Steel plate and its manufacturing method

Abstract

The present invention provides a steel plate having excellent strength and impact toughness even without normalizing heat treatment, and a method for manufacturing the same. [Solution] A steel sheet containing, by weight, carbon (C): over 0.10 to 0.17%, silicon (Si): 0.2 to 0.5%, manganese (Mn): 1.2 to 1.6%, phosphorus (P): 0.012% or less, sulfur (S): 0.003% or less, aluminum (Al): 0.015 to 0.045%, niobium (Nb): 0.03 to 0.05%, vanadium (V): 0.06% or less, titanium (Ti): 0.005 to 0.017%, nitrogen (N): 0.002 to 0.01%, and the remainder consisting of Fe and unavoidable impurities, satisfying the following relational expression 1, and the microstructure of the steel sheet containing, in area fraction, 60 to 85% ferrite and the remainder being pearlite and unavoidable structures. [Relationship 1][C] + [Mn] / 6 + [V] / 5≦0.43 (The above [C], [Mn], and [V] are the contents of each component (wt%).)

Description

[Technical Field]

[0001] The present invention relates to a steel plate and a manufacturing method thereof, and more particularly to a steel plate having excellent strength and toughness that can be used for offshore and onshore wind power generators and the like, and a manufacturing method thereof. [Background technology]

[0002] As wind turbines for both offshore and onshore wind power generators become larger, the load-resistance capacity of wind towers needs to be improved, leading to an increasing demand for stronger, thicker steel materials. However, as steel becomes thicker, its strength decreases, resulting in a vicious cycle of increased thickness. Therefore, if thicker steel materials are made to have the yield strength and tensile strength required for thinner steel materials, this would have many advantages, as it would enable a reduction in shell thickness due to increased design strength. Furthermore, as wind towers are increasingly being installed and operated in extremely cold regions, a guarantee of impact toughness is also required.

[0003] Grain refinement is essential to achieving high strength and excellent low-temperature impact toughness in steel, and rolling is one of the most common methods for this purpose. When rolling is performed at a temperature where recrystallization is possible, new fine austenite grains are generated, driven by the internal stress generated by the rolling force. On the other hand, rolling in the temperature range where recrystallization is not possible induces a grain refinement effect by forming a band structure in the rolling direction due to stress on the grains, generating many dislocations inside and providing more nucleation points for the austenite phase transformation.

[0004] However, as the thickness of a steel material increases, the rolling force that can be applied by rolling is limited, and therefore, it becomes more difficult to form fine crystal grains by rolling in the internal structure, particularly near the center of the steel material. As a result, the yield strength and tensile strength are significantly reduced, making it difficult to achieve the target mechanical properties.

[0005] However, it is often difficult to obtain sufficiently small grains through slab heating and rolling alone, which are the main processes for austenite grain refinement. In particular, the higher the temperature of the steel being rolled, the lower the deformation resistance during rolling. Therefore, slab heating for easy rolling is typically performed at temperatures much higher than the Ae3 temperature, but this results in the austenite grains growing larger. When the grain refinement effect of rolling is insufficient, further austenite grain refinement can be expected through reheat treatment, which is usually referred to as normalizing heat treatment.

[0006] The materials used for wind turbine towers have traditionally been normalized heat-treated steel, but when heat treatment is applied in the manufacturing process, the manufacturing cost increases significantly and it is not commercially viable compared to as-rolled steel or TMCP (Thermo Mechanical Controlled Process) steel.

[0007] Normalized Rolling (NR) is a manufacturing method in which hot rolling is performed within a temperature range that gives steel similar physical properties to normalized heat-treated steel without the need for normalizing heat treatment, followed by air cooling. If it is possible to obtain physical properties similar to normalized heat-treated steel through optimal component design and establishment of manufacturing conditions, it is possible to reduce manufacturing costs by omitting the heat treatment and provide commercially useful steel.

[0008] Patent Document 1 proposes a method for producing a steel material with excellent impact toughness without normalizing heat treatment. However, although the steel material in Patent Document 1 has a low carbon content and is advantageous in terms of ensuring low-temperature impact toughness, it is difficult to achieve sufficient strength. Furthermore, the finish rolling conditions for ensuring strength are not taken into consideration, and as the thickness of the steel material increases, the strength decreases significantly, making it difficult to achieve sufficient yield strength. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Korean Patent Registration No. 10-1917453 Summary of the Invention [Problem to be solved by the invention]

[0010] An object of the present invention is to provide a steel plate having excellent strength and impact toughness even without normalizing heat treatment, and a method for manufacturing the same.

