JPS6015684B2 - Method for manufacturing hot rolled steel materials - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing killed steel materials by hot rolling, particularly rolled steel materials for welded structures with little primary scale tendency, and high-strength steel materials.

In general, it is desired that the scale eaves be minimized as much as possible for the rolled steel materials for welded structures manufactured using Si killed steel or Yamaichi Si killed steel. It is strongly desired that there is no scale habit. However, when we take the above-mentioned section steel as an example, since it is manufactured from Si or N-Si killed steel, it tends to cause scale problems, and the higher the content of Si and peaks, the worse the scale peelability becomes and the scale waves. Since it occurs frequently, research has been conducted on methods to reduce this scale problem, but no method has been established that is economical enough to be adopted on industrial production lines, and the current situation is that scale problems are a problem. Ta. The inventors of the present invention, who have conducted research on an economical manufacturing method that reduces the occurrence of the aforementioned scale habit, first focused on the removability of scale, and as a result,
As is well known, the removability is not only the removability of the upper layer of the scale based on port car tests.
We learned that we must also consider the mixed layer that forms the scale/substrate boundary layer and the peelability of subscales. Research on the releasability of the upper layer of scale or residual scale includes a well-known paper by R. ROlls, but this paper lacks a description of the releasability of the mixed layer and subscale, and the present invention To the best of our knowledge, Sio. There have been no reports of industrial success with high Si content steel of 25% or more.

Now, there are two types of scale, primary scale and secondary scale.The former mainly occurs during the heating process, so it becomes black and gray in thickness, and if it is rolled with this adhered, it will cause the first scale. As shown in the figure, a cloud-shaped scale eaves 15 is formed, which significantly impairs the appearance of the steel surface.

Si-killed steel and AI-Si killed steel are characterized by the remarkable occurrence of the primary scale. The secondary scale occurs after the end of hot rolling, exhibits a pale blue-gray or reddish-gray color, and impairs the appearance. Therefore, the main point of the present invention is to prevent the formation of eaves caused by the primary scale. Therefore, the technical scope of the present invention does not include secondary scale behavior.

Now, the following items [1} to [41] are known as factors that worsen the peelability of scale, especially primary scale.

‘1’ Increase in content of alloying components.

■ Alloying elements (Si, C) in the scale/iron boundary layer
Enrichment of r, Ni, C, As, Sn, etc.).

Glue Alloy components during heating (FeS, Fe2Si04, Mo
o3, NiS2, V2Q, etc.). '4) Formation of a complex mixed layer due to grain boundary penetration of molten alloy components.

Now, as a result of researching each of the above items, the inventors of the present invention have learned that particular attention must be paid to Si.

In other words, Si corresponds to the above m to ■ terms, and especially 120
When the temperature is higher than 0qo, Si enriches in the scale/substrate boundary layer and forms an alloy component with iron oxide Fe○,
04 (Fa scale lite) and the Fe2Si04
The melting point of is around 12060, so if it is heated to a temperature higher than that, the Fe2Si04 enriched in the boundary layer will melt and form grains as shown in the micrograph of the scale and the boundary layer of the steel in Figure 2. This is because in addition to increasing the adhesion of scale in a form similar to that of a naked 16, the melt is estimated to act like an adhesive, and in addition, the melt may cause scale growth. It has been speculated that this inhibits the penetration of oxygen, which is necessary for Fe2Sj04, and inhibits its releasability.First of all, we aimed to solve this problem with Fe2Sj04, and also conducted research on AI. It is an important component as an agent and material modification agent, and
M203 is generated when AI exists, and although the melting point of the eutectic of M203 and Fe○ is as high as 1330q○, it does not viscous the base iron and scale, but also makes Si. This is because it is presumed that it polymerizes with the metal and inhibits the releasability. Now, as a well-known method for solving the above-mentioned problems, there is a method of suppressing the enrichment of [a} Sj.

[b) Method of avoiding melting of Fe2Si04.

