KR102164110B1 - High-strength steel sheet having excellent resistance of sulfide stress crack, and method for manufacturing thereof - Google Patents

Abstract

The present invention relates to a thick steel material suitable for use such as a line pipe and a sour material, and more particularly, to a high-strength steel material excellent in resistance to sulfide stress corrosion cracking, and a manufacturing method thereof.

Description

High strength steel with excellent resistance to sulfide stress corrosion cracking and its manufacturing method {HIGH-STRENGTH STEEL SHEET HAVING EXCELLENT RESISTANCE OF SULFIDE STRESS CRACK, AND METHOD FOR MANUFACTURING THEREOF}

The present invention relates to a thick steel material suitable for use such as a line pipe and a sour material, and more particularly, to a high-strength steel material excellent in resistance to sulfide stress corrosion cracking, and a manufacturing method thereof.

Recently, there is an increasing demand for an upper limit on the surface hardness of line pipe steel.If the surface hardness of line pipe steel is high, it not only causes problems such as uneven roundness during pipe processing, but also due to the high hardness structure of the pipe surface. Cracks occur during pipe processing or problems with insufficient toughness in the use environment. In addition, when the high-hardness structure of the surface portion is used in a sour environment with a lot of hydrogen sulfide, it is highly likely to cause a brittle crack due to hydrogen and cause a large-scale 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 two weeks of operation. There is a case of replacing it with. At this time, as a result of analyzing the cause of SSC occurrence, it is estimated that the formation of a hard spot, which is a high hardness structure on the surface of the pipe, is the cause.

The API standard stipulates a hard spot with a length of 2 inches or more and an Hv of 345 or more. In the DNV standard, the size standard is the same as the API standard, but the upper limit of the hardness is stipulated as HV 250.

On the other hand, steel materials for line pipes are generally manufactured by reheating the steel slabs, hot rolling, and accelerated cooling, and as the surface portion is unevenly rapidly cooled during accelerated cooling, a hard spot (high hardness structure) is formed. ) Is determined 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 portion, and due to the difference in cooling rate, the hardness of the surface portion becomes higher than that of the center portion.

Accordingly, as a method for suppressing the formation of a high-hardness structure on the surface of the steel, it is possible to consider a method of easing the water cooling process, but the reduction of the surface hardness due to the relaxation of the water cooling simultaneously causes the strength of the steel to decrease. It causes problems such as having to add more alloying elements. In addition, the increase of these alloying elements may cause an increase in surface hardness.

An aspect of the present invention is to provide a high-strength steel material having excellent resistance to sulfide stress corrosion cracking and a method of manufacturing the same, effectively reducing the hardness of the surface portion compared to the existing thick plate water-cooled material (TMCP) from optimization of alloy composition and manufacturing conditions. Is to do.

The subject of the present invention is not limited to the above description. Anyone of ordinary skill in the art to which the present invention pertains will not have difficulty in understanding the additional subject of the present invention from the contents throughout the present specification.

One aspect of the present invention, in weight %, carbon (C): 0.02 to 0.06%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 0.8 to 1.8%, 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%, chromium (Cr): 0.05 to 0.3%, molybdenum (Mo): 0.02 to 0.2% and vanadium (V): at least one of 0.005 to 0.1%, Fe and inevitable impurities as the balance And, the Ca and S satisfy the following relational formula 1,

Provides a high-strength steel with excellent resistance to sulfide stress corrosion cracking in which the difference between surface hardness and core hardness (surface hardness-core hardness) is less than 20Hv Vickers hardness.

[Relationship 1]

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

Another aspect of the present invention, heating the steel slab satisfying the above-described alloy composition and relational equation 1 in a temperature range of 1100 ~ 1300 ℃; Manufacturing a hot-rolled sheet by finishing hot rolling the heated steel slab; And cooling after the finishing hot rolling,

The cooling is performed by primary cooling; Air cooling; And secondary cooling, wherein the primary cooling is performed at a cooling rate of 5 to 40°C/s so that the surface temperature of the hot-rolled sheet is Ar1-50°C to Ar3-50°C, and the secondary cooling is It provides a method of manufacturing a high-strength steel material having excellent resistance to sulfide stress corrosion cracking, characterized in that the surface temperature of the hot-rolled sheet is performed at a cooling rate of 50 to 500°C/s so that the surface temperature is 300 to 600°C.

