JP2932969B2 - Cold rolled steel sheet with excellent punching workability and formability without surface defects - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a punching process for manufacturing a chassis or a case of a home electric appliance which has no surface defect and has a complicated press-formed portion such as deep drawing or overhanging. The present invention relates to a cold-rolled steel sheet capable of minimizing the height of burrs formed on the end face of the steel sheet.

[0002]

2. Description of the Related Art Cold rolled steel sheets have been widely put into practical use, including automobile parts and household electric appliances, because of their excellent workability. In particular, when used for chassis and cases of home appliances, it is often applied to sliding parts by punching etc., so that the return of the end face after punching does not exceed a certain height and dimensional accuracy It is desirable not to make it worse. Further, since the components are often laminated in a state of parts, it is necessary that the performance of the final product is not adversely affected, such as an adverse effect on electric circuits due to abrasion powder. As a steel sheet used for such punching, a box-annealed low-carbon Al-killed cold-rolled steel sheet of SPCD or SPCE grade has conventionally been used.

[0003] In a box-annealed low-carbon Al-killed cold-rolled steel sheet, C is coarsely precipitated as fragile cementite in the crystal grain boundaries and in the grains during the annealing process for 10 hours or more, and this cementite cracks during punching. From the starting point of generation, it is known that the generation amount of burrs is small, the die wear is small, and the shear load is small. Mn
It has been confirmed that S precipitates have a similar effect.
However, in order to improve the efficiency of the punching operation, further improvement in the punching workability is desired.

[0004] As means for improving the punching processability, in terms of processing technology, appropriate clearances and punch / die shapes have been devised, and the steel material to be processed is, for example, a steel plate itself. And a method of hardening the surface of a steel sheet have been proposed. As a method of hardening the steel sheet itself, a method of adding a solid solution strengthening element such as P has been proposed, and as a method of hardening the surface of the steel sheet, plastic strain is applied to the surface layer by rolling or the like on the annealed steel sheet. (Japanese Patent Application Laid-Open No. 1-255625), or a method of hardening the surface of a steel sheet by nitriding or carburizing (Japanese Patent Application Laid-Open Nos. 1-255626, 2-133561 and 3-199343). JP-A-3-202442).

[0005] However, although the former method can improve the punching workability to some extent, there is a problem that the mechanical properties such as elongation and r-value, which are the inherent characteristics required of the steel sheet, are remarkably deteriorated. The method requires special equipment and steps for nitriding and carburizing treatments, and it is hardly a good idea in terms of equipment and productivity.

Further, by adding Mn and S, M
A method for producing a large amount of nS precipitates (JP-A-1-230
No. 748 and No. 6-73457) have been proposed, but this method causes hot working cracks at the time of working with an increase in the S, increases a point defect called a blowhole or a blister inside, A problem arises in that point-like surface defects called slivers frequently occur.

For the purpose of reducing the cost of home electric appliances, the punching and assembling steps of chassis parts and the like have been omitted. Conventionally, the parts which were assembled and constituted by a plurality of parts have been replaced with press-formed parts. There are some trends. For example, the pulley shaft to be attached to the chassis plate surface was previously manufactured by a separate process of mechanical cutting, but was inserted into the punched hole and caulked, but recently, the integral molding method of overhang and deep drawing has been adopted. It is becoming mainstream. Therefore, excellent formability is also required for steel sheets used for applications such as chassis. From a metallurgical point of view, punchability is affected by the presence of cementite and MnS, which are the starting points of crack initiation, and the hardness of the steel sheet itself. It was difficult to balance sex.

Further, in order to improve the efficiency of the annealing process, and to improve the surface properties and internal quality, the transition from the conventional box annealing to the continuous annealing capable of short-time annealing is rapidly progressing. There is an urgent need to develop a steel sheet that has both good stamping workability and excellent formability.

