JP2688384B2 - High-strength cold-rolled steel sheet and hot-dip galvanized steel sheet having excellent stretch flange characteristics, and methods for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) This invention has a tensile strength (hereinafter referred to as TS) of 40 kgf / mm.
TECHNICAL FIELD The present invention relates to a cold-rolled steel sheet, a hot-dip galvanized steel sheet, and a method for producing them, which has a ductility of ² or more and is excellent in stretch flangeability.

(Prior art) TS of 40kgf / mm ² or more
For example, in the automobile industry, the demand is increasing from the viewpoint of improving safety and reducing fuel consumption due to weight reduction, and further from the viewpoint of improving rust resistance as a hot-dip galvanized steel sheet,
In addition, there is a demand from the viewpoint of cost reduction due to the thinning of members in the use of building materials.

Needless to say, such a high-tensile steel plate should also satisfy desired workability in the above-mentioned applications.

As a technique for satisfying these requirements, for example, high Mn-Si as disclosed in JP-A Nos. 57-63634 and 56-13437.
There is a method of manufacturing steel as a raw material, but this method mainly depends on solid solution strengthening for strengthening, so a large amount of Si, which is advantageous for strengthening, is added, so the surface properties and chemical conversion treatment And there was a problem that it caused problems in plating and plating.

In addition, as a method not relying on the alloy components as described above, for example, there is a method of utilizing a recovery annealing structure as in JP-A-60-33318, but the material is not stable, the ductility is low, and There are problems such as large internal anisotropy, and although it is low cost, it has not yet been mass produced.

(Problems to be Solved by the Invention) The present invention relates to a high-strength cold-rolled steel sheet having a TS of 40 kgf / mm ² or more, and further relates to a high-strength hot-dip galvanized steel sheet, which eliminates the problems of the conventional technology. It is an object of the present invention to propose a high-strength cold-rolled steel sheet, a high-strength hot-dip galvanized steel sheet and a method for producing them, which are excellent in stretch flange characteristics that meet the above conditions.

(1) Si that deteriorates the surface properties and is harmful to plating
The addition of is avoided and a low alloy system is used.

(2) Improves ductility, especially stretch flange characteristics.

(3) Obtain a stable material.

(4) No particularly severe operating conditions are imposed.

(Means for Solving the Problems) In order to solve the above-mentioned problems, the inventors comprehensively investigated the material and structure of steels of various component systems and various manufacturing conditions, and as a result, When the phase fraction (mainly pearlite) is reduced to form a recrystallized ferrite structure and the structure is made to have uniform fine grains, extremely excellent stretch flange characteristics can be obtained. It was found that the composition and the cold rolling conditions / annealing conditions were optimized and obtained by combining them.

 The present invention is based on the above findings.

That is, the present invention is C :: 0.03 to 0.15 wt% (hereinafter simply referred to as%), Si: 0.05% or less, Mn: 0.5 to 1.2%, Nb: 0.005
~ 0.045% and Al: 0.10% or less, the balance is composed of iron and unavoidable impurities, and a uniform fine recrystallized ferrite structure with an average crystal grain size of 20 μm or less has an area ratio of 95% or more. A high-strength cold-rolled steel sheet with excellent stretch flange characteristics.

C: 0.03 to 0.15%, Si: 0.05% or less, Mn: 0.5 to 1.2
%, Nb: 0.005 to 0.045% and Al: 0.10% or less, and the balance is steel made of iron and unavoidable impurities, and the material is cold-rolled with a reduction of 50% or more after hot rolling. Stretch flange characteristics characterized by rolling, followed by annealing at a heating rate of 5 ° C / s or more in a continuous annealing line, holding for 20 to 60s in the temperature range of 720 ° C to 780 ° C, and then cooling. A method for producing a high-strength cold-rolled steel sheet excellent in

C: 0.03 to 0.15%, Si: 0.05% or less, Mn: 0.5 to 1.2
%, Nb: 0.005 to 0.045% and Al: 0.10% or less, the balance is composed of iron and inevitable impurities, and the average grain size is
Area ratio of uniform fine recrystallized ferrite structure of 20 μm or less
It is a high-strength hot-dip galvanized steel sheet with a structure of 95% or more and excellent stretch flange characteristics.

