JP6910523B1 - Manufacturing method of ultra-soft rolled steel that does not easily rust - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a controlled cooling steel material having high corrosion resistance and extremely softness and a method for producing the same. SOLUTION: Scrap is melted in an arc furnace and oxygen is blown to receive molten steel having C of 0.05% or less in a radle 2 together with slag having FeO of 20% or more and basicity of 1.0 to 1.5. Oxygen-containing gas is blown from the steel receiver through the plug 3 at the bottom of the radle to reduce C to 0.03% or less, and then an airtight cover is attached to the radle, and the inside of the radle is 50 Torr by the vacuum pump 11 while continuing the blowing. After reducing the pressure to 200 Torr or less to make C 0.01% or less and O 0.05% or more and 1.3% or less, the steel pieces are subjected to continuous casting and then subjected to hot rolling to finish processing temperature. Is performed avoiding the γ-α coexistence region, and then slow cooling is performed to obtain a highly corrosion-resistant and extremely soft controlled cooling steel material. The tensile strength is less than 300 MPa. [Selection diagram] Fig. 1

Description

The present invention relates to an extremely soft rolled steel material having excellent corrosion resistance in which the generation of red rust is suppressed by forming so-called black rust, and a method for producing the same.

Iron is a metal that easily rusts. Rust that occurs on cold-rolled steel sheets in the atmosphere is initially a group of punctate rust, but after worm-eaten and pockmarked, it becomes large corrosion holes. It is typical red rust, brown and porous. It is well understood that cold-rolled steel sheets need to be painted.
On the other hand, some of the ancient iron excavated from the tumulus has withstood corrosion for more than 1500 years even though it is not in a corrosion-resistant environment, and is so resistant to rust that it leaves a mark.
Similarly, it is well known that some of the tatara iron does not easily rust. For example, the polished surface of the current tatara iron fragments remains metallic luster indoors for more than 15 years like stainless steel. It is presumed that a thin non-conductor film (semi-transparent) is formed.
The black tile nails of large wooden buildings have been used for more than 1000 years.
The iron pillars of Chandra Balman (440 mm diameter x 9 m long) in India have withstood corrosion in the underground for 1600 years. It has been elucidated that Fe purity is 99.8% and it contains P iron.

Non-Patent Document 1 describes the mechanism by which tatara iron does not easily rust. According to the report, "In Tatara ironmaking, the reduced iron sand comes into contact with charcoal and rapidly absorbs coal to form molten iron grains, which are air-oxidized in front of the tuyere to raise the temperature and form sparks and fall to the bottom of the furnace. And solidify. At that time, a large amount of oxygen is dissolved as a solid solution. 』,
"Even during forging, the temperature rises due to heating and air oxidation, the surface dissolves, and a large amount of oxygen dissolves. A large amount of oxygen is dissolved by immediate solidification. 』,
"In equilibrium theory, when molten iron equilibrates with FeO, O dissolves 0.17% or more at 1530 ° C. Due to rapid solidification, it becomes a supersaturated solid solution. The solid solution of oxygen is triggered by heating and humidity and decomposes to generate "black rust", which is a dense magnetite (non-conductor) on the surface, in a short time. This black rust prevents the progress of corrosion. "
Magnetite, which is a non-conductive film, is almost transparent when it is thin and exhibits a metallic luster, but it is considered that iron black rust shows a dense dark brown color as the film thickness grows. Some online information confuses black rust and black dyeing.
From the above, it is inferred that the purity of steel and the oxygen content are related to the corrosion of steel.

Non-Patent Document 2 abstracts a paper on the influence of alloying elements on the progress of corrosion of steel. According to it, the following can be seen from FIG. V-6 regarding corrosion in an acid solution.
1) C clearly promotes corrosion. Steel close to pure iron does not easily rust.
2) The progress of corrosion is large in cold-rolled steel sheets, but it is greatly reduced in annealed materials.
Unfortunately, there are no studies on the effects of O.
Black rust (non-conductor film) is formed by supersaturated oxygen to exhibit corrosion resistance, but it is considered that no C assists corrosion resistance after the black rust has fallen off.

