KR20000045512A - Method for producing hot rolled-molten zinc plated steel strip having excellent adherence - Google Patents

Abstract

PURPOSE: A method is provided to produce a hot rolled -molten zinc plated steel strip having excellent plating adherence. CONSTITUTION: A hot rolled steel strip contains 0.5 to 2.0wt% of silicon. The steel strip is cleansed by acid and ground a surface oxidized layer in 0.5 to 2micrometer using a brush roll. Then the steel strip is heated in a temperature of 460 to 550°C and dipped in a plating bath which is composed of 0.20 to 0.25wt% of Al, 0.05 to 0.10wt% of Ni and the remain of Zn and incidental impurities. Then, the hot rolled-molten zinc plated steel strip has excellent plating adherence.

Description

Manufacturing method of high strength hot rolled hot dip galvanized steel sheet with excellent plating adhesion

The present invention relates to a method for manufacturing a high strength hot-dip galvanized steel sheet used for automobile reinforcement, etc. More specifically, a high-strength hot-rolled steel sheet containing a large amount of silicon (Si) is pickled to remove insoluble silicate oxide using a brush roll. The present invention relates to a method for manufacturing a hot-rolled hot-dip galvanized steel sheet having excellent plating adhesion to change the plating bath composition and heat treatment conditions.

Hot-dip galvanized steel sheets manufactured by hot-dip galvanizing after hot-rolled steel sheets are shorter in manufacturing process than hot-dip galvanized steel sheets using cold-rolled material, and thus the production cost is low.

In recent years, the use of automobile reinforcing materials, such as high strength other than the conventional building materials is increasing, so that the plating adhesion of hot-rolled hot-dip galvanized steel sheet is required.

However, when manufacturing high-strength hot-dip galvanized steel sheet, since the steel sheet contains a variety of reinforcing elements, there is a problem that zinc plating properties and other soft materials. For example, large additions of silicon, manganese, and phosphorus, which are major reinforcing elements of steel, greatly degrade plating adhesion. In particular, silicon is a promising component that can maintain ductility without significantly reducing the strength of steel, but when it contains 0.5 wt% or more, silicate scale which is not soluble in acid remains, and the critical concentration of silicon on the surface of the steel sheet during heat treatment before plating As it is concentrated above, the plating adhesion is greatly deteriorated, so countermeasures are urgently needed.

Many reports have been reported on the adverse effects of silicon on the adhesion to the plating, and the method of pre-plating Ni, Fe, FeO, etc. and the oxidation-reduction method in the direct heating furnace are suggested as a method of improving the plating adhesion of silicon-added steel. It is becoming.

However, the former method requires a separate electroplating facility and increases the manufacturing cost. The latter does not require annealing heat treatment in the case of a plating process made of hot rolled steel, so that an indirect heating furnace is used instead of a direct heating furnace. It is hard to adopt as we use. In addition, there is a problem that it is very difficult to properly adjust the thickness of the oxide film by the oxidation-reduction method.

In general, the plating adhesion of the hot-rolled hot-dip galvanized steel sheet containing silicon is greatly affected by the pickling degree and heat treatment conditions of the plating material.

Conventionally, descaling has been performed by a chemical method of pickling a scale formed on a hot rolled steel sheet. In general, in the case of pickling treatment, a pickling treatment using an aqueous hydrochloric acid solution or an aqueous sulfuric acid solution as a pickling solution is generally performed.

However, if hydrochloric acid or sulfuric acid is used alone as the pickling solution under industrially limited pickling conditions, complete descaling is not possible.

Therefore, as a method for improving the plating adhesion of silicon-containing hot-rolled steel sheet, in the Korean patent application 96-62029, which the inventors have previously applied, the hot-rolled steel sheet containing 0.3-1.0 wt% of silicon was deposited in 10-15 vol% aqueous hydrochloric acid solution. After continuous electrolytic etching in an aqueous solution of 30-40 vol% sulfuric acid containing 10-50 g / L of sodium fluoride, the steel sheet was heated to 460-550 ° C. in a heating zone, and then an excellent hot-rolled hot-dip galvanized steel sheet was prepared. The method was proposed.

However, if the silicon content exceeds 1.0% by weight, even though the silicon oxide layer remaining on the pole surface of the steel sheet is completely removed during the pickling process, the surface concentration of silicon during the heat treatment process before plating may exceed the threshold value, thereby causing plating wettability and plating adhesion. This deterioration problem occurred, and the water pollution problem occurred when sodium fluoride was contained in the pickling solution.

