JP2005226123A - Method for manufacturing hot-rolled steel sheet having fine ferrite structure - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a hot-rolled steel sheet having finer ferrite crystal grains than ever, specifically an average grain size of less than 2 μm. <P>SOLUTION: The hot-rolled steel sheet having the fine ferrite structure comprises, by mass%, 0.04-0.20% C, 0.01-2.0% Si, 0.5-3.0% Mn and the balance Fe with unavoidable impurities. The manufacturing method comprises: a step (A) including the first rolling of rolling a base steel plate having the above composition, in the temperature range of an Ae3 transformation temperature or higher at a total rolling reduction of 80% or higher; a step (B) including the second rolling of rolling the plate subsequently to the step A by one pass, in the temperature range of the Ae3 transformation temperature or higher at a total rolling reduction of 30 to 55%; a step (C) including the third rolling of rolling the plate by one pass, within 0.3 sec after the second rolling in the step (B), at a rolling reduction of 35 to 60%; and a step (D) of cooling the plate within 0.2 sec after the third rolling in the step (C), to a temperature of (Ae3 transformation temperature -130°C) or lower, at a cooling rate of 600°C/sec or higher. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for producing a hot-rolled steel sheet that refines the ferrite crystal grain size of carbon steel.

  It is known that strength and toughness can be improved by refinement of ferrite crystal grains, and a technology for producing a hot-rolled steel sheet having a fine ferrite structure is an important technique for manifesting material functions of steel materials. In addition, the strength can be enhanced without using special elements, so the recyclability of the product is high and the burden on the global environment is low.

  As a means for obtaining a hot-rolled steel sheet having a fine ferrite structure, a large strain processing method has been extensively studied. For example, Patent Document 1 discloses that a high-strength hot-rolled steel sheet having a fine-grained ferrite structure with a grain size of 3 to 5 μm is obtained from carbon steel by one pass or cumulative large pressure in the transformation region.

  Further, in Patent Document 2, by rolling down at a reduction rate of 40% or more in a temperature range of 650 to 950 ° C., and further applying a reduction at a reduction rate of 40% or more continuously within 2 seconds, about 2 to 3 μm. It is disclosed that a fine-grained ferrite structure can be obtained.

All of these are supposed to utilize the grain refinement mechanism by ferrite transformation and recrystallization of ferrite during rolling.
JP 58-123823 A JP 59-229413 A

  In the method according to the above publication, etc., about 2 to 3 μm was the limit of fine graining. An object of the present invention is to provide a production method for realizing crystal grain refinement more than conventional, specifically, a ferrite crystal grain size of less than 2 μm on average.

  The present invention will be described below. In order to facilitate understanding of the present invention, reference numerals in the accompanying drawings are appended in parentheses, but the present invention is not limited to the illustrated embodiment.

  In the present invention, as schematically shown in the process diagram of FIG. 1, a raw steel plate having a predetermined composition in a high temperature state suitable for hot working is subjected to a first rolling with a total rolling reduction of 80% or more, one pass. The second rolling, the third rolling performed immediately thereafter, and the cooling steps performed immediately thereafter are processed to obtain a hot rolled steel sheet having a fine ferrite structure.

The present inventors have found the following conditions effective for obtaining fine crystal grains from the results of experiments using a multi-stand hot test rolling mill (described later) capable of rolling under high pressure in a short pass time. It was. It has been found that by appropriate combination of these, crystal grain refinement more than that of the conventional method can be obtained, and the present invention has been completed. If this is expressed by focusing on the metal crystal structure,
(1) The ferrite transformation is not performed until the third rolling as the final pass, and the austenite before the ferrite transformation is refined as much as possible and the dislocation density is increased.
(2) In the first rolling, austenite is sufficiently refined and recrystallized.
(3) In the second rolling, while avoiding rolling under ultra-high pressure where dynamic recrystallization and static recrystallization are significantly accelerated, rolling at a sufficient reduction rate is performed to accumulate strain and dislocation density. To increase.
(4) The time between passes between the second rolling and the third rolling, which is the final pass, is shorter than the conventional rolling method in order to minimize the recrystallization and recovery of austenite and increase the strain accumulation effect. To do.
(5) Also in the third rolling, which is the final pass, rolling with a sufficient reduction ratio is performed to accumulate strain and increase the dislocation density.
(6) After the third rolling, it is cooled quickly to promote ferrite transformation and suppress ferrite grain growth.
That is the essence.

