Hot rolled steel sheet, and method for manufacturing same

Abstract

A hot rolled steel sheet, and a method for manufacturing same are provided. The hot rolled steel sheet of the present invention comprises, by wt%, 0.02-0.09% of C, 1.0% or less of Si, 0.3-2.0% of Mn, 0.4-2.0% of Cr, 1.0% or less of Cu, 1.0% or less of Ni, 0.1% or less of Al, 0.02-0.1% of Nb, 0.01-0.1% of Ti, 0.03% or less of P, 0.02% or less of S, 0.015% or less of N and the balance of Fe and inevitable impurities, and satisfies Nb+Ti≤0.12, wherein the microstructure of the steel sheet comprises ferrite or bainite as a main phase and contains the balance (less than 20 area%) of other structures, the weathering index defined by the relation is 6.0 or more, the yield strength, at 30° with respect to the rolling direction, of the steel sheet is 450 MPa or more, and the -5°C impact absorption energy, at 120° with respect to the rolling direction, of the steel sheet, is 47 J or more.

Description

Hot-rolled steel sheet and manufacturing method thereof

[0001] The present invention relates to a hot-rolled steel sheet that can be used in various ways, such as for construction, bridges, and ground support, and a method for manufacturing the same. More specifically, the present invention relates to a high-strength seawater-resistant hot-rolled steel sheet having excellent impact properties and material uniformity and having a YS of 450 MPa or more, and a method for manufacturing the same.

[0002] Steel with excellent seawater resistance is a steel that reduces corrosion by approximately 60% or less compared to general steel when used for structures such as construction and civil engineering in environments affected by seawater salinity, thereby extending the durability of the structure. Accordingly, the safety of the structure can be increased in a seawater environment.

[0003] Seawater-resistant steel for structural use is primarily used as steel pipe piles to support offshore structures. Small-diameter steel pipes are manufactured transversely, while large-diameter steel pipes are manufactured in a spiral shape. Therefore, when steel pipe piles are used to support the weight of a structure, such as a steel pipe pile, it is necessary to ensure that the required properties are met in all directions when tested on the plate to ensure structural stability.

[0004] As an example of such prior art, the invention presented in Patent Document 1 can be cited. In the case of Patent Document 1, only a component system that achieves seawater resistance characteristics by formulating the Cu, Ni, and Cr component system is presented, but no specific mention is made of the mechanical properties.

[0005] Furthermore, in the case of Patent Document 2, the main components are similar—Cu, Ni, and Cr—but the strength level is low, at TS 520 MPa or lower, and the range of components differs. In other words, most of these existing patents do not present a method for simultaneously ensuring material uniformity and seawater resistance in the base material.

[0006] [Prior Art Literature]

[0007] [Patent Document]

[0008] (Patent Document 1) KR 2012-0083855A

[0009] (Patent Document 2) JP 2022-060949A

[0010] The purpose of the present invention is to provide a high-strength seawater-resistant hot-rolled steel sheet having excellent impact properties and material uniformity and having a YS of 450 MPa or more, and a method for manufacturing the same.

[0011] In addition, the technical problems to be achieved in the present invention are not limited to the technical problems mentioned above, and other technical problems not mentioned can be clearly understood by a person having ordinary skill in the technical field to which the present invention belongs from the description below.

[0012] One aspect of the present invention is:

[0013] In weight %, C: 0.02 to 0.09%, Si: 1.0% or less, Mn: 0.3 to 2.0%, Cr: 0.4 to 2.0%, Cu: 1.0% or less, Ni: 1.0% or less, Al: 0.1% or less, Nb: 0.02 to 0.1%, Ti: 0.01 to 0.1%, P: 0.03% or less, S: 0.02% or less, N: 0.015% or less, containing residual Fe and unavoidable impurities, and satisfying Nb+Ti≤0.12.

