JPS60181231A - Manufacture of high-tension hot-rolled steel sheet having superior workability - Google Patents

Abstract

PURPOSE:To obtain a steel sheet having a superior stretch flanging property and improved stength when the titled steel sheet similar in composition to a C-Si- Mn steel sheet for popular use and having a fine bainite-base composite structure is manufactured, by increasing the amount of C. CONSTITUTION:A steel billet consisting of >0.15-0.20% C, <=1.5% Si, 0.3-1.5% Mn, <=0.02% P, <=0.01% S and the balance Fe is prepd. The billet is heated and subjected to continuous hot finish rolling at >= 80% total draft and >=(Ar3+ 50 deg.C) final rolling pass temp. The hot rolled steel sheet is cooled at >=45 deg.C/sec cooling rate and coiled at 300-500 deg.C.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention uses C-3i-Mn steel or C-Mn steel,
It does not require the addition of special elements (carbonitride-forming elements such as Nb, Ti, V, etc.), and by improving the current hot rolling process, it is particularly effective for stretch flanges than conventional precipitation-strengthened high-strength hot-rolled steel sheets. The present invention relates to a method for manufacturing a high-strength hot-rolled steel sheet that has good properties and excellent workability.

(Prior art) In recent years, in the automobile industry, one of the measures to improve the fuel efficiency of automobiles is to reduce the weight of automobile bodies, and from the viewpoint of thinner steel plates and safety, high-strength hot-rolled steel sheets with excellent workability have been developed. Demand is increasing.

Conventionally, in order to obtain high-strength hot-rolled steel sheets with excellent workability, Nb
, Ti, V, etc. to improve strength through solid solution hardening and precipitation strengthening through carbonitride formation, and dual phase (D
There is a manufacturing method that improves the balance of strength and ductility by using a steel plate. The former is Nb
, Ti, V, etc. are expensive and cause an increase in cost, and resource constraints are expected in the future. The latter DP rust steel plate has a good strength-ductility balance, but when applied to wheels, which are automobile parts, there are problems: 1. Poor stretch flangeability, for example, when processing wheel discs, cracks may occur when forming hub holes by burring. Easy to occur. ◎There are two points: the wheel rim has a large softening phenomenon in the weld heat-affected zone, the wall thickness of that part decreases significantly during mold correction, and no improvement in fatigue properties is observed.

One possible way to solve these problems is to manufacture a high-strength hot-rolled steel sheet using C-C-3i steel and having a composite structure of ferrite and bainite. For example, JP-A-57-1459
As shown in No. 85, by controlling the cooling after the final rolling pass and controlling the coiling temperature with C-C-3i-steel, Ferrite I
...A high-strength hot-rolled steel sheet with a bainite composite structure and its production have been shown, and it is inexpensive and has improved cracking during wheel disk processing and hub hole forming by burring, but it is sufficient. isn't it. In addition, there are some automobile suspension parts that particularly require stretch flangeability, and cracking has occurred. This is JP-A-57-14
The ferrite-bainite structure shown in No. 5985 has a bainite ratio of 5 to 80%, and the present inventors have determined that in order to improve the stretch flange, the bainiite ratio should be 70%.
Patent application 1982-2 that it is effective to set the ratio to % or more
485. The present inventors further improved the strength (TS
It has been found that increasing the amount of C is effective in stably securing a bainite ratio of 70% or more (B5Kgf/mm'). Furthermore, it is better than DP steel in terms of wall thinning during mold straightening after flash butt welding.

(Object of the invention) In view of the current situation, the present invention provides a general purpose C-C-3i-
Using steel with similar composition to that of the T series, it has excellent stretch flangeability.
S80, the present invention provides a new manufacturing method for manufacturing bainite-based high-strength hot-rolled steel sheets of gf/mm' or more at a low cost.

(Structure and operation of the invention) The basic components of the present invention include C: more than 0.15 and 0.20
% or less, Si: 1.5% or less, Mn: 0.3-1
．． 5X, P: 0.02% or less, S: 0.01
% or less, and the balance is Fe and unavoidable elements. C and Mn are essential elements for securing the necessary strength and obtaining a composite structure of ferrite and bainite. If C: o, 15x or less, Mn: less than 0.3%, TS 8 (1 Kgf/mm' or more - L) It is difficult to obtain a steel plate with the strength of C: more than 0.20%, Mn: 1.5
C: more than 0.15 and 0.20% or less, Mn: 0.3~
It is set to 1.5X. Preferably, when Si is added in an amount of 0.2% or more, the solid solution [C] in the ferrite grains decreases and promotes the concentration of C element in the untransformed austenite grains, making it easier to obtain a suitable composite structure of ferrite and bainite. It has the function of improving the strength-ductility balance of steel sheets. Si
Si: 1.5% or less, preferably 0.0%, because if it exceeds 1.5%, the concentration of C element in the untransformed austenite grains becomes saturated, which is economically disadvantageous and impairs weldability.
2 to 1.5%. Since P may impair weldability, P:
The content shall be 0.02% or less. Since S forms MnS-based inclusions and reduces stretch-flangeability, in order to reduce MnS-based inclusions and improve stretch-flangeability, S:
The content shall be 0.01% or less. Ca has the function of controlling the shape of inclusions to form fine spheres and improves stretch flangeability, so it is preferably contained up to 0.01%.

