KR20040059356A - Method for manufacturing sheet steel with ferrite and martensite having excellent uniform elongation ratio - Google Patents

Abstract

PURPOSE: A method for manufacturing 45 to 80 kgf/mm¬2 class thin sheet steel having excellent uniform elongation and dual phase of ferrite and martensite by controlling ratio of B/N and controlling cooling rate according to amount of alloy is provided. CONSTITUTION: The method comprises first step of coiling the hot finish rolled steel sheet in the temperature range of 560 to 620 deg.C after hot finish rolling steel comprising 0.04 to 0.18 wt.% of C, 0.1 to 0.3 wt.% of Si, 1.4 to 1.8 wt.% of Mn, 0.005 to 0.03 wt.% of P, 0.005 wt.% or less of S, 0.003 wt.% or less of N, 0.02 to 0.2 wt.% of acid soluble Al, 0.02 to 0.1 wt.% of Mo 0.0007 to 0.004 wt.% of B and a balance of Fe and other inevitable impurities and satisfying the following relation: 0.7<=B/N<=1.5 at a temperature of Ar3 or more; second step of recrystallization annealing the cold rolled steel sheet in the temperature range of 770 to 830 deg.C for 30 to 180 seconds after cold rolling the coiled hot rolled steel sheet to a reduction ratio of 50 to 80%; third step of first cooling the recrystallization annealed steel sheet to a temperature of 650 to 720 deg.C in a cooling rate of 1 to 10 deg.C/sec; fourth step of maintaining the second cooled steel sheet for 10 to 180 seconds, thereby forming a dual phase of ferrite and martensite after second cooling the first cooled steel sheet to a temperature of 200 to 350 deg.C in a cooling rate (deg.C/sec) satisfying the following relation: log(CR)>=£2.4-(Mn(%)+0.3Si(%)+2.7P(%))|, where CR is cooling rate; fifth step of hot dip galvanizing the heated steel sheet for 10 seconds or less by heating the dual phase formed steel sheet to a temperature of 450 to 470 deg.C; and sixth step of cooling the hot dip galvanized steel sheet to an ordinary temperature.

Description

Method for manufacturing sheet steel with ferrite and martensite having excellent uniform elongation ratio}

The present invention relates to a method for manufacturing a thin steel sheet used in automobile panels and structural parts, and more particularly, excellent uniform elongation through control of the B / N ratio and the control of the cooling rate according to the amount of alloy, ferrite and martensite It relates to a manufacturing method of 45 ~ 80kgf / mm grade ² steel sheet having a composite structure of.

Recently, in order to reduce fuel costs of automobiles, it is required to reduce the weight of the vehicle body, and in the case of panels and structural parts, 45-80 kgf / mm grade ² high strength steel sheets having excellent workability have been developed. On the other hand, in the case of automotive structural parts, a steel plate having a good impact absorption is required, and such a steel sheet is suitable for steel sheets having high tensile strength at high speed. In order to increase the high tensile strength of steel grades, steel sheets with high uniform elongation of steel sheets should be used. For steels with high uniform elongation, dual phase (DP) steel of ferritic-martensite and residual austenitic martensite are processed. Transformation Induced Plasticity (TRIP) steel Japanese Patent Application Laid-open No. 57-61819 is a conventional technique for the two-phase structure steel of ferrite-martensite. The prior art relates to a composite tissue steel obtained by annealing Mn-Cr based low carbon steel for a short time, and has a low yield ratio, no yield point elongation, and exhibits excellent ductility compared to solid solution hardening steel or precipitation hardening steel of the same tensile strength. . However, since the prior art has added 1.8 wt% or more of Mn and 0.4 wt% or more of Cr, ductility is insufficient to be applied to a part such as a panel or a member.

