KR20210076482A - Steel sheet having excellent resistance of sulfide stress cracking and method of manufacturing the same - Google Patents

Abstract

The present invention relates to a thick steel material which can be properly used for a line pipe and sour resistant material, and more particularly, to a high strength steel material and a manufacturing method thereof, which simultaneously have excellent sulfide stress corrosion cracking resistance and excellent propagation resistance of the sulfide stress corrosion cracking resistance.

Description

Steel material having excellent resistance to sulfide stress corrosion cracking and manufacturing method thereof {STEEL SHEET HAVING EXCELLENT RESISTANCE OF SULFIDE STRESS CRACKING AND METHOD OF MANUFACTURING THE SAME}

The present invention relates to a thick steel material suitable for use in line pipes and sour materials, and more particularly, to a high-strength steel having excellent resistance to sulfide stress corrosion cracking and a method for manufacturing the same.

In recent years, there has been an increasing demand for limiting the upper limit on the surface hardness of line pipe steels. When the surface hardness of the line pipe steel is high, it not only causes problems such as non-uniformity of roundness during pipe processing, but also cracks during pipe processing due to the high hardness structure of the pipe surface or lack of toughness in the operating environment. . In addition, when the high hardness structure of the surface part is used in a sour environment with a lot of hydrogen sulfide, there is a high possibility of causing a brittle crack by hydrogen to cause a large accident.

In 2013, during a large crude oil/natural gas mining project in the Caspian Sea, Sulfide Stress Cracking (SSC) occurred in the high-hardness part of the pipe surface within 2 weeks of operation, and a 200km subsea pipeline was constructed as a clad pipe. has been replaced by . At this time, as a result of analyzing the cause of the occurrence of SSC, it is estimated that the cause is the formation of a hard spot, which is a high-hardness tissue on the surface of the pipe.

The API standard stipulates that the hard spot is 2 inches or more in length and Hv 345 or more. In the DNV standard, the size standard is the same as the API standard, but the upper limit of hardness is defined as Hv 250.

On the other hand, a steel material for a line pipe is generally manufactured by performing hot rolling by reheating a steel slab and performing accelerated cooling. During the accelerated cooling, as the surface portion is rapidly cooled non-uniformly, a hard spot (hard spot; a portion where a high hardness structure is formed) ) is considered to occur.

In a steel sheet manufactured by conventional water cooling, since water is sprayed on the surface of the steel sheet, the cooling rate of the surface portion is faster than that of the center, and the hardness of the surface portion becomes higher than that of the center portion due to the difference in the cooling rate.

As a method for suppressing the formation of a high-hardness structure on the surface of the steel, a method of easing the water cooling process may be considered. However, since the decrease in surface hardness due to the relaxation of water cooling is accompanied by a decrease in the strength of the steel, it causes problems such as having to add more alloying elements. In addition, the increase of these alloying elements is also a cause to increase the surface hardness.

Korean Publication No. 1998-028324

One aspect of the present invention is to provide a high-strength steel material having excellent resistance to sulfide stress corrosion cracking and a method for manufacturing the same by effectively reducing the hardness of the surface portion compared to the existing heavy plate water-cooling material (TMCP) by optimizing the alloy composition and manufacturing conditions. will do

More specifically, an aspect of the present invention is to provide a high-strength steel with a yield strength of 450 MPa or more and a method for manufacturing the same, having excellent resistance to sulfide stress corrosion cracking in _{a high-pressure H 2 S environment exceeding a partial pressure of 1 bar.}

In addition, one aspect of the present invention is to increase the resistance to sulfide stress corrosion cracking by effectively controlling the hardness of the surface portion to be low through the optimization of alloy composition and manufacturing conditions, and at the same time to prevent the propagation of sulfide stress corrosion cracking in a _{high-pressure H 2 S environment.} By minimizing the accelerating chromium (Cr) content, it is also intended to secure propagation resistance of sulfide stress corrosion cracking.

The subject of the present invention is not limited to the above. Those of ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the additional problems of the present invention from the contents throughout the present specification.

An aspect of the present invention, by weight, carbon (C): 0.02 to 0.06%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 0.8 to 1.8%, chromium (Cr): less than 0.05%, Phosphorus (P): 0.03% or less, sulfur (S): 0.003% or less, aluminum (Al): 0.06% or less, nitrogen (N): 0.01% or less, niobium (Nb): 0.005 to 0.08%, titanium (Ti) : 0.005~0.05%, calcium (Ca): 0.0005~0.005%; Nickel (Ni): 0.05 to 0.3%, molybdenum (Mo): 0.02 to 0.2%, and vanadium (V): at least one of 0.005 to 0.1%, the balance contains Fe and unavoidable impurities,

The Ca and S satisfy the following relation 1,

The steel provides a steel, wherein the surface microstructure is composed of ferrite, or is composed of a composite structure of ferrite and pearlite, and the central microstructure is composed of ash ferrite.

[Relational Expression 1]

0.5 ≤ Ca/S ≤ 5.0 (wherein each element means a weight content)

Another aspect of the present invention comprises the steps of heating a steel slab satisfying the above-described alloy composition and Relation 1 in a temperature range of 1100 to 1300 ° C. for 2 hours or more; manufacturing a hot-rolled sheet material by finishing hot rolling the heated steel slab; and cooling after the finish hot rolling;

The cooling may include primary cooling; air cooling; and secondary cooling,

The primary cooling is performed at a cooling rate of 5-40 °C/s so that the surface temperature of the hot-rolled sheet material is Ar1-50 °C ~ Ar3-50 °C, and the secondary cooling is the surface temperature of the hot-rolled sheet material is 300 It provides a method for manufacturing steel, which is performed at a cooling rate of 50 to 500 °C / s so as to be -600 °C.

