Steel material with excellent hydrogen embrittlement resistance and impact toughness, and manufacturing method thereof

Abstract

To provide a steel material having improved hydrogen embrittlement resistance and impact properties despite being an alloy system that is lower in cost than conventional steels, and a manufacturing method thereof. [Solution] The steel material is composed, by weight, of C: 0.15-0.40%, Si: 0.4% or less (excluding 0%), Mn: 0.3-0.7%, S: 0.01% or less (excluding 0%), P: 0.03% or less (excluding 0%), Cr: 0.6-2.0%, Mo: 0.15-0.8%, Ni: 1.6-4.0%, Cu: 0.30% or less (excluding 0%), Nb: 0.12% or less (excluding 0%), N: 0.015% or less (excluding 0%), Al: 0.06% or less (excluding 0%), B: 0.007% or less (excluding 0%), the balance being Fe and inevitable impurity elements, and is characterized in that the relationship between the sum (SUM) of the contents of C, Cu, Nb, Ni, Cr, and Mo and the impurity elements satisfies the following relational expression 1. [Relationship 1] |(C-SUM) (Cu-SUM) (Nb-SUM) (Ni-SUM) (Cr-SUM) (Mo-SUM)| × 105 > 3.0 (Here, SUM means the total content of specific impurity elements, that is, the total content (wt%) of [W + Nd + Zr + Co].)

Description

[Technical field]

[0001] The present invention relates to a steel material having excellent hydrogen embrittlement resistance and impact toughness, and a method for producing the same. [Background technology]

[0002] The hydrogen economy is a term used to describe an economic system in which hydrogen is used as an energy source to replace existing fossil fuels in daily life and industrial activities. Due to the depletion of fossil fuels and the rise of environmental issues, the full-scale expansion of the hydrogen economy is expected to begin around 2020, and Japan is taking proactive steps, such as announcing country-specific roadmaps with the aim of realizing a hydrogen economy both domestically and internationally. As a means to realise a hydrogen economy, governments around the world are actively promoting not only the popularisation of hydrogen electric vehicles, but also the construction of hydrogen filling stations and other supporting infrastructure to expand hydrogen demand.

[0003] A hydrogen filling station is an infrastructure that stores hydrogen and supplies it to users. The pressure accumulator in a hydrogen filling station is equipment that pressurizes hydrogen to a pressure higher than the filling pressure of the hydrogen fuel tank on board the vehicle, in order to fill the hydrogen fuel tank on board a hydrogen electric vehicle using a differential pressure method. Currently, the filling pressure for hydrogen electric vehicles has increased from 350 bar to 700 bar, and a pressure of 800 bar or more is also required for the pressure tank. One material that can be used for the accumulators in hydrogen filling stations is STS316L austenitic steel, which has hydrogen embrittlement resistance. However, to withstand a pressure of about 900 bar, a thickness of 405 mm is required, which is unrealistic and increases the construction costs of the hydrogen filling stations.

[0004] On the other hand, in the case of high-strength low-alloy steel, there is a risk of phenomena such as a decrease in ductility, notch strength, and impact toughness in a hydrogen gas atmosphere. If this issue can be overcome and the hydrogen embrittlement resistance of high-strength low-alloy steel can be improved, it is expected to become an effective technology that can simultaneously satisfy the safety and cost reduction of hydrogen filling stations. Several techniques have been investigated to improve the hydrogen embrittlement resistance of high strength low alloy steels. As an example, a steel has been presented that utilizes (V, Mo)C precipitates as trap sites for diffusing hydrogen to improve hydrogen resistance (Patent Document 1). Specifically, when quantifying hydrogen embrittlement resistance based on the size of (V, Mo)C precipitates, it is disclosed that the average diameter of the precipitates must be 1 to 20 nm, preferably 1 to 10 nm, and more preferably 1 to 5 nm.

[0005] Furthermore, in order to improve the properties of steel, it has been proposed to further include Cu, Ni, Cr, Nb, W, B, etc. However, if the above Ni is contained at a maximum of 12%, there is a possibility that the manufacturing cost of the steel will increase significantly, and there is a problem that it is not practical to apply to actual environments. It is disclosed that Nb, Ca, Mg, rare earth metals (hereinafter referred to as REM), etc. may be further contained. However, Nb and REM are extremely expensive elements and their prices fluctuate greatly, so there is a risk that a stable supply of the raw materials cannot be secured. Other patent documents disclose steel materials for high-pressure hydrogen use with a tensile strength of 900 to 1100 MPa and a yield ratio of 85% or more, and disclose that they contain W, Co, etc., for the purpose of improving the properties of the steel. However, these also have the drawback of significantly increasing the manufacturing cost because very expensive elements are added. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Korean Patent Publication No. 2018-0038024 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a steel material which has improved hydrogen embrittlement resistance and impact properties despite being made of an alloy system which is less expensive than conventional steels, and a method for producing the same. The object of the present invention is not limited to the above. The object of the present invention can be understood from the entire content of this specification, and a person having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the additional object of the present invention. [Means for solving the problem]