On the other hand, Japanese Patent Application Laid-open No. 58-42246 is a conventional technique using metamorphic organic plastic steel. The prior art has a problem that the annealing temperature is high in the austenite single phase region, and the structure is vulnerable to secondary processing brittleness after press working with a mixed structure of bainite and residual austenite. Another conventional technique using metamorphic organic plastic steel is Japanese Patent Application Laid-open No. Hei 6-145788. In the prior art, a mixed structure steel sheet of ferrite and residual austenite has been proposed, but the steel sheet has excellent ductility due to metamorphic organic plasticity, but has a high Si content, which is problematic in weldability and paintability, and high Al content forms steel inclusions. There is a problem that processing cracks occur during easy processing.

The present invention is to solve the above problems of the prior art, in order to prevent the uniform elongation is lowered by solid solution nitrogen after hot dip galvanizing or zinc alloying process, controlling the B / N ratio and the amount of alloy By controlling the cooling rate according to the excellent uniform elongation and to provide a manufacturing method of 45 ~ 80kgf / mm grade ² steel sheet having a composite structure of ferrite and martensite, its purpose is to.

The present invention for achieving the above object by weight, C: 0.04 ~ 0.18%, Si: 0.1 ~ 0.3%, Mn: 1.4 ~ 1.8%, P: 0.005 ~ 0.03%, S: 0.005% or less, N: 0.003% or less, acid value Al: 0.02 to 0.2%, Mo: 0.02 to 0.1%, B: 0.0007 to 0.004%, remaining Fe and other inevitable impurities, the content of B and N is the following relationship,

0.7 ≤ B / N ≤ 1.5

To satisfy the river,

The first step of finishing hot rolling over A _r3 and then winding at 560 ~ 620 ℃,

After the winding, the second step of cold rolling at 50 ~ 80% reduction rate and then recrystallized annealing at 770 ~ 830 ℃ for 30 to 180 seconds,

After the recrystallization annealing, the third step of first cooling to a cooling rate of 1 ~ 10 ℃ / second to 650 ~ 720 ℃,

After the first cooling, the following relationship,

log (CR) ≥ [2.4- (Mn (%) + 0.3Si (%) + 2.7P (%))]

Fourth step of making a composite structure of ferrite and martensite by second cooling to 200 ~ 350 ℃ at a cooling rate (CR, ℃ / sec) that satisfies and then maintained for 10 ~ 180 seconds,

After making the composite structure, and heated to 450 ~ 470 ℃ a fifth step of hot dip galvanizing and less than 10 seconds and after the hot dip galvanizing, comprising a sixth step of cooling to room temperature.

In addition, the present invention is a weight%, C: 0.04 ~ 0.18%, Si: 0.1 ~ 0.3%, Mn: 1.4 ~ 1.8%, P: 0.005 ~ 0.03%, S: 0.005% or less, N: 0.003% or less, acid Soluble Al: 0.02 to 0.2%, Mo: 0.02 to 0.1%, B: 0.0007 to 0.004%, remaining Fe and other inevitable impurities, and the content of B and N is

0.7 ≤ B / N ≤ 1.5

To satisfy the river,

The first step of finishing hot rolling over A _r3 and then winding at 560 ~ 620 ℃,

After the winding, the second step of cold rolling at 50 ~ 80% reduction rate and then recrystallized annealing at 770 ~ 830 ℃ for 30 to 180 seconds,

After the recrystallization annealing, the third step of first cooling to a cooling rate of 1 ~ 10 ℃ / second to 650 ~ 720 ℃,

After the first cooling, the fourth step of hot-dip galvanizing for 10 seconds or less by heating to 450 ~ 470 ℃,

After the hot dip galvanizing, the following relationship,

log (CR) ≥ [2.4- (Mn (%) + 0.3Si (%) + 2.7P (%))]

5th step of making a composite structure of ferrite and martensite by second cooling to 200 ~ 350 ℃ at a cooling rate satisfying the following (CR, ℃ / sec) and maintaining for 10 ~ 180 seconds.