[0011] The object of the present invention is not limited to the above-mentioned matters. Further object of the present invention is described in the entire content of the specification, and a person having ordinary skill in the art to which the present invention pertains will have no problem in understanding the further object of the present invention from the content described in the specification of the present invention. [Means for solving the problem]

[0012] The present invention relates to a sintered body containing, by weight, more than 0.10 to 0.17% carbon (C), 0.2 to 0.5% silicon (Si), 1.2 to 1.6% manganese (Mn), 0.012% or less phosphorus (P), 0.003% or less sulfur (S), 0.015 to 0.45% aluminum (Al), 0.03 to 0.05% niobium (Nb), 0.06% or less vanadium (V), 0.005 to 0.017% titanium (Ti), and 0.002 to 0.01% nitrogen (N), with the remainder consisting of Fe and inevitable impurities, and satisfying the following relational formula 1: The microstructure of the steel sheet is characterized by an area fraction of 60 to 85% ferrite, with the remainder being pearlite and unavoidable structures.

[0013] [Equation 1] [C] + [Mn] / 6 + [V] / 5 ≦ 0.43 (The above [C], [Mn], and [V] are the contents of each component (wt%).)

[0014] The thickness of the steel plate may be 100 mm or less.

[0015] The ferrite may have an average crystal grain size of 50 μm or less.

[0016] The steel sheet may have a yield strength of 355 MPa or more and a tensile strength of 470 MPa or more, evaluated perpendicular to the rolling direction, at a t / 4 point (t: thickness of the steel sheet, mm).

[0017] The steel plate may have a Charpy impact absorption energy of 100 J or more at -20°C evaluated in the rolling direction at a t / 4 point (t: thickness of the steel plate, mm).

[0018] A method for producing a steel sheet of the present invention includes a step of preparing a steel slab containing, by weight %, more than 0.10 to 0.17% carbon (C), 0.2 to 0.5% silicon (Si), 1.2 to 1.6% manganese (Mn), 0.012% or less phosphorus (P), 0.003% or less sulfur (S), 0.015 to 0.045% aluminum (Al), 0.03 to 0.05% niobium (Nb), 0.06% or less vanadium (V), 0.005 to 0.017% titanium (Ti), and 0.002 to 0.01% nitrogen (N), with the remainder being Fe and inevitable impurities, and satisfying the following relational expression 1: heating the steel slab at a temperature range of 1150 to 1200°C for 3 hours or more; A hot rolling step in which the heated steel slab is rough rolled and finish rolled at a finish rolling start temperature that satisfies the following relational expression 2; The method is characterized by including a step of cooling after the hot rolling.

[0019] [Equation 1] [C] + [Mn] / 6 + [V] / 5 ≦ 0.43 (The above [C], [Mn], and [V] are the contents of each component (wt%).)

[0020] [Equation 2] 800℃≦Finish rolling start temperature≦857℃+(464×[C])+(6445×[Nb])-(644×[Nb] 0.5 ) + (732 × [V]) - (230 × [V]0.5 )+(890×[Ti])+(363×[Al])-(357×[Si]) (The above [C], [Nb], [V], [Ti], [Al], and [Si] are the contents (wt%) of each component.)

[0021] The hot rolling can be performed at a cumulative reduction rate of 45% or more and with eight or fewer rolling passes.

[0022] The finishing temperature of the finish rolling may be Ar3 or higher.

[0023] The cooling may be air cooling.

[0024] After the cooling, the method may further include a step of heat treating the steel at a temperature range of 830 to 930° C. for 1.3t+30 minutes or more (t: thickness of steel, mm). [Effects of the Invention]

[0025] According to the present invention, it is possible to provide a steel plate having excellent strength and low-temperature impact toughness that can be used for offshore and onshore wind power generators, etc., and since normalizing heat treatment can be omitted, it is possible to improve economic efficiency.

[0026] The various beneficial advantages and effects of the present invention are not limited to the above-mentioned contents, but can be more easily understood in the course of describing specific embodiments of the present invention. [Brief explanation of the drawings]

[0027] [Figure 1] 1(a) and 1(b) are photographs of the microstructures at the t / 4 point of Example 1 and Comparative Example 10, respectively, in the examples of the present invention. [Figure 2] 1 is a graph showing the change in yield strength depending on the finish rolling start temperature for producing a hot-rolled steel plate with a thickness of 100 mm for steel type 2 in an example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0028] The terminology used herein is for the purpose of describing the invention and is not intended to limit the invention. Furthermore, as used herein, the singular forms "a," "an," and "the" include the plural forms unless the related definition clearly dictates otherwise.

