JPH02163344A - Cold rolled steel sheet for deep drawing excellent in die galling resistance - Google Patents

Abstract

PURPOSE:To improve the die galling resistance of the steel sheet without deteriorating chemical conversion treating property by regulating the surface of a cold rolled steel sheet in which respective contents of C, Si, Mn, P, S, Al, N, and Ti are specified to a roughness in which the area ration of projecting parts and the average area of one projecting part are specified, respectively. CONSTITUTION:A cold rolled steel sheet having a composition consisting of, by weight, <=0.0040% C, <=0.04% Si, <=0.35% Mn, <=0.1% P, <=0.02% S, <=0.1% Al, <=0.0050% N, 0.01-0.07% Ti, and the balance essentially Fe is continuously annealed and subjected to skin pass rolling by using dull-worked rolls. By the above skin pass rolling, the steel-sheet surface can be regulated to a roughness satisfying the conditions of 25-60% SSr (area ratio of projecting parts) and 2X10<4>-10<5>(mum<2>) SGr (average area per projecting part). If necessary, 0.001-0.01.0% Nb and 0.0001-0.0030% B are incorporated to the above steel-sheet composition.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a cold-rolled steel sheet for deep drawing that has excellent die galling resistance. The aim is to advantageously improve the mold galling resistance without deteriorating it.

(Prior Art) Cold-rolled steel sheets with improved workability, especially deep drawability, are mainly used as interior and exterior plates for automobiles. Therefore, in the past, research has mainly been conducted on the premise of obtaining optimal manufacturing conditions that satisfy the material properties required of steel plates when press-forming them as automobile parts.

In particular, in the case of steel sheets for automobiles, it is necessary to adapt to a wide variety of designs, so it is important to improve the r value corresponding to deep drawability, as well as to lower yield stress and increase work hardening from the viewpoint of shape fixability. It has been.

From this point of view, for example, Japanese Patent Application Laid-Open Nos. 59-193221 and 63-768 have been developed.
There is an ultra-low carbon steel disclosed in Publication No. 48, etc., and recently, the proportion of such ultra-low carbon steel and continuously annealed steel in the production of cold-rolled steel sheets has been increasing.

Amid this trend, "mold galling" has recently become a problem, especially during press forming of automotive steel sheets.
So far, very few technologies focusing on this point have been reported.

Japanese Patent Publication No. 61-266001 proposes a technique for improving press workability by forming a predetermined organic polymer film on the surface of a steel sheet to improve the slidability of the surface of the steel sheet. Stop.

However, after pressing, steel sheets for automobiles are usually subjected to phosphate treatment as a pre-painting treatment, but cold-rolled steel sheets that have been subjected to the above-mentioned lubrication treatment have a phosphate treatment property.
The problem was that it was significantly inferior to untreated cold-rolled steel sheets. In addition, in the case of organic coatings, there is a risk that they may deteriorate over time and turn into books.

(Problem to be solved by the invention) From the viewpoint of workability, especially deep drawability or shape fixability during press forming, elongation (El) and Lankford value (r
It is thought that it is better to improve the Y and S values (lower Y and R values), and the manufacturing technology for this purpose has been realized by ultra-low carbonization. On the other hand, however, a new problem has arisen in such steel sheets: compared to conventional steels, mold galling occurs more frequently during press working.

The present invention aims to advantageously solve the above-mentioned problems, and to propose a cold-rolled steel sheet for deep drawing that has improved die galling resistance by improving the surface roughness of the steel sheet.

(Means for solving problems) First, the background to the elucidation of this invention will be explained.

In order to improve workability, that is, r value and Ef, it is effective to reduce cl, and as a result, the steel becomes softer. However, on the other hand, it has been found that ultra-low carbon steel, especially its continuously annealed material, has greater sliding resistance with the press die than conventional steel.

When the sliding resistance increases in this way, severe mold galling occurs in areas where sliding is severe, such as the bead of the press die, resulting in poor inflow of the steel plate, and this insufficient inflow can lead to breakage. There was a case.

