KR20030052199A - Manufacturing method of retained austenite contained cold rolled steel sheets with good ductility - Google Patents

Abstract

PURPOSE: A manufacturing method of residual austenite contained cold rolled steel sheet having better ductility compared to existing method by controlling cooling of the annealed steel sheet after titrating chemical constituents of high strength cold rolled steel sheet having 50 to 80 kg/mm¬2 class tensile strength and annealing the steel sheet is provided. CONSTITUTION: The method comprises a process of hot rolling a steel slab under the condition that finish rolling temperature is A3 transformation point or more, wherein the steel slab comprises 0.05 to 0.20 wt.% of C, 0.7 to 1.5 wt.% of Si, 1.5 to 1.8 wt.% of Mn, 0.005 wt.% or less of S, 0.003 to 0.007 wt.% or less of N, 0.01 to 0.1 wt.% of Mo, 0.0005 to 0.002 wt.% of B, 0.01 to 0.1 wt.% of P, 0.02 to 0.5 wt.% of Al, 0.02 to 0.10 wt.% of Cr, 0.1 to 0.5 wt.% of Ni and a balance of Fe and inevitable impurities, and Si, P, Al, Cr and Ni satisfy the following relational expression: Si%+9.5P%+0.45Al%+0.3Cr%+0.6Ni%>=1.0; a process of cold rolling the coiled steel slab to a reduction ratio of 30 to 80% after coiling the hot rolled steel slab at a temperature of 630 to 680 deg.C; a process of maintaining the cold rolled steel sheet at a temperature of 780 to 810 deg.C for 5 to 180 seconds; and annealing process comprising the steps of first cooling the steel sheet to a temperature of 620 to 680 deg.C at a cooling rate of 5 deg.C/sec or less, second cooling the first cooled steel sheet to a temperature of 350 to 450 deg.C, maintaining the second cooled steel sheet at a constant temperature for 1 to 20 minutes and cooling the resulting steel sheet to an ordinary temperature, wherein an average cooling rate (CRtot) of the first and second cooling rates in the annealing process satisfies the following relational expression: CRmax£3.0-(Mn+0.3Si%+2.7P%)|>=log(CRtot, deg.C/sec)>=CRmin£2.5-(Mn+0.3Si%+2.7P%)|, where the CRmax is the maximum cooling rate, the CRtot is the total cooling rate, and the CRmin is the minimum cooling rate.

Description

Manufacturing method of retained austenite contained cold rolled steel sheets with good ductility}

The present invention relates to a method for manufacturing a cold rolled steel sheet used in automotive panels and structural components, and more particularly, by optimizing the chemical composition in the tensile strength 50-80kg / mm ² high strength cold rolled steel sheet and controlling the cooling after annealing It relates to a method for producing residual austenite-containing cold rolled steel sheet excellent in ductility compared to the method.

In order to improve fuel efficiency and reduce exhaust gas emissions of automobiles, weight reduction of the vehicle body is being promoted, and in order to increase the safety of passengers, it is required to increase the strength of steel sheets used in panels or structural members. In general, when the strength of the steel sheet increases, the workability is lowered, and thus the conventional high strength steel sheet does not satisfy the workability required for a panel or a structural member.

Meanwhile, in order to increase the ductility of high strength steel sheets, steel grades using transformation organic plasticity of retained austenite have been proposed. Japanese Unexamined Patent Publication No. 61-157625 discloses annealing steel sheets containing one or two or more of P, Ni, Cu, Cr, Ti, Nb, V, and Mo, in addition to 0.4-1.8% Si and 0.2-2.5% Mn. After cooling for 30 seconds to 30 minutes at 500-350 ℃ to provide a method for producing a retained austenite-containing steel sheet. In addition, Japanese Special Office No. 62-35461 has an annealing steel sheet containing 0.7-2.0% Si and 0.5-2.0% Mn, and then maintains it for 10-50 seconds at 650-450 ℃ during cooling. A steel sheet having more than% ferrite and retained austenite was presented. However, these methods did not consider the bainite transformation during cooling after annealing.

