JPS58100622A - Production of silicon-manganese high strength steel plate having excellent suitability to chemical conversion - Google Patents

Abstract

PURPOSE:To obtain an Si-Mn high strength steel plate having improved suitability to chemical conversion and paint corrosion resistance by restricting the contents of Si and Mn in steel, adding specific elements for stabilization of carbides and negative catalyst elements and annealing the steel in a specific atmosphere. CONSTITUTION:Elements for stabilization of carbides which are 0.1-1.0% Cr, 0.03-0.20% Ti, 0.03-0.20% V, 0.03-0.20% Nb, and 0.03-0.20% Zr as well as 1 kind components acting as a negative catalyst in deposition of C which are 0.008-0.025% S, 0.008-0.025% Se, 0.008-0.020% B, 0.01-0.20% Sb, 0.01-0.20% Bi, 0.02-0.01% Sn, etc. are added to Si-Mn steel consisting basically of 0.3- 2.0% Si, 0.5-2.0% Mn and 0.1% C. Further, Si and Mn are restricted to 3.0% Si+Mn. Such steel is annealed in an atmosphere controlled to 1vol% concn. of gaseous hydrogen and dew point -60-0 deg.C, whereby the Si-Mn high strength steel plate for automobiles, etc. having excellent suitability to chemical conversion is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a silicone-manganese-based (hereinafter referred to as 51-Mn-based) high-strength steel sheet having excellent chemical conversion treatment properties (phosphate coating surface treatment properties). It is related to Si0.3-2.0%, Mn0.5-2
．． Steel with a content of 0%, 00.1% or less, Or,
By adding one or more of carbide stabilizing elements such as Ti, V, Nb, and Zr and graphitization suppressing elements such as S, Se, Sb, Bi, and Sn, precipitation of C on the surface during annealing can be suppressed. To obtain a method for manufacturing a 51-Mn-based high-strength steel sheet with excellent chemical conversion treatability, by both suppressing coloring (blueing) on the steel surface by suppressing the coloring (blueing) of the steel surface by simultaneously controlling the annealing gas components. It is in. The present invention was based on the discovery of a new fact that the chemical conversion treatability of 51-Mn steel sheets is controlled by surface C diffused from inside during annealing.

Recently, there has been a demand for the development of steel plates with higher strength, mainly for automobile steel plates, with the aim of making them lighter in weight, and various types of high-strength steel plates (hereinafter referred to as high-strength steel plates) have already been developed.
Although these have been put into practical use, they can be roughly divided into Si, Mn,
Solid solution strengthened type that increases the content of P, etc. and Nb, Ti, V
There is also a precipitation-strengthened type that adds substances such as. The latter type of high-strength steel and p-added high-strength steel usually require less addition of required elements (<0.2%), so chemical conversion treatment properties are not much different from general mild steel sheets. It is known that because it requires a fairly high additive content (0.3% or more), chemical conversion treatment properties deteriorate (phosphate crystals become coarser), and therefore paint corrosion resistance is inferior to that of ordinary mild steel sheets. ing. One of the reasons for this is Si.

When Mn is high, during annealing (700 to 800
During the process of heating and softening at ℃), a thick composite oxide pattern containing Si and Mn is formed on the steel surface, causing the steel plate to become colored and bluish.
It is said that this inhibits the phosphate reaction, and from this point of view, techniques for specifically limiting the amount of Sl are known (% Publication No. 53-73'i'1, JP-A-51-107218).
.

On the other hand, it is well known that the coloration of the steel plate surface is not only caused by the steel components, but also by the atmospheric gas during annealing, and from this point of view it is also known to limit the atmospheric dew point (D, P,). (Japanese Unexamined Patent Publication No. 119708/1983). In other words, annealing is usually heated in a mixed gas of H2 and N2, but the mixed gas contains a small amount of moisture, and the ratio of this N20 to N2 determines whether or not oxidation of the components in the steel will occur. .

Therefore, by selecting an appropriate atmospheric gas, Si
Even steel plates with high Mn content can be manufactured without causing bluing. In particular, in the case of high tensile steel, it is generally manufactured by continuous annealing with a short annealing time (10 minutes), so it is advantageous compared to batch annealing to obtain bright annealed plates without bluing even if the Si and MnO contents are quite high. It is.

Therefore, the present inventors have developed Sl-Mn without blueing.
We found that chemical conversion treatment defects also occur in Si-Mn-based high tensile strength steels, and demonstrated that the chemical conversion treatment defects in Si-Mn-based high tensile strength steels are not solely due to the presence of a composite acid-relaxing film containing Si and Mn, as has been said up until now. I found out. As a result of various investigations into the causes of this, they discovered the fact that Si in steel significantly promotes the precipitation of surface carbon, and found that this surface C was the cause of poor chemical conversion treatment. On the other hand, Mn
It has also become clear that since it suppresses the precipitation of surface C, if the amount is the same, it densifies the phosphate crystal and improves chemical conversion treatment properties. On the other hand, it is widely known that surface C contamination has a negative effect on chemical conversion treatment even in the case of mild steel sheets that are naturally good in chemical conversion treatment.The origin of this surface C is from residual rolling oil and annealing gas (CO). It is assumed that this occurs due to adhesion of carbon dioxide, diffusion from within the steel, etc.

