KR20040059293A - High strength cold rolled steel sheet having superior workability - Google Patents

Abstract

PURPOSE: A cold rolled steel sheet is provided which prevents deterioration of plating property and weldability due to large quantities of silicon and carbon and having 100 kgf/mm¬2 or more of strength and superior workability, and a manufacturing method of the cold rolled steel sheet is provided. CONSTITUTION: The high strength steel product having high yield ratio and high strain hardening coefficient comprises 0.05 to 0.2 wt.% of carbon, 0.3 to 0.7 wt.% of silicon, 4 to 7 wt.% of manganese, 0.03 wt.% or less of phosphorus, 0.005 wt.% or less of sulfur, 0.0015 to 0.002 wt.% of boron, 0.02 to 0.05 wt.% of molybdenum, 0.05 to 0.15 wt.% of aluminum and a balance of Fe and other inevitable impurities, wherein micro-structure of the steel product is consisted of 5 to 20 vol.% of bainite, 5 to 15 vol.% of residual austenite and a balance of ferrite. The manufacturing method of the high strength steel product having high yield ratio and high strain hardening coefficient comprises a step of reheating a steel slab comprising 0.05 to 0.2 wt.% of carbon, 0.3 to 0.7 wt.% of silicon, 4 to 7 wt.% of manganese, 0.03 wt.% or less of phosphorus, 0.005 wt.% or less of sulfur, 0.0015 to 0.002 wt.% of boron, 0.02 to 0.05 wt.% of molybdenum, 0.05 to 0.15 wt.% of aluminum and a balance of Fe and other inevitable impurities in the temperature range of 1,200 to 1,250 deg.C for 60 to 180 minutes; a step of finishing hot rolling of the reheated steel slab at a temperature of Ar3 point or more, and coiling the hot rolled steel sheet at a temperature of 560 to 680 deg.C; and a step of cold rolling the coiled hot rolled steel sheet to a reduction ratio of 40 to 45%, batch annealing the cold rolled steel sheet at a temperature of 620 to 650 deg.C, and air cooling or furnace cooling the batch annealed steel sheet.

Description

High strength cold rolled steel sheet with excellent workability and manufacturing method {HIGH STRENGTH COLD ROLLED STEEL SHEET HAVING SUPERIOR WORKABILITY}

The present invention relates to an ultra-high strength steel sheet of 100 kgf / mm ² used for a bumper reinforcement of a vehicle or an impact absorber in a door. It is about.

Bumper reinforcements of automobiles or shock absorbers in doors are parts that are directly related to passenger safety in the event of a vehicle collision. Ultra high strength cold rolled steel sheets with tensile strength of 80kgf / mm ² or more are mainly used. This cold rolled steel sheet should have high tensile strength, low yield ratio, high work hardening index and high elongation. In addition, the use of high strength steels is increasing in order to increase fuel economy in response to the increasingly severe environmental pollution regulations. Recently, research on commercialization of high strength steel of 100 kgf / mm ² or more has been increasing.

High-strength steels for automobiles include transformation induced plasticity (TRIP) steel and dual phase steel (DP), and as the strength increases, the two steels become more difficult to classify. Also called complex phase steel (CP).

The manufacturing process is divided into hot rolling, cold rolling and annealing. The hot rolled sheet having ferrite and perlite is cold rolled to be processed to the final thickness of the product, and then heated and cooled to an annealing temperature of A ₁ or more transformation point. At this time, the austenite formed during the annealing process is transformed into martensite or bainite by controlling the speed during the cooling process, and when transformed into martensite, this steel is called an ideal tissue steel. As the ratio of martensite increases, the ductility increases as the ratio of martensite increases, and the ductility increases as the ferrite ratio increases. However, when the martensite ratio is too large to increase the strength, the ferrite ratio decreases and the ductility decreases.