[0029] The meaning of "comprises" as used in the specification embodies features and does not exclude the presence or addition of other features.

[0030] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Dictionary-defined terms are to be interpreted as having a meaning consistent with the relevant technical literature and the presently disclosed content.

[0031] The present inventors recognized that as wind structural steel used in onshore and offshore wind towers becomes larger and more cost-effective, it is necessary to develop a method for ensuring the required physical properties of the material.

[0032] In particular, the inventors have conducted extensive research into a method for ensuring high strength and low-temperature impact toughness in steel materials for wind power structures having a certain thickness or more. As a result, they have confirmed that it is possible to provide steel materials for wind power structures having the desired properties by controlling the relationship between the component composition and some components in alloy design and optimizing the manufacturing conditions, which has led to the completion of the present invention.

[0033] The present invention will be described in detail below.

[0034] First, the alloy composition of the steel sheet of the present invention will be described in detail. Unless otherwise specified, the content of each element is based on weight, and the proportion of the structure is based on area.

[0035] The steel plate can contain, by weight, carbon (C): over 0.10 to 0.17%, silicon (Si): 0.2 to 0.5%, manganese (Mn): 1.2 to 1.6%, phosphorus (P): 0.012% or less, sulfur (S): 0.003% or less, aluminum (Al): 0.015 to 0.045%, niobium (Nb): 0.03 to 0.05%, vanadium (V): 0.06% or less, titanium (Ti): 0.005 to 0.017%, and nitrogen (N): 0.002 to 0.01%.

[0036] Carbon (C): More than 0.10~0.17% C is an element effective in improving the strength of steel. To fully obtain this effect, it can be contained in an amount exceeding 0.10%. However, if the C content exceeds 0.17%, it may be advantageous in ensuring strength, but there is a problem in that the band-shaped pearlite fraction increases significantly, significantly impairing low-temperature impact toughness. If the C content is less than 0.10%, it may not be sufficient to ensure strength. Preferably, C is more than 0.100% to 0.170%.

[0037] Silicon (Si): 0.2 to 0.5% The Si element is not only used as a deoxidizer, but is also advantageous in improving the strength of steel. To fully obtain the above-mentioned effects, the Si content can be 0.2% or more. However, if the Si content exceeds 0.5%, there is a risk of excessive formation of island martensite (MA), which may result in a deterioration of low-temperature impact toughness. The Si content is preferably 0.20 to 0.50%.

[0038] Manganese (Mn): 1.2-1.6% Mn is an element advantageous in improving the strength of steel through its solid solution strengthening effect. To fully obtain this effect, the Mn content can be 1.2% or more. However, if the Mn content exceeds 1.6%, it bonds with sulfur (S) in the steel to form MnS, which significantly reduces low-temperature impact toughness. Therefore, the Mn content can be 1.2 to 1.6%, preferably 1.20 to 1.60%, and more preferably 1.45 to 1.6%.

[0039] Phosphorus (P): 0.012% or less The P element is advantageous for improving the strength and ensuring corrosion resistance of steel, but may significantly impair the impact toughness of steel, so it is preferable to limit its content as low as possible. In the present invention, since the target physical properties can be achieved even if the P content is contained up to 0.012%, the P content can be set to 0.012% or less. However, 0% can be excluded in consideration of the level of unavoidable addition.

[0040] Sulfur (S): 0.003% or less S is an element that significantly impairs low-temperature impact toughness by combining with Mn in the steel to form MnS and the like. Therefore, it is advantageous to limit the S content to as low as possible. In the present invention, even if the S content is a maximum of 0.003%, the target physical properties can be achieved without difficulty, so the S content can be set to 0.003% or less. However, 0% can be excluded in consideration of the level of unavoidable addition.

[0041] Aluminum (Al): 0.015 to 0.045% Al is an element capable of inexpensively deoxidizing molten steel. To fully obtain the above-mentioned effects, the Al content can be 0.015% or more. However, if the Al content exceeds 0.045%, not only will nozzle clogging occur during continuous casting, but impact toughness may also be significantly reduced due to the formation of oxidized inclusions, which is undesirable.

