JPH11293396A - High strength hot dip galvanized steel sheet, galvannealed steel sheet, and their production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a high strength hot dip galvanized steel sheet and a galvannealed steel sheet, excellent in stability of material quality, and their production. SOLUTION: The high strength hot dip galvanized steel sheet and the galvannealed steel sheet are produced by using, as a substrate, a cold rolled steel sheet having a composition containing, by weight, 0.06-0.18% C, <=1.5% Si, 1.4-2.3% Mn, 0.2-0.6% Cr, <=0.05% P, 0.010-0.100% Sol. Al, <=0.01% S, and <=0.01% N, and the steel-sheet structure consists of >=5% by area ratio, in total, of martensitic phase, bainitic phase, and retained austenite phase and the balance ferritic phase and pearlitic structure. A steel with the above composition is hot rolled, pickled, and cold rolled. In the successive continuous hot dip galvanizing line, annealing temperature, annealing time, and average cooling rate from the annealing temperature to 570 deg.C are regulated to a temperature not lower than the Ac1 point, 30-90 sec, and >=5 deg.C/sec, respectively, and then, the steel sheet is allowed to stay at 570-390 deg.C for >=40 sec.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet, and a method for producing the same using a cold-rolled steel sheet as a base. More specifically, the present invention relates to a high-strength hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet having excellent press formability and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, there has been a demand for a reduction in the weight of a vehicle body due to the need for improving fuel efficiency and reducing exhaust gas. On the other hand, with the growing need for car safety,
It is desired to maintain a high body strength while reducing the weight of the body. In addition, high corrosion resistance is usually required for parts of the vehicle body. From the above background, the use of high-strength hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets as materials for automotive components is increasing.

[0003] Since many automotive parts have complicated shapes and high productivity is required in production, they are often processed by press molding. However, high-strength steel sheets are inferior in press-formability as compared with soft steel sheets, and pose a problem.

There are two factors as the cause. One is the material of the high-strength steel plate. High-strength steel sheets have low ductility and are liable to crack. In addition, the springback is strong and the shape freezing property is poor. The other is a change in the material. As described above, since a high-strength steel sheet is inferior in press-formability, the range of press conditions that can be formed is generally narrower than that of a soft steel sheet. Therefore, if the material of the steel sheet fluctuates, cracks and shape defects occur immediately, leading to a decrease in productivity. Therefore, in order to achieve high productivity in press forming, it is necessary that the material of the steel sheet is always stable. However, a high-strength steel sheet has a large variation in the material as compared with a soft steel sheet, and in reality, press forming is more difficult.

[0005] Regarding the former problem, a method for producing a steel sheet having an excellent strength-ductility balance and a low yield ratio has been proposed (for example, JP-A-5-179402, JP-A-5-179402).
4-173946, JP-A-8-134591). These steel sheets have a high-strength, high-ductility steel by forming the steel sheet into a composite structure of ferrite phase + martensite phase, ferrite phase + bainite phase, or ferrite phase + martensite phase + bainite phase. A material with a low yield ratio is obtained.

However, regarding the latter problem, conventionally,
No effective solution has been proposed.

[0007]

SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide a high-strength hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet, and a method for producing the same, which are excellent in material stability. Aim.

[0008]

The causes of the change of the material include the change of the steel composition, the change of the hot rolling condition, the change of the cold rolling condition, the annealing temperature and the cooling condition in the continuous galvanizing line. Fluctuation and the like. According to the knowledge obtained by the present inventors, among them, the most difficult to control and the biggest cause of the material fluctuation is the fluctuation of the winding temperature after hot rolling. The winding temperature after hot rolling tends to be different particularly at the leading end and the trailing end of the coil, and also in the width direction, the edge portion tends to be lower in temperature than the central portion.

[0009] In many steel sheets having a composite structure, the material after hot rolling as well as the material after cold rolling, annealing, and hot dip galvanizing are affected by the winding temperature. That is, when the winding temperature is high, the structure after hot rolling becomes a coarse and soft ferrite + pearlite structure, and the material has low strength and high ductility. On the other hand, when the winding temperature is low, a fine and hard bainite structure is formed, and the material has high strength and low ductility.
It is considered that this difference in the hot-rolled structure has a complicated effect on the structure after plating, and changes the material. If the winding temperature can always be maintained at a constant temperature, there is no change in the material due to the winding temperature, but it is very difficult to keep the winding temperature constant in the actual operation.

