KR101585724B1 - A thick plate of pipeline with excellent DWTT at low temperature and YR ratio characteristics, and method of the same - Google Patents

Abstract

There is provided a line pipe steel material excellent in both core fatigue propagation propagation resistance and yield ratio characteristics and a method for manufacturing the same.
The present invention relates to a ferritic stainless steel comprising, by weight, 0.03 to 0.06% of C, 0.05 to 0.50% of Si, 0.5 to 1.6% 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% of Cu, 0.05 to 0.3% of Mo, 0.01% or less of P, 0.001% or less of S, 0.001% or less of Ca, 0.0005% or less of Cr, 0.05 to 0.35% Wherein the microstructure is composed of 60 to 80% of needle-shaped ferrite, 20 to 30% of bainite, less than 10% of equiaxed ferrite and 5% of ground martensite in an area fraction, And the average effective grain size of the acicular ferrite and the equiaxed ferrite is not more than 10 mu m, the average effective grain size of the bainite is not more than 20 mu m, the average effective grain size of the amorphous martensite is not more than 3 mu m, And a method for producing the same.

Description

Technical Field [0001] The present invention relates to a steel pipe having excellent low-temperature fracture propagation resistance and yield ratio at the same time, and a manufacturing method thereof,

The present invention relates to a steel sheet for a downstream line pipe having excellent low temperature 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 case of a steel material having a thickness of 25 mm or more, it is very important to guarantee the fracture propagation resistance at the center of the thickness

In general, low-temperature tough line pipe steels containing phosphorus (P) and sulfur (S), which are impurity elements (P) and sulfur (S), respectively, contain less than 200ppm and 30ppm, respectively, are subjected to low temperature rolling up to the ferrite transformation start temperature It is possible to secure the core fracture propagation resistance (DWTT characteristic) at a guaranteed temperature of -20 ° C. Such a low temperature rolling operation promotes equiaxed ferrite transformation on the microstructure, and relatively low acicular ferrite and bainite fraction deteriorates the tensile strength, so that there is a limit to securing the low resistance property.

SUMMARY OF THE INVENTION Accordingly, the present invention has been made in order to solve the above-mentioned problems of the prior art, and it is an object of the present invention to provide a steel sheet for a downstream line pipe which is excellent in both low temperature breakdown propagation resistance and yield ratio characteristics of a line pipe steel by optimizing a composition component and a manufacturing process .

Another object of the present invention is to provide a method of manufacturing the steel sheet for a line pipe.

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

According to an aspect of the present invention,

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 1.6% of Mn, 0.01 to 0.05% of Al, 0.005 to 0.02% of Ti, 0.002 to 0.01% 0.05 to 0.3%, P: 0.01% or less, S: 0.001% or less, Ca: 0.0005 to 0.004%, Cr: 0.05 to 0.35%, Ni: 0.1 to 0.4% The balance iron (Fe) and other unavoidable impurities,

The microstructure includes an area percentage of 60 to 80% of needle-shaped ferrite, 20 to 30% of bainite, less than 10% of equiaxed ferrite and less than 5% of martensite, and

Wherein the average effective grain size of the acicular ferrite and the equiaxed ferrite is not more than 10 mu m, the mean effective grain size of the bainite is not more than 20 mu m, the average effective grain size of the martensite is not more than 3 mu m, .

Further, according to the present invention,

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 1.6% of Mn, 0.01 to 0.05% of Al, 0.005 to 0.02% of Ti, 0.002 to 0.01% 0.05 to 0.3%, P: 0.01% or less, S: 0.001% or less, Ca: 0.0005 to 0.004%, Cr: 0.05 to 0.35%, Ni: 0.1 to 0.4% A step of providing a steel slab containing residual iron (Fe) and other unavoidable impurities, followed by heating at 1140 to 1180 캜;

Maintaining the heated slab at 1100 to 1140 DEG C for at least 30 minutes, and then extracting the slab;

Performing recrystallization reverse rolling on the extracted slab at Tnr + 20 ° C to Tnr + 60 ° C to produce a recrystallized steel sheet;

Rolling the rolled steel sheet at a rolling start temperature Tnr-50 ° C to Tnr-20 ° C and a rolling finish temperature Ar3 + 80 ° C to Ar3 + 120 ° C; And

Cooling the non-recrystallized steel sheet at a cooling rate ranging from Ar + 20 ° C to Ar3 + 60 ° C at a cooling rate in the range of 15 to 50 ° C / s and terminating the cooling at Ms-40 ° C to Ms + 20 ° C; To a method of producing a steel sheet for a line pipe having a thickness of 25 mm or more.

