KR101639902B1 - Steel having excellent low temperature toughness and hydrogen induced cracking resistance and manufacturing method thereof - Google Patents

Abstract

The present invention relates to a method for producing a steel sheet, which comprises 0.03 to 0.05 wt% of carbon (C), 0.05 to 1.0 wt% of silicon (Si), 1.0 to 1.4 wt% of manganese (Mn) 0.001 to 0.1% by weight of aluminum (Al), 0.1 to 0.3% by weight of copper (Cu), 0.1 to 0.5% by weight of nickel (Ni) (Ti): 0.01 to 0.02% by weight, vanadium (V): 0.02 to 0.06% by weight, calcium (Ca): 0.0005 To about 0.003% by weight, nitrogen (N): 0.001 to 0.01% by weight, balance of iron (Fe) and other unavoidable impurities, and a method for producing the same.

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 provides a method of producing a carbon nanowire comprising: 0.03 to 0.05 wt% carbon; 0.05 to 1.0 wt% silicon; 1.0 to 1.4 wt% manganese; 0.010 wt% 0.001 to 0.1 wt% of aluminum (Al), 0.1 to 0.3 wt% of copper (Cu), 0.1 to 0.5 wt% of nickel (Ni), 0.05 to 0.5 wt% of chromium (Cr) (V): 0.02 to 0.06% by weight, and the amount of calcium (Nb) is 0.03 to 0.055% by weight, The present invention provides a steel material excellent in low temperature toughness and hydrogen organic cracking resistance comprising 0.0005 to 0.003 wt% of Ca, 0.001 to 0.01 wt% of nitrogen (N), balance of Fe and other unavoidable impurities.

On the other hand, the steel may have a Carbon Index (CI) of 9.0 or more expressed by the following formula (1).

(1): ???????? C = ([Cr] + [Mo] + [Nb] + [V]) / [C]

(Wherein the [Cr], [Mo], [Nb], [V], and [C] represent the weight percentages of chromium, molybdenum, niobium, vanadium,

Meanwhile, the steel 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 drop weight test (DWTT) test at 0 ° 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 acicular ferrite and bainite structure.

On the other hand, the steel material may be a steel material for a pipe of 53 mm or less in thickness.

In another aspect, the present invention provides a method of manufacturing a semiconductor device, comprising: 0.03 to 0.05 wt% carbon; 0.05 to 1.0 wt% silicon; 1.0 to 1.4 wt% manganese; 0.010 wt% 0.001 to 0.1 wt% of aluminum (Al), 0.1 to 0.3 wt% of copper (Cu), 0.1 to 0.5 wt% of nickel (Ni), 0.05 to 0.5 wt% of chromium (Cr) (V): 0.02 to 0.06% by weight, and the amount of calcium (Nb) is 0.03 to 0.055% by weight, Reheating a slab containing 0.0005 to 0.003 wt% of Ca, 0.001 to 0.01 wt% of nitrogen (N), remaining Fe (Fe) and other unavoidable impurities; Controlling and rolling the reheated slab; Directly after the controlled rolling; And after the direct 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 1140 ° 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 (Ar3 + 50) to (Ar3 + 100) deg.

On the other hand, the direct quenching may be to start cooling at (Ar 3 + 0) - (Ar 3 + 40) ° C.

On the other hand, the direct quenching may terminate the cooling at a temperature of (Ms-100) ° C or lower.

On the other hand, the bake may be squeezed at a temperature of 630 to 690 ° C. At this time, the calculation may be performed so as to satisfy the following equation (4).

(4): bake 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 pipe having a thickness of 53 mm or less, which is excellent in low temperature toughness and hydrogen organic cracking resistance by optimizing the stiffness and manufacturing conditions.

Fig. 1 is a schematic diagram of the C.I value and the CLR value.

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 that the present inventors have found out that the present inventors have found that the present inventors have found that the above- (Nb), titanium (Ti), vanadium (V), calcium (Ca), nitrogen (N) and iron (Fe) It is possible to obtain a steel material for a steel pipe having a thickness of 53 mm or less, which is excellent in low temperature toughness and hydrogen organic cracking resistance, when the steel material is produced by the quenching and sintering method.

