JP2008189958A - Hot-rolled steel sheet having uniform fine ferrite structure and method for producing the same - Google Patents

Abstract

The present invention provides a hot-rolled steel sheet that improves the deformability in secondary processing or uniform deformability.
SOLUTION: The main phase is ferrite, and C: 0.04 to 0.20% by mass, Si: 0.01 to 2.0%, Mn: 0.5 to 3.0%, and the balance is A hot-rolled steel sheet made of Fe and inevitable impurities, the ferrite crystal grain diameter D2 at a ¼ depth position of the plate thickness from the surface of the steel plate is less than 2.0 μm, and the plate thickness 1 / 2 The relationship between the ferrite crystal grain size D3 at the depth position and the ferrite crystal grain size D1 at the depth position of 50 μm from the surface of the steel sheet satisfies (D3−D1) /D2≦0.4, and 50 μm from the surface of the steel sheet. A rolling direction grain size Dr and a plate thickness direction grain size Dt of the ferrite crystal grains in the depth position satisfy the formula (1). | (Dr−Dt) / ((Dr + Dt) / 2) | ≦ 0.25 (1)
[Selection] Figure 1

Description

  The present invention relates to a hot-rolled steel sheet having a uniform ultrafine ferrite structure in the sheet thickness direction and a method for producing the same.

  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.

Furthermore, Patent Document 3 discloses a method for producing a hot-rolled steel sheet in which the time between passes is set to an extremely short time by rolling of three passes or more, the strain due to rolling is increased, and the steel plate is rapidly cooled after rolling. According to this, the ferrite transformation can be promoted and the grain growth of the ferrite can be suppressed, and a fine ferrite structure having a crystal grain size of less than 2 μm is obtained with a simple composition carbon steel not containing precipitation strengthening elements such as Ti and Nb. It becomes possible.
JP 58-123823 A JP 59-229413 A JP 2005-226123 A

  However, in the invention described in Patent Document 1 or 2, it is difficult to obtain a ferrite structure having a particle size of less than 2 μm. In addition, when the rolling temperature is lowered for the purpose of reducing the crystal grain size, the ferrite becomes a layered processing structure expanded in the rolling direction, and there is a problem that the deformability of the secondary processing of the material is lowered.

  Further, in the invention described in Patent Document 3, an equiaxed structure can be obtained with a ferrite grain size of less than 2 μm, but the rolling reduction of the final pass is 35 to 60%, resulting in large rolling, and thus obtained. The fine-grain hot-rolled steel sheet has a grain size difference in the thickness direction, and in particular when the grain size difference exceeds 40% of the average grain size, it becomes a factor that hinders the uniform deformability during secondary processing of the material.

  As described above, the conventional technology does not contain a precipitation strengthening element, the difference in ferrite grain size in the plate thickness direction is small, the crystal grain size does not extend excessively in the rolling direction, and the ferrite crystal grain size is 2 μm. It was not possible to obtain a hot-rolled steel sheet that satisfies the above condition. For this reason, even if the crystal grains are refined, the strength is increased, but a lamellar structure is formed and the deformability in the secondary processing is reduced, or a large grain size difference is generated in the plate thickness direction, resulting in uniform deformability in the secondary processing. There was a case of decline.

  Then, this invention makes it a subject to provide the hot rolled steel plate which improves the deformability in secondary processing or the uniform deformability in view of the said problem.

  Hereinafter, the hot-rolled steel sheet of the present invention and the manufacturing method thereof will be described. 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.

The invention according to claim 1 has ferrite as a main phase, and in mass%, C: 0.04 to 0.20%, Si: 0.01 to 2.0%, Mn: 0.5 to 3.0% The balance is a hot-rolled steel sheet made of Fe and inevitable impurities, and the ferrite crystal grain size D2 at a ¼ depth position from the surface of the steel sheet is less than 2.0 μm, and the surface of the steel sheet The relationship between the ferrite crystal grain size D3 at the position of the plate thickness ½ depth and the ferrite crystal grain size D1 at the position of 50 μm depth from the surface of the steel sheet satisfies (D3-D1) /D2≦0.4, By providing a hot-rolled steel sheet characterized in that the rolling grain size Dr and the thickness direction grain size Dt of the ferrite crystal grains satisfy a formula (1) at a depth of 50 μm from the surface of the steel sheet. It is a solution.
| (Dr−Dt) / ((Dr + Dt) / 2) | ≦ 0.25 (1)

  Here, the “main phase” is a phase occupying an area of 50% or more with respect to the area of the cross section in an arbitrary cross section of the steel sheet. Moreover, each particle size represented by D1, D2, and D3 represents an average particle size, and the average particle size is a value obtained by the ASTM cutting method. Further, the particle diameters of D1, Dr, and Dt located at a depth of 50 μm from the surface of the steel plate have a relationship of D1 = (Dt + Dr) / 2.

