JPS6119733A - Preparation of super 70kg grade high strength hot rolled steel plate excellent in elongation flange property - Google Patents

Abstract

PURPOSE:To economically prepare a high strength hot rolled steel plate excellent in an elongation flange property, by continuously casting steel with a specific composition which contains C, Si, Mn and Al and reduced in the content of S, P and Ca, and rolling and cooling the cast steel under a specific condition. CONSTITUTION:Steel containing, on a wt. basis, 0.12-0.30% C, Si<1.5%, 1.0- 2.0% Mn, 0.01-0.10% Al, S<0.005% and, if necessary one or more of P<0.008% and Ca<0.005% and comprising the remainder of Fe and inevitable impurities is continuously cast and heated at 1,200 deg.C or less to be rolled under such a condition that a total draft at 1,000 deg.C or less is set to 90% or more and finish rolling is completed in a temp. region of Ar3+50 deg.C-Ar3+150 deg.C. After the completion of finish rolling, the cooling of the steel plate is started within 2sec and the cooling speed in this case is set so as to satisfy formula logCR( deg.C/sec)>=31/10(1- Ceq) (wherein Ceq=C+Mn/5). Subsequently, the steel plate is wound up at 500 deg.C or less and a super 70kg grade high strength hot rolled steel plate having 50% or more of a bainite structure and excellent in an elongation flange property is obtained.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention is directed to a super 7000 steel having excellent stretch flangeability, which is made of a continuously cast steel made of specific components and made of steel as a starting material.
The present invention relates to a method for manufacturing Kgf/mf class high strength hot rolled steel sheets.

(Prior Art) Conventionally, hot-rolled high-strength steel sheets with tensile strength (TS) >70 Kgf/mW have been precipitation-strengthened steels using Ti, Nb, and V. When this precipitation-strengthened steel is used as a relatively thick plate, such as a line pipe, its workability requires only simple bending, and it is fully utilized. However, precipitation-strengthened steel is not suitable for cases where the plate thickness is thin and extremely strict workability is required, such as in the case of automobile suspension parts. In particular, recently there has been an increasing demand for higher strength (lighter weight) in order to improve the fuel efficiency of automobiles, but from the perspective of workability, it is not enough to simply increase the strength of the base material of steel sheets. It's fleeting. Stretch flange formability is particularly required for automotive steel sheets used in suspension parts such as members and wheels, and among these, when workability is taken into consideration, punched hole expandability is the most important. It is. Therefore, when increasing the strength of hot-rolled thin steel sheets, a steel sheet with excellent stretch flangeability is required as it is punched rather than cut.

(Problems with Prior Art) An example of a conventional TS having a strength of 70 Kgf/ma+' or higher is the technology described in Japanese Patent Publication No. 57-47256. This is a precipitation-strengthened steel made of Ti and Nb, but these precipitation-strengthening elements are expensive and high-temperature heating is essential, making it extremely uneconomical. Further, in this known technique, in an embodiment, stretch flangeability is d/do 2.
1 to 2, 3, but from a numerical level, this is due to the cut hole expansion test and not the punched hole expansion test.

Recently, as an example of improvement to the above technology, JP-A-59-18
A technology No. 32 has been proposed. This can be achieved by rolling Ti-added steel at a coiling temperature of 200 to 500°C.0) Preventing the weakening of ferrite grain boundaries by appropriately suppressing the precipitation of TiC within the grains; It has the following characteristics: - Improved bending workability by fixing it as an inclusion of TiS; (1) Strengthening of steel by appropriate precipitation of TiS.

However, in the case of this steel, it is attempted to satisfy these specifications by adding Tll1fi+, and high temperature heating is also essential, so it cannot be said that the economical efficiency is good. In addition, in the case of this steel, attention was focused only on bending at one end face of the steel, and it cannot be said that other severe processing was considered.

Further, as an example of not using precipitation strengthening elements such as T1, there is a technique described in Japanese Patent Application No. 182542/1982. It is composed of a composite structure of borigonal ferrite and martensite, and is characterized by its high strength but stretchability and ductility comparable to that of mild steel. However, in the case of this steel, the stretch flangeability is poor, and the winding temperature is extremely low at 250° C. or less, so the product is likely to have a defective shape and requires a straightening process, so it cannot be said to be economical.

