KR20160078573A - Steel having excellent low temperature toughness and hydrogen induced cracking resistance and manufacturing method thereof - Google Patents

Abstract

The present invention relates to steel having excellent low temperature toughness and hydrogen induced cracking resistance, and a manufacturing method thereof. According to the present invention, the steel comprises: 0.12-0.18 wt% of carbon (C), 0.05-1.0 wt% of silicon (Si), 0.5-1.8 wt% of manganese (Mn), 0.010 wt% or less of phosphorus (P), 0.0020 wt% or less of sulfur (S), 0.005-0.1 wt% of aluminum (Al), 0.02-0.5 wt% of chromium (Cr), 0.02-0.3 wt% of molybdenum (Mo), 0.001-0.005 wt% of niobium (Nb), 0.001-0.005 wt% of titanium (Ti), 0.005-0.05 wt% of vanadium (V), 0.0005-0.003 wt% of calcium (Ca), 0.001-0.01 wt% of nitrogen (N), and the remainder consisting of iron (Fe) and other inevitable impurities.

Description

TECHNICAL FIELD [0001] The present invention relates to a steel material having excellent low-temperature toughness and hydrogen-organic cracking resistance, and a method of manufacturing the steel material.

The present invention relates to a steel material for a pipe used in a petroleum refining facility including H ₂ S at low temperatures and a method of manufacturing the steel material, and more particularly to a steel material having excellent low temperature toughness and hydrogen organic cracking resistance, and a manufacturing method thereof.

Recently, process pipes used in refineries require steel pipes with excellent resistance to hydrogen organic cracking for the treatment of gases or oils containing H ₂ S, and some require low temperature toughness comparable to transport line pipes. The steel pipes generally use a steel pipe having a thickness of 40 mm or more. The post weld heat treatment (PWHT) is required to prevent breakage of the steel pipe due to the residual stress of the welded part during manufacturing. It is important to ensure the resistance to hydrogen organic cracking and low temperature toughness after PWHT because most steel pipe manufactures steel pipes by PWHT as a whole.

Hydrogen organic cracking occurs when hydrogen atoms generated by corrosion in an environment containing H ₂ S enter into the material and reach a critical concentration. The hydrogen atoms that enter the material are diffused in the material and are trapped in regions with high non-uniformity of the material - impurities, segregation zones, inclusions. When the hydrogen atoms are concentrated, the local stress is increased, and the maximum stress that the material can withstand is lowered. When the local stress is greater than the stress that the material can withstand, the crack grows and the fracture progresses. Histologically, highly unstable highly localized pearlite or bainite phases are vulnerable to hydrogen organic cracking. In order to obtain good hydrogen organic crack resistance, the ferrite single phase structure is the most excellent, but since the ferrite single phase structure has low strength, the size and the fraction of the second phase should be strictly controlled when the ferrite is mixed with the ferrite in order to improve the strength.

Low temperature toughness is measured by Charpy Impact Test or DWTT (Drop Weight Tear Test) test. The higher the Charpy impact energy value or the DWTT ductile wavefront ratio, the better the low temperature toughness. Typically, a TMCP process is applied to a low carbon steel having a carbon content of 0.1 wt% or less to produce a steel having excellent low temperature toughness, but the maximum thickness is limited to about 40 mmt due to the appropriate rolling amount condition. Further, when accelerated cooling is performed to improve the strength, a high hardness structure such as bainite or martensite is generated, and the low temperature toughness and hydrogen organic cracking resistance are deteriorated.

On the other hand, in the case of steel of quenching and tempering steel, resistance to hydrogen organic cracking can be improved by the relaxation of Mn segregation, suppression of pearlite band structure, and spheroidization of cementite during the punching process. However, there is a problem that the low temperature toughness is lowered due to a large crystal grain size relative to the TMCP, and when grain boundary cementite is produced in the bake process, low temperature toughness and hydrogen organic cracking resistance deteriorate.

Disclosure of Invention Technical Problem [8] The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to provide a steel material excellent in low temperature toughness and hydrogen organic cracking resistance and a method of manufacturing the same.

On the other hand, the object of the present invention is not limited to the above description. It will be understood by those of ordinary skill in the art that there is no difficulty in understanding the additional problems of the present invention.

In one aspect, the present invention relates to a method of manufacturing a semiconductor device comprising: 0.12 to 0.18% by weight of carbon (C), 0.05 to 1.0% by weight of silicon (Si), 0.5 to 1.8% by weight of manganese (Mn) 0.001 to 0.1% by weight of aluminum (Al), 0.02 to 0.5% by weight of chromium (Cr), 0.02 to 0.3% by weight of molybdenum (Mo) 0.005 to 0.005% by weight of titanium (Ti), 0.001 to 0.005% by weight of vanadium (V), 0.0005 to 0.003% by weight of calcium (Ca), 0.001 to 0.01% (Fe) and other unavoidable impurities, and excellent resistance to hydrogen organic cracking.