'c} Method for measuring the dispersion of Fe2SiQ in steel materials. Three methods have been considered. One way to suppress Si enrichment is to apply paint to the surface of the material to be heated and heat it, but it is not economical, except for special high-grade steel materials, because the material and processing costs increase. Therefore, methods have been adopted to suppress oxidation, that is, to burn at a relatively low air ratio during heating. In addition, as mentioned above, methods have been used to avoid melting of Fe2Si04 by low-temperature heating, and to disperse Si on the surface into the steel billet by calfing the surface of the cold billet. However, both methods have problems in terms of practicality or economy.

As a result of research into a manufacturing method that does not have the above-mentioned problems, the present inventors have developed the extremely practical and economical manufacturing method of the present invention, the gist of which is "CO.05
~0.60%, Nho. 5-2.0%, Sio. 25～
0.60%, No. 05% or less, PO. 03% or less, S
O. 03% or less, the balance being Fe and unavoidable impurities, is soaked for 10 to 8 powders in a temperature range of more than 1250 qo to less than 1320 q0 at an oxygen concentration of 2 to 8% in the soaking zone of a heating furnace, and then ``Method for manufacturing hot rolled steel material by hot rolling.''

Next, the method of the present invention will be explained in detail with reference to the drawings.

FIG. 3 is a schematic vertical cross section of a well-known heating furnace used for heating steel materials, and shows a multi-zone heating furnace consisting of a pre-heating zone 1, a heating zone 2, and a soaking zone 3, and 4a to 4p are heating burners; 5 is a material to be heated (hereinafter simply referred to as steel material), 6 is a flue, and 7a to 7
c is an oxygen meter. In other words, the method of the present invention can be applied even if the heating furnace is of a type in which a pre-heating zone and a heating zone are integrated. Now, the steel material 5 is pushed into the heating furnace from the direction of the arrow 8 by a pusher (not shown) or charged by a walking beam, and undergoes the steps of subheating, heating, and soaking by the heating burners 4a to 4p. It is extracted from the extraction port 9 and sent to the rolling process. The present inventors stopped the conventional combustion control with a low air ratio, that is, a theoretical air amount of around 1.0 parts, and low free oxygen in the atmosphere, and mainly controlled the heating burners 4c, 4n, 4o, and 4p in the soaking zone 3. Air ratio 1.1-1.4
By changing the free oxygen concentration in the atmosphere to 2 to 10%, scale is actively generated.
After that, normal hot rolling including descaling, that is, rough and finish rolling, was performed, and as a result of comparing the occurrence rate of scale rudders and the above-mentioned free oxygen concentration, it was found that the occurrence of conventional scale defects did not necessarily decrease. We obtained new knowledge that an atmosphere with a high free oxygen concentration due to combustion control with a high air ratio actually reduces the occurrence of scale formation, which was believed to be the case.

That is, as shown in Figure 4, the horizontal axis shows the air ratio and free oxygen concentration (%).
), and the scale occurrence rate is plotted on the vertical axis, as shown by broken line 10, the scale occurrence rate decreases sharply from around the air ratio of 1.1, and even above the air ratio of 1.4, the scale occurrence does not occur. Almost never seen. As is clear from this result, the most preferable range of free oxygen concentration is 2 to 8%. In other words, the free oxygen concentration may be increased to 9 to 10%, but heating will become uneven and unnecessarily increasing the amount of scale generated will not be economical, and if it is less than 2%, the prevention of scale eaves will be stable. Therefore, in the present invention, the free oxygen concentration (hereinafter simply referred to as oxygen concentration) is limited to 2 to 8%. Furthermore, according to numerous experiments conducted by the present inventors, the oxygen concentration in the tropical zone and the tropical zone in addition to the scorching zone is 2 to 8%.
However, in terms of heating efficiency, productivity, and scale generation, increasing the oxygen concentration too much is economically disadvantageous, so the oxygen concentration in the preheating zone and heating zone should be lowered. In other words, we performed combustion control with an air ratio of around 1.0, and operated by keeping only the soaking zone at an oxygen concentration of 2 to 8%, but we were still able to achieve good results in terms of scaling, and this did not reduce productivity. I didn't have to. Now, the broken line 10 is when the temperature in the soaking zone is 129,000,
When the temperature is 1260q0, broken lines 11 and 1320q
The case of o is broken line 12, the case of 1350oo is broken line 14,
Further, the case where the temperature is 122,000 is shown by the broken line 13. As is clear from the figure, scale formation occurs frequently even when the temperature is lower than 122,000 and higher than 1,350 pm.

