KR101087420B1 - Hot-rolled steel having high strength, and method for producing the same - Google Patents

Abstract

The present invention relates to a high strength hot rolled steel sheet and a method of manufacturing the same. 0.035 ~ 0.050wt% of carbon (C), 0.01 ~ 0.1wt% of silicon (Si), 0.15 ~ 0.30wt% of manganese (Mn), 0.005 ~ 0.035wt% of phosphorus (P), 0.05wt% of sulfur (S) Hereinafter, it has an alloy composition of 0.01 to 0.10 wt% of aluminum (Al), 0.2 to 0.3 wt% of copper (Cu), 0.35 to 1.0 wt% of nickel (Ni), and the remaining iron (Fe). According to the present invention, it is possible to manufacture a hot rolled steel sheet having improved corrosion resistance and tensile strength by using copper contained in the scrap, thereby reducing the manufacturing cost and having the advantage of being lightweight due to high strength.

Hot rolled steel sheet, corrosion resistance, strength improvement

Description

High strength hot rolled steel sheet and its manufacturing method {Hot-rolled steel having high strength, and method for producing the same}

The present invention relates to a high strength hot rolled steel sheet and a method for manufacturing the same, and more particularly, to a high strength hot rolled steel sheet for a 340MPa class automotive and improved corrosion resistance and tensile strength.

Recently, in the automotive industry, demand for hot rolled steel sheets having high strength and high self-weight reduction effect is increasing for lightening automobiles and satisfying consumer demands. Accordingly, various studies are being conducted to manufacture high strength hot rolled steel sheet according to the weight reduction of automobiles.

In general, the cold rolled steel sheet is mainly used in the part requiring the manufacture of a complex shape in the automobile, and the hot rolled steel sheet is mainly used in the structural member such as its reinforcement, wheel, chassis.

In addition, the 340MPa-grade automotive hot rolled steel sheet, which has been widely used recently, has a copper (Cu) content of 0.1 wt% or less to prevent high temperature cracking, and manganese (Mn), silicon (Si), phosphorus ( It is manufactured by adding alloying elements such as P).

However, copper (Cu) is an impurity element that is inevitably included in an electric furnace process, and thus, high-grade scrap is used to control the content thereof, thereby causing a problem in that steelmaking costs increase.

In addition, as the surface quality of the vehicle becomes an essential item, the chassis parts which are not exposed to the outside of the vehicle are required to be corrosion-resistant, and thus the situation is also required to be improved.

The present invention is to solve the conventional problems as described above, an object of the present invention is to use a high strength hot rolled steel sheet to improve the corrosion resistance and tensile strength by using copper (Cu), which is inevitably included in the electric furnace process And to provide a method for producing the same.

According to a feature of the present invention for achieving the above object, the present invention is carbon (C) 0.035 ~ 0.050wt%, silicon (Si) 0.01 ~ 0.1wt%, manganese (Mn) 0.15 ~ 0.30wt%, phosphorus (P ) 0.005 ~ 0.035wt%, sulfur (S) 0.05wt% or less, aluminum (Al) 0.01 ~ 0.10wt%, copper (Cu) 0.2 ~ 0.3wt%, nickel (Ni) 0.35 ~ 1.0wt% Addition iron (Fe) and other unavoidable impurities.

The content ratio of nickel (Ni) and copper (Cu) is in the range of Ni≥1.45Cu.

0.035 ~ 0.050wt% of carbon (C), 0.01 ~ 0.1wt% of silicon (Si), 0.15 ~ 0.30wt% of manganese (Mn), 0.005 ~ 0.035wt% of phosphorus (P), 0.05wt% or less of sulfur (S), aluminum (Al) 0.01 ~ 0.10wt%, copper (Cu) 0.2 ~ 0.3wt%, nickel (Ni) 0.35 ~ 1.0wt% and alloy composition of the remaining iron (Fe), the content of nickel (Ni) and copper (Cu) A steel slab having a ratio of Ni≥1.45Cu in the condition range is homogenized at 1150 to 1250 ° C, hot finished rolling at 800 to 900 ° C, and wound at a temperature range of 560 to 650 ° C.

The present invention increases the content of copper and phosphorus, which is an alloying element that improves corrosion resistance, and prevents high temperature cracks that may be caused by an increase in the content of copper and phosphorus by adding nickel and silicon to produce hot rolled steel sheets having improved corrosion resistance and tensile strength.

Therefore, it is possible to reduce the weight due to the improved strength, and can be used stably even in a corrosive environment due to improved corrosion resistance.

