High-strength and impact-toughness steel material and method for manufacturing the same

Abstract

Provided is a steel material having excellent strength and impact toughness even when a heat treatment process is omitted, and a method for producing the same. [Solution] The steel sheet is characterized in that, by weight, it is made up of C: 0.12-0.18%, Si: 0.2-0.5%, Mn: 1.0-1.7%, P: 0.012% or less, S: 0.003% or less, Al: 0.015-0.045%, Nb: 0.02-0.05%, V: 0.01-0.08%, Ti: 0.005-0.017%, N: 0.002-0.01%, and the remainder is Fe and inevitable impurities, the carbon equivalent (Ceq) of the following [Relational Formula 1] is 0.48 or less, the microstructure contains, by area fraction, 60-85% ferrite and the remainder pearlite, the microstructure contains one or more precipitates of NbC and VC, and the size of the precipitates is 50 nm or less. [Equation 1] Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15 (In the above Relational Formula 1, C, Mn, Cr, Mo, V, Cu, and Ni are the contents (wt%) of each component.)

Description

Technical Field

[0001] The present invention relates to a steel material having excellent high strength and impact toughness and a method for manufacturing the same, and more particularly, to a steel material that can be used for onshore wind turbines and the like, and relates to a steel material having excellent high strength and impact toughness and a method for manufacturing the same.

Background Art

[0002] In recent years, as the height of onshore wind turbine towers has gradually increased, the demand for thick high-strength steel materials with excellent load resistance has increased, and at the same time, guarantees for impact toughness have also been required. In order to achieve high strength and excellent impact toughness of steel materials, grain refinement is essential, and the rolling process is one of the typical methods for grain refinement. When rolling is performed at a temperature at which recrystallization is possible, new austenite fine grains are generated using the internal stress generated by the rolling pressure as a driving force. On the other hand, rolling in a temperature range that does not reach the temperature at which recrystallization is possible causes the grains to be stressed and form a band structure in the rolling direction. When a large number of dislocations occur inside and austenite undergoes a phase transformation, more nucleation sites are provided, and the grain refinement effect can be induced.

[0003] However, as the thickness of the steel material increases, the rolling pressure applied by rolling is limited, and it becomes more difficult to form fine grains by rolling, especially closer to the center of the steel material. This is because austenite grains tend to grow as the temperature is higher and the heating time is longer at temperatures above Ae3. On the other hand, in the process where austenite grain refinement mainly occurs, it is often difficult to secure sufficiently small grains through slab reheating and rolling alone. In particular, the higher the temperature of the steel being rolled, the lower the deformation resistance during rolling, making rolling easier. For this reason, slab reheating is mainly carried out at a much higher temperature than the Ae3 temperature, but in this case, the austenite grains grow larger. If the grain refinement effect by rolling is insufficient, further austenite grain refinement can be expected through reheat treatment after the rolling process, and this generally corresponds to normalizing heat treatment.

[0004] Traditionally, wind turbine towers have used steel that has undergone normalizing heat treatment. However, due to the manufacturing process, applying such heat treatment significantly increases production costs, making it commercially less feasible compared to as-rolled or TMCP (Thermo Mechanical Controlled Process) steel. Therefore, there is a need to manufacture steel that has similar physical properties to normalized steel without undergoing normalizing heat treatment.

[0005] Patent Document 1 presents a method for producing steel with excellent impact toughness even without normalizing heat treatment. However, while Patent Document 1 may be advantageous in ensuring sufficient low-temperature impact toughness due to its low carbon content, it has limitations in ensuring sufficient strength, and furthermore, the strength may decrease significantly as the thickness increases. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Korean Registered Patent Publication No. 10-1917453 [Overview of the project] [Problems that the invention aims to solve]

[0007] The object of the present invention is to provide a steel material having excellent strength and impact toughness even without a heat treatment process, and a method for manufacturing the same. The problems that the present invention will not address are not limited to those described above. Further problems that the present invention will address are described throughout the specification, and any person with ordinary skill in the art to which the present invention belongs will have no difficulty understanding these further problems from the contents of the specification. [Means for solving the problem]

