JPH03232927A - Production of alloying hot dip galvanized cold-rolled steel sheet for deep drawing excellent in baking hardenability and powdering resistance - Google Patents

Abstract

PURPOSE:To obtain the steel sheet by using a dead-soft carbon steel in which Ti of prescribed composition is added, limiting the amount of S, and also limiting cooling velocities after recrystallization annealing and after alloying treatment, respectively, in a steel sheet for automobile outer and inner sheets. CONSTITUTION:A steel having a composition consisting of, by weight, <=0.005% C, <=1.0% Si, <=1.0% Mn, <=0.1 % P, <=0.002% S, 0.01-0.1% Al, <=0.005% H, Ti in the amount of >=[(C%)-12/32(S%)]X4% by effective Ti(Ti*) represented by an equation and <=0.1% by Ti quantity, and the balance Fe is used. This steel is continuously cast into a slab, and, directly after reheating or casting, finish hot rolling is completed at a temp. of the Ar3 point or above and the resulting plate is coiled, pickled, and cold-rolled by the ordinary method, and the resulting sheet is subjected to recrystallization annealing in the temp. region between 800 deg.C and the Ac3 point for >=1sec. Successively, this steel is cooled at >=50 deg.C/s cooling rate, subjected to hot dip galvanizing and further to alloying treatment, and then cooled at >=10 deg.C/s cooling rate.

Description

[Detailed Description of the Invention] (Field of Industrial Application) In recent years, press formability, which is first required for steel sheets for the interior and exterior of automobiles, has been affected by the diversification of car body designs and the use of plastic materials for automobile parts. Due to the rise in popularity, there is a demand for high processing characteristics, especially deep drawability, into increasingly severe shapes. To satisfy these characteristics, ultra-low carbon steel,
We are forced to use so-called IF steel.

On the other hand, in order to improve the durability of the tip during spot welding and to improve the rust prevention of the car body, automobile manufacturers are increasingly demanding steel sheets in which the plating layer is alloyed after hot-dip galvanizing. Furthermore, among these steel plates, those used as outer plates are required to maintain shape after press forming and have Punto resistance.

In order to meet these demands, the present invention has developed an alloyed hot-dip galvanizing coating for deep drawing that has excellent bake hardenability, powdering resistance, and secondary workability, and is suitable for use in areas where the above characteristics are required. The present invention relates to a method for manufacturing cold-rolled steel sheets.

(Prior Art) As a steel sheet for the purpose of improving corrosion resistance, there has conventionally been an alloyed hot-dip galvanized steel sheet.

On the other hand, in order to ensure deep drawability, it is currently essential to use ultra-low carbon steels with additions of Ti, Nb, etc., but especially in Ti-added ultra-low carbon steels, grain boundaries are Because it is clean, alloying treatment after hot-dip galvanizing rapidly progresses alloying at grain boundaries, resulting in poor powdering resistance.

Studies have been conducted to solve these problems, and methods for manufacturing alloyed hot-dip galvanized steel sheets using ultra-low carbon steel have been published in (1) JP-A-61-27691 and (2) JP-A-61-27691. -27 [There is a publication i92. In both cases, the iron concentration in the surface alloyed galvanized layer is increased to 15 to 35 wt% to ensure powdering resistance.

For this purpose, the alloying treatment temperature is set at 700 to 850°C, which is much higher than the alloying treatment temperature normally performed.

However, when this process is actually carried out industrially, there is a decrease in productivity due to a decrease in the threading speed due to high temperature treatment, an increase in the burden on equipment including rolls, adhesion to the rolls, and alloying. There is a concern that the shape of the plate may become defective due to rapid cooling after the chemical treatment.

In addition, in (2), S and N are used to impart bake hardenability.
i, and in effect BH of 5 kg f/- or more
has been granted, but no consideration has been given to the statute of limitations.

(Problems to be Solved by the Invention) As described above, a method for producing a steel sheet with excellent bake hardenability and powdering resistance by alloying Ti-added ultra-low carbon steel by hot-dip galvanizing has already been disclosed. This significantly raised the conventional alloying treatment temperature. Therefore, the powdering resistance has not been improved by alloying Ti-added ultra-low carbon steel at the usual alloying temperature, nor has the BH property been imparted while taking aging properties into consideration. .

In other words, by applying the alloying treatment in normal hot-dip galvanizing, it has excellent bake hardenability and powdering resistance, excellent secondary workability even without the addition of B, and even deep coating with excellent aging resistance. It is an object of the present invention to establish a method for manufacturing an alloyed hot-dip galvanized cold-rolled steel sheet for drawing.

