WO2008072866A1

WO2008072866A1 - Cold-rolled steel sheet with high yield ratio and excellent weather resistance

Info

Publication number: WO2008072866A1
Application number: PCT/KR2007/006405
Authority: WO
Inventors: Jai Ik Kim; Kee Jo Jeong; Jong Hwa Kim; Soo Hee Lee
Original assignee: Posco
Priority date: 2006-12-12
Filing date: 2007-12-10
Publication date: 2008-06-19
Also published as: KR100815799B1; CN101558178B; CN101558178A; JP5101627B2; JP2010512460A

Abstract

There is provided a cold-rolled steel sheet with a high yield ratio, and excellent weather resistance and workability. The cold-rolled steel sheet including, by weight: 0.08 to 0.20% C, 0.1 to 0.5% Si, 0.9 to 2.0% Mn, <0.02% P, <0.01% S, 0.02 to 0.07% Al, 0.03 to 0.06% Nb, 0.05 to 0.30% Ni, 0.2 to 0.5% Cu, 0.3 to 0.6% Cr, 0.001 to 0.004% B, 0.02 to 0.08% Co, the balance being Fe and unavoidable impurities. Also, there is provided a method of manufacturing the same.

Description

COLD-ROLLED STEEL SHEET WITH HIGH YIELD RATIO AND EXCELLENT WEATHER RESISTANCE

Technical Field

[1] The present invention relates to a high-strength cold-rolled steel sheet with excellent weather resistance for use in construction, railway vehicles and containers, and a manufacturing method thereof, and more particularly, to a high-strength cold- rolled steel sheet with a high yield ratio and excellent weather resistance. Background Art

[2] Conventionally, materials like stainless or aluminum have been utilized to reduce weight or extend useful life of containers or railway vehicles. These products require characteristics such as bending workabiltiy, weldability and durability. Also, transport structures tend to undergo impact when cargos are shipped or loaded thereon so that it is necessary to suppress impact-caused deformation. To this end, a steel with a high yield ratio may be desirably employed.

[3]

[4] A yield ratio is defined as a ratio of tensile strength to yield strength, out of steel values obtained from a tensile test. A higher yield ratio at the same tensile strength means higher yield strength of the steel. That is, a steel with a high yield ratio has greater resistance against deformation when impact is imposed, due to high yield strength which is characteristic of an elastic material, thereby further capable of suppressing deformation. To be employed in containers, the steel may have a yield ratio of at least 80%. Especially, the containers should withstand various climatic conditions on the ground or sea depending on transportation circumstances. This has led to a demand for the steel with excellent weather resistance.

[5]

[6] As an example, conventionally, SPA-C such as Korean standard KS-D3542 and

Japanese standard JIS-G3125 has been adopted as a rolled steel with excellent weather resistance. However, such a steel has a low tensile strength of 50kg/mm and thus is increased in weight when used to manufacture bigger products. This results in higher transportation costs. Also, a high-strength cold-rolled steel sheet having a tensile strength of 60 to 80kg/mm has been utilized for structural parts of a car. However, this type of steel is focused on increasing strength, thus not exhibiting desired excellent weather resistance.

[7]

[8] Recently, in the container industry, attempts have been made to manufacture a larger-sized container by considerably reducing size thereof to ensure smaller costs and address environment problems, thereby enhancing transportation efficiency. Especially, a steel having a high strength of at least 80kg/mm and excellent weather resistance has been required and a manufacturing method thereof has been suggested.

[9]

[10] For example, Japanese Patent Laid-open Publication No. hei 7-207408 discloses a method of manufacturing a hot-rolled steel sheet. In this method, a steel containing < 0.008% C, 0.5 to 2.5% Si, 0.1 to 3.5% Mn, 0.03 to 0.20% P, <0.01% S, 0.05 to 2.0% Cu, 0.005 to 0.1% Al, <0.008% N, 0.05 to 6.0% Cr, 0.05 to 2.0% Ni, 0.05 to 3.0% Mo, and 0.0003 to 0.002% B is heated to 1100 to 1300⁰C. Then, the steel is finish-rolled at 800 to 95O⁰C and coiled at 400 to 700⁰C. However, by this technology, only limited examples produce a steel sheet having a tensile strength of 60 to 70kg/mm , and most examples produce a steel sheet having a tensile strength of 50kg/mm , thereby not achieving a tensile strength of 80kg/mm . Also, hardenability-enhancing components such as Cr, and Mo are added in a great amount, degrading weldability and increasing manufacturing costs.