[0008] The steel material of the present invention, which is excellent in high strength and impact toughness, is characterized by having the following composition by weight %, C: 0.12~0.18%, Si: 0.2~0.5%, Mn: 1.0~1.7%, P: 0.012% or less, S: 0.003% or less, Al: 0.015~0.045%, Nb: 0.02~0.05%, V: 0.01~0.08%, Ti: 0.005~0.017%, N: 0.002~0.01%, with the remainder being Fe and unavoidable impurities, and having a carbon equivalent (Ceq) of 0.48 or less as shown in the following [Relationship Formula 1], and having a microstructure that, by area fraction, contains 60~85% ferrite and the remainder being pearlite, and contains one or more precipitates of NbC and VC within the microstructure, with the size of the precipitates being 50 nm or less. Here, [Relationship 1] is Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, where C, Mn, Cr, Mo, V, Cu, and Ni are the content (weight %) values ​​of each component.

[0009] The present invention relates to a method for producing a steel material with excellent high strength and impact toughness, and is characterized by comprising the steps of: heating a steel slab, which has a carbon equivalent (Ceq) of 0.48 or less as shown in [Relationship Formula 1] below, with the remainder being Fe and unavoidable impurities, under the conditions shown in [Relationship Formula 2] below; and roughly rolling the heated steel slab at a temperature range of 900 to 1100°C, followed by finish hot rolling to Ar3 or higher. Here, [Relationship 1] is Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, where C, Mn, Cr, Mo, V, Cu, and Ni are the content (weight %) values ​​of each component. Furthermore, [Relationship 2] is given by slab extraction temperature (°C) > 10300 / {4.09 - log([Nb][C] 0.24 [N] 0.65 )}-273, and in the above relational equation 2, [Nb], [C], and [N] are the respective contents of the alloy composition. This means (weight %). [Effects of the Invention]

[0010] According to the present invention, a steel material is provided that ensures excellent strength and impact toughness without performing normalizing heat treatment after rolling, and can be widely used for wind power structures and the like. Furthermore, it is possible to reduce manufacturing costs by omitting heat treatment and provide a commercially useful steel material. The diverse and beneficial advantages and effects of the present invention are not limited to those described above and can be more easily understood in the process of describing specific embodiments of the present invention. [Brief explanation of the drawing]

[0011] [Figure 1] This graph shows the relationship between slab extraction temperature and yield strength as shown in the embodiment of the present invention. [Modes for carrying out the invention]

[0012] The terms used herein are for illustrative purposes only and are not intended to limit the invention. Furthermore, the singular form used herein also includes the plural form unless the relevant definition clearly indicates otherwise. As used herein, "includes" specifies a configuration and does not exclude the existence or addition of other configurations. Unless otherwise defined, all terms used herein, including technical and scientific terms, have the same meaning as those generally understood by a person of ordinary skill in the art to which this invention pertains. Terms defined in the dictionary are to be interpreted as having the meaning consistent with the relevant technical literature and the content currently disclosed.

[0013] The inventors of the present invention have come to recognize that the NR (Normalized Rolling) method, which involves hot rolling at a temperature range that provides physical properties equivalent to or better than those of steel treated with normalizing, can be optimized by designing the optimal composition and establishing the manufacturing conditions to ensure physical properties equivalent to or better than those of steel treated with normalizing, even without performing normalizing heat treatment after rolling. In particular, for structural steel used in onshore wind turbines and other structures, the increasing size and demand for economic efficiency necessitate a method for economical manufacturing while ensuring the required physical properties of the material. Therefore, by investigating and optimizing the alloy composition and the relationships between some components in alloy design, and optimizing the manufacturing conditions, we confirmed that it is possible to provide steel materials with the target physical properties, thus completing the present invention. The following describes in detail one example of the steel material of the present invention.

[0014] First, the alloy composition of the above steel material will be described in detail. The steel material of the present invention contains, by weight%, C: 0.12 to 0.18%, Si: 0.2 to 0.5%, Mn: 1.0 to 1.7%, P: 0.012% or less, S: 0.003% or less, Al: 0.015 to 0.045%, Nb: 0.02 to 0.05%, V: 0.01 to 0.08%, Ti: 0.005 to 0.017%, N: 0.002 to 0.01%, and may further contain one or more of Cu: 0.5% or less and Ni: 0.5% or less.

