JPH06108202A - Hot dip-galvanized steel sheet having high tensile strength and excellent in plating suitability and workability and its production - Google Patents

Abstract

PURPOSE:To produce a hot dip-galvanized high tensile strength steel sheet excellent in plating suitability and workability by subjecting a low carbon steel slab to hot- rolling and to cold-rolling under respectively specified conditions, applying normalizing treatment and hot dip-galvanizing to the resulting sheet, and further performing alloying treatment. CONSTITUTION:A slab of a low carbon steel having a composition containing, by weight, 0.05-0.15% C, 0.05-0.15% Si, 1.50-2.00% Mn, 0.050-0.100% P, and <0.10% Al is heated up to 1230-1270 deg.C, hot-rolled at 860-890 deg.C hot-rolling finishing temp., and coiled at 520-560 deg.C coiling temp. The resulting hot-rolled plate is cold-rolled at >=40% draft and then heated up to 700-870 deg.C at >=5 deg.C/sec heating rate by means of a continuous hot dip-galvanizing line and held at the above temp. for 20-60sec to undergo annealing treatment, followed by hot dip-galvanizing. Further, heating is executed to alloy the galvanizing layer with Fe in the steel sheet. By this method, the hot dip galvanized high tensile strength steel sheet can be obtained where fine and uniform recrystallized ferrite structure of <=20mum average crystalline grain size comprises >=95% of the whole area and which has excellent plating suitability and workability and also has <=50kgf/mm<2> tensile strength.

Description

Detailed Description of the Invention

[0001]

This invention relates to tensile strength (hereinafter referred to as TS
Indicates a hot-dip galvanized steel sheet having a high tensile strength of 50 kgf / mm ² or more, and excellent in platability and workability, and a method for producing the same.

[0002]

^2. Description of the Related Art High-strength hot-dip galvanized steel sheets with a TS of 50 kgf / mm ² or more are becoming more and more demanded, for example, in the automobile industry from the viewpoint of improving safety, reducing fuel consumption by reducing weight, and improving rust resistance. is there. In addition, in the above-mentioned applications, strict working is often performed, so that it is required to have excellent workability and good plating property.

As described above, despite the great demands from the users, it is difficult to manufacture a hot-dip galvanized steel sheet having the above-mentioned high tensile strength and workability and plating property by the conventional technique. Has been said. For example, as disclosed in Japanese Patent Publication No. 62-56209, conventionally, in order to increase the tensile strength of a steel sheet, in addition to C, Si, Mn, Nb, Ti, Mo, etc. are used. It is essential to contain many alloy components, and therefore not only the plating properties are significantly deteriorated due to the inclusion of the above alloy components,
There was a problem that workability was also deteriorated.

Further, as disclosed in JP-A-56-13437, there is a method for producing a high-strength hot-dip galvanized steel sheet using a high Mn-Si steel as a raw material. Since it relies mainly on solid solution strengthening, since a large amount of Si is added, it is glad that problems occur in surface properties, chemical conversion treatment and plating properties.

As another method that does not rely on the above alloy components, there is a method of utilizing a recovery annealing structure as disclosed in JP-A-60-33318, but the material is not stable,
There are problems such as low ductility and large in-plane anisotropy, and although it is low cost, it has not yet been mass produced.

[0006]

The present invention has a TS of 50.
Regarding high-strength hot-dip galvanized steel sheet with kgf / mm ² or more, manufacturing the high-workability high-strength hot-dip galvanized steel sheet that solves the problems of the conventional technology and has excellent plating properties that meet the following conditions. Its purpose is to propose with a method. (1) Select the components so as not to impair the plating property and determine the content. (2) Steel with a low yield ratio and excellent ductility in order to obtain good workability. (3) No particularly severe operating conditions are imposed.

[0007]

According to the present invention, in order to obtain extremely excellent platability and workability (low yield ratio),
The phase fraction (mainly pearlite) is reduced to form a recrystallized ferrite structure, and the structure is made into uniform fine grains. In order to obtain such a desired structure, the steel composition and cold rolling conditions are mainly used. -It is a combination of optimized annealing conditions. That is, the gist of the present invention is as follows.

(1) C: 0.05 to 0.15% (hereinafter simply indicated by%), Si: 0.05 to 0.15%, Mn: 1.50 to 2.00%, P: 0.05
0 to 0.100% and Al: 0.10% or less, the balance is composed of iron and inevitable impurities, and the average crystal grain size is 20μ.
Area ratio 95% with uniform fine recrystallized ferrite structure of m or less
A good workability high-strength hot-dip galvanized steel sheet having excellent platability and having the above structure.