[0014] The steel plate microstructure is composed of ferrite or bainite as the main phase and contains other structures with a residual area of ​​less than 20%, and

[0015] The weather resistance index defined by the following relational expression 1 is 6.0 or higher,

[0016] This relates to a hot-rolled steel sheet having a yield strength of 450 MPa or more in a direction 30° to the rolling direction and a -5°C impact absorption energy of 47 J or more in a direction 120° to the rolling direction.

[0017] [Relationship 1]

[0018] Weatherability Index (I) = 26.01 (% Cu) + 3.88 (% Ni) + 1.20 (% Cr) + 1.49 (% Si) + 17.28 (% P) - 7.29 (% Cu)(% Ni) - 9.1 (% Ni)(% P) - 33.39 (% Cu) 2

[0019] Another aspect of the present invention is

[0020] A step of heating a steel slab containing, by weight%, C: 0.02 to 0.09%, Si: 1.0% or less, Mn: 0.3 to 2.0%, Cr: 0.4 to 2.0%, Cu: 1.0% or less, Ni: 1.0% or less, Al: 0.1% or less, Nb: 0.02 to 0.1%, Ti: 0.01 to 0.1%, P: 0.03% or less, S: 0.02% or less, N: 0.015% or less, residual Fe and unavoidable impurities, and satisfying Nb+Ti≤0.12 and a weather resistance index defined by the following relationship 1 of 6.0 or more, in a range of 1100 to 1300°C;

[0021] A step of finishing hot rolling the above-mentioned heated steel slab at a temperature range of Ar3 temperature or higher and 920℃ or lower;

[0022] A step of cooling the above hot-rolled steel sheet at a cooling rate of 10 to 50°C / s; and

[0023] It includes a step of coiling the above cooled hot-rolled steel plate at a temperature range of 500 to 660°C,

[0024] The present invention relates to a method for manufacturing a hot-rolled steel sheet, wherein, in the above-described finishing hot rolling step, the cumulative reduction ratio in a temperature range from Ar3 temperature to Ae3 temperature is controlled to 30% or less.

[0025] [Relationship 1]

[0026] Weatherability Index (I) = 26.01 (% Cu) + 3.88 (% Ni) + 1.20 (% Cr) + 1.49 (% Si) + 17.28 (% P) - 7.29 (% Cu)(% Ni) - 9.1 (% Ni)(% P) - 33.39 (% Cu) 2

[0027] The present invention having the configuration described above can provide a high-strength seawater-resistant hot-rolled steel sheet having YS 450 MPa or higher and excellent impact properties and material uniformity.

[0028] Accordingly, the present invention can be used in various forms such as plates, H-shaped steel, and steel pipes that can be used in various ways for construction, bridges, and ground support, and although the form is not specified, when used as a spiral steel pipe in particular, it can effectively provide a steel material having excellent tensile properties of a steel plate in a direction of 30° to the rolling direction and excellent impact properties of a steel plate in a direction of 120° to the rolling direction.

[0029] FIG. 1 is a graph showing the change in impact properties of steel with respect to the cumulative reduction ratio in the temperature range of Ar3 to Ae3 in one embodiment of the present invention.

[0030] Hereinafter, the present invention will be described.

[0031] The present invention can be used in various forms such as plates, H-beams, and steel pipes, and the form is not specified. However, especially in the case of spiral steel pipes, the tensile performance of the steel plate in the direction of 30° to the rolling direction and the impact characteristics of the steel plate in the direction of 120° to the rolling direction are very important. That is, in terms of the tensile properties of the steel plate in the direction of 30° to the rolling direction and the impact absorption energy of the steel plate in the direction of 120° (here, a 180° difference is regarded as the same direction), a steel plate that satisfies YS of 450 MPa or more and Charpy impact energy of 47 J or more at -5℃ in the direction perpendicular to the tensile direction is required. The reason for this is that the steel plate material in the direction of 30° to the rolling direction and the impact characteristics of the steel plate in the direction of 120° to the rolling direction usually exhibit weak physical properties in the material. If the properties in the above direction are satisfied, a steel plate can be manufactured that satisfies the impact characteristics of having an impact absorption energy of 47 J or more at -5℃ in a Charpy V-notch test while satisfying a yield strength of 450 MPa or more in all directions. In addition, the steel plate of the present invention must satisfy a weather resistance index of 6 or more in order to simultaneously secure seawater resistance.

