JP2005305454A - Method for producing fine-grained hot rolled steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a fine-grained hot rolled steel sheet having excellent workability as-hot rolled and suitable as the stock for a high strength member used for automobiles, various industrial machinery and building. <P>SOLUTION: In the method for producing a hot rolled steel sheet where finish rolling is performed using a tandem rolling mill line after rough rolling, rolling is performed at an Ar<SB>3</SB>point or higher by the rolling mill before one step from the final in the tandem rolling mill line, subsequently, cooling is performed to the temperature range of ≤(Ar<SB>3</SB>point-20)°C at an average cooling rate of ≥50°C/s, further, rolling is performed at a draft of ≤20% by the final rolling mill in the tandem rolling mill line, and within 0.4 s after that, cooling is performed to 720°C. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing a fine-grain hot-rolled steel sheet, and more specifically, a method for producing a fine-grain hot-rolled steel sheet suitable as a material for a high-strength member used for automobiles, various industrial machines or buildings, The present invention relates to a method for producing a fine-grain hot-rolled steel sheet having a fine structure as it is hot rolled and excellent in workability.

  Steel materials used as structural members for transportation machines including automobiles and various industrial machines, or as building materials are required to be excellent in mechanical properties such as strength, workability and toughness. In order to improve these mechanical properties comprehensively, it is effective to refine the structure of steel, and strengthening the structure by refining the structure of steel can also reduce the alloy components, thus reducing the product cost. It is. For this reason, many manufacturing methods for obtaining a fine structure have been studied.

  As the means for refining the structure in the prior art, for example, Patent Documents 1 to 3 propose a technique related to “large rolling rolling”, and Patent Documents 4 and 5 propose a technique related to “controlled rolling / controlled cooling”. .

  That is, in Patent Document 1, in a subsequent stage of continuous hot rolling, a rolling reduction of 40% or more and an average strain rate of 60 seconds −1 are applied, and further, the rolling reduction is continuously 40% within 2 seconds. A technique for refining the structure by the large reduction rolling that applies the above reduction is disclosed.

  Patent Document 2 discloses a technique in which a steel structure is refined by quenching immediately after rolling from a state of accumulating rolling strain within 0.5 seconds.

  Patent Document 3 discloses a technique in which so-called “C—Si—Mn steel” is subjected to multi-pass rolling in a dynamic recrystallization region to obtain a fine grain structure having an average grain size of less than 2 μm.

  Patent Document 4 discloses a technique for forcibly cooling the surface of a so-called “C—Si—Mn steel” before finish rolling to obtain a hot-rolled steel sheet having a fine surface layer.

  Patent Document 5 discloses a first-stage rolling process in which a so-called “C—Si—Mn—Ti steel” is subjected to dynamic recrystallization by rolling at a temperature range of 1100 to 950 ° C. so that the reduction amount is 20% or more. And rolling at a temperature range of less than 950 ° C. and 700 ° C. or higher at a cooling rate of 5 ° C./second or more, with a reduction amount of 20% or more per pass and a cumulative reduction ratio of 50% or more. A technique for obtaining a steel sheet having an average grain size of 2 μm or less by a second stage rolling process in which static recrystallization is repeated is disclosed.

JP 59-229413 A JP 60-243226 A Japanese Patent Laid-Open No. 11-152544 JP-A-9-137248 Japanese Patent Laid-Open No. 11-92859

  An object of the present invention is to provide a method for producing a fine-grain hot-rolled steel sheet having an average grain diameter of ferrite of 5 μm or less, which is suitable as a material for high-strength members used for automobiles, various industrial machines or buildings. .

  In the techniques disclosed in Patent Documents 1 to 5 described above, a fine-grain hot-rolled steel sheet having an average grain size of ferrite of 5 μm or less is not necessarily obtained stably and reliably.

  That is, the technique proposed in Patent Document 1 requires a reduction amount per pass of 40% or more, which is difficult to achieve with a general hot strip mill. Furthermore, it is difficult to control the plate thickness.

  The technique proposed in Patent Document 2 is difficult to stably obtain fine grains when the rolling reduction in the final rolling mill is low.

  Also, the technique proposed in Patent Document 3 is difficult to apply to a general hot strip mill. This is because it is extremely difficult to stably control the rolling temperature in the dynamic recrystallization temperature range in a general hot strip mill.

  The technique proposed in Patent Document 4 has a large particle size of 10 μm or more inside the steel plate, and the contribution to the strengthening of the entire steel material is very slight just by making the surface layer finer.

  In the technique proposed in Patent Document 5, in the case of steel in which the Ti content is lower than the specified value, the first stage of dynamic recrystallization is insufficient, and it is difficult to refine crystal grains. In the case of an additive-free steel, even if this rolling technique is applied, the particle size is only 11 μm or more.

