JPH03294419A - Production of high tensile strength steel plate having high fatigue limit from thin cast slab - Google Patents

Abstract

PURPOSE:To produce a high tensile strength steel plate having high fatigue limit at a low cost by directly and continuously casting a thin slab from a molten steel, applying heat treat ment to this thin cast slab under specific conditions, and carrying out hot rolling and coiling. CONSTITUTION:A molten steel having a composition consisting of, by weight, 0.02-0.4% C, 0.2-3% Mn, and 0.003-0.05% S is directly and continuously cast into a thin slab by a continuous casting method, and this thin cast slab is cooled from solidification down to the Ae3 transformation point at 0.5-30 deg.C/sec average cooling rate and further cooled from the Ae3 transformation point down to the higher temp. between 550 deg.C and a temp. T1 deg.C represented by an equation I or below at <=10 deg.C/sec cooling rate. Successively, the above thin cast slab is heated again up to a temp. in the region between the Ae3 transformation point and 1100 deg.C at >=5 deg.C/sec average temp. rise rate and hot-rolled in the austenite region at 20% reduction of area, and the resulting plate is cooled down to <=650 deg.C at >=20 deg.C/sec average cooling rate and coiled. By this method, the formation of lamellar pearlite can be inhibited and ferrite structure can be refined, and the high tensile strength steel plate having high fatigue limit can be produced at a low cost.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention enables casting of a thin cast strip directly from molten steel, and then simplifies the hot rolling process, yet has a lower fatigue limit than conventional manufacturing methods. The present invention relates to a method for manufacturing high-strength steel sheets with high strength.

(Prior Art) Steel materials that are subject to dynamic loads, such as automobile suspension parts, have important mechanical properties such as fatigue strength. Generally, to increase fatigue strength, it is sufficient to increase tensile strength, but as strength increases, ductility deteriorates, so for parts that require workability, it is recommended to increase fatigue strength simply by increasing tensile strength. Can not.

Therefore, we can increase only the fatigue strength without increasing the tensile strength.
That is, as a method of increasing the fatigue limit (fatigue strength/tensile strength), a composite structure steel sheet having a composite structure of ferrite and martensite is manufactured. Composite structure steel sheets certainly have a higher fatigue limit than ordinary ferrite-pearlite steels, but they have the disadvantage of being complicated in manufacturing method and high in cost due to their composition.

On the other hand, the present inventors have clarified through experiments using various heating, hot rolling, and cooling conditions that the fatigue limit of ferrite-pearlite steel is closely related to the formation state of layered pearlite. It was found that the less pearlite structure formed, the higher the fatigue limit. It was revealed that it is effective to make the austenite into fine grains to obtain such a structure, but in the conventional manufacturing method of hot-rolled sheet, it is difficult to completely eliminate layered pearlite from high-strength steel with a ferrite-pearlite structure. It is not possible.

Incidentally, in recent years, in order to significantly shorten the thin film manufacturing process, attempts have been made to simplify hot rolling by directly casting a thin cast strip from molten steel as a new thin plate manufacturing process.

However, since the slab is thin, this method inevitably results in a small hot rolling reduction, which makes it impossible to sufficiently refine the austenite grains, resulting in coarse ferrite grains after γ/α transformation, resulting in poor strength-ductility balance and fatigue. The reality is that these materials are inferior to conventional hot-rolled steel sheets with the same composition.

(i!!l!i to be solved by the present invention) The problem to be solved by the present invention is to suppress the formation of layered pearlite that deteriorates the fatigue limit, and to minimize the production method to make the ferrite structure as fine as possible. The aim is to realize this at low cost.

(Means for Solving the Problems) As a result of intensive study in view of the above-mentioned circumstances, the inventors of the present invention found that after casting into a thin strip, it is cooled to a predetermined temperature at an appropriate cooling rate as shown in Fig. 1. After the γ/α transformation is carried out, it is heated again to cause the α/γ transformation, and then the shape and surface properties of the slab are improved by rolling, and the austenite is refined, followed by proper cooling. We have discovered that by doing so, we can suppress the formation of layered pearlite and refine the ferrite structure. It has also been revealed that the steel sheet obtained in this manner exhibits a fatigue limit superior to that of conventional hot-rolled steel sheets of the same composition.

The present invention is constructed as follows.

C: 0.02wtk~0.4wt9g, Mn:
0.2 wt zero to 3 wt%; S: 0.003
After casting molten steel containing wt4 to 0.05wt96 by continuous casting, the average cooling rate from solidification to Ae3 transformation point is 0.5
℃/s or more and 30℃/s or less, average cooling rate from the Ae3 transformation point: 10℃/s or more to 550℃ or (
1) Cool to below whichever higher temperature T1 satisfies the formula, and then heat again at an average heating rate of 5°C/s or more to a temperature range of Ae, transformation point or above and 1100°C or below, A method for manufacturing a high tensile strength steel sheet with a high fatigue limit, which comprises rolling 20% or more in an austenite region and then cooling again to a coiling temperature of 650°C or less at an average cooling rate of 20°C/s or more.

T 1 ('e) = Ae3 1000x (Cwt
%i) (1) First, the reasons for limiting the chemical components in the present invention will be described.

C is an important element that determines the strength of steel, and is also the most important element that determines the structural morphology in the transformation from austenite to ferrite. The lower limit of C amount w
The reason why t96 is set to 0.02% is that pearlite is hardly produced when the C content is less than this, and basically there is no problem of fatigue limit deterioration due to the formation of layered pearlite. Further, the reason why the upper limit is set to 04% is that if more C is added, the pearlite ratio increases and becomes close to an all-over pearlite structure, and no layered pearlite structure is generated.

