JPH11236657A - Production of plated steel strip for structural purpose - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a hot dip plated steel strip having sufficient strength as a plated steel for structural purpose on a class of 400 to 540 N/mm<2> and excellent in workability at a low cost. SOLUTION: A steel having a compsn. contg., by weight, 0.01 to 0.20% C, <=0.5% Si, 0.05 to 1.0% Mn, <=0.05% P, <=0.02% S and 0.005 to 0.10% acid soluble Al, and the balance substantially Fe is subjected to continuous casting and is subjected to hot rolling so as to regulate the finish rolling temp. to the Ar3 transformation point or above and the coiling temp. to <=750 deg.C. Next, before pickling, the steel is subjected to cold rolling at a cold rolling ratio of 5 to 50%, is subjected to preannealing at less than the recrystallization temp. of 480 to 650 deg.C in a continuous hot dip plating line and is thereafter introduced into a hot dip plating bath.

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 hot-dip galvanized steel strip having excellent strength and workability, which is used for building materials and the like.

[0002]

2. Description of the Related Art Thin steel sheets used as building structural members, such as lightweight shaped steel, have a predetermined strength and are excellent in corrosion resistance. Characteristics such as excellent workability are required.
Further, in order to secure the dimensional accuracy of the molded product, a steel sheet having good thickness accuracy is desired. As this kind of steel sheet, 400 to 490 N based on a medium carbon Al killed hot rolled steel sheet is used.
/ Mm ² class hot-dip galvanized steel sheet, hot-dip zinc-aluminum alloy coated steel sheet, and the like are used. Although these hot-dip coated steel sheets have properties that satisfy required properties in terms of strength, workability, and corrosion resistance, they have not always been sufficiently satisfactory in terms of sheet thickness accuracy.

[0003]

When a cold-rolled steel sheet is used as a plating base material, very excellent properties can be obtained in terms of workability and thickness accuracy, but the cost is higher than when a hot-rolled steel sheet is used as a material. Get high. In addition, the conventional hot-dip cold-rolled steel sheet requires a heat treatment of being cold-rolled at a high cold-rolling rate of more than 60% and then heating to a temperature higher than the recrystallization temperature before the plating treatment. Consumes energy. As a result, manufacturing costs are further increased. The present invention has been devised to solve such a problem, and adopts cold rolling before pickling and low-temperature annealing before hot-dip coating to obtain sufficient strength and working as a plated steel sheet for a structure. With low cost compared to ordinary hot-dip cold-rolled steel strip,
400 with excellent thickness accuracy compared to hot-dip hot-rolled steel strip
It is intended to obtain a hot-dip coated steel strip of ５540 N / mm ² class.

[0004]

In order to achieve the object of the present invention, C: 0.01 to 0.20% by weight, Si: 0.5% by weight or less, and Mn: 0.05 to 50% by weight. 1.0
% By weight, P: 0.05% by weight or less, S: 0.02% by weight
Hereinafter, steel containing acid-soluble Al: 0.005 to 0.10% by weight and the balance substantially having a composition of Fe is continuously cast,
It is characterized by performing cold rolling before pickling after hot rolling, and then performing pre-annealing at a temperature lower than the recrystallization temperature in a continuous hot-dip plating line. Hot rolling is preferably performed at a finish rolling temperature equal to or higher than the Ar ₃ transformation point and a winding temperature equal to or lower than 750 ° C. The hot-rolled steel strip before pickling is preferably a cold-rolling rate of 5 to 50%.
Cold rolled. The steel strip passed through the continuous hot-dip plating line is 480-650 before being immersed in the hot-dip plating bath.
Annealed at a temperature of less than the recrystallization temperature.

[0005]

When the hot-rolled steel strip before pickling is cold-rolled, the oxide scale is pulverized and the pickling efficiency is improved. Therefore, the line speed of the continuous pickling line can be increased. In addition, the amount of heat energy consumed is reduced by regulating the temperature conditions of hot rolling and pre-plating annealing. as a result,
The manufacturing cost is considerably lower than that of a normal hot-dip galvanized cold-rolled steel sheet, and a hot-dip steel strip excellent in sheet thickness accuracy can be obtained.