  And, as schematically shown in the process diagram of FIG. 1, a raw steel plate that is in a high temperature state suitable for hot working and has a predetermined composition is 80% or more in total rolling reduction, or an average grain in an austenite single phase. Each including the 1st rolling (20) rolled so that a diameter may be 30 micrometers or less, the 2nd rolling (30) of 1 pass, the 3rd rolling (40) performed immediately after that, and the cooling (50) performed immediately after that It processes by a process and obtains said hot-rolled steel plate.

From the results of experiments using a multi-stand hot test rolling mill (10) (see FIG. 3; details will be described later) capable of rolling under high pressure in a short pass time, the present inventors have obtained uniform fine crystals. The following conditions effective for obtaining grains were found. The present inventors have completed the present invention by discovering that by these appropriate combinations, fine ferrite crystal grains more uniform than those obtained by the conventional method can be obtained. This can be expressed as follows by paying attention to the metal crystal structure.
(1) The ferrite is not transformed until the third rolling (40) 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 (20), the austenite is sufficiently refined and recrystallized.
(3) In the second rolling (30), while avoiding ultra-high-pressure rolling that significantly accelerates dynamic recrystallization and static recrystallization, rolling is performed at a sufficient reduction rate to accumulate distortion. Increase dislocation density.
(4) The time between passes between the second rolling (30) and the third rolling (40), which is the final pass, reduces the recrystallization and recovery of austenite as much as possible, and increases the strain accumulation effect, compared with the conventional rolling method. And a short time between passes and a relatively low temperature including the supercooled austenite region.
(5) Also in the third rolling (40) as the final pass, rolling with a sufficient reduction rate is performed to accumulate strain and increase the dislocation density. The outlet temperature at this time is set to a predetermined range.
(6) After the third rolling (40), it is quickly cooled (50) to promote ferrite transformation and suppress ferrite grain growth.
(7) At least in the third rolling (40), rolling is performed in a lubricated state, and the distribution in the plate thickness direction of strain applied by rolling is lowered to give more uniform strain.
(8) At least in the third rolling (40), the rolling is performed in a lubricated state, the temperature rise due to frictional heat generated in high pressure / high speed rolling is suppressed, and the strain accumulation effect is enhanced.
(9) Although the amount of equivalent strain given by lubrication rolling decreases, the effect of refining crystal grains can be maintained and improved by the effect of suppressing the temperature rise.

  Thus, according to the second aspect of the present invention, the main phase of ferrite is C: 0.04 to 0.20%, Si: 0.01 to 2.0%, and Mn: 0.5 to 3.% by mass. The first rolling (20%) is a method for producing a hot-rolled steel sheet containing 0% and the balance being Fe and inevitable impurities, and rolling at a total rolling reduction of 80% or more while maintaining the temperature range above the Ae3 transformation point. And B process including the second rolling (30) for performing one-pass rolling with a rolling reduction of 30 to 55% in the temperature range where the rolling mill entry side temperature is equal to or higher than the Ae3 transformation point following the A process, After the step, the C step including the third rolling (40) for performing one-pass rolling with a rolling reduction of 35 to 70% with the rolling mill entry side temperature as a predetermined temperature range, and continuously within 0.2 sec after the third rolling Cool to a temperature of (Ae3 transformation point -130 ° C) or lower at a cooling rate of 600 ° C / sec or higher. 50) If the predetermined temperature range in the C step is (Ae3 transformation point−60 ° C.) or more and less than (Ae3 transformation point−30 ° C.), the predetermined temperature range is within 0.6 sec. If (Ae3 transformation point−30 ° C.) or more and less than (Ae3 transformation point−5 ° C.), within 0.5 sec, if the predetermined temperature range is (Ae3 transformation point−5 ° C.) or more (Ae3 transformation point + 20 ° C.) The third rolling is performed within 0.3 sec, and at least in the third rolling, rolling oil is supplied between the material to be rolled and the rolling roll, and the Coulomb friction coefficient is rolled at 0.25 or less. .

  Here, the Ae3 transformation point is a thermal equilibrium temperature at which the ferrite transformation starts from a temperature at which the steel is in the austenite region. In addition, the “Coulomb friction coefficient” in rolling was obtained by performing a two-dimensional rolling analysis based on the ONOWAN non-uniform rolling theory, and using the friction coefficient as a variable to calculate backward so that the advanced rate and rolling load agree with the actual measurement values. The advanced rate can be obtained by attaching a mark to the rolling roll in advance and measuring a transfer interval at which the mark is transferred to the material.

