KR20190065040A - Steel material having exellent hydrogen induced crack resistance and low temperature impact toughness and method of manufacturing the same - Google Patents

Abstract

The present invention provides a high strength steel material having excellent hydrogen induced crack resistance and low temperature impact toughness and a manufacturing thereof, wherein the high strength steel material comprises: 0.02-0.12 wt% of C; 0.02-0.60 wt% of Si; 0.6-1.6 wt% of Mn; 0.020 wt% or less (0 wt% exclusive) of P; 0.003 wt% or less (0 wt% exclusive) of S; 0.002-0.060 wt% of soluble Al; 0.005-0.6 wt% of Cu; 0.005-1.0 wt% of Ni; 0.005-0.5 wt% of Cr; 0.01-0.3 wt% of Mo; 0.001-0.10 wt% of Ti; 0.001-0.06 wt% of Nb; 0.001-0.10 wt% of V; 0.001-0.006 wt% of N; 0.0002-0.0060 wt% of Ca; and the remainder being composed of Fe and other unavoidable impurities, with (Ti+V+0.5Nb+0.5Mo) being 0.05% or more and (Ti+V+0.5Nb+0.5Mo)/C being 0.8 or greater, the steel material having a microstructure inclusive of ferrite, and at least one of pearlite, cementite, and MA (martensite-austenite complex phase) wherein the ferrite has a fraction of 90 % by area% or more (100 % exclusive) and at least one of ferrite, cementite, and MA (martensite-austenite complex phase) has a fraction of 10 % by area% or less. The ferrite structure includes 0.03 wt% or more of fine carbonitride precipitates having an average size of 100 nm or less and is 20 μm or less in average grain size.

Description

TECHNICAL FIELD [0001] The present invention relates to a high strength steel material excellent in hydrogen organic cracking resistance and low temperature impact toughness, and a method of manufacturing the same. BACKGROUND ART < RTI ID = 0.0 >

TECHNICAL FIELD The present invention relates to a high strength steel material used for a plant facility for mining, processing, transporting, storing crude oil, and shipbuilding offshore structures and a method of manufacturing the same. More particularly, And more particularly, to an excellent high strength steel material and a manufacturing method thereof

Due to the depletion of petroleum resources and high oil prices, the production of low quality crude oil with a high impurity content such as sulfur gradually increases. As a result, the steel used in all plant facilities for mining, processing, (HIC: Hydrogen Induced Cracking) caused by wet hydrogen sulfide due to the sulfur component in the steel is inevitably required.

In addition, environmental pollution caused by recent safety accidents is a global problem, and as the astronomical cost is required to recover it, the level of demand characteristics of steel used in the energy industry is becoming increasingly strict.

A high strength normalized steel material has been proposed as an example of a plant facility for mining, processing, transporting and storing crude oil and shipbuilding offshore structures.

As a method for obtaining high strength in the normalized steel, the increase of the pearlite fraction by the addition of C or the addition of solid solution strengthening elements such as Cu and Mo and the generation of carbonitride precipitates by the simple addition of Nb and V are utilized.

On the other hand, as a method for improving the resistance to hydrogen organic cracking of steel, there are a method of controlling internal defects such as inclusions and voids inside the steel which can act as a starting point of the accumulation and crack of hydrogen, A method of minimizing a hardened structure of pearlite or the like, or applying a pressure softening to control its shape, and the like are widely proposed and used.

However, as the required strength of the steel material is gradually increased, the amount of C added and the amount of the solid solution strengthening component are increasing for securing strength. However, the increase of the amount of C increases the pearlite fraction and also causes the pearlite to appear as banded streucture, thereby facilitating propagation of the hydrogen organic crack, .

Addition of solid solution strengthening components such as Cu, Ni, and Mo has an effect of increasing the strength, but it causes surface cracks due to the red-hot brittleness or causes a significant decrease in economic efficiency due to high cost.

In addition, if Nb and V are added excessively in order to obtain a strengthening effect by the precipitate, the hardenability of the weld portion is greatly increased to exceed the standard, which causes a decrease in toughness of the weld portion and a cause of hydrogen organic cracking.

As described above, the conventional method has a problem that it is difficult to secure the high strength of the normalized steel, the excellent impact toughness at low temperature, and the excellent hydrogen organic crack prevention property.

Accordingly, there is a demand for the development of a normalized steel having high strength, high impact toughness at low temperature, and excellent hydrogen organic crack prevention characteristics at the same time.

Korean Patent Publication No. 2011-0060449

A preferred aspect of the present invention is to provide a high strength steel excellent in hydrogen organic cracking resistance (HIC characteristic) and low temperature impact toughness.

Another aspect of the present invention is to provide a method for producing a high strength steel excellent in hydrogen organic cracking resistance (HIC characteristic) and low temperature impact toughness.

According to a preferred aspect of the present invention, there is provided a ferritic stainless steel comprising 0.02 to 0.12% of C, 0.02 to 0.60% of Si, 0.6 to 1.6% of Mn, 0.020% 0.001 to 0.6% of Cu, 0.005 to 1.0% of Ni, 0.005 to 0.5% of Cr, 0.01 to 0.3% of Mo, 0.01 to 0.3% of Mo, 0.001 to 0.10 of Ti (excluding 0% 0.001 to 0.06% of N, 0.001 to 0.10% of N, 0.001 to 0.006% of N, 0.0002 to 0.0060% of Ca and the balance of Fe and other unavoidable impurities, and (Ti + V + 0.5Nb + 0.5Mo ) Is not less than 0.05%, (Ti + V + 0.5Nb + 0.5Mo) / C is not less than 0.8, and at least one of ferrite and the remaining pearlite, cementite and MA (martensite-austenite composite phase) , Wherein the percentage of the ferrite is 90% or more (excluding 100%) by area%, and the total fraction of the pearlite, cementite and MA (martensite-austenite composite phase) is 10% And a fine carbonitride precipitate having an average size of 100 nm or less is contained in the ferrite structure in an amount of 0.03% And a high strength steel excellent in hydrogen organic cracking resistance (HIC characteristic) and low temperature impact toughness having an average crystal grain size of the ferrite structure of not more than 20 mu m is provided .