In addition, as an example of no defective shape due to low temperature winding, JP-A-53
There is a technique described in No.-95121. It is said that the ductility was improved by setting the coiling temperature to 400° C. or lower (380° C. in the example) to make both the parent phase and the second phase polygonal. However, this technique does not take into account workability other than ductility at all. Therefore, although the strength is high, there is no consideration of workability, and it is thought that it has not been applied to parts that require severe workability, such as automobile suspension parts.

(Objective of the Invention) The present inventors have developed a simple C-S i-Mn
Ultra-low tensile strength achieved through controlled hot rolling and controlled cooling based on ultra-low P and ultra-low S (+Ca) treatment. OKgf/m
In order to obtain a steel that has extremely good stretch-plunge properties, which is considered to be the most important formability of hot-rolled thin steel sheets, and is also economically viable, we conducted extensive research and study. As a result, although it is a simple C-3i-Mn system, the austenite grains are made relatively large even though they are fine by performing controlled rolling in rough and finish rolling and setting the finish rolling finish temperature to a high value. By starting cooling immediately, controlling the cooling rate, and keeping the coiling temperature below 500°C, a structure consisting mainly of fine bainite structure can be obtained, and this structure has extremely good stretch flangeability. I found out.

(Structure of the invention) That is, the present invention is configured as follows.

C:'0.12-0.30wtX5i<1.5wt% Mn: 1.0-2. Owt% Al: 0.01~0.10wt$ S <0.005wt'X P < 0.008wtZ, Ca <
Contains one or two of Q, O05wtX, and the remainder is Fe.
Continuous casting of steel consisting of and unavoidable impurities, 120
After heating to 0°C or lower, the total reduction rate at 1000°C or lower is 9 (H
With the above conditions, finish rolling is completed in the temperature range of Ar3 + 50°C - Ar3 + 150°C, cooling is started within 2 seconds after finishing rolling, and the cooling rate shall satisfy the following formula. , 1ogcR(”C/5ec) > −(1-Ceq)C
eq= C+ -Mn Then, by winding at 500°C or higher, a super 70 Kgf/mm' class high-strength hot-rolled steel sheet with excellent stretch flangeability, characterized by having a bainite structure of 50% or more, is produced. Method.

The reason for limiting the components of the present invention will be explained in detail below.

C is an important element for causing extremely fine precipitation of iron carbides and strengthening the structure by bainite. 0.1
If it is less than 2%, the bainite structure ratio will be less than 50z, and not only will stretch flangeability not be improved, but the strength will be exceeded.
It cannot be maintained at OKgf/mm'. Also, a large amount of C
is simply advantageous in terms of reinforcement, but the iron carbide becomes coarse and the structure targeted by the present invention cannot be obtained. It also causes problems with deterioration of ductility and weldability. From the above, C is 0.12w
It was set as t%~0.30 approval B. Among them, 70Kgf/
To obtain mm' class, use 0.12-0.18%, maf,
In order to obtain a grade of 80Kgf/l11m' or higher, 0.
A preferred range is 15 to 0.25 wtX of C.

Si forms a substitutional solid solution in the ferrite phase and is effective in increasing the strength. Furthermore, it has the effect of increasing the degree of work hardening of ferrite and increasing its ductility. However, at 1.5z or higher, these effects become saturated, so the upper limit is set to 1.5z.
I made it z. When considering that S1 scale deteriorates pickling properties and surface appearance, and also impairs economic efficiency, it is preferable to add 0.7 wt% or less. This addition amount does not impair strength.

Mn increases the strength of steel and also improves its ductility. In particular, since the steel of the present invention must have a high strength of over 70 kgf/ff1m', 1.0 wt% is necessary. However, if there are too many MnO markings, not only will it be less economical, but special consideration will be required when melting the steel.
The upper limit was set to 2.0 wt%. 70Kgf within this range,
/mm' is 1.2-2.0%, 80Kgf/mm
1.4 to 2.0% is preferable.

