KR20060033489A - Method for manufacturing high strength hot rolled steel sheet having excellent uniformity in coil - Google Patents

Abstract

The present invention relates to a hot rolled steel sheet used for the use of wheels, rims, seat frames of automobiles.

In the present invention, by weight%, C: 0.07 to 0.09%, Si: 0.3% or less, Mn: 1.1 to 1.3%, P: 0.03% or less, S: 0.010% or less, Al: 0.005 to 0.10%, N: 0.0020 to Preparing a steel slab composed of 0.0080%, Ti: 0.025-0.06%, Nb: 0.03-0.045%, and remaining Fe and other unavoidable impurities; reheating the prepared steel slab at a temperature of 1250-1300 ° C and then 1030 ° C. Rough rolling at the above temperature; Finishing hot-rolling the rough rolled steel sheet to a reduction ratio of 150% or more at a temperature of 960 ℃ or more and then finish rolling at a temperature of 850 ~ 900 ℃; And winding the finished rolled steel sheet at a temperature of 550 to 650 ° C. to a method for producing a high strength hot rolled steel sheet having excellent material uniformity in a coil.

The present invention provides a high-strength hot rolled steel sheet which can minimize the longitudinal material deviation of the hot rolled steel sheet by controlling the slab reheating conditions, hot rolling conditions and winding conditions.

Hot rolled steel, precipitation strengthening, yield strength, material deviation, dynamic recrystallization

Description

Method for manufacturing high strength hot rolled steel sheet having excellent uniformity in coil}             

1 is a schematic diagram showing a high temperature compression test method for measuring a temperature capable of dynamic recrystallization.

2 is a stress-temperature curve graph obtained by the high temperature compression test method.

3 is a graph showing the distribution of tensile strength and yield strength for each coil length of Inventive Example 2 and Comparative Example 2. FIG.

The present invention relates to a hot-rolled steel sheet used for wheels, rims, seat frames, etc. of automobiles, and more particularly, high-strength hot-rolled steel sheet capable of reducing coil longitudinal yield strength variation by forming uniform ferrite grains and precipitates. It relates to a manufacturing method of.                         

Recently, various techniques for manufacturing hot rolled steel sheets using transformations during recrystallization, precipitation, and cooling have been developed. Examples of such techniques include US Patent 5,514,227, Japanese Patent Application Laid-Open No. 2000-334466, Japanese Patent Application Laid-Open No. 2001-333376, US Patent Publication 2002-108691, Korean Patent Publication No. 1998-59811, Japanese Patent Application Laid-Open No. 5-308808, Japanese Patent Application Laid-open No. Hei 5-279379, Japanese Patent Application Laid-open No. Hei 9-143570, and US Patent 6,290,784.

US Pat. No. 5,514,227 of the prior arts is characterized by providing a hot rolled steel sheet having a yield strength of 50 ksi or more by precipitating Ti and V to precipitate carbonitrides of these elements. In addition, Japanese Laid-Open Patent Publication No. 2000-334466 is a manufacturing technique of hot rolled steel sheet using precipitation strengthening of these elements by adding a small amount of Ti, Nb and W, and Japanese Laid-Open Patent Publication No. 2001-333376 and US Patent 2002-108691. The arc relates to the production technology of hot rolled steel sheet utilizing the precipitation strengthening of these elements by adding Ti and Mo. In addition, Korean Laid-Open Patent Publication No. 1998-59811 is a technique for manufacturing a hot rolled steel sheet made of ferrite and bainite by adding Ti and Nb and dividing into three stages for cooling. -279379 relates to the production technology of hot rolled steel sheet having a microstructure in which a small amount of Ti and Nb are added to satisfy a specific formula and an equiaxed ferrite is 80% or more. In addition, Japanese Laid-Open Patent Publication Nos. 9-143570 and 6,290,784 are one of the methods for producing a precipitation-reinforced hot rolled steel sheet, and technology for forming fine grains by reducing Nb and Ti additive steel by 40% or more in the unrecrystallized region. It is about.

