Steel material with excellent resistance to sulfide stress corrosion cracking and manufacturing method thereof

Abstract

The present invention relates to a thick steel material suitable for use in applications such as line pipes and sour-resistant materials, and more particularly to a high-strength steel material that has excellent resistance to sulfide stress corrosion cracking and excellent resistance to the propagation of sulfide stress corrosion cracking, and a method for producing the same. [Solution] The steel material of the present invention contains, by weight, carbon (C): 0.02-0.06%, silicon (Si): 0.1-0.5%, manganese (Mn): 0.8-1.8%, chromium (Cr): less than 0.05%, phosphorus (P): 0.03% or less, sulfur (S): 0.003% or less, aluminum (Al): 0.06% or less, nitrogen (N): 0.01% or less, niobium (Nb): 0.005-0.08%, titanium (Ti): 0.005-0.05%, calcium (Ca): 0. and one or more of nickel (Ni): 0.05-0.3%, molybdenum (Mo): 0.02-0.2%, and vanadium (V): 0.005-0.1%, with the balance being Fe and unavoidable impurities, wherein the Ca and S satisfy the following relational expression 1, and the steel material is characterized in that the microstructure of the surface portion is composed of ferrite or a composite structure of ferrite and pearlite, and the microstructure of the center portion is composed of acicular ferrite. [Equation 1] 0.5≦Ca / S≦5.0 (where each element represents its weight content)

Description

[Technical Field]

[0001] The present invention relates to a high-strength steel material having excellent resistance to sulfide stress corrosion cracking and a manufacturing method thereof, and more particularly to a thick steel material suitable for applications such as line pipes and sour-resistant materials. [Background technology]

[0002] Recently, there has been an increasing demand for an upper limit on the surface hardness of linepipe steel. High surface hardness of linepipe steel not only leads to problems such as uneven roundness during pipe processing, but also causes problems such as cracking during pipe processing and insufficient toughness in the operating environment due to the high-hardness structure of the pipe surface. Furthermore, when used in a sour environment with a high concentration of hydrogen sulfide, the high-hardness structure of the surface is likely to induce hydrogen-induced brittle cracking, resulting in a serious accident.

[0003] In 2013, in a huge crude oil / natural gas mining project in the Caspian Sea, sulfide stress corrosion cracking (SSC) occurred in a highly hardened area on the surface of a pipe within two weeks of operation, resulting in the replacement of a 200km subsea pipeline with clad pipe. Analysis of the cause of the SSC revealed that it was caused by the formation of a hard spot, a highly hard structure, on the surface of the pipe.

[0004] The API standard specifies that hard spots must be at least 2 inches long and have a hardness of at least Hv345, while the DNV standard has the same size criteria as the API standard but specifies an upper hardness limit of Hv250.

[0005] On the other hand, steel for line pipes is generally produced by reheating a steel slab, hot rolling it, and then performing accelerated cooling. However, it is believed that hard spots (areas where high-hardness structures are formed) occur due to uneven rapid cooling of the surface during accelerated cooling.

[0006] In steel sheets manufactured by conventional water cooling, water is sprayed onto the surface of the steel sheet, so the cooling rate at the surface is faster than that at the center, and this difference in cooling rate results in the hardness of the surface being higher than that at the center.

[0007] One approach to suppressing the formation of high-hardness structures on the surface of steel is to relax the water cooling process. However, this reduces the surface hardness, which also reduces the strength of the steel, necessitating the addition of more alloying elements. Furthermore, this increase in alloying elements also increases the surface hardness. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Korean Patent Publication No. 1998-028324 Summary of the Invention [Problem to be solved by the invention]

[0009] The object of the present invention is to provide a high-strength steel material and a manufacturing method thereof, which has excellent resistance to sulfide stress corrosion cracking by effectively reducing the surface hardness compared to existing water-quenched thick plate materials (TMCP) through optimization of the alloy composition and manufacturing conditions.

[0010] More specifically, an object of the present invention is to provide a high-strength steel material having a yield strength of 450 MPa or more and excellent resistance to sulfide stress corrosion cracking in a high-pressure H2S environment at a partial pressure exceeding 1 bar, and a method for manufacturing the same.

[0011] In addition, by optimizing the alloy composition and manufacturing conditions, the hardness of the surface is effectively controlled to be low, thereby increasing resistance to sulfide stress corrosion cracking. At the same time, resistance to the propagation of sulfide stress corrosion cracking is also ensured by minimizing the chromium (Cr) content, which accelerates the propagation of sulfide stress corrosion cracking in a high-pressure H2S environment.

