JPH11193419A - Production of galvannealed high strength cold rolled steel sheet excellent in formability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a galvannealed high strength cold rolled steel sheet capable of hot rolling in a temp. region where deformation can easily be done and also having low yield ratio and excellent formability. SOLUTION: In hot-rolling a steel having a composition consisting of, by mass, 0.10-0.25% C, <=0.50% Si, 1.0-3.0% Mn, <=0.010% S, <=0.10% Al, and the balance Fe with inevitable impurities, finish rolling is completed at a finishing temp. higher than the Ar3 point and coiling is performed at >700 deg.C coiling temp. The resultant hot rolled steel plate is acid-pickled and cold-rolled into steel sheet. Subsequently, the galvannealing is applied to the steel sheet by means of a continuous hot dip galvanizingline. At this time, the steel sheet is heated to a temp. between the Ac1 point and 850 deg.C for >=10 sec, cooled down to plating temp. at >=10 deg.C/sec average cooling rate, hot-dip-galvanized, and then subjected to alloying treatment at 450-600 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a galvannealed high-strength cold-rolled steel sheet having excellent workability,
More specifically, the present invention relates to a method for producing an alloyed hot-dip galvanized high-strength cold-rolled steel sheet having a tensile strength of 490 MPa or more and excellent in formability represented by shape freezing and total elongation of a press-formed product.

[0002]

2. Description of the Related Art In recent years, high-strength cold-rolled steel sheets have been adopted as measures for safety and weight reduction of automobiles, and there has been a strong demand for improvement of rust-preventive force of cold-rolled steel sheets for extending the life of automobiles. In particular, recently, a steel plate for a vehicle chassis reinforcing member has been required to have a tensile strength of 440 MPa or more. For this reason, development of an alloyed hot-dip galvanized high-strength cold-rolled steel sheet having excellent formability has been demanded.

Most steel sheets used in automobiles are used for press forming, so that not only cracks and wrinkles do not occur at the time of forming, but also characteristics such that the shape of parts after press forming does not change due to springback. That is, shape freezing properties are required, and it is generally known that these properties are excellent although yield strength and yield ratio (yield strength / tensile strength) are low.

In the field of hot-dip galvanized steel sheet, it is difficult to obtain a high-strength steel sheet having a tensile strength of 490 MPa or more only by solid-solution strengthening which has been conventionally known. It is necessary to strengthen the composite structure including

[0005] However, even in the case of a composite structure, when the ratio of the hard phase in the structure of the steel sheet is increased in order to obtain high strength, not only the yield ratio is increased, but also the hole expansion showing the total elongation and stretch flangeability. Moldability such as moldability also becomes extremely poor. Further, such a composite structure steel sheet has a problem that the martensitic structure is tempered by plating and a subsequent alloying treatment to lower the tensile strength, so that a target high tensile strength cannot be obtained.

As a technique for solving such a problem, Japanese Unexamined Patent Application Publication No.
As disclosed in JP-A-290955, C:
Hot rolling of steel containing 0.005 to 0.25% is completed at a finishing temperature in the temperature range of Ar ₁ point to Ar ₃ points, and 7
Winding at a temperature of 00 ° C. or less, hot-dip galvanizing the cold-rolled steel sheet, and setting the alloying temperature after plating to 450 to
By setting the temperature to 600 ° C., an alloyed hot-dip galvanized high-strength cold-rolled steel sheet having a low yield ratio and good formability can be obtained.

[0007]

SUMMARY OF THE INVENTION The above-mentioned JP-A-2-290 is disclosed.
According to the manufacturing method disclosed in US Pat.
Although low yield ratio and good formability alloyed hot-dip galvanized cold-rolled steel sheets have been obtained, in such a manufacturing method,
Since the finishing temperature needs to be in the range of Ar ₁ point to Ar ₃ point, which is the austenite-ferrite two-phase region, deformation resistance increases during hot rolling. In particular, in the high-strength steel sheet which is the subject of the present invention, since the amount of additional elements such as C and Mn is large, the deformation resistance becomes excessive, the wear of the work roll is remarkable, and it is difficult to roll to the target thickness. However, there is a problem that the shape of the hot-rolled steel sheet is deteriorated.

[0008] In view of the above problems, the present invention can perform hot rolling in a temperature range where deformation is easy, and has a low yield ratio,
An object of the present invention is to produce an alloyed hot-dip galvanized high-strength cold-rolled steel sheet having good formability.

