Steel sheet and manufacturing method therefor

Abstract

The present invention relates to a steel sheet used for a friction component or the like used to protect a structure in a region of a construction vehicle subject to severe wear, and a method for manufacturing same.

Description

Steel plate and method of manufacturing the same

[0001] The present invention relates to a steel plate used for friction parts or automotive parts, etc., to protect structures in areas where wear phenomena are active, such as construction equipment, and a method for manufacturing the same.

[0002] Friction parts are used in construction equipment and vehicles to protect structures in areas where wear is active. These friction parts primarily utilize high-hardness, wear-resistant martensitic hot-rolled steel sheets.

[0003] Conventional high-hardness wear-resistant hot-rolled steel sheets have been utilized for parts requiring wear resistance by primarily employing a martensitic microstructure to achieve high strength and hardness. However, they have a disadvantage in that processing is limited due to inferior bendability resulting from their high strength, which necessitates the use of roll forming methods or minimal processing.

[0004] This acts as a similar limiting factor in high-strength hot-rolled steel sheets utilizing martensite as the main phase, and various technologies have been proposed to overcome this.

[0005] Patent Document 1 suggests that excellent bendability can be secured in 1 GPa grade high-strength martensitic hot-rolled steel sheets when the hard phases of martensite and tempered martensite are reduced compared to the center in the microstructure at positions of 200 µm and 100 µm from the surface. Additionally, Patent Document 2 suggests that in 1.2 GPa grade martensitic steel, low-temperature impact toughness is improved through microstructure refinement, excellent flange formability is obtained by controlling precipitates, and high bendability is obtained by controlling surface roughness.

[0006] However, Patent Document 1 may be perceived as having a small effect on improving bendability because it is difficult to secure sufficient strength, the change between the center and the surface layer of the steel plate is not significant, and there is no control over the microstructure and physical properties of the ultra-surface layer of 100 μm or less. On the other hand, Patent Document 2 has the disadvantage that there are limitations in improving bendability relative to strength through surface roughness control alone, a certain level of surface roughness can be obtained by simply going through the pickling process, and manufacturing conditions must be strictly limited to control precipitates.

[0007] Furthermore, while alloying elements such as Si, Mn, Mo, Cr, Cu, and Ni, which are primarily used to manufacture the aforementioned high-hardness steels, are effective in improving hardness and formability, adding large amounts of these elements to enhance material properties leads to segregation of the alloy components and microstructural non-uniformity, resulting in inferior bendability. In particular, steels with high hardenability are sensitive to microstructural changes during cooling, leading to the non-uniform formation of low-temperature transformation structures; consequently, there is a problem in that it is difficult to achieve even higher bendability.

[0008] Meanwhile, if there are differences in the rate of wear in products manufactured using wear-resistant steel—for example, if the initial wear rate is excessively fast—it becomes difficult to ensure stable product usage or predict replacement timing. Furthermore, since users may lose confidence in the wear-resistant steel due to the relatively noticeable rapid initial wear rate, it is becoming important to ensure a uniform wear rate.

[0009] (Patent Document 1) Korean Published Patent Application No. 10-2023-0085173

[0010] (Patent Document 2) Korean Published Patent Application No. 10-2021-0024135

[0011] According to one embodiment of the present invention, it is possible to secure high hardness and wear resistance while simultaneously securing excellent bending processability, and in particular, to provide a steel plate having a uniform wear rate in the early and late stages of wear and a method for manufacturing the same.

[0012] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0013] A steel sheet according to one embodiment of the present invention comprises, in weight %, C: 0.15~0.24%, Si: 0.1~0.6%, Mn: 0.6~1.9%, Cr: 0.005~0.6%, Mo: 0.002~0.3%, Nb: 0.001~0.03%, Ti: 0.005~0.08%, V: 0.003~0.05%, Al: 0.01~0.5%, P: 0.003~0.03%, S: 0.001~0.01%, N: 0.001~0.01%, B: 0.0002~0.003%, the remainder being Fe and other unavoidable impurities, and

[0014] Satisfying the following [Relation 1],

[0015] The difference between the Mn content at a depth of 5 μm from the surface and the Mn content at a thickness of 1 / 4 is 0.12 wt% or more, and

[0016] The microstructure from the surface to a depth of 5 µm may be ferrite with an area of ​​40% or less.

[0017] [Relationship 1]

[0018] 3 ≤ T ≤ 30

[0019] T=(C / 8) 0.8 *(0.45*Si+1.1)*(4.8*Mn+0.8)*(2.2*Cr+1.2)*(3.5*Mo+0.2)*25

[0020] C, Si, Mn, Cr, and Mo in the above relationship (1) are weight percent of the corresponding alloying elements, and 0 is substituted if they are not added.

[0021] The microstructure of the above steel plate may contain at least one of martensite, tempered martensite, and less martensite in an area of ​​90% or more, and at least one of pearlite and bainite in an area of ​​10% or less.

[0022] The above steel plate may have a difference of 40% or less in the wear rate during the initial 30 minutes compared to the wear rate during the initial 3 minutes in the ASTM G65 standard wear test.

[0023] The above steel plate has a core cross-sectional hardness of 37 to 60 HrC and may have an R / t of 4 or less.

[0024] A method for manufacturing a steel plate according to another embodiment of the present invention comprises, in weight %, C: 0.15~0.24%, Si: 0.1~0.6%, Mn: 0.6~1.9%, Cr: 0.005~0.6%, Mo: 0.002~0.3%, Nb: 0.001~0.03%, Ti: 0.005~0.08%, V: 0.003~0.05%, Al: 0.01~0.5%, P: 0.003~0.03%, S: 0.001~0.01%, N: 0.001~0.01%, B: 0.0002~0.003%, the remainder being Fe and other unavoidable impurities, and heating a steel slab satisfying the following [Equation 1] to 1100~1350℃. Step;

[0025] A step of obtaining a bar by terminating rough rolling of the above heated steel slab at a rough rolling temperature (RDT) satisfying [Relationship 2] with respect to the center of the thickness;

[0026] A step of descaling the surface of the above bar and cooling it to a surface cooling temperature (SFCT) that satisfies [Relationship 3] based on the surface;

[0027] A step of performing finish rolling after the above cooling, and cooling to a surface cooling temperature (SFCT2) satisfying [Equation 3] based on the surface after one pass of finish rolling;

[0028] A step of obtaining a hot-rolled steel sheet by terminating finish rolling at a finish rolling temperature (FDT) of RT+20℃ or higher relative to the center of the thickness of the above bar;

[0029] A step of first cooling the above hot-rolled steel sheet to a cooling stop temperature (CST) satisfying the conditions of [Equation 4] below: and

[0030] The method may include a step of secondarily cooling the first cooled hot-rolled steel sheet to a coiling temperature (CT) between room temperature and 250°C and then coiling it.

