JP2001329343A - Hot rolled steel sheet and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot rolled steel sheet exhibiting good adhesion of scale and to provide its production method. SOLUTION: As to this hot rolled steel sheet, in a hot rolled steel sheet in which the surface layer of ferrite is provided with scale, the average grain size L in the surface layer of ferrite is <=20 μm, the thickness (d) of the scale is <=10 μm, and also, in the case the content of C is defined as C by mass % L2×d<=(logC)2×300 is satisfied. In this method for producing the hot rolled steel sheet, finish rolling is finished at <=(Ar3+50 deg.C), and, within 1s, the rolled stock is cooled by >=80 deg.C at a cooling rate of >=100 deg.C/s and is thereafter coiled at 400 to 600 deg.C, by which the average grain size L in the surface layer of ferrite is controlled to <=20 μm, the thickness (d) of scale is controlled to <=10 μm, and also, in the case the content of C is defined as C by mass%, L2×d<=(logC)2×300 is satisfied.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet having excellent scale adhesion to a hot-rolled steel sheet including a striped plate used for automobile parts, building materials and the like, and a method for producing the same.

[0002]

2. Description of the Related Art Various studies have been made to improve the scale adhesion of a hot-rolled steel sheet. For example, in Japanese Patent Application Laid-Open No. 10-36939, in a steel containing 0.1% or more of Al, cooling is started at a cooling speed of 20 s ° C./s or more (abbreviated to cooling speed, the same applies hereinafter) within 2 s after rolling is completed, A technique of winding at a temperature of not more than ℃ has been proposed.

[0003] In this prior art, the growth of the surface layer scale is suppressed by rapid cooling (abbreviation for immediate cooling, the same applies hereinafter) and rapid cooling after rolling to achieve a thin scale. At the same time, by winding at 550 ° C. or less, with respect to the scale of the base steel vicinity, by promoting the eutectoid transformation by _{4FeO → Fe 3 0 4 + Fe} , it is directed adhesion of the scale increase. Furthermore, the formation of a composite oxide layer of Fe and Al at the scale / base iron interface also contributes to improving the adhesion. Therefore, in this technique, 0.1 to 1 mass% of Al is added.

[0004]

However, with the conventional method, sufficient scale adhesion could not be obtained.
Further, since the adhesion of the scale is improved by the composite oxide layer of Fe and Al at the interface, there is a problem that the addition amount of Al is required to be one digit larger than that of a normal thin steel sheet.

An object of the present invention is to provide a hot-rolled steel sheet exhibiting good scale adhesion and a method for producing the same.

[0006]

The above object is achieved by the following invention. The first invention is a hot-rolled steel sheet having a scale on the surface of the ground iron, wherein the average particle diameter L of the surface of the ground iron is 20 μm.
Hereinafter, a hot-rolled steel sheet characterized by satisfying L ² × d ≦ (logC) ² × 300 when the scale thickness d is 10 μm or less and the C content is C in mass%.

The present invention has been made as a result of intensive studies to solve the above-mentioned problems. In the process, by examining various factors governing the adhesiveness of the scale, it was found that the adhesiveness of the scale was dramatically improved by refining the structure of the base iron. Furthermore, the C content of the ground iron also affects the scale adhesion, and when the C content is large, the adhesion of the scale decreases due to an increase in the amount of carbide present at the interface between the scale and the base iron.

Therefore, by controlling the thickness of the scale and the average grain size of the base steel according to the C content of the base steel, the scale adhesion of the hot-rolled steel sheet can be remarkably improved. The present invention is based on such findings, and the reasons for individual limitations are as follows. Here, the steel sheet targeted by the present invention is:
In addition to the normal smooth hot-rolled steel sheet, it also includes a striped steel sheet.

[0009] The average particle diameter L of the surface iron layer is 20 µm or less.
The progress of the eutectoid transformation by Fe ₃ 0 ₄ + Fe, after transformation Fe ₃ 0 ₄
Also have a large particle size. Therefore, the scale adhesion deteriorates. This tendency is due to the average particle size of
It becomes remarkable when it exceeds. Therefore, the average particle size of
Define it to be 20 μm or less.