As a result of repeated investigations by the present inventors, the degree of dependency of the material on the winding temperature varies depending on the composition of the steel sheet, especially the content of Cr, and the composition at which the dependency on the winding temperature of the material is minimized. That is, it was found that there was a composition that maximized the material stability. In addition to this finding, the present invention has been completed by adding the results of examining the components of the steel sheet and the range of manufacturing conditions in which this effect is effective.

The structure for solving the above-mentioned problem is as follows. (1) In the first invention, C: 0.06 to 0.18%, Si: 1.5% by weight
%, Mn: 1.4-2.3%, Cr: 0.2-0.6%, P: 0.05%
Hereinafter, Sol. Al: 0.010 to 0.100%, S: 0.01% or less, N: 0.
A cold rolled steel sheet containing not more than 01% is used as a base material, and the steel sheet structure has a total area ratio of 5% or more of a martensite phase, a bainite phase and a retained austenite phase, and the balance consists of a ferrite phase and a pearlite structure. High strength hot-dip galvanized steel sheet with excellent material stability.

(2) In the second invention, C: 0.06 to 0.18 by weight%
%, Si: 1.5% or less, Mn: 1.4 to 2.3%, Cr: 0.2 to 0.6
%, P: 0.05% or less, Sol. Al: 0.010 to 0.100%, S: 0.01
% Or less, N: 0.01% or less of the cold rolled steel sheet as a base, the structure of the steel sheet has a martensite phase, a bainite phase and a residual austenite phase having a total area ratio of 5% or more,
A high-strength alloyed hot-dip galvanized steel sheet excellent in material stability, characterized in that the balance consists of a ferrite phase and a pearlite structure.

(3) In the third invention, C: 0.06 to 0.18 by weight%.
%, Si: 1.5% or less, Mn: 1.4 to 2.3%, Cr: 0.2 to 0.6
%, P: 0.05% or less, Sol. Al: 0.010 to 0.100%, S: 0.01
% Or less, N: 0.01% or less, hot-rolled, pickled, cold-rolled, then annealed and galvanized in a continuous hot-dip galvanizing line to produce hot-dip galvanized steel sheet , The annealing temperature in the continuous hot-dip galvanizing line is ₁ point or more of Ac, the annealing time is 30 seconds or more and 90 seconds or less, and the average cooling rate is 5 degrees C / sec or more to 570 degrees C or less after annealing, and then the temperature range is 570 degrees C or less and 390 degrees C or more. A method for producing a high-strength hot-dip galvanized steel sheet having excellent material stability, characterized by staying for 40 seconds or more.

(4) The fourth invention is the method for producing a high-strength alloyed hot-dip galvanized steel sheet having excellent material stability according to the third invention, wherein the coating after plating is alloyed. is there.

Hereinafter, the present invention will be described in detail. First, the reasons for limiting the steel sheet components of the present invention will be described.

C: 0.06 to 0.18% C is an indispensable component for securing the strength of the steel, increasing the hardenability, and forming a composite structure. To obtain the above effects, the lower limit is set to 0.06%. However, excessive addition of C deteriorates weldability and delayed fracture resistance, so the upper limit is made 0.18%.

Si: 1.5% or less Si is a component useful for securing the strength of steel, increasing hardenability, and forming a composite structure. However, if added excessively, the wettability of the plating and the alloying property are deteriorated, so the upper limit is made 1.5%.

Mn: 1.4 to 2.3% Mn is a component necessary for improving the strength and toughness of the steel sheet by solid solution strengthening and grain refinement strengthening. It also has the effect of stabilizing the austenite phase, generating the bainite phase and the martensite phase, and turning the steel sheet structure into a composite structure. In the present invention, in order to obtain the above effect, the lower limit is 1.4%
And Further, even if added in excess of 2.3%, the effect is saturated and the production cost increases, so the upper limit is made 2.3%.

