JP2000282151A - Manufacture of steel sheet excellent in surface characteristics and workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a high strength steel sheet or a hot-dip metal coated steel sheet, excellent in surface characteristics and workability. SOLUTION: A steel slab, which has a composition containing, by weight, 0.0005-0.01% C, <=2.0% Si, 0.05-3.0% Mn, <=0.2% P, 0.0005-0.02% S, 0.5-3.0% Cu, 0.005-0.10% acid-soluble Al, and <=0.007% N and optionally containing 0.01-0.1% Ti, 0.01-0.1% Nb, and 0.2-2.0% Ni, is used. This steel slab is reheated or subjected to hot direct rolling to undergo hot rolling at a finish rolling temperature not lower than the Ar3 transformation point and at a coiling temperature of 650-800 deg.C. Subsequently, cold rolling at 10-60% cold rolling rate and acid pickling are applied, and the resultant steel sheet is annealed at a temperature between the recrystallization temperature and 900 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 steel sheet having excellent surface properties and workability suitable for use in automotive steel sheets, home electric appliances, electric machines, electronic materials and the like.

[0002]

2. Description of the Related Art In a steel sheet to which Cu is added, the effect of strengthening the precipitation of Cu is exhibited only by performing a heat treatment at a relatively low temperature. The Cu-added steel sheet is used as a material that can obtain higher strength than conventional high-strength cold-rolled steel sheet by utilizing this property.
It is widely used in a wide range of fields such as home appliances, electric machines, and electronic materials. In this type of application, excellent deep drawability is required because severe press working may be performed. Also, in view of the recent market where the beauty of products is a problem, having good surface properties is also an important requirement for increasing the product value.

[0003] In this type of steel sheet, a steel whose composition is specified is made into a continuously cast slab or an ingot slab having a thickness of about 250 mm, and the continuous cast slab or the ingot slab is hot-rolled to a thickness of about 2 to 6 mm. After descaled the obtained hot rolled steel strip by pickling,
It is manufactured by cold rolling and then recrystallization annealing. The hot-dip coated steel strip is manufactured by hot-dip plating after recrystallization annealing. The workability of the manufactured cold-rolled steel strip and hot-dip coated steel strip is evaluated based on the elongation obtained by a tensile test and the Rankford value which is an index of deep drawability. However, elongation and Rankford values are greatly affected by manufacturing conditions as well as steel composition. Therefore, various manufacturing conditions have been conventionally set in order to secure required workability.

[0004]

In particular, the coiling temperature during hot rolling greatly affects the elongation of the steel strip and the Rankford value. If the winding temperature is set higher, the elongation and Rankford value will increase, but if the hot-rolled steel sheet is wound at an excessively high temperature, the oxidation of the steel sheet surface will proceed in the cooling process after winding, and a thick oxide scale will form. Generate. As a result, in the subsequent pickling step, the descalability is reduced, which not only greatly impairs the productivity but also adversely affects the surface properties of the steel sheet. In particular, in the Cu-added steel strip, the descaling property is significantly reduced. For this reason, setting the winding temperature to a higher value imposes restrictions from a practical point of view, and there is a limit in improving the elongation and the Rankford value by increasing the winding temperature.

The present invention has been devised in order to solve such a problem. The steel composition and the manufacturing conditions are comprehensively adjusted to set a relatively high winding temperature, and the pre-pickling process is performed. It is an object of the present invention to produce a steel sheet and a hot-dip steel sheet having good surface properties and excellent workability without impairing productivity by performing cold rolling for increasing descaleability.

[0006]

According to the production method of the present invention, to achieve the object, C: 0.0005 to 0.01% by weight, Si: 2.0% by weight or less, Mn: 0.05 to 0.05% by weight. 3.
0% by weight, P: 0.2% by weight or less, S: 0.0005-
A steel slab having a composition containing 0.02% by weight, Cu: 0.5 to 3.0% by weight, acid-soluble Al: 0.005 to 0.10% by weight, and N: 0.007% by weight or less is reused. Heating or direct feeding, finishing rolling temperature Ar ₃ transformation point or higher, winding temperature 650
To 800 ° C., the resulting hot-rolled steel sheet is subjected to cold rolling at a cold rolling rate of 10 to 60%, and after pickling, the obtained steel sheet is annealed at a recrystallization temperature or higher and 900 ° C. or lower. It is characterized by the following. As the steel slab to be used, Ti: 0.0
1 to 0.1% by weight, Nb: 0.01 to 0.1% by weight, N
i: One or more of 0.2 to 2.0% by weight can be contained. B: 0.0002 to 0.002% by weight, Zr: 0.01 to 0.1% by weight, V: 0.01 to
Steel slabs containing 0.1% by weight of one or more can also be used.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, when manufacturing a hot-rolled steel sheet from a steel slab alloy-designed according to the strength of the steel sheet or hot-dip coated steel sheet, the hot-rolled steel sheet is wound around a coil at a relatively high temperature. High elongation and Rankford value are ensured for steel sheets or hot-dip coated steel sheets. Oxidation scale tends to be easily generated due to a relatively high winding temperature, but the descaling property is improved by performing cold rolling at a cold rolling rate of 10 to 60% in a pre-pickling process. . Therefore, even if the oxide scale grows thickly, the oxide scale is easily removed from the steel sheet surface under the ordinary pickling conditions. In addition, cold rolling before pickling,
It is also effective in improving the surface properties of the obtained steel sheet or hot-dip coated steel sheet.