  The invention according to claim 3 has ferrite as a main phase, and in mass%, C: 0.04 to 0.20%, Si: 0.01 to 2.0%, Mn: 0.5 to 3.0% In which the balance is Fe and inevitable impurities, and the first rolling is performed so that the structure at the end of rolling is an austenite single phase and the average grain size is 30 μm or less (20 A 'process including') and B process including second rolling (30) for performing one-pass rolling with a rolling reduction of 30 to 55% in the temperature range where the rolling mill entry side temperature is equal to or higher than the Ae3 transformation point following the A 'process. And after the B process, the rolling of the rolling mill is performed at a rolling temperature of (Ae3 transformation point−60 ° C.) or more and less than (Ae3 transformation point + 20 ° C.) in the third rolling (one rolling at a rolling reduction of 35 to 70%) ( 40) including C), and 600 ° C./sec or more within 0.2 sec after the third rolling. (50) D process which cools to the temperature below (Ae3 transformation point -130 degreeC) with a cooling rate, and the 3rd rolling has the entrance side temperature of this 3rd rolling (Ae3 transformation point-60 degreeC) or more ( If it is less than (Ae3 transformation point -30 ° C), within 0.6 sec after the second rolling, if the entry side temperature is (Ae3 transformation point -30 ° C) or more (Ae3 transformation point -5 ° C), after the second rolling. Within 0.5 sec, if the entry side temperature is (Ae3 transformation point−5 ° C.) or more and less than (Ae3 transformation point + 20 ° C.), it is performed within 0.3 sec after the second rolling, and at least in the third rolling, the material to be rolled Rolling oil is supplied between the steel and the rolling roll, and rolling is performed with a Coulomb friction coefficient of 0.25 or less.

  Invention of Claim 4 is a manufacturing method of the hot-rolled steel sheet of Claim 3, 1st rolling (20 ') is rolling of multiple continuous passes, and the entrance side of this 1st rolling If the temperature is 850 ° C. or more and less than 900 ° C., the total rolling reduction is 65% or more. If the temperature is 900 ° C. or more and less than 950 ° C., the total rolling reduction is 70% or more. If the temperature is 1000 ° C. or higher, the total rolling reduction is 80% or higher.

  Invention of Claim 5 is the manufacturing method of the hot-rolled steel sheet as described in any one of Claims 2-4, and the entrance side temperature of the 3rd rolling (40) is (Ae3 transformation point-60 degreeC) or more. The steel sheet is cooled between the second rolling (30) and the third rolling (40) so as to be less than (Ae3 transformation point + 20 ° C.).

  According to the hot-rolled steel sheet of the present invention, it is possible to improve the deformability and the uniform deformability at the time of secondary processing, which is disadvantageous in the conventional fine-grained ferrite steel sheet. In addition, it does not contain precipitation strengthening elements, and can be made high strength by refining crystal grains with a general-purpose steel plate, so it is excellent in product recyclability and can reduce the burden on the global environment. it can.

  According to the method for producing a hot-rolled steel sheet of the present invention, a steel sheet having the above characteristics can be reliably produced. Specifically, according to the present invention, no precipitation strengthening element is contained, the difference in ferrite grain size in the plate thickness direction is small, the crystal grain size is not excessively extended in the rolling direction, and the ferrite crystal grain size is increased. It is possible to produce a hot-rolled steel sheet satisfying that the thickness is less than 2 μm.

  Such an operation and gain of the present invention will be made clear from the best mode for carrying out the invention described below.

First, the steel plate of the present invention will be described.
<Components contained in steel sheet>
The components contained in the steel plate of the present invention may be the same as those contained in ordinary carbon steel. Specifically, C: 0.04 to 0.20% by mass%, Si: 0.01 to 2. It contains 0%, Mn: 0.5 to 3.0%, and the balance is made of a steel plate made of Fe and inevitable impurities. Each will be described below.

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 deoxidation during steelmaking and has an effect of improving the workability of the steel sheet. If the content exceeds 2.0% by mass, the fine ferrite structure of the present invention is used. Since the toughness as a hot-rolled steel sheet having the above is impaired, its content is 2.0 mass% as the upper limit. 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. Further, hot brittleness due to S is prevented, and the Ae3 transformation 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%.

<Ferrite phase>
The steel sheet of the present invention has a main phase as a ferrite phase. Therefore, the cross-sectional area of the ferrite phase may be 50% or more with respect to the cross-sectional area when the steel plate is cut at an arbitrary cross-section. Preferably it is 70% or more.

<Ferrite crystal grain size>
The ferrite crystals of the steel sheet of the present invention have a predetermined uniform particle size distribution in the thickness direction of the steel sheet. Specifically, it is as follows.
The ferrite crystal grain size at a position of 50 μm in the plate thickness direction from the steel plate surface is D1, the ferrite crystal grain size at a depth of ¼ of the plate thickness from the steel plate surface in the plate thickness direction is D2, and the plate thickness direction from the steel plate surface Further, when the ferrite crystal grain size at a depth of ½ of the plate thickness is D3, the following formula (2) is satisfied.
(D3-D1) /D2≦0.4 (2)

  Here, D1, D2, and D3 represent average particle diameters at the respective positions, and the average particle diameters are obtained by the ASTM cutting method. The distribution ratio in the plate thickness direction can be quantitatively evaluated by the equation (2). By satisfying the equation (2), a predetermined uniform particle size distribution is obtained in the plate thickness direction of the steel plate. means.