[0015] Carbon (C): 0.12 to 0.18% by weight (hereinafter referred to as %). Unless otherwise specified in the present invention, the content of each element is based on weight%. The above C is an element effective for improving the strength of steel. For this purpose, it is preferable to contain the above C at 0.12% or more. However, if the content exceeds 0.18%, there is a problem that the degree of segregation in the center of the steel material increases, an island-like martensite (MA) structure is formed, and the low-temperature impact toughness is greatly inhibited. More preferably, it is contained at 0.17% or less.

[0016] Silicon (Si): 0.2 to 0.5% The above Si is not only used as a deoxidizer but also an element advantageous for improving the strength and toughness of steel. In order to sufficiently obtain such an effect, it is preferable that the above Si is contained at 0.2% or more. However, when the content exceeds 0.5%, there is a risk that excessive MA is formed and the low-temperature impact toughness decreases. Therefore, the above Si is preferably 0.2 to 0.5%.

[0017] Manganese (Mn): 1.0 to 1.7% The above Mn is an element advantageous for improving the strength of steel by the solid solution strengthening effect. In order to sufficiently obtain the effect, it is preferable to contain the above Mn at 1.0% or more. However, if the content exceeds 1.7%, it combines with sulfur (S) in the steel to form MnS, so there is a risk of greatly inhibiting the low-temperature impact toughness. Therefore, the above Mn is preferably contained at 1.0 to 1.7%, and more preferably at 1.35 to 1.65%.

[0018] Phosphorus (P): 0.012% or less While the above-mentioned P is advantageous for improving the strength and ensuring corrosion resistance of steel, it may significantly impair the impact toughness of steel, so it is preferable to limit its content to the lowest possible level. In this invention, even if the above-mentioned P is included at a maximum of 0.012%, it does not hinder the securing of the target physical properties, so it is preferable to keep its content at 0.012% or less. However, considering the level that will inevitably be mixed in, 0% can be excluded.

[0019] Sulfur (S): 0.003% or less The above-mentioned sulfur (S) is an element that significantly impairs the hydrogen-induced cracking resistance and impact toughness of steel by bonding with manganese (Mn) in steel to form MnS, etc. Therefore, it is advantageous to control the content of S to the lowest possible level. In this invention, even if the above-mentioned sulfur is included at a maximum of 0.003%, it does not hinder the securing of the target physical properties, so its content can be limited to 0.003% or less. However, considering the level that will inevitably be introduced, 0% can be excluded.

[0020] Aluminum (Al): 0.015~0.045% The above-mentioned Al is an element that can deoxidize molten steel inexpensively. To obtain the above-mentioned effect sufficiently, it is preferable to include 0.015% or more of the above-mentioned Al. However, if the content becomes excessive and exceeds 0.045%, it is undesirable because it not only induces nozzle clogging during continuous casting but also significantly reduces impact toughness due to the formation of Al-based oxidative inclusions. Therefore, it is preferable to include the above-mentioned Al in an amount of 0.015 to 0.045%.

[0021] Niobium (Nb): 0.02-0.05% The above-mentioned Nb precipitates in the form of NbC or Nb(C,N), significantly improving the strength of the base material. When reheated at high temperatures, the dissolved Nb suppresses the recrystallization of austenite and the transformation of ferrite or bainite, thereby achieving a microstructure refinement effect. For this reason, it is preferable to contain 0.02% or more. However, if the content is excessive, undissolved Nb forms in the form of TiNb(C,N), which can lead to ultrasonic testing (UT) defects and inhibit low-temperature impact toughness. Therefore, the upper limit of the above-mentioned Nb is preferably 0.05%. More advantageously, it contains 0.035 to 0.045%.

[0022] Vanadium (V): 0.01-0.08% The above-mentioned V has a lower solid solution temperature compared to other alloying elements and contributes significantly to increased strength by forming VC during the air cooling process after hot rolling. Steel materials such as those of the present invention may not have sufficient strength after post-weld heat treatment (PWHT). Therefore, a strength improvement effect can be obtained by including 0.01% or more of the above-mentioned V. However, if the content exceeds 0.08%, the fraction of hard phases such as martensite-austenite (MA) increases, which leads to a problem of a significant decrease in low-temperature impact toughness. Therefore, the above-mentioned V content is preferably 0.01 to 0.08%.