[0009]

Means for Solving the Problems The present inventors have set the winding temperature in the hot rolling stage, which has not been noticed at all, to 7 ° C.
By setting the temperature to be higher than 00 ° C, it is possible to obtain an alloyed hot-dip galvanized high-strength cold-rolled steel sheet having both a low yield ratio of 0.50 or less and excellent formability even when the finishing temperature is higher than ₃ points of Ar. The present invention has been found.

That is, the method for producing a galvannealed high-strength cold-rolled steel sheet according to the present invention is as follows.
0 to 0.25%, Si: 0.50% or less, Mn: 1.0
~ 3.0%, S: 0.010% or less, Al: 0.10
%, And when hot rolling is performed on steel comprising the balance of Fe and unavoidable impurities, the finish rolling is finished at a finishing temperature of more than _three points, and the winding temperature is raised to over 700 ° C. The hot-rolled steel sheet obtained is pickled and cold-rolled into a thin steel sheet, and then subjected to alloying hot-dip galvanizing by a continuous hot-dip galvanizing line, the thin steel sheet is heated to a temperature of Ac ₁ point to 850 ° C for 10 seconds or more. After heating, the steel sheet is cooled to a plating temperature at an average cooling rate of 10 ° C./sec or more, hot-dip galvanized, and then alloyed at a temperature in the range of 450 to 600 ° C.

First, the chemical components of steel used in the present invention will be described. C: 0.10 to 0.25% C is an important element that controls the tensile strength of the steel sheet, and generates a hard phase mainly composed of martensite, thereby contributing to high strength. To obtain a high-strength composite structure steel sheet of 490 MPa or more, addition of 0.10% or more is required. However, if the amount of the hard phase is excessive, it becomes difficult to secure a low yield ratio, and the spot weldability also decreases. Therefore, the upper limit of C is set to 0.25%.

Si: 0.50% or less Since Si has the effect of discharging solid solution C in the ferrite phase into the austenite phase, it stabilizes the hard phase, improves the total elongation and lowers the yield ratio. Have. However, an excessive addition impairs the plating wettability, so the upper limit of the addition amount is set to 0.50%.

Mn: 1.0 to 3.0% Mn is added to stabilize the austenite phase, facilitate the formation of a hard phase in the cooling process, and obtain a low yield ratio and high strength. If the addition amount is too small, a hard phase cannot be obtained and a required high strength cannot be obtained, so the lower limit is made 1.0%. However, if added excessively, the ratio of the austenite phase increases and becomes unstable, making it difficult to form a hard phase. Therefore, the upper limit of the addition amount is set to 3.0%.

S: not more than 0.010% S forms nonmetallic inclusions and impairs formability. In particular, in the case of a high-strength steel sheet as in the present invention, the effect is liable to appear remarkably. 010% or less,
Preferably, the content is 0.005% or less.

Al: 0.10% or less Al is added for deoxidizing steel. However, excessive addition not only saturates the effect, but also increases the cost of the steel and causes poor plating. Therefore, the addition amount is limited to 0.10% or less.

The steel used in the present invention contains the above components as basic components and the balance is Fe and unavoidable impurities. To improve the steel properties, the following P, Cr, Ca and REM are used.
One or more of (rare earth elements) can be selectively added, and the following steel compositions (1), (2) and (3) can be obtained. (1) In addition to the basic components, P: 0.10% or less. (2) In addition to the basic component or the component of (1), Cr:
1.0% or less. (3) In addition to the basic component, the component (1) or (2), one or two of Ca and REM: 0.005% or less in total.

P has the same function as Si, and has the function of ensuring the balance between strength and formability.
2% or more is added. However, if it is added in excess of 0.10%, poor plating and spot weldability will be impaired.

Cr has the same action as Mn, and facilitates the formation of a hard phase, and facilitates obtaining a low yield ratio and high strength.
However, excessive addition not only impairs the formability, but also
Is expensive and increases steel costs, so the upper limit is 1.0
%.

Ca and REM improve formability through morphological control of nonmetallic inclusions. The amount of addition may be 0.005% or less in total of one type or two types.

