JPH09143751A - Method for producing hot-rolled steel with excellent surface properties - Google Patents

Abstract

(57) 【Abstract】 PROBLEM TO BE SOLVED: To produce a hot-rolled steel material such as a hot-rolled steel sheet excellent in surface property without cracking, even if a steel piece in which Cu, Sn, etc. remain as a playing card element is used as a raw material. I will provide a. SOLUTION: As a playing card element, C by weight%
u: 0.01 to 0.50% and Sn: 0.001 to 0.05% containing steel pieces, for example, C: 0.01 to 0.20%, Si: 2.0% or less, M
n: 0.05 to 2.0%, Ca: 0 to 0.0100%, Zr: 0 to 0.10.
%, Rare earth element: 0 to 0.10%, Nb: 0 to 0.10%, Ti: 0
~ 0.10%, V: 0 ~ 0.20%, Cr: 0 ~ 1.0%, Mo: 0 ~
1.0%, Cu: 0.01 to 0.50% and Sn: 0.001 to 0.05
%, Balance: A sulfur compound is attached to a steel slab composed of iron and unavoidable impurities so as to have a deposition amount of 10 to 1000 g / m ² in terms of sulfur, followed by heating and hot rolling.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot-rolled steel material having excellent surface properties, and in particular, it has no cracks and is used as a structural member for automobiles or industrial machines.
The present invention relates to a method for manufacturing a hot-rolled steel sheet having excellent surface properties (hereinafter referred to as hot-rolled steel sheet).

[0002]

2. Description of the Related Art In recent years, the amount of scrap iron such as discarded automobiles has been increasing, and it is being reviewed as a raw material for iron making, and it is expected that it will be used in large quantities in the future. However, iron scrap contains Trump elements such as copper (Cu) and tin (Sn) that are difficult to remove by ordinary refining methods. The element remains in the steel. Steel with such a Trump element remains, cracks occur during hot working, and surface defects occur when hot rolling is performed.Therefore, iron scrap containing Trump element is used as a raw material for hot-rolled steel sheets with excellent surface properties. Could not be manufactured.

As a method of preventing surface defects caused by a playing card element, for example, Japanese Patent Laid-Open No. 6-292949 discloses Cu
When casting contained steel, make a thin ingot with a thickness of 75 mm or less,
A method is disclosed in which, after heating so as not to reach a temperature of ℃ or more, rough rolling is omitted and hot rolling is performed. This is Cu
This is a method of rolling at a temperature equal to or lower than the melting point to prevent the occurrence of surface defects. However, in this method, since it causes a decrease in productivity because it is a thin cast piece, or because the heating temperature before rolling is low, the size and amount of precipitates necessary for improving the mechanical properties of the hot-rolled steel sheet can be arbitrarily set. However, there is a problem that the target material cannot be secured because it cannot be controlled.

Further, the present inventors previously added Ni to steel containing Cu, Sn and the like as a playing card element, hot rolled it and wound it at 500 ° C. or lower to obtain ductility, hole expandability, etc. We proposed a method for producing hot-rolled steel sheets with good surface properties without sacrificing the workability (Japanese Patent Application No. 6-32116). According to this method, it is possible to suppress the occurrence of cracks during hot rolling and manufacture a hot rolled steel sheet having excellent surface properties. However, this method requires the addition of Ni, which inevitably increases the manufacturing cost.

Therefore, even when iron scrap is used as a raw material for iron making, a method for producing a hot-rolled steel material having excellent surface properties and free from cracking during hot working, in particular, reduction in productivity and production cost It has been desired to develop a method capable of preventing the occurrence of cracks during hot rolling without increasing the number and manufacturing a hot-rolled steel sheet having excellent surface properties.

[0006]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems in the prior art, and even when a billet containing Cu, Sn, etc. remaining as a playing card element is used as a raw material, it does not crack and has excellent surface properties. Method for producing hot-rolled steel, especially for hot-rolled steel sheets that are widely used as relatively inexpensive structural materials produced by continuous hot-rolling, reducing productivity and increasing production costs. It is an object of the present invention to provide a method for producing a hot-rolled steel sheet having excellent surface properties without any problems. The hot-rolled steel material refers to a steel material manufactured through a hot working (mainly rolling) process such as a hot-rolled steel plate, a seamless steel pipe, a bar steel / wire material, and a shaped steel.