The present inventors have made various studies in order to solve the above problems. As a result, in the so-called “tandem hot rolling” in which finish rolling is performed using a tandem rolling mill row after rough rolling, rolling is performed at a temperature of ₃ or more points in the rolling mill one stage before the end of the tandem rolling mill row, and thereafter Within 0.4 seconds after cooling at an average cooling rate of 50 ° C./second or more to a temperature range of “Ar ₃ points−20 ° C.” or less and further rolling at a rolling reduction of 20% or less in the final rolling mill. It was found that by cooling to 720 ° C., a structure having fine ferrite crystal grains having an average particle diameter of 5 μm or less can be obtained.

  The reason why the microstructure having the fine ferrite is obtained by finish rolling in such “tandem hot rolling” is not necessarily clear, but is presumed to be due to the following (1) to (3).

  (1) The rapid cooling before rolling by the final rolling mill makes it possible to suppress the release of processing strain imparted to austenite by rolling in the rolling mill one stage before the final.

  (2) A large amount of strain is accumulated in order to receive rolling in the next final rolling mill while the release of processing strain is suppressed.

  (3) The rapid cooling immediately after rolling by the final rolling mill reaches a temperature range in which ferrite transformation is actively generated without releasing the strain of austenite, so that nucleation of ferrite is promoted remarkably.

  The present invention has been completed based on the above findings.

  The gist of the present invention resides in a method for producing a fine-grain hot-rolled steel sheet having an average grain size of ferrite of 5 μm or less shown in the following (1) to (5).

(1) A method for producing a fine-grained hot-rolled steel sheet that is subjected to finish rolling using a tandem rolling mill train after rough rolling, and a temperature of Ar ₃ or higher in a rolling mill one stage before the final stage of the tandem rolling mill train And then cooled to a temperature range of “Ar ₃ point−20 ° C.” or less at an average cooling rate of 50 ° C./second or more, and further at a rolling reduction of 20% or less in the final rolling mill of the tandem rolling mill row. A method for producing a fine-grain hot-rolled steel sheet having an average grain size of ferrite of 5 μm or less, which is rolled and then cooled to 720 ° C. within 0.4 seconds.

(2) A method for producing a fine-grain hot-rolled steel sheet, which is subjected to finish rolling using a tandem rolling mill train after rough rolling, and a temperature of Ar ₃ or higher in the rolling mill one stage before the end of the tandem rolling mill train And then cooled to a temperature range of “Ar ₃ point−20 ° C.” or less at an average cooling rate of 50 ° C./second or more, and further at a rolling reduction of 20% or less in the final rolling mill of the tandem rolling mill row. A method for producing a fine-grain hot-rolled steel sheet having an average grain size of ferrite of 5 μm or less, characterized by cooling to 720 ° C. within 0.2 seconds at an average cooling rate of 400 ° C./second or more after rolling.

  (3) The above (1) or (2), wherein the sheet bar is heated by an auxiliary heating device after roughly rolling the steel ingot or steel slab and before performing finish rolling using a tandem rolling mill row A method for producing a fine-grain hot-rolled steel sheet having an average grain size of ferrite of 5 μm or less.

  (4) In mass%, C: 0.02-0.2%, Mn: 0.05-3.0%, Si: 0.001-3.0%, P: 0.001-0.2% And Al: 0.001 to 3%, with the balance having an average particle diameter of 5 μm or less of the ferrite according to any one of the above (1) to (3) for a steel having a chemical composition composed of Fe and impurities Method for producing fine-grain hot-rolled steel sheets.

  (5) By mass%, C: 0.02-0.2%, Mn: 0.05-3.0%, Si: 0.001-3.0%, P: 0.001-0.2% And Al: 0.001 to 3%, and further includes at least one component selected from one or more of the following groups (a) to (c), the balance being substantially Fe and impurities The method for producing a fine-grain hot-rolled steel sheet having an average grain size of ferrite of 5 μm or less according to any one of (1) to (3), wherein the steel has a chemical composition comprising

(A) Ca: 0.0002 to 0.010%, Zr: 0.01 to 0.10%, and REM (rare element): 0.002 to 0.10%,
(B) Nb: 0.005-0.10%, Ti: 0.005-0.20% and V: 0.005-1.0%,
(C) Cr: 0.05-1.0% and Mo: 0.05-1.0%.

  The temperature referred to in the present invention refers to the temperature on the steel sheet surface, and the “average cooling rate” refers to the temperature difference before and after cooling divided by the cooling time.

The rolling reduction (%), when the material to be rolled thickness before rolling n pass t _ni, the material to be rolled thickness after rolling was _{t n0 {(t ni -t n0} ) / t ni} × 100 Means what is required in

  The “average particle diameter” of ferrite refers to a value obtained by multiplying the average intercept length obtained by the cutting method by 1.128.

  “REM (rare earth element)” is a general term for a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM indicates the total content of the above elements.