In addition, Japanese Unexamined Patent Application Publication Nos. 1-230715 and 2-217425 disclose that cementite-free bays are added directly above martensitic transformation temperature after two-phase annealing by adding an appropriate amount of Si and Mn to a low carbon steel below 0.2% C. A method of producing a steel sheet containing a large amount of retained austenite when a nit transformation occurs is proposed and has already been put to practical use in some automobiles. However, the steel sheets shown in Japanese Patent Application Laid-Open Nos. 1-230715 and 2-217425 have suggested the need to control bainite transformation with respect to the cooling rate after annealing, but the evidence is not sufficient, and the residual austenite is obtained. To this end, the addition of an appropriate amount of Si and strict control in the bainite temperature range was required, but the effect of other components such as Si, P, Al, and Cr on the strict control was not mentioned.

Also, Japanese Unexamined Patent Application Publication No. 6-145788 proposes a method of reducing Si content and adding Al and Mo in consideration of the amounts of C and Si for manufacturing in a hot dip plating facility. However, even in the case of the steel sheet produced by this technique, the cooling rate is more than enough to suppress the pearlite transformation and does not mention the relationship with the bainite transformation noise.

Japanese Unexamined Patent Publication No. 5-271857 describes the effect of Si and Al to suppress bainite transformation and presents a relational formula. Although 0.1% of P was added, the effect of stabilizing austenite by P is not included. It wasn't. In addition, nitrogen was limited to 0.01% or less to prevent nitrogen from using Al as AlN in the steel. Also, this method did not suggest the cooling rate after annealing.

In the present invention, in order to solve the above problems and to stably produce a steel sheet containing residual austenite having excellent ductility, components related to bainite stabilization are represented by a Si equivalent equation, limited to a certain range, and cooling rates are controlled according to chemical components. The present invention provides a method for producing a high strength cold rolled steel sheet containing residual austenite having excellent ductility by causing rapid cementite-free precipitation of bainite transformation in the temperature range of 350-450 ° C. and simultaneously moving carbon to austenite. There is this.

Cold rolled steel sheet manufacturing method of the present invention for achieving the above object, by weight% C: 0.05-0.20%, Si: 0.7-1.5%, Mn: 1.5-1.8%, S: 0.005% or less, N: 0.003-0.007 % Or less, Mo: 0.01-0.1%, B: 0.0005-0.002%, P: 0.01-0.1%, Al: 0.02-0.5%, Cr: 0.02-0.10%, Ni: 0.1-0.5%, Said Si, P, Steel slab made of Al, Cr and Ni satisfying the following conditions, Si% + 9.5P% + 0.45Al% + 0.3Cr% + 0.6Ni% ≥1.0 and composed of the remaining Fe and unavoidable impurities. Hot-rolled under the above conditions, wound at a temperature of 630-680 ° C, cold-rolled at a reduction ratio of 30-80%, and then maintained at 780-810 ° C for 5-180 seconds and then up to 620-680 ° C. The average cooling rate (CRtot) of the primary cooling rate and the secondary cooling rate in the annealing of the first cooling to less than ℃ / sec, the second cooling to 350-450 ℃ again and the constant temperature for 1-20 minutes and then cooled to room temperature Where CRmax [3.0- (Mn + 0.3Si% + 2.7P%)] ≥log (CRtot, ° C / sec) ≥ CRmin [2.5- (Mn + 0.3Si% + 2.7P%)] It is configured to include that group.

Hereinafter, the present invention will be described in detail.

The present invention is superior to the conventional manufacturing method of plastic organic transformation steel by continuous annealing by limiting the chemical composition in relation to the thickening process to the austenite as much as possible in the cooling rate and the ostampering process ductility can be obtained excellent workability and stable material This excellent 50-80 kg / mm2 grade high strength cold rolled steel sheet is produced, characterized by.

