Steel material and method for manufacturing same

Abstract

The present invention provides a method for manufacturing a steel material, the method comprising the steps of: hot-rolling a steel material comprising, by wt%, 0.08-0.11% of carbon, 0.10-0.20% of silicon, 1.0-1.2% of manganese, 0.015-0.055% of aluminum, 0.005-0.015% of niobium, 0.012-0.022% of titanium, 0.45-0.55% of chromium, 0.45-0.55% of molybdenum, 1.0-1.4% of nickel, 0.1-0.2% of copper, 0.001-0.002% or less of boron, and the remainder of iron and air-cooling the hot-rolled steel material to room temperature; subjecting the steel material to a first heat treatment at a temperature of 850-950°C; quenching the heat-treated steel material by rapid cooling down to room temperature; subjecting the quenched steel material to a second heat treatment at 790-830°C; air-cooling the heat-treated steel material to room temperature; and tempering the air-cooled steel material at a temperature of 550-650°C, wherein the second heat treatment temperature has a relational expression related to the tempering temperature.

Description

Steel and its manufacturing method

[0001] The present invention relates to steel and a method for manufacturing the same, and more specifically, to a high-strength heat-treatable steel having excellent low-temperature toughness and low yield performance, and a method for manufacturing the same.

[0002] With the tightening of global environmental regulations, interest in the treatment of CO2 gas emitted from industries is increasing. Consequently, the demand for tanks to liquefy, capture, and store CO2 gas is rising, and the demand for steel materials required for their production is also surging. Since CO2 gas has a liquefaction point of approximately -55°C under pressure of 7 to 8 bar, materials suitable for tanks require high strength characteristics capable of withstanding high pressure and low temperature, as well as excellent low-temperature toughness.

[0003] In addition, stress relief in welded joints is crucial when manufacturing gas tanks. Generally, post-weld heat treatment (PWHT) or mechanical stress relief (MSR) using hydraulic pressure are used for stress relief; however, for large tanks, the use of mechanical stress relief is unavoidable because heat treatment is difficult. Nevertheless, since the use of high hydraulic pressure makes material deformation likely, steel materials used for this purpose require a low yield ratio in addition to high strength and low-temperature toughness.

[0004] The problem that the present invention aims to solve is to provide a high-strength heat-treatable steel capable of achieving not only excellent low-temperature toughness but also low-yield performance through a microstructural configuration for realizing a low yield ratio, and a method for manufacturing the same.

[0005] However, these tasks are exemplary and do not limit the scope of the invention.

[0006] A method for manufacturing steel according to one aspect of the present invention comprises the steps of: (a) hot rolling a steel comprising, in weight percent, carbon (C): 0.08%~0.11%, silicon (Si): 0.10%~0.20%, manganese (Mn): 1.0%~1.2%, aluminum (Al): 0.015%~0.055%, niobium (Nb): 0.005%~0.015%, titanium (Ti): 0.012%~0.022%, chromium (Cr): 0.45%~0.55%, molybdenum (Mo): 0.45%~0.55%, nickel (Ni): 1.0%~1.4%, copper (Cu): 0.1%~0.2%, boron (B): 0.001%~0.002% or less, and the remainder being iron and unavoidable impurities, and air cooling to room temperature; (b) a step of first heat treating the steel at a first temperature (T1) within 850℃ to 950℃; (c) a step of quenching the heat-treated steel by rapidly cooling it to room temperature; (d) a step of second heat treating the quenched steel at a second temperature (T2) within 790℃ to 830℃; (e) a step of air-cooling the heat-treated steel to room temperature; and (f) a step of tempering the air-cooled steel at a third temperature (T3) within 550℃ to 650℃; wherein the third temperature (T3) is associated with the second temperature (T2) and satisfies the following Equation 1 or Equation 2.

[0007] Equation 1: T3 - 0.5 × T2 ≤ 235 (where 790℃ ≤ T2 ≤ 810℃)

[0008] Equation 2: T3 + 2 × T2 ≤ 2260 (where 810℃ < T2 ≤ 830℃)

[0009] (Here, T2 is the value of the second temperature (unit: ℃), and T3 is the value of the third temperature (unit: ℃))

[0010] In addition, the steel material after performing steps (a) to (f) above can satisfy a yield strength of 500 MPa or more, a low-temperature toughness of -60°C of 100 J or more, and a yield ratio of 0.8 or less.

[0011] In addition, the microstructure of the steel after performing steps (a) to (f) above may consist of ferrite: 60 to 80% and tempered martensite: 20 to 40% in terms of area fraction.

[0012] In addition, the above steel may further contain one or more of phosphorus (P): greater than 0 and less than or equal to 0.012%, sulfur (S): greater than 0 and less than or equal to 0.003%, and nitrogen (N): greater than 0 and less than or equal to 0.006% in weight%.

[0013] A steel material according to one aspect of the present invention comprises, in weight percent, carbon (C): 0.08%~0.11%, silicon (Si): 0.10%~0.20%, manganese (Mn): 1.0%~1.2%, aluminum (Al): 0.015%~0.055%, niobium (Nb): 0.005%~0.015%, titanium (Ti): 0.012%~0.022%, chromium (Cr): 0.45%~0.55%, molybdenum (Mo): 0.45%~0.55%, nickel (Ni): 1.0%~1.4%, copper (Cu): 0.1%~0.2%, boron (B): 0.001%~0.002% or less, and the remainder being iron and unavoidable impurities, and has a yield strength of 500 MPa or more and a low-temperature toughness of -60℃ It satisfies a yield ratio of 100J or more and a yield ratio of 0.8 or less, and the final microstructure consists of ferrite: 60 to 80% and tempered martensite: 20 to 40% in area fraction.

[0014] In addition, it may further include one or more of phosphorus (P): greater than 0 and less than or equal to 0.012% by weight, sulfur (S): greater than 0 and less than or equal to 0.003%, and nitrogen (N): greater than 0 and less than or equal to 0.006% by weight.

[0015] According to the present invention, a heat-treatable steel with excellent low-temperature (-40°C and -60°C) toughness and low yield performance and a method for manufacturing the same can be realized. For example, a heat-treatable steel with excellent low-temperature toughness and low yield performance satisfying a yield strength of 500 MPa or more, a low-temperature toughness of -60°C of 100 J or more, and a yield ratio of 0.8 or less can be realized and a method for manufacturing the same.

[0016] Of course, the scope of the present invention is not limited by these effects.

[0017] FIG. 1 is a flowchart illustrating a method for manufacturing steel according to one embodiment of the present invention.

[0018] Figure 2 is a graph showing the temperature over time in a method for manufacturing steel according to one embodiment of the present invention.

[0019] FIG. 3 is a graph illustrating the correlation between the second temperature (unit: ℃) at which the second heat treatment is performed and the third temperature (unit: ℃) at which the tempering heat treatment is performed in the comparative example and embodiment according to the first experimental example of the present invention.