The lower limit of the Mn content is set to 0.2% because if the Mn content is less than this, sufficient grain refinement of ferrite cannot be achieved, and if the rolling reduction rate is small, the structure becomes inhomogeneous and the fatigue limit does not become high. . A similar phenomenon is observed when the amount of S is Q,003% or less. Therefore, the lower limit of the amount of S is set to 0.00
It was set at 3%.

Although the cause of this is beyond speculation, it is thought to be related to the precipitation of MnS in austenite, which will be described later. Also,
The upper limit of the Mn content is set at 3.0 to keep the cost of composition control in the steelmaking process low and to prevent deterioration of workability.
% or less. The upper limit of the amount of S is limited to 0.05% in order to prevent hot cracking and maintain workability.

In the present invention, components other than those mentioned above should be kept low in principle, but on the other hand, appropriate amounts of Si, P,
Ti, V, Nb, Cr, Ni, Mo, B, etc. may be added, and in particular, St and P are elements that increase strength and suppress the formation of layered pearlite, so they do not cause deterioration in workability51: Addition of P: 0.15% or less does not impair the spirit of the present invention. Furthermore, Ti, V, and the like may be actively added as elements that promote intragranular ferrite transformation, but the present invention is not limited by this.

Next, the manufacturing method will be described.

In the present invention, after casting steel with the above-mentioned components, Ae, average 6 speeds up to the transformation point are 0.5 t/s or more, 3
The reason why the cooling rate is set to 0° C./s or less is that refinement of the final structure and equalization cannot be achieved outside this cooling rate range. The reason for this is not fully clear, but the Mn depletion zone that occurs near the precipitates due to the precipitation of MnS in austenite promotes the formation of ferrite during γ/α transformation, and the austenite structure remains as cast. This is thought to be because, despite the coarse structure, many ferrite grains are generated near the MnS inside the grains, resulting in a fine ferrite structure.

Next, we limited the average cooling rate to 10°C/s or more when cooling from the Aes transformation point to 550°C or the temperature T1 that satisfies equation (1), whichever is higher, because in 6 speeds below this, This is because the ferrite structure does not become fine. Also, 550℃ or the temperature T1 that satisfies equation (1)
The reason for the limitation of cooling to the higher temperature is that unless it is cooled to a temperature lower than this, a fine ferrite structure cannot be obtained even if it is reheated and cooled to a temperature range of Ae 3 or higher, as described later. be. Further, although the schematic diagram in FIG. 1 shows that reheating is performed immediately after cooling to the above temperature condition, the present invention does not affect the present invention even if it is heated again after being maintained in this temperature range. It is not subject to any restrictions.

Next, when austenitizing again, the temperature increase rate was increased to 5
The reason why the heating rate must be at least .degree. C./s is that if the heating rate is slow, the austenite produced will become coarse. Furthermore, for the same reason, it is desirable that the heating temperature and the retention period at that temperature be as low and short as possible within the range where the structure is completely austenitized, and the upper limit of the heating temperature was limited to 1100°C.

The reason why the rolling reduction in the austenite region is limited to 20% or more is because if the rolling reduction is lower than this, the austenite structure cannot be sufficiently refined and a fine ferrite structure cannot be obtained after the γ/α transformation.

However, although it is not a necessary condition of the present invention to perform rolling even in the austenite region before the -th γ/α transformation after casting, such rolling contributes to refinement of the final structure, so such rolling is not the gist of the present invention. It does not cause any loss.

Finally, the reason why we limited the final cooling conditions to an average cooling rate of 20"C/s or more to a coiling temperature of 650℃ or less is because if the average cooling rate is 20"C/s or less, the ferrite structure will become coarse. In addition, the reason why the winding temperature is set to 650° C. or lower is that if winding is carried out at a higher temperature than this, scale formation will be significant and the pickling cost will increase.

(Example) Example 1 A case (1) in which a material having the chemical components shown in Table 1 was rolled under the normal hot rolling conditions shown in Table 2 and an exemplary manufacturing condition (II) of the present invention shown in Table 3. ) compared the mechanical properties of steel sheets manufactured by The results are shown in Table 4. As is clear from these results, the steel of the present invention exhibits a higher fatigue limit than the conventional hot-rolled material, regardless of the steel type.

Example 2 Table 5 shows the mechanical properties of the steel plate when the manufacturing conditions were changed using material A in Table 1. As is clear from these results, the steel of the present invention exhibits an excellent fatigue limit.

(Effects of the Invention) According to the present invention, by using thin slabs and simplifying hot rolling, it is possible not only to achieve a significant reduction in manufacturing costs, but also to achieve better fatigue than conventional high tensile strength hot rolled steel sheets. The characteristics can be manufactured and its industrial significance is great.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the thermal history after casting. 4 others

Claims (1)

[Claims] 1 After casting molten steel containing C: 0.02wt% to 0.4wt%, Mn: 0.2wt% to 3wt%, and S: 0.003wt% to 0.05wt% by continuous casting. , Average cooling rate from solidification to Ae_3 transformation point 0.5℃/s or more, 30℃/s
Below, further average cooling rate from Ae_3 transformation point: 10
Temperature T that satisfies equation (1) or 550℃ at ℃/s or more
1, and then heated again at an average temperature increase rate of 5°C/s or higher to a temperature range of Ae_3 transformation point or higher and 1100°C or lower, resulting in a temperature of 20% or more in the austenite region. After rolling, the average cooling rate is 20℃/
A method for producing a high tensile strength steel sheet with a high fatigue limit, which comprises cooling again to a coiling temperature of 650° C. or lower at a temperature of 650° C. or higher. T1 (°C) = Ae_3-1000×(Cwt%) (1)