JPH03267314A - Production of hot rolled high tensile strength steel plate excellent in workability - Google Patents

Abstract

PURPOSE:To stably obtain the steel plate having a uniform superfine structure by using a steel with a specific composition as a steel stock for a hot rolled steel plate, carrying out temp. regulation to form a specific structure at the time of hot rolling, and then exerting rolling at a specific reduction of area. CONSTITUTION:A Continuously cast slab or an ingot of a steel which has a composition containing, by weight, 0.03-0.25% C, 0.01-2.00% Si, 0.40-2.00% Mn, and 0.01-0.10% Al or further containing one or more kinds among 0.01-0.10% each of Nb, V, and Ti and <=0.01% Ca and having the balance Fe is cooled from a hot cast slab or a hot ingot. This continuously cast slab or ingot is rolled at a temp. not higher than the Ar3 point at >=30% total reduction of area and successively subjected to temp. rise up to a temp. in the region between the Ac3 point and (Ac3 point +100 deg.C) at >=10 deg.C/sec heating rate, by which the structure is inversely transformed from ferrite to austenite. Then, rolling at >=10% total reduction of area is carried out in the above austenitic phase temp. region.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for manufacturing a hot-rolled high-strength steel plate having a uniform ultra-fine structure and excellent workability.

<Prior art and its challenges> It is generally believed that the strength, ductility, and other properties of steel improve as the structure becomes finer, and there has been a fierce competition to develop technologies to further refine the structure of steel. .

The results of these researches that have been continued for many years are a) controlled rolling.

b) Controlled rolling/accelerated cooling.

C) Large reduction rolling (for example, JP-A No. 62-253733,
JP-A-63-145720, etc.).

This led to the creation of new microstructure techniques such as

However, the following problems have been pointed out with each of these techniques.

In other words, in controlled rolling technology, once the austenite (r) grains produced by hot working called controlled rolling become as fine as possible, it is practically impossible to make them any finer. So, the ferrite (α) grain size is 10! It is difficult to even obtain a uniform microstructure on the order of m.

Even if a technology that combines controlled rolling and accelerated cooling is used, as mentioned above, controlled rolling can achieve sufficient T.
Since the refinement of the structure is not achieved, there is a limit to the attempt to forcibly transform and generate fine α by subsequent accelerated cooling, and therefore it is still possible to obtain a uniform structure in which the α grain size is so fine as to be less than 10 mm. It was extremely difficult.

What's more, if the α grain size is 5 or less (Grain 5ize)
It was absolutely impossible to obtain a uniform ultrafine structure with a diameter of 12 or more.

On the other hand, the microstructure refinement technology by large reduction rolling applies a large reduction with a reduction rate of 30χ/pass or more in the T non-recrystallization temperature range to make the γ grains "work-hardened T containing deformation bands inside the grains", and then −
It aims to refine the structure by causing α transformation,
In this method, the γ grains before the T-α transformation are simply elongated by large reduction rolling and do not become isotropically fine grains, so there is still a limit to the grain size reduction after the transformation. It has not been possible to achieve a uniform ultrafine structure with an α grain size of less than 5 mm.

Therefore, the purpose of the present invention is to provide an industrial means that can stably produce an "ultra-fine, isotropic, uniform structure" in steel sheets, which has been difficult to achieve using conventional methods. The purpose of the present invention was to provide a method that enables high-efficiency production of "hot-rolled high-strength steel sheets exhibiting excellent workability" without relying on the addition of particularly special and expensive alloying elements.

<Means for Solving the Problems> As a result of intensive research from various viewpoints in order to achieve the above object, the inventors of the present invention have developed the following method: In addition to using a material with a specific composition as a material, the temperature is adjusted during hot rolling to expose the structure containing ferrite at the front, the structure is rolled at a predetermined reduction rate, and then heated rapidly. In order to reversely transform the ferrite into austenite, or if the austenite grain size of the raw material steel is 200 mm or more, the raw steel is once subjected to a predetermined reduction in the austenite recrystallization temperature range before the ferrite structure appears. Carrying out rate rolling,
When the ferrite is reversely transformed into austenite by carrying out the processing heat treatment in the above step, the austenite structure that appears becomes an ultra-fine Mi texture that cannot be obtained by conventional controlled rolling or the like. Therefore, when this ultra-fine austenite structure is further subjected to rolling processing and then cooled, the ferrite that is transformed and produced is extremely fine because it is based on the ultra-fine austenite structure, which was previously extremely difficult to achieve. A steel plate having an isotropic and uniform ultrafine structure with a grain size far below 10 rrm can be stably obtained. Moreover, this ultra-fine-structured steel sheet exhibits properties such as strength and ductility that are far superior to conventional steel sheets, and has the potential to become a highly desirable hot-rolled tensile steel sheet for processing. It was.