[0023] Titanium (Ti): 0.005~0.017% The above-mentioned Ti, when included with N, plays a role in reducing the occurrence of surface cracks due to the formation of AlN precipitates by forming TiN, so it is preferable that it be included in an amount of 0.005% or more. However, if the content exceeds 0.017%, coarse TiN is formed during the reheating of the steel slab, which acts as a factor that inhibits low-temperature impact toughness. Therefore, the above-mentioned Ti is preferably 0.005 to 0.017%, and more preferably 0.01 to 0.015%.

[0024] Nitrogen (N): 0.002~0.01% The above-mentioned N, when included with Ti, forms TiN, which is advantageous in suppressing grain growth due to heat effects during welding. When adding Ti, it is preferable to include 0.002% or more of the above-mentioned N in order to fully obtain the above-mentioned effect. However, if the content exceeds 0.01%, coarse TiN is formed, which inhibits low-temperature impact toughness and is therefore undesirable. For this reason, the content of the above-mentioned N is preferably 0.002 to 0.01%.

[0025] In addition to the above composition, it may further contain one or more of the following: copper (Cu): 0.5% or less and nickel (Ni): 0.5% or less.

[0026] Copper (Cu): 0.5% or less The above-mentioned Cu is an element whose strength can be greatly improved by solid solution strengthening. However, if the Cu content is excessive, it not only increases the carbon equivalent and hinders weldability, but also significantly degrades the surface quality of the product. Therefore, it is preferable that the Cu be included at a maximum of 0.5% when added. However, it should be made clear that in this invention, the above-mentioned Cu is not essential because it is not necessary to add it, as the target physical properties can be achieved without any problems even if the Cu is not added.

[0027] Nickel (Ni): 0.5% or less The above-mentioned nickel (NI) is an element that can simultaneously improve the strength and low-temperature impact toughness of the base material. However, it is an expensive element, and if its content exceeds 0.5%, the economic viability decreases significantly. Therefore, it is preferable to include NI at a concentration of 0.5% or less. However, in this invention, it should be made clear that the above-mentioned nickel is not essential, as the target physical properties can be achieved without adding it.

[0028] The remainder consists of iron (Fe) and unavoidable impurities. Since unavoidable impurities can be unintentionally introduced during the normal steel manufacturing process, it is impossible to completely eliminate them. This is easily understood by engineers in the field of normal steel manufacturing. Furthermore, this invention does not completely exclude the addition of compositions other than the steel composition described above.

[0029] In order to ensure impact toughness along with the target level of strength, the steel material of the present invention preferably has its content appropriately adjusted when adding a certain amount of elements that are advantageous for improving such physical properties. Therefore, it is preferable that the carbon equivalent (Ceq) represented by the following [Relationship Formula 1] is 0.48 or less. If the carbon equivalent (Ceq) exceeds 0.48, it is advantageous for ensuring strength, but there is a risk that it will greatly impair the physical properties after welding. Also, if a large amount of alloying elements are included, the cost will increase and economic efficiency will be impaired, so it is preferable that the carbon equivalent (Ceq) is 0.48 or less. [Relationship 1] Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15 (In the above relational equation 1, C, Mn, Cr, Mo, V, Cu, and Ni are the content (weight %) values ​​of each component.)

[0030] The microstructure of the steel material of the present invention contains 60-85% ferrite by area fraction, with the remainder being pearlite. If the ferrite fraction is less than 60% or the remaining pearlite fraction exceeds 40%, it is advantageous for ensuring strength, but there is a risk of a significant decrease in impact toughness. Also, if the ferrite fraction exceeds 85%, it is advantageous for ensuring impact toughness, but it is difficult to ensure sufficient strength. Therefore, it is preferable that the steel material of the present invention contains 60-85% ferrite by area fraction, with the remainder being pearlite. The average grain size of the ferrite is preferably 30 μm or less. If the average grain size of the ferrite exceeds 30 μm, it becomes difficult to secure the yield strength and there is a risk that the impact toughness will decrease significantly; therefore, the average grain size is preferably 30 μm or less.

