KR101482258B1 - Hot Rolled Steel Sheet Having Superior Hot Press Forming Property and High Tensile Strength, Formed Article Using the Steel Sheet and Method for Manufacturing the Steel Sheet and the Formed Article - Google Patents

Abstract

The present invention relates to a hot-rolled steel sheet for use in automotive structural members and parts, and a method of manufacturing the same, and aims to provide a high-strength hot-rolled steel sheet excellent in hot-forming workability, a molded article using the same, and a manufacturing method thereof.

The present invention relates to a steel sheet comprising 0.1 to 0.5% of C, 1.0 to 3.0% of Mn, 0.5 to 4.0% of Si, 0.5 to 0.1% of W, 0.01 to 0.1% of N, 0.01 to 0.1% of Al, 0.001 to 0.1% of Ti, 0.001 to 0.1% of Nb, 0.001 to 0.1% of V, and at least one of Mo and 0.001 to 0.1% of P, , 0.01 to 1.5% of Cr, 0.01 to 1.5% of Cr, 0.005 to 1.0% of Cu and 0.005 to 2.0% of Ni, the balance being Fe and other unavoidable impurities, Characterized in that the structure is composed of 10% or less of pro-eutectoid ferrite, 10% or less of pearlite and the rest of bainite or 10% or less of pro-eutectoid ferrite, 10% or less of pearlite, remaining bainite and martensite A high-strength hot-rolled steel sheet excellent in workability, a molded article using the same, and a manufacturing method thereof.

Hot Forming, Strength, Hot Rolled Steel, Molded Products

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength hot-rolled steel sheet having excellent hot-forming workability, a molded article using the same,

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet used for automobile structural members and parts, and a method of manufacturing the same, and more particularly, to a high-strength hot-rolled steel sheet having excellent hot-forming workability and a molded article using the same.

In the automobile industry, automobile structural materials and parts are becoming lighter and stronger in order to secure environmental regulations and passenger safety.

In order to increase the strength of steel sheets for automobiles, research is underway to develop a method for manufacturing high-strength steel sheets having excellent formability.

Hot press forming is widely applied to obtain high strength and high strength at the same time. The high strength steel sheet used in this hot forming process is a cold rolled steel sheet which is heated and maintained at a high temperature and then pressed with a metal mold to form an automobile structural member and is quenched by a metal mold through which cooling water flows to be subjected to high- Parts.

However, such a hot forming process is low in productivity and the cost of cold-rolled steel sheets used for hot forming is high, so that the burden on cost is inevitably large.

In addition, the hardenability of the cold-rolled steel sheet is not sufficient, and at different cooling rates, different parts have different hardness values.

Examples of conventional techniques for solving such problems include those proposed in Japanese Patent Application Laid-Open Nos. 2006-126733, 2006-152427, 2006-213959, and the like.

First of all, in Japanese Patent Laid-Open No. 2006-126733, there is disclosed a method of producing a steel sheet comprising 0.05 to 0.4% of C, 0.01 to 4.0% of Mn, 0.005 to 5.0% of Cr, 0.1 to 3.0% of Mo + , Ni, and the like, and a cold-rolled steel sheet to which N: 0.01% or less is added is hot-pressed to have a martensite structure of 60% or more.

Japanese Unexamined Patent Publication No. 2006-152427 discloses a steel sheet comprising 0.25 to 0.45% of C, 0.5 to 3.0% of Mn + Cr and alloying elements such as Mo, Nb, Ti, V and B and adding 0.002% And cooling the steel sheet at a cooling rate of 10 to 500 DEG C / s under Ms point to produce a cold-rolled steel sheet.

Japanese Patent Laid-Open No. 2006-213959 also proposes a cold-rolled steel sheet for hot forming similar to the above-mentioned two patents.

It is an object of the present invention to provide a high-strength hot-rolled steel sheet excellent in hot-forming workability, a molded article using the same, and a method of manufacturing the same.

Hereinafter, the present invention will be described.