Hereinafter, alloy components, contents, manufacturing conditions, and the like specified in the present invention will be described. C: 0.01 to 0.20% by weight An alloy component effective for improving the strength of the steel strip, and its effect is seen at 0.01% by weight or more. Further, the C content is set to 0.0
Reduction to less than 1% by weight requires special treatment such as vacuum degassing for extremely low carbon. However, when a large amount of C exceeding 0.20% by weight is added, ductility and weld toughness tend to decrease. Si: 0.5% by weight or less Like C, it is an alloy component effective for improving the strength of a steel strip.
However, when a large amount of Si exceeding 0.5% by weight is added, the plating property in the hot-dip plating line is deteriorated, and defects such as non-plating tend to occur.

Mn: 0.05-1.0% by weight S contained in steel as impurities is fixed as MnS,
It is an alloy component that prevents high-temperature embrittlement due to S, and effectively acts to improve the strength of a steel strip. From the viewpoint of preventing high-temperature embrittlement, a Mn content of at least 0.05% by weight or more is required. However, when a large amount of Mn exceeding 1.5% by weight is added, ductility is greatly deteriorated as compared with an increase in strength, and plating properties are also deteriorated. P: 0.05% by weight or less An alloy component effective for improving the strength of a steel sheet. However, 0.
When a large amount of P exceeding 05% by weight is contained, the low-temperature toughness tends to deteriorate.

S: not more than 0.02% by weight A harmful element which causes embrittlement at high temperatures and deteriorates ductility at room temperature. The lower the lower, the better. However, ultra-low S requires strict selection of raw materials and strict control of refining conditions, which causes an increase in manufacturing cost. Therefore, in the present invention, the upper limit of the S content is set to 0.02% by weight within a range where the adverse effect due to S does not appear. Acid-soluble Al: 0.005 to 0.10% by weight It is an alloy component added as a deoxidizing agent. To obtain a sufficient deoxidizing effect, 0.005% by weight or more of acid-soluble Al must be added. is necessary. The effect of Al deoxidation saturates at 0.10% by weight, and even if it is added more than that, the cost of steel material will increase.

Hot rolling: finishing temperature not less than Ar ₃ transformation point,
Winding temperature of 750 ° C. or less The finishing temperature at the time of hot rolling was set to an Ar ₃ transformation point or more from the viewpoint of securing the thickness accuracy. If the finishing temperature does not reach the Ar ₃ transformation point, the change in hot strength accompanying the transformation is large, and the production troubles such as gauge hunting and width drawing in which the sheet thickness fluctuates greatly in the rolling direction are likely to occur, and It also causes the thickness accuracy to deteriorate. In order to improve ductility, a higher winding temperature is more advantageous. However, 750 ° C
If the coiling temperature exceeds the above, not only the wound coil is deformed by its own weight, but it becomes difficult not only to pass through the process after hot rolling but also to deteriorate the shape accuracy of the manufactured steel strip.

[0010] Cold rolling before pickling: Cold rolling rate of 5 to 50% If the hot-rolled steel strip is cold-rolled prior to pickling, the accuracy of thickness is improved and the surface scale is pulverized. Will be better. The effect of improving pickling properties becomes remarkable at a cold rolling reduction of 5% or more. Cold rolling before pickling is also effective in strengthening the work of the steel strip. However, when the cold rolling ratio exceeds 50%, although the strength is increased by the work strengthening, the ductility is significantly deteriorated.

