Steel plate and its manufacturing method

Abstract

The present invention relates to a steel plate and a manufacturing method thereof, and more particularly to a high-strength steel plate having excellent corrosion resistance and a manufacturing method thereof.

Description

[Technical Field]

[0001] The present invention relates to a steel plate and a manufacturing method thereof, and more particularly to a high-strength steel plate having excellent corrosion resistance and a manufacturing method thereof. [Background technology]

[0002] Steel materials used in port structures or piers of offshore bridges must be made of materials that are highly resistant to corrosion in seawater environments in order to ensure durability.

[0003] Generally, to improve seawater corrosion resistance, methods can be used to increase the Cr and Cu contents in steel. Steels with high Cr and Cu contents form a Cr oxide film and a Cu-enriched layer on the surface of the steel, and both methods are effective in suppressing corrosion of steel in a seawater environment.

[0004] However, the above elements are more expensive than ordinary alloying elements, which has the drawback of increasing the manufacturing cost of steel materials.

[0005] Meanwhile, high-strength steel must be used to improve the architectural stability of structures. Increasing the C or Mn content in steel is effective for increasing the strength of steel. This is because it increases the fraction of hard carbon-enriched regions in steel, which is composed of a composite structure of ferrite and carbon-enriched regions. Here, the carbon-enriched regions can be pearlite or bainite.

[0006] However, since corrosion usually occurs at the boundary between regions with different hardness, as described above, the method of increasing the fraction of pearlite or bainite has the problem of increasing the number of locations where corrosion occurs, thereby impairing corrosion resistance.

[0007] Therefore, in order to manufacture economical and high-strength seawater-resistant steel, a method is required that minimizes the use of Cu and Cr, reduces the fraction of pearlite or bainite, and yet ensures high strength. Summary of the Invention [Problem to be solved by the invention]

[0008] According to one embodiment of the present invention, a steel sheet and a method for manufacturing the same are provided.

[0009] According to one embodiment of the present invention, a high-strength steel plate having excellent corrosion resistance and a method for manufacturing the same are provided.

[0010] The object of the present invention is not limited to the above-mentioned content, and a person skilled in the art will have no difficulty in understanding further object of the present invention from the overall content of this specification. [Means for solving the problem]

[0011] According to one embodiment of the present invention, the alloy contains, by weight, C: 0.030 to 0.070%, Si: 0.50 to 1.30%, Mn: 0.30 to 0.70%, Cr: 0.50 to 1.50%, Al: 0.05% or less, Cu: 0.25 to 0.50%, Ni: 0.05 to 0.50%, Si: 0.0100% or less, Ti: 0.020 to 0.050%, Nb: 0.050 to 0.090%, the balance being Fe and other unavoidable impurities, The R value defined by the following relational expression 1 is 0.75 or more, The microstructure at a 1 / 4 point from the surface in the thickness direction contains, by area%, 85% or more of ferrite and the remainder of the structure, The average crystal grain size of the ferrite is 30 μm or less, It is possible to provide a steel sheet in which the content of solute Cr is 90% or more of the total Cr content.

[0012] [Equation 1] R = [Si] + ([Ni] / [Cu]) (In the formula, [Si], [Ni], and [Cu] are the weight percentages of each element.)

[0013] The remaining structure may include one or more of pearlite and bainite.

[0014] The steel sheet may have a Q value defined by the following relational expression 2 of 0.3 to 8.5.

[0015] [Equation 2] Q = ([Ti] + [Nb]) / [Cr]) (In the formula, [Ti], [Nb], and [Cr] are the weight percentages of each element present in particulate form, such as precipitates or inclusions, in the steel.)

[0016] The steel plate may have a tensile strength of 600 MPa or more and an impact toughness at -5°C of 100 J or more.

[0017] After a corrosion test according to KS D ISO 14993, the steel plate may have a Cr content of 0.50% by weight or more at the interface of the corrosion product formed.

[0018] The steel plate may have a relative corrosion rate of 60% or less compared to KS-SS275 in a corrosion test according to KS D ISO 14993.

