JP3323737B2 - Method for producing high-strength hot-rolled steel sheet having ultrafine structure - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot-rolled steel sheet, and more particularly, to a hot-rolled steel sheet having an ultrafine structure as hot-rolled and having excellent ductility, toughness, fatigue strength, and impact resistance, and in particular, The present invention relates to a method for producing a high-strength hot-rolled steel sheet for automotive materials, structural materials, and pipe materials, which exhibits superiority to the above materials in a high strength range of a tensile strength of 450 MPa or more.

[0002]

2. Description of the Related Art As a means for enhancing the mechanical properties of steel materials used for automobile materials, structural materials, pipe materials, etc., it is considered effective to make the crystal structure (hereinafter, structure) of the steel material ultra-fine. Many techniques for obtaining a fine structure have been sought before. Particularly, in recent years, high-tensile steel sheets have been widely used in order to reduce the thickness and reduce cost, and high-tensile steel sheets have been used for the purpose of suppressing deterioration in workability, toughness, etc. due to the high tensile strength. The miniaturization of the structure in has been an important issue. In addition, this micronization of the structure is particularly achieved by the stretch flangeability (e.g., evaluated by the hole expandability with a conical punch).
Is considered to be an effective means of improving

[0003] As a method of refining the structure, a controlled rolling method, a controlled cooling method, a large rolling reduction method and the like have been conventionally known. Among them, a method widely used as a method for simultaneously achieving high tensile strength and microstructure refinement is a method in which Nb or Ti is included in a material and controlled rolling is performed, and the obtained steel sheet is a so-called precipitation strengthened type. It was a high-tensile hot-rolled steel sheet. The reason that this method has been widely used is that the steel material can be easily increased in tension by the precipitation strengthening action of the contained Nb or Ti, and the recrystallization suppressing action of austenite grains of Nb and Ti gives the steel sheet a An object of the present invention is to promote the strain-induced transformation from γ to α when low-temperature rolling is performed, and to obtain an effect of refining ferrite grains. However, a drawback of the high-strength steel sheet manufactured by the above method is that the mechanical properties are highly anisotropic. For example, in the case of an automotive steel sheet for press forming, the forming limit is the most ductile or stretch flangeable. Although determined by the level in the inferior direction, it is difficult to ensure high press formability because the anisotropy is too large. Further, regarding toughness and fatigue strength which are important for a structural material or a pipe material, the difficulty that this anisotropy is large leads to the same problem.

On the other hand, as a method of refining the structure by the above-mentioned large rolling reduction method, there are proposals represented by, for example, JP-A-58-123823 and JP-A-59-229413. The point of ultra-fine refining in these methods is to promote strain-induced transformation from γ to α by applying a large pressure to austenite grains, which is basically the same as the case of the precipitation-strengthened steel containing Nb and Ti. Typically, they use the same mechanism. However, the difference between the two is that the controlled rolling of precipitation-strengthened steel utilizes the effect of suppressing the recrystallization of austenite grains of Nb and Ti, whereas the large rolling reduction method uses Nb and Ti.
The point is that the crystal grains can be refined without containing b and Ti. For this reason, the steel material obtained by the large rolling reduction method has an advantage that the anisotropy of the mechanical properties is improved as compared with the precipitation strengthened steel, but the rolling reduction per pass is 40%.
The most difficult point is to set the rolling conditions that are difficult to perform with a general hot strip mill, such as the need to make the above.

Further, as a material satisfying both the strength and workability of a high-strength steel sheet, the so-called TR of retained austenite is considered.
IP Effect (Transformation Induc)
A steel sheet utilizing edPlasticyry (transformation-induced plasticity) has recently been proposed. For example, JP-A-60-43
No. 425 discloses that a steel sheet is heated at 450 to 650 ° C. after hot rolling.
The method discloses a method for producing a steel sheet having retained austenite by holding the steel sheet at a temperature of 4 to 20 seconds and then winding it at 350 ° C. or lower. However, in this method, the above-described special cooling control is required, and therefore, there is a problem that a stable and uniform material cannot be obtained.
Further, although the ductility is remarkably improved, there is a disadvantage that the stretch flangeability is inferior.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has been made in consideration of a general hot strip strip.
It can be easily carried out using a mill, and the anisotropy of the mechanical properties is small, and the refinement of the ferrite grain size can be further enhanced as compared with the conventional technology. It is an object of the present invention to provide a method of manufacturing a high-tensile hot-rolled steel sheet which can significantly improve fatigue strength, particularly stretch flangeability.

[0007]

Means for resolving the problem The conventional means for reducing the grain size of the ferrite crystal uses the strain-induced transformation from γ to α in any of the methods as described above.
The present inventor thought that the above problems could not be avoided as long as the strain-induced transformation was used, and sought a new method for refining crystal grains. As a result, they have found the following new means, and have accomplished the present invention.