  The “steel ingot” as used in the present invention includes a slab produced by continuous casting.

  Hereinafter, the inventions relating to the method for producing a fine-grain hot-rolled steel sheet having the average grain size of ferrite of (1) to (5) of 5 μm or less are referred to as “present invention (1)” to “present invention (5)”, respectively. That's it. Also, it may be collectively referred to as “the present invention”.

  According to the present invention, it is possible to easily obtain a fine-grain hot-rolled steel sheet having an excellent workability and an average particle diameter of ferrite of 5 μm or less. Fine-grained hot-rolled steel sheets with an average particle size of ferrite of 5 μm or less produced by the method of the present invention are used as materials for high-strength members of automobiles, especially as materials for automobile undercarriage parts, or for various industrial machines and buildings. It is suitable as a material for a high-strength member to be used for the purpose.

  Hereinafter, each requirement of the present invention will be described in detail.

(A) Structure of fine-grain hot-rolled steel sheet The fine-grain hot-rolled steel sheet of the present invention has a fine structure in which the average particle diameter of ferrite is 5 μm or less.

  Here, when the main phase of the structure is composed of a phase other than ferrite, for example, pearlite, cementite, bainite, martensite, or untransformed austenite (so-called “residual austenite”), it remains excellent in hot rolling. It becomes difficult to ensure the “strength-ductility balance”. Therefore, it is preferable to use ferrite as the main phase.

  Here, the “main phase” refers to a phase whose proportion in the structure exceeds 50% by volume. In addition, it is generally known that the volume ratio of a certain phase is equal to the area ratio. Therefore, the volume ratio occupied by ferrite is determined from the ratio of ferrite obtained by a two-dimensional evaluation method, for example. Can do.

  In the present invention, the proportion of ferrite in the structure is preferably 60% or more. More preferable is 70% or more, and a value close to 100% may be used. In the structure of the fine-grain hot-rolled steel sheet of the present invention, the phase other than ferrite as the main phase is composed of one or more of pearlite, cementite, bainite, martensite, and untransformed austenite.

  Moreover, when the average particle diameter of ferrite is 5 μm or less, the target strength can be ensured with an addition amount of an alloy element which is smaller than that of a conventional steel plate. Furthermore, the so-called “plating property” when the plating process is performed is also good. On the other hand, when the average particle diameter of ferrite exceeds 5 μm, the increase in strength due to refinement is remarkably reduced, so that it is necessary to increase the content of the alloy element, resulting in an increase in cost.

  Therefore, the microstructure of the fine-grain hot-rolled steel sheet of the present invention has been defined such that the average grain size of ferrite is 5 μm or less.

  The average particle diameter of ferrite is preferably 4 μm or less from the viewpoint of increasing the strength, more preferably 3 μm or less, and may be submicron, but from the viewpoint of workability, The average particle size is preferably 1 μm or more, more preferably 1.5 μm or more.

  As described above, the “average particle size” of ferrite refers to a value obtained by multiplying the average intercept length obtained by the cutting method by 1.128.

(B) Production conditions for fine-grain hot-rolled steel sheet
The method for producing a fine-grain hot-rolled steel sheet having an average grain size of ferrite of 5 μm or less according to the present invention is defined as follows with the rolling and cooling conditions in finish rolling by a tandem rolling mill train after rough rolling as essential requirements. To do.

That is, after “rolling at the temperature of Ar ₃ point or higher by the rolling machine one stage before the end of the tandem rolling mill row, the temperature range of“ Ar ₃ point −20 ° C. ”or lower at an average cooling rate of 50 ° C./second or higher. Is further cooled to 720 ° C. within 0.4 seconds after rolling at a rolling reduction of 20% or less in the final rolling mill of the tandem rolling mill row ”.

  Here, it is not necessary to specify in particular about the manufacturing method of the steel ingot or steel piece used as the rolling object of the rough rolling performed before the finish rolling by the said tandem rolling mill row. For example, it may be any of rimmed steel, capped steel, semi-killed steel, killed steel and the like melted by a converter, electric furnace, flat furnace, or the like. Further, the target steel may be a steel ingot using any means of “ingot forming method” or “continuous casting method” injected into a mold. In other words, the rough rolling performed before the finish rolling by the tandem rolling mill may be performed by a normal method using, for example, a steel ingot or steel slab as described below.

That is, the steel ingot or steel slab subjected to rough rolling is once cooled to a temperature range of Ar ₃ point or lower and then reheated to a temperature of Ac ₃ point or higher, in a temperature range higher than Ar ₃ point. Steel that has been cooled and reheated, or a steel ingot that has not been lowered to the temperature range of Ar ₃ point or less after casting, or a steel slab that has not been lowered to the temperature range of Ar ₃ point or less after hot working Either may be sufficient. From the viewpoint of fine graining, it is preferable that the material is once cooled to a temperature range of Ar ₃ point or less and then reheated to a temperature of Ac ₃ point or more. When it is subjected to rough rolling as cast, it may be passed through an auxiliary heating device or charged into a heating furnace for the purpose of heat retention or heating.