[0019] According to one embodiment of the present invention, a step of reheating a steel slab containing, by weight %, C: 0.030 to 0.070%, Si: 0.50 to 1.30%, Mn: 0.30 to 0.70%, Cr: 0.50 to 1.50%, Al: 0.05% or less, Cu: 0.25 to 0.50%, Ni: 0.05 to 0.50%, Si: 0.0100% or less, Ti: 0.020 to 0.050%, Nb: 0.050 to 0.090%, the balance being Fe and other inevitable impurities, and having an R value defined by the following Relation 1 of 0.75 or more; hot rolling the reheated steel slab at a finish rolling temperature of 750 to 900°C; The hot-rolled steel sheet is primarily cooled to a temperature range of 500 to 650°C at a cooling rate of 5 to 20°C / s, and then coiled; and The method for manufacturing a steel sheet may further include a step of secondary cooling the coiled steel sheet to 400°C at a cooling rate of 0.40°C / min or more.

[0020] [Equation 1] R = [Si] + ([Ni] / [Cu]) (In the formula, [Si], [Ni], and [Cu] are the weight percentages of each element.)

[0021] The reheating step can be carried out at a temperature in the range of 1100 to 1300°C. [Effects of the Invention]

[0022] According to one embodiment of the present invention, a steel sheet and a method for manufacturing the same can be provided.

[0023] According to one embodiment of the present invention, it is possible to provide a high-strength steel plate having excellent corrosion resistance and a method for manufacturing the same.

[0024] According to one embodiment of the present invention, it is possible to provide a high-strength steel plate having excellent corrosion resistance in an environment where it comes into contact with seawater, such as a port structure or a floodgate at an estuary embankment, and a method for manufacturing the same. [Brief explanation of the drawings]

[0025] [Figure 1] 10 shows the results of measuring the Cr content at the interface of the corrosion product of Example 17 according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0026] Hereinafter, preferred embodiments of the present invention will be described. The embodiments of the present invention can be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The present embodiments are provided to explain the present invention in more detail to those skilled in the art to which the invention pertains.

[0027] The present invention will be described in detail below.

[0028] The steel composition of the present invention will be described in detail below.

[0029] In the present invention, unless otherwise specified, the percentage representing the content of each element is based on weight.

[0030] A steel sheet according to one embodiment of the present invention may contain, in weight percent, C: 0.030 to 0.070%, Si: 0.50 to 1.30%, Mn: 0.30 to 0.70%, Cr: 0.50 to 1.50%, Al: 0.05% or less, Cu: 0.25 to 0.50%, Ni: 0.05 to 0.50%, S: 0.0100% or less, Ti: 0.020 to 0.050%, Nb: 0.050 to 0.090%, with the balance being Fe and other unavoidable impurities.

[0031] Carbon (C):0.030~0.070% Carbon (C) is the most economical and effective element for strengthening steel, and in the present invention, 0.030% or more may be added. If the carbon (C) content is less than 0.030%, other alloy elements must be added to ensure sufficient strength, which is uneconomical. In one embodiment of the present invention, 0.035% or more may be added. On the other hand, if the carbon (C) content exceeds 0.070%, the pearlite or bainite fraction in the structure may be excessively increased, resulting in reduced corrosion resistance and a risk of reduced impact toughness. In one embodiment of the present invention, the upper limit of the carbon (C) content may be 0.065%.

[0032] Silicon (Si): 0.50 to 1.30% Silicon (Si) is generally added to deoxidize molten steel and is also effective as a solid-solution strengthening element. It also improves corrosion resistance by forming a protective film (H4SiO4) on the surface and reducing the fraction of carbon-enriched regions that eventually differentiate into pearlite or bainite. If the silicon (Si) content is less than 0.50%, the above-mentioned effects and the effect of improving corrosion resistance may be insufficient. In one embodiment of the present invention, the lower limit of silicon (Si) may be 0.55%. On the other hand, if the silicon (Si) content exceeds 1.30%, problems such as reduced weldability may occur. In one embodiment of the present invention, the upper limit may be 1.25%.

