KR20150075319A - High strength steel for hot forming and preparation method thereof - Google Patents

Abstract

The present invention relates to a high-strength structural steel material for hot forming having a low loss of impact toughness and strength even after hot forming. By manufacturing the high-strength structural steel material in accordance to the manufacturing method of the present invention, the high-strength structural steel material has a tensile strength of 700 Mpa or higher even after hot rolling and cooling, and has a yield strength of 460 Mpa or higher; has a tensile strength of 600 Mpa or higher; and an impact absorbing energy of 100 J or higher at -5°C after hot forming at 800°C for 30 minutes may be provided. The high-strength structural steel material comprises: 0.10-0.20 wt% of C, 1.2-2.5 wt% of Mn, 0.2-0.8 wt% of Si, 0.005-0.10 wt% of Nb, 0.01-1.0 wt% of Mo, 0.05-1.0 wt% of Cr, 0.005-0.1 wt% of Ti, 0.005-0.5 wt% of Al, 70-150 ppm of N, 60 ppm or less of Ca (exclusive of 0 ppm), 0.02 wt% or less of P, 0.01 wt% or less of S, at least one selected from a group composed of 0.010-1.0 wt% of Cu, 0.01-2.0 wt% of Ni, 0.005-0.3 wt% of V, and 0.005-0.2 wt% of W, and the remainder consisting of Fe and inevitable impurities.

Description

TECHNICAL FIELD [0001] The present invention relates to a high strength structural steel for hot forming and a manufacturing method thereof. BACKGROUND OF THE INVENTION [0002]

The present invention relates to a high strength structural steel for hot forming used in an axle housing of a heavy truck and a method of manufacturing the same. More particularly, the present invention relates to a high strength steel for hot forming, High strength structural steel and a manufacturing method thereof.

Recently, in the case of structural steels, the use of high-temperature-controlled high-strength steel plates has been increasing. However, in the case of heat-treated control high-strength steel sheet, when the steel sheet is used in a product subjected to a hot-forming step for deep processing such as an axle housing of an automobile,

That is, if the steel sheet is heated to a high temperature of 800 ° C. for hot forming, the lower microstructure introduced by the thermal processing control in the case of the heat treatment control high-strength steel sheet is recovered and the strength and impact toughness are decreased due to the growth of the grain size. Accordingly, it is necessary to develop a high-strength steel sheet having low loss of strength and impact toughness even after hot forming at a high temperature of 800 ° C.

The present invention is intended to provide a high strength structural steel material for hot forming which ensures excellent tensile strength and impact toughness even after hot forming.

The present invention is intended to provide a method for manufacturing a high strength structural steel for hot forming, which ensures excellent tensile strength and impact toughness even after hot forming.

The present invention relates to a ferritic stainless steel comprising 0.10 to 0.20% of C, 1.2 to 2.5% of Mn, 0.2 to 0.8% of Si, 0.005 to 0.10% of Nb, 0.01 to 1.0% of Mo, 0.05 to 1.0% : 0.005 to 0.1%, Al: 0.005 to 0.5%, N: 70 to 150 ppm, Ca: 60 ppm or less (excluding 0 ppm), P: 0.02% or less and S: 0.01% , Ni: 0.01 to 2.0%, V: 0.005 to 0.3% and W: 0.005 to 0.2%, and the balance Fe and other unavoidable impurities, and expressed by the following relational expression 1 And a nitride index (NI) of 0.23 to 0.5, which is a high strength structural steel for hot forming.

[Relation 1]

Nitride index (NI) = 0.293 x Ti + 0.151 x Nb + 0.519 x Al + 0.275 x V + 0.073 x Cu + 0.499 x Si + 0.180 x Cr + 0.076 x W

Here, preferably, the microstructure of the high-strength structural steel for hot forming has an area fraction of not more than 15% of ferrite and the remainder is bainite.