(Means for Solving the Problems) As a result of intensive study in view of the above circumstances, the inventors found that T.
By using ultra-low carbon steel with added I, by limiting the amount of S and the cooling rate after recrystallization annealing and alloying treatment, we have given bake hardenability and secondary workability while taking aging properties into consideration. We have discovered a method for producing an alloyed hot-dip galvanized cold-rolled steel sheet for deep drawing which is alloyed at a normal post-alloying treatment temperature and has excellent so-called powdering properties without peeling of the plating layer.

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

In this experiment, the amount of C and N was set to 0.005νt% or less,
Steels containing 0.04 wt% Ti and varying S levels were produced by vacuum melting. About those steelsA
After hot-rolling at a finishing temperature above rs, it was rolled up at 700°C.

After pickling and cold rolling, this was recrystallized and annealed at 850°C.
After cooling to 400°C at 0°C/s, hot-dip galvanizing, heating to 550°C, alloying treatment, cooling to room temperature at 70°C/s, and 1% temper rolling. Bake hardening 1 (BH) and powdering resistance were investigated.

Note that powdering resistance was evaluated by performing 180° bending, adhering cellophane tape to the bent portion, peeling it off, and checking the presence or absence of a release plating layer attached to the tape.

In other words, it has been found that BHft and powdering resistance change depending on the amount of S. Although the cause of this phenomenon is not clear, T l 4C2S
This is considered to be because the precipitation of TiC is involved and the amount of TiC precipitation or the solid solubility limit changes. That is, BH and powdering resistance can be imparted by utilizing solid solution C obtained by redissolving TiC after annealing.

The present invention imparts bake hardenability, secondary workability, and powdering resistance to Ti-added ultra-low carbon steel by appropriately controlling the amount of S and the cooling rate after recrystallization annealing and alloying treatment. be.

That is, the gist of the present invention is that C; 0.005 wt% or less, S
I: 1.0wt% or less, Mn: 1.0wt
% or less, P; 0.1wt% or less, S; 0.00
2wt% or less, A, 1llO, 01-0. Iwt%, N
; In addition to containing 0.005wt% or less, Ti (1)
The effective Ti amount shown by the formula, (Ti amount) is f (Cwt%)
-" [Swt%) A steel containing lx4wt% or more and 01-wt% or less of Ti, with the remainder consisting of Fe and unavoidable impurity elements, is made into a slab by continuous casting, then reheated or cast. Immediately finish the finish hot rolling at the intended temperature, roll it up, pickle it, cold roll it in the usual way, and recrystallize it for more than 1 second at a temperature range of 800℃ or higher and AC3 points or lower. After applying, the method is cooled at a cooling rate of 50°C/s or more, followed by hot-dip galvanizing, further alloying treatment, and then cooled at a cooling rate of 10°C/s or more. This is a method for producing an alloyed hot-dip galvanized steel sheet for deep drawing that has excellent powdering resistance.

The present invention will be specifically explained below.

First, the reason for limiting the chemical components will be explained.

C is an element that plays an important role in imparting bake hardenability and powdering resistance in the present invention. Formability at room temperature, ie low YP.

In order to ensure a high Eρ and a high 1 value and to achieve non-aging, the addition amount is preferably low. Therefore, the upper limit is 0.005wt
%.

Sl is added to increase the strength of steel, but excessive addition makes the steel hard and deteriorates weldability.

In addition, in order to improve the surface properties of the steel, it is better to add as little as possible, and in particular, in the present invention, in order to ensure the adhesion of the plating layer liquid film during hot-dip galvanizing, the upper limit is set to 1.
It is set to 0wt%.

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

P is an element with a greater solid solution strengthening ability than Si and Mn, and its addition causes less deterioration in ductility and deep drawability, so it is an important element for increasing strength while ensuring formability. It is. In the present invention, it is added when the purpose is to increase the strength, but excessive addition leads to hardening of the steel, which not only deteriorates formability but also reduces the adhesion of the plating layer liquid film during hot-dip galvanizing. This also causes deterioration of secondary workability due to grain boundary segregation of P.
The upper limit is set to 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 provide bake hardening and powdering resistance.

AN is necessary for deoxidizing steel, and in order to improve the yield of Ti, 0.01 wt% or more is required.

On the other hand, excessive addition increases costs and leaves inclusions in the steel, so the upper limit is set to 0.1 wt%.

Since N is fixed with Ti by the hot rolling stage, a large amount of T
Since the formation of iN causes deterioration of workability, the upper limit is set to O.
, QO is 5wt%.

Ti needs to be added in an amount sufficient to fix C, N and S and ensure aging properties. S is Ti with Ti and C
Since Ti precipitates as the amount of C precipitated, if the effective amount of Ti is expressed by equation (1), the lower limit is the amount of C precipitated as TiC, that is, it is necessary to add 32 12 or more. However, excessive addition causes deterioration of the i value due to precipitation of TiP when the amount of P added is high, and at the same time, an increase in solid solution T1 further deteriorates EII, so the upper limit is set to 0.1. Let it be wt%.

