KR100989778B1 - hot-rolled steel sheet having good drawing ability and galvanizing property, and method for producing the same - Google Patents

Abstract

The present invention relates to a high-strength hot rolled steel sheet excellent in workability and plating characteristics and a method of manufacturing the same. In the present invention, by weight% of carbon (C) 0.004 ~ 0.02%, manganese (Mn) 1.0 ~ 2.5%, sulfur (S) 0.005 ~ 0.01%, aluminum (Al) 0.01 ~ 0.1%, nitrogen (N) 0.004 ~ 0.01% , Phosphorus (P) 0.05 ~ 0.1%, boron (B) 0.0005 ~ 0.002%, niobium (Nb) containing 0.06 ~ 0.15%, has an alloy composition consisting of the remaining iron (Fe) and other unavoidable impurities, the average size is 0.2 Fine NbC precipitates and AlN precipitates of less than or equal to μm are distributed. High-strength hot-rolled steel sheet of the present invention can secure the plating characteristics by reducing the addition amount of Mn, Si, and has a strength increase effect through grain refinement through the composite effect of Mn, Nb, B, but due to precipitates generated in precipitation hardening steel There is no increase in yield strength has the advantage of ensuring excellent workability.

Hot rolled steel, high strength, workability, plating

Description

Hot-rolled steel sheet having good drawing ability and galvanizing property, and method for producing the same}

The present invention relates to a high-strength hot-rolled steel sheet excellent in workability and plating characteristics, and a method for manufacturing the same. It is related with a hot rolled steel sheet and its manufacturing method.

Hot rolled steel sheets used as materials for automobile reinforcement and building structural materials are added with alloying elements to secure strength and formability. Generally used hot rolled steel sheet uses high-strength hardening steel, and the use of metamorphic steel for additional workability.

However, precipitation hardened steel has a disadvantage in that a large amount of precipitation occurs during hot rolling, so that the rolling load increases, and the yield strength increases due to the precipitates generated, thereby degrading workability.

In addition, alloy steels, such as manganese (Mn) and silicon (Si), are added to transformed tissue steel to secure residual austenite. It is concentrated to the surface of the ride and preferentially forms oxide layers of manganese oxide (MnO ₂ ) and silicon oxide (SiO ₂ ) on the steel surface. As such, when the oxide layer is formed on the surface of the steel, the portion where the oxide layer is formed is not hot dip galvanized, which causes a decrease in plating property.

In addition, in the case of the metamorphic tissue steel, multistage cooling is required to change the cooling rate in the cooling line in order to form residual austenite for strained organic plasticity.

As such, precipitation hardened steel is easy to secure high strength, but there is a problem of deterioration in workability due to a large amount of precipitates, and deformed steel is degraded in plating property because a large amount of alloying elements such as Mn and Si must be added to secure residual austenite. And manufacturing difficulties such as deterioration of workability and temperature control and low temperature winding for multi-stage cooling. In order to prevent this, the amount of addition of alloying elements such as Mn and Si may be reduced, but this is accompanied by a decrease in strength of the steel and the inability to form sufficient residual austenite.

The present invention is to solve the conventional problems as described above, an object of the present invention is to manufacture a high-strength hot-rolled steel sheet to adjust the content and amount of alloying elements so that the workability and plating characteristics of the high-strength hot-rolled steel sheet can be improved at the same time and the production thereof To provide a way.

The present invention is to solve the conventional problems as described above, the object of the present invention, by weight% carbon (C) 0.004 ~ 0.02%, manganese (Mn) 1.0 ~ 2.5%, sulfur (S) 0.005 ~ 0.01 %, 0.01% to 0.1% of aluminum (Al), 0.004% to 0.01% of nitrogen (N), 0.05% to 0.1% of phosphorus (P), 0.0005% to 0.002% of boron (B), and 0.06% to 0.15 of niobium (Nb) It has an alloy composition composed of iron (Fe) and other unavoidable impurities, and fine NbC precipitates and AlN precipitates having an average size of 0.2 μm or less are distributed.

The NbC and AlN precipitates are 1 × 10 ⁶ / mm ² ~ 1X10 ⁸ / mm ^{2 has} a distribution number.

