WO2009082149A2

WO2009082149A2 - High strength cold-rolled steel sheet having excellent weather resistance and method manufacturing the same

Info

Publication number: WO2009082149A2
Application number: PCT/KR2008/007581
Authority: WO
Inventors: Jai-Ik Kim; Kee-Jo Jeong
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2007-12-24
Filing date: 2008-12-22
Publication date: 2009-07-02
Anticipated expiration: 2010-06-24
Also published as: WO2009082149A3; KR20090068625A; CN101910436A; KR100925639B1; CN101910436B

Abstract

Provided are a high strength cold-rolled steel sheet with excellent weather resistance and a manufacturing method thereof. The cold-rolled sheet includes, by weight, 0.08 to 0.20% C, 0.1 to 0.5% Si, 1.2 to 2.0% Mn, ≤0.03% P, ≤0.01% S, 0.02 to 0.07% Al, 0.02 to 0.06% V, 0.2 to 0.5% Cu, 0.3 to 0.8% Cr, 0.04 to 0.08% Zr, 0.02 to 0.08% Co, 0.02 to 0.07% W, the balance being Fe and unavoidable impurities. According to the cold-rolled steel sheet and the manufacturing method thereof, a high strength cold-rolled steel sheet with excellent weather resistance, mechanical properties and high yield ratio can be obtained by appropriately controlling components and process conditions. Therefore, it is possible to manufacture a high value-added steel sheet for use in outdoor structures requiring high impact resistance. Also, annealing process is performed in a relatively low-temperature range, thereby saving energy and improving annealing efficiency.

Description

HIGH STRENGTH COLD-ROLLED STEEL SHEET HAVING EXCELLENT WEATHER RESISTANCE AND METHOD MANUFACTURING THE SAME

Technical Field

[1] The present invention relates to a high strength cold-rolled steel sheet having excellent impact resistance for use in railway vehicles and containers, and a manufacturing method thereof, and more particularly, to a cold-rolled steel sheet having high strength characteristic as well as weather resistance by controlling components of a steel and manufacturing conditions, and a method of manufacturing the cold-rolled steel sheet.

[2]

Background Art

[3] Conventionally, materials such as stainless steel and aluminum have been used to reduce weight or extend useful life of containers or railway vehicles. These products require bending workability, weldability, durability, etc. Also, transport structures tend to receive impact when cargos are shipped or loaded thereon, and thus it is necessary to suppress impact-caused deformation. To this end, an impact-resistant material may be desirably used. The impact resistance of a working member is a factor that is closely related to a thickness and yield ratio of a material.

[4] A yield ratio is defined as a ratio of tensile strength to yield strength, among material parameters obtained through 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 its high yield strength which is characteristic of an elastic material, thereby further capable of suppressing deformation. To be used for impact-resistant structure, e.g., containers, the steel may have a yield ratio of at least 80%. Furthermore, 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] As an example, in the related art, SPA-C (Korean standard KS-D3542 and Japanese standard JIS-G3125) has been adopted as a rolled steel with weather resistance. However, such a steel has a low tensile strength of 50 kg/mm , thus leading to an increase in the weight of the container when the steel is used to manufacture a large- sized container, e.g., 50-feet container. This results in an increase in transportation cost, and therefore there is still an ongoing demand for reduction of a steel weight. Also, a high-strength cold-rolled steel having a tensile strength of 60 to 80 kg/mm has been used for structural members of a car. However, this type of steel does not have desired excellent weather resistance because much attention is still paid on the increase of strength.

[6] In the container industry, to reduce costs and address environmental problems, attempts have recently been made to enhance transportation efficiency by manufacturing a larger- sized container through reduction of its weight. To this end, there is an increasing demand for the development of a steel that has excellent weather resistance, is lightweight, and has a high strength of at least 80 kg/mm .