[0031] [Relationship 1]

[0032] 3 ≤ T ≤ 30

[0033] T=(C / 8) 0.8 *(0.45*Si+1.1)*(4.8*Mn+0.8)*(2.2*Cr+1.2)*(3.5*Mo+0.2)*25

[0034] C, Si, Mn, Cr, and Mo in the above relationship (1) are weight percent of the corresponding alloying elements, and 0 is substituted if they are not added.

[0035] [Relationship 2]

[0036] RDT ≤ (RT + 100 + Bar thickness * 3.5)

[0037] RT = 902 - 325*C + 34.4*Si - 22.7*Mn - 13.6*Cr + 13.3*Mo - 94.6*Nb + 158*Ti + 194*V

[0038] [Relationship 3]

[0039] FT ≤ SFCT ≤ FT+50℃

[0040] SFCT2 ≤ FT-50℃

[0041] FT = 722 + 135*C + 19.8*Si - 42.1*Mn - 0.5*Cr + 5.86*Mo + 182*Nb - 192*Ti - 94*V

[0042] [Relationship 4]

[0043] Ms-50℃ ≤ CST ≤ Ms+50℃

[0044] Ms = 515 - 264*C - 29.8*Si - 43.3*Mn - 23.9*Cr - 12.9*Mo

[0045] In the above equations 2 to 4, C, Si, Mn, Cr, Mo, Nb, Ti, Al, N, and B represent the weight percent of the corresponding alloying elements, and 0 is substituted if they are not added. Bar thickness (mm) refers to the bar thickness after rough rolling is completed.

[0046] The thickness of the above bar may be 20 to 50 mm.

[0047] The above first cooling can be performed at a first average cooling rate of 60 to 100℃ / sec.

[0048] The above secondary cooling can be performed at a secondary average cooling rate of 1 to 40℃ / sec.

[0049] The result obtained by performing the above rough rolling can be at least 6 times the thickness of the final hot-rolled steel sheet.

[0050] The above-mentioned wound steel plate may further include the steps of pickling and oiling.

[0051] According to one aspect of the present invention, a steel plate having excellent bending formability and hardness and a method for manufacturing the same can be provided.

[0052] In addition, the above steel plate has a small difference between the wear rate at the initial stage of wear and the subsequent wear rate, thereby ensuring a uniform wear rate. This offers the advantage of making it easy to predict the product's replacement time.

[0053] The various and beneficial advantages and effects of the present invention are not limited to those described above, and may be more easily understood in the process of explaining specific embodiments of the present invention.

[0054] Figure 1 shows an image of the surface area in Example 1 of the present invention and the result of analyzing it with EPMA.

[0055] Figures 2(a) and 2(b) are electron microscope images of the microstructure of the cross-section of the surface region of Inventive Example 1 and Comparative Example 1, respectively, among the embodiments of the present invention.

[0056] Figure 3 shows (a) and (b) electron microscope images of the microstructures of the deep cross-sections of Inventive Example 1 and Comparative Example 4, respectively, among the embodiments of the present invention.

[0057] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0058] In addition, embodiments of the present invention are provided to more fully explain the present invention to those with average knowledge in the relevant technical field.

[0059] In drawings, the shapes and sizes of elements may be exaggerated for clearer explanation.

[0060] In describing the embodiments of the present invention, if it is determined that a detailed description of known technology related to the present invention may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined considering their functions in the present invention, and these may vary depending on the intentions or conventions of the user or operator. Therefore, such definitions should be based on the content throughout this specification. The terms used in the detailed description are merely for describing the embodiments of the present invention and should not be limited in any way. Unless explicitly stated otherwise, expressions in the singular form include the meaning of the plural form.

[0061] In this description, expressions such as “include” or “equipped” are intended to refer to certain characteristics, numbers, steps, actions, elements, parts or combinations thereof, and should not be interpreted to exclude the existence or possibility of one or more other characteristics, numbers, steps, actions, elements, parts or combinations thereof other than those described.

[0062] In this specification, terms such as 'top', 'upper', 'upper surface', 'lower', 'lower surface', 'lower surface', and 'side surface' are based on the drawings and may actually vary depending on the direction in which the elements or components are arranged.

[0063] Additionally, throughout the specification, when it is said that one part is 'connected' to another part, this includes not only cases where they are 'directly connected,' but also cases where they are 'indirectly connected' with other elements in between.

[0064] The present invention will be described in detail below through each embodiment or example of the invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may also be combined with other embodiments or examples. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0065]

[0066] Hereinafter, a steel plate according to one embodiment of the present invention will be described.

[0067] First, the alloy composition of the present invention will be described. Unless otherwise noted, the unit % in the alloy composition described below refers to weight % (or indicated as wt.%).

[0068] The steel sheet of the present invention may contain, in weight%, C: 0.15~0.24%, Si: 0.1~0.6%, Mn: 0.6~1.9%, Cr: 0.005~0.6%, Mo: 0.002~0.3%, Nb: 0.001~0.03%, Ti: 0.005~0.08%, V: 0.003~0.05%, Al: 0.01~0.5%, P: 0.003~0.03%, S: 0.001~0.01%, N: 0.001~0.01%, and B: 0.0002~0.003%.