The present invention having such a constitution as described above is characterized in that it has a core DWTT ductile waveguide ratio of 85% or higher at a guaranteed temperature of -30 캜 or lower and is excellent in low temperature fracture propagation resistance and at the same time has a yield strength of 65 ksi It is possible to provide a steel plate for a pipe line of 25 mm or more.

Rolling is terminated by 80 to 120 ° C higher than the Ar3 temperature, and rolling and productivity improvements are expected.

Hereinafter, a steel sheet for a line pipe excellent in low temperature fracture toughness and yield ratio of the present invention and a method for producing the steel sheet will be described in detail so that those skilled in the art can easily carry out the invention.

According to the prior art, the center segregation present in the steel acts as a starting point of the crack, lowering the propagation resistance of the crack and facilitating the cracking. Therefore, if the crystal grains in the center portion are not sufficiently refined through low- It was difficult to expect a satisfactory low temperature toughness of the steel sheet for a downstream line pipe. However, since the low temperature rolling up to the Ar3 straight line increases the yield ratio and requires a lot of time and energy in the manufacture of the steel sheet, there is a problem that it is difficult to expect the resistance reduction of the steel sheet and the manufacturing efficiency is decreased.

Therefore, in order to minimize center segregation, the present invention minimizes impurity elements (P) and sulfur (S) and optimizes carbon (C) and manganese (Mn) to provide excellent core fracture resistance And a method for manufacturing the same. In addition, by completing the rolling at a temperature higher by 80 to 120 ° C than the Ar3 temperature, the cooling is started at a temperature 20 to 60 ° C higher than the Ar3 temperature, thereby suppressing the isometric ferrite transformation and inducing only the needle-shaped ferrite and bainite transformation, .

First, the following line pipe steel plate of the present invention is described, and its composition is required to be controlled as follows.

That is, the steel sheet for a downstream line pipe according to the present invention comprises 0.03 to 0.06% of C, 0.05 to 0.50% of Si, 0.5 to 1.6% of Mn, 0.01 to 0.05% of Al, 0.005 to 0.02% 0.001 to 0.01% of N, 0.02 to 0.07% of Nb, 0.05 to 0.35% of Cr, 0.1 to 0.4% of Ni, 0.05 to 0.4% of Cu, 0.05 to 0.3% of Mo, 0.01% : 0.001% or less, Ca: 0.0005 to 0.004%, the balance iron (Fe), and other unavoidable impurities.

The reason for limiting the numerical values of the above components will be described as follows. Hereinafter, it is necessary to pay attention that the content unit of each component is weight% unless otherwise stated.

Carbon (C): 0.03 to 0.06%

Carbon is the most effective element for improving the strength of steel. However, when the steel is added in an excessively large amount, segregation may be promoted at the center of thickness when casting the slab in the steelmaking process, thereby reducing the toughness, weldability and formability of the steel sheet. If the content of carbon is less than 0.03 wt%, the content of carbon is too low to obtain a desired strength, so that it is necessary to additionally include an expensive alloy element to obtain a desired strength. However, when the content exceeds 0.06% by weight, the content of carbon is too high, and as a result, the core toughness, weldability and moldability are deteriorated as described above.

Silicon (Si): 0.05 to 0.50%

Silicon serves as deoxidizer to deoxidize molten steel and is used as solid solution strengthening element. When the content of silicon is less than 0.05% by weight, deoxidation of molten steel is not sufficient and toughness may be lowered. However, when the content exceeds 0.50% by weight, red scales due to silicon are formed during hot rolling, and the surface shape of the steel sheet becomes very poor and the toughness of the welded portion is lowered.