Therefore, the present inventors have proposed the present invention based on the results obtained through years of research and field application experiments in order to solve the problems such as deterioration of plating adhesion during the hot dip plating of the silicon-containing steel. An object of the present invention is to provide a method for producing a high strength hot rolled hot dip galvanized steel sheet.

The method for producing a high strength hot rolled hot dip galvanized steel sheet of the present invention for achieving the above object is pickling a hot rolled steel sheet containing 0.5 to 2.0% by weight of silicon in the steel, and using a brush roll to the surface oxide layer remaining 0.5 to 2㎛ After grinding, after the heat treatment at the temperature of the steel sheet to 460 ~ 550 ℃, Al: 0.20 to 0.25% by weight, Ni: 0.05 to 0.10% by weight, the rest is plated by plating in a plating bath composed of Zn and unavoidable impurities It is a configuration.

Hereinafter, the present invention will be described in detail with reference to the preferred embodiments.

In the present invention, hydrochloric acid having a concentration of 10 to 15 vol.% Is used as the pickling solution. Since hydrochloric acid reacts and dissolves sequentially on the surface of the scale, it is used as a pickling solution because the pickling time is superior to sulfuric acid and the pickling time is shortened and the pickling surface is beautiful.

As such, the effect of using hydrochloric acid as a pickling solution does not occur under all conditions. In the case of dilute hydrochloric acid having a concentration of hydrochloric acid of less than 10 vol%, the pickling time is prolonged due to the lack of dissolving ability and the occurrence of fine pickling. There is a problem to be solved, and superacids occur at hydrochloric acid concentrations exceeding 15 vol%. Therefore, in the present invention, the concentration of hydrochloric acid was set to 10 to 15 vol%.

If the pickling temperature is less than 70 ℃ pickling time is long, the pickling phenomenon occurs in the continuous pickling line of the limited length of the pickling tank, and if the pickling temperature exceeds 80 ℃, the reaction is active and local pickling phenomenon occurs.

Because pickling time is closely related to pickling concentration and pickling temperature, the optimum pickling time is automatically determined when the concentration of pickling solution and pickling temperature are determined.

However, even under the optimum pickling conditions, when the Si content in steel exceeds 0.5%, a dense silicate-based oxide (FeSiO ₃ ), which is not locally dissolved in the acid, remains on the surface of the steel sheet, thereby deteriorating plating adhesion. In the present invention, by using a brush roll to remove the remaining silicate oxide to improve the plating adhesion.

At this time, when the amount of surface grinding was less than 0.5 μm, the thick silicate oxide could not be completely removed. When the surface grinding amount was more than 2 μm, the iron loss amount was increased, and the improvement of plating adhesion was also reached. Therefore, the grinding amount of the surface oxide layer was made into the range of 0.5-2.0 micrometers.

According to the present invention, which enables descaling of silicon-containing steel in a short time due to the effect of high-speed pickling of hydrochloric acid and removal of silicate by brush roll, in the case of hot-rolled steel sheet having a Si content of less than 1.0 wt% And it is possible not only to obtain a smooth pickling steel plate surface without the occurrence of over-acid pickling, it is possible to produce a hot-dip galvanized steel sheet excellent in surface appearance and hot plating adhesion during hot dip plating.

The reason for limiting the temperature of the steel sheet in the heating zone during the heat treatment before plating to 460 to 550 ° C is as follows.

After pickling, the hot rolled steel sheet is heated to 150 ℃ in the preheating zone, and then heated to 460 ~ 550 ℃ in the heating zone, which is an indirect heating furnace, and passes through the gas cooling zone to enter the plating bath at about 460 ℃. . At this time, the plating film is improved by reducing and removing the oxide film of the steel sheet remaining after hydrochloric acid washing in the furnace of 10 to 20% hydrogen atmosphere. Since the silicon-containing steel of the present invention is very easy to oxidize, it is diffused to the surface of the steel sheet during the hot rolling and continuous annealing heat treatment during the production of ordinary hot-rolled hot-dip galvanized steel sheet, so that the concentration of these elements is 10 to 100 times higher than that of the base material (bulk). It becomes high enough.

As such, the silicon concentrated in the grain boundary and in the mouth reacts with an extremely small amount of moisture or impurities in the furnace atmosphere to form an oxide film such as SiO ₂ , which greatly reduces wettability with molten zinc in the hot dip galvanizing process. As a result, it is difficult to secure the wettability, and thus the unplating phenomenon is frequently caused, or even when hot-dipped, the plating adhesion is deteriorated and plating peeling occurs during processing.