According to the present invention, in providing a thick steel material having a predetermined thickness, it is possible to provide a high-strength steel material having excellent resistance to sulfide stress corrosion cracking by effectively reducing the hardness of the surface portion.

The steel material 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.

1 is a graph showing the relationship between yield strength and surface hardness of an invention steel and a comparative steel according to an embodiment of the present invention.

Currently, TMCP (Thermo-Mechanical Control Process) material supplied to the thick plate material and hot-rolled market is an inevitable phenomenon that occurs during cooling after hot rolling (a phenomenon in which the cooling speed of the surface part becomes faster than the center part). 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 such an increase in the hardness of the surface portion causes cracks during processing or impairs low-temperature toughness, as well as sour ( sour) In the case of a steel material applied to the environment, there is a problem of becoming the starting point of hydrogen embrittlement.

Accordingly, the inventors of the present invention have studied in depth a way to solve the above problems. In particular, by effectively lowering the hardness of the surface portion of a thick steel material having a certain thickness or more, it is intended to provide a steel material having high strength as well as resistance to sulfide stress corrosion cracking.

As a result, in manufacturing the thick steel material, it was confirmed that the intended steel material can be provided by deriving a method for controlling the phase transformation of the surface part and the center part by separating and optimizing and applying it. It came to completion.

Hereinafter, the present invention will be described in detail.

The high-strength steel material having excellent sulfide stress corrosion cracking resistance according to an aspect of the present invention is weight %, carbon (C): 0.02 to 0.06%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 0.8 to 1.8% , 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%, chromium (Cr): 0.05 to 0.3%, molybdenum (Mo): 0.02 to 0.2%, and vanadium (V): 0.005 to 0.1%.

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

On the other hand, unless specifically stated in the present invention, the content of each element is based on the weight, and the ratio of the structure is based on the area.

Carbon (C): 0.02~0.06%

Carbon (C) is an element that has the greatest influence on the properties of steel. When the content of C is less than 0.02%, there is a problem in that the component control cost is excessively generated during the steelmaking process, and the heat-affected zone is softened more than necessary. On the other hand, if the content exceeds 0.06%, it may reduce the resistance to hydrogen organic cracking of the steel sheet and impair weldability.

Therefore, in the present invention, the C may be included in an amount of 0.02 to 0.06%, and more advantageously, it may be included in an amount of 0.03 to 0.05%.

Silicon (Si): 0.1~0.5%

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

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, thereby deteriorating low-temperature toughness, as well as increasing hardenability of the steel and impairing weldability. In addition, Mn center segregation is a factor that causes hydrogen organic cracking.

Therefore, in the present invention, the Mn may be included in an amount of 0.8 to 1.8%, and more advantageously, it may be included in an amount of 1.0 to 1.4%.

Phosphorus (P): 0.03% or less

Phosphorus (P) is an element that is unavoidably added in steel, and when its content exceeds 0.03%, not only the weldability is significantly lowered, but also the low-temperature toughness is reduced. Therefore, it is necessary to limit the P content to 0.03% or less, and it is more preferable to limit it to 0.01% or less in terms of securing low-temperature toughness. However, 0% can 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 in 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. Meanwhile, the S is combined with Mn in the steel to form MnS inclusions, and in this case, the hydrogen-organic cracking resistance of the steel is lowered, and thus it is more preferably limited to 0.002% or less. However, 0% can be excluded in consideration of the load during the steelmaking process.

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

Aluminum (Al) usually reacts with oxygen (O) existing in molten steel to remove oxygen and acts as a deoxidizer. Therefore, the Al can be added to the extent that it can have a sufficient deoxidizing power in the steel. However, when the content exceeds 0.06%, a large amount of oxide-based inclusions are formed, which is not preferable because the low-temperature toughness of the material and the resistance to hydrogen-organic cracking are impaired.

