JPS5974259A - Cold rolled steel plate with superior workability and suitability to phosphating - Google Patents

Abstract

PURPOSE:To obtain a cold rolled steel plate provided with superior workability and suitability to phosphating by forming a thickened layer of a proper thickness contg. C, Mn, S or P on the surface of a dead soft steel or a stabilized steel having a specified composition. CONSTITUTION:This cold rolled steel plate is obtd. by forming a layer of 2- 100mum thickness contg. one or more among C, Mn, S and P on the surface of a dead soft steel consisting of <=0.0050% C, <=0.040% Si, 0.010-0.100% sol. Al and the balance Fe with inevitable impurities or on the surface of a stabilized steel contg. 0.010-0.100% in total of one or more among 0.010-0.100% Ti, 0.015- 0.100% Nb and 0.015-0.100% Zr besides the components in the dead soft steel. The thickened layer contains preferably one or more among 0.01-0.30% C, 0.02-0.30% P and 0.018-0.30% P, and it is formed by cementation or other method. The cold rolled steel plate has improved suitability to chemical conversion treatment while making use of the softness of the dead soft steel, so it is suitable for use as an external plate for a car, etc.

Description

[Detailed Description of the Invention] The present invention provides an extremely low carbon content with good ductility and excellent workability used for automobile outer panels, etc., and furthermore, carbon such as Ti, Nb + Zr, and nitrogen. The present invention relates to cold rolled steel sheets (hereinafter referred to as ultra-low carbon steels) that have been improved in chemical conversion treatment performance by adding fixed elements to improve aging properties and deep drawing performance.

Recently, cold-rolled steel sheets used for automobiles and home appliances have excellent workability by keeping C to an extremely low range (below 0,0050%), and to prevent aging deterioration, TI ( Carbon such as Nb and Zr.

Adding nitrogen fixing elements, and adding Si, Mn, P
, S content is also controlled to be low. When these ultra-low carbon steels are used in automobiles and home appliances, they are treated with phosphates as a base for painting, and the quality of this phosphate treatment determines their ability to be painted immediately. In other words, the steel plates we had up until now were not completely satisfactory.

On the other hand, the present inventors found that when a cold rolled steel sheet is subjected to phosphate treatment, when C*Mn+P+S is present in a certain composition range in the surface layer of the steel, the rate of formation of a chemical conversion film is fast and fine particles are formed. We have found that a crystalline film can be obtained.

The object of the present invention is to obtain a cold-rolled steel sheet that maintains material properties such as formability in ultra-low carbon steel and has excellent phosphate treatment properties.

That is, co, oos% or less, SlO, 040% or less, 8
Ultra-low carbon steel consisting of 0tAtO, 010~0.100%, balance Fe and unavoidable impurities, or these steels further containing one or more of Ti + Nb*Zr in a total amount of 0.010~0.100% This invention relates to a cold-rolled steel sheet that has excellent workability and phosphate treatment properties, in which a concentrated layer consisting of one or more types of C9Mnlp+8 is formed on one side of stabilized steel to which C9Mnlp+8 is added. . Here, the components of C + Mn, P, and S in the concentrated layer are preferably C0.01 to 0.3%, Mn 0.01 to 0.3%.
20-3.0%, PO, 02-0.30%, 5O8
018 to 030%, if the range of these concentration components is below the lower limit, no excellent effect on phosphate treatability will be obtained, and even if added beyond the range of these threshold components, No further increase in the effect on phosphate treatment properties can be expected; on the contrary, the effect may decrease.

Further, the thickness of the layer enriched with these elements is 92 μm or more and 100 μm or less on one side from the surface. The reason why the lower limit was set at 2μ is that if the thickness is less than this, there is little effect contributing to phosphate treatment, and the reason why the upper limit was set at 100μ is that even if the thickness exceeds this, there is little effect on improving chemical conversion treatment properties, and rather the workability etc. This is because the risk of impairing material properties increases.

A generally known method can be used to form a concentrated layer of C+Mn+P+8 on the surface of the steel plate. That is, the cladding method, the infiltration method, etc. are used to produce the cold rolled steel sheet of the present invention, but other effective methods can also be used.