[0020] Figure 4 is a photograph showing the microstructure of a steel material after the tempering process according to the first experimental example of the present invention.

[0021] Figure 5 is a photograph showing the microstructure of a steel material before and after the tempering process according to an embodiment of the second experimental example of the present invention.

[0022] Figure 6 is a photograph showing the microstructure of a steel material before the tempering process according to a comparative example among the second experimental examples of the present invention.

[0023] Figure 7 is a graph showing the change in stress according to the elongation rate in steel materials according to the example and comparative example of the second experimental example of the present invention.

[0024] The present invention will be described in detail below. However, in describing the present invention, if it is determined that a detailed description of related known technologies or configurations may unnecessarily obscure the essence of the present invention, such detailed description will be omitted.

[0025] Furthermore, the terms described below are defined in consideration of their functions in the present invention; since these may vary depending on the intentions or practices of the user or operator, their definitions should be based on the content throughout this specification describing the present invention.

[0026] The present invention provides a heat-treatable steel with excellent low-temperature toughness and low yield performance, satisfying a yield strength of 500 MPa or more, a low-temperature toughness of 100 J or more at -60°C, and a yield ratio of 0.8 or less, and having a final microstructure consisting of ferrite: 60 to 80% and tempered martensite: 20 to 40% in area fraction, and a method for manufacturing the same.

[0027] steel plate

[0028] A steel sheet according to one embodiment of the present invention comprises, in weight percent, carbon (C): 0.08%~0.11%, silicon (Si): 0.10%~0.20%, manganese (Mn): 1.0%~1.2%, aluminum (Al): 0.015%~0.055%, niobium (Nb): 0.005%~0.015%, titanium (Ti): 0.012%~0.022%, chromium (Cr): 0.45%~0.55%, molybdenum (Mo): 0.45%~0.55%, nickel (Ni): 1.0%~1.4%, copper (Cu): 0.1%~0.2%, boron (B): 0.001%~0.002% or less, and the remainder being iron and unavoidable impurities. In addition, the steel plate may further contain one or more of, in weight percent, phosphorus (P): greater than 0 and less than or equal to 0.012%, sulfur (S): greater than 0 and less than or equal to 0.003%, and nitrogen (N): greater than 0 and less than or equal to 0.006%.

[0029] Hereinafter, the role and content of each component included in the steel plate according to one embodiment of the present invention will be described. The following contents refer to weight percent.

[0030] Carbon (C): 0.08%~0.11%

[0031] Carbon (C) is the main element that forms a martensite constituent (MA), which is susceptible to toughness. While excessive carbon content is detrimental to toughness, the addition of an appropriate amount of carbon, a key element for ensuring hardenability, is essential for steel grades that require quenching after heat treatment, as the formation of low-temperature structures such as martensite or bainite is important. If the carbon content is less than 0.08%, sufficient quenching effect may not be achieved, potentially resulting in insufficient strength. If the carbon content exceeds 0.11%, the formation of a large amount of MA phase leads to reduced weld toughness and impact toughness, and there are problems with exceeding the target strength.

[0032] Silicon (Si): 0.10%~0.20%

[0033] Silicon (Si) is added as a deoxidizer to remove oxygen from steel during the steelmaking process along with aluminum, and it can also have a solid solution strengthening effect. In addition, it can be utilized as an element to improve hardenability in steels for heat treatment. On the other hand, silicon (Si) is known as an element that promotes the formation of the MA phase, which is susceptible to poor toughness, by inhibiting the formation of cementite. If the silicon content is less than 0.10%, there is a possibility that the hardenability will be insufficient, and if a large amount of silicon is added exceeding 0.20% of the total weight, toughness decreases due to an increase in the MA phase fraction, and there is a risk of brittle fracture because the MA phase formed in the heat-affected zone (HAZ) does not decompose.

[0034] Manganese (Mn): 1.0%~1.2%

[0035] Manganese (Mn) is an element that increases solid solution strengthening and hardenability. By increasing the hardenability of steel, it can suppress the formation of grain boundary ferrite in the heat-affected zone (HAZ) during cooling after welding, thereby improving the toughness of the HAZ. To ensure sufficient hardenability for thick heat-treated materials, it is necessary to add at least 1.0% manganese. However, if manganese is added excessively beyond 1.2%, microstructural non-uniformity due to Mn segregation in the center may occur, potentially degrading the toughness of the HAZ; therefore, it is desirable to limit the upper limit to 1.2%.

[0036] Aluminum (Al): 0.015%~0.055%

[0037] Since aluminum (Al) is a major deoxidizer for molten steel, it needs to be added in an amount of 0.015% or more to achieve a deoxidation effect. If the content exceeds 0.055%, not only is the deoxidation effect saturated, but the fraction and size of Al2O3 inclusions also increase, which can reduce toughness; therefore, it is desirable to limit the upper limit to 0.055%.

[0038] Niobium (Nb): 0.005%~0.015%

[0039] Niobium (Nb) is a key element utilized to improve strength through Nb(C,N) precipitation hardening during hot rolling, so it needs to be added at a level of 0.005% or more. However, excessive addition leads to a problem where low-temperature toughness decreases due to the formation of coarse Nb(C,N) precipitates. Additionally, while it has a positive effect on micrograin refinement through the pinning effect, it may exceed the upper limit of yield strength resulting from micrograin refinement. Therefore, the Nb content may be limited to less than 0.015%.

[0040] Titanium (Ti): 0.012%~0.022%

[0041] Titanium (Ti) is an important element that refines the grain size of the weld zone by forming carbonitrides. When added at less than 0.012%, the amount of precipitated Ti carbonitrides is small, so the effect on grain suppression is negligible. When added at more than 0.022%, the Ti carbonitrides formed in the weld zone become coarse, which reduces the effect of suppressing grain growth and facilitates the initiation and propagation of brittle cracks, resulting in a decrease in toughness.

[0042] Chrome (Cr): 0.45 ~ 0.55%

[0043] Chromium is an element that increases hardenability, thereby increasing yield strength and tensile strength through solid solution. According to an embodiment of the present invention, the addition of an appropriate amount of chromium is essential for the formation of martensite during quenching, and this effect can be achieved when the chromium content is 0.45% or more. However, if the chromium content exceeds 0.55%, there is a problem that an excessive low-temperature transformation phase structure is formed, which reduces the low-temperature toughness of the weld heat-affected zone and may increase manufacturing costs.

[0044] Molybdenum (Mo): 0.45 ~ 0.55%

[0045] Molybdenum (Mo) has the effect of increasing the strength of steel by increasing its hardenability, similar to the chromium described earlier. Specifically, it has the effect of increasing strength by delaying the transformation during the cooling process after heat treatment. To obtain this effect sufficiently, it is desirable to add at least 0.45%. However, since molybdenum is an expensive element, excessive addition is economically disadvantageous and can reduce toughness by excessively increasing the hardness of the weldment. Therefore, in a steel plate according to one embodiment of the present invention, the upper limit of the molybdenum content is controlled to 0.55%.

