JP2812770B2 - Manufacturing method of alloyed hot-dip galvanized cold-rolled steel sheet for deep drawing with excellent bake hardenability and powdering resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial application field) In recent years, the press formability required as a steel sheet for the inner and outer plates of automobiles is firstly diversified in body design and the use of plastic materials for automobile parts. With the rise, processing characteristics for increasingly severe shapes, especially high deep drawability, are required. In order to satisfy these characteristics, it is necessary to use ultra-low carbon steel, so-called IF steel.

On the other hand, there is a growing demand from automobile manufacturers for a steel sheet in which a plated layer is alloyed after hot-dip galvanizing in order to improve the durability of a chip during spot welding and the rust prevention of a vehicle body.
Furthermore, among such steel plates, those used as outer plates are required to maintain shape after press forming and to have dent resistance.

In order to meet such demands, the present invention is suitable for use in parts where the above properties are required, and is suitable for use in deep-drawing alloyed hot-dip galvanizing with excellent bake hardenability and powdering resistance, and excellent secondary workability. The present invention relates to a method for manufacturing a cold-rolled steel sheet.

(Prior Art) As a steel sheet aimed at improving corrosion resistance, there has been an alloyed hot-dip galvanized steel sheet. On the other hand, in order to ensure deep drawability, it is currently essential to apply it to ultra-low carbon steels added with Ti, Nb, etc.
In the case of Ti-added ultra-low carbon steel, the grain boundaries are not clean.
The problem is that the powdering resistance is poor.

In order to solve these problems, a method for producing an alloyed hot-dip galvanized steel sheet using ultra-low carbon steel has been described so far (1) Japanese Patent Application Laid-Open No. 61-276961 and (2) Japanese Patent Application Laid-Open
No. 61-276962. In each case, the iron concentration in the alloyed galvanized layer on the surface is increased to 15 to 35 wt% to secure powdering resistance. Therefore, the alloying treatment temperature is set to 700 to 850 ° C., and the treatment is performed at a temperature much higher than the usual alloying treatment temperature.

However, when this process is actually performed industrially,
Decrease in productivity due to decrease in threading speed due to high temperature processing,
There is a concern that the load on equipment such as rolls may be increased, and the shape of the plate may be deteriorated due to adhesion to rolls and rapid cooling after alloying treatment. In addition, in (2), the amounts of S and N are regulated in order to impart bake hardenability, and substantially 5 kgf / m
Although m ² or more BH has been granted, considered against aging properties is not performed.

(Problems to be Solved by the Invention) As described above, a method of alloying Ti-added ultra-low carbon steel by hot dip galvanizing and manufacturing a steel sheet excellent in bake hardenability and powdering resistance has already been disclosed. The conventional alloying treatment temperature was greatly increased. Therefore, it does not alloy Ti-added ultra-low carbon steel at the usual alloying treatment temperature to improve powdering resistance.
Further, neither BH property is imparted while further considering the aging effect.

In other words, by performing alloying treatment in normal hot-dip galvanizing, it is excellent in bake hardenability and powdering resistance,
Further, it is an object of the present invention to establish a method for producing a deep drawn alloyed hot-dip galvanized cold-rolled steel sheet which is excellent in secondary workability even without the addition of B, and is also excellent in aging.

(Means for Solving the Problems) The present inventors have conducted intensive studies in view of the above-described circumstances,
By using an ultra-low carbon steel with Ti added, by limiting the amount of S and the cooling rate after recrystallization annealing and alloying treatment, baking hardenability and secondary workability are given while considering aging, zinc plating The present invention has found a method for producing a hot-dip galvannealed cold-rolled steel sheet for deep drawing, which is alloyed at a normal alloying treatment temperature and has excellent so-called powdering properties without peeling of a plating layer.

 FIG. 1 shows the experimental results that led to the establishment of the present invention.

In this experiment, steels in which the contents of C and N were set to 0.005 wt% or less and the contents of Ti were set to 0.04 wt% and S was changed at various levels were melted by vacuum melting. These steels were rolled at 700 ° C. after hot rolling at a finishing temperature of ₃ or more Ar. After pickling and cold rolling this, 8
After recrystallization annealing at 50 ° C and cooling to 400 ° C at 80 ° C / s,
Subject to hot-dip galvanizing, then heated to 550 ° C,
After alloying, cool to room temperature at 70 ° C / s,
%, And the bake hardening (BH) and powdering resistance were investigated.