[7] For example, Japanese Patent Laid-open Publication No. Hei 7-207408 discloses a method of manufacturing a hot-rolled steel sheet. In this related art 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 1,100 to 1,300 ⁰C. Then, the steel is finish-rolled at 800 to 950 ⁰C and then coiled at 400 to 700 ⁰C. However, according to this related art method, only limited examples provide a steel sheet having a tensile strength of 60 to 70 kg/mm , and most examples provide a steel sheet having a tensile strength of 50 kg/mm . Thus, it is difficult to obtain a steel sheet having a tensile strength of 80 kg/mm . Also, hardenability-enhancing components such as Cr and Mo are added in a great amount, degrading weldability and increasing manufacturing costs.

[8] Also, Japanese Patent Laid-open Publication No. Hei 11-21622 discloses a method of manufacturing a steel product. In this related art 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 1,050 to 1,300 ⁰C. Thereafter, the steel is hot-rolled at 950 ⁰C or higher by at least 40%, then finish-rolled at 900 to 750 ⁰C and followed by air cooling. However, even according to this related art method, most steel products have a tensile strength of 50 kg/mm and only limited steel products exhibit a tensile strength of 60 kg/mm . Thus, this related art method is merely applied to the manufacture of a steel sheet having a tensile strength of 50 kg/mm . Moreover, this reference discloses that the addition of 0.03 to 0.15% P may improve corrosion resistance in a sea water atmosphere. However, an excessive amount of P causes a cold-rolled steel sheet to suffer central segregation, thereby rapidly deteriorating workability of the steel sheet. Thus, the above-described related art methods do not suggest an inventive method of achieving a steel sheet having a tensile strength of at least 80 kg/mm as well as excellent impact resistance and weather resistance.

[9] Disclosure of Invention

Technical Problem

[10] An aspect of the present invention provides a high strength cold-rolled steel sheet ha ving a tensile strength of at least 80 kg/mm as well as excellent impact resistance and weather resistance by controlling added amounts of steel components such as Co, V, Zr and W and optimizing rolling and annealing conditions, and a method of manufacturing the high strength cold-rolled steel sheet.

[H]

Technical Solution

[12] According to an aspect of the present invention, there is provided a high strength cold-rolled steel sheet comprising, by weight, 0.08 to 0.20% C, 0.1 to 0.5% Si, 1.2 to 2.0% Mn, <0.03% P, <0.01% S, 0.02 to 0.07% Al, 0.02 to 0.06% V, 0.2 to 0.5% Cu, 0.3 to 0.8% Cr, 0.04 to 0.08% Zr, 0.02 to 0.08% Co, 0.02 to 0.07% W, the balance being Fe and unavoidable impurities, and there is also provided a method of manufacturing a high strength cold-rolled steel sheet with excellent impact resistance and weather resistance, the method including: reheating the high strength cold-rolled steel sheet to 1,150 to 1,300 ⁰C; finish-rolling the reheated steel at a temperature of 800 ⁰C to 950 ⁰C; cooling the finish-rolled steel at a cooling rate of 20 to 40 ⁰C per second; coiling the steel at 500 to 650 ⁰C, and cold-rolling the coiled steel; and continuously annealing the cold-rolled steal at an annealing temperature ranging from 550 ⁰C to an Al transformation point.

Advantageous Effects

[13] According to the present invention, a high strength cold-rolled steel sheet with excellent weather resistance, mechanical properties and high yield ratio can be obtained by appropriately controlling components and process conditions. Therefore, it is possible to manufacture a high value-added steel sheet for use in outdoor structures requiring high impact resistance. In addition, an annealing process is performed in a relatively low-temperature range, thereby saving energy and improving annealing efficiency.

[14]

Best Mode for Carrying out the Invention

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

[16] The inventors have completed the present invention through repeating researches and experiments to obtain a high-tensile strength steel which has excellent impact resistance and weather resistance as well as various workability characteristics, i.e., bendability, weldability, stretchability, etc., and is thus useful for containers and railway vehicles. According to the present invention, steel components may have chemical compositions as follows.

[17] First, carbon (C) is added to increase the strength of a steel sheet. A greater amount of C increases tensile strength and yield strength of the steel sheet. However, an excessive amount of C degrades workability of the steel, and thus an upper limit of C is set to amount of 0.20%. Meanwhile, if the amount of C is 0.08% or smaller, it is impossible to achieve the precipitation- strengthening effect. Therefore, the amount of C may be set to a range of 0.08 to 0.20%.

