JPH1036939A - Hot rolled steel and production of hot rolled steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a hot rolled steel excellent in adhesion for scales and to provide a method for economically producing a hot rolled steel sheet excellent in adhesion for scales without using special equipment. SOLUTION: This hot rolling stock has a compsn. contg. <=0.2% C, <=0.08% Si, 0.05 to 2% Mn, <=0.025% P, <=0.015% S, 0.1 to 1% Al, 0 to 0.1% Nb, 0 to 0.1% Ti, 0 to 0.2% V, 0 to 1% Cr, 0 to 0.5% Mo, 0 to 0.005% B, 0 to 0.004% Ca, 0 to 0.05% Zr, 0 to 0.05% rare earth elements, and the balance iron with ineviatable impurities. As for the method for producing the same, finish rolling is started at <=1100 deg.C and is finished at <=900 deg.C, within 2sec after the completion of the rolling, forced cooling is started, and, it is rapidly cooled at >=20 deg.C/sec cooling rate at least to 700 deg.C, is coiled at <=550 deg.C and is thereafter cooled or it is subjected to forced cooling at >=30 deg.C/sec cooling rate at least to 700 deg.C, is coiled at 650 to 550 deg.C and is thereafter slowly cooled at the average cooling rate of >=20 deg.C/hr from 500 to 400 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a hot-rolled steel material having excellent adhesion to scales, such as automobile parts such as undercarriage parts, building materials, steel plates used for steel pipes and drums, and section steel or bar steel, and The present invention relates to a method for producing a hot-rolled steel sheet having excellent scale adhesion.

[0002]

2. Description of the Related Art Oxidized scale (hereinafter simply referred to as scale) is formed on the surface of a hot-rolled steel sheet. This scale may peel off during the subsequent forming (bending, drawing, etc.). If the scale is partially peeled off, the appearance quality of the product is impaired, and the peeled scale is also a cause of deteriorating the workability when forming a steel material. For this reason, when it is used for an application that requires molding quality and requires appearance quality, it is used after being subjected to descaling treatment by another process such as pickling or shot blasting. On the other hand, from the viewpoint of pursuing economic efficiency, development of a hot-rolled steel sheet having excellent scale adhesion that can be processed without performing descaling treatment is desired.

[0003] The adhesion of a scale is affected by its composition, structure, thickness and the like. Thin and dense scales have excellent adhesion. It is important that the steel is not exposed to a high temperature oxidizing atmosphere to provide such a scale layer. In order to improve the adhesiveness of the scale, there have been proposed methods of rapidly cooling the steel after hot rolling or cooling in a non-oxidizing atmosphere.

Japanese Patent Application Laid-Open No. 61-194112 discloses that
A steel slab to which Si is added by weight% is hot-rolled at a temperature of Ar3 + 50 ° C. or less, and then cooled at a cooling rate of 40 ° C./sec or more.
A method is described in which a scale is thinned by winding at 500 to 360 ° C. to obtain a hot-rolled steel sheet having excellent scale adhesion. However, in a steel containing Si, a low-melting eutectic compound composed of Fe ₂ SiO ₄ (firelite) and FeO is formed at the interface between the steel and the scale at the stage of heating the slab. Bite into. For this reason, scale removal before finish rolling becomes insufficient, and finish rolling may be performed with some scale remaining. In this part, the adhesion of the scale is inferior because a thick scale remains as a result. Further, in this method, since the steel sheet is cooled to a low temperature of 500 ° C. or less at a rate of 40 ° C./second or more after finish rolling, the formability of the steel sheet itself may be sacrificed.

JP-A-59-222533 discloses a hot-rolled steel sheet which is hot-rolled, wound at 700 to 550 ° C., and then cooled to 350 ° C. in a non-oxidizing atmosphere comprising N ₂ gas or Ar gas. Has been proposed. Furthermore, JP-A-62-136529
In the publication, after hot rolling, cooling at 20-40 ° C / sec.
A method for producing a hot-rolled steel sheet which is wound below and cooled to 350 ° C. in a non-oxidizing atmosphere in the same manner as in the above publication is proposed. However, these methods are methods in which cooling after winding is performed in a non-oxidizing atmosphere, and equipment for adjusting the atmosphere by shutting off the air is required, which hinders economic efficiency and productivity.