B is added to improve secondary processability, but
In the case of the present invention, there are no particular restrictions. However, when adding P to increase the strength of steel, P segregates at the grain boundaries, which may cause the grain boundaries to become brittle, leading to deterioration in secondary workability. In that case, B is added to ensure secondary processability.

If it is less than 0.0001wt%, there is no effect, and excessive addition will harden the steel, deteriorate workability, and saturate the secondary workability improvement effect, so the upper limit is set at 0.003νt.
%.

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

1. There are no particular restrictions other than the hot-dip galvanizing pre-treatment process including recrystallization annealing and the alloying treatment process, and steel with the above chemical composition can be obtained as a slab through normal continuous casting, but it can be obtained as a thin slab. It may be manufactured by a continuous casting method. Further, it is made into a cold rolled sheet by ordinary hot rolling, pickling and cold rolling.

Recrystallization annealing, which is necessary before hot-dip galvanizing, is necessary to ensure ductility and deep drawability, to ensure sufficient recrystallization and grain growth, and at the same time to impart bake hardenability and powdering resistance. , 800 for the purpose of redissolving TiC.
It shall be held for 1 second or more in a temperature range of ℃ or higher. However, a temperature exceeding the AC3 point is not preferable because it causes a decrease in the F value due to deterioration of the texture accompanying transformation and roughening of the surface due to coarsening of crystal grains.

The cooling after recrystallization annealing and before the hot-dip galvanizing treatment must be performed at a cooling rate that allows solid solution C to remain sufficiently at the grain boundaries. That is, at a cooling rate of 50° C./s or less, C re-precipitates, resulting in poor powdering resistance even if alloying treatment is performed after hot-dip galvanizing.

Further, cooling to room temperature after the alloying treatment requires a cooling rate of 10° C./s or more in order to ensure the amount of bake hardening. 1
At a cooling rate of less than 0°C/s, the T remelted by recrystallization annealing
iC precipitates again and the amount of bake hardening decreases, which is not preferable. Note that recrystallization may be completed by continuous annealing or box annealing before hot-dip galvanizing.

(Example) Example I C; 0.0027wt%, S i ; 0.15w
t%, Mn; 0.11wt%, P H0,008wt%
, S; 0.0015wt%, AΩ; 0.032wt
%, N; 0.0020wt%, B; 0.000
Steel consisting of 5wt%, Ti: 0.03wt%, balance Fe and unavoidable impurities was tapped from a converter and made into a slab by continuous casting.

After hot rolling was heated at 1100°C, the finishing temperature was 930°C.
It was rolled up at 00°C. After pickling, cold rolling was performed at a reduction rate of 80%, recrystallization and cooling were performed under the conditions shown in Table 1, hot-dip galvanizing (450°C) and alloying treatment (55%
0°C), 1% temper rolling was performed.

3 After that, JIS Z 2201. was used for material evaluation. It was processed into a No. 5 test piece and subjected to a tensile test according to the test method described in No. 2241.

Regarding the bake hardening amount (BH), 1
It was expressed as the amount of increase in yield point stress before and after treatment when held at 70°C for 20 minutes.

In addition, aging properties were evaluated by conducting a tensile test after holding at 100° C. for 60 minutes and evaluating the degree of elongation at yield point.

On the other hand, regarding powdering property, as mentioned above, 180
After performing the bending process and adhering cellophane tape to the bent part, this was peeled off and evaluation was made based on the presence or absence of the peelable plating layer attached to the tape. Table 2 summarizes the results.

4 1st Table No., Recrystallization conditions Cooling rate I ('CX5) ('C/s) 1 750X40 80 2 81, Ox'5 1.03 830X
5 70 4 830×20 90 5 850×3 40 6 850×3 100 7 900×2 30 8 900 Out of range cooling rate I: Cooling rate after recrystallization until entering the hot-dip galvanizing bath Cooling rate ■: Cooling rate after alloying process Recrystallization conditions and cooling rate before hot-dip galvanizing and alloying process, and further alloying BH of 3 kg f /- for No. 4.6 and 8 where the cooling rate after treatment is according to the scope of the present invention
It is possible to obtain a material that has good powdering resistance and no problems with aging.