According to the present invention, in providing a thick steel material having a certain thickness, the hardness of the surface portion is effectively reduced to provide a high strength steel material excellent in resistance to sulfide stress corrosion cracking.

In addition, according to the present invention, it is possible to provide a high-strength steel material that is excellent in resistance to sulfide stress corrosion cracking and is also excellent in propagation resistance of sulfide stress corrosion cracking.

The steel of the present invention can be advantageously applied not only as a pipe material such as a line pipe, but also as a sour material, and has excellent sulfide stress corrosion cracking properties even in _{a high-pressure H 2 S environment exceeding a partial pressure of 1 bar.} High-strength steel can be effectively provided.

1 shows the surface microstructure and hardness of the invention steel and the comparative steel in the experimental example of the present invention.

Currently, the TMCP (Thermo-Mechanical Control Process) material supplied to the heavy plate material and hot rolling market has the hardness of the surface part due to an inevitable phenomenon (the cooling rate of the surface part becomes faster than the central part) that occurs during cooling after hot rolling. has higher characteristics than the center.

For this reason, as the strength of the material increases, the hardness at the surface portion increases significantly compared to the center portion, and this increase in hardness at the surface portion may cause cracks during processing or impair low-temperature toughness. In addition, in the case of steel applied to a sour environment, there is a problem that becomes the starting point of hydrogen embrittlement. In spite of these problems of the prior art, a steel material having excellent resistance to sulfide stress corrosion cracking under a _{high-pressure H 2 S environment is not currently provided.}

Accordingly, the present inventors recognized the problems of the prior art and, as a result of intensive examination, not only can effectively suppress sulfide stress corrosion cracking caused by hard spots, but even if hard spots occur and cracks occur on the surface, this We found a steel material that does not propagate easily and ended up completing it.

Specifically, the present inventors, as an aspect of the invention, in a thick steel material having a certain thickness or more, by effectively lowering the hardness of the surface portion, as well as securing resistance to cracking and propagation resistance of cracks, and at the same time, a steel material having high strength wanted to provide.

The present inventors have come up with a new cooling control technology, not a conventional cooling method according to the prior art, while repeating research and experiments, and thereby a technology that can alleviate the hardness of the surface portion by dualizing the phase transformation of the surface portion and the center portion came to envision

That is, the present invention has developed a technique capable of reducing hardenability of the surface portion and forming ferrite on the surface portion by promoting decarburization of the surface layer portion during heating and rolling process. In addition, the inventors of the present invention found that, when Cr is added as an alloying element to steel, the propagation resistance to sulfide stress corrosion cracking deteriorates, and the conditions of the steel composition and manufacturing process (heating, rolling, cooling, etc.) By optimizing, it is intended to provide a steel sheet manufacturing technology with excellent resistance to sulfide stress corrosion cracking even under _{high pressure H 2 S environment.}

Hereinafter, the component system of the steel according to the present invention will be described preferentially.

Steel according to an aspect of the present invention, by weight, carbon (C): 0.02 to 0.06%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 0.8 to 1.8%, chromium (Cr): 0.05 % or less, phosphorus (P): 0.03% or less, sulfur (S): 0.003% or less, aluminum (Al): 0.06% or less, nitrogen (N): 0.01% or less, niobium (Nb): 0.005 to 0.08%, titanium (Ti): 0.005 to 0.05%, calcium (Ca): 0.0005 to 0.005%; Nickel (Ni): 0.05 to 0.3%, molybdenum (Mo): 0.02 to 0.2%, and vanadium (V): at least one of 0.005 to 0.1%, the balance may include Fe and unavoidable impurities.

Hereinafter, the reason for limiting the alloy composition of the steel provided in the present invention as described above will be described in detail.

Meanwhile, unless otherwise specified in the present invention, the content of each element is based on the weight, and the ratio of the tissue is based on the area.

Carbon (C): 0.02~0.06%

Carbon (C) is the element that has the greatest influence on the physical properties of steel. When the content of C is less than 0.02%, there is a problem in that the component control cost during the steelmaking process is excessively generated, and the heat-affected zone of the weld is softened more than necessary. On the other hand, if the content exceeds 0.06%, the hydrogen-induced cracking resistance of the steel sheet may be reduced and weldability may be impaired. Accordingly, in the present invention, the C may be included in an amount of 0.02 to 0.06%, more preferably 0.03 to 0.05%.

Silicon (Si): 0.1-0.5%

Silicon (Si) is an element that not only serves as a deoxidizer in the steelmaking process, but also increases the strength of steel. When the content of Si exceeds 0.5%, the low-temperature toughness of the material deteriorates, the weldability is impaired, and the scale peelability during rolling is reduced. On the other hand, in order to lower the content of Si to less than 0.1%, the manufacturing cost increases. In the present invention, the content of Si may be limited to 0.1 to 0.5%, and more preferably to 0.2 to 0.4%. have.

Manganese (Mn): 0.8~1.8%

Manganese (Mn) is an element that improves hardenability of steel without impairing low-temperature toughness, and may be included in an amount of 0.8% or more. However, when the content exceeds 1.8%, central segregation occurs and low-temperature toughness is deteriorated, and there is a problem in that the hardenability of the steel is increased and weldability is impaired. In addition, Mn center segregation is a factor inducing hydrogen-induced cracking. Therefore, in the present invention, the Mn may be included in an amount of 0.8 to 1.8%. Alternatively, in terms of central segregation, the Mn may be preferably contained in an amount of 0.8 to 1.6%, more preferably in an amount of 1 to 1.4%.

