KR20090068625A

KR20090068625A - High strength cold-rolled steel sheet having excellent weather resistance and method manufacturing the same

Info

Publication number: KR20090068625A
Application number: KR1020070136313A
Authority: KR
Inventors: 김재익; 정기조
Original assignee: 주식회사 포스코
Priority date: 2007-12-24
Filing date: 2007-12-24
Publication date: 2009-06-29
Also published as: WO2009082149A3; CN101910436B; WO2009082149A2; CN101910436A; KR100925639B1

Abstract

A high strength cold-rolled steel sheet and a manufacturing method thereof are provided to secure high weatherability and high mechanical properties via proper control of components and processes, and realize a high yield ratio. A high strength cold-rolled steel sheet consists of carbon 0.08~0.20wt%, silicon 0.1~0.5wt%, manganese 1.2~2.0wt%, phosphorus 0.03wt% or less, sulfur 0.01wt% or less, aluminum 0.02~0.07wt%, vanadium 0.02~0.06wt%, copper 0.2~0.5wt%, chrome 0.3~0.8wt%, zirconium 0.04~0.08wt%, cobalt 0.02~0.08wt%, tungsten 0.02~0.07wt%, and remnant Fe and other inevitable impurities. The steel sheet is re-heated at 1150~1300°C, and rolled for finishing at 800~950°C. The rolled steel sheet is cooled at 20~40°C per second, and wound at 500~650°C to be cold-rolled. The result is subject to heat treatment at annealing temperature after continuous annealing at 550°C~the transition temperature of aluminum.

Description

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

The present invention relates to a high strength cold rolled steel sheet excellent in impact resistance used in railroad cars, containers, and the like, and to a method for manufacturing the same. The present invention relates to a cold rolled steel sheet having high strength and weather resistance by adjusting steel components and manufacturing conditions.

Conventionally, materials such as stainless steel or aluminum have been used for the purpose of reducing the weight and extending the service life of railroad cars or containers. Characteristics required for such products include bending workability, weldability and durability. In addition, in the case of the transport structure, it is often necessary to suppress the deformation due to the impact of the cargo loading and loading, it is recommended to apply a material having an impact resistance. Impact resistance of the processing member is a factor showing a close relationship with the thickness and yield ratio of the material.

Yield ratio is defined as the ratio of the tensile strength to the yield strength of the material values obtained through the tensile test, and the higher yield ratio at the same tensile strength level means the higher yield strength of the material. In other words, the steel having a high yield ratio increases the ability to suppress the deformation by increasing the resistance to deformation even when subjected to high impact strength, which is a material property of the elastic region. When used in impact-resistant applications such as containers, it is desirable to secure a yield ratio of 80% or more.In addition, when applied to applications such as containers, which must withstand various climatic conditions in the sea or on land, depending on the transport conditions, steel having excellent weather resistance Use is required.

For example, SPA-C (Industrial Standards KS-D3542 and JIS-G3125) materials, which are conventionally weathered rolled steels, have been used. However, these steels have a high tensile strength of 50 kg / mm ² and have a container size of about 50 feet. In the case of manufacturing, the demand for weight reduction of materials is increasing due to the limitation of transportation cost due to the increase of the weight of the container itself. High strength cold rolled steel with a tensile strength of 60 to 80 kg / mm ² is used for structural members of automobiles, but there is a problem in that these materials also have poor weather resistance because it is a challenge to secure strength characteristics.

Recently, in the container industry, in order to cope with cost reduction and environmental problems, attempts have been made to reduce the weight of containers and to produce larger containers, thereby greatly increasing the efficiency of transportation. To this end, in addition to securing weather resistance in terms of materials, the demand for development of high strength steel sheets of 80 kg / mm ² or more is achieved to achieve light weight and tensile strength.