By weight%, carbon (C) 0.004 ~ 0.02%, manganese (Mn) 1.0 ~ 2.5%, sulfur (S) 0.005 ~ 0.01%, aluminum (Al) 0.01 ~ 0.1%, nitrogen (N) 0.004 ~ 0.01%, phosphorus ( P) 0.05 ~ 0.1%, boron (B) 0.0005 ~ 0.002%, niobium (Nb), 0.06 ~ 0.15, alloy composition consisting of the remaining iron (Fe) and other unavoidable impurities, after continuous casting temperature of Ac3 point or more After reheating and holding for 3-4 hours, hot finish rolling is performed at Ar3 or higher and Ar3 + 100 or lower, followed by cooling by forced cooling and winding at 600 ° C or lower.

Forced cooling after the hot finish rolling is carried out at a cooling rate in the range of 30 ℃ / sec ~ 50 ℃ / sec.

The hot-rolled steel sheet of the present invention lowers the content of Mn and Si, which degrades the plating property, than the conventional transformation structure steel, and lowers the content of C, which degrades the workability, than the conventional precipitation hardened steel, thereby improving workability and plating characteristics.

In addition, the strength reduction of the steel sheet generated by lowering the content of the alloying elements of Mn, Si, C is based on IF steel, but Nb and a small amount of solid solution strengthening element are added to the IF steel to increase the strength by grain refinement and solid solution strengthening. To ensure that.

As such, the alloy design that lowers the content of C and the addition amount of the alloying elements (Mn, Si) can be widely commercialized in automobiles and building structures since it can simultaneously secure the improvement of the workability and the plating characteristics compared with the existing high strength hot rolled steel sheet. It works.

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

Hot rolled steel sheet of the present invention by weight% carbon (C) 0.004 ~ 0.02%, manganese (Mn) 1.0 ~ 2.5%, sulfur (S) 0.005 ~ 0.01%, aluminum (Al) 0.01 ~ 0.1%, nitrogen (N) 0.004 ~ 0.01%, phosphorus (P) 0.05 ~ 0.1%, boron (B) 0.0005 ~ 0.002%, niobium (Nb) 0.06 ~ 0.15% and the rest of the iron (Fe) and other unavoidable impurities.

The hot rolled steel sheet having the composition as described above is based on Nb-based IF (Interstitial free steel) steel, but the content of manganese (Mn) and silicon (Si), which deteriorate the plating characteristics, is lower than that of the conventional metamorphic steel. The strength reduction caused by securing the characteristics and lowering the content of the alloying elements of Mn and Si to ensure high strength through the addition of Nb and a small amount of solid solution strengthening element. (Typically, the steel is more than 2% Mn and less than 1% Si.)

More specifically, the high strength is ensured by the composite effect of Nb and Mn and the amount of B added. Nb, when Mn and B are added together, delays recrystallization during hot rolling to refine grains and precipitates solid solution carbon into NbC.

In addition, in the present invention, carbon is added lower than conventional precipitation hardened steel so that there is no solid carbon, which is a characteristic of the IF steel, so that elongation, which is a workability index, may be secured. (Usually, the precipitation hardened steels ensure high strength by forming precipitates by adding Mn within 2% and adding C and 0.05% Nb of about 0.1%.)

Here, C, which is added lower than conventional precipitation hardening steel or transformation tissue steel, combines with Nb added to IF steel to form NbC precipitates. In general, NbC precipitates increase the strength by making the grains finer.

For this purpose, it is important to set the content of the alloying elements contained in the Nb-based IF steel, the condition of their composition ratio, the cooling rate after the hot rolling, and the conditions of the winding temperature.

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

Carbon (C): 0.004 ~ 0.02w%

Carbon is an indispensable element for imparting high strength to steel. If the carbon content exceeds 0.02%, the dissolved carbon will greatly degrade the aging resistance, so a large amount of expensive Nb must be added to remove the dissolved carbon. In this case, manufacturing costs rise and recrystallization temperatures rise. Therefore, the annealing temperature should be increased, otherwise the crystal grains of the annealed steel sheet become fine and the ductility greatly decreases.