[0042] Niobium (Nb): 0.03-0.05% The Nb precipitates in the form of NbC or Nb(C,N), significantly improving the strength of the base metal. Furthermore, the dissolved Nb inhibits the recrystallization of austenite and the transformation of ferrite or bainite during high-temperature reheating, thereby achieving a refined microstructure. However, excessive Nb content causes undissolved Nb to form TiNb(C,N), which reduces low-temperature impact toughness. Therefore, the upper limit of Nb is preferably set to 0.05%. Therefore, in the present invention, the Nb content can be 0.03 to 0.05%, preferably 0.030 to 0.050%, and more preferably 0.035 to 0.045%.

[0043] Vanadium (V): 0.06% or less (including 0%) V has a lower solid solution temperature than other alloying elements and forms VC during the air-cooling process after hot rolling, significantly contributing to increased strength. Therefore, adding 0.01% or more of V can improve strength. However, if the V content exceeds 0.06%, the hardness of polygonal ferrite becomes too high, the proportion of hard phases such as MA increases, and low-temperature impact toughness is significantly reduced. A V content of 0.060% or less is preferable.

[0044] Titanium (Ti): 0.005 to 0.017% When added together with N, Ti forms TiN, which reduces the occurrence of surface cracks due to the formation of AlN precipitates, so adding 0.005% or more is effective. However, if the Ti content exceeds 0.017%, coarse TiN is formed during reheating of the steel slab, which acts as a factor that inhibits low-temperature impact toughness. Therefore, the Ti content is effectively 0.005 to 0.017%, preferably 0.0050 to 0.0170%, and more preferably 0.01 to 0.015%.

[0045] Nitrogen (N): 0.002-0.01% When added together with Ti, N forms TiN, making it an advantageous element for suppressing grain growth due to heat effects during welding. To fully obtain the above-mentioned effects when Ti is added, N can be contained in an amount of 0.002% or more. However, if the N content exceeds 0.01%, coarse TiN is formed, which impairs low-temperature impact toughness, which is undesirable. A content of 0.0020 to 0.010% is preferable.

[0046] The remaining component of the present invention is iron (Fe). However, in a normal manufacturing process, unintentional impurities may be inevitably mixed in from raw materials or the surrounding environment, and this cannot be excluded. Since these impurities are known to anyone skilled in the normal manufacturing process, not all of the contents thereof are specifically mentioned in this specification.

[0047] The above steel sheet can satisfy the following relational expression 1.

[0048] [Equation 1] [C] + [Mn] / 6 + [V] / 5 ≦ 0.43 (The above [C], [Mn], and [V] are the contents of each component (wt%).)

[0049] In the present invention, in order to ensure both the target level of strength and low-temperature impact toughness, it is necessary to appropriately control the content of elements that are advantageous for improving such properties when adding certain amounts of such elements. If the value of the above relational expression 1 exceeds 0.43, it may be advantageous for ensuring strength, but there is a risk that the physical properties after welding will be significantly impaired. Furthermore, since the inclusion of a large amount of alloying elements will increase costs and impair economic efficiency, it is effective to keep the value at 0.43 or less. It is even more effective to keep the value at 0.430 or less.

[0050] The microstructure of the steel sheet may contain, in area %, 60 to 85% ferrite and the remainder pearlite and unavoidable structures.

[0051] In this case, the average crystal grain size of the ferrite may be 50 μm or less.

[0052] If the ferrite fraction is less than 60%, the strength may become excessively high and the low-temperature impact toughness may decrease, and if it exceeds 85%, it may become difficult to ensure the strength.If the average grain size of the ferrite is less than 50 μm, it is difficult to ensure the yield strength and low-temperature impact toughness proposed in the present invention.

[0053] Furthermore, the steel material of the present invention has a yield strength of 355 MPa or more, a tensile strength of 470 MPa or more, and an average Charpy impact energy (CVN) value of 100 J or more at -20°C at a t / 4 point in the thickness direction (where t means the thickness (mm) of the steel material), evaluated perpendicular to the rolling direction, and thus has appropriate strength as well as excellent low-temperature impact toughness.

[0054] On the other hand, the steel material of the present invention can ensure excellent strength and impact toughness even without normalizing heat treatment. Conventionally, normalizing heat treatment has been performed to improve yield strength and low-temperature impact toughness. Normalizing heat treatment further refines crystal grains during the transformation from austenite to ferrite, thereby improving yield strength and low-temperature impact toughness. The steel material of the present invention can ensure sufficient yield strength and low-temperature impact toughness even without the normalizing heat treatment.