[0008] The steel material of the present invention having excellent hydrogen embrittlement resistance and impact toughness contains, by weight, carbon (C): 0.15 to 0.40%, silicon (Si): 0.4% or less (excluding 0%), manganese (Mn): 0.3 to 0.7%, sulfur (S): 0.01% or less (excluding 0%), phosphorus (P): 0.03% or less (excluding 0%), chromium (Cr): 0.6 to 2.0%, molybdenum (Mo): 0.15 to 0.8%, nickel (Ni): 1.6 to 4.0%, copper (C): 0.2 to 0.4%, and manganese (Mn ... u): 0.30% or less (except 0%), niobium (Nb): 0.12% or less (except 0%), nitrogen (N): 0.015% or less (except 0%), aluminum (Al): 0.06% or less (except 0%), boron (B): 0.007% or less (except 0%), the balance being Fe and unavoidable impurity elements, and is characterized in that the relationship between the sum (SUM) of the contents of C, Cu, Nb, Ni, Cr and Mo and the impurity elements satisfies the following relational expression 1. [Equation 1] |(C-SUM)·(Cu-SUM)·(Nb-SUM)·(Ni-SUM)·(Cr-SUM)·(Mo-SUM)|×10 5 >3.0 (Here, SUM is the total content of a specific impurity element, and means the total content (wt%) of [W+Nd+Zr+Co].)

[0009] The method for producing a steel material having excellent hydrogen embrittlement resistance and impact toughness of the present invention includes the steps of preparing a steel slab satisfying the above alloy composition and Relational Formula 1 and heating it in a temperature range of 1000 to 1200°C; hot rolling the heated steel slab to a finish rolling temperature Ar3 or higher to produce a hot-rolled steel sheet; cooling the hot-rolled steel sheet to room temperature; reheating the cooled hot-rolled steel sheet to a temperature range of 800 to 900°C and then holding the sheet for 1 to 2 hours to austenitize the sheet; cooling the austenitized hot-rolled steel sheet to room temperature at a cooling rate of 0.5 to 20°C / s; and tempering the sheet after cooling by heat treating it in a temperature range of 580 to 680°C for 30 minutes or more per 25 mm of the steel sheet thickness. Effect of the Invention

[0010] According to the present invention, it is possible to provide a steel material that is excellent in impact toughness as well as hydrogen embrittlement resistance while constructing an alloy system that is less expensive than existing steel materials. The steel material of the present invention has an effect that it can be advantageously applied to the fields in which hydrogen is utilized, which are gradually increasing. [Brief description of the drawings]

[0011] [Figure 1] 1 is a photograph of an apparatus capable of performing an ultra-low deformation rate tensile test in a hydrogen environment. [Figure 2a] 1 shows electron backscatter diffraction (EBSD) measurement photographs of Comparative Examples 1 to 3 according to an embodiment of the present invention. [Figure 2b] 1 shows electron backscatter diffraction (EBSD) measurement photographs of Examples 1, 3, 5, and 7 according to an embodiment of the present invention. [Diagram 3] 4 shows photographs of Example 3 according to an embodiment of the present invention, in which the distribution of precipitates was measured by a transmission electron microscope (TEM) and an energy spectroscopic analysis method. [Figure 4] The results of Relational Expression 2 for the Comparative Example and the Example of the present invention are shown in a graph. [Figure 5a] In one embodiment of the present invention, the change in phase transformation depending on the cooling rate after austenitization in Comparative Example 1 was measured using a dilatometer. [Figure 5b] In one embodiment of the present invention, the change in phase transformation depending on the cooling rate after austenitization in Comparative Example 2 was measured using a dilatometer. [Figure 5c] In one embodiment of the present invention, the change in phase transformation depending on the cooling rate after austenitization in Comparative Example 3 was measured using a dilatometer. [Figure 5d] In one embodiment of the present invention, the change in phase transformation depending on the cooling rate after austenitization in Examples 1 and 2 was measured using a dilatometer. [Figure 5e] In one embodiment of the present invention, the change in phase transformation depending on the cooling rate after austenitization in Examples 3 and 4 was measured using a dilatometer. [Figure 5f] In one embodiment of the present invention, the change in phase transformation depending on the cooling rate after austenitization in Examples 5 and 6 was measured using a dilatometer. [Figure 5g] In one embodiment of the present invention, the change in phase transformation depending on the cooling rate after austenitization in Examples 7 and 8 was measured using a dilatometer. [Figure 5h] In one embodiment of the present invention, the change in phase transformation depending on the cooling rate after austenitization in Example 9 was measured using a dilatometer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Taking into consideration the gradual expansion of hydrogen use due to economic and environmental factors, the inventors of the present invention have conducted extensive research to develop a steel material that can be suitably used in a hydrogen environment. As a result, the inventors confirmed that by optimizing the manufacturing conditions of steel in an alloy system that is less expensive than conventional steels, it is possible to derive a microstructural structure that is advantageous for ensuring the intended physical properties, and to provide steel material that has excellent hydrogen embrittlement resistance and impact properties, thereby completing the present invention. In particular, the present invention has a technical significance in that it provides a target steel material by forming the structure of the steel material into a martensite matrix structure and refining the effective crystal grains by utilizing niobium (Nb). The present invention will be described in detail below.