[0069] Carbon (C): 0.15~0.24%

[0070] The above-mentioned carbon (C) is the most economical and effective element for strengthening steel and has a significant influence on strength and hardness. As its addition increases, hardenability increases, making it easier to form hard phases such as bainite and martensite in the microstructure, thereby increasing tensile strength. Furthermore, due to the tendency of carbon to accumulate in austenite rather than ferrite, it tends to migrate to the core once ferrite is formed in the surface layer. However, if the carbon content exceeds 0.24%, the hardness of the martensite increases excessively, leading to problems such as reduced bendability along with an excessive increase in strength, and it may be difficult to secure sufficient weldability. On the other hand, if the carbon content is less than 0.15%, it is difficult to obtain a sufficient strengthening effect. Therefore, it is desirable for the carbon content to be in the range of 0.15 to 0.24%. The lower limit of the carbon content is more preferably 0.16%, more preferably 0.17%, and most preferably 0.18%. The upper limit of the above C content is more preferably 0.24%, more preferably 0.23%, and most preferably 0.22%.

[0071] Silicon (Si): 0.1~0.6%

[0072] The above-mentioned silicon is an element advantageous for deoxidizing molten steel, exhibiting a solid solution strengthening effect, and improving formability by delaying the formation of coarse carbides. If the Si content is less than 0.1%, sufficient solid solution strengthening and formability improvement effects cannot be obtained. On the other hand, if the Si content exceeds 0.6%, the removal of red scale formed on the surface of the steel sheet during hot rolling is not easy, and as a result, the surface quality of the steel sheet may deteriorate significantly. In addition, there is a problem of reduced ductility and weldability. Therefore, it is desirable for the Si content to be in the range of 0.1 to 0.6%. The lower limit of the Si content is more preferably 0.15%, more preferably 0.2%, and most preferably 0.25%. The upper limit of the Si content is more preferably 0.55%, more preferably 0.5%, and most preferably 0.45%.

[0073] Manganese (Mn): 0.6~1.9%

[0074] Like Si, the above-mentioned Mn is an effective element for solid solution strengthening of steel. It increases the hardenability of steel, facilitating the formation of hard phases such as bainite and martensite during cooling after hot rolling. Furthermore, if ferrite is formed in the surface layer, it is concentrated in austenite, providing a mechanism for transformation into twinned martensite when the phase transformation to martensite occurs. If the Mn content is less than 0.6%, sufficient effects regarding solid solution strengthening and the formation of bainite and martensite cannot be obtained. On the other hand, if the Mn content exceeds 1.9%, grain boundaries become brittle, causing problems such as low-temperature cracking. Additionally, excessive strength increase may make it difficult to secure sufficient formability. Furthermore, during slab casting in the continuous casting process, segregation develops significantly at the center of the thickness, and upon cooling after hot rolling, the microstructure is formed unevenly in the thickness direction, resulting in inferior bending workability. In particular, it makes it difficult to manufacture a uniform microstructure during cooling across the entire length and width of the hot-rolled steel sheet. Accordingly, it is preferable that the Mn content has a range of 0.6 to 1.9%. It is more preferable that the lower limit of the Mn content be 0.7%, more preferable that it be 0.8%, and most preferable that it be 0.9%. It is more preferable that the upper limit of the Mn content be 1.9%, more preferable that it be 1.8%, and most preferable that it be 1.7%.

[0075] Chrome (Cr): 0.005~0.6%

[0076] The above Cr strengthens the steel through solid solution and delays the ferrite phase transformation upon cooling, thereby facilitating the formation of martensite and bainite. If the Cr content is less than 0.005%, sufficient effects regarding solid solution strengthening and the formation of martensite and bainite cannot be obtained. On the other hand, if the Cr content exceeds 0.6%, segregation zones develop significantly at the center of the thickness, similar to Mn, and the microstructure becomes non-uniform in the thickness direction, thereby reducing bendability. Therefore, it is desirable for the Cr content to be in the range of 0.005 to 0.6%. The lower limit of the Cr content is more preferably 0.007%, more preferably 0.008%, and most preferably 0.01%. The upper limit of the Cr content is more preferably 0.5%, more preferably 0.45%, and most preferably 0.4%.

[0077] Molybdenum (Mo): 0.002~0.3%

[0078] The above Mo increases the hardenability of the steel, facilitating the formation of martensite and bainite. If the Mo content is less than 0.002%, the aforementioned effects cannot be sufficiently obtained. On the other hand, if the Mo content exceeds 0.3%, excessive hardenability leads to the formation of martensite on the surface layer, which drastically degrades bendability, is economically disadvantageous, and may make it difficult to secure sufficient weldability. Furthermore, due to excessively high hardenability, it becomes impossible to form a ferrite structure on the surface layer. Therefore, it is desirable for the Mo content to be in the range of 0.002 to 0.3%. The lower limit of the Mo content is more preferably 0.005%, more preferably 0.01%, and most preferably 0.02%. The upper limit of the Mo content is more preferably 0.25%, more preferably 0.2%, and most preferably 0.15%.

[0079] Niobium (Nb): 0.001~0.03%

[0080] The above Nb is a representative precipitation strengthening element along with Ti and V. It precipitates as a precipitate during hot rolling and exerts a grain refinement effect by delaying recrystallization, thereby effectively improving the strength and impact toughness of steel. If the content of the above Nb is less than 0.001%, the aforementioned effect cannot be sufficiently obtained. On the other hand, if the content of the above Nb exceeds 0.03%, coarse composite precipitates are formed during hot rolling, which impairs bendability. Furthermore, it has the effect of making the aspect ratio of the matrix phase austenite excessively large before the martensite phase transformation. Therefore, it is desirable for the content of the above Nb to be in the range of 0.001 to 0.03%.

