JPS6126726A - Manufacture of over 80 kilo high-strength hot-rolled steel sheet having excellent elongation and tensibility - Google Patents

Abstract

PURPOSE:To obtain the titled high-strength hot-rolled steel sheet having excellent elongation and tensibility at low cost by heating C-Si-Mn continuously-cast billet of specified composition, roughing and finish-rolling under specified conditions, and controlling appropriately the cooling velocity and winding temp. CONSTITUTION:Steel, consisting of 0.12-0.30%, by weight, C, <1.5% Si, 1.0- 2.0% Mn, 0.01-0.10% Al, <0.005% S, >=1 kind among <0.008% P and <0.005% Ca, if necessary, and the remainder of Fe and inevitable impurities, is continuously cast. The billet is heated at <=1,200 deg.C, and broken down at >=90% total draft at <=1,000 deg.C, and finish-rolled at Ar3-Ar3+50 deg.C. Then within 2sec, cooling is started at a cooling velocity shown by equations I and II, and the sheet is wound at <=500 deg.C. Since the steel sheet has minute ferrite-bainite-martensite structure and over 80kgf/mm.<2> high strength, the sheet can be suitably used for automobiles, etc.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention is directed to a super 80% steel having excellent elongation and extensibility, which is made of steel made of specific components and continuously cast as a starting material.
The present invention relates to a method for manufacturing Kgf/mv class high strength hot rolled steel sheets.

(Prior art and its problems) Conventional hot-rolled high-strength steel sheets with a tensile strength of 80 kgf/am'' or higher include precipitation-strengthened steels to which precipitation-strengthening elements such as Ti, Nb, and V are added, and precipitation-strengthened steels containing ferrite and martensite. There are dual phase steels and bainitic steels with composite structures.Precipitation strengthened steels can easily be strengthened even with a small amount of precipitation strengthening elements, but the strength-ductility balance is poor and the yield ratio (YR) is 0.8~ Dual P as high as 0.8
In the case of hase steel, the total elongation is high and the YR is 0.5
~0.6, which is low and has good workability, but there are many problems in terms of manufacturing. If you try to manufacture it by normal rolling, you will end up with a high alloy.
(below), and correction is required in the post-process. Also, bainite steel has tensile strength (TS) =
It can produce high strength of 80-110Kgf/m/ and YR is 0.
B~0.8, which is better than precipitation strengthened steel, but elongation is not very good.

By the way, in recent years there has been a high demand for higher strength steel sheets for automobiles, and this demand is the same for suspension parts such as wheel members that use hot-rolled thin steel sheets, but extremely strict workability is also required. be done. In particular, there are many cases where ε2>o on the forming limit diagram, that is, overhang processing is involved. However, among the various high-strength hot-rolled steel sheets mentioned above, precipitation strengthened steel and bainitic steel have good workability. I can not say. Therefore, some examples of Dual Phase steel, which has a strength of 80 Kgf/■ or more and has good workability, will be presented and the problems thereof will be explained. First, JP-A-54-11442
There is a technique described in No. 5. This is C-C containing Cr
-5i-steel (selective addition elements Cu, Ni).

Nb, Ti, etc.) is cooled from the austenite single phase state to M
The method involves winding at a temperature of 1800°C or less above point S.
This requires the addition of Cr, and high alloy Dual Phas
Since it is e-steel, the alloy cost is high. Also, JP-A-54-1
No. 14426 is one in which Mill is essential added instead of Cr, and the alloy cost is also high.

The technology described in JP-A-54-150318 is C-
C-3i-steel (selective addition elements Ca, REM) is finished at a temperature of Ar3 or higher and a coiling temperature of 550°C or lower. A large amount is added, which increases the alloy cost and the heating temperature is high (causing poor pickling due to Si scale and poor surface appearance.In addition, the technology described in JP-A No. 55-131130 -3i-steel (selective addition elements Cu, Ni,
Cr, Md.

Nb, V, Ti, REM, etc.) are reheated and cooled. In other words, it is tempered steel, which increases manufacturing costs.