And, if the content of carbon (C) is added less than 0.004%, the amount of NbC precipitates are reduced, so that a solid solution strengthening element must be added, thereby increasing the manufacturing cost. In addition, it may indicate a weakness in the plating properties and secondary processing brittleness, there is a problem that the strength is lowered and the in-plane anisotropy is increased by easily coarse grains of the hot-rolled steel sheet. Therefore, the carbon (C) content is preferably adjusted within the range of 0.004 to 0.02%.

Manganese (Mn): 1.0 ~ 2.5w%

Manganese (Mn) is known as a solid solution strengthening element that precipitates sulfur (S) dissolved in steel as MnS to prevent hot shortness caused by solid solution sulfur. Therefore, it is common to add a high content of manganese, but in the present invention, manganese and niobium (Nb) are added together so that crystal grains can be refined while delaying recrystallization during hot rolling. Grain refinement improves the tensile strength of the steel. However, when the content of manganese is high, the plating property may be degraded by the formation of an oxide layer, so the upper limit thereof is limited to 2.5% or less.

And in order to secure these characteristics, the content of manganese should be more than 1.0%. If the content of manganese is less than 1.0%, the strength increase effect of manganese is less than 1.0% is added.

Sulfur (S) 0.005 ~ 0.01w%

Sulfur (S) reacts with manganese (Mn) to form fine MnS precipitates. When the sulfur (S) content is less than 0.005%, not only the amount of precipitates precipitated but also the number of precipitates precipitated is very small. In the present invention, since the Mn content is low, when the sulfur content is more than 0.01%, the content of the dissolved sulfur is large, so that the ductility and moldability are greatly lowered, and there is concern of red brittleness. Therefore, the content of sulfur is preferably between 0.005-0.01%.

Aluminum (Al) 0.01 ~ 0.1w%

Aluminum reacts with nitrogen to form fine AlN precipitates, which have grain-fineness and strength enhancement effect by precipitation strengthening. If the content of aluminum is less than 0.01%, the amount of AlN precipitates is reduced and sufficient strength cannot be secured. If the content of aluminum is more than 0.1%, there is a difficulty in continuous casting, which leads to low productivity and excessive yield strength. have. Therefore, the content of aluminum is preferably between 0.01 and 0.1%.

Nitrogen (N) 0.004 ~ 0.01w%

When the content of nitrogen (N) is less than 0.004%, the number of precipitated AlN is small, so that the effect of grain refinement and precipitation strengthening is small, and when it exceeds 0.01%, it is difficult to guarantee the aging by solid nitrogen. Therefore, the content of nitrogen is preferably between 0.004 ~ 0.01%.

Phosphorus (P) 0.05-0.1w%,

Phosphorus (P) is an element having a high solid solution strengthening effect and a low drop in elongation value, which ensures high strength in steels for controlling precipitates as in the present invention. In general, high-strength hot-rolled steel sheet is preferably limited to less than 0.015% phosphorus content, but P is added in 0.05 ~ 0.1% to improve the strength. However, if P content is increased, secondary processing brittleness may occur. Therefore, the content of P should be added within 0.1%.

Boron (B) 0.0005 ~ 0.002w%,

Boron (B) is added to prevent secondary processing brittleness that may occur due to the addition of phosphorus (P). Generally, boron is preferably added in an amount of 0.0005% or more, and if it is added in an amount of 0.002% or more, a material deviation may occur due to segregation.

Niobium (Nb), 0.06 to 0.15w%

Niobium is added for the purpose of improving the workability of the steel sheet. Niobium is a strong carbide-generating element, added to steel to precipitate NbC precipitates to precipitate carbon in solid solution, thereby securing steel's age and processability.

When niobium is less than 0.06%, the precipitation of NbC precipitates is too small, so the effect of strength hardening due to precipitation hardening cannot be expected.If the amount of addition is over 0.15%, niobium is dissolved to decrease the elongation value and increase the yield strength. Therefore, the workability is weakened. Therefore, the content of niobium is preferably between 0.06 and 0.15%.

In such a component system, the finer the precipitate, the better the strength. In particular, the average size of the NbC precipitate and the AlN precipitate is preferably 0.2 μm or less.