After the hot dip galvanizing, it comprises a sixth step of cooling to room temperature.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The present invention is to control the B / N ratio and to control the cooling rate according to the amount of alloy, in order to prevent the uniform elongation is lowered by the solid solution nitrogen after the alloying process of hot dip galvanized or zinc layer, the ferrite and martensite of It is characterized by improving the uniform elongation of 45 ~ 80kgf / mm grade ² steel sheet having a composite structure, first look at the reasons of the limitation of the present invention.

C: 0.04-0.18 wt%

The C is a strong invasive solid solution strengthening element of the steel, and is added to secure the austenite volume fraction at 5 to 30% at the annealing temperature, and when it is added less than 0.04% by weight, the austenite fraction is small at the two-phase annealing. It is difficult to ensure, and when added in excess of 0.18% by weight, since the spot weldability is lowered, it is preferable to limit the content to 0.04 to 0.18% by weight.

Si: 0.1-0.3 wt%

The Si improves the weldability by improving the fluidity of the molten metal during welding in the Mn-added steel grade as much as possible to reduce the inclusion of metal in the weld zone, and release the carbon in the ferrite as austenite to lower the yield strength and improve the ductility As a component to be added, if less than 0.1% by weight can not be obtained the above effect, if it is added in excess of 0.3% by weight the weld inclusions are formed by the silicon oxide and the paintability and plating properties, etc., the content is 0.1 ~ It is preferable to limit it to 0.3 weight%.

Mn: 1.4-1.8 wt%

The Mn was added to stabilize the austenite to secure martensite and at the same time it is a solid solution strengthening element. When the amount of Mn is less than 1.4 wt%, the cooling rate is too fast to obtain martensite, and thus thermal stress is generated. When the content is more than 1.8% by weight, the addition of more than 1.8% by weight not only adversely affects the weldability, paintability, and plating property, but also forms a band structure in the steel sheet structure, thereby lowering the ductility, and therefore, the content is preferably limited to 1.4 to 1.8% by weight. Do.

P: 0.005 to 0.03 wt%

P is a solid solution strengthening element, and is added as a grain boundary segregation element to concentrate on the surface of the hot-rolled steel sheet to promote pickling. If the content of P is less than 0.005% by weight, the above effect cannot be obtained. If the content of P is more than 0.03% by weight, the secondary processing brittleness and the spot weldability and the alloying reaction of the hot dip galvanized layer are delayed, so the content is limited to 0.005 to 0.03% by weight. It is desirable to.

S: 0.005 wt% or less

When the S is added in excess of 0.005% by weight, coarse MnS is generated in the hot rolled sheet, which is a starting point of the processing crack, and therefore, the content is preferably limited to 0.005% by weight or less.

N: 0.003 wt% or less

Although N is an austenite stabilizing element, when it is added in excess of 0.003% by weight, it is supersaturated during cooling in the alloying process of hot dip galvanized or zinc layer, thereby lowering the uniform elongation. Therefore, the content is preferably limited to 0.003% by weight or less. .

Acid value Al: 0.02-0.2 wt%

The acid value Al is an effective ingredient for preventing cracks in the manufacture of slabs by combining with oxygen in the steel, and when added below 0.02% by weight, the above effect cannot be obtained, and when added in excess of 0.2% by weight, the alloy cost increases. It is preferable to limit the content to 0.02 to 0.2% by weight.

Mo: 0.02% to 0.1% by weight, B: 0.0007% to 0.004% by weight

Mo and B is a component that delays the transformation of austenite into pearlite during the cooling process at the annealing temperature in the same manner as Mn, and when added simultaneously, the effect is greatly increased compared to the case where it is added alone. Add. If the content of Mo is less than 0.02% by weight, the above effect cannot be obtained. If the content of Mo is more than 0.1% by weight, the second phase is coarsened to lower the bendability and the alloy cost increases, so that the content is 0.02 to 0.1% by weight. It is desirable to limit. In addition, when the content of B is less than 0.0007% by weight, the above effect cannot be obtained. When the content of B is more than 0.004% by weight, excessive B is condensed on the surface, thereby degrading the plating adhesion. Therefore, the content is preferably limited to 0.0007 to 0.004% by weight. Do.

