JPS6213520A - Method for inhibiting oxidation of steel material during heating - Google Patents

Abstract

PURPOSE:To inhibit the surface oxidation and grain boundary oxidation of a steel material during heating and to enhance the yield and quality of hot steel sheets by placing solid carbon on or near the surface of the steel material and heating the steel material at a specified temp. CONSTITUTION:When a steel slab is charged into a heating furnace, granular carbon is uniformly distributed on or near the surface of the slab and the slab is heated at >=730 deg.C. The slab is preferably covered with a steel sheet or the like so that generated CO and CO2 stay near the slab. Thus, the surface defects of hot steel sheets due to the surface oxidation or grain boundary oxidation of the slab are reduced to enhance the yield and quality.

Description

[Detailed Description of the Invention] Technical Field> The present invention is aimed at reducing scale loss in forged steel slabs and reducing grain boundary oxidation in high alloy slabs such as stainless steel and high Ni steel when heating steel materials. The present invention relates to an oxidation prevention method that aims to prevent oxidation, prevent flaws in hot rolled steel sheets manufactured therefrom, and improve yield.

<Prior art and its problems> The production yield of hot rolled steel sheets has improved due to the progress and improvement of various technologies.
It has already reached 98.7-99%, but the loss is 1.0-1
．． Scale loss due to surface oxidation is 0.5 to 0 out of 3%
．． 7%, that is, approximately 50%, and reducing scale loss during heating is one of the important issues for improving yield.

In addition, when hot-rolled sheets are manufactured from slabs of stainless steel or high-Nl steel manufactured by continuous casting or ingot-forming, grain boundary oxidation occurs in the surface layer of the slab during heating. Due to field oxidation, blemishes occur during rolling.
Surface flaws such as bald spots and edge cracks occur.

Traditionally to prevent these.

(1) Apply 100 to 300 #LI of antioxidant to the entire surface of the slab.
8 to prevent oxidation of the surface layer.

(2) An inert gas such as At or N2 is introduced into the furnace to prevent oxidation of the surface layer.

After taking the above measures, the steel material to be heated is charged into the heating furnace.
After holding and heating at 1100 to 1400°C for several hours, it is hot rolled.

However, with the method (1), there is a tendency for oxidation to occur due to unevenness in the film thickness that occurs when applying the antioxidant, and for the antioxidant to peel off when kept in a high-temperature furnace for a long time, resulting in localized primary scale. This causes surface defects on the hot-rolled steel sheet after rolling, making it impossible to achieve a sufficient effect in preventing oxidation.

Furthermore, method (2) uses a large amount of expensive gases such as A and N2, which inevitably leads to a significant increase in the price of the product.

In view of the current situation, the present invention provides an inexpensive and powerful antioxidant method.

<Structure of the Invention> According to the present invention, there is provided an oxidation prevention method characterized in that when heating a steel material, solid carbon is placed on or near the surface of the steel material and the steel material is heated to a temperature of 730" or less. Ru.

Further, according to the present invention, when heating the steel material, solid carbon is placed on or near the surface of the steel material, and further covered with a suitable material so that the generated carbon monoxide and carbon dioxide are retained in the vicinity. A method for preventing oxidation is provided, which comprises heating to 730°C or higher.

The carbon used is carbon with a fixed carbon content of 70 to 99.9%,
Solid carbon of petroleum coke or graphite, in either case it is advantageous to use granular carbon.Although the method of the present invention can be applied in principle to any heating situation, in practice When charging the slab into the heating furnace, it is advantageous to uniformly distribute carbon grains on its surface, preferably covering it with a thin steel plate or the like.

<Specific description of the invention> Oxidation of iron and combustion of carbon proceed according to a linear equation.

Fe + 1/202 = Fe O, (1) C +
1/2 o2=co (2) CO+
1/202 = C02 (3) In general, these reactions depend on temperature, oxygen potential, etc., but the oxidation priority at each temperature is given, for example, in "Iron and Steel Smelting" edited by the Japan Institute of Metals.
The standard free energy of formation (ΔG
O). ΔGO is calculated by the following formula.

ΔG, :, = −83310+ 15.827 (c
al) (4) ΔG' = -21370
0-20,95T (cal) (5)
oO From formulas (4) and (5), at temperatures above 730°C.

ΔG Eagle, 〉ΔG Breasts.

Therefore, C is fermented preferentially over Fe.

Further, as can be understood from equations (4) and (5), this tendency becomes more pronounced as the temperature becomes higher, and combustion of carbon occurs first to become CO gas.