  In addition, it is preferable that the heating temperature in reheating a steel ingot or a steel piece shall be 1200 degrees C or less which does not coarsen an austenite crystal grain. The heating temperature is preferably set to 1000 ° C. or higher in order to ensure the rolling temperature and reduce the load on the rolling mill. More preferably, it is 1100 degreeC or more.

Moreover, not lowered the temperature after temperature drop to between steel ingot or heat not processed until Ar ₃ point or less of the temperature range to Ar ₃ point or less of the temperature range after casting any regard to the steel slab, casting and hot working After that, it is desirable to cool the steel ingot and the steel slab to a temperature range of 1200 ° C. or lower and then perform rough rolling to suppress grain growth during rolling. In this case, the rough rolling is preferably started from a temperature range of 1000 ° C. or higher in order to ensure the rolling temperature and reduce the load on the rolling mill. More preferably, it is 1100 degreeC or more.

After the above rough rolling, the above-described finish rolling and cooling, that is, “average cooling of 50 ° C./second or more after rolling at a temperature of Ar ₃ point or higher in a rolling machine one stage before the end of the tandem rolling mill row. It is cooled to a temperature range of “Ar ₃ point−20 ° C.” or less at a speed, and further rolled at a rolling reduction of 20% or less in the final rolling mill of the tandem rolling mill row, and then to 720 ° C. within 0.4 seconds. By performing the process of “cooling”, the release of strain accumulated in austenite is suppressed, and this is used as a driving force to promote nucleation of ferrite from austenite. Therefore, the average grain size of ferrite is as small as 5 μm or less. Organization is obtained.

When the rolling temperature in the rolling machine one stage before the end of the tandem rolling mill is less than the Ar ₃ point, not only does it cause the formation of processed ferrite, but also the transformation to untransformed austenite due to strain concentration on soft ferrite. Since strain accumulation is insufficient, it is impossible to achieve finer ferrite to a desired size.

  When the average cooling rate after rolling in the rolling machine one stage before the final stage of the tandem rolling mill is below 50 ° C./sec, even if the rolling temperature is ensured, the desired size can be achieved. Ferrite refinement cannot be achieved.

Further, even when the cooling temperature after rolling in the rolling machine one stage before the end of the tandem rolling mill row exceeds “Ar ₃ point−20 ° C.”, the refinement of ferrite to a desired size cannot be achieved. Furthermore, if the rolling reduction ratio in the final rolling mill of the tandem rolling mill row exceeds 20%, not only will it cause the formation of processed ferrite, leading to a decrease in workability, but an excessive reduction ratio will result in poor plate thickness. It becomes easy.

  Even when the cooling time until the temperature reaches 720 ° C. exceeds 0.4 seconds after the rolling in the final rolling mill of the tandem rolling mill row is finished, refinement of ferrite to a desired size cannot be achieved. Here, the reason for prescribing the time until cooling to 720 ° C. is that when the cooling is temporarily stopped or slowed down at a temperature exceeding 720 ° C., the strain introduced by rolling is reduced before the fine ferrite is formed. This is because it is released or the existence state of strain is changed and becomes ineffective for nucleation of ferrite, and the ferrite grains are remarkably coarsened.

Therefore, in the present invention (1), after rolling at the temperature of Ar ₃ point or higher with the rolling machine one stage before the end of the tandem rolling mill row, the “Ar ₃ point − The temperature was reduced to a temperature range of “20 ° C.” or lower, and further, rolling was performed at a rolling reduction of 20% or lower with the final rolling mill in the tandem rolling mill row, and then cooled to 720 ° C. within 0.4 seconds.

The temperature in the present invention refers to the temperature on the steel sheet surface, “average cooling rate” refers to the difference between the temperature difference before and after cooling divided by the cooling time, and the reduction ratio (%) is the material to be rolled before rolling in the n-th pass. As described above, the thickness is determined by {(t _ni −t _n0 ) / t _ni } × 100, where t _ni is the thickness and t _{n 0} is the thickness of the rolled material after rolling.

From the viewpoint of accumulating rolling distortion, the rolling temperature in the rolling mill one stage before the end of the tandem rolling mill row should be Ar ₃ point or higher and “Ar ₃ point + 100 ° C.” or lower. preferable. More preferably, it is Ar3 point or higher and “Ar3 point + 60 ° C.” or lower.

  The average cooling rate after rolling in the rolling machine one stage before the end of the tandem rolling mill row is preferably 70 ° C./second or more, and more preferably 100 ° C./second or more. The upper limit of the average cooling rate does not need to be specified in particular, and may be the maximum average cooling rate possible with respect to the size of the material to be rolled in terms of equipment capacity.

