JPS5976824A - Manufacture of high-strength cold rolling steel sheet - Google Patents

Abstract

PURPOSE:To manufacture a high-strength cold rolling steel sheet which is excellent in surface state and cold workability, by hot-rolling low carbon steel which limits the upper limit of Si and Mn, at a finish temperature exceeding an Ar3 point, and thereafter, performing a specified heat treatment, and performing its annealing again after cold rolling. CONSTITUTION:The following treatments are performed to steel which contains <=0.15% C, <=3R Si, <=2% Mn, and contains one or both of <=1% Cu and 0.03- 0.13% P as necessary, 0.5-4% total quantity of Si and Mn, and the balance iron with inevitable impurities. That is to say, this steel is hot-rolled at a finish temperature exceeding an Ar3 transformation point, is cooled at a speed of >=30 deg.C/second, and is wound at >=600 deg.C temperature. Subsequently, after pickling, it is subjected to solution heat treatment at 850-950 deg.C temperature. Subsequently, after quenching, an obtained steel sheet is subjected to continuous or box annealing at >=500 deg.C temperature of, and also, after cold rolling, it is subjected to continuous or box annealing.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a high-strength cold-rolled steel sheet, and particularly to a method for manufacturing a high-strength cold-rolled steel sheet with excellent surface texture and cold workability.

In recent years, the amount of high-strength cold-rolled steel sheets used has been rapidly increasing, mainly in the automobile industry, with the main purpose of improving fuel efficiency by reducing the weight of vehicle bodies. Such cold-rolled steel sheets are required to have sufficient strength for IM to cope with the thinning of automobile bodies, and are also required to withstand various harsh forming conditions such as deep drawing. In order to meet these demands, various manufacturing methods for high-strength cold-rolled steel sheets have been proposed, but a method that satisfies all properties such as workability, strength, and ductility has yet to be found. Not served.

As a result of intensive research to solve the above-mentioned problems, the present inventors have defined 11 elements to be included in the material in the production of cold-rolled steel sheets, and have conventionally By changing the conditions that were considered essential from the viewpoint of manufacturing steel sheets and that would not result in deterioration of workability, it was unexpectedly possible to improve the surface texture and cold workability. The present invention was achieved by discovering that it is possible to produce a high-strength cold-rolled steel sheet with significantly improved properties.

The method for manufacturing high-strength cold-rolled steel sheets according to the present invention includes C0915
% or more, contains 3% or less of Si and 2% or less of Mn, and the total RI amount of Si and Mn is 0.5 to 4%,
The material consisting of the remainder iron and unavoidable impurities is hot-rolled at a rolling temperature of /1r above the transformation point, and then cooled at a rate of not less than 30°C/second to a temperature of 600°C or higher. After winding and pickling, solution treatment is carried out at a temperature of 850 to 950°C, quenching is performed, and the #M plate thus prepared is continuously or box annealed at a temperature of 500°C or higher, and further cold-treated. After rolling,
It is characterized by continuous or box annealing.

In the steel material used in the present invention, 0 imparts high strength to the obtained cold rolled steel sheet, but if its content becomes excessive,
It lowers the elongation, plastic strain ratio r value, stretch flangeability, etc. of the steel plate, and deteriorates other properties such as formability and spot weldability, so it is set at 0.15% or less. In addition, in the present invention,
The chemical composition of the citrus fruit is not shown, and % means weight %. on the other hand,
If the C content is too low, add sufficient strength to the steel plate.
In addition, harmful effects such as coarsening of the steel plate structure and a decrease in the strength of the welded part when spot welded may occur. Therefore, the C content is at least 0.001%, preferably 0.005% or more.

In addition, when annealing is performed continuously, good results can be obtained at a relatively low C content.

Si is a main component that gives high strength to the obtained steel sheet, and has the effect of improving formability.

However, when the content is excessive, formability deteriorates, problems arise in steelmaking operations, and furthermore, when spot welding is performed, the nugget structure of the weld becomes coarse and the strength deteriorates in this area. do. Therefore, in the present invention,
The content of St is 3% or less.

Mn is often mixed into molten steel as an impurity element.
? - While fixing S of Wt, +4 material strength is changed to 11t
Furthermore, it has the effect of suppressing the transformation point from increasing due to the addition of Sl. In order to effectively express these effects, it is necessary to contain 4J, 0.2% 1;), but it is not necessary to contain it in a large amount exceeding 2%.