[0081] Titanium (Ti): 0.005~0.08%

[0082] The above Ti is a representative precipitation strengthening element along with Nb and V, and forms coarse TiN through its strong affinity with nitrogen. The above TiN has the effect of inhibiting grain growth during the heating process for hot rolling. In addition, the Ti remaining after reacting with nitrogen is dissolved in the steel and combines with carbon to form TiC precipitates, making it a useful component for improving the strength of the steel. If the Ti content is less than 0.005%, the effects of inhibiting grain growth and improving strength cannot be sufficiently obtained. On the other hand, if the Ti content exceeds 0.08%, coarse TiN is generated, and the precipitates become coarse, resulting in inferior bendability during forming. Therefore, it is desirable for the Ti content to be in the range of 0.005 to 0.08%. The lower limit of the Ti content is more preferably 0.01%, more preferably 0.015%, and most preferably 0.02%. The upper limit of the above Ti content is more preferably 0.07%, more preferably 0.06%, and most preferably 0.045%.

[0083] Vanadium (V): 0.003~0.05%

[0084] The above-mentioned V is a representative precipitation-strengthening element along with Nb and Ti. Although it hardly precipitates during hot rolling, it forms precipitates after high-temperature coiling, cooling, or tempering to improve the strength of the steel. Therefore, it is effective for further strength enhancement without increasing deformation resistance or rolling load due to delayed recrystallization during hot rolling. If the V content is less than 0.003%, sufficient strength enhancement cannot be obtained. On the other hand, if the V content exceeds 0.05%, coarse precipitates are formed, resulting in poor bendability; similar to Nb, it is impossible to maintain a low aspect ratio of the matrix austenite, and it is also economically disadvantageous. Therefore, it is desirable for the V content to be in the range of 0.003 to 0.05%. The lower limit of the V content is more preferably 0.006%, more preferably 0.008%, and most preferably 0.01%. The upper limit of the above V content is more preferably 0.04%, more preferably 0.03%, and most preferably 0.02%.

[0085] Aluminum (Al): 0.01~0.5%

[0086] The above Al is an element added primarily for deoxidation. In this case, Al refers to Sol-Al. If the content of the above Al is less than 0.01%, a sufficient deoxidation effect cannot be obtained. On the other hand, if the content of the above Al exceeds 0.5%, it combines with nitrogen to form excessive AlN, which makes corner cracks prone to occur in the slab during continuous casting and makes defects caused by the formation of inclusions prone to occur. Therefore, it is desirable for the content of the above Al to be in the range of 0.01 to 0.5%. The lower limit of the above Al content is more preferably 0.015%, and more preferably 0.02%. The upper limit of the above Al content is more preferably 0.1%, more preferably 0.08%, and most preferably 0.05%.

[0087] Phosphorus (P): 0.003~0.03%

[0088] Similar to Si, the above-mentioned P simultaneously possesses solid solution strengthening and ferrite transformation promoting effects. However, controlling the P content to less than 0.003% requires high manufacturing costs, making it economically disadvantageous and insufficient to obtain sufficient strength. On the other hand, if the P content exceeds 0.03%, brittleness may occur due to grain boundary segregation, fine cracks are prone to occur during bending forming, and ductility and impact resistance properties are significantly reduced. Therefore, it is desirable for the P content to have a range of 0.003 to 0.03%. The lower limit of the P content is more preferably 0.005%, more preferably 0.007%, and most preferably 0.01%. The upper limit of the P content is more preferably 0.025%.

[0089] Sulfur (S): 0.001~0.01%

[0090] The above-mentioned sulfur (S) is an impurity present in steel; if its content exceeds 0.01%, it combines with Mn and others to form non-metallic inclusions. Consequently, this causes problems such as the easy occurrence of fine cracks during the bending process of steel and a significant reduction in impact resistance. In the present invention, the lower limit of the S content is not specifically limited; however, since controlling it to less than 0.001% requires a significant amount of time during steelmaking operations and reduces productivity, the lower limit of the S content may be limited to 0.001% in consideration of this. Accordingly, it is preferable for the S content to have a range of 0.001 to 0.01%. It is more preferable for the lower limit of the S content to be 0.002%. It is more preferable for the upper limit of the S content to be 0.008%, more preferable for 0.006%, and most preferable for 0.005%.

[0091] Nitrogen (N): 0.001~0.01%

[0092] The above N is a representative solid solution strengthening element along with C and forms coarse precipitates together with Ti, Al, etc. If the N content is less than 0.001%, it is difficult to obtain sufficient solid solution strengthening and precipitate formation effects; furthermore, controlling the N content to less than 0.001% requires a significant amount of time during steelmaking operations, resulting in reduced productivity. Meanwhile, although the solid solution strengthening effect of N is generally superior to that of carbon, there is a problem where toughness decreases significantly if the N content exceeds 0.01%. Therefore, it is desirable for the N content to be in the range of 0.001 to 0.01%. The lower limit of the N content is more preferably 0.002%, and even more preferably 0.003%. The upper limit of the N content is more preferably 0.008%, even more preferably 0.007%, and most preferably 0.006%.

[0093] Boron (B): 0.0002~0.003%

[0094] When the above B exists in a solid solution state in steel, it mainly segregates at grain boundaries and has the effect of improving the brittleness of the steel by stabilizing grain boundaries. It also plays a role in suppressing the formation of coarse AlN nitrides by stabilizing dissolved N. Furthermore, it delays the ferrite phase transformation, making it effective for the formation of hard phases such as bainite and martensite. If the content of the above B is less than 0.0002%, the effects of improving brittleness, suppressing the formation of coarse AlN nitrides, and forming bainite and martensite cannot be sufficiently obtained. On the other hand, if the content of the above B exceeds 0.003%, the aforementioned effects no longer increase, and there is a disadvantage in that ductility decreases, leading to reduced formability. Therefore, it is desirable for the content of the above B to be in the range of 0.0002 to 0.003%. The lower limit of the above B content is more preferably 0.0004%, more preferably 0.0006%, and most preferably 0.0008%. The upper limit of the above B content is more preferably 0.0025%, and more preferably 0.002%.

[0095] The remaining component is iron (Fe), and it may contain some unintended and unavoidable impurities introduced during the manufacturing process. Since these impurities are known to any skilled person in the ordinary manufacturing process, not all details are specifically mentioned in this specification.