On the other hand, after forming austenite in the annealing process as in the above method, by controlling the cooling rate and the cooling end temperature in the cooling process to maintain a portion of austenite at room temperature, thereby increasing the strength and ductility of the metamorphic tissue steel at the same time There is this. That is, when the residual austenite is transformed into martensite by processing during plastic deformation, the transformation phase formed by the plastic organic transformation along with the strength increases the ductility by relaxing local stress concentration. It is called steel (CAMP-ISIJ vol. 1, p. 877). TRIP steel is widely used as a high strength steel because it has excellent strength and ductility.

In metamorphic organic plastic steel, it is important to maintain metastable austenite over a certain portion at room temperature. To this end, Si, Al, P, etc. are added to increase the carbon activity in the ferrite, and the formation of carbides is suppressed to increase the amount of carbon in the austenite so that the austenite remains. At this time, when the component range is added, carbon is 0.1 to 0.25% and silicon is added to 1.0 to 2.5%, but when it is cold rolled and continuously annealed, a large amount of silicon is added so that austenite is transformed into bainite. It is possible to manufacture the modified organic plastic steel by suppressing the retardation and delaying the bainite transformation to retain the austenite. However, since the above method requires the addition of 0.1% or more of carbon and 1.0% or more of Si to obtain a large amount of retained austenite, the flash-butt weldability is very deteriorated by carbon, silicon, etc. when manufacturing a hot rolled steel sheet. In addition, when welding to connect the cold rolled sheet material for annealing, there is a difficulty in the process as well as a problem that silicon forms an oxide on the surface to deteriorate the plating property.

It is an object of the present invention to provide a cold rolled steel sheet having a strength and excellent workability of 100 kgf / mm ² or more and preventing plating and weldability deterioration due to a large amount of silicon and carbon.

1 is a graph showing the tensile strength and yield strength according to the cooling method for each alloy

Figure 2 is a graph showing the total elongation and uniform elongation according to the cooling method for each alloy

3 is a graph showing the yield ratio and work hardening index according to the cooling method for each alloy

4 is a graph showing tensile strength x yield strength for each alloy.

Steel material of the present invention for achieving the above object, in weight%, carbon: 0.05-0.2%, silicon: 0.3-0.7%, manganese: 4-7%, phosphorus: 0.03% or less, sulfur: 0.005% or less, nitrogen : 0.003% or less, boron: 0.0015 ~ 0.002%, molybdenum: 0.02 ~ 0.05%, aluminum: 0.05 ~ 0.15%, remaining other unavoidable impurities and Fe, and the microstructure is 5-20vol.% Of bainite and 5 Residual austenite of ˜15 vol.% And the remainder are ferrite.

In addition, the steel manufacturing method of the present invention, in weight%, carbon: 0.05 to 0.2%, silicon: 0.3 to 0.7%, manganese: 4 to 7%, phosphorus: 0.03% or less, sulfur: 0.005% or less, nitrogen: 0.003 % Or less, boron: 0.0015 to 0.002%, molybdenum: 0.02 to 0.05%, aluminum: 0.05 to 0.15%, steel slab composed of the other unavoidable impurities and Fe, and reheat the steel slab at 1200 to 1250 ° C for 60 to 180 minutes Hot-rolling the reheated slab at an Ar ₃ or more point, winding at 560-680 ° C, and cold-rolling the wound hot-rolled plate at a reduction ratio of 40-45%, at 620-650 ° C. Heat treatment and air or furnace cooling.

Hereinafter, the present invention will be described in detail.

In the present invention, instead of reducing a large amount of silicon and carbon added to the continuous annealing material to adjust the content of manganese in order to prevent the decrease in strength to have a yield ratio of less than 0.7 and a work hardening index of 0.4 or more with a tensile strength of 100kgf / mm ² or more It is characterized by. The steel of the present invention is made of high strength metamorphic organic plastic steel by a batch annealing process.

