JPH0772298B2 - Method for manufacturing hot rolled high strength steel sheet with excellent workability - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a hot-rolled high-strength steel sheet having a uniform ultrafine structure and excellent workability.

<Conventional technology and its problems> Generally, it is said that various properties such as strength and ductility of steel materials improve as the structure becomes finer, and development of a technology for further refining the steel material structure has hitherto been hardened. .

The results of these studies, which have been continued for many years, are as follows: a) controlled rolling, b) controlled rolling and accelerated cooling, c) large reduction rolling (for example, JP-A-62-253733 and JP-A-63).
-145720, etc.) and other new microstructural refinement technologies have been created.

However, the following problems have been pointed out in each of these technologies.

In other words, in the controlled rolling technique, the "austenite (γ) grains produced by hot working called controlled rolling" cannot be further refined when they become fine to a certain degree. Then, it is difficult to even obtain a uniform fine structure having a ferrite (α) grain size of about 10 μm.

Even with the technique of combining accelerated cooling with the above-mentioned controlled rolling, sufficient γ can be obtained by the controlled rolling as described above.
Since the refinement of the microstructure is not achieved, there is a limit to forcibly producing the transformation of fine α by the accelerated cooling after that, and therefore, it is possible to obtain a uniform microstructure so that the α grain size is less than 10 μm. Was extremely difficult. Furthermore, the α particle size is 5 μm or less (Grain Size N
It was impossible to obtain a uniform ultrafine structure of 12 or more in o.

On the other hand, the structure refining technology by large reduction rolling applies a large reduction with a reduction rate of 30% / pass or more in the γ non-recrystallization temperature range to make γ grains “work hardening γ containing deformation zones within the grains”, and γ → α
This method aims to make the structure finer by causing transformation, but in this method, the γ grains before the γ → α transformation are merely elongated by large-pressure rolling and are not isotropic fine grains. Therefore, there is a limit to the refinement of the structure, and therefore, it was not possible to realize a uniform ultrafine structure in which the α-grain size after transformation is less than 5 μm.

In view of the above, the object of the present invention is to provide an industrial means capable of stably producing a "ultrafine and isotropic uniform structure" on a steel sheet, which was difficult to achieve by the conventional method. It was therefore to provide a method by which a "hot-rolled high-strength steel sheet exhibiting excellent workability" can be produced with high efficiency without resorting to the addition of a special and expensive alloying element.

<Means for Solving the Problems> The inventors of the present invention have conducted extensive studies from various viewpoints in order to achieve the above-mentioned object, and as a result, “material steel of hot-rolled steel sheet (continuous cast slab or ingot etc.) As a material having a specific composition as the material, the temperature is adjusted during hot rolling, and a structure containing ferrite is exposed in advance, and the structure is rolled at a predetermined reduction ratio and then rapidly heated. When the above ferrite is transformed back to austenite, or when the austenite grain size of the above material steel is 200 μm or more, the material steel before the appearance of the ferrite structure is once subjected to a predetermined rolling reduction in the austenite recrystallization temperature range. Rolling of
Then, when the ferrite is reversely transformed into austenite by performing the work heat treatment in the above step, the austenite structure that appears becomes an ultrafine structure that cannot be obtained by conventional controlled rolling or the like. Therefore, when the ultrafine austenite structure is further subjected to rolling and then cooled, the transformation-generated ferrite is also extremely fine because it is based on the ultrafine austenite structure. It is possible to stably obtain a steel sheet having an isotropic uniform ultrafine structure having a grain size far smaller than 10 μm. Moreover, this ultrafine structure steel sheet shows a value far superior to the conventional steel sheets in terms of characteristics such as strength and ductility.
It is possible to obtain a very desirable hot rolled steel tensile steel sheet for processing. "