On the other hand, a so-called IF-based cold rolled steel sheet to which carbon and nitride forming elements such as Ti and Nb are added and C and N are fixed has excellent workability such as high elongation and r value and low yield stress. It is widely used for difficult-to-machine parts such as automobile bodies, and this IF-based cold-rolled steel sheet exerts its properties more than fully by adopting a continuous annealing method. However, although workability is good, there is a problem that punching workability is inferior to low carbon Al-killed cold-rolled steel sheet, and the burr height of the end face after punching becomes large. Not suitable.

[0010]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a conventional IF-based cold-rolled steel sheet by appropriately adjusting the chemical composition of the steel material. Cold-rolling with good punching workability and no burr problem, while ensuring excellent moldability and mechanical properties as obtained in, and without causing problems such as internal defects and surface defects It is intended to provide a steel plate.

[0011]

The structure of the cold rolled steel sheet according to the present invention, which has achieved the above-mentioned objects, is expressed as mass%
C ≦ 0.008% Si ≦ 1.0% 0.025% ≦ Mn ≦ 1.0% P ≦ 0.1% 0.010% ≦ S <0.025% Al ≦ 0.1% N ≦ 0 0.01% Ti ≦ 0.2% and 4 ≦ Ti * / C where Ti * = Ti − [(48/14) N + (48/3)
2) S], and the balance consists of Fe and unavoidable impurities, and has a gist at a place where the punching workability is particularly enhanced without causing deterioration of mechanical properties and the like and surface defects.

[0012]

In the cold-rolled steel sheet according to the present invention, the content and content of each element are specified as described above, and the content of Ti is determined in consideration of the relationship with the content of C, N and S. I have. this is,
The present invention is based on an ultra-low carbon IF steel having excellent press formability to improve the punching workability, which has been pointed out as a disadvantage, while maintaining its characteristics. The reason for determining the content will be described in detail later, and among them, the important technical features are as follows.

That is, in order to improve the punching property while maintaining the excellent formability inherent in the IF-based cold-rolled steel sheet, it is necessary to use a precipitate, which is a starting point of crack generation at the time of punching, specifically Ti Presence of ₄ C ₂ S ₂ is indispensable, and proper control of its size and dispersion state is the key to obtaining a cold-rolled steel sheet that simultaneously satisfies excellent formability, punching workability, and surface properties. Based on the new findings identified by the inventors. Precipitates and inclusions present in ultra-low carbon IF steel sheets include TiC, Ti
₄ C ₂ S ₂ , TiS, TiN, FeTiP, TiO _2, etc., and Nb (C, N) etc. in the Nb-added system. Of these precipitates and inclusions, especially the formability and punching workability Ti ₄ C ₂ has a remarkable favorable effect on the surface properties.
It was confirmed that the S _2. And the Ti ₄ C ₂ S ₂
It was also confirmed that the other precipitates and inclusions had the following tendencies.

TiC, Nb (C, N): Since the size as a precipitate is very small, there is a tendency that the formability is largely deteriorated, and the effect of improving the punchability is small. FeTiP: the size is about the same as Ti ₄ C ₂ S ₂ ,
Although it has the effect of improving the punching property, it greatly deteriorates the formability.

TiS, TiN: It seems that the effect of improving punchability is expected to be slightly larger than Ti ₄ C ₂ S _{2 in} terms of size. However, since it is soft, there is little substantial improvement effect. When the S content is increased, Ti ₄ C ₂
Although the amount of S ₂ increases, a certain amount (C is Ti ₄ C ₂ S ₂
Until Ti) is generated, and TiS is not generated until C is consumed.
iS is not preferable because it causes surface defects. Further, in practical steel, it is difficult to control the amount of N, and it is very difficult to properly control the amount of TiN.

TiO ₂ : The size is one order of magnitude larger than that of Ti ₄ C ₂ S _2, which is not preferable because not only the effect of improving the punching property is small but also a cause of surface defects. MnS: The same effect of improving punchability as Ti ₄ C ₂ S ₂ can be expected.