Non-Patent Document 3 describes a steel sheet for enamel as the only steel type / product in which oxygen is utilized as an alloying element.
There are two types of steel sheets for amber, the older one is high oxygen steel using armco iron or rimmed steel (0.03 to 0.1% O for non-deoxidized steel), and the other is ultra-low carbon Ti. Additive clean steel (O ≦ 0.003% for deoxidized steel).
Carbon-containing high oxygen steels (eg, rimmed steels) are not currently manufactured. This is because a large amount of CO gas bubbles hinders continuous casting.
Both are ultra-low carbon steels to prevent the generation of CO gas bubbles during enamel treatment. In the former case, a large amount of Mn oxide is said to enhance the adhesiveness of the coated glass layer. The steel type is used without any particular problems in strength, toughness, workability, etc., but there is no description that the high oxygen steel sheet has excellent corrosion resistance. This is probably because the enamel-treated material itself has super-corrosion resistance, so there is no problem with the corrosion resistance of the steel used as the fabric.

From the above, it was found that the conditions for iron that does not easily rust are extremely low carbon and high oxygen.
1) What kind of current products are compatible with the steel grade of the relevant component other than enamel steel?
2) Even if it conforms, it will be possible to compete with the current cost of the product.
The first compatible product is inevitably low strength because it is an ultra-low carbon steel, and is limited to products to which it is acceptable.
Similarly, because of its high oxygen content, there are many non-metal inclusions, which are limited to products that do not matter. I can't find it easily.
On the other hand, regarding the second cost problem, vacuum decarburization treatment such as RH method is currently applied to all production of steel for enamel and other ultra-low carbon steel. For electric furnace ordinary steel mills that do not have the processing equipment, the equipment and cost burden is a major obstacle.

Consider the second cost.
In the production of new steel grades and new products, normal melting refining must be loaded with 1) refining to ultra-low carbon and 2) oxygen-enriched refining, which is the opposite of normal deoxidation. Advanced decarburization is done by vacuum treatment represented by the RH method today. Vacuum treatment of molten steel was originally developed for degassing (H), expanded to advanced deoxidation, and expanded to advanced decarburization.

Patent Document 1 discloses a method for producing high-quality enamel steel having extremely low carbon and high oxygen by the RH method. Refining the above ingredients is not particularly difficult except for the cost.
The RH method is widely applied to general deep drawing steel sheets toward ultra-low carbon and low oxygen.
The problem with vacuum decarburization is
1) Reheating must be followed or preceded in order to cope with the temperature drop of the molten steel due to the treatment for a long time (eg, 20 minutes or more).
2) The vacuum treatment is performed at a high degree of vacuum and under slagless to promote degassing. Scattered due to the bursting of air bubbles on the molten steel surface It is inevitable that the molten steel adheres to the inner surface of the vacuum vessel or the inner surface of the upper part of the ladle, and the work load of refractory consumption and the treatment of the adhered bullion is large.

Patent Document 2 discloses a method (AOD process) advantageous for decarburization in melting stainless steel. According to the report, when the molten steel in the arc furnace is transferred to a converter-shaped smelting furnace (AOD furnace) and oxygen mixed gas is blown into it to suppress the oxidation of Cr and proceed with decarburization, the pressure inside the furnace is reduced. Dramatically improve decarburization efficiency up to extremely low carbon. More conveniently, reheating is not required due to partial heat generation of Cr oxidation.
When this method is applied to ordinary steel, since there is no heat source (oxidation of Cr), an AOD furnace and a reheating furnace are required downstream of the arc furnace, and there are no practical examples due to problems of equipment cost and operating cost.

As mentioned above, vacuum treatment is applied to the production and quality enhancement of special steel, but for ordinary steel, the burden of equipment cost, energy cost, refractory cost, etc. is large except for mass production by large converter, especially reheating. The process (reheating equipment) is indispensable. Therefore, no vacuum equipment has been installed. A drastic cost reduction of vacuum processing is expected, but it is extremely difficult with the current vacuum processing method.

Patent Document 3 describes a simple vacuum treatment in ladle refining. According to this, the molten steel pre-deoxidized in the arc furnace is received by the airtight radle on the side wall together with the floating slag with less FeO, the top and bottom of the radle are sealed with a vacuum cover, and the inert gas is depressurized from the bottom of the radle while depressurizing from the top cover. Is blown in, and a mixture of molten steel + bubbles + slag is formed on the upper part of the molten steel in a low vacuum state to induce deoxidation, desulfurization, and non-metal inclusions. Since the stirring energy is large, the reaction rate is high, and the reheating step indispensable for vacuum processing is not required.
High-grade valve spring materials and the like have been manufactured by this method with a small cost burden. The problem is that there is no disclosure as to whether it is possible to produce ultra-low carbon steel. Furthermore, it is unclear whether the equipment is suitable for the production of reverse hyperoxygen steel because it is a deoxidizing facility.