Cr: 0.2-0.6% Cr stabilizes the austenite phase like Mn and facilitates the formation of a composite structure. Further, the feature of the present invention,
The stability of the material with respect to the fluctuation of the winding temperature can be obtained by optimizing the Cr content. The mechanism of this action is unknown, but is presumed to be involved in the continuous cooling transformation behavior of C-Mn-Cr steel. If the Cr content is less than 0.2% or more than 0.6%, material stability against fluctuations in the winding temperature will not be exhibited.
0.6%.

P: 0.05% or less P has the effect of stabilizing the austenite phase.
On the other hand, it deteriorates the workability of the steel sheet, the plating adhesion, and the alloying property of the plating film. If the addition exceeds 0.05%, the workability of the steel sheet, the plating adhesion and the alloying property of the plating film deteriorate, so the upper limit is made 0.05%.

Sol. Al: 0.010% to 0.100% Al is added for the purpose of deoxidizing steel, but the lower limit is set to 0.010% in order to obtain a desired effect, and the upper limit is saturated even if added more. 0.100% in order to perform

S: 0.01% or less S is desirably low in terms of workability, and the upper limit is made 0.01%.

N: 0.01% or less N is desirably low in terms of workability, and the upper limit is made 0.01%.

The components not limited to the above may be contained in the steel sheet as long as they do not significantly change the continuous cooling transformation behavior of the steel.

The total area ratio of the martensite phase, the bainite phase, and the retained austenite phase: 5% or more The steel sheet of the present invention realizes high strength, ductility, and a low yield ratio by forming the steel sheet into a composite structure. I have. 5% of total area ratio of martensite phase, bainite phase and retained austenite phase (hereinafter, referred to as area ratio of hard phase)
If it is less than 5%, the above effect is insufficient, so the lower limit is 5%.
And

Next, the reasons for limiting the manufacturing conditions of the steel sheet of the present invention will be described. Annealing temperature: Ac ₁ point or more In the present invention, in order to obtain a steel sheet having high strength, high ductility and a low yield ratio, the steel sheet structure needs to be a composite structure. For that purpose, an austenite phase needs to be generated during annealing. Part of this austenite phase transforms into a bainite phase and then a martensite phase in the subsequent cooling process,
Becomes a composite organization. If the annealing temperature is lower than the Ac ₁ point, the austenitic phase is not formed and the above-mentioned action does not work, so the lower limit is set to the Ac ₁ point.

Annealing time: 30 to 90 sec As described above, the steel sheet structure during annealing needs to contain an austenitic phase.
The formation of the austenite phase is insufficient, and the effect of the above-described action is reduced. Further, even if annealing is performed for more than 90 seconds, the generation of the austenite phase does not proceed any longer. Therefore, the lower limit is 30 seconds and the upper limit is 90 seconds.

Cooling rate from annealing temperature to 570 ° C: 5 ° C / s
If the cooling rate is less than 5 ° C / sec, the transformation from the austenite phase to the bainite phase and the martensite phase does not occur stably, and the formation of the composite structure becomes unstable.
5 ° C / sec.

Residence time in the temperature range of 570 ° C. or less and 390 ° C. or more: 40 seconds or more In the cooling process after annealing, the steel stays in the temperature range of 570 ° C. or less and 390 ° C. or more for a certain time to advance austenite → bainite transformation, Stabilize the material. If the staying time in the temperature range is less than 40 sec, austenite → austenite before bainite transformation does not sufficiently proceed →
Since the martensitic transformation occurs, the fluctuation of the material due to the fluctuation of the cooling rate after annealing increases. Therefore, the lower limit is 40
sec.

The hot dip galvanized steel sheet of the present invention is subjected to an alloying treatment of the plating film if necessary. The effect of the present invention is not affected by the presence or absence of the alloying treatment, the alloying temperature, and the cooling rate after the alloying treatment.