For example, in Japanese Patent Laid-Open No. 7-207408, C 0.008% or less, Si 0.5 to 2.5%, Mn 0.1 to 3.5%, P 0.03 to 0.20%, S 0.01% or less, Cu 0.05 to 2.0%, Al 0.005 to 0.1% And N 0.008% or less, Cr 0.05-6.0%, Ni 0.05-2.0% and Mo 0.05-3.0%, B 0.0003-0.002% by heating at 1100-1300 ° C. and rolling at 800-950 ° C. to finish 400 A method for producing a hot rolled steel strip, characterized by winding at ˜700 ° C., has been proposed. However, only a few embodiments of this technology satisfy the tensile strength of 60 ~ 70kg / mm ² grade, and in most cases the tensile strength is 50kg / mm ² grade, which can not be obtained 80kg / mm ² tensile strength Appeared. In addition, there is a problem in that weldability is deteriorated and manufacturing cost is increased by adding a large amount of hardenability improving elements such as Cr and Mo among the component components.

As another example, Japanese Patent Laid-Open No. 11-21622 discloses C 0.15% or less, Si 0.7% or less, Mn 0.2-1.5%, P 0.03-0.15%, S 0.02% or less, Cu 0.4% or less, Al 0.01-0.1% And Cr: 0.1% or less, 0.4 to 4.0% of Ni and 0.1 to 1.5% of Mo is heated to 1050 to 1300 ° C to perform hot rolling at 40% or more at 950 ° C or more, and then finish rolling at 900 to 750 ° C We proposed a method for the implementation. However, even at this time, most of the tensile strength is 50kg / mm ² grade, only a few showed the tensile characteristics of 60kg / mm ² grade, this also can be said to be mainly applied to 50kg / mm grade ² tensile strength steel. In addition, although it mentions the effect of improving the corrosion resistance in the seawater atmosphere by adding 0.03 ~ 0.15% P, the addition of a large amount of P causes a central segregation of the cold rolled material, such as a problem that sharply degrades the workability of the steel sheet, etc. In the present invention, there is a problem in that it does not provide a method of obtaining a tensile strength of 80 kg / mm ² or more at the same time excellent in impact resistance and weather resistance.

The present invention by controlling the addition amount of cobalt, vanadium, zirconium, tungsten, etc. among the steel components in order to solve the above problems, by optimizing the rolling and annealing conditions, such as impact resistance and weather resistance as well as tensile strength 80kg / It is an object of the present invention to provide a cold rolled steel sheet having a high strength characteristic of mm ² or more and a method of manufacturing the same.

In order to achieve the above object, the present invention is a weight%, carbon (C) 0.08-0.20%, silicon (Si) 0.1-0.5%, manganese (Mn) 1.2-2.0%, phosphorus (P) 0.03% or less, sulfur ( S) 0.01% or less, aluminum (Al) 0.02-0.07%, vanadium (V) 0.02-0.06%, copper (Cu) 0.2-0.5%, chromium (Cr) 0.3-0.8%, zirconium (Zr) 0.04-0.08% Cobalt (Co) 0.02 to 0.08%, tungsten (W) 0.02 to 0.07%, high strength cold rolled steel sheet having excellent impact resistance and weather resistance containing residual Fe and other unavoidable impurities, and the steel of the above composition to 1150 ~ 1300 ℃ Reheating, finishing rolling the reheated steel at 800 to 950 ° C, cooling the finish rolled steel at a cooling rate of 20 to 40 ° C per second, and a temperature of 500 to 650 ° C after the cooling Winding and cold rolling in the step, and after the cold rolling impact resistance and weather resistance comprising the step of continuous annealing and heat treatment at the annealing temperature of less than 550 ℃ ~ A1 transformation point It provides a method for producing excellent high strength cold rolled steel sheet.

According to the present invention, in the production of high strength cold rolled steel sheet having excellent weather resistance, such as materials for outdoor use requiring impact resistance, as well as securing weather resistance and mechanical properties at the same time and high yield ratio through proper component and process control. Steel sheet with high added value can be produced. In addition, as the annealing operation is performed in a relatively low temperature region, energy saving and annealing workability improvement effects may be simultaneously obtained.

Hereinafter, the present invention will be described in detail.

The inventors of the present invention have completed the present invention by repeating research and experiment on high tensile strength steel, which is useful for containers and railway vehicles, in addition to satisfying impact resistance and weather resistance along with various processing characteristics, that is, bendability, weldability, and stretchability. The steel component is preferably composed as follows.

