JPH06240366A - Production of high strength galvannealed cold rolled steel sheet excellent in deep drawability - Google Patents

Abstract

PURPOSE:To produce a high strength galvannealed steel sheet excellent in deep drawability by subjecting the cold rolled steel strip of an extremely low carbon Cu added steel having a specified compsn. to reducing annealing in a hot dip galvanizing line and thereafter executing galvanizing and alloying treatment. CONSTITUTION:The hot rolling of the slab of the extremely low carbon steel having a compsn. contg., by weight, <0.01% C, <0.3% Si, 0.1 to 2% Mn, <0.1% Al, <0.01% S, <0.1% P, 0.8 to 2.0% Cu and one or >= two kinds among 0.005 to 0.1% Nb, 0.005 to 0.1% Ti and 0.0001 to 0.003% B or furthermore contg. 0.4 to 1.0% Ni is started at 1000 to 1200 deg.C and is finished at (the Ar3 point +20 deg.C) to 950 deg.C, and it is coiled round a coil. This hot rolled steel sheet is subjected to cold rolling at >=60% draft into a cold rolled steel strip. After that, it is subjected to continuous annealing at 750 to 900 deg.C, is thereafter fed to a galvanizing line and is annealed in a reducing atmosphere of 550 to 700 deg.C. Successively, it is subjected to galvanizing treatment, and after that, the galvanized layer is subjected to alloying treatment with Fe in the steel strip at 450 to 550 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength hot-dip galvanized cold-rolled steel sheet having excellent deep drawability, which is used for mechanical structural members such as automobiles and general processing.

[0002]

2. Description of the Related Art In recent years, there has been a strong demand for reducing the thickness of steel sheets and increasing their strength in order to save energy and reduce fuel consumption. On the other hand, so-called Dual Phase steel and the like have been developed, but the Rankford value, which is an index of deep drawability, was not necessarily high. On the other hand, as a technology of Dual Phase cold-rolled steel sheet with excellent Rankford value,
There is a technique described in Japanese Patent No. 6814. This technique is to form a dual phase steel sheet by forming an advantageous texture for the Rankford value by box annealing, and subsequently quenching it from the (α + γ) 2 phase region by continuous annealing. However, the tensile strength of the steel sheet obtained by this invention is 49 at most.
The strength is 0 MPa and does not reach the predetermined strength of the present invention.
Further, there is no description about galvannealing.

As a technique utilizing ultra-low C-based Cu-added steel, there is a technique described in Japanese Patent Publication No. 2-15609. This technique forms a texture that is advantageous for the Rankford value by continuous annealing. However, the tensile strength is at most 400 MPa, which is below the predetermined strength of the present invention. Further, there is no description about galvannealing.

As described above, there is no prior art relating to an alloyed hot-dip galvanized cold-rolled steel sheet having a tensile strength of 550 MPa or higher and an excellent deep drawability.

[0005]

DISCLOSURE OF THE INVENTION The present inventors have paid attention to ultra-low C-based Cu-added steel for producing an alloyed hot-dip galvanized cold-rolled steel sheet having a tensile strength of 550 MPa or more and excellent deep drawability. , Conducted an experiment in the lab. The sample steels shown in Table 1 were vacuum melted-hot rolled-cold rolled, and annealed as shown in FIG.
The tensile strength and Rank Ford value were investigated. The results are shown in Figure 2. As described above, the conventional tensile strength is 400 at most.
Although only cold-rolled steel sheets of 450 MPa class were available, according to the present invention, it is possible to produce a high strength-high Rank Ford value alloyed hot-dip galvanized cold-rolled steel sheet having a tensile strength of 550 MPa or more.

[0006]

[Table 1]

That is, in order to improve the Rankford value, which is an index of deep drawability, an extremely low C system is advantageous, and a continuous annealing line forms a texture structure advantageous to the Rankford value. Then, by prescribing the temperature in the reducing atmosphere on the hot dip galvanizing line, the heat treatment and hardening property of Cu is used to enhance the strength, and at the same time, galvannealing with excellent corrosion resistance and weldability is performed. is there.