[0031] The microstructure of the above-mentioned steel material may contain NbC and / or VC precipitates. The size of these precipitates is preferably 50 nm or less. A size exceeding 50 nm is undesirable because it may significantly reduce impact toughness. The precipitates are preferably located within ferrite crystal grains.

[0032] On the other hand, the steel material of the present invention has a yield strength of 370 MPa or more, a tensile strength of 520 MPa or more, and an average Charpy impact absorption energy (CVN, -20°C) value of 40 J or more, evaluated perpendicular to the rolling direction at the t / 4 point in the thickness direction (where t means the thickness of the steel material (mm)), and possesses excellent strength and low-temperature impact toughness. Next, an embodiment of the steel manufacturing method of the present invention will be described in detail. The above manufacturing method involves heating a steel slab having the alloy composition described above and a carbon equivalent (Ceq) of 0.48 or less in [Relationship Formula 1], and then hot-rolling it. Each step will be described in detail below.

[0033] Heating of steel slabs It is preferable to heat and homogenize the steel slab that satisfies the above alloy composition. At this time, it is preferable to heat it so as to satisfy the temperature conditions specified in [Relational Equation 2] below. On the other hand, it is preferable that the slab extraction temperature in [Relational Equation 2] below does not exceed 1200°C.

[0034] If the heating temperature of the steel slab does not satisfy the conditions of the following relational equation 2, the precipitates (carbon and nitrides) formed in the slab will not be sufficiently redissolved, reducing the formation of precipitates in the post-hot-rolling process, and ultimately making it difficult to satisfy the yield strength and tensile strength presented in this invention. On the other hand, if the slab extraction temperature exceeds 1200°C, the austenite grains may coarseen, potentially impairing the properties of the steel; therefore, it is preferable that the temperature not exceed 1200°C. [Relationship 2] Slab extraction temperature (°C) > 10300 / {4.09 - log([Nb][C] 0.24 [N] 0.65 )}-273 (In the above relational equation 2, [Nb], [C], and [N] each represent the content (by weight) of the alloy composition.)

[0035] Hot rolling The heated steel slab is hot-rolled. It is preferable to roughly roll the heated steel slab at a temperature range of 900 to 1100°C, and then finish hot-roll it to Ar3 or higher. The temperature during the rough rolling is 900If the temperature is below ℃, there is a problem that the temperature during the subsequent finish hot rolling will be excessively low. On the other hand, if the finish hot rolling temperature is below Ar3, the rolling load will be high, which may lead to quality defects such as surface cracks. Ar3=910-310C-80Mn-20Cu-55Ni-80Mo+119V+124Ti-18Nb+179Al (Here, each element represents its content (meaning weight change)) After the hot rolling process described above, the product is cooled by air. The steel material of the present invention, manufactured by the above method, can achieve high strength and excellent impact toughness without subsequent heat treatment, such as normalizing. [Examples]

[0036] Next, embodiments of the present invention will be described. It goes without saying that the following embodiments can be modified in various ways without departing from the scope of the invention, by anyone with ordinary skill in the art to which the invention pertains. The following embodiments are provided to aid in understanding the invention, and the scope of the invention should not be limited to the following embodiments, but should be defined not only by the claims described below, but also by equivalents thereof.

[0037] (Example 1) Slabs were manufactured by continuous casting of molten steel having the alloy composition (by weight %) shown in Table 1 below, with the remainder being Fe and unavoidable impurities. The slabs were manufactured with a thickness of 300 mm. Examples 1 to 4 in Table 1 satisfy both the alloy composition and relational formula 1 specified in the present invention, comparative example 1 is a case where the C content and relational formula 1 deviate from the values ​​specified in the present invention, and comparative example 3 is a case where the Nb content deviates from the value specified in the present invention.

[0038] [Table 1]

[0039] In Table 1 above, relational expression 1 is calculated as follows. [Relationship 1] Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15 (In the above relational equation 1, C, Mn, Cr, Mo, V, Cu, and Ni are the content (weight %) values ​​of each component.)