[0012] The object of the present invention is not limited to the above-mentioned content, and anyone having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding further object of the present invention from the overall content of the specification of the present invention. [Means for solving the problem]

[0013] The steel material of the present invention has, by weight percent, carbon (C): 0.02 to 0.06%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 0.8 to 1.8%, chromium (Cr): less than 0.05%, phosphorus (P): 0.03% or less, sulfur (S): 0.003% or less, aluminum (Al): 0.06% or less, nitrogen (N): 0.01% or less, niobium (Nb): 0.005 to 0.08%, titanium (Ti): 0.005 to 0.05%, calcium (Ca): 0.0005 and one or more of nickel (Ni): 0.05-0.3%, molybdenum (Mo): 0.02-0.2%, and vanadium (V): 0.005-0.1%, with the balance being Fe and unavoidable impurities, wherein the Ca and S satisfy the following relational expression 1, and the steel has a surface microstructure that is composed of ferrite or a composite structure of ferrite and pearlite, and a center microstructure that is composed of acicular ferrite. [Equation 1] 0.5≦Ca / S≦5.0 (where each element represents its weight content)

[0014] The method for producing a steel material of the present invention includes the steps of heating a steel slab that satisfies the above-mentioned alloy composition and Relational Formula 1 in a temperature range of 1100 to 1300°C for two hours or more, hot rolling the heated steel slab to produce a hot-rolled plate material, and cooling the hot-rolled plate material after the hot-rolling, wherein the cooling includes a primary cooling step, an air-cooling step, and a secondary cooling step, and the primary cooling is carried out at a cooling rate of 5 to 40°C / s so that the temperature of the surface of the hot-rolled plate material is Ar1-50°C to Ar3-50°C, and the secondary cooling is carried out at a cooling rate of 50 to 500°C / s so that the temperature of the surface of the hot-rolled plate material is 300 to 600°C. [Effects of the Invention]

[0015] According to the present invention, when 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 provided.

[0016] Furthermore, according to the present invention, it is possible to provide a high-strength steel material that is excellent in resistance to sulfide stress corrosion cracking and also in resistance to the propagation of sulfide stress corrosion cracking.

[0017] The steel material of the present invention can be advantageously applied not only to pipe materials such as line pipes but also to sour-resistant materials, and can effectively provide high-strength steel materials with excellent sulfide stress corrosion cracking properties, especially in high-pressure H2S environments exceeding a partial pressure of 1 bar. [Brief explanation of the drawings]

[0018] [Figure 1] In the examples of the present invention, the microstructure and hardness of the surface portion of the inventive steel and the comparative steel are shown. DETAILED DESCRIPTION OF THE INVENTION

[0019] Thermo-Mechanical Control Process (TMCP) materials currently supplied to the thick plate and hot rolling markets have the characteristic of having higher hardness in the surface area than in the center due to an inevitable phenomenon that occurs during cooling after hot rolling (the cooling rate in the surface area is faster than in the center).

[0020] As a result, as the strength of a material increases, the hardness of the surface becomes significantly higher than that of the center, and this increased hardness in the surface region can lead to cracking during processing and reduce low-temperature toughness. Furthermore, in the case of steels used in sour environments, this can be the starting point for hydrogen embrittlement. Despite these problems with conventional technologies, there is currently no steel that exhibits excellent resistance to sulfide stress corrosion cracking in high-pressure H2S environments.

[0021] Therefore, the present inventors recognized the problems with the conventional technology and conducted extensive research. As a result, they discovered and completed a steel material that not only can effectively suppress sulfide stress corrosion cracking due to hard spots, but also has the property of not easily propagating cracks even if hard spots occur and cracks form on the surface.

[0022] Specifically, as one aspect of the invention, the inventors attempted to provide a steel material having high strength while ensuring resistance to cracking and crack propagation by effectively reducing the hardness of the surface portion of a thick steel material having a certain thickness or more.

[0023] Through extensive research and experimentation, the inventors came up with a new cooling control technology instead of the conventional cooling method, and by using this technology, they were able to achieve a technology that can reduce the hardness of the surface portion by dualizing the phase transformation between the surface portion and the center portion.

[0024] That is, the present invention has developed a technology that can reduce the hardenability of the surface by promoting decarburization of the surface layer during the heating and rolling processes, and can form ferrite in the surface. In addition, the inventors of the present invention have discovered that the addition of Cr to steel as an alloying element reduces the resistance to sulfide stress corrosion cracking propagation, and have therefore provided a technology for manufacturing steel sheets that have excellent resistance to sulfide stress corrosion cracking even in high-pressure H2S environments by optimizing the steel composition and manufacturing process conditions (heating, rolling, cooling, etc.).

[0025] Hereinafter, the chemical composition of the steel material according to the present invention will be described first. The steel material according to one aspect of the present invention may be composed of, by weight percent, 0.02 to 0.06% carbon (C), 0.1 to 0.5% silicon (Si), 0.8 to 1.8% manganese (Mn), less than 0.05% chromium (Cr), 0.03% or less phosphorus (P), 0.003% or less sulfur (S), 0.06% or less aluminum (Al), 0.01% or less nitrogen (N), 0.005 to 0.08% niobium (Nb), 0.005 to 0.05% titanium (Ti), 0.0005 to 0.005% calcium (Ca), and one or more of 0.05 to 0.3% nickel (Ni), 0.02 to 0.2% molybdenum (Mo), and 0.005 to 0.1% vanadium (V), with the balance being Fe and unavoidable impurities.

[0026] The reasons for limiting the alloy composition of the steel material provided in the present invention as described above will be explained in detail below. Meanwhile, in the present invention, unless otherwise specified, the content of each element is based on weight, and the proportion of the structure is based on area.

