KR20200047082A - 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 sheet suitable for a line pipe, a sour-resistant material and the like, and more specifically, to a method for manufacturing a high-strength steel sheet having excellent resistance of sulfide stress crack. The manufacturing method of the present invention includes a step of heating a steel slab in which Ca and S satisfy the following equation 1, 0.5 <= Ca/S <= 5.0, in a temperature range of 1,100 to 1,300 °C; a step of preparing a hot rolled sheet by finishing hot rolling the heated steel slab; and a step of cooling after the finish hot rolling, wherein the cooling includes a primary cooling step and a secondary cooling step, the primary cooling is performed at a cooling rate of 5 to 40 °C/s so that the surface temperature of the holt rolled sheet becomes Ar1-150 to Ar1-50 °C, and the secondary cooling is performed at a cooling rate of 50 to 500 °C/s so that the surface temperature of the hot rolled sheet becomes 300 to 600 °C.

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 as a line pipe, sour material, and more particularly, to a method of manufacturing a high strength steel material having excellent sulfide stress corrosion cracking resistance.

Recently, there is an increasing demand for an upper limit on the surface hardness of line pipe steel materials. When the surface hardness of line pipe steel materials is high, not only causes problems such as unevenness in roundness during pipe processing, but also due to the high hardness structure of the pipe surface. When the pipe is processed, cracks occur or a problem of insufficient toughness in the use environment occurs. In addition, when the high-hardness structure of the surface portion is used in a sour environment with a large amount of hydrogen sulfide, it is highly likely to cause a brittle crack by hydrogen and cause a large accident.

In 2013, during a large 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, cladding a 200km subsea pipeline. There is a case of replacing with. At this time, as a result of analyzing the cause of the occurrence of the SSC, it is assumed that the hard spot, a high hardness structure of the pipe surface portion, is formed.

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

On the other hand, the steel material for line pipes is generally manufactured by reheating a steel slab, performing hot rolling and performing accelerated cooling, and a hard spot (high hardness structure) is formed as the surface portion is rapidly quenched during accelerated cooling. ).

Since the steel plate manufactured by normal water cooling is sprayed with water on the surface of the steel plate, the cooling rate of the surface portion is faster than that of the central portion, and the difference in the cooling rate makes the hardness of the surface portion higher than that of the central portion.

Thus, as a method for suppressing the formation of a high-hardness structure at the surface of the steel material, a method of alleviating the water cooling process may be considered, but the reduction of the surface hardness due to the water cooling relaxation simultaneously reduces the strength of the steel material. It causes problems such as the need to add alloy elements. In addition, the increase of these alloying elements may cause the surface hardness to increase.

One aspect of the present invention, to effectively reduce the hardness of the surface portion compared to the existing thick plate water cooling material (TMCP) from the alloy composition and optimization of manufacturing conditions, thereby providing a method for manufacturing a high strength steel material having excellent sulfide stress corrosion crack resistance Is to do.

The subject of this invention is not limited to the above-mentioned content. Anyone having ordinary knowledge in the technical field to which the present invention pertains will have no difficulty in understanding additional problems of the present invention from the contents throughout the present specification.

One aspect of the present invention, by weight, carbon (C): 0.02 ~ 0.06%, silicon (Si): 0.1 ~ 0.5%, manganese (Mn): 0.8 ~ 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%, at least one of Fe and unavoidable impurities Including, Ca and S heating a steel slab satisfying the following relationship 1 in the temperature range of 1100 ~ 1300 ℃; Preparing a hot rolled sheet material by finishing hot rolling the heated steel slab; And cooling after the finish hot rolling, wherein the cooling includes a primary cooling step and a secondary cooling step,

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 material is Ar1-150 ° C to Ar1-50 ° C, and the secondary cooling has a surface temperature of 300 to 600 ° C. Provided is a method for producing a high strength steel material having excellent sulfide stress corrosion cracking resistance, characterized in that it is performed at a cooling rate of 50 to 500 ° C / s so as to be ° C.

[Relationship 1]

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

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

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 a relationship between yield strength and surface hardness of an invention steel and a comparative steel in an embodiment of the present invention.

Currently, the TMCP (Thermo-Mechanical Control Process) material that is supplied to the heavy plate material and the hot rolling market is caused by the inevitable phenomenon that occurs during cooling after hot rolling (the cooling rate of the surface part becomes faster than the center part), and the hardness of the surface part Has higher characteristics than the center. Due to this, as the strength of the material increases, the hardness at the surface portion becomes significantly higher than the center portion, and such an increase in the hardness of the surface portion not only causes cracking during processing or inhibits low-temperature toughness, but also sour ( sour) In the case of steel applied to the environment, there is a problem that becomes the starting point of hydrogen embrittlement.