The steel material may further include at least one selected from the group consisting of 0.3% by weight or less of copper (Cu) and 0.5% by weight or less of nickel (Ni), if necessary.

Meanwhile, the steel material may have a shear area (SA) of 85% or more and a crack length ratio (CLR) of 10% or less at the HIC (Hydrogen Induced Cracking) test at a DWTT (Drop Weight Tear Test) test at -10 ° C .

On the other hand, the steel may have a yield strength of 420 MPa or more and a tensile strength of 520 MPa or more.

On the other hand, the microstructure of the steel may include tempered bainite and tempered martensite structure.

On the other hand, the steel material may be a steel material for a steel pipe having a thickness of about 40 to 70 mm.

In another aspect, the present invention relates to a method for producing a silicon carbide composite material, which comprises 0.12 to 0.18 weight% of carbon (C), 0.05 to 1.0 weight% of silicon (Si), 0.5 to 1.8 weight% of manganese (Mn) 0.001 to 0.1% by weight of aluminum (Al), 0.02 to 0.5% by weight of chromium (Cr), 0.02 to 0.3% by weight of molybdenum (Mo) 0.005 to 0.005% by weight of titanium (Ti), 0.001 to 0.005% by weight of vanadium (V), 0.0005 to 0.003% by weight of calcium (Ca), 0.001 to 0.01% (Fe) and other unavoidable impurities; Controlling and rolling the reheated slab; Controlling and cooling after the control rolling; Cooling and quenching; And after the quenching; Which is excellent in low temperature toughness and hydrogen organic cracking resistance.

Meanwhile, the reheating may be performed by reheating the steel slab and then extracting the steel slab at 1080 to 1170 ° C. At this time, the reheating temperature may be 1150 to 1250 ° C.

On the other hand, the controlled rolling may be to finish rolling at a temperature of (Tnr) to (Tnr + 60) 占 폚.

On the other hand, the control cooling may be to start control cooling at a temperature of (Ar 3 + 40) to (Ar 3 + 100) ° C and to terminate control cooling at a temperature of Ms ° C or lower.

On the other hand, the quenching may be performed by heat-treating at a temperature of (Ac3 + 60) to (Ac3 + 80) ° C and water cooling to a temperature of Ms ° C or lower. At this time, the quadrature may be performed so as to satisfy the following equation (5).

(5): Duration = 1.6 (min / mm) x thickness (mm) + (10-30) (min)

On the other hand, the bake may be squeezed at a temperature of 630 to 690 ° C. At this time, the above calculation may be performed so as to satisfy the following equation (6).

(6): bending time = 2.4 (min / mm) x thickness (mm) + (10-30) (min)

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. The various features of the present invention and the advantages and effects thereof will be more fully understood by reference to the following specific embodiments.

According to the present invention, it is possible to provide a steel material for a tail pipe excellent in low temperature toughness and hydrogen organic cracking resistance by optimizing the stiffness and manufacturing conditions.

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Further, the embodiments of the present invention are provided to more fully explain the present invention to those skilled in the art.

As a result of extensive research, the inventors of the present invention have found out that the present inventors have found that the present inventors have found out that the inventors of the present invention have found that the above- (Nb), titanium (Ti), vanadium (V), calcium (Ca), nitrogen (N) and iron (Fe) in a specific composition ratio, It is possible to obtain a steel material for a pipe having excellent low-temperature toughness and hydrogen-organic cracking resistance, thereby completing the present invention.

Specifically, the steel material excellent in low-temperature toughness and hydrogen-organic cracking resistance of the present invention is composed of 0.12 to 0.18% by weight of carbon (C), 0.05 to 1.0% by weight of silicon (Si), 0.5 to 1.8% (P): 0.010 wt% or less, sulfur (S): 0.0020 wt% or less, aluminum (Al): 0.005 to 0.1 wt%, chromium (Cr): 0.02 to 0.5 wt%, molybdenum 0.001 to 0.005 wt% of niobium (Nb), 0.001 to 0.005 wt% of titanium (Ti), 0.005 to 0.05 wt% of vanadium (V), 0.0005 to 0.003 wt% of calcium (Ca) ): 0.001 to 0.01% by weight, balance iron (Fe) and other unavoidable impurities.

The reason for adding each component constituting the steel composition of the present invention and the appropriate content range thereof are as follows.

Carbon (C): 0.12 to 0.18 wt%

C is an element added to improve strength, but it can improve strength by increasing its content. However, as the addition amount increases, the pearlite fraction increases, deteriorating low temperature toughness and hydrogen organic cracking resistance. In order to improve low-temperature toughness and hydrogen organic cracking resistance, it is necessary to reduce the C content. However, if the C content is less than 0.12 wt%, it is difficult to secure the strength and cracks occur on the surface of the slag during the continuous casting process, do. If the C content exceeds 0.18% by weight, sufficient toughness can not be ensured and weldability is deteriorated, which is not preferable as a welded structure steel. Therefore, the C content is preferably 0.12 to 0.18% by weight, % ≪ / RTI > by weight.

