JPS63241137A - Hot rolled thin steel plate having excellent workability and scale tight adhesion - Google Patents

Abstract

PURPOSE:To improve the workability and scale tight adhesion of a hot rolled thin steel plate by regulating the limited value of the compsn. of the steel. CONSTITUTION:The titled steel plate contg., by weight, 0.0020-0.10% C, <=0.03% Si, 0.10-0.50% Mn, <=0.010% P, <=0.010% S, as well as <=0.015% P+S, <=0.015% Al, <=0.0030% N as well as >=2.5 (Al/N)X(14/27) and the balance consisting substantially of iron is prepd. By this constitution, the steel plate having the excellent workability and scale tight adhesion can be obtd. Said steel plate can be utilized not only as the material for a thin steel or angle steel by forming working, drum can, pipe, etc., but also as the substitute material for a cold rolled steel plate for an automobile.

Description

Detailed Description of the Invention (Field of Industrial Application) This invention relates to a hot-rolled thin steel sheet with excellent workability and scale adhesion, and is applicable not only to drums and pipes, but also to cold-rolled steel sheets for automobiles. (a) It is also useful as a replacement material.

(Prior art) A thin steel plate (thickness: 2.5 mm) obtained by hot rolling a continuously cast slab.
O+uw or less) has poor workability and adhesion of secondary scale, so it cannot be used as is for applications that require severe molding processing. Therefore, in light processing applications, surface treatment such as bonding treatment after pickling is required. After being applied, it was merely formed into drums, pipes, etc.

Of course, such hot-rolled thin steel sheets have not been used for applications that require forming of automotive steel sheets, and cold-rolled steel sheets have been used exclusively.

(Problems to be solved by the invention) As mentioned above, hot-rolled thin steel sheets manufactured from continuously cast slabs have poor elongation compared to cold-rolled steel sheets, and have not been used for parts that are difficult to form. However, because of poor scale adhesion, pickling treatment was required, but such pretreatment is extremely disadvantageous economically.

・This invention advantageously solves the above problems, and has workability as a substitute for cold-rolled steel sheets even among automotive steel sheets. Moreover, there is no need for pickling or bonding treatment, and it can be formed immediately at the processing manufacturer. The purpose of this study is to propose a hot-rolled thin steel sheet with excellent workability and scale adhesion that can be processed and surface-painted.

(Means for Solving the Problems) That is, the present invention has the following features: C: 0.0020 to 0.10 wt% (hereinafter simply expressed as %).

Si: 0.03 or less, Mn: 0.10-0.50%, P: 0.
0.010% or less, S: 0.010% or less, and P+S: 0
．． A hot-rolled thin steel sheet with excellent workability and scale adhesion, containing 0.015% or less, Ar2: 0.015% or less, N: 0.0030% or less, and the remainder being substantially Fe. It is.

This invention will be explained in detail below.

First, in this invention, the reason why the component composition is limited to the above range will be explained.

C: 0.0020 to 0.10% In order to give the hot rolled thin steel sheet according to the present invention properties suitable for use in automobile molded parts, molded steel, drums, pipes, etc., it is necessary to use ultra-low carbon steel or low carbon steel. Carbon steel is suitable. That is, most of C becomes Fe5C (cementite) in the product, which deteriorates scale adhesion, so it is desirable to reduce it as much as possible. However, on the other hand, strength is required for automobile molded parts, molded steel, drums, pipes, etc., and for this purpose a certain amount of cl is required. Therefore, in this invention, in order to satisfy both conditions, the C content is set to 0.0.
The content was limited to a range of 0.020 to 0.10%.

Si: 0.03% or less If Si is contained in a large amount exceeding 0.03%, Si will concentrate in a film form at the scale interface, impairing the adhesion of the secondary scale, so the St content should be 0.03%. % or less.

Mn: 0.10 to 0.50% If Mn exceeds 0.50%, not only will the workability deteriorate and elongation properties will not be obtained (Mn will be dissolved solidly in FeO, resulting in secondary scale adhesion. (mechanical strength of the scale),
On the other hand, if it is less than 0.10%, the desired strength of the steel sheet that is the objective of this invention cannot be obtained, and Mn/S becomes small and hot kneeling cracking occurs. .50
% range.

P: 0.010% or less There are two reasons why P is set to 0.010% or less. One is secondary scale adhesion, which is preferably lower because P concentrates at the scale interface and deteriorates the adhesion.

Another reason is that the Ar3 transformation point increases as P increases. That is, in the hot-rolled thin steel sheet of the present invention, it is necessary to make the hot-rolled finish exit temperature (F, D, ↑,) higher than the Ar transformation point, but by adding a large amount of P, Ar =
As the transformation point increases, it becomes very difficult to make F, D, and T higher than the Ar3 transformation point. If rolling is carried out at a temperature below the Ar3 transformation point, abnormal grain growth remains in the surface layer, impairing ductility. From the above viewpoint, the P content is 0.0
It was regulated to 10% or less.