(Means for solving the problem) As mentioned above, high-strength hot-rolled steel sheets for automobile suspension parts have high strength, good elongation and stretchability, and low cost. Ta. In view of this current situation, the present inventors developed a simple C-C-3i- system based on ultra-low P and ultra-low S (+Ca) treatment, with a tensile strength of over 80 Kgf/Peng by controlled hot rolling and cooling control. Although it is a male product, we have carried out extensive research and study to obtain a steel that has excellent stretchability and elongation, which are essential for hot-rolled thin steel sheets used in automobile suspension parts, and is also economical. As a result, although it is a simple C-5i-Mn system, it is coarse.

After finishing controlled rolling to make the austenite grains fine, cooling is started immediately, and by controlling the cooling rate and keeping the coiling temperature to 500°C or less, fine ferrite-bainite-martensa, It has been discovered that a super 80 kgf/h grade hot-rolled steel sheet with a structure and excellent elongation and stretchability can be obtained at a low cost. That is, C: 0.12-0. -30wt$ S i < 1.5wt$ Mn: 1.0~2. Owt$ AI: 0.01-0.10wtX 5 < 0.005wt% as necessary TeP < 0.008wt%, Ca <
Contains 0.005wt% of one or two types, the balance being Fe
Continuous casting of steel consisting of and unavoidable impurities, 120
After heating to 0℃ or less, the total reduction rate at 1000℃ or less is 90%.
With the above conditions, finish rolling is completed in a temperature range of Ar3 or more and less than Ar3 +50°C, cooling is started within 2 seconds after finish rolling, and the cooling rate is set to satisfy the following formula, and then 500 Super 80K with excellent elongation and stretchability, characterized by having 5H or less bainite, ferrite, and martensitic structures by winding at temperatures below ℃.
This is a method for producing gfham' class high strength hot rolled steel sheet.

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 and martensite. If it is less than 0.12wt%, the strength will exceed 80Kg.
Unable to maintain F// class. Further, a large amount of C is simply advantageous in terms of reinforcement, but the carbides become coarse and the structure targeted by the present invention cannot be obtained. It also causes deterioration in ductility and causes problems in weldability. That's all CjtO, 12
wt% ~0.30wt$. Among them, 0.1
The preferred range is 5 to 0.25 wt%.

Si forms a 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, since these effects are saturated when the temperature exceeds 1.5 wtX, the upper limit was set to 1.5 wH. When considering that Si 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, this steel must have a high strength of over 80 kgf/mv class, so 1. OwtX is necessary. However, if the amount of Mn added is too large, it not only becomes uneconomical but also
The upper limit has been set to 2.0 because special consideration is required for melting in steel manufacturing. It was set as OwtX. Within this range, a range of 1.4 to 2.0 is preferred.

S must be thoroughly reduced. In particular, the steel of the present invention has Mn added to ensure strength, and in order to effectively use this MJ+, it is necessary to prevent the generation of MnS. Regarding zero point weldability, which causes sulfide-based inclusions to deteriorate workability, the smaller the S content, the better. Therefore, the S content was set to less than 0.005 wt$. If even stricter workability is required, it is important to reduce Si, but in addition Ca should be reduced by 0.005w.
When adding less than t$, it is preferable to use a sulfide with little plasticity.

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

When used for more severe applications from the viewpoint of improving spot weldability and workability, it is preferable to thoroughly lower P to less than 0.008 wtX.

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

The heating temperature needs to be 1200°C or less. In the present invention, Ti and Nb are used from the viewpoint of economy and ductility.

■No addition of must be done. This low-temperature heating is also advantageous for energy saving.

In order to obtain a more thorough microstructure, it is preferable to set the heating temperature to 1100"O or less. In this temperature range, Si scale can be avoided, and it is also preferable from the viewpoint of improving the pickling property 9 surface appearance. The lower limit is preferably as low as possible within a range where hot rolling can be performed, but it is usually about 50°C.

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 reduction rate at 1000° C. or lower to 80 g or more to make the austenite as fine as possible.