For the above reasons, in the present invention, by controlling the amount of Ti ₄ C ₂ S ₂ produced and the state of dispersion in the IF steel sheet with a very low carbon content of Ti, formability and punching properties are improved.
Furthermore, they have succeeded in obtaining a cold-rolled steel sheet satisfying all the surface properties. Incidentally, as a technique for improving the formability and punching property of an ultra-low carbon IF steel sheet, Japanese Patent Application Laid-Open No.
No. 73457, there is a method of adding Ti and Nb in a complex manner. According to the present inventors' investigation, when Nb is added in a range of 0.003 to 0.03%, Nb (C , N) is generated and C in the steel is consumed, so that Ti ₄ C ₂ S ₂ effective for improving the punching property cannot be generated. In addition, in this method, 0.02% or more of S is essential for improving the punching property.
There is a problem that the generation rate of surface defects is increased by the generation of GaN, and the practicability is lacking. Hereinafter, the reason for determining the component composition of the steel material in the present invention will be described in detail.

C contributes to the improvement of the strength of the steel material as a strengthening element. However, if it is too much, the elongation and the r-value etc. decrease and the workability deteriorates. 008%.

Si is an effective element for increasing the strength without lowering the ductility, but if it is too much, an oxide film formed at the time of annealing increases, adversely affecting a chemical conversion treatment often performed thereafter, Further, when performing electroplating, there arises a problem that the plating thickness becomes non-uniform and plating unevenness occurs, so the amount of Si should be suppressed to 1.0% or less. In particular, when the cold-rolled steel sheet is hot-dip galvanized, it is desirable to suppress the Si content to 0.3% or less, because the formation of an oxide film tends to cause an unplated portion.

Since P has a function of strengthening the steel by forming a solid solution in the steel, it is effective to contain P in an appropriate amount according to the required strength. However, if it is too much, segregation occurs and workability is deteriorated. 0.1% is made the upper limit because it has an adverse effect.

Since N has an adverse effect on mechanical properties such as elongation and r-value when its content is increased, it must be suppressed to 0.01% or less. Al effectively acts as a deoxidizing agent, but if it is too much, it becomes a source of nonmetallic inclusions and adversely affects mechanical properties and workability.
% Or less.

S is an extremely important element in the steel sheet of the present invention together with Ti, which will be described later. That is, in order to obtain excellent punchability, an appropriate amount of Ti ₄ C ₂
It is necessary to produce and disperse S ₂ , and for that purpose, S must be contained at least 0.01% or more. However, if the content is excessively larger than 0.025%, the incidence of surface defects increases, so the content must be suppressed below this value.

FIG. 1 shows C: 0.0052%, S
i: 0.01%, Mn: 0.4%, P: 0.014%,
Al: 0.03%, Ti: 0.08%, N: 0.006
5 shows the punching workability and r value when the S content was changed from 0.002% to 0.037% for a cold-rolled steel sheet having a basic composition containing 0.1%. In this figure,
Box-annealed Al-killed cold-rolled steel sheet (C: 0.044%, A) which is considered to have excellent punching properties and mechanical properties.
1: 0.045%) is also shown as a comparative example.
FIG. 2 shows the relationship between the S content and the incidence rate of surface defects.

The physical property test method was as follows. That is,
120 kg of 40 kg steel ingot melted and cast in a laboratory
After heating to 0 ° C., it is roughly rolled to 30 mm ^t . Then
After heating this slab to 1200 ° C., the plate thickness: 4 mm
^t , finishing temperature: 920 ° C or higher, winding temperature: 680 ° C
Hot-rolled under the conditions described above and ground to 3.2 mm ^t .
The sample was cold-rolled to 8 mm ^t , subjected to continuous annealing at a soaking temperature of 800 ° C. for 1 minute, and then temper-rolled at 1% to obtain a test cold-rolled steel sheet. The r-value was determined by measuring the width and thickness of a 10 mm × 110 mm test piece when tensile strains of 0% and 15% were applied. In addition, the punching workability is 0.
An 8 mm ^t cold-rolled steel sheet was punched out with a clearance of 9% to 10 mmφ to measure and evaluate the burr height. The surface flaw was evaluated by visually observing the surface of each of the obtained cold-rolled steel sheets and determining the surface flaw occurrence rate at a fixed length.