Nitrogen (N): 0.01% or less

Since nitrogen (N) is difficult to completely remove industrially from steel, the upper limit is 0.01%, which is an allowable range in the manufacturing process. On the other hand, the N in the steel reacts with Al, Ti, Nb, V, etc. to form nitride, thereby inhibiting the growth of austenite grains, and from this, has an advantageous effect on improving the toughness and strength of the material, but its content is 0.01%. If it is added excessively in excess, N in a solid solution state exists, which adversely affects the low-temperature toughness. Accordingly, the content of 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): 0.005~0.08%

Niobium (Nb) is a solid solution when the slab is heated, suppresses the growth of austenite grains during subsequent hot rolling, and is deposited thereafter to improve the strength of the steel. In addition, it is combined with C in the steel to precipitate as carbide, thereby minimizing the increase in yield ratio and improving the strength of the steel.

If the content of Nb is less than 0.005%, the above-described effects cannot be sufficiently obtained. On the other hand, if the content exceeds 0.08%, the austenite grains are not only refined more than necessary, and the formation of coarse precipitates results in low temperature toughness and hydrogen. There is a problem of deteriorating organic crack resistance.

Therefore, in the present invention, the Nb may be included in an amount of 0.005 to 0.08%, and more advantageously, 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 bonding with N and depositing in the form of TiN when the slab is heated.

When Ti is added in an amount of less than 0.005%, the austenite grains become coarse, reducing the low-temperature toughness, whereas even when the content exceeds 0.05%, coarse Ti-based precipitates are formed, resulting in low-temperature toughness and resistance to hydrogen-organic cracking. Reduce

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

Calcium (Ca): 0.0005~0.005%

Calcium (Ca) plays a role in suppressing segregation of MnS, which causes hydrogen-organic cracking by combining with S during the steelmaking process to form CaS. In order to sufficiently obtain the above-described effect, it is necessary to add the Ca in an amount of 0.0005% or more, but when the content exceeds 0.005%, not only CaS is formed, but also CaO inclusions are formed, causing hydrogen organic cracking by the inclusions. have.

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

As described above, in containing Ca and S, it is preferable that the component ratio (Ca/S) of Ca and S satisfies the following relational expression 1.

The component ratio of Ca and S is an index representing the central segregation of MnS and the formation of coarse inclusions.If the value is less than 0.5, MnS is formed in the center of the steel thickness to reduce the resistance to hydrogen-organic cracking, whereas the value is 5.0. If exceeded, Ca-based coarse inclusions are formed, thereby reducing the resistance to hydrogen-organic cracking. Therefore, it is preferable that the component ratio (Ca/S) of Ca and S satisfies the following relational formula 1.

[Relationship 1]

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

On the other hand, the high-strength 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%, chromium (Cr): 0.05 to 0.3%, Molybdenum (Mo): 0.02 to 0.2% and vanadium (V): 0.005 to 0.1% may further include one or more.

Nickel (Ni): 0.05~0.3%

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

Chrome (Cr): 0.05~0.3%

When the slab is heated, chromium (Cr) is dissolved in austenite to improve the hardenability of steel. In order to obtain the above-described effect, Cr may be added in an amount of 0.05% or more, but when the content exceeds 0.3%, there is a problem that weldability is deteriorated.

Therefore, in the present invention, when the Cr is added, it may be included in an amount of 0.05 to 0.3%.

Molybdenum (Mo): 0.02~0.2%

Molybdenum (Mo) serves to improve the hardenability of steel and increase strength, similar to the Cr. In order to obtain the above-described effect, Mo may 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 cracking resistance is inhibited. There is a problem.

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

Vanadium (V): 0.005~0.1%

Vanadium (V) is an element that improves the strength by increasing the hardenability of the steel material, and it is necessary to add 0.005% or more for this effect. However, if the content exceeds 0.1%, the hardenability of the steel increases excessively, forming a structure vulnerable to low temperature toughness, and reducing the resistance to hydrogen-organic cracking.