Preferably, the high-strength structural steel for hot forming has a tensile strength of 700 Mpa or more, and the hot-formed structural steel for hot forming has a yield strength of 460 MPa or more, a tensile strength of 600 MPa or more after hot- At 5 ℃, the impact absorption energy is over 100J.

The present invention relates to a ferritic stainless steel comprising 0.10 to 0.20% of C, 1.2 to 2.5% of Mn, 0.2 to 0.8% of Si, 0.005 to 0.10% of Nb, 0.01 to 1.0% of Mo, 0.05 to 1.0% : 0.005 to 0.1%, Al: 0.005 to 0.5%, N: 70 to 150 ppm, Ca: 60 ppm or less (excluding 0 ppm), P: 0.02% or less and S: 0.01% , Ni: 0.01 to 2.0%, V: 0.005 to 0.3% and W: 0.005 to 0.2%, and the balance Fe and other unavoidable impurities, and expressed by the following relational expression 1 Reheating the slab having a nitride index (NI) of 0.23 to 0.5 at 1050 to 1250 占 폚; Subjecting the reheated slab to finish rolling under the condition that the cumulative rolling reduction ratio (%) and the finish rolling starting temperature (占 폚) satisfy the following relational expression 2; Cooling the hot-rolled steel material at a cooling rate of 10 to 50 DEG C / s, and ending cooling at a cooling end temperature not higher than the bainite transformation completion temperature (Bf), in a method of manufacturing a high strength structural steel material for hot forming .

[Relation 1]

Nitride index (NI) = 0.293 x Ti + 0.151 x Nb + 0.519 x Al + 0.275 x V + 0.073 x Cu + 0.499 x Si + 0.180 x Cr + 0.076 x W

 [Relation 2]

Rolling roll cumulative rolling reduction ratio (%) / rolling roll starting temperature (캜) ≤0.09

The steel material for high-strength structural steel for hot forming according to the present invention can maintain excellent strength and impact toughness even when heated at a high temperature of 800 DEG C for 30 minutes.

1 is a graph showing an example of a change in ferrite fraction and a change in tensile strength according to a value obtained by dividing a cumulative rolling reduction ratio by a finish rolling starting temperature.
2 is a graph showing an example of a change in tensile strength according to a nitride index (NI) and a change in impact absorption energy at -5 DEG C after holding at a high temperature of 800 DEG C for 30 minutes and then air cooling.

Hereinafter, the high strength structural steel for hot forming according to the present invention will be described in detail.

The high strength structural steel for hot forming according to the present invention comprises 0.10 to 0.20% of C, 1.2 to 2.5% of Mn, 0.2 to 0.8% of Si, 0.005 to 0.10% of Nb, 0.01 to 1.0% of Mo, 0.01 to 1.0% of Cr, : 0.05 to 1.0%, Ti: 0.005 to 0.1%, Al: 0.005 to 0.5%, N: 70 to 150 ppm, Ca: 60 ppm or less (excluding 0 ppm), P: 0.02% .

Hereinafter, the reason for limiting each composition of the steel will be described. However, it should be noted that the content of each component means weight% unless otherwise stated.

C: 0.10 to 0.20%

If the content of C is less than 0.10%, the curing ability can not be sufficiently secured. However, when the content of C exceeds 0.2%, deterioration of the physical properties of the welded part is caused by an increase in the carbon equivalent, so the range of C is preferably limited to 0.10 to 0.20%.

Mn: 1.2 to 2.5%

Since Mn is a useful element for improving the strength by solid solution strengthening, it is necessary to add at least 1.2%. However, when it exceeds 2.5%, the toughness of the welded part is greatly lowered due to an increase in the hardenability, so that the content of Mn is preferably limited to 1.2 to 2.5%.