As is clear from FIGS. 1 and 2, if the S content is less than 0.008%, the height of the burrs at the time of punching becomes so large that the object of the present invention cannot be achieved. On the other hand, when the S content is 0.02% or more, the generation of surface flaws becomes remarkable, which again does not meet the object of the present invention. However, when the S content is in the range of 0.007% or more and less than 0.02%, surface flaws and burr heights can be suppressed as much as possible and a high level of r value can be secured.

When the punched portion of the steel sheet is observed microscopically, a shear fracture is first formed on the front and back surfaces of the steel sheet as the punch descends after the contact between the punch and the steel sheet, and the rest is locally strongly sheared. Upon receiving this, voids are generated and cracks are formed, and the cracks unite from the front and back surfaces to complete the processing. Therefore, in order to reduce burr after punching, it is effective to promote the formation of voids and the progress of cracks described above. However, since Ti ₄ C ₂ S ₂ is a relatively large precipitate, It is considered that when the steel sheet is subjected to strong shear deformation, the formation and development of voids and cracks in the vicinity of the precipitate are promoted, and as a result, burrs are reduced.

On the other hand, Ti combines with C and S to form Ti ₄ C ₂
In addition to producing S ₂ and Ti, it also binds with N, so that an amount that can completely fix N in the above range is necessary. In order to impart formability and aging resistance, it is sufficient to fix C. Quantity of T
i is also needed. From this viewpoint, in the present invention, as described above, the amount of Ti added is 4 ≦ Ti * / C, where Ti * = Ti − [(48/14) N + (48/3)
2) S].

However, the effect of adding Ti as described above is about 0.2.
%, The addition of more than that is not only economically wasteful, but also has the negative effect of increasing the recrystallization temperature, so the upper limit was set to 0.2%.

Mn is also an element which forms coarse precipitates of MnS as described above and contributes to the improvement of punching workability.
% Or more. But too much,
Since the steel sheet becomes too hard and adversely affects workability, the upper limit is set to 1.0%.

[0030] The cold-rolled steel sheet according to the present invention is prepared by melting a steel material satisfying the above-mentioned composition requirements and casting it according to a conventional method, followed by hot rolling, pickling, cold rolling, annealing temper rolling and the like. The cold-rolled steel sheet obtained is free from surface defects such as surface flaws and has excellent mechanical properties, workability, and punching workability. Become. At this time, in order to further enhance the workability, the slab heating temperature during the hot rolling is preferably suppressed to a low temperature of about 1200 ° C. or less. Regarding annealing, continuous annealing is desirable from the viewpoint of material homogeneity and shape.

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

[0032]

EXAMPLES Hereinafter, the structure, operation, and effects of the present invention will be described in detail with reference to examples. However, the present invention is not limited to the following examples, and can be adapted to the above and following points. It is also possible to implement the present invention with appropriate modifications within the scope, and all of them are included in the technical scope of the present invention.

Example 40 kg of the components shown in Table 1 were melted in a laboratory by vacuum.
After heating the steel ingot to 1200 ° C., it was roughly rolled to 30 mm ^t , and then this slab was heated to 1200 ° C., and the sheet thickness was 4 m.
m ^t, finishing temperature 920 ° C. or higher, and hot rolled under conditions of a coiling temperature 680 ° C.. Then pressure cold-rolled from the ground to 0.8 mm ^t to 3.2 mm ^t, was subjected to continuous annealing for 1 minute soaking at 800 ° C., it was carried out 1% temper rolling.