According to the experience of the present inventors, the lower limit of this temperature is 1250o.
o and the upper limit was less than 132,000. That is 125
If it is less than 1,000, scale generation is insufficient, and if it is less than 1,
This seems to be because if it exceeds 320 oo, the scale will melt and the peelability of the scale will not improve. The temperature is a value obtained by correcting the value measured by a temperature meter on the furnace wall close to the steel material by the temperature value measured on the steel material by a pyrometer.

At present, there is no known method of directly contacting the steel material in an operating heating furnace and continuously measuring its temperature, but the method described above or the furnace temperature is the only method available. If adopted, the objective can be achieved. Next, Figure 5 shows the soaking time in the soaking zone and the incidence of scale formation. In Figure 5, the horizontal axis indicates the residence time of steel in the soaking zone, or in other words, the soaking time (
minutes) and scale eaves occurrence rate (%) on the vertical axis.As mentioned above, even if the oxygen concentration is 2 to 8%, if the burning time is too short, it will not be effective in reducing scale eaves. It's faint, I can see it.

That is, it is desirable that the heating time be at least 10 minutes or longer.However, excessively long burning time is not only undesirable in terms of productivity and energy saving, but also tends to increase the rate of scale formation.
It is preferable to set it to about minutes. Furthermore, Fig. 6 shows the results of a study on the correlation between the time the steel material spent in the heating furnace and the scale occurrence rate. Figure 6 shows the furnace time (hours) on the axis and the scale occurrence rate (%) on the vertical axis, and the scale occurrence rate increases when the furnace time exceeds 5 hours. Therefore, when implementing the method of the present invention, care should be taken to ensure that the furnace time does not exceed 5 hours. Next, the composition of the atmospheric gas in the heating furnace will be explained. The composition of the atmospheric gas excluding oxygen in the heating furnace in the above-mentioned embodiments shown in FIGS.
Less than 02,300 leg m, the rest was N2, by the way, H
20, C02, and S02 have oxidizing power similar to oxygen (02), and as the temperature rises, they exhibit an oxidizing power close to that of 02, but in the content range in the future, their influence that leads to scale habit is weak, Its effect on scale generation appears to be different from that of 02.

Although the reason for this is not clear, the atmosphere gas composition in the present invention may be a combustion atmosphere consisting of 022 to 8%, and 030% or less and/or CO2 20% or less, with the balance being N2. However, S02 is preferably 100 or less in any case. Now, as mentioned above, the steel material that has reached the rolling temperature in the heating furnace undergoes normal edging rolling or vertical scale cutting rolling, and then the upper layer of scale is removed by high-pressure descaling water of 100 to 200 kg/ground, and then the upper layer of scale is removed. Residual scale is peeled off by repeated rolling and descaling.

Accordingly, the present inventors estimate that the reason why the scale peelability improves as described above and the scale eaves decrease is because it is said that the scale is enriched in the scale/substrate boundary layer and inhibits the peelability. The resulting alloy compound increases the amount of dispersion into the scale under the 02 conditions and temperature conditions of the present invention, and the remaining part has a strong oxidation expansion force in the subscale even after the upper layer peels off, and a peeling force greater than the adhesion force is exerted. It is thought that the reason for this is that it is easy to peel off scale, just like low-Si steel. Next, the reasons for limiting the components in the present invention will be explained.

As mentioned above, the steel type to which the present invention is applied is Sj killed or N-Si killed steel, and is mainly intended for use in rolled steel materials for welded structures and high-strength steel materials. Each component element is used to maintain the physical and chemical performance of the steel, and the reason for limiting the range of the components is as follows.

