JP3063010B2 - Method for producing hot-rolled Zn-based hot-dip steel sheet having excellent plating surface properties and plating adhesion - Google Patents

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 obtained by subjecting a hot-rolled steel sheet to hot dip galvanizing.

[0002]

2. Description of the Related Art Generally, a hot-rolled Zn-based hot-rolled steel sheet is wound into a coil after hot rolling, cooled naturally to a temperature of less than 100 ° C. for a long time, and then cooled to 80 ° C. in a pickling line.
And then processed in a hot-dip plating line. In the case of a Zn-based hot-dip cold-rolled steel sheet, cold rolling is performed between pickling and hot-dip plating.
Further, as a hot-dip plating line, foreign matters called smut or the like (Fe oxide, Fe + silicate, etc.) adhered to the surface of the steel sheet during the pickling step and the subsequent transportation during heating in an oxidizing atmosphere gas to about 700 to 800 ° C. ), By burning off surface contaminants such as oils and fats, and further reducing in a hydrogen atmosphere gas to ensure the surface cleanliness necessary for plating adhesion, and then near the plating bath temperature in a non-oxidizing or reducing atmosphere. The so-called Zenzimer method of hot-dip plating by cooling to a temperature of about 700 to 800 ° C. in a hydrogen atmosphere gas, and a foreign matter (Fe oxide, Fe
+ Silicate, etc.), removes surface contaminants such as oils and fats, secures the surface cleanliness necessary for plating adhesion, and then cools to near the plating bath temperature in a non-oxidizing or reducing atmosphere, so-called hot-dip plating The non-oxidizing furnace method is adopted.

[0003] In the case of hot-rolled Zn-based hot-rolled steel sheets, cold rolling is not performed before plating, so that the quality of the hot-rolled steel sheet is adjusted.
Although it is not necessary to perform recrystallization annealing at about 00 to 800 ° C.,
Removal of foreign substances (Fe oxides, Fe + silicates, etc.) such as smut adhered to the surface of the steel sheet during the pickling process and subsequent transportation, and surface dirt such as oils and fats by burning and reduction, and surface cleaning necessary for plating adhesion. 700 to ensure
Heating to about 800 ° C. is inevitable.

[0004]

However, heating from a low temperature of less than 100.degree. C. to a high temperature of about 700 to 800.degree. C. in order to secure surface cleanliness required for plating adhesion requires enormous energy and long heating. Need a furnace,
Large operating cost burden. Further, thereafter, cooling to a temperature near the plating bath temperature (about 500 ° C.) which is an intermediate temperature between the two is a great loss of thermal energy. Further, in a high-temperature hydrogen atmosphere, hydrogen is occluded in the steel sheet, and after plating, this hydrogen is released to the interface between the steel sheet and the plating, thereby causing a blister-like defect on the plating surface. Also, when heated to a high temperature, tempering of the transformed structure, re-solid solution of precipitates, grain growth, etc. occur in a short time, deterioration of steel sheet material,
It has the disadvantage of causing fluctuations. As a method of solving the above problem, for example, Ni or Ni
Although a method of coating a system alloy (JP-A-61-44168) is disclosed, it is possible to reduce the heating temperature, but since the pretreatment temperature is 150 ° C. or less, it is necessary to cool the coil once. In addition, not only can the number of days required after winding be reduced, but also the heat retained in the coil cannot be used, energy loss still persists, and operating costs increase due to an increase in the number of pretreatment steps. Further, in order to avoid blistering defects on the plating surface (which leads to deterioration of peeling resistance and corrosion resistance as well as deterioration of appearance), a method of reducing the heating temperature to 600 to 720 ° C. (Japanese Patent Laid-Open No. 52-9)
No. 5543) is disclosed, however, since the temperature of the steel sheet temporarily drops to less than 100 ° C. due to cleaning with an acid, not only the number of days required after winding can not be shortened, but also the heat retained in the coil cannot be used. However, the heating temperature of 600 to 720 ° C. still requires enormous energy and a long heating furnace, so that the operation cost reduction effect is small. Further, tempering of the transformed structure, re-solid solution of precipitates, grain growth, etc. However, deterioration and fluctuation of the steel sheet material cannot be avoided since the occurrence still occurs.