<Vertical and horizontal relationship of ferrite crystal grains>
Furthermore, the steel plate of the present invention satisfies the following formula (1) when the grain size in the rolling direction is Dt and the grain size in the plate thickness direction is Dt in ferrite grains located 50 μm from the steel plate surface in the plate thickness direction.
| (Dr−Dt) / ((Dr + Dt) / 2) | ≦ 0.25 (1)

  Here, Dr and Dt separate the measurement in the rolling direction from the measurement in the plate thickness direction when the ferrite structure is observed with a microscope in a cross section perpendicular to the width direction of the rolled material and the grain size is calculated by a cutting method. To get it. And the aspect ratio of particle | grains can be evaluated quantitatively by Formula (1), and it means that the structure | tissue which is not lamellar but high in equiax degree is formed by satisfy | filling Formula (1).

  With the steel plate of the present invention described above, it becomes possible to improve the deformability and uniform deformability at the time of secondary processing, which is disadvantageous in the conventional fine-grain ferrite steel plate. In addition, it does not contain precipitation strengthening elements, and can be made high strength by refining crystal grains with a general-purpose steel plate, so it is excellent in product recyclability and can reduce the burden on the global environment. it can.

Next, the manufacturing method of the hot-rolled steel sheet of this invention is demonstrated.
FIG. 1 is a flow diagram of a hot-rolled steel sheet manufacturing method S1 (hereinafter sometimes simply referred to as “manufacturing method S1”) according to the first embodiment of the present invention, and is described as appropriate. The manufacturing method S1 includes four steps of step A, step B, step C, and step D in this order. Each step will be described with reference to FIG.

<Process A>
Process A is a process including a first rolling with a total rolling reduction of 80% or more in a temperature range equal to or higher than the Ae3 transformation point to be an austenite single layer. Here, the first rolling is preferably multi-pass rolling, but is not limited thereto. By this first rolling, a material having an austenite particle size of 30 to 600 μm after heating can be rolled into a material to be rolled having a particle size of about 30 μm or less.

<Process B>
In the process B, the second rolling, which is a one-pass rolling with a rolling reduction of 30 to 55%, in the temperature range equal to or higher than the Ae3 transformation point with respect to the material to be rolled obtained in the process A, following the process A. It is a process including. If the rolling reduction is smaller than this range, fine particles cannot be obtained. The reason is not clear, but it is presumed that if the rolling reduction is insufficient, strain accumulation due to rolling will be insufficient. In addition, if the rolling reduction is larger than this range, the rolling load becomes excessive, and problems such as enlarging the equipment, exceeding the equipment limit, and destabilizing rolling such as occurrence of seizure also occur. The entry side temperature is set to a temperature range equal to or higher than the Ae3 transformation point. When the temperature before the second rolling is less than the Ae3 transformation point, the time during which the material to be rolled is in the supercooled austenite region becomes long, leading to the third rolling. This is because the ferrite transformation occurs. If the temperature before the second rolling is too high, recrystallization and recovery are likely to occur, and it becomes difficult to obtain fine-grained ferrite. Therefore, the temperature is preferably less than (Ae3 transformation point + 30 ° C.). The temperature before the second rolling can be adjusted by changing the air cooling / standby time. In addition, when it is necessary to greatly reduce the temperature, water cooling may be performed.

<Process C>
The process C is a process including the 3rd rolling which is 1-pass rolling with a rolling reduction of 35 to 70% within the time specified by the temperature range after the process B. Specifically, it is as follows.
(Condition 1) If the temperature before the third rolling is (Ae3 transformation point−60 ° C.) or more and less than (Ae3 transformation point−30 ° C.), the rolling reduction is 35 to 70% within 0.6 sec after the second rolling. 3rd rolling which is pass rolling is performed.
(Condition 2) If the temperature before the third rolling is (Ae3 transformation point−30 ° C.) or more and less than (Ae3 transformation point−5 ° C.), the rolling reduction is 35 to 70% within 0.5 sec after the second rolling. 3rd rolling which is pass rolling is performed.
(Condition 3) If the temperature before the third rolling is (Ae3 transformation point−5 ° C.) or more and less than (Ae3 transformation point + 20 ° C.), one pass with a rolling reduction of 35 to 70% within 0.3 sec after the second rolling. The 3rd rolling which is rolling is performed.