In addition to the above compositions, the remainder is composed of Fe and other unavoidable impurities.

The present invention is characterized by improving the uniform elongation, and in order to improve the uniform elongation, the B / N ratio must be appropriately controlled. If the B / N ratio is less than 0.7, the effect of improving the uniform elongation is small. If the B / N ratio is greater than 1.5, the effect of improving the uniform elongation is included and the plating adhesion is reduced by excessive addition of B. Therefore, the B / N ratio is limited to 0.7 to 1.5. It is preferable.

The steel formed as described above is finished hot rolled at a temperature of A _r3 or higher, and then wound at 560 to 620 ° C. When the finish hot rolling temperature is less than A _{r3, a} fine hot rolled structure cannot be obtained. Therefore, the finish hot rolling temperature is preferably limited to A _r3 or more. In addition, when the coiling temperature is less than 560 ° C, shape defects occur due to supercooling of the hot rolled sheet, and when it exceeds 620 ° C, the surface thickening of elements deteriorating the wettability of the hot dip galvanizing, such as Si, Mn, and B, is increased. It is preferable to wind up the coiling temperature to 560-620 degreeC.

After the winding, cold rolling at a reduction ratio of 50 to 80%, and then recrystallized annealing at 770 ~ 830 ℃ for 30 to 180 seconds. If the cold reduction rate is less than 50%, the accumulated energy required for recrystallization cannot be secured. If the cold reduction rate is higher than 80%, it is difficult to process by cold rolling and cracking occurs at the edge of the steel sheet during rolling. Is preferably limited to 50-80%. In addition, the recrystallization annealing step is to promote the formation of ferrite and austenite fraction and carbon distribution at the same time as the recrystallization, when the recrystallization annealing temperature is less than 770 ℃, the amount of austenite formation is small, the effect of the composite structure is small, 830 ℃ If it exceeds A, the amount of austenite is excessively formed and ductility decreases. Therefore, the recrystallization annealing temperature is preferably limited to 770 to 830 ° C. In addition, when the recrystallization annealing time is less than 30 seconds, the amount of austenite formation is small, and the effect of the composite structure is small, and when the recrystallization annealing time is longer than 180 seconds, the amount of austenite is excessively formed, so the recrystallization annealing time is limited to 30 to 180 seconds. desirable.

After the recrystallization annealing, the first cooling to 650 ~ 720 ℃ at a cooling rate of 1 ~ 10 ℃ / sec. The primary cooling step is to increase the ductility and strength by making the carbon concentration of ferrite and austenite approach the equilibrium state, the uniform elongation and ductility is insufficient when the primary cooling end temperature is less than 650 ℃, 720 ℃ Since the uniform elongation and ductility are insufficient even if it exceeds, it is preferable to limit the said primary cooling end temperature to 650-720 degreeC. In addition, when the cooling rate to the first cooling end temperature is less than 1 ℃ / second, it is difficult to suppress the pearlite transformation in the cooling process, if it exceeds 10 ℃ / second, there is no effect of carbon redistribution, until the first cooling end temperature The cooling rate of is preferably limited to 1 ~ 10 ℃ / second.

After the first cooling, the following relation 1,

[Relationship 1]

log (CR) ≥ [2.4- (Mn (%) + 0.3Si (%) + 2.7P (%))]

After cooling to 200 ~ 350 ℃ at a cooling rate (CR, ℃ / sec) satisfying to maintain for 10 to 180 seconds to form a complex structure of ferrite and martensite. If the relation 1 is not satisfied, pearlite or bainite structure is obtained, and it is difficult to expect the improvement of strength and ductility, which is an advantage of the ferrite and martensite composite structure.