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

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

In the high-strength steel material of the present invention having the above-described alloy composition, the difference between the surface layer hardness and the center hardness (surface layer hardness-center hardness) may be controlled to be less than or equal to 20Hv Vickers hardness. In this case, it may include a case where the hardness value of the surface layer is lower than the hardness value of the center.

That is, the steel material of the present invention minimizes the difference in hardness between the surface layer and the center while securing the strength equal to or greater than that of the conventional TMCP steel, and suppresses the formation and propagation of cracks during processing, thereby providing excellent resistance to sulfide stress corrosion cracking. Can have. Preferably, the steel material of the present invention may have a yield strength of 450 MPa or more.

Here, the surface layer refers to a point of 0.5 mm from the surface in the thickness direction, which may correspond to both sides of the steel material. In addition, the "center" refers to the remaining area except for the surface layer.

In the present invention, the hardness of the surface layer represents the maximum hardness value measured from the surface to a point of 0.5 mm in the thickness direction with a 1kgf load using a Vickers hardness tester, and the average hardness at the center is the average value of the hardness values measured at the point t/2. Show. Usually, hardness can be measured about 5 times for each location.

In the present invention, the microstructure of the steel material is not specifically limited, and any phase and any fraction range may be used as long as the hardness difference between the surface layer portion and the center portion is 20 Hv or less.

Specifically, the microstructure of the surface layer of the steel material may have the same or softer phase as the microstructure of the center. For example, when the microstructure of the surface layer of the steel material is composed of a composite structure of ferrite and pearlite, the center The microstructure can be composed of acyclic ferrite. However, it should be noted that it is not limited thereto.

Hereinafter, a method of manufacturing the high-strength steel material of the present invention in which the difference in hardness between the surface layer portion and the center portion is minimized as described above will be described in detail.

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

[Slab heating]

After preparing a steel slab that satisfies the alloy composition and component relationship proposed in the present invention, it may be heated, and at this time, it may be performed at 1100 to 1300°C.

When the heating temperature exceeds 1300°C, not only the scale defects increase, but also the austenite grains become coarse, thereby increasing the hardenability of the steel. In addition, there is a problem in that resistance to hydrogen-organic cracking is 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, there is a concern that the re-use rate of the alloying element is lowered.

Therefore, in the present invention, the steel slab may be heated in a temperature range of 1100 to 1300°C, and in terms of securing strength and resistance to hydrogen-organic cracking, it may be performed in a temperature range of 1150 to 1250°C.

[Hot Rolled]

The heated steel slab may be hot-rolled to produce a hot-rolled sheet, and at this time, finish hot rolling may be performed at a cumulative reduction ratio of 50% or more in a temperature range of Ar3+50°C to Ar3+250°C.

When the temperature during the finishing hot rolling is higher than Ar3+250°C, there is a problem in that a structure vulnerable to low temperature toughness such as upper bainite is formed due to an increase in hardenability due to grain growth, and thus hydrogen-organic cracking characteristics are deteriorated. On the other hand, if the temperature is lower than Ar3+50°C, the temperature at which the subsequent cooling is started becomes too low, so that the fraction of air-cooled ferrite may become excessive and the strength may decrease.

If the cumulative reduction ratio during finish hot rolling in the above-described temperature range is less than 50%, recrystallization by rolling does not occur to the center of the steel, resulting in coarsening of crystal grains at the center and deterioration of low temperature toughness.

[Cooling]

It is possible to cool the hot-rolled sheet material manufactured according to the above, and in particular, in the present invention, there will be technical significance in proposing an optimal cooling process capable of obtaining a steel material with a minimum hardness difference between the surface layer and the center.

Specifically, the cooling is performed by primary cooling; Air cooling; And secondary cooling, and each process condition will be described in detail below. Here, the primary cooling and secondary cooling may be performed by applying a specific cooling means, for example, water cooling may be applied.

Primary cooling

In the present invention, primary cooling may be performed immediately after finishing the above-described finish hot rolling, and specifically, it is preferable to start when the surface temperature of the hot-rolled sheet obtained by the finish hot rolling is Ar3-20°C to Ar3+50°C. Do.