  Thus, the present invention is a method for producing a hot-rolled steel sheet having a fine ferrite structure, and in mass%, C: 0.04 to 0.20%, Si: 0.01 to 2.0%, Mn: 0.5 A process including the first rolling in which the steel sheet comprising ~ 3.0% and the balance of Fe and inevitable impurities is rolled at a total rolling reduction of 80% or more in the temperature range of the Ae3 transformation point or more, and the A Following the process, the B process including the second rolling in which a one-pass rolling with a rolling reduction of 30 to 55% is performed in a temperature range equal to or higher than the Ae3 transformation point, and the rolling reduction within 0.3 sec after the second rolling in the B process. C process including 3rd rolling which performs 35 to 60% 1-pass rolling, and at a cooling rate of 600 ° C./sec or more within 0.2 sec after the third rolling in the C process (Ae3 transformation point−130 ° C. ) D process for cooling to the following temperature It provides a method for producing a hot rolled steel sheet in which solves the above problems.

  The sheet surface temperature (finishing temperature) after the third rolling in the C process is (Ae3 transformation point + 30 ° C.) to (Ae3 transformation point−60 ° C.) from the viewpoint of obtaining fine ferrite crystal grains by cooling in the D process. A range is preferable.

  Further, in order to keep the sheet surface temperature after the third rolling within the range of the above finishing temperature, between the first rolling and the second rolling (Step B) and / or between the second rolling and the third rolling ( In step C), cooling may be performed. By doing in this way, the processing heat generated by the first rolling and the second rolling (normally, the material temperature rises by several tens of degrees Celsius) can be cooled. In particular, it is effective when the rolling speed is high, when the high-pressure reduction rate is adopted, when the rolled material is thick, or when rolling is performed in a system with a high friction coefficient.

  According to the present invention, the ferrite crystal grain size of general-purpose carbon steel can be remarkably reduced. As an effect, the strength can be enhanced without using a special element, so that the recyclability of the product is high and the burden on the global environment can be reduced.

Hereinafter, the present invention will be specifically described. The present invention is constituted by the following five points.
(Material steel plate): In mass%, C: 0.04 to 0.20%, Si: 0.01 to 2.0%, Mn: 0.5 to 3.0%, the balance being Fe and inevitable It consists of mechanical impurities.
(Step A): First rolling (basically, but not limited to, multi-pass rolling) with a total rolling reduction of 80% or more in a temperature range equal to or higher than the Ae3 transformation point that becomes an austenite single phase.
(Step B): Subsequently, second rolling, which is one-pass rolling with a rolling reduction of 30 to 55%, is performed in a temperature range equal to or higher than the Ae3 transformation point.
(Step C): Within 0.3 sec after the second rolling, the third rolling which is a one-pass rolling with a rolling reduction of 35 to 60% is performed.
(Step D): Cooling to a temperature of (Ae3 transformation point−130 ° C.) or less at a cooling rate of 600 ° C./sec or more within 0.2 sec after the third rolling.

  Hereinafter, each item will be described in detail.

(Material steel plate)
The component of the material according to the present invention may be an ordinary carbon steel component. Specifically, in terms of mass%, C: 0.04 to 0.20%, Si: 0.01 to 2.0%, Mn: 0.00. 5 to 3.0% is contained, and the balance is made of a steel plate made of Fe and inevitable impurities.