[0046] Nickel (Ni): 1.0 ~ 1.4%

[0047] Nickel (Ni) is a useful element capable of simultaneously increasing strength and toughness, and is an effective element for improving low-temperature impact toughness. When the nickel content is added at 1.0% or more, the above effects can be achieved. In addition, nickel can also improve hardenability when added in large amounts, so it can be utilized as an element to address the problem of reduced hardenability caused by the addition of chromium and molybdenum. However, if the nickel content exceeds 1.4%, it is economically disadvantageous, so the upper limit is controlled to 1.4% by weight.

[0048] Copper (Cu): 0.1 ~ 0.2%

[0049] Copper (Cu) is an effective element for increasing strength and improving toughness, so it is necessary to add more than 0.1%. However, if the copper content exceeds 0.2%, it can cause surface defects, and there is a problem that the carbon equivalent increases with increasing content.

[0050] Boron (B): 0.001 ~ 0.002%

[0051] Boron (B) is an element effective in improving strength by effectively increasing the hardenability of steel even with a small amount of addition, so it is necessary to add at least 0.001%. In heat-treatable steel, the addition of an appropriate amount of boron increases hardenability and helps to form a complete martensite structure. On the other hand, if the content is excessive, it causes problems that impair the toughness and weldability of the steel, so it is desirable not to exceed 0.002%.

[0052] Meanwhile, the above steel plate may further include one or more of phosphorus (P): greater than 0 and less than or equal to 0.012%, sulfur (S): greater than 0 and less than or equal to 0.003%, and nitrogen (N): greater than 0 and less than or equal to 0.006% in weight percent as described below.

[0053] Phosphorus (P): Greater than 0, 0.012% or less

[0054] Phosphorus (P) is an element that causes grain boundary segregation in the base material and weldment, and it is necessary to actively reduce it to prevent the problem of steel embrittlement; however, reducing phosphorus to almost the limit increases the load on the steelmaking process. In this invention, it was confirmed that weldment toughness improves as the P content decreases. Accordingly, in order to secure excellent weldment toughness in high-strength steel, it is necessary to limit it to 0.012% or less.

[0055] Sulfur (S): Greater than 0, 0.003% or less

[0056] Sulfur (S) is one of the representative impurity elements that can impair low-temperature toughness by combining with Mn to form MnS inclusions, and can embrittle steel by causing red-hot brittleness. Therefore, it is desirable to control its content as low as possible, and it is necessary to limit its content to 0.003% or less.

[0057] Nitrogen (N): Greater than 0, 0.006% or less

[0058] Nitrogen (N) forms precipitates such as AlN, Tin, and NbN, which increase iron loss and inhibit grain growth, so it is added as low as possible and limited to 0.006 wt% or less. If the nitrogen content exceeds 0.006 wt%, the problem of increased iron loss appears.

[0059] The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any person skilled in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0060] In order to manufacture a product satisfying composite properties such as strength, toughness, and yield ratio, the present invention utilizes a quenching and tempering (QT) heat treatment method as described below, and by performing an aberration region heat treatment between the quenching and tempering processes followed by an air cooling process, the microstructure and composite properties are to be secured as follows.

[0061] The final microstructure of the steel according to an embodiment of the present invention having the alloy element composition as described above may consist of ferrite: 60 to 80% and tempered martensite: 20 to 40% in area fraction.

[0062] The above steel plate satisfies a yield strength of 500 MPa or more, a tensile strength of 610 to 770 MPa, an elongation of 21% or more, and a yield ratio of 0.80 or less, and can satisfy a low-temperature toughness of -60℃ of 100 J or more.

[0063] Hereinafter, a method for manufacturing steel according to one embodiment of the present invention having the alloy element composition described above will be explained.

[0064] Method of manufacturing steel

[0065] FIG. 1 is a flowchart illustrating a method for manufacturing steel according to one embodiment of the present invention, and FIG. 2 is a graph showing temperature over time in a method for manufacturing steel according to one embodiment of the present invention.

[0066] Referring to FIGS. 1 and 2, the method for manufacturing the steel comprises: (a) hot rolling a steel comprising, in weight percent, carbon (C): 0.08%~0.11%, silicon (Si): 0.10%~0.20%, manganese (Mn): 1.0%~1.2%, aluminum (Al): 0.015%~0.055%, niobium (Nb): 0.005%~0.015%, titanium (Ti): 0.012%~0.022%, chromium (Cr): 0.45%~0.55%, molybdenum (Mo): 0.45%~0.55%, nickel (Ni): 1.0%~1.4%, copper (Cu): 0.1%~0.2%, boron (B): 0.001%~0.002% or less, and the remainder being iron and unavoidable impurities. (S110) a step of air cooling to room temperature; (b) a step of first heat treating the steel at a temperature of 850℃ to 950℃; (S120) (c) a step of quenching the heat-treated steel by rapidly cooling it to room temperature; (S130) (d) a step of second heat treating the quenched steel in an ideal temperature range; (S140) (e) a step of air cooling the heat-treated steel to room temperature; (S150) and (f) a step of tempering the air-cooled steel at a temperature of 550℃ to 650℃; (S160).

[0067] Hot rolling step (S110)

[0068] First, through the slab reheating process of the slab produced by the continuous casting process, the redissolution of segregated components and precipitates may occur during casting. If the reheating temperature of the slab is below 1050°C, there is a problem where the rolling load increases due to insufficient heating temperature. In addition, Nb-based precipitates, for example, Nb carbonitrides (Nb(C,N)), do not reach the solid solution temperature and are not reprecipitated as fine precipitates during hot rolling, which fails to inhibit the growth of austenite grains and causes the austenite grains to coarsen rapidly. Conversely, if the reheating temperature exceeds 1250°C, the austenite grains coarsen rapidly or decarburization occurs, making it difficult to secure the strength and low-temperature toughness of the manufactured steel. Subsequently, the steel is hot-rolled to produce a hot-rolled steel sheet of the required thickness and air-cooled to room temperature.

[0069] The thickness of the hot-rolled steel sheet is preferably 100 mm or less, and more preferably 10 to 50 mm. Hot rolling may be performed using AR (As Rolled), Controlled Rolling, or TMCP. The rolling finish temperature is preferably completed within the range of 900 to 1050°C, the temperature at which ferrite is generated.

[0070] Since the microstructure of the final product is primarily determined by the steps following hot rolling (S120, S130, S140, S150, S160), it may not be affected by the hot rolling process. In the present invention, hot rolling can perform the function of sizing rolling, which determines the size of the sheet metal through an as-roll process to match the required product specifications prior to the subsequent steps (S120, S130, S140, S150, S160).

[0071] 1st heat treatment step (S120)

[0072] Next, the rolled steel is air-cooled, and then reheated at a temperature of 850°C to 950°C for 10 to 120 minutes, i.e., first heat treatment (S120).