Si: 0.2 to 0.8%

Si is used as a deoxidizing agent and is useful because it has an effect on the formation of nitrides. However, when it is added in excess of 0.8%, it lowers the low temperature toughness of the welded portion and deteriorates the weldability at the same time. On the other hand, when the Si content is less than 0.2%, the effect of nitride formation is deteriorated. Therefore, the Si content is preferably limited to 0.2 to 0.8%.

Nb: 0.005 to 0.10%

Nb is the most important element in the production of TMCP steel and precipitates in the form of nitride, which greatly improves the strength of the base material and the welded portion. Further, in the case of reheating at a high temperature, the solid solution Nb has the effect of inhibiting recrystallization of austenite, suppressing ferrite transformation, and making the structure finer. In addition, in the present invention, when the slab is cooled after rough rolling, the bainite is formed at a low cooling rate. For this effect, Nb is preferably added in an amount of 0.005% or more, but if it exceeds 0.10%, the possibility of causing a brittle crack in the edge of the steel is increased. Accordingly, the content of Nb is preferably limited to a range of 0.005 to 0.10%.

Mo: 0.01 to 1.0%

The addition of only a small amount of Mo greatly improves the hardenability and has the effect of suppressing the formation of ferrite. Since the strength can be greatly improved, it is necessary to add at least 0.01%, but when it exceeds 1.0% And toughness may be inhibited. Therefore, the Mo content is preferably limited to a range of 0.01 to 1.0%.

Cr: 0.05 to 1.0%

Since Cr has a great effect on increasing the hardenability by increasing the hardenability, it is necessary to add 0.05% or more in order to obtain such an effect. When it exceeds 1.0%, the weldability may be greatly decreased. Is preferably added.

Ti: 0.005 to 0.1%

The addition of Ti can improve the low-temperature toughness by suppressing the growth of crystal grains during reheating and is effective for nitride formation, so that it is necessary to add 0.005% or more to exhibit the effect. However, if it exceeds 0.1%, there is a problem that clogging of the performance nozzle or low-temperature toughness due to centering is reduced, so that the content of Ti is preferably limited to a range of 0.005 to 0.1%.

Al: 0.005-0.5%

Since Al is an element capable of inexpensively deoxidizing molten steel and is an element useful for nitride formation, it is preferably added in an amount of 0.005% or more, but if it exceeds 0.5%, it causes nozzle clogging during continuous casting, To about 0.5%.

N: 70 ~ 150 ppm

The addition of N increases the strength by nitride formation, but it is necessary to limit its content to 150 ppm or less because it greatly reduces toughness. However, when the N content is 70 ppm or less, sufficient nitride formation is deteriorated, so that the lower limit of the N content is preferably 70 ppm.

Ca: 60 ppm Less than (0 ppm Excluded)

Ca is added as CaS to inhibit the nonmetallic inclusions of MnS. For this purpose, Ca should be added in excess of 0.0005%. However, if the content exceeds 0.006%, it reacts with oxygen contained in the steel to produce CaO which is a nonmetallic inclusion, so that the upper limit is preferably limited to 0.006%.

P: not more than 0.02%

P is an element favorable for strength improvement and corrosion resistance, but it is preferably contained as low as possible since it is an element which greatly deteriorates impact toughness. However, P is an impurity inevitably contained in the manufacturing process. Therefore, it is preferable to limit the upper limit to 0.02%.

S: not more than 0.01%

Since S is an element that significantly inhibits impact toughness by forming MnS or the like, it is preferably contained as low as possible, but it is preferably an impurity which is inevitably contained in the manufacturing process. Therefore, the upper limit is preferably limited to 0.01%.

The present invention may further include at least one selected from the group consisting of 0.010 to 1.0% of Cu, 0.01 to 2.0% of Ni, 0.005 to 0.3% of V, and 0.005 to 0.2% of W in the present invention.

Cu: 0.010 to 1.0%

Cu is an element capable of minimizing toughness deterioration of the base material and increasing the strength at the same time, and therefore, at least 0.01% should be added to exhibit the effect. On the other hand, when it exceeds 1.0%, the surface quality of the product is greatly deteriorated, so that the content of Cu is preferably limited to 0.010 to 1.0%.