Silicon (Si): 0.05 to 1.0 wt%

Since Si acts as a deoxidizer, it should be added in an amount of 0.05 wt% or more. If the content exceeds 1.0 wt%, the toughness and weldability are impaired and the amount of intervening material in the steel is increased to reduce low temperature toughness and hydrogen organic cracking resistance. The range is preferably 0.05 to 1.0% by weight, and may be, for example, about 0.1 to 0.5% by weight.

Manganese (Mn): 0.5 to 1.8 wt%

Mn is required to be 0.5 wt% or more for securing the strength. However, if the content exceeds 1.8% by weight, the weldability is deteriorated and the Mn-centered segregation or MnS inclusions are generated in the slab, thereby deteriorating the hydrogen organic cracking resistance. For example, from about 1.0 to about 1.8% by weight.

Phosphorus (P): 0.010% by weight or less

P is an element which is essential in steel and inhibits weldability and toughness during steelmaking, and is an element easily segregated in the slab central portion and austenite grain boundary during solidification, which lowers the low temperature toughness and hydrogen organic cracking resistance and minimizes its content But it is preferable to limit the content to 0.010% by weight or less, for example, about 0 to 0.008% by weight in consideration of the load generated in the steelmaking process.

Sulfur (S): 0.002 wt% or less

S is an impurity element and generally reacts with Mn to form MnS, which is stretched upon rolling to act as a starting point for generation of hydrogen organic cracks and also inhibits low-temperature toughness. Therefore, it is preferable to reduce S as much as possible, but it is preferable to limit the range to 0.002% by weight or less, for example, from 0 to 0.001% by weight, for reasons such as process constraints for S removal.

Aluminum (Al): 0.005 to 0.1 wt%

Al is an element added for deoxidation in steelmaking, which improves the impact absorption energy, but reacts with oxygen as well as Si to form oxide inclusions. If the content of Al is less than 0.005 wt%, deoxidation is not sufficiently performed. If the content of Al exceeds 0.1 wt%, the impact toughness is deteriorated and a large amount of inclusions are formed to deteriorate hydrogen organic cracking resistance. Is preferably 0.005 to 0.1% by weight, and may be, for example, about 0.02 to 0.05% by weight.

Cr (Cr): 0.02-0.5 wt%

Cr not only increases the strength of the steel by increasing the incombustibility of the steel but also reduces the corrosion rate of the steel to reduce the amount of hydrogen generated. In order to increase the strength and improve the resistance to cracking by hydrogen, it should be added in an amount of 0.02 wt% or more. The strength is increased with an increase in the addition amount. However, when the addition amount is 0.5% by weight or more, the toughness of the steel is inhibited. Therefore, the range is preferably 0.02 to 0.5% by weight and may be, for example, about 0.1 to 0.4% by weight.

Molybdenum (Mo): 0.02 to 0.3 wt%

Mo, like Cr, is an element that improves strength and hydrogen organic cracking resistance, and has a higher incombustibility than Cr, so its effect is higher than that of Cr. In order to obtain the effect according to Mo addition, it is required to add 0.02 wt% or more. When the amount is more than 0.3 wt%, not only the price becomes excessively high but also the generation of the second phase having high hardness is easy, The range is preferably 0.02 to 0.3 wt%, and may be, for example, about 0.1 to 0.3 wt%.

Niobium (Nb): 0.001 to 0.005% by weight

Nb is 1200? (C, N) at the time of hot rolling to increase the strength. Also, Nb (C, N) precipitates inhibit grain growth during the austenitization of austenite at the time of ingestion, thereby improving the low temperature toughness. In order to improve the strength and toughness by the addition of Nb, 0.001% by weight or more of Nb should be added. However, since it can act as a starting point of the generation of hydrogen organic cracks in the production of coarse distillate containing Nb, the upper limit is preferably 0.005 wt% or less. Therefore, the range is preferably 0.001 to 0.005% by weight, and may be, for example, about 0.001 to 0.003% by weight.

Titanium (Ti): 0.001 to 0.005 wt%

Ti is an element which forms carbide or nitride and inhibits crystal growth of austenite phase during reheating, and ultimately forms a fine homogeneous ferrite, thereby improving low-temperature toughness. In order to suppress such austenite grain growth, the lower limit of Ti is preferably 0.001% by weight. The finely dispersed Ti (C, N) precipitate decreases the diffusion coefficient of hydrogen and increases the resistance to hydrogen organic cracking. However, as the addition amount increases, Ti reacts with all of N in the steel to inhibit the formation of Nb (C, N) precipitates, which is effective for low-temperature toughness. . Therefore, the range is preferably 0.001 to 0.005% by weight, and may be, for example, about 0.001 to 0.003% by weight.