[0006] As described above, the method proposed in the past does not disclose a method of economically producing a steel sheet while achieving both the formability of the steel sheet and the adhesiveness of the scale.

[0007]

DISCLOSURE OF THE INVENTION The present invention provides an economical method for producing a steel sheet excellent in scale adhesion, a hot-rolled steel material such as a section steel or a strip steel, and a hot-rolled steel sheet excellent in scale adhesion. The aim is to provide a method.

[0008]

SUMMARY OF THE INVENTION The gist of the present invention is to provide a hot-rolled steel material having excellent scale adhesion as described in (1) below and an excellent scale adhesion as described in (2) and (3). In a method for manufacturing a hot-rolled steel sheet.

(1) By weight%, C: 0.2% or less, Si:
0.08% or less, Mn: 0.05 to 2%, P: 0.025% or less,
S: 0.015% or less, Al: 0.1 to 1%, Nb: 0 to 0.1
%, Ti: 0 to 0.1%, V: 0 to 0.2%, Cr: 0 to 1
%, Mo: 0 to 0.5%, B: 0 to 0.005%, Ca: 0 to 0%
0.004%, Zr: 0-0.05%, and rare earth element: 0-0.
A hot-rolled steel material containing 05% and the balance being iron and unavoidable impurities and having excellent adhesion to scale (first invention).

(2) The finish rolling of the steel having the chemical composition described in the above (1) is started at 1100 ° C. or less and finished at 900 ° C. or less, and two seconds after the finish rolling of the steel sheet after finish rolling. Within the process of starting forced cooling within 200 ° C, quenching at a cooling rate of at least 20 ° C / sec, and winding at 550 ° C or less. Manufacturing method (second invention).

(3) Finish rolling of the steel having the chemical composition described in (1) above is started at 1100 ° C. or less and finished at 900 ° C. or less, and 2 seconds after the finish rolling of the steel sheet after finish rolling. Start forced cooling within 300 ° C, quench at a cooling rate of 30 ° C / sec or more to at least 700 ° C, and wind up at 650-550 ° C.
A method for producing a hot-rolled steel sheet having excellent formability and scale adhesion, comprising a step of gradually cooling a temperature between 550 to 400 ° C at an average cooling rate of 20 ° C / hour or less (third invention). .

In the present invention, in addition to the effect of thinning by rapid cooling after hot rolling, the hot rolling of the present invention for improving the adhesion of the scale by utilizing the following new knowledge on the scale structure is provided. A method for producing steel materials and hot-rolled steel sheets was completed.

[0013] It has been found that a scale layer having excellent adhesion can be obtained by adding an appropriate amount of Al to a steel sheet and selecting conditions for finish rolling and subsequent cooling.

Table 1 shows that a slab having a thickness of 340 mm made of steel containing Al in various proportions is heated to 1220 ° C. to perform rough rolling, and finish rolling starts at 1050 ° C. and ends at 860 ° C. And
Start cooling at a cooling rate of 30 ° C / sec within 2 seconds, 500 ° C
The results of evaluating the adhesiveness of the scale of a hot-rolled steel sheet (sheet thickness 2.3 mm) manufactured by winding in the above manner are shown.

[0015]

[Table 1]

The adhesiveness of the scale was determined by bending the test piece by bending the test piece 90 degrees at an inner bending radius of 1.5 times the plate thickness, and then measuring the scale peeling area ratio at the outside of the bent portion, and evaluating it according to the following criteria.

印 mark (good adhesion): scale peeling area ratio ≦ 2% x mark (poor adhesion): scale peeling area ratio> 2% From the results shown in Table 1, the steel sheet having a high Al content It can be seen that the adhesiveness of the scale is excellent. Although there are still many unknown reasons why the adhesion of the scale of the steel containing Al is improved, it is presumed as follows.