Carbon [C]: 0.05-0.2%

Carbon in steel is the most important component in steel materials and is closely related to all physical and chemical properties such as strength, toughness and corrosion resistance, and is the most important component in steel properties. In the case of TRIP steel, especially when the amount of carbon is less than 0.05%, the stability and ductility of the retained austenite is low and the fraction is decreased, so that the strength and ductility are greatly reduced, and when it exceeds 0.2%, the workability due to the decrease in weldability and the rapid increase of the second phase fraction There are disadvantages such as declining. Therefore, the content of carbon is preferably 0.05 to 0.2%.

Silicon [Si]: 0.3-0.7%

Silicon is a ferrite stabilizing element that is dissolved in ferrite and contributes to strength, and is usually added as a deoxidizer. In the case of TRIP steel, silicon is an important element that increases the amount of carbon in the austenite and increases the carbon content in the austenite, thereby increasing the austenite stability. If the silicon content is less than 0.3%, the deoxidation effect is reduced, and it is effective in the flotation separation of MnS inclusions in steels containing a large amount of manganese as an element to increase the fluidity of molten steel, and the weldable weldability in the range of 5-30 manganese / silicone ratio Improves. If the content of silicon is more than 0.5%, not only causes a hot roll scale, but most importantly, weldability is deteriorated. In addition, when the cooling rate is properly controlled, it inhibits bainite transformation and contributes to the formation of residual austenite.

Manganese [Mn]: 4-7%

Manganese is an austenite stabilizing element that facilitates the formation of low-temperature transformation phases such as acicular ferrite and bainite, that is, increases hardenability and increases strength. When carbon and silicon are contained under the conditions of the present invention, in order to have a strength of 100kgf.mm ² or more, it is preferable to add 4% or more of manganese. Manganese also enhances strength by solid solution strengthening and combines with sulfur (S) in the steel to form MnS. When the content of such manganese is more than 7%, the weldability is lowered, causing hydrogen organic embrittlement by inclusion inclusions, and forms a segregation zone in the center of the sheet during hot rolling.

Phosphorus [P]: Less than 0.003%

In cold rolled annealing steel sheets, phosphorus is an element that increases strength through solid solution strengthening. If a small amount is added, the effect of increasing strength is small. If excessively added, brittleness is increased and weldability is deteriorated.

Sulfur [S]: 0.005% or less

If sulfur is more than 0.005%, coarse TiS and MnS are formed in the hot rolled sheet, which degrades the workability and toughness, so it is effective to reduce it as much as possible.

Nitrogen [N]: 0.003% or less

Nitrogen is preferably managed at less than 0.003% because finer grains are used to increase the amount of solid solution carbon that can be used for harden hardening in steel and to increase grain boundary strength to improve secondary processing brittleness.

Boron [B]: 0.0015-0.002%

Boron increases the hardenability of the steel even when a small amount is added to the steel, thereby facilitating the formation of low temperature transformation phases such as acicular ferrite and bainite. Usually at high temperatures it segregates at the austenite grain boundaries and inhibits ferrite formation, contributing to the hardenability of the steel, for which it is added at least 0.0015%. If the addition amount of boron is more than 0.002%, the weldability is reduced, and the recrystallization temperature is excessively increased to reduce the drawing property. Therefore, the boron content is preferably 0.0015 to 0.002%.

Molybudene [Mo]: 0.02 to 0.05%

Molybudene is added to improve the hardenability of the steel, for this purpose, 0.02% is added, but when it exceeds 0.05%, the weldability is lowered, the manufacturing cost is increased, and the drawing property is lowered. Therefore, the content of molybdenum is preferably 0.02 to 0.05%.

Aluminum (Al) 0.05 ~ 0.15%

Al is also added for deoxidation, and combines with N in the steel to form AlN, which serves to refine the austenite grains at high temperatures. The minimum amount which shows this effect is 0.05% or more, and at most 0.15% is preferable.

