JPS6141722A - Manufacture of ultralow carbon cold rolled steel sheet favorable to phosphate treatment - Google Patents

Abstract

PURPOSE:To manufacture cold rolled steel sheet superior in phosphate treatment, by adding a specified quantity of carbonitride forming element to ultralow carbon steel under the regulation relating to carbon content, hot and cold rolling, annealing continuously said steel. CONSTITUTION:In ultralow carbon steel contg. <=0.010% C, <=0.010% N, Al in 0.010%<=SolAl<=0.080% or >0.0010 B is added, further, at least one kind of Nb, Ti, Zr, V as carbonitride forming element are added so that a formula (1) is satisfied between Xi% (atomic concn. in order, of Nb, Ti, V to i=1-4) and carbon content C% (atomic concn.). The steel is hot rolled at <=880 deg.C finishing temp., then cold rolled to steel sheet, successively, annealed continuously, further temper rolled. the titled steel sheet can be manufactured.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention involves adding carbonitride-forming elements to steel and regulating the amount of addition in relation to carbon content in steel that is continuously annealed. The present invention relates to a method for producing an ultra-low carbon cold-rolled steel sheet that has improved phosphate treatability.

<Conventional technology and its problems> As the manufacturing technology for cold-rolled steel sheets has progressed from the conventional box annealing method to the continuous annealing method, the steel sheet material has been developed using ultra-low-temperature steel that shows good workability even with the continuous annealing method. Demand for carbon steel is increasing. In particular, ultra-low carbon aluminum guild steel sheets to which carbonitride-forming elements such as Nb, Ti, Zr, or V are added are often used for steel sheets for automobile exterior panels that require non-aging properties.

When cold-rolled steel sheets are used for automobile exterior panels, etc., they are subjected to phosphate treatment as a base for painting, but this treatment is not always good for ultra-low carbon steels.

Therefore, as seen in JP-A No. 59-74259, a technique for improving phosphate treatment properties by imparting a 6S layer consisting of C, Mn, P, and S to the surface of ultra-low carbon; M in steel as seen in JP-A-58-52456 and JP-A-51-74280.
Techniques have been devised to improve phosphate treatability by adjusting the n, S, and P contents.

In addition, regarding 51-Mn series high tensile strength steel sheets, Japanese Patent Application Laid-open No. 58-
As seen in No. 1001122, a technique has been devised in which carbonitride-forming elements are added and annealing is performed under a specific atmosphere to suppress surface C precipitation and improve phosphate treatability.

However, for ultra-low carbon steels containing carbonitride-forming elements, the amount of added elements has conventionally been regulated in relation to the carbon content from the perspective of materials; did not exist.

<9. Therefore, Nagisa et al. of Undeveloped IJI published a patent application No. 59-8H1 to improve the phosphate treatability of ultra-low carbon steel by regulating the amount of added elements in relation to c, A2, N, and B. We propose to produce ultra-low carbon cold-rolled steel sheets. Rumor has it that the objective of Kiwasu 1 is to obtain a cold-rolled steel sheet that has excellent workability and phosphate treatment properties for ultra-low carbon aluminum guild steel manufactured by the continuous firing gun method.

<Structure of the invention> Hon Hikari” is C50-010! I! Q%, N≦0
．． 010B11 rl.

% composition of steel Te 0.010B [(1%≦Son An≦
0.080 tonne force% or B 0.0010i
[For steels containing rfi of 1% or more, for steels containing at least one element selected from the group consisting of Nb, Ti, Zr and V in addition to these, Nb, Ti,
The amount of Zr and V added was changed to Xi% (N for i=1 to 4).
B, Ti, Zr, V (atomic concentration in order), carbon/C (Nb, Ti, Zr, V(7)i concentration relative to C) A steel adjusted to have a hot rolling finish temperature (FDT) of less than 880°C The present invention provides a method for producing an ultra-low carbon cold-rolled steel sheet with good phosphate treatability, which comprises continuously annealing the cold-rolled steel sheet after hot rolling.

In other words, conventional FDT has a high Ar3 transformation point in the case of ultra-low carbon steel, so in order to simply maintain FDT≧Ar3, 900
Although it was controlled at about °C ± 20 °C, in the present invention, the FD
It is possible to obtain an ultra-low carbon aluminum guild steel sheet with better phosphate treatability than the previous application by setting T to less than 880°C to promote the precipitation of added carbonitride-forming elements and then performing continuous annealing. That is.

The present invention will be explained in more detail below.

100 kg of vacuum melted steel with the chemical composition shown in Table 1 added with Nb, Ti, Zr, and V was melted in a laboratory and then bloomed, and the hot rolling finish temperature (FDT) was varied in the range of 750 ° C to 920 ° C. After hot rolling, cold rolling and electrolytic degreasing,
Continuous annealing was performed at 830° C. for 20 seconds, and the material, phosphate treatability, and amount of precipitation of the added carbonitride-forming element were examined.