The present inventors believe that the ultimate cause of failure in chemical conversion treatment of 51-Mn-based materials lies in this surface C, and if this surface C is suppressed, chemical conversion treatment of Si-Mn-based materials can be improved. We conducted various experiments based on the idea that defects could also be improved. As a result, Cr, Nb, Ti, V
Carbide stabilizing elements such as S, or S which preferentially diffuses and adsorbs to the surface and acts as a negative catalyst against the precipitation of C;
By adding Ss, Bi, Sb, B, and Sn, the precipitation of C on one surface is suppressed, and at the same time, dense phosphate crystals are formed, and it is possible to obtain a product that is equivalent to or better than ordinary mild steel. This became clear, and this is how the present invention was constructed. The details of the present invention will be explained below using the drawings.

FIG. 1 is a drawing showing that the surface C precipitation increases in proportion to the 81 concentration in steel, which is the discovery that led to the present invention. The vertical axis of the figure is the ion microanalyzer (IMA)
This is the count number of the surface C (integral amount up to 150 thicknesses) measured by .

The surface C is proportional to the Si concentration in the steel, and Mn suppresses this.
It can be seen that the influence of Mn and Mn is greater.

Figure 2 shows 81 in steel when commercially available phosphate treatment is applied.
This shows the relationship between the amount of phosphate treated crystals (the larger the diameter, the coarser the crystals, and the better the film performance). , it can be seen that the negative influence of Si on crystal coarsening is alleviated. Although the cause of the coarsening of phosphate crystals due to 81 in steel cannot be immediately linked to surface C due to the phenomenological similarity between Figures 1 and 2, Figure 3 (
photo) and Figure 4. When performing phosphate chemical conversion treatment, a pretreatment called surface conditioning after degreasing is essential, but this surface conditioning agent is a sodium phosphate dispersion of titanium colloid, and the titanium adsorbed on the steel plate surface is used for subsequent phosphoric acid treatment. It is said to act as a nucleus for salt crystal precipitation and make the crystals more dense. Figure 3 shows this effect.Even for mild steel sheets (batch annealed capped steel sheets) that are generally said to have excellent chemical conversion properties, (a) is a sheet with surface conditioning, and (b) is a sheet with surface conditioning. ), it is obvious at first glance that only coarse crystals are formed.

On the other hand, Figure 4 shows 51-M selected from the samples in Figure 1.
Surface C content of n-based high tensile strength steel and adsorbed titanium (surface conditioning agent)
This figure shows the relationship between titanium as a nucleating agent and surface C.
It can be seen from the table that the higher the fiC, the more difficult it is for titanium to adsorb. In other words, this series of results shows that whether or not phosphate crystals form densely on the steel surface is determined by the adsorption of titanium as a nucleating agent, and the adsorption of titanium is inhibited by the presence of surface C. This and 81 clearly show that this promotes the precipitation of C on the surface, thereby coarsening the phosphate crystals.

Based on the above basic experiments, the present inventors added Or and Ti, which are carbide stabilizing elements, to 51-Mn-based high tensile strength steel.

By adding V, Nb and Zn, S, which also becomes a negative catalyst for C precipitation, 81), Bi,
It has been found that by adding B and Sn, precipitation of C on the surface of Si-Mn high tensile strength steel can be effectively suppressed, thereby preventing deterioration of chemical conversion treatment properties. These elements exhibit some effects even when added alone.

By adding two or more kinds in combination, a remarkable effect of suppressing surface C and therefore improving chemical conversion treatment properties can be achieved.

In particular, when elements from groups with different action mechanisms (carbide stabilization group and negative catalyst group) are combined, excellent effects can be expected with a smaller amount added. However, the effects of T1 and S, which stably form TiS, may cancel each other out, so it is not always advisable to add them in combination.

In addition, in the present invention, the effect can be expected only under the conditions where a bright annealed plate without bluing can be obtained, so from this point of view, the main components Si + Mn are limited to 3.0%, and at the same time, the annealing atmosphere has a hydrogen gas concentration. The hydrogen gas concentration must be controlled to be 1% or more and the dew point (D, P,) to be in the range of -6oC to 0C, preferably to be 3% or more and the dew point to be -10C or less. The limited range of gas atmosphere is wide in S.
This is because the ease of bluing can change depending on the total amount of i+Mn, and the smaller this total amount is, the higher the dew point can be adopted. For example, Si+Mn ==
If it is 2.0%, H25%, D, P -20℃
If so, it is sufficient.

The steel sheet manufactured by the method of the present invention can be easily manufactured by a normal thin sheet manufacturing method. That is, molten steel is made into slabs (steel slabs) by continuous casting or ingot-forming, and heated to 1100 ~
After heating to 1200° C., hot rolling is performed, scale is removed by pickling, and then cold rolling is performed to a desired thickness. Next, annealing is performed, and if the objective is to obtain a composite structure steel, the steel is heated in the α+γ temperature range and then rapidly cooled. The cooling rate is determined by a known method and is determined by the amount of Mn (for example, 50° C./sec or higher for 1.5 Mn).