The steel material is produced by a steel material manufacturing method including a normalizing heat treatment step. The fraction of ferrite, pearlite, cementite and MA in the microstructure after the normalizing heat treatment of the steel is increased by 10% or less compared to that before the normalizing heat treatment , Pearlite decreased by less than 10%, cementite increased by less than 5%, and MA decreased by less than 3%.

According to another aspect of the present invention, there is provided a steel sheet comprising, by weight%, 0.02 to 0.12% of C, 0.02 to 0.60% of Si, 0.6 to 1.6% of Mn, 0.001 to 0.6% of Cu, 0.005 to 1.0% of Ni, 0.005 to 0.5% of Cr, 0.01 to 0.3% of Mo, 0.001 to 0.3% of Ti, (Ti + V + 0.5Nb + 0.5) containing 0.10% of Nb, 0.001 to 0.06% of N, 0.001 to 0.10% of V, 0.001 to 0.006% of N, 0.0002 to 0.0060% of Ca, and the balance of Fe and other unavoidable impurities. Mo) of not less than 0.05% and (Ti + V + 0.5Nb + 0.5Mo) / C of not less than 0.8 to a temperature of 1080 to 1300 占 폚;

The slabs being reheated as described above are controlled and rolled to obtain a hot-rolled steel sheet at a rolling finish temperature of 850 ° C or higher;

Cooling the hot-rolled steel material to a cooling termination temperature of 550 to 750 ° C and then air-cooling it; And

A normalizing heat treatment step in which the cooled steel material is maintained at a temperature of 850 to 950 캜 for 1.3 * t [t is a steel sheet thickness (mm)] + (5 to 60 minutes), followed by air cooling; (HIC characteristic) and a low-temperature impact toughness containing hydrogen-organic cracking resistance .

The method may further include a step of subjecting the steel material, which has been heat-treated according to the normalizing heat-treating step, to tempering treatment at 500 ° C or more for 10 minutes or more.

According to a preferred aspect of the present invention, it is possible to provide a high-strength steel excellent in hydrogen organic cracking resistance and low-temperature impact toughness which can be effectively applied to plant facilities for mining, processing, transporting and storing crude oil, shipbuilding offshore structures and pressure vessels .

1 is a microstructure photograph of a steel material (Comparative Example 3) produced by a conventional method.
2 is a microstructure photograph of a steel material (Inventive Example 1) produced according to the present invention.
3 is a photograph showing the distribution of precipitates of a steel material produced according to a conventional method (Comparative Example 3).
4 is a photograph showing the distribution of precipitates of the steel material (Inventive Example 1) produced according to the present invention.

Hereinafter, preferred embodiments of the present invention will be described.

However, embodiments of the present invention are provided to more fully describe the present invention to those skilled in the art.

In addition, 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.

In addition, to include an element throughout the specification does not exclude other elements unless specifically stated otherwise, but may include other elements.

One of the main concepts of the present invention is to maximize the fraction of fine ferrite and ensure that the hard phase such as cementite, pearlite and MA phase, which are unfavorable microstructures, has a minimum microstructure in order to secure both HIC propagation resistance and strength. In order to suppress the growth of austenite after the formation of austenite, it is necessary to reduce the amount of ferrite produced during cooling in order to produce nano-sized fine precipitates in the ferrite crystal grains by hot rolling (controlled rolling) and water cooling (HIC characteristic), low-temperature impact toughness and strength at the same time by finely controlling the crystal grain size by fine ferrite and precipitate.

In order to produce a high normalized normalized steel having excellent resistance to hydrogen organic cracking and low temperature toughness, the content of C is controlled to be lower than 0.12% so that the content of C is lower than that of the conventional method, However, since the cooling conditions after hot rolling are controlled so that fine precipitates of 100 nm or less are present in the ferrite powder at 0.03% by weight or more, the growth is suppressed after the austenite is formed during the normalizing heat treatment so that the ferrite grains Cementite and MA phase, which is a structure disadvantageous to the low-temperature toughness and the hydrogen-organic crack-preventing property, is controlled to be 10% or less, and a large amount of fine precipitates are controlled not only to increase the strength It also acts as a fixation site for absorbed hydrogen, The stars (in the HIC characteristic) and the low-temperature impact toughness to provide an excellent high-strength steel and a method of manufacturing the same.

Hereinafter, a high strength steel excellent in hydrogen organic cracking resistance (HIC characteristic) and low temperature impact toughness according to a preferred aspect of the present invention will be described in detail.

A high strength steel material excellent in hydrogen organic cracking resistance (HIC characteristic) and low temperature impact toughness according to a preferred aspect of the present invention comprises 0.02 to 0.12% of C, 0.02 to 0.60% of Si, 0.6 to 1.6% of Mn, , P: not more than 0.020% (excluding 0%), S: not more than 0.003% (excluding 0%), Sol.Al: 0.002 to 0.060%, Cu: 0.005 to 0.6% 0.001 to 0.5%, Mo: 0.01 to 0.3%, Ti: 0.001 to 0.10%, Nb: 0.001 to 0.06%, V: 0.001 to 0.10%, N: 0.001 to 0.006% (Ti + V + 0.5Nb + 0.5Mo) / C of not less than 0.8, and the ferrite and the remaining pearlite , Cementite and MA (martensite-austenite composite phase), wherein the fraction of the ferrite is at least 90% (excluding 100%) by area%, and the pearlite, cementite and MA (martensite- Knit composite phase) is not more than 10% by area% A fine carbonitride precipitate having an average size of 100 nm or less is present in the light texture in an amount of 0.03% or more by weight and an average crystal grain size of the ferrite structure is 20 占 퐉 or less.

