JPH0949053A - Hot rolled steel sheet excellent in formability, machinability and blankability and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a hot rolled steel sheet in which precipitates essentially consisting of Ti4 C2 S2 are specifically present by specifying the contents of C, Si, P, S, Mn, N, Ti or the like and the relations among the content of Ti and the contents of C, S and N in a steel. SOLUTION: A slab composed of a steel satisfying, by mass, 0.0010 to 0.010% C, <=0.25% Si, 0.20 to 1.0% Mn, <=0.05% P, 0.010 to <0.025% S, <=0.01% N and <=0.15% Ti, satisfying (Ti*/48)/(C/12+S/32)=1.0 to 3.0, where Ti*=Ti-(48/14)N, and the balance Fe is prepd. This slab is heated at 1100 to 1200 deg.C, and thereafter, hot rolling is finished at (the Ar3 transformation point +80 deg.C) to (the Ar3 transformation point -40 deg.C), which is next coiled at 700 deg.C. Thus, the objective steel sheet in which precipitates with 1000 to 10000Å size essentially consisting of Ti4 C2 S2 are present in the inside by >=5×10<3> pieces/mm<2> can be obtd.

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 formability such as overhanging property and punching workability as well as excellent machinability and a method for producing the hot rolled steel sheet. It can be effectively used as a product and steel sheet for automobiles.

[0002]

2. Description of the Related Art Hot-rolled steel sheets have been widely put into practical use in automobiles and household electric appliances, but recently, they have been adapted to the increasingly complicated shapes of final formed products and pressed by users. In order to meet the demand for process simplification, it has become necessary to have better workability than ever. In order to meet such demands, at present, ultra-low carbon steel is made to contain an appropriate amount of carbon and nitride forming elements such as Ti and Nb, and C and N are contained.
A so-called IF-based hot rolled steel sheet, which is fixed, is often used.

That is, the IF hot rolled steel sheet has a low yield stress and a high elongation, and has excellent workability. Widely used as a material for parts. However, the conventional IF hot rolled steel sheet has poorer machinability and punching workability than the low carbon Al killed cold rolled steel sheet, and has a difficulty in the finished surface roughness and chip disposability during cutting, and after punching work. However, it has a drawback that the burr height of the end face becomes large, and the practicability is significantly impaired depending on the application.

That is, in press forming of household electric appliances, automobile parts and the like, before and after that, in most cases, punching for blanking and the like, and cutting such as drilling and threading are performed. Although the above-mentioned IF hot-rolled steel sheet is generally excellent in workability, it has poor machinability and punching workability as compared with the low carbon aluminum killed cold-rolled steel sheet. The chip disposability is poor, and the burr height of the end face after punching becomes large, and when used as, for example, automobile parts, the electrodeposition coating becomes poorly adhered at the burr portion, which adversely affects the corrosion resistance. This may cause metal powder to be generated due to the abrasion of the burr or give surface scratches to each other when it is given or when the punched plates are stacked and conveyed.

Therefore, in order to improve the cutting workability and the punching workability, it is possible to adopt a method described later, but the portion where the cutting or punching is actually performed is a very small part of the steel sheet. Nevertheless, the mechanical properties of other parts are sacrificed in order to improve the cutting property and punching property of the small part, and the lack of strength becomes a serious problem.

That is, as a means for improving the machinability of steel, a method of adding a so-called free-machining improving element such as Pb, S, Bi, Te, P is generally used. Among them, lead free-cutting steel and sulfur free-cutting steel to which Pb and S are added are widely used. Of these, the former lead free-cutting steel is
Although it has excellent physical properties such as relatively good mechanical properties, it contains harmful lead, which causes environmental hygiene problems.In recent years, it has been gradually replaced by lead free-cutting steel. is there.

On the other hand, the sulfur free-cutting steel is significantly deteriorated in mechanical properties, especially ductility due to the addition of S, as compared with the lead free-cutting steel, and the upper limit of the S addition amount is limited in order to secure the strength characteristics. However, there is a limit to improving machinability. Further, as a general measure of machinability, tool life, finished surface roughness, chip disposability, cutting resistance and the like can be mentioned, especially when it is intended to be applied to the intended use of the present invention. Although excellent chip disposability is required, conventional sulfur free-cutting steel is inferior in chip disposability to lead free-cutting steel, and the problem that chips grow long and become entangled with the work piece or processing machine Frequent.

