DE68903459T2 - METHOD FOR STICKING ELECTROFINE BLANKS. - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION

This invention relates to a pickling process for electrical steel strips.

BACKGROUND OF THE INVENTION

An electrical steel sheet is manufactured by processes of hot rolling a slab, annealing, if necessary, the hot-rolled steel strip to improve magnetic properties, pickling, cold rolling and finish annealing. It is known that this type of steel is inferior in descaling properties during pickling, depending on the content of Si as an essential element. There have been proposals for improving the descaling properties, e.g., in Japanese Patent Laid-Open Nos. 76 422/79, 33 436/81 or 138 014/85.

However, with regard to pickling of the hot-rolled strip, a big problem is involved in intergranular corrosion, regardless of descaling. When the electrical steel strip is wound at high temperatures after hot rolling, the steel surface is subjected to intergranular oxidation. If pickling is continued unnecessarily after descaling is completed, corrosion by intergranular oxidation increases preferentially around grain boundaries as pitting, and this acts as a cause of fine cracks during subsequent cold rolling and deteriorates surface properties after cold rolling. The fine cracks lead to various problems, which not only reduce product value due to external appearance of products, but also deteriorate magnetic properties, particularly iron loss, by generating fine grains in the surface layers during final annealing and further act as causes of unevenness of insulation coatings.

Each of the above conventional proposals only addresses descaling as a problem, none of them specifies the pickling conditions by taking intergranular corrosion into account.

DISCLOSURE OF THE INVENTION

In view of the above-mentioned difficulties, the invention optimizes the pickling conditions not only with regard to the descaling properties but also the intergranular corrosion in order to provide the desired descaling properties and to realize a pickling process that enables accurate control of the intergranular corrosion.

To achieve the object, the invention is characterized by pickling with hydrochloric acid a hot-rolled steel strip containing Si: 0.2 to 4.0 wt. %, wound at a temperature CT or annealed after winding, while fulfilling the following conditions:

In a case of Si ≤ 1.0 wt.%

CT ≥ 270.6(%Si)² - 475.9(%Si) + 915.3

in case of Si > 1.0 wt.%

CT ≥ 5.0(%Si)² - 50.1(%Si) + 755.4

where

CT: coiling temperature (ºC) of the hot rolled strip

(%Si): Si content (wt.%);

for a pickling time that satisfies the following condition

0.48(%Si) + 0.59 ? t.Bexp(-Q/RT) ≤ 0.24(%Si) + 4.00

where

(%Si): Si content (wt.%) of the hot-rolled strip

t: pickling time (sec)

T: Temperature (K) of the pickling liquid

B: -0.48(HCl)² + 15.1(HCl) + 5.03

where (HCl): HCl concentration (wt.%) of the pickling liquid

Q: 5300 cal/mol

R: 1.986 cal/mol.K

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows the presence and absence of intergranular corrosion in the hot-rolled steel strips in relation between the Si contents and the coiling temperatures CT; Figure 2 shows the influence of the Si content and the pickling time on the descaling properties and the intergranular corrosion; Figure 3 shows the influence of the pickling temperature and the pickling time on the descaling properties and the intergranular corrosion; and Figure 4 shows the influence of the HCl concentration of the pickling liquid and the pickling time on the descaling properties and the intergranular corrosion.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described in detail together with the reasons for the limitations.

In the invention, pickling is carried out on hot-rolled silicon steel strips (including coiled and post-coiling annealed) under specified conditions to prevent intergranular corrosion.

During the inventors' experiments, it was apparent that intergranular corrosion occurred in hot-rolled strip with a Si content and a coiling temperature within specified ranges. Therefore, the subject matter of the invention is limited to certain hot-rolled steel strips for which the Si content and coiling temperature are specified.

Figure 1 shows, in relation between the Si content and the coiling temperatures CT, the presence and absence of intergranular corrosion in the steel strips A to I of Table 1 which were hot rolled at different coiling temperatures and subjected to pickling for a period of 100 sec under the condition of 12% HCl and 90°C, accordingly, intergranular corrosions can be controlled by the Si content and coiling temperature, and they appear when the hot rolled strips with Si ≥ 0.2 wt.% are coiled at the temperatures above a - b in the figure. On the other hand, when a steel strip with Si < 0.2 wt.% was coiled at a high temperature such as 850°C, no intergranular corrosion occurs, and when a steel with Si ≥ 0.2 wt.% was coiled at a high temperature such as 850°C, no intergranular corrosion occurs. 0.2 wt.% at less than a - b, no intergranular corrosion occurs despite a strong pickling at 12% HCl, 90ºC and 100 sec. The same results were obtained in the case of hot-rolled strips which were annealed under different conditions after winding.

The area specified in Figure 1 where the intergranular corrosion occurs is expressed

in case of Si ≤ 1.0 wt.%

CT ≥ 270.6(%Si)² - 475.9(%Si) + 915.3 ... (1)

in the case of Si > 1.0 wt.%

CT ≥ 5.0(%Si)² - 50.1(%Si) + 755.4 ... (2)

Therefore, the invention limits the subject matter to hot-rolled steel strips containing Si ≥ 0.2 wt.% and satisfying the above formulas (1) and (2) and carries out pickling under the specified conditions.

When the Si content exceeds 4.0 wt.%, the brittleness of the steel is noticeable, and because cold rolling difficult, the subject matter is limited to hot-rolled steel strips with Si: 0.2 to 4.0 wt.%.

