N are practically possible steelmaking control capability limits, it is preferable that the respective contents are P: 0.001% or more, S: 0.0001% or more, and N: 0.001% or more.

Al: 0.003% to 0.1%
Al is useful as a deoxidizer for steel, and the Al content is 0.003% or more at which a deoxidizing effect is exhibited. However, when the Al content is excessive, alumina inclusions are generated, causing defects in the weld. Therefore, the Al content is 0.003% or more and 0.1% or less.

Nb: 0.035% to 0.1%
Nb is effective for refinement of crystal grains and is a precipitation strengthening element, and the Nb content needs to be 0.035% or more for manifestation of the effect. On the other hand, when the Nb content is excessive, weldability deteriorates. Therefore, the Nb content is 0.035% or more and 0.1% or less.

V: 0.001% to 0.1%
V is a precipitation strengthening element, and in order to make this work effectively, the V content needs to be 0.001% or more. On the other hand, when the V content is excessive, weldability deteriorates. Therefore, the V content is 0.001% or more and 0.1% or less.

Ti: 0.001% to 0.1%
Ti is effective for refining crystal grains and is a precipitation strengthening element, and the Ti content needs to be 0.001% or more in order to achieve the effect. On the other hand, when the Ti content is excessive, weldability deteriorates. Therefore, Ti content shall be 0.001% or more and 0.1% or less.

The thick hot-rolled steel sheet of the present invention preferably further contains Ca: 0.0001% to 0.005% in addition to the above component composition.
Ca: 0.0001% to 0.005%
Ca has the effect of improving toughness by fixing S and suppressing the formation of MnS. In order to exhibit such an effect, the Ca content is preferably 0.0001% or more. On the other hand, when the Ca content is excessive, the toughness is reduced due to the formation of the Ca-based oxide, so the Ca content is preferably 0.005% or less.

  In addition to the above component composition, the thick hot rolled steel sheet of the present invention further includes Cu: 0.001% to 0.5%, Ni: 0.001% to 0.5%, Mo: 0.001% to 0.5%, Cr: One or more selected from 0.001% to 0.5% and B: 0.0001% to 0.004% may be contained.

Cu: 0.001% to 0.5%
Cu is an element effective in controlling the transformation of steel and improving the strength of hot-rolled steel sheets. In order to exhibit such an effect, it is preferable to make Cu content 0.001% or more. However, since Cu reduces hot workability, its content is preferably 0.5% or less.

Ni: 0.001% to 0.5%
Ni is an element effective in controlling the transformation of steel and improving the strength of the hot-rolled steel sheet. In order to exhibit such an effect, the Ni content is preferably 0.001% or more. However, since Ni decreases the hot workability, its content is preferably 0.5% or less.

Mo: 0.001% to 0.5%
Mo is an element effective in controlling the transformation of steel and improving the strength of the hot-rolled steel sheet. In order to exhibit such an effect, the Mo content is preferably 0.001% or more. However, since Mo promotes the formation of martensite and lowers the toughness, its content is preferably 0.5% or less.

Cr: 0.001% to 0.5%
Cr is an effective element that suppresses the deterioration of toughness by the effect of delaying pearlite transformation and the effect of reducing grain boundary cementite. In order to exhibit these effects, the Cr content is preferably 0.001% or more. On the other hand, if the Cr content is excessive, there is a possibility that a hardened structure is formed in the welded portion and the toughness of the welded portion is deteriorated. Therefore, the Cr content is preferably 0.001% or more and 0.5% or less.

  Cu, Ni, Mo, and Cr are all rare metals, and are difficult to ensure stably and are also expensive elements. Therefore, from the viewpoints of securing the stability of raw materials, production costs, etc., it is preferable to avoid the addition of these elements as much as possible, and the respective contents are preferably set to 0.1% or less.