[0008]

The gist of the present invention is as follows. (1) Mass%, C: 0.01% or less, Si:
0.3% or less, Mn: 0.1 to 2%, Al: 0.1% or less, S: 0.01% or less, P: 0.1% or less, Cu:
0.8-2.0%, and Nb: 0.005-
0.1%, Ti: 0.005 to 0.1%, B: 0.00
Of 01-0.003%, 1 type or 2 types or more are included,
Immediately or after heating to 1000 to 1200 ° C., a slab made of steel consisting of the balance Fe and unavoidable impurities,
Finishing temperature (Ar ₃ points + 20 ° C) during hot rolling
Rolling is completed at 950 ° C to 950 ° C, rolled into a hot rolled steel strip, cold rolled at a cold rolling reduction of 60% or more, and subsequently annealed at an annealing temperature of 750 to 900 ° C in a continuous annealing line to form a steel strip. After that, in a hot dip galvanizing line in a reducing atmosphere, 550 to 7
Annealing at 00 ° C, cooling and dipping in a hot dip galvanizing bath, hot dip galvanizing, and subsequent alloying treatment 450-55
A method for producing a high-strength hot-dip galvanized cold-rolled steel sheet having excellent deep drawability and having a tensile strength of 550 MPa or more, which is performed at 0 ° C.

(2) Mass%, C: 0.01% or less, Si: 0.3% or less, Mn: 0.1 to 2%, Al:
0.1% or less, S: 0.01% or less, P: 0.1% or less, Cu: 0.8 to 2.0%, Ni: 0.4 to 1.0%
In addition, Nb: 0.005-0.1%, Ti:
0.005-0.1%, B: 0.0001-0.003
%, Steel containing 1 or 2 or more and the balance Fe and unavoidable impurities is formed into a slab, and immediately or after heating to 1000 to 1250 ° C. and hot rolling, a finishing temperature (Ar Rolling is completed at ₃ points ₊ 20 ° C) to 950 ° C, rolled into _a hot rolled steel strip, cold rolled at a cold rolling reduction of 60% or more, and then an annealing temperature of 75 in a continuous annealing line.
After annealed at 0-900 ° C to form a steel strip, annealed at 550-700 ° C in a reducing galvanizing line in a hot dip galvanizing line, dipped in a hot dip galvanizing bath after cooling, hot dip galvanizing, and then alloying A method for producing a high-strength hot-dip galvanized cold-rolled steel sheet having excellent deep drawability and having a tensile strength of 550 MPa or more, which is characterized in that the heat treatment is performed at 450 to 550 ° C.

[0010]

Next, the reasons for limiting the constituents of the present invention will be described in detail. C is 0.01% or less. If it exceeds this, a texture that is unfavorable to the Rankford value is generated and the deep drawability deteriorates. Mn is an element that imparts strength, and is 0.2 to
Add in the range of 2.0%. If it is less than the lower limit, the target strength cannot be obtained. When added in excess of 2.0%, C is picked up during production and the upper limit of C cannot be satisfied.

Si causes Si scale and lowers the plating adhesion, so the content is made 0.3% or less. Al is necessary as a deoxidizing agent, but if it exceeds 0.1%, alumina-based inclusions increase and the ductility is impaired. S increases the amount of A-type inclusions extending in the rolling direction, cracks propagate from there as origins, and ductility decreases, so the upper limit is made 0.01%.

P is an element that imparts strength and corrosion resistance. However, if it exceeds 0.1%, the ductility decreases. Cu
Is an important element in the present invention. That is, Cu is precipitated during heating in a reducing atmosphere in a hot dip galvanizing line and subsequent alloying treatment to obtain a predetermined strength. 0.8
If it is less than 2.0%, there is no effect, and if it exceeds 2.0%, the effect is saturated and a surface defect called Cu heggling may occur during hot rolling.

Further, in order to prevent the Cu hegging, addition of Ni is desirable. Less than 0.4% has no effect, 1.0%
If it exceeds, not only the effect will be saturated, but also the economical efficiency will be impaired. Ti and Nb fix C and N in the steel sheet and suppress aging. Further, by fixing the solid solution C, it is possible to increase the degree of accumulation of {111} texture that improves the Rankford value, which is an index of deep drawability, when annealing in a continuous annealing line. For that purpose, 0.001% or more is required for each, and if it exceeds 0.1%, the ductility decreases due to the carbonitride formed.

B is an element that prevents deterioration of secondary workability. For that purpose, 0.0001% or more is necessary, and 0.
If it exceeds 003%, the effect is saturated. Next, the hot rolling conditions will be described. The above-mentioned steel is usually melted in a converter and made into a slab by continuous casting. After the converter is melted, various secondary refining may be performed. This slab is charged into the heating furnace as cold, hot or hot pieces.

