KR101531361B1 - High-strength steel plate and high-strength steel pipe having excellent deformability and low-temperature toughness, and manufacturing methods therefor - Google Patents

Abstract

A high strength steel plate and a high strength steel pipe excellent in deformation performance and low temperature toughness capable of suppressing a decrease in thickness at the time of deformation and a method for producing the same are disclosed. , And the ratio of the effective crystal grain size at the center of the thickness is 20 占 퐉 or less and the X-ray random intensity ratio of the {111} plane parallel to the plate surface in the center of the thickness is 0.5 to 5.0, Ray random intensity ratio of the {100} plane is 3.0 or less, the X-ray random intensity ratio of each of the {112} plane and the {223} plane is 0.5 to 4.0, and the thickness is 25 mm or more and a tensile strength of 565 MPa or more.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength steel sheet and a high strength steel pipe excellent in deformation performance and low temperature toughness and a method of manufacturing the same. BACKGROUND OF THE INVENTION 1. Field of the Invention [0001]

The present invention relates to a high-strength steel plate and a high-strength steel pipe which are suitably used as a line pipe for transporting natural gas, crude oil and the like, and particularly excellent in deformation tolerance against ground deformation and the like and low-temperature toughness.

In recent years, the importance of the line pipe as a method of transporting natural gas and crude oil over a long distance has been increasing. The line pipe has various environments to be laid. For example, the line pipe is attached to an environment in which fluctuations of the ground in summer and winter in the frozen soil, external pressure due to currents in the seabed, and stratum fluctuations due to earthquakes occur. Under such an environment, there is a case where the line pipe is bent or displaced due to ground fluctuation or the like, and therefore, a steel pipe excellent in deformation performance, in which buckling is unlikely to occur even when the line pipe is deformed, is desired.

As a steel pipe excellent in deformation performance, there has been proposed a steel pipe which is improved in consideration of the work hardening index (n value) disclosed in Patent Document 1 and a steel pipe which is improved in its yield strength, which is the ratio of the yield strength to the tensile strength A steel pipe has been proposed which focuses on the improvement of the steel pipe.

Patent Document 1: JP-A-11-279700 Patent Document 2: JP-A-2005-15823

Conventionally proposed technology is a technique for improving the deformation performance of a steel plate or a steel pipe used in a line pipe or the like by paying attention to a work hardening index and a yield ratio.

However, in particular, a line pipe used for cold regions such as frozen ground is required to be excellent in low-temperature toughness, but a technique for obtaining a steel plate and a steel pipe excellent in deformation performance and low-temperature toughness has not been sufficiently studied.

The present invention has been made in view of the above problems, and it is an object of the present invention to provide a high strength steel sheet and a high strength steel pipe excellent in deformation performance and low temperature toughness capable of suppressing a decrease in thickness at the time of deformation and a method of manufacturing the same.

Means for Solving the Problems The present inventors have studied extensively in order to solve the above problems. As a result, it has been found that it is possible to improve the deformation performance of a steel plate or a steel pipe used in a pipeline or the like by focusing on the rank pod value.

Conventionally, attention has not been focused on the reduction in the thickness of a steel plate or a steel pipe used for a line pipe or the like when deformed due to a ground change or the like. The rank pod value is known in the field of automotive steel sheet or the like as an index value for evaluating the reduction amount of thickness at the time of deformation. There has not been proposed a technique for attempting to improve the deformation performance of a steel plate or a steel pipe used in a pipeline or the like by focusing on the rank pod value.

The present inventors have made extensive studies in order to obtain a high strength steel sheet and a high strength steel pipe excellent in deformation performance and low temperature toughness. As a result, it has been found that it is particularly effective to optimize the size of the effective grain diameter while appropriately adjusting the amount of the texture having a predetermined crystal orientation in obtaining the high-strength steel sheet and the high-strength steel pipe excellent in deformation performance and low temperature toughness. The present inventors further studied and found that it is particularly effective to control various production conditions including reduction rate in hot rolling at the time of optimizing the amount of the aggregate structure having a predetermined crystal orientation, The reduction rate per pass of the rolling in the temperature range is very important.