[0005]

SUMMARY OF THE INVENTION The present invention utilizes the heat retained in a coil to continuously perform dry descaling, temperature adjustment, and the like.
By performing hot-dip plating and adjusting the atmosphere to non-oxidizing or reducing at least after the completion of dry descaling, the thermal energy required for temperature adjustment of the steel sheet
Minimize equipment, shorten the time required after winding, reduce operating cost, and avoid blister-like defects on the plating surface, waist breaks, deterioration and fluctuation of the steel sheet material while maintaining good plating adhesion. It is a manufacturing method that enables

[0006]

Hereinafter, the present invention will be described in detail.

(1) Chemical composition of steel sheet In the present invention, the chemical composition of the steel sheet is not particularly limited, and can be appropriately selected depending on the required processing characteristics and applications. Low-carbon mild steel containing Mn and Al as basic components, and Nb,
High tensile steel to which Ti, Cr and P are added can be used.

(2) Rolled Material In the present invention, the rolled material is not particularly limited, and for example, a normal continuous cast slab or a thin continuous cast slab can be applied. Further, application of a so-called direct rolling (DR) method is also possible.

(3) Hot Rolling Conditions In the present invention, the hot rolling conditions to be limited are the lower limit of the winding temperature. The upper limit of the winding temperature and other conditions (for example, heating temperature, hot rolling reduction, cooling conditions in a hot run table) can be appropriately selected depending on the required processing characteristics and applications.

The reason for limiting the lower limit of the winding temperature will be described below. If the winding temperature is lower than 350 ° C., a distorted pattern called buckling is generated at the time of rewinding, which impairs the appearance quality after plating, makes it impossible to effectively use the heat retained by the coil, and causes energy loss. Previous 40
Equipment required for temperature adjustment to 0 to 600 ° C becomes long,
The effect of reducing equipment costs cannot be enjoyed. 350 ° C
400-600 ° C before plating when wound at less than
The temperature adjustment to 350 ° C. or more may cause deterioration and fluctuation of the steel sheet material due to tempering of the transformed structure. In order to increase the descaling efficiency in the post-process, it is also possible to adopt a process condition for reducing the scale thickness. For example, rolling in an inert atmosphere such as Ar or N ₂ , between rolling stands with cooling water containing a solvent having a scale formation suppressing action, cooling on a hot run table,
It is possible to adopt cooling of the wound coil in an N ₂ seal atmosphere BOX.

(4) Descaling Conditions Descaling is limited to dry descaling without using an acid solution. As a result, foreign substances (Fe oxide,
Since the generation of Fe + silicate or the like can be avoided, it is possible to ensure surface cleanliness required for plating adhesion without heating and removing foreign matter. As a specific dry descaling method, a vacuum arc (10 ^-1 to 10 ^-6)
Torr), plasma, reaction (reduction), magnetic polishing (number +
Micron to several hundred microns of magnetic powder), shot blast, sand blast, grid blast, wire brush, grinder and the like can be used alone or in combination.

In order to maintain the surface cleanliness required for plating adhesion obtained by dry descaling, at least after dry descaling is completed, an inert gas atmosphere such as argon or nitrogen, or a mixture of inert gas and hydrogen is used. It is necessary to maintain a non-oxidizing or reducing atmosphere such as an atmosphere or a hydrogen atmosphere. It goes without saying that dry descaling may be performed in a non-oxidizing or reducing atmosphere.

In particular, it is desirable to use a reducing atmosphere for a difficult-to-plate material containing a large amount of Si or the like.

Further, dry descaling must be performed at 350 ° C. or higher. The reasons for the limitation are described below.