The structure of the scale generated when steel is hot-rolled usually has a FeO layer on the surface side of the steel sheet and a thin Fe ₃ O ₄ layer on the outermost surface of the FeO layer. However, in the case of the steel containing Al, the interface between the steel sheet surface and the FeO layer is formed with FeAl ₂ O ₄ mainly composed of FeAl ₂ O _4, which is considered to be concentrated during slab heating or hot rolling. Oxidized F
A thin layered composite oxide with e is formed. This composite oxide has a crystal structure (spinel structure) having good adhesion to the base iron. The crystal structure of Fe ₃ O ₄ is the same as the spinel structure, and Fe ₃ O ₄ is superior to FeO in terms of adhesion to ground iron. Furthermore, since FeAl ₂ O ₄ has better crystal lattice consistency with Fe ₃ O ₄ , the adhesive strength with Fe Iron is stronger in FeAl ₂ O ₄ than in Fe ₃ O _4. It is. From these facts, it is considered that this composite oxide layer has an effect of improving the adhesion between the scale layer and the base iron.

Unlike the steel having a high Si content, the removability of the scale before the finish rolling is satisfactory without any problem even if the Al content is high. This is presumably because the melting point of the eutectic compound of FeAl ₂ O ₄ and FeO is high, so that the eutectic compound does not enter the molten state at a normal slab heating temperature and does not penetrate the scale.

[0020] FeO, which accounts for the majority of the scale layer,
At 560 ° C or lower, Fe and Fe ₃ O ₄ have a property of eutectoid transformation, and when the temperature of the steel sheet becomes 560 ° C or lower, Fe
O decomposes to form Fe and Fe ₃ O ₄ layers. A composite compound mainly composed of FeAl ₂ O ₄ having good adhesion to steel is A
l Due to the limited content, it does not cover the entire surface of the steel sheet base, but there are also areas where the scale adhesion is not improved and the area is not partially covered with the composite oxide. However, the Fe ₃ O ₄ layer generated by the eutectoid transformation of FeO described above
It covers not only the FeAl ₂ O ₄ composite oxide layer but also the surface of the steel sheet substrate where no composite oxide is formed. This further strengthens the adhesion between the scale layer and the base steel.

Further, this is the time of transformation Fe is precipitated during the Fe ₃ O ₄ crystals, though wedge Fe is in the form such as to secure the Fe ₃ O ₄ crystals. It is presumed that this also contributes to improving the adhesion of the scale. The above-mentioned eutectoid transformation of FeO is particularly promoted in the temperature range of 550 to 400 ° C. Therefore, by keeping the hot-rolled steel sheet in this temperature range, a Fe ₃ O ₄ layer is uniformly generated at the interface of the steel sheet, and the adhesion of the scale can be further improved.

Even if the coil winding temperature is increased to some extent to improve the formability of the hot-rolled steel sheet, the adhesion of the scale can be maintained at a favorable level by utilizing the effect of the above eutectoid transformation. .

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The constituent features of the present invention and the effects thereof will be described below. The contents of the alloy elements described below are expressed in terms of% by weight.

(A) Chemical composition of steel C: Used to ensure the strength of the steel sheet. But,
If the content exceeds 0.2%, a large amount of cementite (Fe ₃ C) is formed on the surface of the steel sheet, thereby impairing the adhesion of the scale and deteriorating the formability and weldability of the steel sheet. Therefore, the content of C is set to 0.2% or less. When formability of a steel sheet is required, C is preferably low, and the present invention is effective even with an extremely low carbon of about 0.001%. However, it is not economical to reduce C to the extremely low carbon region, and the lower limit of C content is
Preferably it is 0.02%.

Si: has the effect of deoxidizing molten steel during steelmaking, and is also used as an element for increasing the strength of a steel sheet. However, Si forms a low melting point compound with Fe,
It causes poor descaling before hot rolling and poor adhesion of scale when forming a steel sheet. Therefore, the upper limit of the Si content is 0.08%. Preferably 0.04%
The following is the case, and Si may not be used in applications where the emphasis is on moldability over strength.