First, among the steel components, C is an element added to improve the strength of the steel sheet, but the tensile and yield strengths increase as the amount added increases. However, when the excessive amount is added, the upper limit is set to 0.20% because the workability of the material decreases. On the other hand, when C amount is 0.08% or less, there exists a problem that sufficient precipitation strengthening effect cannot be acquired. Therefore, the content of C is preferably set to 0.08 to 0.20%.

Si not only provides molten steel deoxidation and solid solution strengthening effect, but also forms a dense oxide of Fe ₂ SiO ₄ together with Fe in the surface layer of the steel at high temperature to improve corrosion resistance. Addition is required. Therefore, Si should be added to improve weather resistance, but if excessively added, there is a problem in that the weldability is lowered and the plating property is degraded, so it is preferable to add 0.5% or less, so the Si range is limited to 0.1 to 0.5%.

Mn is an element that is effective for strengthening by solid solution and is an important element for increasing the strength of steel and improving hot workability, but is an element that inhibits the ductility and workability of a material due to MnS formation. If the Mn content is low, the workability is improved but it is difficult to secure the strength, so 0.5% or more should be added to secure the target strength. On the other hand, when Mn is added excessively, the upper limit is set at 2.0% because there is a problem of deterioration in economics and weldability due to the addition of a large amount of alloying elements.

Since P plays a role in improving the corrosion resistance of steel, it is preferable to add a large amount in terms of corrosion resistance. However, since P is an element causing central segregation during casting, the content is limited to 0.03% or less since it causes a decrease in weldability and toughness. It is desirable to.

Although S is known to be an effective element for improving corrosion resistance, it is preferable to reduce the content as much as possible because it combines with Mn in steel to form a non-metallic inclusion serving as a corrosion start point. Therefore, the amount of S is limited to 0.01% or less, but preferably controlled to 0.005% or less.

Al is generally an element which is effective in improving deoxidation and corrosion resistance of molten steel, but when it is added excessively, there is a problem of decreasing workability by increasing the amount of inclusions in the steel, so the content is preferably set at 0.02 to 0.07%.

V is an element exhibiting the effect of delaying the recrystallization of ferrite as well as the effect of increasing the strength of the steel sheet by being precipitated in combination with C, N _2, etc. in steel, and addition of 0.02% or more is required to secure the target strength. On the other hand, when V is added at 0.06% or more, the manufacturing cost is increased and the hot rolling workability is deteriorated, so the addition range is limited to 0.02 to 0.06%.

Cu is an element that improves corrosion resistance by forming a stable rust layer in a corrosive atmosphere, and more than 0.2% of Cu is required to secure a target corrosion resistance. However, at 0.5% or more, there is a problem that not only causes grain boundary cracking during playing but also roughens the surface state of the hot rolled steel sheet. Therefore, Cu management range was 0.2 to 0.5%.

Cr is an element that plays a role of forming a stable rust layer, such as Cu, it is required to add more than 0.3% to secure corrosion resistance and obtain strength. In addition, if the addition amount is more than 0.8%, rather than causing the hole corrosiveness, but also increase the manufacturing cost rapidly, it is preferable to add in the range of 0.3 ~ 0.8%.

Zr is an element that retards the recrystallization of ferrite phase and requires addition of 0.04% or more to obtain a target strength level in the low temperature region. On the other hand, when Zr is added in an amount of 0.08% or more, the upper limit thereof is limited to 0.08% because there is a problem in that rolling property cannot be secured.

Co is an element that promotes the formation of a surface layer corrosion inhibiting product by reacting with Cu and Cr, which are added to secure corrosion resistance in steel, and at least 0.02% or more is required to obtain such an effect. However, when Co is added in an amount of 0.08% or more, the range is limited to 0.02% to 0.08% because the Co acts as an increase factor of the manufacturing cost rather than the effect of improving the corrosion resistance.

On the other hand, W is an element added to secure the hardenability and strength characteristics, it is necessary to add 0.02% or more in order to obtain the target strength level in the low temperature region. On the other hand, when W is added at 0.07% or more, there is a problem that the rollability cannot be secured. Therefore, the upper limit thereof is preferably limited to 0.07%, so the addition range is 0.02 to 0.07%.

The steel formed as described above is preferably manufactured under the following conditions.