[0033] Manganese (Mn): 0.30-0.70% Manganese (Mn) is an element effective in solid solution strengthening of steel, but may induce embrittlement of steel through the formation of MnS, resulting in reduced corrosion resistance. If the manganese (Mn) content is less than 0.30%, the effect of strengthening steel is small. In one embodiment of the present invention, the lower limit may be 0.35%. On the other hand, if the Mn content exceeds 0.70%, a large amount of MnS is formed, which excessively increases the pearlite or bainite fraction in the structure, potentially resulting in reduced corrosion resistance. In one embodiment of the present invention, the upper limit may be 0.65%.

[0034] Chromium (Cr): 0.50-1.50% Chromium (Cr) is a basic alloying element that improves seawater corrosion resistance. In the present invention, it has been discovered that chromium (Cr) concentrates at the interface of corrosion products and stabilizes the corrosion layer, thereby improving seawater corrosion resistance. If the chromium (Cr) content is less than 0.50%, the above-mentioned effects may be insufficient. In one embodiment of the present invention, the lower limit may be 0.55%. On the other hand, if the chromium (Cr) content exceeds 1.50%, the seawater corrosion resistance effect may tend to converge. In one embodiment of the present invention, the upper limit may be 1.45%.

[0035] Aluminum (Al): 0.05% or less Aluminum (Al) is an element added to deoxidize molten steel and improve corrosion resistance. However, excessive addition of Al can lead to the formation of many oxide-based inclusions in the steel, which can cause the steel to become brittle. Therefore, in the present invention, the aluminum (Al) content can be limited to 0.05% or less. In one embodiment of the present invention, the upper limit can be 0.045%. In one embodiment of the present invention, the aluminum (Al) content can be 0.001% or more.

[0036] Copper (Cu): 0.25-0.50% Copper (Cu) is an element that improves seawater resistance. In the present invention, copper (Cu) can be added to improve corrosion resistance. However, excessive addition can be cost-effective and can cause surface defects known as Cu shortness. Therefore, the copper (Cu) content can be limited to 0.50% or less. According to one embodiment of the present invention, the copper content can be 0.45% or less. On the other hand, if the copper content is less than 0.25%, the effect of improving corrosion resistance may be insufficient. According to one embodiment of the present invention, the lower limit can be 0.27%.

[0037] Nickel (Ni): 0.05 to 0.50% Nickel (Ni) is generally an essential element in Cu-added steels to prevent surface defects known as Cu hot shortness. It is also an element that, together with Cu, is effective in improving seawater resistance. However, because Ni is an expensive element, excessive addition may be undesirable from an economic standpoint. If the nickel (Ni) content is less than 0.05%, the effects of suppressing Cu shortness and improving corrosion resistance may be insufficient, whereas if the nickel (Ni) content exceeds 0.50%, these effects may tend to converge. In one embodiment of the present invention, the upper limit may be 0.45%. In one embodiment of the present invention, the lower limit of nickel (Ni) may be 0.07%.

[0038] Sulfur (S): 0.0100% or less Sulfur (S) is an element that is detrimental to impact toughness and corrosion resistance. Sulfur (S) combines with Mn in steel to form nonmetallic inclusions (MnS), which act as corrosion initiation sites, so it is preferable to reduce the amount of S as much as possible. Therefore, in the present invention, the sulfur (S) content can be limited to 0.0100% or less. However, 0% is excluded to take into account cases where sulfur is unavoidably contained during the manufacturing process.

[0039] Titanium (Ti): 0.020-0.050% Titanium (Ti) can strengthen steel by precipitating in steel as TiC and the like. Meanwhile, the precipitation of TiC has the effect of consuming C, which forms Cr carbides, thereby increasing the amount of dissolved Cr. If the titanium (Ti) content is less than 0.020%, the above effect may not be significant. In one embodiment of the present invention, the lower limit may be 0.025%. On the other hand, if the titanium content exceeds 0.050%, coarse carbonitrides may be formed, which may reduce the impact value. In one embodiment of the present invention, the upper limit may be 0.045%.