Hereinafter, the components and ranges of components will be described.

C: 0.02 to 0.12% by weight (hereinafter also referred to as "%")

In the normalized steel, C is generally added to form a pearlite, cementite or MA phase to secure tensile strength. In the present invention, however, C is added as an important element for forming fine precipitates of 100 nm or less, and its content is 0.02 To 0.012%. When the content of C is less than 0.02%, the fraction of the carbonitride precipitates and the fraction of pearlite and the like are lowered and the tensile strength of the base phase may be lowered. When the content is more than 0.12%, a hard phase such as pearlite is excessively generated, So that the impact resistance at low temperature as well as hydrogen organic cracking resistance can be lowered. More preferably, the content of C is 0.03 to 0.10%, and the content of C is more preferably 0.05 to 0.08%.

Si: 0.02 to 0.60%

Si is an element to be added for the purpose of strengthening solubility with deoxidation and desulfurization effect, and is preferably added in an amount of 0.02% or more for securing the yield strength and tensile strength.

On the other hand, if the content exceeds 0.60%, the weldability and low-temperature impact properties are deteriorated and the surface of the steel sheet can be easily oxidized to form an oxide film intensively, so that the Si content is preferably limited to 0.02 to 0.60%. More preferably, the content of Si is 0.05 to 0.45%, and the content of Si is more preferably 0.05 to 0.35%.

Mn: 0.6 to 1.6%

Mn is an element having a large strength increasing effect by solid solution strengthening, and is preferably added in an amount of 0.6% or more. On the other hand, if Mn is added excessively, segregation becomes serious in the thickness direction center portion of the steel sheet, and at the same time, formation of MnS as a non-metallic inclusion is promoted together with segregated S. The MnS inclusions produced in the center portion are stretched by the subsequent rolling, and as a result, the low temperature toughness and the hydrogen organic cracking resistance are greatly reduced, so that the content of Mn is preferably limited to 1.6% or less. A more preferable content of Mn is 0.8 to 1.45%, and a more preferable content of Mn is 1.1 to 1.3%.

P: 0.020% or less (excluding 0%)

P has an effect of increasing strength at the time of addition, but it is better to manage as low as possible because it is an element that especially lowers the low-temperature toughness by grain segregation in heat-treated steel. However, it is expensive to remove it excessively in the steelmaking process, so it is desirable to limit it to less than 0.020% (excluding 0%).

S: 0.003% or less (excluding 0%)

S is considered to be a typical factor for promoting the generation and propagation of hydrogen-induced cracks as well as deteriorating the low-temperature toughness by forming MnS inclusions mainly in the thickness direction center of the steel sheet by bonding with Mn. Therefore, in order to secure hydrogen organic cracking resistance, S should be removed as much as possible in the steelmaking process, but excessive cost may be required, so it is preferable to limit the S to 0.003% or less. The more preferable content of S is 0.001% or less.

Sol.Al: 0.002 to 0.060%

In addition to Si and Mn, Al is added as a strong deoxidizer in the steelmaking process, and when added alone or in combination, more than 0.002% should be added to obtain the maximum effect. On the other hand, if it exceeds 0.060%, the effect becomes saturated and the fraction of Al ₂ O ₃ in the oxidative inclusions produced as a result of deoxidation increases more than necessary, so that the size becomes large and may not be removed well during refining. Therefore, Sol. The content of Al is preferably limited to 0.002 to 0.06%. More preferred Sol. The content of Al is 0.005 to 0.04%, and the more preferred Sol. The content of Al is 0.01 to 0.025%.

Cu: 0.005 to 0.6%

Cu is a component added because it has an effect of significantly improving the strength by solidification and precipitation and has an effect of suppressing corrosion in a wet hydrogen sulfide atmosphere. Excessive addition of Cu causes cracks on the surface of the steel sheet, and it is preferable that the content of Cu is limited to 0.005 to 0.6% from the viewpoint of economy as an expensive element . More preferably, the content of Cu is 0.02 to 0.4%, and the content of Cu is more preferably 0.05 to 0.3%.

Ni: 0.005 to 1.0%

Ni has little effect of increasing the strength, but is effective in improving the low-temperature toughness. In particular, when Cu is added, it has an effect of suppressing surface cracking due to selective oxidation occurring at reheating of the slab. Ni is added to obtain such an effect, and Ni is also an expensive element, and its content is preferably limited to 0.005 to 1.0% from the viewpoint of economy. More preferably, the content of Ni is 0.02 to 0.5%, and the content of Ni is more preferably 0.05 to 0.3%.

Cr: 0.005-0.5%

Cr has a small effect of increasing the yield strength and tensile strength due to the employment, but it is effective in preventing degradation of the cementite by slowing the decomposition rate of the cementite during the tempering or post-welding heat treatment, so 0.005% or more is added. On the other hand, if Cr is added in an amount exceeding 0.5%, not only the cost increases, but also the weldability is deteriorated. Therefore, the content of Cr is preferably limited to 0.5% or less. More preferably, the content of Cr is 0.01 to 0.4%, and the content of Cr is more preferably 0.02 to 0.2%.

Mo: 0.01 to 0.3%

Mo is generally an element effective for prevention of strength drop during tempering or post-welding heat treatment like Cr, and has an effect of preventing toughness deterioration due to intergranular segregation of impurities such as P and the like. In the present invention, Mo has the effect of greatly increasing the strength by generating nano-sized carbonitride.