[0007]

Means for Solving the Problems The present inventors have conducted various studies to solve the above-mentioned problems, and as a result, the following (1)
The findings of (2) were obtained.

(1) Crack Generation Mechanism in Cu- and Sn-Containing Steels When Cu is contained in the steel, surface cracks, edge cracks, etc. occur in the hot-rolled steel sheet due to cracks generated during hot working, and the surface quality deteriorates. . This hot work crack occurs when Cu, which is not oxidized at the interface between the scale and the steel and is deposited in the molten state, enters the crystal grain boundaries of the base material during heating of the steel slab, and the steel surface becomes brittle.

FIG. 1 is a diagram schematically showing a crack generation mechanism during hot rolling in Cu-containing steel. Generally, a hot rolled product is manufactured by charging a steel slab as a rolling material into a heating furnace in an oxidizing atmosphere, heating the slab for a long time, and then hot rolling the slab. By heating the steel piece in the furnace, as shown in FIG. 1 (a), thickness of 1mm consists base iron side surface of the steel piece 1 FeO, with Fe ₃ O ₄ and Fe ₂ O ₃ The above scale 2 is generated. In addition, this scale 2 is a FeO-based scale in which the amount of FeO in the scale 2 is 90% or more.

The FeO-based iron scale is produced by Fe being diffused in the scale and selectively oxidized by heating for a long time in a heating furnace, and is contained in steel. The Cu that is melted is concentrated in the molten state at the interface between the scale and steel (that is, the surface of the base iron) and is precipitated as liquid Cu3. After that, when hot rolling is applied,
As shown in (b), Cu in the molten state penetrates deeply into the grain boundaries 4 of the steel, and the steel surface becomes brittle and cracks occur.

The occurrence of this crack is further promoted when Sn coexists in the steel. As a result of the investigation, in the Cu-Sn composite-containing steel, a molten Cu-Sn alloy (liquid Cu-Sn alloy) was precipitated at the interface between the iron scale and steel produced by selective oxidation, and the melting point of this alloy was Cu. It is considered that the cracking is promoted because it is lower than that of the case alone. On the other hand, cracking does not occur in steel containing only Sn. The reason for this is not clear, but since Sn precipitation is not observed when only Sn is contained, it is considered that Sn is oxidized in the same manner as Fe and forms scale.

From the above results, in order to prevent the occurrence of cracks during hot rolling, it is effective to suppress the precipitation of Cu or Cu-Sn alloy on the surface of the billet when heating the billet. Considered to be the target.

(2) By depositing a sulfur (S) compound on the surface of the steel slab before heating in advance, Cu becomes a sulfide and the precipitation of Cu or Cu-Sn alloy on the surface of the steel slab is suppressed. . As a result, cracking can be prevented.

FIG. 2 is a diagram schematically showing the mechanism of suppressing the precipitation of Cu or Cu-Sn alloy by the sulfur compound. When the steel slab to which the sulfur compound is attached is charged into the heating furnace, the sulfur compound first reacts with the thin scale with a thickness of 100 to 200 μm already formed on the surface of the slab immediately after continuous casting. As shown in FIG. 2 (a), a low melting point FeS—FeO 2 eutectic mainly composed of FeS is formed. This eutectic is in a semi-molten state.

By the heating for a long time in the heating furnace, the oxidation proceeds and the FeO-based scale is formed. However, since Fe diffuses in the FeS-FeO eutectic and is selectively oxidized, FIG. As shown in b), the FeS-FeO 2 eutectic remains at the scale-steel interface (base iron surface). Cu is
Since it has an extremely high affinity with S as compared with Fe, the precipitation of liquid Cu or Cu-Sn alloy by the selective oxidation of Fe and the sulfurization reaction of Cu occur simultaneously to form Cu sulfide (Cu ₂ S). , FeS
-Dissolves in the FeO eutectic. Therefore, Cu or Cu-Sn
No alloy precipitation occurs (see FIG. 2 (c)).