B: 0.0001% or more and 0.004% or less
B is effective in suppressing the ferrite transformation at a high temperature in the cooling process after finishing the finish rolling during the production of the hot-rolled steel sheet and preventing the strength of the hot-rolled steel sheet from being reduced due to the decrease in the hardness of the ferrite. In order to exhibit such an effect, the B content is preferably 0.0001% or more. On the other hand, if the B content is excessive, a hardened structure may be formed in the weld. Therefore, the B content is preferably 0.0001% or more and 0.004% or less.

The thick hot-rolled steel sheet of the present invention preferably has a composition that satisfies the component indices shown in the following formulas (1) and (2). By satisfying these formulas, it is possible to further increase the strength, toughness and improve the weldability.
Pcm = [% C] + [% Si] / 30 + ([% Mn] + [% Cu] + [% Cr]) / 20 + [% Ni] / 60
+ [% V] / 10 + [% Mo] / 7 + 5 × [% B] ≦ 0.25 (1)
Px = 701 × [% C] + 85 × [% Mn] ≧ 181 (2)

  Here, in the formulas (1) and (2), [% C], [% Si], [% Mn], [% Cu], [% Cr], [% Ni], [% V], [% % Mo] and [% B] are the contents (% by mass) of each element. Moreover, when a steel plate does not contain Cu, Pcm value shall be calculated by setting [% Cu] in Formula (1) to zero. The same applies to [% Cr], [% Ni], [% V], [% Mo], and [% B].

  Pcm shown in the equation (1) is an index of weld zone toughness. When the Pcm value exceeds a certain value, the toughness of the welded portion tends to decrease when, for example, pipe forming and welding a hot-rolled steel sheet. Therefore, the Pcm value is preferably 0.25 or less.

  On the other hand, Px shown in the formula (2) is an index of strength, in order to ensure sufficient hot-rolled steel sheet strength as a material for X80 grade steel pipe without using rare elements such as Cu, Ni, Mo, Cr. Preferably has a Px value of 181 or more. However, if the Px value becomes excessively high, the elongation characteristics deteriorate, pipe making becomes difficult, and the deformability against forced deformation associated with crustal movements such as earthquakes may be reduced. The following is preferable.

  In the thick hot-rolled steel sheet of the present invention, components other than the above are Fe and inevitable impurities. Examples of inevitable impurities include Co, W, Pb, and Sn.

Next, the structure of the thick hot-rolled steel sheet of the present invention will be described.
In the thick hot-rolled steel sheet of the present invention, the ratio of precipitated Nb to the total amount of added Nb is 5% or more and less than 35%, the volume fraction of bainitic ferrite at the center position of the sheet thickness is 95% or more, and The tempered martensite and / or tempered bainite has a volume fraction of 95% or more at a position of 1 mm in the plate thickness surface layer.

Ratio of precipitated Nb to total added Nb amount: 5% or more and less than 35% In the thick hot-rolled steel sheet of the present invention, the ratio of precipitated Nb to the total Nb amount is 5% or more and less than 35%. If the Nb precipitation ratio is less than 5%, the hot rolled steel sheet tends to have insufficient strength, and it may be difficult to satisfy the X80 grade steel pipe strength characteristics. On the other hand, if it exceeds 35%, precipitation strengthening becomes excessive, and the yield ratio tends to be high, whereby the buckling resistance tends to deteriorate. Therefore, the ratio of precipitated Nb to the total Nb amount is 5% or more and less than 35%.

Here, the ratio (mass ratio) of Nb precipitated in the steel sheet is obtained by measuring the mass of Nb precipitated in the steel sheet by extraction residue analysis and calculating the ratio (mass%) of this measured value with respect to the total Nb content. Can do. In the extraction residue analysis, the steel plate was subjected to constant current electrolysis (about 20 mA / cm ² ) in 10% acetylacetone-1% tetramethylammonium-methanol, and the dissolved residue was collected with a membrane filter (pore diameter: 0.2 μmφ). Melting with a mixed flux of sulfuric acid, nitric acid and perchloric acid, the amount of precipitation can be quantified by ICP emission spectrometry.