[0055] Next, the method for producing a steel sheet according to the present invention will be described in detail. In the method, a steel slab that satisfies all of the alloy components and Relational Formula 1 described above is prepared, and the steel slab is produced through heating, hot rolling, and cooling.

[0056] The conditions for each step will be described in detail below.

[0057] Steel slab heating The steel slab is preferably subjected to a step of heating and homogenizing, and the heating step can be carried out at a temperature range of 1150 to 1200° C. for 3 hours or more.

[0058] If the heating temperature of the steel slab is less than 1150°C, the precipitates (carbonitrides) formed in the slab will not be sufficiently redissolved, and the formation of precipitates will decrease in processes after hot rolling, ultimately making it difficult to achieve the yield strength and tensile strength proposed in the present invention. On the other hand, if the temperature exceeds 1200°C, the austenite grains will become coarse, which may impair the physical properties of the steel. Furthermore, if the heating time of the steel slab is less than 3 hours, the target temperature will not be reached all the way to the center, which may make it difficult to redissolve the precipitates (carbonitrides).

[0059] hot rolling The steel slab heated as described above can be hot-rolled to produce a hot-rolled steel sheet. In this case, the heated steel slab is rough-rolled in a temperature range of 900 to 1100°C, and then finish-rolling is started in the temperature range shown in Relation 2, and finish-hot-rolling can be performed at Ar3 or higher.

[0060] [Equation 2] 800℃≦Finish rolling start temperature≦857℃+(464×[C])+(6445×[Nb])-(644×[Nb] 0.5 ) + (732 × [V]) - (230 × [V] 0.5 )+(890×[Ti])+(363×[Al])-(357×[Si]) (The above [C], [Nb], [V], [Ti], [Al], and [Si] are the contents (wt%) of each component.)

[0061] If the temperature during the rough rolling is less than 900°C, the temperature at the start of the subsequent finish rolling will be too low. If the temperature at the start of the finish rolling is less than 800°C, the temperature at the start of the subsequent finish rolling will be too low. If the finish rolling is started at a temperature higher than that of Relational Formula 2, a sufficient rolling force will not be transmitted to the unrecrystallized region, and the size of polygonal ferrite will not be sufficiently reduced. As a result, it will be difficult to ensure the yield strength and impact toughness that the present invention aims to provide.

[0062] Furthermore, if the finish hot rolling temperature is less than Ar3, the rolling load will be large, which may result in poor quality such as surface cracks.

[0063] In the present invention, Ar3 can be expressed as follows:

[0064] Ar3=910-310C-80Mn-20Cu-55Ni-80Mo+119V+124Ti-18Nb+179Al (Here, each element means its weight content.)

[0065] The cumulative reduction rate during the finish rolling can be 45% or more, and the number of rolling passes can be 8 or less. If the cumulative reduction rate is less than 45%, the austenite is not sufficiently pressed during rolling, which can cause the final ferrite structure to coarsen and reduce strength and impact toughness. If the number of rolling passes exceeds 8, the reduction amount per pass decreases, which can also cause the ferrite to coarsen.

[0066] cooling After the hot rolling, the steel sheet is cooled, and the cooling method is not particularly limited in the present invention. For example, the steel sheet can be air-cooled to room temperature.

[0067] Normalizing heat treatment The above steel sheet can be subjected to normalizing heat treatment as needed, at a temperature and time of 830 to 930°C for 1.3t+30 minutes or more (t: thickness of steel, mm). [Example]

[0068] Examples of the present invention will now be described. It goes without saying that those skilled in the art will appreciate that various modifications to the following examples are possible without departing from the scope of the present invention. The following examples are provided for the purpose of understanding the present invention, and the scope of the present invention should not be limited to the following examples, but should be defined by the claims below as well as equivalents thereof.

[0069] (Example) Molten steel having the composition (wt %) shown in Table 1 below, the remainder being Fe and unavoidable impurities, was continuously cast to produce a steel slab having a thickness of 300 mm. The steel slab was heated under the conditions shown in Table 2, then rough rolled at 900°C or higher, and then finish rolled under the conditions shown in Table 2 to produce a hot-rolled steel sheet.