The reason for providing the above-mentioned concentrated layer in ultra-low carbon steel is to take advantage of the excellent formability unique to ultra-low carbon steel.
If the value exceeds 0.0050% + St exceeds 0040%, the elongation value deteriorates in both cases. At is used as a deoxidizer, but
If 5otAt is less than 0.010%, the deoxidizing ability is insufficient and 0.
．． When it exceeds 100%, the elongation value and r value decrease. T.I.
Nb and Zr have the function of fixing carbon and nitrogen in steel, and are elements added to prevent aging deterioration.If the lower limit of each element is below, it has no effect, and if it exceeds the upper limit, it becomes expensive. In addition, it undergoes precipitation hardening and becomes hard.

Note that the total amount of TI, Nb, and Zr also has a large effect on the material, and the lower limit value has no effect of preventing aging deterioration, and the upper limit value causes precipitation hardening, so a limited range of 0.010% to 0.100% was set. It is.

The present invention will be explained in detail below using examples.

Table 1 shows the chemical components of the internal and surface concentrated layers for the copper of the present invention and comparative steel. This is a comparative study.

These pots are made from slabs obtained by continuous casting, heated to 1250℃ in a heating furnace, and then hot-rolled to a thickness of 3 to 3.5 mm at a finishing temperature of around 900℃ in a hot rolling mill. ,
After pickling, it is cold-rolled in a cold rolling mill, and then rolled in a continuous annealing furnace at 65°C.
Annealed at 0 to 800℃ and further rolled at a rolling reduction of 0.2 using a cold rolling mill.
Approximately 3% dry rolling was performed.

When forming a surface enriched layer by the cladding method, a steel billet of ultra-low carbon material and a predetermined C
, Mn, P, and S, the pressing surfaces of the steel pieces were polished and cleaned, and then stacked, welded around the four circumferences, and heated and rolled.

In addition, when forming a surface concentrated layer by the infiltration method,
(b) When concentrating carbon, Co-H,
, -CH4-based atmosphere to (b) enrich sulfur, NaCA-BaCt2-C after annealing.
aCt2- 300-500℃ in FeS-based neutral salt bath
It was soaked in water.

As is clear from Table 1, the appearance of the steel of the present invention after phosphate treatment is almost the same as that of the comparative steel.The grain size (R density) of the IR coating crystals is fine, and the phosphate coating The formation speed is fast, and the quality of the film remains unchanged even if the treatment is carried out for up to the normal treatment time (120 seconds).

In addition, as shown in Table 1, C and M are added to the surface of the ultra-low carbon material.
The steel of the present invention, which has a concentrated layer of n, p, and sulfur, is superior to comparative steels in terms of material quality.

As described above, the steel of the present invention has a wider range of time in which a good phosphate film can be formed than comparative steels, and for example, when used as a steel plate for automobiles, it is possible to apply phosphate treatment to the automobile body after assembly. The phosphate treatment is characterized by a uniform phosphate treatment finish on the entire car body even if the treatment time varies depending on the part, and it is a cold treatment material that takes advantage of the soft properties of ultra-low carbon steel and has excellent chemical conversion treatment properties. It has the ability to kill two birds with one stone in obtaining rolled steel plates.

Note that the surface enriched layer made of the steel of the present invention is usually present on both surfaces, but it can also be provided only on one side by taking a sealing means or the like depending on the condition.

Claims (4)

[Claims] (1) C: 0.0050% or less, St: 0
．． 040% or less. aotAt: 0.010 to 0.100%, on the surface of ultra-low carbon steel consisting of the balance Fe and unavoidable impurities, enriched J consisting of one or more types of CI Mn + S I P in an amount of 2 μ to 100 μ per side. A cold-rolled steel sheet with @ and excellent workability and phosphate treatment properties. (2) C: 0.0050% or less, Si: 0.04
0% or less, 8oLAL: o, o io ~ 0.100
%, TiO, 010-0100%, Nb: 0
．． 015~0.100%, Zr 0.015~0.1
The surface of the stabilized steel contains at least one member selected from the group consisting of CrMn+S+P
A cold-rolled steel sheet having excellent workability and phosphate treatment properties and having a concentrated layer consisting of one or more of the following. (3) Concentrated layer contains C: 0.01 to 0.30%, Mn:
'0.20~3.0%, P:0.02~0.30
%, S: 0.018 to 0.30% of one or more types of cold rolled steel sheet according to claim 1. (4) # layer contains C: 0.01 to 0.30%, Mn:
0.20-3.0%, P: 0.02-0.30%, S
: The cold-rolled steel sheet according to claim 2, comprising one or more of 0.018 to 030%.