Chromium (Cr): less than 0.05%

Chromium (Cr) is dissolved in austenite during reheating of the slab to increase the hardenability of the steel and contribute to securing the strength of the steel sheet. However, the present inventors have found that when Cr is added in an amount of 0.05% or more, the propagation of sulfide stress corrosion cracks can be promoted. That is, in the steel, by controlling the content of Cr to less than 0.05%, the effect of securing resistance to propagation of sulfide stress corrosion cracking is exhibited. On the other hand, the steel material according to an aspect of the present invention may contain Cr in an amount of more than 0% and less than 0.05%, more preferably 0.01% or less, and if it is possible to secure strength, Cr may not be added. have.

Phosphorus (P): 0.03% or less

Phosphorus (P) is an element that is unavoidably added to steel, and when its content exceeds 0.03%, not only weldability is remarkably reduced, but also low-temperature toughness is reduced. Therefore, it is necessary to limit the P content to 0.03% or less, and more preferably, the P content may be included to 0.01% or less in terms of securing low-temperature toughness. However, 0% may be excluded in consideration of the load during the steelmaking process.

Sulfur (S): 0.003% or less

Sulfur (S) is an element that is unavoidably added to steel, and when its content exceeds 0.003%, there is a problem of reducing the ductility, low-temperature toughness and weldability of the steel. Therefore, it is necessary to limit the content of S to 0.003% or less. On the other hand, S is combined with Mn in the steel to form MnS inclusions, and in this case, the hydrogen-induced cracking resistance of the steel is reduced, and more preferably, it may be included in an amount of 0.002% or less. However, 0% may be excluded in consideration of the load during the steelmaking process.

Aluminum (Al): 0.06% or less (excluding 0%)

Aluminum (Al) typically reacts with oxygen (O) present in molten steel to act as a deoxidizer to remove oxygen. Accordingly, the Al may be added to an extent to have sufficient deoxidation power in the steel. However, when the content exceeds 0.06%, a large amount of oxide-based inclusions are formed, which is undesirable because low-temperature toughness, hydrogen-induced cracking resistance, and sulfide stress corrosion cracking resistance of the material are impaired. Therefore, the Al may be included in an amount of 0.06% or less, and more preferably, it may be included in an amount of 0.04% or less.

Nitrogen (N): 0.01% or less

Since nitrogen (N) is industrially difficult to completely remove from steel, 0.01%, which is an allowable range in the manufacturing process, is set as the upper limit. On the other hand, N inhibits the growth of austenite grains by reacting with Al, Ti, Nb, V, etc. in steel to form a nitride, and from this, has a beneficial effect on the improvement of toughness and strength of the material, but the content is 0.01% When it is added excessively in excess, N in a solid solution state is present, which adversely affects low-temperature toughness. Therefore, the N may be limited to 0.01% or less, and 0% may be excluded in consideration of the load during the steelmaking process.

Niobium (Nb): 0.005 to 0.08%

Niobium (Nb) is dissolved during heating of the slab, suppresses the growth of austenite grains during subsequent hot rolling, and is then precipitated to improve the strength of steel. In addition, it serves to improve the strength of the steel while minimizing the increase in the yield ratio by bonding with C in the steel and precipitating it as carbide.

If the content of Nb is less than 0.005%, the above-mentioned effects cannot be sufficiently obtained, whereas if the content exceeds 0.08%, not only the austenite grains are refined more than necessary, but also low-temperature toughness and hydrogen due to the formation of coarse precipitates There is a problem in that organic cracking resistance deteriorates.

Therefore, in the present invention, the Nb may be included in an amount of 0.005 to 0.08%, and more preferably, it may be included in an amount of 0.02 to 0.05%.

Titanium (Ti): 0.005~0.05%

Titanium (Ti) is effective in inhibiting the growth of austenite grains by combining with N during heating of the slab and precipitating in the form of TiN.

When Ti is added in an amount of less than 0.005%, austenite grains become coarse and low-temperature toughness is reduced. On the other hand, even when the content exceeds 0.05%, coarse Ti-based precipitates are formed, resulting in low-temperature toughness and hydrogen-induced cracking resistance. to reduce

Therefore, in the present invention, the Ti may be included in an amount of 0.005 to 0.05%, more preferably 0.03% or less in terms of securing low-temperature toughness.

Calcium (Ca): 0.0005~0.005%

Calcium (Ca) binds with S during the steelmaking process to form CaS, thereby inhibiting segregation of MnS, which causes hydrogen-induced cracking. In order to sufficiently obtain the above-described effect of inhibiting the segregation of MnS, it is necessary to add Ca in an amount of 0.0005% or more, but when the content exceeds 0.005%, not only CaS but also CaO inclusions are formed to induce hydrogen inclusion by the inclusions. There is a problem causing cracks.

Therefore, in the present invention, the Ca may be included in an amount of 0.0005 to 0.005%, and more preferably in an amount of 0.001 to 0.003% in terms of securing hydrogen-induced cracking resistance.

In the steel material according to an aspect of the present invention, in containing Ca and S as described above, it is preferable that the component ratio (Ca/S) of Ca and S satisfies the following relational expression (1).