Annealing before plating: 480 to 650 ° C., less than recrystallization temperature The cold-rolled steel strip is passed through a pickling process to a continuous hot-dip galvanizing line. In a continuous hot-dip galvanizing line, a steel strip is introduced into a hot-dip bath after performing pre-plating annealing in a reducing atmosphere gas at a temperature of 480 to 650 ° C. and a temperature lower than a recrystallization temperature. At this time, the annealing temperature is 480 ° C.
If not reached, the surface of the steel strip is not sufficiently reduced, so that non-plating is likely to occur. However, at an annealing temperature exceeding 650 ° C., the temperature condition is the same as that of a normal hot-dip cold-rolled steel sheet, and the effects of energy saving and cost saving targeted by the present invention cannot be obtained. Further, when annealing is performed at a temperature higher than the recrystallization temperature, the steel strip is softened, and the strength given by cold rolling before pickling decreases.

[0012]

EXAMPLES Example 1 Various steel materials shown in Table 1 were continuously cast and then hot-rolled at a finishing temperature of 900 ° C. and a winding temperature of 580 ° C. to obtain a hot-rolled steel having a thickness of 2.3 to 5.75 mm. Obi was manufactured. A part of the obtained hot-rolled steel strip was subjected to cold rolling at a cold-rolling rate of 65% or less to obtain a steel strip having a thickness of 2.3 mm, followed by pickling. The remaining hot-rolled steel strip was sent to the pickling step without cold rolling. In pickling, a continuous pickling line is used for 9
The steel strip was passed through a 9% HCl aqueous solution maintained at 0 ° C.
The line speed was changed at the time of passing, and the immersion time of the steel strip in the aqueous HCl solution was changed, and the removal state of the hot-rolled scale was visually observed.

[0013]

Each of the pickled steel strips is sent to a continuous hot-dip galvanizing line, subjected to pre-plating annealing at 600 ° C., and then Zn-5% A
Then, the steel strip was immersed in a single bath to produce a hot-dip zinc-aluminum alloy-plated steel strip having a basis weight of 90 g / m ² per side. The tensile properties, sheet thickness deviation, weld toughness, pickling properties of hot-rolled sheet pickling and hot-dip plating properties of the obtained hot-dip coated steel strip were investigated, and the results of the investigation were compared with those of cold rolling before pickling. The results are shown in Table 2 in relation to the cold rolling reduction. In the tensile test, a JIS No. 5 test piece sampled in parallel with the rolling direction was used, and the tensile strength, 0.2% proof stress,
The mechanical properties such as elongation were investigated. Regarding the toughness of the weld, the test piece was butt-welded using a MAG welding machine, and the excess portion of the weld bead was ground and removed, and then a 2 mm U cut out in a direction perpendicular to the welding direction centering on the weld heat affected zone.
A notched impact test piece (JIS No. 4 subsize) was prepared. This impact test piece was subjected to a Charpy impact test,
The Charpy impact value was measured. When the Charpy impact value is 50 J / cm ² or more, the toughness of the weld is good.
Less than cm ² was evaluated as poor weld toughness.

Regarding pickling properties, scale removal was possible even if the line speed was increased by 30% or more compared to a steel strip which was not cold-rolled before pickling. The pickling performance was evaluated as good, and the one with less than 30% of the unlined line speed of the scale removal name was evaluated as "no pickling performance improvement". Regarding the plating properties, the surface appearance of the obtained hot-dip galvanized steel strip was visually observed, and those in which no plating was not detected were evaluated as good 性, and those in which no plating was detected were evaluated as poor x. In the visual observation, non-plating with a diameter of several hundred μm to several mm is detected, and in the case of assembled non-plating, even non-plating in units of several μm is detected. Thickness accuracy is measured by measuring the thickness of each part of the hot-dip steel strip. If the thickness deviation is within ± 50 μm, the sheet thickness is good. ○ If the thickness deviation exceeds ± 50 μm, The thickness accuracy was evaluated as poor.