In other words, the steel with the above chemical composition is reheated at 1150 ~ 1300 ℃, finish hot rolling at 800 ~ 950 ℃, cooled at a cooling rate of 20 ~ 40 ℃ / sec and cold rolled after winding to 500 ~ 650 ℃ Tensile strength of 80kg / mm ² with excellent weather resistance and impact resistance by heat treatment in the temperature range below 550 ~ A ₁ transformation point It relates to a method of manufacturing the above high strength cold rolled steel sheet.

If the reheating temperature is 1150 DEG C or lower, the breakage of the solidified structure formed during casting is insufficient, so that the center segregation is well developed, resulting in mixing of the finally formed crystal grains, which significantly reduces the workability and impact toughness. In addition, if the reheating temperature exceeds 1300 ℃, the formation of scale by oxidization is promoted, and the thickness reduction of slab is large and grain coarsening occurs when reheating.The economical loss due to the increase of the heating unit is large, so the management range is 1150. It was limited to -1300 degreeC.

If the finish hot rolling temperature is higher than 950 ° C, uniform hot rolling is not performed throughout the thickness, resulting in insufficient grain refinement, resulting in a drop in impact toughness due to grain coarsening. On the contrary, when the hot rolling is finished at a temperature of less than 800 ° C. as the hot rolling is finished in the low temperature region, it is preferable to limit the finish hot rolling temperature to 800 ° C. to 950 ° C. as the hybridization of crystal grains proceeds rapidly, resulting in deterioration of corrosion resistance and workability.

On the other hand, if the cooling rate in the run-out-table (ROT) after finishing hot rolling is less than 20 ° C / sec, coarse grains are formed due to the promotion of grain growth, resulting in a decrease in strength. The lower limit was limited to 20 ° C / sec. On the other hand, if the cooling rate is 40 ℃ / sec or more to form a hard second phase, such as bainite to significantly reduce the cold rolling property, the cooling rate was set to 20 ~ 40 ℃ / second.

In addition, when the hot rolled winding temperature is 650 ° C. or more, the sufficient precipitation effect is not obtained, and thus the material strength decreases, making it difficult to secure a stable target strength of 80 kg / mm ² . On the other hand, at the winding temperature below 500 ℃, the hard phase is generated during cooling and holding, and the roll force of the rolling mill is rapidly increased in the cold rolling process, and thus rolling property cannot be secured. It was limited to -650 degreeC.

The hot rolled material is subjected to rolling under normal cold rolling conditions and subjected to a continuous annealing process. At this time, it is necessary to properly manage the annealing temperature in order to secure the target material properties. When the annealing temperature is lower than 550 ° C. in the continuous annealing process, the deformation grains during cold rolling remain as it is, and thus the ductility drops sharply. On the other hand, at the annealing temperature above the A ₁ transformation point, the hard martensite phase is formed by transformation during cooling after annealing, and the yield strength is lowered, and the impact resistance is lowered as the yield ratio is lowered to 60% or less, so the upper limit of the annealing temperature is A ₁ side. It was made into a thick point.

The steel ingots prepared by melting to satisfy the composition shown in Table 1 were reheated for 1 hour in a 1200 to 1260 ° C. heating furnace, followed by hot rolling. The hot rolling finish temperature was set at 860 ~ 910 ℃ and the coiling temperature was divided into two conditions of 560 ℃ and 620 ℃, and the final thickness was 1.1mm considering the thickness of customers. The evaluation results of the standardized corrosion resistance index (CI) value and the weather resistance test are shown in [Table 2].

The weather resistance test is the result of a salt spray test (SST) for 480 hours in a 5% salt solution (NaCl solution) at 30 ℃. Here, the corrosion resistance index (CI) is a weather-related evaluation index specified in ASTM G101, and the higher this value, the better the weather resistance of the steel, which is mainly calculated based on alloying elements.

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 .