[0096] It is desirable that the above steel plate satisfies the following relationship 1.

[0097] [Relationship 1]

[0098] 3 ≤ T ≤ 30

[0099] T=(C / 8) 0.8 *(0.45*Si+1.1)*(4.8*Mn+0.8)*(2.2*Cr+1.2)*(3.5*Mo+0.2)*25

[0100] C, Si, Mn, Cr, and Mo in the above relationship (1) are weight percent of the corresponding alloying elements, and 0 is substituted if they are not added.

[0101] The above Equation 1 is a factorization of the combination of alloying elements contributing to strength, considering the solid solution strengthening effect and maintaining the formation of the hard phases, bainite and martensite, in the microstructure of the steel of the present invention at an appropriate level. As the value of "T" in Equation 1 increases, the formation of the hard phases, bainite and lath martensite, increases, and the hardness and strength of each hard phase increase. Therefore, a larger value is advantageous for securing strength and hardness; however, if it is excessive, it is impossible to form ferrite and the resulting Mn content reduction region on the surface during the intermediate process of rolling and surface cooling, which leads to inferior bending workability of the steel and an increase in material variation in the overall length and width of the hot-rolled steel sheet. Considering these points, it is preferable that the above T be between 3 and 30.

[0102] The microstructure of the above steel plate is described in detail.

[0103] The above steel plate contains 90% or more (including 100%) of martensite by area fraction and may contain one or more of bainite and ferrite, or 10% or less (including 0%). The measurement standard is based on the core of the steel plate, and more specifically, may be based on the point at 1 / 4 of the thickness. The above martensite may be any of martensite (fresh martensite), tempered martensite, or less martensite. More precisely, the microstructure of the core of the above steel plate consists mainly of a martensite phase, and carbides such as tempered martensite may be observed due to the self-tempering effect depending on the coiling temperature control. By sufficiently securing the martensite, tempered martensite, or less martensite in the core of the present invention, excellent strength and hardness can be secured.

[0104] It is preferable that the difference between the Mn content at a depth of 5 μm from the surface and the Mn content at a depth of 1 / 4 of the thickness of the above steel plate be 0.12 weight% or more. If the difference in Mn content between the surface layer region at a depth of 5 μm and the center is too small, the surface layer region, which is hard due to the high Mn content, lacks deformation resistance during bending processing. This leads to the easy formation of surface irregularities caused by deformation concentration, eventually resulting in fracture. The presence of a surface layer with reduced Mn content is essential to prevent excessive deformation concentration and ensure appropriate processability. Conversely, if the change in Mn content is too large, deformation that should be distributed across the entire thickness is concentrated only in the limited surface layer, potentially forming surface irregularities despite a large bending radius. Therefore, the difference in content may be 0.6 weight% or less. The distribution of Mn content in the thickness direction can be confirmed and interpreted using FE-SEM EDS or FE-EPMA. As an example, Figure 1 shows the Mn distribution of Invention Example 1 among the examples confirmed by EPMA.

[0105] The microstructure from the surface of the steel plate up to 5 μm may have a ferrite fraction area % of 40% or less. The remainder must be a hard phase formed at low temperatures, such as bainite, martensite, or tempered bainite or martensite, and it is preferable to exclude a pearlite structural phase as it would significantly impede bendability. The ferrite may be 5% or more.

[0106] For the above steel plate, the difference between the wear rate during the initial 30 minutes and the wear rate during the initial 3 minutes in the ASTM G65 standard wear test may be 40% or less. In other words, this means that the wear rate during the initial 3 minutes is not faster than the wear rate during the subsequent 30 minutes by more than 40%; for instance, a rate difference of 0% means that the wear rate during the initial 3 minutes is the same as the wear rate during the subsequent 30 minutes. As an example of calculating the difference in wear rate, it can be calculated as follows: Wear Rate Difference (%) = (Initial 3-minute wear rate per minute (g / min) / Initial 30-minute wear rate per minute (g / min) - 1) * 100

[0107] When comparing the initial wear amount converted to a wear rate per minute with the subsequent wear amount converted to a wear rate per minute, the absence of a difference in wear rate implies that the steel possesses uniform wear resistance throughout its entire lifespan, to the extent that the total wear amount can be predicted based on the initial wear amount. Therefore, from the user's perspective, it is easy to have confidence in the wear resistance of the steel because the initial wear rate does not differ significantly from the overall wear rate of the steel.

[0108] The above steel plate has a core cross-sectional hardness of 37 to 60 HrC and excellent bendability relative to the core cross-sectional hardness, so that the bendability (ratio of bending radius to steel plate thickness at which no crack occurs when bent 90 degrees, R / t) may be 4 or less. The strength of the above steel plate may have a tensile strength (TS) of 950 MPa or more and a yield strength of 900 MPa or more.

[0109] Hereinafter, a method for manufacturing a steel plate according to another embodiment of the present invention will be described.

[0110] First, the above manufacturing method may include heating, hot rolling, cooling, and coiling a steel slab satisfying the aforementioned alloy composition and Equation 1.

[0111] Steel slab heating

[0112] A steel slab satisfying the aforementioned alloy composition and Equation 1 is heated to a temperature range of 1100 to 1350°C. If the steel slab heating temperature is below 1150°C, precipitates are not sufficiently re-dissolved, resulting in reduced precipitate formation in processes after hot rolling, the presence of coarse TiN, and insufficient curing of the slab, making it difficult to consistently control the temperature of the steel sheet during hot rolling. Conversely, if the slab heating temperature exceeds 1350°C, strength is reduced due to abnormal grain growth of austenite grains. Therefore, it is preferable for the slab heating temperature to have a range of 1100 to 1350°C. It is more preferable that the lower limit of the slab heating temperature be 1150°C, and even more preferable that it be 1160°C. The upper limit of the above slab heating temperature is more preferably 1340℃, more preferably 1330℃, and most preferably 1320℃.

[0113] The steel slab used in the manufacturing method of the present invention may be refined and cast through a converter process or an electric furnace process.