[0040] Niobium (Nb): 0.050-0.090% Niobium (Nb) not only has a precipitation strengthening effect like Ti, but also induces non-recrystallization rolling to refine grain size, thereby improving strength and impact toughness. Meanwhile, when NbC precipitates, it consumes C, which forms Cr carbides, resulting in an increase in solute Cr. If the niobium (Nb) content is less than 0.050%, the above effect may be insignificant. The lower limit according to one embodiment of the present invention may be 0.045%. If the Nb content exceeds 0.090%, the above effect tends to converge. In one embodiment of the present invention, niobium (Nb) may be contained in an amount of 0.085% or less.

[0041] In addition to the above-described composition, the steel of the present invention may contain the remaining iron (Fe) and inevitable impurities. The inevitable impurities may be unintentionally mixed in during the normal manufacturing process, and therefore cannot be excluded. Since such impurities are known to any engineer in the field of normal steel manufacturing, the present specification will not specifically mention all of the contents thereof.

[0042] The steel sheet according to one embodiment of the present invention may have an R value defined by the following relational expression 1 of 0.75 or more.

[0043] [Equation 1] R = [Si] + ([Ni] / [Cu]) (In the formula, [Si], [Ni], and [Cu] are the weight percentages of each element.)

[0044] As a result of research into simultaneously ensuring mechanical properties and corrosion resistance, the inventors have confirmed that corrosion resistance may be poor depending on the relationship between the Si, Ni, and Cu contents. When the R value defined by the above-mentioned Relational Formula 1 is less than 0.75, the surface quality may be poor. In particular, surface deterioration due to Cu-hot shortness is a phenomenon that leaves cracks of tens to hundreds of μm in the surface layer, which may cause a decrease in impact value. There is no need to particularly limit the R value, but in consideration of economy in terms of the cost of alloy addition, the upper limit of the R value in the present invention may be 3.00.

[0045] In the following, the microstructure of the steel of the present invention will be described in detail.

[0046] In the present invention, unless otherwise specified, the percentage representing the fraction of the microstructure is based on the area.

[0047] According to an embodiment of the present invention, the microstructure at a 1 / 4 point from the surface in the thickness direction of the steel plate may include, in area %, 85% or more of ferrite and the remainder, and the remainder may include at least one of pearlite and bainite.

[0048] In the present invention, the fraction of the microstructure is observed using an optical microscope at a quarter point from the surface of the steel sheet in the thickness direction.

[0049] Ferrite is a structure that can ensure corrosion resistance and impact toughness, and in the present invention, it can be contained in an amount of 85% or more. If the ferrite fraction is less than 85%, there is a possibility that a problem will arise in that the desired level of corrosion resistance or impact toughness cannot be ensured.

[0050] Pearlite and bainite affect the strength and impact value of steel, and in the surface layer, they are factors that affect seawater corrosion resistance in terms of the degree of structural uniformity. In other words, a higher pearlite and bainite fraction increases strength but decreases impact value, potentially reducing seawater corrosion resistance. When the area fraction of one or more of pearlite and bainite is 15% or less, not only can the target strength and impact value be achieved, but the pearlite fraction in the surface layer is also appropriately controlled, ensuring corrosion resistance. However, if the area fraction exceeds 15%, the aforementioned effects cannot be achieved.

[0051] According to one embodiment of the present invention, the ferrite may have an average crystal grain size of 30 μm or less.

[0052] The average grain size in the present invention is expressed as the average value of the ferrite grain sizes measured at five randomly selected points by the circular transverse line method described in KS D 0205. The average ferrite grain size in the present invention is measured at a quarter point from the surface in the thickness direction, similar to the microstructure fraction.

[0053] Fine ferrite grains are advantageous for ensuring the strength and impact value of steel. Based on the assumption that ferrite grain boundaries can serve as pathways for Cr to migrate to corrosion products, the present invention conducted research and found that when the average ferrite grain size at the 1 / 4 point in the thickness direction of a steel sheet is 30 μm or less, not only can the target strength and impact value be ensured, but also corrosion resistance can be ensured by properly controlling the ferrite grain size in the surface layer. On the other hand, if the average ferrite grain size exceeds 30 μm, the above-mentioned effects cannot be achieved. According to one embodiment of the present invention, the average ferrite grain size may be 8 μm or more.