[0032] In this respect, the hot-rolled steel sheet of the present invention contains, in wt%, C: 0.02 to 0.09%, Si: 1.0% or less, Mn: 0.3 to 2.0%, Cr: 0.4 to 2.0%, Cu: 1.0% or less, Ni: 1.0% or less, Al: 0.1% or less, Nb: 0.02 to 0.1%, Ti: 0.01 to 0.1%, P: 0.03% or less, S: 0.02% or less, N: 0.015% or less, residual Fe and unavoidable impurities, and satisfies Nb+Ti≤0.12, and the steel sheet microstructure has ferrite or bainite as the main phase and contains other structures of less than 20 area% remaining, and the weather resistance index defined by the following relationship is 6.0 or more, and the yield strength of the steel sheet in the direction of 30° to the rolling direction is The steel plate having a strength of 450 MPa or higher and a direction of 120° to the rolling direction can satisfy an impact absorption energy of -5°C of 47 J or higher.

[0033] First, the content of steel components forming the hot-rolled steel sheet of the present invention and the reasons for limiting the content are explained. Here, "%" means "weight %" unless otherwise specified.

[0034] C: 0.02~0.09%

[0035] Carbon (C) is the most effective element for securing the strength of steel plates, and to secure strength, it can be added in amounts of 0.02% or more. However, if the content exceeds 0.09%, playability deteriorates, and the corrosion resistance of the steel may deteriorate due to excessive formation of cementite and pearlite structures. From this perspective, it is preferable to control the C content in the present invention within the range of 0.02 to 0.09%. More preferably, the C content is controlled within the range of 0.03 to 0.07%, and most preferably, within the range of 0.04 to 0.06%.

[0036] Si: 1.0% or less

[0037] Silicon (Si) is an element that improves corrosion resistance by inhibiting the formation of cementite or pearlite structures. In the present invention, its content may be limited to 1.0% or less. If the content exceeds 1.0%, impact and welding properties may be degraded. More preferably, Si is controlled to 0.6% or less.

[0038] Mn: 0.3~2.0%

[0039] Manganese (Mn), along with carbon (C), is the most commonly used element to enhance the strength of steel plates. In the present invention, its content can be controlled within a range of 0.3 to 2.0%. If the content is less than 0.3%, the aforementioned effects cannot be achieved. If it exceeds 2.0%, it can induce center segregation and excessively increase the carbon equivalent, thereby deteriorating weldability. More preferably, Mn is controlled within a range of 0.3 to 1.5%, and most preferably within a range of 0.5 to 1.2%.

[0040] Cr: 0.4~2.0%

[0041] Chromium (Cr) is a basic alloying element that improves seawater corrosion resistance, and it acts to stabilize the corrosion layer by concentrating at the interface of corrosion products, thereby improving seawater corrosion resistance. In the present invention, the Cr content is controlled to be in the range of 0.4 to 2.0%. If the content is less than 0.4%, the aforementioned effect cannot be obtained, and if it exceeds 2.0%, the Ar3 temperature can be increased to coarsen the steel plate structure. More preferably, Cr is controlled to be in the range of 0.5 to 1.5%, and most preferably, in the range of 0.5 to 1.3%.

[0042] Cu: 1.0% or less

[0043] Copper (Cu) is well known as an element that enhances corrosion resistance. In the present invention, its content is controlled to 1.0% or less. If the content exceeds 1.0%, a liquid phase is formed at the base metal interface at high temperatures, causing high-temperature embrittlement. To prevent this, a large amount of expensive nickel must be added, which is undesirable. More preferably, the Cu content is contained in the range of 0.5% or less.