Specifically, the steel material having excellent low-temperature toughness and hydrogen-organic cracking resistance according to the present invention contains 0.03 to 0.05 wt% of carbon (C), 0.05 to 1.0 wt% of silicon (Si), 1.0 to 1.4 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%, copper (Cu): 0.1 to 0.3 wt%, nickel (Nb): 0.03 to 0.055 wt%, titanium (Ti): 0.01 to 0.02 wt%, vanadium (V) ): 0.02 to 0.06 wt%, calcium (Ca): 0.0005 to 0.003 wt%, nitrogen (N): 0.001 to 0.01 wt%, 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): 0.03 to 0.05 wt%

C is the most effective element to strengthen the steel, but it decreases the weldability and low temperature toughness when added in large amounts. In particular, in terms of hydrogen organic cracking resistance, C increases the amount of C-Mn segregation in the center portion or increases the fraction of pearlite, thereby deteriorating hydrogen organic cracking resistance as the addition amount increases. When the C content is less than 0.03% by weight, it is not economical to add a large amount of expensive elements such as Mo, Cr and Ni in order to secure a desired strength. When the C content exceeds 0.05% by weight, sufficient toughness can not be secured In addition, since the weldability is deteriorated and it is not preferable as a welded structure steel, the range is preferably 0.03 to 0.05 wt%, and may be, for example, about 0.03 to 0.045 wt%.

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): 1.0 to 1.4 wt%

Mn is required to be 1.0 wt% or more for securing strength. If the content is more than 1.4% by weight, the weldability is deteriorated and the Mn-centered segregation or MnS inclusions are generated in the slab to deteriorate the hydrogen organic cracking resistance. Therefore, the range is 1.0 To 1.4% by weight, and may be, for example, about 1.2 to 1.4% 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): 0.05 to 0.3 wt%

Cr not only increases the strength of the steel by increasing the penetration of the steel but also reduces the corrosion rate of the steel to reduce the amount of generated hydrogen. In order to increase the strength and improve the resistance to hydrogen organic cracking, it is necessary to add 0.05 wt% or more. The strength is increased with an increase in the addition amount. However, when the addition amount is 0.3% by weight or more, the toughness of the steel is deteriorated. Therefore, the range is preferably 0.005 to 0.1% by weight and may be, for example, about 0.1 to 0.3% 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%.

Copper (Cu): 0.1 to 0.3 wt%

Cu is added to improve the strength and toughness of the steel and to improve corrosion resistance. Cu should be added in an amount of 0.1% by weight or more, since it is dissolved in steel and improves strength and forms a protective coating on the surface in an atmosphere containing hydrogen sulfide to reduce the corrosion rate of steel and reduce the amount of hydrogen diffused into steel. However, since Cu is an element which causes cracks on the surface during hot rolling and hinders the surface quality, it is preferably contained in an amount of 0.3 wt% or less. Therefore, the range is preferably 0.1 to 0.3% by weight, and may be, for example, about 0.1 to 0.2% by weight.

Nickel (Ni): 0.1 to 0.5 wt%

Ni is an element that improves the toughness of steel and is added to reduce surface cracking that occurs during hot rolling of 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 lower limit of Ni is 0.1 wt% or more, but the upper limit is 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. Therefore, the range is preferably 0.1 to 0.5% by weight, and may be, for example, about 0.1 to 0.3% by weight.

Niobium (Nb): 0.03 to 0.055 wt%

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 addition, Nb, which has not been precipitated in the quenching process and has been solidified in the matrix, precipitates during the bake process, thereby reducing or even increasing the strength drop. 0.03% by weight or more should be added in order to obtain the effect of finely pulverizing austenite grains by Nb addition and securing strength through precipitation in sandpaper. However, since it can serve as a starting point of generation of hydrogen organic cracking in the production of coarse distillate containing Nb, the upper limit is preferably 0.055 wt% or less. Therefore, the range is preferably 0.03 to 0.055% by weight, and may be, for example, about 0.03 to 0.05% by weight.

Titanium (Ti): 0.01 to 0.02 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 0.01 wt%. The finely dispersed Ti (C, N) precipitate decreases the diffusion coefficient of hydrogen and increases the resistance to hydrogen organic cracking. However, if the addition amount is increased, the Ti reacts with N in the steel to inhibit formation of Nb (C, N) precipitates which are effective for low-temperature toughness, so that the low-temperature toughness is inhibited and the upper limit is preferably 0.02% . Therefore, the range is preferably 0.01 to 0.02% by weight, and may be, for example, about 0.010 to 0.015% by weight.

Vanadium (V): 0.02 to 0.06 wt%

V is precipitated in the ferrite region in the form of VC, although VN is formed when N is sufficiently present in the steel. 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 0.02% by weight. However, if it is added in an amount of 0.06 wt% or more, coarse V precipitates are formed to deteriorate toughness as well as reduce the resistance to hydrogen organic cracking due to hydrogen accumulation in the steel. Therefore, the range is preferably 0.02 to 0.06% by weight, and may be, for example, about 0.03 to 0.04% by weight.

