JPH10219350A - Production of direct hot rolled steel sheet excellent in plating blistering resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a hot rolled steel sheet for the substrate of hot dip galvanizing excellent in plating blistering resistance even in direct rolling. SOLUTION: In direct rolling in which a steel contg., by mass, <=0.2% C, <=0.1% Si, 0.05 to 2.0% Mn, 0.003 to 0.050% S, <=0.030% P, <=0.1% Sol.Al, <=0.01% N, and the balance Fe with inevitable impurities is subjected to continuous casting and is subsequently, as it is or after heat holding treatment, subjected to hot rolling, a rough bar after rough rolling is rapidly heated by >=50 deg.C at the average temp. rising rate of >=10 deg.C per sec, furthermore, the temp. of the rough bar after the rapid heating is regulated to <=1000 deg.C, and subsequently, finish rolling is executed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a hot-rolled hot-rolled steel sheet used for building materials such as automobiles and home electric appliances, and steel houses and the like, which can be manufactured by using direct-feed rolling at a low manufacturing cost. The present invention relates to a method for producing a hot-rolled steel sheet for hot dip galvanizing that does not generate plating defects such as plating swelling.

[0002]

2. Description of the Related Art In recent years, in order to reduce the cost of hot-dip galvanized steel sheets, hot-rolled sheets have a thickness of 0.8-1.
Hot-dip galvanized steel sheets of about 8 mm have attracted attention. This hot-dip galvanized hot-rolled steel sheet is increasingly used not only for automobiles and home appliances, but also for civil engineering and building materials such as steel houses. The production cost of this hot-rolled steel sheet for plating base is low, and therefore, production by direct-feed rolling in which hot rolling is performed as it is after continuous casting without reheating by heating furnace charging is optimal.

However, if the hot rolled steel sheet is hot-dip galvanized and left to stand for several days, blisters, called blisters, are generated, which not only impairs the appearance but also deteriorates the adhesion of the plating and the corrosion resistance. The problem arises. This is due to the following mechanism. That is, in the oxide film reduction cleaning step of the continuous galvanizing line, the steel sheet is heated to a high temperature and absorbs hydrogen in the atmosphere. Thereafter, when the steel sheet is cooled to room temperature, the absorbed hydrogen is released from the steel and accumulates at the interface between the plating layer and the steel sheet, which pushes up the plating to form blisters.

As a method for preventing such plating blisters, Japanese Patent Application Laid-Open No. 54-026928 discloses a method of increasing the Al concentration in a plating bath and activating the plating bath to reduce the occurrence of blisters. .

Japanese Patent Application Laid-Open No. 1-316444 discloses a method of increasing the Al concentration in a plating bath, reducing the bath temperature and the temperature of a steel sheet before plating, and preventing the occurrence of blisters.

However, in all of these conventional methods, the Al concentration in the plating bath is increased in order to improve the plating adhesion. Although the frequency of blister generation is reduced, a large amount of dross is generated, and the surface properties of the plating are reduced. There is a problem of deterioration.

Therefore, as a solution, hydrogen is not used for reducing the oxide film on the steel sheet surface, or even if hydrogen enters the steel sheet using hydrogen, the hydrogen is trapped in the steel sheet and released from the steel sheet even after cooling to room temperature. It is conceivable that this is not done.
Here, as a method of removing the oxide film without using hydrogen, a chemical method using an aqueous solution or a physical method using a grindstone can be considered, but in any case, there is a significant change in the continuous galvanizing equipment or a uniform method. There is a problem that difficult processing is difficult.

On the other hand, in order to keep the hydrogen that has infiltrated into the steel sheet trapped in the steel sheet even after cooling, coarse precipitates in the steel found in the steel sheet for enamel are effective. For example,
On page 97 of the "Study of Enamelling of Continuously Cast Steel" published by Japan Enamel Industry Association, it is stated that precipitates such as MnO, TiN, TiS and BN and voids around them are the main hydrogen storage sources. There is. However, in direct-feed rolling, the precipitate enters the rolling process in a state of almost solid solution, and is finely precipitated by strain-induced precipitation, so that it is difficult to coarsen the precipitate like a rolled material by ordinary slab heating. is there.
No mention is made of the relationship between enamelness and plating swelling. However, as a method for coarsening the precipitates of a hot-rolled hot-rolled steel sheet, Japanese Patent Application Laid-Open No. 4-253501 discloses a method in which only the temperature of the surface layer is increased by 700 after continuous casting. Once cooled to ~ 1000 ° C,
A method is disclosed in which MnS is finely precipitated and the MnS is coarsened in a recuperation process by recuperating by heat inside the slab. However, in this method, it is possible to coarsen the MnS in the surface layer, but since the MnS inside the steel sheet is still fine, the hydrogen storage capacity is not sufficient, and plating blisters may occur depending on production conditions and components. And
There was a problem that stable production was not possible.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to coarsen MnS which has been finely precipitated by conventional direct rolling,
An object of the present invention is to provide a method for producing a hot-rolled steel sheet for hot-dip galvanizing, which has excellent resistance to blistering even in direct-feed rolling by using a hydrogen storage site.