[0114] In the converter process, molten iron supplied from a blast furnace is primarily used; however, depending on the supply and demand status of hot metal, some scrap or other iron sources may be added for refining to produce molten steel. In particular, when implementing low HMR operations that reduce the amount of molten iron used to meet requirements such as carbon neutrality, the amount of scrap used may increase, and as a result, elements not intended in this invention may be included in the molten steel within the allowable limits.

[0115] In the electric furnace process, molten steel can be obtained by primarily charging scrap, melting it using arc heat, and refining it. In some cases, molten iron may be added in addition to the scrap. As a result of including a large amount of scrap in this manner, elements not intended in this invention may be included in the molten steel within permissible limits.

[0116] Molten steel that has undergone the converter or electric furnace process may undergo an additional refining (secondary refining) process to adjust its composition and other properties.

[0117] Hot rolling

[0118] The above heated steel slab is rough rolled at the rough rolling temperature (RDT) of the following Equation 2 to obtain a bar. It is preferable to perform the rough rolling at the above rough rolling temperature (RDT) based on t / 2 (t: thickness of the steel). At this time, it is preferable that the thickness of the steel be the thickness of the bar. The above rough rolling can be performed within a temperature range of 900 to 1100°C. It is preferable that the thickness of the bar be at least 6 times the thickness of the final hot-rolled steel plate, and specifically, the thickness of the bar may be 20 to 50 mm.

[0119] In the following equations 2 to 4, C, Si, Mn, Cr, Mo, Nb, Ti, Al, N, and B represent the weight percent of the corresponding alloying elements, and 0 is substituted if they are not added.

[0120] [Relationship 2]

[0121] RDT ≤ (RT + 100 + Bar thickness * 3.5)

[0122] RT = 902 - 325*C + 34.4*Si - 22.7*Mn - 13.6*Cr + 13.3*Mo - 94.6*Nb + 158*Ti + 194*V

[0123] After obtaining the above bar, the surface is descaled and cooled to a surface cooling temperature (SFCT) that satisfies [Equation 3] based on the surface.

[0124] The above descaling can be performed by spraying high-pressure water, and at this time, the water pressure can be 150 bar or higher.

[0125] In this case, if the surface cooling temperature is too low, changes in the Mn content occur up to excessively deep thicknesses, which may lead to an inappropriate decrease in strength and hardness, resulting in non-uniformity of the wear-resistant steel material. On the other hand, if the surface cooling temperature is too high, changes in the Mn content do not occur, and thus the effect intended by the present invention cannot be achieved.

[0126] After the above rough rolling, finish rolling is performed, and after the first pass of finish rolling, the bar surface is cooled to a surface cooling temperature (SFCT2) that satisfies [Equation 3] below.

[0127] [Relationship 3]

[0128] FT ≤ SFCT ≤ FT+50℃

[0129] SFCT2 ≤ FT-50℃

[0130] FT = 722 + 135*C + 19.8*Si - 42.1*Mn - 0.5*Cr + 5.86*Mo + 182*Nb - 192*Ti - 94*V

[0131] Additional cooling during rolling at the temperature of the above SFCT2 has the effect of applying additional cooling to the surface, making it easier to form a low-temperature structure even in a situation where the Mn content is deficient.

[0132] Afterwards, hot-rolled steel sheets are manufactured by finishing rolling at a finishing rolling temperature (FDT) of RT+20°C or higher, based on the center of the thickness of the above bar. It is preferable to perform the finishing rolling temperature (FDT) based on t / 2 (t: thickness of the steel). At this time, if the finishing rolling temperature is too low, it is difficult for Mn released from ferrite to diffuse into the core of the thickness of the untransformed austenite, so it may be difficult to create a difference in Mn content between the surface region and the core in order to improve bendability relative to hardness.

[0133] After manufacturing the above hot-rolled steel sheet, the hot-rolled steel sheet is first cooled to a cooling stop temperature (CST) that satisfies the conditions of [Equation 4] below.

[0134] [Relationship 4]

[0135] Ms-50℃ ≤ CST ≤ Ms+50℃

[0136] Ms = 515 - 264*C - 29.8*Si - 43.3*Mn - 23.9*Cr - 12.9*Mo

[0137] After the first cooling, the material is cooled a second time to a winding temperature (CT) between room temperature and 250°C, and then wound.

[0138] The aforementioned first cooling stop temperature is intended to ensure a balanced reduction in Mn content in the surface layer and sufficient core martensite. If the first cooling is stopped at a temperature higher than Ms+50℃ and switched to second low-speed cooling, the cooling is insufficient, resulting in a failure to form sufficient martensite in the core. Consequently, the core cross-sectional hardness becomes insufficient, making it difficult to secure the desired high-strength steel sheet. Furthermore, if high-speed cooling is performed to excessively low temperatures, the same composition as the core is obtained without reducing the Mn content in the surface layer, making it difficult to secure excellent bendability.

[0139] Meanwhile, regarding the cooling rates of the first and second stages, an excessive cooling rate causes the surface layer composition to be formed differently from the intended purpose, and an insufficient cooling rate causes the core structure to be formed differently from the intended purpose. Taking this into consideration, the cooling rate during the first stage can be performed at an average cooling rate of 60 to 100°C / sec, and the cooling rate during the second stage can be performed at an average cooling rate of 1 to 40°C / sec. Lowering the second cooling rate compared to the first cooling rate is done to consider productivity, as once martensite has already been formed in the core, further high-speed cooling only degrades the shape quality of the plate and offers no further advantages.

[0140] Mountain terrain and oiling

[0141] After the above-mentioned coiling, the method may additionally include a step of pickling and oiling the coiled hot-rolled steel sheet. The present invention does not specifically limit the pickling and oiling processes, and any method commonly used in the relevant technical field may be used.

[0142] The present invention will be described in detail below through examples. However, it should be noted that the examples described below are intended merely to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0143] (Example)

[0144] A steel slab satisfying the composition of Table 1 below was prepared. At this time, the content of each component is weight%, and the remainder is Fe and unavoidable impurities.