[0044] Ni: 1.0% or less

[0045] Nickel (Ni) is an element added to prevent high-temperature embrittlement caused by Cu when adding Cu, and its content is controlled to 1.0% or less in the present invention. Typically, it is added at a ratio of 0.5 to 1 times that of Cu in weight percent, and since it is an expensive element, it is desirable to minimize the amount added in line with Cu. More preferably, Ni is controlled to a range of 0.4% or less.

[0046] Al: 0.1% or less

[0047] Aluminum (Al) is used as a deoxidizer, and a certain amount is added to enhance the amount of deoxidation and solid solution strengthening. However, excessive use can cause slab cracks during casting and intergranular oxidation in the final product. Therefore, the present invention controls the Al content to 0.1% or less. More preferably, the Al content is controlled to 0.05% or less.

[0048] Nb: 0.02 ~0.1%

[0049] Niobium (Nb) is used for precipitation strengthening and grain refinement, but if its content is less than 0.02%, the aforementioned effects cannot be expected, and if it exceeds 0.1%, the precipitates become coarser, which may worsen the impact properties of the steel sheet. Therefore, in the present invention, the Nb content is controlled to be in the range of 0.02 to 0.1%. More preferably, Nb is controlled to be in the range of 0.03 to 0.07%.

[0050] Ti: 0.01~0.1%

[0051] Titanium (Ti) is primarily used to increase the strength of steel and suppress coarsening in the heat-affected zone of slabs or welds at high temperatures. In the present invention, the Ti content is controlled within the range of 0.01 to 0.1%. If the Ti content is less than 0.01%, the aforementioned effects cannot be expected, and if it exceeds 0.1%, the impact properties of the steel plate may be degraded. More preferably, the Ti content is controlled within the range of 0.02 to 0.09%.

[0052] Nb+Ti≤0.12

[0053] In the present invention, the sum of the addition amounts of niobium and titanium (Nb+Ti) is limited to 0.12% or less. However, if the sum exceeds 0.12%, precipitates may become coarse, which may deteriorate the impact properties of the steel sheet. More preferably, the Nb+Ti content is controlled to 0.10% or less.

[0054] P: 0.03% or less

[0055] In the present invention, P is contained in a range of 0.03% or less. This P is an impurity that is inevitably contained in steelmaking, and it is desirable to contain as little P as possible because it causes brittleness due to segregation.

[0056] S: 0.02% or less

[0057] In the present invention, S is contained in a range of 0.02% or less. S is an impurity that is inevitably contained in steelmaking, and is an element that can cause grain boundary embrittlement during hot rolling by forming inclusions or FeS compounds with a low melting point, so it is desirable to contain it as little as possible.

[0058] N: 0.015% or less

[0059] In the present invention, the N content is controlled to a range of 0.015% or less, which is the range controlled in a conventional steel.

[0060] Weatherability index

[0061] In the present invention, the weather resistance index (I) according to the following relational expression 1 may be 6.0 or more.

[0062] This weather resistance index (I) is a guarantee required by KS standards KSD3003 and KSD3300, etc., and to meet the standards, the weather resistance index (I) must be 6.0 or higher. If the weather resistance index (I) is less than 6.0, a problem may arise in which the corrosion resistance characteristics required by the standards are not met.

[0063] [Relationship 1]

[0064] Weatherability Index (I) = 26.01 (% Cu) + 3.88 (% Ni) + 1.20 (% Cr) + 1.49 (% Si) + 17.28 (% P) - 7.29 (% Cu)(% Ni) - 9.1 (% Ni)(% P) - 33.39 (% Cu) 2

[0065] The remaining component of the present invention is iron (Fe). However, during the typical manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of typical manufacturing, their full details are not specifically discussed in this specification.

[0066] Meanwhile, the hot-rolled steel sheet of the present invention has a steel sheet microstructure containing ferrite or bainite as a main phase and other structures of less than 20 area% remaining.