Manganese (Mn): 0.5 to 1.6%

Manganese is an element that can improve the strength due to the solid solution strengthening effect. Manganese should be contained in an amount of 0.5 wt% or more to obtain an increase in incombustibility and a high strength required in a 65 ksi steel having a yield strength. However, if the content exceeds 1.6% by weight, segregation may occur at the center of the steel sheet when the slab is cast in the steelmaking process, which may deteriorate the toughness and weldability of the steel sheet.

Aluminum (Al): 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 aluminum is contained in an amount of less than 0.01% by weight, the deoxidizing effect described above is insufficient and toughness is lowered. However, when it exceeds 0.05% by weight, impact toughness is deteriorated.

Titanium (Ti): 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% by weight, 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 excessively, so that TiN is precipitated at the time of heating the slab, which is not suitable for fineness of grain size.

Nitrogen (N): 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 is known to degrade toughness. However, in the present invention, TiN is formed by reacting titanium with 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 wt%, the content of TiN precipitates is small and the effect of suppressing grain growth is not so significant. On the other hand, when the content of nitrogen exceeds 0.01% by weight, nitrogen is present as solute nitrogen and the toughness is significantly lowered.

Niobium (Nb): 0.02 to 0.07%

Niobium is an extremely useful element for finely graining the crystal grains, and is an effective element for enhancing the strength by promoting the formation of needle-shaped ferrite or bainite, which is a high-strength texture. If less than 0.02% by weight is added, the above effect is insignificant. However, if it exceeds 0.07% by weight, the weldability may be deteriorated.

Cr (Cr): 0.05 to 0.35%

Cr is effective to increase the strength of the material, but when it is added in an amount less than 0.05% by weight, the effect of improving the strength is insignificant. Adding too much amount may cause deterioration of weldability, so the upper limit is 0.35%.

Nickel (Ni): 0.1 to 0.4%

Nickel is an element capable of improving strength and toughness at the same time, and in the present invention, it plays a role of improving the strength and brittle fracture stopping property of the post material. When the content of nickel is less than 0.1% by weight, the above-mentioned effect is insignificant. And since nickel is a very expensive element, it is not desirable to unconditionally increase the amount of nickel even though it has the above effect. The reason for this is that the price-strength and toughness improvement effects are relatively small. Therefore, it is preferable to limit the upper limit to 0.4 wt% in consideration of the price, strength, and toughness improving effect.

Copper (Cu): 0.05 to 0.4%

Copper is an element added to improve strength and toughness of a steel and to improve corrosion resistance. Cu is added to the steel to improve its strength and to form a protective coating on the surface in an atmosphere containing hydrogen sulfide to lower 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% by weight, cracks are generated on the surface during hot rolling to lower the surface quality, so that the upper limit is preferably limited to 0.4% by weight.

Molybdenum (Mo): 0.05 to 0.3%

Molybdenum is a useful element for improving the strength of steel. However, when the content is less than 0.05% by weight, the effect of improving the strength is insignificant. If it is added in an amount exceeding 0.3% by weight, the DWTT characteristics can be lowered by forming coarse bainite and ground martensite structure at the center of the thickness. However, since molybdenum is an expensive element and the content thereof is high, the weldability is lowered. Therefore, it is more preferable to limit the upper limit to 0.3 wt%.

Phosphorus (P): not more than 0.010%

Phosphorus is an element which is inevitably contained in a forced quenching. 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 phosphorus content to 0%, but it is inevitably added as an impurity inevitably to the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the phosphorus content is preferably limited to 0.010% by weight.

Sulfur (S): Not more than 0.001%

Sulfur is inevitably contained in the steel and reacts with Mn 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 being rolled 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 impurity inevitably to the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the sulfur content is preferably limited to 0.001 wt%.

Calcium (Ca): 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% by weight, the effect of spheroidizing the MnS inclusions does not appear. However, when the content exceeds 0.004% by weight, a large amount of CaO-based inclusions is produced to lower impact toughness.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

It is desirable to control the microstructure of the steel sheet as follows in order to obtain a steel sheet having the above-mentioned component system, which is a steel sheet excellent in low-temperature fracture toughness and yield ratio characteristics

First, in the present invention, it is preferable that the microstructure of the steel sheet comprises 60 to 75% of needle-shaped ferrite, 20 to 30% of bainite, less than 10% of equiaxed ferrite and less than 5% of martensite in an area fraction.