The above-mentioned prior arts have a technical gist of improving the strength of a hot rolled steel sheet by adding a small amount of elements such as Ti, Nb, V, Mo, and W to generate precipitates during cooling after hot rolling by these elements. However, these prior arts lack the consideration of dynamic recrystallization during rough rolling and finishing rolling, and there is a problem that the material deviation in the coil after rolling is relatively large due to insufficient formation of element formation of precipitates. That is, in the case of such steels containing Nb and Ti, the recrystallization phenomenon during rolling is delayed and the austenite grains are severely deformed and stretched, resulting in uneven structure in the rolling thickness direction, resulting in large material deviation between the hot rolled coil tip and the rear end. There is a problem.

The present invention is to solve the problems of the prior art, to provide a method of manufacturing a high strength hot rolled steel sheet which can minimize the longitudinal material deviation of the hot rolled steel sheet by controlling the slab reheating conditions, hot rolling conditions and winding conditions. , Its purpose is.

The present invention for achieving the above object, in weight%, C: 0.07 ~ 0.09%, Si: 0.3% or less, Mn: 1.1 ~ 1.3%, P: 0.03% or less, S: 0.010% or less, Al: 0.005 ~ Preparing a steel slab composed of 0.10%, N: 0.0020% to 0.0080%, Ti: 0.025% to 0.06%, Nb: 0.03% to 0.045%, and remaining Fe and other unavoidable impurities;

Reheating the prepared steel slab at a temperature of 1250-1300 ° C. and then roughly rolling the steel slab at a temperature of 1030 ° C. or more;

Finishing hot rolling of the rough rolled steel sheet to a reduction ratio of 150% or more at a temperature of 960 ° C. or higher and then finishing rolling at a temperature of 850 ° C. to 900 ° C .; And

It relates to a method of manufacturing a high-strength hot rolled steel sheet excellent in material uniformity in the coil comprising a; winding the finished rolled steel sheet at a temperature of 550 ~ 650 ℃.

Hereinafter, the emphasis component of the present invention and the reason for limitation thereof will be described.

C: 0.07% to 0.09% by weight (hereinafter, referred to only as '%')

The C is the most economical and effective element to strengthen the steel, if the content is less than 0.07% to add the same alloying elements in order to secure the same strength, if it exceeds 0.09% weldability, formability and toughness is deteriorated There is a problem. Therefore, the content of C is preferably limited to 0.07 to 0.09%.

Si: 0.3% or less

The Si is an effective component for deoxidizing molten steel and exerting a solid solution strengthening effect. If the content exceeds 0.3%, a red scale formed by Si is formed on the surface of the steel sheet during hot rolling. Since there is a problem of deterioration, it is preferable to limit the content to 0.3% or less.

Mn: 1.1-1.3%

The Mn is an effective element to solidify the steel, and if the content is less than 1.1%, the effect of the addition cannot be obtained. If the content exceeds 1.3%, the segregation part is greatly developed at the center of thickness during slab casting in the steelmaking process. There is a problem that damages the weldability of the product. Therefore, the content of Mn is preferably limited to 1.1 ~ 1.3%.

P: 0.03% or less

The P is an impurity present in steel, and when its content exceeds 0.03%, it combines with Mn to form a non-metallic inclusion, thereby greatly reducing the toughness and strength of the steel. Therefore, the P is preferably limited to 0.03% or less.

S: 0.010% or less

S is an impurity present in steel, and if its content exceeds 0.010%, it combines with Mn to form a non-metallic inclusion. Therefore, there is a problem of greatly reducing the toughness and strength of the steel, thereby limiting the content to 0.010% or less. It is preferable.

Al: 0.005 ~ 0.10%

Al is a component added for deoxidation. If the content is less than 0.005%, the effect of addition is insufficient. If the content exceeds 0.10%, the effect is saturated and the manufacturing cost is increased. Therefore, the content is 0.005. It is recommended to limit it to ~ 0.10%.