It is necessary to thoroughly reduce S, but in particular the steel of the present invention has Mi+ added to ensure strength, and this M
In order to use n effectively, MnS must be generated. Furthermore, these sulfide-based inclusions including MnS extremely deteriorate stretch flangeability. The stretch flangeability, which is the main objective of the steel of the present invention, is improved because sulfide-based inclusions are not generated. Therefore, S is 0.005X
less than

As described above, in order to improve stretch flangeability, it is necessary to reduce sulfide inclusions, and therefore it is necessary to reduce the amount of S. For more demanding stretch flange applications, to reduce sulfide inclusions, ca 10.00
It is preferable to add less than 5% to form a sulfide with less plasticity.

A1 is necessary as a deoxidizing agent. If it is less than 0.01%, there is no effect, and if it exceeds 0.10%, alumina inclusions will increase and the ductility of the steel will deteriorate.

In addition, from the viewpoint of improving spot weldability and workability, when used for more severe applications, P must be thoroughly reduced to 0.
It is preferable to set it as less than 0.008g.

Next, hot rolling conditions are a very important component in combination with the ingredients in the steel of the present invention.

First, the heating temperature needs to be 1200° C. or lower.

In the present invention, from the viewpoint of economy and ductility, Ti, Nb
, V was not added. Therefore, austenite during hot rolling is less likely to form a trickle. Therefore, unless the austenite grains in the state before rolling are made small, a fine final structure cannot be obtained. Therefore, it is necessary to heat it at a low temperature. This low-temperature heating is also advantageous for energy saving.

To obtain a more thorough microstructure, increase the heating temperature to 110°C.
The temperature is preferably 0°C or lower. In this temperature range, S
It is also preferable from the viewpoint of avoidance of scale and improvement of pickling properties and surface appearance. The lower limit of the heating temperature is preferably about 1000°C, although it is better to be as low as possible in the range where hot rolling can be performed.

Next, the reduction rate is 8oz in the temperature range below 1000℃.
It is necessary to do more than that. As described above, since the present invention does not contain precipitation-strengthening elements, austenite is less likely to become fine grains during hot rolling. Therefore, it is necessary to set the total rolling reduction rate at 100 Q'O or less to 80z or more and make the austenite as fine as possible.

Next, the finish rolling end temperature is Ar3 transformation point + 50'O ~
It is necessary to set the Ar3 transformation point to +150'C. (Here Ar3 = H5,0? -455,04C+ 38.
IS 1-62.5Mn +472.13F) The feature of the present invention is that the content of fine bainite is 50% or more, and this feature leads to improvement in stretch flangeability. However, when the finishing temperature exceeds Ar3 + +50°C, the austenite before transformation becomes coarse, and the product exhibits a coarse bainite-like structure, resulting in deterioration of ductility. Also Ar3+50
If the temperature is less than 0.degree. C., the rolling of austenite promotes ferrite notes, making it impossible to obtain 50% or more of the fine bainis (which is the main focus of the present invention). The preferred range is Ar
3 +50°C to Ar3 +100°C. This temperature range provides the most stable and preferable structure.

It is necessary to start cooling within 2 seconds after finish rolling. In order to make the austenite as fine as possible in the previous stage, the reduction ratio was set high, so the strain inside the grains was large. Therefore, various phenomena such as strain-induced precipitation, transformation, and recrystallization occur one after another, but it is especially necessary to suppress grain growth accompanying recrystallization as much as possible, and it is better to start water cooling as early as possible. If there are no limitations on process capacity, water cooling should be started as soon as possible, preferably within 1 second after finish rolling.

The cooling rate must satisfy the following formula:

1ogcR('O/5ee) > -(1-Ceq)C
eq=C+ -Mn If the speed is slower than this, pearlite transformation occurs during cooling and the iron carbide becomes coarse, resulting in poor stretch flangeability.

The winding temperature must be 500'C or less. The target structure of the steel of the present invention can only be obtained by setting the cooling rate specified above and the coiling temperature to 500° C. or less, and pearlite transformation must not occur. When considering rolling yield etc., 350-400
°C is preferred. Moreover, this temperature range can be said to be a suitable range because it provides the best stretch flangeability. The lower limit is the upper limit at which a correction step is required due to the occurrence of shape defects, and is usually expected to be 250°C.