[0054] The steel sheet according to one embodiment of the present invention may have a Q value defined by the following relational expression 2 of 0.3 to 8.5.

[0055] [Equation 2] Q = ([Ti] + [Nb]) / [Cr]) (In the formula, [Ti], [Nb], and [Cr] are the weight percentages of each element present in particulate form, such as precipitates or inclusions, in the steel.)

[0056] If the Q value defined by the above relational expression 2 is less than 0.3, the amount of Cr precipitated increases, the amount of dissolved Cr decreases, and there is a possibility that corrosion resistance cannot be ensured. On the other hand, if the Q value exceeds 8.5, there is a possibility that the Ti and Nb precipitates become coarse and act as corrosion sites, impairing corrosion resistance, or as crack initiation points, impairing impact value.

[0057] In the present invention, a method was used in which precipitates and inclusions present in particulate form in steel were extracted using electrolytic extraction, and then the contents of Ti, Nb, and Cr in the particles were measured using inductively coupled plasma optical emission spectroscopy.

[0058] In a steel sheet according to an embodiment of the present invention, the content of solute Cr may be 90% or more of the total Cr content.

[0059] According to one embodiment of the present invention, the solute Cr content may refer to the value obtained by measuring the amount of Cr present in particulate form, such as precipitates or inclusions, in a steel and subtracting the amount of solute Cr from the total Cr content of the steel. More specifically, in this invention, particulate precipitates and inclusions present in a steel were extracted using electrolytic extraction, and the Cr content of the particles was measured using inductively coupled plasma optical emission spectroscopy. The solute Cr content was calculated by subtracting the amount of Cr present in particulate form from the Cr content of the steel.

[0060] In the present invention, it was confirmed that seawater corrosion resistance is determined by the Cr content at the interface of corrosion products, and that excellent seawater corrosion resistance is achieved when the Cr content at the interface of corrosion products is 0.50% or more. It was also confirmed that in order for Cr to concentrate at the interface of corrosion products to a certain level or more, sufficient Cr must be dissolved in the steel before the corrosion reaction occurs. In the present invention, analysis of steels with excellent seawater corrosion resistance confirmed that when the dissolved Cr content is 90% or more of the total Cr content, the Cr content at the interface of corrosion products can be 0.50% or more, resulting in excellent seawater corrosion resistance. Therefore, in the present invention, the dissolved Cr content is limited to 90% or more of the total Cr content.

[0061] In the present invention, the interface of the corrosion product may refer to a region that exists in the form of an interface inside the corrosion layer formed on the surface of the steel sheet after the corrosion test, and the Cr content can be expressed as the average value of measured values ​​obtained by EDS analysis at three locations.

[0062] The steel plate according to one embodiment of the present invention has a tensile strength of 600 MPa or more, an impact toughness of 100 J or more at -5°C, a Cr content of 0.50% or more at the interface of corrosion products formed on the surface after a corrosion test according to KS D ISO 14993, and a relative corrosion rate of 60% or less compared to KS-SS275 during a corrosion test according to KS D ISO 14993, ensuring excellent corrosion resistance.

[0063] In the present invention, when the interface of the corrosion product is observed after a corrosion test, it is confirmed that seawater corrosion resistance is improved when the Cr content is 0.50% or more, and seawater corrosion resistance can be evaluated based on this. In the present invention, the interface of the corrosion product refers to a region that exists in the form of an interface inside the corrosion layer formed on the surface of the steel sheet after a corrosion test, and the Cr content can be expressed as the average value of measured values ​​obtained by EDS analysis at three locations.

[0064] The method for producing steel of the present invention will be described in detail below.

[0065] The steel sheet according to one embodiment of the present invention can be produced by reheating a steel slab having the above-mentioned alloy composition, hot rolling it, first cooling it, second coiling it, and cooling it.

[0066] reheating Steel slabs meeting the alloy composition of the present invention can be reheated.