[0073] The above heat treatment is intended to homogenize the components within the structure by re-dissolving carbides remaining in the rolled steel through austenitization, which reverses the transformation of a hot-rolled steel sheet composed of ferrite and pearlite into a single austenite phase. If the first temperature (T1) at which the above first heat treatment is performed is less than 850°C, austenitization is not sufficiently achieved, resulting in the presence of coarse soft ferrite, which causes a problem of reduced hardness in the final product. On the other hand, if the first temperature (T1) at which the above first heat treatment is performed exceeds 950°C, the austenite grains become coarse, which has the effect of increasing hardenability, but there is a problem of reduced low-temperature toughness of the steel. Meanwhile, if the above heating time is less than 10 minutes, austenitization does not occur sufficiently, so the phase transformation by subsequent rapid cooling cannot occur. On the other hand, if it exceeds 120 minutes, the austenite grains become coarse, which has the effect of increasing hardenability, but there is a problem of unfavorable productivity.

[0074] Quenching step (S130)

[0075] Next, the above-mentioned first heat-treated steel is quenched to room temperature (S130).

[0076] As one embodiment, high strength can be obtained by quenching at a cooling rate of, for example, 10 to 40°C / s. According to an embodiment of the present invention, a fully martensite matrix structure with supersaturated carbon can be formed by quenching the steel to room temperature. After quenching, the austenite grain size may be at the level of 20 to 25 μm.

[0077] Second heat treatment step (S140)

[0078] A secondary heat treatment is performed on the above-mentioned quenched steel to perform an abnormal region heat treatment (S140).

[0079] In one embodiment of the present invention, heat treatment (Intercritical annealing) can be performed on steel in an intercritical region between ferrite and austenite to secure a low yield ratio. In this case, the second heat treatment in the present invention can be performed in an intercritical region where ferrite and austenite can coexist. More strictly, the second temperature (T2) for performing the second heat treatment can be performed within the range of 790°C to 830°C. If the second temperature (T2) for performing the second heat treatment is performed at a temperature lower than 720°C, which is the lower limit of the intercritical region range, the final microstructure realized by the process of the present invention described later becomes a single phase of tempered martensite, resulting in a problem where the yield ratio increases. Furthermore, if the second temperature (T2) for performing the second heat treatment is performed at a temperature lower than 790°C, the fraction of tempered martensite in the final microstructure is low, resulting in a problem where the strength decreases. Conversely, if the second temperature (T2) at which the second heat treatment is performed exceeds 830℃, the second heat treatment is performed in the austenite single-phase region, so there may be difficulties in achieving an abnormal structure after performing the second heat treatment.

[0080] When the above secondary heat treatment is performed within the range of 790℃ to 830℃, partial transformation into austenite occurs, and at the same time, carbon (C) is enriched. This is a principle that arises due to the difference in carbon solubility between austenite and ferrite. Therefore, the untransformed remaining portion remains as ferrite because carbon (C) has diffused into austenite, and accordingly, the final austenite fraction can be secured, for example, at a fraction of 50 to 90%.

[0081] The range of 790°C to 830°C applied in the present invention is a temperature range in which the area fraction of phase transformation into austenite is 50 to 90%. If rapid cooling is performed after the second heat treatment, the 50 to 90% fraction of austenite transforms into martensite as is, but if air cooling is performed after the second heat treatment, some of the 50 to 90% fraction transforms into ferrite again, and only about 20 to 40% remains as martensite.

[0082] Air cooling stage (S150)

[0083] The above secondary heat-treated steel is air-cooled to room temperature (S150).

[0084] In order to achieve a low yield ratio in steel, it is generally necessary to have a microstructure composed of two phases, a soft phase and a hard phase. However, as a comparative example, heat-treated steel subjected to a conventional quenching and tempering process has the disadvantage of having a very high yield ratio of 0.95 or higher because it is composed of a single structure of tempered martensite. Furthermore, if quenching is performed instead of air cooling after the second heat treatment, the phases that were transformed into austenite in a fraction of 50 to 90 percent by the second heat treatment step exist as hard martensite in a similar fraction at room temperature. Consequently, if the fraction of tempered martensite in the final structure is maintained at the aforementioned fraction, the yield ratio rises again, which may be disadvantageous in securing a low yield ratio of 0.8 or lower.

[0085] In one embodiment of the present invention, by performing an air cooling step after the second heat treatment, some of the phases that were transformed into austenite by the second heat treatment step in the microstructure are transformed into ferrite, and the remaining area fraction remains as a martensite austenite constituent (MA) in which carbon is more supersaturated. The cooling rate in the air cooling step may be less than 3°C / s, and strictly speaking, may be less than 1°C / s, for example. If the cooling rate in the air cooling step is 3°C / s or more, the yield ratio increases because the tempered martensite fraction in the final microstructure increases, making it difficult to satisfy a yield ratio of 0.8 or less. In addition, the air cooling holding time in the air cooling step may be, for example, 50 to 120 minutes, and the thicker the steel, the longer the holding time.

[0086] Tempering step (S160)

[0087] Next, the air-cooled steel is tempered at a temperature of 550°C to 650°C (S160). Tempering is an essential process for removing residual stress formed by quenching and improving toughness. If tempering is not performed, the martensite structure or MA phase formed during the air-cooling step acts as a factor in the reduction of toughness. Accordingly, toughness is improved by transforming the martensite structure or MA phase into a tempered martensite structure.

[0088] Therefore, the ideal microstructure satisfying the low yield ratio can be composed of ferrite: 60 to 80% and tempered martensite: 20 to 40% in area fraction.

[0089] Tempering heat treatment is performed at a temperature below Ac1, at which phase transformation begins, and is characterized by lower strength and higher toughness as the temperature increases. In the present invention, the third temperature (T3) at which the tempering process is performed may be 550℃ to 650℃. When heat treatment is performed at a temperature below 550℃, strength increases but low-temperature toughness decreases, which is a disadvantage. When tempering at a temperature exceeding 650℃, carbides coarsen and material softening occurs, resulting in problems where the yield ratio and strength are insufficient.

[0090] Furthermore, in the present invention, under the premise that the second temperature (T2) for performing the second heat treatment satisfies the range of 790℃ to 830℃, the third temperature (T3) for performing the tempering heat treatment is associated with the second temperature (T2) for performing the second heat treatment and satisfies the following Equation 1 or Equation 2.

[0091] Equation 1: T3 - 0.5 × T2 ≤ 235 (where 790℃ ≤ T2 ≤ 810℃)

[0092] Equation 2: T3 + 2 × T2 ≤ 2260 (where 810℃ < T2 ≤ 830℃)

[0093] Here, T2 is the value of the second temperature (unit: ℃) for performing the second heat treatment, and T3 is the value of the third temperature (unit: ℃) for performing the tempering heat treatment.