The powdering resistance was evaluated by performing a 180 ° bending process, attaching a cellophane tape to the bent portion, peeling the cellophane tape, and removing the peeling plating layer adhered to the tape.

That is, it has been found that the BH amount and the powdering resistance change depending on the S amount. Although the cause of such a phenomenon is not clear, it is considered that the precipitation of Ti ₄ C ₂ S ₂ is involved and the TiC precipitation amount or solid solubility limit changes. That is, BH and powdering resistance can be imparted by utilizing solid solution C obtained by re-dissolving TiC after annealing.

The present invention imparts bake hardenability, secondary workability, and powdering resistance to a Ti-added ultra-low carbon steel by appropriately setting the S content and the cooling rate after recrystallization annealing and alloying treatment. is there.

That is, the gist of the present invention is as follows: C: 0.005 wt% or less, Si: 1.0 wt%
Below, Mn; 1.0 wt% or less, P; 0.1 wt% or less, S; 0.002 wt% or less, Al; 0.01 to 0.1 wt%, N; 0.005 wt% or less,
i is the effective Ti amount (Ti ^* ) expressed by equation (1) Above, containing less 0.1 wt% in Ti content, the balance finishing a steel consisting of Fe and unavoidable impurity elements, after the slab at the continuous casting, reheating or immediately Ar ₃ point or more temperature after casting heat After finishing the rolling, winding, cold rolling is performed by a usual method after pickling, and after recrystallization annealing for 1 second or more in a temperature range of 800 ° C. or more and ₃ points of Ac or less, 50 ° C./s or more A deep drawing alloy with excellent bake hardenability and powdering resistance, characterized in that it is cooled at a cooling rate, then hot-dip galvanized, and then alloyed, and then cooled at a cooling rate of 10 ° C / s or more. This is a method for producing a galvannealed steel sheet.

Hereinafter, the present invention will be described specifically.

 First, the reasons for limiting the chemical components will be described.

C is an element that plays an important role in imparting baking hardenability and powdering resistance in the present invention. To ensure moldability at room temperature, that is, low YP, high El and high value, and to make it non-aging, it is better that the amount of addition is low. Therefore, the upper limit is made 0.005 wt%.

Si is added to increase the strength of steel, but the addition of corners hardens the steel and deteriorates weldability. Further, in order to improve the surface properties of steel, it is better that the addition amount is small. In particular, in the present invention, the upper limit is set to 1.0 wt.
%.

Mn also contributes effectively to increasing the strength of steel, but excessive addition is added with an upper limit of 1.0 wt% to harden the steel.

P is an element that has a large solid solution strengthening ability compared to Si and Mn, and is an element that is less likely to deteriorate in ductility and deep drawability due to its addition. It is. In the present invention, it is added for the purpose of increasing the strength, but excessive addition leads to hardening of the steel, not only deteriorating the formability, but also the adhesion of the plating layer coating in hot-dip galvanizing. To deteriorate the secondary workability due to grain boundary segregation of P,
The upper limit is 0.1 wt%.

S is an element that plays the most important role in the present invention. As shown in FIG. 1, S is set to 0.002 wt% or less in order to impart a bake hardening amount and powdering resistance.

Al is necessary for deoxidation of steel, and 0.01 wt% or more is required for improving the yield of Ti. On the other hand, excessive addition increases the cost and leaves inclusions in the steel, so the upper limit is 0.1 wt%.

N is fixed by Ti by the hot rolling stage, so a large amount of Ti
Since the formation of N causes deterioration in workability, the upper limit is 0.005.
wt%.

Ti needs to be added in a sufficient amount to fix C, N and S and to ensure aging. S is Ti ₄ C ₂ S ₂ with Ti and C
Assuming that the effective Ti amount is expressed by equation (1), the lower limit is the amount of C to be precipitated as TiC,
In other words, the effective Ti amount The above addition is necessary. However, excessive addition of P causes a deterioration in the value due to precipitation of TiP when the amount of P added is high, and at the same time increases the amount of solid-solution Ti, which further changes El.

