KR101647226B1 - Steel plate having excellent fracture resistance and yield ratio, and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a steel material used as a line pipe or the like, and more particularly to a steel material excellent in fracture propagation resistance and yield ratio characteristics, and a method for producing the same.

Description

TECHNICAL FIELD [0001] The present invention relates to a steel material excellent in fracture propagation resistance and yield ratio and a method of manufacturing the steel material. [0002]

The present invention relates to a steel material used as a line pipe or the like, and more particularly to a steel material excellent in fracture propagation resistance and yield ratio characteristics, and a method for producing the same.

Recently, as the oil development has been centered on the cold regions of Siberia and Alaska, which have poor weather conditions, projects are being actively carried out to transport the rich gas resources of the oilfields to the consumption areas through the line pipes. This line pipe project is mainly applied to steel materials which have both low-temperature fracture toughness and yield ratio characteristics as well as high-temperature materials and durability in consideration of cryogenic temperature and ground deformation as well as high transport gas pressure. Particularly, in the case of superfine steel having a thickness of 30 mm or more, it is very important to guarantee the fracture propagation resistance at the center of the thickness.

Generally, steel for low-temperature toughness line pipe requires low-temperature extraction at 1140 ° C or below for fineness of propagation resistance at the core, or it is necessary to make the initial austenite grain size finer or perform low-temperature rolling to the ferrite transformation start temperature (Ar 3) It is possible to secure the resistance to the propagation of the central fracture at the level of 30 ° C.

However, since the steel sheet produced by the low temperature extraction operation can not sufficiently solidify the Nb carbonitride (Nb (C, N)) precipitated or crystallized in the slab, the effect of precipitation strengthening through re-precipitation of Nb carbonitride is insufficient . Therefore, a manufacturing method in which expensive Mo or Ni is added as a large amount of alloy element to compensate for strength and toughness is generally applied. In addition, the low temperature rolling operation promotes the isometric ferrite transformation on the microstructure, and the ferrite and bainite fractions are relatively low to deteriorate the strength, so that there is a limit to securing the resistance-to-brittleness characteristic.

One aspect of the present invention is to provide a steel material having excellent fracture propagation resistance and yield ratio characteristics of a steel material by optimizing a composition component and a manufacturing process, and a method of manufacturing the same.

The problems to be solved by the present invention are not limited to the above-mentioned problems, and other matters not mentioned can be clearly understood by those skilled in the art from the following description.

An aspect of the present invention is a steel sheet comprising, by weight%, 0.03 to 0.06% of C, 0.05 to 0.50% of Si, 0.5 to 1.8% of Mn, 0.01 to 0.05% of Al, 0.005 to 0.02% 0.05 to 0.4%, P: 0.02% or less, S: 0.005% or less, Ca: 0.0005 to 0.004%, the balance being Fe and unavoidable impurities and,

The microstructure includes an area percentage of 60 to 75% of needle-shaped ferrite, 20 to 30% of bainite, 10% or less of equiaxed ferrite and 5% or less of ground martensite,

The present invention provides a steel material excellent in fracture propagation resistance and yield ratio characteristics in which the average effective grain size of the acicular ferrite and the equiaxed ferrite is 10 占 퐉 or less and the average effective grain size of the bainite is 20 占 퐉 or less.

Still another aspect of the present invention is a method for manufacturing a steel slab, comprising: reheating a steel slab containing the composition to a temperature of 1160 to 1200 캜;

Maintaining the reheated steel slab at a temperature of 1160 DEG C or higher for at least 30 minutes and then extracting the steel slab;

Performing recrystallization reverse rolling in which the steel slab is finished in a range of Tnr-10 ° C to Tnr + 20 ° C;

Rolling after the recrystallization reverse rolling, starting from the range of Tnr-80 ° C to Tnr-40 ° C and ending in the range of Ar3 + 80 ° C to Ar3 + 120 ° C; And

Cooling at a cooling rate of 15 to 50 ° C / s, which starts in the range of Ar 3 + 20 ° C. to Ar 3 + 60 ° C. and ends in the range of Ms-40 ° C. to Ms + 20 ° C. after the non- Which is excellent in fracture propagation resistance and yield ratio characteristics.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a steel material excellent in low temperature fracture toughness or fracture propagation resistance, that is, excellent in DWTT characteristics and excellent in yield ratio even at a low temperature without adding an expensive alloying element. Therefore, a steel material such as a line pipe used at a very low temperature can be economically produced.