C: 0.03 to 0.15%, Si: 0.05% or less, Mn: 0.5 to 1.2
%, Nb: 0.005 to 0.045% and Al: 0.10% or less, the balance is steel made of iron and unavoidable impurities, and the material is cold-rolled with a reduction of 50% or more after hot rolling. Rolled, then heated in an in-line annealing type continuous hot dip galvanizing line at a heating rate of 5 ° C / s or higher to 720 ° C to 780 ° C.
A method for producing a high-strength hot-dip galvanized steel sheet having excellent stretch-flange characteristics, which comprises performing annealing for 20 to 60 s in a temperature range of ℃, followed by cooling and subsequent plating treatment.

Furthermore, C: 0.03 to 0.15%, Si: 0.05% or less, Mn: 0.5 to 1.2
%, Nb: 0.005 to 0.045% and Al: 0.10% or less, the balance is steel made of iron and unavoidable impurities, and the material is cold-rolled with a reduction of 50% or more after hot rolling. Rolled, then heated in an in-line annealing type continuous hot dip galvanizing line at a heating rate of 5 ° C / s or higher to 720 ° C to 780 ° C.
A method for producing a high-strength hot-dip galvanized steel sheet having excellent stretch flange characteristics, characterized by performing annealing for 20 to 60 s in the temperature range of 0 ° C, followed by cooling, followed by plating treatment and then alloying treatment. .

(Function) First, the reason why the composition of the steel sheet of the present invention is limited to the above range will be described.

C: 0.03 to 0.15% C is a desirable component because it is most effective as a strengthening component and is also inexpensive, but when it is added in excess of 0.15%, the fraction of the second phase such as pearlite is remarkable. In particular, the deterioration of the stretch flange characteristic is remarkable.
Further, the deterioration of the weldability becomes remarkable. On the other hand, the addition amount of C is 0.
If it is less than 03%, sufficient TS cannot be obtained even if other components are added. Therefore, the range is 0.03 to 0.15%.

Si: 0.05% or less 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, but due to the nature of scale, the surface Since it is a component that significantly deteriorates the properties and significantly impairs the plating property, it is necessary to set it to 0.05% or less in order to obtain a beautiful surface.

Mn: 0.5-1.2% Mn is an effective strengthening component although its solid solution strengthening ability does not reach that of C, Si, etc. In addition, Mn has an action of suppressing excessive formation and coarsening of pearlite and refining the crystal substance.
In order to exert these effects, it is necessary to add 0.5% or more of Mn. On the other hand, when Mn is added in excess of 1.2%, the strengthening effect saturates, and the distribution of the second phase tends to be connected in a layered manner, which deteriorates the stretch flange characteristic and also affects the recrystallization behavior. Stable manufacturing becomes difficult,
Further, the plating property deteriorates. Therefore, the range is 0.5 to 1.2%.

Nb: 0.005-0.045% The addition of Nb and the control of the addition amount are one of the important requirements of the present invention. In the present invention, due to the effect of adding Nb, an extremely fine and uniform recrystallized ferrite structure is finally obtained, thereby improving the strength and ductility, especially the stretch flange characteristic. It is thought that Nb will bring about the above-mentioned desirable effect by precipitating as carbonitride, but the details thereof are unknown. Such a desired effect cannot be obtained unless the Nb content is 0.005% or more. Also, even if added over 0.045%, the effect will be saturated, so it can be said that it is uneconomical. Further, excessive addition makes stable production difficult. Therefore, the range is 0.005 to 0.045%.

Al: 0.10% or less Al must be added as a deoxidizing component and for cleaning steel. For this, Al is at least 0.005%
It is preferred to add. 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.