First, it has long been known that austenite grains undergo a rolling-recrystallization process in order to make them finer in hot rolling. However, in such miniaturization by recrystallization, the ferrite crystal grain size that can be reached is at most 2
The limit is 0 μm. On the other hand, according to the above-mentioned controlled rolling method or large rolling reduction method, fine grains of about 10 μm can be obtained. It has been considered unsuitable as a method for obtaining a microstructure. Therefore, the present inventors conducted intensive research, and before the start of hot rolling, that is, at the time of heating the slab,
When rolling is performed after extremely reducing the austenite grain size of the material, the subsequent rolling and recrystallization will occur at an extremely accelerated rate, and the recrystallization grains after rolling will greatly advance. Was discovered. Although the reason for this miniaturization is not always clear, it is considered to be due to the mechanism described below.

[0009] Recrystallization of austenite grains by rolling includes dynamic recrystallization and static recrystallization. The former involves a high rolling temperature, a low strain rate, and when a large reduction is applied.
That is, in the case of rolling in a hot strip mill, it is a recrystallization that can occur only under rolling conditions corresponding to the initial-middle stage of rough rolling, in which the rolling temperature is reduced and the strain rate is reduced. It has been thought that in the finish rolling stage, which is faster, this dynamic recrystallization becomes difficult to occur, and instead, static recrystallization will occur.

[0010] However, dynamic recrystallization is a phenomenon in which strain is released by moving grain boundaries or forming new grain boundaries at an extremely high speed, and the refinement of crystal grains facilitates the movement of grain boundaries. Therefore, it acts in a direction that facilitates dynamic recrystallization. Furthermore, since nucleation of dynamic recrystallization occurs at the old grain boundary, the finer the old crystal grain, the more dynamic crystallization
The frequency of nucleation increases, and the refinement of new crystal grains after recrystallization proceeds.

Thus, the inventor of the present invention has found that if the initial austenite grain size is extremely reduced, dynamic recrystallization occurs in a lower temperature range, a higher strain rate range, and a lower strain range. It has been speculated that dynamic recrystallization also occurs in the finish rolling stage, which could not be generated by hot rolling, and that refinement by dynamic recrystallization proceeds with an increase in the rolling reduction, and thus completed the present invention.

That is, in the present invention, C: 0.05-0.
10 wt%, SiO _2: 0.30~2.0 weight%, M
n: 1.0% by weight or less, Al: 0.003 to 0.100
Wt%, Ti: 0.05 to 0.30 contained by weight%, Ri Nana than the remainder Fe and unavoidable impurities, the initial austenite
The continuous casting slab having extremely reduced grain size is heated to a temperature of 950 ° C. or more and 1100 ° C. or less, and then subjected to at least two or more reductions in which a reduction rate of 20% or more is performed at one time. Is hot-rolled so as to be not less than the Ar ₃ transformation point, and then cooled at a cooling rate of 20 ° C./sec or more.
This is a method for producing a high-tensile hot-rolled steel sheet having an ultrafine structure, characterized in that it is wound in a temperature range of 0 ° C. to 550 ° C.

[0013] The present invention is also a method for producing a high-tensile hot-rolled steel sheet having an ultrafine structure, characterized by containing twice the amount of Nb in place of all or part of the Ti. In the present invention, the continuous cast slab having the specified composition is hot-rolled and cooled under the above conditions, so that it can be easily performed by a general hot strip mill, and has anisotropic mechanical properties. In addition, it is possible to produce a high-strength hot-rolled steel sheet with much less ferrite grain size than in the prior art, and with remarkably high ductility, toughness, and fatigue strength, particularly excellent stretch flangeability.

It is needless to say that the most important point in the ultra-fine structure mechanism is to extremely refine the initial austenite grains in the slab heating step. In the present invention, this was achieved by allowing a large amount of TiC to be present. However, even in the above-mentioned precipitation strengthened steel, Ti
Is used as a hyperfine element. However, the effect of Ti in the present invention is clearly different from that in the precipitation strengthened steel. That is, Ti in Ti-based precipitation strengthened steel
In the slab heating step, austenite grains are solutionized, a refining effect is exhibited by utilizing the recrystallization suppression effect as solid solution Ti, and, at the same time, ferrite grains after transformation are reprecipitated as fine TiC. Work to bring out precipitation strengthening. In Nb-based precipitation strengthened steel, Nb plays this role. Therefore, in the above-mentioned conventional precipitation strengthened steel, TiC or N
It is necessary to dissolve bC, and a relatively high heating temperature is an essential requirement. On the other hand, in the present invention, it is an important element that Ti is not dissolved in austenite in the slab heating step but is present as TiC. The first reason is that solid-solution Ti inhibits recrystallization and makes it difficult to cause dynamic recrystallization, which is a miniaturization process of the present invention, and the second is TiC which suppresses the growth of initial austenite. The amount is reduced by its dissolution. Therefore, in the present invention, the slab heating temperature is set to T
It is a necessary requirement to set the temperature in a low temperature range where iC does not dissolve.

