KR101086330B1 - Ultra-Strength Steel with Homogeneous Material and Method for Manufacturing Thereof - Google Patents

Abstract

The present invention optimizes the alloy design with low-cost elements instead of expensive elements, and adjusts the cooling and winding temperatures to have a predetermined temperature difference between the center and the edge, so that the material is particularly uniform in the width direction. About

By weight%, C: 0.05 ~ 0.10%, Si: 0.4% or less (excluding 0%), Mn: 1.0 to 2.5%, Mo: 0.1% or less (excluding 0%), Cr: 0.01 to 0.5%, Al: 0.01 to 0.1%, S: 0.01% or less (excluding 0%), Nb: 0.01 to 0.1%, Ti: 0.05 to 0.15%, V: 0.01 to 0.1%, N: 0.01% or less (excluding 0%), Mo + Cr : Steel is 0.5% or less (except 0%), and the rest is steel with composition including Fe and other unavoidable impurities. Ferrite grain size is 20um or less after cooling and winding, and its ferrite fraction is 70% by volume fraction. Above, the material in which the fine precipitate of 20 nm or less is distributed inside the steel provides a uniform high strength steel.

High strength steel, precipitation hardening, alloy, hot rolling, cooling and winding temperature, ferrite

Description

High-strength steel with uniform material and its manufacturing method {Ultra-Strength Steel with Homogeneous Material and Method for Manufacturing Thereof}

The present invention relates to a high-strength steel with a uniform material and a method of manufacturing the same, and more particularly, to optimize the alloy design with a low-cost element instead of an expensive element, and to adjust the cooling and winding temperatures so that the central part and the edge part have a predetermined temperature difference. The present invention relates to a hot rolled high strength steel and a method of manufacturing the same, which makes the material very uniform.

In general, hot-rolled high strength steel with a tensile strength of 780 MPa or more is manufactured by one of the following methods: first, using precipitation strengthening using fine precipitates, or second, using a bainite + ferrite + austenite composite structure by controlling transformation phase. Or thirdly, martensite strengthening by quenching to below the martensite transformation temperature during ROT cooling.

Precipitated tempered steel produced by the first method is added to the alloying elements Ti, Nb, Mo and the like advantageous for precipitation. The production conditions of hot rolled steel in which the formation of such fine precipitates change very sensitively depending on the amount of alloying elements added. In order to manufacture high strength steel with uniform material in the longitudinal and width directions, the manufacturing conditions during the hot rolling process, especially temperature control It should be precise.

Recently, due to a sharp increase in the price of alloying elements, especially Mo, the burden on the manufacturing cost of high strength steel is increasing. Due to this situation, the demand for low-cost hot-rolled high strength steel is increasing, and there is no choice but to reinforce the insufficient strength due to the reduction of alloying elements by deriving optimum hot-rolling manufacturing conditions that can maximize the generation of fine precipitates.

Therefore, in order to manufacture low-cost high-strength steel with a uniform material, a technology capable of utilizing microprecipitation most efficiently through optimization of alloy design and hot rolled manufacturing conditions should be developed.

Conventional representative techniques for producing high strength steels include the following.

First, in Japanese Patent Laid-Open Publication No. 1995-286243, the strength is 780 N / mm 2 or more, and in weight% C: 0.07 to 0.12, Si≤0.60, Mn: 2.0 to 2.5, P: 0.020 to 0.080, S≤0.010, Al: 0.004 The following formula (1) is implied, implying ~ 0.050, Nb: 0.025 ~ 0.060%, Ti: 0.10 ~ 0.15%, and the remaining material is composed of Fe and inevitable impurities, and the average grain size in the rolling direction is 5 microns or less A method for producing a high strength steel of a structure in which either one or both of the two types of fine grain ferrite structures and fine grain ferrite structures having an average grain diameter of 5 microns or less including cementite having a diameter of 1.3 microns or less is proposed.