Next, the manufacturing conditions of the present invention will be described.
First, a steel slab having the above-mentioned chemical composition is obtained by ordinary ingot or continuous casting, and then hot-rolled through hot rolling. At the time of hot rolling, the heating temperature of the slab may be a conventional method, and is usually 1100 to 1250 ° C. The finishing temperature in hot rolling is more than _three points of Ar. If the Ar is less than ₃ points, high-strength steel will have excessive deformation resistance during hot rolling, increase the rolling load, intensify the wear of the work rolls, and it will be difficult to obtain the desired plate thickness and plate shape. In rolling at a finishing temperature of more than _three points of Ar, the structure of the hot-rolled steel sheet is ferrite + pearlite. However, since the carbides constituting pearlite are spheroidized by high-temperature winding described below, there is a possibility that the formability may be impaired. Absent.

The winding temperature after finish rolling is an important condition in the present invention, and is set to more than 700 ° C. Preferably 730
It is good to be more than ° C. When the winding temperature is higher than 700 ° C., carbides (layered Fe ₃ C constituting pearlite) in the structure of the hot-rolled steel sheet become spherical, and Mn or the like dissolves in the carbides. This carbide becomes austenite as a nucleus during soaking in the hot-dip galvanizing line and becomes stable (high hardenability) austenite due to molten Mn and the like. For this reason, after the alloying heat treatment, it transforms into a hard phase composed of very hard and small martensite, and a structure composed of ferrite and a hard phase mainly composed of martensite (ferrite + martensite is 95% by volume).
Above), and a low yield ratio is realized. When the winding temperature is 700 ° C. or lower, the concentration of Mn or the like is insufficient, so that it becomes difficult to form stable martensite at the time of soaking.
Pearlite and bainite are easily precipitated from austenite, and the yield ratio is increased.

After winding, pickling and cold rolling are performed according to a conventional method to obtain a thin steel sheet. Next, the thin steel sheet is subjected to annealing, hot-dip galvanizing, and alloying treatment in a hot-dip galvanizing line. For annealing, it is necessary to heat the steel sheet at a temperature of Ac ₁ point to 850 ° C. for 10 seconds or more. If the annealing heating temperature is lower than the Ac ₁ point, a hard phase is not generated, and high strength cannot be obtained. On the other hand, when the annealing heating temperature is higher than 850 ° C., a structure in which a large amount of a hard phase is formed is obtained, the yield ratio is increased, and the formability is deteriorated.

The cooling from annealing to hot-dip galvanizing requires an average cooling rate of 10 ° C./sec or more in order to suppress the transformation of austenite into pearlite during cooling. The cooling means at this time may be constant-speed cooling, or may change the cooling rate during the cooling.

The cold-rolled steel sheet after the hot-dip galvanizing is subsequently subjected to an alloying treatment at a temperature in the range of 450 to 600 ° C. When the alloying temperature is lower than 450 ° C., the desired alloy phase of Fe and Zn is not easily formed. On the other hand, 6
When the temperature is higher than 00 ° C., the hard phase is tempered, and it becomes difficult to secure a low yield ratio and high strength. Cooling after the alloying treatment may be performed at a rate of 5 ° C./sec or more according to a conventional method.

[0025]

Hereinafter, the present invention will be described with reference to examples.
Of course, the present invention is not limited by these examples.

Example A A steel having the following components was melted and
The slab was mm thick. This slab is heated at 1200 ° C
After finishing the finish rolling at a finishing temperature of 870 ° C., cooling at an average cooling rate of 30 ° C./sec until winding, winding at a winding temperature of 500 to 750 ° C.,
A 2.6 mm hot rolled steel sheet was obtained. -Steel component (mass%, balance substantially Fe) C: 0.15%, Si: 0.05%, Mn: 2.63
%, P: 0.012%, S: 0.006%, Al:
0.044%,

Next, the hot-rolled steel sheet is pickled and cold-rolled to a thickness of 1.2 mm according to a conventional method.
Annealing for 10 seconds, and the cooling rate until hot-dip galvanizing
After hot-dip galvanizing was performed on both surfaces of the cold-rolled steel sheet at a temperature of 460 ° C./sec and a plating temperature of 460 ° C., an alloying treatment was performed at 500 ° C.

The yield strength (YS), tensile strength (TS), and so on of the alloyed hot-dip galvanized cold-rolled steel sheet thus obtained,
The elongation (El) and the yield ratio (YR) were investigated. FIG. 1 shows the relationship between these values and the hot-rolling winding temperature. From FIG. 1, it is possible to realize a low yield strength while maintaining a high strength by winding at a winding temperature exceeding 700 ° C., and as a result, the yield ratio is reduced to 0.5 or less and the elongation is improved. It is recognized that.