[Table 1] Comparison of Chemical Compositions of Invented Steels and Comparative Steels

Steel grade Chemical composition (% by weight) C Si Mn P S Al Zr Co V Cu Cr W Inventive Steel 1 0.12 0.31 1.45 0.015 0.002 0.035 0.048 0.04 0.03 0.41 0.71 0.04 Inventive Steel 2 0.14 0.42 1.36 0.019 0.003 0.032 0.053 0.06 0.05 0.35 0.64 0.03 Comparative Steel 1 0.05 0.25 2.54 0.010 0.006 0.033 0.024 0.04 0.03 0.35 0.86 -0.0030 Comparative Steel 2 0.13 0.65 1.44 0.094 0.005 0.042 - - 0.12 0.75 1.05 0.03 Comparative Steel 3 0.12 0.05 0.56 0.064 0.004 0.034 0.044 - - 0.36 - - Comparative Steel 4 0.13 0.21 1.34 0.009 0.024 0.051 - - 0.05 0.31 0.76 -

[Table 2] Corrosion evaluation of invention steels and comparative steels

River bell Corrosion Resistance Index (CI) Corrosion degree (weight loss, gr / cm ² ) Weatherability Assessment Inventive Steel 1 6.624341 0.0275 Good Inventive Steel 2 6.735345 0.0268 Good Comparative Steel 1 6.520325 0.0289 Good Comparative Steel 2 4.578445 0.0841 Defective Comparative Steel 3 6.216676 0.0481 Defective Comparative Steel 4 6.234741 0.0428 Defective

The results of examining the salt spray test and the corrosion resistance index are shown in [Table 2]. As shown in [Table 2], <Comparative Steel 2>, <Comparative Steel 3>, and <Comparative Steel 4> have a low corrosion resistance index and a large corrosion loss due to salt spray test of 0.030 g / cm ² or more. It was difficult to apply in terms of weather resistance, whereas the inventions of <Inventive Steel 1>, <Inventive Steel 2>, and <Comparative Steel 1> showed excellent weather resistance in terms of corrosion loss and corrosion resistance index.

[Table 1] using the invention steel (1), (2) and comparative steel (1) ~ (4), by working under the same conditions as [Table 3] to produce a cold rolled steel sheet mechanical for each material The results of evaluating the properties and the processing characteristics are shown in [Table 4].

[Table 3] Manufacturing Conditions of Steel Sheet

division Used steel grade Reheating Temperature (℃) Hot rolled finish temperature (℃) Cooling rate (℃ / s) Winding temperature (℃) Annealing Temperature (℃) Invention 1 Inventive Steel 1 1220 870 25 620 630 Invention 2 1260 910 25 620 660 Invention 3 Inventive Steel 2 1200 860 30 560 570 Invention 4 1250 880 30 560 650 Comparative Material 1 Inventive Steel 1 1080 700 25 450 650 Comparative Material 2 1220 870 25 620 800 Comparative Material 3 1220 870 60 500 650 Comparative Material 4 Inventive Steel 2 1250 880 25 620 450 Comparative Material 5 1250 880 25 620 780 Comparative Material 6 Comparative Steel 1 1250 940 30 560 780 Comparative Material7 1250 920 25 560 650 Comparative Material 8 Comparative Steel 2 1230 910 30 620 650 Comparative Material 9 Comparative Steel 3 1190 860 25 600 650 Comparative Material 10 Comparative Steel 4 1220 870 20 620 650 Comparative Material 11 1220 890 25 620 850

[Table 4] Material Characteristics by Manufacturing Conditions

division Yield strength (kgf / mm ² ) Tensile Strength (kgf / mm ² ) Elongation (%) Yield Ratio (%) Impact resistance Bandability Machinability Invention 1 82.7 89.6 14 92.3 Good Good Invention 2 81.9 88.7 15 92.3 Good Good Invention 3 84.9 95.4 12 89.0 Good Good Invention 4 80.6 93.2 13 86.5 Good Good Comparative Material 1 96.4 105.5 2 91.4 Good Crack occurrence Comparative Material 2 41.4 79.7 18 51.9 Bad Good Comparative Material 3 89.9 99.7 3 90.2 Good Crack occurrence Comparative Material 4 93.2 101.4 2 91.9 Good Crack occurrence Comparative Material 5 52.6 88.3 10 59.6 Bad Crack occurrence Comparative Material 6 47.3 79.4 19 59.6 Bad Good Comparative Material7 96.1 105.2 3 91.4 Good Crack occurrence Comparative Material 8 80.1 84.4 3 94.9 Good Crack occurrence Comparative Material 9 78.1 83.2 3 93.9 Good Crack occurrence Comparative Material 10 85.2 88.1 2 96.7 Good Crack occurrence Comparative Material 11 39.7 70.4 21 56.4 Bad Good