Vanadium (V): 0.005 to 0.05 wt%

When the amount of N is sufficiently present in the steel, VN is formed, but it generally precipitates in the form of VC in the ferrite region. The vacancy carbon concentration is lowered during transformation into austenite-ferrite, and VC provides a nucleation site for cementite formation. Therefore, rather than forming cementite continuously in the grain boundary, it has a discontinuous structure and increases the resistance to hydrogen organic cracking. In order to obtain such an effect, it is necessary to add at least 0.005% by weight. However, if it is added in an amount of 0.05 wt% or more, a coarse V precipitate is formed, which not only hinders toughness but also becomes a hydrogen accumulation place in the steel and lowers the resistance to hydrogen organic cracking. Therefore, the range is preferably 0.005 to 0.05 wt%, and may be, for example, about 0.01 to 0.03 wt%.

Calcium (Ca): 0.0005 to 0.003 wt%

Ca plays a role in neutralizing MnS inclusions. MnS is an inclusion with a low melting point and is stretched when rolled to serve as a starting point of hydrogen organic cracking. The added Ca reacts with MnS and surrounds MnS, which interferes with the stretching of MnS. The MnS spheroidizing action of Ca is closely related to the S content, but it must be added in an amount of 0.0005 wt% or more in order to exhibit the spheroidizing effect. Ca is an element having a high yield due to its high volatility, and the upper limit of Ca is preferably 0.003 wt% or less in consideration of the load generated in the steelmaking process. Therefore, the range is preferably 0.0005 to 0.003% by weight, and may be, for example, about 0.001 to 0.003% by weight.

Nitrogen (N): 0.001 to 0.01 wt%

Since it is difficult to completely remove N from the steel industrially, it is preferable that N is set to a lower limit of 0.001% by weight which is the allowable range in the production process. N forms nitrides with Al, Ti, Nb, V and the like to interfere with the growth of austenite grains to help improve toughness and strength, but contains N in excess of 0.01 wt% And N in these employment states adversely affect the toughness. Therefore, the range is preferably 0.001 to 0.01% by weight, and may be, for example, about 0.002 to 0.005% by weight.

Copper (Cu): not more than 0.3 wt%

If necessary, Cu may be added to improve the strength and toughness of the steel and to improve corrosion resistance. Cu is employed in steel to improve the strength and form a protective coating on the surface in an atmosphere containing hydrogen sulfide to lower the corrosion rate of the steel and reduce the amount of hydrogen diffused into the steel. However, since Cu is an element which causes cracks on the surface during hot rolling and hinders the surface quality, it is preferable that Cu is contained in an amount of 0.3 wt% or less.

Nickel (Ni): 0.5 wt% or less

If necessary, Ni is added as an element to improve the toughness of the steel in order to reduce surface cracking which occurs during hot rolling of the Cu-added steel. In order to reduce surface cracking due to Cu addition, it is preferable to add (for example, about 1.5 times or more) more than Cu-added steel. Therefore, the upper limit is set at 0.5 wt%. In addition, the addition of Ni in an amount exceeding 0.5% by weight hinders the improvement of the hydrogen embrittlement property by Cu addition, and the price of the steel is also increased.

The remainder of the present invention is iron (Fe). However, in the ordinary steel 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 steel making.

On the other hand, it is preferable that the microstructure of the steel of the present invention includes tempered bainite and tempered martensite structure. At this time, it is preferable that the tempered martensite has an area fraction of 15% or more for securing the tensile strength stably.

On the other hand, the steel of the present invention may have a shear area (SA) of 85% or more, more preferably 95% or more at a DWTT (Drop Weight Tear Test) test at -10 ° C. The larger the DWTT ductile wavefront ratio is, the better the low-temperature toughness is, and if it has such a range, it can have excellent low-temperature toughness characteristics.

In addition, the steel of the present invention may have a crack length ratio (CLR) of 10% or less, more preferably 3% or less, in HIC (Hydrogen Induced Cracking) test. As the CLR becomes smaller, it is evaluated as a material excellent in hydrogen organic cracking resistance, and when it has such a range, it can have excellent hydrogen organic cracking resistance.

The steel of the present invention may have a yield strength of 420 MPa or more and a tensile strength of 520 MPa or more. The higher the yield strength and the tensile strength, the higher the strength of the material.

On the other hand, the steel of the present invention can be manufactured to a thickness of 40 to 70 mm, and has excellent low temperature toughness and hydrogen organic cracking resistance as described above, and can be used in a petroleum refining plant including steel for pipes, such as H ₂ S It can be used effectively as steel plate for pipe.

The steel material having excellent low temperature toughness and hydrogen organic cracking resistance according to the present invention can be manufactured by various methods, and the production method thereof is not particularly limited. However, as a preferable embodiment, it can be produced by the following method.