[18] Silicon (Si) is effective for deoxidizing molten steel and strengthening solid solution.

Furthermore, Si forms a compact Fe SiO oxide with Fe on a surface layer of the steel at a high temperature, thus 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 necessarily added to improve weather resistance, but an excessive amount of Si degrades weldability and coating properties. Thus, Si may be added in an amount of up to 0.5%, and therefore the addition amount of Si is limited to a range of 0.1 to 0.5%.

[19] Manganese (Mn) is effective for solid-solution strengthening and significant for increasing strength and hot-rolling workability of the steel. However, Mn also deteriorates 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 should be added in an amount of at least 0.5% to achieve desired strength. On the other hand, an excessive amount of Mn degrades economic efficiency and causes poor weldability. Thus, the upper limit of Mn amount is set to 2.0%.

[20] Phosphorous (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 during casting of the steel. Thus, a great amount of P degrades weldability and tensile strength. Therefore, the addition amount of P may be limited to 0.03% or less.

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

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

[23] Vanadium (V) delays recrystallization of ferrite and also enhances the strength of the steel sheet because it binds with C and N in the steel to be precipitated. To obtain desired strength, V should be added in an amount of 0.02% or more. Meanwhile, the addition amount of V exceeding 0.06% may increase manufacturing costs and deteriorate hot-rolling workability. Therefore, the addition amount of V is limited to a range of 0.02 to 0.06%.

[24] Copper (Cu) forms a stable rust layer in a corrosion atmosphere to thereby improve corrosion resistance. To attain desired corrosion resistance, Cu may be added in an amount of 0.02% 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 as well. Therefore, the addition amount of Cu is limited to a range of 0.2 to 0.5%.

[25] Chromium (Cr) serves to form a stable rust layer like Cu. To achieve corrosion resistance and strength, Cr may be added in an amount of 0.3% or more. Meanwhile, if the addition amount of Cr is 0.8% or more, crevice corrosion is generated and manufacturing cost is dramatically increased. Therefore, Cr may be added in an amount of 0.3 to 0.8%.

[26] Zirconium (Zr), which delays recrystallization of ferrite, may be added in an amount of 0.04% or more to obtain desired strength. Meanwhile, Zr amount exceeding 0.08% cannot ensure rolling property of the steel, and therefore, the addition amount of V is limited up to 0.08%.

[27] Cobalt (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 in an amount of 0.02% or more. However, the amount of Co exceeding 0.08% leads to higher manufacturing costs rather than brings about better corrosion resistance. Therefore, the addition amount of Co is limited to a range of 0.02 to 0.08%.

[28] Meanwhile, tungsten (W), which is added to ensure hardenability and strength characteristics, should be added in an amount of at least 0.02% to attain a desired strength in a low temperature range. On the contrary, if the amount of W exceeds 0.07%, it is difficult to ensure rolling property. Therefore, the upper limit of W amount may be limited to 0.07%, and thus the addition amount of W is set to a range of 0.02 to 0.07%.

[29] The steel having the above-described composition may have a corrosion resistance index (CI) of 6.62 or higher. Furthermore, the steel may have a tensile strength of at least 80 kgf/mm , a yield ratio of at least 80%, and an elongation ratio of at least 10%.

[30] The steel having the above composition may be manufactured under following conditions.

[31] That is, the steel having the above chemical composition is reheated at 1,150 to 1,300

⁰C, and finish hot-rolled at 800 to 950 ⁰C. Then, the steel is cooled at a cooling rate of 20 ⁰C to 40 ⁰C per second. Subsequently, the steel is coiled at 500 ⁰C to 650 ⁰C and followed by cold-rolling. Thereafter, the steel is thermally treated at 550 ⁰C to an A transformation point, thereby manufacturing a high strength cold-rolled steel sheet with excellent weather resistance and impact resistance having a high tensile strength of at least 80kg/mm .