Main phase of thick-walled hot-rolled steel sheet In order to obtain a thick-walled hot-rolled steel sheet with high strength, high toughness and low yield elongation characteristics, the structure of the thick-walled hot-rolled steel sheet is made of bainitic ferrite over the entire thickness. It is preferable. However, when manufacturing a thick hot-rolled steel sheet having a sheet thickness of 12 mm or more, for example, when the cooling rate is adjusted so that bainitic ferrite is generated at the center position of the sheet thickness after hot rolling, the sheet thickness surface layer The cooling rate at the part becomes extremely large. Therefore, in the case of a thick hot-rolled steel sheet, it is extremely difficult to obtain a bainitic ferrite main phase structure over the entire plate thickness.

Therefore, in the thick hot-rolled steel sheet according to the present invention, the main phase of the plate thickness surface layer portion (region from the steel plate surface to the plate thickness direction 1 mm) is tempered martensite and / or tempered bainite, and the main region in the region other than the above surface layer portion. The phase is bainitic ferrite.
Although tempered martensite and tempered bainite are inferior to bainitic ferrite, they have far superior elongation properties and toughness than as-quenched martensite and bainite. Therefore, by making only the plate thickness surface layer part a tempered martensite and / or tempered bainite main phase and the other region a bainitic ferrite main phase, the characteristic deterioration in the plate thickness surface layer part is minimized, and the entire steel plate Can be a thick hot rolled steel sheet having high strength, high toughness and low yield elongation.

Volume fraction of main phase: 95% or more When the total volume fraction of tempered martensite and / or tempered bainite is less than 95% at the plate thickness surface layer position of 1 mm (position of 1.0 mm from the steel plate surface), The toughness of the full thickness is greatly reduced. In addition, when the volume fraction of bainitic ferrite is less than 95% at the center of the plate thickness, the toughness is greatly reduced. Therefore, in the present invention, the volume fraction of the main phase at each position is set to 95% or more.

  In addition, in the plate thickness surface layer portion, examples of the structure other than tempered martensite and / or tempered bainite include bainitic ferrite, pearlite, martensite, retained austenite, etc., and the total volume fraction thereof is 5% or less. It is preferable that Further, in the region other than the surface layer portion of the plate thickness, examples of the structure other than bainitic ferrite include tempered martensite, tempered bainite, pearlite, martensite, retained austenite, etc., and their volume fraction is 5% in total. The following is preferable.

  The thick-walled hot-rolled steel sheet of the present invention has the above composition and structure, and is characterized in that the yield elongation in a tensile test in the longitudinal direction of the hot-rolled steel sheet is 0.1% or more and 3.0% or less. The yield elongation is a yield elongation measured by conducting a tensile test using a round bar specimen taken from a coiled hot-rolled steel sheet (a hot-rolled steel sheet that has not been bent back). Value.

  When the yield elongation is less than 0.1%, the stress-strain curve becomes a round house type, and the yield strength YS may be extremely lowered. On the other hand, if the yield elongation exceeds 3.0%, the yield elongation does not disappear even if pipe forming strain is applied, and the buckling resistance after pipe forming deteriorates. Therefore, the yield elongation in the tensile test in the longitudinal direction of the hot-rolled steel sheet is 0.1% to 3.0%.

Next, the manufacturing method of the thick hot rolled steel sheet according to the present invention will be described.
The thick hot-rolled steel sheet of the present invention is subjected to rough rolling and finish rolling after reheating the slab (steel piece) having the above composition, and then accelerated cooling under predetermined conditions and winding at a predetermined temperature. Can be manufactured.
In addition, melting of the steel used for this invention is possible by any of well-known melting methods, such as a converter method and an electric furnace. The molten steel can be made into a slab (steel piece) by continuous casting or ingot-making / bundling rolling.