On the other hand, when Mo is added in an amount of more than 0.3%, the curing ability is excessively increased to promote a light image which deteriorates toughness and hydrogen organic cracking resistance, so that the content thereof is preferably limited to 0.01 to 0.3% Do. A more preferable content of Mo is 0.02 to 0.2%, and a more preferable content of Mo is 0.05 to 0.15%.

Ti: 0.001 to 0.10%

When Ti is added, the effect of increasing the nano-sized carbonitride greatly increases. It is added in an amount of 0.001% or more, but when it is added in an amount exceeding 0.10%, it is present as a precipitate of coarse TiN hexagonal body, But also as a starting point for hydrogen organic cracking. Therefore, it is preferable to limit the content of Ti to 0.10% or less. More preferably, the content of Ti is 0.002 to 0.05%, and the content of Ti is more preferably 0.005 to 0.03%.

Nb: 0.001 to 0.06%

Nb is dissolved in the austenite during the reheating of the slab to increase the hardenability of the austenite and precipitate as a carbonitride which matches with the matrix at high temperature during hot rolling to inhibit recrystallization and to make the final microstructure fine. Further, there is an effect of generating a fine precipitate having a size of 100 nm or less even after being transformed after cooling, thereby greatly increasing the strength.

However, if Nb is added in an excessively large amount, coarse precipitates are easily formed in the center of the thickness direction, and the hardenability of the welded portion is increased more than necessary to lower the low temperature toughness and the crack resistance of hydrogen organic cracking. To 0.06%. More preferably, the content of Nb is 0.002 to 0.05%, and the content of Nb is more preferably 0.005 to 0.04%.

V: 0.001 to 0.10%

V is almost completely reheated when the slab is reheated, and there is almost no strengthening effect due to precipitation or solidification in the rolling state, but there is an effect of improving the strength by precipitating very fine carbonitride during tempering or post-welding heat treatment. However, the content of V is preferably 0.001 to 0.10% because of the high cost. More preferably, the content of V is 0.001 to 0.06%, and the content of V is more preferably 0.005 to 0.02%.

N: 0.001 to 0.006%

N may form precipitates together with added Nb, Ti, Mo and V to refine the crystal grains of the steel to improve the strength and toughness of the base material. However, when added excessively, N is present in a surplus atomic state and promotes embrittlement at high temperatures . Also, the reusable fraction of carbonitride is reduced during reheating of the slab, which causes the amount of fine precipitate, which is an important means of the present invention, to be greatly reduced. Accordingly, it is preferable that the content of N is limited to 0.001 to 0.006% in consideration of the contents of Nb, V, Mo and Ti and the reheating temperature of the slab. More preferably, the content of N is 0.002 to 0.005%, and the content of N is more preferably 0.002 to 0.0045%.

Ca: 0.0002 to 0.0060%

When Ca is added after Al-deoxidation, it is combined with S present in MnS to inhibit MnS formation, and at the same time, it forms CaS in spherical form and exerts an effect of suppressing hydrogen organic cracking. Therefore, Ca is preferably added in an amount of 0.0002% or more in order to sufficiently form added S into CaS. On the other hand, when Ca is excessively added, surplus Ca bonds with O (oxygen) to form coarse oxidative inclusions, which are then stretched and fractured at the subsequent rolling, promoting hydrogen organic cracking. Therefore, the upper limit of the Ca content is preferably limited to 0.0060%. More preferably, the content of Ca is 0.0005 to 0.0060%, and the content of Ca is more preferably 0.001 to 0.0025%.

(Ti + V + 0.5Nb + 0.5Mo) / C: not less than 0.8 (Ti + V + 0.5Nb + 0.5Mo)

 The reason for limiting the contents of Ti, Nb, V and Mo, which are elements forming denitrified precipitates, to the above formula is that the nano-sized fine precipitates are sufficiently generated in the normalizing heat treatment to suppress the growth of austenite, The content of these elements must be added at a certain level or more because the effect of not only fineening but also increasing the strength greatly due to the fine precipitate itself.

 In addition, a fine precipitate can be sufficiently generated only when the ratio of carbon to carbon, which is an important constituent element of the carbonitride, is high.

Therefore, it is preferable that (Ti + V + 0.5Nb + 0.5Mo) be 0.05% or more and (Ti + V + 0.5Nb + 0.5Mo) / C be 0.8 or more.

According to a preferred aspect of the present invention, a high strength steel excellent in hydrogen organic cracking resistance (HIC characteristic) and low temperature impact toughness is composed of ferrite as a microstructure and one of the remaining pearlite, cementite and MA (martensite- austenite composite phase) , Wherein the fraction of the ferrite is at least 90% (excluding 100%) by area%, and the total fraction of the pearlite, cementite and MA (martensite-austenite composite phase) is at least 10% And a fine carbonitride precipitate having an average size of 100 nm or less is present in an amount of 0.03% or more by weight in the ferrite structure, and the average crystal grain size of the ferrite structure is 20 탆 or less.

Said steel being at least 90% (excluding 100%) of ferrite; And the remaining 10% or less of at least one of pearlite, cementite and MA (martensite-austenite composite phase).

The pearlite, cementite and MA (martensite-austenite composite phase) are effective for improving the strength and particularly tensile strength of the steel sheet. However, when the steel sheet is excessively large, the pearlite, cementite and MA , Impact toughness deteriorates at a low temperature, and in particular, it acts as a path for generation and propagation of hydrogen organic cracks, and the hydrogen organic cracking resistance may be deteriorated drastically. Therefore, it is preferable that the total fraction of at least one of the pearlite, cementite and MA (martensite-austenite composite phase) is limited to 10% or less by area%.

