JPH04168246A - Cold rolled steel sheet for automobile having low corrosion rate and high formability - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a cold rolled steel sheet having a low corrosion rate and high formability suitable for automobile panels and the like.

(Conventional technique fr) The number of years of use of steel plates for automobiles and the like is increasing, and customers' demands for longer lifespans are also becoming stronger. On the other hand, the corrosive environment is becoming increasingly severe, such as the use of magnesium chloride for snow melting in the winter.Normally, a sufficient coating is applied, but this coating breaks down due to the crushing action of these salts, and then corrosion occurs on the steel itself. progress by invading the Therefore, the corrosion resistance of the base material is important for the total corrosion life of the steel plate until a hole is formed. Of course, if the steel plate does not have sufficient paintability, the corrosion resistance due to painting will deteriorate and the total corrosion life will be greatly reduced.

On the other hand, cold-rolled steel sheets used in automobile panels and the like undergo complicated forming processes to become products. Therefore, cold-rolled steel sheets for automobiles must naturally have good formability. This is especially true for panels that undergo complex forming processes, such as fenders and side panels, which this cold-rolled steel sheet aims to achieve.

There are several prior art techniques for increasing the base metal corrosion resistance of cold-rolled steel sheets. Below, we will list them and explain how these prior art differ from the technology targeted by the present invention.

The prior patent, JP-A No. 63-50447, describes a method of diffusing P onto the surface of a steel plate and coating a non-ferrous metal on the diffusion layer, but this method requires a special diffusion treatment and also Because of the P diffusion layer, it becomes hard and the moldability is reduced. Therefore, it is not suitable for automotive applications such as panels that undergo advanced molding.

Also, JP-A-63-1868F! Publication No. 0 attempts to provide high P-high S and corrosion resistance by Ti sulfide and high P. This steel plate is hard due to its high P content and cannot secure formability. Furthermore, high S results in an increase in inclusions, which induces hot embrittlement of the steel and further deteriorates the formability of the product. From this point of view, it is natural that it is not suitable for high-level molding applications such as those for automobiles.

Further, JP-A-63-203747 discloses a hot-rolled steel sheet intended for automobile suspension, which is different from the cold-rolled steel sheet mainly used for panels of the present invention. Although a large amount of Cr is used, panels such as those of the present invention become hard and press formability deteriorates, and since the panels are used after chemical conversion treatment, they are probably unsuitable in terms of chemical conversion treatment properties.

Furthermore, Japanese Patent Application Laid-Open No. 1-152241 increases P and N
i is added. Ni is expensive and has economical problems.

Furthermore, since Ti and Nb are not added, the F value, which is a major index of formability, is low, and there still remains a problem with press formability. If P is too high, it becomes hard, which also results in poor moldability. There are problems with both of these prior art techniques.

(Problems to be Solved by the Invention) An object of the present invention is to achieve both high formability and low base material corrosion rate.

Moreover, it maintains paintability and weldability at a level comparable to conventional materials. Here, the low base metal corrosion rate refers to the time it takes to form holes in the steel plate, and the challenge is to reduce the corrosion rate. The former index of formability is the F value and elongation. The target value is F≧1.8. E Q≧48% (
(equivalent to t=0.8m: EQ depends on the plate thickness), and with this level it is possible to form panels with complex shapes such as fenders.

(Means for Solving the Problems) The gist of the present invention is as follows: (1) Mass ratio (hereinafter the same applies to steel components), C
: 0.0030% or less, N: 0.0025% or less, Si: 0.02% or less, Mn: 0.03 to 0.1
5%, P: 0,018-0.045%, S:
0.005% or less, Al: 0.005-0.0
35%.

Nb: 0.005-0.025%, Ti: 0.02
% or less, with the remainder consisting of Fe and unavoidable impurities, a cold-rolled steel sheet for automobiles having a low corrosion rate and high formability.

(2) C: 0.0030% or less, N: 0.00
25% or less, Si: 0.02% or less, Mn: 0.0
3-0.15%, P: 0.018-0.060%, S
: 0.005% or less, A1: 0.005-0
．． 035%, Nb: 0.005-0.025%, T
i: Contains 0.02% or less, and Cu: 0
．． 05-0.35% and/or B: 0.00
01 to 0.0010%, with the remainder consisting of Fe and unavoidable impurities, a cold rolled steel sheet for automobiles having a low corrosion rate and high formability.

(3) C: 0.0030% or less, N: 0.00
25% or less, Si: 0.02% or less, Mn: 0.0
3-0.15%, P: 0.018-0.060%, S
: 0.005% or less, Al: 0.005-0
．． 035%, Nb: 0.005-0.025%, T
i: 0.02% or less, Cu: 0.05-0.3
5%, B: 0.0001-0.0010%, Ni:
A cold-rolled steel sheet for automobiles containing 0.03 to 0.25%, with the remainder consisting of Fe and inevitable impurities, having a low corrosion rate and high formability.