The method for producing a steel material excellent in low temperature toughness and hydrogen organic cracking resistance according to the present invention is characterized in that it comprises 0.12 to 0.18 wt% of carbon (C), 0.05 to 1.0 wt% of silicon (Si), 0.5 to 1.8 wt% of manganese (Mn) (P): 0.010 wt% or less, sulfur (S): 0.0020 wt% or less, aluminum (Al): 0.005 to 0.1 wt%, chromium (Cr): 0.02 to 0.5 wt%, molybdenum 0.001 to 0.005 wt% of niobium (Nb), 0.001 to 0.005 wt% of titanium (Ti), 0.005 to 0.05 wt% of vanadium (V), 0.0005 to 0.003 wt% of calcium (Ca) ): 0.001 to 0.01 wt%, reheating the slab containing the remaining iron (Fe) and other unavoidable impurities; Controlling and rolling the reheated slab; Controlling and cooling after the control rolling; A step of cooling after the control cooling step; And after the quenching; . ≪ / RTI >

Reheating process

Mn and P segregation on the slab is relaxed by reheating of the slab. When the reheating temperature is low, the diffusion does not occur sufficiently, so segregation of Mn and P is left. Therefore, low temperature toughness and hydrogen organic cracking resistance are deteriorated do. In case of Nb added steel, Nb added to steel is sufficiently solidified during reheating and fine precipitation during rolling or heat treatment improves strength and low temperature toughness. When the heating temperature is high, relaxation of the segregation portion and solidification of Nb are easy, but if the heating temperature exceeds 1250 ° C, the low-temperature impact toughness may be lowered due to coarse TiN precipitation. This lowering of the impact toughness at low temperatures may cause the coarse TiN to function as a crack initiation point, which may lower the impact toughness. On the other hand, when the heating temperature is lower than 1150 DEG C, the slab is not heated sufficiently, and alloying elements such as Nb are not sufficiently solved. This leads to a decrease in the tensile strength of the steel. Therefore, it is preferable that the reheating of the slab is performed in the range of 1150 to 1250 ° C, and then the reheated slab is preferably retained at 1080 to 1170 ° C before extraction.

Controlled rolling process

For general annealed annealed annealed materials, after reheating, rolling is carried out at a high temperature to the target thickness without restriction on the rolling temperature and air-cooled. However, in the present invention, the control rolling is performed in order to miniaturize the grain size of the plate material before the quenching. However, in order to maximize the effect of refining the ferrite grain in the TMCP process, the control rolling is divided into the austenite recrystallization region and the non-recrystallization region, whereas in the present invention, the finishing rolling is terminated at the lowest temperature interval of the austenite recrystallization region, Of the ozokerite crystal grains. To accomplish this, finishing rolling is terminated in the temperature range from (Tnr) to (Tnr + 60) ° C by using the following equation (1), which is a formula for estimating the austenite recrystallization quenching temperature (Tnr).

Tnr = 887 + (464xC) + ((6445xNb) - (644xNb)) + ((732xV) - (230xV)) + (890xTi) + (363 x Al) - (357 x Si)

Controlled cooling process

Control cooling should begin in the austenite single phase region to inhibit the growth of finely recrystallized austenite grains after rolling has ended. For this purpose, the control cooling start temperature is preferably in the range of (Ar3 + 40) to (Ar3 + 100) ° C based on the Ar3 temperature, which is the theoretical temperature at which austenite is transformed into ferrite expressed by the following formula (2). The control cooling termination temperature is controlled to be not more than the martensitic transformation starting temperature (Ms) shown by the following formula (3) in order to suppress further crystal grain change even after the control cooling is terminated. On the other hand, the cooling rate is not particularly limited, and may be about 5 to 10 占 폚 / s.

(2): Ar3 = 910- (273xC) - (74xMn) - (57xNi) - (16xCr) - (9xMo) -

Mx = 561- (474 x C) - (33 x Mn) - (17 x Ni) - (17 x Cr) - (21 x Mo)

Quenching process

The most important factors that determine low temperature toughness are grain uniformity and grain size. During reheating in the quenching process, the austenite grains are further reduced during the course of the phase transformation from ferrite to austenite, which can be expected to improve the low temperature toughness. In addition, C-Mn segregation remained in the plate due to repetitive heat treatment is homogeneously distributed to improve hydrogen organic cracking resistance. Therefore, the quenching temperature is higher than the Ac3 temperature, which is the temperature at which the ferrite begins to transform into austenite, as shown in the following equation (4), so that the austenite transformation occurs at all sites of the steel during quenching. It is preferable to determine it according to the thickness of the steel. However, if quenching is carried out at too high a temperature, the austenite grain growth will occur and the low temperature toughness will be deteriorated. Therefore, the quenching temperature is preferably set to (Ac3 + 60) - (Ac3 + 80) C, which is a temperature at which austenite grain growth does not actively occur. In order to obtain excellent low temperature toughness and hydrogen organic cracking resistance after quench heat treatment, water cooling is performed to a temperature lower than the ferrite Ms temperature. If the transformation is not completed during water cooling, the untransformed austenite undergoes transformation into pearlite during air-cooling after water-cooling, thereby deteriorating low temperature toughness and hydrogen organic cracking resistance.