The slabs formed as described above obtain the molten steel through the steelmaking process and then make the slab through the ingot or continuous casting process. The slab is manufactured from cold rolled steel sheet having a target mechanical property through a reheating process, a hot rolling process, a winding process, a cold rolling process, and an annealing process, and manufacturing conditions for each process will be described in detail.

Reheating process

The slabs are formed as described above and reheated at a temperature of 1200 to 1250 ° C. for 60 to 180 minutes.

Hot rolling process

Next, the heated slab is hot rolled under the condition of finishing rolling at a temperature of Ar ₃ or higher. Finish rolling below Ar ₃ introduces a lot of dislocations into the ferrite formed during hot rolling, and the ferrite grows during cooling or winding to form surface coarse grains, and when rolling at low temperature, the load of the rolling roll increases significantly.

Winding process

Although the hot-rolled rolled sheet is wound, the winding temperature is preferably performed at 560 to 680 ° C. If the coiling temperature is less than 560 ℃, the precipitation of carbon in the steel is delayed, the drawability is lowered, and if the temperature is higher than 680 ℃ carbide is too coarse, it is difficult to be re-used uniformly, causing a non-uniform structure inside, greatly reduce the workability.

Cold rolling process

Cold rolling reduction is a variable that affects the microstructure in addition to the hot rolling and cold rolling conditions. If the cold rolling reduction is low, the texture is not uniform and the workability is low. It is preferable to set it as -45%. Looking at the effect of cold reduction rate on the microstructure, as the cold reduction rate increases, the lattice form formed in the hot rolling is decomposed and changed into granular form, thus the secondary phase morphology (size, shape and distribution map) formed after annealing-cooling. ) Also changes, so the morphology of the retained austenite, the secondary phase formed at this time, also changes, resulting in a change in mechanical properties. In general, the residual austenite enclosed in the hard phase is not transformed to martensite even if sufficient deformation is applied, but it does not contribute to the ductility. Is being reported.

Recrystallization annealing process

If the annealing temperature is too low, sufficient reusability is difficult, so that the coarse structure at the time of hot rolling is maintained, and the workability is lowered. If the annealing temperature is too high, the microstructure is coarse and the strength is reduced. Therefore, for the steel in the present invention, the annealing conditions are usually in the range of 600-700 ° C. The annealing time is preferably 12 to 15 hours. Cooling is preferably furnace cooling and air cooling in consideration of the characteristics of BAF.

Cold rolled steel sheet of the present invention is to reduce the content of silicon and carbon present steel components for the production of TRIP-type composite structure cold rolled steel sheet used in automotive structural steel sheet having excellent weldability and plating properties.

The present invention will be described in more detail with reference to the following Examples.

EXAMPLE

The ingot of Table 1 was heated at 1200 ° C. for 1 hour, hot rolled at 900 ° C. for finishing, and then wound at 620 ° C. for 1 hour, and then cooled. The obtained hot rolled sheet was pickled, cold rolled to a reduction ratio of 45%, and then annealed at a temperature of 620 and 645 ° C for 12 hours, followed by air cooling or furnace cooling.

Yield ratio and work hardening index indicating strength, elongation, and workability were calculated using the specimens prepared as described above, and the results are shown in Table 2. In the steel materials shown in Table 2, those satisfying a tensile strength of 100kgf / mm ² or more, a yield ratio of 0.7 or less, and a work hardening index of 0.4 or more are marked as invention materials.