The pearlite may be present in a finely dispersed form that is not present in banded streutures and has a degree of orientation (Omega 12) defined in ASTM E1268 of 0.1 or less. The finely dispersed pearlite is between the fine ferrites.

The steel material is produced by a steel material manufacturing method including a normalizing heat treatment step. The fraction of ferrite, pearlite, cementite and MA in the microstructure after the normalizing heat treatment of the steel is increased by 10% or less compared to that before the normalizing heat treatment , Pearlite decreased by less than 10%, cementite increased by less than 5%, and MA decreased by less than 3%.

In the ferrite structure, a fine carbonitride precipitate having an average size of 100 nm or less is present in an amount of 0.03% by weight or more.

When the content of the fine carbonitride precipitates is less than 0.03 wt%, coarse austenite is generated due to the inhibition of the austenite growth during the normalizing heat treatment, coarse pearlite is formed during the subsequent cooling, and at the same time, There is a possibility that the hydrogen fixing effect is greatly reduced.

The mean grain size of the ferrite structure is 20 mu m or less.

When the average crystal grain size of the ferrite structure exceeds 20 탆, there is a fear that strength and hydrogen organic crack resistance due to microstructure are lowered.

The steel may have a tensile strength of at least 485 MPa, an impact absorption energy value of at least 120 J at -40 DEG C, and a longitudinal crack ratio (CLR) of 5% or less in HIC evaluation.

The steel may have a thickness of 6 to 133 mm.

Hereinafter, a method for producing a high strength steel excellent in hydrogen organic cracking resistance (HIC characteristic) and low temperature impact toughness according to another preferred embodiment of the present invention will be described.

A method for producing a high strength steel excellent in hydrogen organic cracking resistance (HIC characteristic) and low temperature impact toughness according to another preferred aspect of the present invention comprises 0.02 to 0.12% of C, 0.02 to 0.60% of Si, (Excluding 0%), S: 0.003% or less (excluding 0%), Sol.Al: 0.002-0.60%, Cu: 0.005-0.6%, Ni: 0.005-1.0 0.001 to 0.10% of N, 0.001 to 0.006% of N, 0.001 to 0.006% of Ca, 0.001 to 0.5% of Cr, 0.01 to 0.3% of Mo, 0.001 to 0.10% (Ti + V + 0.5Nb + 0.5Mo) / C of 0.8 or more and (Ti + V + 0.5Nb + 0.5Mo) of 0.05% or more and a balance of Fe and other unavoidable impurities, ≪ / RTI >

The slabs being reheated as described above are controlled and rolled to obtain a hot-rolled steel sheet at a rolling finish temperature of 850 ° C or higher;

Cooling the hot-rolled steel material to a cooling termination temperature of 550 to 750 ° C and then air-cooling it; And

And a normalizing heat treatment step in which the cooled steel material is maintained at a temperature of 850 to 950 캜 for 1.3 * t [t is a steel sheet thickness (mm)] + (5 to 60 minutes), followed by air cooling.

Reheat step

The slab having the above composition is reheated at 1080 to 1300 占 폚.

When the reheating temperature is lower than 1080 ° C, reutilization of the carbides generated in the slab during the performance becomes difficult. When the reheating temperature exceeds 1300 ° C, the austenite grain size becomes too coarse and the tensile strength and low temperature toughness And the like can be significantly deteriorated. In particular, since the added Ti, Nb, V, Mo, and the like must be reheated to a temperature at which sufficient reuse can be performed, the slab reheating temperature is preferably limited to 1080 to 1300 ° C. A more preferable slab reheating temperature is 1150 to 1280 占 폚.

Hot-rolled steel  Steps to Obtain

As described above, the reheated slab is controlled and rolled to a hot rolled steel material at a rolling finish temperature of 850 DEG C or higher.

When the reheated slab is generally rolled, the rolling is terminated at an excessively high temperature and the effect of grain refinement is small. Further, when the rolling is controlled to an excessively low temperature, reused Nb and the like are precipitated as carbonitride, The size of the deposited precipitates is so large that the effect of suppressing the austenite crystal growth during normalization is greatly reduced. Also, the coarse complex inclusions generated in the refining process must accommodate the deformation due to rolling as the steel sheet is increased in strength as the rolling temperature is lowered, thereby separating the inclusions into small-sized inclusions or elongating the amorphous inclusions. Such an elongated or segmented inclusion is a direct cause of generation and propagation of hydrogen organic cracks. In view of such phenomena, it is preferable to limit the rolling finish temperature to 850 DEG C or more. A more preferable rolling finish temperature is 850 to 1050 占 폚. The rolling finish temperature may be Ar ₃ + 70 ° C to Ar ₃ + 270 ° C.

The hot-rolled steel may have a thickness of 6 to 133 mm.

Control cooling stage

The hot-rolled steel is water-cooled to a cooling termination temperature of 550 to 750 ° C and then air-cooled.

Control cooling is performed by water-cooling and air-cooling the controlled-rolled steel as described above. When water cooling is performed after controlled rolling, most of the generation of ferrite at high temperature during the water cooling can be largely prevented, and the effect of miniaturizing is generated at a lower temperature than when air is cooled. Particularly, when the cooling is terminated within the range of 550 to 750 ° C, fine carbonitride is produced at the same time as the ferrite is produced. The precipitate produced at this time is as fine as 100 nm or less. If the cooling termination temperature exceeds 750 ° C, ferrite is produced at high temperature and becomes coarse, and precipitates are formed coarsely, and the effect is greatly reduced. On the other hand, if the cooling termination temperature is too low to be lower than 550 占 폚, fine precipitates are not generated and sufficient strength can not be ensured or when the cooling rate is high, ferrite formation is inhibited and bainite phases are formed, The toughness at low temperature is greatly reduced. Therefore, it is preferable that the cooling termination temperature of the control cooling is limited to 550 to 750 占 폚 in consideration of the components and rolling conditions. The more preferable cooling end temperature is 580 to 670 캜.