700≥-396 * C + 26.8 * Si-68.1 * Mn + 858 (1)

However, rather than producing high-strength steel by precipitation strengthening, the focus is on a method of manufacturing high-strength steel, in which grain size is fine and the bainite structure is formed to improve strength.

Such fine grain high-strength steel has a disadvantage in that weld characteristics are particularly degraded. In the weld heat affected zone (HAZ), it is difficult to expect an increase in strength due to fine grains because grains are coarsened.

In addition, Korean Patent Publication No. 1996-0014367 discloses a tensile strength of 80 kg with excellent processability, which has a residual austenite-ferrite-bainite three-phase composite structure with a volume ratio of 5% or more by controlling steel composition and controlling hot rolling, cooling, and winding conditions. To provide a manufacturing method of / mm2 grade hot rolled steel sheet, in order to achieve the above object by weight% C: 0.15 ~ 0.3%, Si: 1.0 ~ 2.5%, Mn: 1.0 ~ 2.5%, P: 0.04 ~ 0.15%, S: 0.01% or less, Al: 0.01 ~ 0.08%, balance Fe and other unavoidable impurity elements are finish-rolled in the temperature range of 800 to 850 degrees, and water cooled to the temperature range of 700 to 750 degrees. In addition, the present invention proposes a method for producing a high tensile strength 80kg / mm2 hot rolled steel sheet, which is characterized by being air cooled to a temperature range of 630 to 680 degrees, and then water-cooled again to be wound at a temperature range of 370 to 430 degrees. Hot roll fixed conditions are very demanding There are difficulties, there is a problem becomes defective material.

The present invention is to solve the problems as described above, the purpose is to produce a high-strength steel using precipitation strengthening, while minimizing the addition of expensive alloying elements to reduce the manufacturing cost and maximize the fraction of fine precipitates, for example, The present invention provides a precipitation strengthening type high strength steel having a uniform material in a longitudinal direction and a width direction and controlling the cooling and winding temperature conditions by separating them into a central portion and an edge portion of a steel.

In order to solve the above-mentioned problems,

By weight%, C: 0.05 ~ 0.10%, Si: 0.4% or less (excluding 0%), Mn: 1.0 to 2.5%, Mo: 0.1% or less (excluding 0%), Cr: 0.01 to 0.5%, Al: 0.01 to 0.1%, S: 0.01% or less (excluding 0%), Nb: 0.01 to 0.1%, Ti: 0.05 to 0.15%, V: 0.01 to 0.1%, N: 0.01% or less (excluding 0%), Mo + Cr : Steel having a composition of 0.5% (excluding 0%) or less, and the remainder is a composition containing Fe and other unavoidable impurities. The ferrite grain size is 20um or less by cooling and winding, and the ferrite fraction is 70% or more by volume fraction. The fine precipitate of 20 nm or less is distributed within the steel material to provide a uniform high strength steel.

At this time, the difference in the longitudinal volume fraction and the widthwise volume fraction of the fine precipitate is within 20%, the longitudinal tensile strength deviation of the steel material is 50MPa or less, characterized in that the widthwise tensile strength deviation is 30MPa or less.

In the present invention, C: 0.05 to 0.10%, Si: 0.4% or less (excluding 0%), Mn: 1.0 to 2.5%, Mo: 0.1% or less (excluding 0%), Cr: 0.01 to 0.5% , Al: 0.01 to 0.1%, S: 0.01% or less (excluding 0%), Nb: 0.01 to 0.1%, Ti: 0.05 to 0.15%, V: 0.01 to 0.1%, N: 0.01% or less (excluding 0%) And Mo + Cr: a steel material having a relationship of 0.5% or less (excluding 0%) and having a composition containing Fe and other unavoidable impurities at a temperature of 1200 ° C .;

 Hot rolling the heated steel above the Ar3 transformation point;

Cooling and winding the central temperature of the steel to a temperature range of 550 to 650 ° C., while maintaining the edge temperature of the steel at 50 ° C. lower than the central temperature of the steel; Provided with the preparation method.