[0013] A steel material having excellent hydrogen embrittlement resistance and impact toughness according to one embodiment of the present invention preferably contains, by weight percent, 0.15 to 0.40% carbon (C), 0.4% or less (excluding 0%) silicon (Si), 0.3 to 0.7% manganese (Mn), 0.01% or less (excluding 0%) sulfur (S), 0.03% or less (excluding 0%) phosphorus (P), 0.6 to 2.0% chromium (Cr), 0.15 to 0.8% molybdenum (Mo), 1.6 to 4.0% nickel (Ni), 0.30% or less (excluding 0%) copper (Cu), 0.12% or less (excluding 0%) niobium (Nb), 0.015% or less (excluding 0%) nitrogen (N), 0.06% or less (excluding 0%) aluminum (Al), and 0.007% or less (excluding 0%) boron (B).

[0014] The reasons for restricting the alloy composition of the steel material provided by the present invention as described above will be explained in detail below. Unless otherwise specified in the present invention, the content of each element is based on weight, and the proportion of the structure is based on area.

[0015] Carbon (C): 0.15~0.40% Carbon (C) is an austenite stabilizing element that can adjust the Ae3 temperature and martensite formation start temperature (Ms) depending on its content. It is also an interstitial element that is very effective in ensuring high strength by adding asymmetric distortion to the lattice structure of the martensite phase. It is also an essential element for ensuring hardening ability and securing the martensite structure. In order to fully obtain the above effects, it is necessary to add 0.15% or more of C. However, if the C content exceeds 0.40%, there is a risk that carbides will be excessively formed, resulting in a significant decrease in impact toughness and weldability. Therefore, the C content should be 0.15 to 0.40%.

[0016] Silicon (Si): 0.4% or less (excluding 0%) Silicon (Si) is an element that is added not only for solid solution strengthening but also as a deoxidizer during casting. Such Si plays a role in suppressing the formation of carbonitrides, but in the present invention, it is necessary to improve hydrogen embrittlement resistance and impact properties by forming fine carbonitrides, so taking this into consideration, it is preferable to contain 0.4% or less of Si. However, 0% can be excluded considering the level of unavoidable contamination.

[0017] Manganese (Mn): 0.3-0.7% Manganese (Mn) is an austenite stabilizing element that greatly improves the hardening ability of steel and favors the formation of hard phases such as martensite. It also reacts with sulfur (S) to precipitate MnS, which is effective in preventing hot cracking due to sulfur (S) segregation. In order to fully obtain the above effects, it is preferable to include 0.3% or more of Mn. However, since an excessive content of Mn causes a problem of excessively increasing the stability of austenite, it is preferable to limit the Mn content to 0.7% or less in consideration of this. Therefore, the Mn content is preferably 0.3 to 0.7%.

[0018] Sulfur (S): 0.01% or less (excluding 0%) Sulfur (S) is an impurity that is inevitably contained in steel, and if its content exceeds 0.01%, there is a problem that the ductility and weldability of the steel are deteriorated. Therefore, the S content should be limited to 0.01% or less, and 0% can be excluded considering the level of unavoidable contamination.

[0019] Phosphorus (P): 0.03% or less (excluding 0%) Phosphorus (P) has a solid solution strengthening effect, but if its content exceeds 0.03%, it causes brittleness in steel and deteriorates weldability. Therefore, it is better to limit the P content to 0.03% or less, and 0% can be excluded considering the level of unavoidable P contamination.