  In order to enhance the strain accumulation effect, the interval between the second rolling and the third rolling, that is, the time between passes should be as short as possible. However, in order to shorten the time between passes, in terms of the installation space of the rolling mill group and the rolling speed. There are limitations. If the time between passes is equal to or greater than the above value, the effect of crystal grain refinement is clearly reduced. The reason is that the longer the time between passes between the second rolling in the B process and the third rolling in the C process, and the higher the temperature before the third rolling, the more static recrystallization occurs. It is assumed that the accumulation of is insufficient. It is assumed that the lower the temperature before the third rolling, the longer the time between the second rolling and the third rolling may be because the lower the temperature, the more recrystallization is suppressed. Further, if the temperature before the third rolling is made too low, ferrite transformation before the third rolling is likely to occur, so in the present invention, it is set to (Ae3 transformation point −60) ° C. or higher. The lower limit temperature is considered to be related to the time required for cooling performed in the C stroke and the subsequent D stroke. In order to effectively perform “accumulation of strain in an unrecrystallized region” which is estimated to be effective for grain refinement, it is necessary to set the range of the above condition 1, condition 2, or condition 3 .

  Further, as a means for controlling the temperature before the third rolling in the above-mentioned step C to be (Ae3 transformation point−60 ° C.) or more and less than (Ae3 transformation point + 20 ° C.), heat generation and temperature rise in the second rolling are predicted. However, it is conceivable to adjust the temperature before the second rolling so that the temperature after rolling is in the above temperature range, but the second pre-rolling temperature has a constraint that it is not less than the Ae3 transformation point in order to avoid transformation before rolling. is there. On the other hand, as a means for suppressing the temperature increase in the second rolling, there is a method of increasing the heat removal from the roll by lowering the speed of the second rolling, but it is necessary to shorten the time between passes until the third rolling. There is a limit, and the temperature after rolling may not be adjusted. Therefore, a means for cooling the steel sheet between the second rolling and the third rolling is required. From the viewpoint of increasing the degree of freedom in equipment layout, it is desirable to use a rapid cooling device that can provide a large temperature drop over a short distance. For example, if a temperature drop of 10 ° C is required, the time between passes is 0.6 sec at the longest. In order to cool inside, a cooling rate of 17 ° C./sec or more is required. From the viewpoint of reducing recrystallization and recovery between passes as much as possible and enhancing the strain accumulation effect, it is better to complete the temperature adjustment by interpass cooling within a short time after the second rolling as much as possible. It is desirable to complete the cooling immediately after the second rolling using a cooling means having a speed.

  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, when the rolling reduction of the third rolling exceeds 70%, the generation of recrystallization / transformation during processing and processing heat generation that affects the subsequent cooling occur, so the effect of refining the crystal grains is reduced. In addition, the rolling load becomes excessive, causing problems such as enlarging equipment, exceeding equipment limits, and unstable rolling.

  Further, in the third rolling, rolling oil is supplied between the material to be rolled and the rolling roll, and rolling is performed with a Coulomb friction coefficient of 0.25 or less. When the first to third rolling is performed without lubrication, a large shear strain is generated on the surface side of the plate particularly in rolling under high pressure. In many cases, a difference in structure in the thickness direction is caused by the difference in strain. In particular, in high-speed rolling under high pressure, heat generated by friction is so great as to affect crystal refinement. This increase in temperature may inhibit ferrite crystal refinement.

  On the other hand, at least in the third rolling, if the rolling is performed by reducing the friction coefficient by lubrication, the strain amount in the plate thickness direction is equalized, and the structure in the plate thickness direction is equalized along with this, and the frictional heating is generated. It is reduced and excessive heat generation can be suppressed. This is advantageous for crystal grain refinement.

  In addition, since the rolling load can be reduced by lubrication rolling, the upper limit of the rolling reduction restricted by the equipment surface and the heat generation surface can be raised. For example, in the case of 50% reduction, if the lubrication rolling with the friction coefficient μ = 0.15 is performed with respect to the non-lubricating rolling with the friction coefficient μ = 0.4, the rolling load can be reduced by 40% or more. The temperature rise of the material can be reduced by 50 ° C. or more. For this reason, temperature control on the entrance side and the exit side of the third rolling becomes easy, and the scale and load of the cooling facility can be reduced. In order to obtain the above effects sufficiently, the friction coefficient needs to be 0.25 or less. Moreover, as an incidental effect, the effect is great from the viewpoint of practical use, such as the expansion of the range in which the current hot rolling equipment can be used without modification.

  Since the final product ferrite structure is greatly affected by the processing of the steel sheet, it is essential to lubricate by the third rolling, but lubrication rolling may also be performed by the first rolling and the second rolling. Further, if the friction coefficient is smaller than 0.1, the biting property of the material tip during rolling may be remarkably deteriorated. Therefore, the friction coefficient is desirably 0.1 or more.

<Process D>
Step D is a step of cooling to a temperature of (Ae3 transformation point-130 ° C.) or lower at a cooling rate of 600 ° C./sec or higher within 0.2 seconds after Step C. Thereby, a hot-rolled steel sheet in which a fine ferrite structure having an average particle diameter of 2.0 μm or less occupies 50% 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.