· C: 0.05 ~ 0.20%

C determines the fraction of austenite in the annealing temperature range and also increases the amount of austenite remaining at room temperature to improve ductility by moving to austenite and stabilizing austenite during cracking and holding at 350-450 ° C. If it is less than 0.05%, the austenite fraction is small, so that it is difficult to secure the strength, and more than 3% of retained austenite, which may contribute to ductility, is not obtained. If it exceeds 0.20%, the weldability is limited.

· Si: 0.7 ~ 1.5%

Si increases strength by solid solution strengthening and inhibits cementite precipitation when it is maintained at 350-450 ℃ and promotes the enrichment of carbon to austenite, contributing to the improvement of ductility, but less than 0.7%, the effect is small, 1.5% If it exceeds, since pickling property, weldability, coating property, corrosion resistance, etc. fall, it was limited.

· Mn: 1.5 ~ 1.8%

Mn is an austenite forming element, which affects the amount of austenite in the annealing temperature range, is a solid solution strengthening element, and suppresses transformation during the cooling process. When the addition amount is less than 1.5%, the cooling rate for the bainite inhibition increases, If it exceeds 1.8%, the bainite transformation is delayed during the 350-450 ° C. holding, so that the carbon enrichment is insufficient for the given constant temperature holding time, and as a result, it is difficult to obtain a large amount of retained austenite, thereby reducing the ductility.

· P: 0.01 ~ 0.1%

P is a component that increases the strength by solid solution strengthening and adds Si and promotes carbon enrichment with austenite while maintaining at 350-450 ℃. If P is less than 0.01%, it is not effective. Increases.

· S: 0.005% or less

S is limited to 0.005% or less because segregation in the slab causes cracking and MnS is formed in steel to provide a starting point for cracking of the steel sheet.

, Acid-soluble Al: 0.02 ~ 0.5%

Al is usually added for deoxidation of steel, but deoxidation and bonding with N which is not bonded with B and remaining Al contribute to residual austenite stabilization as a ferrite stabilizing element like Si, which is ineffective at less than 0.02% and casting at more than 0.5%. Hot brittleness and ductility are degraded by Al clusters and Al ₂ O ₃ .

· Ni: 0.1 ~ 0.5%

Ni is an austenite stabilizing element, which increases the stability of austenite during annealing, but less than 0.1%, the effect is small, and the alloy cost increases when it exceeds 0.5%.

· Cr: 0.02 ~ 0.10%

Cr is added to prevent surface decarburization during slab heating, and plays the same role as Si as a ferrite stabilizing element.It has little effect at less than 0.02%, and if it is more than 0.10%, it forms a large amount of Cr carbide to decrease ductility, so 0.02-0.10% Shall be.

· N: 0.003 ~ 0.007%

N is less than 0.003% of the austenite stabilizing element, the effect is small, if it is more than 0.007%, the manufacturing cost increases, AlN is formed to reduce the ductility, and also reduces the effect of Al, which acts as austenite carbon enrichment.

· Mo: 0.01 ~ 0.1%, B: 0.0005 ~ 0.002%

Mo and B are added to delay the transformation of austenite after annealing, and the addition of Mo and B can slow the cooling rate even at a low Mn content, thereby improving melt plating property and ductility. For this effect, Mo is added at least 0.01% and B is added at 0.0005%, but if Mo is at least 0.1%, the increase in manufacturing cost is excessive. If B is at least 0.002%, the ductility is rather reduced by solid solution B. .

· Si% + 9.5P% + 0.45Al % + 0.3Cr% + 0.6Ni% ≥1.0

In the present invention, the components that affect the stability of the austenite in the process of the non-precipitated bainite transformation in the 350-450 ℃ temperature range is adjusted to satisfy the Si equivalent equation, such as the equation (1).