[0010]

According to the present invention, there is provided a method for producing a hot rolled steel sheet which is excellent in resistance to blistering by direct rolling.
C ≦ 0.2%, Si ≦ 0.1%, 0.05 ≦ Mn in s%
≦ 2.0%, 0.003 ≦ S ≦ 0.050%, P ≦ 0.
030%, Sol. After continuous casting of steel containing Al ≦ 0.1%, N ≦ 0.01% and the balance being Fe and unavoidable impurities, as it is, or in direct-feed rolling in which hot rolling is performed by heat treatment, rough rolling is performed. The rough bar is rapidly heated at an average temperature rise rate of 10 ° C. or more per second at 50 ° C. or more, and finish rolling is performed after the temperature of the rough bar after the rapid heating is set to 1000 ° C. or more.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention has been made based on the following findings. In the direct rolling, since most of the precipitates start rolling in a solid solution state, the precipitates are finely precipitated due to strain energy caused by the large pressure of the rough rolling. Furthermore, even if high temperature winding is performed, these precipitates do not become so coarse as to increase the amount of absorbed hydrogen.

Therefore, the present inventors have conducted intensive studies and found that after rough rolling by direct rolling in steel containing Mn and S, the rough bar was rapidly heated at 50 ° C. or more at an average temperature rising rate of 10 ° C. or more per second. It was found that by setting the temperature to 1000 ° C. or higher, only MnS among the precipitates capable of absorbing hydrogen became coarse, hydrogen was trapped around the coarsened MnS, and the plating blistering resistance was improved. As a result, it has become possible for the first time to manufacture a hot-rolled steel sheet for hot dip galvanizing excellent in plating swelling resistance even in direct rolling.

The following is a description of the experimental results on which this is based. mass = C = 0.03%, Si = 0.0
1%, Mn = 0.18%, S = 0.010%, P = 0.
012%, Sol. Al = 0.035%, N = 0.00
A steel containing 30% was cast, and then hot-rolled without cooling. After rough rolling, the rough bar temperature is 900 ° C
When the temperature became 1
It heated to 050 degreeC and performed the finish rolling. Then 6
A rolled hot rolled sheet was formed at 50 ° C.

The obtained hot rolled sheet was pickled, subjected to continuous hot-dip galvanizing, and the occurrence of blisters was examined. The plating conditions were such that the Al concentration was 0.13% and the bath temperature was 470 ° C. The evaluation was performed based on the number of blisters existing per 10 cm × 10 cm of the plating surface.

FIG. 1 shows the results. In FIG. 1, the horizontal axis represents the heating rate of the coarse bar heating (° C./second), and the vertical axis represents the plating surface 1.
It is a graph which shows the result of having investigated the number of the blister which generate | occur | produced per 0 cm x 10 cm, and investigated the relationship between both. As is apparent from the figure, the number of blisters generated decreased as the heating rate increased, and no blisters were generated at a heating rate of 10 ° C. or more per second. That is, it was found that the rapid heating suppressed the generation of blisters.

Next, the lower limit of the heating temperature of the coarse bar was investigated. The steel having the above composition was cast and then hot-rolled without cooling. After rough rolling, the coarse bar temperature is 9
When the temperature reached 00 ° C., the temperature was raised to each temperature at 20 ° C./second by an induction heating furnace, and finish rolling was performed. Then 60
The rolled hot rolled sheet was taken at 0 ° C.

The obtained hot-rolled sheet is pickled, subjected to continuous hot-dip galvanizing, and the occurrence of blisters is determined on the plating surface 10c.
The evaluation was based on the number existing per m × 10 cm. The result is shown in FIG. In FIG. 2, the horizontal axis shows the coarse bar heating temperature (° C. /
FIG. 6 is a graph showing the result of examining the relationship between the number of blisters generated per 10 cm × 10 cm of the plating surface on the vertical axis. As is clear from the figure, when the heating temperature of the coarse bar was 1000 ° C. or higher, blisters were not generated.