As mentioned above, in order to suppress the transformation point increase caused by the addition of Si with Mn and give maximum strength to the cold-rolled plate obtained by both, the total amount of Si and Mn should be 0.
A preferable range is 5 to 4.5%. When it is contained in excess beyond this range, it is not preferable because the processability is rather deteriorated.

Further, according to the present invention, in addition to the above-mentioned elements, Cu is 196 or less and P is 0.03 to 0.03.
Contains within the range of 0.13%! By doing so, it is possible to produce a cold-rolled 4M plate with even higher strength.

That is, in addition to Co, 15% or less, S13% or less, and Mn 2% or less, Cu 1% or less and/or po.

3 to 0.13%, and the total amount of Si and Mn is 0.5 to 4%, and 1. A steel consisting of the balance iron and unavoidable impurities is used.

In the past, various studies have been conducted on the effects of the addition of Cu on various monthly properties, mainly in mild steel sheets, by changing the texture to 'PJJ', but there are still many aspects of the effects that are not fundamentally clear. It has been considered that there is no effect of adding Cu in the low carbon region where the C content is 0.02% or less. In particular, the addition of Cu to a high-strength cold-rolled steel sheet as used in the present invention, which contains relatively large amounts of Si and Mn, has not been studied in the past. However, when treating cucumber containing Cu according to the method of the present invention, C is 0.
．． Even in the extremely low carbon region of 0.1% or less, extremely high r
This is one important feature of the method of the present invention.

That is, according to the method of the present invention, Cu has the effect of increasing strength due to the solid solution effect, has the effect of increasing the work hardening rate, and further lowers the transformation point, increases toughness, and at the same time improves weather resistance. It is also markedly improved. In order to effectively express this effect, the content GJ must be at least 0.2
%is necessary. On the other hand, the addition of excessive CIJ during rolling may cause surface defects called tear cracks, which may deteriorate the surface properties, and may also cause the loss shrinkage properties in the hot rolling state. It is set at 1% in consideration of the influence it may have on behavior, etc.

In the present invention, P has the effect of further increasing the strength when combined with Si and lv'ln, which are the main reinforcing elements for improving the strength of the slope, and optionally improves the workability. Also improve. In order to effectively exhibit such an effect, the content must be 0.03% or more, and as the content increases, strength, workability, and weather resistance also improve. ! Improved by i. However, the presence of an excessive amount causes a deterioration in impact secondary workability and a decrease in the cross tensile strength and shear tensile strength due to a decrease in the toughness of the spot welded portion, so it is set to 0.1396 or less.

According to the present invention, in addition to the above-mentioned specification ratio, Nb
By containing a predetermined amount of at least one element selected from .

That is, CO, 15% or less, S'! 3% or less and M n2
% or less, Cu l 5'6 or less and/or Po
.

03 to 0.13%, and the total amount of Si and Mn is 0.5 to 4%, 'ri 0.3% or less, N b O, 2% or less, V O, % or less and Z
A net containing at least one element in the range of 0.3% or less, with the balance consisting of iron and unavoidable impurities is used.

The lower limit of the content of the additional elements Ti, Nb, V, and Zr mentioned above is all 0.01%.On the other hand, even if they are contained in large amounts exceeding the above upper limit, the effect will not increase commensurate with the addition. It is only recognized slowly and is economically disadvantageous.

In the present invention, /l can be used as a deoxidizing element if necessary.
can be added. A7! Due to its deoxidizing effect, it not only improves the addition yield of Si and Mn, which are the main reinforcing elements in the present invention, but also suppresses or prevents the tendency for impact secondary workability to decrease due to the inclusion of P in the material. In particular, A7! Contains 0.01% or more of -U
By doing so, various qualities for cold rolling can be further improved.

However, if the content is too large, the amount of alumina produced increases and surface defects such as slipper scratches may appear. If required, it should be 0.08% or less.

In addition, most of the oxygen in steel exists as non-metallic inclusions, and as the number of oxygen increases, the impact secondary workability decreases, and it also decreases the impact secondary workability. ,'ri
, the effect of adding Zr, etc. is reduced, so the amount of oxygen is 0.0
It is preferable to set it to 15% or less.

In addition, impurities such as S that are inevitably mixed in reduce the +A'4'i characteristic value represented by stretch flangeability.
Therefore, it is preferable to reduce the S content as much as possible. For example, it is desirable that the S content be 0.03% or less.

As described above, the method LSI of the present invention uses steel whose type and amount of contained elements are specified, and is processed under predetermined conditions as described below to improve the surface texture and cold workability. ] It is possible to obtain high-strength cold-rolled specimens with unique properties.