During hot rolling, the heating temperature is preferably 1100°C or less. This is done in order to make the austenite grains as small as possible during heating, and when the heating temperature is high, the rolling speed must be reduced or a delay of 1 hour at the entry side of rolling must be applied to ensure the final rolling pass temperature. This is to prevent sexual decline. The reason why the total rolling reduction ratio in finish rolling (reduction ratio before and after finishing rolling) is set to 80% or more is to reduce the austenite grain size and prevent deterioration of elongation as fine bainite. The cooling rate is set to 45°0/s or more, preferably around 100°C/s, and the temperature at the end of cooling is 300 to 550°C. This is the cooling rate and temperature necessary to obtain a bainite structure of 70% or more.

The characteristics of the hot-rolled steel sheet with a composite structure of bainiite (bohnite) and ferrite produced by the method described above are that in terms of strength-ductility balance, ferrite I...
Although it is inferior to bainite composite structure hot-rolled steel sheets, it has excellent properties in terms of hole expansion ratio, which is one of the evaluation criteria for stretch flangeability. The reason for this is that the bainite phase of a hot-rolled steel sheet with a bainite-ferrite composite structure consisting mainly of bainite is obtained by rapid cooling from the γ region, so it is softer than the bainite phase obtained by rapid cooling from a two-phase separated state. This is because homogeneous deformation occurs up to the high deformation region. Also D
The poor stretch flangeability of P steel is due to the large hardening of the punched end face and the destruction of hard martensite or the generation of voids from the interface with ferrite.

FIG. 1 compares and organizes the strength-ductility balance of these hot rolled steel sheets. The strength-ductility balance of the hot-rolled steel sheet of the present invention having a complex structure of bainite φ ferrite mainly composed of bainite is inferior to that of the hot-rolled steel sheet with a complex structure of polygonal fine-grain ferrite and bainite, but the precipitation-strengthened high-strength hot-rolled steel sheet is equivalent to

Figure 2 compares and organizes the hole expansion ratios of these hot rolled steel sheets. The hot-rolled steel sheet of the present invention having a bainite-ferrite composite structure mainly composed of bainite is superior in hole expansion rate to the hot-rolled steel sheet having a ferrite-bainite composite structure.

(Examples) Examples according to the present invention are shown in Table 1. Inventive Examples 1 to 7 are obtained by hot rolling steel having predetermined components according to the present invention. Comparative Example 8 was obtained by hot rolling a component system with a low C content in accordance with the present invention, and its strength was low. Comparative example 9
is a predetermined component, but the winding temperature during hot rolling was too high, and it has a ferrite/pearlite structure, low strength, and a low hole expansion ratio. Similar to 9, Comparative Example 10 has the prescribed components, but the cooling rate during hot rolling was low, and it had a ferrite/pearlite structure with low strength and a low hole expansion ratio. Comparative Example 11 was hot-rolled in accordance with the present invention using a component system with a high C content, and had a low strength-ductility balance, with a particularly large drop in elongation.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a comparison of strength-ductility balance, and FIG. 2 is a diagram showing a comparison of hole expansion ratio. Patent applicant Representative patent attorney Tomoyuki Yafuki (and 1 other person) Figure 2 TS ('Man□・) Procedural amendment (voluntary) April 27, 1980 Commissioner of the Japan Patent Office Kazuo Wakasugi 1, Case Indication of 1982 Patent Application No. 36148 2, Title of invention: Method for manufacturing high-strength hot-rolled steel sheet with excellent workability 3, Relationship with the person making the amendment case Applicant address: 2-chome Otemachi, Chiyoda-ku, Tokyo 6 number 3 name (6
65) Nippon Steel Corporation 4, Agent 5, Column 6 for detailed explanation of the invention in the specification subject to amendment, Contents of the amendment (1) “Good balance” on page 2, lines 19-20 of the specification be corrected by saying that the balance is good. (2) "Wind-up temperature" on page 8, line 11 of the specification is corrected to "wind-up temperature." (3) Correct the "coiling temperature" in the hot rolling conditions item of Table 1 on page 9 of the specification to "coiling temperature" (see attached sheet).

Claims (1)

[Claims] C: Contains Q, more than 15 and 0.20% or less Si: 1.5X or less Mn: 0.3 to 1.5% P: 0.02% or less S: 0.01% or less , the remaining part consists of Fe and unavoidable elements, the total reduction rate of continuous hot finish rolling is 80% or more, the temperature of the final rolling pass is finished at (Ar3 150°C) or more, and then 45 A method for producing a high-strength hot-rolled steel sheet with excellent workability and having a fine bainite-based composite structure, characterized by cooling at a cooling rate of ℃/S or more and winding at 300 to 550℃.