The present invention was made based on the above-mentioned findings, etc. rc:o, 03 to 0.25% (hereinafter, % representing the component ratio is expressed as weight %).

St: 0.01-2.00%, Mn:
0.40-2.00%.

Contains A1: 0.01 to 0.10%, or further contains Nb: 0.01 to 0.10% if necessary, V: 0
．． 01-0.10%.

Ti: 0.01-0.10%, Ca: 0
．． Continuously cast slabs (slabs, blooms) containing one or more of the following: 0.1% or less, with the remainder consisting of Fe and unavoidable impurities.

Billets) or ingots (hereinafter collectively referred to as "steel slabs") are cooled as they are from the hot slab state as shown in Figure 1, or as hot slabs or placed in a heating furnace as shown in Figure 2. Once the total reduction rate is 30% in the recrystallization temperature range.
After performing the above rolling, it is cooled, and then the above-mentioned first
As shown in Fig. 2 and Fig. 2, (al Ar, 3 points - [Ac 3 points - rolling with a total reduction of 30% or more in the temperature range below the plate (b). Then, Ac 3 points - [Ac 3 points - (Ac 3 point~
The temperature is raised to a temperature range of [Ac3 point + 100° C.] at a heating rate of 10° C./sec or more to cause reverse transformation from ferrite to austenite.

(C) By sequentially heat-treating and cooling in the process of rolling at a total reduction rate of 10% or more in the austenite phase temperature range, hot-rolled high tensile strength with a uniform ultra-fine structure and excellent workability is obtained. It is characterized by the ability to efficiently and stably manufacture steel plates.

Next, the reasons for limiting the manufacturing conditions (chemical composition of the raw material steel, processing conditions, etc.) of the hot-rolled high-strength steel sheet according to the present invention as described above will be explained in detail together with their effects.

A) Chemical composition of the material steel If the C content is less than 0.03%, the grain growth tendency of α and T becomes remarkable, and even if reverse transformation occurs and the grains become fine, they immediately become coarse due to grain growth. Putting it away causes inconvenience. On the other hand, when C is contained in an amount exceeding 0.25%, weldability deteriorates. Therefore, the C content is 0.03-0.25%
It was determined that

i Solid solution Si contributes greatly to increased strength and ductility, and S
Since the addition of i increases retained austenite, it can be said that it is a preferable element for improving ductility from this point of view as well. This effect is particularly preferable when low-temperature winding (approximately 400° C. or less) is performed during hot rolling. However, if the Si content is less than 0.01%, the desired effect of the above action cannot be secured, while if the Si content exceeds 2.00%, it will conversely cause a decrease in ductility and deterioration of weldability. The content was determined to be 0.01 to 2.00%.

Mn Mn is a preferable element that has the effect of improving hot workability of steel as well as increasing strength by improving hardenability, etc. However, if it is contained in an amount exceeding 2.00%, it will cause problems in melting and This will lead to a cost disadvantage. On the other hand, M
If the n content is less than 0.40%, the growth of α grains becomes significant, which adversely affects the production of ultrafine textured needles. For this reason, the Mn content was determined to be 0.40 to 2.00%.

In addition to acting as a good deoxidizing agent for steel, Al also has the effect of improving the strength of the steel by forming carbonitrides, but if its content is less than 0.01%, the desired effect due to the above effect is not achieved. effect cannot be obtained. In addition, since the carbonitrides mentioned above are basically formed in ferrite (α), when added in a small amount, Al does not pose a major hindrance to the formation of α from T by processing, but when added in a large amount exceeding 0.10%, When added, it remains in the form of coarse precipitates in the steel, causing property deterioration. Therefore, the Al content was determined to be 0.01 to 0.10%.