It is not clear why the Sn left behind by the sulfurization reaction of Cu does not precipitate on the surface of the base metal, but it becomes an oxide or a sulfide (SnS) as described above.
It is considered to dissolve in the eS-FeO eutectic.

The present invention has been made on the basis of the above findings, and the gist thereof is the following and the following method for producing a hot rolled steel material.

% As a playing card element
Then, in a steel slab containing Cu: 0.01 to 0.50% and Sn: 0.001 to 0.05%, a sulfur compound is converted to sulfur to be 10 to 1000 g /
A method for producing a hot-rolled steel product having excellent surface properties, which is characterized by performing heating so that the coating amount becomes m ² and then heating and then performing hot rolling.

Steel billet, in weight%, C: 0.01 to 0.20
%, Si: 2.0% or less, Mn: 0.05 to 2.0%, Ca: 0 to 0.01
00%, Zr: 0-0.10%, rare earth elements: 0-0.10%, Nb:
0 to 0.10%, Ti: 0 to 0.10%, V: 0 to 0.20%, Cr: 0
.About.1.0% and Mo: 0 to 1.0%, and as a playing card element, Cu: 0.01 to 0.50% and Sn: 0.0
The method for producing a hot-rolled steel sheet (hot-rolled steel sheet) having excellent surface properties as described above, which is a steel slab containing 01 to 0.05% and the balance being iron and unavoidable impurities.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. It should be noted that here, the manufacturing method of the hot-rolled steel sheet will be described, but other than this, for example, the manufacturing of hot-rolled steel using a steel strip for seamless steel pipe, a steel strip for bar steel, etc. is also different in the basic way of thinking. There is no. Further, “%” of the alloy element means “% by weight”.

The above invention is a method for producing a hot-rolled steel sheet using a steel slab containing Cu: 0.01 to 0.50% and Sn: 0.001 to 0.05% as a rolling material as a playing card element.
When heating a steel slab containing Cu and Sn in the above range and then hot rolling the steel slab, it is necessary to convert the sulfur compound into sulfur in advance so that the deposition amount is 10 to 1000 g / m ^2. By making it adhere, it becomes possible to manufacture a hot-rolled steel sheet having no cracks and excellent surface properties. In addition, "converting to sulfur" means that the amount of adhesion is represented by the amount of sulfur contained therein. As described above, this is because the sulfur compound has a low melting point of FeS-Fe at the interface between the scale and steel (base iron surface).
This is due to the fact that it forms an O 2 eutectic and has the effect of suppressing the precipitation of Cu or Cu—Sn alloy at this interface and the infiltration into the crystal grain boundaries.

When the amount of deposited sulfur is less than 10 g / m ^{2, the} amount of sulfur required for the formation of Cu sulfide is insufficient and Cu or Cu—Sn alloy remains on the surface of the base metal, resulting in deterioration of hot workability. Invite. On the other hand, if the amount of adhesion exceeds 1000 g / m ² , the effect of suppressing the precipitation of Cu or Cu-Sn alloy due to the formation of the FeS-FeO eutectic is saturated, not only causing an economical disadvantage, but also the hot-rolled steel sheet. Adversely affects the surface properties of.

It is sufficient that the above-mentioned sulfur compound decomposes in the heating furnace as the temperature rises and reacts with scale to become FeS to form a FeO-FeS layer at the interface between the scale and steel. For example, not only iron sulfide (FeS) but also a sulfur compound such as calcium sulfide (CaS) and calcium sulfate (CaSO ₄ ), a compound in which two or more kinds of these sulfur compounds are compounded, or a mixture thereof can be used. Further, it is a compound or a mixture of two or more kinds of compounds (at least one of which should contain S), which is decomposed or oxidized during heating to generate F 2
It may form an eO-FeS layer. Examples of such a compound include a compound composed of FeO and FeS, or a mixture of FeO and FeS.