If the dry descaling temperature is lower than 350 ° C., a distortion pattern called waist break occurs due to deformation of the steel sheet due to bending and returning during passing, and the appearance quality is impaired. In addition, the scale is degraded due to the biting of the scale due to the hip break. Furthermore, since the heat retained in the coil cannot be used effectively and energy is lost,
The equipment required for adjusting the sheet temperature before plating to 600 ° C. becomes long, and the effect of reducing equipment cost cannot be enjoyed.

(5) Process conditions after dry descaling In order to maintain the surface cleanliness required for plating adhesion obtained by dry descaling, after dry descaling, continuously in a non-oxidizing or reducing atmosphere. At 400
~ 600 ° C Pre-plating temperature adjustment and hot-dip plating. In particular, it is desirable to use a reducing atmosphere for difficult-to-plate materials containing a large amount of Si or the like. The reasons for limiting the sheet temperature before plating are described below. If the temperature is lower than 400 ° C., so-called “wettability” cannot be secured, and non-plating or plating adhesion is deteriorated. Further, when the temperature is lower than 350 ° C., a distortion pattern called waist break is generated due to deformation of the steel sheet due to bending and bending back at the time of threading, and the appearance quality may be impaired. On the other hand, when the temperature exceeds 600 ° C., tempering of the transformed structure, re-dissolution of precipitates, deterioration and fluctuation of the steel sheet material due to grain growth, and hydrogen in the reducing atmosphere are easily absorbed into the steel sheet. In addition, blistering defects on the plating surface are likely to occur. Further, the alloying reaction between Zn and Fe excessively proceeds,
(4) A brittle plating layer such as a phase appears and deteriorates plating adhesion. Further, from the viewpoint of energy cost and equipment cost required for temperature adjustment, if the temperature exceeds 600 ° C., the loss becomes large. Further, according to the present manufacturing method, since the material is formed in the hot rolling step, it goes without saying that recrystallization annealing for adjusting the material is not necessary.

After completion of plating, required characteristics
Various post-treatments such as a skin pass, a chromate treatment, a bond treatment, and a painting can be appropriately selected depending on the application.

[0018]

EXAMPLE A slab having the chemical composition shown in Table 1 was manufactured by continuous casting, and a coil was manufactured under the hot rolling conditions shown in Table 2.
Dry descaling and hot-dip plating were performed under the following conditions.

[0019]

[Table 1]

[0020]

[Table 2]

(1) Example 1 Using coils manufactured under the conditions shown in Tables 1 and 2, dry descaling and hot-dip plating were performed using a vacuum arc, and the plating adhesion was measured using a DUPONT impact tester. The surface bulging defects and hip break were visually observed, and the material deterioration was evaluated by a tensile test. In addition, evaluation is 30 after plating.
The test was performed after the passage of days. Table 3 shows the results. Steel symbol A
It shows excellent plating adhesion to various component systems of ~ E, and no blister-like defects, waist breaks, and material deterioration are observed.
Good plated products were obtained.

Further, the bare corrosion resistance and the coating corrosion resistance were evaluated by a salt spray test, the chemical conversion treatment property was evaluated by a conversion film adhesion amount, and the coating adhesion was evaluated by an Erichsen test, and all properties were good.

[0023]

[Table 3] Plating bath temperature: 470 ° C., pre-plating plate temperature: 480 ° C. Plating bath component: Zn-0.2% Al Atmosphere: 80% N ₂ -20% H ₂ Plating weight: 190 g / m ² Dry descaling temperature: 350 ° C

(2) Example 2 Dry descaling using a combination of magnetic polishing and reduction using coils manufactured under the conditions shown in Tables 1 and 2,
Hot-dip plating was performed, and the plating adhesion was evaluated with a DUPONT impact tester by visually inspecting the plating surface for blister-like defects and hip breakage, and the material deterioration was evaluated by a tensile test. The evaluation was performed 30 days after plating. Table 4 shows the results. No. In No. 3, since the dry descaling temperature was too low, the hip was broken at the time of coil rewinding. No. In No. 4, since the sheet temperature before plating was too high, blister-like defects occurred and the material deteriorated. Further, since the alloying of Zn and Fe proceeded excessively, the plating adhesion also deteriorated. No. In the case of No. 6, the plating temperature is too low, so that the plating adhesion is deteriorated,
An unplated portion occurred. No. 7, the winding temperature is too low,
An appropriate descaling temperature could not be secured, resulting in buckling and deterioration of the material. No. 1 and No. 2 and No. 5
Satisfies the conditions of the present invention, exhibits excellent plating adhesion, shows no blister-like defects, breaks in hips, and deteriorates in material, and provides a good plated product. (Legend is the same as Table 3)