Hereinafter, the steel material and the manufacturing method of the present invention will be described in detail so that those skilled in the art can readily carry out the present invention.

Conventionally, when a general slab high temperature extraction operation is applied, the Nb solubility can be increased, and the precipitation strengthening effect by Nb can be maximized, however, the problem of the initial austenite grain size being coarsened. In particular, since the temperature at the center of the slab is higher than that at the surface portion, even if the rolling proceeds in the recrystallization zone, the austenite grains are relatively coarser than the surface portion. Therefore, there is a limit in improving the fracture propagation resistance at the center of the steel sheet. Therefore, in order to miniaturize the crystal grains in the center portion, a low-temperature extraction operation or a low-temperature rolling up to just above Ar3 is selectively applied.

However, in the case of low-temperature extraction operation, expensive alloy design is inevitable, and low-temperature rolling up to the top of Ar3 requires an increase in yield ratio and requires much time and energy in the manufacture of steel plates. , There is a problem that the manufacturing efficiency is reduced.

Therefore, in the present invention, there is provided a manufacturing technique of a superalloy steel material having excellent resistance to core fracture propagation by controlling the cooling process of the slab without performing the low temperature extraction or low temperature rolling. In addition, rolling is started at 80 to 120 ° C higher than Ar3 temperature and cooling is started at 20 to 60 ° C higher than Ar3 temperature, thereby suppressing the isometric ferrite transformation and inducing only the needle-like ferrite and bainite transformation to give the resistance reduction ratio.

First, the composition of the present invention will be described in detail. The unit of the following composition is% by weight unless otherwise specified.

The content of carbon (C) is preferably 0.03 to 0.06%. The carbon is the most effective element for improving the strength of steel. However, when added in an excessively large amount, segregation may be promoted at the center of thickness at the time of slab casting in the steelmaking process, thereby reducing the toughness, weldability and moldability of the steel sheet. If the content of carbon is less than 0.03%, the content of carbon is too low to obtain a desired strength, so that it is necessary to further include an expensive alloy element to obtain a desired strength. However, when the content exceeds 0.06%, the content of carbon is too high, which causes a problem that core toughness, weldability and moldability are deteriorated as described above.

The content of silicon (Si) is preferably 0.05 to 0.50%. The silicon serves as a deoxidizer for deoxidizing molten steel and is used as a solid solution strengthening element. When the content of silicon is less than 0.05%, deoxidation of molten steel is not sufficient and toughness may be lowered. However, if it exceeds 0.50%, the red scale formed by the silicon is formed during hot rolling, the surface shape of the steel sheet becomes very weak and the toughness of the welded portion is lowered.

The content of manganese (Mn) is preferably 0.5 to 1.8%. Manganese is an element that can improve the strength due to the solid solution strengthening effect. Manganese should be contained more than 0.5% to increase the ingotability and to obtain the high strength required for the yield strength 65 ksi steel. However, if it exceeds 1.8%, segregation may occur at the center of the steel sheet when the slab is cast in the steelmaking process, which may damage the toughness and weldability of the steel sheet.

The content of aluminum (Al) is preferably 0.01 to 0.05%. Aluminum is added as a deoxidizer in the steelmaking step together with silicon, and is an element capable of improving strength by solid solution strengthening. When the content of aluminum is less than 0.01%, the aforementioned deoxidizing effect is insufficient and the toughness is lowered. However, if it exceeds 0.05%, there is a problem that impact toughness is lowered.

The content of titanium (Ti) is preferably 0.005 to 0.02%. Titanium can bond with N at the solidification stage of the steel to form TiN precipitates, thereby suppressing the growth of austenite grains and improving the toughness of the steel by making the grain size of the final structure finer. When the content of titanium is less than 0.005%, the TiN precipitates are insufficient and it is difficult to suppress the growth of the austenite grains. However, when it exceeds 0.02%, the solute Ti is usually present in an excessively large amount, and TiN is precipitated in a large amount during the heating of the slab, which is not suitable for miniaturization of the grain size.