  The steel sheet of the present invention can be reliably manufactured by the manufacturing process S1 as described above. Specifically, according to the present invention, no precipitation strengthening element is contained, the difference in ferrite grain size in the plate thickness direction is small, the crystal grain size is not excessively extended in the rolling direction, and the ferrite crystal grain size is increased. It is possible to produce a hot-rolled steel sheet satisfying that the thickness is less than 2 μm.

  FIG. 2 is a flow diagram of the method for manufacturing a hot-rolled steel sheet according to the second embodiment of the present invention S2 (hereinafter sometimes simply referred to as “manufacturing method S2”), and the description is described as appropriate. The manufacturing method S2 includes four steps of step A ′, step B, step C, and step D in this order. That is, in the manufacturing method S2, the process A in the manufacturing process S1 is changed to the process A ', and the processes B, C, and D that are processes after the process A' are common. Therefore, only step A 'will be described here, and the other steps will be omitted.

  Step A ′ is a step including first rolling in which the material is rolled so that the structure at the end of rolling is an austenite single phase and the average particle size is 30 μm or less. This is because the smaller the austenite grain size and the larger the grain interfacial area per unit volume, the more efficiently the strain is accumulated in the second and third rolling processes in the subsequent process, and the nucleation site of transformation during the subsequent ferrite transformation It is because it is thought that it contributes to refinement | miniaturization of a ferrite grain more. If a ferrite structure is mixed at this point, it is stretched by a subsequent rolling process and finally remains in the processed structure, which is not preferable in terms of mechanical properties of the steel sheet.

  In order to make the austenite grain size 30 μm or less, specifically, rolling consisting of a plurality of continuous passes is performed, and if the inlet temperature is 850 ° C. or more and less than 900 ° C., the total rolling reduction is 65% or more, 900 ° C. or more and 950 Rolling at a total rolling reduction of 70% or more when it is less than ℃, rolling at a rolling reduction of 75% or more when it is 950 ° C or more and less than 1000 ° C, or rolling at 80% or more when it is 1000 ° C or more.

  In the basic experiment relating to the present invention, the number of passes is 2 to 4 passes, the total rolling reduction is 60 to 80%, the temperature before rolling is 830 ° C. to 1050 ° C., the rolled material is frozen after the rolling, and the austenite grain size is determined. As a result of the measurement, it was found that the austenite average particle diameter is 30 μm or less if the temperature and the total rolling reduction condition are included.

  The conditions for setting the average austenite grain size to 30 μm or less are not particularly limited, but rolling with one pass is not preferable because it requires rolling under one pass and a rolling load becomes excessive. If the reduction rate is limited and the number of passes is increased too much, the reduction rate per pass is lowered, and it becomes difficult to obtain a refining effect by recrystallization of austenite grains. The rolling reduction per pass is preferably 27% or more.

  In the present invention, since the material before the first rolling may be rolled, the total number of passes of rolling from the cast state is not limited. In addition, after the first rolling, the second rolling in the B step may be performed within a short time. On the contrary, if the time until the second rolling is long, austenite grains grow, which is not preferable. In the case where the entire process is continuously performed in the basic experiment, the second rolling was performed within about 1 to 10 seconds after the end of the final pass of the first rolling. There was no big difference.

  The steel plate of the present invention can be reliably manufactured also by the manufacturing process S2 as described above. Specifically, according to the present invention, no precipitation strengthening element is contained, the difference in ferrite grain size in the plate thickness direction is small, the crystal grain size is not excessively extended in the rolling direction, and the ferrite crystal grain size is increased. It is possible to produce a hot-rolled steel sheet satisfying that the thickness is less than 2 μm.

The production facility includes a heat treatment facility, a tandem rolling facility composed of two or more stands, and a cooling device disposed on the exit side of the rolling facility. Each stand of the rolling equipment is required to realize a rolling reduction of a predetermined value or more, and the time between passes between the second rolling and the third rolling is kept within 0.6 sec at most, so that the predetermined rolling Speed is required, and the distance between rolling mills needs to 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. Moreover, when performing water cooling between 2nd rolling and 3rd rolling, it is necessary to arrange | position a water cooling header in a rolling mill housing or between housings.

  In addition, the material steel plate used in the production methods S1 and S2 of the present invention may be a cast material, but in order to reduce internal defects during casting and miniaturize the austenite diameter, one or more hot workings are performed. It is preferable to obtain an austenite structure having a particle size of 600 μm or less. 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.

Next, the embodiment will be described in more detail. However, the present invention is not limited to this embodiment.
The materials adjusted to the components indicated by A to D in Table 1 were cut into cut plates having a width of 100 mm and a length of 70 to 200 mm to obtain test materials. 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. In addition, as described in the table, the Ae3 transformation points of steel types A, B, C, and D, which are the test materials, are 830 ° C., 800 ° C., 770 ° C., and 750 ° C., respectively. The Ae3 transformation point is a thermal equilibrium temperature at which the ferrite transformation starts from a temperature at which the steel is in the austenite region.