The present invention has been made based on the above findings and the like, and "C: 0.03 to 0.25% (hereinafter,% representing a component ratio is% by weight).
, Si: 0.01 to 2.00%, Mn: 0.40 to 2.00%, Al: 0.01 to 0.10%, and Nb: 0.01 to 0.10%, V: 0.01 to 0.10%, Ti: 0.01 to 0.10% , Ca: 0.01% or less, continuously cast slabs (slabs, blooms, billets) or ingots (hereinafter collectively referred to as “steel slabs”) that contain at least one of Fe and unavoidable impurities. As shown in Fig. 1, it is cooled from the hot piece state as it is, or as shown in Fig. 2, the hot piece is left as it is or charged into a heating furnace and then once in the recrystallization temperature range, the total reduction rate is 30% or more. And then cooled, and then the same as in FIGS. 1 and 2 above.
As shown in the figure, (a) rolling with a total reduction of 30% or more is performed in a temperature range of ₃ points or less of Ar, (b) then 10 points in a temperature range of Ac ₃ point to [Ac ₃ point + 100 ° C].
The temperature is raised at a heating rate of ℃ / sec or more to cause the reverse transformation of ferrite to austenite, (c) and the rolling is performed at a total reduction of 10% or more in the austenite phase temperature range. Then, by cooling, it is possible to efficiently and stably produce a hot-rolled high-tensile steel sheet having a uniform ultrafine structure and excellent workability ”.

Next, the reason why the manufacturing conditions (the chemical composition of the raw steel, the processing conditions, etc.) of the hot-rolled high-strength steel sheet according to the present invention are limited as described above will be described in detail together with its action.

<Operation> A) Chemical composition of the raw material steel When the C content is less than 0.03%, the tendency of α and γ grain growth becomes remarkable, and even if the reverse transformation occurs and the grains become finer, the grains immediately become coarse. It causes the inconvenience of ending up. On the other hand, when C is contained in excess of 0.25%, the weldability is deteriorated. Therefore, the C content is set to 0.03 to 0.25%.

Si Solid solution Si greatly contributes to strength increase and ductility improvement, and residual austenite increases due to addition of Si. From this point as well, it can be said that it is a preferable element for ductility improvement. And this effect is low temperature winding by hot rolling (about 400 ℃ or less)
Is especially preferred when doing. However, the Si content is
If it is less than 0.01%, the desired effect due to the above-mentioned action cannot be secured, while if it exceeds 2.00%, ductility is lowered and weldability is deteriorated. Therefore, the Si content is 0.01 to 2.00%.
I decided.

Mn Mn is a preferable element that has the effect of improving the hot workability of steel as well as the effect of increasing the strength by improving the hardenability, but if it is contained in excess of 2.00%, it will cause melting and cost. Will bring disadvantages in. On the other hand, when the Mn content is less than 0.40%, the growth of α-grains becomes remarkable, which adversely affects the production of the ultrafine network. Therefore, the Mn content is
It was set at 0.40 to 2.00%.

Al Al not only acts as a good deoxidizer for steel, but also has the effect of improving the strength of steel by forming carbonitrides,
If the content is less than 0.01%, the desired effect due to the above action cannot be obtained. Since the carbonitrides are formed in ferrite (α) in principle, if a small amount is added, Al
Does not significantly affect the formation of γ to α by processing, but if added in excess of 0.10%, it remains in the form of coarse precipitates in the steel, causing specific deterioration. Therefore, the Al content is set to 0.01 to 0.10%.

Nb, V, Ti, and Ca Since these elements are effective for strengthening steel or improving workability, they are components that require one or more additions.
The reason for limiting the content numerically is as follows.

a) Nb, V and Ti All have the action of forming carbonitrides to improve the strength of the steel, but if the content is less than 0.01% the desired effect due to the above action cannot be obtained. On the other hand, if the content exceeds 0.10%, the α ₃ transformation is delayed and the Ar ₃ transformation point becomes low, so that the processing stress at the time of secondary rolling becomes high and the rolling becomes difficult. Therefore, when Nb, V or Ti is contained, the content of each is set to 0.01 to 0.10%.

b) Ca Ca has the effect of changing the shape of inclusions after rolling (spheroidizing) and improving the workability of steel. However, if the content exceeds 0.01%, inclusions increase and reverse The characteristics will be impaired. Therefore, when Ca is included, its content should be
It was set to 0.01% or less.