[0067] In the present invention, pearlite may be included as the above-mentioned other structure, but if the area fraction thereof exceeds 20%, problems may arise in the corrosion resistance properties.

[0068] The hot-rolled steel sheet of the present invention having the steel composition and microstructure as described above can satisfy a yield strength of 450 MPa or more in a direction of 30° to the steel sheet rolling direction, and a -5°C impact absorption energy of 47 J or more in a direction of 120° to the steel sheet rolling direction.

[0069]

[0070] Next, a method for manufacturing a hot-rolled steel sheet according to one embodiment of the present invention will be described.

[0071] The method for manufacturing a hot-rolled steel sheet of the present invention comprises the steps of: heating a steel slab containing, in wt%, C: 0.02 to 0.09%, Si: 1.0% or less, Mn: 0.3 to 2.0%, Cr: 0.4 to 2.0%, Cu: 1.0% or less, Ni: 1.0% or less, Al: 0.1% or less, Nb: 0.02 to 0.1%, Ti: 0.01 to 0.1%, P: 0.03% or less, S: 0.02% or less, N: 0.015% or less, residual Fe and unavoidable impurities, and satisfying Nb+Ti≤0.12 and a weather resistance index of 6.0 or more defined by the relationship 1, in a temperature range of 1100 to 1300°C; subjecting the heated steel slab to finish hot rolling in a temperature range of Ar3 temperature or higher and 920°C or lower; A step of cooling the hot-rolled steel sheet that has been finished hot-rolled at a cooling rate of 10 to 50°C / s; and a step of coiling the cooled hot-rolled steel sheet at a temperature range of 500 to 660°C, wherein in the finishing hot-rolling step, the cumulative reduction ratio in the temperature range from the Ar3 temperature to the Ae3 temperature is controlled to 30% or less.

[0072] [heating]

[0073] First, the present invention heats a steel slab satisfying the above-described steel composition and weather resistance index in a temperature range of 1100 to 1300°C.

[0074] The heating process of these steel slabs is carried out for the purpose of making the alloying elements available for use during rolling and of improving the surface quality of the material being rolled.

[0075] In the present invention, during the heating, the temperature range is controlled to be 1100 to 1300°C.

[0076] If the above heating temperature is less than 1100℃, the alloying elements may not dissolve well, and cracks may occur due to the low temperature at the edge of the material width during rolling. In addition, if it exceeds 1300℃, the scale layer that has grown on the slab during heating may not be removed well by high-pressure water, and the black skin of the roll may be peeled off, causing a problem of scale intrusion defects on the surface of the rolled material during the slab rolling.

[0077] [Final rolling]

[0078] Next, in the present invention, the heated steel slab is subjected to finishing hot rolling at a temperature range of Ar3 temperature or higher and 920°C or lower.

[0079] This finishing hot rolling is performed for the purpose of securing material and impact properties.

[0080] In the present invention, the finishing hot rolling temperature is controlled to a temperature range of Ar3 temperature or higher and 920°C or lower during the finishing hot rolling. If the finishing hot rolling temperature is lower than the Ar3 temperature, abnormal rolling occurs, forming an uneven structure, which deteriorates impact properties. If it exceeds 920°C, the structure becomes coarse, which may cause problems in securing material and impact properties.

[0081] Meanwhile, in the present invention, in the finishing hot rolling process, it is required that the finishing hot rolling be performed at a cumulative reduction ratio of 30% or less in a temperature range from the Ar3 temperature to the Ae3 temperature.

[0082] In this way, the reason for controlling the cumulative reduction ratio in the above temperature range is to minimize the anisotropy of the material properties, and to guarantee the properties in a spiral steel pipe by guaranteeing the tensile properties at 30 degrees in the rolling direction and the impact properties at 120 degrees in the rolling direction.

[0083] In the present invention, the Ar3 temperature and the Ae3 temperature are terms well known in the technical field to which the present invention belongs, and can be easily obtained by using JMatPro, a commercial software.

