JPS60149719A - Manufacture of hot-rolled high-tension steel sheet - Google Patents

Abstract

PURPOSE:To improve the tensile strength, workability and toughness by hot rolling a cast billet contg. prescribed percentages of C, Si, Mn, S, Ti, etc. under prescribed conditions without reducing the temp. CONSTITUTION:A steel consisting of 0.04-0.18% C, 0.05-0.8% Si, 0.5-2% Mn, <=0.005% S, 0.005-0.045% Ti, 0.003-0.015% N, 0.01-0.08% Al and the balance Fe is continuously cast into a billet. This hot billet is hot extruded without reducing the temp. to the Ar3 point or below. The hot rolling is started at <=1,100 deg.C and finished at the Ar3 point or above.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to hot-rolled high-strength steel sheets, particularly those having a tensile strength of 50 kgf/
The present invention relates to a method for manufacturing a high tensile strength steel plate having a strength of ++++++2 or more and having excellent workability and toughness.

In recent years, as part of energy-saving measures in the hot rolling process, a direct rolling method has been developed in which hot slabs obtained by continuous casting are hot rolled directly or after being lightly heated so that the slab surface temperature is the same as the center temperature. However, in the direct rolling method, there is no step of reheating the slab that has been cooled to room temperature to a high temperature of 1200 to 1300°C, which is the conventional method, and therefore the enormous amount of thermal energy required for heating is required. It has the advantage of saving money. However, when this direct rolling method is viewed from the viewpoint of material quality, there is a problem in that the workability and toughness of the rolled material deteriorate. In other words, the finer the structure of the rolled material, the better the workability and toughness, but in the direct rolling method, the coarse austenite grains immediately after solidification are rolled while being preserved, so the structure of the rolled material is not coarse. Workability and toughness deteriorate.

Therefore, at present, the direct rolling method is only applied to applications where these characteristics do not pose any problems. In addition, a method of producing high ductility and high yield steel by directly rolling a small amount of T1 added steel has been known for a long time, for example, as disclosed in Patent Application Publication No. 194214 of 1981,
A method has been proposed in which a trace amount of Ti is added to low N steel and the steel is directly rolled. However, it is difficult to reduce the N content in steel using normal manufacturing methods, requiring special treatment such as vacuum degassing treatment, which increases manufacturing costs.In addition, low-N steel has problems with direct rolling. There are some drawbacks that cannot be taken as effective countermeasures against certain poor toughness11.

In order to solve these drawbacks, the present invention aims to solve the problem of 50 kgf/1M?, which requires good workability and toughness. The purpose is to manufacture the above hot rolled high steel plate by direct rolling method, O:
no 4-118%, Si: more than 1105-0.8
0%, Mn: 150-2.00%, S:<l:o, o
o 5%, T1: o, o o s~0.045%
, N:Q, 0030-0.0150%, At:Q, 0
1 to [L08% included, further Ou: 1.0 as necessary
Ni: 1.0% or less, Or: 1.0% or less, N
b: 0.1% or less, V: [L1% or less, MO20, 5%
Below, a hot slab obtained by continuous casting of steel containing one or more of Oa: 0.0050% or less, the balance consisting of iron and inevitable impurities, is cooled to a temperature lower than 3 points of Ar. Kotona (Start rolling at a temperature of 1100℃ or less, J
This method is characterized in that hot rolling is finished at a temperature of r3 or higher.

Hereinafter, the reasons for limitations on the constituent elements of the present invention will be explained.

(1) Component C is an effective element for strengthening steel; if Q is less than 0.4%, the required strength of 50k19f/- or more cannot be obtained. On the other hand, if it exceeds 0.18%, workability, toughness and even weldability deteriorate, so in the present invention it is set to 0.04% or more and 0.18% or less.

Sl is an element that strengthens steel without deteriorating its ductility, but if it is less than 0.05%, it has no effect.

On the other hand, increasing the amount of S1 causes so-called island-like scale, which is generally restricted to a low level, but in direct rolling, so-called island-like scale occurs, which impairs the appearance of the steel plate surface and deteriorates weldability.

Mn is effective as a steel strengthening element along with C, but 0
．． If it is less than 501, the desired strength cannot be obtained. On the other hand 2.0
If the OS is too high, workability and weldability will deteriorate.

At is added to improve the soundness of steel through its deoxidizing effect, but it has no effect at less than 0.08% [L01%].
%, the effect becomes saturated.

Ti, N, and Ni are important constituent factors in the present invention, and improve the strength, workability, and toughness of hot rolled sheets obtained on the premise of the direct rolling method that combines continuous casting and hot rolling, which will be described later. In order to achieve this, the following restrictions apply.

