KR102164097B1 - 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 method of manufacturing a high strength steel material having excellent resistance to sulfide stress corrosion cracking.

Description

Manufacturing method of high-strength steel with excellent sulfide stress corrosion cracking resistance {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 method of manufacturing a high strength steel material having excellent resistance to sulfide stress corrosion cracking.

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 2 weeks of operation, and a 200km submarine pipeline was clad pipe. There is a case of replacing it with. At this time, as a result of analyzing the cause of the occurrence of SSC, the formation of a hard spot, which is a high hardness structure on the surface of the pipe, is estimated as the cause.

The API standard stipulates hard spots with a length of 2 inches or more and a Vickers hardness of 345 Hv or more, whereas the DNV standard stipulates the size standard as the API standard, but the upper limit of the hardness is stipulated as a Vickers hardness of 250 Hv.

On the other hand, steel materials for line pipes are generally manufactured by reheating the steel slab, 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 the 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 is significantly 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, a solution to the water cooling process may be considered, but the reduction of the surface hardness due to the relaxation of the water cooling process simultaneously reduces the strength of the steel. It causes problems such as having to add more alloying elements. In addition, the increase of these alloying elements rather increases the surface hardness.

An aspect of the present invention is to provide a method of effectively reducing the hardness of the surface portion compared to the conventional water-cooled thick plate (TMCP) from optimization of alloy composition and manufacturing conditions, and thereby manufacturing a high-strength steel material having excellent resistance to sulfide stress corrosion cracking. 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 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 ℃; Rough rolling the heated steel slab to produce a bar; Cooling and reheating the bar obtained by rough rolling; Manufacturing a hot-rolled sheet by finishing hot-rolling the cooled and reheated bar; And cooling after the finishing hot rolling,

It provides a method of manufacturing a high-strength steel material having excellent resistance to sulfide stress corrosion cracking, wherein the bar is cooled to Ar3 or less, and the reheating is performed so that the temperature of the bar is in the austenite single-phase region. .

[Relationship 1]

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

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

The high-strength steel produced by 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, as well as resistance to sulfide stress corrosion cracking, it is intended to provide a means to manufacture a steel material having high strength.

As a result, in manufacturing the thick steel material, it was confirmed that the intended steel material can be manufactured by deriving a method that can control the phase transformation of the surface part and the center part by separating it, and applying it by optimizing 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% of steel slabs containing at least one of [ It can be manufactured through the process of heating-rough rolling-finishing hot rolling-cooling].

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

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 welding heat affected zone (HAZ) 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 the low-temperature toughness, as well as increasing the hardenability of the steel and impairing the weldability. In addition, the central segregation of Mn becomes a factor causing hydrogen organic cracking.

Therefore, in the present invention, the Mn may be included in an amount of 0.8 to 1.8%, and it is advantageous to include it in an amount of 0.8 to 1.6% in terms of controlling central segregation.

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. On the other hand, the S is combined with Mn in the steel to form MnS inclusions, thereby reducing the resistance to hydrogen-organic cracking of the steel, and 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 nitrides, thereby inhibiting the growth of austenite grains, and thereby 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 of, N in a solid solution state is present, 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 solid solution when the slab is heated, suppresses the growth of austenite grains during subsequent hot rolling, and then precipitates, thereby playing an effective role in improving 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.

Hereinafter, the object to be achieved in the present invention, that is, a method of manufacturing a steel material in which the difference in hardness between the surface layer portion and the center portion is minimized 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 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.

[Cooling and reheating of coarse rolled bar]

It is preferable to roughly roll the heated steel slab according to the above to produce a bar under normal conditions, and then perform a process of cooling and reheating the bar.

In the present invention, before the bar is finished hot-rolled to produce a hot-rolled sheet, it is attempted to refine the austenite grains on the surface of the steel by cooling to a specific temperature and reheating. For this reason, it is possible to effectively lower the hardenability of the surface portion of the steel during the final cooling (referred to as the cooling step after hot rolling), and the effect of reducing the hardness of the surface portion of the final steel can be remarkably obtained.

Specifically, in order to refine the austenite grains on the surface of the steel through the cooling and reheating, it is necessary to cool only the surface portion under conditions capable of selectively generating transformation-reverse transformation, and preferably, the surface portion temperature is Cooling can be performed at least once or more, regardless of the cooling means, until Ar3 or less. More specifically, the cooling may be performed up to a temperature range in which the surface portion is transformed into ferrite .

As mentioned above, the cooling means is not particularly limited, but water cooling may be performed as an example.

As described above, after cooling the surface portion to Ar3 or less, reheating occurs at the surface portion by the center with a relatively high temperature, and the reheating is a temperature range in which ferrite transformed by cooling is reversely transformed into austenite single phase. Bar, the temperature range is not particularly limited.

[Finish hot rolling]

According to the above, the cooled and reheated bar can be finished hot-rolled to produce a hot-rolled sheet material, and at this time, finish hot rolling can be performed with a cumulative reduction ratio of 50% or more in the temperature range of Ar3+50℃~Ar3+250℃. I can.

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]

The hot-rolled sheet manufactured according to the above may be cooled, and it is preferable to start when the average temperature in the thickness direction of the hot-rolled sheet or the temperature at the point t/4 in the thickness direction is Ar3-50°C to Ar3+50°C.

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

In addition, the cooling is preferably performed at a cooling rate of 20 to 100°C/s so as to be 300 to 650°C.

The temperature at which the cooling is terminated can be based on the average temperature in the thickness direction or the temperature at the point t/4 in the thickness direction. On the other hand, if the temperature exceeds 650°C, the phase transformation in the center cannot be completed, making it difficult to secure strength.