[0027] Carbon (C): 0.02~0.06% Carbon (C) is the element that has the greatest effect on the physical properties of steel. If the C content is less than 0.02%, excessive costs for controlling the composition during the steelmaking process will be incurred, and the weld heat-affected zone will be softened more than necessary. On the other hand, if the C content exceeds 0.06%, the hydrogen-induced cracking resistance of the steel sheet will be reduced, and weldability may be impaired. Therefore, in the present invention, the C content may be 0.02 to 0.06%, and more preferably 0.03 to 0.05%.

[0028] Silicon (Si): 0.1 to 0.5% Silicon (Si) is an element that is used not only as a deoxidizer in the steelmaking process but also plays a role in increasing the strength of steel. If the Si content exceeds 0.5%, the low-temperature toughness of the material deteriorates, weldability is impaired, and scale spallability during rolling is reduced. However, reducing the Si content to less than 0.1% increases manufacturing costs. Therefore, in the present invention, the Si content can be limited to 0.1 to 0.5%, and more preferably to 0.2 to 0.4%.

[0029] Manganese (Mn): 0.8-1.8% Manganese (Mn) is an element that improves the hardenability of steel without impairing low-temperature toughness, and can be contained in an amount of 0.8% or more. However, if the Mn content exceeds 1.8%, center segregation occurs, which not only deteriorates low-temperature toughness but also increases the hardenability of the steel and impairs weldability. Furthermore, Mn center segregation can induce hydrogen-induced cracking. Therefore, in the present invention, Mn can be contained in an amount of 0.8 to 1.8%. Alternatively, from the viewpoint of center segregation, Mn can be preferably contained in an amount of 0.8 to 1.6%, more preferably 1 to 1.4%.

[0030] Chromium (Cr): Less than 0.05% Chromium (Cr) dissolves in austenite during slab reheating, improving the hardenability of steel and contributing to the strength of steel sheets. However, the present inventors discovered that adding 0.05% or more of Cr may accelerate the propagation of sulfide stress corrosion cracking. Specifically, controlling the Cr content in steel to less than 0.05% ensures resistance to sulfide stress corrosion cracking. Meanwhile, a steel according to one aspect of the present invention may contain more than 0% but less than 0.05%, more preferably 0.04% or less, and most preferably 0.02% or less. However, the lower limit of the Cr content may be set to 0%, since Cr may not be added if sufficient strength can be ensured. More preferably, the lower limit of the Cr content may be set to 0.0005%.

[0031] Phosphorus (P): 0.03% or less Phosphorus (P) is an element that is inevitably added to steel, and if its content exceeds 0.03%, not only will weldability be significantly reduced, but low-temperature toughness will also be reduced. Therefore, the P content must be limited to 0.03% or less, and from the viewpoint of ensuring low-temperature toughness, P can be contained more preferably in an amount of 0.01% or less. However, taking into account the load during the steelmaking process, the lower limit of the P content can be excluded from 0%, and more preferably, the lower limit of the P content can be set to 0.0001%.

[0032] Sulfur (S): 0.003% or less Sulfur (S) is an element that is inevitably added to steel, and if its content exceeds 0.003%, it reduces the ductility, low-temperature toughness, and weldability of the steel. Therefore, the S content must be limited to 0.003% or less. However, S combines with Mn in steel to form MnS inclusions, which reduces the hydrogen-induced cracking resistance of the steel. Therefore, the S content is preferably 0.002% or less. However, the lower limit of the S content can be excluded from 0% in consideration of the load during the steelmaking process, and the lower limit of the S content can be more preferably 0.0001%.

[0033] Aluminum (Al): 0.06% or less (excluding 0%) Aluminum (Al) typically functions as a deoxidizer, reacting with oxygen (O) present in molten steel to remove oxygen. Therefore, Al can be added to a level that provides sufficient deoxidizing power in the steel. However, an Al content exceeding 0.06% is undesirable because it results in the formation of a large amount of oxide-based inclusions, which impairs the low-temperature toughness, hydrogen-induced cracking resistance, and sulfide stress corrosion cracking resistance of the material. Therefore, Al can be contained in an amount of 0.06% or less, more preferably 0.04% or less. However, considering that Al is an essential addition as a deoxidizer, the lower limit of the Al content can be excluded from 0%, and more preferably, the lower limit of the Al content can be set to 0.005%.

[0034] Nitrogen (N): 0.01% or less (excluding 0%) Because it is difficult to completely remove nitrogen (N) from steel industrially, the upper limit of its content is set to 0.01%, which is within the range allowable in the manufacturing process. N reacts with Al, Ti, Nb, V, and other elements in steel to form nitrides, thereby inhibiting the growth of austenite grains and improving the toughness and strength of the steel. However, excessive addition of N exceeding 0.01% results in the presence of dissolved N, which adversely affects low-temperature toughness. Therefore, N can be limited to 0.01% or less, and more preferably, to 0.009% or less. However, considering the load during the steelmaking process, the lower limit of N content can be excluded from 0%, and more preferably, the lower limit of N content can be set to 0.0005%.