Ni: 0.01 to 2.0%

Ni is an element capable of simultaneously improving the strength and toughness of the base material, and at least 0.01% should be added in order to exhibit the effect. However, since Ni is an expensive element, when it exceeds 2.0%, the economical efficiency is lowered and the weldability is deteriorated. Therefore, the content of Ni is preferably limited to a range of 0.01 to 2.0%.

V: 0.005 to 0.3%

V is an element capable of effectively forming nitride during hot forming because the temperature employed is low compared with other fine alloys and it is effective to prevent precipitation of the nitride in the weld heat affected portion and it is preferable to add 0.005% . However, if it exceeds 0.3%, impact toughness may be lowered, and therefore it is preferable that V is limited to 0.005 to 0.3%.

W: 0.005 to 0.2%

W is an effective element for preventing softening of the base structure and precipitating nitride, but it is expensive, and therefore it is preferable to be added in the range of 0.005 to 0.2%.

The remainder of the invention is iron (Fe) and other inevitable impurities. However, in the ordinary steel manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of steel making.

The steel material of the present invention satisfies the above-mentioned component conditions and has a nitride index (NI) expressed by the following relational expression 1 in the range of 0.23 to 0.5.

[Relation 1]

Nitride index (NI) = 0.293 x Ti + 0.151 x Nb + 0.519 x Al + 0.275 x V + 0.073 x Cu + 0.499 x Si + 0.180 x Cr + 0.076 x W

When the nitride index (NI) is lower than 0.23, the tensile strength after hot forming can not be secured to 600 MPa. If the nitride index (NI) exceeds 0.5, the impact absorption energy is lower than 100 J at -5 DEG C due to formation of coarse nitride. Therefore, the nitride index (NI) is preferably 0.23 to 0.5.

For example, the steel material was maintained at a high temperature of 800 DEG C for 30 minutes and then air-cooled. Then, the change of the tensile strength according to the nitride index NI and the change of the impact absorption energy at -5 DEG C were examined. . As shown in FIG. 2, when the nitride index (NI) is lower than 0.23, the tensile strength after hot forming can not be secured to 600 Mpa. When the nitride index exceeds 0.5, Is lower than 100J.

The microstructure of the high strength structural steel of the present invention preferably has an area fraction of ferrite of 15% or less and the balance of bainite. If the ferrite fraction exceeds 15% by area, the tensile strength before hot working becomes 700 MPa or less, and the yield strength after hot working of 460 MPa and the tensile strength of 600 MPa or more can not be secured.

The tensile strength of the high-strength structural steel for hot forming according to the present invention is 700 MPa or more. The hot-formed high-strength structural steel for hot forming is hot-molded for 30 minutes at a temperature of 460 MPa or higher, The impact absorption energy is preferably 100 J or more.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a method for manufacturing a high strength structural steel for hot forming according to the present invention will be described in detail.

In order to produce a steel material according to the production method of the present invention, a steel material satisfying the above-described composition components and component relations is subjected to reheating, preferably reheating at 1050 to 1250 ° C.

In order to solidify Ti, Nb carbide and complex carbide formed during casting, it is preferable to perform reheating at a temperature of 1050 캜 or higher. However, when reheating at an excessively high temperature, the austenite may be coarsened, so that the upper limit of the reheating temperature is preferably limited to 1250 캜.

Next, the reheated steel is subject to ordinary rough rolling and finish rolling. At this time, the finishing rolling is carried out under the condition that the cumulative rolling reduction ratio (%) and the finish rolling starting temperature (캜) satisfy the following relational expression (2).

[Relation 2]

Rolling roll cumulative rolling reduction ratio (%) / rolling roll starting temperature (캜) ≤0.09

In the case of a high-strength steel plate used for an axle housing or the like, it is usually a steel sheet having a thickness of 20 mm or less. Therefore, in the case of ordinary rolling, the cumulative rolling reduction ratio is very high.