In the invention, pickling is carried out by fulfilling the following formula

0.48(%Si) + 0.59 ? t.Bexp(-Q/RT) &le; 0.24(%Si) + 4.00 ... (3)

where

(%Si): Si content (wt.%) of the hot-rolled strip

t: pickling time (sec)

T: Temperature (K) of the pickling liquid

B: -0.48(HCl)² + 15.1(HCl) + 5.03

where (HCl): concentration (wt.%) of HCl in the pickling liquid

Q: 5300 cal/mol

R: 1.986 cal/mol.K.

The pickling conditions were specified as follows.

Figure 2 shows the influence of Si content and pickling time on descaling properties and intergranular corrosion, where steels C, E, G, I of Table 1 were hot rolled at a coiling temperature of 780ºC and pickled with a pickling liquid of 11.8% HCl and 85ºC for different times to study the surface properties, according to which, when the pickling time is short, the scale remains, and when it is long, intergranular corrosion occurs. The critical pickling time t with respect to the completion of descaling and the occurrence of intergranular corrosion is expressed by the straight lines (I) and (II), that is, the linear formulas (4) and (5) of Si content

t = 7.12(%Si) + 8.75 ... (4)

t = 3.56(%Si) + 59.35 ... (5).

Figure 3 shows the influence of pickling temperature and pickling time on descaling properties and intergranular corrosion, where the hot-rolled steel strips (CT = 780ºC) of steel G (Si: 2.18 wt%) of Table 1 were pickled under a constant concentration of 11.8% HCl at different temperatures to study the surface properties, after which the critical pickling time t with respect to the completion of descaling and the occurrence of intergranular corrosion are expressed by the following Arrhenius equations (6) and (7).

t = 0.0141 exp(5300/RT) .... (6)

t = 0.0389 exp(5300/RT) .... (7).

Figure 4 shows the influence of HCl concentration of the pickling liquid and pickling time on the descaling properties and intergranular corrosion, whereby the hot-rolled steel strips (CT = 780ºC) of steel G (Si: 2.18 wt%) of Table 1 were pickled at the constant temperature of 85ºC with different HCl concentrations to study the surface properties, after which the critical pickling time t with respect to the completion of descaling and the occurrence of intergranular corrosion are expressed by the following equations (8), (9), where B is a parameter of HCl concentration

t = 2825/B .... (8)

t = 7808/B .... (9)

where

B = -0.48(HCl)² + 15.1(HCl) + 5.03.

The same tests as indicated in Figure 2 were carried out on the hot rolled strips with CT = 730ºC (except for steel C) and CT = 850ºC, and the same results as in Figure 2 were obtained, and when the winding temperature CT was above a - b in Figure 1, the influence of the winding temperature on the critical pickling time was not observed.

The above formula (3) is obtained from the formulas (4) to (9) in terms of the optimum pickling time t at which descaling is completed and no intergranular corrosion is caused. That is, if the value of t.Bexp(-5300/RT) is less than 0.48 (%Si) + 0.59, pickling would be insufficient and scale would remain, and if the value is more than 0.24 (%Si) + 4.00, intergranular corrosion would be generated.

Because an inhibitor would have the effect of suppressing intergranular corrosion in addition to the effect of suppressing corrosion of the matrix iron of the steel strip, it may be added to the pickling liquid. In this case, the inhibitor should be added in an amount of more than 0.2 wt% to the amount of HCl because otherwise satisfactory effects cannot be achieved, but if it is added in an amount of more than 1.0 wt%, the effect is saturated and the pickling rate is reduced.

According to the invention, it is possible to produce electrical steel sheet having excellent surface properties with high economic effect by only optimizing the pickling time without requiring special treatment and increasing costs. In addition, since the surface properties are excellent, no fine grains are formed in the surface layers during the final annealing, and therefore the products are excellent in magnetic properties, particularly iron loss, and are uniform in insulation coating.

EXAMPLE

Slabs of steels B, D, F, H from Table 1 were heated to 1200ºC and rolled to a thickness of 2.0 mm at a final temperature of 930ºC. After pickling, their surface properties were examined. The results (the descaling properties and the presence and absence of intergranular corrosion) are shown in Table 2 together with the pickling conditions.

According to the above, when the pickling time is within the range specified in the invention, the descaling is completed and no intergranular corrosion occurs. On the other hand, when the pickling time is shorter than the range specified in the invention, the scale remains, and when it is longer, intergranular corrosion occurs. Table I (wt%) Steels

INDUSTRIAL APPLICABILITY

The The present invention can be applied to a pickling treatment of hot-rolled steel strips in the production of electrical steel sheet. Table 2 Steels Pickling liquid Optimum pickling time* Pickling time Descaling Intergranular corrosion Inventive method Comparative method * Pickling time range of the invention

Claims (1)

Method for pickling electrical steel strip, comprising pickling a hot-rolled steel strip wound at a temperature of CT or annealed after winding and having a Si content: 0.2 to 4.0 wt.% with hydrochloric acid while fulfilling the following conditions: in the case of Si ≤ 1;0 wt.% CT ≥ 270.6(%Si)² - 475.9(%Si) + 915.3 in case of Si > 1.0 wt.% CT ≥ 5.0(%Si)² - 50.1(%Si) + 755.4 where CT: coiling temperature (ºC) of the hot rolled strip (%Si): Si content (wt.%) for a pickling time that satisfies the following condition duration 0.48(%Si) + 0.59 ? t.Bexp(-Q/RT) &le; 0.24(%Si) + 4.00 where (%Si): Si content (wt.%) of the hot-rolled strip t: pickling time (sec) T: Temperature (K) of the pickling liquid B: -0.48(HCl)² + 15.1(HCl) + 5.03 where (HCl): concentration (wt.%) of HCl in the pickling liquid Q: 5300 cal/mol R: 1.986 cal/mol.K