Steel grade C Mn Si P S Sol. Al N Mo B A 0.15 6.5 0.5 0.02 or less 0.002 or less 0.11 0.0017 B 0.15 6.5 0.5 0.02 or less 0.002 or less 0.11 0.0017 0.05 0.0015 C 0.1 4 0.5 0.02 or less 0.002 or less 0.11 0.0017 0.05 0.0015 D 0.15 4 0.5 0.02 or less 0.002 or less 0.11 0.0017 0.05 0.0015 E 0.2 4 0.5 0.02 or less 0.002 or less 0.11 0.0017 0.05 0.0015

Steel grade Annealing Temperature Cooling method YS (kgf / mm ² ) TS (kgf / mm ² ) Uniform elongation (%) % Total elongation YR N TS * El. Remarks A 620 Air cooling 43.8 61.6 15.3 30.0 0.71 0.19 1849.9 Comparative example Ronin 42.2 62.3 15.6 30.4 0.68 0.14 1892.4 Comparative example 645 Air cooling 43.4 62.2 16.3 31.1 0.70 0.19 1936.5 Comparative example Ronin 32.8 69.2 16.0 23.4 0.47 0.13 1620.2 Comparative example B 620 Air cooling 78.4 87.9 32.6 39.7 0.89 0.20 3486.7 Comparative example Ronin 77.9 93.6 30.2 37.2 0.83 0.29 3483.6 Comparative example 645 Air cooling 68.9 108.0 28.2 32.5 0.64 0.54 3509.7 Inventive Example Ronin 70.7 109.1 28.3 30.9 0.65 0.52 3364.9 Inventive Example C 620 Air cooling 80.5 93.6 33.2 40.0 0.86 0.24 3739.8 Comparative example Ronin 80.1 94.2 31.1 39.2 0.85 0.26 3694.9 Comparative example 645 Air cooling 72.9 109.7 27.5 34.3 0.66 0.53 3767.1 Inventive Example Ronin 68.4 119.9 21.0 24.0 0.57 0.43 2881.5 Inventive Example D 620 Air cooling 43.0 57.8 21.2 35.2 0.74 0.10 2035.3 Comparative example Ronin 41.2 57.5 21.5 34.5 0.72 0.21 1983.7 Comparative example 645 Air cooling 38.6 57.7 24.0 37.4 0.67 0.21 2156.5 Comparative example Ronin 34.7 67.0 21.3 32.9 0.52 0.34 2206.8 Comparative example E 620 Air cooling 42.3 60.4 18.4 34.5 0.70 0.19 2084.6 Comparative example Ronin 42.6 61.4 20.4 34.6 0.69 0.22 2126.2 Comparative example 645 Air cooling 39.5 59.9 22.5 37.2 0.66 0.20 2228.5 Comparative example Ronin 34.8 70.4 18.4 28.9 0.49 0.30 2035.9 Comparative example

The workability of the plate is closely related to the work hardening index of the material, and in order to have excellent workability, the deformation concentration part as the deformation proceeds to yield sufficient resistance to deformation after the occurrence of yield, that is, to suppress local necking. Work hardening should occur continuously. Therefore, when the work hardening index increases, the work hardening index has excellent workability, and the work hardening index is assuming that the deformation behavior of the material follows the Hollomon equation. Is proportional to the uniform elongation. In addition, the work hardening index has an inverse relationship with the yield ratio. As the work hardening index increases, the yield ratio decreases. Therefore, in order to increase the workability, it is necessary to develop a steel having a low yield ratio and a high work hardening index.

Along with strength, work hardening index and yield ratio also depend on the microstructure of the material. In general, ferritic-perlite steels have a high yield ratio, and as the steels increase in strength, the work hardening index increases, resulting in poor workability. The ideal structure steel with martensite in the second phase has better yield property and higher work hardening index than the workable ferrite-perlite steel, but the uniform elongation required for processing complex shapes such as press forming is not high. . On the other hand, TRIP steel has excellent work hardening properties, which means that when local strain concentration occurs, the residual austenite transforms into martensite, causing volume expansion and relaxation of concentrated local stress, thereby transferring strain concentration to other sites. Because. High strength steels of 100kgf / mm ² or more require a yield ratio of 0.7 or less and a work hardening index of 0.4 or more for workability.