The present invention achieves an alloy design with low cost and at the same time adjusts the hot rolled manufacturing conditions so that the cooling / winding temperature of the center and the edge of the steel maintains a predetermined temperature difference so that the fine precipitates are uniformly distributed in the steel, thereby lengthening the steel. Directional tensile strength deviation is 50MPa or less, the width direction tensile strength variation is 30MPa or less material has the advantage of producing a very uniform high tensile strength of 780MPa or more.

Hereinafter, the characteristic technique of this invention is demonstrated in detail.

In the present invention, the chemical composition of the high strength steel in weight% C: 0.05 ~ 0.10%, Si: 0.4% or less, Mn: 1.0 ~ 2.5%, Mo: 0.1% or less, Cr: 0.01 ~ 0.5%, Al: 0.01 ~ 0.1%, S: 0.01% or less, Nb: 0.01 to 0.1%, Ti: 0.05 to 0.15%, V: 0.01% to 0.1%, N: 0.01% or less, and Mo + Cr: 0.5% or less, and the rest are Fe and Steel having a composition containing other unavoidable impurities, the temperature for hot rolling, that is, heated sufficiently for 1 hour or more at 1200 ℃ or more, and finish hot rolling at the Ar3 transformation point or more, the center of the steel is 550 ~ 650 ℃ Cool down to a section and wind up.

At this time, the edge temperature of the steel is cooled and wound so as to be maintained at a temperature section 50 ° C lower than the central temperature, so that the ferrite grain size is 20um or less, the ferrite fraction is 70% or more, and the fine precipitate of 20nm or less is made of steel. It is uniformly distributed in the inside, and the material has a longitudinal tensile strength deviation of 50 MPa or less and a tensile strength variation of 30 MPa or less in a longitudinal direction where the difference in fractions in the longitudinal and width directions of such fine precipitates is within 20%. It is a technical point to manufacture high strength steel of tensile strength 780 Mpa or more.

By weight% C: 0.05-0.10%, Si: 0.4% or less, Mn: 1.0-2.5%, Mo: 0.1% or less, Cr: 0.01-0.5%, Al: 0.01-0.1%, S: 0.01% or less, Nb : 0.01 to 0.1%, Ti: 0.05 to 0.15%, V: 0.01% to 0.1%, N: 0.01% or less, Mo + Cr: 0.5% or less, and the rest contains a composition containing Fe and other unavoidable impurities Although the reason for numerical limitation of each component is demonstrated as follows.

C is limited to 0.05-0.10%. If the carbon content is low, the fraction of fine precipitates is low, and thus high strength cannot be obtained. If the carbon content is high, other properties such as weldability are inferior, so the lower limit and the upper limit are limited to 0.05% and 0.10%, respectively.

Si is an element having an effect of improving ferrite strength by solid solution strengthening, but when added in a large amount, Si causes a decrease in surface quality, thereby limiting the upper limit to 0.4%.

The Mn is an element having an effect of improving ferrite strength by solid solution strengthening. If the added amount is too small, it is difficult to produce high strength steel, and if it is too high, segregation such as central segregation or micro segregation becomes severe. For this reason, the upper and lower limits are limited to 1.0% and 2.5%, respectively.

Mo is limited to 0.1% or less. Mo is an element that forms carbide and improves strength through precipitation strengthening. Since it is an expensive element, Mo may be added if there is an alternative element that can secure strength, and if the amount is too large, the strength of steel is increased more than necessary. The upper limit is limited to 0.1% since there is a problem of steel unit increase.

The Cr is also an element for forming carbide, and if the amount is too small, it is difficult to obtain high strength, and if the amount is too high, the weldability is adversely affected, so the lower limit and the upper limit are limited to 0.01% and 0.5%, respectively. Cr may be used instead of expensive Mo to secure high strength.

The Al is added for two purposes, one to remove oxygen present in the steel to prevent the formation of non-metallic inclusions during solidification, and the other to refine the grain size by fixing nitrogen in the steel to AlN. For sake.