(4): Ac3 = 910-203 x sqrt (C) -15.2 x Ni + 44.7 x Si + 104 x V + 31.5 x Mo

(5): Duration = 1.6 (min / mm) x thickness (mm) + (10-30) (min)

Blowing process

After completion of the above-mentioned quenching process, the steel material having a thickness of 40 to 70 mmt may have a secondary phase of martensite and bainite in a ferrite base. Since the secondary phases have high hardness and low temperature toughness and hydrogen organic cracking resistance, they improve the toughness and hydrogen organic cracking resistance through the punching process. When the annealing temperature is lower than 630 ° C, the contrast strength is not lowered immediately after the quenching, but the dislocation loosening is insufficient and it is difficult to secure the DWTT toughness and hydrogen organic cracking resistance. On the other hand, when the bending temperature exceeds 690 ° C, the strength drop is large and the yield strength is less than 420 MPa, resulting in coarsening of the cementite and deterioration in toughness. The bake temperature is preferably set to 630 to 690 DEG C, and the bake time is preferably set to the following formula (6).

(6): bending time = 2.4 (min / mm) x thickness (mm) + (10-30) (min)

As described above, the present invention can provide a steel material having both low-temperature toughness and hydrogen-organic cracking resistance at the same time by optimizing the steel components and restricting the rolling process to uniformly refine the ferrite grains before quenching,

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

Example

Steel slabs were prepared using continuous casting after providing molten steel having the composition (% by weight) as shown in Table 1 below. In Table 1, the steel grades A to B are component systems for applying the pre-pre-control rolling applied to the present invention, and the steel grade C is a component system for general application of the quenching and blooming process.

River C Si Mn P S Al Cr Mo Nb Ti V N Ca A 0.15 0.23 1.31 0.0046 0.0005 0.028 0.22 0.16 0.002 0.003 0.018 0.0033 0.0019 B 0.13 0.23 1.42 0.0051 0.0005 0.034 0.25 0.14 0.002 0.002 0.017 0.0034 0.0020 C 0.15 0.19 1.53 0.0045 0.0009 0.025 0.25 0.15 0.002 0.001 0.019 0.0044 0.0019

Steels made from steel slabs were prepared by re-heating after controlled rolling-controlled cooling or ordinary rolling-air cooling as shown in Table 2 below. The produced steel material was subjected to a quenching process according to the time shown in the above-mentioned formula (5) and subjected to a blanching process according to the time indicated in the above-mentioned formula (6).

division River thickness
(mm) T0
(° C) T5
(° C) SCT
(° C) FCT
(° C) T _Q
(° C) T _T
(° C) Tnr
(° C) Ar3
(° C) Ac3
(° C) Ms
(° C) Inventory 1 A 41 1145 899 825 292 924 690 856 773 849 440 Inventory 2 A 41 1141 900 836 291 917 670 856 773 849 440 Inventory 3 A 56 1139 897 862 152 915 690 856 773 849 440 Honorable 4 A 56 1139 897 862 152 920 670 856 773 849 440 Inventory 5 A 56 1141 863 819 119 921 690 856 773 849 440 Inventory 6 A 56 1141 863 819 119 916 670 856 773 849 440 Honorable 7 A 56 1138 853 807 120 914 650 856 773 849 440 Honors 8 B 56 1139 853 814 137 919 650 856 769 852 445 Proposition 9 B 71 1140 899 839 280 922 670 856 769 852 445 Inventory 10 B 71 1141 862 843 162 914 650 856 769 852 445 Exhibit 11 B 71 1141 862 843 162 926 630 856 769 852 445 Comparative Example 1 C 41 1153 992 - 911 647 869 756 847 432 Comparative Example 2 A 41 1140 899 828 146 916 710 856 773 849 440 Comparative Example 3 A 41 1146 859 776 327 924 610 856 773 849 440 Comparative Example 4 C 56 1138 1002 - 929 621 869 756 847 432 Comparative Example 5 A 56 1139 853 807 319 915 620 856 773 849 440 Comparative Example 6 B 56 1184 853 814 307 917 625 856 769 852 445 Comparative Example 7 C 71 1140 1056 - 903 600 869 756 847 432 Comparative Example 8 B 71 1139 950 889 164 911 630 856 769 852 445 Comparative Example 9 B 71 1138 863 842 286 916 700 856 769 852 445 Comparative Example 10 A 71 1139 901 865 283 940 690 856 773 849 440

* T0: Extraction temperature by reheating, T5: Rolling finish temperature, SCT: Cooling start temperature, FCT: Cooling end temperature, T _Q : Quenching temperature, T _T :