In No. 1, the temperature for recrystallization was low, and it was a little hard, and at the same time, TiC was not sufficiently redissolved, so there was no BH, and the powdering resistance was poor, and peeling of the plating layer was observed. In Nos. 2.5 and 7, in which the cooling rate (cooling rate I) to the hot-dip galvanizing bath after recrystallization was low, the re-melted TiC precipitated during cooling, and the solid solution C was transferred to the grain boundary before alloying treatment. As a result, the alloying process is insufficient and the plating layer peels off. Also, B
There is almost no amount of H. In addition, the cooling rate after alloying treatment (
For N083, the cooling rate (■) was low, even if the cooling rate I was sufficiently fast, TiC still formed during cooling after the alloying process.
is precipitated and solid solution C is reduced, so although the plating layer does not peel off, the amount of BH is small.

7 No. 9, because the temperature for recrystallization exceeded the A c a point, the crystal grains became coarser and the texture deteriorated, resulting in rough skin after the tensile test. The value is showing a low value.

Example 2 Steel having the chemical composition shown in Table 3 was tapped from a converter, continuously cast, and then subjected to normal hot rolling and cold rolling, recrystallization and cooling under certain conditions within the scope of the present invention, Hot-dip galvanizing (450
) and alloying treatment (550°C). That is, the hot rolling was heated at 1150°C, finish rolling was completed at 930°C, and the material was rolled up at 650°C. After pickling, 80% cold rolling was performed, and recrystallization annealing was performed at 850°C for 10 seconds before hot-dip galvanizing.
and cooled at 1009 C/s. Moreover, 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 quality was evaluated in the same manner as in Example 1, and the secondary workability was also evaluated.

Regarding secondary workability, as shown in Figure 2, the sample was
A cup 1 punched to 00φ and squeezed into a cylinder with a drawing ratio of 2.0 is immersed in ethanol 2 at -50°C, placed on a taper punch 3 and subjected to a load P, and judged by the presence or absence of brittle fracture due to expansion. ○: No brittle fracture pillars, ×: Brittle fracture cracks.

Table 4 summarizes the results.

3 kg of A, B, C, D and E steel according to the scope of the invention
A material can be obtained which has a BH of about f/-, has good secondary workability and powdering resistance, and has no problem with aging properties.

The amounts of C and N deviate high, and as a result, the amount of effective Ti (Ti*
), which is also outside the scope of the present invention, has a high BH content of 5 kgf/-, has no peeling plating layer, and has good powdering resistance, but it has a large amount of solute C remaining. It is inferior to the statute of limitations. Since the G steel had a high Si content, adhesion during hot-dip galvanizing was poor, and the plating layer peeled off. It also becomes hard and has a low F value. H steel with high Mni becomes hard and EΩ and ? value is low. (2) The steel has Si that is high outside the range of the present invention, and has poor powdering resistance, as well as lack of BHJiL and poor secondary workability.

In steel J, the effective Ti m (Ti*) was 9, which was lower than the range of the present invention, so a large amount of solid solute C remained, the amount of BH was high, and the powdering resistance was good, but
It is inferior to the statute of limitations.

(Effects of the Invention) The present invention can increase the strength of steel sheets used for the interior and exterior panels of automobiles by baking the paint after forming while maintaining excellent deep drawability. This study clarified a method for manufacturing alloyed hot-dip galvanized steel sheets that have excellent ringability, secondary workability, and aging properties. According to the present invention, it becomes possible to increase the strength of a steel plate after press forming, and at the same time, it becomes possible for automobile manufacturers to improve the durability of tips during spot welding and to improve the rust prevention of car bodies.

[Brief explanation of drawings]

FIG. 1 is a chart showing the range of the amount of S added according to the present invention, and FIG. 2 is an explanatory diagram of the test method for investigating secondary processability used in the present invention. Agent Patent Attorney Tatsuo Chanoki 3rd

Claims (1)

[Claims] C: 0.005wt% or less, Si: 1.0wt% or less, Mn: 1.0wt% or less, P: 0.1wt% or less, S: 0.002wt% or less, Al: 0. In addition to containing 01 to 0.1 wt% and N: 0.005 wt% or less, Ti is the effective Ti amount (Ti^*) shown by formula (1) {
[Cwt%] - 12/32 [Swt%]} × 4wt% or more, containing 0.1wt% or less of Ti, the balance consisting of Fe and unavoidable impurity elements, was made into a slab by continuous casting. Immediately after reheating or casting, finish hot rolling is completed at a temperature of Ar_3 or higher, and the product is rolled up, pickled, and then cold rolled in the usual manner.
After performing recrystallization annealing for 1 second or more in a temperature range of ℃ or higher and Ac_3 points or lower, cooling at a cooling rate of 50℃/s or higher, then hot-dip galvanizing, and further alloying treatment.
A method for producing an alloyed hot-dip galvanized steel sheet for deep drawing with excellent bake hardenability and powdering resistance, characterized by cooling at a cooling rate of 0° C./s or more. Ti^*=[Tiwt%]-48/14[Nwt%]-
2.48/32 [Swt%]・・・・・・・・・(1)