The present invention has been made based on the above findings, and the gist of the present invention is as follows.

(1) A steel sheet comprising, by mass%, 0.03 to 0.08% of C, 0.01 to 0.50% of Si, 1.50 to 2.50% of Mn, 0.015% or less of P, 0.0050% or less of S, 0.001 to 0.080% 0.005 to 0.030% of Ti, 0.010 to 0.050% of Nb, and the balance of Fe and unavoidable impurities, Ceq represented by the following formula (A) is 0.35 to 0.50% B) of 0.15 to 0.19% and having a composite structure of ferrite and bainite or martensite, wherein the effective grain diameter at the center of the thickness is 20 μm or less, The X-ray random intensity ratio of the {111} plane parallel to the printing surface is 0.5 to 5.0, the X-ray random intensity ratio of the {554} plane is 1.0 to 3.0, the X- Surface and a {223} plane of 0.5 to 4.0, an X-ray random intensity ratio of 25 mm or more, and a tensile strength of 565 MPa or more and 710 M Pa, the rank-pod value rD in the direction of 45 ° with respect to the rolling direction of the steel sheet, and the rank-pod value rC in the plate-width direction are 1.0 or more, respectively, and excellent in low-temperature toughness.

(A + Ce + Cd) / (Ce + Cd)

Mo / 15 + V / 10 + 5B (B) Pcm = C + Si / 30 + Mn + Cu +

In this case, C, Mn, Ni, Cu, Cr, Mo, V, Si, and B are contents represented by mass% of each element.

delete

(2) The steel sheet according to any one of (1) to (3), wherein the steel sheet further contains, by mass%, 0.010 to 0.100% of V, 1.0% or less of Ni, 1.0% or less of Cu, 1.0% or less of Cr, 1.0% % Or less, Mg: 0.0010% or less, and REM: 0.005% or less.

(3) A high-strength steel pipe excellent in deformation performance and low-temperature toughness, comprising the steel sheet of (1) or (2).

(4) A ferritic stainless steel comprising, by mass%, 0.03 to 0.08% of C, 0.01 to 0.50% of Si, 1.50 to 2.50% of Mn, 0.015% or less of P, 0.0050% or less of S, 0.001 to 0.080% 0.005 to 0.030% of Ti, 0.010 to 0.050% of Nb, and the balance of Fe and unavoidable impurities, Ceq represented by the following formula (A) is 0.35 to 0.50% B) of 0.15 to 0.19% is heated to a heating temperature of 1000 to 1150 占 폚, and then a reduction rate per pass is set to be 1000 占 폚 or more and 1050 ℃ 10 to 15% when 5 to 10%, the heating temperature is above 1050 ℃ 1150 ℃ or less when less, and further, and rolled to a cumulative reduction ratio of over 35%, followed by a temperature range of less than Ar ₃ transformation point or re-crystallization temperature the cumulative rolling reduction in the rolling as 70 to 80%, then, Ar ₃ A temperature range of Ar ₃ transformation point is less than -50 ℃ taejeom least at a cooling start temperature, and the method of producing a high deformation performance and low temperature toughness characterized in that the water-cooled high-strength steel sheet to the temperature range of 200 to 500 at a cooling end temperature ℃ .

(A + Ce + Cd) / (Ce + Cd)

Mo / 15 + V / 10 + 5B (B) Pcm = C + Si / 30 + Mn + Cu +

In this case, C, Mn, Ni, Cu, Cr, Mo, V, Si, and B are contents represented by mass% of each element.

(5) The steel according to any one of the above items (1) to (4), further comprising, by mass%, 0.010 to 0.100% of V, 1.0% or less of Ni, 1.0% or less of Cu, 1.0% or less of Cr, 1.0% (4) and a method for producing a high strength steel sheet excellent in low temperature toughness, characterized by containing at least one element selected from the group consisting of Ca: 0.0040% or less, Mg: 0.0010% or less, and REM: 0.005% .

(6) A method of manufacturing a high-strength steel pipe excellent in deformation performance and low-temperature toughness, characterized by forming the steel sheet obtained by the manufacturing method of (4) or (5) into a tube shape and welding the butt portion.