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

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 to Ar3-50°C.

That is, when the end temperature of the first cooling exceeds Ar3-50°C, the fraction of phase transformation into ferrite in the surface of the first cooled hot-rolled sheet is low, so that the effect of reducing the hardness of the surface cannot be effectively obtained. When is lower than Ar1-50°C, ferrite phase transformation occurs excessively 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, such as less than 5°C/s, it is difficult to secure the above-described primary cooling end temperature. On the other hand, if it exceeds 40°C/s, it is harder than ferrite at the surface, such as ash. It is difficult to secure a soft structure compared to the central part because the proportion of transformation into the circular ferrite phase increases.

After completion of the primary cooling, it is preferable that the temperature of the center of the hot-rolled sheet is controlled to Ar3-30°C to Ar3+30°C.

When the temperature of the center portion exceeds Ar3+30°C after completion of the primary cooling, the temperature of the surface portion cooled to a specific temperature range increases, and the ferrite phase transformation fraction of the surface portion decreases. On the other hand, if the temperature of the center portion is less than Ar3-30°C, the center portion 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 in turn reduces the effect of reducing the hardness of the surface portion .

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, it is possible to obtain the effect of reheating the surface portion by the relatively high temperature center portion.

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

If the temperature of the surface portion after completion of the air cooling is lower than Ar3-50° C., the time for forming the air-cooled ferrite is insufficient, and the tempering effect by reheating the surface portion is insufficient, which is disadvantageous in reducing the hardness of the surface portion. On the other hand, when the temperature exceeds Ar3-10° C., the air cooling time becomes excessive and the 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 completion of the air cooling in the above-described temperature range (based on the surface temperature), and the secondary cooling is performed at a cooling rate of 50 to 500°C/s so that the temperature of the surface is 300 to 600°C. It is desirable to do.

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 securing low-temperature toughness and suppressing hydrogen embrittlement. On the other hand, when the temperature exceeds 600°C, phase transformation in the center It is not complete and it becomes difficult to secure strength.

In addition, when the cooling rate is less than 50°C/s during the secondary cooling in the above-described temperature range, crystal grains in the center become coarse, making it difficult to secure the target level of strength. On the other hand, when it exceeds 500°C/s, This is not preferable because the fraction of phases that are vulnerable to low-temperature toughness such as bainite increases, which deteriorates the resistance to hydrogen-organic cracking.

The steel material of the present invention manufactured through the series of processes described above may have a thickness of 5 to 50 mm. As described above, the steel material of the present invention has excellent resistance to hydrogen-organic cracking and resistance to sulfide stress corrosion cracking by controlling the difference in hardness between the surface layer portion and the center portion to 20 Hv or less despite the thick thickness.

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

( Example )

A steel slab having the alloy composition of Table 1 was prepared. At this time, the content of the alloy composition is% by weight, and the remainder includes Fe and inevitable impurities. The prepared steel slabs were heated, hot-rolled, and cooled under the conditions shown in Table 2 below to prepare respective steel materials.

Steel grade Alloy composition (% by weight) Relationship 1 Ar3
(℃) Ar1
(℃) C Si Mn P* S* Al N* Ni Cr Mo Nb Ti V Ca* Invention Lesson 1 0.04 0.24 1.09 60 7 0.024 30 0.21 0.18 0.08 0.043 0.012 0.02 18 2.6 798 722 Invention Lesson 2 0.038 0.25 1.25 60 9 0.023 40 0.14 0.12 0.06 0.041 0.013 0 16 1.8 788 723 Invention Lesson 3 0.042 0.23 1.22 90 8 0.025 40 0.15 0.16 0.07 0.046 0.011 0 11 1.4 789 723 Comparative Steel 1 0.11 0.25 1.44 80 8 0.031 50 0.21 0.12 0.06 0.050 0.011 0.02 15 1.9 751 716 Comparative lecture 2 0.036 0.24 2.11 80 8 0.029 60 0 0.1 0 0.035 0.012 0.02 11 1.4 737 717 Comparative lecture 3 0.037 0.22 1.22 60 10 0.038 40 0.16 0.19 0 0.044 0.013 0 4 0.4 797 722 Comparative lecture 4 0.04 0.24 1.09 60 7 0.024 30 0.21 0.18 0.08 0.043 0.012 0.02 18 2.6 798 722

(In Table 1, P*, S*, N*, and Ca* are expressed in ppm.