Accordingly, the inventors of the present invention have studied in depth a solution to the above problems. Particularly, it was intended to provide a means for manufacturing a steel material having a high strength as well as resistance to sulfide stress corrosion cracking by effectively lowering the hardness of the surface portion in a thick steel material having a certain thickness or more.

As a result, in manufacturing the thick steel material, a method capable of separating and controlling the phase transformation of the surface part and the center part is drawn, and it is confirmed that the intended steel material can be manufactured by optimizing and applying the present invention. Came to completion.

Hereinafter, the present invention will be described in detail.

A high strength steel material having excellent sulfide stress corrosion cracking resistance according to an aspect of the present invention is in weight%, carbon (C): 0.02 to 0.06%, silicon (Si): 0.1 to 0.5%, and 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 ~ 0.3%, chromium (Cr): 0.05 ~ 0.3%, molybdenum (Mo): 0.02 ~ 0.2% and vanadium (V): 0.005 ~ 0.1% of the steel slab containing one or more of the slab [ Heating-hot rolling-cooling].

First, the reason for limiting the alloy composition of the steel material provided in 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 weight, and the proportion of tissue is based on area.

Carbon (C): 0.02 ~ 0.06%

Carbon (C) is an element that has the greatest influence on the properties of steel. If the content of C is less than 0.02%, there is a problem in that the cost of controlling the components during the steelmaking process is excessive, and the welding heat-affected zone (HAZ) softens more than necessary. On the other hand, if the content exceeds 0.06%, the hydrogen organic crack resistance of the steel sheet may be reduced and weldability may be impaired.

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

Silicon (Si): 0.1 ~ 0.5%

Silicon (Si) is not only used as a deoxidizing agent in the steelmaking process, but also an element that serves to increase the strength of the steel. When the Si content exceeds 0.5%, the low-temperature toughness of the material deteriorates, inhibits weldability, and degrades the scale peelability during rolling. 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 can be limited to 0.1 to 0.5%.

Manganese (Mn): 0.8-1.8%

Manganese (Mn) is an element that improves the quenching property of steel without inhibiting low-temperature toughness, and may be included in 0.8% or more. However, when the content exceeds 1.8%, central segregation occurs, and the low-temperature toughness is deteriorated, as well as the hardenability of the steel and the weldability is impaired. In addition, the central segregation of Mn is 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 more advantageously, in an amount of 1.0 to 1.4%.

Phosphorus (P): 0.03% or less

Phosphorus (P) is an element that is inevitably added in steel, and when its content exceeds 0.03%, there is a problem that not only the weldability is significantly lowered, but also the low-temperature toughness is reduced. Therefore, it is necessary to limit the content of P to 0.03% or less, and in terms of securing low-temperature toughness, it is more preferable to limit it to 0.01% or less. However, 0% can be excluded considering the load during the steelmaking process.

Sulfur (S): 0.003% or less

Sulfur (S) is an element that is inevitably added to steel, and when its content exceeds 0.003%, there is a problem of reducing the ductility, low-temperature toughness, and weldability of 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 an MnS inclusion, thereby reducing the hydrogen organic crack resistance of the steel, it is more preferable to limit to 0.002% or less. However, 0% can be excluded considering the load during the steelmaking process.

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

Aluminum (Al) usually acts as a deoxidizer to remove oxygen by reacting with oxygen (O) present in molten steel. 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%, it is not preferable because a large amount of oxide-based inclusions is formed, thereby inhibiting low-temperature toughness and hydrogen-organic crack resistance of the material.

Nitrogen (N): 0.01% or less

Nitrogen (N) is difficult to industrially completely remove from the steel, so the upper limit is 0.01%, which is an allowable range in the manufacturing process. On the other hand, the N inhibits the growth of austenite grains by reacting Al, Ti, Nb, V, etc. in steel to form a nitride, from which it has a favorable effect on the toughness and strength of the material, but its content is 0.01%. If it is excessively added in excess, solid solution N is present, which adversely affects low-temperature toughness. Therefore, 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 to 0.08%

Niobium (Nb) is employed during slab heating to suppress the growth of austenite grains during subsequent hot rolling, and then precipitate to play an effective role in improving the strength of the steel. In addition, it combines with C in the steel to precipitate as a carbide, thereby minimizing the increase in yield ratio, while improving the strength of the steel.

When the content of Nb is less than 0.005%, the above-described effect cannot be sufficiently obtained, whereas when 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 that the organic crack resistance deteriorates.