The heating temperature at this time is 1000 to 1200 ° C.
And The lower limit was set to the range where the current continuous hot rolling equipment can operate without lowering productivity. The upper limit value is 1200 ° C.
If it exceeds this, Cu heggling occurs and the surface properties deteriorate. Further, when Ni is added within the above range, the upper limit of the heating temperature is set to 1250 ° C. Although the upper limit value is improved by adding Ni, it is unavoidable that Cu heggling is generated if the upper limit value is exceeded. Similarly, the lower limit was set to a value that can be adopted by the current equipment.

The finishing temperature in hot rolling is (Ar ₃ points +20
C.) to 950.degree. If it is less than the lower limit, a strong texture is formed in the hot-rolled sheet, and it becomes difficult to form a texture advantageous to the Rankford value during continuous annealing, and the Rankford value decreases. The upper limit is a value that can be manufactured in an actual machine in consideration of the heating temperature. The cold rolling rate is 60% or more. Below this, {111} is advantageous for Rankford value
The degree of aggregation of the organization is reduced, and the Rankford value is reduced.

The annealing temperature in the continuous annealing line is 750 to 9
Set to 00 ° C. If it is less than the lower limit, the processed structure during cold rolling remains, and the Rankford value and ductility decrease. The upper limit value is a value that can be taken with the current equipment. The temperature in the reducing atmosphere in the hot dip galvanizing line is important in the present invention. This is 550-700 degreeC. If it is less than the lower limit, the precipitation strengthening of Cu is insufficient and a predetermined strength cannot be obtained. If the upper limit is exceeded, the solid solubility limit of Cu will expand and Cu will form a solid solution.
Precipitation strengthening of u cannot be obtained and a predetermined strength cannot be obtained.

[0018]

Example Steels having the components shown in Table 2 were tapped in a converter and subjected to secondary refining such as vacuum degassing to obtain slabs. Table 2
The steels indicated by the symbols A to F are within the scope of the present invention, and G
The steels indicated by N are outside the scope of the present invention.

Cu is less than the lower limit for G steel, Cu is over the upper limit for H steel, C is over the upper limit for I steel, Si is over the upper limit for J steel, and N is for K steel.
b is above the upper limit, L steel is above P, M steel is above S, Al is above the upper limit, and N is below Ti. These steels were subjected to hot rolling, cold rolling, pickling, continuous annealing, and hot dip galvanizing under the conditions shown in Table 3. The characteristic values of the obtained steel sheet are also shown in Table 3.

As a tensile test piece, a No. 5 test piece according to JIS Z 2201 was used, and the test was carried out according to the method described in JIS Z2241. Further, the r value was calculated by the following formula after applying 10% strain, measuring the rolling direction, the 90 ° direction and the 45 ° direction. Rankford value = {(r value in rolling direction + r value in 90 ° direction + (r value in 45 ° direction × 2)} / 4 Plating adhesion was evaluated by impact test. Was considered good.

Incidentally, Cu hegging was generated during hot rolling, and the steel sheet having remarkably poor surface properties was not subjected to the steps after cold rolling. Table 3
No. 1-No. No. 6 is a steel of the present invention, which has the objective of the present invention of tensile strength of 550 MPa or more and good deep drawability of Rankford value of 1.4 or more, and does not generate Si scale and Cu heggling. The surface quality after plating was also good.

No. 7-No. 19 is a comparative steel. N
o. In No. 7, the amount of Cu is less than the lower limit value, so that the amount of precipitation strengthening in the hot dip galvanizing line is insufficient and the target strength is not obtained. No. In No. 8, since the Cu content was more than the upper limit, Cu hegging was generated during hot rolling, and the surface quality was significantly deteriorated.

No. In No. 9, since the amount of C was above the upper limit, solid solution C was present during annealing, and the degree of accumulation of {111} texture decreased. Therefore, the Rankford value is lower than that of the steel of the present invention.
No. In No. 10, since the amount of Si exceeds the upper limit, the plating adhesion was reduced. No. 11 has Ti and Nb exceeding the upper limits, so T
Carbonitrides of i and Nb were generated, and the ductility was lowered.

No. In No. 12, the P content was more than the upper limit, so the ductility was reduced. No. In No. 13, since the amounts of S and Al exceeded the upper limits, inclusions increased and ductility decreased. No. 14 is Ti, Nb
Is less than the lower limit, C and N cannot be fixed, and the Rankford value is lowered due to the presence of solute C during annealing. Also, due to the presence of solid solution C and N, aging deterioration occurred.

