JPH0621291B2 - Manufacturing method of high strength hot rolled steel sheet - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a high strength hot rolled steel sheet.

BACKGROUND OF THE INVENTION Recently, in the automobile industry, in order to reduce the weight of a vehicle body, it is required to reduce the plate thickness without changing the design strength. We cannot meet such demands because of poor moldability and problems with weldability.

Therefore, as a steel sheet that replaces the conventional precipitation hardening high-strength steel sheet, a composite microstructure high-strength steel sheet composed of two phases of ferrite and martensite is increasingly used.

However, such a composite structure type steel sheet is not always preferable in terms of workability.

Therefore, by further adding Mn as a basic component and Si and Cr in a large amount, a composite structure type steel sheet having characteristics of low yield ratio and good ductility even after the hot rolling process-coiling process has been developed. Has been done.

However, such a composite structure type steel sheet has a problem that the cost is high because a large amount of Si and Cr are used.

On the other hand, it is already disclosed in JP-A-58-167750 that the press workability is significantly improved by substituting a part or most of the martensite of the composite structure hot-rolled steel sheet of ferrite + martensite structure type with bainite. It is disclosed in Japanese Unexamined Patent Publication No. Sho 56-180456.
Nb is effective not only in the formation of bainite but also in preventing the reduction of the heat-affected zone (HAZ portion) strength of flash butt welding when an oil rim or the like is applied. N
The promotion of bainite formation in b-added composite structure type high strength hot rolled steel sheet is due to solid solution Nb in the austenite phase after hot rolling.
The effect of suppressing ferrite transformation due to the above, and the prevention of softening of the flash butt welded portion are the effects of NbC precipitated during cooling of the ferrite region after welding of Nb which was in solid solution before welding.

As described above, in the Nb-added composite microstructure hot-rolled steel sheet, not only Nb needs to be in a solid solution state at the stage of slab before hot rolling, but also at the stage of final product after one winding of hot rolling. It is important to keep Nb in a solid solution state, and to prevent NbC from precipitating in the winding step, in order to achieve a low yield ratio, which is a characteristic of a composite structure type steel sheet.

By the way, in the conventional method, the hot-rolled steel sheet is a slab made by ingoting a steel ingot by a normal ingot method or a slab made by a continuous casting method, once cooled to room temperature, and then heated in a heating furnace. It is manufactured by heating it at a high temperature of 1200 to 1300 ° C. for a long time, and then biting it in a continuous hot rolling mill.

As described above, conventionally, a slab that has become a cold ingot is reheated and then subjected to rough rolling, but in a slab whose temperature has once dropped to room temperature, Nb precipitates and this is re-dissolved. For example, at a high temperature of 1200 ° C. or higher, 1
It must be heated for more than an hour.

That is, even if the slab once cooled to room temperature is reheated to 1100 ° C., complete re-dissolution of precipitates cannot occur, and therefore it has no effect on the control of the structure.

However, if the slab is heated at a high temperature for a long time of 1,200 ° C. or higher as in the conventional method, a huge loss of heating amount occurs.

[Object of the Invention] An object of the present invention is to provide a high-strength hot-rolled steel production method capable of producing a hot-rolled steel sheet having good workability at low heating cost.

[Summary of the Invention] The above-mentioned object is, in% by weight, C: 0.03 to 0.2%, Si: 0.02.
~ 1.5%, Mn: 0.6-2.5%, S: 0.01% or less,
solAl: 0.01-0.06% and at least one of Nb and Ti: Nb: 0.01-0.1%, Ti: 0.01-0.1%, and a transformation strengthening type containing bainite composed of balance iron and unavoidable impurities. In the method for producing a high-strength hot-rolled steel sheet, after continuous casting to obtain a high temperature slab, before the slab reaches a temperature of 550 ° C, by reheating the slab to a temperature of 1050 ° C to 1150 ° C. The precipitate is re-dissolved, hot rolling is started, the hot rolling is finished at a temperature of Ar ₃ point or higher, and the vicinity of the ferrite nose is gradually cooled at 10 ° C / s or lower, and then 600 ° C or lower. It is achieved by a method for producing a high-strength hot-rolled steel sheet, characterized by performing rapid cooling of 20 ° C./s or more up to the coiling temperature.