Therefore, it has been attempted to add an appropriate amount of calcium to sulfur free-cutting steel to control the morphology of sulfides to improve mechanical properties without impairing machinability.
With this method, an increase in manufacturing cost becomes a heavy economic burden. Attempts have also been made to add Ti, Zr, Te, rare earth elements, etc. as morphology controlling elements for sulfides, but these methods are not in the research stage, and the suitable addition amount including the relationship with workability It has not been fully examined, and it is difficult to resolve the economic burden of high costs.

When applied to household electric appliances, automobile parts, etc., since it is often applied to a sliding portion after punching, the return (burr) of the end face after punching has a constant height. In addition to the above, it is also an important characteristic that the dimensional accuracy is not deteriorated, and since it is also often laminated as an intermediate component, it does not cause adverse effects on the electric circuit or deterioration of product performance due to generation of abrasion powder. Will also be required. Therefore, as a steel plate applied to the use where such a punching process is applied, the low carbon A as described above is mainly used.
1 killed steel sheet is used. In this low carbon Al-killed steel sheet, carbon is coarsely precipitated as fragile cementite in the grain boundaries and in the grains, and becomes a starting point of crack generation during punching. It has been confirmed that the mold wear is suppressed and the shear load is also reduced. However, further improvement in punching workability is desired in order to improve punching workability and rationalize.

In order to improve punching workability, attempts have been made to optimize clearances and devise punch and die shapes from the viewpoint of processing technology. Further, with regard to the steel plate to be processed, for example, the steel plate itself is hardened. Alternatively, it has been proposed to harden the surface of the steel sheet. The concrete method of the former is P
There is a method of adding solid solution strengthening elements such as, and the concrete method of the latter is to introduce plastic strain into the surface layer portion by rolling etc. to the annealed steel sheet or nitriding or carburizing the surface. Hardening method by
No. 255626, No. 2-133561, and No. 3
-199343 gazette, the same 3-202442 gazette) etc. are known. However, in the former method, mechanical properties, which are the original required properties such as elongation and r-value, are largely deteriorated for the improvement effect of punching workability. In the latter method, nitriding treatment or carburizing treatment is required. Special equipment is required. Furthermore, methods for producing a large amount of MnS precipitates by adding Mn and S have also been proposed (Japanese Patent Laid-Open Nos. 1-230748 and 6-73457), but the amount of S in steel can be reduced by these methods. If it is increased, not only hot cracking tends to occur, but also a new problem arises in that point defects called blowholes and blisters and surface defects frequently occur when called sliver.

From the above circumstances, it is applied to household electric appliances, automobile parts, etc., which not only require good formability as a whole but also require machinability and punching workability as important requirements. For steel plate applications, a normal ultra-low carbon IF without adding a large amount of free-machining improving elements such as S
There is a strong demand for the development of a hot rolled steel sheet having a composition similar to that of a hot rolled steel sheet and having improved machinability and punching workability while maintaining excellent formability.

[0012]

The present invention has been made in view of the above circumstances, and its purpose is to obtain a conventional IF type steel sheet by properly adjusting the chemical composition of the steel material. The hot-rolled steel sheet has excellent machinability and mechanical properties, is free from problems such as internal defects and surface defects, has good machinability and punching workability, and does not cause burrs. And to establish a manufacturing method capable of reliably obtaining such a hot-rolled steel sheet.

[0013]

[Means for Solving the Problems] The above-mentioned problems can be achieved.
The composition of the hot-rolled steel sheet according to the present invention is
0.0010% ≦ C ≦ 0.010% Si ≦ 0.25% 0.20% ≦ Mn ≦ 1.0% P ≦ 0.05% 0.010 ≦ S <0.025% Al ≦ 0.1 % N ≦ 0.01% Ti ≦ 0.15%, and (Ti * / 48) / (C / 12 + S / 32) = 1.
0 to 3.0 However, Ti * = Ti- (48/14) N is satisfied, and the balance is Fe and inevitable impurities.
Made of Ti_Four C ₂ S₂ Size 1 mainly
5,000-10,000Å precipitates are 5 × 10^Three Pieces / mm
² It has a gist at the place where it exists above.