The tensile, DWTT and HIC tests were carried out on a part of the finally prepared steel sheet in accordance with the above conditions, and the results are shown in Table 3. The tensile test was performed by machining a full thickness plate test piece, and the yield strength (Yield Strength, YS) and tensile strength (TS) of 0.5% unload were measured. The DWTT specimen was processed to a thickness of 19 mm at the fourth quartile of the thickness direction and was performed at -27 ° C., which is -17 ° C. lower than the property guarantee temperature of -10 ° C., based on the temperature correction conditions of the API standard for small test pieces. After fracturing the test specimens with a DWTT tester, the fracture surface ratio was measured by image analysis of the fracture surface, and the cleavage surface by the reverse surface was calculated as the soft fracture surface. In the HIC test, 30 mm thick specimens were taken from the upper, middle, and lower parts of the specimen in the thickness direction and immersed in a solution A solution of NACE standard for 96 hours. The length ratio of the internal cracks was measured by ultrasonic testing, (Crack Length Ratio).

division After rolling After quenching FC
(Mpa) TS
(Mpa) DWTT SA
(%) CLR
(%) FC
(Mpa) TS
(Mpa) DWTT SA
(%) CLR
(%) Inventory 1 635 716 34 79.5 443 552 100 0 Inventory 2 630 711 31 56.5 464 582 100 1.8 Inventory 3 558 660 38 45 440 546 100 0.3 Honorable 4 589 686 42 22.5 451 567 100 0 Inventory 5 493 604 60 50 436 549 100 0 Inventory 6 507 614 54 69.5 458 572 100 0 Honorable 7 563 669 21 70.5 460 569 100 0 Honors 8 492 604 39 38.5 458 576 100 0 Proposition 9 545 656 59 0 432 560 100 One Inventory 10 538 661 28 26 434 562 100 0 Exhibit 11 551 654 51 46.5 447 572 100 0 Comparative Example 1 - 548 654 79 11 Comparative Example 2 510 620 79 66.5 386 527 100 0 Comparative Example 3 589 684 49 84 484 598 100 13.1 Comparative Example 4 - 497 615 51 8 Comparative Example 5 575 679 31 57 498 613 100 11.1 Comparative Example 6 491 604 9 6 470 592 65 3.5 Comparative Example 7 - 458 586 40 6 Comparative Example 8 573 672 23 28.5 467 586 72 2.7 Comparative Example 9 471 589 47 0 393 528 23 0 Comparative Example 10 555 683 10 One 495 603 68 0

As shown in Table 3, Inventive Examples 1 to 11 are examples in which a posterior steel sheet having excellent DWTT and HIC was produced even after the quenching and sintering process by performing controlled rolling-controlled cooling in the steel plate manufacturing process before quenching, , Examples 3 to 8 are 56 mm, and Inventive Examples 9 to 11 are 71 mm. On the other hand, they include tempered bainite and tempered martensite structures, where the tempered martensite is at least 15% in area fraction.

Comparative Example 1, Comparative Example 4, and Comparative Example 7 are comparative examples corresponding to the products of 41 mm, 56 mm, and 71 mm, respectively, which are ordinary quenching furnaces to which the general rolling-air cooling process is applied in the manufacturing process of the thick plate before quenching. As shown in the comparative example, when a general quenching process is applied, a DWTT ductile waveguide ratio of 85% can not be obtained in the final product.

On the other hand, when controlled rolling-controlled cooling is performed in the thick plate process before the pre-firing as in Examples 1 to 11, the yield strength is higher than 500 MPa, tensile strength is higher than 600 MPa, DWTT SA is lower than 60% due to development of bainite and martensite structure, CLR has a strength of 10% or more, low phosphorus and low hydrogen organic cracking resistance. However, when it is subjected to heat treatment, the strength is somewhat reduced, but the bainite / martensite-> austenite-> bainite / martensite , The DWTT characteristics are improved and the segregated elements such as C and Mn are homogenized at a high temperature and the HIC characteristic is also improved so that a material for a steel pipe having excellent fracture propagation resistance and hydrogen organic cracking resistance can be manufactured.

Comparative Examples except for Comparative Example 1, Comparative Example 4, and Comparative Example 7 were the cases where the target properties were not achieved beyond the limit conditions of the thick plate manufacturing process and the quenching process before the quenching.

The brewing temperatures of Comparative Example 2 and Comparative Example 9 were higher than 700 DEG C and the same thickness of Inventive Example 1 and Inventive Example 9, respectively. If the annealing temperature is higher than 700 ℃, the yield strength will be lowered to less than 420Mpa due to excessive loosening of the potential. In the case of Comparative Example 9 having a thickness of 71 mm, the long shedding treatment is performed in comparison with Comparative Example 2, and consequently, the tangent containing cementite is coarsened and the DWTT SA is worsened.

Unlike Comparative Example 2 and Comparative Example 9, Comparative Example 3 and Comparative Example 5 had a softening temperature of less than 630 DEG C, so that the softness of the structure with high strength and hardness was not sufficient and the CLR had a value exceeding 10%.