N: 0.0020-0.0080%

The component content of N is due to the content of Ti below. In general, N is dissolved in the steel and then precipitates to increase the strength of the steel, which is superior to carbon. However, on the other hand, as the amount of nitrogen in the steel increases, the toughness falls significantly. Therefore, in the present invention, by providing an appropriate amount of nitrogen in the steel, TiN is formed to play a role of suppressing grain growth in the reheating process. If the content of N is less than 0.0020%, it is insufficient to form TiN and causes an increase in manufacturing cost during steelmaking. If the content of N exceeds 0.0080%, toughness is greatly reduced and workability is deteriorated. Therefore, it is desirable to limit the content to 0.0020 to 0.0080%.

Ti: 0.025 ~ 0.06%

The Ti is an effective component for miniaturizing the grains, and is present as TiN in the steel, thereby suppressing the growth of the grains during heating for hot rolling. In addition, Ti remaining after reacting with nitrogen is dissolved in the steel and bonded with carbon to form TiC precipitates, which is a useful component for greatly improving the strength of the steel. If the Ti content is less than 0.025%, the austenite grain growth inhibition effect and the strength increase effect due to TiC formation cannot be obtained. If the Ti content exceeds 0.06%, the TiN is rapidly melted to the melting point when manufacturing the steel pipe by welding the steel sheet. Since the toughness of the weld heat affected zone is lowered by this re-use, it is preferable to limit the content to 0.025 to 0.06%.

Nb: 0.03-0.045%

Nb is not only effective for miniaturizing crystal grains, but is also an effective component for greatly improving the strength of steel. If the content of Nb is less than 0.03%, the effect of addition cannot be obtained. If the content of Nb is more than 0.045%, there is a problem of increasing material anisotropy by excessively increasing the austenite recrystallization temperature due to excessive precipitation of Nb carbonitride. . Therefore, the content of Nb is preferably limited to 0.03 ~ 0.045%. In addition, in the present invention, the reason why Nb and Ti are used simultaneously as the precipitation strengthening element is that the precipitate formation temperature of the two elements is different, so that the high precipitation strengthening effect can be obtained by being relatively insensitive to the cooling and winding conditions after hot rolling. to be.

In addition to the above components, the remainder is composed of Fe and other unavoidable impurities.

Hereinafter, the steel plate manufacturing process of this invention is demonstrated in detail.

In the present invention, the slab formed as described above is reheated and hot rolled and wound, wherein the longitudinal yield strength deviation of the hot rolled coil is determined by the microstructure generated during hot rolling and the precipitate formed before and after winding. By controlling the reheating temperature, hot rolling and winding conditions of the slab, the variation in longitudinal yield strength of the hot rolled coil is reduced.

First, a steel slab formed as described above is prepared.

Thereafter, the prepared steel slab is reheated at a temperature of 1250 ~ 1300 ℃ and then roughly rolled at a temperature of 1030 ℃ or more. In this case, if the reheating temperature is less than 1250 ° C., the precipitates are not sufficiently reused, and precipitates such as NbC are reduced in the process after hot rolling. When the reheating temperature is higher than 1300 ° C., the strength decreases due to abnormal grain growth of the austenite grains. Reheating temperature is preferably limited to 1250 ~ 1300 ℃. In addition, when the rough rolling temperature is 1030 ° C. or higher, dynamic recrystallization may occur in the austenite grains during hot rolling, and thus, uniform and fine austenite grains may be formed.

More preferably, after reheating as described above, and extracting the slab at a temperature of 1190 ~ 1250 ℃ or more, it is possible to further improve the strength level of the material by promoting the re-use of the extract and maintaining the grain size of austenite of the appropriate size In addition, it is more advantageous to obtain a uniform microstructure in the longitudinal direction of the coil.

Subsequently, the rough-rolled slab is finished by hot rolling at a reduction ratio of 150% or more at a temperature of 960 ° C. or higher, and then rolling is finished at a temperature of 850˜900 ° C. Since the steel of the composition proposed in the present invention has a temperature capable of dynamic recrystallization of 960 ° C., when the steel sheet has a rolling ratio of 150% or more at a temperature higher than that, it is possible to manufacture a hot rolled plate having well developed dynamic recrystallization.                     