Therefore, in order to secure the plating adhesion of silicon-containing steel in a conventional zinc bath, the silicon content in the steel should be lowered to less than 1.0%, and the steel sheet temperature in the heating zone should be lowered as much as possible.

However, if the steel plate temperature is less than 460 ℃, the plating reaction is deteriorated due to insufficient diffusion reaction between iron and zinc when it is deposited in the plating bath, and if it exceeds 550 ℃, silicon is segregated on the surface of the steel sheet above the critical content to generate unplated steel plate temperature. It limited to 460-550 degreeC.

The reason for limiting the Al content to 0.20 to 0.25% by weight in the hot dip galvanizing bath is to reduce the silicon oxide partially when the Al content is increased to 0.20% or more, but the plating adhesion is improved when the Al content exceeds 0.20%. This is because there is a problem in that the surface quality is deteriorated due to excessive formation of (top dross).

In addition, the reason why Ni was additionally added to the hot dip galvanizing bath containing 0.20 to 0.25 wt% of Al is that the hot rolled steel sheet having a silicon content of more than 1.0 wt% greatly reduces the surface tension of the hot dip zinc bath to contain silicon. In order to improve the plating wettability and plating adhesion of a steel sheet, in the case of steel containing 1.0 to 2.0% by weight of the silicon of the present invention, even if the remaining silicon oxide is completely removed by pickling and brush roll, the zinc This is because in the plating bath, there is a problem in that unplated is generated locally because the concentration of dense silicon oxide on the steel surface exceeds the threshold even during the pre-plating heat treatment at 460 to 550 ° C. At this time, if the Ni content is less than 0.05% by weight, the improvement of plating wettability due to the decrease of the surface tension of the plating bath is insignificant. When the content of Ni is more than 0.10% by weight, the solid solution limit in the zinc plating bath is exceeded. Since the loss is mostly consumed by forming a top dross, the Ni content is limited to 0.05 to 0.10% by weight because the Fe-Zn alloying reaction is promoted, thereby degrading the adhesion and increasing the manufacturing cost.

In this case, impurities, such as Fe and Pb, are inevitably mixed, so that the surface quality is degraded by increasing dross generation in the plating bath.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example 1

Table 1 used the hot rolled steel sheets from which the silicon content of steel differs as a test material. 12 vol% hydrochloric acid was used as the pickling solution, and the pickling temperature and the pickling time were set at 70 ° C. and 30 seconds, which is a normal operating condition. The amount of surface grinding by the brush roll was changed by changing the applied current of the brush roll.

After pickling, the thickness of the oxide film was measured by oxygen depth analysis by Auger Electron Spectroscopy. After pickling, the plate was heated for 60 seconds after changing the temperature of the steel plate, and then deposited for 3 seconds in a 460 ℃ plating bath composed of 0.2 wt% Al, 0.05 wt% Ni, and the remaining Zn and unavoidable impurities. The plating was carried out so as to be 200 g / m 2 (cross section basis).

The adhesion evaluation of the plating layer plated as described above was evaluated for the degree of plating peeling and cracking of the processed portion (bending portion) after the 180 ° bending test by using a scanning electron microscope as shown in Table 2 below.

division Si concentration in steel (wt%) Surface grinding amount (㎛) Heating steel sheet temperature (℃) Plating adhesion evaluation Inventive Example 1 0.5 0.5 500 5 Inventive Example 2 0.8 1.5 550 4 Comparative Example 1 0.01 - 800 5 Comparative Example 2 0.3 - 550 5 Comparative Example 3 0.5 0.5 600 3 Comparative Example 4 0.5 - 500 3 Comparative Example 5 0.8 1.0 600 3 Comparative Example 6 0.8 0.3 550 3 Comparative Example 7 1.0 1.5 550 2 Comparative Example 8 2.0 2.0 460 2

Rating Plating adhesion state One Plating peeling occurs 2 When grain boundary fracture or cleavage fracture is more than 10㎛ in width without plating peeling 3 When the width of grain boundary fracture and cleavage fracture is less than 10㎛ 4 When there is grain boundary destruction of plating layer but no cleavage 5 When there is no grain boundary breakdown or cleavage breakdown of plating layer

* Level 4 or above is the acceptance criteria

Inventive Example 1, in which a hot rolled steel sheet having a silicon content of 0.5% by weight or more but less than 1.0% by weight was subjected to a hydrochloric acid pickling, and then ground with a brush roll, and then heated to a steel plate temperature of 460 to 550 ° C., and then deposited in a zinc plating bath. 2, the thickness of the remaining oxide film was less than 500Å, and the insoluble silicate oxide film was completely removed. In addition, the plating adhesion was excellent because the silicon content concentrated on the surface was lower than the critical value during the heat treatment before plating.