[0035] Niobium (Nb): 0.005-0.08% Niobium (Nb) is an effective element that dissolves during slab heating, suppresses the growth of austenite grains during subsequent hot rolling, and then precipitates to improve the strength of steel. Furthermore, by combining with C in steel and precipitating as carbides, Nb improves the strength of steel while minimizing the increase in the yield ratio. If the Nb content is less than 0.005%, the above-mentioned effects cannot be fully achieved. On the other hand, if the Nb content exceeds 0.08%, not only will the austenite grains become unnecessarily fine, but the formation of coarse precipitates will also result in deterioration of low-temperature toughness and hydrogen-induced cracking resistance. Therefore, in the present invention, Nb can be contained in an amount of 0.005 to 0.08%. The lower limit of the Nb content can be more preferably 0.02%, and the upper limit of the Nb content can be 0.05%.

[0036] Titanium (Ti): 0.005 to 0.05% Titanium (Ti) combines with N and precipitates in the form of TiN during slab heating, effectively suppressing the growth of austenite grains. If Ti is added in an amount less than 0.005%, the austenite grains become coarse, reducing low-temperature toughness. On the other hand, if the Ti content exceeds 0.05%, coarse Ti-based precipitates are formed, reducing low-temperature toughness and hydrogen-induced cracking resistance. Therefore, in the present invention, Ti can be contained in an amount of 0.005 to 0.05%. The lower limit of the Ti content can be more preferably 0.006%, and the upper limit of the Ti content can be more preferably 0.03% in order to ensure low-temperature toughness.

[0037] Calcium (Ca): 0.0005-0.005% Calcium (Ca) combines with S to form CaS during the steelmaking process, thereby suppressing the segregation of MnS, which induces hydrogen-induced cracking. To fully achieve the effect of suppressing MnS segregation, 0.0005% or more of Ca must be added. However, if the Ca content exceeds 0.005%, not only will CaS form, but CaO inclusions will also form, which can cause hydrogen-induced cracking. Therefore, in the present invention, the Ca content can be 0.0005 to 0.005%, and from the viewpoint of ensuring hydrogen-induced cracking resistance, the Ca content can be more preferably 0.001 to 0.003%.

[0038] When the steel material according to one aspect of the present invention contains Ca and S as described above, it is preferable that the component ratio of Ca to S ([Ca] / [S]) satisfies the following relational expression 1. [Equation 1] 0.5≦[Ca] / [S]≦5.0

[0039] (In Relational Formula 1, [Ca] represents the average Ca content in the steel material (wt%), and [S] represents the average S content in the steel material (wt%).) That is, the Ca / S ratio is an index representing the formation of central segregation of MnS and coarse inclusions. If the [Ca] / [S] value is less than 0.5, MnS may form in the center of the steel material's thickness, resulting in a problem of reduced hydrogen-induced cracking resistance. On the other hand, if the [Ca] / [S] value exceeds 5.0, Ca-based coarse inclusions are formed, reducing hydrogen-induced cracking resistance. Therefore, it is preferable that the Ca / S ratio ([Ca] / [S]) satisfies Relational Formula 1. To further improve the above-mentioned effects, the [Ca] / [S] value can more preferably be set in the range of 1.4 to 3.2.

[0040] Meanwhile, the steel material of the present invention may further contain elements other than the above-mentioned alloy composition that can further improve physical properties. Specifically, the steel material may further contain one or more of nickel (Ni): 0.05 to 0.3%, molybdenum (Mo): 0.02 to 0.2%, and vanadium (V): 0.005 to 0.1%. In this case, it is sufficient to contain one or more of Ni, Mo, and V within a range that can achieve the object of the present invention, and the present invention is not necessarily limited to containing all of Ni, Mo, and V.

[0041] Nickel (Ni): 0.05-0.3% Nickel (Ni) is an effective element for improving the strength of steel without deteriorating its low-temperature toughness. To achieve this effect of increasing strength without deteriorating its low-temperature toughness, Ni can be added in an amount of 0.05% or more. However, Ni is an expensive element, and if its content exceeds 0.3%, there is a problem that the manufacturing cost increases significantly. Therefore, in the present invention, Ni can be added in an amount of 0.05 to 0.3%. On the other hand, the lower limit of the Ni content can be preferably 0.08%, more preferably 0.1%. Alternatively, the upper limit of the Ni content can be preferably 0.28%, more preferably 0.21%.

[0042] Molybdenum (Mo): 0.02-0.2% Molybdenum (Mo), like Cr, improves the hardenability of steel and increases its strength. To achieve the aforementioned effect of improving hardenability, 0.02% or more of Mo can be added. However, if the Mo content exceeds 0.2%, a structure that is weak in low-temperature toughness, such as upper bainite, is formed, which impairs hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance. Therefore, in the present invention, when Mo is added, it can be contained in an amount of 0.02 to 0.2%. The lower limit of the Mo content can be more preferably 0.05%, and the upper limit can be 0.15%.

[0043] Vanadium (V): 0.005 to 0.1% Vanadium (V) is an element that increases the hardenability of steel and improves its strength. To achieve this effect, 0.005% or more of vanadium can be added. However, if the V content exceeds 0.1%, the hardenability of the steel increases excessively, resulting in the formation of a structure that is brittle in low-temperature toughness and a decrease in hydrogen-induced cracking resistance. Therefore, in the present invention, V can be added in an amount of 0.005 to 0.1%. Meanwhile, the lower limit of the V content can be more preferably 0.005%, and the upper limit of the V content can be more preferably 0.05%.