[Relational Expression 1]

0.5 ≤ Ca/S ≤ 5.0 (wherein each element means a weight content)

That is, the component ratio of Ca and S is an index representing the central segregation of MnS and the formation of coarse inclusions. When the value is less than 0.5, MnS is formed in the center of the thickness of the steel to reduce hydrogen-induced cracking resistance, whereas the value If it exceeds 5.0, Ca-based coarse inclusions are formed to lower the hydrogen-induced cracking resistance. Therefore, it is preferable that the component ratio of Ca and S (Ca/S) satisfies the above relational expression 1.

On the other hand, the steel of the present invention may further include elements capable of further improving physical properties in addition to the above-described alloy composition, specifically nickel (Ni): 0.05 to 0.3%, molybdenum (Mo): 0.02 to 0.2% and vanadium (V): It may further include one or more of 0.005-0.1%.

Nickel (Ni): 0.05-0.3%

Nickel (Ni) is an effective element for improving strength without deterioration of low-temperature toughness of steel. In order to obtain the effect of increasing strength without such deterioration of low-temperature toughness, Ni may be added in an amount of 0.05% or more, but the Ni is an expensive element, and when its content exceeds 0.3%, there is a problem in that the manufacturing cost is greatly increased. .

Therefore, in the present invention, when the Ni is added, it may be included in an amount of 0.05 to 0.3%, and more preferably, it may be included in an amount of 0.1 to 0.3%.

Molybdenum (Mo): 0.02~0.2%

Molybdenum (Mo) serves to improve the hardenability of steel and increase strength, similarly to Cr. In order to obtain the effect of improving the hardenability described above, Mo can be added in an amount of 0.02% or more, but when the content exceeds 0.2%, a structure vulnerable to low temperature toughness such as upper bainite is formed, and hydrogen organic There is a problem of inhibiting crack resistance and sulfide stress corrosion cracking resistance.

Therefore, in the present invention, when the Mo is added, it may be included in an amount of 0.02 to 0.2%, and more preferably, it may be included in an amount of 0.05 to 0.15%.

Vanadium (V): 0.005-0.1%

Vanadium (V) is an element that improves the strength by increasing the hardenability of the steel, and may be added in an amount of 0.005% or more to obtain this effect. However, if the content exceeds 0.1%, the hardenability of the steel is excessively increased to form a structure vulnerable to low-temperature toughness, and hydrogen-induced cracking resistance is reduced.

Therefore, in the present invention, when V is added, it may be included in an amount of 0.005 to 0.1%, more preferably in an amount of 0.005 to 0.05%.

The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. Since these impurities are known to anyone skilled in the art of manufacturing processes, all details thereof are not specifically mentioned in the present specification.

The steel material according to an aspect of the present invention having the above-described alloy composition is characterized in that the microstructure of the surface portion is composed of ferrite or a composite structure of ferrite and pearlite, whereby the Vickers hardness of the surface portion is controlled to 200 Hv or less can be

On the other hand, in the present specification, the surface portion means up to a point of 1000㎛ in the thickness direction from the surface, which may correspond to both surfaces of the steel. In addition, the central part means the remaining area except for the surface part.

In addition, in the present invention, the hardness of the surface portion represents the maximum hardness value measured from the surface to a point of thickness of 1000 μm using a Vickers hardness with a load of 1 kgf. In general, hardness can be measured about 5 times for each location.

That is, in the steel material according to the present invention, the microstructure of the surface part is composed of ferrite or the complex structure of ferrite and pearlite, and the microstructure of the central part is composed of ash ferrite, so that a soft microstructure is formed in the surface part compared to the central part. can be formed, thereby providing a steel material having a lower hardness than the conventional TMCP steel material on the surface.

Specifically, the steel material according to one aspect of the present invention has a yield strength of 450 MPa or more by securing equal or greater strength compared to the conventional TMCP steel, while remarkably lowering the hardness of the surface portion and minimizing the content of Cr during processing by sulfide The formation and propagation of stress corrosion cracks can be effectively suppressed.

On the other hand, the method for manufacturing the steel according to the present invention described above will be described in detail below.

The steel of the present invention can be manufactured through the process of [slab heating - hot rolling - cooling], and below, each process condition will be described in detail.

[Slab heating]

After preparing a steel slab that satisfies the alloy composition and component relationship proposed in the present invention, it can be heated, and at this time, it can be carried out at 1100 to 1300 ° C. for 2 hours or more.

When the heating temperature exceeds 1300° C., not only scale defects increase, but also austenite grains are coarsened to increase the hardenability of steel. In addition, there is a problem in that hydrogen-induced cracking resistance and low-temperature toughness resistance are deteriorated by increasing the fraction of tissues vulnerable to low-temperature toughness, such as upper bainite, in the center.

On the other hand, if the temperature is less than 1100 ° C or the heating time is less than 2 hours, the decarburization of the surface part is insufficient, and not only adversely affects the formation of ferrite on the surface part in a later process, but also there is a possibility that the re-dissolution rate of the alloying element may be reduced. . Therefore, in the present invention, it is preferable to heat the above-described steel slab in a temperature range of 1100 to 1300 ° C. for 2 hours or more. On the other hand, the upper limit of the slab heating time is not particularly limited, and since the component uniformity increases as the heating time increases, the higher the better, and it may be 50 hours or less, 20 hours or less, or 6 hours or less.

[Hot rolling]

The heated steel slab may be hot-rolled to manufacture a hot-rolled sheet material. At this time, the finish hot rolling may be performed at a cumulative reduction ratio of 50% or more in a temperature range of Ar3+80°C to Ar3+200°C, and may be maintained (air-cooled) for 30 seconds or more after the finish hot rolling.

When the temperature during the finish hot rolling is higher than Ar3+200°C, a structure vulnerable to low-temperature toughness such as upper bainite is formed due to an increase in hardenability due to grain growth, and there is a fear that hydrogen-induced cracking characteristics and low-temperature toughness may be reduced.