S: 0.010% or less If S is contained in a large amount exceeding 0.010%, not only will workability deteriorate, but a FeS film will be formed at the scale interface, impairing the adhesion of secondary scale. , S content is 0
．． 0.010% or less.

P + S: 0.015% or less As mentioned above, both P and S concentrate at the scale interface,
This leads to a decrease in secondary scale adhesion.

FIG. 1 shows the results of an investigation into the effects of P and S in steel on secondary scale adhesion. The scale adhesion was evaluated as follows: O: No scale flaking, Δ: Slight scale flaking, x: Significant scale flaking.

As is clear from the figure, when P+S exceeds 0.015%, secondary kale adhesion deteriorates, so P+'sl is 0.
．． It was limited to 0.015% or less.

Al: 0.015% or less Al is a useful element that fixes N@A/N, improves ductility, and lowers y and s. However, an amount commensurate with the N level is required, and it must be contained on the plate necessary to fix the solid solution N.

On the other hand, since Al2 concentrates as Al zOs at the scale interface and deteriorates secondary scale adhesion, it is desirable to reduce it as much as possible from the viewpoint of adhesion.

In this way, workability and secondary scale adhesion have a contradictory relationship. Therefore, in this invention, as described later, the amount of N is reduced as much as possible to reduce the amount of A2, and the amount is limited to 0.015% or less as the amount that satisfies both of the above characteristics.

N: 0.0030% or less As mentioned above, it is desirable that N be as low as possible from the viewpoint of workability, and considering the allowable amount of A2 mentioned above, 0.0030% or less.
．． The limit is 0.030%. If a large amount of N is contained in excess of this amount, the amount of solid solute N increases, which not only reduces ductility but also deteriorates strain aging resistance.

As mentioned above, Af is required in proportion to Nl, and A2 is required in an amount that can sufficiently fix solid solution N.

Therefore, in this invention, the stoichiometric equivalent of 2.5
It was made to contain double the amount of Af.

Next, a method for manufacturing this invention steel will be explained.

The molten steel having the above-mentioned preferred composition is made into a slab by a continuous casting method.

Then, after heating to about 1200 to 1300°C, hot rolling is performed, preferably to form a plate K: 2. Hot rolled thin steel sheet with Oaim or less.

(Example) Molten steel having various compositions shown in Table 1 was made into slabs by continuous casting, heated to 1240'C, and then hot rolled under the conditions also shown in Table 1. A hot-rolled steel plate with a thickness of 1.4h was obtained.

Table 1 also shows the results of investigating the workability and secondary scale adhesion of each hot rolled sheet thus obtained. Note that the evaluation of secondary scale adhesion was the same as in the case of FIG. 1.

Comparative Example A is a base material for conventional hot-rolled steel sheets for automobiles, but it has problems in workability and secondary scale adhesion. As a guideline for workability, since we are considering replacing cold-rolled steel sheets, we aim for a total elongation of 48% or more, and to evaluate secondary scale adhesion, we observe and evaluate the state of scale peeling through actual press forming. did.

Comparative Example B has high p and p+S, and has problems in workability and secondary scale adhesion.

Comparative Example C has high s and p+S, and also has difficulties in workability and secondary scale adhesion.

In the comparative example, both S and P are within the range, but P and N are high (and there are difficulties in workability and secondary scale adhesion.

Comparative Example E has a high AN and has difficulties in both workability and secondary scale adhesion.

On the other hand, adaptation examples F, G and H according to the present invention all contain c, in order to improve workability and secondary scale adhesion.
Those with lower Mn and P exhibited the effects mentioned above (particularly because lowering P raised the Ar3 transformation point and prevented P from concentrating on the scale interface),
A hot-rolled thin steel sheet with a total elongation value of 48% or more, which is a workability guideline for cold-rolled steel sheets, and excellent secondary scale adhesion that does not cause scale to peel off even during press forming (deep drawing) can be manufactured. Ta.

(Effects of the Invention) Thus, according to the present invention, a hot-rolled thin steel sheet with excellent workability and scale adhesion can be obtained, and it can be used as a material for forming groove steel, angle steel, drums, pipes, etc. In addition, it can also be used as a substitute material for cold-rolled steel sheets for automobiles.

[Brief explanation of drawings]

FIG. 1 is a graph examining the effects of P and S in steel on secondary scale adhesion.

Claims (1)

[Claims] 1. C: 0.0020 to 0.10 wt%, Si: 0.03 wt% or less, Mn: 0.10 to 0.50 wt%, P: 0.010 wt% or less, S: 0. 010wt% or less, P+S: 0.015wt% or less, Al: 0.015wt% or less, N: 0.0030wt% or less, and contains Al/N・14/27≧2.5, with the remainder being substantially A hot-rolled thin steel sheet with excellent workability and scale adhesion, characterized by having a composition of Fe.