The Iron and Steel Institute of Japan, Ferham Vol24 (2019), No.9, P.16, Kazuhiro Nagata, "Chemistry is Suspicious-6 Tatara and Rust-resistant Iron" The Iron and Steel Institute of Japan, Steel Materials and Alloy Elements, P.223, Fig.V-6 The Iron and Steel Institute of Japan, Steel Materials and Alloy Elements, P.751, Fig.19.34 The Iron and Steel Institute of Japan, Steel Materials and Alloy Elements, P.741, Fig.19.12

Published Patent Publication 2001-271179 Published Patent Publication 2010-156021 Published Patent Publication No. 57-192214

It is well known that there are similar irons and steels that are easy to rust and hard to rust, and it is also known that red rust progresses but black rust stagnates. It is presumed to be extremely low carbon and high oxygen.
The first problem to be solved by the present invention is to search for an application that can effectively utilize a steel grade whose component is extremely low carbon and high oxygen, which is characterized by being resistant to rust.
The second is to manufacture the appropriate products that have been searched for without any cost problems.
Specifically, it is to provide a method that can economically process advanced decarburization refining, which has a particularly large cost burden, as much as ordinary steel.

In the first invention, when the composition component is mass%, C is 0.01% or less, Si is 0.01% or less, O is 0.05% or more and 0.13% or less, Mn is 0.5% or less, High corrosion resistance and ultra-soft controlled cooling characterized by P of 0.03% or less, S of 0.03% or less, tensile strength after hot working of less than 300 MPa, and uniform ferrite metal structure. It is a steel material.

In the second invention, the control cooling steel material is a wire rod for a serpentine that does not require Zn plating, a flat steel for glazing that does not require Zn plating, a gas pipe that does not require Zn plating, a steel plate that does not require Zn plating, a thick plate / thin plate / flat steel / bar steel. The highly corrosion-resistant and extremely soft controlled cooling steel material described in the first invention, which is one of the steel materials for an electromagnetic iron core that does not require surface insulation treatment using any of the wire rods as a material.

The third invention is characterized in that the means for making the tensile strength less than 300 MPa and making the metal structure uniform ferrite is performed in a single ferrite region at a hot working temperature of 910 ° C. or lower after intermediate rolling, and then slowly cooled. This is a method for producing a highly corrosion-resistant and extremely soft controlled cooling steel material according to the first invention or the second invention.

In the fourth invention, the method of setting C to 0.01% or less and O to 0.05% or more and 0.13% or less oxidizes the C concentration in molten steel to 0.05% or less in a melting furnace and FeO. The molten steel is discharged into a radle together with floating slag having a concentration of 20% or more and a basicity of 1.0 or more and 1.5 or less, and an oxygen-containing gas is blown from the bottom of the radle to promote the CO reaction and promote the oxidation of Fe. Next, an airtight cover is attached to the upper opening of the radle, and the airtight cover inner space is reduced to 50 torr or more and 200 torr or less by a vacuum pump while blowing is continued to control the amount of C and the amount of O within the range described in the first invention. This is a method for producing a controlled cooling steel material having high corrosion resistance and extremely softness.

Here, as the definition of the predicate, since steel is originally defined as an alloy of iron and carbon, the "steel material" of the present invention is substantially C-free, so that it is not steel but an iron-oxygen alloy. However, even iron with 0.01% C or less is commonly called "ultra-low carbon steel", and its use overlaps with ordinary steel materials, so it followed suit.
"Hot working" is substantially hot rolling, and includes hot extrusion for steel pipes.
The "hot working temperature" is the temperature immediately before machining.
"Slow cooling" is a cooling rate lower than that of air cooling, and for a wire rod with a diameter of 5.5 mm, the air cooling is about 7 ° C / s, but the air cooling is set to 4 ° C / s or less as a guide. If the diameter is large, it will be slowly cooled even with air cooling.
(The cooling rate is the average from 900 ° C to 500 ° C)
"Controlled cooling" is usually the control of cooling conditions to obtain the desired metallographic structure and mechanical properties from austenite. In the third invention, it can be said that the heat treatment corresponds to direct annealing.
In the present specification, "%" indicating the concentration of the component is all mass%.

The first effect of the ultra-soft controlled cooling steel material of the present invention is that as a result of containing oxygen in hypersaturation, red rust (iron hydroxide) is less likely to occur, and black rust (magnetite) is generated and subsequent rust progresses. It is suppressed and becomes a highly corrosion-resistant steel material.
An example of an economical application of a highly corrosion-resistant steel material is an iron wire for a gabion. In the product, the wire is drawn to a desired diameter and then galvanized. However, if the hot-rolled wire of the present invention is used as it is, or if the wire diameter is smaller than that of a normal wire, hot-rolled again to obtain the desired diameter. The iron wire can be used as an iron wire for gabion without galvanizing, which is a great advantage in terms of cost.
The same effect can be obtained with grating, gas pipes, thin plates, etc.
Depending on future improvements, some stainless steel products may be replaced by the steel materials of the present invention.