The present invention relates to a steel sheet comprising 0.1 to 0.5% of C, 1.0 to 3.0% of Mn, 0.5 to 4.0% of Si, 0.5 to 0.1% of W, 0.01 to 0.1% of N, 0.01 to 0.1% of Al, 0.001 to 0.1% of Ti, 0.001 to 0.1% of Nb, 0.001 to 0.1% of V, and at least one of Mo and 0.001 to 0.1% of P, , 0.01 to 1.5% of Cr, 0.01 to 1.5% of Cr, 0.005 to 1.0% of Cu and 0.005 to 2.0% of Ni, the balance being Fe and other unavoidable impurities, Characterized in that the structure is composed of 10% or less of pro-eutectoid ferrite, 10% or less of pearlite and the rest of bainite or 10% or less of pro-eutectoid ferrite, 10% or less of pearlite, remaining bainite and martensite To a high-strength hot-rolled steel sheet excellent in workability.

The present invention also relates to a steel sheet comprising a steel sheet containing 0.1 to 0.5% of C, 1.0 to 3.0% of Mn, 0.5% or less of Si, 0.1% or less of W, 0.01 to 0.1% of N, 0.01 to 0.1% 0.001 to 0.1% of Ti, 0.001 to 0.1% of Nb and 0.001 to 0.1% of V, and 0.001 to 0.01% of B, And at least one of at least one selected from the group consisting of Mo: 0.01 to 1.5%, Cr: 0.01 to 1.5%, Cu: 0.005 to 1.0% and Ni: 0.005 to 2.0%, and the balance Fe and other unavoidable impurities , And the structure thereof includes at least one of martensite of 80% or more and bainite, pearlite and ferrite of 20% or less in an area fraction, and has a tensile strength of 1470 MPa or more.

The present invention also relates to a steel sheet comprising a steel sheet containing 0.1 to 0.5% of C, 1.0 to 3.0% of Mn, 0.5% or less of Si, 0.1% or less of W, 0.01 to 0.1% of N, 0.01 to 0.1% 0.001 to 0.1% of Ti, 0.001 to 0.1% of Nb and 0.001 to 0.1% of V, and 0.001 to 0.01% of B, And a steel material comprising 0.01 to 1.5% of Mo, 0.01 to 1.5% of Cr, 0.005 to 1.0% of Cu and 0.005 to 2.0% of Ni and at least one of Fe and other unavoidable impurities To a high-temperature hot-rolled steel sheet excellent in hot forming workability, characterized in that hot rolling at the Ar3 transformation point or more is completed and cooling is carried out at a cooling rate of 10 DEG C / s or higher and winding is carried out at Bs (bainite transformation start temperature) .

In the present invention, the hot-rolled steel sheet produced as described above is used to hold the steel sheet at a temperature equal to or higher than Ac3 point, subject to hot forming processing, and quenched at a cooling rate of 1 deg. C / s or higher to Ms (martensitic transformation start temperature) And a method for producing a molded article.

According to the present invention, it is possible to obtain a hot-formed structural member having excellent strength even if it is cooled at a low cooling rate in the hot forming process as compared with a conventional cold-rolled steel sheet for hot forming, Can be provided.

Hereinafter, the present invention will be described in detail.

The steel sheet according to the present invention contains 0.1 to 0.5% of C, 1.0 to 3.0% of Mn, 0.5% or less of Si, 0.1% or less of W, 0.01 to 0.1% of N, 0.01 to 0.1% 0.001 to 0.1% of Ti, 0.001 to 0.1% of Nb and 0.001 to 0.1% of V, and 0.001 to 0.01% of B, And at least one of at least one of Fe, 0.01 to 1.5% of Cr, 0.01 to 1.5% of Cr, 0.005 to 1.0% of Cu and 0.005 to 2.0% of Ni, and the balance of Fe and other unavoidable impurities. The reason for limiting the numerical value of each component is as follows.

C is limited to 0.1 to 0.5%. Carbon is an essential element for increasing the strength of a steel sheet. In order to obtain a high-strength martensite phase in hot-formed components, the lower limit of the C content is 0.1%. When carbon is added in an amount exceeding 0.5%, the weldability deteriorates, which may cause defects in assembling the automobile. In addition, since the strength of the steel sheet in the plating process becomes too high and the steel sheet becomes difficult to reach, the upper limit is 0.5% Limit.