Although these phenomena have not yet been completely elucidated, the results of the above experiments will be described below in connection with the state of precipitates. MnS starts to precipitate during the rough rolling and precipitates finely at the end of the rough rolling. However, when rapid heating is performed from this state, finer MnS of the fine MnS is obtained.
nS re-dissolves. Although the cause is not clear, the solid solution limit is rapidly increased by rapid heating, and in a state where the driving force for solid solution is extremely large, finer MnS having a relatively large interfacial energy is preferentially dissolved in the solid solution. It is thought to be. Therefore, after heating the coarse bar, since the re-dissolved MnS is re-precipitated with slightly larger MnS as a nucleus among the fine precipitates selectively remaining during the coarse bar heating, the number of MnS in the hot-rolled sheet decreases, and the coarse precipitate Things. Thus, by performing rapid heating of the coarse bar, it is possible to coarsen MnS even in direct-feed rolling, and since the coarse MnS becomes a trap site for hydrogen in steel, it has excellent plating blister resistance. It is possible to manufacture a hot-rolled hot-rolled steel sheet for hot dip galvanizing.

The reasons for limiting each component will be described below. 1) C: 0.2% or less When C exceeds 0.2%, a large amount of carbide precipitates, lowers the ductility and impairs the formability. 2) Si: 0.1% or less When Si is excessively added, the plating property is deteriorated, and at the same time, the strength is increased and the formability is deteriorated.
It was as follows. 3) Mn: 0.05% to 2.0% Since Mn plays an important role in the present invention and serves as hydrogen trapping sites as coarse MnS, it is necessary to add 0.05% or more. However, excessive addition causes hardening of the steel and deteriorates the formability, so the upper limit was made 2.0%. 4) S: 0.003% to 0.050% S also becomes a hydrogen trap site as MnS similarly to Mn. If the amount is too small, MnS does not grow sufficiently, so it is necessary to add 0.003% or more. However, if added excessively, the amount of MnS becomes excessive and moldability deteriorates, so the upper limit was made 0.050%. 5) P: 0.030% or less P is a solid solution strengthening element, and an excessive addition causes hardening of steel, so the upper limit was made 0.030%. 6) Sol. Al: 0.1% or less Al fixes the deoxidizing agent and N in the form of AlN.
It is an indispensable element. However, since excessive addition of Al generates a large amount of fine precipitates in steel and deteriorates workability, the upper limit is set to 0.1%. Deacidification and A
In order to sufficiently exert the effect of fixing 1N, 0.00
It is desirable to add 5% or more. 7) N: 0.01% or less It is desirable to reduce N as much as possible, but it is not necessarily 0 from the viewpoint of cost. However, if N is present in excess, the crystal grains become finer and workability deteriorates, so the upper limit was made 0.01%. 8) Other optional components In the present invention, in addition to the above-mentioned elements, C
a, Zr, V, Nb, Ni, Co, Cr and the like can be added. In addition, even if an element mixed at the time of scrap recycling, particularly Sn or Cu, is added, the effect of the present invention does not change and there is no problem at all.

Furthermore, even if B is added to steel, BN becomes a trap site for hydrogen, so that the blister generation rate decreases. Therefore, B may be added to the steel of the present invention. Further, manufacturing conditions will be described below.

In the present invention, the temperature of each step has an important meaning, and if any one of them is missing, the effect of the present invention cannot be obtained. 9) Rough Bar Heating Rate In the present invention, the rough bar heating rate is important. When the average heating rate is 10 ° C. or more per second, the finer ones of the finely precipitated MnS after the rough rolling are selectively dissolved, and the precipitation nuclei of the MnS after the coarse bar heating are reduced, whereby the hot-rolled sheet is reduced. The coarsening of MnS therein is promoted. On the contrary,
If the average heating rate is less than 10 ° C. per second, all of the fine MnS precipitated during the rough rolling will be uniformly melted.
The effect of coarsening cannot be obtained. Therefore, the lower limit of the average heating rate was set to 10 ° C. per second. 10) Rough Bar Heating Temperature Difference The rough bar heating temperature difference also plays an important role in the present invention.
In order to coarsen MnS remaining after heating the coarse bar by reprecipitation, a certain amount of MnS must be dissolved during the heating of the coarse bar. Therefore, the lower limit of the temperature difference before and after heating was set to 50 ° C. 11) Coarse Bar Heating Temperature Coarse bar heating requires a sufficient temperature to dissolve the MnS once finely precipitated. If the temperature is lower than 1000 ° C., the amount of re-dissolved MnS is small and the MnS cannot be coarsened. 12) Finishing Temperature The finishing temperature is desirably Ar ₃ or more in order to ensure workability, but the effect of the present invention does not change even if α region rolling or annealing after α region rolling is performed. As the annealing method at that time, continuous annealing in a continuous galvanizing line is preferable.
However, there is no problem even if the box annealing is performed. Since the winding temperature does not affect the form of MnS, there is no particular definition for the winding temperature. Further, there is no limitation on the condition of the temper rolling, but if it is too high, the ductility is drastically reduced. In addition,
For the composition adjustment of the steel of the present invention, both a converter and an electric furnace can be used. In the direct rolling, heating may be performed by an edge heater or the like for the purpose of temperature compensation without using a conventional heating furnace.