Mn: It has the effect of securing the strength of the steel sheet and suppressing S, which is present as an impurity in the steel, as MnS to suppress cracks generated during hot rolling. When the Mn content is less than 0.05%, the effect of suppressing cracking by the above-mentioned action cannot be obtained. If the content exceeds 2%, the effect is saturated and the adhesiveness of the scale is impaired. Therefore M
The n content is in the range of 0.05 to 2%.

Al: Conventionally used as a deoxidizing agent in steel making, and its amount is less than 0.1%. However, Al is the most important element for obtaining the scale adhesion of the hot-rolled steel sheet as the object of the present invention. That is, if Al is contained in a large amount, the solid solution of A
During the heating or hot rolling, l is concentrated at the interface between the scale and the steel sheet base to form a complex oxide of Fe and Al. This improves the adhesion of the scale. Al content is 0.
If it is less than 1%, the effect of improving the adhesion of the scale is weak. On the other hand, when Al is excessively contained, the effect of improving the adhesion is saturated and the economic efficiency is impaired. For this reason, the content range is set to 0.1 to 1%.

P, S: P and S concentrate on the surface of the steel material during heating and during hot rolling, and reduce the adhesion of the scale. The content of P is set to 0.025% or less,
When the content of is 0.015% or less, deterioration of the adhesion of the scale can be prevented. For this reason, P is set to 0.025% or less,
S should be 0.015% or less.

The hot-rolled steel sheet constituting the present invention is basically composed of the above-mentioned chemical composition. However, if the strength or formability of the steel sheet is required, the following elements may be added. Good.

When a high-strength hot-rolled steel sheet is required, N
One or more selected from b, Ti, V, Cr, and Mo may be added. Nb, Ti and V precipitate on the ferrite ground as carbonitride and increase the strength of the steel sheet.
If less than 0.005%, the effect is weak. Therefore, when any of these elements is used, 0.005
% Is preferably added. Even if it is contained excessively, its effect is saturated and the economy is low. Therefore, the upper limit of the addition is 0.1% for Nb and Ti, and 0.1% for V.
0.2%.

Since Cr and Mo have the effect of increasing the strength through transformation strengthening, they can be added as required. In the case of adding, if the content is less than 0.01%, there is no effect. Therefore, when these elements are used, it is preferable to add 0.01% or more of any of the elements. Cr is 1
% And Mo are 0.5%, which are the upper limits of the contents in which each element effectively contributes to the strength. Therefore, when these elements are used, the upper limit is set to 1% for Cr and 0.5% for Mo.

In the present invention, when a hot-rolled steel sheet having better formability is required, B may be added. B can be expected to have an effect of lowering the yield strength of the steel sheet through the action of lowering the Ar3 point of the steel and improving the formability. In order to exhibit the effect, it is preferable to add 0.001% or more. Even if added in excess of 0.005%, the effect is saturated, so the upper limit of the addition is 0.005% or less.

Further, in the present invention, when a hot rolled steel sheet having better bendability or the like is required, Ca, Zr or a rare earth element can be used for the purpose of controlling the form of inclusions. However, in order to obtain the effect, in the case of Ca, 0.00
It is preferred to add 02% or more, 0.01% or more for Zr, and 0.002% or more for rare earth elements. On the other hand, if Ca is contained in an amount exceeding 0.004%, in the case of Zr in an amount exceeding 0.05% or in the case of a rare earth element in an amount exceeding 0.05%, on the contrary, the number of inclusions in the steel is increased and formability is deteriorated. Therefore, when these elements are used, the upper limit of the content is 0.00 in the case of Ca.
4%, 0.05% for Zr, 0.05% for rare earth elements
And For controlling the form of the inclusions, any one of these elements may be used, but two or more of these elements may be added in combination.

The other components are Fe and inevitable impurities.

(B) Manufacturing Conditions of Hot-Rolled Steel Sheet In the present invention, when hot-rolling a steel having the chemical composition described in (A) above, finish rolling starts at 1100 ° C. or less and ends at 900 ° C. or less. Cooling is started within 2 seconds after finishing rolling, cooling is performed rapidly at a cooling rate of 20 ° C / sec or more up to at least 700 ° C, and winding at 550 ° C or less is included. To manufacture hot rolled steel sheets.