As a result of numerous experiments and studies to solve the above problem, the inventors conducted skin bath rolling using a roll whose surface had been subjected to dull processing, preferably laser dull processing, to improve the surface roughness of the steel sheet. By increasing y and s and limiting the area of the surface protrusions, y and s cause a decrease in r value and Ef.

It has been found that mold galling resistance can be advantageously improved without causing an increase in

In other words, it has been found that sliding resistance can be advantageously reduced by increasing the surface roughness of the steel plate and reducing the contact area between the steel plate and the press die.

This invention is based on the above findings.

That is, this invention has C: 0.0040 wtχ (
(hereinafter simply expressed in %), Si: 0.04% or less,
Mn2 0.35% or less, P: 0.1% or less, S: 0
．． 0.02% or less, At: 0.1% or less, N: 0.02% or less, At: 0.1% or less, N: 0.
0.050% or less and Ti: 0.01-0.07%
, Nb: 0.001-0.010% as necessary
Or a cold-rolled steel sheet containing B: 0.0001 to 0.0030%, with the remainder essentially having a composition of Fe, the surface of which has a convex area ratio SSr: 25 to 60%
Average area per large convex portion SGr: 2 × 10
This cold-rolled steel sheet for deep drawing has excellent die galling resistance and satisfies the conditions of 4 to 105 (μ layer 2).

This invention will be specifically explained below.

First, the experimental results that formed the basis of this invention will be explained.

Low carbon steel and ultra-low carbon steel cold-rolled steel sheets whose compositions are shown in Table 1 were used as test steels.

Table 1 A) These cold-rolled steel sheets were subjected to skin bath rolling at a rolling reduction ratio of 0.8% using a skin bath roll whose surface was dulled by a laser. At this time, the roughness pattern of the steel sheet surface after skin bath rolling was changed by variously changing the laser dulling method.

FIG. 1 shows the relationship between the friction coefficient and surface roughness (SRa) during a sliding test conducted by the method described later.

The sliding test at this time was conducted under the conditions that the sample thickness was 0.7 mm and the presser load was 100 kgf. Note that the surface roughness SRa is the average roughness of the center surface, and a portion of area S is extracted from the roughness curved surface on the center surface, and orthogonal coordinate axes, X axis, Y axis, and Place the axis,
With the axis perpendicular to the center plane as the Z axis, the roughness curved surface is Z = f
When expressed as (X, Y), the value is given by the following formula, where L=SN (unit: μ resistance).

As is clear from the figure, the friction coefficient of the unoiled steel plate decreases as SRa increases, and SRa = 2.0.
When reaching ttm, the friction coefficient of ultra-low carbon steel has decreased to the same level as low carbon steel.

The reason for the decrease in the friction coefficient due to the increase in SRa is considered to be as follows. In other words, the larger the surface roughness, the
Since the contact area between the steel plate and the press mold becomes smaller and the actual contact area is divided into smaller pieces, the occurrence of mold galling is suppressed and the coefficient of friction is reduced. Furthermore, the increase in SRa is
This also prevents a drastic increase in the contact area caused by the convex portion being collapsed due to contact pressure with the press die.

In this way, the die galling resistance of a cold rolled steel sheet is determined by the surface roughness SRa
However, the inventors conducted numerous experiments to realize more stable mold galling resistance, and found that the convex area ratio SSr on the steel sheet surface and the average per convexity It has been found that excellent mold galling resistance can be obtained by regulating the area ratio SGr within a predetermined range.

Figure 2 shows the effect of the area ratio (SSr) of the convex part on the friction coefficient during the sliding test using the ultra-low carbon steel used in Figure 1 and the convex part 1.
The results of an investigation regarding the relationship with the average area per unit (SGr) are shown.

The sliding test was conducted with the test piece having a thickness of 0.8 mm and a pressing force of 100 kgf, and the lubrication conditions at this time were no oil. Further, the area ratio of the convex portions (SSr) and the average area per convex portion (SGr) were determined using a three-dimensional surface roughness meter. Furthermore, the numerical values in the figure are the friction coefficients during sliding at each point.

As is clear from the figure, 25%≦SSr≦60% and 2 × 104 pta”≦SGr≦105 ate
”, the It! friction coefficient was 0.3 or less, and good mold galling resistance was obtained.