[0067] In the present invention, the reheating temperature is not particularly limited, but a reheating temperature commonly used in the same technical field can be applied. According to one embodiment of the present invention, the reheating can be performed in a temperature range of 1100 to 1300°C.

[0068] hot rolling The reheated steel slab can be hot rolled at a finish rolling temperature of 750 to 900°C.

[0069] The finish rolling temperature is a factor that affects the grain size. A lower finish rolling temperature is more advantageous for grain refinement, but temperatures above 900°C can make it difficult to achieve the desired fine grain size. On the other hand, temperatures below 750°C can cause problems with excessive rolling load, making it difficult to apply to manufacturing processes.

[0070] Primary cooling and winding The hot-rolled steel sheet can be primarily cooled to a temperature range of 500 to 650°C at an average cooling rate of 5 to 20°C / s, and then coiled.

[0071] During primary cooling, the faster the average cooling rate and the lower the coiling temperature, the smaller the crystal grain size. In the present invention, the average cooling rate can be controlled to 5°C / s or more and the coiling temperature to 650°C or less to ensure the target crystal grain size. On the other hand, if the average cooling rate exceeds 20°C / s or the coiling temperature is less than 500°C, not only is it difficult to achieve a thickness of 10 mm or more, but there is also the problem that the material may be locally supercooled, making it difficult to apply.

[0072] Secondary cooling The coiled steel sheet can be secondarily cooled to 400°C at an average cooling rate of 0.40°C / min or more.

[0073] If the average cooling rate after coiling is less than 0.40°C / min, the formation of Cr carbides is promoted, which may result in a decrease in the amount of dissolved Cr. In the present invention, it was confirmed that when the average cooling rate to 400°C after coiling is 0.40°C / min or more, the formation of Cr carbides is sufficiently suppressed and the amount of dissolved Cr becomes 90% of the total amount added. Although there is no particular limit on the maximum cooling rate to 400°C after coiling, considering the reproducibility of the ferrite grain size and pearlite fraction proposed in the present invention, the upper limit can be set to 0.70°C / min. [Example]

[0074] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention.

[0075] (Example) Steel slabs having the alloy compositions shown in Table 1 below were manufactured. Each steel slab was heated to 1250°C and then rolled into 16 mm thick hot-rolled steel sheets under the conditions shown in Table 2 below. In this case, the cooling end temperature during secondary cooling was set to 400°C or less for all steel slabs.

[0076] [Table 1]

[0077] [Equation 1] R = [Si] + ([Ni] / [Cu]) (In the formula, [Si], [Ni], and [Cu] are the weight percentages of each element.)

[0078] [Table 2]

[0079] The ferrite, pearlite, and bainite fractions, as well as the ferrite grain size, were observed for each steel plate. The precipitate contents of Ti, Nb, and Cr were measured, and the solute Cr content was calculated using Relational Formula 2 and shown in Table 3 below. The tensile strength, impact toughness at -5°C, Cr content at the interface of the corrosion product after corrosion, corrosion rate, and relative corrosion rate to KS-SS275 were also measured and shown.

[0080] The microstructure fraction was measured using an optical microscope after nital etching of the specimens, and the ferrite grain size was measured at five points using the circular transverse line method described in KS D 0205 and the average value was calculated. The microstructure characteristics were observed at a quarter point from the surface of the steel sheet in the thickness direction.

[0081] To measure Relation 2 and the solute Cr content, particulate precipitates and inclusions in the steel were extracted using electrolytic extraction, and the Ti, Nb, and Cr contents of the particles were measured using inductively coupled plasma optical emission spectroscopy. The solute Cr content was calculated by subtracting the particulate Cr content from the Cr content of the steel.

[0082] The corrosion test was performed for 120 cycles (960 hours) as specified in KS D ISO 14993. The corrosion product interface refers to the region that exists in the form of an interface within the corrosion layer formed on the surface of the steel sheet after the corrosion test, and the Cr content at the interface can be measured at that location using EDS analysis. After removing the corrosion product using the KS D ISO 8407 method, the corrosion rate was measured using the following equation. Specifically, the test was performed for a total of 960 hours, repeating the following conditions: 2 hours of 5% NaCl salt spray at 35°C, 4 hours of drying at 60°C, and 2 hours of wetting at 50°C.