Further, in weight%, C: 0.03 to 0.2%, Si: 0.02 to 1.
5%, Mn: 0.6-2.5%, S: 0.01% or less, solA
l: 0.01 to 0.06% and at least one of Nb and Ti is N
b: 0.01 to 0.1%, Ti: 0.01 to 0.1%, Cr:
In the method for producing a transformation-strengthened high-strength hot-rolled steel sheet containing bainite, which comprises 0.1-1.0% of balance iron and unavoidable impurities, after continuous casting to obtain a high-temperature slab, Before the slab reaches a temperature of 550 ° C., the slab is reheated to a temperature of 1050 ° C. to 1150 ° C. to redissolve the precipitates, hot rolling is started, and the hot rolling is performed by Ar.
High-strength heat characterized by terminating at a temperature of ₃ points or more, then gradually cooling the vicinity of the ferrite nose at 10 ° C / s or less, and then rapidly cooling at 20 ° C / s or more to a winding temperature of 600 ° C or less. This is achieved by the method of manufacturing a rolled steel sheet.

The structure of the present invention will be described below.

C: 0.03 to 0.2% C is an essential element for maintaining necessary strength and forming low temperature transformation products such as bainite and martensite, but if it exceeds 0.2%, workability and weldability deteriorate. In addition, the low yield ratio characteristic, which is one of the characteristics of the steel sheet of the present invention, is impaired. The lower limit is 0.03% in order to exert the effects of strengthening and hardening.

Si: 0.02 to 1.5% Si is an element necessary for deoxidizing molten steel and is the most effective substitutional solid solution element for obtaining high strength and high ductility. Further, it has a function of favoring normal polygonal ferrite formation. In order to exert such characteristics, the lower limit is 0.02%. In addition, the upper limit was 1.5% to prevent embrittlement (increasing transition temperature) of the weld and prevent deterioration of the surface oxide scale state.

Mn: 0.6 to 2.5% Mn is an essential element for increasing hardenability and obtaining a desired structure. In order to exert its effect, 0.6% or more is required. If it exceeds 2.5%, it becomes difficult in welding and at the same time ductility deteriorates, and the price of steel sheet becomes high, so the upper limit is set.
2.5%

S: 0.01% or less S forms sulfides and deteriorates workability, so it is desirable that the content be as low as possible. However, if the content is 0.01% or less, the desired workability can be secured, so the S content. The upper limit of
It was set at 0.01%.

sol Al: 0.01 to 0.06% sol Al is effective as a deoxidizing agent for steel, but if its content is less than 0.01, the deoxidizing effect cannot be expected,
On the other hand, the content of sol Al is limited to 0.01 to 0.06% because the effect of deoxidation is saturated even if the content exceeds 0.06% and no further effect can be expected.

Nb: 0.01 to 0.1%, Ti: 0.01 to 0.1% Nb and Ti are generally precipitation strengthening elements, but the main purpose of the present invention is to serve as a solid solution element, and coexist with Mn and the like to perform hot rolling. It has a function of affecting the later transformation structure and making it easy to obtain a bainite structure or the like. Further, the structure is made finer, the stretch flangeability is improved, the hardening of the heat-affected zone after welding is prevented from decreasing, and it is useful for improving the fatigue strength not only in the base plate but also in the wheel desk. From the above viewpoints, the range of these components is Nb: 0.01 to 0.1%, T
i: 0.01 to 0.1%. One of Nb and Ti may be added, but both may be added.

Incidentally, Cr may be added in an amount of 0.1 to 1.0%. Unlike other elements, Cr has no solid solution strengthening ability by itself, but is a preferable element for improving hardenability and obtaining a bainite structure. The lower limit is 0 from the amount that can exert the effect.
The upper limit is set to 1% and the upper limit is set to 1.0% from the amount at which the effect reaches saturation and becomes uneconomical.

The reason why the slab is kept at 550 ° C. or higher after the slab is melted is as follows. Even if the temperature is as low as 550 ° C., since the transformation into the ferrite phase has hardly progressed in the steel type, the reheating is performed at the stage where the precipitation of carbonitride is not completed. For this reason, it can be easily decomposed, and for example, at a low temperature such as 1050 to 1150 ° C
It is possible to re-dissolve the precipitate even by heating for a short time. Such a solid solution state works effectively for the formation of a low temperature transformation product after hot rolling.