Further, in the construction of the manufacturing method according to the present invention, the slab made of steel satisfying the requirements of the component composition is 1,100 to
After heating in the temperature range of 1,200 ° C., hot rolling is completed in the temperature range of (Ar ₃ transformation temperature + 80 ° C.) to (Ar ₃ transformation temperature −40 ° C.), and then rolled in a temperature range of 700 ° C. or less. By taking it, 5 × 1 of a precipitate having a size of 1,000 to 10,000 Å mainly composed of Ti ₄ C ₂ S ₂ is taken inside.
It has the gist of producing more than 0 ³ pieces / mm ² .

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION As described above, the hot-rolled steel sheet according to the present invention specifies the component composition of the steel material and, in particular, contains T
It is a material in which a specific amount of a precipitate of i ₄ C ₂ S ₂ as a main component is allowed to exist in a specific amount, which basically makes it an excellent press by having a composition similar to that of a conventional ultra-low carbon IF steel sheet. It has succeeded in greatly improving the machinability and punching workability while ensuring the formability and the like.

More specifically, in addition to containing the minimum amount of S necessary for improving the machinability and punching workability, S is produced as a Ti ₄ C ₂ S ₂ -based precipitate, and its size and By controlling the amount of the hot rolled steel sheet, the hot rolled steel sheet satisfying both excellent formability and machinability / punching workability at the same time can be obtained without the drawbacks as pointed out in the conventional example.

In ordinary press molding and punching, it is considered that the material undergoes processing at substantially room temperature without raising the temperature so much that precipitates capable of exhibiting the above-described modifying effect at room temperature. It is necessary that the inclusions be in the form, size, and dispersion state, and that the cutting heat causes a considerably high temperature. it is necessary that such inclusions may melt at a processing temperature range, but the Ti ₄
It has been confirmed that the C ₂ S ₂ -based precipitate has all of these characteristics, and that the machinability and punching workability are significantly improved.

On the other hand, from a microscopic view of the punching condition of the steel sheet, a shear fracture surface is first formed on the front and back surfaces of the steel sheet as the punch lowers after the contact between the punch and the steel sheet, and stress concentration on the fracture surface Strong shear deformation occurs and voids occur,
This becomes a crack and the crack progresses from the front and back,
It is considered that the cracks coalesce to obtain a punched state. Therefore, in order to reduce the burr after punching, it is effective to promote the formation of voids and the development of cracks, but since Ti ₄ C ₂ S ₂ is a relatively large precipitate, It is considered that when the steel sheet undergoes strong shear deformation, formation of voids and cracks near the precipitates and their progress are promoted, the machinability is remarkably improved, and burrs are reduced.

The precipitates and inclusions present in the ultra low carbon IF steel sheet are MnS and Al ₂ O ₃ contained in ordinary steel.
In addition, TiC, Ti ₄ C ₂ S ₂ , Ti
S, TiN, FeTiP, Ti ₂ O, etc., and Nb-added steels, such as Nb (C, N), have their respective sizes and distribution states, but they are characteristic of formability and machinability of hot-rolled steel sheets. After repeated examinations on the influence on the punching workability, the following facts were confirmed.

Ti ₄ C ₂ S ₂ : The size as a precipitate is 1,000 to 10,000Å, and the amount thereof is 5 × 10.
^{When it is 3} pieces / mm ² or more, the machinability and punching workability are remarkably improved, but the adverse effect on the formability caused by the presence of other precipitates is hardly recognized. And the T
The i ₄ C ₂ S ₂ -based precipitate is a slab made of steel satisfying the requirements for the composition as described later in detail.
After heating in the temperature range of 1,200 ° C., hot rolling is completed in the temperature range of (Ar ₃ transformation temperature + 80 ° C.) to (Ar ₃ transformation temperature −40 ° C.), and then rolled in a temperature range of 700 ° C. or less. By taking it, it was confirmed that it was efficiently generated. If any of the above requirements is not satisfied, TiC, TiS, etc., which have a poor effect of improving the machinability and punching workability and adversely affect the formability, are produced as described below, and the object of the present invention can be achieved. Disappear.