The Mn is an element having a very strong solid solution strengthening effect in the steel, and at the same time, it delays the transformation from austenite to ferrite and lowers the Ar3 temperature. If the addition amount is too small, hot forming processing becomes difficult in a single phase of austenite during hot forming. If the amount is too large, there is a problem in manufacturing hot-rolled coils due to weldability and increase in rolling load during hot rolling, 1.0 to 3.0%.

The Si is an element having an effect of enhancing the ferrite strength by solid solution strengthening. However, when the Si is added in a large amount, the increase of the scale defects results in lowering the surface quality and lowering the plating ability.

The W is the only element capable of improving the heat-curing ability during hot forming and improving the heat resistance. In addition, W is an important element in the present invention since grain growth of crystal grains can be suppressed in reheating the steel sheet and the effect of reducing the grain size can be expected. However, when the addition amount of W exceeds 0.1%, this effect is not only saturated but also the manufacturing cost is increased due to the high price, so that the upper limit is limited to 0.1%.

Al is added for two purposes. One of them is to remove the oxygen present in the steel to prevent the formation of non-metallic inclusions during solidification, and the other is to fix the nitrogen existing in the steel to AlN to fix the grain size To make it finer. If the content of Al is too low, the above-mentioned purpose can not be attained. On the other hand, when the content of Al is too high, there is a problem of increasing the strength of the steel and a problem of raising the steel basic unit. %.

Since S is an impurity which precipitates in the form of MnS and increases the amount of precipitate, it is necessary to control the amount of S to be low, and the upper limit is limited to 0.03%. The reason why the lower limit is not defined is that the lower the S amount is, the better the moldability becomes for the same reason as described above.

If P is excessively added, the weldability and adverse effects are exerted in the production of hot rolled steel. Also, since the workability deteriorates, the upper limit is limited to 0.1%.

N is a very important element in the present invention. N is a solid solution strengthening element and at the same time, it is an element which forms a nitride by binding with Ti, Nb, V, Al and the like. In the present invention, sufficient N is added for heat treatment and strength enhancement. If the N content is less than 0.01%, such an effect can not be expected. If the N content exceeds 0.1%, problems in the steelmaking and casting process may be caused.

Therefore, the content of N is limited to 0.01 to 0.1%.

The above B plays a role of suppressing the transformation of austenite into ferrite or bainite by segregating the grain boundaries and lowering the grain boundary energy. When the addition amount is too small, the effect of addition can not be sufficiently secured. , There is a fear that the deterioration of toughness and the reduction of the entanglement due to the precipitation of a large amount of the B precipitate at the grain boundary may occur, and therefore the content of B is limited to 0.001 to 0.01%.

The hot-rolled steel sheet of the present invention contains one or more of Ti, Nb, and V.

Ti, Nb, and V are effective elements for enhancing the toughness of the steel sheet by enhancing the precipitation of the carbonitride, and improving the toughness of the hot-formed structural component through grain refinement. If the addition amount is less than 0.001%, such an effect can not be obtained. If the addition amount exceeds 0.1%, the manufacturing cost may be increased and excessive carbonitride may be precipitated, which may cause problems in the hot rolling manufacturing process.

The hot-rolled steel sheet of the present invention contains 0.01 to 1.5% of Mo, 0.01 to 1.5% of Cr, 0.005 to 1.0% of Cu, and 0.005 to 2.0% of Ni.

The Mo greatly improves the hardenability, and after the hot forming process, the martensite structure is ensured smoothly, thereby making it possible to manufacture high-strength hot formed parts. Further, precipitation of fine carbides is promoted, and the increase of the strength and the miniaturization of the grain size affect the improvement of toughness. In order to obtain this effect, the lower limit is limited to 0.01%, and when the content is 1.5%, the effect is saturated and the steelmaking cost increases.

The Cr is an element that promotes hardenability and carbide formation, and is an important element for the production of high-strength hot-formed structural components. In order to obtain the effect, the lower limit is limited to 0.01%, and when the content is 1.5%, the effect is saturated and there is a problem of an increase in steelmaking cost, and the upper limit is limited to 1.5%.