In addition, after rolling in the rolling machine one stage before the end of the tandem rolling mill row, it is preferable to cool to a temperature range of “Ar ₃ point −40 ° C.” or lower, and “Ar ₃ point −60 ° C.” or lower. It is more preferable to cool to a temperature range of. This cooling may be performed up to a temperature in the vicinity of 740 ° C., for example.

  Note that the rolling in the final rolling mill of the tandem rolling mill row is such that the cooling water after rolling in the rolling mill one stage before the final rolling mill does not flow out to the outlet side (that is, the tandem outlet side) of the final rolling mill. In addition, the cooling water for rapid cooling immediately after rolling on the exit side of the final rolling mill has a so-called “draining” function so that it does not flow backward to the entry side of the final rolling mill. This process is necessary to prevent interference between cooling waters poured from above and below the steel sheet, which is a material to be rolled, to perform precise cooling control and uniform cooling. And the last rolling mill gives tension to the steel plate which is a material to be rolled with the rolling mill of the previous stage, and prevents the deterioration of the sheet passing property and the plate thickness shape due to cooling and roll heat removal. It also has a cooling function for steel plates. For this reason, if each said function is ensured, in the final rolling mill of a tandem rolling mill row, it is possible to make the rolling reduction ratio 0% only by bringing the material to be rolled into contact with the roll. However, from the viewpoint of sufficiently performing strain accumulation and making the ferrite crystal grains finer, the rolling reduction in the final rolling mill of the tandem rolling mill row is preferably 1% or more, and more preferably 5% or more.

  The cooling after the end of rolling in the final rolling mill of the tandem rolling mill row is 0 at an average cooling rate of 400 ° C./second or more after the end of rolling in the final rolling mill from the viewpoint of suppressing the release of rolling distortion. It is preferable to carry out to 720 ° C. within 2 seconds.

Therefore, in the present invention (2), after rolling at a temperature of Ar ₃ point or higher with a rolling mill one stage before the end of the tandem rolling mill row, “Ar ₃ point − at an average cooling rate of 50 ° C./second or higher”. After cooling to a temperature range of “20 ° C.” or less and further rolling at a rolling reduction of 20% or less with the final rolling mill in the tandem rolling mill row, within 0.2 seconds at an average cooling rate of 400 ° C./sec or more. It was prescribed to cool to 720 ° C.

  It is more preferable that the cooling performed within 0.2 seconds at the above average cooling rate of 400 ° C./second or higher is performed up to 700 ° C.

  When the temperature of the steel sheet reaches 720 ° C. after finishing rolling in the final rolling mill, ferrite transformation starts and transformation to ferrite becomes active at a temperature between 720 ° C. and 580 ° C. Therefore, in the present invention (1), the lower limit temperature when cooling to 720 ° C. within 0.4 seconds after rolling at a rolling reduction of 20% or less in the final rolling mill of the tandem rolling mill row is 640 ° C. Is good. Further, in the present invention (2), after rolling at a rolling reduction of 20% or less with a final rolling mill in a tandem rolling mill row, the sheet is cooled to 720 ° C. within 0.2 seconds at an average cooling rate of 400 ° C./second or more. The lower limit temperature in this case is also preferably 640 ° C.

  Alternatively, in both cases of the present invention (1) and the present invention (2), after reaching 720 ° C., the cooling is temporarily stopped or the cooling rate is slowed down for 1 second in the temperature range of 720 to 580 ° C. It is preferable to stay above. The residence time in the temperature range is preferably 2 seconds or longer, and more preferably 4 seconds or longer.

  From the viewpoint of suppressing the growth of crystal grains during finish rolling, it is preferable to lower the start temperature of finish rolling in the tandem rolling mill row. For this purpose, it is preferable to roughly roll a steel ingot or steel slab having a temperature of 1200 ° C. or less, and then perform finish rolling using a tandem rolling mill. However, if the temperature before the rolling-side front end portion of the material to be rolled enters the tandem rolling mill row is lowered, the temperature drop at the rear end portion and the edge portion increases. Therefore, after rough rolling and before finish rolling using a tandem rolling mill row, it is a material to be rolled by an auxiliary heating device in order to prevent a temperature drop at the rear end portion or edge portion of the material to be rolled. The seat bar should be heated.

  For the reasons described above, in the present invention (3), after roughly rolling a steel ingot or steel slab, the sheet bar is heated by an auxiliary heating device before finishing rolling using a tandem rolling mill row. .

  In order to refine the ferrite, the heating temperature by the auxiliary heating device is preferably 1100 to 950 ° C. The upper limit of the heating temperature is more preferably 1050 ° C, still more preferably 1000 ° C.