Slab heating temperature: 1000 ° C or higher and 1250 ° C or lower If the slab (steel slab) reheating temperature is lower than 1000 ° C, the precipitation strengthening elements Nb, V, and Ti are not sufficiently dissolved, and cooling and winding after finishing rolling is completed. Does not form precipitates with C in the process. As a result, the amount of solute C contained in the finally obtained hot-rolled steel sheet increases, yield elongation increases, and strength decreases. On the other hand, when the temperature exceeds 1250 ° C., the austenite coarsens and the toughness of the finally obtained hot-rolled steel sheet deteriorates, and the precipitation form of the precipitates changes, so that it becomes impossible to secure the X80 grade steel pipe strength. Therefore, the slab (steel slab) reheating temperature is set to 1000 ° C. or more and 1250 ° C. or less.

  In addition, when hot-rolling a slab (steel piece), when the slab after casting has a temperature of 1000 ° C. or higher and 1250 ° C. or lower, direct rolling may be performed without heating the slab. The slab after reheating is subjected to rough rolling and finish rolling to be adjusted to an arbitrary plate thickness, but the conditions for rough rolling are not particularly limited in the present invention.

Reduction ratio in non-recrystallization temperature range during finish rolling: 20% or more and 85% or less By performing finish rolling in the non-recrystallization temperature range (about 940 ° C or less in the case of the steel composition of the present invention), the austenite phase Recrystallization is delayed and strain accumulates, and bainitic ferrite is refined during γ / α transformation, improving strength and toughness. Here, when the rolling reduction in the non-recrystallization temperature region during finish rolling is less than 20%, these effects are not sufficiently exhibited. On the other hand, if the rolling reduction exceeds 85%, the deformation resistance increases and hinders rolling. Therefore, in the present invention, the rolling reduction is set to 20% or more and 85% or less. Preferably they are 35% or more and 75% or less.

Finishing rolling end temperature: (Ar ₃ −50 ° C.) or more (Ar ₃ + 100 ° C.) or less In order to finish rolling with a uniform particle size and structure, the finishing rolling end temperature is set to (Ar ₃ −50 ° C.) or more. There is a need. When the finish rolling finish temperature is lower than (Ar ₃ -50 ° C.), ferrite transformation occurs inside the steel plate during finish rolling, the structure becomes non-uniform, and desired characteristics cannot be obtained. On the other hand, when the finish rolling finish temperature exceeds (Ar ₃ + 100 ° C.), the crystal grains become coarse and the toughness deteriorates. Accordingly, the finish rolling finish temperature is set within the range of (Ar ₃ −50 ° C.) to (Ar ₃ + 100 ° C.).
The finish rolling end temperature is a measured temperature value of the steel sheet surface on the exit side of the finish rolling mill.

Average cooling rate in the temperature range of 750 ° C or lower and 650 ° C or higher at the center of the plate thickness: 5 ° C / s or more and less than 50 ° C / s To suppress the formation of pearlite transformation and polygonal ferrite and to ensure strength or toughness It is necessary to forcibly cool the steel sheet after finish rolling, and to set the average cooling rate in the temperature range of 750 ° C. or lower and 650 ° C. or higher at the center of the plate thickness to 5 ° C./s or higher. On the other hand, if the cooling rate at the central portion of the plate thickness in the above temperature range becomes too large, martensite or bainite is formed in the central portion of the plate thickness and the toughness deteriorates, so the average cooling rate at the central position of the plate thickness in the above temperature range Must be less than 50 ° C / s. Preferably, it is 5 ° C./s or more and 40 ° C./s or less.

  The cooling conditions in the temperature range of 650 ° C. or less at the center of the plate thickness are not particularly limited. For example, after the plate thickness center position has been cooled to 650 ° C., forced cooling may be stopped at a predetermined temperature of 650 ° C. or lower, and the plate may be allowed to cool to a winding temperature described later. In addition, forced cooling is continued to the coiling temperature even after the center position of the plate thickness is cooled to 650 ° C, and the temperature range from 650 ° C to the coiling temperature is maintained at an average cooling rate of 5 ° C / s or more and less than 50 ° C / s. It may be cooled.