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 0.003 to 0.007% by weight.

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, when the Mn content is 1.4% by weight or less like the steel of the present invention, the Carbon Index (CI) represented by the following formula (1) is preferably 9.0 or more, more preferably 10.0 or more. This is because in this case, excellent hydrogen organic crack resistance can be obtained. HIC (Hydrogen Induced Cracking) is strongly influenced by segregation and inclusions in the steel. C and Mn are representative elements for increasing hardenability during the solidification process of the steel. When the content is increased, C-Mn is segregated and hardness is high There is an increased likelihood that tissue will form and HIC will deteriorate. Also, when a large amount of C is added, grain boundary cementite may be formed during the panning process, or carbides such as Cr, Mo, Nb and V may be coarsened and adversely affect HIC.

(1): ???????? C = ([Cr] + [Mo] + [Nb] + [V]) / [C]

(Wherein the [Cr], [Mo], [Nb], [V], and [C] represent the weight percentages of chromium, molybdenum, niobium, vanadium,

On the other hand, it is preferable that the microstructure of the steel material of the present invention comprises acicular ferrite and bainite structure. At this time, the bainite may cause a decrease in the toughness of the DWTT due to the coarsening of the crystal grains, so that the area fraction thereof is preferably 15% or less.

Meanwhile, the steel of the present invention may have a shear area (SA) of 85% or more, more preferably 90% or more at the time of DWTT (Drop Weight Tear Test) test at 0 ° 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 material of the present invention may have a crack length ratio (CLR) of 10% or less, more preferably 5% 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 present invention steel is, for example, or less, and the thickness 53㎜ be made of 40 ~ 53mm, including a bar, steel, such as H ₂ S for a pipe having an excellent low-temperature toughness and hydrogen-induced crack resistance, as described above It can be used as a steel plate for pipes used in oil refining facilities.

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 comprises 0.03 to 0.05 wt% of carbon (C), 0.05 to 1.0 wt% of silicon (Si), 1.0 to 1.4 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%, copper (Cu): 0.1 to 0.3 wt%, nickel (Nb): 0.03 to 0.055 wt%, titanium (Ti): 0.01 to 0.02 wt%, vanadium (V) Reheating the slab containing 0.02 to 0.06 wt% of calcium, 0.0005 to 0.003 wt% of calcium, 0.001 to 0.01 wt% of nitrogen, balance of Fe and other unavoidable impurities; Controlling and rolling the reheated slab; Directly after the controlled rolling; And after the direct 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, the reheating of the slab is preferably performed in the range of 1150 to 1250 ° C, and the reheated slab is retained at 1080 to 1140 ° 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, it is preferable to perform control rolling to reduce the grain size of the plate before direct quenching. In order to maximize the effect, the finish temperature of the finish rolling is limited to (Ar3 + 50) to (Ar3 + 100) . When the finish temperature of the finish rolling exceeds (Ar3 + 100) 占 폚, the austenite refining effect is insufficient and the low temperature toughness deteriorates. When the finish temperature is less than (Ar3 + 50) 占 폚, ferrite development is promoted, Can not be achieved. Here, the Ar3 temperature means a temperature at which austenite is transformed into ferrite, and is represented by the following formula (2).

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

Direct Filling Process

Next, after completion of rolling, direct quenching is performed on-line. Control cooling must be initiated in the austenite single-phase region to ensure ingenuity. For this purpose, the control cooling start temperature is in the range of (Ar3 + 0) to (Ar3 + 40) ° C. On the other hand, when the quenching end temperature is higher than (Ms-100) ° C based on the martensitic transformation starting temperature Ms given by the following formula (3), the target strength can not be achieved, Do.

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

Blowing process

Next, after the above-mentioned quenching step, the plate has a composite structure of needle-like ferrite, bainite and martensite. Since the hardness of the above-mentioned tissues is high and the hydrogen organic cracking resistance is low, the hydrogen organic cracking resistance must be improved through the pouring 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 annealing temperature is preferably 630 to 690 DEG C, and the baking time is preferably as shown in the following formula (4) so as to obtain sufficient effect over the entire thickness.

(4): bake time = 2.4 (min / mm) x thickness (mm) + (10-30) (min)

INDUSTRIAL APPLICABILITY 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 performing direct quenching and post-annealing after rolling.

