JPS6293002A - Production of high strength hot rolled steel sheet - Google Patents

Abstract

PURPOSE:To produce a high-strength hot rolled steel sheet having good workability at a low cost by subjecting the high-temp. slab which has the spe cific compsn. consisting of C, Si, Mn, S, Al, B, Ti and Fe and which is subjected to quick cooling and continuous casting to high-temp. hot rolling under adequate conditions. CONSTITUTION:A steel contg. 0.03-0.2wt% C, 0.02-1.5% Si, 0.6-2.5% Mn, <=0.01% S, 0.01-0.6% solAl, 0.0005-0.01% B, and 0.01-0.1% Ti, added with 0.1-1.0% Cr according to need and consisting of the balance Fe and unavoidable impurities is continuously cast at a cooling rate of >=50 deg.C/min in the stage of solidification. The resulted high-temp. slab is held at >=500 deg.C according to need and the hot rolling thereof is started after heating in the case of <1,050 deg.C or as it is in the case of >=1,050 deg.C before the slab temp. falls down to 550 deg.C. Such hot rolling is ended at the temp. of the Ar3 point or above and thereafter the steel sheet is subjected to the prescribed controlled cooling. The transformation strengthened high-strength hot rolled steel sheet contg. bainite is thus obtd. at a low heating cost.

Description

[Detailed description of the invention] [Field of application of the invention] 2 The present invention is directed to a method for producing high-strength hot-rolled steel sheets.

[The backbone of invention FTL in recent years! ! In the world of cars, due to the quantification of single bodies,
There is a desire to be able to reduce the plate thickness without changing the design strength, but conventional precipitation-hardened high-strength steel plates have poor press formability and problems with weldability. We are unable to respond to such requests.

Therefore, as a steel plate to replace the conventional precipitation hardening type high tensile strength steel plate, a composite structure type high tensile steel plate consisting of two phases of ferrite and martensite is increasingly being adopted.

However, such a composite layer ajl! 'm board is also easy to work with.

Therefore, Mn is further used as a basic component, and Sl.

By adding Cr to multi-FJ, hot rolling process
A composite structure steel sheet has been developed that has the characteristics of a low yield ratio and long ductility even after the rolling process.

However, such a composite structure steel sheet has a problem in that it is expensive because Si and Cr are used as the bulk material.

On the other hand, B is known as an element that improves the hardenability of steel, and can improve hardenability at low cost, but for this purpose, B needs to be in a solid solution state.

By the way, in the conventional method, a hot-rolled steel plate is produced by cooling a slab made by blooming a steel ingot by a continuous casting method or a slab made by a continuous casting method to -(1 room temperature).
After that, it is heated in a heating furnace at a high temperature of 1200 to 1300° C. for a long time, and then rolled into a continuous hot rolling mill to produce it.

In this way, conventionally, /'XJ lumped slabs were
The slab is heated to F and then subjected to rough rolling, but once the temperature has fallen to room temperature, B precipitates as BN, and in order to dissolve it again, it is necessary to
The material must be heated for an hour at a high temperature of 0°C or higher.

In other words, once the slab has cooled down to room temperature, it is heated to 1100℃ for 1 time.
Even with rg heating, complete solid solution of the precipitates cannot occur, and therefore it has no effect on controlling the structure.

However, subjecting the slab to high-temperature, long-term heating of 1200° C. or higher in this manner results in a huge loss in heating amount.

[Object of the Invention] An object of the present invention is to provide a method for producing high-strength hot-rolled steel that can produce a hot-rolled steel sheet with good workability at low oil and heat costs.

[Requirements for the invention] 1] Target, ball 1%, C: 0.03 to 0.2%
.

Si: 0.02-1.5%, Mn
: 0.8 to 2.5%, S: 0.01
% or less, sol Al: 0-01 to 0.06%,
B: 0.0005-0.01%, Ti: 0.01-0.
In a method for manufacturing transformation-strengthened high-strength hot-rolled steel sheets containing bainite containing 1% iron and unavoidable impurities, continuous casting is performed at a cooling rate of 50°C/min or higher during solidification to form a high-temperature slab. If the temperature of the slab is lower than 1050°C, heating is performed before the temperature of the slab reaches 600°C.
℃ or higher, hot rolling is started without heating, the hot rolling is rolled at a temperature of Ar 3 or higher, and then predetermined controlled cooling is performed. This is achieved by the manufacturing method of steel plates.