If the area percentage of the microstructure is less than 60% of the needle-like ferrite, the DWTT characteristics deteriorate, and if it exceeds 80%, the strength and yield ratio may deteriorate. Also, if the bainite content is less than 20%, the strength and yield ratio may be inferior and the DWTT characteristics may deteriorate if the bainite content exceeds 30%. If the area percentage of the equiaxed ferrite exceeds 10%, the strength and yield ratio are deteriorated. If the area of the martensite exceeds 5%, the DWTT characteristics may deteriorate.

On the other hand, in order to safely use the steel for the line pipe at a low temperature, the drop weight tearing test (DWTT) characteristic which exhibits the brittle fracture arresting property must be superior. Basically, it is possible to use DWTT ductile wave rate of more than 85% at -20 ℃ in pipe state. Steel plates supplied to these pipes should basically have a DWTT ductile wave fracture rate of 85% or more at less than -20 ° C. Since DWTT characteristics are required up to the guarantee temperature of -30 ℃, the steel pipe must basically guarantee the DWTT characteristic to below -30 ℃. In general, the DWTT characteristics are closely related to the effective grain size of the steel.

The effective grain size is defined as the size of grains with high grain boundaries. When cracks are initiated and crack propagated, the propagation path changes at the effective grain boundaries. Therefore, the smaller the effective grain size, the greater the crack propagation resistance. Here, the high hardness angle means a case where the bearing difference between crystal grains is 15 degrees or more, and the grains having high hard grain boundaries are referred to as effective grains.

In the steel sheet of the present invention, the average effective grain size of the ferrite is not more than 10 mu m, the average effective grain size of the bainite is not more than 20 mu m, and the average effective grain size of the amorphous martensite is not more than 3 mu m It is preferable to maintain good crack propagation resistance.

The steel sheet of the present invention having the above-described composition and microstructure can secure a DWTT ductile waveguide ratio of 85% or more at a guaranteed temperature of -30 占 폚 or lower and effectively have a yield ratio of less than 88% at the same time.

Next, a method for manufacturing a steel sheet for a pipe line pipe having excellent low temperature fracture toughness and yield ratio at the center of the present invention will be described. The method for manufacturing a steel according to the present invention comprises the steps of: preparing a steel slab composed as described above and then heating at 1140 to 1180 캜; Maintaining the heated slab at 1100 to 1140 DEG C for at least 30 minutes, and then extracting the slab; Performing recrystallization reverse rolling on the extracted slab at Tnr + 20 ° C to Tnr + 60 ° C to produce a recrystallized steel sheet; Rolling the rolled steel sheet at a rolling start temperature Tnr-50 ° C to Tnr-20 ° C and a rolling finish temperature Ar3 + 80 ° C to Ar3 + 120 ° C; Cooling the non-recrystallized steel sheet at a cooling rate ranging from Ar + 20 ° C to Ar3 + 60 ° C at a cooling rate in the range of 15 to 50 ° C / s and terminating the cooling at Ms-40 ° C to Ms + 20 ° C; .

First, in the present invention, a steel slab having a composition as described above is prepared and then heated to a temperature ranging from 1140 to 1180 ° C. By reheating the steel slab to a temperature of 1140 캜 or higher, NbC can be dissolved to exist in the Nb atom state. However, when the reheating temperature exceeds 1180 ° C, coarse TiN precipitates are formed during reheating. Therefore, in the present invention, the slab reheating temperature range is preferably limited to 1140 to 1180 ° C.

In the present invention, the reheated slab is maintained at a temperature of 1100 to 1140 DEG C for at least 30 minutes in the crack zone, and then extracted. When the slab extraction temperature is lower than 1100 ° C, the workability such as rolling property may not be easy. When the temperature exceeds 1140 ° C, the workability is easy, but the slab extraction temperature is not good. It is desirable to manage. In addition, when the steel sheet is kept in the crack zone for less than 30 minutes, 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.