In addition to the above components, in the present invention, N, O and S are 0.0050%, 0.0070% and unavoidable impurity components, respectively.
Acceptable up to 0.010%. Above all, by reducing S, the stretch flange characteristic is remarkably improved. Such a degree becomes remarkable on the higher strength side where TS exceeds 45 kgf / mm ² . Therefore, the higher the strength is, the more advantageous the reduction of S is.

 Next, the reasons for limiting the crystal structure will be described.

As described above, the object of the present invention is to improve ductility, particularly stretch flange characteristics.

Such a property is obtained by obtaining an excellent stretch flange property when the second phase fraction (mainly pearlite is reduced to make the recrystallized ferrite fraction 95% or more and the structure thereof is an average fine grain of 20 μm or less). To be

In this case, the larger the fraction of pearlite (especially coarse one), which is the origin of the stretch flange cracking, the more disadvantageous, and the more ununiform or coarse recrystallized ferrite structure is also disadvantageous. . Therefore, the recrystallized ferrite fraction is 95% or more, and the average crystal grain size of the recrystallized ferrite is 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, heating temperature 1280 to 1180
℃, hot rolling finishing temperature 900 ~ 800 ℃, winding temperature 650 ~ 500 ℃
Is mentioned.

In cold rolling, a higher reduction ratio is generally advantageous for refining the structure after recrystallization annealing. Taking this into consideration, the lower limit of the cold rolling reduction was set to 50%.
However, increasing the cold reduction more than necessary causes problems such as an increase in the thickness of the hot-rolled base plate although it is not harmful in terms of material.

Regarding the heating rate of annealing in the continuous annealing line for cold-rolled steel sheets and in-line annealing method continuous hot-dip galvanizing line for hot-dip galvanized steel sheets, the higher the heating rate, the more advantageous the refinement of recrystallized grains. In order to obtain uniform and fine recrystallized grains, 5 ° C / s or more, preferably 10 ° C / s or more is required. In addition, the upper limit of the heating rate is about 100 ° C / s from the industrial technical range of equipment.

The annealing temperature shall be 720-780 ℃. At a temperature lower than 720 ° C, recrystallization does not proceed sufficiently, elongation is reduced, stretch flangeability is deteriorated, and a satisfactory material cannot be obtained. Again 78
If the temperature exceeds 0 ° C, softening due to grain growth slightly proceeds, which is not desirable. In this invention, because Nb is added, the Nb carbonitride suppresses the abnormal grain growth of the recrystallized grains, and therefore a uniform and fine recrystallized ferrite grain structure can be obtained in a relatively wide temperature range. .

The holding time of such annealing may be substantially zero, but it is advantageous to perform it for 20 s or more in terms of the stability of the material. 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.

By the way, in the use of the steel sheet of the present invention, since the yield stress of the original sheet is the most important as the strength after member molding,
It may be desired that the yield ratio (YR) be 70% or more even if the formability is sacrificed to some extent. In order to obtain such high strength and an appropriate yield ratio, at least 700% is required in the cooling process following annealing. It is preferable to perform rapid cooling at a cooling rate of 20 ° C / s or more in a temperature range of to 500 ° C.

Further, in the production of the hot-dip galvanized steel sheet, 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 be performed or not. The change in characteristics due to this alloying treatment is small, and the materials are substantially the same.

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

These steel slabs were hot-rolled, cold-rolled and then annealed in a continuous annealing line under the conditions shown in Table 2.

The tensile properties of the steel sheet thus obtained and the side bend elongation properties corresponding to the stretch flange properties were measured and the evaluation results are shown in Table 3. The tensile test was evaluated using JIS No. 5 test piece. The side bend elongation property was evaluated according to Japanese Examined Patent Publication No. 50-35438, and a strip-shaped test piece with a width of 40 mm and a length of 170 mm was taken into consideration in order to obtain an appropriate clearance especially when shearing during manufacturing. Then, the sheared surface was lightly finished with emery paper, subjected to a test, subjected to in-plane bending deformation, and evaluated by the elongation of the flange portion immediately after the occurrence of cracks.