[Relationship 1]

Sieq = Si% + 9.5P% + 0.45Al% + 0.3Cr% + 0.6Ni% ≥1.0

When the amount of ingredient added according to relation 1 is 1.0 or more, a large amount of stable residual austenite can be obtained and a steel sheet having excellent ductility can be manufactured. If the component addition amount is less than 1.0 by Si equivalent, the austenite is not stabilized in the 350-450 ° C. temperature range, and thus the amount of retained austenite obtained at room temperature decreases, thus reducing the ductility of the steel sheet.

The slabs formed as described above are obtained by ingot or continuous casting process after obtaining molten steel through steelmaking process. The slab is hot rolled, wound, cold rolled, and annealed to produce a steel sheet having the desired mechanical properties. The manufacturing conditions for each process will be described in detail.

· Hot rolling

The slab is hot rolled, wherein the hot rolling conditions are such that the finish rolling temperature is A3 transformation point or more in order to obtain a fine hot rolled structure.

· Winding process

The hot rolled hot rolled sheet is wound, wherein the winding temperature is performed at 630 to 680 ° C. This winding temperature is for concentrating an austenite stabilizing element such as Mn around the perlite of the hot rolled sheet in which the austenite nucleus is generated during reverse transformation during heating for annealing.

- cold rolling

The wound hot rolled sheet is cold rolled, wherein the cold reduction rate is preferably 30 to 80%. Deformation of hot-rolled structure and its deformation energy is the energy of recrystallization process, so if the cold reduction rate is less than 30%, this deformation effect is small. If it is over 80%, it is difficult to process by cold rolling and cracking occurs at the edge of steel sheet during rolling. There is a problem.

· Annealing Process

The cold rolled sheet is then annealed, at which time the annealing is first maintained at 780-810 ° C. for 5-180 seconds. This is to obtain a fine recrystallized structure and at the same time to form the amount of austenite required for strength and the amount of ferrite necessary for ductility, and below 780 ° C, the cementite of the hot rolled sheet is not completely re-dissolved, so the carbon content in the austenite is low and 810 ° C. In the case of excess, the amount of ferrite is reduced to reduce the ductility.

Next, the first cooling is performed at a rate of 5 ° C./sec or less to 620-680 ° C., and the second cooling is performed again to 350-450 ° C., where the average cooling rate (CRtot) of the first cooling and the second cooling satisfies relation 2 Do it.

[Relationship 2]

CRmax [3.0- (Mn + 0.3Si% + 2.7P%)] ≥log (CRtot, ° C./sec) ≥ CRmin [2.5- (Mn + 0.3Si% + 2.7 P%)]

If the total cooling rate (CR tot) at the annealing temperature is outside the condition of Equation 2, which is expressed as the range of the upper limit speed (CR max) and the lower limit speed (CR min), that is, if the cooling rate is too fast, the cement is maintained at 350-450 ℃. Tight, non-deposited bainite transformation is delayed, resulting in a coarse quenching structure and ductility deterioration, and if the cooling rate is too slow, the bainite transformation with precipitated perlite or cementite will not occur, resulting in no residual austenite. Ductility is lowered.

In the present invention, the cooling process is divided into primary cooling and secondary cooling to quench after slow cooling in order to improve the mail flow and shape of the steel sheet, and to sufficiently disperse the carbon distribution to austenite in the primary cooling process. This is to satisfy the relation 2 in the secondary cooling process.

In addition, the cooled steel sheet is maintained for 1-20 minutes in the temperature range of 350-450 ° C. At this time, carbon is concentrated and stabilized with austenite. In less than 1 minute, the thickening does not occur sufficiently. The knight was encroached and ductility decreased, so it was limited.