In the present invention, the slab is subjected to recrystallization back-rolling that finishes at Tnr + 20 ° C to Tnr + 60 ° C to produce a recrystallized back-rolled steel sheet. Subsequently, the recrystallization back-rolled steel sheet is subjected to non-recrystallization back-rolling at the rolling start temperature Tnr-50 ° C to Tnr-20 ° C and at the rolling finish temperature Ar3 + 80 ° C to Ar3 + 120 ° C.

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 a recrystallization back-rolling process in which the extracted slab is rolled and finished at Tnr + 20 ° C to Tnr + 60 ° C to produce a recrystallized steel sheet.

Tnr = 887 + (464 * C) + (6445 * Nb) - (644 * SQRT (Nb))) + (890 * Ti) + Tnr where Tnr is the temperature at which the recrystallization station of austenite is stopped. (363 * Al) - (357 * Si). The crystal grains of old austenite can be made fine through recrystallization back-rolling.

The average rolling reduction during recrystallization reverse rolling is preferably limited to 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. Even when the rolling finish temperature is less than Tnr + 20 占 폚 or Tnr + 60 占 폚, the DWTT characteristics may be greatly reduced due to the above reasons.

The second rolling is a non-recrystallized reverse rolling process in which the recrystallized reverse-rolled steel sheet is subjected to a non-recrystallization reverse rolling at a temperature between Tnr-160 ° C and Tnr-130 ° C.

The non-recrystallized reverse rolling starting temperature is preferably limited to a range of Tnr-50 ° C to Tnr-20 ° C. The Ar3 temperature refers to the temperature at which austenite is transformed into ferrite and is not necessarily limited thereto. In theory, Ar3 = 910 - (273xC) - (74xMn) - (57xNi) - (9 x Mo) - (5 x Cu) + (0.35 x (t-8)) where t is the thickness of the steel sheet. If the rolling start temperature exceeds Tnr-20 占 폚, there is a problem that a coarse transformation texture is formed. When Tnr is less than 50 占 폚, there is a problem that the rolling finish temperature is less than Ar3 + 80 占 폚 and the cooling start temperature is less than Ar + 20 占 폚, so that the strength and yield ratio are deteriorated.

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

The non-recrystallized reverse rolling end temperature is preferably limited to a range of Ar 3 + 80 ° C to Ar 3 + 120 ° C. When the non-recrystallized reverse rolling end temperature exceeds Ar 3 + 120 캜, a coarse transformation texture is formed. In addition, when the temperature is lower than Ar 3 + 80 ° C, the strength and yield ratio are deteriorated.

In the present invention, the non-recrystallized reverse-rolled steel sheet is cooled at a cooling rate ranging from Ar + 20 ° C to Ar3 + 60 ° C at a cooling rate in the range of 15 to 50 ° C / s and is cooled to a temperature range of Ms-40 ° C to Ms + The cooling is terminated. Controlling the cooling start temperature is an important factor for suppressing the formation of equiaxed ferrite. In the present invention, the cooling start temperature is preferably limited to a range of Ar 3 + 20 ° C to Ar 3 + 60 ° C. When the cooling start temperature is lower than Ar3 + 20 占 폚 or exceeds Ar3 + 60 占 폚, the needle-like ferrite content fraction does not satisfy 60 to 85%.

The cooling end temperature is limited to the range of Ms-40 ° C to Ms + 20 ° C.

In the present invention, Ms = 539-423 × (% C) -30.4 × (% Mn) -17.7 × (% Ni) -12.1 × (% Cr) -7.5 × (% Mo) When the cooling end temperature exceeds Ms + 20 占 폚, a carbide is formed and the DWTT characteristic is deteriorated. When the cooling end temperature is less than Ms-40 占 폚, parcel of martensite is formed and the DWTT characteristic is deteriorated.

At this time, the cooling rate is preferably limited to a range of 15 to 50 DEG 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 sheet produced by the above method 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.

Concretely, it is possible to provide a core steel for a pipe line having a yield strength of less than 88% and a yield strength of at least 65 ksi and a thickness of 25 mm or more, while securing a core DWTT ductile waveguide ratio of 85% have.

Hereinafter, the present invention will be described in detail with reference to Examples. However, the following examples are only for illustrating the present invention in more detail and do not limit the scope of the present invention.