No. 15 is because the heating temperature exceeds the upper limit,
Cu hot spots were generated during hot rolling, and the surface properties were significantly deteriorated.
No. In No. 16, the finishing temperature was less than the lower limit value, so a texture harmful to the Rankford value developed during hot rolling, and the Rankford value decreased. No. No. 17 has a cold rolling rate less than the lower limit, so {111} is necessary to improve the Rankford value.
The degree of aggregation of the organization has decreased, and the Rankford value has decreased.

No. In No. 18, since the annealing temperature in the continuous annealing line was less than the lower limit value, the degree of accumulation of {111} texture required for improving the Rankford value was lowered, and the Rankford value was lowered. No. No. 19 is Cu because the temperature in the reducing atmosphere in the continuous hot dip galvanizing line is less than the lower limit value.
However, the target strength could not be obtained because the strength of the steel sheet was not sufficiently increased by the precipitation or cluster strengthening.

No. In No. 20, the temperature in the reducing atmosphere in the continuous hot-dip galvanizing line was above the upper limit value, so that Cu was solid-dissolved, Cu was not precipitated or strength was not improved by cluster strengthening, and the target strength was not obtained.

[0028]

[Table 2]

[0029]

[Table 3]

[0030]

As described above, according to the present invention, it is possible to provide a high-strength hot-dip galvanized cold-rolled steel sheet having a deep drawability of 550 MPa or more without impairing the appearance and the plating adhesion. Thus, by applying the steel sheet according to the present invention to a member that requires severe deep drawing, for example, the weight of an automobile can be easily reduced, fuel consumption can be improved, and resources can be saved, and the industrial value is extremely high.

[Brief description of drawings]

FIG. 1 is a diagram showing a heat treatment cycle of a continuous annealing line (CAPL) and a heat treatment cycle of a continuous hot-dip galvanizing line (CGL).

FIG. 2 is a diagram showing a Rankford value and a tensile strength of a Cu-added steel subjected to the heat treatment cycle shown in FIG.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI Technical indication location C23C 2/06 2/28 (72) Inventor Eiko Mitsumasa 1 Kimitsu, Kimitsu-shi, Chiba New Nippon Steel Stock Company Kimitsu Works

Claims (2)

[Claims] 1. Mass% C: 0.01% or less, S
i: 0.3% or less, Mn: 0.1 to 2%, Al: 0.1
% Or less, S: 0.01% or less, P: 0.1% or less, C
u: 0.8-2.0%, Nb: 0.00
5 to 0.1%, Ti: 0.005 to 0.1%, B: 0.
Of 0001 to 0.003%, one or two or more kinds of steel are made into a slab containing the balance Fe and unavoidable impurities, and immediately or thereafter heated to 1000 to 1200 ° C.
Finishing temperature (Ar ₃ points + 20 ° C) during hot rolling
Rolling is completed at 950 ° C to 950 ° C, rolled into a hot rolled steel strip, cold rolled at a cold rolling reduction of 60% or more, and subsequently annealed at an annealing temperature of 750 to 900 ° C in a continuous annealing line to form a steel strip. After that, in a hot dip galvanizing line in a reducing atmosphere, 550 to 7
Annealing at 00 ° C, cooling and dipping in a hot dip galvanizing bath, hot dip galvanizing, and subsequent alloying treatment 450-55
A method for producing a high-strength hot-dip galvanized cold-rolled steel sheet having excellent deep drawability and having a tensile strength of 550 MPa or more, which is performed at 0 ° C. 2. C: 0.01% or less by mass%, S
i: 0.3% or less, Mn: 0.1 to 2%, Al: 0.1
% Or less, S: 0.01% or less, P: 0.1% or less, C
u: 0.8 to 2.0%, Ni: 0.4 to 1.0%, Nb: 0.005 to 0.1%, Ti: 0.0
0 to 0.1%, B: 0.0001 to 0.003%, containing 1 or 2 or more kinds, and immediately after the steel containing the balance Fe and unavoidable impurities as a slab or 1000 to 1250 ° C Heated to
Finishing temperature (Ar ₃ points + 20 ° C) during hot rolling
Rolling is completed at 950 ° C to 950 ° C, rolled into a hot rolled steel strip, cold rolled at a cold rolling reduction of 60% or more, and subsequently annealed at an annealing temperature of 750 to 900 ° C in a continuous annealing line to form a steel strip. After that, in a hot dip galvanizing line in a reducing atmosphere, 550 to 7
Annealing at 00 ° C, cooling and dipping in a hot dip galvanizing bath, hot dip galvanizing, and subsequent alloying treatment 450-55
A method for producing a high-strength hot-dip galvanized cold-rolled steel sheet having excellent deep drawability and having a tensile strength of 550 MPa or more, which is performed at 0 ° C.