[0044] The remaining component of the present invention is iron (Fe). However, in a normal manufacturing process, unintentional impurities from raw materials or the surrounding environment may be inevitably mixed in, and this cannot be excluded. Since these impurities are known to anyone skilled in the normal manufacturing process, the contents of all of them will not be specifically mentioned in this specification.

[0045] A steel material according to one aspect of the present invention having the above-described alloy composition is characterized in that the microstructure of the surface portion is composed of ferrite or a composite structure of ferrite and pearlite, and thus the Vickers hardness of the surface portion can be controlled to 200 Hv or less.

[0046] In this specification, the term "surface region" refers to the region from the surface to a depth of 1000 μm in the thickness direction, which may correspond to both surfaces of the steel material. The term "center region" refers to the remaining region excluding the surface region. In the present invention, the hardness of the surface region refers to the maximum hardness value measured from the surface to a depth of 1000 μm using a Vickers hardness tester under a load of 1 kgf. Typically, the hardness can be measured approximately five times at each position.

[0047] That is, the steel material according to the present invention has a surface microstructure that is composed of ferrite or a composite structure of ferrite and pearlite, and a central microstructure that is composed of acicular ferrite, which allows for the formation of a softer microstructure in the surface region than in the central region. As a result, it is possible to provide a steel material with a lower hardness in the surface region than existing TMCP steel materials.

[0048] Specifically, the steel material according to one aspect of the present invention has a strength equal to or greater than that of existing TMCP steel materials, and has a yield strength of 450 MPa or more, but by significantly reducing the hardness of the surface and minimizing the Cr content, it is possible to effectively suppress the formation and propagation of sulfide stress corrosion cracking during processing.

[0049] Meanwhile, a method for manufacturing the steel material according to the present invention will be described in detail below. The steel material according to the present invention can be manufactured through the steps of "slab heating-hot rolling-cooling", and the conditions for each step will be described in detail below.

[0050] [Slab heating] After preparing a steel slab satisfying the alloy composition and chemical relationship proposed in the present invention, it can be heated. This can be done at 1100 to 1300°C for 2 hours or more. Heating temperatures above 1300°C can increase scale defects and coarsen austenite grains, potentially reducing the hardenability of the steel. Furthermore, increasing the fraction of structures vulnerable to low-temperature toughness, such as upper bainite, in the center can lead to problems such as reduced hydrogen-induced cracking resistance and low-temperature toughness resistance.

[0051] On the other hand, if the temperature is less than 1100°C or the heating time is less than 2 hours, decarburization of the surface portion will be insufficient, which will not only adversely affect the formation of ferrite in the surface portion in subsequent processes but may also reduce the rate of redissolution of alloying elements. Therefore, in the present invention, when heating the above-mentioned steel slab, heating can be performed at a temperature range of 1100 to 1300°C for 2 hours or more, and more preferably at a temperature range of 1145 to 1250°C for 3.0 hours or more. On the other hand, there is no particular upper limit to the slab heating time; since the longer the heating time, the higher the degree of component uniformity, the better, and the longer the heating time, the better. It can be 50 hours or less, 20 hours or less, or 6 hours or less.

[0052] [Hot rolling] The heated steel slab can be hot-rolled to produce a hot-rolled sheet material. At this time, hot rolling can be performed in the temperature range of Ar3+80°C to Ar3+200°C with a cumulative reduction of 50% or more, and the hot rolling can be held (air-cooled) for 30 seconds or more after hot rolling.

[0053] If the hot rolling temperature is higher than Ar3+200°C, the increased hardenability due to grain growth may result in the formation of a structure such as upper bainite that is vulnerable to low-temperature toughness, potentially reducing hydrogen-induced cracking resistance and low-temperature toughness. On the other hand, if the hot rolling temperature is lower than Ar3+80°C, the temperature at which subsequent cooling begins may be too low, resulting in an excessive proportion of air-cooled ferrite, potentially reducing strength and inhibiting decarburization of the surface, potentially preventing it from contributing to the formation of ferrite in the surface in subsequent processes. Therefore, in the present invention, the finish hot rolling temperature is preferably set to Ar3+80°C to Ar3+200°C.

[0054] If the cumulative reduction rate during hot rolling in the above-mentioned temperature range is less than 50%, recrystallization due to rolling does not occur to the center of the steel material, causing coarsening of crystal grains in the center and deteriorating low-temperature toughness. Therefore, in the present invention, it is preferable that the cumulative reduction rate during hot rolling is 50% or more.

[0055] On the other hand, if the holding time after hot rolling is less than 30 seconds, the time for surface decarburization will be insufficient and will not contribute to the formation of ferrite in the surface in the subsequent steps, so in the present invention, the holding time after finish hot rolling is preferably 30 seconds or more. There is no particular upper limit to the holding time after finish hot rolling, but it is preferably 30 minutes or less, 10 minutes or less, or 5 minutes or less. By having such a holding time, it is possible to ensure the cooling start temperature described below from air cooling.