The present invention will be explained in detail below.

C: 0.03 to 0.20 C not only maintains the required strength and forms low-temperature transformation products such as bainite and martensite, but also deteriorates the properties and weldability of the steel sheet. One of the characteristics of low yield ratio is impaired. The lower limit is set to 0.03% in order to exhibit the 1-effect of strengthening and hardenability.

Si: 0.02-1.5' 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. Furthermore, it has the function of favoring normal polygonal ferrite formation. In order to exhibit such characteristics, the lower limit was set to 0.02%. In addition, the L limit was set at 1.5% in order to prevent embrittlement (increase in transition temperature) of the weld and to prevent deterioration of the surface oxidation scale condition.

Mn: 0.6-2.5'.

Mn is an essential element for increasing hardenability and obtaining a desired structure. In order to exhibit its effect, 0.8% or more is required; if it exceeds 2.5%, it becomes difficult to weld and at the same time deteriorates ductility, making the steel plate expensive.
The limit is 2.5% + S, Σ-' January Bovine Suck J S produces sulfides and deteriorates workability, so it is desirable to have as little as possible, but the content is 0.01%. The desired processability can be ensured if the S content is -
The F limit was set at 0.01%.

sol Al: Q, 01~0.06゜5olAl
is effective as a deoxidizing agent for steel, but if the content is less than 0.01%, no deoxidizing effect can be expected; on the other hand, even if the content exceeds 0.06%, the deoxidizing effect is not expected. golAl because it is saturated and no further effect can be expected.
The content was limited to 0.01-0.06%.

Ti: 0.01-0.10 Ti is a precipitation-strengthening element and is useful for preventing a decrease in the hardness of the heat-affected zone after welding.In addition to this effect, the present invention maximizes the hardenability improvement effect of B. : Contained for the main purpose of producing a low effect, the lower and upper limits are set at 0.01 to 0.1% from the viewpoint of this effect.

B: 0.0005 to 0.01 B is an element that improves hardenability, and is an advantageous element in reducing the amount of addition of other expensive elements and obtaining a structure of a certain ψ. The lower limit is 0.0005 to 0.0 from the amount at which the effect can be exhibited, and the upper limit is from the amount at which the effect reaches saturation and is no longer economical. Let's be Ol.

Note that Cr may be added in an amount of 0.1 to 1.0%.

Unlike other elements, Cr itself does not have a hardening ability, but it is a preferable element for improving hardenability and obtaining bainite fi1m. The lower limit is set at 0.1% from the amount that can produce the effect, and the upper limit is set at 1.0% from the amount at which the effect reaches saturation and becomes uneconomical.

β, 50℃ - In the current continuous casting slab, the cooling rate during solidification is 3~b near the center, and the distribution of solute components such as Mn, S, and P between molten steel and solid iron occurs completely. Therefore, the concentration of solute elements is large in the final solidified zone between the secondary dendrite and the dendrite resin after solidification. This tendency is remarkable in steel types with high M n Ji such as composite structure type steel sheets. The shading is maintained even after hot rolling, and in extreme cases b
It exhibits a laminated structure called anded structure. Therefore, after performing controlled cooling, M n
In the high region of No. 3, martensite grains are densely packed, and in severe cases, a layered martensite layer is formed. On the other hand, M
In a region where the n content is small, no martensitic phase exists at all, and such a heterogeneous structure deteriorates the ductility, which is a characteristic of a composite structure type steel sheet.

By setting the cooling rate during solidification to 50°C/min or more, the distribution coefficient becomes large, and the difference in concentration of Mn, etc. is small to the extent that it does not pose a problem in obtaining a composite structure in steel with 2.5% Mn or less. found. Conversely, if continuous casting is performed at a cooling rate of 50° C./min during solidification, the ductility of the composite muff steel strength hot-rolled steel sheet will be greatly improved. It also reduces coagulation and improves material quality.