And, when cooling to the above-described temperature range, if the cooling rate is less than 20°C/s, crystal grains become coarse, making it difficult to secure the strength of the target level. On the other hand, when it exceeds 100°C/s, it is difficult to obtain a low temperature like upper bainite. This is not preferable because the fraction of the phase that is vulnerable to toughness increases and deteriorates the resistance to hydrogen-organic cracking.

The high-strength steel manufactured by the above-described alloy composition and manufacturing process may have a thickness of 5 to 50 mm, and even if it has a certain thickness, the difference between the surface layer hardness and the center hardness (surface layer hardness-center hardness) is less than 20 Hv of Vickers hardness. Can be controlled by 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 present invention can obtain a high-strength steel material that 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 of such high-strength steel It can have excellent resistance to organic cracking and resistance to sulfide stress corrosion cracking. 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 high-strength steel manufactured by the above-described bar is not specifically limited, and any phase and any fraction range may be used as long as the hardness difference between the surface layer and the center is 20 Hv or less.

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

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. At this time, rough rolling was performed on the heated steel slab under normal conditions to produce a bar, and then hot rolling was performed after cooling the bar for some steel types, and the hot rolling It was performed after the cooled bar was reheated to the austenite single-phase region.

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 Comparative Steel 1 0.11 0.25 1.44 80 8 0.031 50 0.21 0.12 0.06 0.05 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
(℃) Bar cooling Finish hot rolled Cooling Water cooling
Count End temperature
(℃) Temperature
(℃) Reduction rate
(%) Starting temperature
(℃) End temperature
(℃) speed
(℃/s) Invention Lecture 1 Invention Example 1 30.9 1128 2 755 888 75 822 471 26 Invention Lecture 2 Invention Example 2 19.5 1129 2 742 920 75 824 448 42 Comparative lecture 1 Comparative Example 1 30.9 1134 0 - 845 75 785 456 24 Comparative lecture 2 Comparative Example 2 30.9 1138 0 - 822 75 770 442 30 Comparative lecture 3 Comparative Example 3 30.9 1124 2 744 899 75 826 486 26 Comparative lecture 4
Comparative Example 4 30.9 1136 0 - 892 75 829 477 24 Comparative Example 5 30.9 1129 0 - 879 75 822 475 28

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 188 192 -4 488 Not occurring Invention Example 2 F+P AF 186 194 -8 478 Not occurring Comparative Example 1 UB AF+UB 289 256 33 544 Occur Comparative Example 2 UB AF+UB 286 254 32 549 Occur Comparative Example 3 F+P AF 188 193 -5 477 Occur Comparative Example 4 AF AF 221 195 26 493 Not occurring Comparative Example 5 AF AF 219 192 27 491 Not occurring

(In Table 3, F denotes ferrite, P denotes pearlite, AF denotes acyclic ferrite, and UP denotes Upper Bainite.)

As shown in Tables 1 to 3, Inventive Examples 1 and 2, which satisfy all of the alloy composition and manufacturing conditions proposed in the present invention, have a significantly lower surface hardness compared to the center, and resistance to sulfide stress corrosion cracking It can also be confirmed that it is excellent (see FIG. 1).

On the other hand, in Comparative Examples 1 and 2, which did not satisfy the alloy composition proposed in the present invention, and the manufacturing process was also out of the conditions of the present invention, the hardness of the surface portion was excessively higher than that of the center portion, and the difference exceeded 30Hv, and the SSC characteristic It was also inferior.

Comparative Example 3 was able to obtain the effect of reducing the hardness of the surface portion as it was prepared by the manufacturing process proposed in the present invention, but the SSC characteristics were inferior as the content of Ca and the component ratio of Ca/S in the alloy composition deviated from the present invention. .

In Comparative Examples 4 and 5, the alloy composition satisfies the present invention, but the manufacturing process, in particular, when the rough-rolled bar was not cooled, the hardness of the surface portion was excessively higher than that of the center portion, and the difference was 20 Hv. Exceeded.

Claims (7)

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 ℃;
Rough rolling the heated steel slab to produce a bar;
Cooling and reheating the bar obtained by rough rolling;
Manufacturing a hot-rolled sheet by finishing hot rolling the cooled and reheated bar at a temperature range of Ar3+50°C to Ar3+250°C; And
Comprising the step of cooling immediately after the finish hot rolling,
The method of manufacturing a high-strength steel material having excellent resistance to sulfide stress corrosion cracking, wherein the bar is cooled to Ar3 or less, and the reheating is performed so that the temperature of the bar is in the austenite single-phase region.

[Relationship 1]
0.5 ≤ Ca/S ≤ 5.0 (where each element means weight content)
The method of claim 1,
The method of manufacturing a high-strength steel material having excellent resistance to sulfide stress corrosion cracking, wherein the bar is cooled by water cooling at least once or more.
The method of claim 1,
The finish 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.
The method of claim 1,
The step of cooling after the finish hot rolling is performed up to 300 to 650°C at a cooling rate of 20 to 100°C/s. Method for producing a high-strength steel material having excellent resistance to sulfide stress corrosion cracking.
The method of claim 4,
The cooling is initiated at Ar3-50 ℃ ~ Ar3 + 50 ℃ sulfide stress corrosion cracking resistance is excellent high-strength steel manufacturing method.
The method of claim 1,
The high-strength steel is a method of manufacturing a high-strength steel having excellent resistance to sulfide stress corrosion cracking, characterized in that the difference between the hardness of the surface layer and the hardness of the center (surface hardness-center hardness) is less than or equal to 20Hv.
The method of claim 1,
The high-strength steel has a yield strength of 450 MPa or more, and has a thickness of 5 to 50 mm. A method of manufacturing a high-strength steel having excellent resistance to sulfide stress corrosion cracking.

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