The reason why the above appropriate range exists for the surface convex portions of the steel plate is considered to be as follows.

That is, the lower the convex area ratio, the smaller the contact area, and therefore the lower the friction coefficient. However, if the area ratio of the convex portions is low on the negative side, when the pressing force becomes large, the convex portions will be completely crushed, and the contact area will conversely become large and the coefficient of friction will become high. Therefore, in order to compensate for the strength of the protrusions, it is considered that a minimum line of the average area per protrusion exists because the protrusion area ratio is low. However, on the other hand, if the area of each convex part is too large, the sliding distance becomes longer and galling occurs between the protrusions, resulting in an increase in the coefficient of friction. Furthermore, due to the precision of the processing technology, it is thought that there is a minimum line of the area ratio of the convex portions.

Furthermore, it has been found that chemical conversion treatment properties are improved by providing the above-described irregularities to the surface of the steel sheet.

Figure 3 shows the area ratio of convex portions (SSr) and the average area per convex portion (SGr) that affect the P ratio value after chemical conversion treatment using the ultra-low carbon steel used in Figure 1. The results of an investigation into the relationship between

As is clear from the figure, 25%≦SSr≦60% and 2 × 104 tts”≦SGr≦l □, s u
In the case of m t, the P ratio is 0.9 or more, and the chemical conversion treatment properties are improved.

The reason why the above appropriate range exists is considered to be as follows.

In other words, by imparting irregularities to the surface of a steel sheet, non-uniform areas (areas where the oxide film is thin or areas where the oxide film is broken) are created in the oxide film on the surface of the steel sheet, and these non-uniform areas are difficult to form during the chemical conversion reaction. This is considered to be a crystal precipitation site.

However, if both SSr and SGr are too small, the non-uniformity of the oxide film will become coarse, and therefore the precipitation sites will also become coarse, resulting in poor chemical conversion treatment properties. Also SSr and SG
If both r becomes large, the number of non-uniform parts of the oxide film, that is, the parts where iron is eluted, occupies the entire surface of the steel sheet.

Here, the phosphate treatment reaction will be explained as follows, taking the typical case of zinc phosphate treatment as an example.

That is, when a steel plate is immersed in an acid solution with a ρ of about 3.0 for zinc phosphate treatment, Fe on the surface of the steel plate is dissolved as shown by the following equation (1).

Fe + 2HsPO4→Fe(HzPOa)z +
Hz ↑ ・(1) This Fe dissolution reaction causes an increase in pH at the solution-steel plate interface (reduction in H" ions) and an increase in Fe" concentration, which leads to the following (2)
The phosphate precipitation reaction shown by the equation (3) occurs, and a phosphorus and salt film is formed on the surface of the steel sheet.

2Zn(HzPO4)z + Fe(HzPO4)z
+48zO-Zn2Fe(Po4)z” 4Hz
O+ 4HzPO4”'(2)3Zn()IzPO4)
z + 4) 1zO→Zn5(PO4)z H4Hz
O+ 4) 13PO4・”(3) In the above phosphate treatment reaction, the presence of P in steel has the effect of lowering the hydrogen overpotential of steel in acid and promoting acid solubility of steel. It is known that the effect of improving phosphatability by increasing the P content in steel is due to the effect of promoting the acid solubility of steel. The contribution of P promotes the Fe dissolution reaction of equation (1), and accordingly increases the pH and Fe concentration near the steel plate-solution interface. In addition, ZnzFe(PO4)t 4HzO, which is a phosphate coating by the reaction of formula (2), that is, Phos
The phophyllite coating is a phosphate coating produced by the reaction of equation (3) Zn+(Po4)z ・4)1
It is known that the coating properties are superior to those of 20, that is, the opeite coating.

If there is excessive iron elution, Zn(H1PO4)z necessary for phosphate crystal precipitation cannot be supplied in time, and phosphate treatment performance is inhibited.

When SSr or SGr increases, the above phenomenon occurs, and it is thought that chemical conversion treatment properties deteriorate.

Therefore, it is considered that there is an appropriate range of chemical conversion treatability.