As for the means to increase the cooling rate during solidification, strong cooling may be used in continuous casting of the conventional thickness, or continuous casting may be carried out to form a thin slab with a thickness commensurate with the cooling rate.In this case, reducing the slab thickness The accompanying reduction in rolling ratio does not affect the material quality at all because the difference in concentration of solute elements is originally small.

The reason why the slab is maintained at 550° C. or higher after melting is as follows. Even at a low temperature of 550°C, transformation to ferrite phase has hardly progressed in this steel type, so BN, Fe 23 (C
B) 6 etc. 17) B(7) Reheating is required at a stage where the precipitation of carbonitrides is not completed. Therefore, since it can be easily decomposed, solid solution can be re-dissolved even by heating at a low temperature of 1050° C. to 1150° C. for a short time. Such a solid solution state works effectively for the formation of low-temperature transformation products after hot rolling.

In addition, for example, heat retention at a temperature of 550℃ or higher is done using a blunt kk.
L-? 41) In addition, hot rolling may be started at a temperature of, for example, 1050 to 1150°C.

After hot rolling is completed, predetermined controlled cooling is performed.

[Example] Steels shown in Table 1 were melted. Al, A2゜A6, Bl,
B2 is an example, and the others are comparative examples.

AI, A2, A3, Bl, B2. B3.

C1 and C2 were solidified using a continuous casting machine with a slab thickness of 50 mm. Furthermore, AI and A2.

Bl, B2. For CI, the high-temperature slab that comes out of the continuous casting machine is heat-insulated with heat insulating material, etc., and then lightly heated and hot-rolled under the conditions shown in Table 2 to obtain a plate with a thickness of 2.8.
It was made into a hot rolled coil of mm. A3. B3. Regarding C3, after cooling to room temperature, it was reheated in the conventional process and hot rolled.

Also, A4, A5, B4. B5 is a slab obtained by conventional continuous casting, and A4. B4 is directly rolled, and A5. As for B5, it was heated to ++j and hot rolled.

In this embodiment, the following cooling was performed as an example of controlled cooling. That is, in order to ensure a ferrite body M ratio of 50% or more in terms of strength and ductility balance, the area around the ferrite nose is slowly cooled to 10°C/S or less, and then cooled to 600°C.
It was rapidly cooled at 20°C/S or more to the following coiling temperature.

As shown in Table 3, all of the hot-rolled steel sheets according to this example have dramatically improved workability, especially the strength-ductility balance (TSXEJI), and have also achieved a reduction in the current capacity of the heating furnace. It is clear that Therefore, according to this embodiment,
A composite structure steel strength hot-rolled steel sheet with good workability can be manufactured at low cost.

[Effect of the invention] According to the present invention, the following effects can be obtained.

■It is possible to save on heating costs.

■It is possible to produce high-strength hot-rolled steel sheets that have good workability and are particularly suitable for application as steel sheets for automobile bodies without using expensive elements.

Table 1 ○marks are examples Table 2 O marks are examples. Table 3 (Part 1) ○marks are examples Table 3 (Part 2) O marks are examples.

Claims (1)

[Claims] 1% by weight, C: 0.03-0.2%, Si: 0.0
2-1.5%, Mn: 0.6-2.5%, S: 0.01
% or less, solAl: 0.01-0.06%, B: 0.
0005 to 0.01%, Ti: 0.01 to 0.1%, and the balance is iron and inevitable impurities, including bainite. After obtaining a high-temperature slab by continuous casting at a speed of 50°C/min or more, before the slab reaches a temperature of 550°C, or after heating if the temperature of the slab is less than 1050°C, If the temperature of the slab is 1050°C or higher, hot rolling is started without heating, the hot rolling is finished at a temperature of Ar_3 or higher, and then predetermined controlled cooling is performed. Manufacturing method for high-strength hot-rolled steel sheets. 2. The method for producing a high-strength hot-rolled steel sheet according to claim 1, wherein 0.1 to 1.0% of Cr is added. 3. The method for producing a high-strength hot-rolled steel sheet according to claim 1 or 2, which comprises obtaining a high-temperature thin slab and then keeping the slab warm.