[0070] [Table 1]

[0071] Relation 1 is calculated as follows:

[0072] [Equation 1] [C] + [Mn] / 6 + [V] / 5 ≦ 0.43 (The above [C], [Mn], and [V] are the contents of each component (wt%).)

[0073] [Table 2]

[0074] Here, the relational expression 2 is a value calculated as follows:

[0075] [Equation 2] 800℃≦Finish rolling start temperature≦857℃+(464×[C])+(6445×[Nb])-(644×[Nb] 0.5 ) + (732 × [V]) - (230 × [V] 0.5 )+(890×[Ti])+(363×[Al])-(357×[Si]) (The above [C], [Nb], [V], [Ti], [Al], and [Si] are the contents (wt%) of each component.)

[0076] The microstructures of the steel sheets manufactured as shown in Tables 1 and 2 were observed and their mechanical properties were evaluated. The microstructures were observed using an optical microscope, and the ferrite fraction and diameter were measured using an analysis program. The microstructures were measured at t / 4 (t: steel sheet thickness, unit: mm) in the thickness direction of each steel, and the results are shown in Table 3 below.

[0077] The mechanical properties of each steel were evaluated at a quarter-tip point in the thickness direction. Tensile test specimens were taken from each thickness direction in a direction perpendicular to the rolling direction to measure tensile strength (TS), yield strength (YS), and elongation (El). Impact test specimens were taken from JIS No. 4 standard test specimens at a quarter-tip point in the thickness direction in the rolling direction to measure average impact toughness (CVN) at -20°C. The results are shown in Table 4.

[0078] Meanwhile, in order to examine the changes in mechanical properties before and after normalizing heat treatment for the invention examples and comparative examples, the hot-rolled steel sheets were subjected to normalizing heat treatment at 890°C for 1.3t+30 minutes (t: thickness of steel sheet, mm) and then air-cooled to room temperature, and the evaluation results of the mechanical properties before and after the heat treatment are shown in Table 4 below.

[0079] [Table 3]

[0080] [Table 4]

[0081] The tensile properties and low-temperature impact toughness before and after normalizing are shown in Table 4. In the case of Inventive Examples 1 to 9, the composition ranges, Relations 1 and 2, and microstructural properties proposed by the present invention are satisfied, and both the tensile properties and low-temperature impact toughness are satisfied. In particular, even when the thickness is increased, it is confirmed that the yield strength proposed by the present invention of 355 MPa or more is exhibited.

[0082] Specifically, in the case of Inventive Examples 1 to 9, when the results of as-rolled and normalizing heat treatment are compared, the impact toughness after heat treatment increases slightly, and although there is a difference from the physical properties before heat treatment, it still meets the impact toughness proposed by the present invention, indicating that the method of the present invention (normalized rolling method) can ensure sufficient physical properties without performing an additional normalizing heat treatment. Normalized rolling is a rolling process in which rolling conditions are controlled so that effects similar to those obtained by normalizing can be obtained (rolling process in which the final deformation is carried out at a certain temperature range leading to material conditions equal to values ​​obtained by normalizing).

[0083] In contrast, in the case of Comparative Examples 1 to 3, the Nb content and Relational Formula 2 were outside the values ​​proposed in the present invention, and it was confirmed that the coarse ferrite grains and low Nb content prevented sufficient NbC precipitates, and the yield strength, tensile strength, and impact toughness proposed in the present invention were not met.In the case of Comparative Examples 4 to 6, the C content and Relational Formula 1 were outside the ranges proposed in the present invention, and it was confirmed that the excessive addition of C content resulted in sufficient yield / tensile strength, but deteriorated impact toughness.

[0084] Although Comparative Examples 7 to 9 all satisfy the compositional ranges proposed in this invention, the total number of rolling passes during finish rolling is excessive, and even if the ferrite and pearlite fractions satisfy the values ​​proposed in this invention, the ferrite grain size is large, and it can be seen that the yield strength and impact toughness are not satisfied. In the case of Comparative Examples 10 to 12, although the compositional ranges are satisfied, Relational Formula 2 and the cumulative reduction rate during finish rolling are outside the values ​​proposed in this invention, and it can be confirmed that the yield strength and impact toughness are not satisfied due to the coarsening of the ferrite grain size.

[0085] In the cases of Comparative Examples 1 to 12, although the impact toughness after the normalizing heat treatment was improved, the results showed that the yield strength and impact toughness were not satisfactory.