TiC, Nb (C, N): Since the size of the precipitate is about 100Å and is too fine, not only the formability is adversely affected, but also the effect of improving the machinability and punching workability is poor. A method of adding Ti and Nb in the steel as a cold-rolled steel sheet having improved formability and punching workability among ultra-low carbon IF steel sheets (Japanese Patent Laid-Open No. 6-73457) is known. According to a study conducted by the present inventors, when Nb is added to steel in a range of 0.003 to 0.03%, Nb (C, N) is generated and C in the steel is consumed. Ti effective in improving machinability and punching workability
It was confirmed that the object of the present invention could not be achieved as a result of the ₄ C ₂ S _{2 not} being produced.

FeTiP: The size of the precipitate is Ti
It is about the same as ₄ C ₂ S ₂ and effectively acts to improve the machinability and punching workability, but it precipitates in the form of a film at the crystal grain boundaries, which significantly deteriorates the formability.

TiS, TiN: The size of the precipitate is
Ti_Four C₂ S₂ It's slightly larger, and it's size
Seems to have the same effect of modification, but
In that case, it is too soft, so there is almost no improvement in punching workability.
It does not work. In addition, the temperature rise process during cutting process
Satisfactory machinability is also demonstrated because the shape is not effective for machinability
Not done. Also, Ti_Four C₂ S₂ Aiming to increase the amount of precipitation
When a large amount of S is added to the steel, the S content exceeds a certain level.
In particular, when the component system with a small amount of C is used, Ti _Four C₂ S
₂ TiS is generated at the final stage of the generation of Al and causes surface defects.
It is not preferable. Also, it is very difficult to control the N content in practical steel.
Therefore, these Ti compounds are used to improve machinability.
Not easy to use.

Ti ₂ O: The size of the precipitate is Ti ₄
It is larger than C ₂ S ₂ by an order of magnitude, and the effect of improving the machinability and punching workability is poor, and further it causes a surface defect, which is not preferable.

The form of MnS: MnS includes inclusions and precipitates, the former having a size of micron and the latter having a size of 1/10 thereof. When the amount of S increases, the amount of inclusions increases and contributes to the improvement of machinability and punching workability. However, as shown in the above-mentioned JP-A-6-73457, it improves punching workability. Requires 0.02% or more of S, and a large amount of S not only impairs moldability but also causes surface defects.

As described above, various compounds are conceivable as the inclusions and precipitates that can be formed in the ultra-low carbon Ti-containing IF steel sheet, and among them, 1,000 to 10,000 Å
Ti ₄ C ₂ S ₂ having the size of the precipitates has a favorable effect on the machinability and punching workability without adversely affecting the formability and the presence of the precipitates mainly composed of the Ti ₄ C ₂ S ₂ Based on the new finding that the characteristics can be exhibited extremely effectively if the composition and processing conditions of the steel material are properly adjusted so that the amount becomes 5 × 10 ³ pieces / mm ² or more, This is the completion of the invention.

Next, the reason for defining the composition of the steel material in the present invention will be clarified. C is an element that is indispensable for forming Ti ₄ C ₂ S ₂ by combining with Ti as described later, and must be contained at least 0.0010% or more. However, when the amount of C becomes too large, the amount of TiC produced increases and the elongation decreases, and the yield stress increases and the formability deteriorates. Therefore, the upper limit of 0.010 is set to ensure good formability. Defined as%.

Si is an effective element for increasing the strength without lowering the ductility, but if it is too much, the oxide film formed during annealing increases, which adversely affects the chemical conversion treatment often performed thereafter. Further, when electroplating is performed, there arises a problem that the plating thickness becomes uneven and uneven plating occurs. Therefore, the amount of Si should be suppressed to 0.25% or less. In particular, when the hot-rolled steel sheet is alloyed and hot-dip galvanized, an unplated portion is likely to be formed due to the formation of the oxide film.
It is desirable to suppress it to 0% or less.

Mn is an element which produces MnS and acts as an auxiliary to the machinability and punching workability, and 0.20% or more must be contained in order to exert its effect effectively, but it is too much. If so, the formability will be adversely affected, so it must be suppressed to 1.0% or less.

P may form a solid solution in steel or form FeTiP-based precipitates. In the former case, P acts as a strengthening element and adversely affects the formability, and the latter precipitates. In the form of a product, some improvement in machinability and punching workability is recognized, but the adverse effect on formability becomes noticeable, so it must be suppressed to 0.05% or less.