On the other hand, if the temperature is lower than Ar3+80°C, the temperature at which the subsequent cooling starts becomes too low, and there is a risk that the strength of the air-cooled ferrite is excessive due to an excessive fraction of air-cooled ferrite, and the surface portion decarburization is suppressed to prevent subsequent processes. There is a fear that it cannot contribute to the formation of ferrite on the surface of the Therefore, in the present invention, it is preferable to set the finishing temperature of the finish hot rolling to Ar3+80°C to Ar3+200°C.

At the time of finish hot rolling in the above temperature range, if the cumulative reduction ratio is less than 50%, recrystallization by rolling to the center of the steel does not occur, so that the grains in the center are coarsened, and there is a problem in that the low-temperature toughness is deteriorated. Therefore, in the present invention, it is preferable that the cumulative reduction ratio during finish hot rolling is 50% or more.

On the other hand, if the holding time after finish hot rolling is less than 30 seconds, the time for surface decarburization becomes insufficient, and since it cannot contribute to surface ferrite formation in a later process, in the present invention, the holding time after finish hot rolling is 30 seconds or more It is preferable to The upper limit of the holding time after finish hot rolling is not particularly limited, but it is preferably 30 minutes or less, or 10 minutes or less, or 5 minutes or less. The onset temperature can be ensured.

[Cooling]

It is possible to cool the hot-rolled sheet material manufactured according to the hot rolling, and in particular, in the present invention, it will be said that there is technical significance in proposing an optimal cooling process to obtain a steel material having effectively lowered the hardness of the surface portion.

Specifically, the cooling comprises the steps of primary cooling; air cooling; and secondary cooling, and each process condition will be described in more detail below.

Here, the above-mentioned primary cooling and secondary cooling may be performed by applying a specific cooling means, and water cooling may be performed as an example.

primary cooling

In the present invention, after the above-mentioned [finishing hot rolling - hold for 30 seconds or more], primary cooling may be performed. Specifically, it is preferable to start the primary cooling when the surface temperature of the hot-rolled sheet material obtained through the above-described process is Ar3-20 ℃ ~ Ar3 + 50 ℃.

When the starting temperature of the primary cooling exceeds Ar3+50°C, the phase transformation from the surface portion to ferrite during the primary cooling is not sufficiently achieved, so that the effect of reducing the hardness of the surface portion cannot be obtained. On the other hand, when the starting temperature of the primary cooling is less than Ar3-20°C, excessive ferrite transformation occurs to the center, which causes a decrease in the strength of the steel.

In addition, the primary cooling is preferably performed at a cooling rate of 5 to 40 °C / s so that the surface temperature of the hot-rolled sheet material is Ar1-50 °C ~ Ar3-50 °C.

That is, when the end temperature of the primary cooling exceeds Ar3-50 ℃, the fraction of the phase transformation into ferrite in the surface part of the primary cooled hot-rolled sheet material is low, so that the effect of reducing the hardness of the surface part cannot be sufficiently obtained. On the other hand, if the temperature is lower than Ar1-50°C, excessive ferrite phase transformation occurs to the center, making it difficult to secure the target level of strength.

In addition, if the cooling rate at the time of the primary cooling is too slow, less than 5°C/s, it is difficult to secure the above-mentioned primary cooling end temperature, whereas when it exceeds 40°C/s, the surface portion undergoes phase transformation into ash ferrite. Because of the high fraction, a soft tissue cannot be formed on the surface.

At the end of the primary cooling, the temperature of the center of the hot-rolled sheet material is preferably controlled to Ar3-30 ℃ ~ Ar3 + 30 ℃.

That is, when the temperature of the center of the hot-rolled sheet material exceeds Ar3+30℃ at the end of the primary cooling, the temperature of the surface portion cooled to a specific temperature range is increased, and the ferrite phase transformation fraction of the surface portion is lowered.

On the other hand, if the temperature of the center of the hot-rolled sheet material is less than Ar3-30°C, the temperature of the center of the hot-rolled sheet material is excessively cooled and the temperature at which the surface portion can be reheated during subsequent air cooling is lowered, so that the tempering effect cannot be obtained, which Consequently, the effect of reducing the hardness of the surface portion is reduced.

air cooling

It is preferable to air-cool the hot-rolled sheet material that has completed the primary cooling under the above-described conditions, and through the air cooling process, the effect of recuperating the surface portion by the relatively high-temperature central portion can be obtained.

The air cooling is preferably terminated when the surface temperature of the hot-rolled sheet material is in the range of Ar3-50°C to Ar3-10°C.

If the surface temperature of the hot-rolled sheet material is lower than Ar3-50 ℃ after completing the air cooling, not only the time for forming air-cooled ferrite is insufficient, but also the tempering effect by recuperating the surface portion is insufficient, which is disadvantageous in reducing the hardness of the surface portion . On the other hand, when the surface temperature of the hot-rolled sheet material exceeds Ar3-10°C after completing air cooling, the air cooling time becomes excessive and ferrite phase transformation occurs in the center, making it difficult to secure the target level of strength.

secondary cooling

It is preferable to perform secondary cooling immediately after the air cooling is completed in the above temperature range (based on the surface temperature), and the surface temperature at the end of the air cooling is the same as the starting temperature at the time of the secondary cooling.

Meanwhile, the secondary cooling is preferably performed at a cooling rate of 50 to 500° C./s so that the temperature of the surface portion becomes 300 to 600° C.