The content of nitrogen (N) is preferably 0.002 to 0.01%. Nitrogen is dissolved in the steel and precipitates to increase the strength of the steel. The presence of nitrogen in the steel degrades toughness. However, in the present invention, TiN is formed by reacting with titanium using an appropriate amount of nitrogen to control grain growth to be suppressed during reheating of the slab. When the content of nitrogen is less than 0.002%, the content of TiN precipitates is small and the effect of suppressing crystal grain growth is not so significant. On the other hand, when the content of nitrogen exceeds 0.01%, nitrogen is present as solute nitrogen and the toughness is greatly lowered.

The content of niobium (Nb) is preferably 0.04 to 0.07%. Niobium is a very effective element for securing strength through precipitation strengthening effect which forms fine precipitates by bonding with carbon or nitrogen. In addition, it is an element which is also very useful for making crystal grains finer, and is an element effective for enhancing the strength by promoting the formation of acicular ferrite or bainite which is a high strength texture. If less than 0.04% is added, the above effect is insignificant. However, if it exceeds 0.07%, the weldability can be lowered.

The content of chromium (Cr) is preferably 0.05 to 0.35%. Since chromium is effective for raising the strength of the material, when it is added at less than 0.05%, the effect of improving the strength is insignificant. On the other hand, if it is added in an excessively large amount, the weldability may deteriorate, so the upper limit is 0.35%.

The content of copper (Cu) is preferably 0.05 to 0.4%. Copper is an element added to improve strength and toughness of a steel and to improve corrosion resistance. Copper is employed in the steel to improve its strength and to form a protective coating on the surface in an atmosphere containing hydrogen sulphide to reduce the corrosion rate of the steel and to reduce the amount of hydrogen diffused into the steel. However, when the content of copper exceeds 0.4%, cracks are generated on the surface during hot rolling and the surface quality is lowered. Therefore, it is preferable to set the upper limit to 0.4%.

The content of phosphorus (P) is preferably 0.02% or less. Phosphorus is an element that is inevitably contained in steel production, and as described above, the content of phosphorus in the present invention should be controlled as low as possible. When phosphorus is added, it segregates at the center of the steel sheet and can be used as crack initiation or propagation path. Theoretically, it is advantageous to limit the content of phosphorus to 0%, but it is inevitably added as an inevitable impurity in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, it is preferable to manage the upper limit of the phosphorus content to 0.02%.

The content of sulfur (S) is preferably 0.005% or less. Sulfur is inevitably contained in the manufacture of steel, and reacts with manganese to form MnS, which is a nonmetallic inclusion, and is controlled to be as low as possible in order to lower the toughness of the steel after stretching in rolling and to secure the core brittle fracture- . Theoretically, it is advantageous to limit the content of sulfur to 0%, but it is inevitably added as an inevitable impurity in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, it is preferable to control the upper limit of the sulfur content to 0.005%.

The content of calcium (Ca) is preferably 0.0005 to 0.004%. Calcium is an element useful for spheroidizing MnS non-metallic inclusions and can inhibit cracking around the MnS inclusions. When the calcium content is less than 0.0005%, the effect of spheroidizing the MnS inclusions does not appear. However, when the content exceeds 0.004%, a large amount of CaO-based inclusions is produced to lower impact toughness.

The remainder may include iron (Fe) and impurities that are not intended from the raw material or the surrounding environment during normal manufacturing processes.

Hereinafter, the microstructure of the steel material of the present invention will be described in detail. The microstructure of the steel material of the present invention preferably contains 60 to 75% of needle-shaped ferrite, 20 to 30% of bainite, 10% or less of equiaxed ferrite and 5% or less of martensite.