[0020] Chromium (Cr): 0.6~2.0% Chromium (Cr) is a ferrite stabilizing element and increases hardening ability. The Ae3 temperature and the temperature range where delta ferrite forms are adjusted according to the Cr content. In addition, Cr reacts with oxygen (O) to form a dense and stable protective film of Cr2O3, which can improve corrosion resistance in a hydrogen environment, but can also expand the temperature range where delta ferrite forms. The higher the Cr content, the greater the possibility of delta ferrite forming during the steel casting process, which remains even after heat treatment and adversely affects the properties of the steel. Therefore, in order to obtain the effects of improving hardenability and corrosion resistance due to Cr, the Cr content should be 0.6% or more, while from the viewpoint of suppressing the formation of delta ferrite, the Cr content should preferably be limited to 2.0% or less. Therefore, the Cr content should be 0.6 to 2.0%.

[0021] Molybdenum (Mo): 0.15~0.8% Molybdenum (Mo) increases the hardening ability of steel and is known as a ferrite stabilizing element. Mo improves the strength of materials through strong solid solution strengthening. In order to fully obtain the above effects, it is preferable that the Mo content is 0.15% or more. On the other hand, if the Mo content is excessive, the temperature range in which delta ferrite is formed may be expanded, and there is a risk that delta ferrite is formed and remains during the steel casting process. In consideration of this, it is preferable to limit the Mo content to 0.8% or less. Therefore, the Mo content should be 0.15 to 0.8%.

[0022] Nickel (Ni): 1.6-4.0% Nickel (Ni) is an effective element for improving the impact toughness of steel, and can be added to improve the strength of steel without deteriorating its low-temperature toughness. It can also suppress hydrogen diffusion into the steel, improving its resistance to hydrogen embrittlement. In order to fully obtain the above effects, it is preferable that the Ni content be 1.6% or more. However, Ni is an expensive element, and if the Ni content exceeds 4.0%, there is a problem that the production costs increase significantly. Therefore, it is preferable that the Ni content is 1.6 to 4.0%.

[0023] Copper (Cu): 0.30% or less (excluding 0%) Copper (Cu) is an element that improves the hardening ability of materials and is added to ensure that the steel material has a homogeneous structure after heat treatment. If the Cu content exceeds 0.30%, there is a high risk of cracks occurring in the steel material. Therefore, it is preferable that the Cu content be 0.30% or less, except for 0%.

[0024] Niobium (Nb): 0.12% or less (excluding 0%) Niobium (Nb) is one of the elements that form carbonitrides in the form of M(C,N) (where M stands for metal), and can improve hydrogen embrittlement resistance by forming fine carbonitrides. As will be described in detail later, the present invention is characterized by providing a method for suppressing hydrogen embrittlement by forming the base structure of a steel material from martensite and trapping diffusible hydrogen using Nb-based precipitates that are semi-coherent with the martensite. In addition, Nb is in solid solution when the slab is reheated, but it suppresses the growth of austenite crystal grains during hot rolling, and then precipitates to improve the strength of the steel. If the Nb content exceeds 0.12%, the weldability of the steel may decrease, and the crystal grains may become finer than necessary. Therefore, the Nb content should be 0.12% or less, except for 0%.

[0025] Nitrogen (N): 0.015% or less (excluding 0%) Nitrogen (N) is difficult to completely remove from steel during manufacturing, but it is effective in stabilizing austenite and forming carbonitrides. If the N content exceeds 0.015%, it bonds with boron (B) in the steel to form BN, which increases the possibility of defects occurring in the steel. Therefore, the N content should be 0.015% or less, except for 0%.

[0026] Aluminum (Al): 0.06% or less (excluding 0%) Aluminum (Al) expands the ferrite region and is added as a deoxidizer during casting. In the present invention, since elements other than Al that are effective for stabilizing ferrite are contained, the Ae3 temperature may rise excessively as the Al content increases. Also, if the Al content exceeds 0.06%, a large amount of oxide-based inclusions are formed, which causes a problem of deteriorating the physical properties of the material. Therefore, the Al content should be 0.06% or less, and 0% can be excluded in consideration of the level of unavoidable inclusion.

[0027] Boron (B): 0.007% or less (excluding 0%) Boron (B) is a ferrite stabilizing element, and even a very small amount of it contributes greatly to improving the hardenability of steel. It also easily segregates at grain boundaries, and is effective in strengthening the grain boundaries. If the B content exceeds 0.007%, there is a high possibility that BN will be formed, which undesirably has an adverse effect on the physical properties of the steel. Taking this into consideration, it is preferable that the B content be 0.007% or less, although 0% can be excluded.