As a result of repeated research based on the above basic data, the inventors discovered that by regulating the surface condition as shown below, a cold-rolled steel sheet with excellent galling properties on both sides can be obtained. .

According to this invention, 25%≦SSr≦60% and 1
05 am”≧SGr≧2×105 pm”. SSr >60%, SSr <25% or SGr<2X105 pmz, SGr >105
pm'', it is not possible to obtain excellent mold galling resistance and chemical conversion treatment properties.

Although laser dulling has been mainly explained as a dulling method, it goes without saying that other methods such as plasma processing and electrical discharge machining can also be used, and it is important to keep the surface roughness within the appropriate range mentioned above. be.

(Function) Next, the reason why the steel composition is limited to the above range in this invention will be described.

When these interstitial solid solution elements such as C and N are contained in excess, they cause deterioration of ductility and drawability. 0.00 for C
If it exceeds 40%, or 0.0050% in the case of N, the deterioration of ductility and drawability will be significant. Therefore, C is 0.004
0% or less, and N was 0.0050% or less.

Si, Mn+P; These substitution type solid solution elements are
Compared to C9N, it has less adverse effect on ductility and drawability, so it can be added depending on the desired strength level. In the case of this invention, the upper limits of the material are 0.04% for Si, 0.35% for Mn, and 0.1% for P.

S: From the standpoint of descaling properties and chemical conversion treatment properties, the more Sl the better, but if C4 exceeds 0.02%, the corrosion resistance of the steel sheet deteriorates. Furthermore, since addition of a large amount of S causes deterioration of the material, the upper limit of sl was set to 0.02%.

A1: It is effective in deoxidizing steel, but if it exceeds 0.1%, it causes deterioration of surface properties due to inclusions and deterioration of chemical conversion treatment properties, so 0.1% is set as the upper limit.

Ti: It not only effectively contributes to the fixation of solid solution components such as C, N, and S, but also has a great effect on improving the material quality by forming precipitates with C, N, S, and the like.

In particular, even if each of C, N, and S is at the upper limit of the limited range, in order to completely fix it, it is desirable to add the amount that satisfies the following formula (1). The upper limit was set at 0.07%.

On the other hand, if the content is less than 0.01%, the effect of its addition is poor. Therefore, the amount of Ti added is 0.01 to 0.07
% range.

Next, the additive element N described in claims 2 to 3 of this invention
b. We will discuss the reasons for limiting B.

Nb is effective in improving in-plane anisotropy while ensuring a high E11 and high r value by adding Nb in combination with Ti. However, if the content is less than 0.001%, the effect of its addition will be poor, while if it exceeds 0.010%, the anisotropy improvement effect will be saturated, conversely causing a decrease in Effi. Therefore, the amount of Nb added was 0.001 to 0.010%.

B: A useful element that improves secondary processing brittleness when added in a small amount. However, if the amount of B is less than 0.0001%, the addition effect will be poor, while if it is added in a large amount exceeding 0.0030%, it will cause deterioration of the material.
It was supposed to be added in a range of 1 to 0.0030%.

(Example) A steel slab having the chemical composition shown in Table 2 was manufactured by a converter continuous casting method, heated to 1250°C, soaked, and hot rolled to a thickness of 3.2 mm by rough rolling and finishing rolling. Made of steel plate. After pickling, the steel plate was cold-rolled to a thickness of 0.8 mm, subjected to continuous annealing (soaking temperature 750 to 850°C), and then skin-pass rolled (reduction ratio 0.8%). Ta.

Here, the skin bath roll used was a roll with dull edges formed by laser processing (a roll subjected to laser dull processing).

The surface roughness of the steel plate was measured in the rolling direction, and the average surface roughness SRa was determined. JIS No. 5 test pieces for tensile testing were cut out from each cold-rolled sheet thus obtained, and the mechanical properties were examined. Table 3 shows the results. Here, the T value was measured by a three-point method after applying 15% tensile prestrain, and was measured in the L (rolling) direction, the C (90° to the rolling direction), and the D (45° to the rolling direction). The average value in the direction was calculated as rt + rC + 2rn.