According to the present invention, it is possible to obtain a high-strength steel sheet and a high-strength steel pipe excellent in deformation performance and low-temperature toughness capable of suppressing a reduction in thickness at the time of deformation.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a photograph of the structure at the center of thickness of the steel sheet of the present invention. Fig.

First, the reason why the numerical ranges of the composition of the steel sheet and the steel pipe according to the present invention are limited will be described. In the following, "% " represents " mass% ".

C is an element necessary for securing the strength of the steel. If the C content is less than 0.03%, the strength of the final product is insufficient. If the C content exceeds 0.08%, the low-temperature toughness of the base material and the HAZ is remarkably lowered. Therefore, the amount of C is 0.03 to 0.08%.

Si acts as a deoxidizing agent and contributes to the improvement of strength. If the amount of Si is less than 0.01%, the strength of the final product may be insufficient. If the amount of Si exceeds 0.50%, the toughness of HAZ remarkably decreases. Therefore, the amount of Si is set to 0.01 to 0.50%.

Mn is an element contributing to improvement of steel strength. If the Mn content is less than 1.50%, the strength of the final product may be insufficient. If the Mn content exceeds 2.50%, the low-temperature toughness of the base material and the HAZ is remarkably lowered. Therefore, the amount of Mn is 1.50 to 2.50%. It is preferably 1.50 to 2.00%.

Al is an element that acts as a deoxidizing element and contributes to the refinement of the metal structure. If the amount of Al is less than 0.001%, this effect can not be sufficiently obtained. When the amount of Al exceeds 0.080%, Al-based non-metallic inclusions increase in the steel and deterioration of the steel is deteriorated. Therefore, the amount of Al is limited to 0.080% or less. The preferred range is 0.001 to 0.050% or less.

Ti precipitates as TiN in the steel, thereby restraining the coarsening of the austenite grains in the HAZ during reheating of the slab, thereby finely reducing the metal structure and improving the low temperature toughness of the base material and the HAZ. However, if the amount of Ti is less than 0.005%, this effect can not be sufficiently obtained. If the amount of Ti is more than 0.030%, the coarsening of TiN or precipitation hardening by TiC deteriorates the low-temperature toughness. Therefore, the amount of Ti is 0.005 to 0.030%.

Nb has an effect of suppressing recrystallization of austenite at the time of hot rolling and making the structure finer and improving the low temperature toughness of the base material and HAZ. However, if Nb content is less than 0.010%, this effect can not be sufficiently obtained. On the other hand, if the amount of Nb exceeds 0.050%, the toughness of the HAZ and the local weldability are adversely affected. Therefore, the amount of Nb is 0.010 to 0.050%.

P is an impurity inevitably contained in the steel. By causing grain boundary segregation or center segregation, the low temperature toughness of the base material and the HAZ deteriorates. When the P content is 0.015% or less, the range is acceptable for low temperature toughness. Therefore, the amount of P is limited to 0.015% or less.

S is an impurity inevitably contained in the steel, and a sulfide which is stretched by hot rolling is formed in the steel, thereby decreasing ductility and toughness. When the S content is 0.0050% or less, the allowable range for ductility and toughness is acceptable. Therefore, the amount of S is limited to 0.0050% or less.

N is precipitated as TiN in the steel, thereby restraining coarsening of the austenite grains during the slab reheating and the HAZ, and improving the low temperature toughness of the base material and the HAZ. When the N content is less than 0.0010%, this effect can not be sufficiently obtained. If the N content exceeds 0.0060%, the toughness is lowered by the increase of the amount of solid solution N. Accordingly, the N content is 0.0010 to 0.0060%.

In the present invention, the carbon equivalent Ceq expressed by the following formula (A), which is calculated from the content expressed by mass% of C, Mn, Ni, Cu, Cr, Mo and V, is set to 0.35 to 0.50%. The carbon equivalent Ceq is a value which is an index of hardenability.

If the Ceq value is less than 0.35%, the target tensile strength of 565 MPa or more can not be obtained. On the other hand, if the Ceq value is more than 0.50%, generation of MA (Martensite-Austenite Constituent: a mixture of martensite and austenite) deteriorating toughness is remarkable and toughness is deteriorated. In the following equation (A), elements not contained in the steel are calculated as zero.