[Example B] Steel having the chemical components shown in Table 1 was melted into a slab having a thickness of 20 mm. This was made into a hot-rolled steel sheet having a thickness of 2.6 mm under the conditions shown in Table 2. The obtained steel sheet is pickled and cold-rolled to a thickness of 1.2 mm (a reduction of 54%).
Was obtained. With respect to these cold-rolled steel sheets, galvannealed cold-rolled steel sheets (double-sided plating) were manufactured under the continuous plating conditions shown in Table 2. The tensile properties, uneven plating properties and powdering properties of the obtained plated steel sheets were examined. The plating unevenness was determined by visually observing the plating surface.
The powdering property was determined by bending a sample steel sheet into a V-shape, peeling off the bent portion with an adhesive tape, and visually checking the plating adhesion state on the tape adhesion surface. Table 3 shows the results.
Shown in In the table, with respect to the plating unevenness and powdering properties, も の indicates that both were good, and X indicates that one of them was poor.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

[Table 3]

From Table 3, it can be seen that the alloyed hot-dip galvanized cold-rolled steel sheets of the invention examples have a yield ratio of 0.5 or less and 20% even though they have high strength of 500 MPa or more.
It can be seen that the above-described excellent total elongation is exhibited. In addition,
Each of the structures was a composite structure mainly composed of ferrite and martensite and partially containing bainite (several percent or less).

On the other hand, even in the case where the chemical components are within the range specified by the present invention, Sample Nos. 2, 4, 5, and 7 whose production conditions are out of the range specified by the present invention generally have a high yield ratio. ,
Some have even worse total elongation. Further, even if the manufacturing conditions are in the range specified in the present invention, in Sample Nos. 10 to 13 using the comparative steels F to I whose chemical components are out of the range of the present invention, the yield ratio is generally high and the yield ratio is low. In the case of No. 11 (No. 11), the total elongation is low. Further, in Nos. 11 to 13, the surface properties are inferior.

[0035]

According to the present invention, particularly in hot rolling, by setting the finishing temperature to be more than _three points of Ar and the winding temperature to be more than 700 ° C., the high-strength hot-rolled steel sheet can be heated in a temperature range where deformation resistance is low. Rolling can be easily performed, and the load on the rolling equipment can be reduced, and a galvannealed cold-rolled steel sheet having a tensile strength of 490 MPa or more having a low yield ratio and excellent formability can be easily manufactured. In addition, according to the present invention, since the alloying treatment can be performed at a low temperature, it is possible to reduce the energy cost in addition to improving the surface properties such as uneven plating and powdering properties.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between yield strength, tensile strength, elongation, yield ratio and winding temperature of a galvannealed cold-rolled steel sheet according to an example.

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

Claims (3)

[Claims] 1. C .: 0.10 to 0.25 in mass%
%, Si: 0.50% or less, Mn: 1.0 to 3.0%,
When hot rolling a steel containing S: 0.010% or less and Al: 0.10% or less, the balance being Fe and unavoidable impurities, the finish rolling is completed by setting the finishing temperature to more than _three points of Ar. After winding at a temperature higher than 700 ° C., the obtained hot-rolled steel sheet is pickled and cold-rolled into a thin steel sheet. From Ac ₁ point to 850
After heating to a temperature of 10 ° C for 10 seconds or more, the average cooling rate is 10 ° C
After cooling to the plating temperature at a rate of at least / sec and subjecting to hot-dip galvanizing, alloying is performed at a temperature in the range of 450 to 600 ° C. Production of alloyed hot-dip galvanized high-strength cold-rolled steel sheets with excellent formability Method. 2. The composition according to claim 1, further comprising at least one element selected from the group consisting of P: 0.10% or less and Cr: 1.0% or less, in addition to the components described in claim 1. A method for producing an alloyed hot-dip galvanized high-strength cold-rolled steel sheet with excellent formability. 3. The moldability according to claim 1 or 2, further comprising at least 0.005% in total of one or two elements of Ca and REM in addition to the components described in claim 1 or 2. Excellent alloying hot dip galvanized high strength cold rolled steel sheet manufacturing method.