Hereinafter, alloy components, contents, production conditions, and the like contained in steel to which the present invention is applied will be described. C: 0.0005 to 0.01% by weight C in the steel of the present invention is a component fixed as a carbide such as TiC and NbC. The lower the C content, the more the Rankford value and elongation are improved. The contents of Ti and Nb as the fixing elements can be reduced. Therefore, the upper limit of the C content is restricted to 0.01% by weight. However, 0.
If the C content is less than 0005% by weight, excessive decarburization refining is required in the steel making process, resulting in an increase in production costs.

Si: 2.0% by weight or less Si is an element that has an adverse effect on workability and plating property, and the elongation and the Rankford value tend to decrease as the Si content increases. However, since the reduction in workability is not so large compared to the improvement in strength, it can be used as an effective alloy component as a strengthening element for steel. In order to obtain the strength required as a high-strength steel sheet, it is preferable to contain 0.2% by weight or more of Si. However, when a large amount of Si exceeding 2.0% by weight is contained, even if cold rolling is performed as a pre-pickling step, the surface properties are not improved, and the workability is also reduced.

Mn: 0.05 to 3.0% by weight An effective component for preventing hot brittleness induced by S during hot rolling and increasing the strength of steel. To prevent hot brittleness, a Mn content of 0.05% by weight or more is required. When the Mn content is 0.50% by weight or more, the effect of improving the strength of the steel sheet becomes remarkable as in the case of Si. However, when a large amount of Mn exceeding 3.0% by weight is contained, workability is deteriorated. P: 0.2% by weight or less Like Si, it is a strengthening element that does not significantly deteriorate workability, and a necessary amount according to the strength required for a product is added. However, a large P content exceeding 0.2% by weight tends to reduce the secondary work brittleness resistance.

S: 0.0005 to 0.02% by weight Since the component induces cracking during hot working, the upper limit is restricted to 0.02% by weight. However, it forms sulfides with Mn, Ti, etc., affecting the formation of carbide-based precipitates,
Has the effect of improving the Rankford value. Also, 0.
Reducing the S content to less than 0005% by weight would require enormous costs for desulfurization refining in the steelmaking process. Therefore, in the present invention, the lower limit of the S content is set to 0.0
005% by weight.

Cu: 0.5-3.0% by weight Cu is an important alloy component for improving the strength of steel, and a necessary amount corresponding to the required strength is added. Cu is precipitated even by heat treatment at a relatively low temperature, and has an effect of strengthening the matrix. Such an effect becomes remarkable at a Cu content of 0.5% by weight or more. However, since it is also a component that induces hot embrittlement during hot working, the Cu content is restricted to 3.0% by weight or less. When a large amount of Cu exceeding 3.0% by weight is contained, even if Ni effective for preventing hot brittleness is added, the adverse effects caused by Cu cannot be prevented.

Al: 0.005 to 0.10 wt% Al is added as a deoxidizing agent and has an effect of fixing N. Such an effect is obtained when the Al content of 0.005% by weight or more
It becomes remarkable in the content. However, when a large amount of Al exceeding 0.10% by weight is contained, oxide inclusions increase and workability and surface properties deteriorate. N: 0.007% by weight or less N is an unavoidable component and is fixed with Ti or the like.
However, when a large amount of N exceeding 0.007% by weight is contained, an increase in the amount of Ti, Nb, etc. necessary for fixing N is required,
Workability is degraded due to the increase in precipitates.