Steel grades A ~ E were heated at 1200 ° C for 1 hour, finished hot rolled at 900 ° C, wound at 680 ° C, maintained for 1 hour, then cold-pressed, pickled and cold-rolled to 45% reduction rate for 12 hours at 645 ° C. Tensile strength (TS), yield strength (YS), elongation (EI), yield ratio (YR), and work hardening index (N) for air-cooled and furnace-cooled steels are shown in FIGS. 1, 2, and 3.

As shown in Table 2, the strength of B and C steels is high, and especially when annealed at 645 ° C. among B and C steels, they show excellent characteristics regardless of the cooling rate such as air cooling or furnace cooling.

As shown in Fig. 1.3, the strength of the basic composition A steel is low. Manganese, molybdenum, and boron-added steels B and C have a high work hardening index and high strength, and excellent elongation and yield ratio. However, in the case of D and E steels, the strength is not high, because when carbon is added, carbides are easily formed to form pearlite, and thus the fraction and stability of residual austenite are reduced, resulting in low work hardening index. The work hardening index of the D and E steels is low.

In Fig. 2, the uniform elongation is high in the high strength B and C steels because the stability and the fraction of the retained austenite are high, so that work hardening after the yield point is effectively progressed and necking is suppressed. However, in the case of D and E steels, although the overall elongation is high, the uniform elongation is low. Unlike the B and C steels, the uniform elongation was not sufficiently hardened, so the local necking progressed quickly. From the above results, it can be seen that B and C steels, which have high strength and elongation, have high tensile strength X elongation value, which can be confirmed from Fig. 4.

As a result of performing hot rolling on the steel species of A, B, C, D, and E and changing the cold reduction rate, annealing time, and cooling method, it can be seen that B and C steels have excellent processing characteristics together with strength.

As described above, according to the present invention, a high-strength metamorphic organic steel having excellent workability having a high tensile strength of 100 kgf / mm ² or more and having a yield ratio of 0.7 or less and a work hardening index of 0.4 or more is obtained. In addition, since silicon and carbon are added in an amount less than the transformation organic plastic steel produced through continuous annealing, it has excellent properties of weldability and hot dip galvanizing property, and is excellent in workability and is used in automobile structural parts using presses.

Claims (6)

By weight%, carbon: 0.05-0.2%, silicon: 0.3-0.7%, manganese: 4-7%, phosphorus: 0.03% or less, sulfur: 0.005% or less, boron: 0.0015-0.002%, molybdenum: 0.02-0.05 %, Aluminum: 0.05 ~ 0.15% The remaining other unavoidable impurities and Fe, and the microstructure is 5 ~ 20vol.% Bainite and 5 ~ 15vol.% Residual austenite and the rest is made of ferrite. High strength steel with machining hardness index. The high strength steel having a high yield ratio and a high work hardening paper, characterized in that the carbon is 0.1 ~ 0.15%. The high strength steel having high yield ratio and high work hardening index according to claim 1, wherein the manganese is 4.0 to 6.5%. By weight%, carbon: 0.05-0.2%, silicon: 0.3-0.7%, manganese: 4-7%, phosphorus: 0.03% or less, sulfur: 0.005% or less, boron: 0.0015-0.002%, molybdenum: 0.02-0.05 %, Aluminum: 0.05% to 0.15%, reheating the steel slab composed of the remaining other unavoidable impurities and Fe for 60 to 180 minutes in the 1200 to 1250 ℃ section, Hot-rolling the reheated slab at Ar ₃ or higher and winding it at 560 ~ 680 ℃ A method of producing a high strength steel having a high yield ratio and a high work hardening index, comprising: cold-rolling the wound hot rolled sheet at a reduction ratio of 40 to 45%, and subjecting it to annealing at 620 to 650 ° C. and to air cooling or furnace cooling. The high strength steel having a high yield ratio and a high work hardening paper, characterized in that the carbon is 0.1 ~ 0.15%. The high strength steel having a high yield ratio and a high work hardening index according to claim 4, wherein the manganese is 4.0 to 6.5%.