B is added to improve the secondary workability, but is not particularly restricted in the present invention. However, when P is added to increase the strength of the steel, P segregates at the grain boundaries, so that the grain boundaries are embrittled and the secondary workability may be deteriorated. In that case, to ensure secondary workability, B
Is added. If the content is less than 0.0001 wt%, the effect is not obtained, and excessive addition hardens the steel, deteriorates workability and saturates the effect of improving secondary workability, so the upper limit is 0.003 wt%.
And

 Next, a manufacturing method according to the present invention will be described.

There is no particular restriction except for the hot-dip galvanizing pretreatment step including recrystallization annealing and the alloying treatment step.Steel having the above-mentioned chemical components can be obtained as slab by ordinary continuous casting, It may be manufactured by a casting method. Further, a cold-rolled sheet is formed by ordinary hot rolling, pickling and cold rolling.

The recrystallization annealing required before applying the hot-dip galvanizing process,
In order to ensure ductility and deep drawability, sufficient recrystallization and grain growth are performed, and at the same time, at temperatures over 800 ° C for the purpose of re-dissolving TiC to impart bake hardenability and powdering resistance. It shall be held for 1 second or more. But Ac
_A temperature exceeding ₃ points is not preferred because it causes a decrease in the value due to the deterioration of the texture accompanying the transformation and a rough surface due to the coarsening of the crystal grains.

After the recrystallization annealing, the cooling rate until the hot dip galvanizing treatment is performed needs a cooling rate enough to allow the solid solution C to sufficiently remain at the grain boundaries. That is, at a cooling rate of 50 ° C./s or less, C is reprecipitated, and even if an alloying treatment is performed after hot-dip galvanizing, the powdering resistance is poor.

Further, cooling to room temperature after the alloying treatment requires a cooling rate of 10 ° C./s or more in order to secure the bake hardening amount. 10 ℃
If the cooling rate is less than / s, the TiC re-dissolved by the recrystallization annealing is applied again, and the bake hardening amount is undesirably reduced. The recrystallization may be terminated in continuous annealing or box annealing before hot-dip galvanizing.

(Examples) Example 1 C; 0.0027 wt%, Si; 0.15 wt%, Mn; 0.11 wt%, P; 0.008 w
t%, S; 0.0015 wt%, Al; 0.032 wt%, N; 0.0020 wt%, B;
A steel containing 0.0005 wt%, Ti; 0.03 wt%, balance Fe and unavoidable impurities was spot-fired and slab-formed by continuous casting. After hot rolling at 1100 ° C, the finishing temperature was set to 930 ° C and winding was performed at 700 ° C. After pickling, cold rolling is performed at a rolling reduction of 80%, recrystallization and cooling are performed under the conditions shown in Table 1, and after hot-dip galvanizing (450 ° C) and alloying (550 ° C), 1% Temper rolling.

Thereafter, as a material evaluation, the test piece was processed into a JIS Z 2201, No. 5 test piece, and a tensile test was performed in accordance with the test method described in 2421.

For the bake hardening (BH), a 2% pre-strain was applied.
The amount of increase in yield point stress before and after treatment when holding at 0 ° C. for 20 minutes is shown.

Further, the aging property was evaluated by the degree of elongation at the yield point after performing a tensile test after holding at 100 ° C. for 60 minutes. On the other hand, the powdering property was evaluated by performing a 180 ° bending process as described above, attaching a cellophane tape to the bent portion, peeling the cellophane tape, and removing the peeling plating layer adhered to the tape. Table 2 summarizes the results.

Recrystallization conditions and cooling speed before galvanizing and alloying treatment, further and have No.4,6 and 8, 3 kgf / mm ² BH in accordance with the scope of the present invention is the cooling rate after the alloying treatment A material having good powdering resistance and having no problem with aging can be obtained.

No. 1 has a low temperature for recrystallization and is slightly hard. At the same time, the re-dissolution of TiC is insufficient and there is no BH. Further, the powdering resistance is poor and the plating layer is peeled off. Nos. 2, 5, and 7 in which the cooling rate (cooling rate I) to the hot-dip galvanizing bath after recrystallization was low, re-dissolved TiC was precipitated during cooling, and solid solution C was bound before the alloying treatment. Therefore, the alloying treatment is insufficient and the plating layer is peeled off. Also, there is almost no BH amount. In the case of No. 3 where the cooling rate (cooling rate II) after the alloying treatment was low, even if the cooling rate I was sufficiently high, TiC was precipitated during the cooling after the alloying treatment and solid solution C decreased. Therefore, although the plating layer does not peel off, the amount of BH is small. In No. 9, since the temperature for recrystallization exceeded the Ac ₃ point, the crystal grains became coarse and the texture deteriorated, and the surface became rough after the tensile test, indicating a low value.