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, steel types A to H are component systems for applying direct quenching, and steel type I is a component system for general quenching and blooming processes. On the other hand, as shown in Table 1, the components A to C satisfy all of the conditions of the alloy components restricted in the present invention, and the components D to I are C, Mn, Cu, Ni, Nb and V And the like were prepared differently from the limitations of the present invention.

The steel slabs were manufactured in the following manner. As shown in Table 2, a steel sheet having a thickness of 53 to 56 mm was produced through a direct quenching process or a general quenching / sintering process. The bake time was determined through the above equation (4).

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. The specimens were tested at -17 ° C, -17 ° C lower than 0 ° C. 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).

* T0: Extraction temperature by reheating, T5: Rolling finish temperature, SCT: Cooling start temperature, FCT: Cooling end temperature, _TT :

Comparative Example 11 does not satisfy the DWTT characteristic at 0 캜 because the effect of grain refinement is insufficient as a plate material after the usual quenching / poring treatment. On the other hand, Examples 1 to 3, in which the slab of the alloy component limited in the present invention is directly etched, have a yield strength of not less than 450 MPa, a DWTT SA of not less than 85% and a CLR of not more than 10% Respectively. On the other hand, the microstructure of the steel according to the present invention includes acicular ferrite and bainite structure, wherein the area fraction of bainite was 15% or less.

However, even if manufactured by a direct quenching / sintering process, the strength, DWTT and CLR characteristics were degraded when the alloy component and the manufacturing conditions were restricted in the present invention, and will be described in detail below.

In order to improve HIC characteristics, it is necessary to suppress formation of a pale light band which is vulnerable to HIC. Thus, the development and comparative examples of the present invention are designed such that the SCT is higher than the Ar3 temperature in order to suppress the formation of the pearlite band. Also, when martensite-Austenite Constituents (MA constituents) are formed by cold rolling, the yield strength of the steel is lowered and the HIC characteristic is deteriorated. However, in the present invention, because the martensite is transformed into ferrite and cementite by the cold-rolling process, the amount of change in the strength and the HIC characteristic due to the martensite is less than that of the normal TMCP. Accordingly, the strength of the product and the HIC characteristics showed a greater dependence on the component content than the manufacturing process. From the strength perspective, D, G and H steels yield and tensile strength higher than the target by controlling the amount of carbon and manganese added, but the comparison made with E and F steels without Cu and Ni In Examples 5 to 8, the yield strength or the tensile strength has failed to reach the target strength value.

Except for the histological aspect, HIC is greatly influenced by segregation and inclusion in the river. In the solidification process of steel, C and Mn are representative elements for increasing hardenability, and as the content of segregation is increased, C-Mn segregation occurs, resulting in a structure with high hardness, which increases the possibility of deterioration of HIC. In addition, when C is added in a large amount, grain boundary cementite may be formed during the bake process, or carbides such as Cr, Mo, Nb and V may be coarsened to adversely affect HIC. For example, in the case of Comparative Examples 3 to 8 using D, E, and F steels having a C content of 0.05 wt% or more, cracks occurred in the HIC test even though the Mn content was 1.0 to 1.35 wt%. 1 showing the CI value and the CLR value, FIG. 1 shows that when the Mn content is 1.4 wt% or less, the CI is 9.0 or more, and more preferably 10.0 or more, the HIC characteristics can be stably satisfied. On the other hand, in Fig. 1, Mn content is different from 1.4 wt% for QT steel and Mn content exceeding 1.5 wt%, which is caused by the difference of C effect by HIC generation mechanism and manufacturing method by Mn.

Mn combines with S during solidification to form MnS, which is stretched and crushed during plate rolling and acts as a starting point for HIC cracking. In the steel types A to H of the present invention, Ca / S was adjusted to 2.0 or more in order to suppress the formation of MnS by controlling the addition amount of S to 10 ppm or less and sphericity of the produced MnS. However, as shown in Comparative Examples 8 to 9 in which the steel grades G and H were used, when the Mn addition amount was more than 1.5% by weight, MnS stretched during rolling occurred and the CLR value of the HIC test deteriorated to 20 or more. %.