In addition, if the secondary cooling end temperature is less than 200 ℃ ferrite and martensite composite structure can not be obtained stably, even if it exceeds 350 ℃ ferrite and martensite composite structure can not be obtained stably, the secondary cooling end It is preferable to limit temperature to 200-350 degreeC. In addition, since the ferrite and martensite complex structure cannot be stably obtained when the holding time is less than 10 seconds or more than 180 seconds after the secondary cooling, the holding time after the secondary cooling is preferably limited to 10 to 180 seconds. .

After making the composite structure, it is heated to 450 ~ 470 ℃ hot-galvanized 10 seconds or less. If the hot-dip galvanizing temperature is less than 450 ℃ zinc plating is insufficient, if it exceeds 470 ℃ excessive zinc plating is made, the hot-dip galvanizing temperature is preferably limited to 450 ~ 470 ℃. In addition, since the zinc plating becomes excessive when the hot dip galvanizing time exceeds 10 seconds, the hot dip galvanizing time is preferably limited to 10 seconds or less.

After the hot dip galvanizing, it is cooled to room temperature.

In addition, the present invention may further comprise the step of heat-treating the alloying heat treatment for less than 30 seconds by heating to 500 ~ 580 ℃ between the hot dip galvanizing step and the step of cooling to room temperature. The alloying heat treatment is for alloying the hot dip galvanized layer formed in the hot dip galvanizing step, if the alloying heat treatment temperature is less than 500 ℃ alloying is unstable, even if it exceeds 580 ℃ alloying is unstable, the alloying heat treatment temperature is 500 ~ It is desirable to limit to 580 ° C. Further, since the alloying becomes excessive when the alloying heat treatment time exceeds 30 seconds, the alloying heat treatment time is preferably limited to 30 seconds or less.

In the above production method, a composite structure of ferrite and martensite is formed, followed by hot dip galvanization or a composite structure of ferrite and martensite, followed by hot dip galvanization, followed by alloy heat treatment. However, in the present invention, after forming the composite structure of ferrite and martensite after hot dip galvanizing or after hot-dip galvanizing and alloying heat treatment, the same effect can be obtained even if the composite structure of ferrite and martensite is formed.

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

EXAMPLE

Steel prepared as shown in Table 1 was hot-rolled at 900 ° C. to prepare a steel plate of 3.0 mm. After cooling, it was wound up at 580 ° C., pickled, and cold rolled to a thickness of 1.2 mm. The cold rolled steel sheet is subjected to annealing and primary cooling under the conditions shown in Table 2, and then cooled to 460 ° C. and maintained for 10 seconds, and then heated to 540 ° C. and further heated to 540 ° C. for 20 seconds, and 300 ° C. Secondary cooling (cooling method (4)). The mechanical properties of the steel sheet manufactured as described above were examined, and the results are shown in Table 2 below.

As can be seen in Table 2, the steel grade that satisfies the components of the present invention and the annealing conditions showed a very excellent uniform elongation and ductility at the same level of tensile strength than the steel grade outside the scope of the present invention. In particular, the comparative steel (G ~ I) is a steel that does not add Mo or B at the same time different from the steel of the present invention, satisfies the relation 1, but the uniform elongation and ductility relative to the strength appeared lower than the steel of the present invention, which is the present invention It shows the austenite stabilizing effect of Mo and B complex addition of steel. Comparative steel (J) is a steel in which the B / N ratio is outside the scope of the present invention, and showed a lower uniform elongation than the inventive steel having a B / N ratio of 0.7 or more.

On the other hand, after the first cooling to the invention steel (C) after the second cooling to 250 ℃ and maintained for 120 seconds and then again heated to 460 ℃ and maintained for 5 seconds, the final cooling to room temperature (1), 250 ℃ Secondary cooling, holding for 120 seconds, heating to 460 ℃ again, holding for 5 seconds, heating to 540 ℃ for 20 seconds, final cooling to room temperature (2), cooling to 460 ℃, holding for 5 seconds, then to room temperature Steel sheets were manufactured by the secondary cooling method (3) and the cooling method (4), and the results are shown in Table 3 below.