[0145]

[0146] The above steel slab was heated and rough rolled at the rough rolling temperature (RDT) to obtain a bar, scale was removed and the bar was cooled to the surface cooling temperature (SCFT), then finish rolling was performed, and after one pass of finish rolling, the bar was cooled to the surface cooling temperature (SCFT2), and then finish rolling was performed at the center finish rolling temperature (FDT) to produce a hot-rolled steel sheet. The above hot-rolled steel sheet was first cooled to the cooling stop temperature (CST) and secondarily cooled to the coiling temperature (CT), and then coiled to produce a steel sheet specimen. The specific process conditions are shown in Table 2 below.

[0147]

[0148] The microstructure and physical properties of the steel plate specimens manufactured as described above were analyzed, and the results are shown in Table 3 below.

[0149] First, the microstructure was observed at a point 1 / 4 of the thickness of the steel plate using a general electron microscope, and the results were presented based on measurements taken at magnifications of 1000x and 3000x, respectively. Meanwhile, the difference between the average Mn content measured at five or more locations at a thickness of 5㎛ from the surface and the average Mn content measured at five or more locations at a thickness of 1 / 4 was calculated. At this time, it is preferable to perform quantitative analysis of the Mn content using EPMA, an additional analysis instrument installed on the electron microscope (FE-SEM) equipment; however, when analyzing the Mn content at a thickness of 5㎛ from the surface, it can be replaced by performing the GDS component analysis method multiple times.

[0150] Meanwhile, to determine the corrosion rate, the amount of wear was analyzed. The ASTM G65 test method (standardized test tool for measuring wear using rubber wheels) was used for the wear amount. Unground silica (AFS 50-70) was used, and with a reference diameter of 228.3 mm and a rotation speed of 200 rpm, the initial wear amount was measured as the decrease in specimen weight (g) after 3 minutes of wear testing, and the total wear amount was measured as the decrease in specimen weight (g) after 3 minutes followed by 30 minutes of wear testing; the results are shown in Table 3. The wear rate (g / min) was determined by calculating the amount of wear per minute and then determining how much faster the wear rate was during the initial 3 minutes compared to the wear rate during the subsequent 30 minutes. For example, the wear rate during the initial 3 minutes of Invention Example 1 is about 0.079 g / min (0.238 / 3) and the wear rate during the initial 30 minutes is 0.072 g / min (2.168 / 30), and it can be confirmed that the wear rate during the initial 3 minutes is only about 10% faster.

[0151] Bending ductility was expressed based on the value (t) obtained by dividing the minimum bending radius (R) at which no crack occurs by the thickness of the steel plate, obtained by sequentially reducing the bending radius during the V-bending test. Yield strength (YS) and tensile strength (TS) were obtained by performing a conventional tensile test on a tensile specimen conforming to JIS 5 standards. Core cross-sectional hardness was measured using Rockwell hardness and was measured at the center of the thickness at the 1 / 4 point of the steel plate thickness.

[0152]

[0153] As shown in Table 1-3 above, it can be confirmed that when the steel composition and manufacturing process conditions satisfy the scope of the present invention, they all have excellent mechanical properties.

[0154] Meanwhile, Figure 1 is the result of analyzing the surface area of ​​the above invention example 1 with EPMA, and a deficiency of Mn in the surface area was confirmed.

[0155] Figures 2(a) and 2(b) are electron microscope images of the cross-sectional microstructures of the surface region of Inventive Example 1 and Comparative Example 1, respectively, among the embodiments of the present invention, and it can be seen that a large amount of ferrite is observed in the microstructure of the surface region of Comparative Example 1 compared to Inventive Example 1.

[0156] Figure 3 shows electron microscope images of the microstructures of the core cross-sections of Inventive Example 1 and Comparative Example 4, respectively, of the embodiments of the present invention. It can be seen that Comparative Example 4 has difficulty securing hardness because a sufficient martensite structure is not formed.

[0157] In Comparative Example 1, surface cooling was not sufficiently performed after the first pass of the finish rolling, and consequently, the SFCT2 temperature was higher than that of SFCT due to double heating. As a result, the reduction in surface layer Mn content occurred sufficiently, and the bendability was excellent at 1.8; however, the initial wear rate was 56% faster than the overall wear rate, exhibiting a characteristic of non-uniform wear resistance behavior between the initial and overall stages. Although excellent bendability was exhibited despite high hardness while controlling the surface layer Mn content, it was not included in the inventive examples because it did not possess uniform wear resistance.

[0158] In the case of Comparative Example 2, the bar thickness was excessively thin in the state before cooling the surface layer after finishing rough rolling. In addition, the finish rolling temperature was not sufficiently high. As a result, it can be seen that the reduction in surface layer Mn content did not occur sufficiently, and although the difference in wear rate was greater than that of Comparative Example 1, it was not sufficiently large, and the bendability also showed inferior results.

[0159] In the case of Comparative Example 3, the Mn and Cr components were excessively high, so an alloy amount exceeding the T range was used. Although all other conditions were satisfied, a significant difference in wear rate was observed despite a large decrease in surface layer Mn content. In the case of Comparative Example 4, conversely to Comparative Example 3, the component content was excessively low, so an alloy amount falling below the T range was used. All other conditions were satisfied, and although the bendability was excellent, this was not due to differences in Mn content or wear rate, but rather exhibited the characteristic of low core cross-sectional hardness.

[0160] In the case of Comparative Example 5, the range of T was exceeded, similar to Comparative Example 3, but the degree was weaker than that of Comparative Example 3, and this was the result of insufficient Si content and Mo content exceeding the target range. Although there was a difference in Mn content, the degree was insufficient, and consequently, the bendability showed inferior results.

[0161] In the case of Comparative Example 6, the rough rolling was terminated at an excessively high temperature during the manufacturing process, and although Nb and Ti were added in high amounts to compensate for this, the two elements exceeded the content range and did not play an effective role. Regarding the rough rolling temperature and bar thickness, it appears that having an appropriate temperature and thickness range is important for effectively controlling the difference in Mn content. Consequently, the bendability became inferior.