Average cooling rate in the temperature range of 750 ° C or higher and 650 ° C or higher at the plate thickness surface of 1mm position: 50 ° C / s or higher and 300 ° C / s or lower If it is less than 50 ° C / s, the cooling rate at the center of the thickness of the thick hot-rolled steel sheet having a thickness of, for example, 12 mm or more and 30 mm or less becomes insufficient, pearlite and polygonal ferrite are generated, and the thick hot-rolled steel sheet is formed. The strength and toughness of the steel sheet deteriorate. On the other hand, if the average cooling rate in the temperature range of 750 ° C or lower and 650 ° C or higher at the 1mm position of the plate thickness exceeds 300 ° C / s, the surface layer hardness increases and the elongation characteristics deteriorate, so the upper limit is 300 ° C / s There is a need to. Preferably, it is 50 ° C./s or more and 200 ° C./s or less.

Winding temperature: 200 ° C. or higher and 550 ° C. or lower In the present invention, setting the winding temperature to a predetermined temperature range is extremely important for securing low yield elongation. When the coiling temperature exceeds 550 ° C., the yield elongation is extremely increased to deteriorate the buckling resistance, and the precipitation strengthening is excessive to increase the yield ratio. Therefore, the coiling temperature is 550 ° C. or lower. In order to exhibit particularly excellent buckling resistance, the temperature is desirably 400 ° C. or lower. On the other hand, if the winding temperature is too low, hot rolling winding will be hindered, so the lower limit of the winding temperature is 200 ° C. Here, the winding temperature is the surface temperature of the steel plate immediately before winding, which is substantially the same as the temperature at the plate thickness surface layer of 1 mm.

  For example, the steel sheet after finish rolling is cooled with water, and the amount of water is adjusted so that the average cooling rate in the temperature range of 750 ° C. or lower and 650 ° C. or higher at the center of the plate thickness is 5 ° C./s or higher and lower than 50 ° C./s. If the water cooling is stopped when the sheet thickness central position is cooled to an arbitrary temperature of 650 ° C. to the coiling temperature, the average cooling rate in the temperature range of 750 ° C. or lower and 650 ° C. or higher at the plate thickness 1 mm position is naturally 50 ℃ / s or more. Further, if water cooling is performed as described above, the 1 mm position of the plate thickness surface layer is once cooled to a temperature below the Ms point or below the Bs point, and then reheated and tempered after the winding step.

In the present invention, it is preferable to satisfy the following formula (3) in order to suppress the yield elongation to a low level and improve the buckling resistance.
0.0 ≦ 7.09 × [% C] −1.31 × [% Mn] + 0.00910 × CT ≦ 3.3 (3)
Here, in the formula (3), [% C] and [% Mn] are the content (mass%) of each element,
CT is the coiling temperature (° C).

  In the above formula (3), 7.09 × [% C] −1.31 × [% Mn] + 0.00910 × CT (= YE) is an index of yield elongation. The smaller the YE value, the lower the yield elongation of the hot-rolled steel sheet, and the tendency to improve the buckling resistance of the steel pipe. In order to ensure sufficient buckling resistance, the YE value is preferably 3.3 or less. On the other hand, when the yield elongation of the hot-rolled steel sheet becomes extremely small, the stress-strain curve becomes a round house type, and the yield strength YS may be extremely lowered. Therefore, the YE value is preferably 0.0 or more.

  Using a slab (steel piece) (thickness: 215 mm) having the composition shown in Table 1, hot rolling is performed under the hot rolling conditions shown in Table 2, and after completion of the hot rolling, cooling is performed under the cooling conditions shown in Table 2. And it wound up in the shape of a coil at the coiling temperature shown in Table 2, and made it the hot-rolled steel plate (steel strip) of the plate thickness shown in Table 2. Furthermore, using these hot-rolled steel sheets as the raw material, open tubes were formed by continuous roll forming in the cold, and the end faces of the open tubes were electro-welded to form electric-welded steel tubes (outer diameter 610 mmφ). The finish rolling finish temperature described in Table 2 is the temperature (measured value) on the steel sheet surface on the exit side of the finish rolling mill, and the winding temperature shown in Table 2 is the temperature (measured value) on the steel sheet surface immediately before winding. Value). The cooling after the hot rolling was finished by water cooling, and the water cooling was stopped when the plate thickness center position was cooled to near the target winding temperature. The temperature at the center position of the plate thickness and the cooling rate are calculated values by heat transfer calculation.