The Cu is an element effective for promoting fine precipitates to increase the strength. If it is less than 0.005%, the effect can not be obtained. If it exceeds 1.0%, the workability is deteriorated. Therefore, the content of Cu is limited to 0.005-1.0%.

The Ni is an element effective for enhancing solubility as well as improving the hardenability and improving the heat treatability. If the content is less than 0.005%, the effect can not be obtained. If the content exceeds 2.0%, problems of workability deterioration, hot rolling scale formation, and manufacturing cost increase, and the Ni content is limited to 0.005 to 2.0%.

Fig. 1 schematically shows the change in the continuous cooling phase change due to the addition of the alloying element.

FIG. 1 (a) is a graph showing the microstructure obtained by cooling a general steel from a high temperature (for example, finish rolling finish temperature) to a different temperature at different cooling rates (cooling rate 1> 2> 3) And FIG. 1 (b) is a graphical representation of the microstructure as a continuous cooling state diagram when an alloy element for improving hardenability is added to a general steel.

1 (a), a single phase of martensite is obtained upon cooling at a cooling rate of 1, and a ferrite + bainite + martensite structure is obtained at a cooling rate of 2, and when cooled at a cooling rate of 3, ferrite + Structure of pearlite + bainite is obtained.

As shown in Fig. 1 (b), it can be seen that the ferrite and pearlite bainite transformation curves are shifted to the right in the time axis as compared with the position in Fig. 1 (a), which means that the transformation is delayed will be. The effect of this alloying element is to obtain a microstructure different from that of a normal steel at the same cooling rate, that is, at a cooling rate of 1 in FIG. 1 (b), martensite is obtained, At the cooling rate of 3, microstructure of bainite and martensite is obtained. That is, there is an advantage that the cooling rate is enhanced without increasing the cooling rate.

In the present invention, an alloying element for improving hardenability is added to a general steel to obtain an effect of enhancing the cooling rate without increasing the cooling rate.

The hot-rolled steel sheet of the present invention is characterized in that the structure thereof is composed of 10% or less pro-eutectoid ferrite, 10% or less of pearlite, remaining bainite, or 10% or less of periostatic ferrite, 10% or less of pearlite, .

When the structure contains martensite, it is preferable that the bainite content is 50% or more and the martensite content is 30% or less.

Hereinafter, a method for manufacturing a steel sheet according to the present invention will be described.

In the present invention, steel having the above composition is reheated to a slab and subjected to hot rolling to finish hot rolling at a temperature higher than the Ar3 transformation point and cooling at a cooling rate of 10 DEG C / s or higher to obtain Bs (bainite transformation Hot-rolled steel sheet excellent in hot-forming workability is produced by winding the steel sheet at a temperature not higher than the initial temperature (usually not higher than 600 ° C).

The reason why the hot rolling finishing temperature is defined to be higher than the Ar3 transformation point is to prevent the two-phase rolling from occurring. In the case of the steel according to the present invention, when two-phase rolling is performed, It is impossible to obtain a bainite structure over the entire structure pursued by the present invention.

The reason why the cooling rate after hot rolling is limited to not less than 10 DEG C / s is that the precipitation of ferrite and pearlite is carried out at a cooling rate of not more than 10 DEG C / s, and therefore the hot bainite intended by the present invention or bainite and martensite . As can be seen from FIG. 1, the upper limit is not limited because the higher the cooling rate, the more difficult it is to obtain bainite and martensite structure.

The reason why the hot rolled coiling is limited to the coiling temperature below the Bs (bainite transformation start temperature) is that the coiling at a higher temperature causes pearlite transformation, so that the low temperature structure intended by the present invention can not be obtained, If the actual hot-rolled coil is excellent in performance and can be wound, it is not limited because it is advantageous to obtain a fine bainite or martensite structure contemplated by the present invention.

When the hot-rolled steel sheet thus produced is maintained at a temperature equal to or higher than the Ac3 point, hot-formed, and quenched at a cooling rate of 1 DEG C / s or higher to the Ms (martensitic transformation starting temperature), the martensite area fraction Of 80% or more and a tensile strength of 1470 MPa or more can be obtained.