It is in.

In other words, C and N are extremely reduced, and S and M
Based on high-purity steel with n reduced to the limit,
A trace amount of Ti is added to the extent that it does not inhibit the corrosion rate for formability.
and Nb. The former is hot-rolled TiN to remove harmful N.
Used for fixing. The latter acts as NbC during hot rolling to make the hot rolled sheet into fine grains, and since it is harmful during subsequent annealing, it is coarsened at the hot rolling winding stage to reduce the number of grains. After the carbide is put to useful use in this way, it becomes harmless.

(Effects) Next, we will discuss the effects of each component and the reasons for numerical limitations.

C, N: Interstitial impurity elements that, if present in hot-rolled sheets, hinder the development of an orientation with a favorable F value during cold rolling and annealing. In addition, if it remains in the product plate, it will deteriorate the ductility of the steel due to strain aging. Furthermore, in this method, a small amount of P is added in order to reduce the corrosion rate, but it is necessary to compensate for the hardening. Therefore, in this method, C and N are reduced as much as possible. 0.0 each
The upper limit is 0.030% and 0.0025%.

Si: When Si exists on the steel surface layer, it forms a stable oxide film and deteriorates chemical conversion treatment properties and paintability. Therefore, it should be kept below the impurity level of 0.02%.

Mn, S: Mn combines with S and is mainly MnS in steel.
becomes. This MnS becomes the core of corrosion and accelerates it.

Furthermore, Mn hardens the steel through solid solution strengthening and impairs ductility, and also has an adverse effect on the F value. Therefore, it should be reduced as much as possible to 0.15% or less. And S is set to a high purity level of 0.005% or less, preferably 0.002% or less. M
The lower limit of n is the current state of converter melting that produces large quantities and FeS
The content was set at 0.03% in consideration of preventing brittleness.

P: P forms stable rust during corrosion and reduces the corrosion rate. However, it makes the steel hard and has an adverse effect on formability. Therefore, it should be added at 0.045% or less. The lower limit is required to be 0.018% even when the above-mentioned M n S lowering effect is combined from the viewpoint of reducing the corrosion rate.

Preferably 0.030% is required.

AQ: AQ is necessary as a deoxidizing agent, but if the amount added is too large, inclusions will increase and the ductility and formability of steel will deteriorate. Therefore, the amount added is 0.005 to 0.035%.

Nb: Nb, as NbC, plays a role in controlling the crystal grains of the hot-rolled sheet during hot rolling and fixing harmful C.

However, if it is too large, it acts as fine precipitates, deteriorating the F value and hardening the steel. Therefore, the amount added is 0.
005 to 0.025%.

Ti: Ti is added mainly to fix harmful N as TiN. Therefore, add 0.02% or less.

Preferably Ti/N is 2 to 5. If more than this is added, TiC will be formed and the moldability will be slightly deteriorated. In addition, solid solution Ti
This increases corrosion resistance and chemical conversion treatment properties.

To further stably reduce the corrosion rate, add Cu from 0.05 to
Add 0.35%. Addition of less than 0.05% has no effect, and addition of more than 0.35% hardens the steel through solid solution strengthening of Cu or precipitation of ε-Cu, reducing formability.

Further, when adding Cu, cracks may occur during hot rolling, which is an intermediate process, and in that case, it is preferable to add 0.03 to 0.25% of Ni. Below the lower limit, there is no cracking prevention effect, and the effect is saturated near the upper limit.

Furthermore, when this steel is subjected to severe forming, it may exhibit forming defects called secondary work brittleness or longitudinal cracking. This occurs because in high-purity steels such as wood grains, there are no grain boundary strengthening elements such as solid solute carbon in the grain boundaries, resulting in a decrease in grain boundary strength.To compensate for this, B is added by 0.000
Add 1 to 0.0010%.

If it is less than the lower limit, there is no effect, and if it exceeds o,ooto%, the F value and ductility will deteriorate significantly.

Wood-like melting is done in a converter. After converter refining, it is decarburized by vacuum degassing. Then, it is made into a slab by ingot formation, blooming, or continuous casting, and then hot rolled. Hot rolling conditions are heating: 1
050-1250℃, rolling end temperature = 880-950℃
However, the winding temperature is about 600 to 780°C.

In order to further improve the moldability, it is desirable that the heating temperature is ≦1120°C and the winding temperature is ≧690°C.

After hot rolling, it is pickled and then cold rolled, but the cold rolling rate is usually 70.
-85%, but in the steel of the present invention, it is preferable to have a high cold rolling rate of 78% or more in order to ensure formability.

Annealing is performed by box annealing or continuous annealing. Box annealing is normally performed at 650 to 750°C for 5 to 20 hours, but in order to further improve formability, 700°C is used.
A high temperature of 10 hours or more and a long time of 10 hours or more are preferred.