At this time, if the reduction ratio is set to the sum of the reduction ratios from the first to the third rolling stands of the finishing rolling process, it is possible to manufacture a hot rolled sheet having better dynamic recrystallization. At this time, the rolling reduction rate of each rolling stand may be set larger than the value Ec obtained from the following formulas (1) to (3).

Ec = 4.75 × 10 ^-4 (Strain Rate × Exp (Q / RT)) ^0.17

Where Q is the activation energy, R is the rolling roll radius (mm) and T is the rolling temperature.

Strain Rate = [(strain × U) / {(α × 60 × R) / 1000}]

(Where U is rolling speed and R is rolling roll radius (mm))

α = cos ^-1 [1-{(Hh) / 2R}]

(Where H is the thickness of the rolled sheet before rolling, h is the thickness of the rolled sheet after rolling, R is the rolling roll radius (mm))

In addition, the rolling end temperature contributes to the steel sheet of the present invention, the composite addition of Nb and Ti exhibits the effect of increasing the strength by the precipitation strengthening of the precipitation element.                     

When the rolling finish temperature is less than 850 ℃, a strong deformation set structure is formed in the austenite during hot rolling, NbC precipitates are processed organic precipitates to grow into coarse precipitates can not contribute to the precipitation strengthening, if the austenite exceeds 900 ℃ Deformed tissues do not develop in the grains, so the grains of the ferrite cannot be refined, and as a result, the desired strength level cannot be obtained.

At this time, it is possible to use the following method as a method for determining the dynamic recrystallization possible temperature. The high temperature compression test according to FIG. 1 is a method for confirming recrystallization and transformation of flow stress in a general hot rolling temperature region, and continuously cooling the specimen temperature heated at 1200 ° C. from 1100 ° C. to 600 ° C. At the same time, an effective strain E of 0.8 is applied at a constant strain rate. The stress-temperature curve obtained from this compression test is shown in FIG. 2. In FIG. 2, the stress increases with decreasing temperature, and if there is no metallurgical change, the stress increases with a constant slope. However, when metallurgical changes such as grain size, dislocation density, and phase change occur, the slope changes. The first inflection point shown in FIG. 2 is the temperature at which recrystallization begins to be delayed, that is, the temperature at which dynamic recrystallization does not occur (T _ndrx ), and the second inflection point is the temperature (A _r3 ) at which phase transformation starts to occur from austenite to ferrite phase. . The effective strain E can be obtained from Equation 4 below.

E = (2/3 ^1/2 ) × Ln (H / h)

(Where H is the thickness of the rolled sheet before rolling and h is the thickness of the rolled sheet after rolling)

Then, the finish rolled steel sheet is wound at a temperature of 550 ~ 650 ℃. At this time, if the winding temperature is less than 550 ℃ precipitates do not form crazy, the low temperature structure bainite transformation occurs to increase the strength, but the ductility is lowered, if it exceeds 650 ℃ microstructure is formed of coarse ferrite and pearlite and precipitates This coarsening has a problem of falling strength.

On the other hand, the cooling history of the leading end (within 30 m from the coil end), the rear end (within 30 m from the coil end), and the central part (between the leading end and the rear end) in the longitudinal direction of the steel sheet during hot rolling are different. That is, the cooling rate of the front end and the rear end of direct contact with the atmosphere is faster than the cooling rate of the central part not in contact with the atmosphere. As a result, the amount of fine Nb and Ti precipitates after coiling of the hot rolled coil is changed for each coil part. Therefore, in the present invention, it is more preferable to control the front end portion and the rear end portion in the longitudinal direction of the hot rolled steel sheet to be maintained at 30 ° C. or more higher than the central portion.

Hereinafter, the present invention will be described in more detail with reference to Examples.

EXAMPLE

Steel slabs having a composition as shown in Table 1 were reheated, hot rolled, and wound under the conditions of Table 2 to prepare steel sheets.