That is, in the method of the present invention, after molten steel containing the above-mentioned elements is made into a steel billet by an in-air method, this is first heated in Ar.
Hot rolling is carried out at a temperature of -1- above the transformation point. A
This is because if finish hot rolling is performed at a temperature 1ζ lower than the r transformation point, the hot-rolled structure becomes mixed grains and various characteristic values after cold rolling and annealing become low. In addition, this P (rolling process) includes a process of immediately hot rolling a hot steel billet without cooling it after applying a hot solvent (pot direct rolling), a ?MLM charging hot rolling process, etc. The above-mentioned Bot Direct U-ring is an effective means for preventing the occurrence of surface flaws in Cu-containing steel, thereby making it possible to obtain products with excellent surface properties.

The hot-rolled steel sheet thus hot-rolled is then cooled at a rate of 30° C./second or more. This is because if the cooling rate is slower than 30° C./sec, a desirable hot-rolled sheet structure cannot be obtained in order to obtain an m-slope having excellent properties after cold rolling and annealing.

The thus cooled plate is then rolled up at a temperature of 600° C. or higher. The temperature during this winding has a great effect on the r value of the rope characteristics, and according to the present invention 60 +
An improvement in the r value is achieved only by employing a temperature of 1'c or higher. The reason why the winding temperature has a dominant effect on improving the r value is not necessarily clear, but it is assumed that it is due to the purification of the ferrite 1 phase.

After winding, it is pickled according to the usual method, and then 850 to 950
After performing solution treatment at a temperature of °C, it is rapidly cooled. This treatment is advantageously carried out in a continuous annealing furnace containing a quenching zone. The cooling rate during this quenching changes the properties of the hot-rolled sheet, but it is generally accepted that the higher the cooling rate, the more distorted the characteristic values will be obtained.

After solution treatment, the quenched fruit is then reheated by annealing. This round! Either box annealing or continuous annealing may be used for this. This reheating improves the thermal grain structure, and at the same time, in the case of cucumbers containing Cu, this step actively precipitates Cu, thus obtaining a desirable texture and crystal structure as a product. .The reheating temperature is 50°C in the case of box annealing.
600-800 in case of 0-750゛C1 continuous annealing
It is preferable to set it in the range of ℃. In either case, cooling after soaking does not affect the quality of the solid solution carbon and removes the negative effects.
Alternatively, it is preferable to reduce the speed for the purpose of reduction. This cold rolling can be carried out according to a conventional method, but it is preferable that the cold rolling rate is as high as possible within equipment constraints from the viewpoint of improving the mechanical properties of the obtained product.

The reheated cypress is cold rolled and then further recrystallized and annealed. This recrystallization annealing may be performed by box annealing or continuous annealing, but in the case of box annealing, the temperature is preferably 650 to 900°C, and in the case of continuous annealing, the temperature is preferably in the range of 700 to 900°C.

Furthermore, according to the present invention, when this recrystallization annealing is performed by the box annealing method, cold workability and strength can be further improved by performing continuous annealing in which soaking is maintained at a temperature of 700 to 900°C. An improved cold-rolled mesh board can be obtained.

As described above, according to the present invention, by selecting hot rolling conditions, processing conditions for the coil after hot rolling, annealing conditions, etc., using t4rl that specifies the types of elements to be included and the proviso, It is possible to obtain a high-strength cold-rolled test plate with excellent surface properties and cold workability, and such a steel plate has a strength that is compatible with the thinning of the steel plate used in automobile bodies, for example. It also has excellent moldability.

Examples of the present invention are listed below, but the present invention is not limited to these Examples in any way.

Example 1 A test sample tM having the chemical composition shown in Table 1 was prepared according to the high method.
890~ by pot direct rolling after ζ melting
Hot rolled at a finishing temperature of 910°C and coiled at a temperature of 40 to 80°C to obtain a hot rolled coil with a fin thickness of 2.5]
child.

After pickling, the hot-rolled coil was continuously annealed, heated at 930° C. for 3 minutes, and then rapidly cooled. Thereafter, the hot rolled sheet was reheated at 650°C for 10 hours using the box annealing method.

Next, cold rolling was performed to obtain a cold rolled steel plate having a thickness of 0.7 mm.

After box annealing the product at 700°C for 3 hours, it was subjected to about 1% temper rolling to obtain a cold-rolled product according to the present invention. Ta. The r values of these products are also shown in Table 1. Incidentally, no surface flaws were observed on these products, and the surface texture was uneven.

As is clear from Table 1, the cold-rolled steel sheet obtained by the method of the present invention has extremely high r-value while having high strength. The strength class of the material used in this example corresponds to 45 to 50 kilograms.