  The cooling conditions until winding, the winding temperature, and the cooling conditions after winding after the above-described cooling performed after rolling in the final rolling mill of the tandem rolling mill row are not particularly limited, and the heat to be manufactured What is necessary is just to determine suitably according to the structure | tissue of a rolled steel plate.

  For example, when ferrite is the main phase and the structure other than ferrite, which is the main phase, is collectively referred to as the second phase, and a structure containing pearlite or cementite as the second phase is desired, a low-temperature transformation phase such as bainite or martensite What is necessary is just to perform cooling and winding-up on the conditions which avoid formation. In addition, when it is desired to obtain a composite structure such as “DP steel (duplex steel)” or “TRIP steel” as the second phase, cooling is performed so as to pass through the nose of the ferrite region on the cooling curve. After the ferrite transformation is promoted, the pearlite transformation is avoided and the bainite or martensite region is quenched, and then winding is performed.

  If you want to make a composite structure containing 3-30% of martensite by volume as the second phase, after cooling with the final rolling mill of finish rolling, cool to 350 ° C within 30 seconds from the end of rolling. It is good to wind up. In addition, it is more preferable to cool to 250 ° C. within 20 seconds from the end of finish rolling from the viewpoint of further improving the workability by promoting the formation of martensite. The volume ratio of martensite is more preferably 5 to 30%, and further preferably 7 to 30%.

  Furthermore, when it is desired to form a composite structure containing 3 to 30% of untransformed austenite (so-called “residual austenite”) as the second phase, after completion of rolling after finishing rolling in the final rolling mill of finish rolling, rolling is completed. It is good to cool to 500 degreeC within 30 second and to wind up at 500-350 degreeC. From the viewpoint of further improving the workability by increasing the proportion of retained austenite, it is more preferable to cool to a temperature range of 480 to 370 ° C. within 20 seconds from the end of finish rolling. The cooling rate is more preferably 50 ° C./second or more. The volume ratio of retained austenite is more preferably 5 to 30%, and still more preferably 7 to 30%.

  The hot rolling is preferably performed using a lubricant for the purpose of reducing rolling load. Further, cooling may be performed between the rolling mills from the final tandem rolling mill row of “tandem hot rolling” to the rolling mill two stages before to suppress the temperature rise of the material to be rolled due to the rolling. Lubricating rolling is preferably performed from the last to the first rolling mill from the viewpoint of sheet feeding.

  When the hot-rolled steel sheet manufactured by the method of the present invention is subjected to surface treatment such as hot-dip galvanizing, alloying hot-dip galvanizing, and electroplating, a surface-treated steel sheet having excellent surface properties and ductility is obtained. Can do.

(C) Chemical composition of target steel The present invention (1) to present invention (3) are not particularly defined with respect to the chemical composition of steel to be manufactured. However, in order to obtain a fine-grained hot-rolled steel sheet having an excellent “strength-ductility balance” and excellent workability and having an average grain size of ferrite of 5 μm or less, the chemical composition of the steel to be manufactured is defined by 4) and the present invention (5). In the following description, “%” of the content of each element means “mass%”.

C:
C is a preferable component for increasing the strength of the steel sheet, and the content is preferably 0.02% or more. However, if its content exceeds 0.20%, workability and weldability may be deteriorated. Therefore, the C content is preferably 0.02 to 0.20%.

Mn:
Mn secures the strength of the steel sheet and fixes S present as an impurity in the steel as MnS to suppress cracks that occur during hot working such as continuous casting or hot rolling. Have. However, when the content of Mn is less than 0.05%, the above-mentioned effect is difficult to obtain. On the other hand, the content exceeding 3.0% not only saturates the action but also causes a decrease in workability. Sometimes. For this reason, the content of Mn is preferably 0.05 to 3.0%. A more preferable Mn content is 0.1 to 2.5%. When it is desired to leave the austenite as the second phase as it is, that is, when it is desired to have a structure containing so-called “residual austenite”, the lower limit of the Mn content is preferably 0.5%. Preferably it is 0.8%. When martensite is to be generated as the second phase, the value of “Mn + Si”, which is the sum of the Si content described below, is preferably 1% or more, more preferably 1.5%. That's it.

Si:
Si is a preferred component that improves the strength and ductility of the steel sheet through solid solution strengthening. Further, it has an effect of increasing the amount of ferrite and increasing the amount of austenite remaining untransformed in so-called “TRIP steel” (so-called “residual austenite”). However, even if Si is contained in an amount exceeding 3.0%, the effect of the above action is saturated and weldability may be deteriorated. On the other hand, the lower limit may be 0%, but 0.001% is preferably the lower limit from the viewpoint of cost required for reduction. Therefore, the Si content is preferably 0.001 to 3.0%. The Si content is more preferably 0.03 to 2.0%. When it is desired to leave the austenite untransformed as the second phase, that is, when a structure containing so-called “residual austenite” is desired, the value of “Si + Al”, which is the sum of the Al content described later, is 1 % Or more is preferable. Moreover, when it is desired to generate martensite as the second phase, the value of “Mn + Si” which is the sum of the Mn content is preferably 1% or more, more preferably 1.5% or more. is there.