Wherein the steel after controlled cooling has a microstructure in which the ferrite is 90% or more (excluding 100%) and the total fraction of at least one of pearlite, cementite and MA phase is 10% or less, 0.03% by weight or more of the precipitate may be present.

Normalizing  Heat treatment step

The cooled steel material is maintained at a temperature of 850 to 950 캜 for 1.3 * t [t is the steel sheet thickness (mm)] + (5 to 60 minutes), followed by air cooling and normalizing heat treatment.

If the holding temperature is lower than 850 캜, it is difficult to secure the strength because the solid solute elements are difficult to be reused. On the other hand, when the holding temperature is higher than 950 캜, grain growth occurs and low temperature toughness is deteriorated.

The reason for limiting the holding time in the normalizing heat treatment is that if the holding time is shorter than the reference time, the homogenization of the tissue is difficult and the productivity is deteriorated if the holding time is longer than the reference time.

Therefore, the holding time in the normalizing heat treatment is preferably limited to 1.3 * t [t is the steel sheet thickness (mm)] + (5 to 60 minutes).

The steel microstructure after the normalizing heat treatment is composed of ferrite; Cermetite and MA (martensite-austenite composite phase), wherein the percentage of the ferrite is 90% or more (excluding 100%) in area%, and the pearlite, cementite and MA Site-austenite composite phase) is 10% or less by area%, fine carbonitride precipitates of 100 nm or less are present in 0.03% or more by weight in the ferrite structure, and the average grain size of the ferrite structure The size may be 20 μm or less.

The percentage of ferrite, pearlite, cementite and MA in the steel microstructure after the normalizing heat treatment is increased by 10% or less, the pearlite is decreased by 10% or less, the cementite is decreased by 5% or less, 3% or less.

Thus, by optimally controlling the steel composition, controlled rolling, controlled cooling, and normalizing heat treatment methods according to the present invention, it is possible to minimize hardened structures such as pearlite, cementite and MA, which are unfavorable to toughness and hydrogen organic cracking resistance, It is possible to effectively manufacture a high-strength steel excellent in high strength, low temperature toughness and hydrogen organic cracking resistance at the same time through microfabrication of the structure.

Tempering  Heat treatment step

The normalized steel can be subjected to tempering treatment at 500 ° C or more for 10 minutes or more as necessary.

The tempering heat treatment can be carried out for the purpose of increasing toughness and strength at low temperatures. The tempering temperature is preferably 500 캜 or higher. If the tempering temperature is lower than 500 캜, the removal of the residual stress of the steel is not smooth or the desired fine precipitate is not generated. When the tempering heat treatment time is less than 10 minutes, generation of precipitates is insignificant, and therefore, it is preferable to be limited to 10 minutes or more.

By the above tempering heat treatment, the fraction of ferrite in the microstructure of the steel can be increased by 3% or less by area%, and the total fraction of pearlite, cementite and MA (martensite-austenite composite phase) % ≪ / RTI >

By the above tempering heat treatment, the content of the carbonitride precipitates in the steel can be increased by 0.001 to 0.005% by weight.

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

 (Example)

The slabs having the steel compositions (chemical composition) shown in the following Table 1 were reheated under the manufacturing conditions shown in Table 2, and then the reheated slabs were controlled and rolled to obtain hot-rolled steel sheets having the thicknesses shown in Table 2, The steel was heat treated to produce a steel.

At this time, the normalizing heat treatment time was 1.3 * t [t is the steel sheet thickness (mm)] + 20 minutes.

In the following Table 1, the inventive materials (a to c) are in conformity with the steel composition of the present invention, and the comparative materials (d to h) are out of the steel composition of the present invention.

(%) And average grain size (占 퐉) of the ferrite, the content (weight%) and the average size (nm) of the fine precipitate and the pearlite fraction (area%), and the yield strength (MPa), tensile strength (MPa), impact absorption energy (J, -45 캜) and HIC CLR (%) were measured. The results are shown in Tables 3 and 4 below.

In Table 3, the microstructure other than ferrite and pearlite is at least one of cementite and MA (martensite-austenite composite phase).

On the other hand, in Inventive Example 1 and Comparative Example 3, the distribution of microstructure and precipitate was observed, and the results are shown in Figs. Fig. 1 is a microstructure photograph of Comparative Example 3, Fig. 2 is a microstructure photograph of Inventive Example 1, Fig. 3 is a precipitate distribution photograph of Comparative Example 3, and Fig. 4 is a photograph of a precipitate distribution of Inventive Example 1.

Component
Chemical composition (% by weight) C Si Mn P S Sol.Al Cu Ni Cr Mo Ti Nb V N Ca Ti + V +
0.5 Nb +
0.5Mo (Ti + V +
0.5 Nb +
0.5Mo)
/ C foot
persons
ashes

a 0.055 0.32 1.14 0.003 0.0005 0.027 0.23 0.15 0.06 0.08 0.023 0.008 0.009 0.0032 0.0016 0.076 1.38 b 0.035 0.22 1.45 0.003 0.0008 0.031 0.01 0.14 0.02 0.12 0.043 0.002 0.002 0.0041 0.0012 0.106 3.03 c 0.081 0.43 0.91 0.005 0.0004 0.015 0.01 0.01 0.12 0.03 0.083 0.021 0.012 0.0029 0.0013 0.121 1.49 ratio
School
ashes



d 0.041 0.32 1.45 0.007 0.001 0.032 0.01 0.02 0.04 0.05 0.011 0.002 0.003 0.0042 0.0013 0.040 0.98 e 0.145 0.38 1.35 0.007 0.0009 0.022 0.12 0.33 0.02 0.11 0.012 0.003 0.002 0.0032 0.0021 0.071 0.49 f 0.089 0.33 0.82 0.006 0.0008 0.035 0.02 0.01 0.02 0.42 0.001 0.003 0.002 0.0045 0.0012 0.215 2.41 g 0.102 0.15 1.08 0.005 0.001 0.021 0.11 0.02 0.03 0.05 0.15 0.005 0.006 0.0034 0.0023 0.184 1.80 h 0.061 0.31 1.25 0.008 0.0009 0.035 0.01 0.03 0.02 0.09 0.021 0.003 0.003 0.0088 0.0018 0.071 1.16