[0056] [cooling] It is possible to cool the hot-rolled plate material produced by hot rolling, and in particular, the present invention is technically significant in that it proposes an optimal cooling process that can obtain steel material with effectively reduced surface hardness.

[0057] Specifically, the cooling includes a primary cooling step, an air cooling step, and a secondary cooling step, and the conditions for each step are described in more detail below. Here, the primary cooling and secondary cooling described above can be performed by applying a specific cooling means, and water cooling can be used as an example.

[0058] Primary cooling In the present invention, primary cooling can be carried out after the above-mentioned hot rolling and holding for 30 seconds or more. Specifically, it is preferable to start primary cooling when the surface temperature of the hot-rolled sheet material obtained through the above-mentioned process is Ar3-20°C to Ar3+50°C.

[0059] If the starting temperature of the primary cooling exceeds Ar3+50°C, the phase transformation to ferrite does not occur sufficiently in the surface during the primary cooling, and the effect of reducing the hardness of the surface cannot be obtained.On the other hand, if the starting temperature of the primary cooling is less than Ar3-20°C, excessive ferrite transformation occurs in the center, causing a decrease in the strength of the steel.

[0060] Furthermore, the primary cooling is preferably carried out at a cooling rate of 5 to 40°C / s so that the surface temperature of the hot-rolled sheet material is between Ar1-50°C and Ar3-50°C. That is, if the end temperature of the primary cooling exceeds Ar3-50°C, the fraction of ferrite phase transformation in the surface of the primarily cooled hot-rolled sheet material is low, and the effect of reducing the hardness of the surface cannot be fully achieved. On the other hand, if the end temperature is lower than Ar1-50°C, excessive ferrite phase transformation occurs even in the center, making it difficult to ensure the target level of strength.

[0061] Furthermore, if the cooling rate during primary cooling is too slow, such as less than 5°C / s, it is difficult to ensure the primary cooling end temperature. On the other hand, if the cooling rate exceeds 40°C / s, the fraction of the surface region that undergoes phase transformation to acicular ferrite is high, making it impossible to form a soft structure in the surface region. Therefore, during primary cooling, the average cooling rate can be controlled to 5 to 40°C / s, and more preferably 17 to 40°C / s, based on the surface region temperature.

[0062] At the end of the primary cooling, the temperature of the center of the hot-rolled sheet material can be controlled to Ar3-30°C to Ar3+30°C. That is, if the temperature of the center of the hot-rolled sheet material exceeds Ar3+30°C at the end of the primary cooling, the temperature of the surface portion cooled to a specific temperature range will rise, and the fraction of ferrite phase transformation in the surface portion will decrease. Therefore, at the end of the primary cooling, the temperature of the center of the hot-rolled sheet material can more preferably be controlled to 730-810°C.

[0063] On the other hand, if the temperature of the center of the hot-rolled sheet material is less than Ar3-30°C, the center temperature of the hot-rolled sheet material will be excessively cooled, and the temperature to which the surface can be reheated during subsequent air cooling will be low, making it impossible to obtain the tempering effect, which ultimately reduces the effect of reducing the hardness of the surface.

[0064] air-cooled The hot-rolled sheet material that has completed the primary cooling under the above conditions is preferably air-cooled, and the air-cooling process has the effect of reheating the surface portion by the relatively high temperature center portion. Air-cooling is preferably completed when the temperature of the surface portion of the hot-rolled sheet material reaches a temperature range of Ar3-50°C to Ar3-10°C.

[0065] If the surface temperature of the hot-rolled sheet material is lower than Ar3-50°C after air-cooling is complete, not only will there be insufficient time to form air-cooled ferrite, but the tempering effect of the surface reheating will be insufficient, which will be detrimental to the reduction of surface hardness.On the other hand, if the surface temperature of the hot-rolled sheet material is higher than Ar3-10°C after air-cooling is complete, the air-cooling time will be excessive, causing ferrite transformation in the center, making it difficult to achieve the target level of strength.

[0066] Secondary cooling It is preferable to carry out secondary cooling immediately after air cooling is completed within the above temperature range (based on the surface temperature), and the surface temperature at the end of air cooling is the same as the starting temperature of secondary cooling. On the other hand, it is preferable to carry out secondary cooling at a cooling rate of 50 to 500°C / s so that the surface temperature becomes 300 to 600°C.

[0067] If the secondary cooling end temperature is less than 300°C, the proportion of the MA phase in the center will increase, adversely affecting low-temperature toughness and suppression of hydrogen embrittlement.On the other hand, if the secondary cooling end temperature exceeds 600°C, the phase transformation in the center will not be completed, making it difficult to ensure strength.

[0068] Furthermore, if the cooling rate during secondary cooling within the above-mentioned temperature range is less than 50°C / s, the crystal grains in the center will become coarse, making it difficult to achieve the target level of strength. On the other hand, if the cooling rate exceeds 500°C / s, the proportion of phases that are vulnerable to low-temperature toughness, such as upper bainite, in the microstructure of the center will increase, undesirably deteriorating hydrogen-induced cracking resistance. Therefore, during secondary cooling, the average cooling rate based on the surface temperature can be controlled to 50 to 500°C / s, more preferably 245 to 500°C / s. Meanwhile, according to one aspect of the present invention, the steel material manufactured through the above-mentioned series of processes can have a thickness of 5 to 50 mm.