S is combined with Ti as described later to form Ti ₄
It is an essential element for forming precipitates mainly composed of C ₂ S ₂ , and in order to effectively exert the effect of improving the machinability and punching workability due to the precipitation, 5 × 10 ³ pieces / mm ² or more in the steel It is necessary to form a Ti ₄ C ₂ S ₂ -based precipitate of 1., and for this purpose, S must be contained in an amount of 0.010% or more. However, these effects are saturated at about 0.025%, and if contained excessively, they adversely affect the press formability and cause increase in surface defects, so the upper limit is 0.025%.

N combines with Ti to form TiN, but the TiN has almost no effect of enhancing the machinability and punching workability, and like C, it deteriorates the tensile properties of the steel material. The upper limit is 0.01 to prevent defects.
%.

Al is inevitably mixed as a deoxidizer or the like.
It is an element that comes, but if too much is Al ₂ O_Three Non-metallic system
It becomes a generation source of inclusions and adversely affects mechanical properties and workability.
Since it will affect, it is desirable to suppress it to 0.1% or less.
New

Ti is the same as Ti ₄ C ₂ S _{2 in} steel as described above.
It is the most important element for improving the machinability and punching workability by forming the main precipitate, and the lower limit of the content should be decided according to the content of N, S, C in the steel as described later. is necessary. However, the effect of Ti is saturated at an absolute amount of 0.15%, and addition of more than that is economically useless.

Ti produces Ti ₄ C ₂ S ₂ and TiN, as well as TiS depending on the conditions.
Also has the effect of fixing C, and in determining the lower limit value, C and S in steel should be taken into consideration in consideration of those effects.
It is indispensable to add an amount satisfying the requirement of the following formula (1) depending on the contents of N and N. (Ti * / 48) / (C / 12 + S / 32) = 1.0 to 3.0 (1) However, Ti * = Ti- (48/14) N

However, when the value of the expression (1) is less than 1.0,
An amount of Ti ₄ C ₂ S that is sufficient to contribute to improvement of machinability and punching workability due to insufficient amount of Ti combined with C and S
₂ does not form, and if it exceeds 3.0, the elongation deteriorates in either case due to the increase of solid solution C when the amount of C is excessive, and to the formation of MnS when the amount of S is excessive. And satisfactory moldability cannot be obtained.

As described above, in the hot-rolled steel sheet of the present invention, Ti ₄ C ₂ S ₂ -based precipitates having a size of precipitates in the range of 1,000 to 10,000Å are contained in the steel in an amount of 5 × 10 5. ^By forming ³ pieces / mm ² or more, machinability and punching workability are remarkably enhanced without adversely affecting the formability. However, such precipitates mainly composed of Ti ₄ C ₂ S ₂ are formed in the steel. In order to produce an appropriate amount of the hot rolled steel sheet, the manufacturing conditions of the hot rolled steel sheet (hot rolling temperature, winding conditions, etc.) are important. In order to achieve the object, after heating a slab made of steel satisfying the requirements of the above-mentioned composition in the temperature range of 1,100 to 1,200 ° C, (Ar ₃ transformation temperature + 80 ° C) to (A 3
It was confirmed that the hot rolling should be completed in the temperature range of r ₃ transformation temperature −40 ° C. and then wound in the temperature range of 700 ° C. or lower.

If the heating temperature at this time is less than 1,100 ° C.
Increases the amount of TiS and MnS produced,_Four C₂ S₂ 
Insufficient amount of system precipitates and exceeding 1,200 ℃
At high temperature, most of the precipitates dissolve and a proper amount of Ti
_Four C₂ S₂ No system precipitate can be obtained. After hot rolling
When the coiling temperature of T exceeds 700 ℃, Ti
_Four C₂ S₂ The system precipitates are changed to FeTiP series precipitates,
Effect of improving machinability and punching workability as intended by the invention
Will not be obtained. The lower limit of the winding temperature is particularly limited
No, but if it becomes too low, Ti-based precipitates will form
Does not occur, or even if it is generated, it precipitates as TiC and Ti_Four 
C₂ S₂ Since it may not be possible to obtain system precipitates,
More preferably, the temperature is set to 500 ° C. or higher.