B) The reason why the steel billet is cooled from the hot billet and rolled at a total reduction of 30% or more in a temperature range of Ar ₃ points or less is the reason why the hot billet is once cooled to the Ar ₃ point or less and then rolled. This is because the method of the invention has the main requirement of "reverse transformation of the ferrite phase into the austenite phase after plastic working of the structure containing ferrite", and for that purpose it is necessary to generate the ferrite phase. is there. The cooling temperature at this time is not particularly limited as long as it is ₃ Ar points or less, but from the viewpoint of practical operability,
It can be said that it is preferable to make the temperature region as high as possible near the Ar ₃ point or less {Ar ₃ point to (Ar ₃ point −100 ° C.)}. However, when the ferrite phase is reversely transformed into the austenite phase after the plastic working is applied to the structure containing ferrite, the γ grains after the reverse transformation become finer as the volume ratio of ferrite (α) during the plastic working increases. From the viewpoint of product performance, therefore, it is desirable that the cooling temperature is set to Ar ₁ point or lower in order to increase the volume fraction of ferrite.

And the total rolling reduction of the rolling process performed in the temperature range of Ar ₃ points or less is set to 30% or more because the rolling reduction at this time is 30% in total.
This is because the stable formation of fine γ grains due to reverse transformation can be achieved only when the above conditions are met.

That is, when the rolling process is performed in a temperature range of Ar ₃ point or lower, the ferrite (α) is work hardened by the rolling and the reverse transformation nuclei to austenite (γ) increase, and the number of the reverse transformation nuclei is extremely high. If there is a large amount, extremely fine γ grains are generated by the subsequent rapid temperature rise to the austenite region. However, the number of reverse transformation nuclei shows a remarkable rapid increase tendency only when the reduction rate becomes 30% or more in total, and the desired ultrafine γ grains can be stably formed, so that the total reduction rate at Ar ₃ points or less is achieved. Although the rate is set to 30% or more, it is preferably 50% or more.

By the way, if the γ grain size of continuous cast or ingot cast slab (material steel) is 200 μm or more, the hot slab is cooled to below Ar ₃ point and rolled, and then reverse transformation is performed. However, the desired uniform ultrafine structure may not be obtained. However, even in such a case, before cooling the hot steel slab as it is or after charging into the heating furnace, it is once processed in the recrystallization temperature range of austenite to be finely grained by recrystallization (γ grain size: 200 μm The above problems will be eliminated.

However, in order to recrystallize the hot steel slab in the recrystallization temperature range of austenite so that the γ grain size is less than 200 μm, it is necessary to add working at a reduction rate of 30% or more in the recrystallization temperature range.

C) opposite from the "work hardened ferrite of temperature from rising above reasons Ac ₃ point of raising the temperature in a temperature range up to 10 ° C. / sec or higher heating rate (alpha) of the Ac ₃ point - [Ac ₃ point + 100 ° C.] The transformation produces very fine gamma grains. "
This is for sufficiently exhibiting the characteristic actions and effects of the method according to the present invention. In this case, the upper limit of the temperature rise is set to [Ac ₃ point + 100 ° C], because if the temperature is raised above this temperature, γ will undergo grain growth and finally the desired uniform ultrafine structure steel sheet can be obtained. Therefore, the desired strength and workability cannot be ensured.

If the heating rate when raising the temperature range from Ac ₃ point to [Ac ₃ point + 100 ° C] is less than 10 ° C / sec, the strain due to processing that causes the introduction of reverse transformation nuclei is α → γ reverse transformation. Therefore, the desired fine γ grain structure cannot be realized. Therefore, the heating rate is set to 10 ° C./sec or more.

Any method such as “utilization of processing heat” or “active heating from the outside”, or a combination of both may be adopted as the means for raising the temperature.

D) Reason for performing rolling with a total reduction of 10% or more in the austenite phase temperature region The γ grains generated by reverse transformation are made finer, and the ferrite (α) -containing structure generated by subsequent cooling is made into a desired ultrafine structure, which is excellent. In order to secure the characteristics, it is necessary to add rolling processing with a total reduction rate of 10% or more (preferably 30% or more) to the steel rapidly heated to the austenite phase temperature range. If the ratio is less than 10%, the desired uniform ultrafine structure cannot be stably realized.