From Table 3, within the composition range according to the present invention, good elongation (E1.) And side bend elongation (that is, stretch flange characteristic) can be obtained despite high strength (TS ≧ 40 kgf / mm ² ). I understand. Moreover, an appropriate yield ratio is obtained.

Example 2 Steel A of Table 1 was used to manufacture cold-rolled steel sheets under various manufacturing conditions shown in Table 4, and the results of investigating tensile properties and side bend elongation properties in the same manner as in Example 1 are shown in Table 5. .

As is clear from Table 5, it is understood that good strength-elongation balance and stretch flange characteristics can be obtained within the range of the manufacturing conditions according to the present invention.

Example 3 In order to investigate the influence of the structure on the ductility and stretch flange characteristics, Table 6 shows the results of an examination of the correspondence relationship between the compositions and the treatment conditions.

From the table, the second phase fraction, recrystallized ferrite average grain size,
It can be seen that a good material can be obtained by appropriately controlling the recrystallized ferrite area ratio. Among the comparative examples, TS has a TS of 40 kgf / mm ² or less and has good elongation and side bend elongation characteristics, but it cannot be said to be a sufficient material. The test materials in Table 6 were manufactured under the components and manufacturing conditions shown in Table 7.

(Example) Example 4 Steel slabs having various compositional compositions shown in Table 1 were melted and cast according to a conventional method. These steel slabs were hot-rolled under the conditions shown in Table 8, cold-rolled, then annealed in a line in a continuous hot-dip galvanizing line, followed by plating and alloying, An alloyed hot-dip galvanized steel sheet was produced.

Table 9 shows the results of evaluation and evaluation of the tensile properties of the steel sheet thus obtained and the side-hand elongation properties corresponding to the stretch flange properties. The tensile test was carried out using JIS No. 5 test pieces, and the side bend elongation characteristics were evaluated in the same manner as in Example 1.

From Table 9, within the component composition range according to the present invention, good elongation (E1.) And side bend elongation (that is, stretch flange characteristic) can be obtained despite high strength (TS ≧ 40 kgf / mm ² ). You can see that

Example 5 Steel A in Table 1 was used to produce alloyed hot-dip galvanized steel sheets and non-alloyed hot-dip galvanized steel sheets under various production conditions shown in Table 10, and tensile properties and side properties were obtained in the same manner as in Example 1. Table 11 shows the results of the investigation on the bend elongation characteristics.

As is apparent from Table 11, it is understood that good strength-stretch balance and stretch flange characteristics can be obtained within the range of the manufacturing conditions according to the present invention.

The influence of the presence or absence of alloying treatment on the material was small, and the material was substantially the same.

Example 6 In order to investigate the influence of the structure on the ductility and stretch flangeability, Table 12 shows the results of an examination of the correspondence relationship between the compositions and treatment conditions.

From the table, the second phase fraction, recrystallized ferrite average grain size,
It can be seen that a good material can be obtained by appropriately controlling the recrystallized ferrite area ratio. Among the comparative examples, TS has a TS of 40 kgf / mm ² or less and has good elongation and side bend elongation characteristics, but it cannot be said to be a sufficient material. The test materials in Table 12 were manufactured under the components and manufacturing conditions shown in Table 13.

Example 7 The hot rolling finishing temperature was set to 800 by using the steel of the composition system shown in Table 14.
After performing hot rolling at ~ 850 ° C and cold rolling at a cold reduction of 65%, annealing is performed by heating at 10 ° C / s and soaking at 740 ° C for 30s, and then plating treatment, and further After alloying treatment, stretch flange properties were investigated according to Example 1.

In this test, in order to make the conditions more severe than in Example 1, the clearance was made larger than usual upon shearing, and the end face was not treated at all. Table 15 shows the results.