  Test pieces were collected from the obtained hot-rolled steel sheet before pipe making, and subjected to extraction residue analysis, tensile test, impact test, DWTT test, and structure observation. In addition, the tensile test has two types of test pieces: a full-thickness tensile test piece collected from a flat plate obtained by bending back a hot-rolled coil, and a round bar test piece described above from a steel plate that is bent without bending back the hot-rolled coil. It carried out in. In addition, a DWTT test was carried out on ERW steel pipes. In addition, a buckling resistance test was conducted on the ERW steel pipe. The test method was as follows.

(1) Extraction residue analysis (measurement method of precipitated Nb ratio)
Test specimens were collected from the center of the thickness of the obtained hot-rolled steel sheet and the surface thickness of 1 mm, and the precipitates extracted by electrolytic extraction using a maleic acid electrolyte were analyzed by ICP emission spectrometry. The amount of Nb was measured and expressed as mass% with respect to the total Nb mass of the test piece as “a ratio (%) of precipitated Nb to the total Nb amount”. The composition of the maleic acid electrolyte was 10% maleic acid-2% acetylacetone-5% tetramethylammonium chloride-methanol. In the electrolytic extraction, constant current electrolysis (about 20 mA) is performed, and the residue is collected with a membrane filter. After that, the filter and residue are crushed and then melted using a mixed flux of lithium borate and sodium peroxide. The molten product is dissolved in hydrochloric acid, diluted to a certain volume with water, and the thickness is measured by ICP emission spectrometry. The Nb deposition rate at the center position and the 1 mm thickness surface layer shall be quantified. The case where the Nb precipitation ratio was in the range of 5% or more and less than 35% was evaluated as “Nb precipitation ratio excellent in buckling resistance”.

(2) Tensile test The tensile test was performed with the following two types of test pieces.
(A) The obtained hot-rolled coil was straightened and sampled so that the rolling direction (L direction) was the longitudinal direction, a plate-like full-thickness test piece (parallel part width: 25 mm, distance between gauge points: 50 mm) , Board thickness: total thickness)
(B) Round bar test specimen (parallel part 30mm, gauge length 25mm, gauge part taken from the center part of the hot-rolled coil plate thickness that remains curved so that the rolling direction (L direction) is the longitudinal direction. (Diameter 6mmφ)

  Using the above two types of tensile test pieces, in accordance with ASTM E8M-04, a tensile test is performed at room temperature, yield strength YS (MPa), tensile strength TS (MPa), and yield elongation (%). Asked. A case where the yield strength was 550 MPa or more and the tensile strength was 650 MPa or more was evaluated as “good tensile properties”. However, if the strength becomes too high, the elongation characteristics deteriorate, so it is desirable that the yield strength is 690 MPa or less and the tensile strength is 760 MPa or less. Moreover, the case where the yield elongation in the round bar tensile test piece was 0.1% or more and 3.0% or less was evaluated as “low yield elongation characteristics were good”.

(3) Charpy impact test V-notch specimens were taken from the center of the thickness of the obtained hot-rolled steel sheet so that the direction perpendicular to the rolling direction (C direction) was the longitudinal direction, and stipulated in JIS Z 2242 In accordance with the Charpy impact test, the absorbed energy vE _-60 (J) and the ductile-brittle fracture surface transition temperature vTrs (° C) at -60 ° C were determined. The test temperature was a multiple temperature condition including −60 ° C., and three test pieces were used at each test temperature. The arithmetic average of the obtained absorbed energy value at −60 ° C. was obtained and was defined as the absorbed energy value vE- _{60 of the} steel sheet. Moreover, the average value of the brittle fracture surface ratio at each test temperature was also obtained, and vTrs was obtained by plotting these on a graph (horizontal axis: test temperature, vertical axis: brittle fracture surface ratio). A case where vE- ₆₀ was 100 J or more and vTrs was −80 ° C. or less was evaluated as “good toughness”.