 (A + Ce + Cd) / (Ce + Cd)

In the present invention, the Pcm represented by the following formula (B) calculated from the content expressed by mass% of C, Si, Mn, Cu, Cr, Ni, Mo, V and B is set to 0.15 to 0.19%. Pcm is a value which is an index of weldability.

If the Pcm exceeds 0.19%, the low temperature toughness of the base material and the HAZ deteriorates. If the Pcm is less than 0.15%, deterioration of the low temperature toughness of the base material and the HAZ is suppressed, but the target tensile strength can not be obtained. In the following equation (B), the element not contained in the steel is calculated as zero.

 Mo / 15 + V / 10 + 5B (B) Pcm = C + Si / 30 + Mn + Cu +

The above is the reason for limiting the basic elements of the steel sheet and the steel pipe according to the present invention. The steel sheet and the steel pipe according to the present invention are made of Fe and inevitable impurities in addition to the base element.

Further, the steel sheet and the steel pipe according to the present invention may further include one or more elements selected from V, Ni, Cu, Cr, Mo, B, Ca, Mg and REM, . Even if these elements are contained in the following ranges, the X-ray random intensity ratio in the steel sheet and the steel pipe, and the rank pod value are within the ranges specified in the present invention.

V has almost the same effect as Nb, but its effect is weaker than Nb. It also has an effect of suppressing the softening of the welded portion. If the V content is less than 0.010%, the effects of improving the low temperature toughness of the base material and the HAZ and suppressing softening of the welded portion become insufficient. If the V amount exceeds 0.100%, the toughness of the HAZ and the local weldability are adversely affected. Therefore, the amount of V is 0.010 to 0.100%.

Ni, Cu, Cr, and Mo contribute to the strengthening of steel by increasing the hardenability. However, if the content is too large, not only the economical efficiency is lowered but also the toughness of the HAZ and the weldability at the site are deteriorated. Therefore, the contents of Ni, Cu, Cr, and Mo are each 1.0% or less.

B is an element contributing to the strengthening of steel by increasing the hardenability. If the B content is less than 0.0001%, this effect can not be sufficiently obtained. If the amount of B exceeds 0.0020%, the toughness and local weldability of HAZ decrease. Therefore, the amount of B is 0.0001 to 0.0020%.

Ca and REM are elements that control the shape of sulfides and contribute to the improvement of low temperature toughness. If the amount of Ca exceeds 0.0040% and the amount of REM exceeds 0.005%, CaO-CaS or REM-CaS will precipitate in large quantities to form large clusters and large inclusions, which may degrade the degree of steel and adversely affect local weldability. Therefore, the amount of Ca is 0.0040% or less, and the amount of REM is 0.005% or less.

Mg is an element that disperses and precipitates as a fine oxide and contributes to improvement of low-temperature toughness by suppressing particle diameter coarsening of HAZ. If the amount of Mg exceeds 0.0010%, toughness is deteriorated due to coarsening of the oxide. Therefore, the amount of Mg is 0.0010% or less.

Next, the reasons for limiting the metal structure, texture, thickness, tensile strength, and rank-pod value (r value) of the steel sheet and steel pipe according to the present invention will be described.

In order to improve the work hardening property, the metal structure needs to have a composite structure of one or two of soft ferrite and hard bainite or martensite.

Further, the metal structure needs to have an effective grain diameter of 20 mu m or less at the center of the thickness. If the effective grain diameter is more than 20 占 퐉, the low-temperature toughness deteriorates. The effective grain diameter refers to the grain diameter at the circle equivalent diameter of the portion surrounded by the boundary of the structure having an azimuth difference of 15 degrees or less as measured by an EBSP (Electron Backscatter Diffraction Pattern) method.

In order to obtain a desired rank pod value for the steel sheet and the steel pipe, the texture must satisfy the conditions described below for the X-ray random intensity ratio. In the present invention, the rank pod value rD in the direction of 45 DEG and the rank pod value rC in the plate width direction with respect to the rolling direction of the steel sheet are noted. By increasing the rank-pod value, the deformation performance of the steel plate and the steel pipe may be increased.