(Example)

Steel grade
Composition Component (% by weight) C Si Mn P S Nb Cr Ni Mo Al Cu Ca Ti N A 0.032 0.209 1.38 0.0057 0.0008 0.034 0.226 0.289 0.103 0.024 0.181 0.0007 0.015 0.0044 B 0.039 0.18 1.40 0.005
3 0.0007 0.034 0.267 0.306 0.102 0.033 0.201 0.0010 0.011 0.0042 C 0.034 0.182 1.40 0.0055 0.0005 0.032 0.261 0.300 0.100 0.032 0.203 0.0009 0.013 0.0041 D 0.036 0.18 1.41 0.0150
0.0005 0.032 0.251 0.302 0.101 0.031 0.202 0.0010 0.012 0.0043 E 0.032 0.19 1.45 0.0063 0.0015 0.034 0.253 0.295 0.102 0.033 0.204 0.0080 0.012 0.0045

Steel slabs having the components shown in Table 1 were prepared. The slabs were made of a plate having a thickness of 33 mm according to the manufacturing conditions described in Table 2 below.

No. River
Bell extraction
Temperature
(° C) Redetermination
Rolling
End
Temperature
(° C) theory
Tnr
(° C) Unrecognition
Rolling
Starting temperature (캜) Unrecognition
Rolling
Termination temperature
(° C) Unrecognition
accumulate
Reduction rate
(%) Acceleration
Cooling
Start
Temperature
(° C) theory
Ar3
(° C) Cooling
speed
(° C / s) Accelerated cooling
Termination temperature
(° C) theory
Ms
(° C) foot
persons
Yes One A 1125 985 950 905 870 75 805 765 35 445 475 2 B 1133 999 963 913 875 78 811 760 29 466 471 3 C 1105 974 952 891 845 80 789 762 23 465 473

ratio
School
Yes


4 A 1165 1025 950 903 866 75 800 765 34 455 475 5 A 1095 946 950 899 864 75 799 765 32 473 475 6 B 1115 988 963 943 913 70 845 760 41 454 471 7 B 1125 999 963 872 811 85 751 760 19 466 471 8 C 1133 990 952 909 866 80 742 762 18 490 473 9 C 1122 982 952 911 875 78 799 762 13 542 473 10 C 1114 984 952 905 862 80 788 762 51 389 473 11 D 1121 988 952 908 873 78 810 760 35 451 472 12 E 1115 984 957 901 873 78 805 759 33 455 472

The DWTT and the tensile test were conducted on the inventive examples and comparative examples through the above-described manufacturing process. The DWTT test was carried out at room temperature at -50 ° C, and the softened wave-face 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 3 below.

The optical microscope analysis of the inventive examples and comparative examples through the manufacturing process of Table 2 above, and the fraction fraction of the microstructure was measured and shown in Table 3 below. Electron Backscatter Diffraction (EBSD) analysis was performed to measure the average size of the effective grain boundaries having the high-hardness grain boundaries, and the results are shown in Table 3. The yield strength, tensile strength and yield ratio are also shown in Table 3 below.

  No. River
Bell couch
ferrite
Fraction (%)
/Average
Size (㎛) Isometric
ferrite
Fraction (%)
/Average
Size (㎛) roadbed
Martensite
Fraction (%)
/Average
Size (㎛) Bay knight
Fraction (%)
/Average
Size (㎛) alpha beta gamma δ foot
persons
Yes One A 69 / 7.8 5 / 8.1 1 / 1.2 25 / 17.8 -33 521 625 83 2 B 72 / 8.2 3 / 9.1 2 / 2.2 23 / 17.3 -37 524 619 85 3 C 70 / 7.1 7 / 7.9 1 / 0.9 22 / 14.1 -45 522 609 86

ratio
School
Yes


4 A 47 / 12.1 6 / 9.9 1 / 1.7 46 / 25.6 -19 522 629 83 5 A 58 / 9.5 8 / 8.5 2 / 0.9 32 / 21.5 -23 515 615 84 6 B 42 / 12.6 - 6 / 2.8 52 / 20.1 -15 499 631 79 7 B 10 / 8.5 58 / 8.5 1 / 1.6 31 / 16.2 -35 505 569 89 8 C 8 / 9.5 61 / 15.4 4 / 3.8 28 / 23.5 -21 488 555 88 9 C 57 / 9.9 13 / 12.4 5 / 4.1 27 / 19.1 -24 467 558 84 10 C 38 / 6.8 4 / 7.5 2 / 2.4 56 / 21.5 -15 495 633 78 11 D 61 / 8.3 4 / 6.9 2 / 2.6 33 / 22.2 -27 515 629 82 12 E 66 / 7.2 5 / 6.8 1 / 1.7 28 / 17.5 -24 505 612 83