[32] The steel, when reheated to less than 1,150 ⁰C, 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 1,300 ⁰C, accelerates scale formation due to oxidization, resulting in significant decrease in thickness of a slab and coarsening grains. Also, in this case, much heat should be consumed to lead to a great economic loss, and thus the reheating temperature may be in a range of 1,150 to 1,300 ⁰C.

[33] In the case where the finish hot-rolling temperature is above 950 ⁰C, the steel is not hot-rolled uniformly across an entire thickness thereof. Thus, grains are not sufficiently refined so that the steel is degraded in impact toughness due to the coarse grains. On the contrary, in the case where the finish hot-rolling temperature is below 800 ⁰C, the steel is finish hot-rolled at a low temperature, thereby accelerating the admixture of grains thereof. This deteriorates corrosion resistance and workability, and therefore the finish hot-rolling temperature may be set to a range of 800 to 950 ⁰C.

[34] Meanwhile, in the case where the steel is cooled at a cooling rate of less than 20 ⁰C 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, which leads to a decrease in strength. Therefore, the cooling rate of the steel may be set to 20 ⁰C or less per second. On the contrary, the cooling rate exceeding 40 ⁰C per second leads to formation of a 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 40 ⁰C per second.

[35] A hot-rolling coiling temperature exceeding 650 ⁰C does not bring about sufficient precipitation effects, thereby making it difficult to stably ensure a desired strength of 80 kg/mm . Meanwhile, in the case where the coiling temperature is below 500 ⁰C, a hard phase is formed while the steel is cooled and maintained so that a roll force of a rolling mill sharply increases during the cold-rolling. This makes it difficult to roll the steel and thus the coiling temperature of the steel may be set to a range of 500 to 650 0C.

[36] 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 the case where the annealing temperature is lower than 550 ⁰C during the continuous annealing, deformed grains in the cold-rolling still remain, sharply decreasing ductility and thus deteriorating workability. In contrast, an annealing temperature exceeding an A transformation point results in formation of martensite because the steel is transformed when cooled after being annealed. This decreases yield strength of the steel to 60% or less, and thus the impact resistance of the steel becomes poorer. Therefore, the upper limit of the annealing temperature of the steel is set to the A transformation point.

[37] Steel ingots satisfying following Table 1 were reheated for 1 hour at 1,200 to 1,260 0C in a heating furnace, and then hot-rolled. A finish hot-rolling temperature was set to a range of 860 to 910 ⁰C. One group of steels were coiled at 560 ⁰C and the other group of steels were coiled at 620 ⁰C. A final target thickness of each steel sheet is set to 1.1 mm in consideration of user's available thickness range. Steels having compositions as listed in Table 1 were measured for a standardized corrosion resistance index (CI) and weather resistance. The results are shown in Table 2.

[38] For the weather resistance test, the steels underwent a salt spray test (SST) for 480 hours at a temperature of 30 ⁰C 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. The corrosion resistance index (CI) is derived primarily based on alloy elements and defined as following Equation.

[39] 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 [40] Table 1 [Table 1]

Comparison of Chemical Composition in Invention Steels and Comparative Steels

[41] [42] Table 2 [Table 2]

[Table ]

Evaluation of Corrosion in Invention Steels and Comparative Steels

[43] The results of salt spray test and corrosion resistance indices are shown in Table 2.

As shown in Table 2, Comparative steels 2, 3 and 4 are low in corrosion resistance index and their corrosion-induced weight losses were 0.030 g/cm or more in the salt spray test, and therefore they are not suitable for weather-resistant steels. In contrast, it can be observed that Inventive steels 1 and 2 and Comparative steel 1 exhibit excellent weather resistance characteristics in terms of corrosion-induced weight loss and corrosion resistance index.

[44] Cold-rolled steel sheets were manufactured under conditions listed in Table 3 below using Inventive steels 1 and 2, and Comparative steels 1 to 4 in Table 1, and then mechanical properties and workability were measured. The results are shown in Table 4 below.