The crystal orientations described below all refer to the plane parallel to the plane. The X-ray random intensity ratio is a numerical value representing the degree of integration of the crystal planes having respective orientations, and represents the ratio of the X diffraction intensities of the crystal planes having respective orientations to the random standard sample without aggregate structure.

It is preferable that the aggregate structure having the crystal orientation of the {111} plane is extremely developed because the rC and rD can be increased as it is developed. From the viewpoint of obtaining preferable rC and rD, it is necessary to set the X-ray random intensity ratio of the {111} plane to 0.5 or more. If the X-ray random intensity ratio of the {111} plane is more than 5.0, there is a possibility that a desired value can not be obtained with respect to the X-ray random intensity ratio of the other crystal orientations. Should be 5.0 or less.

It is preferable that the aggregate structure having the {554} plane crystal orientation is extremely developed because the rC can be increased as it is developed. From the viewpoint of obtaining a preferable rC, it is necessary to set the X-ray random intensity ratio of the {554} face to 1.0 or more. In addition, when the X-ray random intensity ratio of the {554} plane is more than 3.0, there is a possibility that a desired value can not be obtained with respect to the X-ray random intensity ratio of the other crystal orientations. The random intensity ratio should be 3.0 or less.

It is preferable that the texture having the crystal orientation of the {100} plane causes rC and rD to decrease as the development progresses, so that the development is extremely suppressed. From the viewpoint of obtaining a preferable rC, it is necessary to set the X-ray random intensity ratio of the {100} plane to 3.0 or less.

It is desirable that the texture of the {212} plane and the {223} plane of the texture having the crystal orientation is extremely developed because the rD can be increased as it is developed. From the viewpoint of obtaining a preferable rD, it is necessary to set the X-ray random intensity ratio of each of {112} plane and {223} plane to 0.5 or more. If the X-ray random intensity ratio of each of the {112} plane and the {223} plane is more than 4.0, there is a possibility that a desired value can not be obtained with respect to the X-ray random intensity ratio of the other crystal orientations. Plane and the {223} plane is 4.0 or less.

For the X-ray random intensity ratio of the present invention, the measured value measured by X-ray diffraction at the center of the thickness is used. This is because the texture that can raise the rC and rD such as the {111} plane easily develops in the thickness surface layer and is difficult to develop at the center of thickness, so that the X-ray random intensity ratio at the center of thickness is evaluated, So that it is possible to exhibit a deformation performance more than a certain level in the entire direction.

The steel sheet should have a thickness of 25 mm or more and a tensile strength of 565 MPa or more (grade of X70 or more in API specification) from the viewpoint of preventing breakage due to internal pressure when used as a line pipe while securing necessary strength as a final product There is a need.

In the present invention, the larger the rank-pod value rD in the direction of 45 DEG and the rank-pod value rC in the plate-width direction with respect to the rolling direction of the steel sheet, the better the deformation performance. RD and rC are preferably 1.0 or more, and 1.1 or more is more preferable in order to reduce the possibility of occurrence of buckling or the like due to reduction in thickness when the steel plate or the steel pipe is deformed.

Next, a method of manufacturing a steel sheet according to the present invention will be described.

First, molten steel having the above-mentioned composition is melted by a known solvent method using a converter or the like, and then a steel piece is obtained from molten steel obtained by a known casting method such as continuous casting.

Then, the obtained slab is heated to a temperature of 1000 to 1150 캜. If the heating temperature is less than 1000 占 폚, sufficient recrystallization of austenite can not be achieved, and sufficiently high temperature toughness can not be obtained. If the heating temperature is higher than 1150 DEG C, the austenite grains are coarsened and the effective grain diameter is increased to lower the low temperature toughness.

Then, the reduction rate per pass, that is, the cumulative reduction ratio / number of passes is set to 5 to 10% when the heating temperature is 1000 ° C or more and less than 1050 ° C, and the heating temperature is 1050 ° C Or more and 1150 占 폚 or less, the rolling is carried out at 10 to 15%, and the cumulative rolling reduction is 35% or more. If the cumulative rolling reduction is less than 35%, the fineness of the austenite grain size due to recrystallization can not be sufficiently achieved, and the effective grain diameter is increased and the low temperature toughness is lowered.