[0040] The above slab was heated and roughly rolled under the conditions shown in Table 2, then finished hot-rolled at a temperature range of 880-900°C to produce a 5mm thick hot-rolled steel sheet, which was then air-cooled to room temperature. Invention Examples 1-4 and Comparative Examples 1 and 3 satisfy the process conditions specified in the present invention, but Comparative Example 2 did not satisfy the conditions of the following relational formula 2.

[0041] [Table 2]

[0042] In Table 2 above, relational equation 2 is as follows: [Relationship 2] Slab extraction temperature (°C) > 10300 / {4.09 - log([Nb][C] 0.24 [N] 0.65 )}-273 (In the above relational equation 2, [Nb], [C], and [N] each represent the content (by weight) of the alloy composition.)

[0043] The microstructure and mechanical properties of the steel materials manufactured as described above were evaluated. The microstructure was observed using an optical microscope, and the fraction and diameter of ferrite were measured using an analysis program. The size of the precipitates was measured using a transmission electron microscope to determine the average diameter. The microstructure was measured at t / 4 (where t is the thickness of the steel, in mm) in the thickness direction of each steel material, and the results are shown in Table 3 below. The mechanical properties of each steel material were evaluated at the 1 / 4th t point in the thickness direction. Tensile test specimens were taken at points in each thickness direction perpendicular to the rolling direction, and the tensile strength (TS), yield strength (YS), and elongation (El) were measured. Impact test specimens conforming to JIS No. 4 standards were taken at the 1 / 4th t point in the thickness direction in the rolling direction, and the average impact toughness (CVN) at -20°C was measured. The results are shown in Table 4 below.

[0044] [Table 3]

[0045] As shown in Table 3 above, the inventive steels 1 to 4 produced according to the proposed alloy composition, component relationships, and manufacturing conditions satisfy the polygonal ferrite fraction, crystal grain size, and precipitate size specified in the present invention. In contrast, while Comparative Examples 1 and 3 satisfy the polygonal ferrite fraction, their ferrite crystal grain size deviates from the value specified in the present invention. Furthermore, Comparative Example 1 deviates from the precipitate size specified in the present invention.

[0046] [Table 4]

[0047] Table 4 above shows the tensile properties and low-temperature impact toughness before and after normalizing. The normalizing process involved holding the material at 870°C for 128 minutes, followed by air cooling.

[0048] In the cases of Invention Examples 1 to 4, the component range, relational formulas 1 and 2, and microstructure characteristics specified in the present invention are satisfied, and both tensile properties and low-temperature impact toughness are met. Specifically, in the cases of Invention Examples 1 to 4, when comparing the results after as-rolling and normalizing heat treatment, the yield strength and tensile strength after heat treatment decrease slightly, but still meet the strengths specified in the present invention. In the case of impact toughness, it is also confirmed that it increases slightly after heat treatment compared to when manufactured by the NR method, and that it meets the impact toughness specified in the present invention.

[0049] On the other hand, in the case of Comparative Example 1, the C content and relational formula 1 are outside the range specified in the present invention. Due to the excessive addition of C, although the yield / tensile strength meets the values ​​specified in the present invention, the impact toughness does not meet the specified values. Comparative Example 2 satisfies all the component ranges specified in the present invention, but fails to satisfy relational formula 2. This is because the slab extraction temperature is very low, and it is clear that the yield strength and tensile strength do not meet the values ​​specified in the present invention after both hot rolling (as-rolled) and normalizing heat treatment. Furthermore, it can be seen that the decrease in yield strength is much larger compared to Invention Examples 1 to 4. This is judged to be because Nb was not sufficiently dissolved in the slab, and NbC did not precipitate sufficiently during rolling, resulting in a significant decrease in strength. In contrast, Comparative Example 3 shows that when the Nb content deviates from the value specified in the present invention, even though the Nb was heated to a temperature at which it was sufficiently dissolved in the slab, the Nb content itself was very low, and NbC precipitates did not precipitate sufficiently, resulting in a failure to meet the yield strength and tensile strength specified in the present invention. It can be confirmed that Comparative Example 3 also shows a significant decrease in yield strength after normalizing heat treatment.