No special method is necessarily required to pre-deposit sulfur on the steel billet. A method such as coating or spraying may be appropriately used. For example, the sulfur compound may be applied in the form of a slurry, or may be dissolved or suspended in water or another liquid and then sprayed or applied. In particular,
For example, a method may be mentioned in which iron sulfide (FeS) is made into a powder and suspended in water, and a dispersion is applied by spraying. Depending on the sulfur compound used, it is also possible to spray in the form of powder and attach it to the surface of the steel slab.

The heating after the above treatment with the sulfur compound and the subsequent hot rolling are not particularly limited and may be carried out by a commonly used method.

As described above, the contents of Cu and Sn as the Trump element contained in the steel billet which is the rolling material are 0.01 to 0.50% for Cu and 0.001 to 0.001 for Sn.
The reason for limiting the value to 0.05% is as follows.
That is, if the content of these playing cards is more than 0.50% for Cu and 0.05% for Sn, it is possible to prevent the occurrence of surface defects after hot rolling, even if the treatment specified in the above invention is performed. No, while the content of these elements is less than 0.01% for Cu and Sn
This is because if it is less than 0.001%, the occurrence of cracks, that is, the deterioration of workability remains within the allowable limit even if hot rolling is performed under normal conditions.

As described above, the steel slab used as the rolling material in the invention is a steel slab (Cu: 0.01 to 0.50% and Sn) in which Cu and Sn remain as the Trump element as a result of using iron scrap as a raw material for iron making. : 0.001 to 0.05%)
However, there is no limitation on other components. That is, a material to which various alloying elements are added in order to impart the properties required according to the use as a hot rolled steel sheet can be used as a rolling material. Further, unavoidable impurities mixed in during the manufacturing process may be contained.

In the above invention, in the invention of 1, the steel billet as a rolling material has C: 0.01 to 0.20% and Si: 2.0%.
Below, Mn: 0.05 to 2.0%, Ca: 0 to 0.0100%, Zr: 0 to
0.10%, rare earth elements: 0 to 0.10%, Nb: 0 to 0.10%, T
i: 0 to 0.10%, V: 0 to 0.20%, Cr: 0 to 1.0% and Mo: 0 to 1.0%, and further, as a trump element, Cu: 0.01 to 0.50% and Sn: 0.001 to 0.05% It is a method of using a steel slab containing iron and the balance consisting of iron and unavoidable impurities.

When the components contained in the hot-rolled steel sheet produced according to the invention are limited as described above, the reasons for the limitation are as follows.

C: C is a preferable component for increasing the strength of the steel sheet, but if it is less than 0.01%, its effect is not exhibited, while if its content exceeds 0.20%, the workability deteriorates and the welding Cause deterioration of sex. Therefore, the content of C is 0.01-
It was set at 0.20%.

Si: Si is a preferred component that improves the strength and ductility of the steel sheet through solid solution strengthening.

However, if the content exceeds 2.0%, the effect is saturated and the weldability deteriorates. Therefore, the Si content was set to 2.0% or less. The lower limit is not particularly limited,
Since it is also used as a deoxidizer, it is preferably 0.01%.

Mn: Mn is a preferable component for improving the strength of the steel sheet, and further contains S existing as an impurity in the steel.
It is fixed as MnS and also has the effect of suppressing cracking that occurs during hot rolling. However, if the content is less than 0.05%, the effect of suppressing cracking is not recognized, while if the content exceeds 2.0%, the workability decreases and the weldability deteriorates. Therefore, the content of Mn is set to 0.05 to 2.0%.

Ca, Zr, and rare earth elements: All of these components have the effect of adjusting the shape of inclusions to improve cold workability, so they are added as necessary. But Ca
For 0.10% for Zr, 0.10% for Zr, and 0.10% for the total of rare earth elements, the workability deteriorates due to too many inclusions in the steel. , Ca: 0 to 0.0100%, Zr: 0
.About.0.10% and rare earth elements: 0 to 0.10%.