If the needle-like ferrite is less than 60%, the drop weight tearing test (DWTT) characteristic deteriorates. If it exceeds 85%, the strength and yield ratio deteriorate. When the bainite content is less than 20%, the strength and yield ratio are inferior. When the bainite content exceeds 30%, the DWTT characteristics are deteriorated. When the proportion of the equiaxed ferrite exceeds 10%, the strength and the yield ratio deteriorate. When the martensite content exceeds 5%, the DWTT characteristics deteriorate.

On the other hand, in order to safely use the steel material of the present invention at a low temperature with a line pipe or the like, the Drop Weight Tearing Test (DWTT) characteristic exhibiting brittle fracture stop characteristics must be superior. In particular, pipes used in extreme environments are required to have a DWTT ductile wave fracture rate of at least 85% at a temperature below -20 ° C. The steel materials supplied to these pipes are basically required to have a DWTT ductile wave fracture rate of more than 85% . This DWTT characteristic is closely related to the effective grain size of the steel.

The effective grain size is defined as the size of grains having a high grain boundary, and when cracks are initiated and propagated, the cracks change the propagation path at the effective grain boundaries. Therefore, as the effective grain size becomes finer, the crack propagation resistance increases. The term "high hardness grain boundary" means a case where the difference in orientation between crystal grains is 15 degrees or more, and a grain having high hardness grain boundaries is referred to as an effective grain.

In the steel material of the present invention, the average effective grain size of the acicular ferrite and the equiaxed ferrite is preferably 10 占 퐉 or less, and the average effective grain size of the bainite is preferably 3 占 퐉 or less.

It is also preferable that the difference in average effective grain size between the surface layer (within 5 mm from the surface) and the center of thickness of the needle-shaped ferrite, the equiaxed ferrite and the bainite is less than 5 탆. This is to maintain the good crack propagation resistance of the superfine steel. When the difference in average effective grain size between the surface layer and the center portion exceeds 5 μm, the crack develops intensively along the coarse central grain, so that crack propagation resistance And is rapidly deteriorated. On the other hand, the average martensite average effective grain size is preferably 3 m or less throughout the entire thickness.

Hereinafter, a method for manufacturing the steel material of the present invention will be described in detail. In the present invention, in addition to the above-mentioned composition, a method for manufacturing a uniform and fine structure by optimizing the rolling process is proposed in order to secure the low-temperature fracture toughness and the yield ratio at the same time.

First, a steel slab satisfying the alloy and the composition range is prepared and reheated at 1160 to 1200 ° C. Before the hot rolling, the slab is reheated to a temperature of 1160 DEG C or higher so that the Nb carbonitride dissolves to exist in the Nb atom state. When the reheating temperature is less than 1160 ° C, the Nb carbonitride is low in solubility and the strength deterioration occurs. When the reheating temperature is higher than 1200 ° C, coarse TiN precipitates are formed during reheating, and the initial austenite grains grow rapidly Therefore, the slab reheating temperature is preferably 1160 to 1200 ° C.

The reheated slab is maintained at a temperature of 1160 DEG C or more for 30 minutes or more in the crack zone and then extracted. If the slab holding temperature is less than 1160 DEG C, the workability such as rolling property may be lowered. In addition, when the steel sheet is held for less than 30 minutes in the crack zone, the slab thickness and the cracking degree in the longitudinal direction are low, so that the rolling property is poor and the physical property deviation of the final steel sheet may be caused. Therefore, while it is preferable to hold it for as long as possible, it is preferable to conduct it for 90 minutes or less in consideration of productivity.

The extracted slab is cooled to a temperature range of 1100 DEG C or less at a cooling rate of 5 DEG C / s or more. As a result, it is possible to control so that recrystallization reverse rolling can be performed at a low temperature. Recrystallization The low-temperature rolling is an effective method to miniaturize the grain size even if the initial austenite is coarsened by high-temperature extraction. In particular, since the temperature at the center of the thickness of the slab can be lowered, the center particle size can be finely controlled. Ultimately, fine uniform recrystallization austenite conditioning is possible throughout the thickness. If the temperature of the slab immediately after cooling is 1100 ° C or higher, the effect of recrystallization and low-temperature rolling is insufficient, so that a coarse bainite can be formed at the center of the thickness of the steel sheet, thereby securing the DWTT characteristics of the present invention I can not. The termination temperature of the cooling is not particularly limited, but it is preferable to perform the cooling to a temperature exceeding Tnr + 20 占 폚 in consideration of the subsequent recrystallization reverse rolling.