In addition, 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.)

Steel grade division thickness
(mm)
heating
Temperature
(℃) Finish rolling 2nd stage
Cooling
The presence or absence Primary cooling After air cooling
surface
Temperature
(℃) Secondary cooling Temperature
(℃) Reduction rate
(%) Initiate
Temperature
(℃) Cooling
speed
(℃/s) Surface
End
Temperature
(℃) center
End
Temperature
(℃) Surface
End
Temperature
(℃) Cooling
speed
(℃/s) Invention Lesson 1 Invention Example 1 30.9 1128 893 80 ○ 825 22 710 802 722 466 345 Invention Lesson 2 Inventive Example 2 19.5 1158 918 77 ○ 815 13 699 799 754 489 321 Invention Lesson 3 Invention Example 3 25.7 1145 905 77 ○ 822 17 703 799 765 443 245 Comparative Steel 1 Comparative Example 1 30.9 1129 850 75 × 803 245 492 495 - - - Comparative lecture 2 Comparative Example 2 30.9 1127 848 75 × 780 255 488 494 - - - Comparative lecture 3 Comparative Example 3 30.9 1133 895 77 × 823 261 503 495 - - - Comparative lecture 4
Comparative Example 4 30.9 1131 888 80 × 823 359 465 483 - - - Comparative Example 5 30.9 1132 895 77 ○ 825 25 611 754 642 455 324 Comparative Example 6 30.9 1145 879 75 ○ 830 123 724 789 777 454 333

Yield strength (YS), Vickers hardness at the surface and center, and resistance to sulfide stress cracking were measured for each steel material manufactured according to the above, and the microstructure was observed, and the results are shown in Table 3 below. Done.

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

The hardness of each steel position is measured with a load of 1kgf using a Vickers hardness tester. Measured. At this time, the hardness at the center was measured at the t/2 position after cutting the steel material in the thickness direction, and the hardness at the surface was measured at the steel surface.

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

In addition, the resistance to sulfide stress cracking is obtained after applying an applied stress of 90% yield strength to the specimen in a standard solution of strong acid (5% NaCl + 0.5% acetic acid) saturated with 1 bar of H ₂ S gas according to NACE TM0177. The fracture was observed within 720 hours.

division Microstructure Hardness (Hv) Yield strength
(MPa) SSC Surface center Surface center Hardness difference Invention Example 1 F+P AF 173 186 -13 464 Not occurring Invention Example 2 F+P AF 181 196 -15 489 Not occurring Invention Example 3 F+P AF 177 191 -14 481 Not occurring Comparative Example 1 UB AF+UB 284 235 49 534 Occur Comparative Example 2 UB AF+UB 275 244 31 545 Occur Comparative Example 3 AF AF 224 194 30 483 Occur Comparative Example 4 AF AF 228 191 37 478 Not occurring Comparative Example 5 F+P AF+F+P 175 181 -6 421 Not occurring Comparative Example 6 AF AF 224 198 26 475 Not occurring

(In Table 3, F represents ferrite, P represents pearlite, AF represents acyclic ferrite, and UP represents upper bainite.)

As shown in Tables 1 to 3, Inventive Examples 1 to 3, which satisfy all of the alloy composition and manufacturing conditions proposed in the present invention, have a significantly lower hardness in the surface portion than in the center, and resistance to sulfide stress corrosion cracking It can also be confirmed that the excellent (see Fig. 1).

On the other hand, the alloy composition proposed in the present invention is not satisfied, and the cooling process is also out of the conditions of the present invention, Comparative Examples 1 to 3, and the alloy composition satisfies the present invention, but the cooling process is out of the present invention. The hardness was excessively higher than that of the center, and the difference was more than 30Hv. Among these, Comparative Examples 1 to 3 were also inferior in SSC characteristics.