In order to fully exert the above-mentioned effect of improving the cold workability, Ca: 0.0002 to 0.01%, Zr: 0.01 to 0.
10% and rare earth elements: 0.002 to 0.10% are preferable.

Nb, Ti and V: Nb, Ti and V have the action of precipitating as carbonitrides on the ferrite base and increasing the strength of the steel sheet, so they are added as necessary. However, Nb and Ti are 0.10% and V is 0.20%.
If it is contained in excess of 10%, its effect is saturated and it is not economical. Therefore, the content of each Nb: 0 ~ 0.
10%, Ti: 0 to 0.10%, and V: 0 to 0.20%.
Nb: 0.005-0.
10%, Ti: 0.005 to 0.10%, and V: 0.005 to 0.20%
It is preferred that

Cr and Mo: Since Cr and Mo have the effect of strengthening the steel sheet by transformation strengthening, they are added as necessary. However, the content of both Cr and Mo is 1.0%.
If it exceeds, the effect will be saturated and it is not economical. Therefore, the contents of Cr and Mo are both 0 to 1.0.
%. In order to sufficiently exert the above-mentioned strengthening action, it is preferable that both Cr and Mo be 0.01 to 1.0%.

Regarding Cu and Sn, for the same reason as in the above-mentioned invention, the content of Cu was set to 0.01 to 0.50%, and the content of Sn to 0.001 to 0.05%.

The hot-rolled steel sheet produced by the invention of the above is a steel sheet which, in addition to the above-mentioned components, has the balance of iron (Fe), acid-soluble aluminum (sol.Al) added as a deoxidizing agent, and inevitable impurities. Phosphorus (P) and sulfur (S) are important as impurities. It is desirable that the content of each of these components be suppressed to a low value as described below.

Sol.Al: Al is a component added as a deoxidizing agent for steel, but its content is sol.Al from the viewpoint of the cleanliness of steel.
The Al content is preferably 0.30% or less. .

P: P is an impurity element which adversely affects the weldability, and its content should be set in order to secure the desired weldability.
It is desirable to set it to 0.05% or less. In particular, in order to uniformly disperse ferrite, the content is preferably 0.010% or less.

S: S is an impurity element that forms a sulfide-based inclusion and reduces the workability. Therefore, the content of S is
0.05% or less, 0.003 to obtain particularly excellent workability
% Or less is preferable.

The treatment performed when hot rolling after heating the steel slab containing each of the above components is the same as in the case of the invention.

As described above, and the inventions (hereinafter, these inventions are referred to as "the method of the present invention".
The method of the present invention means the invention of or)) is a method of depositing a sulfur compound on a steel slab having the above composition to a predetermined deposition amount, heating, and then hot rolling. is there. The steel used may be steel melted by a converter, an electric furnace or the like, and the steel type may be any of rimmed steel, capped steel, semi-killed steel or killed steel. Further, the steel slab may be obtained by any means of ingot-casting, slab-rolling and continuous casting.

According to the method of the present invention, it is possible to produce a hot-rolled steel sheet which is free from cracks and has excellent surface properties even when using iron scrap containing Cu and Sn trump elements.

The hot-rolled steel sheet produced by the method of the present invention may be further hot-dip galvanized, alloyed hot-dip galvanized, or
The surface-treated steel sheet obtained by performing a surface treatment such as electroplating has a good workability because the base material is a steel sheet excellent in workability in which cracking is unlikely to occur.

[0047]

EXAMPLE A slab having the chemical composition shown in Table 1 was added to Table 2
The iron sulfide (FeS) powder is sprinkled so that the amount shown in Fig. 1 (sulfur equivalent amount) is sprayed, charged into a heating furnace and heated at 1250 ° C for 4 hours.
Heat for an hour, then rough and finish roll,
Rolling was completed at 00 ° C. After that, control cooling is performed and 500
The hot rolled steel sheet with a thickness of 2.3 mm was manufactured by winding at a temperature of ℃. Steel types A to Y are steels satisfying the conditions specified in the method of the present invention, steel type Z is a conventional steel not containing a playing card element,
α and β are comparative steels in which the content of the Trump element exceeds the range defined by the method of the present invention.