[0015] In addition, the inventor of the present invention has found that, when the amounts of C and Mn added to the raw material are restricted, the formation of a structure that inhibits stretch flangeability such as a pearlite phase, a bainite phase and a martensite phase is suppressed, and In addition to newly finding that a structure composed of a fine ferrite phase and a retained austenite phase is formed, such a structure includes a ferrite having a total volume ratio of pearlite, bainite and martensite of 5% or less and an average grain size of 10 μm, It contains 5 to 20% of retained austenite, confirming that the ductility and stretch flangeability are excellent.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the present invention are as described in the preceding section. The reasons for limiting the chemical composition of the material and the conditions for limiting the hot rolling and cooling conditions will be described below. C: First, C is required to be 0.05% by weight or more in order to generate necessary austenite to obtain a necessary strength and to secure a sufficient amount of TiC in a heating step which is important for refining the structure. It is. However, when the amount exceeds 0.10% by weight, the ratio of the second phase such as pearlite, bainite, martensite, etc. increases, and the stretch flangeability deteriorates.
Therefore, C is set in the range of 0.05 to 0.10% by weight.

Si: Si is an effective element for improving strength while improving the strength-elongation balance by solid solution strengthening, and is effective for promoting ferrite transformation to obtain the structure aimed at by the present invention. It is necessary to add 0.3% by weight or more in order to exert its effect.
On the other hand, the addition of a large amount of SiO ₂ tends to produce scales with poor descaling properties during hot rolling, which tends to adversely affect the surface properties of the product. In the present invention, since the heating temperature is set to a low temperature range for ultrafine structure, the upper limit of the amount of Si in which the surface texture is reduced can be higher than that of ordinary steel, but it exceeds 2.0%. And adverse effects become apparent, so 2.0
% By weight.

Mn: Mn is an element effective for improving the strength. However, if it exceeds 1.0% by weight, the transformation of ferrite is remarkably delayed, and a transformation phase of pearlite, bainite and martensite is easily formed, The stretch flangeability deteriorates. Therefore, Mn was set to 1.0% by weight or less. Al: Al is an element that is extremely effective for deoxidation in the molten steel stage, but its effect cannot be obtained at 0.01% by weight or less.
On the other hand, if it exceeds 0.10% by weight, crystal grains become coarse and internal defects due to inclusions are caused. Therefore, Al was set in the range of 0.003 to 0.10% by weight.

Ti: Ti is, as mentioned earlier, Ti
C is an element essential for refining the initial austenite grains in the slab heating step and causing dynamic recrystallization in the subsequent rolling process. In order to exhibit this effect, at least 0.05% by weight or more is required. The effect of miniaturization increases with an increase in TiC, but becomes saturated when it exceeds 0.3% by weight. Therefore, T
i was in the range of 0.05 to 0.30% by weight.

The above-mentioned limited composition is used as the basic composition of the steel material used in the production method according to the present invention. Nb is an element having the same action as that of Ti. Good results can also be obtained by substituting this with Ti or by adding it in an overlapping manner with the addition of Ti. The effect of Nb is almost equivalent to the effect of Ti when viewed at an atomic ratio equivalent to C. Therefore, the addition of Nb is required to be approximately twice as much as that of Ti in terms of weight ratio, and there is no economic merit. However, when used for applications such as ERW welding or flash butt welding, residual oxide at the weld joint interface may be problematic. Nb may be added for such uses and purposes.

Next, the hot rolling conditions in the present invention will be described. In the present invention, first, it is an important technical point that the effect of refining the initial austenite grains by TiC is maximized, and the lower limit of the heating temperature is limited to 950 ° C. and the upper limit is limited to 1100 ° C. I do. If the lower limit temperature is lower than 950 ° C., it is difficult to exceed the finish rolling in the austenite region, and it is difficult to secure the desired microstructure and mechanical properties. When the upper limit temperature exceeds 1100 ° C., the amount of TiC dissolved in austenite increases, the effect of refining austenite grains is lost, and dynamic recrystallization during rolling is unlikely to occur due to the increase in solid solution Ti. This is because the desired high ferrite volume ratio, ultrafine ferrite grains, and retained austenite structure are difficult to obtain, and only mechanical properties comparable to those of the prior art can be obtained .