Component Product thickness
(mm) Slab
Reheating
Temperature
(Degree) Rolling
Wrap-up
Temperature
(Degree) Cooling
End
Temperature
(Degree) Normalizing
Temperature
(Degree) Inventory 1 a 30.0 1230 880 635 891 Inventory 2 b 9.5 1190 862 712 903 Inventory 3 c 95.0 1163 982 623 888 Comparative Example 1 b 25.0 1062 885 642 916 Comparative Example 2 c 45.0 1163 936 513 913 Comparative Example 3 c 80.0 1134 981 - 905 Comparative Example 4 d 51.0 1223 963 635 899 Comparative Example 5 e 30.0 1198 924 621 913 Comparative Example 6 f 12.0 1178 872 602 900 Comparative Example 7 g 78.0 1192 974 655 897 Comparative Example 8 h 25.0 1153 875 638 887

Component product
thickness
(mm) Ferrite fraction
(%) Fine precipitate content (%) Pearlite
Fraction (%) Average diameter of ferrite grain (mu m) Precipitate average diameter
(nm) Inventory 1 a 30.0 96.2 0.046 2.2 17 35 Inventory 2 b 9.5 98.9 0.065 0.7 13 48 Inventory 3 c 95.0 92.1 0.074 4.6 15 41 Comparative Example 1 b 25.0 98.9 0.022 0.9 23 55 Comparative Example 2 c 45.0 92.1 0.024 4.6 22 37 Comparative Example 3 c 80.0 88.2 0.015 8.5 31 120 Comparative Example 4 d 51.0 98.4 0.024 1.0 16 45 Comparative Example 5 e 30.0 83.7 0.043 9.6 25 113 Comparative Example 6 f 12.0 89.9 0.131 3.7 17 63 Comparative Example 7 g 78.0 88.2 0.112 6.9 26 145 Comparative Example 8 h 25.0 95.4 0.009 2.7 23 36

Component Yield strength after normalizing
(MPa) After normalizing
Tensile Strength (MPa) Shock absorption energy after normalizing (J, -49 degrees) HIC CLR after normalizing
(%) Inventory 1 a 348 501 253 0.0 Inventory 2 b 380 515 314 0.0 Inventory 3 c 314 500 157 0.0 Comparative Example 1 b 319 447 301 0.0 Comparative Example 2 c 305 458 198 0.0 Comparative Example 3 c 281 412 67 52.0 Comparative Example 4 d 321 446 272 0.0 Comparative Example 5 e 354 522 49 12.0 Comparative Example 6 f 365 592 34 23.0 Comparative Example 7 g 337 517 61 7.0 Comparative Example 8 h 344 455 275 0.0

As shown in Tables 3 and 4, the steel material (Inventive Example 1-3) produced according to the steel composition and manufacturing conditions of the present invention had a tensile strength of 485 MPa or more, a shock absorption energy value of 120 J or more at -40 캜, (CLR) of less than 5%. Such a steel material of the present invention is very effective for use as a steel material for shipbuilding, marine and pressure vessels.

On the other hand, in Comparative Example 1, the composition satisfies the range of the invention, but since the reheating temperature of the slab is lower than the range of the invention, the coarse carbonitride precipitates produced during the slab production are not sufficiently reused and are produced as fine precipitates upon subsequent rolling and cooling The fraction is greatly decreased and the effect of increasing the strength after normalizing is hardly exhibited.

The composition of Comparative Example 2 also satisfies the range of the invention, but the cooling end temperature in the cooling step after rolling is lower than that of the invention, so that the rate at which the precipitate is produced becomes too slow, resulting in a failure to generate sufficient carbonitride. Even in this case, the strength after normalizing can not be sufficiently secured.

Comparative Example 3 is a conventional manufacturing method in which the component satisfies the scope of the invention but is not subjected to control cooling after rolling and is air-cooled. However, due to a too slow cooling rate, ferrite transformation is initiated at a high temperature to generate coarse ferrite But also the generation of fine precipitates is not sufficient. In this case, the final microstructure is not only coarse in the ferrite (white portion in the photograph) as shown in FIG. 1, but also in the form of pearlite (black portion in the photograph) containing a little MA structure, Toughness, and HIC characteristics are not sufficiently secured.

Comparative Example 4 shows a case where the added carbonitride generating elements are not sufficient. Even though all of the production conditions are within the range of the present invention, the amount of micro precipitates produced is not sufficient and sufficient strength can not be secured.

Comparative Example 5 shows a case where the addition amount of C exceeds the range of the present invention. As the content of C increases, the amount of carbonitride to be deposited increases, but when the ratio of C is higher than that of the alloying elements, the generated carbonitrides are formed to a great extent so that the ability to inhibit the growth of austenite during the subsequent normalizing is decreased , The effect of increasing the strength as a whole is also greatly reduced, and consequently, the toughness at the low temperature and the toughness at the low temperature are simultaneously deteriorated.