(2) C: 0.05 to 0.15 wt%, Si: 0.05 to 0.15
wt%, Mn: 1.50 to 2.00 wt%, P: 0.050 to 0.100 wt% and Al: 0.10 wt% or less, with the balance being steel consisting of iron and unavoidable impurities, and hot rolling this material. After that, cold rolling with a rolling reduction of 40% or more, and then heating at a heating rate of 5 ° C / s or more in a continuous hot dip galvanizing line
Manufacture of high-workability, high-strength hot-dip galvanized steel sheet with excellent plateability, which is characterized by annealing for 20 to 60 s in the temperature range of 700 ℃ to 870 ℃, and subsequent plating after cooling. Method.

(3) The method for producing a good-workability high-strength hot-dip galvanized steel sheet having excellent plating properties as described in (2) above, which comprises subjecting a plating treatment to an alloying treatment.

[0011]

First, the reason why the composition of the steel sheet and the starting material in the present invention is limited to the above range will be described. C: 0.05 to 0.15% C is a desirable component because it is most effective as a strengthening component and is inexpensive, but when it is added in excess of 0.15%, the fraction of the second phase such as pearlite becomes remarkable. And the ductility deteriorates significantly. On the other hand, if the C content is less than 0.05%, sufficient TS cannot be obtained even if other components are added. Therefore, the range is 0.05 to 0.15%.

Si: 0.05 to 0.15% Si is effective for strengthening steel and has little adverse effect on deterioration of ductility, so it is a component to be added in a large amount from the viewpoint of mechanical properties. It has the drawback of significantly impairing sex. Therefore, the lower limit of the amount of Si added is limited to 0.05% as the minimum for obtaining the target low yield ratio, and the upper limit is limited to 0.15% so as not to impair the plating property.

Mn: 1.50-2.00% The strengthening ability of Mn is an effective strengthening component although it does not reach that of C, Si and the like. Moreover, Mn has an action of suppressing excessive generation of pearlite and refining the crystal grains. In order to exert these effects, it is necessary to add Mn in an amount of 1.50% or more. On the other hand, even if Mn is added in excess of 2.00%, not only the improvement of strengthening is not seen, but also the plating property is deteriorated. Therefore, the amount of Mn should be in the range of 1.50 to 2.00%.

P: 0.050 to 0.100% P is effective for strengthening steel and is a component to be added in a large amount. On the other hand, if added in a large amount, it easily causes embrittlement, so the upper limit of P is 0.100%. did. Regarding the lower limit of the P amount, it is 0 as an indispensable amount to obtain the desired strength.
It was 050%.

Al: 0.10% or less Al must be added as a deoxidizing component and for cleaning steel. For this purpose, it is preferable to add at least 0.005% of Al. However, if added in excess of 0.10%, there is a high risk of causing surface defect troubles due to alumina clusters and the like. Therefore, 0.10% or less.

Next, the reasons for limiting the crystal structure will be described.
One of the objects of the present invention is to provide good workability, especially a low yield ratio as described above. The inventors comprehensively investigated the material and structure of steel sheets manufactured under various component systems and under various conditions, and as a result, in the composition of the steel sheet of the present invention, the second phase fraction (mainly pearlite) was 5%.
If it is reduced to below, in other words, the ferrite fraction is 95% or more and the structure is made into uniform fine grains with an average of 20 μm or less, excellent workability can be obtained even though it is a high-strength steel. I found out that. This low yield ratio is disadvantageous as the fraction of the pearlite (particularly coarse) structure is higher, and is similarly undesirable when the ferrite structure is nonuniform or coarse. Therefore, the recrystallized ferrite fraction is set to 95% or more, and the average crystal grain size of the recrystallized ferrite is set to 20 μm or less.

Next, the reasons for limiting the manufacturing conditions will be described.
The steps from melting to hot rolling can be carried out according to a method usually used without particular limitation. As an example of typical hot rolling conditions, the heating temperature is 1230 to 1270.
℃, hot rolling finish temperature 860-890 ℃, coiling temperature 520-56
An example is 0 ° C.

A higher reduction ratio in cold rolling is generally advantageous in that the structure after recrystallization is refined. From the viewpoint of such crystal refinement, the lower limit of the cold rolling reduction was set to 40%. It should be noted that increasing the cold reduction more than necessary causes no problem in terms of material, but poses a problem such as an increase in the thickness of the hot-rolled base plate.

A higher heating rate for annealing in a continuous hot-dip galvanizing line is advantageous for refining recrystallized grains, and is preferably 5 ° C./s or more in order to obtain uniform and fine recrystallized grains. Requires 10 ° C / s or more. The upper limit of heating rate is 20 from the industrial technical range of equipment.
℃ / s.

The annealing temperature is 700 to 870 ° C. 70
At temperatures lower than 0 ° C, recrystallization does not proceed sufficiently and a satisfactory material cannot be obtained. On the other hand, if the temperature exceeds 870 ° C, softening due to grain growth will proceed a little, which is not desirable.