Further, when a hot-rolled steel sheet which is excellent in the formability such as the ductility of the steel as well as the adhesiveness of the scale is required, the forced rolling to at least 700 ° C. which starts within 2 seconds after the finish rolling is completed. Cooling is performed at a cooling rate of 30 ° C./sec or more, and then winding is performed at 650 ° C. to 550 ° C., and an average of 20 ° C.
It can be manufactured by slow cooling at a cooling rate less than or equal to the hour.

In any case, slab heating before finish rolling may be performed, or slab heating may be omitted if the finish rolling temperature can be secured. Further, when the slab is thin, the rough rolling can be omitted.

Usually, before rough rolling and finish rolling, the scale is removed with high-pressure water or the like. If finish rolling is performed without sufficient removal of the scale, the remaining scale is pushed into the steel sheet by rolling and remains as a biting scale. As a result, the scale in this portion becomes thick, and the adhesion of the scale during molding processing is impaired. Therefore, in order to produce a hot-rolled steel sheet having excellent scale adhesion, it is necessary to sufficiently remove the scale before finish rolling.
Finish rolling is preferably performed at a high temperature from the viewpoint of deformation resistance and the quality of the steel sheet, but there are also restrictions such as economy,
Usually, the finish rolling start temperature is 1150 ° C or less, and the finish rolling end temperature is 950 ° C or less.

When steel is present in an oxidizing atmosphere such as the atmosphere, the scale grows exponentially faster as the temperature of the steel increases. Also, as the thickness of the scale increases, it tends to peel off. Therefore, in order to improve the adhesion of the scale, the time during which the steel sheet stays at a high temperature must be shortened.

Therefore, in the present invention, the finish rolling is performed at 1100 ° C.
Start below and end below 900 ° C. The finish temperature of the finish rolling is preferably 860 ° C or lower. It is preferable that the lower limit of the start temperature of the finish rolling is 900 ° C. and the lower limit of the finish temperature is 750 ° C. From the viewpoint of improving the adhesion of the scale, it is preferable that these temperatures, especially the finish rolling end temperature, be lower, but if it is lower than this, the formability of the steel sheet is impaired and the deformation resistance of the steel increases, so that The load becomes excessive and rolling becomes difficult.

It is desirable that the stay time of the steel sheet in the high temperature region after the finish rolling is as short as possible in order to suppress scale growth. Therefore, at least 2 hours after finishing rolling
The forced cooling is started within seconds, preferably within one second.

If the steel sheet is mainly used for bending and the formability such as ductility of the steel sheet is not so strict, at least a cooling rate of 20 / sec or more is required until the temperature of the steel sheet reaches 700 ° C. After cooling, the film is wound at a temperature of 550 ° C. or lower in order to avoid slow cooling in a temperature range (560 ° C. or higher) where FeO 2 with poor adhesion is generated. Even if the cooling rate is excessively high, the effect of suppressing scale growth saturates,
The upper limit is preferably set to 80 ° C./sec, since the cooling device becomes excessively large and impairs economic efficiency. The cooling rate in the temperature range from 700 ° C. to the winding is optional, but a higher cooling rate is preferred. For a transformation-strengthened high-strength steel sheet or the like, it may be wound after being forcibly cooled to around room temperature. After winding, it can be cooled by a usual cooling method such as cooling in the air, water spray for shortening the cooling time, or forced cooling by immersion in cooling water.

When a steel sheet having better formability is required, for example, due to severe drawing, etc., sufficient recovery of the rolling strain caused by hot rolling is performed, and the crystal grains are enlarged.
To grow pearlite and cementite sufficiently,
It is effective to raise the winding temperature of the rolled steel sheet to 550 ° C or higher. However, if the winding temperature is too high, the scale becomes thick and the adhesion is impaired, so the upper limit is set to 650 ° C.