In Comparative Example 6, the addition amount of Mo exceeds the range of the present invention. Mo is an important element to make fine precipitates, and at the same time, the molten Mo greatly increases the hardenability of the steel, thereby promoting the formation of residual austenite as a MA constituent during cooling after normalizing. As a result, the yield strength of the steel is lowered, the tensile strength is greatly increased, and the toughness at low temperature is significantly lowered.

Comparative Example 7 is a case where the added amount of Ti exceeds the range of the present invention. Ti is the most important element for making fine precipitates, but when added excessively, precipitates are produced in a very coarse size, which acts as a starting point of fracture, resulting in a significant deterioration of toughness at low temperature and resistance to organic cracking .

Comparative Example 8 is a case where the addition amount of N exceeds the range of the present invention. However, if N is added excessively, the storage capacity of the crude carbonitride produced in slab manufacturing is greatly reduced even if the reheating temperature is increased. Therefore, even if the subsequent production conditions satisfy the range of the present invention, the fraction of the produced fine carbonitride is significantly lowered, so that a sufficient strengthening effect can not be obtained.

On the other hand, the steel material (Comparative Example 3) produced according to the conventional method has a banded structure of pearlite as shown in Fig. 1, whereas the steel material (Inventive Example 1) (Fig. 4) produced according to the present invention (Fig. 4) as well as the pearlite existed in a finely dispersed form without being present in the form of strips (Comparative Example 3 ) (Fig. 3). It can be seen that more fine precipitates are distributed.

As described above, when a steel material is manufactured according to the steel composition and manufacturing conditions of the present invention, hardening tissues such as pearlite, which is unfavorable to toughness and hydrogen organic cracking resistance, are minimized, and high strength and low temperature toughness And hydrogen organic cracking resistance at the same time.

Claims (11)

(Excluding 0%), S: not more than 0.003% (excluding 0%), Sol: 0.02 to 0.12%, Si: 0.02 to 0.60%, Mn: 0.6 to 1.6% 0.001 to 0.06% of Al, 0.005 to 0.6% of Cu, 0.005 to 1.0% of Ni, 0.005 to 0.5% of Cr, 0.01 to 0.3% of Mo, 0.001 to 0.10% of Ti, 0.001 to 0.06% of Nb, (Ti + V + 0.5Nb + 0.5Mo) is contained in an amount of 0.05% or more, and Ti (Ti) is contained in an amount of 0.001 to 0.10%, 0.001 to 0.006% of N, 0.0002 to 0.0060% of Ca and the balance of Fe and other unavoidable impurities. + V + 0.5Nb + 0.5Mo) / C is not less than 0.8 and contains at least one of ferrite as a microstructure and remaining pearlite, cementite and MA (martensite-austenite composite phase) Wherein the total percentage of at least one of the pearlite, cementite and MA (martensite-austenite composite phase) is 10% or less by area%, and the average size of the ferrite structure is 100 nm or less Or less of the fine carbonitride precipitates are present in an amount of 0.03% or more by weight, High-strength steels are bacteria grain size is not more than 20㎛ hydrogen induced crack resistance and low temperature impact toughness excellent.
The method of claim 1, wherein the pearlite is present in a finely dispersed form having a degree of orientation (Omega-12) as defined in ASTM E1268 of 0.1 or less. This excellent high strength steel.
The method of claim 1, wherein the steel has a tensile strength of at least 485 MPa, an impact absorption energy value of at least 120 J at -40 캜, and a longitudinal crack ratio (CLR) of less than or equal to 5% High strength steel with excellent impact toughness at low temperatures.
The high strength steel material according to claim 1, wherein the steel material has a thickness of 6 to 133 mm.
0.001 to 0.06% of Nb, 0.001 to 0.10% of V, 0.001 to 0.10% of Sol of Al, 0.002 to 0.060% of Sol. Al, 0.001 to 0.10%, 0.001 to 0.6% of Cu, 0.005 to 1.0% of Ni, 0.005 to 0.5% of Cr, 0.01 to 0.3% of Mo, 0.0002 to 0.0060% of Ca, 0.001 to 0.006% (Ti + V + 0.5Nb + 0.5Mo) is not less than 0.05% and not more than 0.020% (excluding 0%), S: not more than 0.003% (excluding 0%), the balance Fe and other unavoidable impurities. Ti + V + 0.5 Nb + 0.5 Mo) / C of 0.8 or more to 1080 to 1300 占 폚;
Hot rolling the reheated slab to a rolling finish temperature of 850 DEG C or higher to obtain a hot-rolled steel sheet;
Cooling the hot-rolled steel sheet to a cooling end temperature of 550 to 750 ° C by water cooling; And
A step of subjecting the cooled steel sheet to a normalizing heat treatment at 850 to 950 캜 for 1.3 * t [t: steel sheet thickness (mm)] + (5 to 60 minutes); Which is excellent in hydrogen-induced crack resistance and low-temperature toughness.
6. The method of manufacturing a high strength steel plate according to claim 5, wherein the reheating temperature in the step of reheating the slab is 1150 to 1300 占 폚.
The method of manufacturing a high strength steel sheet according to claim 5, wherein the rolling finish temperature is 850 to 1050 ° C.
6. The method of manufacturing a high strength steel plate according to claim 5, wherein the rolling finish temperature is Ar ₃ + 70 ° C to Ar ₃ + 270 ° C.
6. The method of manufacturing a high strength steel sheet according to claim 5, wherein the hot-rolled steel has a thickness of 6 to 133 mm.
6. The method of manufacturing a high strength steel sheet according to claim 5, further comprising a step of tempering the steel material heat-treated at 500 DEG C or more for 10 minutes or more according to the normalizing heat treatment step.
The method of manufacturing a high strength steel plate according to claim 10, wherein the content of carbonitride precipitates in the steel is increased by 0.001 to 0.005 wt% by the tempering heat treatment.

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