JPS5976825A - 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 bringing low carbon steel which limits the upper limit of Si and Mn, to hot rolling and heat treatment under a specified condition, and bringing it to box annealing after pickling and cold rolling. CONSTITUTION:Steel which contains <=0.15% C, <=3% Si, <=2% Mn, and contains one or both of <=1% Cu and 0.03-0.13% P as necessary, and 0.5-4% Si+Mn, and the balance iron with inevitable impurites is treated as mentioned below. That is to say, this steel is hot-rolled at a finish temperature exceeding an Ar3 transformation point, is cooled at >=30 deg.C/second speed, and is wound. Subsequently, it is held at a temperature exceeding 500 deg.C. Subsequently, after pickling and also cold rolling, its box annealing is executed at 650-800 deg.C temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a high-strength cold-rolled steel sheet, and more particularly to a method for manufacturing a high-strength cold-rolled steel sheet with poor 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 to cope with the thinning of automobile bodies, and
Formability that can withstand various harsh forming conditions such as deep drawing is also required. In order to meet these demands, various manufacturing methods for high-strength cold-rolled steel sheets have been proposed, but no method has yet been found that satisfies all properties such as workability, strength, and ductility. do not have.

As a result of intensive research in order to solve the above-mentioned problems, the present inventors have defined the species 1 and 1 of the elements contained in the material in the N construction of cold-rolled VI sheets, and
31. It is unexpected from the viewpoint of manufacturing steel sheets with poor workability that changing the conditions that have been considered to cause a decline in workability. The inventors have now discovered that it is possible to obtain a high-strength cold-rolled steel sheet with significantly improved surface properties and cold workability, leading to the present invention.

The method for producing a high-strength cold-rolled grain board according to the present invention includes CO,1
5% or less, Si 3% or less, and Mn 2% or less, and the total amount of Si and Mn is 0.5 to 4%, with the balance consisting of iron and inevitable impurities. The material was hot rolled at a finishing temperature of 100° C. or higher, wound up while cooling at a rate of 30° C./sec or higher, then heat treated to hold at a temperature of 500° C. or higher, pickled, and further cold rolled. After that, box annealing is performed at a temperature of 650 to 800°C.

In the steel material used in the present invention, C imparts high strength to the obtained cold-rolled steel sheet, but if its content becomes excessive,
It reduces the elongation, plastic strain ratio r value, stretch flangeability, etc. of the steel plate, and deteriorates other properties such as formability and spot weldability.
Therefore, it is set to 0.15% or less. In the present invention, % indicating the chemical composition of steel means weight %. On the other hand, when the C content is too low, sufficient strength cannot be imparted to the steel plate, and harmful effects such as coarsening of the steel plate structure and a decrease in the strength of the welded part when spot welded occur. Sometimes. Therefore, the C content is at least 0.001%, preferably 0.005% or more.

Note that when annealing is performed continuously, good results can be obtained with 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 and there are problems in steelmaking operations.Furthermore, when spot welding is performed, the nucket structure of the weld becomes coarse and the strength deteriorates in this area. do. Therefore, in the present invention, 'C' is defined as having a Si content of 394 or more.

Mn has the effect of fixing a small amount of S, which is often mixed as an impurity element in the melt tank, and improving the strength of the single material. Furthermore, it has the effect of suppressing the rise in the transformation point due to the addition of St. have In order to effectively express these effects,
Although it is necessary to contain 0.2% or more, it is not necessary to contain a large amount exceeding 2%.

As mentioned above, in order to suppress the increase in transformation point due to the addition of Si with Mn and give maximum strength to the cold rolled steel sheet obtained by both, the total amount of Si and Mn should be 0.81%.
A preferable range is 5 to 4%. When the content exceeds this range, it is not preferable because it actually reduces processability.

According to the present invention, a cold-rolled steel sheet with higher strength is obtained by containing Cu in an amount of 1% or less and/or P in a range of 0.03 to 0.13% in addition to the above-mentioned elements. be able to.

That is, CO,l 5 ! '6 or less, S13!1 (in addition to 1 or less and Mn2% or less, contains Cl11% or less and/or PO103~0.13%, and St and Mn
In that case, the amount of iron used is 0.5 to 49 °C, the balance consisting of iron and unavoidable impurities.

Conventionally, the effects of adding Cu on various material properties, including texture, have been examined in various ways, mainly on mild steel plates, but there are still many aspects of the effects that are fundamentally unclear. It was considered that there was no effect of adding Cu in the low carbon region where the C content was 0.02% or less. In particular, the addition of Cu to a high-strength cold-rolled steel sheet as in the present invention, which contains relatively large amounts of Si and Mn, has not been studied in the past. However, when treating Cu-containing steel according to the method of the present invention, C is 0.0
An extremely high r value can be obtained even in the extremely low carbon range of 1% or less, and 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 the strength due to the hardening effect, and also has the effect of increasing the work hardening rate.Furthermore, Cu has the effect of lowering the transformation point, increasing the toughness, and especially improving the weather resistance. 11 was also significantly improved. In order to effectively express such an effect, the content must be at least 0.
．． 2% is Rishi/Kaname. On the other hand, addition of excessive Cu to the phase may cause surface defects called cavities, which may change the surface texture (1N), and may also affect the scale properties and behavior during hot rolling. Considering the impact, it is set at 1%.

