KR20110000400A - Steel sheet having excellent low yield ratio property, and method for producing the same - Google Patents

Abstract

PURPOSE: A steel sheet having excellent low yield ratio property and a manufacturing method thereof are provided to improve the strength of a steel sheet by controlling the contents of Al, Si, and V. CONSTITUTION: A steel sheet having excellent low yield ratio property comprises C 004~015 weight%, Si 0~02 weight% or less, Mn 18~25 weight%, P 0~0015 weight% or less, S 0~0005 weight% or less, Al 0~020 weight% or less, Mo 001~005 weight%, V 001~005 weight%, N 0~60 ppm or less, and Fe and inevitable impurities of the rest amount. The micro-structure is composed of including ferrite 70~95% and martensite 5~30%.

Description

TECHNICAL FIELD [0001] The present invention relates to a high strength steel sheet having excellent resistance to abrasion resistance and a manufacturing method thereof,

The present invention relates to a high-strength steel sheet excellent in low-yield ratio characteristics and a method for producing the same, and more particularly, to a high-strength steel sheet excellent in press formability and excellent in resistance to bending characteristic having a tensile strength of 490 to 780 MPa .

Since most of automobile parts are formed by press working, excellent press workability is required. Especially, in recent years, automobile design has become complicated and consumers' desires are diversified, so a steel having high strength and excellent plating processability is required .

For example, high-strength steel plates using composite structure (CP) steels are used for parts that require high energy absorption performance in the event of a vehicle collision such as members, pillars, and bumper reinforcements.

In order to optimize the fraction of martensite and ferrite, tensile strength and yield strength are obtained. In addition, Mn, Cr, Mo and B, which are minerals, are added.

However, because the austenite is transformed into ferrite, pearlite (cementite) and bainite in the anomalous zone during the overshoot and the alloying heat treatment which are present immediately before and after the plating in the manufacture of the galvannealed steel sheet, There arises a problem of elongation at yield point and lowering of strength.

Therefore, in recent years, a large amount of Mn and Mo have been added to suppress the transformation of austenite into ferrite, pearlite and bainite, thereby securing the martensite fraction.

However, excessive use of Mn and Mo has a problem of restricting the resistance ratio, which causes a problem of component twisting due to springback phenomenon during press working.

In addition, over-addition of Mn inhibits formability by forming streaks such as Mn bands in the final product due to center segregation and resin segregation at the time of playing, and manganese oxide (Mn ₂ SiO _4, etc.) So that the hot dip galvanizing property is deteriorated.

In addition, over-addition of Mo can secure the strength, but since Mo is an expensive element, the manufacturing cost is increased and commercial production becomes difficult.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and it is an object of the present invention to provide a high strength steel sheet having a strength of 490 to 780 MPa or more and excellent press formability, There is.

In order to achieve the above-mentioned object, the present invention provides a ferritic stainless steel comprising 0.04 to 0.15 wt% of C, 0.2 wt% or more of Si, 0.2 wt% or less of Mn, 1.8 to 2.5 wt% of Mn, Mo: 0.01 to 0.05 wt%, V: 0.01 to 0.05 wt%, N: more than 0 and not more than 60 ppm, Al: more than 0 and not more than 0.20 wt% The balance Fe and other unavoidable impurities,

The microstructure is characterized by being composed of two phases including ferrite and martensite.

The microstructure has a volume fraction of 70 to 95% of ferrite and 5 to 30% of martensite.

A manufacturing method which is another characteristic element of the present invention is a manufacturing method which comprises: 0.04 to 0.15 wt% of C, 0.2 wt% or more of Si, 0.2 to 2.0 wt% of Mn, 0.015 wt% or less of P, Al: more than 0 and not more than 0.20 wt%, Mo: 0.01 to 0.05 wt%, V: 0.01 to 0.05 wt%, N: more than 0 and not more than 60 ppm. A steel slab having an alloy composition of the remainder Fe and other unavoidable impurities

Rolled at 550 to 650 占 폚, cold rolled, maintained in a temperature range of Ar1 to Ar3 for 5 to 120 seconds, and then heat-treated at a temperature of 10 to 50 占 폚 / sec Cooled to 400 to 500 ° C at a cooling rate, and then subjected to hot-dip galvanizing treatment.

The present invention relates to a high strength steel sheet having a tensile strength of 490 to 780 MPa and excellent in press formability and a method of manufacturing the same. According to the present invention, the amount of Mn and Mo is controlled, So that it is possible to improve the press formability due to the low yield ratio characteristics while satisfying the strength.

Hereinafter, preferred embodiments of a steel sheet and a method of manufacturing the steel sheet having excellent resistance to brittle characteristics according to the present invention will be described in detail.