The reduction rate per pass is particularly important for obtaining the aggregate structure of the desired crystal orientation. Conventionally, there has been no case in which the reduction rate per one pass is increased due to restriction of facilities. However, in order to obtain a desired structure in the steel sheet or steel pipe of the present invention, the reduction rate per pass needs to be within the above range. If the reduction rate per pass is out of the above range, the target distribution of the texture can not be obtained.

The reduction ratio of the individual passes may be out of the range due to the convenience of the path schedule or the like. However, in the case of a path having a half or more of the number of passes, the reduction rate is preferably within the above range, good.

Subsequently, rolling is carried out at a temperature lower than the Ar ₃ transformation point and above the recrystallization temperature, with the cumulative rolling reduction being 70% or more. If the cumulative rolling reduction is less than 70%, the development of the texture of {554} planes is suppressed, and the target of X-ray random intensity ratio can not be obtained, and the rC value is lowered.

Then, the water-cooled to a temperature range of Ar ₃ transformation point or more than -50 ℃ start cooling the temperature range of Ar ₃ transformation point temperature is less than 200 to 500 ℃ at a cooling end temperature. When the cooling start temperature is lower than the Ar ₃ transformation point -50 占 폚, generation of ferrite is promoted, and the desired strength can not be obtained. When the cooling start temperature is not lower than the Ar ₃ transformation point, the development of the individual texture of the {112} plane and the {223} plane is suppressed so that the desired X-ray random intensity ratio can not be obtained. do.

If the cooling end temperature is less than 200 占 폚, it may cause a decrease in productivity or a hydrolytic defect. If the cooling termination temperature exceeds 500 deg. C, the desired strength can not be obtained. The cooling rate is not particularly limited, but is about 1 to 10 DEG C / s.

The Ar ₃ transformation point can be obtained from the following equation (C). C, Si and the like in the following formula (C) mean the content of each element expressed as% by mass in the steel.

Ar ₃ = 868-396 × C + 24.6 × Si-69.1 × Mn-36.1 × Ni-20.7 × Cu-24.8 × Cr + 29.6 × Mo (C)

The thus produced steel sheet is further formed into a tube shape, and the butt portion is joined to obtain a steel pipe. As a method of forming the steel sheet into a tube shape, known UOE method, bending roll method or the like is used, and the butt welding method is arc welding, laser welding, or the like.

While the embodiments of the present invention have been described in detail above, it should be understood that the foregoing embodiments are merely illustrative of specific embodiments of the present invention and that the technical scope of the present invention is construed to be limited no.

Example

Hereinafter, the effects of the present invention will be described by way of examples.

Molten steel having compositions of the respective steel types A to F shown in the following Table 1 was melted in a converter, and was made into a piece by continuous casting. The obtained slabs were subjected to hot rolling and cooling under the conditions shown in Table 2 below to form the steel sheets and steel sheets Nos. 1 to 5 and 8 to 15 in the form of a tube, and the steel sheets obtained by joining the buttocks. 6 to 7 steel pipes were obtained. No. The diameters of steel pipes 6 to 7 are 48 inches (1219.2 mm).

The obtained steel sheet and steel pipe were measured for tensile strength and the like described below. The results are shown in Table 3.

The tensile strength was measured by cutting a JIS No. 5 plate test piece whose longitudinal direction was parallel to the rolling direction from the obtained steel plate and using this test piece to perform a tensile test according to the method described in JIS Z2241. Tensile strength was also determined for grades in API technical specifications. For the steel pipe, the tensile strength was measured with a test piece having a total thickness in the longitudinal direction of the steel pipe, based on the API technical standard.

Metallic tissues were observed by optical microscope. The effective grain diameter is measured by the EBSP method and the boundary of the structure having an azimuth difference of 15 degrees or more is regarded as grain boundary to obtain the area inside one single crystal and the area converted to the circle equivalent diameter is evaluated as the effective grain diameter Respectively.