[H]

[12] Also, Japanese Patent Laid-open Publication No. hei 11-21622 discloses a method of manufacturing a steel product. In this method, a steel having a composition consisting of, by weight, < 0.15% C, <0.7% Si, 0.2 to 1.5% Mn, 0.03 to 0.15% P, <0.02% S, < 0.4% Cu, 0.01 to 0.1% Al, <0.1% Cr, 0.4 to 4.0% Ni and 0.1 to 1.5% Mo is heated to 1050 to 1300⁰C. Then, the steel is hot-rolled at 95O⁰C or higher by at least 40%, finish-rolled at 900 to 75O⁰C and air cooled. However, even by this technology, most steel products have a tensile strength of 50kg/mm and only limited steel products demonstrate a tensile strength of 60kg/mm . This technology is generally applied to manufacture a steel sheet having a tensile strength of 50kg/mm .

[13]

[14] Moreover, P is added in a great amount of 0.03 to 0.15% to improve corrosion resistance in a sea water atmosphere. However, P added in a great amount causes a cold rolled steel product to suffer central segregation, thereby rapidly deteriorating workability thereof.

[15]

[16] Moreover, Japanese Patent Laid-open Publication No. hei 6-104858 discloses a method of manufacturing a steel sheet. In this method, a steel containing 0.02 to 0.12% C, < 0.5% Si, 0.1 to 2.0% Mn, 0.07 to 0.15% P, < 0.02% S, 0.25 to 0.55% Cu, 0.01 to 0.05% Al, 0.3 to 1.25% Cr, <0.006% N₂, 0.06 to 0.20% Ti has a composition regulated to satisfy a formula of 12.1X ti.eff (%)/Mn(%)>1.0. The steel is re-heated at 118O⁰C or higher, hot-rolled at 880 to 95O⁰C and coiled at 65O⁰C or lower. By this technology, Ti is added in connection with an Mn content to control a precipitate. However, examples of this technology produce a steel sheet having a tensile strength of 60kg/mm , lower than 80kg/mm , which the present invention aims to achieve.

[17]

Disclosure of Invention Technical Problem

[18] The present invention has been made to solve the foregoing problems of the prior art and therefore an aspect of the present invention is to provide a steel sheet with a high yield ratio and excellent weather resistance, particularly having a high tensile strength of at least 80kg/mm .

[19]

Technical Solution

[20] According to an aspect of the invention, the invention provides a cold-rolled steel sheet with a high yield ratio and excellent weather resistance including, by weight: 0.08 to 0.20% C, 0.1 to 0.5% Si, 0.9 to 2.0% Mn, <0.02% P, <0.01% S, 0.02 to 0.07% Al, 0.03 to 0.06% Nb, 0.05 to 0.30% Ni, 0.2 to 0.5% Cu, 0.3 to 0.6% Cr, 0.001 to 0.004% B, 0.02 to 0.08% Co, the balance being Fe and unavoidable impurities.

[21] The cold-rolled steel sheet may have a tensile strength of at least 80kg/mm .

[22]

[23] According to another aspect of the invention, the invention provides a method of manufacturing a cold-rolled steel sheet with a high yield ratio, the method including: reheating a steel to 1150 to 1300⁰C, the steel comprising, by weight, 0.08 to 0.20% C, 0.1 to 0.5% Si, 0.9 to 2.0% Mn, <0.02% P, <0.01% S, 0.02 to 0.07% Al, 0.03 to 0.06% Nb, 0.05 to 0.30% Ni, 0.2 to 0.5% Cu, 0.3 to 0.6% Cr, 0.001 to 0.004% B, 0.02 to 0.08% Co, the balance being Fe and unavoidable impurities; hot-rolling the steel at a finish rolling temperature of 850 to 95O⁰C; cooling the steel at a cooling rate of 20 to 4O⁰C per second; coiling the steel at 500 to 65O⁰C; cold-rolling the coiled steel; and continuously annealing the steal at a temperature ranging from 500⁰C to an A transformation point.