According to the experimental results, the strength is lowered when the average size of the precipitate exceeds 0.2㎛. Furthermore, in the component system of the present invention, a large amount of precipitates of 0.2 μm or less are distributed, and the number of distribution thereof is 1 × 10 ⁶ / mm ² ~ 1 X ^{10 8} pcs / mm ² Should be

If the number of distribution of the precipitate mm ² 1X10 ⁶ fewer than sugar is difficult to be achieved because a sufficient strength compensating effect securing strength if it exceeds 1X10 ⁸ gae / mm ^2, the It is not preferable because the yield strength increases as in precipitation hardening steel.

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 method of manufacturing a high strength hot rolled steel sheet having excellent workability and plating characteristics having an alloy composition according to the present invention will be described in detail with reference to the following examples.

 -Manufacturing method of hot rolled steel sheet

Hot rolled steel sheet of the present invention by weight% carbon (C) 0.004 ~ 0.02%, manganese (Mn) 1.0 ~ 2.5%, sulfur (S) 0.005 ~ 0.01%, aluminum (Al) 0.01 ~ 0.1%, nitrogen (N) 0.004 Hot steel slabs containing ~ 0.01%, phosphorus (P) 0.05-0.1%, boron (B) 0.0005-0.002%, niobium (Nb), 0.06-0.15% and the composition of the remaining iron (Fe) and other unavoidable impurities Through rolling, the average size of NbC and AlN precipitates is less than 0.2㎛.

At this time, the average size of NbC precipitates and AlN precipitates of the hot rolled steel sheet is affected by the manufacturing process such as reheating temperature and winding temperature together with the component design, but in particular, the cooling rate after hot rolling.

Hot Rolling Condition

The steel slab to which the alloying element is added is reheated at a sufficiently high temperature and hot rolled. At this time, if the reheating temperature is low, the coarse precipitates generated during continuous casting remain in a completely insoluble state, and many coarse precipitates remain even after hot rolling, and thus must be reheated at a sufficiently high temperature.

That is, the steel slab to which the alloying element is added is reheated to a temperature of at least Ac 3 point and maintained for 3-4 hours, and hot finished rolling at a temperature of Ar 3 or more to Ar 3 + 100 and then cooled by forced cooling. And it is wound at less than 600 ℃ to produce a fine hot rolled steel sheet structure.

Thus, hot finishing based on the Ar3 transformation temperature is because when the hot finishing rolling temperature is less than the Ar3 transformation temperature, the workability is lowered due to the formation of the rolled grain and the strength is lowered. Therefore, it is preferable to perform hot finishing rolling at an Ar3 point or more and an Ar3 + 100 or less.

And after the hot rolling of the hot rolled steel sheet to increase the final reduction rate so that the steel sheet can enter the cooling line as soon as possible to prevent the growth of grains, the cooling rate at this time is 30 ℃ / sec or more 50 ℃ / sec or less. desirable.

Even if the component ratio is controlled to obtain a fine precipitate according to the present invention, if the cooling rate is less than 30 ℃ / sec, the average size of the precipitate may exceed 0.2㎛. And if the cooling rate is faster than 50 ℃ / sec there is a fear that surface cracking occurs.

-Winding condition-

As mentioned above, although hot rolling is performed, it is preferable that a coiling temperature is 600 degrees C or less. This is because when the coiling temperature exceeds 600 ° C., the precipitate grows too coarsely and the grain refining effect is reduced, making it difficult to secure the strength.

Hereinafter, the high-strength hot-rolled steel sheet and its manufacturing method excellent in workability and plating characteristics according to the present invention will be described in detail in comparison with the prior art. To help understand the invention, the data values according to the examples according to the present invention and the conventional comparative examples are shown as a table.

Table 1 shows the present invention and the conventional alloy design is divided into Examples and Comparative Examples, Table 2 shows the results of the mechanical properties according to the alloy design of Table 1.