Scale thickness d: 10 μm or less As the scale thickness increases, the amount of strain applied to the scale at the time of steel plate processing increases, and accordingly, the scale is easily peeled. This tendency becomes remarkable when the scale thickness exceeds 10 μm. Therefore, the scale thickness is 10 μm
It is specified as follows.

Relationship with C content C (mass%): L ² × d ≦ (lo
gC) ² × 300 Further, as described below, the scale adhesion is affected by the C content of the base iron in addition to the average particle size and the scale thickness. This is considered to be because, when the amount of C is large, the amount of carbide present at the interface between the scale and the base iron increases, thereby reducing the adhesion of the scale. This inequality was obtained while organizing the experimental results.

The ratio of the left side / right side of this inequality, that is, [L ² × d]
/ [(logC) ² × 300] is considered to be an index (hereinafter, referred to as an inconsistent strain index) indicating the ease of peeling of the scale. When the mismatch index exceeds 1.0, the adhesiveness of the scale starts to decrease. Therefore, it is necessary to control the scale thickness and the average grain size of the base iron according to the C content of the base iron, and to satisfy the mismatch strain index of 1.0 or less, that is, L ² × d ≦ (logC) ² × 300.

[0013] A second invention provides a method of manufacturing a hot-rolled steel sheet,
Finish rolling is completed at (Ar ₃ + 50 ° C) or less and 100 ° C /
After cooling at 80 ° C or more at a cooling rate of s or more, 400-600 ° C
The average particle diameter L of the surface iron layer is 20 μm or less, the scale thickness d is 10 μm or less, and the C content is mass%.
A method of manufacturing a hot-rolled steel sheet, wherein L ² × d ≦ (log C) ² × 300 is satisfied when C is set to C.

The present invention improves the adhesion of the scale of a hot-rolled steel sheet by controlling the average grain size and the scale thickness of the surface layer of the base iron by quenching after finish rolling and winding at a predetermined temperature. is there. The reason for limiting the production conditions of the present invention will be described below.

In the finish rolling, when the rolling end temperature is high, the scale becomes thick and the Fe particle size becomes large. Along with this, since FeO that is greater the particle diameter of the Fe ₃ 0 ₄ after eutectoid transformation as described below, the adhesion of the scale is deteriorated, the trend is finishing temperature exceeds 50 ° C. than the Ar ₃ point It becomes remarkable. Therefore, the upper limit of the final rolling is set to (Ar ₃ + 50 ° C.).

After the rolling is completed, the formation of scale is suppressed by cooling to 80 ° C. or more at a cooling rate (cooling rate) of 100 ° C./s or more within 1 s, and the scale is reduced by making the ground iron finer. To improve the adhesion. In particular, if any one of the time from the end of rolling to the start of cooling, the cooling rate, and the amount of temperature drop due to quenching is out of this range, the grain size of the ground iron increases, resulting in good scale adhesion. You can't get sex.

When the winding temperature is higher than 600 ° C.,
Scale proceeds and eutectoid transformation by only 4FeO → Fe ₃ 0 ₄ + Fe reaction from the surface layer side, eutectoid transformation in the scale / base steel interface does not proceed. If the winding temperature is below 400 ° C,
For eutectoid transformation at the scale / base iron interface does not proceed, eutectoid transformation by 4FeO → Fe ₃ 0 ₄ + Fe proceeds randomly island inside the scale, it is impossible to obtain a good scale adhesion. That is, the coiling temperature is, for promoting eutectoid transformation by 4FeO → Fe ₃ 0 ₄ + Fe reaction in the base iron side of the scale must be substantially taken up at 400 to 600 ° C..

According to a third aspect of the present invention, a finishing mill comprising a plurality of rolling stands is used, and at least between two or more stands including a stand from a first stand to a third stand, a unit is provided between these stands. Per width (1m)
A method for producing a hot-rolled steel sheet according to a second invention, characterized in that cooling water at a rate of 100 L / min or more is injected, and 30% or more of the steel sheet area between stands is substantially covered with the cooling water.

