JP2787366B2 - Manufacturing method of hot-dip galvanized high-tensile cold-rolled steel sheet - Google Patents

Description

The present invention relates to a method for producing a high-strength steel sheet having excellent workability and corrosion resistance.

(Prior Art) molten zinc plating of the conventional high strength steel sheet steel plate A ₁ transformation point or more in a continuous plating line, and heated to a temperature below A ₃ transformation point, melt plating zinc bath heated around 450 ° C. (JP-A-55-131168). When an alloying treatment is added, reheating is performed for a short time at a temperature of about 560 ° C. (for example, JP-A-56-142821, JP-A-57-57827, and JP-A-5-57827).
After that, cooling is performed without holding treatment or the like.

(Problems to be Solved by the Invention) The present invention provides a method for producing a high-strength steel sheet having a completely unprecedented strength-ductility balance and excellent corrosion resistance by drastically reviewing a hot-dip plating process. It is.

(Means for Solving the Problems) The gist of the present invention is that C: 0.06 to 0.45%, Mn:
0.5 to 2.5%, Si: 2.5% or less, P: 0.1% or less, solAl: 0.2%
Hereinafter, the slab consisting of the remaining Fe and inevitable impurities is referred to as Ar.
_After hot rolling at ₃ transformation point or more, and performing normal pickling and cold rolling of the material, in the continuous plating line, Ac ₁ temperature + 20 ° C or more, A
c ₃ Heat to -20 ° C for 10 seconds or more and within 5 minutes, then 650 ° C
The average cooling speed from 20 to 450 ° C until entering the hot-dip zinc bath or at least 20 ° C / s, before or after hot-dip galvanizing
A method for producing a high-strength steel sheet excellent in workability, characterized in that the steel sheet is maintained at a temperature range of 300 ° C to 450 ° C for 60 seconds to 600 seconds.

 Hereinafter, the present invention will be described in detail.

In the present invention, the lower limit of the added amount of C is set to 0.06% because if the amount of C is smaller than this, the generation of residual austenite described later is suppressed, and an excellent strength-ductility balance cannot be obtained. The upper limit is set to 0.45% because if C is added any more, it becomes hard and the workability is reduced.

The reason why the lower limit of the Mn content is set to 0.5% is that addition of less than this suppresses the generation of sufficient retained austenite, and the upper limit is set to 2.5% because workability deteriorates.

Si is known as an effective alloying element for improving strength and ductility because it increases strength and hardly deteriorates ductility. On the other hand, the addition of Si is also known to impair the plating property, and in a normal continuous hot-dip plating process, it is considered to be 0.
Plating failure becomes remarkable by addition of 1% or more of Si. However, plating can be improved if pre-plating or surface grinding is performed before continuous hot-dip plating. Therefore, Si in the present invention
The upper limit of the amount was set to 2.5% at which ductility was deteriorated.

Although the addition of P is preferable as a strengthening element, the workability is deteriorated when the addition amount is large.

Al needs to be added as a deoxidizing element.
If l increases, workability and plating property deteriorate, so the upper limit was made 0.2%.

In the present invention, as another component of steel, a component included for improving strength and hardenability, that is, Cu: 1.5
%, Ni: 1.5% or less, Mo: 1.0% or less, Cr: 1.5% or less,
V: 1.0% or less, B: 0.005% or less, Ti: 0.1% or less, Nb: 0.1%
The inclusion of the following does not impair the spirit of the present invention.

 Next, the manufacturing process will be described.

The reason for setting the finishing temperature of hot rolling at or above the Ar ₃ transformation point is that if hot rolling is performed at this temperature or lower, the structure becomes coarse, and the strength-ductility balance is significantly deteriorated even after plating.

Heating condition in continuous plating line is Ac ₁ temperature + 20 ° C
As described above, the reason why heating to Ac ₃ temperature of −20 ° C. is limited to 10 seconds or more and within 5 minutes is that a two-phase structure of ferrite and austenite is formed in this temperature range, and the lower limit of the heating time is set to 10 seconds. If the time is shorter, the distribution of the alloy element to each phase does not occur sufficiently, so that the generation of residual austenite described later is suppressed. In addition, the upper limit of the heating time was set to 5 minutes because the distribution of the alloying elements to each phase was almost completed within this time, and it was not necessary to further extend the holding time, and the ferrite grains caused grain growth. This is to cause deterioration of the material.

The reason why the average cooling rate from 650 ° C to entering the molten zinc bath or from 450 ° C to 20 ° C / s or more is limited is that if the average cooling rate is slower than this, an excellent strength-ductility balance can be obtained after plating. This corresponds to the fact that a large amount of pearlite is generated during cooling, and the generation of residual austenite is difficult.