(Final alloy component wt% of steel sheet: balance Fe) division
C Si Mn P S Al N Co Nb Cu Ni B Remarks
× 10000 × 100 × 10000 × 100 × 10 × 10000 Comparative Example 1 1000 100 150 700 30 700 50 30 - One - - Metamorphic tissue Comparative Example 2 1500 50 150 400 50 10000 40 35 - One - - Metamorphic tissue Comparative Example 3 1000 50 200 400 50 10000 70 - - 2 2 - Metamorphic tissue Comparative Example 4 800 - 250 200 80 400 100 - 8 - - - Precipitation hardening steel Comparative Example 5 1000 - 200 200 80 300 80 - 7 - - - Precipitation hardening steel Example 1 60 10 150 900 68 400 43 - 8 One - 6 Example 2 55 10 150 700 63 500 55 - 8 One - 7 Example 3 70 10 150 710 57 400 54 - 6 - - 7 Example 4 80 10 159 500 61 200 61 - 7 - - 9 Example 5 80 10 158 820 60 400 44 - 7 - - 7

division TS (MPa) YS (MPa) EL (%) Plating Comparative Example 1 559 460 34 × Comparative Example 2 580 431 33 × Comparative Example 3 586 481 32 × Comparative Example 4 600 545 28 △ Comparative Example 5 594 539 29 △ Example 1 615 476 32 ○ Example 2 601 459 31 ○ Example 3 608 396 36 ○ Example 4 612 425 32 ○ Example 5 623 481 33 ○

[TS (MPa): Tensile Strength, YS (MPa): Yield Strength, EL (%): Elongation]

Table 2 shows the steel slab having the alloy design of Table 1 heated at 1250 ℃ for 1 hour, hot rolled at 900 ℃ for finishing, then quenched to 560 ℃, rolled at 70% of cold rolling rate and annealed at 800 ℃. Tensile strength, yield strength, elongation, etc. of the manufactured steel plate were measured.

Through the examples and comparative examples of Table 1 and Table 2, even if the alloying elements (C, Si, Mn) is reduced, it can be seen that the strength is improved and the workability is improved by the composite effect of Nb, Mn, and B added in trace amounts.

In addition, by reducing the content of the alloying element that inhibits the plating property, it is possible to ensure the advantageous properties for plating.

As described above, the hot-rolled steel sheet of the present invention may have the characteristics of the hot-rolled steel sheet having improved plating property and workability while securing high strength of 540 MPa or more according to the component design. In particular, the present invention can secure the characteristics of high strength with low elongation of carbon, which is a characteristic of IF steel.

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.

Claims (4)

By weight%, carbon (C) 0.004 ~ 0.02%, manganese (Mn) 1.0 ~ 2.5%, sulfur (S) 0.005 ~ 0.01%, aluminum (Al) 0.01 ~ 0.1%, nitrogen (N) 0.004 ~ 0.01%, phosphorus ( P) 0.05 ~ 0.1%, boron (B) 0.0005 ~ 0.002%, niobium (Nb) 0.06 ~ 0.15%, alloy composition composed of the remaining iron (Fe) and other unavoidable impurities, the average size of 0.2㎛ or less High strength hot rolled steel sheet, characterized in that the fine NbC precipitates and AlN precipitates are distributed. The method according to claim 1, The NbC and AlN precipitates are 1 × 10 ⁶ / mm ² High strength hot rolled steel sheet characterized by having a distribution number of ~ 1X10 ⁸ / mm ² . By weight%, carbon (C) 0.004 ~ 0.02%, manganese (Mn) 1.0 ~ 2.5%, sulfur (S) 0.005 ~ 0.01%, aluminum (Al) 0.01 ~ 0.1%, nitrogen (N) 0.004 ~ 0.01%, phosphorus ( P) 0.05 to 0.1%, boron (B) 0.0005 to 0.002%, niobium (Nb), 0.06 to 0.15, and has an alloy composition consisting of the remaining iron (Fe) and other unavoidable impurities, High strength hot rolled steel sheet characterized by reheating to a temperature of at least Ac3 point after continuous casting and maintaining for 3-4 hours, performing hot finish rolling at Ar3 point or more and below Ar3 + 100, cooling by forced cooling, and winding up to 600 ° C or less. Manufacturing method. The method according to claim 3, Forced cooling after the hot finish rolling is a method of manufacturing a high strength hot rolled steel sheet, characterized in that carried out at a cooling rate of 30 ℃ / sec ~ 50 ℃ / sec range.