[0094] In the case where the second temperature (T2) at which the above second heat treatment is performed is 790℃ ≤ T2 ≤ 810℃, if Equation 1 above is not satisfied, a problem may arise in that a yield strength of 500MPa or more cannot be achieved.

[0095] Meanwhile, in the case where the second temperature (T2) at which the second heat treatment is performed is 810℃ < T2 ≤ 830℃, if Equation 2 is not satisfied, a problem may arise in which a yield ratio of 0.80 or less cannot be achieved.

[0096] The final microstructure of the steel material achieved by performing the steps described above consists of ferrite: 60 to 80% and tempered martensite: 20 to 40% in terms of area fraction, and the steel material satisfies a yield strength of 500 MPa or more, a tensile strength of 610 to 770 MPa, an elongation of 21% or more, and a yield ratio of 0.80 or less, and satisfies a low-temperature toughness of -60°C of 100 J or more.

[0097] Experimental Example

[0098] Hereinafter, the structure and operation of the present invention will be explained in more detail through preferred embodiments and comparative examples. However, these are presented as merely examples of the present invention and should not be interpreted in any way as limiting the present invention. Details not described herein can be sufficiently technically inferred by those skilled in the art, so such descriptions are omitted.

[0099] Experimental Example 1

[0100] Table 1 shows the composition (unit: weight%) of the steel according to the experimental example of the present invention. The remainder consists of iron (Fe) and impurities inevitably contained in the steelmaking process, etc.

[0101] CSiMnPSAlNb0.09470.1681.0840.0080.0030.0330.011TiCrMoNiCuNB0.0160.480.481.030.130.0050.0013

[0102] Referring to Table 1, the composition (unit: weight%) of the steel according to the experimental example of the present invention is, in weight%, Carbon (C): 0.08%~0.11%, Silicon (Si): 0.10%~0.20%, Manganese (Mn): 1.0%~1.2%, Aluminum (Al): 0.015%~0.055%, Niobium (Nb): 0.005%~0.015%, Titanium (Ti): 0.012%~0.022%, Chromium (Cr): 0.45%~0.55%, Molybdenum (Mo): 0.45%~0.55%, Nickel (Ni): 1.0%~1.4%, Copper (Cu): 0.1%~0.2%, Boron (B): 0.001%~0.002% or less, Phosphorus (P): greater than 0% and 0.012%. It satisfies the composition range consisting of: sulfur (S): greater than 0 and less than or equal to 0.003%, nitrogen (N): greater than 0 and less than or equal to 0.006%, and the remainder being iron.

[0103] The measured phase transformation temperatures applied to the experimental example of the present invention having the composition described above are Ac1= 720℃ and Ac3= 840℃.

[0104] Table 2 shows the process conditions applied to the first experimental example of the present invention.

[0105] In Table 2, SRT is the reheating temperature before hot rolling, and FRT (Finish Rolling Temperature) is the finishing rolling temperature of hot rolling. The temperature and time of the first heat treatment are the temperature (T1) and holding time (unit: minutes) in the first heat treatment step (S120), the temperature and time of the second heat treatment are the temperature (T2) and holding time (unit: hours) in the second heat treatment step (S140), and the tempering temperature and tempering time are the temperature (T3) and holding time (unit: hours) in the tempering step (S160). In the first experimental example, quenching corresponds to a cooling rate of 3℃ / sec or higher, and air cooling corresponds to a cooling rate of less than 3℃ / sec. Other process conditions satisfy the process range according to the manufacturing method of the present invention described above with reference to FIGS. 1 and 2, but a single condition was applied for each step.

[0106] Rolling Conditions (As-roll) Heat Treatment Conditions SRTFRT (S110) Cooling 1st Heat Treatment (S120) Cooling (S130) 2nd Heat Treatment (S140) Cooling (S150) Tempering (S160) Scope of the Invention 1050-1250 900-1050 Air Cooling 850-950℃ Quenching 790-830℃ Air Cooling 550-650 Comparative Example A1 1110℃ 960℃ Air Cooling 904℃ @ 33 Min. Quenching 770℃ 1h Air Cooling 580℃ @ 1h Comparative Example A2 1110℃ 960℃ Air Cooling 904℃ @ 33 Min. Quenching 770℃ 1h Air Cooling 600℃ @ 1h Comparative Example A3 1110℃ 960℃ Air Cooling 904℃ @ 33 Min. Quenching 770℃ 1h Air Cooling 620℃ @ 1h Comparative Example A4 1110℃ 960℃ Air Cooling 904℃ @ 33 Min. Quenching 770℃ 1h Air Cooling 630℃ @ 1h Comparative Example A5 1110℃ 960℃ Air Cooling 904℃ @ 33 Min. Quenching 770℃ 1h Air Cooling 640℃ @ 1h Example A1 1110℃ 960℃ Air Cooling 904℃ @ 33 Min. Quenching 790℃ @ 1h Air Cooling 580℃ @ 1h Example A2 1110℃ 960℃ Air Cooling 904℃ @ 33 Min. Quenching 790℃ @ 1h Air Cooling 600℃ @ 1h Example A3 1110℃ 960℃ Air Cooling 904℃ @ 33 Min. Quenching 790℃ @ 1h Air Cooling 620℃ @ 1h Example A4 1110℃ 960℃ Air Cooling 904℃ @ 33 Min. Quenching 790℃ @ 1h Air Cooling 630℃ @ 1h Comparative Example A6 1110℃ 960℃ Air Cooling 904℃ @ 33 Min. Quenching 790℃ @ 1h Air Cooling 640℃ @ 1h Example A5 1110℃ 960℃ Air Cooling 904℃ @ 33 Min. Quenching 810℃ @ 1h Air Cooling 580℃ @ 1h Example A6 1110℃ 960℃ Air Cooling 904℃ @ 33 Min. Quenching 810℃ @ 1h Air Cooling 600℃ @ 1h Example A7 1110℃ 960℃ Air cooling 904℃ @ 33 Min. Quenching 810℃ @ 1h Air cooling 620℃ @ 1h Example A8 1110℃ 960℃ Air cooling 904℃ @ 33 Min. Quenching 810℃ @ 1h Air cooling 630℃ @ 1h Example A9 1110℃ 960℃ Air cooling 904℃ @ 33 Min. Quenching 810℃ @ 1h Air cooling 640℃ @ 1h Example A10 1110℃ 960℃ Air cooling 904℃ @ 33 Min. Quenching 830℃ @ 1h Air cooling 580℃ @ 1h Example A1 11110℃ 960℃ Air cooling 904℃ @ 33 Min. Quenching 830℃ @ 1h Air cooling 600℃ @ 1h Comparative Example A7 1110℃ 960℃ Air cooling 904℃ @ 33 min. Quenching 830℃ @ 1 h Air cooling 620℃ @ 1 h Comparative Example A8 1110℃ 960℃ Air cooling 904℃ @ 33 min.Quenching 830℃@1h Air cooling 630℃@1h Comparative Example A91110℃960℃ Air cooling 904℃@33Min. Quenching 830℃@1h Air cooling 640℃@1h.