When the cumulative rolling reduction ratio is high and the finish rolling finish temperature is low and the cumulative rolled rolling reduction (%) / rolling finish temperature (° C) value is higher than 0.09, ferrite transformation is promoted and the bainite fraction is reduced. As a result, the tensile strength is lower than 700 MPa, and the tensile strength of the steel after hot forming can not be secured over 600 MPa. Therefore, in the present invention, finishing rolling is performed under the condition that the cumulative rolling reduction ratio (%) / the finish rolling starting temperature (占 폚) is 0.09 or less.

FIG. 1 shows an example of a change in ferrite fraction and a change in tensile strength according to a value obtained by dividing the cumulative rolling reduction ratio by the finish rolling starting temperature.

As shown in FIG. 1, when the cumulative rolling reduction ratio (%) / the rolling start temperature (° C) is higher than 0.09, ferrite transformation is promoted and the bainite fraction is reduced. As a result, And in this case, the tensile strength of the steel material after hot forming can not be secured at 600 MPa or more.

Next, the hot-rolled steel is cooled at a cooling rate of 10 to 50 DEG C / s, and cooling is terminated at a cooling end temperature not higher than the bainite transformation completion temperature (Bf). If the cooling end temperature is higher than the bendite transformation completion temperature (Bf) or the cooling rate is lower than 10 ° C / s, the bainite fraction can not be sufficiently secured. When the cooling rate is higher than 50 DEG C / s, a martensite structure is formed and the strength becomes excessively high, and the hot-moldability may be lowered.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following examples are intended to illustrate the present invention more specifically and do not limit the scope of the present invention.

[ Example ]

The steel slabs having the components listed in Table 1 below and the thicknesses shown in Table 2 were reheated to 1200 占 폚 at which the alloying elements could be sufficiently melted and subjected to rough rolling and finish rolling under the conditions shown in Table 2 and cooled to obtain the thickness Lt; / RTI > Thereafter, the steel sheet was maintained at a high temperature of 800 ° C for 30 minutes and then air-cooled to measure the mechanical properties of the steel. The results are shown in Table 3 below. The tensile strength of the steel material before the holding at a high temperature of 800 DEG C and the bainite fraction of the steel material produced as described above were measured, and the results are shown in Table 3 below. The recommended conditions in Table 2 mean that they were produced according to the production conditions of the present invention.

C Si Mn P S Al Ni Cu Cr Mo Ti Nb V W N Ca Nitride index (NI) 0.17 0.45 1.5 0.013 0.002 0.03 0 0 0 0 0.015 0.05 0.07 0 0.01 0.271 0.16 0.4 1.4 0.01 0.001 0.015 0 0 0 0 0.01 0.045 0.065 0 0.008 0.0015 0.235 0.18 0.5 1.6 0.02 0.003 0.05 0.1 0.1 0.1 0.08 0.01 0.055 0.075 0 0.012 0.333 0.12 0.55 1.7 0.021 0.0015 0.11 0.2 0.2 0.15 0.2 0.021 0.07 0.083 0.2 0.011 0.428 0.19 0.53 1.8 0.017 0.0014 0.16 0 0 0 0.4 0.25 0 0.077 0 0.0078 0.0013 0.376 1.18 0.3 1.25 0.015 0.0017 0.3 0.3 0.3 0.25 0.28 0.05 0.065 0.055 0.03 0.0098 0.435 0.14 0.25 1.78 0.018 0.002 0.25 0 0 0 0 0.019 0.035 0.1 0 0.0079 0.293 0.05 0.3 1.6 0.02 0.003 0.015 0.1 0.1 0.1 0.08 0.01 0.01 0 0 0.011 0.187 0.15 0.7 1.3 0.023 0.001 0.1 0 0 0.4 0.3 0.15 0 0.1 0.4 0.0055 0.59 0.12 0.1 1.2 0.02 0.003 0.05 0.2 0.2 0.1 0 0.01 0.01 0 0 0.0095 0.113 0.08 0.15 1.3 0.02 0.003 0.02 0 0.1 0.1 0 0.01 0.015 0.02 0 0.012 0.121