[0069] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0070] (Example) Steel slabs were prepared having the alloy compositions and properties shown in Tables 1 and 2 below. The contents of the alloy compositions are in wt. %, with the remainder consisting of Fe and other unavoidable impurities. The prepared steel slabs were subjected to heating, hot rolling, and cooling processes under the conditions shown in Tables 3 and 4 below to produce the respective steel materials. The inventive steels and comparative steels shown in Tables 1 and 2 below were produced by the same process except for the manufacturing conditions shown in Tables 3 and 4.

[0071] Specifically, the steel materials of the invention steel and the comparative steel were prepared by heating slabs having the compositions shown in Table 1 below under the conditions shown in Table 3, performing rough rolling under normal conditions, and then finish hot rolling under the conditions shown in Table 3, holding for a certain period of time, and then water cooling. The cooling shown in Table 4 was controlled by performing intermediate air cooling after primary cooling, and then secondary cooling.

[0072] [Table 1]

[0073] [Table 2]

[0074] Ar3 = 910 - 310 x C - 80 x Mn - 20 x Cu - 15 x Cr - 55 x Ni - 80 x Mo + 0.35 x (thickness [mm] - 8) Ar1=742-7.1×C-14.1×Mn+16.3×Si+11.5×Cr-49.7×Ni

[0075] [Table 3]

[0076] [Table 4]

[0077] The steel materials manufactured by the above manufacturing process were observed for yield strength, surface Vickers hardness, sulfide stress corrosion cracking resistance, and microstructure, and the results are shown in Table 5 below. Here, yield strength means 0.5% under-load yield strength, and tensile test specimens were taken from API-5L standard specimens in a direction perpendicular to the rolling direction before testing.

[0078] The hardness of the steel was measured on a cross section in the thickness direction using a Vickers hardness tester under a load of 1 kgf, and the hardness of the surface portion was measured from the surface to 100 μm, and the results are shown in Table 5. Meanwhile, the microstructure was measured using an optical microscope, and the type of phase was observed using an image analyzer.

[0079] In addition, sulfide stress corrosion cracking (SSC) characteristics were evaluated using a 4-point bent beam test in accordance with the NACE standard test method (TM-0177), in which 90% of the yield strength of each steel plate was applied to a strong acidic Sol. A solution, and the steel was exposed to a 10 bar H2S environment for 720 hours, after which the occurrence of cracks was evaluated.

[0080] [Table 5]

[0081] In Tables 1 to 4, the inventive steels satisfy all of the composition and manufacturing conditions of the present invention, while the comparative steels do not satisfy one or more of the composition and manufacturing conditions of the present invention. Specifically, comparative steels 1 to 3 do not satisfy all of the composition and manufacturing conditions of the present invention, and in particular, the two-stage cooling method proposed in the present invention is not applied to the cooling.

[0082] On the other hand, Comparative Steels 4 to 9 use steel slabs with the same composition as Inventive Steel 1, but do not satisfy the manufacturing conditions of the present invention. That is, Comparative Steel 4 does not use the two-stage cooling method proposed in the present invention, and Comparative Steel 5 has a surface temperature at the end of primary cooling and a surface temperature after intermediate air cooling that are outside the ranges proposed in the present invention.

[0083] Furthermore, Comparative Steel 6 is a case in which the primary cooling rate of the surface region is outside the range proposed by the present invention, and Comparative Steel 7 is a case in which the finishing temperature of the hot rolling is lower than the lower limit range proposed by the present invention, and as a result of the low finishing temperature of the hot rolling, the primary cooling start temperature, the primary cooling end temperature of the surface region and central region, and the temperature of the surface region after intermediate air cooling are all outside the ranges proposed by the present invention.

[0084] Comparative steel 8 is a case where the slab heating temperature is outside the lower limit range proposed by the present invention, and comparative steel 9 is a case where the holding time after finish hot rolling is outside the lower limit range proposed by the present invention.

[0085] The above-mentioned comparative steels 1 to 4 were not subjected to the two-stage cooling proposed in the present invention, and the microstructure in the surface did not form the ferrite or composite structure of ferrite and pearlite proposed in the present invention. As a result, the hardness of the surface of the steel material in comparative steels 1 to 4 exceeded 200 Hv, and sulfide stress corrosion cracking occurred due to the high hardness of the surface.

[0086] Comparative steel 5 did not undergo the two-stage cooling proposed in the present invention, but the primary cooling end temperatures of the surface and center parts and the surface temperature after intermediate air cooling were low, and ferrite transformation occurred before secondary cooling. Although sulfide stress corrosion cracking did not occur in comparative steel 5, the yield strength did not satisfy the range of 450 MPa or more specified in the present invention.

[0087] In Comparative Steel 6, the primary cooling rate in the surface region exceeded the upper limit proposed by the present invention, preventing ferrite from being formed in the surface region, resulting in sulfide stress corrosion cracking. In Comparative Steel 7, the finishing temperature in hot rolling did not satisfy the lower limit proposed by the present invention, and the cooling temperature after hot rolling also did not satisfy the range proposed by the present invention, resulting in ferrite transformation occurring all the way to the center, resulting in insufficient yield strength.