A JIS No. 5 tensile test piece was sampled from the steel plate having a plate thickness of 2.3 mm to investigate the mechanical properties and the surface properties of the steel plate.

In the investigation of the surface texture, 10 strips with a width of 1 m and a length of 1 m were randomly sampled in the rolling direction, and a range (face) of 1 m × 1 m for each strip was divided into 100 mm intervals and divided into 100 cells. 1m ² after dividing and counting the number of cracked cells
The number of hits was calculated, and the results of 10 sheets were averaged and displayed according to the following criteria.

◯: Crack occurrence frequency is 0.3 pcs / m ² or less ... Surface quality is good X mark: 〃 exceeds 0.3 pcs / m ²・ ・ ・ Surface quality deterioration due to cracks.

As is clear from this result, test No. 1
In the invention examples (steel types A to Y) of Nos. 25 to 25, similar to the conventional example of Test No. 26 (steel plate Z), cracking due to hot rolling does not occur, workability is excellent, and good surface properties are obtained. ing.

On the other hand, Test No. 27, in which the amount of deposited sulfur was less than the range defined by the method of the present invention, and Test No. 29 in which the concentration of the Trump element exceeded the range defined by the method of the present invention.
In Nos. 30 and 30 (steel types α and β), the precipitation of Cu-Sn alloy could not be suppressed, cracking occurred during hot rolling, and the surface quality deteriorated. Also, in Test No. 28 in which the amount of deposited sulfur exceeds the range defined by the method of the present invention, deterioration of the surface properties was observed.

In addition, although the above-mentioned example is the result of the investigation on the hot-rolled steel sheet, a round billet was used as a steel piece and
The same effect was confirmed for the seamless steel pipe manufactured by the treatment of spraying FeS powder.

[0054]

[Table 1]

[0055]

[Table 2]

[0056]

EFFECTS OF THE INVENTION According to the method of the present invention, even if a steel piece in which Cu, Sn, etc. remain as a playing card element is used as a raw material, there is no cracking and a hot rolled steel sheet, a seamless steel pipe, a bar steel or the like having excellent surface properties It is possible to manufacture the hot-rolled steel material. For example, the hot-rolled steel sheet produced by this method has excellent surface properties, and is therefore suitable as a structural component, an industrial equipment member such as an automobile component, and moreover, since addition of Ni is not required, it is advantageous in terms of cost. is there.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a crack generation mechanism during hot rolling in Cu-containing steel.

FIG. 2 is a diagram schematically showing a Cu precipitation suppressing mechanism by a sulfur compound in Cu-containing steel.

[Explanation of symbols]

 1: Steel billet 2: Scale 3: Liquid Cu 4: Grain boundary

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C22C 38/28 C22C 38/28 38/38 38/38

Claims (2)

[Claims] 1. As a playing card element, in% by weight, C
u: 0.01 to 0.50% and Sn: 0.001 to 0.05% to a steel slab containing so as to convert the sulfur compound to sulfur so as to have an adhesion amount of 10 to 1000 g / m ² , and then heat, Then, hot rolling is performed, which is a method for producing hot-rolled steel having excellent surface properties. 2. By weight%, C: 0.01 to 0.20%, Si: 2.0%
Below, Mn: 0.05 to 2.0%, Ca: 0 to 0.0100%, Zr: 0 to
0.10%, rare earth elements: 0 to 0.10%, Nb: 0 to 0.10%, T
i: 0 to 0.10%, V: 0 to 0.20%, Cr: 0 to 1.0% and Mo: 0 to 1.0%, and further, as a trump element, Cu: 0.01 to 0.50% and Sn: 0.001 to 0.05% To a steel slab containing, with the balance consisting of iron and unavoidable impurities,
Manufacture of hot-rolled steel sheet with excellent surface properties, which is characterized in that the sulfur compound is converted to sulfur so that the adhesion amount becomes 10 to 1000 g / m ² and then heated and then hot-rolled. Method.