It should be noted that the heat retention to a temperature of 550 ° C. or higher may be performed, for example, with a heat insulating material.

The hot rolling may be started at a temperature of 1050-1150 ° C, for example.

On the other hand, the reason why the temperature is 1150 ° C. or lower is that the heating cost is reduced, and further, the martensite grains are finely and uniformly dispersed, and a high ductility composite structure type high strength hot rolled steel sheet is obtained. .

After the hot rolling is finished, predetermined controlled cooling is performed.

[Example] The steel shown in Table 1 was melted. A1, A2, A4, B1,
B2. C1. C2 is an example and the others are comparative examples.

A1, A2, A3, A4, B1, B2, B3, C1, C
No. 2 was solidified with a continuous casting machine at a slab thickness of 230 mmt. For A1, A2, A4, B1, B2 and C1, heat is applied to the high temperature slab coming out of the continuous casting machine with heat insulating material, etc. It was rolled into a hot rolled coil having a plate thickness of 2.8 mm.

In this example, the following cooling was performed as an example of controlled cooling. That is, in order to secure a ferrite volume ratio of 50% or more from the balance of strength and ductility, the ferrite nose is gradually cooled at 10 ° C / s or less, and then 600 ° C.
It was rapidly cooled to the following winding temperature at 20 ° C./s or more.

As shown in Table 3, all of the hot-rolled steel sheets according to the present examples have dramatically improved workability, particularly the strength-ductility balance (TS × El), and have a reduction in the heating furnace unit consumption. It is clear that they are entangled. Therefore, according to this embodiment,
A composite structure steel strength hot-rolled steel sheet having good workability can be manufactured at low cost.

[Effects of the Invention] According to the present invention, the following various effects can be obtained.

Heating costs can be saved.

It is possible to produce a high-strength hot-rolled steel sheet which has good workability and is particularly suitable for application as a steel sheet for automobile bodies.

A hot-rolled steel sheet can be obtained in which the strength of the heat-affected zone is not reduced by welding.

Claims (2)

[Claims] 1. C: 0.03 to 0.2% by weight, Si: 0.02
~ 1.5%, Mn: 0.6-2.5%, S: 0.01% or less,
solAl: 0.01-0.06% and at least one of Nb and Ti: Nb: 0.01-0.1%, Ti: 0.01-0.1%, and a transformation strengthening type containing bainite composed of balance iron and unavoidable impurities. In the method for producing a high-strength hot-rolled steel sheet, after continuous casting to obtain a high temperature slab, before the slab reaches a temperature of 550 ° C, by reheating the slab to a temperature of 1050 ° C to 1150 ° C. The precipitate is re-dissolved, hot rolling is started, the hot rolling is finished at a temperature of Ar ₃ point or higher, and the vicinity of the ferrite nose is gradually cooled at 10 ° C / s or lower, and then 600 ° C or lower. A method for producing a high-strength hot-rolled steel sheet, which comprises rapidly quenching at 20 ° C / s or more to the coiling temperature. 2. C: 0.03 to 0.2% and Si: 0.02 in% by weight.
~ 1.5%, Mn: 0.6-2.5%, S: 0.01% or less,
solAl: 0.01-0.06% and at least one of Nb and Ti is Nb: 0.01-0.1%, Ti: 0.01-0.1%, and C
r: 0.1 to 1.0% is added, and in a method for producing a transformation-strengthened high-strength hot-rolled steel sheet containing bainite, which comprises the balance iron and unavoidable impurities, after continuous casting to obtain a high-temperature slab Before the slab reaches a temperature of 550 ° C., the slab is re-solidified by reheating the slab to a temperature of 1050 ° C. to 1150 ° C., hot rolling is started, and the hot rolling is performed. High strength characterized by finishing at a temperature of Ar ₃ points or higher, then gradually cooling around the ferrite nose at 10 ° C / s or less, and then rapidly cooling at 20 ° C / s or more to a winding temperature of 600 ° C or less. Manufacturing method of hot rolled steel sheet.