[24]

Advantageous Effects

[25] According to the invention, a steel sheet can attain weather resistance, mechanical properties and high yield ratio. Therefore, this steel sheet with subsequently high added values can be utilized in a case where impact resistance is required. In addition, continuous annealing is performed in a relatively low-temperature range, thereby saving energy and improving annealing efficiency.

[26] Best Mode for Carrying Out the Invention

[27] Exemplary embodiments of the present invention will now be described in detail.

[28] The inventors have completed the present invention through repeated researches and experiments aimed at obtaining a steel sheet having various workability characteristics and weather resistance and also ensuring high strength and a high yield ratio. According to the present invention, in the weather resistant component system of Cu- Co, an addition amount of Cr, B, and Nb is regulated to attain high strength characteristics and a high yield ratio.

[29]

[30] Carbon (C) is preferably in a range of 0.08 to 0.20% by weight (hereinafter, only referred to as percentage %).

[31] C is added to increase strength of a steel sheet. A greater amount of C increases tensile strength and yield strength of the steel sheet. However, C, when added in an excessive amount, degrades workability of the steel. Thus, C may be added in an amount of up to 0.20%. Meanwhile, the C amount less than 0.08% does not sufficiently serve to strengthen precipitation.

[32]

[33] Silicon (Si) is preferably in a range of 0.1 to 0.5%.

[34] Si is effective for deoxidizing molten steel and strengthening solid solution. Si forms a compact Fe SiO oxide with Fe on a surface layer of the steel at a high temperature, serving to improve corrosion resistance. To attain these effects sufficiently, Si may be added in an amount of at least 0.1%. Therefore, Si should be added to improve weather resistance, but an excessive amount thereof degrades weldability and coating properties. Thus, Si may be added in an amount of up to 0.5%.

[35]

[36] Manganese (Mn) is preferably in a range of 0.9 to 2.0%.

[37] Mn is effective for strengthening solid solution and significant for increasing strength and hot-rolling workability of the steel. However, Mn also hampers ductility and workability of the steel due to formation of MnS. A small amount of Mn is advantageous for workability but leads to insufficient strength of the steel. Therefore, Mn may be added in an amount of at least 0.9% to achieve desired strength. On the other hand, an excessive amount of Mn, which is an expensive alloy element, degrades economic efficiency and impairs weldability. Thus, Mn may be added in an amount of up to 2.0%.

[38]

[39] Phosphor (P) is preferably in a range of at least 0.02%.

[40] P enhances corrosion resistance of the steel and thus may be added in a great amount to increase corrosion resistance. However, P creates central segregation most considerably during casting of the steel. Thus, a great amount of P degrades weldabiliy and tensile strength. Therefore, P may be added in an amount of up to 0.02%.

[41]

[42] Sulfur (S) is preferably in a range of up to 0.01%.

[43] S is known to be effective for increasing corrosion resistance, but when binding with Mn in the steel, forms a nonmetallic inclusion which initiates corrosion. Therefore, Si may be added in as small an amount as possible. Accordingly, S is added in an amount of up to 0.01%, more particularly, up to 0.005%.

[44]

[45] Aluminum (Al) is preferably in a range of 0.02 to 0.07%.

[46] Al is generally effective for deoxidizing molten steel and increasing corrosion resistance, but an excessive amount thereof increases an amount of inclusion in the steel, thereby deteriorating workability. Thus, the Al amount may be set to a range of 0.02 to 0.07%.

[47]

[48] Niobium (Nb) is preferably in a range of 0.03 to 0.06% .

[49] Nb delays re-crystallization of ferrite and binds with C and N in the steel to be precipitated, thereby enhancing strength of the steel sheet. To obtain desired strength, Nb may be added at 0.03% or more. Meanwhile, Nb amount exceeding 0.06% may increase manufacturing costs and hamper hot rolling workability.

[50]

[51] Nickel (Ni) is preferably in a range of 0.05 to 0.3%.