[0028] The remaining component of the present invention is iron (Fe). However, in a normal manufacturing process, unintended impurities may be inevitably mixed in from the raw materials or the surrounding environment, and it is not possible to exclude them. These impurities are known to any technician in a normal manufacturing process, so the entire contents of them will not be mentioned in this specification. However, it is preferable that the steel material of the present invention satisfies the following relational expression 1 with respect to specific impurity elements. [Equation 1] |(C-SUM)·(Cu-SUM)·(Nb-SUM)·(Ni-SUM)·(Cr-SUM)·(Mo-SUM)|×10 5 >3.0 (Here, SUM is the total content of a specific impurity element, and means the total content (wt%) of [W+Nd+Zr+Co].)

[0029] In order to satisfy the above-mentioned alloy component system, the steel material provided by the present invention must satisfy the contents of C, Cu, Nb, Ni, Cr and Mo in the steel presented, while controlling impurity elements that may inhibit the beneficial effects of these elements so as not to be contained in the steel material of the present invention. Specifically, when the value (relationship 1) specifying the relationship between the sum (SUM) of the contents of tungsten (W), neodymium (Nd), zirconium (Zr) and cobalt (Co) and the above-mentioned main elements of the present invention exceeds 3.0, the effect of the above-mentioned main elements described in the present invention can be obtained.

[0030] On the other hand, in the present invention, the elements W, Nd, and Zr constituting the above "SUM" are expensive elements, which significantly increase the manufacturing cost of steel materials, making it difficult to apply them to actual usage environments. Furthermore, Co reduces hardenability, so when it is contained in steel, it may not be possible to obtain the intended structure (preferably a martensite structure) in the process of normalizing or quenching a hot-rolled steel sheet that has been austenitized by reheating and then cooling it to room temperature under specific conditions. Therefore, the sum of the weight percents of alloy elements that should not be contained in the steel material provided by the present invention is limited as "SUM". The steel material of the present invention can ensure both excellent hydrogen embrittlement resistance and excellent impact properties by having the following microstructure and precipitate configuration, which will be described in detail below.

[0031] In the steel material of the present invention, the matrix structure is preferably composed of a tempered martensite phase, and the effective crystal grain size of the tempered martensite is preferably 5 μm or less in average diameter, more preferably 3 μm or less. Here, the effective grain size is the average value of the width of martensite blocks measured using electron backscatter diffraction (EBSD). Since the blocks in martensite have high-angle grain boundaries with each other, they can be considered as the smallest unit that affects the mechanical properties of steel.

[0032] The steel material of the present invention has a matrix structure in which precipitates with a diameter of 20 nm or less are present at a density of 20 / μm. 2 If the number of precipitates with a diameter of 20 nm or less is 20 / μm 2 If it is less than this, the distance between the fine carbonitrides becomes considerably large, and therefore there is a risk that the desired effect of improving hydrogen embrittlement resistance will not be obtained. In the present invention, the above-mentioned precipitates having a diameter of 20 nm or less are fine carbonitrides composed of Nb, and preferably contain Nb(C,N).

[0033] The steel material of the present invention which satisfies the above alloy component system, Relational Formula 1 and microstructure has high strength and excellent impact properties, specifically, a tensile strength of 900 MPa or more and an impact absorption energy value of 100 J or more at -20°C. In addition, the steel material of the present invention has an excellent effect of exhibiting hydrogen embrittlement resistance by satisfying the relationship between the notch tensile strength ratio (RNTS, the ratio of the notch tensile strength (MPa) in an atmosphere in which hydrogen is charged into the sample to the notch tensile strength (MPa) in a general air atmosphere) and the tensile strength (GPa) of the steel material, which is expressed by the following relational expression 2. [Equation 2] (Notched tensile strength in hydrogen charged atmosphere (MPa) ÷ Notched tensile strength in normal air atmosphere (MPa)) × Tensile strength of steel (GPa) ≧ 0.7

[0034] The method for producing a steel material having excellent hydrogen embrittlement resistance and impact toughness according to the present invention will be described in detail below. In simple terms, the present invention can produce the desired steel through the process of "heating steel slab - hot rolling - cooling - reheating (austenitization) - cooling - tempering." However, it should be made clear that this is not limited to this.

[0035] The conditions for each stage are explained in detail below. First, a steel slab that satisfies the above alloy component system and Relational Formula 1 is prepared, and then the steel slab is heated. At this time, the heating step is for facilitating the subsequent hot rolling step, and the heating temperature is not particularly limited, but can be in the range of 1000 to 1200°C. The steel slab heated as described above can be hot-rolled to obtain a hot-rolled steel sheet. In this case, the hot rolling is preferably performed so that the finish rolling temperature is Ar3 or higher. In this way, the uniformity of the structure can be increased by performing hot rolling at a temperature in the austenite single phase region.