[45] Table 3

[Table 3]

[Table ]

Manufacturing Conditions of Steel Sheets

[46]

[47] Table 4

[Table 4]

[Table ]

Material properties under manufacturing conditions

[48] 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 80 kgf/mm , a yield ratio of at least 80%, and a ductility of at least 10%, respectively. Further, they were not cracked during a bending process, thus making it possible to manufacture high strength cold-rolled steel sheets with excellent impact resistance and weather resistance.

[49] In contrast, 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 temperatures were higher than the annealing condition of the present invention attained desired tensile strength, but were reduced in a yield ratio to 70% or less. This is because the second phase is formed by transformation in a cooling process due to the high annealing temperature and resultantly yield strength is lowered. That is, Comparative Example 2 and Comparative Example 5 did not achieve a yield ratio of at least 80%, leading to a decrease in impact resistance. In Comparative Example 4 whose annealing temperature was lower than the annealing condition of the present invention, most deformed grains produced during cold-rolling are not recovered, thus not ensuring desired workability. Moreover, Comparative Example 1 whose finish hot-rolling temperature and coiling temperature fall out of the ranges of the present invention and Comparative Example 3 whose cooling rate is higher than the cooling condition of the present invention demonstrated a ductility of less than 5%, thus not achieving appropriate workability.

[50] In the case where Comparative steel 1 of which W and Mn compositional ranges fell out of the requirement of the present invention but which had relatively excellent weather resistance was manufactured under the conditions of the present invention (Comparative example 7), the Comparative steel 1 did not achieve ductility and workability. Furthermore, Comparative example 6, when increased in the annealing temperature to ensure ductility and workability, was reduced in yield strength due to the creation of dual phase such as martensite, thus making it difficult to attain a yield ratio of 80% or higher.

[51] Also, even in the case where Comparative steels 2 and 3 whose chemical compositional ranges deviated 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), it was difficult to achieve target ranges of workability and impact resistance that the present invention can provide, as shown in Table 4.

[52] While the present invention has been shown and described in connection with the exemplary embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

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

0.08 to 0.20% carbon (C), 0.1 to 0.5% silicon (Si), 1.2 to 2.0% manganese (Mn), <0.03% phosphorous (P), <0.01% sulfur (S), 0.02 to 0.07% aluminum (Al), 0.02 to 0.06% vanadium (V), 0.2 to 0.5% copper (Cu), 0.3 to 0.8% chromium (Cr), 0.04 to 0.08% zirconium (Zr), 0.02 to 0.08% cobalt (Co), 0.02 to 0.07% tungsten (W), the balance being iron (Fe) and unavoidable impurities.

[2] The high strength cold-rolled steel sheet of claim 1, wherein the cold-rolled sheet has a corrosion resistance index (CI) of 6.62 or higher, where the corrosion resistance index (CI) is defined as following Equation: 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.

[3] The high strength cold-rolled steel sheet of claim 1, wherein the cold-rolled steel sheet has a tensile strength of at least 80 kgf/mm , a yield ratio of at least 80%, and an elongation ratio of at least 10%.

[4] A method of manufacturing a high strength cold-rolled steel sheet with excellent impact resistance and weather resistance, the method comprising: reheating a steel to 1,150 to 1,300 ⁰C, the steel comprising, by weight, 0.08 to 0.20% C, 0.1 to 0.5% Si, 1.2 to 2.0% Mn, <0.03% P, <0.01% S, 0.02 to 0.07% Al, 0.02 to 0.06% V, 0.2 to 0.5% Cu, 0.3 to 0.8% Cr, 0.04 to 0.08% Zr, 0.02 to 0.08% Co, 0.02 to 0.07% W, the balance being Fe and unavoidable impurities; finish-rolling the reheated steel at a temperature of 800 ⁰C to 950 ⁰C; cooling the finish-rolled steel at a cooling rate of 20 to 40 ⁰C per second; coiling the steel at 500 to 650 ⁰C, and cold-rolling the coiled steel; and continuously annealing the cold-rolled steal at an annealing temperature ranging from 550 ⁰C to an Al transformation point.