As shown in Table 4, in the case of the inventive materials (1) to (4) in which the chemical composition and the manufacturing conditions satisfy the scope of the present invention, the tensile strength is 80 kgf / mm ² As mentioned above, the yield ratio of 80% or more and the ductility of 10% or more were secured, and no cracking occurred during the bending process, thereby making it possible to manufacture a weather resistant cold rolled steel sheet having excellent impact resistance with high strength.

On the other hand, the comparative materials (1) to (5), which satisfy the chemical composition range of the inventive steel but whose manufacturing conditions are out of the range of the invention method, failed to obtain the desired properties. That is, for <Comparative Material 2> and <Comparative Material 5> where the annealing temperature is higher than the present method, the tensile strength criterion is satisfied, but the yield strength is increased by the second phase generated by transformation in the cooling step as the annealing temperature is high. The yield ratio was lowered to 70% or less. That is, as the yield ratio of more than 80% of the target can not be obtained, the impact resistance was inferior. <Comparative Material 4> whose annealing temperature was lower than the range of the invention method could not secure the target workability because no recovery of the deformed grains produced during cold rolling occurred. In addition, even in the case of <Comparative Material 1> where the hot rolling finish temperature and the coiling temperature were out of the range of the present invention and <Comparative Material 3> whose cooling rate was higher than the invention range, the ductility was less than 5%, so that proper workability was not obtained.

Although W and Mn deviated from the composition of the present invention, when the manufacturing conditions were made in the present invention with respect to the <Comparative Steel 1>, which had relatively excellent weather resistance (Comparative Material 7), it was difficult to secure ductility and workability. In addition, when the annealing temperature was increased to secure the ductility and workability (Comparative Material 6), the yield strength was lowered due to the formation of a composite structure such as martensite.

In the case where the chemical composition was evaluated after changing the manufacturing conditions for the comparative steels (2) to (3) in which the weather resistance could not be secured beyond the composition range of the inventive steel (Comparative Materials 8 to 11), [Table 4] As can be seen in the present invention could not set the target range of workability and impact resistance to be secured.

Claims

By weight%, carbon (C) 0.08 to 0.20%, silicon (Si) 0.1 to 0.5%, manganese (Mn) 1.2 to 2.0%, phosphorus (P) 0.03% or less, sulfur (S) 0.01% or less, aluminum (Al ) 0.02 to 0.07%, Vanadium (V) 0.02 to 0.06%, Copper (Cu) 0.2 to 0.5%, Chromium (Cr) 0.3 to 0.8%, Zirconium (Zr) 0.04 to 0.08%, Cobalt (Co) 0.02 to 0.08% , High strength cold rolled steel with excellent impact resistance and weather resistance, containing 0.02 ~ 0.07% of tungsten (W), balance Fe and other unavoidable impurities.

By weight%, carbon (C) 0.08 to 0.20%, silicon (Si) 0.1 to 0.5%, manganese (Mn) 1.2 to 2.0%, phosphorus (P) 0.03% or less, sulfur (S) 0.01% or less, aluminum (Al ) 0.02 to 0.07%, Vanadium (V) 0.02 to 0.06%, Copper (Cu) 0.2 to 0.5%, Chromium (Cr) 0.3 to 0.8%, Zirconium (Zr) 0.04 to 0.08%, Cobalt (Co) 0.02 to 0.08% Reheating the steel containing 0.02% to 0.07% tungsten (W), balance Fe and other unavoidable impurities to 1150-1300 ° C;

Finishing rolling the reheated steel at 800 ° C. to 950 ° C .;

Cooling the finish-rolled steel at a cooling rate of 20 to 40 ° C. per second;

Winding and cold rolling at a temperature of 500 to 650 ° C. after the cooling;

After the cold rolling step of continuous annealing at an annealing temperature of 550 ℃ ~ A1 transformation point or less heat treatment

Method for producing a high strength cold rolled steel sheet excellent impact resistance and weather resistance comprising a.