[0025]

[Table 4] Plating bath temperature: 470 ° C, Plating bath component: Zn-0.10
% Al atmosphere: 20% N ₂ -80% H ₂ , plating adhesion amount: 90
g / m ²

(3) Example 3 Using the coils manufactured under the conditions shown in Tables 1 and 2, dry descaling and hot-dip plating were performed by vacuum arc, and the plating adhesion was measured using a DUPONT impact tester. The surface bulging defects and hip break were visually observed, and the material deterioration was evaluated by a tensile test. In addition, evaluation is 30 after plating.
The test was performed after the passage of days. Table 5 shows the results. Shows excellent plating adhesion to various bath components,
A good plated product was obtained without any breakage of the waist and deterioration of the material. (Legend is the same as Table 3)

[0027]

[Table 5] Steel symbol: B Descaling temperature: 600 ° C Atmosphere: 100% N ₂ plating coating weight: 150 g / m ²

(4) Embodiment 4 FIG. 1 shows an example of the production equipment of the present invention.

[0029]

According to the present invention, the thermal energy and equipment required for manufacturing a hot-rolled Zn-based hot-rolled steel sheet are minimized, the number of days required after winding is shortened, the operating cost is reduced, and good plating adhesion is obtained. Maintenance and blistering defects on the plating surface
It is possible to avoid bending and deterioration of the steel plate material and fluctuation, and the effect is extremely large in industry.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a production facility of the present invention.

FIG. 2 is a schematic diagram comparing an example of a thermal history of the manufacturing method of the present invention with a conventional method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Hot rolling 2 Hot run table 3 Coil 4 Heating trolley 5 Dry descaling 6 Sheet temperature adjustment before plating 7 Zn-based hot dip plating

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Toshiyuki Higuchi 1 Nishi-no-Su, Oita-shi, Oita Pref. Nippon Steel Corporation Inside Oita Works (72) Inventor Koichiro Tanaka 1-Nishi-No-Su, Oita-shi, Oita Nippon Steel Co., Ltd. Inside the Oita Works (72) Inventor Masami Ogura, Oita, Oita, Nishi-no-Shi, 1 Nippon Steel Corporation Inside Oita Works (72) Inventor Jiro Yamazaki, Oita, Oita, O-shi, 1 Nishi-no-Su, Nippon Steel Corporation Oita Inside the steelworks (72) Inventor Hideo Kato Oita-shi, Oita-shi, Oishi-shi, Oita 1 Nippon Steel Corporation In-house Oita Works (72) Inventor Kazuaki Esaka Oita-shi, Oita-shi, Oaza-shi, Oita 1 Nippon Steel Corporation Oita Inside the steelworks (72) Inventor Junji Hashi Oita, Oita, Oita 1 Osamu Kono Oishi-shi, Oita, Oita Prefecture 1 Nippon Steel Corporation Oita Works (56) References JP-A-54-107434 (JP, A) JP-A-63-312959 (JP, A)

Claims (1)

(57) [Claims] 1. A steel slab is hot-rolled, cooled and then coiled.
Winding, unwinding, dry descaling at 350 ° C or higher, pre-plating temperature adjustment at 400 to 600 ° C , Z
The n type hot dipping is performed continuously in sequence, and, in the method of the least dry descaling after completion to the atmosphere adjusted to a non-oxidizing or reducing, the steel sheet after the cooling
A method for producing a hot-rolled Zn-based hot-rolled steel sheet having excellent plating surface properties and plating adhesion, wherein the hot-rolled steel sheet is coiled and unwound at 350 ° C or higher .