[0107] Table 3 shows the physical properties and microstructure fractions of the steel material implemented in the first experimental example of the present invention. In Table 3, the microstructure fraction (%) corresponds to the area fraction, where F is ferrite and TM is tempered martensite. The microstructure area fraction was calculated using optical microscope images according to the ISO 9042 standard. Yield strength (YS), tensile strength (TS), and elongation (EL) were measured by the metal tensile test method according to the ASTM E8 / E8M standard. Impact toughness was evaluated by the Charpy impact test method according to the JIS Z 2242 standard.

[0108] Mechanical Properties Microstructure Fraction (%) YS (MPa)TS (MPa)El (%) YRRatio Impact Toughness (J @ -60℃) Impact Toughness (J @ -80℃) FTM Scope of the Invention 500↑ 610-770 21.0↑ 0.8↓ 100 J↑ -60~80 20~40 Comparative Example A1 49 869 321.6 0.7 215 48 881~90 10~19 Comparative Example A2 49 268 221.2 0.7 219 212 881~90 10~19 Comparative Example A3 48 867 422.4 0.7 220 216 281~90 10~19 Comparative Example A4 49 067 222.7 0.7 319 416 681~90 10~19 Comparative Example A547965423.60.7322018481~9010~19 Example A152271221.50.7318813270~8020~30 Example A251669722.30.7421017270~8020~30 Example A350768223.90.7425221370~8020~30 Example A450467823.50.7423521870~8020~30 Comparative Example A649666423.40.7524423370~8020~30 Example A554572421.70.7519414565~7525~35 Example A653771622.50.7522016765~7525~35 Example A752668322.80.7721419165~7525~35 Example A852467423.40.7822421965~7525~35 Example A951466223.20.7823522865~7525~35 Example A1057373820.40.7818812860~6535~40 Example A1156271421.20.7920117460~6535~40 Comparative Example A757670021.70.8221919860~6535~40Comparative Example A856668922.10.8221119460~6535~40Comparative Example A955466422.60.8323420460~6535~40

[0109] Referring to Tables 2 and 3, Examples A1 to 10 include a step (S120) of performing a first heat treatment on a hot-rolled steel at a first temperature (T1) within 850°C to 950°C; a step (S130) of rapidly cooling the first heat-treated steel to room temperature to quench it (S140); a step (S150) of performing a second heat treatment on the quenched steel at a second temperature (T2) within 790°C to 830°C; a step (S150) of air-cooling the heat-treated steel to room temperature; and a step (S160) of tempering the air-cooled steel at a third temperature (T3) within 550°C to 650°C; wherein the third temperature (T3) is designed to satisfy Equation 1 or Equation 2 below in relation to the second temperature (T2).

[0110] Equation 1: T3 - 0.5 × T2 ≤ 235 (where 790℃ ≤ T2 ≤ 810℃)

[0111] Equation 2: T3 + 2 × T2 ≤ 2260 (where 810℃ < T2 ≤ 830℃)

[0112] (Here, T2 is the value of the second temperature (unit: ℃), and T3 is the value of the third temperature (unit: ℃))

[0113] The final microstructure of the steel material implemented according to Examples A1 to 10 consists of ferrite: 60 to 80% and tempered martensite: 20 to 40% in terms of area fraction, and it can be confirmed that the steel material satisfies a yield strength of 500 MPa or more, a tensile strength of 610 to 770 MPa, an elongation of 21% or more, and a yield ratio of 0.80 or less, and satisfies a low-temperature toughness of -60°C of 100 J or more.

[0114] In contrast, Comparative Examples A1 to A5 satisfy the range of 550°C to 650°C for the third temperature (T3) of the tempering step (S160), but fail to satisfy the range of 790°C to 830°C for the second temperature (T2) of the second heat treatment step (S140), and thus fall below the range of 60 to 80% for ferrite in the final microstructure, fail to satisfy the range of 20 to 40% for tempered martensite, and fail to satisfy the range of 500 MPa or more for yield strength.

[0115] Comparative Example A6 satisfies the range of the second temperature (T2) of the second heat treatment step (S140): 790℃ ~ 830℃, and the third temperature (T3) of the tempering step (S160): 550℃ ~ 650℃, but does not satisfy Equation 1 above, so it can be confirmed that it does not satisfy the range of yield strength: 500MPa or higher and falls below.

[0116] Comparative examples A7, A8, and A9 satisfy the range of the second temperature (T2) of the second heat treatment step (S140): 790℃ ~ 830℃ and the third temperature (T3) of the tempering step (S160): 550℃ ~ 650℃, but do not satisfy Equation 2 above, so it can be confirmed that they do not satisfy the range of yield ratio: 0.80 or less and exceed it.

[0117] The results of the first experimental example are viewed from a different perspective as follows.

[0118] Comparative Examples A1, A2, A3, A4, and A5 are cases in which a phase transformation of about 25% was performed, followed by a phase transformation heat treatment (secondary heat treatment) at 770°C, and then tempering at 580°C, 600°C, 620°C, 630°C, and 640°C, respectively. The microstructure fractions fall outside the range specified in the present invention, and the high ferrite content does not satisfy a yield strength of 500 MPa or higher.

[0119] Examples A1, A2, A3, A4, and Comparative Example A5 are cases in which a phase transformation of approximately 50% was performed, followed by a phase-phase heat treatment (secondary heat treatment) at 790°C, and then tempering at 580°C, 600°C, 620°C, 630°C, and 640°C, respectively. Since the microstructure fraction is 70–80% ferrite and 20–30% tempered martensite, it satisfies the scope of the present invention. However, due to the characteristic that strength decreases as the tempering temperature increases, in the case of tempering at 640°C, a yield strength of 500 MPa or more is not satisfied due to excessive softening.

[0120] Examples A5, A6, A7, A8, and A9 are cases in which a phase transformation of approximately 80% was performed, followed by a phase transformation heat treatment (secondary heat treatment) at 810°C, and then tempering at 580°C, 600°C, 620°C, 630°C, and 640°C, respectively. The physical properties are sufficiently satisfied under all tempering conditions.

[0121] Examples A10, A11, Comparative Example A7, Comparative Example A8, and Comparative Example A9 are cases in which a phase transformation of 90% or more was performed at 830°C, followed by tempering at 580°C, 600°C, 620°C, and 640°C, respectively. As the phase transformation heat treatment temperature increases, the yield ratio tends to increase as the tempered martensite fraction increases. Therefore, the overall yield ratio is high, and the material properties are satisfied only when tempering is performed at 580°C and 600°C, respectively. When tempering at 620°C or higher, the yield ratio exceeds the range of 0.8 or less due to an excessive decrease in tensile strength.