The cooling is preferably performed at a cooling rate of 5 DEG C / s or more. If the cooling rate is less than 5 DEG C / s, the degree of cracking in the thickness direction of the slab is low, which may cause physical property variations of the final steel sheet. Particularly, the temperature reduction in the center portion of the slab is insufficient, so that the effect of the cold rolling of the recrystallization temperature can not be expected. As a result, coarse bainite is formed at the center of the thickness of the final steel sheet, and the DWTT characteristics are deteriorated. In addition, it takes a long time to control the above-described recrystallization low-temperature rolling start temperature to 1100 ° C or less, which can deteriorate the productivity. On the other hand, it is preferable that the cooling rate does not exceed 20 ° C / s in view of the characteristics of the equipment. The cooling is preferably performed by a water-cooling method.

It is preferable to control the austenite grains to a fine size in order to improve the low-temperature toughness of the steel sheet. This is possible by controlling the rolling temperature and the reduction rate. In the present invention, it is preferable that the rolling is performed in two temperature ranges. Since the recrystallization behavior is different in the two temperature ranges, it is preferable to set the respective conditions.

The first rolling is recrystallization reverse rolling, rolling the cooled slab and terminating at Tnr-10 ° C to Tnr + 20 ° C. Tnr is the temperature at which the recrystallization station of austenite is stopped. Tnr = 887 + (464 * C) + (6445 * Nb) - (644 * SQRT (230 * SQRT (V))) + (890 * Ti) + (363 * Al) - (357 * Si). The unit of the content of each component in the Tnr formula is% by weight, and the SQRT denotes a mathematical code root. The crystal grains of old austenite can be made fine through the recrystallization reverse rolling.

The average rolling reduction during recrystallization reverse rolling is preferably 10% or more. When the average reduction rate is less than 10%, a transformed structure such as coarse bainite which can drastically lower the DWTT characteristic may be caused. The upper limit of the reduction rate is not particularly limited, but it is preferable that the upper limit is not more than 25% when the operating equipment is taken into consideration. Even if the rolling finish temperature is less than Tnr-10 占 폚 or Tnr + 20 占 폚, the DWTT characteristics may be lowered for the same reason as described above.

The second rolling is a non-recrystallized reverse rolling in which the recrystallized reverse-rolled steel sheet is subjected to a non-recrystallization reverse rolling at a temperature lower than the Tnr temperature. The non-recrystallized reverse rolling starting temperature is preferably in the range of Tnr-80 ° C to Tnr-40 ° C. The non-recrystallized reverse rolling end temperature is preferably in a range of Ar 3 + 80 ° C to Ar 3 + 120 ° C. In this case, the Ar3 temperature refers to the temperature at which the austenite is transformed into ferrite, and it is not necessarily limited thereto, but in theory Ar3 = 910- (310 * C) - (80 * Mn) - (55 * Ni) Cr) - (80 * Mo) - (20 * Cu) + (0.35 * (steel sheet thickness-8)). In the Ar3 formula, each component is expressed in terms of% by weight.

When the non-recrystallized reverse rolling starting temperature is lower than Tnr-80 占 폚, the rolling finish temperature is lower than Ar3 + 80 占 폚 and the cooling starting temperature is lower than Ar3 + 20 占 폚, , There is a problem that a coarse transformational structure is formed. If the non-recrystallized annealing finish temperature exceeds Ar3 + 120 deg. C, there is a problem that a coarse transformation texture is formed. When the temperature is lower than Ar3 + 80 deg. C, the strength and yield ratio are deteriorated.

The cumulative rolling reduction of the non-recrystallized reverse rolling step is preferably 73 to 80%. If the cumulative rolling reduction exceeds 80%, the effect of recrystallization back-rolling is weakened and coarse unrecrystallized austenite remains, whereas when the cumulative rolling reduction is less than 73%, austenite is not sufficiently crushed and a fine transformation structure can not be obtained.