Secondly, the strength of the steel material of the present invention is as follows: 1) The purity of iron is 99% or more and does not contain C and Si which are reinforcing elements. 2) In normal hot rolling and air cooling of mild steel, γ- The strength is the lowest when rolled in the α coexistence region. 3) Since there is no precipitation of carbides, there is no need for time for spheroidization for softening, and there is no further softening. Annealed steel material can be obtained. If the strength does not reach the general standard strength, but insufficient strength becomes a problem, it can be dealt with by thickening.

Thirdly, since the controlled cooling steel sheet of the present invention is substantially pure iron, the magnetic hysteresis loss is extremely small, which is not much different from that of a high-grade electromagnetic steel sheet, and because it is a hot-rolled finish, an oxide film is formed on the surface. Although the insulation is insufficient, the induced current loss is not large, so it can be replaced with ordinary high-grade and expensive electromagnetic steel sheets depending on the situation, and the cost reduction effect is great. Since the electromagnetic performance is somewhat low, it can be used as an iron core for an electromagnetic coil, a motor, a DC motor, etc., which have a low operating rate. It is also possible to cut out from a thick plate or steel bar and directly form it into an iron core without laminating thin plates.

Fourth, the only refining device newly required for producing the ultra-soft controlled cooling steel material of the present invention is a simple decompression device having a low degree of vacuum, which is the same as the conventional decarburization device such as the RH method. In comparison, the equipment cost is less than a fraction. Regarding operating costs, there is no reheating process, high energy costs such as steam ejectors for high vacuum are not required, vacuum pumps with low power costs, and refractory durability is good due to high-speed refining. It is overwhelmingly advantageous compared to the current refractory cost of ultra-low carbon steel.

Fifth, the decarburization method in the ladle of the present invention reduces the pressure in the same manner as the conventional vacuum treatment, but the pressure is slightly reduced and slag is interposed unlike the conventional method, so that the molten steel is scattered due to bubble burst and refractory. There is no bare metal adhering to the wall surface of the object, and it is extremely excellent in terms of workability and refractory treatment.

The schematic structure of the refining apparatus for melting the highly corrosion-resistant, ultra-soft controlled cooling steel material of the present invention is shown. The cooling method after hot rolling is shown for the wire rod, and A is the slow cooling of the present invention, and the cooling rate is about 4 ° C./s for the wire rod having a diameter of 5.5 mm. B is 7 ° C./s with normal air cooling. It is a phase diagram of the Fe-P system, and shows the part of the γ loop. The effect of C% on the corrosion rate in 0.1 molar hydrochloric acid of iron is shown. The horizontal axis shows C% and the vertical axis shows the corrosion rate. Source: The Iron and Steel Institute of Japan, Steel Materials and Alloy Elements (New Edition), P.223, Fig. V-6 The relationship between C% and O% in molten steel at the time of steel ejection is shown. The axis is written in English, but the horizontal axis is C% and the vertical axis is O amount (ppm). Source: The Iron and Steel Institute of Japan, 217,218 Nishiyama Memorial Technical Course, P.164, Fig. 2 The effects of decarburization rate and FeO% on the foaming height of slag during the decarburization reaction are shown. Source: Kazumi Ogino, Agne Technology Center, High Temperature Surface Chemistry Vol. 2, p.41, Fig. 18.55 _{The effect of slag basicity (CaO / SiO 2} ) on the foaming height of slag during the decarburization reaction is shown. Source: Kazumi Ogino, Agne Technology Center, High Temperature Surface Chemistry Vol. 2, p.41, Fig. 18.54 The effect of stirring energy density on the uniform mixing time of molten steel (an alternative factor for the reciprocal of the reaction rate) is shown. The vertical axis is the uniform mixing time (sec) and the horizontal axis is the stirring energy density (W / ton). The data of the present invention was added to the source. Source: Hiroyuki Kajioka, published by Chijin Shokan, "Tori Nabe Smelting Method", P.94, [Fig. 2.51]

Hereinafter, a method for producing a controlled cooling steel material having high corrosion resistance and extremely softness of the present invention will be described with reference to the drawings.
In FIG. 1, scrap of raw material is melted by an arc furnace (not shown), oxygen is blown to reduce the C concentration in the molten steel to 0.05% or less, and then the molten steel is placed in the ladle 2 on the ladle trolley 1. Scrap out with slag. By oxidative refining, Si becomes 0.01% or less, Mn becomes 0.2% or less, P and S become about 0.03%, and O becomes about 0.06%. The slag composition has a basicity of 1.0 or more and 1.5 or less by adjusting the amount of lime input. The FeO concentration in the slag is about 20% or more by oxygen blowing.