Therefore, Al should also be added in an appropriate range. If the content is too low, the purpose of the addition cannot be achieved. On the contrary, if Al is too high, there is a problem of increasing the strength of the steel and a problem of increasing the steelmaking unit. Restrict.

Since S is an impurity that increases the amount of precipitates by precipitation in the form of MnS, it is necessary to manage the amount of S low. The upper limit is limited for this reason, but the lower limit is not limited because the lower the amount of S for the same reason, the better the moldability.

The Nb is added for two purposes. One is to delay the recrystallization of the austenite grains during hot rolling, thereby reducing the grain size and improving the strength. The other is to increase the strength through precipitation strengthening by forming fine precipitates. . For this purpose, it is necessary to contain 0.01% or more, but when it exceeds 0.1%, the workability is inferior due to the increase of the rolling load during hot rolling, and the effect of precipitation strengthening is also saturated, thereby limiting the lower limit and the upper limit.

The Ti is an element that is effective in forming a fine precipitate and improving strength through precipitation strengthening. For this purpose, it is necessary to contain 0.05% or more, but when it exceeds 0.15%, the workability is inferior due to the increase of the rolling load during hot rolling, and the lower limit and the upper limit are limited because the effect of precipitation strengthening is also saturated.

V is an element effective for improving the strength through the formation of fine precipitates in the ferrite. To this end, it is necessary to contain 0.01% or more, but when it exceeds 0.1%, the workability is inferior due to the increase of the rolling load during hot rolling, and the lower limit and the upper limit are limited because the effect of precipitation strengthening is also saturated.

N forms coarse TiN or Ti (C, N) precipitates with Ti, thereby reducing the precipitation strengthening effect by reducing the amount of Ti that can precipitate into fine precipitates. Accordingly, the upper limit of N was limited to 0.01%, and the lower the limit, the smaller the amount of fine precipitates.

The Mo + Cr was limited to 0.5% or less, which is an alloying element of Mo and Cr to form carbides, so that the effect can be similar, so that the combined content of the two elements is maintained at 0.5% or less to form carbides. In order to control the increase of strength due to more than necessary, at the same time, it is limited to 0.5% or less to solve the problem of steelmaking unit rise.

In the present invention, the steel containing the above composition is sufficiently heated at 1200 ° C. or higher for at least 1 hour, and hot rolling is finished at an Ar3 transformation point or higher, and the central temperature of the steel is cooled and wound to a temperature section of 550 to 650 ° C. .

At this time, the temperature of the edge portion of the steel (edge) is cooled and wound to maintain 50 ℃ lower than the central temperature, the ferrite grain size is 20um or less, such a ferrite fraction is 70% or more, the fine precipitate of 20nm or less steel Make it evenly distributed inside.

As such, when the central temperature of the steel is maintained at 550 to 650 ° C. while the cooling and winding is performed, the edge temperature of the steel is maintained at 500 to 600 ° C. lower than that of 50 ° C., in the longitudinal and width directions of the fine precipitates. Within 20% of the fractional difference, the tensile strength variation of the steel in the longitudinal direction is 50MPa or less, and the tensile strength variation in the width direction of 30MPa or less can be made of high strength steel with a very uniform tensile strength of 780MPa or more.

Here, the reason for limiting the reheating conditions of the steel to more than 1 hour at 1200 ° C or more is because the coarse precipitate of (Ti, Nb, Mo) (C, N) generated during the steel casting is re-dissolved at a temperature of 1200 ° C or more This is because the coarse precipitate should be sufficiently re-dissolved at, and the coarse precipitate should be sufficiently re-dissolved in order to maximize the fraction of fine precipitates using Ti, Nb, Mo and the like to manufacture high strength steel.

In addition, the reason for defining the hot rolling finish temperature of the steel to more than Ar3 transformation point is to prevent the two-phase reverse rolling is performed, in the case of the present invention, when the two-phase reverse rolling is performed, a large amount of cornerstone ferrite is generated in which no carbide exists By doing so, material deviation may be caused.