[0084] [cooling]

[0085] And in the present invention, the hot-rolled steel sheet subjected to the above finishing hot rolling is cooled at a cooling rate of 10 to 50°C / s.

[0086] Cooling of these hot-rolled hot-rolled sheets is performed for the purpose of refining ferrite grains.

[0087] At this time, in the present invention, the cooling rate for the hot-rolled sheet can be controlled to a range of 10 to 50°C / s. If the cooling rate is less than 10°C / s, ferrite transformation may occur at high temperatures, forming a coarse structure and resulting in material deficiencies. If the cooling rate exceeds 50°C / s, problems such as shape deterioration due to residual stress may occur. More preferably, the cooling rate is controlled to 20 to 45°C / s.

[0088] Meanwhile, in the present invention, the cooling rate is based on the temperature of the surface of the steel plate.

[0089] [Winding]

[0090] Finally, in the present invention, the cooled hot-rolled steel plate is coiled at a temperature range of 500 to 660°C.

[0091] If the coiling temperature is below 500°C, there is a problem of poor impact properties due to the formation of a hard phase such as martensite due to deviation during cooling. On the other hand, if the coiling temperature exceeds 660°C, the ferrite grains and precipitates may become coarse, resulting in a problem of not satisfying the material quality. More preferably, coiling is performed at 550 to 650°C.

[0092] Hereinafter, the present invention will be described in detail through examples.

[0093] (Example)

[0094] Steel gradeCMnSiAlPSNbTiCuNiCrNNb+TiI10.050.70.50.030.010.0030.050.020.30.11.30.0060.077.620.050.70.50. 030.010.0030.050.020.20.050.50.0060.075.630.050.70.50.030.010.0030.040.090.30.11.30.0060.137.6

[0095] *In Table 1, the content unit is weight%, and the remainder is iron and inevitable impurities. And I is the weatherability index. Weatherability index (I) = 26.01 (% Cu) + 3.88 (% Ni) + 1.20 (% Cr) + 1.49 (% Si) + 17.28 (% P) - 7.29 (% Cu)(% Ni) - 9.1 (% Ni)(% P) - 33.39 (% Cu) 2

[0096] No. Steel grade Finishing Hot rolling temperature (℃) Cooling speed (℃ / s) Coiling temperature (℃) Ae3 or less Cumulative reduction ratio (%) Note 1 187 342 6190 Experimental example 1 2 186 63 56165 Experimental example 2 3 185 921 6249 Experimental example 3 4 185 246 62314 Experimental example 4 5 184 543 62120 Experimental example 5 6 183 85 0 62225 Experimental example 6 7 183 149 62131 Comparative example 1 8 182 437 62036 Comparative example 2 9 181 735 61739 Comparative example 3 10 1810 3962245Comparative Example 4 111875415820Experimental Example 7 121872415220Experimental Example 8 131877424470Comparative Example 5 141921386190Comparative Example 6 151872286680Comparative Example 7 162876326200Comparative Example 8 172875336220Comparative Example 9 182877316800Comparative Example 10 193873366360Comparative Example 11

[0097] A steel slab having the same composition and weather resistance index as those in Table 1 above was prepared. Specifically, in Table 1 above, steel type 1 is an inventive steel, steel type 2 is a comparative steel whose weather resistance index is outside the scope of the present invention, and steel type 3 is a comparative steel whose Nb+Ti content is outside the scope of the present invention.

[0098] Next, the prepared steel slab was heated to 1110℃, and under the conditions shown in Table 2, final hot rolling, cooling, and coiling were performed to manufacture a hot-rolled steel sheet. At this time, during the final hot rolling, the cumulative reduction ratio at a temperature below Ae3 was controlled under the conditions shown in Table 2.

[0099] The mechanical properties and microstructural properties of each hot-rolled steel sheet manufactured as described above were evaluated, and the results are shown in Table 3 below. Specifically, the microstructure or fraction of the hot-rolled steel sheet was determined by analyzing the matrix structure at the 1 / 4t point of the sheet thickness of each hot-rolled steel sheet and using the results. Specifically, the fractions of ferrite, bainite, and other structures were measured using FE-SEM and an image analyzer after nickel corrosion.