[0083] Corrosion rate (mm / yr) = 87.6W / DAT (Where, W is the weight loss after corrosion (mg), D is the specific gravity of the metal (g / cm 3 ), A is the area exposed to the corrosive environment (cm 2), T is the corrosion test time (hours), where 960 was used as a fixed value.

[0084] The relative corrosion rate was calculated by dividing the corrosion rate of the target material by the corrosion rate of the comparative material and expressing the result as a percentage (%). The comparative material KS-SS275 had a composition, by weight, of 0.13% C, 0.01% Si, 0.9% Mn, 0.02% Al, 0.01% Cu, 0.01% Ni, 0.005% S, 0.013% Nb, 0.005% Ti, and the balance Fe, and was manufactured under the manufacturing conditions proposed in this invention.

[0085] [Table 3]

[0086] [Equation 2] Q = ([Ti] + [Nb]) / [Cr]) (In the formula, [Ti], [Nb], and [Cr] are the weight percentages of each element present in particulate form, such as precipitates or inclusions, in the steel.)

[0087] As shown in Table 3, in the case of Examples 1 to 25, which satisfy the alloy composition and manufacturing conditions of the present invention, the characteristics of the microstructure proposed in the present invention are satisfied and the physical properties aimed at in the present invention are secured.

[0088] Figure 1 shows the results of measuring the Cr content at the interface of the corrosion product of Example 17, an embodiment of the present invention. Figure 1 is a photograph of the corrosion product, and it can be seen that the average Cr content at the interface of the corrosion product was 1.15%, which meets the requirements of the present invention, and that the corrosion weight loss was 60% or less compared to KS-SS275, demonstrating its superiority.

[0089] On the other hand, in Comparative Examples 1 and 2, the carbon content exceeded the range proposed by the present invention, and it was confirmed that the formation of ferrite was insufficient, and as a result, the desired impact toughness could not be secured and the corrosion rate was also increased.

[0090] In Comparative Examples 3 and 4, the silicon content did not reach the range proposed by the present invention, resulting in insufficient ferrite formation. In particular, Comparative Example 4 did not satisfy Relational Formula 1, resulting in impact toughness and corrosion rate that did not reach the proposed levels.

[0091] In Comparative Examples 5 and 6, the manganese content exceeded the range proposed by the present invention, resulting in excessive pearlite formation and insufficient ferrite formation, which resulted in an excessively fast corrosion rate.

[0092] In Comparative Examples 7 and 8, the sulfur content exceeded the range of the present invention, and it was confirmed that the impact toughness decreased and corrosion progressed at a rapid rate due to the influence of MnS inclusions.

[0093] In Comparative Examples 9 and 10, the chromium content did not reach the range proposed by the present invention, resulting in a lack of chromium at the internal interface of the corrosion product, and thus a decrease in corrosion resistance.

[0094] Comparative Examples 11 to 14 are cases where the titanium content is outside the range proposed by the present invention. In Comparative Examples 11 and 12, the titanium content was insufficient, which inhibited the formation of TiC and relatively promoted the formation of Cr carbides, resulting in an insufficient content of solute Cr and, as a result, reduced corrosion resistance. In Comparative Examples 13 and 14, the titanium content was excessive, resulting in reduced impact toughness.

[0095] In Comparative Examples 15 and 16, the niobium content did not reach the range proposed by the present invention, and the formation of NbC was suppressed and the formation of Cr carbide was relatively promoted, resulting in an insufficient content of solute Cr. As a result, not only was the corrosion resistance reduced, but the non-recrystallization rolling did not work, the crystal grain size became coarse, and the impact toughness was reduced.

[0096] The nickel content was insufficient in Comparative Examples 17 and 18, resulting in reduced corrosion resistance. In particular, Comparative Example 17 did not satisfy Relational Formula 1 and was also poor in impact toughness.