Example 2 A test steel having the chemical composition shown in Table 2 was made by rolling according to the ', pi method.
It was hot-rolled at a finishing temperature of 80 to 900°C and wound at a temperature of 42 to 70°C to obtain a hot-rolled coil with a thickness of 2.5 mm.

After pickling, the hot-rolled coil was continuously annealed, heated at 930 C for 3 minutes, and then rapidly cooled. Thereafter, the hot-rolled sheet was subjected to a rolling die treatment at 750° C. for 3 minutes using a continuous jfl method.

Next, cold rolling was performed to form a cold rolled # grain board with a thickness of 0.7 (q
Ta. This was connected and annealed at 880° C. for 3 minutes and then subjected to about 1% interstitial rolling to yield 1 q of cold-rolled products according to the present invention.

The r values of these products are shown in Table 2.

In addition, as in Example 1, these products also had no surface flaws, and the surface texture was uneven.

As is clear from Table 2, the cold-rolled steel sheet obtained by the method of the present invention has an extremely high r value even though it is used at an initial strength level. In this example, the strength class of the cold-rolled product (q) corresponds to 50 to 55 grade I.

Example 3 The same test steel as in Example 2 was melted by the lightning method, made into a slab, and finished at a temperature of 880 to 90 (FC). Cooling at a rate of 66 °C/s
A hot-rolled steel plate with a thickness of 2.5 gates is rolled up at 0 to 680°C (J
Ta.

After pickling, this hot rolled coil was continuously annealed, heated at 930° C. for 3 minutes, and then rapidly cooled. Next, reheating treatment was performed at 750° C. for 3 minutes using a continuous annealing method, followed by cold rolling to obtain a cold rolled steel plate having a thickness of 0.7 mm.

Thereafter, this cold-rolled steel plate was box-annealed at 700°C for 3 hours, then continuously annealed at 870°C for 3 minutes, and further subjected to 1% steel rolling to obtain a cold-rolled steel plate according to the present invention.

The r values of the obtained cold rolled steel sheets are shown in Table 3.

Table 3 After recrystallization annealing is performed using the box annealing method, the strength of the product is further improved by continuous annealing, and the strength class of the product obtained here is 50 to 55 kg. Equivalent to. Moreover, compared to its strength, the r value of the product obtained by the method of the present invention is extremely high.

Claims (1)

[Claims] (llco, 15% or less, Si 3% or less, and M n 2
% or less, and the total amount of Si and Mn is 0.
5 to 4%, the balance consisting of iron and unavoidable impurities, is hot rolled at a finishing temperature above the Ar transformation point,
While cooling at a rate of 30°C/second or more, the steel plate is wound up at a temperature of 600°C or higher, pickled, then solution treated at a temperature of 850 to 950°C, and rapidly cooled.
A method for producing a high-strength cold-rolled steel sheet, which comprises continuous or box annealing at a temperature of 0.degree. C. or higher, further cold rolling, and then continuous or box annealing. (In addition to 21C0, 15% or less, 313% or less, and Mn 2% or less, Cu 1% or less and/or PO, 03-0.
13%, and the total amount of Sl and Mn is 0.
5 to 4%, the balance consisting of iron and unavoidable impurities, is hot rolled at a finishing temperature of ^r3 transformation point or higher,
While cooling at a rate of 0°C/second or more, the steel plate is wound up at a temperature of 600°C or higher, pickled, then solution treated at a temperature of 850 to 950°C, and rapidly cooled.
1. A method for producing a high-strength cold-rolled plate, which comprises continuous or box annealing at a temperature of 100 ml or higher, further cold rolling, and then continuous or box annealing. (31' GO, 15% or less, S13% or less, and Mn
In addition to 2% or less, Cu 1% or less and/or P 0.03
~0.13%, and the combined amount of Si and Mn is 0.5 to 4%, and ri is 0.3% or less.
N b O, 2% or less, Vo, 2% or less, and Z r 0
．． A citrus fruit containing at least one element in a range of 3% or less, with the balance consisting of iron and unavoidable impurities is hot-rolled at a finishing temperature of ^r3 transformation point or higher, and cooled at a rate of 30'C/sec or higher. After winding and pickling at a temperature of 600°C or higher, solution treatment is performed at a temperature of 850 to 950°C, rapid cooling, and the thus obtained mesh plate is continuously or box annealed at a temperature of 500°C or higher. A patented method for producing a cold rolled steel sheet, which comprises further cold rolling and then continuous or box annealing.