[0122] FIG. 3 is a graph illustrating the correlation between the second temperature (unit: ℃) at which the second heat treatment is performed and the third temperature (unit: ℃) at which the tempering heat treatment is performed in the comparative example and embodiment according to the first experimental example of the present invention.

[0123] In the embodiments according to the first experimental example of the present invention, the ordered pair consisting of the second temperature (unit: ℃) for performing the second heat treatment and the third temperature (unit: ℃) for performing the tempering heat treatment can be seen to be located inside a polygon with vertices at coordinates A (790℃, 580℃), B (790℃, 630℃), C (810℃, 640℃), D (830℃, 600℃), and E (830℃, 580℃) in the graph shown in FIG. 3.

[0124] Figure 4 is a photograph of the final microstructure of a steel material according to Example 5A of the first experimental example of the present invention.

[0125] Referring to Figure 4, it can be seen that the final microstructure of the steel consists of ferrite and tempered martensite, satisfying the range of ferrite: 60 to 80% and tempered martensite: 20 to 40% in terms of area fraction.

[0126] 2nd Experimental Example

[0127] In the second experimental example, the composition of the steel material was applied as disclosed in Table 1.

[0128] Table 4 shows the process conditions applied to the second experimental example of the present invention. In Table 4, SRT is the reheating temperature before hot rolling, and FRT is the finishing rolling temperature of hot rolling. The temperature and time of the first heat treatment are the temperature and time in the first heat treatment step (S120), the temperature and time of the second heat treatment are the temperature and time in the second heat treatment step (S140), which is an ideal region heat treatment, and the tempering temperature and tempering time are the temperature and time in the tempering step (S160). Item "X" indicates that the corresponding process was not performed. The measured values ​​of the phase transformation temperatures applied to the experimental example of the present invention are Ac1 = 720℃ and Ac3 = 840℃. Other process conditions satisfy the process range according to the manufacturing method of the present invention described above with reference to FIGS. 1 and FIGS. 2, but a single condition was applied for each step.

[0129] Process Rolling Conditions (As-roll) Heat Treatment Conditions SRTFRT Cooling 1st Heat Treatment Cooling 2nd Heat Treatment Cooling Tempering Example B1 1120℃ 934℃ Air Cooling 900℃ @1h Quenching 810℃ @1h Air Cooling 600℃ @1h Comparative Example B1 900℃ @1h Quenching 810℃ @1h Quenching 600℃ @1h Comparative Example B2 XX790℃ @1h Quenching X Comparative Example B3 XX840℃ @1h Quenching X Comparative Example B4 900℃ @1h Quenching XXX Comparative Example B5 XX790℃ @1h Quenching 600℃ @1h Comparative Example B6 XX840℃ @1h Quenching 600℃ @1h Comparative Example B7 900℃ @1h Quenching XX600℃ @1h

[0130] Table 5 shows the material properties according to the second experimental example of the present invention. F is ferrite, M is martensite, and TM is tempered martensite. The area fraction of the microstructure was calculated using optical microscope images according to the ISO 9042 standard. Yield strength (YS), tensile strength (TS), and elongation (EL) were measured by the metal tensile test method according to the ASTM E8 / E8M standard. Impact toughness was evaluated by the Charpy impact test method according to the JIS Z 2242 standard.

[0131] Material Properties Mechanical Properties Microstructure Fraction (%) YS(MPa) TS(MPa) El(%) YRR Ratio Impact Toughness (J@-40℃) Impact Toughness (J@-60℃) FM™ Example B 15 387 2325.4 0.7 421 118 465~75 -25~35 Comparative Example B 16 63 768 24.8 0.8 620 319 230~40 -60~70 Comparative Example B 26 95 103 418.3 0.6 778 6145~5 545~5 5-Comparative Example B3791108716.20.73825210 Less than 90 or more-Comparative Example B4873115713.10.752815-100-Comparative Example B564175821.60.8525322045~55-45~55 Comparative Example B677283919.70.9225722410 Less than -90 or more Comparative Example B784288618.20.9518884--100

[0132] Example B1 sequentially performed the steps of: hot rolling the steel and air cooling it to room temperature; first heat treating the steel at a temperature of 850°C to 950°C; quenching the heat-treated steel by rapidly cooling it to room temperature; second heat treating the rapidly cooled steel at 790°C to 830°C; air cooling the heat-treated steel to room temperature; and tempering the air-cooled steel at a temperature of 550°C to 650°C.

[0133] In Example B1, the ordered pair consisting of the second temperature (unit: ℃) for performing the second heat treatment and the third temperature (unit: ℃) for performing the tempering heat treatment can be seen to be located inside a polygon with vertices at coordinates A (790℃, 580℃), B (790℃, 630℃), C (810℃, 640℃), D (830℃, 600℃), and E (830℃, 580℃) in the graph shown in FIG. 3. In Example B1, the third temperature (T3) for performing the tempering heat treatment can be seen to be associated with the second temperature (T2) for performing the second heat treatment and satisfy the following Equation 1.

[0134] Equation 1: T3 - 0.5 × T2 ≤ 235 (where 790℃ ≤ T2 ≤ 810℃)

[0135] Comparative Example B1 sequentially performed the steps of: hot rolling the steel and air cooling it to room temperature; first heat treating the steel at a temperature of 850°C to 950°C; rapidly cooling the heat-treated steel to room temperature and quenching it; second heat treating the rapidly cooled steel at 790°C to 830°C; then quenching and rapidly cooling the steel; and tempering the quenched steel. The difference from Example B1 is the cooling method after the second heat treatment.

[0136] Comparative Examples B2 and B3 are steel materials manufactured by quenching only after a second heat treatment at temperatures of 790°C and 840°C, respectively, after rolling is completed, without performing a subsequent tempering process, and Comparative Example B4 is a steel material manufactured by quenching only after a first heat treatment at a temperature range of 900°C, respectively, after rolling is completed.

[0137] Comparative Examples B5 and B6 are steel materials to which a tempering process in a temperature range of 600°C is additionally applied to Comparative Examples B2 and B3, and Comparative Example B7 is a steel material to which a tempering process in a temperature range of 600°C is additionally applied to Comparative Example B4.

[0138] Referring to Table 5, Example B1 satisfies a yield strength of 500 MPa or more, a tensile strength of 610 to 770 MPa, an elongation of 21% or more, and a yield ratio of 0.80 or less, and satisfies a low-temperature toughness of -40°C of 200 J or more and a low-temperature toughness of -60°C of 100 J or more.

[0139] Comparative Example B1 does not satisfy the requirement as it exceeds the yield ratio range of 0.80 or less, and Comparative Examples B2 to B4 do not satisfy the requirement as they exceed the tensile strength range of 610 to 770 MPa and fall below the -60℃ low-temperature toughness range of 100 J or more. In addition, Comparative Examples B5 to B7 do not satisfy the requirement as they exceed the yield ratio range of 0.80 or less.

[0140] FIG. 5 is a photograph showing the microstructure of steel before and after the tempering process according to an embodiment of the present invention, and FIG. 6 is a photograph showing the microstructure of steel before the tempering process according to a comparative example of the present invention.