The reason for finishing the hot rolling and cooling and winding so that the central portion (ie, the central portion formed long along the longitudinal direction of the steel) in the width direction of the steel having a predetermined length is maintained at a temperature section of 550 to 650 ° C is presented. This is because the production of fine precipitates of 20 nm or less in the temperature range is maximized to obtain hot rolled high strength steel having a small variation in tensile strength in the longitudinal direction.

In addition, the reason for cooling and winding so that the temperature of the edge portion (the edge portion parallel to the longitudinal direction of the steel) of the steel is maintained at a temperature range of 500 to 600 ° C. 50 ° C. lower than the central temperature is the tensile strength in the width direction of the steel. This is to obtain hot rolled high strength steel with small deviation.

If the temperature difference between the center and the edge of the steel is large, the microprecipitation behavior is different, and the fraction of fine precipitates of 20 nm or less is increased at the center, and the fraction of fine precipitates at the edge is small, which increases the variation in the width direction. By maintaining, cooling, and winding, high strength steel having a small variation in strength in the width direction can be obtained.

Hereinafter, preferred examples for aiding in understanding the present invention are presented.

This embodiment is provided so that the present invention can be more easily understood, the present invention is not necessarily limited to the following examples.

[Example]

Steels containing the composition shown in Table 1 were prepared in a thickness of 60 mm and a width of 175 mm through vacuum melting, and after reheating at 1200 ° C. for 1 hour or more, hot rolling was performed to obtain a hot rolled thickness of 6.0 mm.

The hot rolling finish temperature was performed at an Ar3 transformation point or more, and cooled and wound while varying the cooling and winding temperatures of the steel at a temperature range of 400 to 700 ° C. to simulate the temperature of the center and the edge of the steel. The results are shown in Table 2.

[Table 1] Chemical Composition Table by Steel Type (Unit: wt%)

Steel grade C Si Mn Mo Cr Al S Nb Ti V N One
0.071 0.21 1.57 <0.001 0.28 0.04 0.0033 0.04 0.010 0.010 0.0043 2
0.076 0.014 1.66 0.08 0.24 0.029 0.0029 0.029 0.0097 0.009 0.0032 3
0.065 0.13 1.54 0.19 0.25 0.036 0.0022 0.036 0.0093 0.009 0.0025

[Table 2] Tensile strength according to hot rolled manufacturing conditions

Steel grade copy
Steel part Cooling and
Winding temperature (℃) Tensile Strength (TS)
(MPa) (Center-edge)
Strength difference (MPa)
One Edge
450 671



TS <780 MPa Edge
550 682 center
550 682 center
650 709 center
700 685 2 Edge 400 722 TS <780 MPa
Edge
450 783

(823-800) = 23 MPa
Edge
500 800 Edge
550 823 center
550 823 center
600 820 center
650 818 center 700 752 TS <780 MPa
3 Edge 400 782 (848-782) = 66 MPa
Edge
450 801







(848-845) = 3 MPa
Edge
500 822 Edge
550 843 center
550 843 Edge, center
600 845 center
650 848 center 700 764 TS <780 MPa

As shown in Table 1, steel grade 1 is a comparative material which does not belong to the scope of the present invention, and steel grades 2 and 3 are steel grades which belong to the composition range of this invention.

 Hot rolling manufacturing conditions (for example, cooling and winding temperature conditions) for each steel having a composition ratio shown in Table 1 were set in a predetermined temperature range, and after measuring the tensile strength at that time, the difference in strength between the center and the edge of the steel was noted. Only representative ones to be calculated are shown in Table 2.

At this time, the simulated steel part shown in Table 2 is a part that can be estimated by the cooling and winding temperature set for the experiment. If the set temperature range is 550 ~ 650 ° C, it can be estimated as the center of the steel, and the set temperature range is 500 If it is ~ 600 ℃, it can be estimated as the edge of the steel, where the common temperature range, that is, 550 ℃ and 600 ℃ can be seen as the center or the edge.

In Table 2, steel grade 1 is a steel whose Mo + Cr content is less than the composition, and it is impossible to obtain tensile strength of 780 MPa or more required for high strength steel even through cooling and winding up to a temperature range of 450 to 700 ° C.