Unlike HIC, the DWTT characteristics are heavily dependent on thick plate operation factors. In Comparative Examples 1 and 2, slabs having the same constituents as in Inventive Examples 2 to 3 were used. The CLR value, which was influenced by the composition and the slab manufacturing conditions, showed similar values to those of the inventive examples. In Comparative Example 1, the finishing rolling finish temperature was as high as Ar3 + 140 ° C, In Example 2, the extraction temperature was as high as 1167 캜 and coarse crystal grains were generated during reheating, resulting in deterioration of DWTT SA by 84% and 65%. Similar to Comparative Example 1, Comparative Example 3 and Comparative Example 9 showed high DWTT due to the finish rolling finish temperature, and in Comparative Example 8, the DWTT characteristics were degraded due to the effect of the extraction temperature. In particular, in the case of Comparative Example 5, it can be seen that the DWTT SA at 0 ° C deteriorated to 21% due to the combination of high-temperature extraction and high-temperature finish rolling.

In Comparative Example 6 and Comparative Example 8, the cooling end temperature of the direct quenching is 400 占 폚 higher and the tensile strength is low. In the general TMCP material, the cooling end temperature is controlled in the range of 400 to 500 ° C to recover the hardened structure by cooling. However, in the present invention, it is necessary to reduce the C and Mn contents in order to secure HIC characteristics, The recovery effect of the tissue can be anticipated. Therefore, it is preferable to set it to Ms-100 占 폚 or less in order to secure the strength.

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 (14)

0.03 to 0.05% by weight of carbon (C), 0.05 to 1.0% by weight of silicon (Si), 1.0 to 1.4% by weight of manganese (Mn), 0.010% (Al): 0.005 to 0.1 wt%, Cu: 0.1 to 0.3 wt%, Ni: 0.1 to 0.5 wt%, Cr: 0.05 to 0.3 wt%, molybdenum (Mo) ): 0.02 to 0.3 wt%, niobium (Nb): 0.03 to 0.055 wt%, titanium (Ti): 0.01 to 0.02 wt%, vanadium (V): 0.02 to 0.06 wt%, calcium (Ca) %, Nitrogen (N): 0.001 to 0.01% by weight, balance iron (Fe) and other unavoidable impurities,
(Shear Area) of 85% or more at DWTT (Drop Weight Tear Test) test at 0 ° C, a crack length ratio (CLR) of 10% or less at HIC (Hydrogen Induced Cracking) Highly resistant to toughness and hydrogen organic cracking.
The method according to claim 1,
Wherein the steel material has a Carbon Index (CI) of 9.0 or more as expressed by the following formula (1), which is excellent in low temperature toughness and hydrogen organic cracking resistance.
CI = ([Cr] + [Mo] + [Nb] + [V]) / [C]
(Wherein the [Cr], [Mo], [Nb], [V], and [C] represent the weight percentages of chromium, molybdenum, niobium, vanadium,
delete 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,
A steel material excellent in low temperature toughness and hydrogen organic cracking resistance, wherein the microstructure of the steel contains acicular ferrite and bainite structure.
delete 0.03 to 0.05% by weight of carbon (C), 0.05 to 1.0% by weight of silicon (Si), 1.0 to 1.4% by weight of manganese (Mn), 0.010% (Al): 0.005 to 0.1 wt%, Cu: 0.1 to 0.3 wt%, Ni: 0.1 to 0.5 wt%, Cr: 0.05 to 0.3 wt%, molybdenum (Mo) ): 0.02 to 0.3 wt%, niobium (Nb): 0.03 to 0.055 wt%, titanium (Ti): 0.01 to 0.02 wt%, vanadium (V): 0.02 to 0.06 wt%, calcium (Ca) %, Nitrogen (N): 0.001 to 0.01 wt.%, The balance iron (Fe) and other unavoidable impurities;
Controlling and rolling the reheated slab;
Directly after the controlled rolling; And
After the direct finishing,
The controlled rolled slab has a thickness of 40 to 53 mm,
Wherein the steel slab is reheated at a temperature of 1150 to 1250 占 폚 and then extracted at a temperature of 1080 to 1140 占 폚 in the reheating step.
delete delete 8. The method of claim 7,
Wherein the control rolling finishes rolling at a temperature of (Ar3 + 50) to (Ar3 + 100) 占 폚, which is excellent in low temperature toughness and hydrogen organic cracking resistance.
8. The method of claim 7,
Wherein the direct quenching is started at a temperature of (Ar3 + 0) to (Ar3 + 40) deg. C, and is excellent in low temperature toughness and hydrogen organic cracking resistance.
8. The method of claim 7,
Wherein the direct quenching is completed at a temperature of (Ms-100) 占 폚 or less, and wherein the cold quenching is excellent at low temperature toughness and hydrogen organic cracking resistance.
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, wherein the sintering is carried out so as to satisfy the following formula (4).
(4): bake time = 2.4 (min / mm) x thickness (mm) + (10-30) (min)

Images