Example 2 A steel having the chemical composition shown in Table 3 was output from a converter and continuously cast, then subjected to ordinary hot rolling and cold rolling, and recrystallization and cooling were performed under certain conditions within the scope of the present invention. Hot dip galvanizing (450
℃) and alloying treatment (550 ℃). That is, hot rolling is finished at 930 ° C. after heating at 1150 ° C., and 650 ° C.
Was wound up. After pickling, 80% cold rolling was performed, and recrystallization annealing before hot-dip galvanizing was performed at 850 ° C. for 10 s and cooled at 100 ° C./s. After the alloying treatment, it was cooled to room temperature at 80 ° C./s. After the alloying treatment, 1% temper rolling was performed, and the material was evaluated by the same method as in Example 1 and the secondary workability was also evaluated.

As for the secondary workability, as shown in FIG.
A cup 1 punched out to 100φ and squeezed into a cylinder at a draw ratio of 2.0 is immersed in ethanol 2 at −50 ° C., placed on a tapered punch 3 and given a load P, and is judged by the presence or absence of brittle fracture by spreading. No fracture, ×: brittle fracture.

 Table 4 summarizes the results.

A in accordance with the scope of the present invention, B, C, D and E steel and has a 3 kgf / mm ² approximately BH, it is also good secondary workability and powdering resistance, no problem for aging property The material is obtained.

The C and N contents deviate from high, resulting in effective Ti amount (Ti ^* )
The steel F, which has deviated from the scope of the present invention, has a BH amount of 5k.
It is as high as gf / mm ² , has no peeling plating layer, and has good powdering resistance, but is inferior in aging because a large amount of dissolved C remains. G steel had a high Si content and thus had poor adhesion in hot-dip galvanization, and peeling of the plating layer occurred. Moreover, it becomes hard and the value is low. H steel with high Mn content is hardened
El and value are low. Steel I has a high S content deviating from the range of the present invention and has poor powdering resistance,
There is no BH amount and the secondary workability is poor. For steel J, the effective Ti amount (Ti
^* ) Is out of the range of the present invention, so that a large amount of dissolved C remains, the BH amount is high, and the powdering resistance is good, but the aging property is poor.

(Effects of the Invention) The present invention can enhance the strength of a steel sheet used for an inner / outer panel of an automobile by baking after painting, while maintaining excellent deep drawability, and at the same time, powder resistance. The present invention clarifies a method for producing an alloyed hot-dip galvanized steel sheet having excellent ringing properties, secondary workability, and aging properties. According to the present invention, it is possible to increase the strength of the steel sheet after press forming, and at the same time, it is possible to improve the durability of the chip and the rust prevention of the vehicle body at the time of spot welding in an automobile manufacturer.

[Brief description of the drawings]

FIG. 1 is a chart showing the range of the present invention in the amount of S added, and FIG. 2 is an explanatory diagram of a test method for investigating secondary workability used in the present invention.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI C23C 2/28 C23C 2/28

Claims (1)

(57) [Claims] C: 0.005 wt% or less, Si: 1.0 wt% or less, Mn: 1.0 wt% or less, P: 0.1 wt% or less, S: 0.002 wt% or less, Al: 0.01 to 0.1 wt%, N: 0.005wt% or less, and Ti as the effective Ti amount (Ti ^* ) shown in equation (1) As described above, steel containing 0.1% by weight or less of Ti and the balance consisting of Fe and an unavoidable impurity element is made into a slab by continuous casting, and then heated again at a temperature of ₃ or more points of Ar immediately after reheating or casting. Finish rolling, take up, cold-roll in the usual way after pickling, 800 ℃ or more A
c After performing recrystallization annealing for 1 second or more in a temperature range of ₃ points or less,
Cooling at a cooling rate of 50 ° C / s or more, then hot-dip galvanizing, further alloying, and then cooling at a cooling rate of 10 ° C / s or more. An excellent method for producing galvannealed steel sheets for deep drawing.