Table 1 below shows the effect of adding T1 in the direct rolling method. T1-added steel and non-additive steel were made into continuous slabs with a thickness of 250 Wm, and the results were obtained by rolling them directly or by cooling them to room temperature and then reheating them. be.

Note that the rolling conditions are as follows: a rolling start temperature of 1080°C, a finishing temperature of 830°C (>Ar5), and a winding temperature of 560°C.

As is clear from the table above, direct rolling of T1 additive-free steel results in deterioration of workability and toughness;
1 additive steel shows good properties equivalent to or better than the reheating method, and at the same time 10 kl? f/rW? It shows a slight increase in intensity. The reason for this is thought to be as follows.

In the direct rolling method, the hot slab is hot-rolled while being held in the austenite region without lowering its temperature to the ferrite region, so in steel with a small amount of T1 added, the added Ti is not in a solid solution state at the start of rolling. , T1 carbonitrides precipitate during rolling or during winding after rolling. T1 carbonitrides precipitated during rolling have the effect of suppressing austenite recrystallization and grain growth, and refine the structure of the rolled material. This avoids deterioration in workability and toughness due to coarsening of the structure, which is a drawback of the direct rolling method. On the other hand, T1 carbonitrides precipitated during winding increase the strength of the steel through precipitation strengthening. Therefore, adding a small amount of T1 in the direct rolling method is an effective means of simultaneously achieving strengthening of steel and improvement of properties.

From the above, in order to simultaneously achieve property improvement and strengthening in the direct rolling method, (1) T1 added at the start of rolling must be in solid solution. , (2) Some T during rolling
Three conditions are required: (3) T1 of the residue is finely precipitated as carbonitrides during winding after rolling. First, in order to satisfy condition (1), the hot slab must be kept in the austenite region with a large solubility product, that is, A
It goes without saying that the temperature should not fall below the r3 point, but it is also necessary to limit the upper limits of T1 and N, respectively.
O45chi, 0.0150. This is more than this
If i or N is present, TiN will be generated during cooling from solidification, impairing the effect of adding Ti.In addition,
It is necessary to eliminate precipitation sites that induce the precipitation of TiH, and in the steel of the present invention, MnS becomes an effective precipitation site for TIN. Therefore, in order to prevent the precipitation of MnE+, S is no 0.
It needs to be 5% or less, preferably 0.002% or less.

In addition, lowering S improves cold workability, so K
is also valid. Next, regarding the condition (2), in order to maintain workability in the present invention, it is necessary to set the hot rolling finishing temperature to Ar3 point or higher. Precipitates that suppress crystal and grain growth are T.
iN becomes valid. Therefore, the lower limit of N amount is 0.0030%.
, preferably aoosos. If the amount of N is less than this, it is possible to strengthen the steel by precipitation of T1 carbonitride during winding, but the structure of the rolled material cannot be refined, leading to deterioration of workability and toughness. For the same reason, the lower limit of the Ti amount is also set to 0.005%.

Further, if the Ti amount is less than 1L005, precipitation strengthening during winding cannot be expected, and condition (3) cannot be satisfied.

The above-mentioned limited composition is the basic composition of the steel of the present invention, but Ou, Ni, Or, Nb, V, and Mo may be added as necessary.

By adding one or more types of Oa,
The object of the present invention is more effectively achieved.

The reasons for limiting these additive elements are as follows.

O is an element that increases strength without deteriorating low-temperature toughness, but if it exceeds 1.0%, red brittle defects will occur, so the upper limit is set at 1.0%.

N1 increases low temperature toughness and is effective as a reinforcing element. However, in the range of low temperature toughness required for hot rolled steel sheets, it is not necessary to add a large amount exceeding 1.0%, and the upper limit is 1.0%.
．． I will be in charge of 0.

Or is added to increase the strength, but if it exceeds 1.0 t, the low temperature toughness deteriorates, so the upper limit is t0%.

Nl) is effective for grain refinement and precipitation hardening due to retardation of austenite recrystallization, but if it exceeds 1.1%, the effect is saturated and workability deteriorates, so the upper limit is set by Q.
, 1%.

(2) is effective as a precipitation-strengthening element, but if rl exceeds 1 inch, workability deteriorates, so the upper limit is set at 0.1%.

Mo, like Cu, is an element that increases strength without deteriorating low-temperature toughness, but its effect is saturated when the temperature exceeds α5, so the upper limit is set at 0.5%.

Ca has the effect of controlling the shape of sulfides and has the effect of improving absorbed energy in the direction perpendicular to the rolling direction, but
Since internal defects occur frequently when α0 exceeds 150%, the upper limit is set to o, o o so o%.