[0097] In Comparative Examples 19 and 20, the copper content was insufficient, resulting in reduced corrosion resistance.

[0098] Comparative Example 21 satisfied the alloy composition requirements of the present invention, but the finish rolling temperature during hot rolling was excessively high, resulting in an average ferrite grain size exceeding the range proposed in the present invention, and as a result, the impact toughness and corrosion resistance were poor.

[0099] In Comparative Examples 22 and 23, the cooling rate was excessively slow, resulting in insufficient ferrite formation and coarse average ferrite grain size, which reduced corrosion resistance.

[0100] In Comparative Examples 24 and 25, the coiling temperature exceeded the temperature range proposed by the present invention, and the ferrite grain size became coarse, resulting in a decrease in corrosion resistance.

[0101] In Comparative Examples 26 and 27, the cooling rate during secondary cooling did not reach the range proposed by the present invention, and the content of solute Cr was insufficient, resulting in poor corrosion resistance.

[0102] Although the present invention has been described in detail using examples, other embodiments are possible, and the technical spirit and scope of the claims set forth below are not limited to the examples.

Claims

1. Containing, by weight, C: 0.030 to 0.070%, Si: 0.50 to 1.30%, Mn: 0.30 to 0.70%, Cr: 0.50 to 1.50%, Al: 0.05% or less, Cu: 0.25 to 0.50%, Ni: 0.05 to 0.50%, S: 0.0100% or less, Ti: 0.020 to 0.050%, Nb: 0.050 to 0.090%, the balance being Fe and other unavoidable impurities, The R value defined by the following relational expression 1 is 0.75 or more, The microstructure at a quarter point from the surface in the thickness direction contains, in area %, 85% or more of ferrite and the remainder of the structure, The average crystal grain size of the ferrite is 30 μm or less, A steel plate having a solute Cr content of 90% or more of the total Cr content. [Relationship 1] R=[Si]+([Ni] / [Cu]) (In the formula, [Si], [Ni], and [Cu] are the weight percentages of each element.)

2. The steel plate according to claim 1 , wherein the remaining structure includes at least one of pearlite and bainite.

3. The steel sheet according to claim 1, wherein the Q value defined by the following relational expression 2 is 0.3 to 8.

5. [Relationship 2] Q=([Ti]+[Nb]) / [Cr]) (In the formula, [Ti], [Nb], and [Cr] are the weight percentages of each element present in the steel in the form of particles, such as precipitates or inclusions.)

4. The steel plate according to claim 1, wherein the steel plate has a tensile strength of 600 MPa or more and an impact toughness at -5°C of 100 J or more.

5. The steel plate according to claim 1, wherein the steel plate has, after a corrosion test according to KS D ISO 14993, a Cr content at an interface of generated corrosion products of 0.50% or more by weight.

6. The steel plate according to claim 1, wherein the steel plate has a relative corrosion rate of 60% or less compared to KS-SS275 in a corrosion test according to KS D ISO 14993.

7. a step of reheating a steel slab containing, by weight %, C: 0.030 to 0.070%, Si: 0.50 to 1.30%, Mn: 0.30 to 0.70%, Cr: 0.50 to 1.50%, Al: 0.05% or less, Cu: 0.25 to 0.50%, Ni: 0.05 to 0.50%, S: 0.0100% or less, Ti: 0.020 to 0.050%, Nb: 0.050 to 0.090%, the balance being Fe and other inevitable impurities, and having an R value defined by the following relational expression 1 of 0.75 or more; hot rolling the reheated steel slab at a finish rolling temperature of 750 to 900°C; The hot-rolled steel sheet is primarily cooled to a temperature range of 500 to 650°C at a cooling rate of 5 to 20°C / s, and then coiled; and a step of secondary cooling the coiled steel sheet to 400°C at a cooling rate of 0.40°C / min or more. [Relationship 1] R=[Si]+([Ni] / [Cu]) (In the formula, [Si], [Ni], and [Cu] are the weight percentages of each element.)

8. The method for manufacturing a steel sheet according to claim 7, wherein the reheating step is performed at a temperature in the range of 1100 to 1300°C.

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