[0100] And the yield strength of the steel plate forming a 30° direction with respect to the rolling direction of the hot-rolled steel plates was measured, and further, the -5℃ impact absorption energy of the steel plate forming a 120° direction with respect to the rolling direction of the hot-rolled steel plates was measured, and the results are shown in Table 3 below. Here, the yield strength of each hot-rolled steel plate was evaluated by taking a specimen at the 1 / 4t point of the steel plate forming a 30° direction with respect to the rolling direction of the hot-rolled steel plates, and the tensile test was performed using a method of measuring a plate-shaped specimen of the JIS No. 5 specimen size using the JIS standard to evaluate the tensile properties.

[0101] And the impact absorption energy of each hot-rolled steel sheet was measured by taking a specimen at the 1 / 4t point of the steel sheet in the direction of 120° to the rolling direction of the hot-rolled steel sheets, and the impact toughness of each specimen was measured three times at -5℃ using the average of the absorption energy values ​​through the Charpy V-Notch Test.

[0102] No. Steel gradeMicrostructureYield strength in 30° rolling direction (Mpa)Impact absorption energy in 120° rolling direction (J)Non-high structureOther structure (type and Area fraction) 11 Ferrite pearlite (3.2%) 503201 Experimental example 121 Ferrite pearlite (3.0%) 492189 Experimental example 231 Ferrite pearlite (2.8%) 491179 Experimental example 341 Ferrite pearlite (2.7%) 508158 Experimental example 451 Ferrite pearlite (2.8%) 50489 Experimental example 561 Ferrite pearlite (2.9%) 51151 Experimental example 671 Ferrite pearlite (2.9%) 50742 Comparative example 181 Ferrite pearlite (3.0%) 49738 Comparative example 291 Ferrite pearlite (3.2%) 49231 Comparative example 3101 Ferrite pearlite (3.1%) 49727 Comparative example 4111 Ferrite pearlite (4.2%) Bainite (3.2%) 481223 Experimental example 7 121 Bainite (100%) - 499228 Experimental example 8 131 Bainite (55%) Martensite (45%) 51239 Comparative example 5 141 Ferrite pearlite (3.1%) 47644 Comparative example 6 151 Ferrite pearlite (0.4%) 442164 Comparative example 7 162 Ferrite pearlite (3.0%) 386223 Comparative example 8 172 Ferrite pearlite (3.0%) 388221 Comparative example 9 182 Ferrite pearlite (0.2%) 352126 Comparative example 10 193 Ferrite pearlite (0.1%) 75622 Comparative example 11

[0103] As shown in Table 1-3 above, in the case of Experimental Example 1-8 that satisfies the steel alloy composition and manufacturing process conditions of the present invention, the cumulative reduction ratio in the finishing hot rolling below Ae3 temperature was controlled to 30% or less, resulting in good material and impact properties. This is because, as the cumulative reduction ratio increases below Ae3 temperature, the anisotropy of the material increases, resulting in poor impact properties of the steel sheet in the 120° direction of the rolling direction.

[0104] In contrast, Comparative Example 1-4 had a cumulative reduction ratio of 30% or more in the finishing hot rolling at a temperature of Ae3 or lower, and the impact properties of the hot-rolled steel sheet in the direction of 120° to the rolling direction did not satisfy 47J at -5°C.

[0105] In the case of Comparative Example 5, the coiling temperature (CT) was low, so the impact properties were poor due to the formation of a hard phase, and in the case of Comparative Example 6, the finishing hot rolling temperature was too high, so the structure was coarse, and the impact properties were not satisfied.

[0106] In addition, Comparative Example 7 satisfied the impact characteristics, but the yield strength of the steel plate was inferior due to high temperature CT.