Ti: 0.01 to 0.1% by weight C and N, which form a solid solution in steel as an interstitial type, are fixed as carbonitrides and are combined with S to form sulfides. In order to improve the workability by forming sulfides and sufficiently reducing solid solution C and solid solution N, 0.03
A Ti content of 1% by weight or more is required. But,
Even if a large amount of Ti exceeding 0.1% by weight is contained, the effect of improving properties by adding Ti is saturated, and the production cost is rather increased. Nb: 0.01 to 0.1% by weight Carbonitride forming element similar to Ti, and C and N in steel
And has the effect of improving workability. Also, T
When added in combination with i, a composite precipitate is formed, and a relatively large precipitate is formed, thereby improving workability. Such effects become remarkable when the Nb content is 0.01% by weight or more, and is saturated at 0.1% by weight.

Ni: 0.2 to 2.0% by weight An alloy component added as necessary, exhibits an action of preventing hot brittleness caused by Cu, and is particularly effective in a system to which Cu is added. is there. In order to obtain such an effect, the Ni content is required to be at least about 1/3 or more of the Cu content. However, a large amount of Ni exceeding 2.0% by weight
When added, the recrystallized and annealed steel sheet has a hard microstructure, and the workability deteriorates.

B: 0.0002 to 0.002% by weight An alloy component added as necessary, which segregates preferentially at crystal grain boundaries and exhibits an effect of suppressing grain boundary embrittlement caused by P. . It also has the effect of preventing secondary processing cracks during press forming. Such an effect becomes remarkable at a B content of 0.0002% by weight or more. However, 0.002
A large B content exceeding% by weight impairs the growth of crystal grains and causes a reduction in workability. Zr, V: 0.01 to 0.1% by weight An alloy component added as necessary, and has an effect of forming carbonitride and fixing C and N. Also, Ti,
When combined with Nb, it also has the effect of further improving workability. These effects can be obtained by using 0.01% by weight or more of Zr.
And / or V content, but saturates at 0.1% by weight.

Hot rolling condition: finishing rolling temperature Ar ₃ transformation point or higher, winding temperature 650 to 800 ° C. In the present invention, both continuous casting slabs and bulk slabs can be used. Further, the hot slab after the continuous casting or the ingot may be directly conveyed to the hot rolling step, or may be reheated before the hot rolling step. The reheating temperature is to be set according to the steel composition, required characteristics, and the like, and is not particularly specified in the present invention.
Low temperature heating around 100 ° C. is preferred. Hot rolling is performed at a finish rolling temperature of the Ar ₃ transformation point or higher. When the finish rolling temperature is lower than the Ar ₃ transformation point, not only a hot-rolled texture disadvantageous to the Rankford value is formed, but also
It becomes difficult to wind up in the temperature range specified in the present invention. Hot rolled steel sheet is relatively high 650-800 ° C
It is wound up in the temperature range. By setting the winding temperature as high as 650 ° C. or higher, elongation and Rankford value are improved by actions such as coarsening of precipitates such as Ti-based carbides. Further, when performing mechanical descale in combination with cold rolling, the scale thickness increases within a certain range due to the progress of oxidation after winding, and the descalability is improved.
However, in high-temperature winding at a temperature higher than 800 ° C., the scale thickness becomes too large, and even if cold rolling is performed before pickling in a subsequent process, the descalability is significantly deteriorated.

Cold Rolling: Cold Rolling Rate 10-60% A hot rolled coil wound at a temperature of 650-800 ° C.
Oxidation proceeds after winding, so that descaling becomes difficult only by ordinary pickling or pickling combined with a tension leveler or the like. For this reason, the surface quality of the product steel sheet is greatly deteriorated by scale remaining after the pickling, or it is necessary to remarkably reduce the sheet passing speed at the time of pickling in order to obtain a sufficient descale, thereby lowering productivity. In the present invention,
In order to obtain a product having good surface quality without lowering the productivity, cold rolling is performed before pickling, and the scale is delaminated while mechanically pulverizing. As a result, descaling is sufficiently performed under ordinary pickling conditions. In order to improve the descalability by pickling, it is necessary to cold-roll a hot-rolled steel sheet at a cold-rolling rate of 10% or more. A cold rolling reduction of 10% or more is also effective in improving product elongation and Rankford value. However, in the cold rolling in which the cold rolling reduction exceeds 60%, although the elongation and the Rankford value are expected to be improved, the improvement in the descalability corresponding to the increase in the cold rolling reduction is not observed. In addition, the load on the cold rolling mill increases, which causes an increase in manufacturing cost.