That is, if the end temperature of the secondary cooling is less than 300° C., the fraction of the MA phase increases in the center, which adversely affects the securing of low-temperature toughness and suppression of hydrogen embrittlement. On the other hand, when the end temperature of the secondary cooling exceeds 600° C., the phase transformation in the center is not completed, making it difficult to secure strength.

In addition, when the cooling rate is less than 50° C./s during secondary cooling to the above-described temperature range, it is difficult to secure the strength of the target level because the crystal grains in the center are coarsened. On the other hand, when it exceeds 500 °C / s, the fraction of the phase vulnerable to low temperature toughness such as upper bainite increases as the central microstructure, which is not preferable because the hydrogen-induced cracking resistance is deteriorated.

Meanwhile, according to one aspect of the present invention, the steel material manufactured through the above-described series of processes may have a thickness of 5 to 50 mm.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only for illustrating the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

(Example)

Steel slabs having the alloy compositions and properties of Tables 1 and 2 were prepared. At this time, the content of the following alloy composition is weight %, and the remainder includes Fe and other unavoidable impurities. The prepared steel slabs were heated, hot rolled and cooled under the conditions shown in Tables 3 and 4 to prepare each steel.

The invention steels and comparative steels shown in Tables 1 and 2 below were manufactured by the same process except that the manufacturing conditions described in Tables 3 and 4 were followed.

Specifically, the steel materials of the invention steel and comparative steel are heated to the conditions described in Table 3 by heating the slab having the composition shown in Table 1 below, rough rolling under the usual conditions, and finish hot rolling under the conditions shown in Table 3, After holding for a certain period of time, it is cooled with water. The cooling described in Table 4 is controlled by performing intermediate air cooling after primary cooling, followed by secondary cooling.

C Si Mn P S Al N Ni Cr Mo Nb Ti V Ca Invention lecture 1 0.043 0.25 1.32 0.006 0.0007 0.024 0.003 0.21 0.002 0.12 0.043 0.012 0.02 0.0018 Invention lecture 2 0.044 0.24 1.31 0.008 0.0005 0.023 0.004 0.18 0.007 0.14 0.041 0.013 0 0.0016 Invention lecture 3 0.043 0.23 1.33 0.009 0.0008 0.025 0.004 0.15 0.02 0.12 0.046 0.011 0 0.0011 Comparative lecture 1 0.11 0.25 1.44 0.008 0.0008 0.031 0.005 0.21 0.03 0.06 0.05 0.011 0.02 0.0015 Comparative lecture 2 0.036 0.24 1.55 0.008 0.0008 0.029 0.006 0 0.21 0 0.035 0.012 0.02 0.0011 Comparative lecture 3 0.037 0.22 1.22 0.006 0.001 0.038 0.004 0.16 0.19 0 0.044 0.013 0 0.0004 Comparative lecture 4 0.043 0.25 1.32 0.006 0.0007 0.024 0.003 0.21 0.002 0.12 0.043 0.012 0.02 0.0018 Comparative Steel 5 0.043 0.25 1.32 0.006 0.0007 0.024 0.003 0.21 0.002 0.12 0.043 0.012 0.02 0.0018 Comparative lecture 6 0.043 0.25 1.32 0.006 0.0007 0.024 0.003 0.21 0.002 0.12 0.043 0.012 0.02 0.0018 Comparative lecture 7 0.043 0.25 1.32 0.006 0.0007 0.024 0.003 0.21 0.002 0.12 0.043 0.012 0.02 0.0018 Comparative steel 8 0.043 0.25 1.32 0.006 0.0007 0.024 0.003 0.21 0.002 0.12 0.043 0.012 0.02 0.0018 Comparative lecture 9 0.043 0.25 1.32 0.006 0.0007 0.024 0.003 0.21 0.002 0.12 0.043 0.012 0.02 0.0018

Ca/S Ar3 (℃) Ar1 (℃) Invention lecture 1 2.6 778 717 Invention lecture 2 3.2 775 718 Invention lecture 3 1.4 776 719 Comparative lecture 1 1.9 752 715 Comparative lecture 2 1.4 780 726 Comparative lecture 3 0.4 797 722 Comparative lecture 4 2.6 777 717 Comparative Steel 5 2.6 777 717 Comparative lecture 6 2.6 777 717 Comparative lecture 7 2.6 777 717 Comparative steel 8 2.6 777 717 Comparative lecture 9 2.6 777 717

Ar3 = 910 - 310×C - 80×Mn - 20×Cu - 15×Cr - 55×Ni - 80×Mo + 0.35×(thickness[mm]-8)

Ar1 = 742 - 7.1 × C - 14.1 × Mn + 16.3 × Si + 11.5 × Cr - 49.7 × Ni

thickness (mm) slab heating finish hot rolling Holding time after finishing rolling (sec) Heating temperature (℃) Heating time (hr) Finishing temperature (℃) Cumulative rolling reduction (%) Invention lecture 1 30.5 1166 4.3 893 80 72 Invention lecture 2 21.5 1158 4 918 77 135 Invention lecture 3 19.5 1145 3.9 905 77 188 Comparative lecture 1 30.5 1129 4.3 850 75 122 Comparative lecture 2 30.5 1127 4.2 875 75 135 Comparative lecture 3 30.5 1133 3.9 895 77 138 Comparative lecture 4 30.5 1131 4.5 888 80 180 Comparative Steel 5 30.5 1132 3.7 895 77 185 Comparative lecture 6 30.5 1145 4.3 879 75 194 Comparative lecture 7 30.5 1155 3.6 834 75 171 Comparative steel 8 30.5 1050 3.1 870 75 139 Comparative lecture 9 30.5 1145 4.4 865 75 11