C is necessary to increase strength, but if the content increases and exceeds 0.60%, it will impair ductility, so 0.60%
% or less.

On the other hand, if the content is less than 0.05%, sufficient strength cannot be obtained, so the lower limit is set to 0.05%. Mn is an element that has a great effect on improving strength and toughness, and even if it becomes an oxide like Mn○, it is unlikely to become a solid solution in the scale and cause problems, but 2
, 0%* or more, there is a problem in that the toughness rapidly decreases, and if it is less than 0.5%, no strength can be obtained. Si is necessary to make mechanical properties uniform, but if it is less than 0.25%, the strength will be poor and scale will be less likely to occur, so 0.25% is the lower limit.
．． If it exceeds 6%, the toughness will drop sharply, so the upper limit is 0.6.
%. AI is effective in grading steel crystals and improving toughness, but if it exceeds 0.05%, inclusions tend to form, so it is best to keep it at 0.05% or less, but the lower limit is very limited. However, it has some effect, and S
In some cases, such as in i-killed, it is not added intentionally, and the lower limit is not particularly limited.

The lower the P content for killed steel, the better, 0.03%.
If the amount exceeds this amount, surface roughness tends to occur, causing problems in terms of plowability and ductility. S is FeS, C ship, NiS2
The less the element, the better, as it creates low-melting point sulfides such as sulfides, which tend to cause high-temperature brittleness and worsen rutting.
If the content is 3% or less, there will be no problem in the present invention.

Examples regarding the composition and scale reduction rate of steel materials to which the present invention is applied are shown in Table 1 below.

Table 1 The scale reduction rate in Table 1 is calculated based on the number of steel products with scale defects among the steel products heated and rolled using normal low air ratio combustion control, and the number of steel products produced by the manufacturing method according to the present invention. This is the reduction rate by comparing the number of steel materials with defects.

As mentioned above, it was applied to welded structural steel and was found to be effective in reducing scale habit, so C, S;, Mn
As a result of conducting tests on high-strength or high-hardness steel materials with a high content of , good results were obtained as shown in Table 2.

The reason for this is thought to be that C, Mn, etc. have good solid solubility in gasification or scale and do not become a factor that inhibits peeling. Table 2 The present inventors have achieved good results with steel materials having the above-mentioned components, but in addition to the above-mentioned components, Cu, V, Ni,
The effect is also recognized in steel materials to which small amounts of Cr, Nb, Ti, Zr, Mo, Rem, etc. are added.

As explained in detail above, the method of the present invention is a useful manufacturing method that can drastically reduce the occurrence of scale eaves in Si or N-Si killed steel materials.

[Brief explanation of drawings]

Figure 1 is a schematic explanatory diagram of scale waves, Figure 2 is a micrograph (200x magnification) showing the boundary layer between scale and steel, Figure 3 is a schematic vertical cross-sectional view of the heating furnace, and Figure 4 is a diagram showing the oxygen concentration and Figure 5 is a graph showing the relationship between the scale eaves occurrence rate, Figure 5 is a graph showing the relationship between the soaking time and the scale eaves generation rate, and Figure 6 is a graph showing the relationship between the furnace time and the scale occurrence rate. . 1... child tropical zone, 2... heating zone, 3... soaking zone, 4
a~4p... Heating burner, 5... Heated material, 6.
... Flue, 7a-7c...Oxygen meter. Gar figure O 2 figure Power 3 figure Sai 4 figure Z 5 figure Spear 6 figure

Claims (1)

[Claims] 1 C0.05-0.60%, Mn0.5-2.0%,
Si0.25-0.60%, Al0.05% or less, P0
．． A killed steel material consisting of 0.03% or less, S0.03% or less, and the balance being Fe and unavoidable impurities is heated in a soaking zone of a heating furnace.
Production of hot-rolled steel products with few scale defects, characterized in that normal hot rolling is carried out after soaking for 10-80 minutes at an oxygen concentration of 2-8% in a temperature range of more than 1250°C and less than 1320°C. Method.