Therefore, in the present invention, the Nb may be included as 0.005 to 0.08%, and more advantageously as 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 TiN in the form of slab heating.

When Ti is added in an amount of less than 0.005%, austenite grains become coarse to reduce low-temperature toughness, whereas coarse Ti-based precipitates are formed even when their content exceeds 0.05%, resulting in low-temperature toughness and hydrogen organic crack resistance. 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 0.03% or less.

Calcium (Ca): 0.0005 ~ 0.005%

Calcium (Ca) combines with S during the steelmaking process to form CaS, thereby inhibiting segregation of MnS that causes hydrogen organic cracking. 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%, there is a problem of forming CaS inclusions as well as formation of CaS and causing hydrogen organic cracking by the inclusions. have.

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

As described above, when 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. When the value is less than 0.5, MnS is formed in the center of the steel thickness to reduce hydrogen organic crack resistance, while its value is 5.0. If it exceeds, a Ca-based coarse inclusion is formed to degrade hydrogen organic crack resistance. Therefore, it is preferable that the component ratio (Ca / S) of Ca and S satisfies the following relational expression 1.

[Relationship 1]

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

On the other hand, the high-strength steel of the present invention may further include elements that can further improve the physical properties in addition to the above-described alloy composition, specifically nickel (Ni): 0.05 ~ 0.3%, chromium (Cr): 0.05 ~ 0.3%, Molybdenum (Mo): 0.02 ~ 0.2% and vanadium (V): may further include at least one of 0.005 ~ 0.1%.

Nickel (Ni): 0.05 ~ 0.3%

Nickel (Ni) is an effective element for improving 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 Ni is an expensive element, and when its content exceeds 0.3%, there is a problem in that manufacturing cost increases significantly.

Therefore, in the present invention, when Ni is added, it may be included at 0.05 to 0.3%.

Chromium (Cr): 0.05 ~ 0.3%

Chromium (Cr) is employed in austenite during slab heating to improve the quenching properties of steel. Cr may be added in an amount of 0.05% or more in order to obtain the above-described effect, but if the content exceeds 0.3%, there is a problem that weldability is deteriorated.

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

Molybdenum (Mo): 0.02 ~ 0.2%

Molybdenum (Mo), similar to the Cr, improves the quenching properties of the steel and serves to increase the strength. In order to obtain the above-described effect, Mo may be added at 0.02% or more, but when the content exceeds 0.2%, a tissue vulnerable to low-temperature toughness such as upper bainite is formed, and hydrogen organic crack resistance is inhibited. There is a problem.

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

Vanadium (V): 0.005 ~ 0.1%

Vanadium (V) is an element that improves the strength by increasing the quenching properties of steel materials, and for this effect, it is necessary to add at least 0.005%. However, when the content exceeds 0.1%, the hardenability of the steel is excessively increased to form a structure vulnerable to low-temperature toughness, and the resistance to hydrogen-organic cracking is reduced.

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

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

Hereinafter, the object to be achieved in the present invention will be described in detail with respect to a method of manufacturing a steel material having a small difference in hardness between the surface layer portion and the center portion.

[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 performed at 1100 to 1300 ° C.

When the temperature exceeds 1300 ° C during heating, not only does the scale defect increase, but there is a fear that the austenite grains become coarse and increase the quenching properties of the steel. In addition, there is a problem in that hydrogen-organic crack resistance 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 fear that the re-use rate of the alloying element is lowered.

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

[Hot rolling]

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

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

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

[Cooling]

It is possible to cool the hot-rolled sheet material manufactured according to the above, and in the present invention, there will be technical significance to propose an optimal cooling process for obtaining a steel material having a small difference in hardness between the surface layer portion and the center portion.

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

1st cooling

In the present invention, the primary cooling can be performed immediately after the above-mentioned finish hot rolling is completed, and specifically, when the surface temperature of the hot-rolled sheet material obtained by the hot rolling is finished is Ar3-20 ° C to Ar3 + 50 ° C. desirable.

When the starting temperature of the primary cooling exceeds Ar3 + 50 ° C, phase transformation from the surface portion to ferrite is not sufficiently achieved during the primary 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 Ar3-20 ° C, excessive ferrite transformation to the center occurs, 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 Ar1-150 ° C to Ar1-50 ° C.

That is, when the end temperature of the primary cooling exceeds Ar1-50 ° C, the fraction of phase transformation from the surface portion of the primary cooled steel to ferrite is low, so that the effect of reducing the hardness of the surface portion cannot be effectively obtained, whereas the temperature is When it is lower than Ar1-150 ° C, ferrite phase transformation occurs excessively to the center, making it difficult to secure a target level of strength.