Then, the tensile strength and yield strength of each wound specimen was measured, and the results are shown in Table 3 below. The tensile test used at this time was performed according to JIS5 standard.

Steel grade C Si Mn P S Al N Ti Nb A 0.072 0.02 1.19 0.012 0.002 0.029 0.0038 0.032 0.036 B 0.082 0.02 1.22 0.013 0.003 0.030 0.0035 0.030 0.035 C 0.088 0.03 1.19 0.011 0.001 0.028 0.0041 0.048 0.041

 division  Steel grade Reheating Temperature (℃) Slab Extraction Temperature (℃) Rough rolling temperature (℃) Finish rolling Rolling end temperature (℃) Coiling temperature (℃) Rear end ~ Central temperature difference ≥30 ℃ Rolling temperature (℃) Rolling reduction (%) Inventive Example 1 A 1280 1200 1037 975 192 885 620 ㅇ Comparative Example 1 1285 1210 1035 980 150 890 610 ㅇ Inventive Example 2  B 1290 1210 1040 970 180 880 615 ㅇ Comparative Example 2 1280 1195 1035 968 145 885 608 ㅇ Comparative Example 3 1295 1205 1040 983 175 875 612 × Inventive Example 3  C 1280 1200 1030 980 178 880 614 ㅇ Comparative Example 4 1290 1200 1040 975 130 890 622 ㅇ Comparative Example 5 1296 1205 1035 980 180 885 618 ×

division Tensile Strength (MPa) Yield strength (MPa) Material uniformity Tip Center Rear end Tip Center Rear end Inventive Example 1 587 589 585 530 529 537 Great Comparative Example 1 570 575 600 492 510 538 Bad Inventive Example 2 605 595 600 546 538 550 Great Comparative Example 2 578 585 615 515 535 561 Bad Comparative Example 3 580 595 620 524 535 580 Bad Inventive Example 3 622 631 645 562 560 572 Great Comparative Example 4 605 610 639 530 550 585 Bad Comparative Example 5 600 628 650 543 566 610 Bad

As can be seen in Table 3, in the case of Inventive Examples (1 to 3) satisfying the scope of the present invention, the tensile strength and the yield strength at the front end, the center and the rear end are uniform, so that the material uniformity is excellent. You can see that it appears.

However, in Comparative Examples (1 to 5), it can be seen that the values of tensile strength and yield strength at the front end, the center and the rear end are not uniform.

As described above, according to the present invention has a useful effect of providing a method for producing a high strength hot rolled steel sheet that can minimize the longitudinal material deviation of the hot rolled steel sheet by controlling the slab reheating conditions, hot rolling conditions and winding conditions.

Claims (3)

By weight%, C: 0.07 to 0.09%, Si: 0.3% or less, Mn: 1.1 to 1.3%, P: 0.03% or less, S: 0.010% or less, Al: 0.005 to 0.10%, N: 0.0020 to 0.0080%, Preparing a steel slab composed of Ti: 0.025-0.06%, Nb: 0.03-0.045%, remaining Fe and other unavoidable impurities; Reheating the prepared steel slab at a temperature of 1250 ~ 1300 ℃ and then rough rolling at a temperature of 1030 ℃ or more; Finishing hot rolling of the rough rolled steel sheet to a reduction ratio of 150% or more at a temperature of 960 ° C. or higher and then finishing rolling at a temperature of 850 ° C. to 900 ° C .; And And winding the finished rolled steel sheet at a temperature of 550 to 650 ° C .; The method of manufacturing a high strength hot rolled steel sheet having excellent material uniformity in coils according to claim 1, wherein the extraction temperature is controlled to be 1190 to 1250 ° C when the slab is extracted after the reheating. The temperature of the leading end (within 30 m from the end of the coil) and the rear end (within 30 m from the end of the coil) in the longitudinal direction of the hot rolled sheet during the winding is 30 than the temperature of the center (between the leading end and the rear end). Method for producing a high strength hot rolled steel sheet excellent in uniformity of material in the coil, characterized in that it is controlled higher than ℃.

Images