2nd stage
Cooling
The presence or absence Primary cooling start temperature (℃) surface
Primary
Cooling
end temperature
(℃) center
primary cooling
End
Temperature
(℃) surface
Primary
cooling rate
(℃/s) middle
after air cooling
surface temperature
(℃) surface
Secondary cooling end temperature
(℃) surface
Secondary
cooling rate
(℃/s) Invention lecture 1 O 825 710 802 22 750 466 345 Invention lecture 2 O 815 699 799 13 754 489 321 Invention lecture 3 O 822 703 799 17 748 443 245 Comparative lecture 1 X 815 492 495 245 - - - Comparative lecture 2 X 780 488 494 255 - - - Comparative lecture 3 X 823 503 495 261 - - - Comparative lecture 4 X 823 465 483 359 - - - Comparative Steel 5 O 823 611 732 25 642 455 324 Comparative lecture 6 O 820 718 789 123 760 444 359 Comparative lecture 7 O 743 616 702 21 688 466 321 Comparative steel 8 O 818 698 794 16 754 455 324 Comparative lecture 9 O 820 700 795 15 755 454 333

The yield strength, Vickers hardness of the surface portion, resistance to sulfide stress corrosion cracking, and microstructure were observed for the steels manufactured through the above-described manufacturing process, and the results are shown in Table 5 below.

At this time, yield strength means 0.5% under-load yield strength, and tensile specimens were tested after taking API-5L standard specimens in a direction perpendicular to the rolling direction.

The hardness of the steel was measured for the cross section in the thickness direction under a load of 1 kgf using a Vickers hardness tester.

Meanwhile, the microstructure was measured using an optical microscope, and the type of phase was observed using an image analyzer.

In addition, for the evaluation of sulfide stress corrosion cracking (SSC) characteristics, a 4 Point Bent Beam Test was performed according to the NACE standard test method (TM-0177), and Sol. After applying 90% of the yield strength of each steel sheet to solution A, _{and exposing it to an H 2} S environment of 10 bar for 720 hours, the occurrence of cracks was evaluated.

surface tissue core organization surface
Hardness (Hv) Yield strength (MPa) Sulfide Stress Corrosion Cracking (SSC) invention Invention lecture 1 F+P AF 172 478 non-occurring Invention lecture 2 F+P AF 183 489 non-occurring Invention lecture 3 F+P AF 178 490 non-occurring comparative steel Comparative lecture 1 UB AF + UB 284 534 Occur Comparative lecture 2 UB AF + UB 275 545 Occur Comparative lecture 3 AF AF 224 483 Occur Comparative lecture 4 AF AF 228 478 Occur Comparative Steel 5 F+P AF+F+P 175 421 non-occurring Comparative lecture 6 AF AF 224 475 Occur Comparative lecture 7 F+P F+P 175 411 Occur Comparative steel 8 F+AF AF+F 202 452 Occur Comparative lecture 9 F+AF AF+F 205 475 Occur

F: Ferrite, P: Pearlite, AF: Acicular Ferrite, UB: Upper Bainite

In Tables 1 to 4, the invention steel satisfies both the composition and manufacturing conditions of the present invention, and the comparative steel does not satisfy any one or more of the composition and manufacturing conditions of the present invention.

Specifically, Comparative Steels 1 to 3 do not satisfy both the composition and manufacturing conditions of the present invention, and in particular, do not apply the two-stage cooling method proposed in the present invention in cooling.

Meanwhile, Comparative Steels 4 to 9 use steel slabs having the same composition as Inventive Steel 1 of the present invention, but do not satisfy the manufacturing conditions of the present invention. In other words, Comparative Steel 4 does not apply the two-stage cooling method proposed in the present invention, and Comparative Steel 5 is the case where the primary cooling end temperature of the surface part and the surface part temperature after intermediate air cooling are outside the ranges suggested by the present invention. to be.

In addition, in Comparative Steel 6, the surface portion primary cooling rate is outside the range suggested in the present invention, and in Comparative Steel 7, the hot rolling finishing temperature is lower than the lower limit range suggested in the present invention, and the hot rolling finishing temperature This is a case where the primary cooling start temperature, the primary cooling end temperature of the surface part and the central part, and the surface part temperature after intermediate air cooling are all out of the range suggested by the present invention.

Comparative Steel 8 is a case in which the heating temperature of the slab is out of the lower limit range suggested by the present invention, and Comparative Steel 9 is a case in which the holding time after finish hot rolling is out of the lower limit range suggested in the present invention.

The aforementioned Comparative Steels 1 to 4 did not apply the two-stage cooling proposed in the present invention, and the microstructure of the surface portion did not form ferrite or a composite structure of ferrite and pearlite proposed in the present invention. Accordingly, in Comparative Steels 1 to 4, the surface hardness of the steel exceeded 200 Hv, and sulfide stress corrosion cracking occurred due to the high surface hardness.

Comparative Steel 5 did not perform the two-stage cooling proposed in the present invention, but the primary cooling end temperature of the surface part and the center part and the surface part temperature after intermediate air cooling were low, so that ferrite transformation occurred before secondary cooling. In Comparative Steel 5, sulfide stress corrosion cracking did not occur, but the yield strength did not satisfy the 450 MPa or higher grade, which is the range stipulated in the present invention.

In Comparative Steel 6, when the primary cooling rate of the surface portion exceeded the upper limit suggested in the present invention, ferrite could not be formed on the surface portion, resulting in sulfide stress corrosion cracking.