In the present invention, it is possible to efficiently manufacture the steel plate containing the residual austenite in the continuous annealing equipment consisting of a slow cooling zone-quenching zone after the annealing and a molten plating equipment, and by suggesting a Si equivalent equation and a cooling rate relation that can minimize Si, Mn, etc. It is to manufacture 50-80 kg / mm grade ² high strength cold rolled steel sheet with excellent workability and excellent ductility with stable materials. The cold rolled steel sheet produced according to the present invention can be used as an original for hot dip galvanized steel sheet because of its excellent hot-dip galvanizing properties.

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

EXAMPLE

Table 1 shows the invention steel and the comparative steel together.

Inventive steels (steels 1-13) and comparative steels (steels 14-21) having the chemical composition shown in Table 1 were hot-rolled and finish rolled at 900 ° C. to a thickness of 3.0 mm and then cooled and wound up at 650 ° C. One hot rolled sheet was cold rolled to 1.2 mm and annealed to investigate mechanical properties. The annealing condition was maintained at 790 ℃ for 90 seconds and then first cooled to 650 ℃ at 3 ℃ / second, and then again secondary cooling at 10 ℃ / second and maintained at 400 ℃ for 10 minutes and then cooled to room temperature At annealing temperature (T1), primary cooling temperature (T2), secondary cooling temperature (T3), total cooling rate, that is, average speed of primary cooling rate and secondary cooling rate (CRt), and secondary cooling temperature. The holding time t1 was changed. Table 2 shows the annealing conditions and the mechanical properties of the steel sheet obtained by these conditions.

As can be seen from Table 2, the steel sheet that satisfies the annealing conditions of the present invention is a high-strength steel sheet having excellent ductility of at least 35% elongation at the same time as the tensile strength 50-80kg / mm2, even if the comparative steel and the invention steel outside the range of the present invention It has much higher ductility than steel sheets heat treated under conditions outside the scope of the present invention.

This feature is possible by forming a stable austenite as much as possible through the control of the components of the present invention steel using the Si equivalent equation of relation 1 and the first and second cooling conditions and the cooling rate control according to the relation 2 after the annealing. an original plate of the residual austenite by containing a nitro tensile ductility, excellent 50-80kg / mm ² class high strength cold rolled steel sheet and hot-dip galvanized steel sheet can be efficiently produced as compared with the conventional method.

As described above, the present invention can provide 32-40kgf / mm2 grade cold rolled steel sheet excellent in drawing property, hardening hardening resistance and secondary processing brittleness, and this cold rolled steel sheet can be applied to automobile panels and structural parts and melted. Because of its excellent galvanizing properties, it can be used as a master plate for hot-dip galvanized steel sheet.

Claims (1)

By weight% C: 0.05-0.20%, Si: 0.7-1.5%, Mn: 1.5-1.8%, S: 0.005% or less, N: 0.003-0.007% or less, Mo: 0.01-0.1%, B: 0.0005-0.002 %, P: 0.01-0.1%, Al: 0.02-0.5%, Cr: 0.02-0.10%, Ni: 0.1-0.5%, Si, P, Al, Cr, Ni are the following conditions, Si% + 9.5P A steel slab satisfying% + 0.45Al% + 0.3Cr% + 0.6Ni% ≥1.0 and composed of the remaining Fe and unavoidable impurities is hot-rolled at a condition above the finishing rolling temperature A3 transformation point and wound at a temperature of 630-680 ° C. After cold rolling at a reduction ratio of 30-80%, it is maintained at 780-810 ° C. for 5-180 seconds, followed by primary cooling up to 620-680 ° C. up to 5 ° C./sec and back to 350-450 ° C. In the annealing process after cooling to room temperature for 2 to 20 minutes, the average cooling rate (CRtot) of the primary cooling rate and the secondary cooling rate is the following condition, CRmax [3.0- (Mn + 0.3Si). % + 2.7 P%)] ≥ log (CRtot, ° C / sec) ≥ high strength ductility, including satisfying CRmin [2.5- (Mn + 0.3 Si% + 2.7 P%)] Method of producing a cold-rolled steel sheet.