With the above-mentioned components and hot rolling conditions, it is possible to obtain the steel of the present invention having fine bainite of 501 or more. Due to this structure, it has high strength despite being a relatively low alloy.
A hot-rolled thin steel sheet with QKgf/m1 or more provides a steel sheet with extremely excellent stretch flangeability, which is important.

In the following, I explained the reason for the numerical limitation of the structural requirements, but the steel slab used here was designed with economic efficiency in mind.

Continuous casting. Although the slab may be charged into the heating furnace as a cold slab, it is preferable to charge it warmly or hotly to save energy.

(Example) Steel having the components shown in Table 1 was melted in a converter, made into a slab by continuous casting, and then hot rolled. In Table 1,
The steel codes A, B, and C are according to the present invention, and the steel code C is different from the steel code E, and the steel code S is different from the present invention. Table 2 uses steels A, B, and C, heating temperature 1050℃, finishing temperature A
This figure shows the change in tensile strength depending on the cooling rate when r3 +80°C and one winding temperature are kept constant at 350°C. The case where the cooling rate is O'504" is the case according to the present invention, and in Table 2, the cooling rate is 80°C/s to 60°C/s.
ec is according to the present invention. (The steel strips obtained after manufacturing, including those in Table 3, were pickled and then cut into plates on a cutting line. After that, they were subjected to one-carriage rolling. The thickness of all plates was 2.9111 m. The tensile test was conducted according to JIS5 No. 3 test piece was used.) When the cooling rate was 30°0/sec, pearlite transformation occurred and the strength was not sufficient.

Table 3 shows the influence of heating temperature, finishing temperature, and winding temperature when steel A is used and the cooling rate is kept constant at 60° C./see. The calculated Ar3 transformation point is 750°C.
It is. No. 3.4, 5.6 are according to the present invention, N091 is the heating temperature, No. 2 is the finishing temperature, N0
No. 97 has a winding temperature different from that of the present invention. Here, d/do indicates a hole expansion test value, (double) indicates a hole expansion test value using a punched hole, and (cut) indicates a hole expansion test value using a cut hole. In the hole expansion test, a punched hole (dice 20.6 mm) with a diameter of 20FF111 is pressed in the case of (drilling), and a cut hole with a diameter of 20+lIl is expanded in the case of (cut), and the hole expansion test is stopped when a crack penetrates through the plate thickness. Hole diameter and original hole diameter (20.6 for (stroke), 2 for (cut)
0.0). As is clear from Table 3, 1
The steel of the present invention has a punched hole expansion ratio of @ 70 Kgf/mm' and exhibits highly desirable stretch flangeability with d/do > 1.5.

Table 4 shows the hot rolling conditions and material test results for comparative steel H. D had a large amount of C, so the carbides agglomerated, and E had a large amount of S, so the stretch flangeability was extremely poor.

The copper strip according to the present invention may be used as a black leather as it is,
It may also be used after pickling. Alternatively, the plate may be cut on a shearing line. At that time, the shape may be adjusted using a leveler or temper rolling, and curling may be corrected.

Claims (1)

[Claims] C: 0.12-0.30wt% Si<1.5wt% Mn: 1.0-2.0wt% Al: 0.01-0.10wt% S<0.005wt% Necessary P<0.008wt%, Ca<0.005 accordingly
Continuously cast steel containing one or two types of wt%, the balance consisting of Fe and unavoidable impurities, and after heating to 1200°C or less, the total reduction rate at 1000°C or less is 90% or more,
Finish rolling is completed in the temperature range of r_3 + 50°C - Ar_3 + 150°C, and cooling is started within 2 seconds after the end of finish rolling, at which time the cooling rate shall satisfy the following formula, logCR (°C/sec) ≧21/10 (1-Ceq)
Ceq=C+(1/5)Mn Then, by winding it at 500℃ or less, it has a super 70Kgf/mm^2 class high strength heat with excellent stretch flangeability, which is characterized by having a bainite structure of 50% or more. Method of manufacturing rolled steel plate.