[52] Ni generally prevents a Cu-added steel from being cracked during casting and improves corrosion resistance. Ni may be added in an amount of at least 0.05% to achieve these effects. However, the Ni amount exceeding 0.3% undermines corrosion resistance and increases costs due to excessive use of such an expensive alloy element.

[53]

[54] Copper (Cu) is preferably in a range of 0.2 to 0.5%.

[55] Cu forms a stable rust layer in a corrosion atmosphere, thereby improving corrosion resistance. To attain desired corrosion resistance, Cu may be added at 0.2% or more. However, the Cu amount exceeding 0.5% may result in grain boundary cracks during continuous casting and roughen a surface of a hot-rolled steel sheet.

[56]

[57] Chrome (Cr) is preferably in a range of 0.3 to 0.6%.

[58] Cr serves to form a stable rust layer like Cu. To achieve corrosion resistance and strength, Cr may be added at 0.3% or more. Meanwhile, the Cr amount exceeding 0.6% causes crevice corrosion and dramatically increases manufacturing costs. [59]

[60] Boron (B) is preferably in a range of 0.001 to 0.004%.

[61] B enhances hardenablility of the steel and also delays recrystallization of ferrite phase. To achieve desired strength at a low temperature, B may be added in an amount of at least 0.001%. Meanwhile, the B amount exceeding 0.004% facilitates growth of hard phase such as bainite in a hot-rolling process due to increase in hardenability, thereby hampering cold-rolling workability.

[62]

[63] Cobalt (Co) is preferably in a range of 0.02 to 0.08%.

[64] Co reacts with Cu and Cr added to attain corrosion resistance of the steel thereby to facilitate formation of a product inhibiting corrosion of a surface layer. To achieve this effect, Co may be added at 0.02% or more. However, the Co amount exceeding 0.08% leads to higher manufacturing costs rather than brings about better corrosion resistance.

[65]

[66] The steel of the present invention contains the aforementioned components, with the balance being Fe and unavoidable impurities. Optionally, an alloy element may be added to improve characteristics of the steel with excellent weather resistance. Here, although the alloy elements which are not described in the exemplary embodiments of the present invention are added to the composition of the cold-rolled steel sheet, it is not construed that the alloy elements depart from the scope of the present invention.

[67]

[68] The cold-rolled steel of the present invention has both high strength and a high yield ratio as well. According to an exemplary embodiment of the invention, the steel sheet has a tensile strength of at least 80kg/mm , for example, 80 to 110 kg/mm . The steel sheet has a yield strength of at least 80%, for example, 85 to 94%. Also, the steel sheet has a ductility of at least 10%.

[69]

[70] These characteristics satisfy the aforesaid component system and are obtained by low-temperature annealing of the present invention. The low-temperature annealing allows deformed grains to partially remain in microstructures in place of being entirely recovered. The microstructure of the present invention may be ferrite or an admixture of ferrite and perlite, however not limited thereto.

[71]

[72] Hereinafter, a method of manufacturing a cold-rolled steel sheet will be described.

[73] The method of manufacturing the steel having the above composition will be explained according to an exemplary embodiment of the invention. The steel having the above chemical composition is re-heated to 1150 to 1300⁰C, and finish hot-rolled at 850 to 95O⁰C. Then, the steel is cooled at a cooling rate of 20 to 4O⁰C per second and coiled at 500 to 650⁰C. Subsequently, to be cold-rolled and heat-treated, the steel is continuously annealed at 500 to an A transformation point. This manufacturing method produces a cold-rolled steel sheet with a high yield ratio and excellent weather resistance, particularly having a strong tensile strength of at least 80kg/mm .

[74]

[75] The steel, when reheated to less than 1150⁰C, is prone to central segregation due to insufficient destruction of a solidification grain structure formed during casting. Therefore, the finally formed grains are mixed to significantly degrade workability and impact toughness of the steel. Meanwhile, the steel, when reheated to above 1300⁰C, facilitates scale formation due to oxidization, resulting in significant decrease in thickness of a slab. Also, in this case, the steel is degraded in impact toughness due to coarse grains.

[76]

[77] Moreover, such a higher reheating temperature inflicts economic loss. Accordingly, the reheating temperature of the steel may be set to a range of 1150 to 1300⁰C.