In Comparative Examples 1 and 2 having a Si content of less than 0.5% by weight as a comparative example, even if no grinding treatment was performed or the heat treatment temperature before plating exceeded 550 ° C, the plating adhesion was excellent. However, for steel grades with a silicon content of more than 0.5% and less than 1.0% by weight, the steel sheet temperature is higher than 550 ° C (Comparative Example 3, Comparative Example 5), or the surface grinding amount is less than 0.5㎛ (Comparative Example 4, Comparative Example 6) It turns out that plating adhesiveness falls.

In addition, when the silicon content is 1.0% by weight or more (Comparative Example 7, Comparative Example 8), even though the surface grinding amount and the plating pretreatment temperature are within the scope of the present invention, the plated adhesion is greatly reduced, and thus further measures are required.

Example 2

Table 3 used the hot rolled steel sheets containing 1.0 wt%, 1.5 wt%, and 2.5 wt% of silicon in steel as test materials. 12 vol% hydrochloric acid was used as the pickling solution, and the pickling temperature and the pickling time were set at 70 ° C. and 30 seconds under normal operating conditions. In addition, the surface grinding amount by the brush roll was made constant at 1.5 micrometers. After surface grinding, heat-treat the steel plate temperature to 550 ℃ and heat-treat it for 60 seconds before plating. Then, it is deposited for 3 seconds in 460 ℃ plating bath having different Al and Ni content in the plating bath, and the plating amount is 200g / m2 (cross section basis). It was. The adhesion evaluation of the plated plating layer was evaluated for the degree of plating peeling and cracking of the processed part (bending part) after the 180 ° bending test by using a scanning electron microscope as shown in Table 2 below.

In the case of the inventive steel having a silicon content of 1.0 to 2.0% by weight, the surface tension of the plating bath is 400dyne / cm at 600 dyne / cm when Ni is added 0.05 to 0.10% by weight to the plating bath with Al content of 0.20 to 0.25% by weight. Significantly reduced to cm can be produced a high strength hot-rolled hot-dip galvanized steel sheet excellent in plating wettability and plating adhesion.

division Si content (% by weight) Al concentration (% by weight) Ni concentration (% by weight) Plating adhesion evaluation Inventive Example 3 1.0 0.20 0.05 5 Inventive Example 4 2.0 0.25 0.10 4 Comparative Example 9 1.0 0.25 - 3 Comparative Example 10 1.0 0.15 0.05 3 Comparative Example 11 1.5 0.15 0.10 2 Comparative Example 12 2.0 0.25 0.03 2 Comparative Example 13 2.5 0.25 0.10 2

However, when the Ni content in the plating bath is less than 0.05% by weight (Comparative Example 9, Comparative Example 12) or when the Al content is less than 0.2% by weight (Comparative Example 10, Comparative Example 11), due to excessive segregation of silicon oxide on the steel sheet surface Unplating occurs and plating adhesion is significantly degraded.

On the other hand, when the silicon content in the steel exceeds 2.0% by weight (Comparative Example 13) it can be seen that even if the plating bath components are changed, unplated generation and plating adhesion is greatly deteriorated.

Therefore, according to the manufacturing method of the present invention as described above, in the manufacture of hot-rolled hot-dip galvanized steel sheet to plate the high-strength hot-rolled steel sheet containing a large amount of silicon after pickling, using a pickling and brush roll to remove the insoluble silicate oxide, plating By changing the bath composition and heat treatment conditions to secure plating adhesion, the result is an economical and quality improvement.

Claims (1)

After pickling a hot rolled steel sheet containing 0.5 to 2.0 wt% of silicon, grinding the remaining surface oxide layer using a brush roll of 0.5 to 2 µm, and heat-treating the steel sheet at a heating table at 460 ° C to 550 ° C. 0.20 to 0.25% by weight, Ni: 0.05 to 0.10% by weight, the remainder is deposited by plating in a plating bath made of Zn and unavoidable impurities to produce hot rolled hot dip galvanized steel sheet having excellent plating adhesion