C: 0.04-0.20 mass%
C is an element mainly required for securing the strength of steel, but if contained in a large amount, C causes weldability deterioration of the steel material, significant reduction in toughness, and formability deterioration during press forming. Therefore, the upper limit of the C content of the hot rolled steel sheet having the fine ferrite structure of the present invention is 0.20% by mass. Further, when the C content is less than 0.04% by mass, it becomes difficult to ensure the effect of crystal grain refinement, so the lower limit of the C content is 0.04% by mass. Preferable C content is 0.07 mass%-0.16 mass%.

Si: 0.01-2.0 mass%
Si is an alloy element that is necessary for successful deoxidation and has the effect of improving the workability of the steel sheet, but when the content exceeds 2.0 mass%, the fine ferrite structure of the present invention. Since the toughness as a hot-rolled steel sheet having the above is impaired, its content is set to 2.0% by mass. On the other hand, if the content is too small, deoxidation during steelmaking is not sufficiently performed, so the lower limit of the Si amount is 0.01% by mass. A preferable Si content is 0.01% by mass to 1.5% by mass.

Mn: 0.5 to 3.0% by mass
Mn is an inexpensive element and has an effect of increasing the strength of steel. Moreover, hot brittleness due to S is prevented, and the Ae3 point is lowered. If the Mn content is less than 0.5% by mass, such an effect cannot be sufficiently exhibited, so the lower limit of the Mn content is 0.5% by mass. On the other hand, when the content of Mn exceeds 3.0% by mass, such an effect is saturated. Rather, the workability of the hot-rolled steel sheet is deteriorated and the surface properties of the hot-rolled steel sheet are deteriorated. Therefore, the Mn content is 3.0 mass% or less. A preferable Mn content is 0.5 mass% to 2.0 mass%.

  The material steel plate may be a cast material. However, in order to reduce internal defects during casting and to reduce the austenite diameter, one or more hot workings are performed to obtain an austenite structure having a particle size of 600 μm or less. It is preferable to keep. Specifically, in the continuous casting-hot rolling process, it may be in a state where rough rolling for one pass or more has been completed. In a basic experiment relating to the present invention, a material having a ferrite structure with a crystal grain size of about 30 μm is subjected to a predetermined time (for example, 1 to 2 hours) at a predetermined temperature (for example, 1000 to 1200 ° C.) before entering Step A below. The experiment was conducted with the austenite grain size of 30 to 600 μm.

(Process A)
The first rolling with a total rolling reduction of 80% or more is performed on the material in a temperature range equal to or higher than the Ae3 transformation point that becomes an austenite single phase. The first rolling is not limited to the number of passes as long as the total rolling reduction is 80% or more. By this first rolling, a material having an austenite grain size of 30 to 600 μm after heating can be rolled into a material to be rolled having an austenite grain size of about 30 μm or less.

(Process B)
Continuously to the first rolling in the step A, the material to be rolled obtained by the rolling is subjected to one-pass rolling with a rolling reduction of 30 to 55% in a temperature range equal to or higher than the Ae3 transformation point (second rolling). If the rolling reduction is outside this range, fine particles cannot be obtained. The reason is not clear, but if the rolling reduction is insufficient, strain accumulation due to rolling will be insufficient, and if the rolling reduction is excessive, recrystallization and recovery will occur due to heat generation due to rolling and excessive increase of dislocation density. This is presumed to be easier.

(Process C)
Continuously after the 1-pass rolling (second rolling) in the above-mentioned process B, the material to be rolled obtained by the second rolling is rolled in one pass with a rolling reduction of 35 to 60% in a time between passes of 0.3 sec or less ( (3rd rolling) is performed. If the time between passes is longer than this, the crystal grain refining effect is lowered. The reason for this is not clear, but if the time between passes between the second rolling in the B process and the third rolling in the C process exceeds 0.3 sec, static recrystallization occurs, and therefore, accumulation of strain occurs. Is presumed to be insufficient. In order to effectively perform “accumulation of strain in the unrecrystallized region”, which is estimated to be effective for grain refinement, the time between passes until the third rolling needs to be 0.3 sec or less. is there. If the rolling reduction of the third rolling is less than 35%, the accumulation of strain is insufficient, and the effect of promoting ferrite transformation in the subsequent cooling process is insufficient. On the other hand, if the rolling reduction ratio of the third rolling exceeds 60%, the generation of recrystallization and transformation during processing and heat generation that affects the subsequent cooling are generated, so the effect of refining the crystal grains is reduced. In addition, the rolling load becomes excessive, and problems such as enlarging equipment, exceeding equipment limits, and unstable rolling occur.