[78]

[79] In a case where the finish hot rolling temperature is above 950⁰C, the steel is not heat-rolled uniformly across an entire thickness thereof. Thus, the steel is degraded in impact toughness resulting from the coarse grains. On the contrary, in a case where the finish hot rolling temperature is below 850⁰C, the steel is finish hot-rolled at a low temperature, thereby undergoing the admixture of grains thereof. This undermines corrosion resistance and workability, and therefore the finish hot-rolling temperature may be set to a range of 850 to 950⁰C.

[80]

[81] After being hot-rolled as described above, the steel may be cooled at 20 to 40⁰C.

That is, in a case where the steel is cooled at a cooling rate of less than 20⁰C per second on a run-out-table (ROT) after being finish hot-rolled, the steel has relatively coarse grains formed due to the accelerated growth of the grains, thereby degraded in strength. Therefore, the cooling rate of the steel may be set to 20⁰C or less per second. Meanwhile, the cooling rate exceeding 40⁰C per second leads to formation of hard second phase like bainite, thereby significantly deteriorating cold-rolling properties. Therefore, the cooling temperature of the steel may be set to a range of 20 to 40⁰C per second.

[82]

[83] Cooling of the steel as described above is followed by coiling. The steel may be coiled at 500 to 650⁰C. A hot-rolling coiling temperature exceeding 650⁰C does not bring about sufficient precipitation effects, thereby degrading strength of the steel. This renders the steel unlikely to attain a desired strength of 80kg/mm . Meanwhile, in a case where the coiling temperature is below 500⁰C, hard phase is formed while the steel is cooled and maintained, increasing a roll force of a rolling mill during the cold- rolling. This hampers rolling of the steel and thus the coiling temperature of the steel may be set to a range of 500 to 65O⁰C.

[84]

[85] The hot-rolled steel is rolled under general cold-rolling conditions and then subjected to continuous annealing. Here, to attain desired steel characteristics, the steel needs to be annealed at a proper temperature. In a case where the annealing temperature is lower than 500⁰C during the continuous annealing, deformed grains in the cold-rolling still remain, sharply decreasing ductility and thus undermining workability.

[86]

[87] Meanwhile, an annealing temperature of an A transformation temperature or higher results in formation of martensite phase since the steel is transformed when cooled after being annealed. This decreases yield strength of the steel to 60% or less, rendering the steel vulnerable to deformation. Therefore, the steel may be annealed at up to an A transformation point.

[88]

[89] According to the present invention, the cold-temperature annealing may allow the deformed grains in the cold-rolling process to partially remain in the cold-rolled steel sheet.

[90]

Mode for the Invention

[91] Hereinafter, the present invention will be described in more detail by way of

Examples.

[92]

[93] [Example 1]

[94] Steels having compositions as noted in Table 1 were measured for a standardized corrosion resistance index (CI) and weather resistance. The results are shown in Table 2.

[95] For the weather resistance test, the steels underwent a salt spray test (SST) for 480 hours at a temperature of 3O⁰C in 5% saline (NaCl solution). Here, the corrosion resistance index (CI) is an indicator for estimating weather resistance in accordance with ASTM GlOl. A higher corrosion resistance index means stronger weather resistance.

[96] The corrosion resistance index is derived chiefly based on alloy elements and satisfies a following equation. [98] Corrosion resistance index (CI) < 26.01(%Cu) + 3.88(%Ni) + 1.2(%Cr) + 1.49(%Si) + 17.28(%P) - 7.29(%Cu)(%Ni) - 9.10(%Ni)(%P) - 33.39(%Cu)²

[99] [100] Table 1

[101] Table 2

[102] [103] As shown in Table 2, Comparative steel 2 is low in the corrosion resistance index and high in its weight loss by corrosion, thus hard to be applicable as a steel with a weather resistance. Comparative steel 3 and Comparative steel 4 are slightly high in the corrosion resistance index but their weight losses by corrosion were 0.030g/cm or more in the salt spray test.

[104] [105] Accordingly, Comparative steel 3 and Comparative steel 4 exhibit weak weather resistance. Meanwhile, Inventive steels 1 and 2, and Comparative steel 1 demonstrate superior weather resistance considering corrosion-induced weight losses and the corrosion resistance index.