The structure of the molded article includes at least 80% of martensite and 20% or less of bainite, pearlite and ferrite in an area fraction.

The hot-rolled steel sheet of the present invention can obtain a molded article having a martensite area fraction of 80% or more and a tensile strength of 1470 MPa or more even at a relatively low cooling rate of 1 to 30 ° C / s.

If the cooling rate after the hot forming is more than 1 ° C / s, the austenite transforms into ferrite or pearlite which is a high temperature phase during cooling, so that a sufficient martensite fraction can not be secured and it is difficult to obtain a tensile strength of 1470 MPa or more The lower limit of the cooling rate is preferably limited to 1 占 폚 / s, and the higher the cooling rate, the more advantageous for securing the martensite structure, so the upper limit is not limited.

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

[Example]

A steel ingot having the composition shown in Table 1 was produced by vacuum induction melting to have a thickness of 60 mm and a width of 175 mm and reheated at 1200 캜 for 1 hour and then hot rolled to a thickness of 1.6 mm. The hot rolling finish temperature was above the Ar3 transformation point. The cooling rate of the ROT was 5 ° C / s and 50 ° C / s, cooled to the target hot rolled coiling temperature, maintained at 400 to 650 ° C for 1 hour, The hot rolled coils were simulated by Sikim. The hot forming simulation was conducted at a cooling rate of 20 ° C / s using a dilatometer and the hardness was measured and converted into tensile strength. The results are shown in Table 2 below.

The steel types 1, 2 and 3 among the steel types in Table 1 fall within the scope of the invention, while the steel types 4 and 5 are steel types deviating from the constituent conditions of the invention steel. Table 2 shows the production conditions for the steel of Table 1, that is, the cooling rate after the finishing rolling (ROT cooling rate), the presence of pro-eutectoid ferrite depending on the coiling temperature and the converted tensile strength after the final hot-

The criterion of pro-eutectoid ferrite / pearlite oil and no pro-eutectic shown in the following Table 2 is indicated in the case where the amount of pro-eutectoid ferrite / pearlite is each 10% or more, and when it is 10% / This corresponds to the invention steel only when pearlite is zero.

Steel grade
Chemical composition (wt%) C Mn Si Al S P N W B Ti Etc One 0.22 2.2 0.25 0.032 0.002 0.012 0.012 0.03 0.0055 0.025 Ni 0.2, Mo 0.1 2 0.21 2.16 0.29 0.024 0.003 0.01 0.016 0.03 0.0028 0.023 Cr 0.2, Nb 0.01 3 0.21 2.31 0.23 0.042 0.002 0.01 0.011 0.028 0.0037 0.022 Cu 0.03, V 0.01 4 0.07 0.76 0.20 0.030 0.0026 0.011 0.0014 - 0.0018 0.021 Cr 0.21, Mo 0.1 5 0.23 0.41 0.20 0.037 0.0031 0.012 0.0050 0.25 0.0021 0.017 Nb 0.011, V 0.098

Steel grade Grade - Manufacturing Conditions ROT cooling rate (° C / s) Coiling temperature
(° C) Ferrite / Pearlite
The presence or absence After hot forming simulation
Tensile Strength (MPa) Remarks One 1-1 5 450 U 1270 Comparative River 1 1-2 50 450 radish 1610 Inventive Steel 1 2 2-1 50 550 radish 1580 Invention river 2 2-2 50 450 radish 1670 Invention steel 3 2-3 50 650 radish 1320 Comparative River 2 3 3-1 50 550 radish 1540 Inventive Steel 4 3-2 5 450 U 1280 Comparative Steel 3 3-3 5 650 U 1140 Comparative Steel 4 4 4-1 5 550 U 1210 Comparative Steel 5 4-2 50 550 U 1270 Comparative Steel 6 5 5-1 50 500 U 1260 Comparative Steel 7 5-2 50 650 U 1220 Comparative Steel 8

As shown in Table 2, in the case of steels of the invention in which the steel types 1, 2 and 3 were quenched (cooling rate: 50 ° C / s) during ROT cooling and rolled at a temperature not higher than Bs, bainite structure And formed a martensite even at a relatively low cooling rate (20 ° C / s) after hot forming simulation, resulting in a tensile strength of more than 1470 MPa.