As long as the appropriate heating temperature and winding temperature are satisfied as described above, even if the hot rolling finishing temperature slightly changes, Ti
_Although there is not much influence on the amount of ₄ C ₂ S ₂ -based precipitates produced, in order to carry out hot rolling smoothly and obtain hot rolled steel sheet with stable mechanical properties and product shape, the finishing temperature is (Ar
It is desired to set in the range of ( ₃ transformation temperature + 80 ° C) to (Ar ₃ transformation temperature -40 ° C). However, if the hot finishing temperature is too low, FeTiP is produced, and the workability deteriorates due to the presence of itself and solid solution C. On the contrary, if it is too high, the amount of TiS produced increases and satisfactory cutting is achieved. This is because there are difficulties such as not being able to obtain sex.

The hot-rolled steel sheet of the present invention thus obtained can of course be commercialized as it is, but for the purpose of rust prevention, electroplating, hot dip galvanizing, phosphate treatment, chromate treatment, etc. Chemical treatment,
Further, it is of course effective to apply a surface treatment such as an organic resin coating treatment.

[0041]

EXAMPLES The constitutions and effects of the present invention will be specifically described below with reference to examples, but the present invention is not limited by the following examples and can be adapted to the gist of the preceding and the following. It is also possible to make appropriate modifications within the scope of the invention, and all of them are included in the technical scope of the present invention.

Example 1 After heating a steel slab having the composition shown in Table 1 to 1180 ° C., the hot rolling end temperature was about 920 ° C. and the winding temperature was 630.
The temperature was set to ° C and hot rolling was performed to obtain a hot rolled steel sheet having a thickness of 3.2 mm. For each of the obtained hot-rolled steel sheets, the type and amount of precipitates formed in the steel were examined by the following method, and the tensile properties, machinability and punching workability were examined.

Method of confirming kind and amount of precipitates : Transmission electron microscope observation by extraction replica method. That is, determine the number of precipitates of the size in the field of view of approximately 0.015 mm ² at 10,000 times magnification, finding it in terms of the number of the 1 mm ^2. Punching workability : Each test plate is punched into a circle having a diameter of 30 mm with a clearance of about 12%, and the height of burrs generated on the punching end face is measured and evaluated. Machinability: to produce a pipe having an outer diameter of 60mm by high frequency induction heating welding method using the test plates, performed using a high-speed steel byte SKD9, cutting speed 125m / min, cutting at feed speed 0.06 m / rev , Evaluated by the number of cut-offs before cutting becomes impossible (tool life) and the state of chips (good: short curled chips, bad: long curled chips). Table 2 shows the results.

[0044]

[Table 1]

[0045]

[Table 2]

From Tables 1 and 2, the following can be considered. Steel grades A, B, and C are mild steel grades, and steel grade D is a high-strength steel grade, and are examples that satisfy the requirements of the present invention. All of them have excellent tensile properties as well as excellent machinability and punching workability. It has excellent performance.

On the other hand, all of the steel types E to J are S
Since the amount is insufficient, the machinability and punching workability are poor, and moreover, the steel type E has too much C amount and TiC is produced, so that the tensile property is bad, and the steel type I has too much P amount, and therefore the tensile property is bad. Further, since the steel type F has an excessively large amount of Si, although not shown in Table 2, the occurrence of plating unevenness due to the Si oxide film during electrogalvanizing and the plating adhesion during hot dip galvanizing were poor. It was confirmed that the steel type K has a large amount of S and therefore has good punching workability but poor tensile properties, and that it causes surface defects such as blisters and slivers on the surface of the steel sheet after rolling. Furthermore, steel type L is
Since the balance between the amount of Ti and the amount of (C + S) was poor and the requirement of the formula (1) was not satisfied, the tensile properties were poor and the aging resistance was also poor. In the steel type M, a sufficient amount of Ti ₄ C ₂ S ₂ is not deposited because the amount of C is insufficient, the tensile strength is inferior, and both punching workability and machinability are poor.

Example 2 The steel type B shown in Table 1 (Ar ₃ point is 877 ° C.) was used, and the heating temperature, hot rolling finishing temperature and winding temperature were changed as shown in Table 3 above. Hot rolling was performed in the same manner as in Example 1 to produce a hot rolled steel sheet having a thickness of 3.2 mm, and the type and number of precipitates were investigated in the same manner as in Example 1, and tensile properties and cutting were performed. And punching workability were investigated. The results are shown in Table 4.