(2) Rolling conditions In the present invention, the direct rolling method is adopted as mentioned above, but the so-called direct rolling method means that the hot slab obtained by continuous casting is directly rolled without passing through a heating furnace. The rolling method is not limited to the above, but also includes a method in which the slab is lightly heated so that the surface temperature is the same as the center temperature, and then rolled.

Needless to say, the heating time in this case is much shorter than in the conventional reheating method, and is within 90 minutes at most.

In the direct rolling method of the present invention, rolling is started without lowering the temperature of the continuously cast hot slab to a temperature lower than the Ar3 point, but once the temperature is lowered to the ArB point or lower, the added T1 This is because TiN precipitates as TiN, and even if the temperature is increased to the austenite region by reheating, TiN does not dissolve into solid solution again. Further, the rolling start temperature needs to be 1100° C. or lower in order to suppress the aforementioned austenite recrystallization and grain growth. In high-temperature rolling exceeding 1100°C, recrystallization and grain growth are rapid and the desired refinement of austenite cannot be achieved. This is used to start rolling. On the other hand, the finishing temperature is set to Ar3 or higher because if it is lower than that, ferrite will be processed and the workability will deteriorate. Furthermore, it is necessary to effectively refine the grains of austenite. For this purpose, the finishing temperature is Ar3~Ar3+50℃
is preferred, so in the present invention hot rolling is completed at a temperature of Ar3 or higher, and there are no particular restrictions on the coiling temperature, but in order to effectively utilize precipitation strengthening by Ti carbonitrides, it is preferably around 550°C. .

Next, examples of the present invention will be shown.

Example 1 A hot slab obtained by continuously casting a test steel having a composition shown in Table 2 below was directly rolled under the conditions shown in the same table to form a coil with a thickness of 55 mm. Table 3 shows the mechanical properties of the produced hot rolled sheets.

As is clear from Table 3, the method of the present invention produces 50 kgf/l.
A strength of 11 m2 or more and good workability and toughness can be obtained, but since the comparative steel E has a N content or both N and Ti content exceeding the upper limit of the present invention, coarse Ti is already formed at the start of rolling.
N is precipitated, so strengthening and properties have not been improved.

Since steel F has an N content below the lower limit of the present invention, although precipitation strengthening occurs during coiling, grain refinement due to precipitation during rolling is insufficient, and improvements in workability and toughness cannot be expected.

In Steel G, the Ti content is below the lower limit of the present invention, so the addition effect is not apparent. Further, since steel H has a high S content, induced precipitation of TiN by MnS occurs, and the object of the present invention cannot be achieved.

Example 2 Hot slabs obtained by continuous casting of steel having the composition shown in Table 4 below were rolled under various rolling conditions to produce coils with a thickness of 3.811+1. The mechanical properties of the manufactured hot rolled sheets are also listed in the same table. The basic composition of the steel of the present invention includes ci3 Ni, O
r, N'b, V, Mo.

Although the object of the present invention is achieved even if one or more types of Ca are added, the object is not achieved if even one of the rolling conditions deviates from the scope of the present invention.

As described above, according to the present invention
- It is possible to obtain hot-rolled high-strength steel sheets of 50 kgf/- or more by low-temperature rolling of high N steel, improving workability and toughness by improving the deterioration of properties, which is the biggest drawback in the direct rolling method, which has a large energy-saving effect. The range of application can be expanded to include hot-rolled high-strength steel sheets of 50 kgf/- or more, which require a high tensile strength of 50 kgf/-, so it is of great industrial value.

Claims (1)

[Claims] to: 0.04 to 0.18%, Sl: more than α05 to a80
%, Mn: 0.50-2.00%, S; <α005
Chi, T1: α005-0.045%. N: 0.0030~o, o1so%, ht
: A hot slab obtained by continuous casting of steel containing 1 to 0.08% o, o, and the balance consisting of iron and unavoidable impurities.
A method for producing a hot-rolled high-strength steel sheet, which comprises starting rolling at a temperature of 1100° C. or lower without lowering the temperature to a temperature lower than the r3 point, and finishing hot rolling at a temperature of the Ar3 point or higher. 2, O:Q, 04~α18%, Si: More than 105~0,8
0%, Mn: 0.50-2.00%, S:<α
005%, Ti: 0.005-0.045%. N: 0.0030~ao 1so%, ht: n
o1~[108%, further (!u: 1.0% or less, Ni: 1.0% or less, ar: tO% or less, Ml):
0.1% or less, V: 0.1% or less, Mo: 0.5
Below, a hot slab obtained by continuous casting of steel containing one or more of 10050% or less and the remainder iron and unavoidable impurities is cooled to a temperature lower than 3 points of Ar. A method for producing a hot-rolled high-strength steel sheet, characterized in that rolling is started at a temperature of 1100° C. or lower, and hot rolling is finished at a temperature of Ar3 or higher.