Pickling Cold rolling partially removes scale from the steel sheet surface. Particularly, in the case of cold rolling at a large cold rolling reduction, the scale removal rate is high. However, the descale is not complete only by cold rolling, and the scale remains on the steel sheet surface. Since the surface quality of the product is deteriorated as it is, the cold-rolled coil is passed through a pickling tank, and the scale is sufficiently removed by pickling.

The cold rolling ratio in the present invention is obtained by subtracting the thickness of the final product after cold rolling from the thickness of the hot rolled steel sheet,
It is calculated by the value divided by the thickness of the hot-rolled steel sheet.

Annealing after cold rolling: 90 at or above the recrystallization temperature
0 ° C. or less The cold-rolled steel strip is work-hardened and has extremely low workability. Therefore, annealing is performed at a recrystallization temperature or higher to obtain the deep drawability required for a steel sheet.
The annealing method is set according to the steel composition, required characteristics, and the like, and any of continuous annealing and box annealing can be employed. However, in any of the annealing methods, at an annealing temperature exceeding 900 ° C., α → γ transformation occurs and the crystal orientation is randomized, so that the deep drawability is significantly deteriorated. The steel sheet manufactured in this way is also used as a plating base sheet for electroplating, hot-dip plating, vapor deposition plating and the like. Also in this case, a plated steel sheet having excellent workability can be obtained. In the specification of the present application, "steel plate" is used in the sense of including the use as a plating base sheet of this kind.

For example, a hot-dip coated steel sheet is manufactured by applying hot-dip plating made of Zn, Al or an alloy thereof. In the hot-dip plating equipment, the steel sheet before being immersed in the plating bath is heated to the recrystallization temperature or higher, and the same effect as the above-described annealing of the steel sheet is obtained. However, in any of the annealing methods, α → γ at an annealing temperature exceeding 900 ° C.
Since transformation occurs and the crystal orientation is randomized, the workability is significantly deteriorated. Also in this case, the annealing conditions, the plating conditions, and the like are not particularly limited, and conditions that are generally employed industrially are selected.

[0023]

EXAMPLES Example 1 Steel having the composition shown in Table 1 was melted in an electric furnace to obtain a 50 kg ingot.

[0024]

[Table 1]

Each ingot was hot forged into a 35 mm thick steel slab, heated to 1130 ° C., and then hot rolled. The finishing temperature of the hot rolling was set in the range of 890 to 950 ° C. so that any steel had an Ar ₃ transformation point or higher. The finished sheet thickness takes into account the rolling rate in the subsequent cold rolling process,
It was set in the range of 2.2 to 7.0 mm. After hot rolling,
Charge into a salt bath furnace heated to 475-725 ° C,
By maintaining at a predetermined temperature for 2 hours, a process corresponding to winding of a hot-rolled steel sheet was performed. Next, the cold rolling rate is 5-45%.
And then descaled by pickling. Further, the steel sheet was annealed to be heated to a temperature not lower than the recrystallization temperature and not higher than 900 ° C. The production conditions at this time are shown in Table 2 for each steel type.

[0026]

[Table 2]

Table 2 shows the results of the determination of the mechanical properties and surface skin of the annealed steel sheet. The mechanical properties were measured using JIS No. 5 tensile test pieces. Rankford value is 15
After giving the% tensile pre-strain, it is measured by the three-point method, and the average value in the L direction (rolling direction), T direction (direction inclined at 45 degrees to the rolling direction) and D direction (direction orthogonal to the rolling direction) ( rL +
2rT + rD) / 4. The surface skin was judged by visual observation, and the case where the scale was completely removed was evaluated as good, and the case where one or more scale residues were recognized was evaluated as poor. In addition, as the embrittlement temperature for evaluating the secondary work cracking property, the temperature measured as follows was used. That is, a blank punched to a diameter of 90 mm is formed into a flat-bottomed cylindrical cup having a diameter of 33 mm by three-stage drawing with a drawing ratio of 2.7 at three stages, and immersed in refrigerants of various temperatures consisting of liquid nitrogen and an organic solvent, while having a tip angle of 60 mm. The punch was pushed in from the top of the cylinder, and the lowest temperature at which no brittle cracking occurred was measured.

As can be seen from the investigation results in Table 2, the steel sheets satisfying the steel composition and the manufacturing conditions specified in the present invention have good surface properties, high elongation and Rankford value. It turns out that it is excellent in deep drawability and workability. On the other hand, those using steel types O and P whose steel compositions are out of the range specified in the present invention exhibited low elongation, Rankford value, and high secondary working crack temperature. Further, even if the composition satisfies the conditions of the present invention, if the manufacturing conditions are out of the range specified in the present invention, any one of the surface properties, elongation, Rankford value, and secondary working crack temperature is used. Or several showed bad values. From this, it was confirmed that a steel sheet having excellent surface properties and workability can be manufactured by combining steel compositions and manufacturing conditions in a specified manner.