Nb, V, Ti, and Ca These elements are effective in strengthening steel or improving workability, so one or more of these elements are added as necessary, but the content is numerically limited. is due to the following reason.

a) Nb, V and Ti All of these have the effect of forming carbonitrides and improving the strength of steel, but if their content is less than 0.01%, the desired effect due to the above effect cannot be obtained. On the other hand, 0.1
If the content exceeds 0%, Ar is added to retard α transformation.
3 The transformation point becomes low and the processing stress during secondary rolling becomes high (making rolling difficult. Therefore, Nb, V or Ti
When containing, the content is 0.01~
It was determined to be 0.10%.

b) Ca Ca has the effect of changing the shape of inclusions after rolling (spheroidization) and improving the workability of steel, but if it is contained in an amount exceeding 0.01%, the number of inclusions increases. On the contrary, the characteristics will be impaired. Therefore, when Ca is contained, the content is set to be 0.01% or less.

Rolling is performed after cooling a hot steel piece to below the Ar= point.
This is because the main requirement of the method of the present invention is "to apply plastic working to a structure containing ferrite and then reversely transform the ferrite phase into an austenite phase", and for this purpose, it is necessary to generate a ferrite phase. It is. The cooling temperature at this time is not particularly limited as long as it is below the Ar+ point, but from the viewpoint of practical operability, it is in the high temperature range below the Ar3 point (from the Ar3 point to (Ar:+ It can be said that it is preferable to set the temperature to -100°C). However, when plastic working is applied to a structure containing ferrite and the ferrite phase is reversely transformed into an austenite phase, the ferrite (α
) The higher the volume fraction of ferrite, the finer the T grains after reverse transformation, so from the standpoint of product performance, in order to increase the volume fraction of ferrite, the cooling temperature should be between 3 points Ar and 3 points below [ac 3 points]. is desirable.

The reason for setting the total rolling reduction rate of 30% or more in the rolling process performed in the temperature range below the Ar+ point is that the fine T grains are stabilized by reverse transformation only when the total rolling reduction rate at this time is 30% or more. This is because formation can be achieved.

That is, when rolling is performed in a temperature range of Ar3 or lower, ferrite (α) is work-hardened by this rolling, and the number of reverse transformation nuclei to austenite (T) increases. If the number of these reverse transformation nuclei is extremely large, extremely fine T grains will be generated by the subsequent rapid temperature rise to the austenite region. However, the number of reverse-transformed nuclei shows a remarkable tendency to rapidly increase only when the total rolling reduction rate reaches 30% or more, and the desired ultrafine γ
In order to achieve stable grain formation, the total rolling reduction ratio at the Ar point of 1 or less was set at 30% or more, but it is preferably 50% or more.

By the way, continuous casting is an ingot cast copper piece (
If the single grain size of the raw material steel is 200p or more, the desired uniform ultrafine structure cannot be obtained even if the hot steel piece is cooled to below the Ar3 point and reverse transformation is caused after rolling. There is a possibility that you may not get it. However, even in such cases,
Before cooling the hot wire piece, either as it is or after charging it into the heating furnace, it can be processed in the austenite recrystallization temperature range to make the grain finer (γ grain size: less than 200Q+) by recrystallization. The problem will be eliminated.

However, in order to recrystallize a hot steel slab in the austenite recrystallization temperature range and make the grain size less than 200 mm, it is necessary to apply processing at a reduction rate of 30% or more in the recrystallization temperature range.

Raising the temperature above the Ac= point is because "very fine γ grains are generated from work-hardened ferrite (α) through reverse transformation."
This is to fully exhibit the characteristic actions and effects of the method according to the present invention. In this case, the upper limit of the heating temperature was set to [Ac3 point + 100°C] because if the temperature was raised above this temperature, γ grains would grow and the desired uniform ultra-fine structure steel sheet would ultimately be obtained. First, it becomes impossible to secure the desired strength and workability.

If the heating rate is less than 10°C/sec when increasing the temperature to the temperature range from Ac = point to (AC, point +100°C), the strain caused by processing that causes the introduction of reverse transformation nuclei will cause α-ding reverse transformation. As a result, the desired fine T-grain structure cannot be achieved.Therefore, the above heating rate is set to 10°C/sec.
It was set as C or higher.