For the hot rolling, a three-stand hot rolling mill 10 that is continuously arranged in the heating furnace 11 as shown in FIG. 3 was used. The distance between the first stand (F1) 1 and the second stand (F2) 2 is 2.1 m, and the distance between the second stand (F2) 2 and the third stand (F3) 3 is 1.0 m. In addition, rolling with a time between passes of 0.6 seconds or less is possible. An inter-stand water-cooled header 13 was disposed between the second stand (F2) 2 and the third stand (F3) 3. The rolling reduction of each rolling stand was set to be 40% or more. The specimen 4 that has passed through the stands 1 to 3 from the heating furnace 11 is guided to the cooling device 12. The lubrication header 14 is installed on the entry side of each stand, and can inject the lubricant toward the work roll. Table 2 shows the rolling mill specifications and rolling conditions.

  As shown in Table 2, the specimen 4 was rolled in 4 to 5 passes in the first stand (F1) 1 so that the total rolling reduction was 70 to 80%. Then, 2nd rolling and 3rd rolling were implemented by 2nd stand (F2) 2 and 3rd stand (F3) 3, respectively.

  Table 3 shows the conditions of each process in the test conducted in this example. Here, the average γ (austenite) particle size described in the table is prepared by preparing a test piece different from the test piece to be used in the post-process, and rapidly quenching the test piece subjected to the first rolling under the same conditions to room temperature. And it measured by structure | tissue observation.

  As shown in Table 3, in each test represented by trial numbers 1 to 32, the trial test indicated by “Manufacturing Method S1” shown in the remarks column is based on the requirements of the manufacturing method S1 described above. A steel sheet is manufactured by a manufacturing method that is satisfied. Similarly, the test of the trial number indicated by “Manufacturing method S2” is a steel plate manufactured by a manufacturing method that satisfies the requirements of the manufacturing method S2. Furthermore, in the test of the trial number corresponding to any of the production methods, “production methods S1 and S2” are indicated. If the remarks column is blank, the manufacturing method does not satisfy any of the requirements.

  Table 4 shows the results of analysis on the hot-rolled steel sheets manufactured by the manufacturing methods shown in trial numbers 1 to 32.

  As can be seen from Table 4, in the example of the present invention appropriately lubricated and rolled in the C process, it is possible to obtain a steel sheet having good uniformity in particle size distribution in the sheet thickness direction and a small particle shape aspect ratio. . That is, it has a non-layered structure. As a result, a hot-rolled steel sheet having good elongation and excellent secondary workability can be obtained. About the mechanical property, it actually measured only about the example of trial numbers 1-7 and trial numbers 30-32. About the trial numbers 1-7, although it is based on the steel type A, the direction of this invention has shown the high elongation in any case. On the other hand, since the contained components are different for the trial numbers 30 to 32, it cannot be directly compared with the results of the trial numbers 4 to 7, but has the effect of the present invention obtained from the structure. About the trial number 31, although the value of elongation is small compared with others, this originates in the steel type C containing many C (carbon). From the above, it can be seen that the mechanical properties conceivable from the structure appear remarkably.

  FIG. 4 shows an enlarged view of the structure of a steel plate as one of the examples of the present invention and a steel plate as one of the comparative examples. The example of the present invention is based on trial number 3, and the comparative example 1 is based on trial number 4. This also shows that a steel sheet having an equiaxed structure having a uniform ferrite crystal grain size distribution in the thickness direction can be obtained by the manufacturing method of the present invention.

  Although the present invention has been described with reference to the most practical and preferred embodiments at the present time, the present invention is not limited to the embodiments disclosed herein. The hot-rolled steel sheet having a uniform fine ferrite structure and a method for producing the same can be changed as appropriate without departing from the scope or spirit of the invention that can be read from the claims and the entire specification. It should be understood as being included in the technical scope of the invention.

It is a flowchart of the manufacturing method of the present invention concerning a first embodiment. It is a flowchart of the manufacturing method of the present invention concerning a second embodiment. It is a figure which shows the example of a rolling apparatus. It is a structure enlarged view of the steel plate which shows an example of the result of an Example.