The steel sheet of the present invention contains 0.04 to 0.15 wt% of C, more than 0.2 wt% of Si, 1.8 to 2.5 wt% of Mn, more than 0 to 0.015 wt% of P, up to 0.005 wt% of S, 0 to 0.2 wt%, Mo: 0.01 to 0.05 wt%, V: 0.01 to 0.05 wt%, N: more than 0 and 60 ppm or less. The balance Fe and other unavoidable impurities, and the microstructure is composed of two phases including ferrite and martensite.

Specifically, in the present invention, the content of Mn and Mo is adjusted so as to secure the target strength, vanadium is added, carbon deposited in the ferrite is precipitated in the ferrite during the annealing process to increase the tensile strength, So that the characteristics are secured.

Hereinafter, reasons for limiting the function and content of the alloying elements of the present invention will be described.

C: 0.04 to 0.15 wt%

C is for the purpose of strength improvement. When C is added in an amount less than 0.04 wt%, the strength is lowered and the austenite is transformed into ferrite, making it difficult to secure a martensite fraction. If it is added in an amount exceeding 0.15 wt%, the weldability is deteriorated and the ductility and stretch-flangeability with the increase in strength are lowered.

Silicon (Si): more than 0 and not more than 0.2 wt%

Si is a solid solution strengthening element which contributes to the purification of steel and promotes carbon enrichment in austenite. In addition, Si is added to the steel to which the proper Mn is added to improve the flowability of the molten metal during welding, thereby minimizing the inclusion residue in the weld and improving the strength without hindering the balance between yield ratio and strength and ductility.

In addition, Si slows the diffusion rate of carbon in ferrite, thereby suppressing carbide growth and stabilizing ferrite to improve elongation.

When Si is added in an amount exceeding 0.2 wt%, surface defects due to plating and scale are generated, and weldability is deteriorated. Therefore, the Si content is preferably in the range of 0.2 wt% or less.

Manganese (Mn): 1.8 to 2.5 wt%

Mn is added as a solid solution strengthening element. Mn stabilizes the austenite to lower the bimetallic annealing temperature, and martensite is easily produced even at a low critical cooling rate.

When Mn is added in an amount less than 1.8 wt%, the austenite is transformed into bainite and pearlite in the annealing heat treatment section, the strength is lowered and the yield strength is increased. When Mn is added in an amount exceeding 2.5 wt%, the hardenability is increased and the workability is poor, and the manganese band develops at the center of the thickness during the slab casting, thereby lowering the bending workability.

Phosphorus (P): more than 0 and not more than 0.015 wt%

P guarantees a high strength with an element having a high solid solution strengthening effect and a small decrease in elongation value. P serves to stabilize the retained austenite due to the increase of the solid carbon by suppressing the formation of carbide.

When P is added in an amount exceeding 0.015 wt%, not only the workability is lowered but also the weldability is greatly lowered.

Sulfur (S): more than 0 and not more than 0.005 wt%

S inhibits toughness and weldability and increases MnS nonmetallic inclusions, causing cracks during steel working, and excessive additions increase coarse inclusions, deteriorating fatigue characteristics and are limited to less than 0.005 wt%.

Aluminum (Al): more than 0 and not more than 0.2 wt%

Al is an element mainly used as a deoxidizer. Al stabilizes the ferrite grains to improve the elongation and improve the plating ability.

Here, Al represents the amount remaining in the molten steel after aluminum deoxidation. When Al exceeds 0.04 wt%, the strength is lowered.

Molybdenum (Mo): 0.01 to 0.05 wt%

Mo is an element that can be added as an ingot element to secure the martensite fraction and improve the strength. When the Mo content is less than 0.01 wt%, it is difficult to secure the martensite fraction. When the Mo content exceeds 0.05 wt%, the Mn banding layer and the MnS inclusions are promoted in the high manganese steel.

Vanadium (V): 0.01 to 0.05 wt%

V bonds with carbon in the ferrite, forms a grain carbide to improve the strength, and reduces the yield ratio by reducing the solid carbon.

V is required to be added in an amount of 0.01 wt% or more in order to reduce the amount of solid carbon, and when the amount is over 0.05 wt%, the yield strength is increased by solid solution strengthening.

N: more than 0 and not more than 60ppm

N is made into fine grains by the formation of AlN, but in the galvannealing process during hot dip galvanizing, it becomes supersaturated upon cooling in the alloying process, and the uniform elongation is lowered, so it is limited to 60ppm or less.

The present invention includes the elements of the above steel sheet, the rest being substantially iron (Fe) and inevitable elements, and it is possible to allow fine incorporation of unavoidable impurities of 0.01% or less as an element contained according to the conditions of raw materials, materials, do.