[0036] Although there is no particular upper limit to the finish rolling temperature, if the temperature is too high, there is a problem that the austenite grains become coarse, so in consideration of this, it is preferable to limit the temperature to 1000° C. or less. More preferably, the finish rolling is performed at 900 to 1000° C. The hot-rolled steel sheet manufactured as described above is cooled (air-cooled) to room temperature, and then reheated to a high temperature to convert it to austenite. At this time, it is preferable that the reheating is carried out in the temperature range of 800 to 900° C., and that the temperature is maintained for at least 1 hour and up to 2 hours.

[0037] If the reheating temperature is less than 800°C, unintended carbides formed during the cooling process after hot rolling may not be sufficiently remelted, whereas if the temperature exceeds 900°C, the crystal grains may become coarse, deteriorating the physical properties of the steel. If the austenitizing time is less than 1 hour, the unavoidable carbides formed during cooling after hot rolling may not be sufficiently remelted, whereas if the austenitizing time exceeds 2 hours, the grains may become coarse, resulting in deterioration of the properties of the steel. Thereafter, the hot-rolled steel sheet that has been austenitized as described above is cooled to room temperature at a cooling rate of 0.5 to 20° C. / s. This cooling process is a normalizing or quenching process. The above cooling step can form a martensite phase as the steel structure, but care must be taken during this process to avoid the formation of ferrite and pearlite structures that significantly reduce the strength of the matrix.

[0038] Since the steel of the present invention contains elements that are advantageous in improving hardenability, such as Cr, Mo, and B, it is preferable to control the cooling rate to suppress the formation of ferrite, pearlite, etc. Specifically, the cooling is preferably performed at a cooling rate of 0.5°C / s or more. However, if the cooling rate exceeds 20°C / s, there is a risk of cracks occurring due to a thermal gradient caused by the difference in cooling rate between the center and surface of the steel plate. Subsequently, the hot-rolled steel sheet that has been normalized or quenched as described above can be subjected to a tempering treatment. At this time, the tempering treatment can be performed by heat treating the steel sheet at a temperature range of 580 to 680°C for 30 minutes or more per 25 mm of thickness. If the tempering temperature is less than 580°C, the precipitation of fine carbonitrides may not be induced within the heat treatment time due to the excessively low temperature, whereas if the temperature exceeds 680°C, the material may soften or become a dual phase region, resulting in the formation of an unintended structure and thus reducing the strength.

[0039] On the other hand, when tempering in the above temperature range, if the time is less than 30 minutes based on a steel plate thickness of 25 mm, heat is not sufficiently injected into the steel, and there is a risk that the intended precipitates will not be sufficiently formed. The above tempering time can be carried out for a time period during which the target precipitates are sufficiently formed, so there is no particular upper limit, but it is advantageous for the time not to exceed 120 minutes. Meanwhile, it should be made clear that the preferred thickness range of the steel material provided by the present invention may be 25 to 100 mm.

[0040] After the tempering heat treatment is completed, the steel sheet can be cooled to room temperature by air cooling. The steel material targeted by the present invention can be obtained through the above-mentioned series of steps. Preferably, the steel structure is composed of a tempered martensite phase, in which specific carbonitrides are uniformly distributed, thereby achieving improved hydrogen embrittlement resistance and impact properties.

[0041] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are provided to illustrate the present invention in more detail and are not intended to limit the scope of the present invention. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom. (Example)

[0042] A steel slab having the alloy composition shown in Table 1 below was prepared, heated at 1000 to 1200° C., and then finish hot-rolled at Ar3 or higher to produce a hot-rolled steel sheet having a thickness of 30 mm. Then, each hot-rolled steel sheet was reheated at various temperatures in the range of 800 to 900°C for at least 1 hour to a maximum of 2 hours to be austenitized, and then normalized or quenched and cooled to room temperature. At this time, the cooling by normalizing or quenching was performed at a cooling rate in the range of 0.5 to 20°C / s. Each of the hot-rolled steel sheets cooled as described above was tempered at various temperatures in the range of 580 to 680°C for at least 30 minutes per 25 mm of steel sheet thickness, and then air-cooled to room temperature to produce the final steel material. At this time, the tempering time was not more than 2 hours. Meanwhile, in the following Table 1, steel types 1 to 3 are existing ASTM A723 steel types, and the other steel types all satisfy the alloy composition proposed in the present invention.