[0031]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is to manufacture a hot-dip galvanized steel sheet by using a cold-rolled steel sheet as a base material in a continuous hot-dip galvanizing line or to alloy a galvanized film thereafter. The case is targeted.
The smelting, hot rolling, pickling, cold rolling, hot-dip galvanizing conditions and alloying treatment conditions and the like not specified above of the steel of the present invention are not particularly limited, and may be conventional methods.

[0032]

Embodiments of the present invention will be described below. Steel containing the components shown in Table 1 and the balance being unavoidable impurities (Example of the present invention: N
oc, d, e, f, g, Comparative Example: No. a, b, h, i) were melted in a laboratory by a vacuum melting method, and an ingot obtained by casting was obtained with a sheet thickness of 2.6 mm.
Hot-rolled. In the hot rolling, the heating temperature was 1250 ° C, and the finish rolling temperature was 900 ° C. Immediately after the finish rolling, the steel sheet was held in a furnace at a constant temperature for 1 hour, and then cooled in the furnace to simulate coil winding in an actual machine. Here, the furnace temperature was regarded as the winding temperature. Furnace temperature is 480
° C, 580 ° C and 640 ° C. Thereafter, the steel sheet was pickled and cold-rolled to obtain a steel sheet having a thickness of 1.0 mm (cooling ratio 61.5%).

[0033]

[Table 1]

Next, the steel sheet obtained above was subjected to hot-dip galvanizing in a laboratory. The production conditions simulate a continuous hot-dip galvanizing line, annealing, cooling, holding, hot-dip galvanizing on both sides, and gas wiping under the conditions shown in Tables 2 and 3. An alloying treatment was performed. The temperature of the plating bath was 465 ° C. The coating weight was set to be about 60 g / m ² per one side. The alloying conditions were such that the Fe concentration in the coating was 9%.
It was adjusted to be about 11% by weight. After the alloying treatment, it was cooled to room temperature.

The material of the test material thus manufactured is
It was measured using a JIS-5 tensile test piece cut out at right angles to the rolling direction. Further, the structure of the steel sheet was investigated, and the area ratio of the hard phase was investigated.

The evaluation of the dependence of the material on the winding temperature was performed as follows. A hot-dip galvanized steel sheet was produced using the same steel, except that all the production conditions except the winding temperature were the same. Of these, the winding temperature was 640 ° C and 480 ° C.
Difference in tensile strength of hot-dip galvanized steel sheet at ℃ (= ΔTS)
And used as an index of the dependence of the material on the winding temperature. That is, the smaller the absolute value of ΔTS is, the smaller the dependency of the material on the winding temperature is, and the more excellent the material stability is against the fluctuation of the winding temperature. In the present invention, the tensile strength (TS) is 45 kg / mm ² or more, the value of tensile strength (TS) × total elongation (El) is 1650 (kg / mm ² ·%) or more, and the yield ratio (YR) is 65%. A steel sheet having a value of not more than 18 and an absolute value of ΔTS of not more than 18 kg / mm ² was accepted.

Tables 2 and 3 show the manufacturing conditions and the measured values of the materials and the area ratio of the hard phase in the examples.

[0038]

[Table 2]

[0039]

[Table 3]

FIG. 1 shows the relationship between the Cr content in steel and the dependence of the tensile strength on the winding temperature. Experiment Nos. 7 and 2 in Tables 2 and 3
1, 28 to 36 are examples of the present invention. In each case, the area ratio of the hard phase is 5% or more, the tensile strength (TS) is 45 kg / mm ² or more, and the product of the tensile strength (TS) and the total elongation (El) is 1650 (kg / mm ² ·%) or more. ,
The yield ratio (YR) was less than 65%. Further, the absolute value of ΔTS was 18 kg / mm ² or less.

Experiment Nos. 7 to 21 are examples in which the Cr content in steel was changed. The value of ΔTS increases as the Cr concentration in the steel increases, and the absolute value of ΔTS is the test material No.e (Cr concentration: 0.40wt
%) Was the smallest.

Experiment Nos. 28 to 30 are the cases where alloying of the film was not performed. The material (TS, El, YP, YR) and the winding temperature dependency (ΔTS) were almost the same as compared to the case where alloying was performed under the same conditions (Experiment Nos. 13 to 15).