When receiving steel, a mixed gas such as oxygen and an inert gas is blown into the molten steel 4 through a refractory blowing plug 3 provided at the bottom of the ladle 2. When C-containing unacidified steel is ejected, violent CO boiling occurs due to the CO reaction, and the ladle 2 may overflow. Therefore, gas blowing is also used to stabilize the boiling. C in the molten steel burns about 0.02% during the receiving steel for 1 to 3 minutes and becomes 0.03% or less.
After receiving the steel, the ladle carriage 1 is guided to the decompression device, and during that time, the blowing is continued to proceed with the CO reaction.

The decompression device includes a vacuum cover 5 joined to an airtight flange 6 provided on the outer periphery of the upper part of the iron skin of the radle 2, an auxiliary material input hopper 7 attached to the vacuum cover 5, and a suction pipe 8 connected to the vacuum cover 5. A cooling tower 9 that is connected to the suction pipe 8 to cool the suction gas, a filter-type dust collector 10 that follows the cooling tower 9 and removes dust in the gas, a vacuum pump 11 that follows the cooling tower 9 and sucks the refined gas, and the above. It is composed of a gas supply system 12 for blowing.
The gas supply system 12 sucks a part of the exhaust gas of the vacuum pump 11 and pressurizes it by the compressor 13. Gas mixing of the exhaust gas pipe 14, the oxygen gas pipe 15, the inert gas pipe 16 and the above three types of gases as appropriate. It consists of a machine 17 and a plug 3 that blows mixed gas into molten steel.

When the ladle trolley 1 reaches the decompression device, the vacuum cover 5 is lowered and the lower end of the vacuum cover 5 comes into contact with the airtight flange 6 to form the upper space of the ladle into an airtight chamber.
Immediately, the vacuum pump 11 is operated to depressurize the space toward about 100 Torr. As the pressure is reduced, the surface of the molten steel floating in the slag changes from a locally ejected shape due to gas injection to a flat foamed shape, and the entire liquid level rises by 500 mm or more.
A mixture of molten steel, slag, and air bubbles is formed in the rising portion. It can be seen from the large amount of iron particles mixed in the ejecta when it overflows. Vigorous agitation between molten steel, slag, and bubbles accelerates the chemical reaction that should occur.

The residual C in the molten steel _{reacts with the dissolved O, O 2} in the bubbles, and Fe O in the slag and is released as CO, and the concentration can be easily reduced to 0.01% or less.
When the C% is high, the blown O ₂ mainly reacts with C and hardly reacts with Fe, but when the C% is 0.05% or less, the reaction shifts to Fe priority and the O% in molten steel increases. The temperature rise of the molten steel due to the heat of oxidation develops.
If necessary, an Mn alloy or the like is added to the molten steel from the auxiliary material hopper 7 to obtain a predetermined component.
After refining for a predetermined time, O% is measured by measuring the temperature of molten steel with a thermocouple and an oxygen sensor based on an oxygen concentration cell of solid electrolyte, and after fine adjustment, transfer the radle 2 to a continuous casting machine for billets for continuous casting. To serve.

During casting, undeoxidized steel usually produces and rises air bubbles due to the CO reaction that occurs at the solidification interface during solidification, and ingot steel becomes rimmed steel, but continuous casting makes it impossible to cast due to the occurrence of blow-up.
However, in the present invention, C% is 0.01% or less, the CO reaction is suppressed, and casting can be performed without any problem.

Hot working will be described by taking wire rod rolling as an example. The obtained steel pieces are subjected to a normal wire rod mill and are made into wire rods by hot rolling. If the rolling temperature is set loosely according to ordinary ordinary steel, it will be 900 to 950 ° C. after intermediate rolling. This range is the γ-α coexistence range for this steel grade. In the present invention, the region is avoided for softening, and the region is used in a high temperature γ region or a low temperature α region.
After rolling, as shown in FIG. 2, the wire mill is usually equipped with a controlled cooling device (eg, Stelmore process). In the present invention A, the controlled cooling by the device is applied, but the ventilation is stopped, the ring pitch of the parallel ring row is reduced, the cooling rate is made smaller than that of the air cooling B, and the softening is assisted.