[0107] Meanwhile, Fig. 1 is a graph showing the change in impact properties of steel with respect to the cumulative reduction ratio in the temperature range of Ar3 to Ae3 in one embodiment of the present invention. As shown in Fig. 1, it can be confirmed that the impact properties are improved when the cumulative reduction ratio is lower than 30% in the finishing hot rolling at a temperature of Ae3 or lower.

[0108] As described above, the detailed description of the present invention has described preferred embodiments of the present invention. However, it will be apparent to those skilled in the art that various modifications may be made without departing from the scope of the present invention. Therefore, the scope of the rights of the present invention should not be limited to the described embodiments, but should be determined not only by the claims described below but also by equivalents thereof.

Claims

1. In weight%, C: 0.02 to 0.09%, Si: 1.0% or less, Mn: 0.3 to 2.0%, Cr: 0.4 to 2.0%, Cu: 1.0% or less, Ni: 1.0% or less, Al: 0.1% or less, Nb: 0.02 to 0.1%, Ti: 0.01 to 0.1%, P: 0.03% or less, S: 0.02% or less, N: 0.015% or less, containing residual Fe and unavoidable impurities, and satisfying Nb+Ti≤0.12, The steel sheet microstructure is composed of ferrite or bainite as the main phase and contains other structures with a residual area of ​​less than 20%, and The weather resistance index defined by the following relational expression 1 is 6.0 or higher, Hot rolled steel sheet having a yield strength of 450 MPa or more when the steel sheet is oriented at 30° to the rolling direction, and a -5°C impact absorption energy of 47 J or more when the steel sheet is oriented at 120° to the rolling direction. [Relationship 1] Weatherability Index (I) = 26.01 (% Cu) + 3.88 (% Ni) + 1.20 (% Cr) + 1.49 (% Si) + 17.28 (% P) - 7.29 (% Cu)(% Ni) - 9.1 (% Ni)(% P) - 33.39 (% Cu) 2 2. A hot-rolled steel sheet in accordance with paragraph 1, wherein the other organization is pearlite.

3. A step of heating a steel slab containing C in wt% of 0.02 to 0.09%, Si in wt% or less, Mn in wt% or less, Mn in wt% or less, Cr in wt% or less, Cu in wt% or less, Ni in wt% or less, Al in wt% or less, Nb in wt% or less, Nb in wt% or less, Ti in wt% or less, P in wt% or less, S in wt% or less, N in wt% or less, N in wt% or less, residual Fe in wt% or less, and unavoidable impurities, and satisfying Nb+Ti≤0.12 and a weather resistance index of 6.0 or more defined by the following equation 1, in a range of 1100 to 1300°C; A step of finishing hot rolling the above heated steel slab at a temperature range of Ar3 temperature or higher and 920℃ or lower; A step of cooling the hot-rolled steel sheet that has been finished by hot rolling at a cooling rate of 10 to 50°C / s; and It includes a step of coiling the cooled hot rolled steel plate at a temperature range of 500 to 660℃, A method for manufacturing a hot-rolled steel sheet, wherein, in the above-mentioned finishing hot rolling step, the cumulative reduction ratio in a temperature range from the Ar3 temperature to the Ae3 temperature is controlled to 30% or less. [Relationship 1] Weatherability Index (I) = 26.01 (% Cu) + 3.88 (% Ni) + 1.20 (% Cr) + 1.49 (% Si) + 17.28 (% P) - 7.29 (% Cu)(% Ni) - 9.1 (% Ni)(% P) - 33.39 (% Cu) 2 4. A method for manufacturing a hot-rolled steel sheet in the third paragraph, wherein the coiled hot-rolled steel sheet has a steel sheet microstructure consisting mainly of ferrite or bainite and contains other structures of less than 20 area%, and a yield strength of the steel sheet formed in a direction of 30° to the rolling direction is 450 MPa or more, and a -5°C impact absorption energy of the steel sheet formed in a direction of 120° to the rolling direction is 47 J or more.

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