Experiment Nos. 31 to 33 are the cases where the annealing temperature was 770 ° C. In the case of annealing temperature 850 ° C under the same conditions (Experiment No.
Compared with 13-15), TS was higher, but ΔTS was almost the same. It is considered that the reason why the TS was increased as compared with the case where the annealing temperature was 850 ° C. was that the crystals of the steel sheet did not grow sufficiently during annealing and the crystal grain size after cooling became small.

Experiment Nos. 34 to 36 are examples in which the cooling rate after annealing was changed. The TS was slightly higher than when the cooling rate was 7 ° C./sec under the same conditions (Experiment Nos. 13 to 15). This is presumably because the area ratio of the hard phase has increased.

Experiment Nos. 1 to 6, 22 to 27 and 37 are comparative examples. In Experiments Nos. 1 to 3, since the steel did not contain Cr, the steel sheet after annealing did not have a composite structure, and no hard phase was observed. The material has a YR higher than 65% and a TS × El value of 1650 (kg /
mm ² %).

In Experiments Nos. 4 to 6, the absolute value of ΔTS was greater than 18 kg / mm ² because the Cr content in the steel was less than 0.2%.

In Experiments Nos. 22 to 27, the Cr content in steel was 0.6%.
Therefore, the absolute value of ΔTS was larger than 18 kg / mm ² .

In Experiment No. 37, since the cooling rate after annealing was low, the steel sheet did not have a composite structure, and no hard phase was observed. The material has a YR higher than 65% and a TS × El value of 16
It was less than 50 (kg / mm ² ·%).

[0049]

As described above, according to the present invention,
Since the steel sheet has a composite structure, a high strength hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet having excellent strength-ductility balance, low yield ratio, and excellent press formability can be obtained stably. Since the steel sheet of the present invention has a hot-dip galvanized film or an alloyed hot-dip galvanized film on its surface, it has excellent corrosion resistance, and has high strength and excellent press formability. Can be used for

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the Cr content in steel and the dependence of tensile strength on the winding temperature.

Claims (4)

[Claims] C .: 0.06 to 0.18%, Si: 1.5% by weight%
Mn: 1.4 to 2.3%, Cr: 0.2 to 0.6%, P: 0.05% or less, Sol. Al: 0.010 to 0.100%, S: 0.01% or less, N: 0.01
% Of a cold-rolled steel sheet containing at most 5% by weight of a martensite phase, a bainite phase, and a retained austenite phase, and the balance consists of a ferrite phase and a pearlite structure. High strength galvanized steel sheet with excellent material stability. 2. In% by weight, C: 0.06-0.18%, Si: 1.5%
Mn: 1.4 to 2.3%, Cr: 0.2 to 0.6%, P: 0.05% or less, Sol. Al: 0.010 to 0.100%, S: 0.01% or less, N: 0.01
% Of a cold-rolled steel sheet containing at most 5% by weight of a martensite phase, a bainite phase, and a retained austenite phase, and the balance consists of a ferrite phase and a pearlite structure. High strength alloyed hot-dip galvanized steel sheet with excellent material stability. 3. C: 0.06 to 0.18%, Si: 1.5% by weight
Mn: 1.4 to 2.3%, Cr: 0.2 to 0.6%, P: 0.05% or less
Bottom, Sol. Al: 0.010 to 0.100%, S: 0.01% or less, N: 0.01
%, Hot-rolled, pickled, cold-rolled
After that, it is annealed in a continuous hot-dip galvanizing line,
A process to manufacture hot-dip galvanized steel sheet by applying hot-dip galvanizing
The annealing temperature in the continuous hot-dip galvanizing line
Ac degree ₁Annealing point, annealing time 30sec or more and 90sec or less, annealing
The average cooling rate from temperature to 570 ° C is 5 ° C / sec or more,
After that, stay in the temperature range below 570 ℃ and above 390 ℃ for more than 40sec
High strength molten zinc with excellent material stability
Manufacturing method of plated steel sheet. 4. The method for producing a high-strength alloyed hot-dip galvanized steel sheet according to claim 3, wherein the film after plating is alloyed.