In order to secure the spot weldability of the steel sheet, the carbon equivalent (Ceq) is limited to 0.24 or less.

The slab having a composition as described above is produced through a continuous casting process after obtaining molten steel through a steelmaking process. Here, the slab is manufactured through a hot-rolling process and a hot-rolling process in which the slab is rolled to a desired thickness, The surface of the steel sheet is subjected to the following steps of hot dip galvanizing.

 Each process is as follows.

[Heating furnace process]

The slab having the above composition is reheated at 1150 to 1250 占 폚 to recycle the segregated components when casting.

If the reheating temperature is lower than 1150 ° C, the segregated components can not be reused. If the reheating temperature is higher than 1250 ° C, the austenite grains are coarsened and it is difficult to secure the strength.

In this case, the reheating time is preferably 1 to 3 hours, and if it is maintained for more than 1 hour, it can be economically damaged, and if it is too short, the segregated components may not be reused.

[Hot rolling process]

Finish the reheated slab by hot rolling at a temperature above Ar3, the austenite zone. After the hot rolling is finished, the steel sheet is rolled at 550 to 650 ° C to prevent surface enrichment of Mn and Si and coarsening of carbide.

If the coiling temperature is lower than 550 ° C, pearlite is formed instead of ferrite. If the coiling temperature is higher than 650 ° C, it is difficult to secure a ferrite content.

After hot rolling is completed, the steel sheet is cooled to a coiling temperature of 20 to 30 DEG C / sec, which is a normal cooling rate. The hot-rolled steel sheet is pickled to remove scale and coated with oil to prevent oxidation.

[Cold Rolling Process]

Cold rolling to achieve the final desired thickness of the steel sheet and cold rolling to obtain the desired material is carried out by cold rolling at a reduction rate of 50 to 70% at room temperature.

[Annealing Process]

And maintained at a temperature range of Ar1 to Ar3 for 5 to 120 seconds and then cooled to 400 to 500 DEG C at a cooling rate of 10 to 50 DEG C / sec.

The vanadium carbide carbide precipitates in the ferrite grain at 500 to 700 ° C in the annealing process to improve the tensile strength and improve the elongation.

At this time, if the cooling rate is too low, the austenite is transformed into ferrite pearlite (cementite) or bainite during the cooling process, and when the cooling rate is too high, the problem of material unevenness occurs.

Further, if the temperature is maintained for 5 seconds or more in the range of Ar1 to Ar3, a sufficient amount of martensite can not be obtained because austenite phase is not sufficiently formed during heating, and if it exceeds 120 seconds, the productivity is lowered.

The microstructure of the steel sheet produced by the production method of the present invention has a volume fraction of 70 to 95% of ferrite and 5 to 30% of martensite.

When the content of ferrite is less than 70%, the ductility and pressability are lowered. When the content of ferrite exceeds 95%, it is difficult to secure strength, so that the volume fraction of ferrite is limited to 70 to 95%.

When the martensite content is less than 5%, it is difficult to secure strength. When the martensite content exceeds 30%, the ductility and press workability are lowered, so that the martensite volume fraction is limited to 5 to 30%.

That is, the steel of the present invention is improved to control the content of Mn and Mo and to reduce the yield ratio by adding V.

Further, by adjusting the fraction of ferrite and martensite through the hot rolling control technique and the annealing heat treatment temperature control, it is possible to obtain a steel sheet excellent in tensile strength in the range of 490 to 780 MPa and yield ratio in the yield ratio of less than 60%.

Hereinafter, a steel sheet having excellent resistance to bending property and a manufacturing method thereof will be described in comparison with other comparative examples.

Table 1 shows the composition ratios of the Examples of the present invention and Comparative Examples.

The examples and comparative examples in Table 1 were reheated at 1,200 DEG C for 2 hours, and then subjected to finish hot rolling at 2.8 mm at a temperature of 890 DEG C (Ar3 or higher) and then rewound at 580 DEG C. [ After the pickling treatment, cold rolling was carried out at a thickness of 1.2 mm, and the cold-rolled steel sheet was annealed at 700 to 800 ° C., then quenched to 460 ° C., immersed in a zinc plating bath, annealed at 500 ° C., Respectively.