(4) DWTT test DWTT test piece (size: full plate thickness) so that the direction (C direction) perpendicular to the rolling direction is the sample long side direction from the base material part of the obtained hot rolled steel sheet and ERW steel pipe (Thickness x width 3in. X length 12in.) Was collected and subjected to a DWTT test in accordance with ASTM E 436 to determine the lowest temperature (DWTT SA85% TT) at which the ductile fracture surface ratio was 85%. . It was evaluated that DWTT SA85% TT had “excellent DWTT characteristics” when the hot-rolled steel sheet was −30 ° C. or lower and the ERW steel pipe was −10 ° C. or lower. In addition, when extracting a test piece from the base material part of an ERW steel pipe, it extract | collected so that a test piece longitudinal direction might turn into a pipe peripheral direction, and implemented the test similarly to the steel plate.

(5) Pipe shaft compression test (buckling resistance test)
Pressure-resistant plates were attached to both ends of the obtained electric resistance welded steel pipe (steel pipe length 1,800 mm), and an axial compression test was conducted using a large compression tester. The amount of strain at the point where the compressive load was maximized was defined as the critical buckling strain. The case where the critical buckling strain was 0.5% or more was evaluated as “good buckling resistance”.

(6) Microstructure observation From the obtained hot-rolled steel sheet, a block-shaped test piece that can observe all positions in the thickness direction is collected, and an L cross section is obtained using a scanning electron microscope (magnification: 2000 to 5000 times). Observation (the hot-rolled steel sheet width direction is perpendicular to the observation surface) was performed. In order to obtain average tissue information, observe at least 3 fields of view at each thickness position for the thickness 1/2 (center) position and 1 mm thickness surface layer, and the volume fraction of each constituent tissue is the average of these values. Asked. A structure in which the volume fraction of bainitic ferrite at the center of the plate thickness is 95% or more and the volume fraction of tempered martensite and / or tempered bainite at the plate thickness 1mm position is 95% or more “Excellent organization”.
Table 3 shows the results of the above (1) to (6).

  As shown in Table 3, the hot-rolled steel sheet of the inventive example was good in tensile properties (strength, low yield elongation) and toughness (low temperature toughness). In addition, any ERW steel pipe made of the hot-rolled steel sheet of the inventive example had good toughness (low temperature toughness) and showed excellent buckling resistance. On the other hand, the hot-rolled steel sheet of the comparative example could not obtain sufficient characteristics in either one or both of tensile properties and toughness (low temperature toughness). In addition, the electric resistance welded steel pipe made of the hot-rolled steel sheet of the comparative example did not have sufficient characteristics in either toughness (low temperature toughness) or buckling resistance.

Claims (8)