The X-ray random intensity ratio was obtained by cutting a test piece having a 10 mm width in the rolling direction and a 10 mm width in the rolling direction from the obtained steel sheet, polishing the test piece to the vicinity of the center of thickness by mechanical polishing, polishing it with a mirror surface by buffing, And adjusting the thickness of the center core layer to be the measurement plane and measuring the diffraction intensity of each crystal orientation by X-ray diffraction.

The rank pod value was obtained by cutting a JIS No. 5 plate test specimen from the obtained steel plate so that a test piece whose longitudinal direction was parallel to the rolling direction and a test piece parallel to the direction of 45 DEG with respect to the rolling direction and parallel to the plate width direction were prepared, Was subjected to a tensile test in accordance with the method described in JIS Z2241, and from the ratio of the width deformation and the thickness deformation of each test piece when a 3% single axis tensile deformation was applied to the test piece, And the pod values rC, rD and rL. rL is the rank pod value in the rolling direction.

The Charpy impact test was carried out by preparing a V-notch Charpy test piece from the obtained steel sheet in the thickness direction at 1/4 position and evaluating the Charpy absorbed energy at a test temperature of -40 ° C according to the method described in JIS Z2242.

In each example, the tensile strength was determined to be equal to or greater than 565 MPa, while the low temperature toughness was determined to be equal to or greater than 200 J. In addition, underlined portions in the respective tables indicate that they are outside the scope of the invention.

Manufacturing no. 1 to 7 are examples; 1 to 5 are steel plates, and No. 6 to 7 are examples of steel tubes. These ratios satisfy the conditions of the present invention, rD value is 1.0 or more, rC value is 1.0 or more, Charpy absorbed energy is 200 J Thus, a high-strength steel sheet excellent in low-temperature toughness can be obtained.

It was also found that the metal structure, the effective crystal grain diameter, the X-ray random intensity ratio, and the tensile strength satisfied the conditions of the present invention, because their compositions, thicknesses, and manufacturing methods satisfied the conditions of the present invention.

Fig. 1 shows an example of a photograph of a structure at the center of thickness of a steel sheet according to the invention. The photograph of Fig. 2. Among the photographs of the structure, the portion where the fine structure is not present in the inside and the fine structure in the inside is ferrite, and in the portion other than the ferrite, the portion which is entirely gray and has the fine structure inside is bainite or martensite.

Manufacturing no. 8 to 15 are comparative examples. Manufacturing no. 8 is an example in which the effective grain size is large and the low-temperature toughness is deteriorated because the heating temperature is high.

Manufacturing no. 9 is an example in which the distribution of the desired texture can not be obtained because the reduction rate per one pass at the temperature range higher than the recrystallization temperature is low and the rD value and the rC value, which are indexes of the deformation characteristics, are deteriorated.

Manufacturing no. 10 is an example where degradation of low-temperature toughness is caused by an increase in the effective grain diameter, since the cumulative reduction ratio at a temperature range higher than the recrystallization temperature is low, and thus the fineness of the austenite grain size by recrystallization can not be sufficiently achieved.

Manufacturing no. 11 is an example in which the cumulative rolling reduction at a temperature range lower than the Ar ₃ transformation point ideal recrystallization temperature is low and the rC value is deteriorated because the development of the texture of {554} plane is suppressed.

Manufacturing no. 12 is an example in which development of the texture of each of {112} and {223} planes is suppressed and the rD value is deteriorated because the cooling start temperature is high.

Manufacturing no. 13 is an example in which the strength is lowered because the cooling end temperature is high.

Manufacturing no. 14 is an example in which the strength, the rD value and the rC value are deteriorated because Ceq and Pcm are all low and the reduction rate per pass at the temperature range higher than the recrystallization temperature is low.

Manufacturing no. 15 is an example in which Ceq and Pcm are all high, the effective crystal grain size is large because the heating temperature is high, and the strength is increased, whereby the toughness is lowered.