On the other hand, bainitic structure containing the eutectoid ferrite / pearlite was obtained by controlling the ROT quenching and the coiling temperature of the steel types 4 and 5, and based on this, the hot forming was cooled and cooled at a cooling rate of 20 ° C / s , A tensile strength of 1470 MPa could not be obtained.

When the coiling temperature was higher than Bs, the final hot rolled steel showed ferrite + pearlite texture and the tensile strength after hot forming was as low as 1200 MPa.

The coiling temperature was kept below Bs by using the steel types 1, 2 and 3. However, when the cooling rate was lower than the range of the present invention, the composite structure of ferrite / perlite + bainite was observed as a whole, Was less than 1470 MPa.

The presence or absence of pro-eutectoid ferrite depends on the case where the final hot rolling is performed at a temperature lower than the Ar3 transformation point and also depends on the cooling rate after the finish rolling (ROT cooling rate) .

That is, although the Ar3 transformation temperature mainly depends on the cooling rate after the start of cooling in the austenite region, rolling at the Ar3 transformation point or below implies generation of pro-eutectoid ferrite, which causes microstructure heterogeneity.

The slower the ROT cooling rate promotes the ferrite and pearlite transformation, and the higher the cooling rate, the more the bainite and martensite transformation occur.

Further, the lower the coiling temperature at which the hot rolling transformation is completed, the lower the probability of the presence of pro-eutectoid ferrite. As shown in Table 2, as the coiling temperature is higher under the same composition and cooling conditions, coercive ferrite is produced more.

In the present invention, even if cooling is carried out at a low cooling rate after hot forming, it is intended to secure a high strength of 1470 MPa or more in tensile strength. In the hot-rolled steel sheet, by inhibiting cornstone ferrite and pearlite and obtaining bainite or martensite structure, It is possible to dissolve the carbide more quickly and uniformly upon heating and holding, and to prevent micro-segregation in the hot-rolled sheet, thereby achieving the above-mentioned target.

Fig. 1 is a schematic view showing control of continuous phase transformation by addition of an alloying element, wherein (a) shows a case of a general steel and (b) shows a case where an alloy element is added.

Claims (4)

0.1 to 0.5% of Mn, 1.0 to 3.0% of Mn, 0.5 to 4.0% of Si, 0.1 to 0.1% of W, 0.01 to 0.1% of N, 0.01 to 0.1% of Al, : 0.001 to 0.1% of Ti, 0.001 to 0.01% of B, 0.001 to 0.1% of Ti, 0.001 to 0.1% of Nb and 0.001 to 0.1% of V, Wherein at least one of Fe and other unavoidable impurities is added in an amount of 0.1 to 1.5%, Cr: 0.01 to 1.5%, Cu: 0.005 to 1.0%, and Ni: 0.005 to 2.0% Or less, or 10% or less of perovskite ferrite, 10% or less of pearlite, and the remaining bainite, or 10% or less of perovskite ferrite, 10% or less of pearlite, High-strength hot-rolled steel sheet excellent in molding processability delete 0.1 to 0.5% of Mn, 1.0 to 3.0% of Mn, 0.5 to 4.0% of Si, 0.1 to 0.1% of W, 0.01 to 0.1% of N, 0.01 to 0.1% of Al, : 0.001 to 0.1% of Ti, 0.001 to 0.01% of B, 0.001 to 0.1% of Ti, 0.001 to 0.1% of Nb and 0.001 to 0.1% of V, At least one of Fe and other unavoidable impurities is added at a temperature of not less than the Ar3 transformation point and not more than 1.5%, Cr: 0.01 to 1.5%, Cu: 0.005 to 1.0% and 0.005 to 2.0% A method of manufacturing a high-strength hot-rolled steel sheet excellent in hot-forming workability, characterized in that hot rolling is completed and cooling is carried out at a cooling rate of 10 DEG C / s or higher and winding is carried out at Bs (bainite transformation start temperature) delete

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