% By mass
C: 0.04% to 0.15%, Si: 0.01% to 0.55%,
Mn: 1.0% to 3.0%, P: 0.03% or less,
S: 0.01% or less, Al: 0.003% or more and 0.1% or less,
N: 0.006% or less, Nb: 0.035% or more and 0.1% or less,
V: 0.001% or more and 0.1% or less, Ti: 0.001% or more and 0.1% or less, with the balance being Fe and inevitable impurities, and the ratio of precipitated Nb to the total amount of added Nb is 5% or more and 35% The volume fraction of bainitic ferrite at the center of the plate thickness is 95% or more, and the volume fraction of tempered martensite and / or tempered bainite at the plate thickness 1 mm position is 95% or more. A thick-walled hot-rolled steel sheet having a structure and having a yield elongation of 0.1% to 3.0% in a tensile test in the longitudinal direction of the hot-rolled steel sheet. The thick hot-rolled steel sheet according to claim 1, wherein the composition satisfies the following formulas (1) and (2).
Record
Pcm = [% C] + [% Si] / 30 + ([% Mn] + [% Cu] + [% Cr]) / 20
+ [% Ni] / 60 + [% V] / 10 + [% Mo] / 7 + 5 × [% B] ≦ 0.25 (1)
Px = 701 × [% C] + 85 × [% Mn] ≧ 181 (2)
Here, in the formulas (1) and (2), [% C], [% Si], [% Mn], [% Cu], [% Cr], [% Ni], [% V], [% % Mo] and [% B] are the contents of each element (% by mass).   The thick hot-rolled steel sheet according to claim 1 or 2, further comprising Ca: 0.0001% or more and 0.005% or less by mass% in addition to the composition.   In addition to the above composition, Cu: 0.001% to 0.5%, Ni: 0.001% to 0.5%, Mo: 0.001% to 0.5%, Cr: 0.001% to 0.5%, B: 0.0001 The thick hot-rolled steel sheet according to any one of claims 1 to 3, wherein the thick hot-rolled steel sheet contains one or two or more selected from% to 0.004%. % By mass
C: 0.04% to 0.15%, Si: 0.01% to 0.55%,
Mn: 1.0% to 3.0%, P: 0.03% or less,
S: 0.01% or less, Al: 0.003% or more and 0.1% or less,
N: 0.006% or less, Nb: 0.035% or more and 0.1% or less,
V: 0.001% or more and 0.1% or less, Ti: 0.001% or more and 0.1% or less, and a steel slab having a composition composed of Fe and unavoidable impurities in the remainder is reheated to a temperature range of 1000 ° C to 1250 ° C, Following rough rolling and rough rolling, the rolling reduction in the non-recrystallization temperature range is 20% or more and 85% or less, and the finish rolling finish temperature is set to a temperature range of (Ar ₃ −50 ° C.) to (Ar ₃ + 100 ° C.). After finish rolling, after finishing the finish rolling, the average cooling rate in the temperature range of 750 ° C or lower and 650 ° C or higher at the center of the plate thickness is 5 ° C / s or higher and less than 50 ° C / s, and the thickness of the surface layer 1mm position is 750 ° C or lower Production of thick-walled hot-rolled steel sheet, which is cooled at an average cooling rate of 50 ° C / s or more and 300 ° C / s or less in a temperature range of 650 ° C or more and wound at a winding temperature of 200 ° C or more and 550 ° C or less Method. The method for producing a thick hot-rolled steel sheet according to claim 5, wherein the composition satisfies the following formulas (1) and (2), and the winding temperature satisfies the following formula (3).
Record
Pcm = [% C] + [% Si] / 30 + ([% Mn] + [% Cu] + [% Cr]) / 20
+ [% Ni] / 60 + [% V] / 10 + [% Mo] / 7 + 5 × [% B] ≦ 0.25 (1)
Px = 701 × [% C] + 85 × [% Mn] ≧ 181 (2)
0.0 ≦ 7.09 × [% C] −1.31 × [% Mn] + 0.00910 × CT ≦ 3.3 (3)
Here, in the formulas (1), (2) and (3), [% C], [% Si], [% Mn], [% Cu], [% Cr], [% Ni], [% % V], [% Mo], and [% B] are the contents (% by mass) of each element.
In equation (3), CT is the coiling temperature (° C).   The method for producing a thick hot-rolled steel sheet according to claim 5 or 6, further comprising Ca: 0.0001% or more and 0.005% or less by mass% in addition to the composition.   In addition to the above composition, Cu: 0.001% to 0.5%, Ni: 0.001% to 0.5%, Mo: 0.001% to 0.5%, Cr: 0.001% to 0.5%, B: 0.0001 The method for producing a thick hot-rolled steel sheet according to any one of claims 5 to 7, comprising one or more selected from 1% or more and 0.004% or less.