There are no particular conditions for the plating, and there is no particular designation for the heating temperature before plating, the bath temperature, the Al concentration in the plating bath, or the like. Further, as for the hot-rolled sheet before plating, the effect of the present invention does not change whether the hot-rolled black scale material or the pickling material is used.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Various embodiments of the present invention will be described below. (Example 1) Steel having the components shown in Table 1 was cast, and immediately thereafter hot-rolled. The finishing temperature was 860 ° C and the winding temperature was 650 ° C. The resulting hot-rolled steel sheet was pickled, heated to 700 ° C. in a hydrogen-nitrogen atmosphere, and then heated at a bath temperature of 470 ° C.
It was immersed in a galvanized steel sheet having a concentration of 0.12% to obtain a galvanized steel sheet. Several days after plating, the presence or absence of blisters was visually inspected. In the evaluation, those in which blisters did not occur were indicated by symbols (circle mark), and those in which blisters occurred were indicated by crosses (cross marks).

From Table 1, it was found that blisters did not occur under the conditions of each example. On the other hand, any one of the temperature difference before and after the coarse bar heating, the coarse bar heating rate, and the coarse bar heating temperature
It was found that the effect of the present invention could not be obtained under the condition lacking at least one (Comparative Example). (Example 2) Steel having the components shown in Table 2 was cast, and immediately thereafter hot-rolled. The finishing temperature was 850 ° C and the winding temperature was 600 ° C. Without pickling the obtained hot rolled sheet,
Hot-dip galvanizing was performed with the black scale. After heating to 850 ° C., it was immersed in zinc plating with an Al concentration of 0.14% at a bath temperature of 480 ° C. to obtain a galvanized steel sheet. Several days after plating, the presence or absence of blisters was visually inspected. In the evaluation, those that did not generate blisters are marked with a circle (circle mark).
, And those having blisters are indicated by crosses (cross marks).

From Table 2, it was found that blisters did not occur under the conditions of each example. On the other hand, it was found that the effect of the present invention could not be obtained under the condition (comparative example) in which any one of the temperature difference before and after the heating of the rough bar, the heating rate of the rough bar, and the heating temperature of the rough bar was missing.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

By using the hot-rolled steel sheet produced by the direct-feed rolling obtained by the present invention, blister does not occur even when hot-dip galvanizing is performed, and a hot-dip galvanized steel sheet can be manufactured at low cost.

[Brief description of the drawings]

FIG. 1 Coarse bar heating rate (° C./sec) and plating surface 1
The graph which shows the result of having investigated about the relationship with the number of blisters generated per 0 cm x 10 cm.

Fig. 2 Rough bar heating temperature (° C) and plating surface 10cm x
FIG. 9 is a graph showing the results of an investigation on the relationship with the number of blisters generated per 10 cm.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Ken Nakahara, Inventor: 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Jun Taniai 1-2-1, Marunouchi, Chiyoda-ku, Tokyo, Japan Inside Honko Tube Co., Ltd.

Claims (1)

[Claims] 1. A mass% of C ≦ 0.2% and Si ≦ 0.
1%, 0.05 ≦ Mn ≦ 2.0%, 0.003 ≦ S ≦
0.050%, P ≦ 0.030%, Sol. Al ≦ 0.
After continuous casting of steel containing 1%, N ≦ 0.01% and the balance consisting of Fe and unavoidable impurities, in the direct-feed rolling in which hot rolling is performed as it is or after heat treatment, the rough bar after rough rolling is removed every second. Rapidly heating at 50 ° C or more at an average heating rate of 10 ° C or more, and raising the temperature of the coarse bar after the rapid heating to 1000 ° C.
A method for producing a directly-rolled hot-rolled steel sheet having excellent resistance to plating swelling, wherein finish rolling is performed after the above.