The non-recrystallized steel sheet is cooled. The cooling start temperature is preferably Ar 3 + 20 ° C to Ar 3 + 60 ° C. If the cooling start temperature is lower than Ar3 + 20 占 폚 or exceeds Ar3 + 60 占 폚, there is a problem that the needle-like ferrite titration fraction can not be secured. The cooling end temperature is preferably set in the range of Ms-40 ° C to Ms + 20 ° C. The Ms means the martensitic transformation temperature and can be derived from Ms = 539-423 * C-30.4 * Mn-17.7 * Ni-12.1 * Cr-7.5 * Mo. In the above Ms formula, each component is in weight%. When the cooling end temperature exceeds Ms + 20 ° C, coarse martensite and carbide are formed to deteriorate the DWTT characteristics. When the cooling end temperature is lower than Ms-40 ° C, the coarse bainite fraction exceeds 30% The DWTT characteristic deteriorates.

The cooling rate is preferably 15 ° C / s or higher, and more preferably 15 to 50 ° C / s. When the cooling rate is less than 15 ° C / s, equiaxed ferrite is formed and the grain size of the structure is increased, so that both strength and toughness may be deteriorated. When the cooling rate exceeds 50 DEG C / s, the ferrite transformation is suppressed and the DWTT characteristics are deteriorated.

The steel of the present invention is excellent in low-temperature fracture toughness or fracture propagation resistance, that is, in DWTT characteristics even at a low temperature, and has excellent yield ratio. More specifically, the present invention provides a steel having a core DWTT ductile wave fracture rate of 85% or more at a guaranteed temperature of less than -30 占 폚, and at the same time a yield strength of 65 ksi with a yield ratio of less than 88% and a thickness of 30 mm or more.

Hereinafter, embodiments of the present invention will be described in detail. The following examples are for the purpose of understanding the present invention and are not intended to limit the present invention.

(Example)

A slab satisfying the composition shown in the following Table 1 was prepared, and a plate having a thickness of 38 mm was produced according to the production conditions shown in Table 2 below. It is obvious to a person skilled in the art that the composition in Table 1 is expressed in terms of% by weight and the remainder not shown in Table 1 is Fe and unavoidable impurities.

Steel grade C Si Mn P S Nb Cr Al Cu Ca Ti N A 0.042 0.22 1.58 0.0097 0.0008 0.048 0.246 0.024 0.181 0.0007 0.015 0.0044 B 0.049 0.19 1.55 0.0103 0.0007 0.054 0.266 0.033 0.201 0.0010 0.011 0.0042 C 0.045 0.18 1.53 0.0088 0.0005 0.045 0.259 0.032 0.203 0.0009 0.013 0.0041

No. Steel grade Extraction temperature (캜) Slab cooling stop temperature (℃) Slab cooling rate (° C / s) Recrystallization rolling finish temperature (캜) Theory Tnr (占 폚) Non-recrystallization rolling start temperature (캜) Non-recrystallization rolling finish temperature (캜) Cumulative reduction ratio of non-recrystallization (%) Accelerated cooling start temperature (캜) Theory Ar 3 (° C) Cooling rate (° C / s) Accelerated cooling end temperature (℃) Theory Ms
(° C) One A 1175 1081 11 1025 1018 951 874 76 810 774 22 475 470 2 B 1182 1092 13 1072 1062 985 890 80 831 773 31 435 468 3 C 1167 1079 9 1015 1020 960 876 77 800 776 25 465 470 4 A 1205 1135 16 1060 1018 958 873 78 799 774 29 455 470 5 A 1123 1065 9 1001 1018 948 861 78 810 774 26 464 470 6 A 1161 1095 3 1036 1018 965 866 78 805 774 22 471 470 7 B 1185 1095 9 1075 1062 1030 943 70 865 773 42 458 468 8 B 1162 1079 10 1053 1062 922 819 88 777 773 26 466 468 9 C 1171 1075 11 1011 1020 962 861 82 751 776 19 486 470 10 C 1177 1081 12 1015 1020 966 875 78 805 776 11 555 470 11 C 1174 1089 13 1021 1020 961 879 80 811 776 53 359 470

The plate thus prepared was subjected to an optical microscope analysis to determine the fraction of each phase. The results are shown in Table 3. Electron backscatter diffraction (EBSD) analysis was performed on the surface portion (5 mm from the surface) and the center portion, and the average size of the effective grain having high hardness grain boundaries was measured and shown in Table 3.