[0043] For each of the steel materials manufactured as described above, bar-shaped tensile test pieces (total length 120 mm, parallel part 32 mm, gauge diameter 6.25 mm) of JIS No. 4 sub-size were prepared in the rolling direction. Then, using the ASTM E23 standard, impact test pieces with a V-notch in the middle of the test pieces with lengths of 10 mm horizontal x 55 mm vertical were prepared in the same rolling direction, and the impact properties were evaluated using a Charpy impact tester, and the results are shown in Table 3 below. At this time, the higher the absorbed energy, the better the toughness, and the average value was shown after three measurements (however, in the case of steel type 8, the measured values ​​were two times). In addition, to evaluate the strength and hydrogen embrittlement resistance of each steel material, notched tensile test pieces (notch diameter 3.6 mm, notch angle 60°) for hydrogen embrittlement experiments conforming to ASTM G142 in the rolling direction were prepared for each steel. The ultimate tensile strength (UTS) of the JIS No. 4 sub-size bar-shaped tensile test pieces and notched tensile test pieces was then measured in the general atmosphere, and the results are shown in Table 3 below.

[0044] Meanwhile, to create an environment in which hydrogen can be injected, the test specimen was placed in a cell that could hold 1N NaOH + 3g / L NH4SCN solution, and hydrogen was injected into the test specimen by continuous negative hydrogen charging. At the same time, a slow strain rate tensile test (SSRT, Fig. 1) was performed at a tensile speed of 1×10 -5 The hydrogen embrittlement resistance was evaluated using equipment capable of performing the test (measurement of the temperature at which the specimen is subjected to the hydrogen embrittlement test). The results are shown in Table 3. An index showing the strength and hydrogen embrittlement resistance of a material is the relationship between the notch tensile strength ratio (RNTS = notch tensile strength in hydrogen charging atmosphere (MPa) ÷ notch tensile strength in general air atmosphere (MPa)) and the tensile strength (GPa) of the steel material, which in the present invention is shown in Relational Formula 2. This applies the ratio of strength deterioration when hydrogen is charged to each test piece, and by multiplying the RNTS value by the tensile strength (GPa) of the material, the strength and hydrogen embrittlement resistance can be intuitively judged at the same time.

[0045] The same test pieces as the bar-shaped tensile test pieces were observed for the type of microstructure using a scanning electron microscope (SEM), and the results are shown in Table 3. The effective grain size was confirmed using electron backscatter diffraction (EBSD), and the results are shown in Figure 3. Furthermore, the distribution of precipitates in the observed microstructure was observed using a transmission electron microscope (TEM) and energy spectroscopy, and the results are shown in FIG. 4.

[0046] [Table 1]

[0047] [Table 2]

[0048] [Table 3]

[0049] As shown in Table 3, Examples 1 to 9 of the present invention, which satisfy the alloy composition and manufacturing conditions according to the present invention, are not only superior in hydrogen embrittlement resistance compared to Comparative Examples 1 to 3, which correspond to conventional steels, but also have excellent impact toughness, as they have an impact absorption energy value of 100 J or more (maximum 195 J or more) at -20°C.

[0050] FIG. 2a shows the EBSD measurement results for Comparative Examples 1-3, and FIG. 2b shows the EBSD measurement results for Invention Examples 1, 3, 5 and 7, from which the effective crystal grain size can be confirmed. As shown in Fig. 2b, the effective grain size of the inventive example is less than 3μm, which is significantly finer than the comparative example shown in Fig. 2a. Although no separate measurement photographs are shown for inventive example 9, the results were similar to those of the inventive examples.

[0051] FIG. 3 is a photograph of the distribution of precipitates in Example 3 observed by TEM and energy spectroscopy. In FIG. 3(a), Nb precipitates are indicated by white arrows, and it can be seen that their size is approximately 20 nm or less. On the other hand, although not shown explicitly, in the cases of Comparative Examples 1 to 3, cementite containing Fe was observed, and some cementite was also observed in Invention Example 3, which corresponds to the present invention (FIG. 3(b)). However, compared with such cementite, the size of the Nb precipitates was remarkably smaller and they were finely distributed, and therefore they were distinguishable (FIG. 3(c)).

[0052] Fig. 4 is a graph showing the values ​​of Relational Formula 2 for Comparative Examples 1 to 3 and Invention Examples 1 to 9. As shown in Fig. 4, it can be seen that the values ​​of Relational Formula 2 for Comparative Examples 1 to 3 are all less than 0.7, whereas the values ​​for Invention Examples are all 0.7 or more. On the other hand, in order to confirm the change in phase transformation according to the cooling rate after austenitization for each steel type, the hot-rolled steel sheets obtained by hot rolling were austenitized (reheating temperature in Table 2), and then cooled at different cooling rates (0.25, 0.5, 1.0, 2.5, 4.3, 10, 20 (℃ / s)) to confirm the phase transformation using a dilatometer. The results are shown in Figures 5a to 5h.

[0053] Comparative Examples 1 to 3 are examples that deviate from the alloy composition proposed in the present invention, and as shown in Figures 5a to 5c, transformation behavior to bainite is confirmed. On the other hand, all of the inventive examples according to the present invention (Figures 5d to 5h) show martensitic transformation behavior in the cooling rate range of the present invention (0.5 to 20°C / s), and the transformation temperature is about 300 to 400°C.