Table 2 shows the results of measurement of the mechanical properties of the examples and comparative examples in Table 1. < tb > < TABLE >

(The remainder Fe, unit: wt%), division
Chemical composition (wt%) Remarks
C Si Mn P S Mo Al V N (ppm) One 0.058 - 1.8 0.015 0.003 0.04 0.04 - 60 Comparative Example 2 0.059 0.15 1.8 0.015 0.003 0.04 0.20 - 60 Comparative Example 3 0.057 0.14 1.8 0.015 0.003 0.04 0.20 0.05 59 Example 4 0.058 0.14 1.8 0.015 0.003 0.04 0.20 0.10 61 Comparative Example 5 0.058 - 2.1 0.014 0.003 0.04 0.04 - 58 Comparative Example 6 0.060 0.16 2.1 0.015 0.003 0.04 0.20 - 59 Comparative Example 7 0.062 - 2.1 0.014 0.003 0.04 0.20 0.05 58 Example 8 0.063 0.16 2.1 0.014 0.003 0.04 0.20 0.10 57 Comparative Example 9 0.058 - 2.4 0.015 0.003 0.04 0.04 - 60 Comparative Example 10 0.057 0.16 2.4 0.014 0.003 0.04 0.20 - 61 Comparative Example 11 0.061 0.15 2.4 0.014 0.003 0.04 0.20 0.05 57 Example 12 0.061 0.15 2.4 0.014 0.003 0.04 0.20 0.10 57 Comparative Example

division
Hot rolling condition Cold annealing condition Mechanical property Organization characteristic Plating property Remarks FDT (占 폚) CT (° C) AT (占 폚) YS TS Hand YR Vm (%) One 890 600 790 369 461 36 80 0 Good Comparative Example 2 890 600 790 325 491 33 66 6 Good Comparative Example 3 890 600 790 300 515 33 58 6 Good Example 4 890 600 790 325 520 30 63 6 Good Comparative Example 5 890 600 790 453 562 30 81 8 Good Comparative Example 6 890 600 790 391 592 28 66 12 Good Comparative Example 7 890 600 790 352 605 28 58 12 Good Example 8 890 600 790 380 615 26 62 12 Good Comparative Example 9 890 600 790 561 744 24 75 20 Good Comparative Example 10 890 600 790 515 785 21 66 26 Good Comparative Example 11 890 600 790 480 799 21 60 26 Good Example 12 890 600 790 500 805 19 62 26 Good Comparative Example

TS: tensile strength, El: elongation ratio, YR: resistance ratio Vm: martensite volume fraction] [0050] The hot-

That is, in the case of the inventive steels 3, 7 and 11 according to the present invention, manganese bands are suppressed as a result of adding Al and Si, so that the formation of martensite bands after annealing can be suppressed and the elongation can be improved. As a result of the addition, the tensile strength is satisfied and a resistance ratio of less than 60 is formed.

In addition, when vanadium carbide carbide is precipitated in the ferrite ingot during the annealing process at 500 to 700 ° C in the annealing step, the tensile strength is improved and the yield strength is lowered. However, when vanadium exceeds 0.05 wt% 8, 12), the ferrite solid solution strengthened, resulting in an increase in the yield strength of the ferrite magnet with the resistance ratio exceeding 60%.

In other words, in the embodiments of the present invention, compared to the comparative steels of the comparative examples, even when Mo and Mn are added in the same amounts, the reduction of manganese bands and the formation of MnS inclusions are suppressed according to the difference in the amount of Si input, In addition, it can be seen that the tensile strength is increased and the yield strength is lowered by controlling the amount of vanadium input, so that the low resistance characteristic can be obtained.

Claims (3)

P: more than 0 and not more than 0.015 wt%, S: more than 0 and not more than 0.005 wt%, Al: more than 0 and not more than 0.20 wt%, C: 0.04 to 0.15 wt% 0.01 to 0.05 wt% of Mo, 0.01 to 0.05 wt% of V, and N: more than 0 and 60 ppm or less. The balance Fe and other unavoidable impurities, Wherein the microstructure is composed of two phases including ferrite and martensite. The method according to claim 1, Wherein the microstructure has a volume fraction of 70 to 95% of ferrite and 5 to 30% of martensite. P: more than 0 and not more than 0.015 wt%, S: more than 0 and not more than 0.005 wt%, Al: more than 0 and not more than 0.20 wt%, C: 0.04 to 0.15 wt% 0.01 to 0.05 wt% of Mo, 0.01 to 0.05 wt% of V, and N: more than 0 and 60 ppm or less. A steel slab having an alloy composition of the remainder Fe and other unavoidable impurities Rolled at 550 to 650 ° C, cold rolled, maintained in a temperature range of Ar1 to Ar3 for 5 to 120 seconds, and then heat-treated at a temperature of 10 to 50 ° C / sec Wherein the steel sheet is cooled to a temperature of 400 to 500 占 폚 at a cooling rate, and is then subjected to hot-dip galvanizing treatment.