[0050] (Example 2) On the other hand, as another embodiment, a steel material with a thickness of 75 mmt was manufactured by rolling a slab having the components of Invention Example 1 of Example 1 above. In this case, in order to confirm the yield strength with respect to the slab extraction temperature, the result of the above relational equation 2 for the extraction temperature was changed, and the results of the relationship between the extraction temperature and the yield strength are shown in Figure 1. It can be seen that when the extraction temperature does not satisfy relational equation 2, the yield strength specified in the present invention is not met, whereas when relational equation 2 is satisfied, all samples show excellent yield strength.

Claims

1. In weight percent, the composition is as follows: C: 0.12-0.18%, Si: 0.2-0.5%, Mn: 1.0-1.7%, P: 0.012% or less, S: 0.003% or less, Al: 0.015-0.045%, Nb: 0.02-0.05%, V: 0.01-0.08%, Ti: 0.005-0.017%, N: 0.002-0.01%, with the remainder being Fe and unavoidable impurities. A steel material using a steel slab heated under the temperature conditions of [Relationship Formula 2] below, wherein the carbon equivalent (Ceq) in [Relationship Formula 1] below is 0.48 or less. The microstructure of the aforementioned steel material consists of 60-85% ferrite and 15-40% pearlite in terms of area fraction. The microstructure contains one or more precipitates of NbC and VC, A steel material exhibiting high strength and impact toughness, characterized in that the size of the precipitates is 50 nm or less. [Relationship 1] Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15 (In the above relational equation 1, C, Mn, Cr, Mo, V, Cu, and Ni are the content (weight %) values ​​of each component.) [Relationship Equation 2] Slab extraction temperature (°C) > 10300 / {4.09 - log([Nb][C] 0.24 [N] 0.65)} - 273 (In the above relational equation 2, [Nb], [C], and [N] each represent the content (by weight) of the alloy composition.)

2. The steel material is characterized by further comprising one or more of Cu: 0.5% or less and Ni: 0.5% or less, as described in claim 1, which is a steel material with excellent high strength and impact toughness.

3. The steel material with high strength and impact toughness according to claim 1, characterized in that the ferrite crystal grain size is 30 μm or less.

4. The steel material according to claim 1, characterized in that one or more of the aforementioned NbC and VC precipitates are present within the ferrite crystal grains.

5. The steel material is characterized in that, at a point t / 4 in the thickness direction (where t means the thickness of the steel material (mm)), the yield strength evaluated perpendicular to the rolling direction is 370 MPa or more, the tensile strength is 520 MPa or more, and the Charpy impact absorption energy (CVN, -20°C) value at -20°C is an average of 40 J or more, making it a high-strength and impact-tough steel material as described in claim 1.

6. In weight percent, the composition is as follows: C: 0.12-0.18%, Si: 0.2-0.5%, Mn: 1.0-1.7%, P: 0.012% or less, S: 0.003% or less, Al: 0.015-0.045%, Nb: 0.02-0.05%, V: 0.01-0.08%, Ti: 0.005-0.017%, N: 0.002-0.01%, with the remainder being Fe and unavoidable impurities. The process involves heating a steel slab with a carbon equivalent (Ceq) of 0.48 or less, as shown in [Relationship Formula 1] below, under the conditions of [Relationship Formula 2] below, The heated steel slab is roughly rolled at a temperature range of 900 to 1100°C, and then finished hot-rolled to an Ar3 or higher, The microstructure consists of 60-85% ferrite and 15-40% pearlite by area fraction, and the microstructure contains one or more precipitates of NbC and VC. A method for producing steel with high strength and impact toughness, characterized in that the size of the precipitates is 50 nm or less. [Relationship 1] Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15 (In the above relational equation 1, C, Mn, Cr, Mo, V, Cu, and Ni are the content (weight %) values ​​of each component.) [Relationship Equation 2] Slab extraction temperature (°C) > 10300 / {4.09 - log([Nb][C] ０．２４ [N] ０．６５ )}-273 (In the above relational equation 2, [Nb], [C], and [N] each represent the content (by weight) of the alloy composition.)

7. The method for producing a steel material with high strength and impact toughness according to claim 6, characterized in that the steel slab further contains one or more of Cu: 0.5% or less and Ni: 0.5% or less.

8. The method for producing a steel material with high strength and impact toughness according to claim 6, characterized in that the slab extraction temperature is 1200°C or lower.

Images