In addition, in the present invention, P has the effect of further increasing the strength by combining with Si and Mn, which are the main reinforcing elements for improving the strength of the steel sheet. Also improve. In order to effectively express such effects, the content must be 0.03% or more, and as the content increases, strength, workability, and even
The t-threshold property is also significantly improved. However, the presence of an excessive amount causes a deterioration in impact secondary workability and a decrease in cross tensile strength and shear tensile strength due to a decrease in the toughness of spot welds, so the content is set at 0.13% or less.

According to the present invention, in place of CIJ and/or P& described in "1", or in conjunction with Cu and/or P, Nb,
By containing a predetermined amount of at least one element selected from T+ and Zr, a cold-rolled steel sheet with even higher strength can be obtained.

That is, in addition to C015% or less, St3% or less, and Mn2% or less, Cu1% or less and/or PO103 to 0.
13% or not, and the total M amount of 3t and Mn is 0.5 to 4%, and T t 0.3
% or less, N b 0.2% or less, ■ 0.2% or less, and Z
r Contains at least one species in a range of 0.3% or less,
We use citrus which consists of residual iron and unavoidable impurities.

The lower limit of the content of the above-mentioned additional elements T1, Nb, The effect increases only slowly and is economically disadvantageous.

In the present invention, 7i is used as a deoxidizing element if necessary.
, x can be added. Due to its deoxidizing effect, 80 not only improves the addition yield of 3i and Mn, which are the main reinforcing elements in the present invention, but also suppresses the decrease in impact secondary workability due to the inclusion of P in the material. In particular, by containing Δp in an amount of 1.01% or more, various qualities of cold rolled steel can be further improved.

However, if the content is too large, the amount of alumina produced increases and surface defects such as sintering scratches may appear. 0.0 if particularly good surface quality is required.
It is considered to be 8% or less.

In addition, most of the oxygen in m exists as nonmetallic inclusions, and when the amount increases, f #j impact secondary workability decreases, and C, S t, Mn, P, Cu, Nb ,V,Ti
, 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 unavoidably mixed in (as typified by stretch flangeability) (represented by stretch flangeability), are best kept as low as possible since they reduce the material's hardness.For example, S is 0. 0
It is desirable that it be 3% or less.

The method of the present invention uses a citrus fruit whose type and amount of contained elements have been specified, and by treating it under predetermined conditions as explained below, the surface texture and cooling are improved. 7 can be achieved by obtaining a high-strength cold-rolled steel sheet with excellent workability.

That is, in the method of the present invention, after molten steel containing the above-mentioned elements is made into a steel billet by the usual method, this is first subjected to finish hot rolling at a temperature of 2' above the Ar transformation point. This is because the hot-rolled structure becomes mixed grains, and various characteristic values after cold rolling and baking decrease. In addition, this hot rolling process includes a process of hot rolling a hot steel billet immediately after hot melting (hot direct rolling), a hot billet charging hot rolling process, etc.
Hot direct rolling prevents the occurrence of surface flaws on the Cu-containing net, so it is possible to obtain a product with excellent surface properties.

The hot-rolled steel sheet thus hot-rolled is then cooled at a rate of 30°C/sec or more. When the cooling rate is slower than 30°C/sec, cold rolling, hardening #11! This is because a preferable hot-rolled sheet structure cannot be obtained for later obtaining a steel sheet having excellent properties. The temperature during winding is preferably 500°C or higher.

The U-shaped rolled coil wound in this way was then
Perform reheating treatment by holding at an altitude of 'c or above. In order to effectively exhibit the effects described below, a holding time of 1 hour or more is required, but a holding time of about 3 to 10 hours is most appropriate. In this reheating treatment, CIJ precipitates, and the strength and cold workability are improved by the precipitation, especially for thick plates! ! ) The bending properties of the rolled steel plate are improved, and hot-rolled steel plates can also be used. still,
[Pretreatment] Even in the single culm winding process, increasing the winding temperature promotes the precipitation of Cu in the coil-cooled culm after winding, so the higher the winding temperature is, the better. Furthermore, according to the present invention, once after S[12],
It may be cooled to room temperature and then subjected to the above reheating treatment, or it may be directly subjected to the reheating treatment.

The steel plate reheated as described above is then cold rolled and box annealed at a temperature of 650 to 800°C. In general, box annealing requires longer annealing time than continuous annealing, so it is possible to produce products with good workability and uneven grain growth.
In particular, in the method of the present invention, as described above, high-temperature winding and/or reheating treatment is used to precipitate CIJ and improve workability, and this box annealing improves the workability of Cu.
By further promoting the precipitation of , it is possible to obtain a product with even better properties such as strength and workability.