[0141] Referring to Table 5 and Fig. 5(a), it can be seen that in Example B1, island-shaped MA phases are mixed within the ferrite matrix structure with an area fraction of 25–35%. The MA phases were austenite phases with an area fraction of 50–60% during the phase-phase heat treatment, but through the air cooling process, some of them transform into ferrite, and the remaining fractions exist as MA phases at room temperature in a state where the components are concentrated. As shown in Fig. 5(b), when tempering is performed thereafter, the MA phases transform into tempered martensite, and the final structure consists of ferrite and tempered martensite.

[0142] Referring to Table 5 and Figure 6 (a), Comparative Example B1 shows a pattern in which the austenite formed during the second heat treatment of the abnormal region transforms into martensite at room temperature with a similar fraction of 60–70% after quenching. Accordingly, when tempering is subsequently performed, the tempered martensite fraction is higher compared to Example B1, resulting in good strength and low-temperature toughness, but the yield ratio increases to 0.86, which can be understood as making it difficult to comply with the low yield ratio characteristics according to the embodiment of the present invention.

[0143] Referring to Table 5 and Figures 6(b), 6(c), and 6(d), it can be seen that Comparative Examples B2, B3, and B4 have a martensite structure formed with an area fraction greater than that of ferrite, which increases the ultimate tensile strength and makes it easy to secure a low yield ratio, but the low-temperature toughness values ​​are very low, at 61, 52, and 15, respectively, because they do not undergo a tempering process.

[0144] Referring to Table 5, it can be seen that Comparative Examples B5, B6, and B7 transformed from a martensite structure into a tempered martensite structure through tempering. As a result, while toughness increased, the yield ratios increased to 0.85, 0.92, and 0.95, respectively, as the ultimate tensile strength decreased significantly. This indicates that it is difficult to comply with the low yield ratio characteristics according to the embodiment of the present invention.

[0145] FIG. 7 is a graph showing the change in stress according to the elongation in steel materials according to the embodiments and comparative examples of the present invention. More specifically, FIG. 7 shows the tensile graphs of Example B1 and Comparative Example B1, and it can be seen that there is a clear difference in the yield ratio, that is, the margin of tensile stress from yield to fracture, depending on the cooling method after the second heat treatment.

[0146] Referring to Fig. 7, Example B1, which was cooled by air cooling after a second heat treatment, has a microstructure composed of ferrite: 60 to 80% and tempered martensite: 20 to 40% in terms of area fraction. Consequently, the ferrite with the higher area fraction yields first, and subsequently, the tempered martensite increases the ultimate tensile strength, thereby enabling the realization of a low yield ratio. In other words, it can be confirmed that a margin of tensile stress is secured from yield to fracture, making it possible to realize a low yield ratio.

[0147] On the other hand, Comparative Example B1, which was cooled by quenching after a second heat treatment, has a microstructure composed of ferrite: 30–40% and tempered martensite: 60–70% in area fraction, and it can be confirmed that it is difficult to achieve low yield ratio characteristics as the ultimate tensile strength is significantly reduced due to the relatively high area fraction of tempered martensite.

[0148] It will be understood that the present invention includes not only the disclosed embodiments but also various modifications and equivalent alternative embodiments that can be derived from the disclosed embodiments by those skilled in the art. Accordingly, the technical scope of protection of the present invention should be determined by the claims below.

Claims

1. (a) A step of hot rolling a steel material comprising, in weight percent, carbon (C): 0.08%~0.11%, silicon (Si): 0.10%~0.20%, manganese (Mn): 1.0%~1.2%, aluminum (Al): 0.015%~0.055%, niobium (Nb): 0.005%~0.015%, titanium (Ti): 0.012%~0.022%, chromium (Cr): 0.45%~0.55%, molybdenum (Mo): 0.45%~0.55%, nickel (Ni): 1.0%~1.4%, copper (Cu): 0.1%~0.2%, boron (B): 0.001%~0.002% or less, and the remainder being iron and unavoidable impurities, and air cooling to room temperature; (b) a step of performing a first heat treatment on the steel at a first temperature (T1) within 850℃ to 950℃; (c) a step of rapidly cooling the heat-treated steel to room temperature to quench it; (d) a step of performing a second heat treatment on the quenched steel at a second temperature (T2) within 790℃ to 830℃; (e) a step of air-cooling the heat-treated steel to room temperature; and (f) a step of tempering the air-cooled steel at a third temperature (T3) within 550℃ to 650℃; comprising, The above third temperature (T3) is characterized by being associated with the above second temperature (T2) and satisfying the following Equation 1 or Equation 2, Method of manufacturing steel. Equation 1: T3 - 0.5 × T2 ≤ 235 (where 790℃ ≤ T2 ≤ 810℃) Equation 2: T3 + 2 × T2 ≤ 2260 (where 810℃ < T2 ≤ 830℃) (Here, T2 is the value of the second temperature (unit: ℃), and T3 is the value of the third temperature (unit: ℃)) 2. In Paragraph 1, The steel material after performing steps (a) to (f) above satisfies a yield strength of 500 MPa or more, a low-temperature toughness of -60°C of 100 J or more, and a yield ratio of 0.8 or less, Method of manufacturing steel.

3. In Paragraph 1, The microstructure of the steel after performing steps (a) to (f) above consists of ferrite: 60 to 80% and tempered martensite: 20 to 40% in area fraction, Method of manufacturing steel.

4. In Paragraph 1, The above steel further comprises, in weight percent, one or more of phosphorus (P): greater than 0 and less than or equal to 0.012%, sulfur (S): greater than 0 and less than or equal to 0.003%, and nitrogen (N): greater than 0 and less than or equal to 0.006%, Method of manufacturing steel.

5. In wt%, carbon (C): 0.08%–0.11%, silicon (Si): 0.10%–0.20%, manganese (Mn): 1.0%–1.2%, aluminum (Al): 0.015%–0.055%, niobium (Nb): 0.005%–0.015%, titanium (Ti): 0.012%–0.022%, chromium (Cr): 0.45%–0.55%, molybdenum (Mo): 0.45%–0.55%, nickel (Ni): 1.0%–1.4%, copper (Cu): 0.1%–0.2%, boron (B): 0.001%–0.002% or less, and the remainder being iron and unavoidable impurities, Satisfying a yield strength of 500 MPa or more, a low-temperature toughness of -60℃ of 100 J or more, and a yield ratio of 0.8 or less, and The final microstructure consists of ferrite: 60–80% and tempered martensite: 20–40% by area fraction, Steel.

6. In Paragraph 5, In weight percent, further comprising one or more of Phosphorus (P): greater than 0 and less than or equal to 0.012%, Sulfur (S): greater than 0 and less than or equal to 0.003%, and Nitrogen (N): greater than 0 and less than or equal to 0.006%, Steel.

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