P:
P has the effect | action which raises the intensity | strength of a steel plate. However, if P is contained in excess of 0.2%, embrittlement or weldability may be deteriorated due to grain boundary segregation. On the other hand, the lower limit may be 0%, but 0.001% is preferably the lower limit from the viewpoint of cost required for reduction. Therefore, the P content is preferably 0.001 to 0.2%. In order to more reliably prevent embrittlement and weldability deterioration, the upper limit of the P content is more preferably 0.1%. In order to further improve the workability, the upper limit value of P is more preferably 0.05%, and extremely preferably 0.02%.

Al:
Al has a deoxidizing action, an increase in the proportion of ferrite as a main phase in the structure, and an action of increasing the amount of “residual austenite” in so-called “TRIP steel”. In order to obtain such an effect reliably, the Al content is preferably 0.001% or more. However, even if Al is contained in excess of 3%, the above effect is saturated and the cost is increased. Therefore, the Al content is preferably 0.001 to 3%. In addition, when adding Al only for the purpose of deoxidation, the upper limit of the Al content is preferably 0.10%, and from the economical viewpoint, the upper limit is more preferably 0.05%. preferable.

  The chemical composition of the steel according to the present invention (4), which is the production object of the present invention (1) to the present invention (3), is composed of the above elements from C to Al, with the balance being Fe and impurities. Is.

  The chemical composition of the steel according to the present invention (5) to be produced by the present invention (1) to the present invention (3) is replaced with a part of Fe of the steel according to the present invention (4). And at least one component selected from one or more of the following groups (a) to (c).

(A) Ca: 0.0002 to 0.010%, Zr: 0.01 to 0.10%, and REM (rare element): 0.002 to 0.10%,
(B) Nb: 0.005-0.10%, Ti: 0.005-0.20% and V: 0.005-1.0%,
(C) Cr: 0.05-1.0% and Mo: 0.05-1.0%.

  Here, since any element from Ca to REM (rare earth element) described in the group (a) has an effect of improving the cold workability by adjusting the shape of inclusions, the elements from Ca to REM are In the range described below, each of them may be contained alone or in combination of two or more.

  As described above, REM indicates a total of 17 elements of Sc, Y and lanthanoid, and the content of REM indicates the total content of the above elements.

  Since any element from Nb to V described in the group (b) is precipitated as a carbonitride on the ferrite ground and has the effect of further increasing the strength by precipitation strengthening, the elements from Nb to V are described below. Each may be contained alone within the range, or two or more of them may be combined and contained.

  In addition, since Cr and Mo described in the group (c) have the effect of improving the hardenability and increasing the ratio of so-called “residual austenite”, Cr and Mo are within the ranges described below. It may be contained alone or in combination.

(A) Group (Ca, Zr and REM):
Ca, Zr, and REM are all elements that have an effect of improving the cold workability by adjusting the shape of inclusions. In order to reliably obtain this effect, it is preferable that Ca is 0.0002% or more, Zr is 0.01% or more, and REM is 0.002% or more. However, when the contents of Ca, Zr, and REM exceed 0.010%, 0.10%, and 0.10%, respectively, the inclusions in the steel increase so that the workability may deteriorate. Therefore, when adding Ca, Zr, and REM, the content should be 0.0002 to 0.010%, 0.01 to 0.10%, and 0.002 to 0.10%, respectively. .

(B) Group (Nb, Ti and V):
Nb, Ti, and V are all elements that have the effect of precipitating as a carbonitride on ferrite ground and further increasing the strength by precipitation strengthening. In order to obtain this effect with certainty, it is preferable that the content of Nb, Ti and V is 0.005% or more. However, even if Nb, Ti, and V are contained in amounts exceeding 0.10%, 0.20%, and 1.0%, the above effects are saturated and the cost is increased. Therefore, when Nb, Ti and V are added, their contents should be 0.005 to 0.10%, 0.005 to 0.20% and 0.005 to 1.0%, respectively. .

(C) Group (Cr and Mo):
Cr and Mo are both elements that have an effect of increasing the strength of the steel sheet by transformation strengthening. In order to reliably obtain this effect, it is preferable that both Cr and Mo have a content of 0.05% or more. However, even if both Cr and Mo are contained in excess of 1.0%, the above effects are saturated and the cost is increased. Therefore, when adding Cr and Mo, the content is preferably 0.05 to 1.0%.

  About the element of said (a) group to (c) group, you may contain the element chosen from a some group in combination.

  Examples of impurities mixed in steel include S, N, and Sn. For example, the content of S and N is preferably regulated as follows.