In the case of the usual method, the adhesion of the scale is impaired by an amount corresponding to the increase in the winding temperature. However, in the present invention, forced cooling is started within at least 2 seconds, preferably within 1 second after finishing rolling, and the cooling rate up to 700 ° C. is reduced.
The average cooling rate between 550 to 400 ° C is set to 20 ° C / hour or less when the coil wound at a temperature of 30 ° C / sec or more and the coil wound in the range of 650 to 550 ° C is cooled. Ensure adhesion. The reason why the cooling rate to 700 ° C. is 30 ° C./sec or more is to shorten the residence time of the steel sheet in a high temperature region where scale growth is fast. If the cooling rate is excessively high, the formability of the steel sheet will be impaired, and the cooling equipment will be excessively large and large amounts of cooling water will be consumed. Is preferred.

Slow cooling at a cooling rate of not more than 20 ° C./hour between 550 ° C. and 400 ° C. after winding takes place at 560 ° C. or less.
This is because eutectoid transformation of eO 2 (4FeO → Fe ₃ O ₄ + Fe) is sufficiently advanced, and the scale is made to have a composition mainly composed of Fe ₃ O ₄ . When the cooling rate exceeds 20 ° C./hour, eutectoid transformation does not proceed sufficiently and the adhesion of the scale is not improved. Preferably, slow cooling is performed at 15 ° C./hour or less. If the temperature of the coil is less than 400 ° C, the eutectoid transformation rate will decrease, so there will be no slow cooling effect, and the time required for cooling will be longer and the economy will be impaired. .

The method of forced cooling performed between finish rolling and winding up may be ordinary spray cooling with cooling water, laminar flow cooling, or the like, but further cooling is promoted by spraying high-pressure water or the like. Means can also be used. When gradually cooling between 550 and 400 ° C after winding, if necessary, use a heat retention cover with auxiliary heat input means such as gas heating or electric heating, or use the coil end or inside or outside. A method of auxiliaryly heating or keeping the temperature of the peripheral portion may be used.

[0047]

Example 1 Table 2 shows the chemical composition of the sample used for the test.

[0048]

[Table 2]

The steels A to M satisfy the chemical composition defined in the first invention, whereas the steels N to S are steels selected for comparison with the present invention. After heating each steel slab with a thickness of 340 mm in a heating furnace, rough rolling is performed, and the finish rolling temperature, the forced cooling condition by pouring water and the winding temperature are changed to 2.3 mm in thickness,
A 1250 mm wide hot rolled steel sheet was obtained. After winding, it was allowed to cool to room temperature in the air. Samples of 50 mm width and 120 mm length were taken from these steel plates and subjected to the bending test specified in JIS Z2248,
The bending was performed at a bending radius of 1.5 times the plate thickness and a bending angle of 90 degrees.
After removing the floating scale by applying adhesive tape to the outside of the bent part of the test piece, take an enlarged photograph of the outside of the bent part and measure the area ratio of the part where the scale has peeled off, according to the following criteria The scale adhesion was evaluated.

○ (good adhesion): scale peeling area ratio ≦ 2% x (poor adhesion): scale peeling area ratio> 2% Table 3 shows the production conditions of each test material and their evaluation results. Show.

[0051]

[Table 3]

As shown in Table 3, the steel sheets (test numbers 1 to 13) manufactured by the method satisfying the conditions specified in the second invention have excellent scale adhesion.

On the other hand, when steel such as Al whose chemical composition is out of the conditions specified by the present invention is used (Trial Nos. 19 to 24), and even when the chemical composition is within the range of the present invention, the production conditions are set to the second condition. When the conditions deviated from the conditions stipulated by the invention (test numbers 14 to 17), the adhesion of the scale was reduced. In Test No. 18, the winding temperature was high and the film was not gradually cooled after winding, so that the adhesion was not favorable.

[0054]

Example 2 The steel having the chemical composition shown in Table 4 had a thickness of 340.
After heating the slab with a heating furnace, rough rolling is performed, and the hot-rolled steel sheet with a thickness of 2.0 mm and a width of 950 mm is changed by changing the finishing rolling temperature, forced cooling conditions by pouring water, winding temperature and cooling conditions after winding. I got Here, it was manufactured with the intention of improving the moldability as well as the adhesiveness of the scale.