In addition, when the cooling rate during the primary cooling is too slow to be 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 is harder than ferrite, such as ash. The proportion of transformation into a circular ferrite phase increases, making it difficult to secure a soft structure compared to the central portion.

On the other hand, after completing the primary cooling, it is preferable that the central temperature of the hot-rolled sheet material is controlled to Ar3-50 ° C to Ar3 + 10 ° C.

After the primary cooling is completed, when the temperature of the central portion exceeds Ar3 + 10 ° C, the primary cooling termination temperature of the surface portion rises and the ferrite phase transformation fraction of the surface portion decreases. On the other hand, when the temperature of the central portion is less than Ar3-50 ° C, the central portion is excessively cooled, so that the tempering effect of the surface portion by the relatively high temperature central portion cannot be obtained, which in turn lowers the effect of reducing the hardness of the surface portion.

Secondary cooling

It is preferable to perform the secondary cooling immediately after completing the above-described primary cooling, and the secondary cooling is preferably performed at a cooling rate of 50 to 500 ° C / s so that the temperature of the surface portion is 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 in the center increases, which adversely affects low temperature toughness and suppresses hydrogen embrittlement, whereas when the temperature exceeds 600 ° C, phase transformation in the center occurs Since it cannot be completed, it becomes difficult to secure strength.

In addition, when the cooling rate is less than 50 ° C / s during the second cooling to the above-described temperature range, it is difficult to secure a target level of strength due to coarsening of the crystal grains in the center, whereas when it exceeds 500 ° C / s, the upper part is formed into the central microstructure. The fraction of phases vulnerable to low-temperature toughness, such as bainite, increases, which degrades hydrogen-organic cracking resistance, which is undesirable.

The high-strength steel produced 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 Vickers hardness 20 Hv or less the word may be a. At this time, it may also include a case where the hardness value of the surface layer portion is lower than the hardness value of the center portion.

That is, the present invention can obtain a high-strength steel material that minimizes the difference in hardness between the surface layer portion and the center portion while securing the strength compared to the conventional TMCP steel material at an equal or higher level, and the formation and propagation of cracks during processing of the high-strength steel material are suppressed, and hydrogen is suppressed. It can have excellent resistance to organic cracking and sulfide stress corrosion cracking resistance. Preferably, the steel material of the present invention may have a yield strength of 450 MPa or more.

Here, the surface layer part means up to a point in the thickness direction of 0.5 mm from the surface, which may correspond to both sides of the steel material. In addition, the center means the rest of the area except the surface layer.

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

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

Specifically, the microstructure of the surface layer portion may have the same or a softer phase than the central microstructure, for example, when the microstructure of the surface layer portion of the steel is composed of a composite structure of ferrite and pearlite, the core microstructure The tissue may be composed of acyclic ferrite. However, it should be noted that the present invention is not limited to this.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate 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 items described in the claims and the items reasonably inferred therefrom.

( Example )

A steel slab having an alloy composition of Table 1 below was prepared. At this time, the content of the alloy composition is weight%, and the rest includes Fe and unavoidable impurities. The prepared steel slabs were manufactured under the conditions shown in Table 2, respectively, through heating, hot rolling and cooling.

Steel Alloy composition (% by weight) Relation 1 Ar3
(℃) Ar1
(℃) C Si Mn P * S * Al N * Ni Cr Mo Nb Ti V Ca * invent
River 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 invent
River 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 invent
River 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 compare
River 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 compare
River 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 compare
River 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 compare
River 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 x Mn + 16.3 x Si + 11.5 x Cr-49.7 x Ni].)

Steel division thickness
(mm)
heating
Temperature
(℃) Finish rolling 2-stage
Cooling
The presence or absence 1st cooling Secondary cooling Temperature
(℃) Rolling reduction
(%) Initiate
Temperature
(℃) Cooling
speed
(℃ / s) Surface
End
Temperature
(℃) center
End
Temperature
(℃) Surface
End
Temperature
(℃) Cooling
speed
(℃ / s) Invention Steel 1 Inventive Example 1 30.9 1139 888 77 ○ 820 18 584 795 471 288 Invention Steel 2 Inventive Example 2 19.5 1148 920 75 ○ 811 14 595 780 448 274 Invention Steel 3 Inventive Example 3 25.7 1142 911 77 ○ 818 16 587 788 452 249 Comparative Steel 1 Comparative Example 1 30.9 1142 845 75 × 789 245 456 495 - - Comparative Steel 2 Comparative Example 2 30.9 1138 822 75 × 765 255 489 494 - - Comparative Steel 3 Comparative Example 3 30.9 1121 899 77 × 822 261 499 495 - - Comparative steel 4
Comparative Example 4 30.9 1148 892 77 × 823 281 465 483 - - Comparative Example 5 30.9 1125 867 77 ○ 821 25 711 780 467 281 Comparative Example 6 30.9 1129 879 75 ○ 826 77 494 689 475 276

The yield strength (YS), the Vickers hardness at the surface and the center, and the resistance to sulfide stress cracks were measured for each steel material prepared according to the above, microstructures were observed, and the results are shown in Table 3 below. Did.