Next, the finish rolling finishing temperature needs to be equal to or higher than the Ar3 transformation point and lower than the Ar3 transformation point +50°C. (Here, Ar3 =
905.07-455.04C+38. ISi-82
, 5Mn +472.13F), the steel of the present invention is characterized by having a fine ferrite, bainite, and martensitic structure, and this structure allows a high-strength steel plate with excellent elongation and stretchability to be obtained. 'The finishing temperature is Ar
When the temperature exceeds 3+50°C, the austenite grains after finish rolling become large and are likely to be hardened, and the amount of bainite increases to 50% or more, resulting in poor extensibility. If the temperature is lower than the Ar3 transformation point, ferrite transformation progresses too much and a portion becomes a processed ferrite structure (fine ferrite, bainite, or martensitic structure cannot be obtained), resulting in deterioration of ductility.

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.

In JP-A No. 53-9512, finish rolling of C-C-3i steel is completed at an Ar point of 3 or more, and the austenite volume fraction is preferably between 3 and 1 Ar (preferably, the austenite volume fraction is 0.4, preferably 0.2
A technique is described in which the material is rapidly cooled from a temperature below 400° C. to 400° C. or less and then rolled up, but since this attempts to obtain a polygonal IIHI Phase structure only in the cooling stage, the grains become large. On the other hand, in the present invention, the austenite is made into fine grains and subjected to a large strain during the rolling stage, so cooling can be carried out immediately after the rolling is completed.

In other words, the steel of the present invention has a new Dual Ph
It should be called ase steel, and the grains of the final product are extremely fine (ferrite grain size number 13 or higher), so it was published in Japanese Patent Publication No. 5
It has better growth than the description in No. 3-11512.

The cooling rate must satisfy the following formula:

If the speed is slower than this, pearlite transformation occurs during cooling, the iron carbide becomes coarse, and ductility and stretchability deteriorate.

The coiling 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. must not be allowed to occur. When considering rolling yield etc., 300~400℃
is preferred. 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.

The reason for the numerical limitation of the configuration requirements has been described above, but
The steel slab used here will be continuously cast for economic reasons. 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 is A
, B, C steel, heating temperature 1050°C, finishing temperature Ar
This figure shows the change in tensile strength depending on the cooling rate when the winding temperature is constant at 3+20°C and 350°C. If the cooling rate is greater than 10.6 (+-CeB), it is according to the present invention, and in Table 2, it is 60 ° C / sec,
A temperature of 80° C./see is suitable for the present invention. (3rd and 4th
The steel strip obtained after manufacturing, including the front, was pickled and then cut into plates on a plate line. After that, it was subjected to one-part temper rolling to produce a product. The thickness of all products is 2. θ■. A JIS No. 5 test piece was used for the tensile test. ) At a cooling rate of 30"O/sec, the strength was not sufficient.

Table 3 shows the influence of heating temperature, finishing temperature, and winding degree when steel A is used and the cooling rate is kept constant at 80'0/see. The calculated Ar3 transformation point is 75
It is 0°C. No. 3.4 is according to the present invention,
No. 1 is the heating temperature, No. 2.5 is the finishing temperature, N0
The winding temperature of No. 06 is different from that of the present invention. As is clear from Table 3, the steel of the present invention exhibits excellent elongation and stretchability even though it is in the 80 Kgf/sr class.

Table 4 shows the hot rolling conditions and material test results for comparative steel H. D has a large amount of C, so the carbide is coarse and has poor elongation, and E has a large amount of S, so it has poor stretchability.

The copper strip according to the present invention may be used as a skin as it is,
It may also be used after being pickled, or it may be cut into plates on a shearing line. At that time, the shape may be adjusted using a leveler or temper rolling, and curling may be corrected.

Procedural amendment stamp system August 1981 λo (3

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 a temperature range of r_3 or more and less than Ar_3 + 50°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) ≧31/10 (1-Ceq)
Ceq = C + 1/5Mn Then, by winding at 500°C or less, a super 80 with excellent elongation and extensibility, characterized by having 50% or less bainite, ferrite, and martensite structures.
A method for manufacturing Kgf/mm^2 class high strength hot rolled steel sheet.