It is correct to regard this steel grade as an Fe-P alloy, not a carbon steel. Because C is 0.01% or less, which is close to pure iron. On the other hand, P is present at an average of about 0.03%, and the P concentration differs between the dendritic crystals at the time of solidification, and the latter is usually 2 to 3 times that of the former. At the time of rolling, the microsegregation zones of P are distributed in a bundle.
FIG. 3 shows a portion of the γ loop in the Fe-P phase diagram. Unlike carbon steel, A ₃ temperature rises rather than falls as the amount of the alloy. Assuming 0.06% P concentration in the enrichment section from FIG., A ₃ temperature of about 950 ° C., dilute portions of about 910 ° C.. Therefore, when rolling is performed at the above-mentioned coexistence region temperature of 910 to 950 ° C., the strip-shaped high P portion precedes and collects P while discharging a trace amount of C to grow an α phase having no C high P, and the remaining portion is a trace amount of C. It has a low P γ phase. Clarify the segregation of P. In the final metallographic structure, high P-ferrite and low-P-ferrite are distributed in a band shape when observed closely, and they are not strictly homogeneous. Includes microstructure problems specific to γ-loop forming elements.
In the present invention, in order to avoid the differentiation of C and P and to make the material softer, the hot rolling temperature is set to a single region of α or γ, that is, 910 ° C. or lower or 950 ° C. or higher to obtain a more uniform ferrite. It has some advantages in softening and corrosion resistance.

The reason for specifying the tensile strength to be less than 300 MPa is that 1) the annealed lumber has excellent corrosion resistance as compared with the processed lumber, and 2) the cold workability is maximized. The upper limit was the strength level that could not be obtained. It is significant to achieve below the level of sufficient annealing of mild steel wire only by rolling. In addition to being advantageous for secondary processing due to its softness, there are various applications that do not require strength.

The behavior of the oxygen concentration [O%] in the molten steel will be described.
From the theory of equilibrium, [C%] and [O%] are in inverse proportion to each other.
FIG. 5 shows the relationship between [C%] and [O%] after oxygen smelting and steel removal. Decarburization refining to low carbon is a prerequisite for melting high oxygen steel.
From the figure, in order to set [O%] to 500 ppm (0.05%), [C%] must be about 0.05% or less. When decompression treatment is performed under the [C%] and [O%] concentrations, the reaction of [C] and [O] is reactivated, and the molten steel overflows from the ladle due to intense boiling, which is dangerous. For this problem, decarburization must be done to 0.03% C or less before decompression. As the [C%] decreases, the boiling intensity also decreases.

For that purpose, as described above, oxygen-containing gas having an appropriate concentration is blown into the steel receiving material to promote decarburization.
The decarburization reaction proceeds even if the composition of the blown gas is inert, but in that case, a deoxidation reaction is involved. Oxygen-containing gas adds new oxidation to deoxidation by CO reaction and not only translates decarburization and oxidation, but also heats molten steel by oxidation of Fe, and the molten steel temperature in refining, which is a cooling process. Compensate for the decline. If the steel output temperature is set slightly higher, reheating becomes unnecessary toward continuous casting, which is very convenient.

The specification of the component of the steel material of the present invention will be described.
To ensure ultra-softness, as appropriate components, C; 0.01% or less, Si; 0.01% or less, Mn; 0.5% or less, O; 0.05 or more and 0.13% or less, P; 0.03% or less, S; 0.03% or less, and the rest are Fe and unavoidable impurities.
It does not contain C and Si, which are the reinforcing elements of steel, and Mn is also kept to the minimum necessary.
With the above components and the above-mentioned controlled cooling, the tensile strength can be reduced to less than 300 MPa.

Regarding C%, at least 0.02% or less is required so that boiling (Rimming action) due to continuous generation of CO bubbles does not occur in casting of undeoxidized steel, but it is set to a lower level due to softening.
Another reason is that as shown in FIG. 4 and as described in paragraph [0004], the corrosion rate of the cold-rolled steel sheet increases in accordance with the increase in C%, so the smaller the C%, the better.
Another major conclusion from FIG. 4 is that the corrosion rate of the annealed material is extremely low. This is the basis for slow cooling after hot rolling in paragraph [0032].