Example 2 A steel having a composition shown in Table 3 was refined in a converter and a degassing furnace, and was continuously cast to a thickness of 250 mm.
A slab with a unit weight of 13 tons was manufactured.

[0030]

[Table 3]

After each slab was reheated to 1130 ° C. in a heating furnace, it was hot-rolled in a hot rolling mill, and finished to a sheet thickness of 1.2 to 7.7 mm at a finishing temperature in the range of 880 to 940 ° C.
Next, the hot-rolled steel sheet was wound around a coil in a temperature range of 450 to 725 ° C. The hot-rolled steel sheet was cold-rolled at a cold-rolling rate of 5 to 45%, and then passed through a continuous pickling line having a hydrochloric acid-based pickling solution tank to be descaled. Next, this steel sheet was passed through an annealing line and subjected to annealing in which the steel sheet was heated to a recrystallization temperature or higher to obtain a steel sheet as a product. Table 4 shows the production conditions and the properties of the obtained steel sheet at this time.

[0032]

[Table 4]

As is clear from Table 4, even in the case of steel sheets manufactured from the same steel material, the test pieces 23, 28 and 30 in which the manufacturing conditions specified in the present invention are not satisfied, have low elongation,
It showed a Rankford value and a high secondary cracking temperature.
In addition, the surface skin was poor, and the productivity decreased due to the decrease in the pickling rate. On the other hand, it can be seen that the steel sheets manufactured according to the present invention have good surface properties, and have high elongation and Rankford value, and thus are excellent in deep drawability and workability.

[0034]

As described above, in the present invention, in order to improve the surface properties, deep drawability, and workability of a steel sheet, hot rolling is performed by setting the winding temperature to a relatively high temperature. Before pickling the obtained hot-rolled steel sheet, cold rolling is performed at a specific cold-rolling rate. The peelability of the scale is improved by cold rolling before pickling, and even in the case of a hot-rolled steel sheet in which the winding temperature is set to a relatively high temperature, productivity does not deteriorate in the descale process due to pickling, A high-strength steel sheet of a Cu precipitation strengthened type having excellent surface properties and workability can be manufactured.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification FI theme coat ゛ (reference) C22C 38/16 C22C 38/16 F term (reference) 4K032 AA01 AA02 AA04 AA14 AA15 AA16 AA17 AA21 AA22 AA23 AA24 AA27 AA29 AA35 AA36 AA39 CC04 CE02 CG01 CG02 CH04 CH05 4K037 EA01 EA02 EA04 EA13 EA15 EA16 EA18 EA19 EA20 EA23 EA25 EA27 EA28 EA31 EA32 EA35 EB02 EB03 EB06 EB09 FC05 FE03 FG01 FG03

Claims (4)

[Claims] 1. C: 0.0005 to 0.01% by weight, S
i: 2.0% by weight or less, Mn: 0.05 to 3.0% by weight, P: 0.2% by weight or less, S: 0.0005 to 0.0
2% by weight, Cu: 0.5 to 3.0% by weight, acid-soluble Al:
A steel slab having a composition containing 0.005 to 0.10% by weight and N: 0.007% by weight or less is reheated or directly fed, and the finish rolling temperature is higher than the transformation point of Ar ₃ and the winding temperature is 650 to 800.
° C hot rolling, and the resulting hot-rolled steel strip is subjected to a cold rolling rate of 10 to 10.
A method for producing a steel sheet having excellent surface properties and workability, comprising performing 60% cold rolling, pickling, and annealing the obtained steel sheet at a recrystallization temperature of 900 ° C. or less. 2. The steel slab according to claim 1, further comprising:
i: 0.01 to 0.1% by weight and / or Nb: 0.01
A method for producing a steel sheet having excellent surface properties and workability using a steel slab having a composition containing 0.1 to 0.1% by weight. 3. The steel slab according to claim 1 or 2,
Further, a method for producing a steel sheet having excellent surface properties and workability using a steel slab having a composition containing Ni: 0.2 to 2.0% by weight. 4. The steel slab according to claim 1, further comprising: B: 0.0002 to 0.002% by weight, Zr: 0.0
A method for producing a steel sheet having excellent surface properties and workability using a steel slab having a composition containing one or more kinds of 1 to 0.1% by weight, V: 0.01 to 0.1% by weight.