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

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

Then, the resistance to sulfide stress cracking was applied 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 regulations. It was observed whether the fracture occurred within 720 hours.

division Microstructure Hardness (Hv) Yield strength
(MPa) SSC Surface center Surface center Hardness difference Inventive Example 1 F + P AF 178 186 -8 477 Not occurring Inventive Example 2 F + P AF 182 192 -10 480 Not occurring Inventive Example 3 F + P AF 176 190 -14 468 Not occurring Comparative Example 1 UB AF + UB 284 255 29 544 Occur Comparative Example 2 UB AF + UB 280 245 35 555 Occur Comparative Example 3 AF AF 216 192 24 483 Occur Comparative Example 4 AF AF 222 194 28 486 Not occurring Comparative Example 5 AF + F AF 212 191 21 479 Not occurring Comparative Example 6 F + P + AF AF + F + P 172 178 -6 421 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 satisfying both the alloy composition and manufacturing conditions proposed in the present invention can be confirmed that the hardness of the surface portion is significantly lower than the center portion, and resistance to sulfide stress corrosion cracking It can also be confirmed that it is excellent (see Fig. 1).

On the other hand, Comparative Examples 1 to 3 that do not satisfy the alloy composition proposed in the present invention, and the cooling process also deviates from the conditions of the present invention, and Comparative Example 4 in which the alloy composition satisfies the present invention but the cooling process deviates from the present invention The hardness was excessively higher than the center, and the difference exceeded 20Hv. Among them, Comparative Examples 1 to 3 also had poor SSC characteristics.

In Comparative Examples 5 and 6, although multi-stage cooling was applied as in the present invention, Comparative Example 5 in which the end temperature of the surface portion was excessively high during primary cooling, so that the ferrite phase, which is a soft tissue compared to the center portion, was not sufficiently formed in the surface portion. The hardness of the surface was higher than that of the center. In Comparative Example 6, the cooling rate was excessive during the primary cooling, and the termination temperature of the surface portion was excessively low, so that the central termination temperature was also low. As a result, ferrite and pearlite were formed in the central region, so that the yield strength was less than 450 MPa. It was difficult.

Claims (6)

In weight percent, 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% ; 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%, at least one of Fe and unavoidable impurities Including, Ca and S heating a steel slab satisfying the following relationship 1 in the temperature range of 1100 ~ 1300 ℃; Preparing a hot rolled sheet material by finishing hot rolling the heated steel slab; And cooling after the finish hot rolling,
The cooling includes a primary cooling step and a secondary cooling step,
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 material is Ar1-150 ° C to Ar1-50 ° C, and the secondary cooling has a surface temperature of 300 to 600 ° C. Method for producing a high-strength steel excellent in sulfide stress corrosion cracking resistance, characterized in that it is performed at a cooling rate of 50 to 500 ° C / s so as to be ° C.

[Relationship 1]
0.5 ≤ Ca / S ≤ 5.0 (where each element represents the weight content)
According to claim 1,
The finishing hot rolling is a method of manufacturing a high strength steel with excellent sulfide stress corrosion cracking resistance, which is performed at a cumulative rolling reduction rate of 50% or more in a temperature range of Ar3 + 50 ° C to Ar3 + 250 ° C.
According to claim 1,
The primary cooling is a method of manufacturing a high strength steel material having excellent sulfide stress corrosion cracking resistance, which is initiated when the surface temperature of the hot rolled sheet is Ar3-20 ° C to Ar3 + 50 ° C.
According to claim 1,
A method of manufacturing a high strength steel material having excellent sulfide stress corrosion cracking resistance, characterized in that the central temperature of the hot rolled sheet material is Ar3-50 ° C to Ar3 + 10 ° C after completing the primary cooling.
According to claim 1,
The high-strength steel is a method of manufacturing a high-strength steel with excellent sulfide stress corrosion cracking resistance, characterized in that the difference between the surface layer hardness and the center hardness (surface layer hardness-center hardness) is Vickers hardness of 20 Hv or less.
According to claim 1,
The high-strength steel material has a yield strength of 450 MPa or more, and has a thickness of 5 to 50 mm.

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