Explanation of symbols

1 First stand (F1)
2 Second stand (F2)
3 Third stand (F3)
4 Co-test material 10 3 Stand hot rolling mill device 11 Heating furnace 12 Cooling device 13 Inter-stand cold water header 14 Lubrication header 20 First rolling 30 Second rolling 40 Third rolling 50 Cooling

Claims (5)

Ferrite as the main phase, C: 0.04 to 0.20% by mass, Si: 0.01 to 2.0%, Mn: 0.5 to 3.0%, the balance is Fe and inevitable A hot-rolled steel plate made of impurities,
While the ferrite crystal grain size D2 at the 1/4 depth position of the plate thickness from the surface of the steel plate is less than 2.0 μm,
The relationship between the ferrite crystal grain size D3 at the position of the plate thickness 1/2 depth from the surface of the steel sheet and the ferrite crystal grain size D1 at the position of 50 μm depth from the surface of the steel sheet is (D3-D1) / D2 ≦ 0. 4
A hot rolled steel sheet, wherein a rolling grain size Dr and a plate thickness direction grain size Dt of the ferrite crystal grains satisfy a formula (1) at a depth of 50 μm from the surface of the steel plate.
| (Dr−Dt) / ((Dr + Dt) / 2) | ≦ 0.25 (1) Ferrite as the main phase, C: 0.04 to 0.20% by mass, Si: 0.01 to 2.0%, Mn: 0.5 to 3.0%, the balance is Fe and inevitable A method for producing a hot-rolled steel sheet made of impurities,
A process including the first rolling that rolls at a total rolling reduction of 80% or more while maintaining the temperature range above the Ae3 transformation point;
B process including 2nd rolling which performs 1 pass rolling of rolling reduction 30-55% in the temperature range whose rolling mill entrance side temperature is more than an Ae3 transformation point following said A process,
After Step B, Step C includes third rolling for performing one-pass rolling at a rolling reduction of 35 to 70% with a rolling mill entry side temperature as a predetermined temperature range;
And D step of cooling to a temperature of (Ae3 transformation point-130 ° C) or lower at a cooling rate of 600 ° C / sec or higher within 0.2 seconds after the third rolling,
In step C, if the predetermined temperature range is (Ae3 transformation point−60 ° C.) or more and less than (Ae3 transformation point−30 ° C.), the predetermined temperature range is (Ae3 transformation point−30 ° C.) within 0.6 sec. If it is less than (Ae3 transformation point−5 ° C.) within 0.5 sec, and if the predetermined temperature range is (Ae3 transformation point−5 ° C.) or more (Ae3 transformation point + 20 ° C.), it will be within 0.3 sec. While performing 3 rolling,
A method for producing a hot-rolled steel sheet, comprising rolling oil at least in the third rolling and supplying a rolling oil between a material to be rolled and a rolling roll with a Coulomb friction coefficient of 0.25 or less. Ferrite as the main phase, C: 0.04 to 0.20% by mass, Si: 0.01 to 2.0%, Mn: 0.5 to 3.0%, the balance is Fe and inevitable A method for producing a hot-rolled steel sheet made of impurities,
A ′ step including the first rolling that is rolled so that the structure at the end of rolling is an austenite single phase and the average particle size is 30 μm or less;
B process including 2nd rolling which performs 1 pass rolling of rolling reduction 30-55% in the temperature range whose rolling mill entrance side temperature is more than Ae3 transformation point following said A 'process,
After the step B, the rolling mill entry side temperature includes a third rolling that performs one-pass rolling with a rolling reduction of 35 to 70% in a temperature range of (Ae3 transformation point−60 ° C.) or more and less than (Ae3 transformation point + 20 ° C.). C process;
And D step of cooling to a temperature of (Ae3 transformation point-130 ° C) or lower at a cooling rate of 600 ° C / sec or higher within 0.2 sec after the third rolling,
In the third rolling, if the entry temperature of the third rolling is (Ae3 transformation point−60 ° C.) or more and less than (Ae3 transformation point−30 ° C.), the entry temperature is within 0.6 sec after the second rolling. If it is (Ae3 transformation point-30 ° C) or more and less than (Ae3 transformation point-5 ° C), the entry side temperature is (Ae3 transformation point-5 ° C) or more (Ae3 transformation point + 20 ° C) within 0.5 sec after the second rolling. ) If less than 0.3 second after the second rolling,
A method for producing a hot-rolled steel sheet, comprising rolling oil at least in the third rolling and supplying a rolling oil between a material to be rolled and a rolling roll with a Coulomb friction coefficient of 0.25 or less.   The first rolling is continuous multi-pass rolling, and if the entry temperature of the first rolling is 850 ° C. or higher and lower than 900 ° C., the total rolling reduction is 65% or higher and 900 ° C. or higher and lower than 950 ° C. 4. The heat according to claim 3, wherein the total rolling reduction is 70% or more, 950 ° C. or more and less than 1000 ° C., the total rolling reduction is 75% or more, and 1000 ° C. or more, the total rolling reduction is 80% or more. A method for producing rolled steel sheets.   The steel sheet is cooled between the second rolling and the third rolling so that the entrance temperature of the third rolling becomes (Ae3 transformation point−60 ° C.) or more and less than (Ae3 transformation point + 20 ° C.). The method for producing a hot-rolled steel sheet according to any one of claims 2 to 4.

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