Si% was set to 0.01% or less in order to eliminate the deoxidizing action, and Mn% was set to the same upper limit as that of enamel steel.
O% was set to 0.05%, which is a problem of enamel adhesion, with reference to the range of enamel steel, and the upper limit was extended from corrosion resistance.
Non-Patent Document 5 shows the influence of O on the mechanical properties of pure iron, and it is interpreted that there is no particular inconvenience up to 0.013% O, and the range is set as such.
As described above, as conditions for ensuring high corrosion resistance, 1) high oxygen (O; 0.05 to 0.13% O) following Tatara, 2) low carbon steel (≦ 0.01% C), 3) The three factors of the annealed structure (<300 MPa) were combined and identified.

The concentration of unavoidable impurities depends on the grade of the raw material, but it is about 0.4% or less in total for ordinary scrap. The purity of iron can be 99.0% or higher.

The identification of the slag composition will be described.
The slag composition affects the decarburization / oxidation of molten steel and assists the decarburization / oxidation by blowing oxygen-containing gas. For that purpose, the FeO concentration in the slag needs to be 15% or more.
Another role of the slag in the present invention is to prevent the metal from adhering to the airtight cover and the ladle side wall due to the vacuum treatment.
In normal vacuum decarburization, a high vacuum (1 Torr or less) is applied under slagless, so the molten steel scatters due to the bursting of bubbles on the surface of the molten steel, and the metal deposits accumulate on the vacuum vessel and the side wall of the ladle, resulting in quality and work. The problem is big.

The presence of slag and low vacuum prevent this scattering, but there are also problems. The decarburization process generates a large amount of gas, and it is dangerous if the foaming height becomes excessive. The foamability of slag varies greatly depending on the composition.
FIG. 6 shows the effects of the amount of gas and FeO% on the foaming height during oxygen blowing in a converter.
From the figure, it can be seen that the foaming height naturally increases in proportion to the amount of gas, while it decreases as the FeO concentration increases. From here, the FeO concentration was set to 20% or more.

Similarly, FIG. 7 shows the effect of slag basicity on foaming height. From the figure, when the basicity exceeds 1.5, the foaming height increases rapidly. From this, the basicity (= CaO / SiO ₂ ) was specified to be 1.0 or more and 1.5 or less.

The decompression treatment conditions will be described.
In ordinary ultra-low carbon steel, high vacuum is a condition because it is 0.005% C or less. The amount of agitated gas is also limited and the reaction rate is not high.
In the present invention, the stirring energy density (kW / ton), which is proportional to the reaction rate, is emphasized, and the amount of gas is emphasized rather than the degree of vacuum.

FIG. 8 shows the effect of stirring energy density on the reaction rate (substitute by uniform mixing time). In the depressurization treatment of the present invention, the degree of vacuum is about 100 Torr with respect to 1 Torr of the RH method, the amount of blown gas is 10 times or more, and the stirring power is several tens of times.
The lower limit of the appropriate range of vacuum is 50 Torr, which can be easily obtained by a one-stage vacuum pump, and the upper limit is 200 Torr, which causes effective foaming, although the lower limit differs slightly depending on the pump model. Vacuum equipment and its power costs are extremely economical. By the way, in high vacuum, a multi-stage (3 to 6) exhaust system is required for all types.
The stirring energy density is inversely proportional to the molten steel capacity, proportional to the amount of gas, and proportional to the logarithm of the pressure ratio above and below the molten steel up to about 0.1 atm.

Regarding the composition of the blown gas, if the purpose is only decarburization by promoting the CO reaction, only the inert gas may be used. In the present invention, an oxygen mixed gas is used to translate decarburization and oxidation. Pure oxygen is not good for plug durability. Since argon gas is expensive, a part of the exhaust gas of the vacuum pump is returned as a measure to reduce the amount used. Exhaust gas consists of Ar, O ₂ , CO, CO ₂ and some N ₂ .

The controlled cooling steel material having high corrosion resistance and extremely softness of the first invention has been described above. It is desirable to apply it to products where low strength is acceptable.
As an example, in the manufacturing process of an iron wire for a gabion, a mild steel wire is used as a material, drawn to a predetermined diameter, and then galvanized to obtain a product. The same applies to some wire meshes. The low-carbon, high-oxygen annealed steel material of the present invention has excellent corrosion resistance, so that zinc plating is not required.
Similarly, grating, gas pipes, steel plates, etc., which are usually Zn-plated, do not need to be plated.