As can be seen from Table 3, excellent mechanical properties of the composite tissue steel was obtained at a cooling rate of 10 ℃ / second satisfying the relational expression 1, the second cooling rate formula.

As described above, the present invention relates to a thin steel sheet for hot-dip galvanized steel sheet excellent in uniform elongation of 45-80kgf / mm ² grade tensile strength having a composite structure of ferrite and martensite, can be applied to automotive panels and structural parts It is effective in lightening automobiles and improving crash absorption.

Claims (4)

By weight%, C: 0.04 to 0.18%, Si: 0.1 to 0.3%, Mn: 1.4 to 1.8%, P: 0.005 to 0.03%, S: 0.005% or less, N: 0.003% or less, acid value Al: 0.02 to 0.2%, Mo: 0.02% to 0.1%, B: 0.0007% to 0.004%, remaining Fe and other unavoidable impurities, and the content of B and N is 0.7 ≤ B / N ≤ 1.5 To satisfy the river, The first step of finishing hot rolling over A _r3 and then winding at 560 ~ 620 ℃, After the winding, the second step of cold rolling at 50 ~ 80% reduction rate and then recrystallized annealing at 770 ~ 830 ℃ for 30 to 180 seconds, After the recrystallization annealing, the third step of first cooling to a cooling rate of 1 ~ 10 ℃ / second to 650 ~ 720 ℃, After the first cooling, the following relationship, log (CR) ≥ [2.4- (Mn (%) + 0.3Si (%) + 2.7P (%))] Fourth step of making a composite structure of ferrite and martensite by second cooling to 200 ~ 350 ℃ at a cooling rate (CR, ℃ / sec) that satisfies and then maintained for 10 ~ 180 seconds, After making the composite structure, the ferrite and martensite excellent in uniform elongation comprising a fifth step of heating to 450 ~ 470 ℃ and hot dip galvanizing 10 seconds or less and the sixth step of cooling to room temperature after the hot dip galvanizing Method for manufacturing a composite tissue steel sheet consisting of. According to claim 1, After the fifth step is heated to 500 ~ 580 ℃ further comprising the step of alloying heat treatment for 30 seconds or less, characterized in that the composite sheet steel sheet made of excellent ferrite and martensite excellent elongation Manufacturing method. By weight%, C: 0.04 to 0.18%, Si: 0.1 to 0.3%, Mn: 1.4 to 1.8%, P: 0.005 to 0.03%, S: 0.005% or less, N: 0.003% or less, acid value Al: 0.02 to 0.2%, Mo: 0.02% to 0.1%, B: 0.0007% to 0.004%, remaining Fe and other unavoidable impurities, and the content of B and N is 0.7 ≤ B / N ≤ 1.5 To satisfy the river, The first step of finishing hot rolling over A _r3 and then winding at 560 ~ 620 ℃, After the winding, the second step of cold rolling at 50 ~ 80% reduction rate and then recrystallized annealing at 770 ~ 830 ℃ for 30 to 180 seconds, After the recrystallization annealing, the third step of first cooling to a cooling rate of 1 ~ 10 ℃ / second to 650 ~ 720 ℃, After the first cooling, the fourth step of hot-dip galvanizing for 10 seconds or less by heating to 450 ~ 470 ℃, After the hot dip galvanizing, the following relationship, log (CR) ≥ [2.4- (Mn (%) + 0.3Si (%) + 2.7P (%))] 5th step of making a composite structure of ferrite and martensite by second cooling to 200 ~ 350 ℃ at a cooling rate satisfying the following (CR, ℃ / sec) and maintaining for 10 ~ 180 seconds. After the hot-dip galvanizing, a method for producing a composite structured steel sheet consisting of ferrite and martensite excellent in uniform elongation comprising a sixth step of cooling to room temperature. According to claim 1, After the fourth step is heated to 500 ~ 580 ℃ further comprising the step of alloying heat treatment for 30 seconds or less, characterized in that the composite sheet steel sheet composed of excellent ferrite and martensite excellent elongation Manufacturing method.