[106] [107] [Example 2] [108] The steels, i.e., Inventive steels 1 and 2, and Comparative steels 1 to 4 in Table 1 according to Example 1 were utilized to manufacture cold-rolled steel sheets under conditions set forth in Table 3. Each steel was measured for mechanical properties and workability characteristics and the results are shown in Table 4.

[109] [HO] Table 3

Table 4

[112] [113] As shown in Table 4, Inventive examples 1 to 4 whose chemical compositions and manufacturing conditions satisfy the conditions of the present invention attained a tensile strength of at least 80kgf/mm , a yield ratio of at least 80%, and a ductility of at least 10%, respectively. The inventive examples were not cracked during a bending process, and thereby manufactured into steel sheets with high strength and high yield ratios.

[114] [115] Meanwhile, Comparative examples 1 to 5 whose chemical compositions satisfy the conditions of Inventive steels but whose manufacturing conditions fall out of the range of the present invention did not achieve desired characteristics. That is, Comparative Example 2 and Comparative Example 5 whose annealing temperature is higher than that of the present invention attained desired tensile strength, but were reduced in a yield ratio to 70% or less. This is because second phase is formed by transformation in a cooling process due to the high annealing temperature, thereby lowering yield strength. That is, Comparative Example 2 and Comparative Example 5 did not attain a yield ratio of at least 80%, thereby rendered less resistant to deformation.

[116]

[117] In Comparative Example 4 whose annealing temperature was lower than that of the present invention, most deformed grains formed during cold-rolling remain, thus not ensuring ductility and bending workability. Moreover, Comparative Example 1 whose hot-rolling finishing temperature and coiling temperature fall out of the ranges of the present invention and Comparative Example 3 whose cooling rate is higher than the condition of the present invention demonstrated a ductility of less than 5%, thus not achieving appropriate workability.

[118]

[119] In a case where Comparative steel 1 whose Mn and Cr compositional range fell out of the requirement of the present invention but which had relatively excellent weather resistance was manufactured under the condition of the present invention (Comparative Example 7), the Comparative Example 1 did not achieve ductility and workability. Furthermore, Comparative Example 1, when increased in the annealing temperature to ensure ductility and workability, was reduced in yield strength due to creation of dual phase, thereby not attaining a yield ratio of at least 80%.

[120]

[121] In a case where Comparative steels 2 to 3 whose chemical compositional ranges departed from the requirement of the present invention and which could not achieve weather resistance were tested under different manufacturing conditions (Comparative Examples 8 to 11), workability and an adequate compositional range of the steels could not be determined.

Claims

[1] A cold-rolled steel sheet with a high yield ratio and excellent weather resistance comprising, by weight:

0.08 to 0.20% C, 0.1 to 0.5% Si, 0.9 to 2.0% Mn, <0.02% P, <0.01% S, 0.02 to

0.07% Al, 0.03 to 0.06% Nb, 0.05 to 0.30% Ni, 0.2 to 0.5% Cu, 0.3 to 0.6% Cr,

0.001 to 0.004% B, 0.02 to 0.08% Co, the balance being Fe and unavoidable impurities. [2] The cold-steel sheet of claim 1, wherein the cold-rolled steel sheet has a tensile strength of at least 80kg/mm . [3] A method of manufacturing a cold-rolled steel sheet with a high yield ratio, the method comprising: reheating a steel to 1150 to 1300⁰C, the steel comprising, by weight, 0.08 to

0.20% C, 0.1 to 0.5% Si, 0.9 to 2.0% Mn, <0.02% P, <0.01% S, 0.02 to 0.07%

Al, 0.03 to 0.06% Nb, 0.05 to 0.30% Ni, 0.2 to 0.5% Cu, 0.3 to 0.6% Cr, 0.001 to

0.004% B, 0.02 to 0.08% Co, the balance being Fe and unavoidable impurities; hot-rolling the steel at a finish rolling temperature of 850 to 95O⁰C; cooling the steel at a cooling rate of 20 to 4O⁰C per second; coiling the steel at 500 to 65O⁰C; cold-rolling the coiled steel; and continuously annealing the steal at a temperature ranging from 500⁰C to an A transformation point.