In the present invention, the adhesion of the scale is further improved by appropriately adjusting the cooling method of the steel sheet between the rolling stands in the finish rolling, that is, the cooling method between the stands. In general, in finish rolling, rolling is started after the scale is completely removed once with high-pressure water on the entry side, but also between the finishing stands after descaling, oxidation of the steel sheet progresses, and the scale grows.
For the adhesion of the scale, if fine the scale (FeO) to generate between finishing stand, after eutectoid transformation Fe ₃ 0 ₄
Can also be made finer, and the adhesion can be improved.

In a fourth aspect of the present invention, P: 0.03% or less in mass%, S:
i: A method for producing a hot-rolled steel sheet according to the second or third invention, wherein a steel satisfying either or both of 0.02% or more is used.

In the present invention, the adhesiveness of the scale is improved by appropriately adjusting the chemical components of the steel sheet. Next, the reasons for limiting the chemical components will be described.

P: 0.03% or less P is an element which increases the compressive stress acting inside the scale by randomizing the growth direction of the scale in the scale growth between the finishing stands. for that reason,
During finish rolling, if the scale is once separated from the ground iron due to an increase in compressive stress, it is not possible to apply sufficient rolling distortion to the scale itself. The tendency is that the P content is 0.03wt%
It becomes remarkable when it exceeds. Therefore, by reducing the amount of P added, strain due to rolling is imparted to the scale, thereby making the scale finer. From the above, it is preferable that the P content be 0.03 wt% or less.

Si: 0.02% or more Si is an element that suppresses the scale from once peeling off the ground iron in the scale growth between the finishing stands.
Therefore, by increasing the amount of Si added, strain due to rolling is imparted to the scale, whereby the scale can be refined. This effect cannot be obtained if the amount of Si is less than 0.02 wt%. Therefore, the amount of Si is preferably set to 0.02 wt% or more.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION In practicing the present invention, steel of a target chemical composition is melted in a steel refining device such as a converter or an electric furnace, and a slab is cast by continuous casting or the like. The slab is directly or temporarily cooled and then reheated to perform hot rolling.
The descaling is desirably performed immediately before the start of finish rolling in order to suppress the subsequent generation of scale. It is desirable to inject cooling water between the finishing rolling stands of hot rolling to cover a wide steel plate area between the finishing stands with the cooling water.

Here, the scale that grows between the finishing stands grows in accordance with the crystal grains of the base iron at the time of scale generation.
The scale can also be refined. Since the particle size after scale growth has a large effect on the scale particle size in the initial stage of production, it is particularly effective to reduce the scale particle size in the initial stage of production. Therefore, it is effective to perform water cooling from the previous stage and to refine the ground iron structure between the stands of the finishing rolling mill row including a plurality of rolling stands.

Further, by performing water cooling between such finishing stands, scale growth can be suppressed at the same time. From this, when each finishing stand is set to F1, F2, F3 ... from the entry side, at least between the two stands including the preceding stage between F1 and F2, between F2 and F3 is a unit for each stand By injecting cooling water at a rate of 100 L / min or more per width (1 m), and by covering substantially 30% or more of the steel plate area between stands with cooling water, the ground iron structure between the finishing stands can be refined to reduce the scale. The particle size can be reduced, and at the same time, scale growth can be suppressed.

After the end of the rolling, the cooling is immediately started and the winding is performed. Thus, by carrying out the invention, a hot-rolled steel sheet having excellent scale adhesion can be obtained.

The reason for this is not intended to limit the scope of the present invention, but is considered as follows. That is, the scale (Fe
Relates O), after winding, in that it promotes the eutectoid transformation by 4FeO → Fe ₃ 0 ₄ + Fe reaction, FeO is Fe is discharged at the base steel side, Fe ₃ 0 ₄ is produced thereon. Here, the Fe exhaled, precipitated while keeping the steel matrix and integrity in response to the particle size of the base iron (azimuth), precipitated in response to the orientation of the Fe ₃ 0 ₄ is also discharged Fe.