Electrical steel sheets are a type of steel / product that does not require strength and does not pose a problem with non-metal inclusions. Since the controlled cooling steel material of the present invention is substantially pure iron, the magnetic hysteresis loss is extremely small, and since it is a hot-rolled finish, an oxide film is formed on the surface and the insulating property is insufficient, but the induced current loss is not large. In some cases, it can be replaced with ordinary high-grade and expensive high-Si electromagnetic steel sheets. Although the electromagnetic performance is somewhat low, the electromagnetic loss is small even when used as an iron core for electromagnetic coils, motors, DC devices, etc., which have a low operating rate. It is advantageous for high-grade electrical steel sheets at the total cost.
When forming the iron core, a thick plate or flat steel may be secondarily hot-rolled to obtain a desired thickness, and thin plates may be laminated as before. It may be post-annealed. The cost is reduced more than before.

Regarding slow cooling after rolling, if the steel bar has the same diameter or thickness of 10 mm or more, it is substantially cooled by air cooling. No special equipment required.
Flat steel, shaped steel, planks, and steel pipes may be treated in the same manner as wire rods and steel bars. There is no particular problem.

If the hot coiled thick plate of the present invention is processed into a cold-rolled steel plate as usual and annealed by brilliance, a thin plate that can replace the stainless steel plate may be obtained.

Consider the amount of high-grade waste such as new press as a melting material. The melted components are C; 0.1 to 0.2%, Si; 0.01% or less, Mn; about 0.2%, and Cu + Ni + Cr; about 0.3%. Fits the present invention.

The specifications of the decompression device for ladle refining following the arc furnace with a capacity of 30 tons are as follows.
Exhaust capacity; 500 Nm ³ / h x 3 units Blow-in gas amount; 0.2 to 0.5 Nm ³ / minute Processing pressure; 70 to 200 Torr
Stirring energy density; 0.1 to 0.3 kW / t
Processing time: 5 to 7 minutes reached C%; 0.010% or less It can be operated at a lower cost than general LF (ladle smelting equipment with arc heating).

The main points of hot rolling conditions for wire rods are as follows.
Finish rolling temperature; 920 ° C
Controlled cooling; slow cooling Under the above conditions, the tensile strength of the steel material is 280 MPa.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to produce an extremely soft steel material, and it is possible to impart high corrosion resistance to various existing products.

1; Radle trolley 2; Radle 3; Blow-in plug 4; Molten steel 5; Vacuum cover 6; Airtight flange 7; Secondary material hopper 8; Suction pipe 9; Cooling tower 10; Dust collector 11; Vacuum pump 12; Gas supply system 13; Compressor 14; Pressurized exhaust gas pipe 15; Oxygen gas pipe 16; Inactive gas pipe 17; Mixer

Claims (3)

A method for producing a steel material that does not easily rust. When the composition component is mass%, C is 0.01% or less, Si is 0.01% or less, O is 0.05% or more and 0.13% or less, and Mn is 0. 5% or less, P 0.03% or less, S 0.03% or less, the balance is Fe and unavoidable impurities (mainly Cu + Ni + Cr), the purity of iron is 99.0% or more, and the rolling temperature is 910 ° C. It is characterized in that uneven distribution of P is alleviated by rolling in the γ or α single-phase region while avoiding the range of 950 ° C. or lower, the tensile strength is reduced to less than 300 MPa by slowly cooling after rolling, and zinc plating is not required. A method for manufacturing a hot-rolled wire rod for a rust, a hot-rolled flat steel for grazing, a hot-rolled gas pipe, or a substitute steel plate for a Zn-plated steel plate. A method for producing a steel material that does not easily rust. When the composition component is mass%, C is 0.01% or less, Si is 0.01% or less, O is 0.05% or more and 0.13% or less, and Mn is 0. 5% or less, P is 0.03% or less, S is 0.03% or less, the balance is Fe and unavoidable impurities (mainly Cu + Ni + Cr) , the purity of iron is 99.0% or more, and the rolling temperature is 910 ° C. It is characterized by alleviating uneven distribution of P by rolling in the γ or α single-phase region while avoiding the range of 950 ° C. or lower, slowly cooling after rolling to reduce the tensile strength to less than 300 MPa, and the surface remaining as an oxide film. A method for manufacturing an electromagnetic iron core material . In claim 1 or 2, the C concentration of molten steel in the melting furnace is oxidized to 0.05% or less, and the floating slag having a FeO concentration of 20% or more and a basicity of 1.0 or more and 1.5 or less is said to be present. The molten steel is discharged to the radle, oxygen-containing gas is blown from the bottom of the radle to promote the CO reaction and the oxidation of Fe, then an airtight cover is attached to the upper opening of the radle, and the space inside the airtight cover is 50 torr or more by a vacuum pump. Control of the amount of C and the amount of O, which is characterized by continuously inducing the amount of C to 0.01% or less and the amount of O to 0.05% or more and 0.13% or less while reducing the pressure to 200 torr or less. Method .

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