If necessary, an overaging treatment is performed at around 300° C., and a product is obtained by performing skin pass rolling at a rolling rate of around 1%.

In the present invention, the addition level of the component added to suppress surface C, when added alone, is the lower limit at which the effect of suppressing surface C has been recognized. For example, S
The lower limit is as low as 0.008 inches for ``A'' and ``B''. On the other hand, in Cr, where the effect is not noticeably large, it is 0.1
%, and for other carbide stabilizing elements, it cannot be determined strictly as it depends on the level of C, but for example, C0,0
At 0.05%, carbide stabilizing elements such as Ti, V, Nb, and Zr are effective even at about 0.03%. However, the appropriate level for normal high tensile strength is 00.05 to 0.
In the range of Section 07, the amount of the above elements added is 0.1-0.2%
is desirable. When one or more of these additive components are added in combination, the level of addition of each component may be low. On the other hand, the upper limits of these components are, in principle, based on their effects (
The concentration is such that the effect of improving phosphate treatment properties is saturated.
In addition, when S is 0.025- or more, surface defects occur during hot rolling, and T1. V, Nb, Zr
The upper limit was set in consideration of the fact that the strength of the material would become too high and the price would become too high. The lower limit of Sl is 0.3% because if it is less than that, Si will not significantly precipitate C on the surface, and therefore the phosphate treatment property will not deteriorate. Mn is
Mainly from the material standpoint of obtaining high strength and surface C
The lower limit of Y was set at 0.5 cm for two reasons: the suppressing effect of The upper limit of C is set to 0.1% because if it is 0.1% or more, it becomes difficult to suppress the concentration of surface C even if a C suppressing element is added.

Examples of the present invention will be described below.

Example Steels having the composition shown in Table 1 were melted by vacuum melting at 2 Q Kf, heated at 1250°C, and hot rolled to about 2.3 kg.
A hot rolled steel plate having a thickness of 1111 was obtained. After removing scale on the surface of the hot-rolled steel sheet by grinding with a grinder, the hot-rolled steel sheet was cold-rolled to a thickness of 0.7 mm using a mineral oil emulsion as a lubricant. After cleaning by solvent degreasing, heating was performed at 770°C for 1 minute in the gas atmosphere shown in Table 1.
The steel plate was rapidly cooled to 300° C. at a cooling rate of 50° C./SeC by blowing atmospheric gas jets onto both sides of the steel plate. Thereafter, it was cooled in a furnace to 150°C and then taken out into the atmosphere.

The samples thus obtained were on the one hand subjected to surface analysis and on the other hand subjected to chemical conversion treatment followed by electrodeposition coating. Chemical treatment is Nippon Barker Rising Co., Ltd. B
t 3004 (dip type), and cathodic electrodeposition of Power Top U-30 manufactured by Nippon Paint Co., Ltd. was performed to a thickness of 20 μm. Chemical conversion treatment properties are evaluated by classifying the size of phosphate crystals into 1 to 5 grades (5: good;
l poor), and paint corrosion resistance was determined by salt spray test (S
S T ) 480 hr The swelling width of the scratch area after 1 to 5 steps (5: good, l poo
r).

From the results in Table 1, 51-Mn steel sheets to which carbide stabilizing elements and preferential adsorption elements (negative catalysts) are added have improved chemical conversion treatment properties and improved paint corrosion resistance to the same level as ordinary steel. It is clear that there are

[Brief explanation of the drawing]

FIG. 1 is a drawing showing the relationship between the amount of Si and Mn in steel and surface concentration of C. Figure 2 shows Haganechu 81. FIG. 3 is a relationship diagram between the amount of Mn and phosphate treatability (crystal size). FIG. 3 is a micrograph (1000x magnification) of the state of precipitation of phosphate crystals in mild steel sheets with and without surface conditioner (titanium colloid) treatment. (a) With surface adjustment, (b) Without surface adjustment. FIG. 4 is a diagram showing the influence of surface C on eyewear of titanium colloid (nucleating agent) (51-Mn high-tensile steel). 4 λ mouth S L z Wt9

Claims (1)

[Claims] (1) st 0.3-2.0%, Mn 0.5%-2
．． 0%. Based on silicon-manganese steel with CO11% or less, Or 0.1-1.0%, Ti 0.03-0.
20%, VO, 03~0.20%, Nl), 0
．． 03-0.20%, ZrO, 03-0.20%
,80.008~O,,02,5%,Se0.008~
0.025%. BO,008~0.020%, SbO,01~
0.20%, Bi 0.01-0.20% and Sn
Each contains one or more components in the range of 0.02 to 0.10%, and Si-1-MnS2.0
% or less, and the balance consists of iron and unavoidable impurities.
A method for manufacturing a silicon-manganese reinforced steel sheet with excellent chemical conversion properties, characterized by annealing in an atmosphere controlled at ℃.