[0162] Comparative Example 7 showed a special case where the rolling end temperature was too low and only the bendability was inferior, even though all other conditions were satisfied. Through this, it was confirmed that the rules of manufacturing conditions are also important.

[0163] In the case of Comparative Example 8, the first cooling temperature CST was too high and exceeded, and the coiling temperature CT was also high, resulting in insufficient strength and hardness. Under these conditions, despite the difference in Mn content, the bendability was inferior.

Claims

1. In weight %, containing C: 0.15~0.24%, Si: 0.1~0.6%, Mn: 0.6~1.9%, Cr: 0.005~0.6%, Mo: 0.002~0.3%, Nb: 0.001~0.03%, Ti: 0.005~0.08%, V: 0.003~0.05%, Al: 0.01~0.5%, P: 0.003~0.03%, S: 0.001~0.01%, N: 0.001~0.01%, B: 0.0002~0.003%, and the remainder being Fe and other unavoidable impurities, Satisfying the following [Relation 1], The difference between the Mn content at a depth of 5 μm from the surface and the Mn content at a thickness of 1 / 4 is 0.12 wt% or more, and A steel plate in which the microstructure from the surface to a depth of 5 µm is ferrite with an area % of 40% or less. [Relationship 1] 3 ≤ T ≤ 30 T=(C / 8) 0.8 *(0.45*Si+1.1)*(4.8*Mn+0.8)*(2.2*Cr+1.2)*(3.5*Mo+0.2)*25 C, Si, Mn, Cr, and Mo in the above relationship (1) are weight percent of the corresponding alloying elements, and 0 is substituted if they are not added.

2. In Paragraph 1, A steel plate in which the microstructure of the above steel plate comprises at least one of martensite, tempered martensite, and less martensite in an area of ​​90% or more, and at least one of pearlite and bainite in an area of ​​10% or less.

3. In Paragraph 1, A steel plate containing 5% or more of the above ferrite.

4. In Paragraph 1, The above steel plate is a steel plate in which the difference in wear rate during the initial 30 minutes compared to the wear rate during the initial 3 minutes in the ASTM G65 standard wear test is 40% or less.

5. In Paragraph 1, The above steel plate includes a core cross-sectional hardness of 37 to 60 HrC and an R / t of 4 or less.

6. A step of heating a steel slab to 1100~1350℃, comprising, in weight %, C: 0.15~0.24%, Si: 0.1~0.6%, Mn: 0.6~1.9%, Cr: 0.005~0.6%, Mo: 0.002~0.3%, Nb: 0.001~0.03%, Ti: 0.005~0.08%, V: 0.003~0.05%, Al: 0.01~0.5%, P: 0.003~0.03%, S: 0.001~0.01%, N: 0.001~0.01%, B: 0.0002~0.003%, the remainder being Fe and other unavoidable impurities, and satisfying the following [Equation 1]; A step of obtaining a bar by terminating rough rolling of the above heated steel slab at a rough rolling temperature (RDT) satisfying [Relationship 2] with respect to the center of the thickness; A step of descaling the surface of the above bar and cooling it to a surface cooling temperature (SFCT) that satisfies [Relationship 3] based on the surface; A step of performing finish rolling after the above cooling, and cooling to a surface cooling temperature (SFCT2) satisfying [Equation 3] based on the surface after one pass of finish rolling; A step of obtaining a hot-rolled steel sheet by terminating finish rolling at a finish rolling temperature (FDT) of RT+20℃ or higher relative to the center of the thickness of the above bar; A step of first cooling the above hot-rolled steel sheet to a cooling stop temperature (CST) satisfying the conditions of [Equation 4] below: and A step of secondarily cooling the above first-cooled hot-rolled steel sheet to a coiling temperature (CT) between room temperature and 250°C, and then coiling it. A method for manufacturing a steel plate including [Relationship 1] 3 ≤ T ≤ 30 T=(C / 8) 0.8 *(0.45*Si+1.1)*(4.8*Mn+0.8)*(2.2*Cr+1.2)*(3.5*Mo+0.2)*25 C, Si, Mn, Cr, and Mo in the above relationship (1) are weight percent of the corresponding alloying elements, and 0 is substituted if they are not added. [Relationship 2] RDT ≤ (RT + 100 + Bar thickness * 3.5) RT = 902 - 325*C + 34.4*Si - 22.7*Mn - 13.6*Cr + 13.3*Mo - 94.6*Nb + 158*Ti + 194*V [Relationship 3] FT ≤ SFCT ≤ FT+50℃ SFCT2 ≤ FT-50℃ FT = 722 + 135*C + 19.8*Si - 42.1*Mn - 0.5*Cr + 5.86*Mo + 182*Nb - 192*Ti - 94*V [Relationship 4] Ms-50℃ ≤ CST ≤ Ms+50℃ Ms = 515 - 264*C - 29.8*Si - 43.3*Mn - 23.9*Cr - 12.9*Mo In the above equations 2 to 4, C, Si, Mn, Cr, Mo, Nb, Ti, Al, N, and B represent the weight percent of the corresponding alloying elements, and 0 is substituted if they are not added. Bar thickness (mm) refers to the bar thickness after rough rolling is completed.

7. In Paragraph 6, A method for manufacturing a steel plate in which the thickness of the above bar is 20 to 50 mm.

8. In Paragraph 6, A method for manufacturing a steel plate in which the above-mentioned first cooling is performed at a first average cooling rate of 60 to 100℃ / sec.

9. In Paragraph 6, A method for manufacturing a steel plate in which the above secondary cooling is performed at a secondary average cooling rate of 1 to 40℃ / sec.

10. In Paragraph 6, A method for manufacturing a steel plate having a thickness of at least 6 times that of the final hot-rolled steel plate, obtained by performing the above rough rolling.

11. In Paragraph 6, A method for manufacturing a steel plate, further comprising the steps of pickling and oiling the above-mentioned wound steel plate.

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