In particular, the present invention improves the corrosion resistance by using the copper contained in the scrap, so it is not necessary to use high-quality scrap or to reduce the amount of use can be produced at a low cost compared to the equivalent hot-rolled steel sheet.

Therefore, a hot rolled steel sheet having improved corrosion resistance and tensile strength can be manufactured even at a relatively low cost, thereby reducing the manufacturing cost.

Hereinafter, a preferred embodiment of the high strength hot rolled steel sheet according to the present invention and a manufacturing method thereof will be described in detail.

0.035 ~ 0.050wt% of carbon (C), 0.01 ~ 0.1wt% of silicon (Si), 0.15 ~ 0.30wt% of manganese (Mn), 0.005 ~ 0.035wt% of phosphorus (P), 0.05wt% of sulfur (S) Hereinafter, it contains 0.01 to 0.10 wt% of aluminum (Al), 0.2 to 0.3 wt% of copper (Cu), and 0.35 to 1.0 wt% of nickel (Ni), and has an alloy composition composed of residual iron (Fe) and other unavoidable impurities.

The steel slab having the alloy composition is reheated and subjected to hot finish rolling to produce a hot rolled steel sheet.

The present invention ensures corrosion resistance by increasing the content of copper (Cu) and phosphorus (P), which are inevitably included in an electric furnace process, and may be generated during hot rolling by copper (Cu) and phosphorus (P). Surface cracks are prevented through the addition of nickel (Ni) and silicon (Si).

Copper (Cu) is an element having an effect of improving corrosion resistance. However, when the copper content is high, there is a problem that a surface defect occurs because a copper concentrate phase is formed at an iron scale interface by selective oxidation of iron during reheating of the cast solidified steel slab. To solve this problem, nickel (Ni) and silicon (Si) are added.

Nickel (Ni) increases the solubility of copper in austenite and increases the absorption of copper in the hot rolled scale. In addition, the melting point of the Cu-rich phase is increased to prevent high temperature cracking due to liquid copper.

Silicon (Si) improves the absorption of copper in the hot rolled scale and increases the tension required for liquid to penetrate the grain boundaries to prevent hot cracking by copper.

The content ratio of nickel (Ni) and copper (Cu) is in the range of Ni≥1.45Cu. In other words, when nickel is added at 1.45 times or more of copper, the occurrence of high temperature cracks by copper is significantly reduced.

The function and content of the alloying elements of the present invention are as follows.

Carbon (C): 0.035 ~ 0.050wt%

Carbon aims to improve strength. If the carbon content is less than 0.035wt%, it is difficult to secure the strength. Therefore, the solid solution strengthening element should be added.The manufacturing cost increases, and if it is added more than 0.050wt%, the weldability decreases and the ductility and stretch-flangeness of the strength increase. Is lowered.

Silicon (Si) 0.01 ~ 0.1wt%

Silicon is a solid solution element that contributes to the cleansing of steels. When silicon is added to steel to which the appropriate manganese is added, it improves the flowability of molten metal during welding, thereby reducing the inclusions in the weld as much as possible and improving the strength without compromising yield ratio and balance of strength and ductility.

This silicon improves the absorption of copper in the hot rolled scale and increases the tension required for liquid to penetrate the grain boundaries to prevent hot cracking by copper.

Silicon does not have an effect when the content is added less than 0.01wt%, there is a problem that the surface defects due to the plating property and red scale when added in excess of 0.1wt%.

Manganese (Mn) 0.15 ~ 0.30wt%

Manganese (Mn) secures strength through strengthening employment and improving hardenability.

If the amount of manganese is less than 0.15wt%, the effect of increasing strength is small, and if it is added in excess of 0.30wt%, the grain boundary segregation of manganese rapidly increases during slab casting, which impairs the workability and weldability of steel.

Therefore, manganese (Mn) is contained in the range of 0.15 ~ 0.30wt% without defects and weldability is not reduced.

Phosphorus (P) 0.005 ~ 0.035wt%

Phosphorus is an element that has a high solid solution effect and a low elongation. Phosphorus lowers the grain boundary energy due to segregation of the austenite grain boundaries in steel at high temperatures to mitigate grain embrittlement by the copper thickening phase.

If the phosphorus content is less than 0.005wt%, the effect of improving the corrosion resistance is insufficient, and if it exceeds 0.035wt%, it is segregated at the grain boundary to generate secondary processing brittleness. Therefore, phosphorus is contained in the range of 0.005 ~ 0.035wt% which does not generate secondary processing brittleness while improving the corrosion resistance.