In Comparative Examples 5 and 6, although multi-stage cooling was applied as in the present invention, in Comparative Example 5, ferrite and pearlite were formed in the center due to the excessively low end temperature of the surface portion during the first cooling, and the yield strength was 450 MPa. It was difficult to secure the intended strength below. In Comparative Example 6, the cooling rate was excessively fast during the first cooling, so that a soft structure compared to the center portion was not formed as a matrix structure of the surface portion, so that the hardness of the surface portion was higher than the center portion by exceeding 20 Hv.

Claims (9)

In% by weight, carbon (C): 0.02 to 0.06%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 0.8 to 1.8%, phosphorus (P): 0.03% or less, sulfur (S): 0.003% Below, 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% and ; Nickel (Ni): 0.05 to 0.3%, chromium (Cr): 0.05 to 0.3%, molybdenum (Mo): 0.02 to 0.2% and vanadium (V): at least one of 0.005 to 0.1%, Fe and inevitable impurities as the balance Including,
The Ca and S satisfy the following relationship 1,
The microstructure of the surface layer is composed of a complex structure of ferrite and pearlite, and the microstructure of the central part is composed of acyclic ferrite,
High-strength steel with excellent resistance to stress corrosion cracking of sulfides with a Vickers hardness of 20Hv or less (surface hardness-core hardness) between the surface hardness value and the center hardness value.

[Relationship 1]
0.5 ≤ Ca/S ≤ 5.0 (where each element means weight content)
delete The method of claim 1,
The steel is a high-strength steel having excellent resistance to sulfide stress corrosion cracking having a yield strength of 450 MPa or more.
The method of claim 1,
The steel is a high-strength steel having excellent resistance to sulfide stress corrosion cracking having a thickness of 5 to 50mm.
In% by weight, carbon (C): 0.02 to 0.06%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 0.8 to 1.8%, phosphorus (P): 0.03% or less, sulfur (S): 0.003% Below, 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% and ; Nickel (Ni): 0.05 to 0.3%, chromium (Cr): 0.05 to 0.3%, molybdenum (Mo): 0.02 to 0.2% and vanadium (V): at least one of 0.005 to 0.1%, Fe and inevitable impurities as the balance Including, wherein Ca and S are the steps of heating a steel slab satisfying the following relational formula 1 in a temperature range of 1100 ~ 1300 ℃; Manufacturing a hot-rolled sheet by finishing hot rolling the heated steel slab; And cooling after the finishing hot rolling,
The cooling is performed by primary cooling; Air cooling; And secondary cooling,
The first cooling is performed at a cooling rate of 5 to 40°C/s so that the surface temperature of the hot rolled sheet is Ar1-50°C to Ar3-50°C, and the secondary cooling is performed at a surface temperature of the hot rolled sheet of 300 to 600 A method of manufacturing a high-strength steel material having excellent resistance to sulfide stress corrosion cracking, characterized in that it is carried out at a cooling rate of 50 to 500°C/s so that it becomes °C.

[Relationship 1]
0.5 ≤ Ca/S ≤ 5.0 (where each element means weight content)
The method of claim 5,
The finishing hot rolling is a method of manufacturing a high-strength steel material having excellent resistance to sulfide stress corrosion cracking, which is performed with a cumulative reduction ratio of 50% or more in a temperature range of Ar3+50°C to Ar3+250°C.
The method of claim 5,
The primary cooling is initiated when the surface temperature of the hot-rolled sheet is Ar3-20°C to Ar3+50°C. The method of manufacturing a high-strength steel material having excellent resistance to sulfide stress corrosion cracking.
The method of claim 5,
After completion of the primary cooling, the central temperature of the hot-rolled sheet is Ar3-30°C to Ar3+30°C. Method for producing a high-strength steel material having excellent resistance to sulfide stress corrosion cracking.
The method of claim 5,
After completion of the air cooling, the method of manufacturing a high-strength steel material having excellent resistance to sulfide stress corrosion cracking, wherein the temperature of the surface portion of the hot-rolled sheet is Ar3-10°C to Ar3-50°C.

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