[0029] At that time, Fe ₃ 0 ₄ is used to precipitate while attempts to keep consistency with Fe, this eutectoid transformation, to improve the adhesion of some degree scale. However, Fe ₃ 0 ₄ and Fe does not completely aligned because the lattice constant of the crystal is different, Fe ₃ 0 ₄ and F
Distortion occurs at the interface of e, and this distortion lowers the adhesion of the scale. Therefore, the Fe ₃ 0 ₄ and comminuted, if it is possible to absorb the distortion at the grain boundaries, it is possible to greatly improve the adhesion of the scale.

[0030] Therefore, as a method for grain refining of Fe ₃ 0 ₄ to be precipitated in this eutectoid transformation, if fine the base steel for Fe precipitates in response to this base steel orientation, the precipitated Fe it can be Fe ₃ 0 ₄ also grain refining precipitated in response to. Further, the FeO before transformation if grain refining can be further Fe ₃ 0 ₄ also comminuted after transformation.

Here, the scale adhesion will be considered.
Interface base steel surface layer and scale, mutual dimensional change due to Fe0 / Fe ₃ 0 ₄ phase transformation of gamma / alpha phase transformation and scale of the base steel,
Alternatively, stress is generated due to a difference in coefficient of thermal expansion. In general,
In a polycrystalline substance, stress concentrates on the grain boundary, and the size of the crystal grain increases as the crystal grain increases, and the crystal is easily broken. Conversely, by making the crystal grains of the scale finer, stress concentration can be reduced, and breakage of the scale, that is, separation from the surface of the ground iron can be prevented.

An index indicating the ease of peeling of the aforementioned scale.
For [L ² × d] / [(logC) ² × 300] (inconsistent strain index),
It is considered as follows. The numerator [L ² ×
d] indicates that the scale is easily peeled off due to an increase in the scale particle diameter and the scale thickness d that increase with an increase in the average particle diameter L of the ground iron surface layer. It is considered that the average particle diameter L is squared because the ease of peeling of the scale is related to the cross-sectional area of the crystal grains of the scale or the contact area at the interface.

Regarding the denominator [(logC) ² × 300] of the inconsistent strain index, the logC of the denominator is a negative number at a normal C content (1% or less), and the absolute value decreases as the C increases. Become. Since this logC is in the denominator, the mismatched strain index increases with increasing C, indicating that the scale is easily peeled.
Here, the contribution of the C amount is logarithmically, the ease of peeling of the scale is not proportional to the increase amount of the C amount, but tends to gradually change every time the C amount increases at a constant rate. It reflects.

[0034]

EXAMPLES The present invention will be described with reference to examples. Note that, needless to say, the present invention is not limited to these examples.

Steels having the components shown in Table 1 were melted in a laboratory vacuum melting furnace and subjected to laboratory hot rolling. Immediately before the start of rolling, perform descaling once and then start rolling.
Rolled to a thickness of 3.Omm by pass. Immediately after the first pass and the second pass, 200 L / min of cooling water per unit width (1 m) is injected for 1 s for 3 s of the inter-pass time, so that 33% of the steel plate area between the finishing stands Was simulated to be covered with cooling water. After the end of the rolling, the cooling was started immediately, and the cooling was gradually performed in a furnace simulating the winding.

[0036]

[Table 1]

The evaluation of the scale adhesion was performed by bending the U-bent steel sheet cooled to room temperature and then performing a tape peeling test at the bent portion to measure the scale adhesion area ratio on the peeled tape surface. Here, the bending radius (the radius of the U-bending punch / the thickness of the steel plate) was set to 2.0, and the tape peeling test was performed outside the bent portion of the steel plate. Table 2 shows the experimental conditions, the average particle size of the surface iron layer, the scale thickness, and the results of the tape peeling test.

[0038]

[Table 2]

Here, the rolling end temperature is FT, the time from the end of rolling to the start of cooling is ST, the average cooling rate during rapid cooling is CR,
The temperature drop due to rapid cooling is ΔT, the winding temperature is CT,
The scale area ratio is RA.

FIG. 1 shows the effect of the inconsistent strain index L ² × d / (log C) ² × 300 on the scale area ratio RA. When the inconsistent strain index is 1 or less as claimed in the present invention, RA is 5% or less, indicating good scale adhesion.