Here, the sliding test method will be described. Using the slidability measuring device shown in FIG. 4a, the pull-out load when pulling out the test piece 5 held between the chuck 4 was measured using a tensile tester 1. The presser load of punch 2 due to cylinder 3 is 100
kgf. Test piece 5 is for the unoiled one, 20
A piece measuring 300 mm x 0.7 mm was prepared.

In addition, the speed at which the test piece 5 is pulled out is 500 mm/1li.
n, the sliding distance was 50 mm.

In addition, a vertical load N was applied to the test piece 5 using a punch 2 having a shape shown in Fig. 4, D was 15, and h was 1, and the load F (kgf) when pulled upward was measured. From /2N,
The coefficient of friction was determined.

Further, the chemical conversion treatment was performed as follows.

Treatment liquid Nippon Parker Rising Co., Ltd. Palbond L30
20 Treatment liquid treatment method i FULL DIP method Treatment conditions: Immersion at 42°C for 120 seconds Evaluation method 2 P ratio Here, P ratio is Phosphophyllite (
(100) plane of ZnzFe(POn)z' 4H20) and Hopeite (Zni(POn)z I 4Hz
This is the X-ray diffraction intensity ratio of the (020) plane of O), and the better the coating is.

This value is high table

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the surface roughness SRa of a steel plate and the coefficient of friction during sliding. Figure 2 is the effect of the area ratio SSr of convex parts on the coefficient of friction and the average area per convex part. Figure 3 is a graph showing the influence of SGr. Figure 3 is a graph showing the relationship between the area ratio SSr of convex parts and the average area SGr per convex part on the P ratio. Figures 4 a and b are, respectively, FIG. 1 is a schematic diagram of a sliding property measuring device and an explanatory diagram of a punch used therein. As is clear from Table 4, all the steel plates according to the present invention exhibit superior galling properties in both types compared to the comparative examples. (Effects of the Invention) Thus, according to the present invention, it is possible to obtain a cold-rolled steel sheet for deep drawing, particularly an ultra-low carbon steel sheet, which has excellent galling properties in both molds without impairing chemical conversion treatment properties, and can be used, for example, as a steel sheet for automobiles. It is suitable for use in applications such as Diagram 2 6IJ SRa (μ-) 2.0 56 (-(xto'pm2) Diagram 3 O sq, (xyo4μ expression) Diagram 4 (a) (b)

Claims (1)

[Claims] 1. C: 0.0040wt% or less, Si: 0.04wt% or less, Mn: 0.35wt% or less, P: 0.1wt% or less, S: 0.02wt% or less, Al: 0.1 wt% or less, N: 0.0050 wt% or less, and Ti: 0.01 to 0.07 wt%, with the remainder being substantially Fe, and the surface thereof is convex. Part area ratio SS_r: 25-60
% and average area per convex portion SG_r: 2×10^
A cold-rolled steel sheet for deep drawing with excellent die galling resistance, which satisfies the condition of 4 to 10^5 (μm^2). 2, C: 0.0040wt% or less, Si: 0.04wt% or less, Mn: 0.35wt% or less, P: 0.1wt% or less, S: 0.02wt% or less, Al: 0.1wt% or less, A cold-rolled steel sheet containing N: 0.0050 wt% or less and Ti: 0.01 to 0.07 wt% together with Nb: 0.001 to 0.010 wt%, with the remainder being substantially Fe. , whose surface has a convex area ratio SS_r: 25~
60% and average area per convex portion SG_r: 2×1
A cold-rolled steel sheet for deep drawing with excellent die galling resistance, characterized by satisfying the condition of 0^4 to 10^5 (μm^2). 3, C: 0.0040wt% or less, Si: 0.04wt% or less, Mn: 0.35wt% or less, P: 0.1wt% or less, S: 0.0wt% or less, Al: 0.1wt% or less, A cold-rolled steel sheet containing N: 0.0050 wt% or less and Ti: 0.01 to 0.07 wt%, together with B: 0.0001 to 0.0030 wt%, with the remainder being substantially Fe. , whose surface has a convex area ratio SS_r: 25~
60% and average area per convex portion SG_r: 2×1
A cold-rolled steel sheet for deep drawing with excellent die galling resistance, characterized by satisfying the condition of 0^4 to 10^5 (μm^2).