On the other hand, the plate material was subjected to DWTT and tensile tests, and the results are shown in Table 4. DWTT is referred to as Drop Weight Tear Test and is one of the test methods for API standard certification designed to evaluate the brittle fracture propagation resistance of line pipe steels. The DWTT test was carried out from room temperature to -60 ° C. The ductile wave fracture ratio of the notched portion was measured for each specimen, and the lowest temperature at which the value was 85% or more is shown in Table 4.

No. Steel grade Average area fraction of thickness center tissues / Average size (%) / (占 퐉) Average Size of Thickness Core Tissues - Average Size of Surface Tissues (탆) Remarks Needle ferrite Equiaxed ferrite Martensite Bay knight Needle ferrite Equiaxed ferrite Bay knight One A 71 / 7.8 5 / 8.2 1 / 1.6 23 / 15.5 2.2 3.3 2.6 Inventory 1 2 B 68 / 8.2 3 / 9.1 3 / 2.4 26 / 18.3 2.6 2.1 2.8 Inventory 2 3 C 69 / 6.8 8 / 6.9 1 / 0.8 22 / 13.8 2.1 3.2 2.9 Inventory 3 4 A 41 / 13.1 6 / 9.6 3 / 1.4 50 / 36.6 3.2 5.5 10.9 Comparative Example 1 5 A 58 / 9.4 7 / 8.5 2 / 0.9 33 / 22.5 3.9 4.9 5.3 Comparative Example 2 6 A 62 / 7.2 2 / 7.9 3 / 2.2 33 / 25.6 3.5 4.1 15.6 Comparative Example 3 7 B 42 / 12.6 0/- 6 / 2.9 52 / 26.1 3.4 - 6.6 Comparative Example 4 8 B 15 / 8.1 55 / 8.5 1 / 1.4 29 / 16.5 2.1 3.8 6.9 Comparative Example 5 9 C 9 / 9.5 59 / 17.4 4 / 3.8 28 / 21.5 3.7 5.6 9.9 Comparative Example 6 10 C 55 / 9.6 14 / 12.1 6 / 4.2 25 / 29.1 3.1 5.7 14.5 Comparative Example 7 11 C 45 / 6.6 3 / 7.1 4 / 2.4 48 / 23.5 2.3 2.6 3.9 Comparative Example 8

No. Steel grade DWTT Lowest temperature to satisfy ductile wave rate 85% or more (℃) Yield strength in the direction perpendicular to the rolling (MPa) Tensile strength in the direction perpendicular to the rolling (MPa) Yield ratio in the direction perpendicular to the rolling direction (%) Remarks One A -41 525 615 85 Inventory 1 2 B -35 519 629 83 Inventory 2 3 C -46 510 601 85 Inventory 3 4 A -15 529 631 84 Comparative Example 1 5 A -24 493 599 82 Comparative Example 2 6 A -27 505 615 82 Comparative Example 3 7 B -11 489 635 77 Comparative Example 4 8 B -29 503 561 90 Comparative Example 5 9 C -28 486 551 88 Comparative Example 6 10 C -21 477 558 85 Comparative Example 7 11 C -17 505 636 79 Comparative Example 8

As shown in Table 3, Inventive Example 1 to Inventive Example 3 were obtained by rolling and cooling the steel sheet according to the present invention using steel sheets A, B and C satisfying the composition range of the present invention, An acicular ferrite having an average grain size of about 60 to 75%, an equiaxed ferrite having a fraction of less than about 10%, a bainite of 20 to 30% having an average grain size of not more than 20 mu m, the acicular ferrite, Knit made of steel sheet with surface martensite composite steel having a difference in average effective grain size of less than 5 탆 and a fraction of less than 5% of average grain size of 3 탆 or less at the surface layer (inside 5 mm from the surface) , And the minimum temperature that satisfies DWTT ductile wave rate of 85% or more is less than -30 ° C and the yield ratio is less than 88%.