In Table 3, α is the minimum temperature (%) satisfying the DWTT ductile waveguide ratio of 85% or more, β is the tensile strength in MPa direction perpendicular to the rolling direction, (%).

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As shown in Table 3, Inventive Examples 1-3 were obtained by rolling and cooling the steel sheet according to the present invention using steel grades A, B and C satisfying the composition range of the present invention, A ferrite of about 60 to 75% in size and an equiaxed ferrite in a fraction of less than about 10%, a bainite in a fraction of 20 to 30% with an average grain size of less than 20 탆, and a ferrite having an average grain size of less than 3 탆, And the minimum temperature at which the DWTT ductility factor exceeds 85% is less than -30 ° C and the yield ratio is less than 88%. And satisfied the physical properties.

In accordance with the API-5L standard, yield strength of API-X65 (65 ksi) steel pipe is 448 MPa, and yield strength of steel sheet for manufacturing steel pipe is usually required to be about 460 to 560 MPa. The yield strengths of the inventive examples in the above Table 3 show values which can be used as a steel sheet for a line pipe of 65 ksi class.

On the other hand, Comparative Example 4-12 showed the results for the DWTT characteristic or the yield ratio in the manufacturing process and the like which were outside the scope of the present invention.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. It will be possible.

Claims (5)

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 1.6% of Mn, 0.01 to 0.05% of Al, 0.005 to 0.02% of Ti, 0.002 to 0.01% 0.05 to 0.3%, P: 0.01% or less, S: 0.001% or less, Ca: 0.0005 to 0.004%, Cr: 0.05 to 0.35%, Ni: 0.1 to 0.4% The balance iron (Fe) and other unavoidable impurities,
The microstructure includes an area percentage of 60 to 80% of needle-shaped ferrite, 20 to 30% of bainite, less than 10% of equiaxed ferrite and less than 5% of ground martensite,
Wherein the average effective grain size of the acicular ferrite and the equiaxed ferrite is not more than 10 mu m, the average effective grain size of the bainite is not more than 20 mu m, the average effective grain size of the amorphous martensite is not more than 3 mu m,
A steel plate for a line pipe with a thickness of 25 mm or more having a yield ratio of less than 88% and having a DWTT ductile wave fracture ratio of 85% or more at a temperature below -30 ° C.
delete 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 1.6% of Mn, 0.01 to 0.05% of Al, 0.005 to 0.02% of Ti, 0.002 to 0.01% 0.05 to 0.3%, P: 0.01% or less, S: 0.001% or less, Ca: 0.0005 to 0.004%, Cr: 0.05 to 0.35%, Ni: 0.1 to 0.4% A step of providing a steel slab containing residual iron (Fe) and other unavoidable impurities, followed by heating at 1140 to 1180 캜;
Maintaining the heated slab at 1100 to 1140 DEG C for at least 30 minutes, and then extracting the slab;
Performing a recrystallization reverse rolling at a temperature of Tnr + 20 ° C to a temperature of Tnr + 60 ° C for the slab extracted, to an average rolling reduction of 10% or more to produce a recrystallization-backed steel sheet;
A step of performing non-recrystallization reverse rolling of the recrystallization-back-rolled steel sheet at a rolling start temperature Tnr-50 ° C to Tnr-20 ° C at a rolling finish temperature Ar3 + 80 ° C to Ar3 + 120 ° C in the range of 73 to 80% ; And
Cooling the non-recrystallized steel sheet at a cooling rate ranging from Ar + 20 ° C to Ar3 + 60 ° C at a cooling rate in the range of 15 to 50 ° C / s and terminating the cooling at Ms-40 ° C to Ms + 20 ° C; Wherein the steel plate has a thickness of 25 mm or more.
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