In addition, continuous annealing is performed at 720-870℃, 0.5-3
m1n is the normal condition, but it is also 830℃ or higher,
Conditions of 1.5 m1n or more are preferable. In the steel of the present invention, final large crystal grains are obtained to impart formability such as F value and elongation, but from this point of view, it is preferable to anneal by a continuous annealing method that allows higher temperature annealing. After annealing, skin pass rolling is performed, but in order to ensure the quality of the material, it is preferable to keep the skin pass rolling ratio to a minimum value of 0.5% or less for wood-like finishes.

(Example) Steels shown in Table 1 were melted in a converter. All steels were refined in a converter and then decarburized by RH degassing.

Among these, steels with steel codes A, B, E and F are steels according to the present invention. Steel with steel code C has high Mn and S. Similarly, D steel has low P. Steels with steel codes G and H have high P, and H steel also has high Ti. C, Mn for steel weight
, S, and Cu are high and P is low.

For steels J and K, Nn+S/T is used to form TiS.
The amount of i is limited. Addition of Si and Cr to steel M
In this case, Si and P are added and high Mn and S are added. Furthermore, steels N and O are ordinary ultra-low carbon steel and low carbon steel;
The formability level specified by the present invention is comparable to steel N.

The conditions after hot rolling are shown below.

Hot rolling heating temperature: 1100-1135°C Finishing temperature: 895-918°C Coiling temperature near 05-730°C Hot-rolled plate thickness = 4ml Cold-rolled/annealed cold-rolled plate thickness: 0.8mm (cold rolling ratio = 80% ) Continuous annealing: 8
50℃, 1,8m1n Steel code E and box annealing other than 0 20℃, 12h Steel code E.

Temper rolling elongation: 0.2 to 0.4% The obtained steel plates were sampled and examined for mechanical test values and corrosion rate. The mechanical test was conducted using a JISS No. test piece. The results are shown in Table 2.

Next, in order to simulate the corrosion rate of an actual automobile, bonding treatment with zinc phosphate was performed, followed by electrodeposition coating, followed by intermediate coating and top coating.

A corrosion test was then conducted by making a cross cut.

Corrosion test is CCT (cyclic corrosion)
test). This consists of the cycle shown below. (SST: Salt water jet test) SST → Dry → Wet relative humidity: 40% Relative humidity: 98% 35°C
60℃ 50℃ 4h 2h
In this test, the condition after 1500 hours was investigated. As an index, after CCT, the rust layer was removed and the plate thickness was measured. The remaining board thickness was determined by taking 10 points from the one with the largest reduction in board thickness and using the average board thickness. The test was conducted with 3 repetitions. The respective values are also shown in Table 2. In addition, the remaining plate thickness is illustrated in FIG.

The steel according to the invention has a very small corrosion loss, generally within 20%. In addition, the workability is good and the F value is 2.0.
．． The level of EQ≧49% is ensured. On the other hand, other steels have a large corrosion loss or have low workability, and do not meet the object of the present invention. In this example, the paintability (adhesion state of the zinc phosphate film and paint adhesion) of the steel according to the present invention was good without any problems.

(Effects of the Invention) As described above, the present invention has an extremely slow corrosion rate, excellent workability, and good paintability, making it ideal for automotive panels that undergo complex molding processes. can be provided.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the average remaining plate thickness of Meisho.

Claims (3)

[Claims] (1) Mass percentage (hereinafter the same applies to steel components), C
: 0.0030% or less, N: 0.0025% or less, Si
: 0.02% or less, Mn: 0.03 to 0.15%, P:
0.018-0.045%, S: 0.005% or less, A
l: 0.005-0.035%, Nb: 0.005-0
．． 025%, Ti: 0.02% or less, balance Fe
A cold-rolled steel sheet for automobiles, which has a low corrosion rate and high formability, and is composed of unavoidable impurities. (2) C: 0.0030% or less, N: 0.0025% or less, Si: 0.02% or less, Mn: 0.03 to 0.15
%, P: 0.018-0.060%, S: 0.005%
Below, Al: 0.005-0.035%, Nb: 0.0
05 to 0.025%, Ti: 0.02% or less,
Furthermore, Cu: 0.05 to 0.35% and/or
B: Contains 0.0001 to 0.0010%, balance Fe
A cold-rolled steel sheet for automobiles, which has a low corrosion rate and high formability, and is composed of unavoidable impurities. (3) C: 0.0030% or less, N: 0.0025% or less, Si: 0.02% or less, Mn: 0.03 to 0.15
%, P: 0.018-0.060%, S: 0.005%
Below, Al: 0.005-0.035%, Nb: 0.0
05-0.025%, Ti: 0.02% or less, Cu: 0
．． 05-0.35%, B: 0.0001-0.0010
%, Ni: 0.03 to 0.25%, and the balance consists of Fe and inevitable impurities, and has a low corrosion rate and high formability.