Phosphate treatment property was determined by the pinhole area ratio (expressed as PHE) when a steel plate was degreased, washed with water, and subjected to phosphate treatment and then subjected to the pinhole test described below. The pinhole test is a method to detect areas where phosphate crystals are not attached after phosphate treatment.A filter paper soaked with potassium ferricyanide solution is placed on the test surface, and phosphate crystals are eluted from the areas where phosphate crystals are not attached. Color is developed by reacting with iron ions, which are then subjected to image analysis to determine the color change.1. part 11
Δlit is defined as PHE. According to conventional knowledge, if PHE≦3, the phosphate treatment property is good.

The precipitated amounts of added Nb, Ti, Zr, and (2) were determined by electrolytic extraction, and the solid solution amounts were determined by subtracting the precipitated amounts from their respective contents. The precipitation amount (atomic concentration) of Nb, Ti, Zr,
ol −X2.1nsol =X3.1nsol −X
4, and the solid solution ff1 (j7 concentration) is sequentially 5ol-X
l, saI-X2, sol-X3, so
Let it be I-x4.

In addition, the contents (atomic concentrations) of Nb, Ti, Zr, and
The phosphate treatment properties are shown in FIG. 1, with C being the atomic concentration.

When FDT is less than 880°C, Σ X i / C≦+-
1 1,8, the PHE was almost O, indicating good phosphate treatment properties. In addition, when X i /C>+-1, the phosphate treatment properties are good.

On the other hand, when FDT is set to 880°C or higher, Σ+−I X i /C≦2.5 Temo-specific i, :, PHE>
3. ! :*There are cases where this occurs.

So it's good! 11 In order to stably obtain a cold-rolled steel plate having salt treatability by continuous annealing, Σ+-1 X i /C≦2.5, and the hot rolling end temperature is 88
The temperature may be lower than 0°C.

However, if the carbon content exceeds 0.01%, even if the added elements satisfy the above conditions and the phosphate treatment properties are good, the quality of the steel sheet itself deteriorates, which is not preferable.

In addition, if the N content at 11°C exceeds 0.01%, a non-aging steel sheet cannot be made unless a large amount of A is added, and in that case, both the phosphorus and salt treatment properties of the material deteriorate. The content of AfL is preferably 0.608% or less; however, if the content of Ai is small, the ability to remove oxygen is insufficient, so the content of /l must be 0.010% or more.

B is also used to prevent aging deterioration of steel sheets, but its effect is not observed when the amount added is less than 0.0010%.

The added carbonitride-forming element may be present as a solid solution or as a precipitate depending on the content of C and N. However, in the case of aluminum guild steel, N is preferentially combined with human materials, so the state of solid solution and precipitation is greatly influenced by the C content.

The same can be said of B-added steel. In particular, in continuously annealed materials, the annealing time is short and the surface concentrated layer is thin, so it is presumed that the states of solid solution and precipitation of added elements in the steel are reflected on the steel surface.

When additive elements exist on the steel surface in a solid solution state, these elements, other than C and N, have a strong bonding force with 0, so they tend to form oxides. However, if a strong oxide film is formed over a wide area, it will inhibit the elution of the base iron necessary for phosphate crystal precipitation, and the phosphate treatment will be affected. It is considered to be one move against the opponent.

Therefore, it is presumed that a cold-rolled steel sheet with good processability as described in Section 11i was obtained due to these synergistic effects, but the details thereof are not clear.

<Example> Hereinafter, the effects of the present invention will be specifically explained with reference to Examples.

The slag that was continuously cast and tapped from a converter with the chemical composition shown in Table 2 was hot rolled and then cold rolled at FDT = 750°C to 880°C (Samples 1 to 6) or FDT 910°C to 920°C (Sample 7.8). 1. After annealing using the g-sequential annealing method, the specimens were subjected to pre-rolling, and their phosphate treatability was investigated. For the phosphate treatment, BT3112 manufactured by Nippon Parriki Rising 131 was used, and the results were evaluated using PHE and are shown in Table 2.

From the results in Table 2, it can be seen that the tears produced by the method of the present invention always have P
It is clear that the HE value is less than 3, indicating good phosphorus 111 treatment properties.

Therefore, when producing ultra-low carbon aluminum guild steel containing carbonitride-forming elements by the continuous annealing method, the hot rolling end temperature should be lower than 880°C, and carbonitride elements such as Nb and T
The amounts of i, Zr, and ■ added are determined below in relation to the carbon content.
Good phosphate treatment properties can be obtained by adjusting i/C≦2.5.

[Brief explanation of the drawing]

:fS1 diagram shows the pinhole evaluation value of phosphate treatment. be. FIG.1

Claims (1)

[Claims] Steel with a composition of C≦0.010% by weight, N≦0.010% by weight, 0.010%≦Sol Al≦0.080% by weight, or steel containing 0.0010% by weight or more of B. Regarding steel containing at least one element selected from the group consisting of Nb, Ti, Zr and V in addition to these, the amount of Nb, Ti, Zr and V added is Xi% (with respect to i = 1 to 4). When the carbon content is C% (atomic concentration), Σ^4_i_=_1Xi/C (Nb, Ti with respect to C)
, Zr, and V) is Σ^4_i_=_1X
A phosphoric acid method characterized by continuously annealing a cold rolled steel plate after hot rolling a steel adjusted to satisfy i/C≦2.5 at a hot rolling finish temperature (FDT) of less than 880°C. A method for producing ultra-low carbon cold-rolled steel sheets with good salt treatment properties.