The limitation of performing in-line heat treatment for 60 seconds to 600 seconds at a temperature range of 300 ° C to 450 ° C before or after hot-dip galvanizing is to hold for 60 seconds or more and 600 seconds or less in this temperature range. This is because the strength-ductility balance is significantly improved. This is because bainite transformation proceeds by this treatment, and C, which is an austenite-stable element, is concentrated in the remaining austenite, and exists as residual austenite after cooling.
sticity) and increased ductility.
The lower limit of the retention time is limited by the time required for bainite transformation to proceed sufficiently, and the upper limit is defined as the time during which the retained austenite undergoes transformation and does not decrease significantly.

Whether this in-line heat treatment is performed before or after hot-dip galvanizing is related to the presence or absence of alloying treatment. The alloying treatment can be achieved by performing in-line heat treatment after hot-dip galvanizing.

(Example) The steel sheet shown in Table 1 was used as a raw material, and plating was performed in a continuous hot-dip galvanizing line. Steel types AD are the steels of the present invention, and E and F are comparative steels, and the amounts of C and Mn do not satisfy the range of the present invention. Table 2 shows the mechanical properties and hot-rolling finishing temperature of the material after plating when these steels were plated in a continuous galvanizing line. The plating material was pickled, heated in a reducing atmosphere, and then passed through a plating bath. At that time, the temperature of the plating bath was 460 ° C. In addition, materials having poor plating properties were subjected to pre-plating or surface grinding before entering a continuous galvanizing line.

Materials 11 and 13 were subjected to in-line heat treatment at 400 ° C. for 300 seconds before hot-dip galvanizing zinc, and the other materials were subjected to in-line heat treatment after plating.

Materials 1,2,10,11,12,13,14,15 satisfying the scope of the present invention
Is the strength-ductility balance index TS × El is 2400 (kg / mm
×%) or more, showing excellent characteristics as a high tensile strength steel for working.

On the other hand, Material 3 having a finishing temperature of Ar ₃ or less was lower in both strength and ductility than the steel of the present invention, and the material was clearly deteriorated.

Material 4 whose average cooling rate from 650 ° C. to the molten zinc bath or 450 ° C. is lower than the claimed range cannot prevent the formation of pearlite, and the strength-ductility balance is reduced accordingly. As compared with the steel of the present invention.

Further, the material 5 having a low holding temperature after cooling produced martensite and increased the strength, but markedly deteriorated ductility, whereas the material 6 having a high holding temperature exhibited a ferrite-pearlite structure. , The strength was low. In the material 7 having a short holding time, the retained austenite is not sufficiently generated,
High ductility was not obtained.

Material 8 having an in-line heating temperature of 670 ° C. and lower than the Ac ₁ temperature + 20 ° C. exhibited a ferrite-pearlite structure and low strength. On the other hand, the in-line heating temperature was 850 ° C and the Ac ₃ temperature −
Material 9 having a temperature higher than 20 ° C. produced martensite and increased the strength, but markedly deteriorated ductility.

Materials whose material components do not satisfy the scope of the present invention
However, pearlite is formed even though other production conditions are satisfied, and an excellent strength-ductility balance cannot be obtained.

(Effect of the Invention) According to the present invention, a high-strength steel with excellent workability can be manufactured in a continuous hot-dip galvanizing line, and high-tensile steel can be applied to parts requiring workability and corrosion resistance. This is an industrially valuable invention.

Continuation of front page (56) References JP-A-2-93051 (JP, A) JP-A-58-39770 (JP, A) JP-A-56-163219 (JP, A) (58) Fields studied (Int .Cl. ⁶ , DB name) C23C 2/00-2/40 C21D 8/02-8/04

Claims (2)

(57) [Claims] (1) C: 0.06-0.45%, Mn: 0.5-2.5%, Si: 2.5
% Or less, P: 0.1% or less, solAl: 0.2% or less, slab consisting of the balance Fe and unavoidable impurities is hot rolled at the Ar ₃ transformation point or higher, and the material is subjected to ordinary pickling and cold rolling, followed by continuous plating In the line, it is heated to Ac ₁ temperature + 20 ° C or higher and Ac ₃ temperature −20 ° C for 10 seconds or more within 5 minutes, and then the average cooling rate from 650 ° C to entering the molten zinc bath is 30 ° C / s or more, and hot dip galvanizing Thereafter, a method for producing a high-strength steel sheet excellent in workability, characterized in that the steel sheet is held at a temperature range of 300 ° C. and 450 ° C. for 60 seconds to 600 seconds. 2. C: 0.06-0.45%, Mn: 0.5-2.5%, Si: 2.5
% Or less, P: 0.1% or less, solAl: 0.2% or less, slab consisting of the balance Fe and unavoidable impurities is hot rolled at the Ar ₃ transformation point or higher, and the material is subjected to ordinary pickling and cold rolling, followed by continuous plating In the line, it is heated to Ac ₁ temperature + 20 ° C or higher and Ac ₃ temperature −20 ° C for 10 seconds to 5 minutes, and then the average cooling rate from 650 ° C to 450 ° C is set to 20 ° C / s or more. A method for producing a high-strength steel sheet excellent in workability, characterized by performing hot-dip galvanizing after holding for 60 seconds to 600 seconds in a temperature range.