[0016]

As can be seen from the survey results in Table 2, the plated steel strip manufactured under the conditions specified in the present invention was 400 to 54 strips.
In addition to having sufficient workability required as a 0N / mm ² class plated steel sheet for general structural use, it was excellent in plate thickness accuracy and weld toughness. In addition, it was found that the pickling property and the plating property were good and the production was easy. On the other hand, the comparative material which had not been subjected to cold rolling at a cold rolling rate of 5% or more before pickling was inferior in thickness accuracy and pickling properties despite the use of the same steel material. However, in the case of cold rolling under high pressure exceeding the cold rolling reduction specified in the present invention, 400 to 540 N
It had too high strength and was inferior in ductility as a plated steel sheet for general structural use in the class of / mm ² . Further, in the hot-dip coated steel sheet obtained from steel type No. 5 containing excess C, although the tensile properties, surface properties, and the like were good, the toughness of the weld was inferior. The hot-dip coated steel sheet obtained from steel type No. 6 containing excessive Si had good tensile properties, weld toughness, etc., but had no plating. In the hot-dip coated steel sheet obtained from steel type No. 7 containing excessive Mn,
The tensile properties and pickling properties were good, but the plating properties and weld toughness were poor.

Example 2 After continuously casting steels of steel types 1 and 2, hot rolling was performed at a finishing temperature of 900 ° C. and a winding temperature of 580 ° C. to produce a hot-rolled steel strip having a sheet thickness of 2.88 mm. After the thickness of the hot-rolled steel strip was set to 2.3 mm by cold rolling at a cold rolling rate of 20%, the strip was passed through a continuous hot-dip galvanizing line via an acid washing step. In the continuous hot-dip galvanizing line, pre-plating annealing was performed while changing the annealing temperature in the range of 450 to 700 ° C., and then hot-dip Zn-5% Al plating with a basis weight of 90 g / m ² per side was applied. The tensile properties, thickness deviations, weld toughness, pickling properties during hot-rolled sheet pickling and hot-dip plating properties of the resulting hot-dip steel strip were investigated, and the results were compared with the annealing temperature of pre-plating annealing. Table 3 shows the relation.

[0019]

As can be seen from the results of the examination in Table 3, the steels annealed before plating under the temperature conditions specified in the present invention exhibited good tensile properties and good plating properties. On the other hand, when the annealing temperature was as low as 450 ° C. during the pre-plating annealing, non-plating occurred and it could not be used as a plated steel sheet for a structure. Conversely, when the annealing temperature was as high as 700 ° C., the material was greatly softened by recrystallization in both steel type Nos. 1 and ² , and a steel strip having a target value of 400 N / mm ² or more could not be obtained.

[0021]

As described above, in the present invention, by combining cold rolling before pickling and annealing at a relatively low temperature before plating, the sheet thickness can be reduced as compared with the hot-rolled hot-rolled steel strip. excellent accuracy, in comparison to low manufacturing cost conventional melt plated cold rolled steel strip has a sufficient strength as a structural plated steel sheet 400~540N / mm ² class, dip plated steel strip is excellent in workability Manufactured.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Mikio Soshita 11-1, Showa-cho, Kure-city, Hiroshima Pref.

Claims (4)

[Claims] C: 0.01 to 0.20% by weight, Si:
0.5% by weight or less, Mn: 0.05 to 1.0% by weight,
Continuously cast steel containing P: 0.05% by weight or less, S: 0.02% by weight or less, acid-soluble Al: 0.005 to 0.10% by weight, and the balance being substantially Fe. A method for producing a plated steel strip for a structure, comprising: performing cold rolling before pickling after hot rolling, and then performing pre-annealing at a temperature lower than a recrystallization temperature in a continuous hot-dip galvanizing line. 2. The method according to claim 1, wherein the hot rolling is performed at a finish rolling temperature not lower than the Ar ₃ transformation point and a winding temperature not higher than 750 ° C. 3. The method according to claim 1, wherein cold rolling is performed at a cold rolling rate of 5 to 50% before pickling. 4. The method according to claim 1, wherein the steel strip passed through the continuous hot-dip galvanizing line is subjected to pre-plating annealing at 480 to 650 ° C. and lower than the recrystallization temperature.