Claims

1. In weight percent, it is composed of carbon (C): 0.15 to 0.40%, silicon (Si): 0.4% or less (excluding 0%), manganese (Mn): 0.3 to 0.7%, sulfur (S): 0.01% or less (excluding 0%), phosphorus (P): 0.03% or less (excluding 0%), chromium (Cr): 0.6 to 2.0%, molybdenum (Mo): 0.15 to 0.8%, nickel (Ni): 1.6 to 4.0%, copper (Cu): 0.30% or less (excluding 0%), niobium (Nb): 0.12% or less (excluding 0%), nitrogen (N): 0.015% or less (excluding 0%), aluminum (Al): 0.06% or less, boron (B): 0.007% or less, the balance being Fe and inevitable impurity elements, The relationship between the sum (SUM) of the contents of C, Cu, Nb, Ni, Cr and Mo and the impurity elements satisfies the following Relation 1: The microstructure is composed of tempered martensite, and the effective grain size is an average diameter of 5 μm or less; The microstructure contains 20 precipitates with a diameter of 20 nm or less per μm. ２ There are more than A steel material having excellent hydrogen embrittlement resistance and impact toughness, characterized in that the precipitates having a diameter of 20 nm or less are Nb(C,N). [Relationship 1] |(C-SUM)・(CMSUM)・(Nb-SUM)・(Ni-SUM)・(CMSUM)・(M / -SUM)|×1 ５ >3.0 (Here, SUM means the total content of a specific impurity element, that is, the total content (wt%) of [W+Nd+Zr+Co].)

2. 2. The steel material having excellent hydrogen embrittlement resistance and impact toughness according to claim 1, characterized in that the steel material has a tensile strength of 900 MPa or more and an impact absorption energy value of 100 J or more at -20°C.

3. 2. The steel material having excellent hydrogen embrittlement resistance and impact toughness according to claim 1, characterized in that the relationship between the notch tensile strength ratio (RNTS, the ratio of the notch tensile strength in an atmosphere in which hydrogen is charged into a sample to the notch tensile strength in a general air atmosphere) and the tensile strength (GPa) of the steel material satisfies the following relational expression 2. [Relationship 2] (Notched tensile strength in hydrogen charging atmosphere (MPa) ÷ Notched tensile strength in general air atmosphere (MPa)) × tensile strength of steel (GPa) ≧ 0.7

4. In weight percent, carbon (C): 0.15-0.40%, silicon (Si): 0.4% or less (except 0%), manganese (Mn): 0.3-0.7%, sulfur (S): 0.01% or less (except 0%), phosphorus (P): 0.03% or less (except 0%), chromium (Cr): 0.6-2.0%, molybdenum (Mo): 0.15-0.8%, nickel (Ni): 1.6-4.0%, copper (Cu): 0.30% or less (except 0%) , niobium (Nb): 0.12% or less (except 0%), nitrogen (N): 0.015% or less (except 0%), aluminum (Al): 0.06% or less, boron (B) 0.007% or less, the balance being Fe and unavoidable impurity elements, and the relationship between the sum (SUM) of the contents of C, Cu, Nb, Ni, Cr and Mo and the impurity elements satisfies the following Relational Expression 1, and heating the steel slab at a temperature range of 1000 to 1200°C; hot rolling the heated steel slab to a finish rolling temperature Ar3 or higher to produce a hot-rolled steel sheet; Cooling the hot-rolled steel sheet to room temperature; an austenitizing step of reheating the cooled hot-rolled steel sheet to a temperature range of 800 to 900 ° C. and holding the reheated steel sheet for 1 to 2 hours; cooling the austenitized hot-rolled steel sheet to room temperature at a cooling rate of 0.5 to 20° C. / s; The method for manufacturing a steel material having excellent hydrogen embrittlement resistance and impact toughness according to claim 1, further comprising a tempering step of heat treating the steel plate at a temperature range of 580 to 680°C for 30 minutes or more per 25 mm of thickness after the cooling. [Relationship 1] |(C-SUM)・(CMSUM)・(Nb-SUM)・(Ni-SUM)・(CMSUM)・(M / -SUM)|×1 ５ >3.0 (Here, SUM means the total content of a specific impurity element, that is, the total content (wt%) of [W+Nd+Zr+Co].)

5. 5. The method of claim 4, wherein the step of cooling the austenitized hot-rolled steel sheet is performed by a normalizing or quenching process.

6. 5. The method of claim 4, further comprising the step of air-cooling the steel to room temperature after the tempering step.

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