[0088] In Comparative Steel 8, the slab heating temperature was outside the range proposed by the present invention, and in Comparative Steel 9, the holding time after hot rolling was outside the range proposed by the present invention. In both Comparative Steels 8 and 9, the ferrite transformation in the surface region was insufficient, and a composite structure of ferrite and acicular ferrite was formed. As a result, the effect of reducing the hardness in the surface region was not fully exerted, and sulfide stress corrosion cracking occurred.

[0089] As described above, the invention steels 1 to 3, which satisfy all of the alloy compositions and manufacturing conditions proposed in the present invention, all have a surface hardness of 200 Hv or less. Although the surface hardness is significantly low, it is possible to ensure a yield strength of 450 MPa or more, and it can also be confirmed that they have excellent resistance to sulfide stress corrosion cracking.

[0090] In contrast, in Comparative Steels 1 to 9, which do not satisfy the alloy composition of the present invention or the manufacturing conditions of the present invention, the hardness of the surface of the steel material was not sufficiently reduced, and sulfide stress corrosion cracking occurred or a yield strength of 450 MPa or more could not be secured.

[0091] Among the experimental examples described above, photographs of the microstructures of the surface regions of Inventive Steel 2 and Comparative Steel 3, measured with an optical microscope, and the measured hardness values ​​of the surface regions, are shown in Figure 1. Specifically, the photograph on the left in Figure 1 shows the hardness values ​​measured using a Vickers hardness tester up to 100 μm from the surface, and the photograph on the right shows the hardness values ​​measured up to 500 μm from the surface.

[0092] As can be seen from FIG. 1, the steel material according to the present invention has a surface hardness of 200 Hv or less, whereas the comparative steel 3, which does not undergo the two-stage cooling proposed in the present invention, has a surface hardness of more than 200 Hv.

Claims

1. The steel material contains, by weight, one or more of carbon (C): 0.02 to 0.06%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 0.8 to 1.8%, chromium (Cr): 0.02% or less, phosphorus (P): 0.03% or less, sulfur (S): 0.003% or less, aluminum (Al): 0.06% or less, nitrogen (N): 0.01% or less, niobium (Nb): 0.005 to 0.08%, titanium (Ti): 0.005 to 0.05%, calcium (Ca): 0.0005 to 0.005%; nickel (Ni): 0.05 to 0.3%, molybdenum (Mo): 0.02 to 0.2%, and vanadium (V): 0.005 to 0.1%, with the remainder being Fe and inevitable impurities, The Ca and S satisfy the following relational expression 1: The steel material has a surface microstructure composed of ferrite or a composite structure of ferrite and pearlite, and a central microstructure composed of acicular ferrite, A steel material characterized in that the Vickers hardness of the surface portion is 200 Hv or less. [Relationship 1] 0.5≦Ca / S≦5.0 (wherein each element represents a weight content)

2. 2. The steel material according to claim 1, wherein the steel material has a yield strength of 450 MPa or more.

3. In weight percent, carbon (C): 0.02-0.06%, silicon (Si): 0.1-0.5%, manganese (Mn): 0.8-1.8%, chromium (Cr): less than 0.05%, phosphorus (P): 0.03% or less, sulfur (S): 0.003% or less, aluminum (Al): 0.06% or less, nitrogen (N): 0.01% or less, niobium (Nb): 0.005-0.08%, titanium (Ti): 0.005-0.05%, calcium (Ca): 0.0005-0.0005% and one or more of nickel (Ni): 0.05-0.3%, molybdenum (Mo): 0.02-0.2%, and vanadium (V): 0.005-0.1%, with the balance being Fe and inevitable impurities, wherein the Ca and S satisfy the following relational expression 1: The cooling step includes a primary cooling step, an air 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 temperature of the surface of the hot-rolled plate material is Ar1-50 ° C. to Ar3-50 ° C., and the secondary cooling is performed at a cooling rate of 50 to 500 ° C. / s so that the temperature of the surface of the hot-rolled plate material is 300 to 600 ° C., A method for producing a steel material, characterized in that the microstructure of the surface portion is composed of ferrite or a composite structure of ferrite and pearlite, and the microstructure of the center portion is composed of acicular ferrite. [Relationship 1] 0.5≦Ca / S≦5.0 (wherein each element represents a weight content)

4. 4. The method for producing a steel material according to claim 3, wherein the hot rolling is carried out in a temperature range of Ar3+80°C to Ar3+200°C at a cumulative reduction rate of 50% or more.

5. The method for manufacturing a steel material according to claim 3, further comprising the step of holding the steel material for 30 seconds or more before cooling after the hot rolling.

6. 4. The method for manufacturing a steel material according to claim 3, wherein the primary cooling is started when the temperature of the surface of the hot-rolled plate material is in the range of Ar3-20°C to Ar3+50°C.

7. 4. The method for producing a steel material according to claim 3, wherein the temperature of the central portion of the hot-rolled plate material is Ar3-30°C to Ar3+30°C after the primary cooling is completed.

8. 4. The method for producing a steel material according to claim 3, wherein the temperature of the surface of the hot-rolled plate material is Ar3-10°C to Ar3-50°C after the air cooling is completed.

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