(D process)
The material to be rolled obtained by one-pass rolling (third rolling) in step C is cooled to a temperature range of (Ae3 transformation point-130 ° C) or lower at a cooling rate of 600 ° C / sec or higher within 0.2 seconds. I do. As a result, a hot-rolled steel sheet in which a fine ferrite structure having an average particle diameter of 2.0 μm or less occupies 60% or more is obtained. By performing cooling under the above conditions, recrystallization / recovery of austenite is suppressed and ferrite transformation is promoted. Preferably, cooling is performed to a temperature range of (Ae3 transformation point−130 ° C.) or lower and (Ae3 transformation point−200 ° C) or higher.

  In Step D, it is preferable that the time from the end of the third rolling in Step C to the start of cooling be 0.1 sec or less. Furthermore, it is desirable that the cooling rate is 900 ° C./sec or more. As a result, a hot-rolled steel sheet can be obtained in which the fine ferrite structure having an average particle diameter of 1.5 μm or less occupies 50% or more.

(production equipment)
The equipment for producing a hot-rolled steel sheet having a fine ferrite structure according to the present invention comprises a heat treatment equipment, a tandem rolling equipment comprising two or more stands, and a cooling device disposed on the outlet side of the rolling equipment. Each stand of the rolling equipment is required to realize a reduction ratio of a predetermined value or more, and the time between passes between the second rolling and the third rolling is kept within 0.3 seconds. Therefore, the distance between the rolling mills must be set within a predetermined value. Moreover, it is necessary to arrange the cooling device in the vicinity of the exit side of the tandem rolling facility so that the material to be rolled after the third rolling can be immediately cooled.

  The material adjusted to the components shown in Table 1 was cut into a cut plate having a width of 100 mm and a length of 70 to 200 mm to obtain a test material. After holding this test material in a heating furnace with a furnace temperature of 1000 ° C. for 1 hour, hot rolling and cooling were performed.

  For hot rolling, a three-stand hot rolling mill as shown in FIG. 2 was produced and used. The distance between the first stand and the second stand is 2.1 m, the distance between the second stand and the third stand is 1.0 m, and the time between passes can be rolled for 0.3 seconds or less. It is. The rolling reduction of each rolling stand was set to be 40% or more. Table 2 shows the rolling mill specifications and rolling conditions.

  Table 3 shows the average grain size and ferrite fraction of each test material after hot rolling and cooling, together with the rolling conditions and cooling conditions of the first to third rolling. In addition, the measurement of the average particle diameter of a crystal grain was performed by the ASTM cutting method.

  Moreover, the microscope picture of the fine ferrite structure | tissue obtained by the manufacturing method of the hot rolled sheet steel of this invention is shown in FIG.