The slab heating furnace temperature in the hot rolling line is ΔG0 > Δ
Since the temperature is GO FiOoO, carbon is present on or near the surface of the heated steel material, and the c generated by the reaction of equation (2)
By creating an atmosphere of o gas or CO2 gas generated by equation (3), the oxidizing atmosphere on the surface of the steel material can be weakened, and surface oxidation and grain boundary oxidation can be prevented.

The present inventors conducted the following basic experiment to prove the above theory.

Using high Ni alloy steel having the composition shown in Table 1,
A block of one corner of the intestine was prepared, and after surface polishing, Fig. 1&b
The solid carbon particles were arranged as shown in ,c. A shows the case where no carbon grains are arranged, b shows the case where carbon grains with a grain size of 2 to 51 square meters are arranged on the upper surface, and C shows the case where the same carbon grains are arranged and the upper surface is covered with a thin steel plate.

A block treated in this way was exposed to air for 12 hours.
Surface oxidation depth after holding at 50℃ for 90 minutes (h + Homaro)
and the grain boundary oxidation depth (h2ts) were measured. FIG. 2 is a 100 times enlarged photograph of the cross section of the block shown in Table 3, showing that subscales and primary scales are formed on its surface. hl and h2 are defined as shown in this figure.

The experimental results are shown in Figure 3. When carbon grains a are not arranged, h 1 = 0.41 mm, h 2 = 0.85 m
-, whereas in the case of b, h = 0.08m
m, h2 = 0.22■, and by simply arranging carbon grains, the surface and grain boundary oxidation depth is approximately 175
It was observed that the Furthermore, when the carbon grains are covered with a thin steel plate as shown in C, h = 0.02 m layer, h2 = 0.05 m layer, the oxidation depth is reduced to about 1/10, and the oxidation prevention is It was confirmed that this method was even more effective than case b.

As is clear from these results, surface oxidation and grain boundary oxidation are significantly prevented by arranging solid carbon grains at or near the surface of the heated steel material. In particular, it is effective to surround the steel material to be heated with a thin steel plate in order to retain monoxide and carbon dioxide generated by the combustion of carbon on the surface of the steel material and to enhance the protective effect thereof.

<Example> High Ni alloy steel slab with the composition of 3 heats shown in Table 2 (thickness 200 mm x width i ooo intestine - x length 7000 mm
), according to the method of the present invention, 2 to 5 cm of carbon grains C are arranged as shown in Figure 4, and the whole is covered with a steel plate S having a thickness of 0.5 mm and folded into a gutter shape. The seams of the cover were welded. Eight slabs treated in this way and three untreated slabs were charged into a walking beam heating furnace.
After holding at 250° C. for 2 hours, hot rolling was performed to produce a hot rolled coil (thickness: 5 mm x width: 1000 mm). The occurrence of surface flaws in the hot-rolled coil thus obtained was investigated, and the inventive example and the comparative example were compared. Regarding surface flaws, each test material coil was rewound, and the front and back surfaces of the hot-rolled coil were visually observed, and the following ranking was performed based on the number of surface flaws generated per 105 m2.

A: 0-10 surface flaws per 10m layer 2 B:
10-30 pieces C: ”
30-50 pieces D: Ri 5
As can be seen in Table 3, which shows the survey results of 0 or more pieces, all of the comparative examples are ranked D, whereas the examples of the present invention are ranked A. Therefore, the present invention The effectiveness of the method is demonstrated.

<Effects of the Invention> As stated above, the method of the present invention is a simple means of causing solid carbon to exist on or near the surface of the steel material to be heated.
It has achieved a significant reduction in surface defects in hot-rolled sheets due to surface oxidation and grain boundary oxidation of steel materials, and can greatly contribute to improving the yield and quality of hot-rolled steel sheets, and its industrial value is enormous.

[Brief explanation of drawings]

Figures 1A, 1B, and 1C are perspective views showing the state in which carbon particles are arranged on the surface of a steel material. Figure 2 is a 100x enlarged photograph of the cross section of the heated steel material.
The situation of scale occurrence and the definition of "oxidation depth" are shown. FIG. 3 is a graph showing a comparison of the fermentation depths that occur under the various carbon arrangement conditions shown in FIG. 1. FIG. 4 is a perspective view showing the arrangement and covering of carbon grains in a preferred embodiment of the present invention.

Claims (1)

[Claims] 1. An oxidation prevention method characterized by placing solid carbon on or near the surface of the steel material and heating it to 730°C or higher when heating the steel material. 2. The method for preventing oxidation according to claim 1, characterized in that granular carbon is used. 3. When heating steel materials, place solid carbon on or near the surface of the steel material, cover it with an appropriate material so that the generated carbon monoxide and carbon dioxide stay in the vicinity, and heat it to 730°C or higher. An antioxidant method characterized by: 4. The method for preventing oxidation according to claim 2, characterized in that granular carbon is used.