When the cooling and coiling temperature of steel grade 2 is controlled to be relatively low at 400 ° C., the temperature of the fine precipitate is too low to precipitate, and thus it is difficult to maximize the precipitate.

In addition, when the cooling and coiling temperature of steel grade 2 were controlled to be relatively high at 700 ° C, coarse precipitates of 100 nm or more were formed rather than fine precipitates of 20 nm or less, resulting in low tensile strength of 752 MPa.

In steel grade 2, when the cooling and winding temperature range is set to 450 ~ 650 ℃, the tensile strength is all higher than 780MPa.

At this time, the steel cooled and wound at a relatively low temperature (450 ~ 550 ℃) can be estimated as the edge portion, the steel cooled and wound at a relatively high temperature (550 ~ 650 ℃) can be seen as the center, their strength The difference is based on 823 MPa at 550 ° C, the maximum tensile strength of steel grade 2, and the edge temperature is 500 ° C, which is 50 ° C lower than this, and the tensile strength is 800MPa, so the variation in tensile strength is 23MPa. The present invention in which the direction tensile strength variation is set to 30 MPa or less is satisfied.

On the other hand, when the cooling and winding temperature range of steel grade 3 is set to 450 to 650 ° C, the tensile strength is obtained at 800 MPa or more, and the tensile strength deviation is also the maximum difference in strength based on 848 MPa at 650 ° C where the maximum tensile strength is shown. Appeared to be 3 MPa.

However, when the cooling and winding temperature of steel grade 3 was adjusted to 400 ° C, although the tensile strength was obtained as high as 782MPa, the tensile strength deviation with 650 ° C simulated as the center was obtained as 66MPa, resulting in a large variation in tensile strength with the center. .

Through this, if the center and edge temperatures of the actual hot rolled steel are maintained at temperature ranges of 550 to 650 ° C and 500 to 600 ° C, respectively, high-strength steels of 780 MPa or more with a uniform width direction of less than 30 MPa can be obtained. .

Claims (3)

By weight%, C: 0.05 ~ 0.10%, Si: 0.4% or less (excluding 0%), Mn: 1.0 to 2.5%, Mo: 0.1% or less (excluding 0%), Cr: 0.01 to 0.5%, Al: 0.01 to 0.1%, S: 0.01% or less (excluding 0%), Nb: 0.01 to 0.1%, Ti: 0.05 to 0.15%, V: 0.01 to 0.1%, N: 0.01% or less (excluding 0%), Mo + Cr : Steel is 0.5% or less (except 0%), and the rest is steel with composition including Fe and other unavoidable impurities. Ferrite grain size is 20um or less after cooling and winding, and its ferrite fraction is 70% by volume fraction. High-strength steel with a uniform material in which fine precipitates of 20 nm or less are distributed inside the steel. According to claim 1, wherein the longitudinal volume fraction and the widthwise volume fraction difference of the fine precipitate is within 20%, the longitudinal tensile strength deviation of the steel material is 50MPa or less, the widthwise tensile strength deviation is 30MPa or less High strength steel with uniform material. By weight%, C: 0.05 ~ 0.10%, Si: 0.4% or less (excluding 0%), Mn: 1.0 to 2.5%, Mo: 0.1% or less (excluding 0%), Cr: 0.01 to 0.5%, Al: 0.01 to 0.1%, S: 0.01% or less (excluding 0%), Nb: 0.01 to 0.1%, Ti: 0.05 to 0.15%, V: 0.01 to 0.1%, N: 0.01% or less (excluding 0%) and Mo + Cr: Heating a steel material having a relationship of 0.5% or less (except 0%) and a balance containing Fe and other unavoidable impurities;  Hot rolling the heated steel above the Ar3 transformation point; Cooling and winding the central temperature of the steel to a temperature range of 550 to 650 ° C., while maintaining the edge temperature of the steel at 50 ° C. lower than the central temperature of the steel; Manufacturing method.