[0086] 1(a) and 1(b) are photographs showing the optical microstructures at the thickness t / 4 point of Example 1 and Comparative Example 10, respectively. When the finish rolling start temperature does not satisfy Relational Formula 2, it can be predicted that the ferrite grains will become large and coarse due to the growth of austenite during rolling, making it difficult to ensure the yield strength and impact toughness that the present invention aims to achieve. On the other hand, Example 1, which satisfies Relational Formula 2, has very fine ferrite grains.

[0087] FIG. 2 is a graph showing the change in yield strength as a function of the finish rolling start temperature for a 100 mm thick hot-rolled steel sheet using steel type 2 that satisfies the composition range proposed in the present invention. It can be seen that in the temperature range that satisfies Relational Formula 2, the yield strength proposed in the present invention is satisfied, while when the temperature is too high, a decrease in yield strength occurs.

Claims

1. The alloy contains, by weight, carbon (C): more than 0.10 to 0.17%, silicon (Si): 0.2 to 0.5%, manganese (Mn): 1.2 to 1.6%, phosphorus (P): 0.012% or less, sulfur (S): 0.003% or less, aluminum (Al): 0.015 to 0.045%, niobium (Nb): 0.03 to 0.05%, vanadium (V): 0.06% or less, titanium (Ti): 0.005 to 0.017%, and nitrogen (N): 0.002 to 0.01%, with the remainder consisting of Fe and inevitable impurities, and satisfies the following relational expression 1: The microstructure of the steel sheet is characterized by containing, in terms of area fraction, 60 to 85% ferrite and the remainder pearlite and unavoidable structures. [Relationship 1] [C]+[Mn] / 6+[V] / 5≦0.43 (The above [C], [Mn], and [V] are the contents (wt%) of each component.)

2. 2. The steel plate according to claim 1, wherein the thickness of the steel plate is 100 mm or less.

3. 2. The steel sheet according to claim 1, wherein the average grain size of the ferrite is 50 μm or less.

4. 2. The steel sheet according to claim 1, wherein the steel sheet has a yield strength of 355 MPa or more and a tensile strength of 470 MPa or more, evaluated perpendicular to the rolling direction at a point t / 4 (t: thickness of the steel sheet, mm).

5. The steel plate according to claim 1, characterized in that the Charpy impact absorption energy at -20 ° C. evaluated in the rolling direction at a t / 4 point (t: thickness of the steel plate, mm) is 100 J or more.

6. preparing a steel slab containing, in weight percent, carbon (C): over 0.10 to 0.17%, silicon (Si): 0.2 to 0.5%, manganese (Mn): 1.2 to 1.6%, phosphorus (P): 0.012% or less, sulfur (S): 0.003% or less, aluminum (Al): 0.015 to 0.045%, niobium (Nb): 0.03 to 0.05%, vanadium (V): 0.06% or less, titanium (Ti): 0.005 to 0.017%, nitrogen (N): 0.002 to 0.01%, and the remainder being Fe and inevitable impurities, and satisfying the following relational expression 1; heating the steel slab at a temperature in the range of 1150 to 1200°C for at least 3 hours; A step of rough rolling the heated steel slab and hot rolling the same at a finish rolling start temperature that satisfies the following relational expression 2; cooling after the hot rolling A method for manufacturing a steel sheet, comprising: [Relationship 1] [C]+[Mn] / 6+[V] / 5≦0.43 (The above [C], [Mn], and [V] are the contents (wt%) of each component) [Relationship 2] 800 ° C ≦ Finish rolling start temperature ≦ 857 ° C + (464 × [C]) + (6445 × [Nb]) - (644 × [Nb] ０．５ )+(732×[V])−(230×[V] ０．５ )+(890×[Ti])+(363×[Al])−(357×[Si]) (The above [C], [Nb], [V], [Ti], [Al], and [Si] are the contents (wt%) of each component.)

7. 7. The method for manufacturing a steel sheet according to claim 6, wherein the hot rolling is performed at a cumulative reduction rate of 45% or more and at a number of rolling passes of 8 or less.

8. The method for producing a steel sheet according to claim 6, wherein the finish rolling is finished at a temperature of Ar3 or higher.

9. The method for producing a steel sheet according to claim 6, wherein the cooling is air cooling.

10. The method of claim 6, further comprising the step of heat treating the steel sheet at a temperature range of 830 to 930°C for 1.3t+30 minutes or more (t: thickness of the steel sheet, mm) after the cooling.

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