In Table 3, the trial numbers 1, 4, 8, 9, 14, 17, 18, 19, and 22 deviating from the range defined by the present invention have an average particle size after hot rolling / cooling exceeding 2.0 μm. It was a thing.
Among these, in trial No. 1, the total rolling reduction in the first rolling in the process A was 75%, which is less than “80% or more” defined by the present invention, so the average particle diameter exceeds 2.0 μm. It became.
In Test No. 4, the rolling reduction in the second rolling in the B step was 20% which is less than “30 to 55%” defined by the present invention, and thus the average particle size exceeded 2.0 μm. It is presumed that, due to the second rolling with a rolling reduction of 20%, the accumulation of strain and the high density of dislocations were not sufficient.
In the trial No. 8, the rolling reduction in the second rolling in the B step was 60% exceeding “30 to 55%” defined by the present invention, and thus the average particle size exceeded 2.0 μm. It is presumed that the recrystallization and recovery of austenite occurred due to the heat generated by the rolling because the rolling reduction in the second rolling exceeded the range defined by the present invention.
In Test No. 9, the rolling reduction ratio in the third rolling of the C process was 30%, which is less than “35-60%” defined by the present invention, and thus the average particle size exceeded 2.0 μm. It is estimated that the third rolling with a rolling reduction of 30% did not provide sufficient strain accumulation and high density of dislocations.
In test No. 14, in the C process, the time between passes after the second rolling to the third rolling was 0.5 sec exceeding “within 0.3 sec” defined by the present invention. The thickness exceeded 2.0 μm. This is probably because the time between passes exceeded 0.3 sec defined in the present invention, and static recrystallization occurred and the accumulation of strain was not sufficient.
Since the test numbers 17 and 18 were 0.3 sec and 0.5 sec exceeding the “within 0.2 sec” defined by the present invention in the D process, the time until the cooling start after the third rolling in the C process was The average particle size exceeded 2.0 μm. In these trial numbers, in the D process, since the time until the start of cooling after the third rolling in the C process exceeded “within 0.2 sec” defined by the present invention, recrystallization and recovery were not sufficiently suppressed, This is presumably because the ferrite transformation was insufficiently promoted.
In Test No. 19, the cooling rate in the D step was 300 ° C./sec, which is less than “600 ° C./sec or more” defined by the present invention, so the average particle size exceeded 2.0 μm. In this case, since the cooling rate was less than “600 ° C./sec or more” defined by the present invention, it is presumed that the recrystallization and recovery were not sufficiently suppressed and the ferrite transformation was not sufficiently promoted.
In the trial No. 22, the cooling stop temperature in the step D was 740 ° C. exceeding the “Ae 3 transformation point −130 ° C. or lower, in this example, the Ae 3 transformation point is 830 ° C., which is 700 ° C. or lower” defined by the present invention It was considered that the ferrite transformation promotion by cooling was insufficient, and the average particle size exceeded 2.0 μm.

  On the other hand, in the trial numbers 2, 3, 5-7, 10-13, 15, 16, 20, 21, 23 in which hot rolling and cooling were performed within the range specified in the present invention, the average particle size was A hot-rolled steel sheet having a fine ferrite structure of less than 2.0 μm occupying 50% or more was obtained. In particular, in the trial numbers 12, 13, 15, 16, 20, and 21, hot-rolled steel sheets in which a fine ferrite structure having an average grain size of 1.5 μm or less occupies 60% or more were obtained.

It is a figure which shows the process of the manufacturing method of this invention. It is a figure which shows facilities, such as a hot rolling mill used for the Example. It is the microscope picture of the fine ferrite structure | tissue obtained by the manufacturing method of the hot rolled sheet steel of this invention.

Claims (1)

A method for producing a hot-rolled steel sheet having a fine ferrite structure,
A material containing, by mass%, C: 0.04 to 0.20%, Si: 0.01 to 2.0%, Mn: 0.5 to 3.0%, the balance being Fe and inevitable impurities A process including the first rolling, in which the steel sheet is rolled at a temperature range equal to or higher than the Ae3 transformation point at a total rolling reduction of 80% or more;
Subsequent to the A step, the B step including the second rolling for performing one-pass rolling with a rolling reduction of 30 to 55% in a temperature range equal to or higher than the Ae3 transformation point;
C process including the 3rd rolling which performs 1-pass rolling with a rolling reduction of 35 to 60% within 0.3 sec after the second rolling in the B process;
After the third rolling in the step C, the step D is cooled to a temperature of (Ae3 transformation point-130 ° C) or less at a cooling rate of 600 ° C / sec or more within 0.2 seconds;
A method for producing a hot-rolled steel sheet, comprising:

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