In Table 15, the side bend elongation of 55% of steel Q is a sufficiently good value as compared with the conventional material, but it can be seen that this property is further improved by further reducing S. The indication of> 60% in the table indicates that the test piece slipped through the jig and did not cause cracks, indicating that it has extremely excellent side bend elongation characteristics (stretch flange characteristics). .

(Effects of the Invention) The present invention has high strength and ductility, which has never been achieved.
The present invention relates to a high-strength cold-rolled steel sheet or hot-dip galvanized steel sheet having excellent stretch flange characteristics. Conventionally, TS is 40kgf /
High-strength steel sheets in the mm ² or higher class often suffer from press cracking problems mainly due to the lack of stretch flange characteristics, and it is difficult to obtain a material with a high yield ratio to show sufficient strength after forming parts. Although it was a further obstacle in the case of surface treatment of steel sheet, according to the present invention, such a problem was solved by applying a fine and uniform ferrite phase. It has also become possible to use rust-preventive steel plates mainly as strength members for automobiles.

Claims (5)

(57) [Claims] 1. C: 0.03 to 0.15 wt%, Si: 0.05 wt% or less, Mn: 0.5 to 1.2 wt%, Nb: 0.005 to 0.045 wt% and Al: 0.10 wt% or less, the balance being iron and A high-strength cold-rolled steel sheet with excellent stretch flange characteristics, which has a composition of inevitable impurities and has a uniform fine recrystallized ferrite structure with an average crystal grain size of 20 μm or less with an area ratio of 95% or more. 2. C: 0.03-0.15 wt%, Si: 0.05 wt% or less, Mn: 0.5-1.2 wt%, Nb: 0.005-0.045 wt% and Al: 0.10 wt% or less, the balance being iron and Steel made of unavoidable impurities is used as a material, and this material is hot-rolled, then cold-rolled at a reduction rate of 50% or more, and then heated at a heating rate of 5 ° C / s or more in a continuous annealing line. A method for producing a high-strength cold-rolled steel sheet having excellent stretch flange characteristics, which comprises performing annealing for 20 to 60 s in a temperature range of ℃ to 780 ℃, and then cooling. 3. C: 0.03 to 0.15 wt%, Si: 0.05 wt% or less, Mn: 0.5 to 1.2 wt%, Nb: 0.005 to 0.045 wt% and Al: 0.10 wt% or less, with the balance being iron and A high-strength hot-dip galvanized steel sheet with excellent stretch flange characteristics, which has a composition of inevitable impurities and has a uniform fine recrystallized ferrite structure with an average crystal grain size of 20 μm or less with an area ratio of 95% or more. 4. C: 0.03-0.15 wt%, Si: 0.05 wt% or less, Mn: 0.5-1.2 wt%, Nb: 0.005-0.045 wt% and Al: 0.10 wt% or less, the balance being iron and Steel made of unavoidable impurities is used as a material, and this material is hot-rolled, then cold-rolled at a reduction rate of 50% or more, and then heated at a heating rate of 5 ° C in an in-line annealing system continuous hot dip galvanizing line. High-strength hot-dip zinc with excellent stretch flange characteristics, characterized by performing annealing at a temperature of 720 ° C to 780 ° C for 20 to 60 s in the temperature range of 720 to 780 ° C, and then performing cooling followed by plating. Manufacturing method of plated steel sheet. 5. C: 0.03 to 0.15 wt%, Si: 0.05 wt% or less, Mn: 0.5 to 1.2 wt%, Nb: 0.005 to 0.045 wt% and Al: 0.10 wt% or less, with the balance being iron and Steel made of unavoidable impurities is used as a material, and this material is hot-rolled, then cold-rolled at a reduction rate of 50% or more, and then heated at a heating rate of 5 ° C in an in-line annealing system continuous hot dip galvanizing line. Excellent in stretch-flange characteristics, characterized by heating at above s / s and holding it in the temperature range of 720 ° C to 780 ° C for 20 to 60s, followed by cooling, followed by plating and then alloying. Method for manufacturing high-strength hot-dip galvanized steel sheet.