[0055]

[Table 4]

From these steel sheets, a No. 5 tensile test piece specified in JIS Z 2201 was sampled in parallel with the rolling direction, and the mechanical properties were evaluated. In addition, from these steel plates, width 50mm, length 120m
A test piece of m was sampled and subjected to a bending test specified in JIS Z 2248 at a bending radius of 1.5 times the plate thickness and a bending angle of 90 degrees. After removing the floating scale by applying an adhesive tape to the outside of the bending portion of the bending test piece, take an enlarged photograph of the outside of the bending portion and measure the area ratio of the portion where the scale has peeled off, The adhesiveness of the scale was evaluated based on the standard.

印 (good adhesion): scale peeling area ratio ≦ 2% x (poor adhesion): scale peeling area ratio> 2% Table 5 shows the production conditions of each test material and their evaluation results. Show.

[0058]

[Table 5]

As shown in Table 5, the steel sheets (test numbers 25 to 31) manufactured by the method satisfying the conditions specified in the third invention have excellent scale adhesion and good ductility. On the other hand, in Test Nos. 32 to 34 and Test Nos. 37 to 39, although the ductility is good as a result of raising the winding temperature, steels whose alloy contents such as Al are out of the range specified by the first invention are used. Test number 37-39
Also, even if the chemical composition is within the range of the present invention, the cooling conditions and the winding conditions are different from the conditions specified by the third invention in the test numbers 32 to
In No. 34, the adhesiveness of the scale is reduced. Test Nos. 35 and 36, in which the winding temperature was too low, had good adhesiveness of the scale, but had poor ductility of the steel sheet. Table 2 is for reference.
The values of TS × El used as an index for comparing ductility when the strength of the steel sheet is different are shown.
It is understood that these values are not preferable even at these values.

[0060]

The hot-rolled steel material of the present invention has excellent adhesiveness of scale, so that it can be processed as it is with a scale, and can be applied to applications requiring formability, so that it can be used around automobiles. It is suitable for parts, frames, steel building materials, drums and the like. Further, the method for manufacturing a hot-rolled steel sheet having excellent scale adhesion according to the present invention can be manufactured without using special equipment such as an atmosphere adjusting device.

Claims (3)

[Claims] (1) By weight%, C: 0.2% or less, Si: 0.08%
Mn: 0.05 to 2%, P: 0.025% or less, S: 0.01
5% or less, Al: 0.1 to 1%, Nb: 0 to 0.1%, Ti
: 0 to 0.1%, V: 0 to 0.2%, Cr: 0 to 1%, Mo
: 0 to 0.5%, B: 0 to 0.005%, Ca: 0 to 0.004
%, Zr: 0 to 0.05%, and rare earth elements: 0 to 0.05%, the balance being iron and unavoidable impurities, the balance being excellent in hot-rolled steel. 2. The finish rolling of a steel having the chemical composition according to claim 1 is started at 1100 ° C. or less and finished at 900 ° C. or less, and the finish-rolled steel sheet is finished within 2 seconds after finishing rolling. Start forced cooling and cool at least to 700 ° C at a cooling rate of 20 ° C /
A method for producing a hot-rolled steel sheet having excellent scale adhesion, comprising a step of quenching at a temperature of 550 ° C. or less after quenching in seconds or more. 3. The finish rolling of the steel having the chemical composition according to claim 1 is started at 1100 ° C. or less and finished at 900 ° C. or less, and the finish-rolled steel sheet is finished within 2 seconds after finishing rolling. Start forced cooling and cool at least 30 ° C / 700 ° C
After quenching for more than 2 seconds, wind up at 650-550 ° C, 550-40
A method for producing a hot-rolled steel sheet having excellent formability and scale adhesion, comprising a step of gradually cooling a temperature between 0 ° C. at an average cooling rate of 20 ° C./hour or less.