In addition, as a means for raising the temperature, any method such as "utilization of processing heat", "active heating from the outside", or a combination of both may be adopted.

In order to make each grain produced by reverse transformation even finer and to make the ferrite (α)-containing structure produced by subsequent cooling into the desired ultra-fine structure and to secure excellent properties, the temperature must be rapidly raised to the austenite phase temperature range. It is necessary to apply rolling to the steel with a total rolling reduction of 10% or more (preferably 30% or more), and if the total rolling reduction at this time is less than 10%, the desired uniform ultrafine structure cannot be stably formed. It cannot be realized.

By cooling the steel made into a plate material through the above-mentioned processing heat treatment, it has an isotropic uniform ultrafine structure with an α grain size of 10 or less, and furthermore, 5R1 or less, and has excellent strength. It becomes possible to industrially and stably manufacture a steel plate that exhibits good workability.

EXAMPLES Hereinafter, the present invention will be explained in more detail with reference to 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 slabs of 2011 m thick and 600 mm thick.

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

A test piece was taken from the steel plate thus obtained, and the grain size number, yield strength, elongation, n value (work hardening coefficient), and warp cracking transition temperature were examined.

Here, the "vertical crack transition temperature" means the brittle crack stop temperature of a cup drawn at 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 sheets manufactured according to the conditions specified in the present invention have a stable ultra-fine uniform structure, and have excellent strength and workability (elongation, n-value,
On the other hand, if the manufacturing conditions do not meet the provisions of the present invention, a sufficient microstructure cannot be achieved, and the properties of the obtained steel sheet will be worse than those produced by the method of the present invention. It can be seen that the result is inferior.

<Summary of Effects> As explained above, according to the present invention, it is possible to produce hot-rolled high-strength steel sheets that have an ultra-fine uniform structure and exhibit excellent strength and workability, which was practically impossible to achieve using conventional techniques. It brings about extremely useful effects industrially, such as making it possible to stably supply hot-rolled high-strength steel sheets for processing that can be stably manufactured, have excellent toughness, and are inexpensive.

[Brief explanation of drawings]

FIGS. 1 and 2 are diagrams each showing a heat pattern according to the method of manufacturing the ultrafine structure control plate of the present invention.

Claims (2)

[Claims] (1) Weight percentage: C: 0.03-0.25%, Si: 0.01-2.00
%, Mn: 0.40-2.00%, Al: 0.01-0
．． 10% or further Nb: 0.01-0.10%, V: 0.01-0.10
%, Ti: 0.01 to 0.10%, Ca: 0.01% or less, and the balance is Fe and unavoidable impurities. Cool from the block state, (a) Roll at a total reduction rate of 30% or more in a temperature range of Ar_3 points or less, (b) Then apply rolling at a temperature of 10°C/sec or more in a temperature range of Ac_3 points to [Ac_3 points + 100°C]. (c) Then, in the austenite phase temperature range, rolling is performed at a total reduction rate of 10% or more, followed by sequential heat treatment and cooling. A method for manufacturing hot-rolled high-strength steel sheets with excellent workability. (2) C: 0.03-0.25%, Si: 0.01-2.00 in weight percentage
%, Mn: 0.40-2.00%, Al: 0.01-0
．． 10% or further Nb: 0.01-0.10%, V: 0.01-0.10
%, Ti: 0.01 to 0.10%, Ca: 0.01% or less, and the balance is Fe and unavoidable impurities. After rolling in a hot lump state or charging it into a heating furnace at a total reduction rate of 30% or more in a recrystallization temperature range, it is cooled and (a) Total reduction in a temperature range of Ar_3 points or less. rolling at a rate of 30% or more; (b) then increasing the temperature to a temperature range of Ac_3 point to [Ac_3 point + 100°C] at a heating rate of 10°C/sec or more to cause reverse transformation from ferrite to austenite; (c) A method for producing an ultra-fine-structured steel sheet, comprising the steps of: rolling at a total reduction rate of 10% or more in the austenite phase temperature range; and sequential heat treatment and cooling.