The holding time of such annealing is substantially zero.
However, from the viewpoint of the stability of the material, it is more advantageous to perform it for 20 s or more. On the other hand, if it exceeds 60 s, material deterioration may occur due to abnormal grain growth, so it was set to 20-60 s.

The plating treatment subsequent to the annealing is not particularly limited, and the usual plating treatment may be performed. Further, in the present invention, the alloying treatment may or may not be performed. The change in characteristics due to this alloying treatment is small, and the materials are substantially the same.

[0023]

【Example】

Example 1 Steel slabs having various component compositions shown in Table 1 were melted and cast according to a conventional method.

[0024]

[Table 1]

These steel slabs were hot-rolled, cold-rolled under the conditions shown in Table 2 and then annealed in a continuous hot-dip galvanizing line, followed by hot-dip galvanizing treatment with an adhesion amount of 45 g / m ^2. And an alloying treatment of heating to 600 ° C. was performed to produce an alloyed hot-dip galvanized steel sheet.

[0026]

[Table 2]

Table 3 shows the results of measuring, investigating and evaluating the tensile properties and plating properties of the steel sheet thus obtained. The tensile test was carried out using JIS No. 5 test pieces, and the evaluation of the plating property was performed by visual observation, and the steel plate obtained a result without non-plating was judged to be good.

[Table 3]

From Table 3, it can be seen that, within the composition range of the component according to the present invention, despite the high strength (TS ≧ 50 kgf / mm ² ),
It can be seen that a steel sheet having a good plating property and a low yield ratio can be obtained.

Example 2 Steel A in Table 1 was used for hot rolling at a slab heating temperature of 1250 ° C., hot finish rolling temperature of 870 ° C. and winding temperature of 540 ° C., and then shown in Table 4. Alloyed hot-dip galvanized steel sheet and non-alloyed hot-dip galvanized steel sheet (deposition amount: 45 g / m ² ) were produced under various production conditions, and the results of investigating tensile properties and plating properties in the same manner as in Example 1 are shown in Table 5 Shown in.

[0030]

[Table 4]

[0031]

[Table 5]

As is clear from Table 5, it is understood that a steel sheet having good elongation characteristics and a low yield ratio can be obtained within the range of manufacturing conditions according to the present invention. Further, as is clear from the comparison between the steel types 1a and 1b, the influence of the presence or absence of the alloying treatment on the material is small, and only the elongation property is deteriorated by about 2% by the alloying treatment, but the substantially same material is obtained. Was given.

Example 3 In order to investigate the influence of the structure on the ductility and the yield ratio, Table 6 shows the results of the investigation of the correspondence relationship between the compositions and the treatment conditions.

[0034]

[Table 6]

It can be seen from the table that a good material can be obtained by properly controlling the second phase fraction, recrystallized ferrite average grain size, and recrystallized ferrite area ratio. Among the comparative examples, TS has a TS of 50 kgf / mm ² or less, and has a good El. And yield ratio, but is not a sufficient material.

[0036]

EFFECTS OF THE INVENTION The hot-dip galvanized steel sheet of the present invention has high strength and good elongation characteristics and a low yield ratio, which makes it possible to use a rust-preventive steel sheet mainly as a strength member for automobiles. It was

Further, according to the hot-dip galvanized steel sheet of the present invention, a fine and uniform ferrite phase can be obtained by specifying the component composition and the manufacturing conditions. It is possible to obtain a high-strength hot-dip galvanized steel sheet having excellent ratio) and plating properties.

Claims (3)

[Claims] 1. C: 0.05 to 0.15 wt%, Si: 0.05 to 0.15 wt%, Mn: 1.50 to 2.00 wt%, P: 0.050 to 0.100 wt% and Al: 0.10 wt% or less, the balance being iron. And the composition of unavoidable impurities,
A high-strength hot-dip galvanized steel sheet with excellent workability and excellent plating properties, in which a uniform and fine recrystallized ferrite structure with an average crystal grain size of 20 μm or less becomes an area ratio of 95% or more. 2. C: 0.05 to 0.15 wt%, Si: 0.05 to 0.15 wt%, Mn: 1.50 to 2.00 wt%, P: 0.050 to 0.100 wt% and Al: 0.10 wt% or less, the balance being iron. And steel made of unavoidable impurities is used as a raw material, this material is hot-rolled, then cold-rolled with a reduction rate of 40% or more, and then heated at a heating rate of 5 ° C / s or more in a continuous hot dip galvanizing line. High-strength hot-dip galvanized steel sheet with excellent plateability, characterized by performing annealing for 20 to 60 s in the temperature range of 700 ° C to 870 ° C, followed by cooling and subsequent plating treatment. Manufacturing method. 3. The method for producing a good workability high-strength hot-dip galvanized steel sheet having excellent plating properties according to claim 2, wherein an alloying treatment is performed after the plating treatment.