In Comparative Example 6, Comparative Example 8, and Comparative Example 10, all of the DWTT SAs were less than 85%. In the case of Comparative Example 6, it was conducted at 1184 ° C in the reheating furnace before rolling. In this case, the austenite grains are grown to cause miniaturization in subsequent rolling and heat treatment, resulting in deterioration of DWTT. In the case of Comparative Example 8, the rolling finishing temperature was at Tnr + 100 占 폚, and crystal grain growth appeared to occur during the time period from the rolling to the cooling facility. Finally, in Comparative Example 10, the austenite grain size was larger than that of Examples 9 to 11 in which the quenching treatment was carried out at 940 占 폚, which is about 90 占 폚 higher than the Ac3 temperature and the quenching treatment was conducted at 930 占 폚 or lower, , The DWTT SA characteristic deteriorated rather.

From the above examples, it can be seen that the afterglow steel sheet for pipe excellent in fracture propagation resistance and hydrogen organic cracking resistance can be produced even after the quenching treatment, if the limited component range and the manufacturing condition range of the present invention are satisfied.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, but, on the contrary, It will be obvious to those of ordinary skill in the art.

Claims (15)

0.1 to 0.18% by weight of carbon (C), 0.05 to 1.0% by weight of silicon (Si), 0.5 to 1.8% by weight of manganese (Mn), 0.010% 0.001 to 0.1% by weight of aluminum (Al), 0.02 to 0.5% by weight of chromium (Cr), 0.02 to 0.3% by weight of molybdenum, 0.001 to 0.005% by weight of niobium (Nb) 0.001-0.005% by weight of vanadium, 0.005-0.05% by weight of vanadium, 0.0005-0.003% by weight of calcium, 0.001-0.01% of nitrogen, balance of iron and other unavoidable impurities, Which is excellent in low temperature toughness and hydrogen organic cracking resistance.
The method according to claim 1,
Wherein the steel material further comprises at least one selected from the group consisting of copper (Cu): 0.3 wt% or less and nickel (Ni): 0.5 wt% or less.
The method according to claim 1,
The steel has a low temperature toughness of not less than 85% in SA (Shear Area) and 10% in CLR (Hydrogen Induced Cracking) test at a drop weight test (DWTT) Steel with excellent hydrogen organic crack resistance.
The method according to claim 1,
The steel material has a yield strength of 420 MPa or more and a tensile strength of 520 MPa or more, which is excellent in low temperature toughness and resistance to organic cracking.
The method according to claim 1,
Wherein the microstructure of the steel comprises tempered bainite and tempered martensite structure, wherein the steel has excellent low-temperature toughness and hydrogen-organic cracking resistance.
The method according to claim 1,
Wherein the steel material is a steel material for a pipe for a pipe having a thickness of 40 to 70 mm, which is excellent in low temperature toughness and hydrogen organic cracking resistance.
0.1 to 0.18% by weight of carbon (C), 0.05 to 1.0% by weight of silicon (Si), 0.5 to 1.8% by weight of manganese (Mn), 0.010% 0.001 to 0.1% by weight of aluminum (Al), 0.02 to 0.5% by weight of chromium (Cr), 0.02 to 0.3% by weight of molybdenum, 0.001 to 0.005% by weight of niobium (Nb) 0.001-0.005% by weight of vanadium, 0.005-0.05% by weight of vanadium, 0.0005-0.003% by weight of calcium, 0.001-0.01% of nitrogen, balance of iron and other unavoidable impurities, Reheating the slab including the slab;
Controlling and rolling the reheated slab;
Controlling and cooling after the control rolling;
A step of cooling after the control cooling step; And
After the quenching;
Which is excellent in low temperature toughness and hydrogen organic cracking resistance.
8. The method of claim 7,
Wherein the reheating is carried out at a temperature of 1080 to 1170 占 폚 after reheating the steel slab, wherein the steel slab is extracted at a temperature of 1080 to 1170 占 폚.
9. The method of claim 8,
Wherein the reheating temperature is 1150 to 1250 占 폚.
8. The method of claim 7,
Wherein the controlled rolling is completed at a temperature of (Tnr) to (Tnr + 60) 占 폚. The method of producing a steel material excellent in low temperature toughness and hydrogen organic cracking resistance.
8. The method of claim 7,
Wherein the control cooling is started at a temperature of (Ar3 + 40) to (Ar3 + 100) 占 폚, and control cooling is terminated at a temperature of Ms 占 폚 or lower. .
8. The method of claim 7,
Wherein the quenching is performed at a temperature of (Ac3 + 60) to (Ac3 + 80) deg. C to water-cool to a temperature of Ms deg. C or lower.
8. The method of claim 7,
Wherein the quenching is performed so as to satisfy the following formula (5): " (1) "
(5): Duration = 1.6 (min / mm) x thickness (mm) + (10-30) (min)
8. The method of claim 7,
A method for producing a steel material excellent in low-temperature toughness and hydrogen-organic cracking resistance, which is carried out at a temperature of 630 to 690 ° C.
8. The method of claim 7,
The method of manufacturing a steel material excellent in low temperature toughness and hydrogen organic cracking resistance is carried out so as to satisfy the following equation (6).
(6): bending time = 2.4 (min / mm) x thickness (mm) + (10-30) (min)