As described above, according to the present invention, by using a ml material that specifies the type and amount of elements to be included and selecting hot rolling conditions, processing conditions for the coil after hot rolling, annealing conditions, etc. Therefore, it is possible to obtain a high-strength cold-rolled steel sheet with excellent surface properties and cold workability, and such a steel sheet has, for example, strength suitable for thinning of automobile bodies, and has excellent formability. has.

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

Example 1 A steel slab was obtained according to a conventional method from the chemically produced steel samples shown in Table 1, and hot rolled at a finishing temperature of 890 to 905 °C to form a steel slab of 40 to 45° (: After cooling at a speed of 7 seconds, it was coiled at a temperature of 640 to 660°C to obtain a hot rolled coil with a thickness of 2°7. This was immediately charged into a furnace maintained at a temperature of 650°C. After soaking and holding for 10 hours, it was cooled. Then, it was pickled and cold-rolled to obtain a cold-rolled steel plate with an opening thickness of 0.8 (q).

This cold-rolled steel plate was heated at a heating rate of 55"C/hour, and
After box plugging at 0° C. for 3 hours, about 196 open mold rolls were performed to obtain a cold rolled product according to the present invention. The r values of these products are also shown in Table 1.

As is clear from Table 1, the cold-rolled sheet rolled by the method of the present invention has an extremely high r-value even though it is made of high-strength steel. The strength class of the materials used in this example all corresponds to 50: + mouth grade polyhedral.

Example 2 A steel billet with the chemical composition shown in Table 2 was prepared according to a conventional method, and then hot rolled at a finishing temperature of 890 to 900°C. The coil was wound up at a temperature of .degree. C. and once cooled to room temperature to obtain a hot-rolled coil with a thickness of 2.81 mm. Next, this steel plate was charged into a furnace maintained at a temperature of 650°C, kept soaked for 10 hours, and then cooled. Pickle this and
A cold rolled steel plate with a fin thickness of 0.8 was obtained by cold rolling.

This cold-rolled steel plate was heated at a heating rate of 25°C/hour to 700°C.
After box annealing at ℃ for 3 hours, about 1% temper rolling was performed to obtain a cold rolled product according to the present invention. The weights and values of these products are not shown in Table 2. For comparison, we also calculated the r value of a cold-rolled product obtained by cooling a wound hot-rolled coil to room temperature, pickling it, cold rolling it, and box annealing it without reheating. Shown in Table 2.

As is clear from these results, the r value of the cold-rolled steel sheet obtained by the method of the present invention can be dramatically increased by applying reheating treatment after hot rolling, regardless of whether it is made of high-Si or iFIiMn-based materials. Thus, although it is a high-strength steel, it has an extremely high r value, which corresponds to 50 kg class.

Claims (1)

[Claims] (11C0, 15% or less, Si 3% or less, and M r+
2% or less, 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 above the static transformation point,
Wind it up while cooling at a rate of 0°C/sec or more, then
After performing heat treatment at a temperature of 00°C or higher,
A method for producing high-strength cold-rolled Wl temporary, characterized by pickling, further cold rolling, and then box annealing at a temperature of 650 to 800'C. +21C0, 15% or less, S13% or less and Mn 2% or less, Cu 1% or less and/or Po, 03-0.
13%, 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 of ^r transformation point or higher, and
Wind it up while cooling at a speed of 0°C/sec or less, and then
A method for manufacturing a high-strength cold-rolled steel sheet, which comprises performing heat treatment to maintain the steel plate at a temperature of 500°C or higher, pickling, further cold rolling, and box annealing at a temperature of 650 to 800°C. (31C0, 15% or more r', S13% or less and Mn2%
Contains the following, and the total amount of Si and Mn is 0.5
~4%, Ti0.3% or less, NbO22%
Below, ■ At least 1f in the range of 0.2% or less and Z r 0.3% or less! Hot-rolled at a finishing temperature of 3 transformation points or higher, and coiled while cooling at a rate of 30°C/sec or higher, , after heat treatment at a temperature of 500'C or higher, pickling, and further cold rolling, 65
A method for producing a high-strength cold-rolled steel sheet, comprising box annealing at a temperature of 0 to e o'c. +41c 0.15% or less, 313% or less and Mn 2% or less, Cu 1% or less and/or Po, 03-0.
13%, and Si and Mn have a total score of 0.
5 to 4%, and i' i 0.3 or more
N b 0.2% or less, V 0.2% or less, and Z r
O, containing at least l fffi in the range of 3% or less, hot rolled at the balance iron and finishing temperature, and rolled at 30°C/sec or less.
After being cooled at a speed of A method for producing a high-strength cold-rolled steel sheet, which comprises annealing.