For each of the temper-rolled materials obtained, the surface properties and the presence / absence of surface defects were examined, the tensile properties were examined using JIS No. 5 tensile test pieces, and the yield point rise after further treatment at 100 ° C. for 60 minutes. Aging resistance (A.
I) was evaluated. The r-value was determined by measuring the width and thickness of a 10 mm × 110 mm test piece when tensile strains of 0% and 15% were applied. Furthermore, the punching workability is 0.
8mm ^t of each cold rolled steel sheet with 9% clearance and 10mmφ
The height of the burr of the end face punched out was measured and evaluated. In addition, as an evaluation of plating properties, zinc electroplating with an attached amount of 10 g / m ^{2 on} one side was performed, and the surface properties were examined. Table 2 shows the results.

[0035]

[Table 1]

[0036]

[Table 2]

The following can be considered from Tables 1 and 2. Steel types A, B, and I are mild steels and steel type E is a high-strength steel, all of which satisfy the requirements of the present invention, and have strength (TS), elongation (El), r-value, On average, good results were obtained in all of I (aging resistance), burr height, surface defects, and surface properties.

On the other hand, C is a comparative example in which the C content is larger than the steel type A and exceeds the specified range of the present invention, and the elongation and the r value are low. Surface defects such as blisters and slivers are observed. Steel type F
Since the amount of Si exceeds the specified range with respect to the steel type E, the plating unevenness occurs due to the formation of an oxide film when electroplating is performed.

Since the amount of Si exceeds the specified range, the steel type F has poor surface properties, and may impair the chemical conversion treatment performed thereafter or the uniformity of electroplating. Steel type G is a comparative example in which the amount of S is insufficient with respect to steel type A,
The burrs generated by the punching process are high and do not meet the purpose of the present invention, and the steel type H has satisfactory elongation, r value and aging resistance because the ratio of Ti * / C does not satisfy the requirements of the present invention. Not obtained. Steel type J has a poor elongation and r-value because the N content exceeds the specified range with respect to steel type I, and steel type K has a small Mn content relative to steel type A, and the specified amount according to the present invention. Therefore, the punching workability is poor and the purpose of reducing the burr height has not been fulfilled.

The steel type L is a comparative example in which the amount of Mn is too large relative to the steel type E, and the elongation and the r value are poor, and the steel type M is a comparative example in which the P amount is too large relative to the steel type E. Low elongation and r-value. Steel type N is a comparative example in which the amount of Al is too large with respect to steel type A, and elongation and r value are low and surface defects are observed. It's getting worse.

[0041]

The present invention is constituted as described above and regulates the composition of the steel material, especially the content of S, Mn, Ti and the like, and the relationship between the S, N and C content and the Ti content. By properly controlling, it is possible to provide a cold-rolled steel sheet free from surface defects and excellent in all of the strength characteristics and punching workability.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of the S content in a steel material on the burr height and r value formed during punching.

FIG. 2 is a graph showing the relationship between the S content in a steel material and the incidence of surface defects on a cold-rolled steel sheet.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Seiichi Miki 1 Kanazawacho, Kakogawa City, Hyogo Prefecture Kobe Steel, Ltd. Kakogawa Works (56) References JP-A-6-306532 (JP, A) JP-A Heihei 6-73457 (JP, A) JP-A-3-199343 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C22C 38/00 301 C22C 38/14

Claims (1)

(57) [Claims] 1. Mass% C ≦ 0.008% Si ≦ 1.0% 0.25% ≦ Mn ≦ 1.0% P ≦ 0.1% 0.01 ≦ S <0.025% Al ≦ 0 .1% N ≦ 0.01% Ti ≦ 0.2%, and 4 ≦ Ti * / C where Ti * = Ti − [(48/14) N + (48/3)
2) A cold-rolled steel sheet which satisfies S] and is excellent in punching workability and formability free from surface defects, with the balance being Fe and inevitable impurities.