Comparative Steel 7 is a case in which the finishing temperature of hot rolling does not satisfy the lower limit suggested by the present invention, and the cooling temperature after hot rolling also did not satisfy the range suggested by the present invention, thereby causing ferrite transformation to the center As a result, the yield strength was insufficient.

In Comparative Steel 8, the heating temperature of the slab is outside the range suggested by the present invention, and in Comparative Steel 9, the retention time after hot rolling is outside the range suggested by the present invention. was not sufficient, and a composite structure of ferrite and ash ferrite was formed, and due to this, the effect of reducing surface hardness was not sufficiently exhibited, resulting in sulfide stress corrosion cracking.

As such, the Inventive Steels 1 to 3, which satisfy both the alloy composition and the manufacturing conditions proposed in the present invention, all had a surface hardness of 200 Hv or less, which was remarkably low in surface hardness, and was able to secure a yield strength of 450 MPa or more, and also sulfide It can be seen that the resistance to stress corrosion cracking is also excellent.

On the other hand, in Comparative Steels 1 to 9 that do not satisfy the alloy composition of the present invention or do not satisfy the manufacturing conditions of the present invention, the hardness of the steel surface part is not sufficiently low, so that sulfide stress corrosion cracking occurs, or yield strength of 450 MPa or more is secured couldn't

Meanwhile, among the above-described experimental examples, for Inventive Steel 2 and Comparative Steel 3, a photograph of the microstructure in the surface portion measured through an optical microscope and a hardness measurement value of the surface portion are shown in FIG. Specifically, in FIG. 1, the left photo shows the value measured from the surface part to 100 µm using a Vickers hardness tester, and the right photo shows the measured hardness value from the surface part to 500 µm. .

As can be seen in Figure 1, the hardness of the surface portion of the steel according to the present invention is 200 Hv or less, whereas in Comparative Steel 3 without performing two-stage cooling proposed in the present invention, it can be confirmed that the hardness of the surface portion exceeds 200 Hv.

Claims (9)

By weight%, carbon (C): 0.02 to 0.06%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 0.8 to 1.8%, chromium (Cr): less than 0.05%, phosphorus (P): 0.03% or less, sulfur (S): 0.003% or less, aluminum (Al): 0.06% or less, nitrogen (N): 0.01% or less, niobium (Nb): 0.005 to 0.08%, titanium (Ti): 0.005 to 0.05%, calcium (Ca): 0.0005 to 0.005%; Nickel (Ni): 0.05 to 0.3%, molybdenum (Mo): 0.02 to 0.2%, and vanadium (V): at least one of 0.005 to 0.1%, the balance contains Fe and unavoidable impurities,
The Ca and S satisfy the following relation 1,
In the steel, the surface microstructure is composed of ferrite, or is composed of a composite structure of ferrite and pearlite, and the central microstructure is composed of ash ferrite.
[Relational Expression 1]
0.5 ≤ Ca/S ≤ 5.0 (wherein each element means a weight content)
The method of claim 1,
The Vickers hardness of the surface portion is 200 Hv or less, steel.
The method of claim 1,
The steel material will have a yield strength of 450 MPa or more.
By weight%, carbon (C): 0.02 to 0.06%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 0.8 to 1.8%, chromium (Cr): less than 0.05%, phosphorus (P): 0.03% or less, sulfur (S): 0.003% or less, aluminum (Al): 0.06% or less, nitrogen (N): 0.01% or less, niobium (Nb): 0.005 to 0.08%, titanium (Ti): 0.005 to 0.05%, calcium (Ca): 0.0005 to 0.005%; Nickel (Ni): 0.05 to 0.3%, molybdenum (Mo): 0.02 to 0.2%, and vanadium (V): at least one of 0.005 to 0.1%, the balance contains Fe and unavoidable impurities;
The Ca and S heating the steel slab satisfying the following relation 1 at a temperature range of 1100 ~ 1300 ℃ for 2 hours or more; manufacturing a hot-rolled sheet material by finishing hot rolling the heated steel slab; and cooling after the finish hot rolling;
The cooling comprises the steps of primary cooling; air cooling; and secondary cooling,
The primary cooling is performed at a cooling rate of 5-40 °C/s so that the surface temperature of the hot-rolled sheet material is Ar1-50 °C ~ Ar3-50 °C, and the secondary cooling is the surface temperature of the hot-rolled sheet material is 300 A method of manufacturing steel, which is performed at a cooling rate of 50 to 500 °C/s so that it becomes -600 °C
[Relational Expression 1]
0.5 ≤ Ca/S ≤ 5.0 (wherein each element means a weight content)
5. The method of claim 4,
The finish hot rolling is to be performed at a cumulative reduction ratio of 50% or more in the temperature range of Ar3 + 80 ℃ ~ Ar3 + 200 ℃, the method of manufacturing a steel.
5. The method of claim 4,
After the finish hot rolling and before cooling, the method of manufacturing a steel material further comprising the step of maintaining for 30 seconds or more.
5. The method of claim 4,
The primary cooling is a method of manufacturing a steel material to be started when the surface temperature of the hot-rolled sheet material is Ar3-20 ℃ ~ Ar3 + 50 ℃.
5. The method of claim 4,
After completing the primary cooling, the core temperature of the hot-rolled sheet material is Ar3-30 ℃ ~ Ar3 + 30 ℃ method of manufacturing a steel material.
5. The method of claim 4,
After completing the air cooling, the surface temperature of the hot-rolled sheet material is Ar3-10 ℃ ~ Ar3-50 ℃ method of manufacturing a steel material.

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