[0049]

[Table 3]

[0050]

[Table 4]

In Tables 3 and 4, reference numerals B _{1 to} B ₄ indicate that the hot rolling finishing temperature and the coiling temperature are both within an appropriate temperature range, and particularly the influence of the heating temperature is investigated. When performed in an appropriate range of 100 to 1,200 ° C. (reference symbols B ₂ and B ₃ ), Ti ₄ C ₂ S of an appropriate size is used.
Proper amount of ₂ type precipitate is generated, and tensile property, machinability,
While having excellent properties in any of the punching workability, in the comparative example (reference numeral B ₁ ) in which the heating temperature exceeds 1,200 ° C., the precipitates are mainly MnS,
A comparative example in which the heating temperature is less than 1,100 ° C. (reference B ₄ ).
Then, the precipitates mainly consist of TiS, and both have poor machinability and punching workability.

Reference numerals B _{5 to} B ₆ show that the heating temperature and the winding temperature are both in the proper temperature range, and particularly the influence of the hot rolling finishing temperature is investigated, and the hot rolling finishing temperature is in the proper range. In the case of the sample (reference B ₆ ), an appropriate amount of Ti ₄ C ₂ S ₂ -based precipitate was formed, and it had excellent properties in terms of tensile properties, machinability, and punching workability. On the other hand, in the comparative example (reference B ₅ ) in which the hot rolling finishing temperature is too high, the precipitates are mainly composed of TiS, and the comparative example (reference B in which the hot rolling finishing temperature is too low).
_{In 7} ), the precipitates mainly consist of FeTiP, and both have poor machinability and punching workability.

[0053] The reference numeral B ₈ .about.B _10, both the heating temperature and the hot rolling finishing temperature was appropriate temperature range, which was particularly studied the effect of coiling temperature, subjected to coiling temperature in an appropriate range Titanium oxides (reference symbols B ₉ and B ₁₀ ) produced an appropriate amount of Ti ₄ C ₂ S ₂ based precipitates of appropriate size, and had excellent properties in terms of tensile properties, machinability, and punching workability. On the other hand, in the comparative example (reference numeral B ₈ ) in which the winding temperature is too high, the precipitates are mainly FeTiP, and satisfactory machinability is not obtained.

[0054]

The present invention is constituted as described above, and regulates the composition of the steel material, in particular the contents of Ti, S, Mn, N, etc., and the S, N and C contents and the Ti contents. Ti ₄ C ₂ S of proper size in steel by controlling relation properly
By generating an appropriate amount of ₂ type precipitates, it is possible to provide a hot rolled steel sheet having extremely excellent machinability and punching workability without impairing formability and mechanical properties, and also the method of the present invention. According to this, a hot-rolled steel sheet having such characteristics can be reliably manufactured industrially.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masakazu Miki 1 Kanazawa-machi, Kakogawa-shi, Hyogo Kamido Steel Works Kakogawa Works

Claims (2)

[Claims] 1. Mass% 0.0010% ≦ C ≦ 0.010% Si ≦ 0.25% 0.20% ≦ Mn ≦ 1.0% P ≦ 0.05% 0.010 ≦ S <0. 025% N ≦ 0.01% Ti ≦ 0.15%, and (Ti * / 48) / (C / 12 + S / 32) = 1.
0 to 3.0 However, Ti * = Ti- (48/14) N is satisfied, and the balance is Fe and inevitable impurities.
Made of Ti_Four C ₂ S₂ Size 1 mainly
5,000-10,000Å precipitates are 5 × 10^Three Pieces / mm
² Formability, machinability, and
Hot rolled steel sheet with excellent punching workability. 2. A slab made of steel satisfying the compositional requirements described in claim 1 at 1,100-1,200 ° C.
After heating in the temperature range of (Ar ₃ transformation temperature + 80 ° C)
~ (Ar ₃ transformation temperature -40 ° C), the hot rolling is completed in the temperature range, and then the coil is wound in the temperature range of 700 ° C or less, so that the size mainly composed of Ti ₄ C ₂ S ₂ is 1.
000 to 10,000Å precipitates 5 × 10 ³ / mm
A method for producing a hot rolled steel sheet having excellent formability, machinability and punching workability, which is characterized by generating ^two or more.