Next, in the present invention, the aim is to repeatedly generate dynamic recrystallization of austenite grains by rolling to make the austenite grains finer. The initial conditions for generating dynamic recrystallization are as follows: Since the requirements are compensated by satisfying the requirements, the rolling reduction requirements are such that the rolling reduction per operation is an important factor. That is, if the rolling reduction is less than 20%, the desired reduction by dynamic recrystallization does not occur, so the lower limit of the rolling reduction for each rolling stand is set to 20%. The upper limit does not need to be particularly limited from the viewpoint of the refining effect, but in reality, there is a limit due to the rolling capacity of the rolling mill, and it depends on the rolling temperature, the chemical composition of the steel, the rolling dimensions, and the like. In general, a rolling mill capable of applying the above-described reduction is preferable, and therefore, the present invention is preferably carried out within the range.

Further, in the present invention, the austenite grains are refined to substantially equiaxed grains at the stage of finishing rolling, and fine ferrite grains begin to be formed if the γ → α transformation is performed as it is. However, at a cooling rate of 20 ° C./sec or less, the ferrite transformation proceeds due to the growth of ferrite grains generated in a high temperature region, and the ferrite transformation accelerating effect and the miniaturization effect are reduced. I do. In addition, from the viewpoint of metallurgy, the upper limit of the cooling rate is not specified, but is preferably 100 ° C./sec or less in order to keep the flatness of the steel sheet good. When the winding temperature exceeds 550 ° C,
The generation of residual austenite is small and the effect of improving ductility is lost, and it is not possible to achieve high tensile strength.In addition, the self-annealing effect after winding is increased, and the ferrite grains that have become ultrafine are grown. Cause undesired results. In addition, when the winding temperature is lower than 350 ° C., a martensite phase is formed, and the amount of retained austenite decreases. Furthermore, the range of the winding temperature is limited to 350 ° C. to 550 ° C. because the flatness of the steel sheet is deteriorated . Hereinafter, examples of the present invention will be described.

[0024]

EXAMPLE A steel material having the chemical composition shown in Table 1 was melted.
A hot-rolled steel sheet having a thickness of 3.2 mm was manufactured under the hot rolling conditions shown in Table 1. Then, samples were taken from these hot-rolled steel sheets, and the volume ratio of polygonal (polygonal) ferrite and the crystal grain size of the ferrite were measured, as well as the tensile properties, the hole expansion ratio with a conical punch (stretch flangeability), Fatigue properties by a swing plane bending test method were examined for a ductile-brittle transition temperature (vTrs) using a 2 mm V-notch Charpy test piece.
Table 3 shows these results collectively.

As is clear from the results of these Examples and Comparative Examples, the steel sheet obtained by applying the production method according to the present invention has excellent strength-elongation balance, stretch flangeability,
It can be seen that toughness and fatigue strength are also excellent.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

[Table 3]

[0029]

As described above, according to the present invention, an ultrafine ferrite structure containing retained austenite can be easily obtained by using a general hot strip mill, and ductility, stretch flangeability, fatigue strength, Production of high-tensile steel sheets with excellent toughness has become possible. In addition, workability and yield in manufacturing a product using this steel sheet have been significantly improved.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between (TS × EL) value, hole expansion ratio (λ), (vTrs) value, and polygonal ferrite volume ratio of steel sheets manufactured by a manufacturing method according to the present invention and a comparative example. is there.

FIG. 2 is a diagram showing a relationship between (TS × EL) values, hole expansion ratios (λ), (vTrs) values, and retained austenite ratios of steel sheets manufactured by a manufacturing method according to the present invention and a comparative example.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C21D 9/46-9/48 C21D 8/00-8/04 C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. C: 0.05 to 0.10% by weight, Si: 0.30 to 2.0% by weight, Mn: 1.0% by weight or less, Al: 0.003 to 0.100% by weight, Ti: 0.05 to 0.30 contained by weight%, Ri name than the remainder Fe and unavoidable impurities, the initial O
After heating a continuously cast slab having extremely fine austenite grain size to a temperature of 950 ° C. or more and 1100 ° C. or less,
The rolling reduction at which the rolling reduction per turn becomes 20% or more is performed at least twice or more, and hot rolling is performed so that the finish rolling temperature becomes the Ar ₃ transformation point or more, and then cooled at a cooling rate of 20 ° C./sec or more, A method for producing a high-tensile strength hot-rolled steel sheet having an ultrafine structure, wherein the method is performed in a temperature range of 350 ° C to 550 ° C. 2. In place of all or part of the Ti,
2. The method for producing a high-tensile hot-rolled steel sheet having an ultrafine structure according to claim 1, wherein the steel sheet contains twice the amount of Nb.