In addition, according to the API-5L standard, yield strength of API-X65 (65 ksi) steel pipe is 448 MPa, and the yield strength of the steel sheet for manufacturing steel pipe is generally required to be about 460 to 560 MPa. The yield strengths of the inventive examples in the above Table 4 show the values that can be used as a steel sheet for a line pipe of 65 ksi class.

On the other hand, Comparative Examples 1 to 8 are examples in which the composition ranges of the present invention are all satisfied but the manufacturing conditions deviate from the conditions controlled by the present invention, and as a result, microstructures to be controlled by the present invention can not be obtained, Or yield ratio.

Claims (8)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.03 to 0.06% of C, 0.05 to 0.50% of Si, 0.5 to 1.8% of Mn, 0.01 to 0.05% of Al, 0.005 to 0.02% of Ti, 0.002 to 0.01% 0.05 to 0.4%, P: 0.02% or less, S: 0.005% or less, Ca: 0.0005 to 0.004%, the balance being Fe and unavoidable impurities,
The microstructure includes an area percentage of 60 to 75% of needle-shaped ferrite, 20 to 30% of bainite, 10% or less of equiaxed ferrite and 5% or less of ground martensite,
Wherein the average effective grain size of the acicular ferrite and the equiaxed ferrite is 10 占 퐉 or less and the average effective grain size of the bainite is 20 占 퐉 or less.
The method according to claim 1,
A steel material excellent in fracture propagation resistance and yield ratio characteristics in which the difference between the average effective crystal grains of the needle-like ferrite, the equiaxed ferrite and the surface layer of the bainite (within 5 mm from the surface) and the center average effective grain size is 5 탆 or less.
The method according to claim 1,
Wherein the on-road martensite average effective grain size is excellent in fracture propagation resistance and yield ratio characteristics of not more than 3 mu m over the entire thickness.
The method according to claim 1,
The steel material is excellent in fracture propagation resistance and yield ratio characteristics with a drop weight tearing test (DWTT) ductility factor of 85% or more at less than -30 캜 and a yield ratio of less than 88%.
The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.03 to 0.06% of C, 0.05 to 0.50% of Si, 0.5 to 1.8% of Mn, 0.01 to 0.05% of Al, 0.005 to 0.02% of Ti, 0.002 to 0.01% The steel slab containing 0.07% of Cr, 0.05 to 0.35% of Cr, 0.05 to 0.4% of Cu, 0.02% or less of P, 0.005% or less of S, 0.0005 to 0.004% of Ca and balance of Fe and unavoidable impurities, Reheating to a temperature of 1200 ° C;
Maintaining the reheated steel slab at a temperature of 1160 DEG C or higher for at least 30 minutes and then extracting the steel slab;
Performing recrystallization reverse rolling in which the steel slab is finished in a range of Tnr-10 ° C to Tnr + 20 ° C;
Rolling after the recrystallization reverse rolling, starting from the range of Tnr-80 ° C to Tnr-40 ° C and ending in the range of Ar3 + 80 ° C to Ar3 + 120 ° C; And
Cooling at a cooling rate of 15 to 50 ° C / s, which starts in the range of Ar 3 + 20 ° C. to Ar 3 + 60 ° C. and ends in the range of Ms-40 ° C. to Ms + 20 ° C. after the non-
And a fracture propagation resistance and a yield ratio characteristic.
The method of claim 5,
And cooling the steel slab to 5 ° C / s or higher to a temperature of 1100 ° C or lower after the steel slab is extracted, wherein the fracture propagation resistance and yield ratio characteristics are excellent.
The method of claim 5,
Wherein said recrystallization reverse rolling is excellent in fracture propagation resistance and yield ratio characteristics at an average rolling reduction of 10% or more.
The method of claim 5,
Wherein the non-recrystallized reverse rolling is excellent in fracture propagation resistance and yield ratio characteristics at a cumulative reduction ratio of 73 to 80%.