Sulfur (S) 0.05 wt% or less

Sulfur (S) inhibits toughness and weldability, increases MnS non-metallic inclusions, causes cracks during processing of steel, and increases coarse inclusions when excessively added to deteriorate fatigue characteristics, thus limiting its upper limit to 0.05 wt% or less.

Aluminum (Al) 0.01 ~ 0.10wt%

Aluminum is added as a deoxidizer to remove oxygen in the steel. When aluminum (Al) is added in small amounts, residual oxygen in steel combines with manganese (Mn) and silicon (Si) to form oxides during steelmaking, making it difficult to control components, lowering hardenability, and oxides are present as nonmetallic inclusions. This reduces the mechanical and fatigue properties of the steel.

If the aluminum is added below 0.01wt%, the effect is insignificant, and when it exceeds 0.10wt%, the workability is inhibited. Therefore, aluminum is contained in the range of 0.01 ~ 0.10wt%, which contributes to the deoxidation effect but does not inhibit the workability of the steel.

Copper (Cu) 0.2 ~ 0.3wt%

Copper induces finer densification of scale layer crystal grains, thereby improving corrosion resistance of the steel. If the copper content is less than 0.2wt%, it is difficult to expect the effect of improving the corrosion resistance.If the copper content is more than 0.3wt%, the low melting point copper penetrates into the grain boundary when the slab is reheated for rolling. May occur.

Therefore, copper (Cu) is contained in the range of 0.2 to 0.3 wt% while improving corrosion resistance and preventing defects or brittleness of the hot rolling process.

Nickel (Ni) 0.35 ~ 1.0wt%

Nickel is added as an element for preventing the red brittleness generated when copper is added for increasing strength and improving corrosion resistance.

Usually, copper (Cu): nickel (Ni) = 1: 1 to 2 is the best effect when added. When it is difficult to secure quality such as redness and brittleness by adjusting process variables when adding copper (Cu), it is added within the range of 0.35 ~ 1.0 wt% according to the content of copper (Cu).

The present invention contains the above-described component system, and the rest are substantially iron (Fe) and unavoidable elements, and fine amounts of unavoidable impurities are also allowed as elements contained according to the situation of raw materials, materials, manufacturing facilities, and the like.

Hereinafter, a high strength hot rolled steel sheet having an alloy composition according to the present invention and a manufacturing method thereof will be described in detail with reference to Examples.

-Manufacturing method of hot rolled steel sheet

The steel slab having the alloy composition described above is homogenized for 2 to 3 hours at 1100 to 1250 ° C, which is the temperature of the austenitic region, and hot-rolled at 800 to 900 ° C to produce a hot rolled steel sheet, followed by 560 to 650 ° C. Wind up in the temperature range of. Cooling is performed by general water cooling.

At this time, when the homogenization treatment temperature is less than 11000 ° C., segregated components are not reusable. When the homogenization temperature is higher than 1250 ° C., the austenite grain size increases and the ferrite grain size is coarsened, resulting in a decrease in strength.

In addition, if the homogenization treatment holding time is shorter than 2 hours, the degree of homogenization may be insufficient and the quality of the steel may deteriorate. If the homogenization treatment holding time is longer than 3 hours, it is not economically efficient.

If the hot finish rolling temperature is less than 800 ℃ the rolling load is increased to have an adverse effect on the rolling sheetability, and if the hot finish rolling temperature exceeds 900 ℃, the structure becomes too coarse to bring down the strength and ductility.

If the coiling temperature is less than 560 ℃ there is a problem that the shape of the hot-rolled steel sheet is bad. When the temperature exceeds 650 ° C., coarse pearlite is formed on the hot-rolled steel sheet, thereby making it difficult to secure high strength.

Table 1 below shows the number of cracks generated on the surface of the specimen prepared with 0.2% copper.

(Unit: wt%) division C Si Mn P S Cu sol-Al Ni Cracks One 0.045 0.02 0.23 0.009 0.008 0.2 0.030 0.03 8 or more 2 0.045 0.02 0.23 0.009 0.008 0.2 0.030 0.2 3 ~ 5 3 0.045 0.02 0.23 0.009 0.008 0.2 0.030 0.3 1 or less

[Number of cracks: unit (per cm ₂ )]

Looking at Table 1, it is confirmed that when the nickel is added at 1.45 times or more of copper, the occurrence of cracks due to copper is greatly reduced.