Claims (7)

In terms of% by mass,
0.03 to 0.08% of C,
Si: 0.01 to 0.50%
Mn: 1.50 to 2.50%
P: not more than 0.015%
S: 0.0050% or less,
Al: 0.001 to 0.080%,
N: 0.0010 to 0.0060%,
Ti: 0.005 to 0.030%
0.010 to 0.050% of Nb, the balance being Fe and inevitable impurities,
Ceq represented by the following formula (A) is 0.35 to 0.50%
The Pcm represented by the following formula (B) is 0.15 to 0.19%
And a composite structure of one or two of ferrite and bainite or martensite,
The effective grain diameter at the center of the thickness is 20 占 퐉 or less,
An X-ray random intensity ratio of a {111} plane parallel to the plate surface is 0.5 to 5.0, a {554} plane has an X-ray random intensity ratio of 1.0 to 3.0, a {100} 112} plane and the {223} plane is 0.5 to 4.0,
A tensile strength of not less than 565 MPa and not more than 710 MPa, a rank pod value rD in the direction of 45 degrees with respect to the rolling direction of the steel sheet, and a rank pod value rC in the plate width direction of 1.0 or more, High strength steel with excellent low temperature toughness.
(A + Ce + Cd) / (Ce + Cd)
Mo / 15 + V / 10 + 5B (B) Pcm = C + Si / 30 + Mn + Cu +
In this case, C, Mn, Ni, Cu, Cr, Mo, V, Si, and B are contents represented by mass% of each element. The steel sheet according to claim 1, further comprising, by mass%
V: 0.010 to 0.100%,
Ni: 1.0% or less,
Cu: 1.0% or less,
Cr: 1.0% or less,
Mo: 1.0% or less,
B: 0.0001 to 0.0020%,
Ca: 0.0040% or less,
Mg: not more than 0.0010%
And 0.005% or less of REM. The high strength steel sheet excellent in deformation performance and low temperature toughness. A high-strength steel pipe excellent in deformation performance and low-temperature toughness, characterized by comprising the steel sheet according to claim 1 or 2. In terms of% by mass,
0.03 to 0.08% of C,
Si: 0.01 to 0.50%
Mn: 1.50 to 2.50%
P: not more than 0.015%
S: 0.0050% or less,
Al: 0.001 to 0.080%,
N: 0.0010 to 0.0060%,
Ti: 0.005 to 0.030%
0.010 to 0.050% of Nb, the balance being Fe and inevitable impurities,
Ceq represented by the following formula (A) is 0.35 to 0.50%
A steel sheet having a Pcm of 0.15 to 0.19% represented by the following formula (B) is heated to a heating temperature of 1000 to 1150 占 폚,
The reduction rate per pass is in the range of 5 to 10% when the heating temperature is lower than 1000 DEG C and lower than 1050 DEG C, and 10 to 15% when the heating temperature is 1050 DEG C or more and 1150 DEG C or lower in the temperature region above the recrystallization temperature of the above- , The rolling reduction ratio is set to 35% or more,
Subsequently, rolling was performed at a temperature lower than the Ar ₃ transformation point and above the recrystallization temperature to a cumulative rolling reduction of 70 to 80%
Then, Ar ₃ transformation point or higher and -50 ℃ the temperature range of Ar ₃ transformation point or less at a cooling start temperature, excellent deformation capacity and low temperature toughness characterized in that the water-cooled to a temperature range of from 200 to 500 ℃ cooling end temperature high-strength A method of manufacturing a steel sheet.
(A + Ce + Cd) / (Ce + Cd)
Mo / 15 + V / 10 + 5B (B) Pcm = C + Si / 30 + Mn + Cu +
In this case, C, Mn, Ni, Cu, Cr, Mo, V, Si, and B are contents represented by mass% of each element. 5. The steel sheet according to claim 4, wherein the steel strip further comprises, by mass%
V: 0.010 to 0.100%,
Ni: 1.0% or less,
Cu: 1.0% or less,
Cr: 1.0% or less,
Mo: 1.0% or less,
B: 0.0001 to 0.0020%,
Ca: 0.0040% or less,
Mg: not more than 0.0010%
REM: 0.005% or less
, And a method for producing a high strength steel sheet excellent in deformation performance and low temperature toughness. A method of manufacturing a high strength steel pipe excellent in deformation performance and low temperature toughness, characterized by forming a steel sheet obtained by the manufacturing method according to claim 4 or 5 into a tube shape and welding the butt portion. delete

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