As shown in Table 3, the inventive steel produced according to the present invention had a tensile strength of 700 MPa or more after hot rolling and cooling, a yield strength of 460 MPa or more, a tensile strength of 600 MPa or more, It is possible to manufacture a high tensile steel having a shock absorption energy of 100 J or more at -5 ° C.

While the illustrative embodiments of the present invention have been shown and described, various modifications and alternative embodiments may be made by those skilled in the art. Such variations and other embodiments will be considered and included in the appended claims, all without departing from the true spirit and scope of the invention.

Claims (4)

0.1 to 0.20% of Mo, 1.2 to 2.5% of Mn, 0.2 to 0.8% of Si, 0.005 to 0.10% of Nb, 0.01 to 1.0% of Mo, 0.05 to 1.0% of Cr, 0.005 to 1.0% of Cr, Wherein the composition contains 0.1 to 0.1% of Al, 0.005 to 0.5% of Al, 70 to 150 ppm of N, 60 ppm or less of Ca (excluding 0 ppm), P of 0.02% or less and S of 0.01% 0.01 to 2.0%, V: 0.005 to 0.3%, and W: 0.005 to 0.2%, and the balance Fe and other inevitable impurities, and has a nitride index (NI) of 0.23 to 0.5.
[Relation 1]
Nitride index (NI) = 0.293 x Ti + 0.151 x Nb + 0.519 x Al + 0.275 x V + 0.073 x Cu + 0.499 x Si + 0.180 x Cr + 0.076 x W
The high strength steel for hot forming according to claim 1, wherein the microstructure of the high strength structural steel for hot forming has an area fraction of not more than 15% of ferrite and the balance of bainite.
The steel sheet for high-strength structural steel for hot forming according to claim 1, wherein the tensile strength of the high-strength structural steel for hot forming is 700 MPa or more, the yield strength after hot forming at 800 占 폚 for 30 minutes is 460 MPa or more and the tensile strength is 600 MPa or more And the impact absorption energy at -5 DEG C is 100 J or more.
0.1 to 0.20% of Mo, 1.2 to 2.5% of Mn, 0.2 to 0.8% of Si, 0.005 to 0.10% of Nb, 0.01 to 1.0% of Mo, 0.05 to 1.0% of Cr, 0.005 to 1.0% of Cr, Wherein the composition contains 0.1 to 0.1% of Al, 0.005 to 0.5% of Al, 70 to 150 ppm of N, 60 ppm or less of Ca (excluding 0 ppm), P of 0.02% or less and S of 0.01% 0.01 to 2.0%, V: 0.005 to 0.3%, and W: 0.005 to 0.2%, and the balance Fe and other inevitable impurities, and has a nitride index (NI) of 0.23 to 0.5 at a temperature of 1050 to 1250 占 폚;
Subjecting the reheated slab to finish rolling under the condition that the cumulative rolling reduction ratio (%) and the finish rolling starting temperature (占 폚) satisfy the following relational expression 2;
Cooling the hot-rolled steel material at a cooling rate of 10 to 50 占 폚 / s and ending cooling at a cooling end temperature not higher than the bainite transformation completion temperature (Bf).
[Relation 1]
Nitride index (NI) = 0.293 x Ti + 0.151 x Nb + 0.519 x Al + 0.275 x V + 0.073 x Cu + 0.499 x Si + 0.180 x Cr + 0.076 x W
[Relation 2]
Rolling roll cumulative rolling reduction ratio (%) / rolling roll starting temperature (캜) ≤0.09

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