Table 2 below shows examples and comparative examples of the present invention in which each component element is different.

(Unit: wt%) division C Si Mn P S Cu sol-Al Ni Comparative Example 1 0.040 0.02 0.22 0.009 0.007 0.10 0.018 0.03 Example 1 0.045 0.02 0.23 0.009 0.008 0.19 0.030 0.30 Example 2 0.042 0.07 0.21 0.03 0.008 0.31 0.025 0.45

Table 3 shows the results of the mechanical properties of the hot rolled steel sheet produced according to the alloy design of Table 2.

In other words, the steel slabs prepared as shown in Table 2 were reheated at 1200 ° C. for 2 hours, hot rolled at 900 ° C., and specimens of the hot rolled steel wound at 600 ° C. were collected to test tensile strength, elongation and water resistance.

division Tensile Strength (MPa) Elongation (%) Hardness (Hv) Water resistance Salt water spray resistance Comparative Example 1 342.2 44.6 104 M1.5 or higher Rust and swelling Example 1 363.2 43.1 114 M1.5 or higher Good Example 2 372.6 42.0 119 M1.5 or higher Good

[Watertightness criteria: M1.5 or higher, salt water spray resistance: no rust and swelling at least 3mm on the cross-cut side]

According to the results of Table 3, it can be seen that the tensile strength of the Example was improved by about 30 to 40 MPa compared with the comparative example, and the corrosion resistance was also improved. Elongation also satisfied the standard range.

In other words, by increasing the content of copper (Cu) and phosphorus (P) elements Tramp, and adding nickel (Ni) and silicon (Si) through the embodiment it can improve the tensile strength and corrosion resistance of hot-rolled steel sheet You can check it.

Since the method of this embodiment uses copper contained in the scrap, it is not necessary to use high-quality scrap or the amount of use can be reduced, which has the effect of reducing the cost in producing hot-rolled steel sheets of the same class.

In this way, when the copper (Cu), phosphorus (P), silicon (Si), nickel (Ni) content is adjusted to confirm that the corrosion resistance is improved, the water resistant adhesion test and the salt spray test as shown in FIG. The surface of was observed.

Referring to FIG. 1, in the comparative example, rust and swelling occurred in a cross-cut direction, whereas in the example, rusting and swelling were not observed.

For reference, in the water tightness test, phosphate treatment, electrodeposition coating, middle coating, and top coating were sequentially performed on the material corresponding to the surface of the car body, and then immersed in distilled water at about 40 ° C. for 10 days and taken out, and the surface of the coating was 2 mm It is to evaluate the coating adhesiveness to the extent of the coating film peeled off the tape when inserting the checker mark graduation and attaching the vinyl tape.

Within the scope of the basic technical idea of the present invention, many other modifications are possible to those skilled in the art, and the scope of the present invention should be interpreted based on the appended claims. will be.

1 is a surface photograph of the water tightness test and the salt spray test of the specimens of the comparative examples and examples of Table 1.

Claims (3)

0.035 ~ 0.050wt% of carbon (C), 0.01 ~ 0.1wt% of silicon (Si), 0.15 ~ 0.30wt% of manganese (Mn), 0.005 ~ 0.035wt% of phosphorus (P), 0.05wt% or less of sulfur (S), aluminum (Al) 0.01 ~ 0.10wt%, copper (Cu) 0.2 ~ 0.3wt%, nickel (Ni) 0.35 ~ 1.0wt%, the balance is made of iron (Fe) and other unavoidable impurities, The content ratio of the nickel (Ni) and copper (Cu) is a high strength hot rolled steel sheet, characterized in that the Ni ≥ 1.45Cu condition range. delete 0.035 ~ 0.050wt% of carbon (C), 0.01 ~ 0.1wt% of silicon (Si), 0.15 ~ 0.30wt% of manganese (Mn), 0.005 ~ 0.035wt% of phosphorus (P), 0.05wt% or less of sulfur (S), aluminum (Al) 0.01 ~ 0.10wt%, copper (Cu) 0.2 ~ 0.3wt%, nickel (Ni) 0.35 ~ 1.0wt% and alloy composition of the remaining iron (Fe), the content of nickel (Ni) and copper (Cu) Steel slab with a ratio of Ni≥1.45Cu Homogenizing treatment at 1150 ~ 1250 ℃, hot finish rolling at 800 ~ 900 ℃, winding method in the temperature range of 560 ~ 650 ℃ characterized in that the manufacturing method of high strength hot rolled steel sheet.

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