Then, the α-particle diameter is 10 μm by cooling the steel which has been subjected to the above-mentioned thermomechanical treatment into a plate material by an arbitrary means.
Hereinafter, it becomes possible to industrially stably manufacture a steel sheet having an isotropic uniform ultrafine structure of 5 μm or less and exhibiting excellent strength and workability.

Hereinafter, the present invention will be described more specifically by way of examples.

Example First, each steel having the chemical composition shown in Table 1 was melted in a 50 kg vacuum melting furnace and cast into hot cast pieces of 20 mm thickness and 60 mm thickness.

Next, these hot cast pieces were rolled and heat-treated under the conditions shown in Table 2 and then rapidly cooled, and then annealed at 680 ° C. for 1 hour to produce hot-rolled steel sheets.

Then, a test piece was sampled from the steel plate thus obtained, and the grain size number, yield strength, elongation, n The value (work hardening coefficient) and the warp resistance transition temperature were examined.

Here, the “vertical crack transition temperature” means the brittle crack stop temperature of the cup drawn with a drawing ratio of 2.0.

The results are also shown in Table 2.

As is clear from the results shown in Table 2, the steel sheet manufactured according to the conditions specified in the present invention stably has an ultrafine uniform structure, and has excellent strength and workability (elongation, n value, and durability). Cracking transition temperature), on the other hand, when the manufacturing conditions do not meet the requirements of the present invention, sufficient microstructure cannot be achieved, and the properties of the obtained steel sheet are inferior to those obtained by the method of the present invention. I see.

<Summary of Effects> As described above, according to the present invention, a hot-rolled high-strength steel sheet having an ultrafine uniform structure and excellent strength and workability that cannot be realized by the conventional technology is provided. It is possible to produce stably, and it is possible to stably supply an inexpensive hot-rolled high-strength steel sheet for processing which is excellent in toughness.

[Brief description of drawings]

FIG. 1 and FIG. 2 are diagrams showing heat patterns according to the method for producing an ultrafine structure steel sheet of the present invention.

Claims (2)

[Claims] 1. By weight ratio, C: 0.03 to 0.25%, Si: 0.01 to 2.00%, Mn: 0.40 to 2.00%, Al:
0.01 to 0.10%, Nb: 0.01 to 0.10%, V: 0.01 to 0.10%, Ti: 0.01 to 0.10%, Ca:
A continuous cast slab or ingot containing 0.01% or less of the following, with the balance consisting of Fe and unavoidable impurities, is cooled from a hot piece or hot lump state. (A) In a temperature range of ₃ points or less of Ar Roll at a total reduction of 30% or more. (B) Then, in the temperature range from Ac ₃ point to [Ac ₃ point + 100 ° C], 10
The temperature is raised at a heating rate of ℃ / sec or more to cause the reverse transformation of ferrite to austenite, (c) and the rolling is performed at a total reduction of 10% or more in the austenite phase temperature range. A method for producing a hot-rolled high-tensile steel sheet having excellent workability, which comprises cooling and cooling. 2. By weight ratio, C: 0.03 to 0.25%, Si: 0.01 to 2.00%, Mn: 0.40 to 2.00%, Al:
0.01 to 0.10%, Nb: 0.01 to 0.10%, V: 0.01 to 0.10%, Ti: 0.01 to 0.10%, Ca:
Recrystallizing continuously cast slabs or ingots containing at least one of 0.01% or less, with the balance being Fe and unavoidable impurities, in the state of hot slabs or hot lumps, or after charging into a heating furnace. After rolling at a total rolling reduction of 30% or more in the temperature range, this is cooled, and (a) rolling at a total rolling reduction of 30% or more is performed in the temperature range of Ar ₃ points or less, (b) subsequently Ac ₃ 10 in the temperature range from point to [Ac ₃ points + 100 ° C]
The temperature is raised at a heating rate of ℃ / sec or more to cause the reverse transformation of ferrite to austenite, (c) and the rolling is performed at a total reduction of 10% or more in the austenite phase temperature range. A method for producing a steel sheet having an ultrafine structure, which comprises cooling and cooling the steel sheet.