FIG. 2 shows the inconsistent strain index L ² × d / (log C) ² of steel type B.
The effect of ST on the x300 value is shown. When ST, which is a claim of the present invention, is 1 s or less, the mismatched strain index is 1 or less, but when ST exceeds 1 s, the mismatched strain index exceeds 1.

FIG. 3 shows the inconsistent strain index L ² × d / (log C) ² of steel type B.
The effect of CR on × 300 is shown. When CR as claimed in the present invention is 100 ° C./s or more, the mismatched strain index is 1 or less, but when CR is less than 100 ° C./s, the mismatched strain index exceeds 1.

FIG. 4 shows the inconsistent strain index L ² × d / (log C) ² of steel type B.
The effect of ΔT on × 300 is shown. When ΔT is 80 ° C. or more, which is the claim of the present invention, the inconsistent strain index is 1 or less, but when ΔT is less than 80 ° C., the inconsistent strain index exceeds 1.

[0044]

As described above, in the hot-rolled steel sheet of the present invention, the scale thickness and the average grain diameter of the ground iron are controlled in accordance with the C content of the ground iron, and the scale adhesion is dramatically improved. are doing. As a manufacturing method, by rapidly cooling after finish rolling and winding at a predetermined temperature, it is possible to control the average grain size and the scale thickness of the surface layer of the base iron, thereby improving the adhesiveness of the scale of the hot-rolled steel sheet. Thus, a hot-rolled steel sheet having excellent scale adhesion is provided, and an industrially useful effect is provided.

[Brief description of the drawings]

FIG. 1 is a diagram showing the effect of a mismatched strain index L ² × d / (logC) ² × 300 on the scale adhesion area ratio (RA).

FIG. 2 is a diagram showing the effect of the time (ST) from the end of rolling to the start of cooling on the inconsistent strain index L ² × d / (logC) ² × 300.

FIG. 3 is a diagram showing the influence of an average cooling rate (CR) during quenching after the end of rolling on a mismatched strain index L ² × d / (logC) ² × 300.

FIG. 4 is a diagram showing the effect of a temperature drop (ΔT) due to quenching after the end of rolling on the inconsistent strain index L ² × d / (logC) ² × 300.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C21D 9/52 102 C21D 9/52 102 // C22C 38/14 C22C 38/14 (72) Inventor Toru Inazumi 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Yoichi Honashiki 1-1-2 Marunouchi, Chiyoda-ku, Tokyo F-term in Nihon Kokan Co., Ltd. 4E002 AA07 AD04 BC07 BD03 BD07 CB01 4K037 EA01 EA04 EA05 EA06 EA15 EA23 EA25 EA27 EA31 FC03 FC04 FC07 FD04 FD06 FE01 FE02 FE06 HA05 JA06 JA07 4K043 AA01 AB01 AB03 AB04 AB15 AB20 AB25 AB26 AB27 AB29 FA03 BA04 BA03

Claims (4)

[Claims] Claims 1. In a hot-rolled steel sheet having a scale on a surface layer of a base iron, an average particle diameter L of the surface layer of the base iron is 20 µm or less, a scale thickness d is 10 µm or less, and a C content is C by mass%. A hot-rolled steel sheet characterized by satisfying L ² × d ≦ (logC) ² × 300. 2. In the production of a hot-rolled steel sheet, finish rolling is performed by (A
r ₃ + 50 ° C) or less, and after cooling within 1s at a cooling rate of 100 ° C / s or more at 80 ° C or more, winding at 400 to 600 ° C to reduce the average particle size L of the ground iron surface layer to 20 μm Hereinafter, when the scale thickness d is 10 μm or less and the C content is C in mass%, L ²
× d ≦ (logC) ² × 300. 3. Using a finishing mill comprising a plurality of rolling stands, at least between two or more stands including between the first to third stands,
3. The heat according to claim 2, wherein cooling water of 100 L / min or more per unit width (1 m) is injected between each of the stands, and 30% or more of a steel plate area between the stands is substantially covered with the cooling water. Manufacturing method of rolled steel sheet. 4. The production of a hot-rolled steel sheet according to claim 2, wherein steel satisfying either or both of P: 0.03% or less and Si: 0.02% or more by mass% is used. Method.