S:
Since S forms sulfide inclusions and degrades workability, the content is preferably suppressed to 0.05% or less. In order to secure further excellent workability, the S content is more preferably 0.008% or less, and very preferably 0.003% or less.

N:
Since N deteriorates workability, the content is desirably suppressed to 0.01% or less. The N content is preferably 0.006% or less.

  Moreover, you may make Cu and Ni contain 0.05-1.0% of all from a viewpoint of transformation strengthening or a corrosion-resistant improvement.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  Steels having chemical compositions shown in Tables 1 to 3 were heated, rough-rolled, finish-rolled, cooled and wound up under the conditions shown in Tables 4 to 6 using an experimental rolling mill, and the plate thickness was 2.3 mm. A steel plate was obtained.

  Test pieces were collected from the obtained steel plates, and the microstructure and tensile properties at room temperature were investigated.

  The structure was determined using an optical microscope or an electron microscope, and the average particle diameter and area ratio (hence, volume ratio) of ferrite were determined. Here, the “average particle diameter” of the ferrite was obtained by multiplying the average intercept length obtained by the cutting method by 1.128 as described above.

  Tensile properties at room temperature were investigated using JIS No. 5 test pieces.

  Tables 7 to 9 collectively show the results of the above investigations.

  As apparent from Tables 7 to 9, the test numbers 1 to 11 (see Table 7), test numbers 15 to 17 (see Table 8), and test numbers 18 to 20 (see Table 9) were produced by the method of the present invention. Each of the hot-rolled steel sheets has a fine structure with ferrite as the main phase and an average grain size of ferrite of 5 μm or less, and is the product of “TS ×” which is the product of tensile strength (TS) and total elongation (El). This is a hot-rolled steel sheet having an excellent “strength-ductility balance” with a value of “EL” of 14000 MPa ·% or more.

  On the other hand, since the hot rolled steel sheets having test numbers 12 to 14 whose manufacturing conditions deviate from the conditions specified in the present invention have an average grain size of ferrite exceeding 5 μm, compared with the hot rolled steel sheets of the present invention examples. The strength has decreased.

  That is, in test number 12, since the average cooling rate after rolling in the rolling mill one stage before the last and the temperature after cooling deviate from the specified conditions of the present invention, the average grain size of ferrite is as large as 6.1 μm, The strength of the steel sheet is reduced as compared with test numbers 1 to 4 which are examples of the present invention.

  In Test No. 13, since the reduction amount in the final rolling mill exceeded the specified condition of the present invention, processed ferrite was generated, and the average grain size of the ferrite was as large as 6.71 μm. And compared with the test number 6, while the intensity | strength of a steel plate falls, desired "TS * EL" is not obtained and "strength-ductility balance" is also inferior.

  In Test No. 14, the time up to 720 ° C. after rolling in the final rolling mill deviates from the prescribed conditions of the present invention, so the average grain size of ferrite is as large as 5.4 μm. Compared to 1-4, the strength of the steel sheet is reduced.

According to the present invention, it is possible to stably provide a fine-grained hot-rolled steel sheet that is excellent in workability while being hot-rolled, and is applied to automobile undercarriage parts, various industrial machines and architectural members. The performance and life of these products can be further improved, resulting in extremely useful effects in the industry.

Claims (5)

A method for producing a fine-grain hot-rolled steel sheet, which is subjected to finish rolling using a tandem rolling mill row after rough rolling, and rolled at a temperature of Ar ₃ or higher by a rolling mill one stage before the end of the tandem rolling mill row Thereafter, it is cooled to a temperature range of “Ar ₃ point −20 ° C.” or less at an average cooling rate of 50 ° C./second or more, and further rolled at a rolling reduction of 20% or less in the final rolling mill of the tandem rolling mill row, Then, it cools to 720 degreeC within 0.4 second, The manufacturing method of the fine grain hot rolled sheet steel whose average particle diameter of a ferrite is 5 micrometers or less. A method for producing a fine-grain hot-rolled steel sheet, which is subjected to finish rolling using a tandem rolling mill row after rough rolling, and rolled at a temperature of Ar ₃ or higher by a rolling mill one stage before the end of the tandem rolling mill row Then, after cooling to a temperature range of “Ar ₃ point−20 ° C.” or less at an average cooling rate of 50 ° C./second or more and further rolling at a rolling reduction of 20% or less in the final rolling mill of the tandem rolling mill row A method for producing a fine-grain hot-rolled steel sheet having an average grain size of ferrite of 5 μm or less, characterized by cooling to 720 ° C. within 0.2 seconds at an average cooling rate of 400 ° C./second or more.   3. The ferrite average according to claim 1, wherein the sheet bar is heated by an auxiliary heating device after roughly rolling the steel ingot or the steel slab and before performing finish rolling using the tandem rolling mill row. A method for producing a fine-grain hot-rolled steel sheet having a particle size of 5 μm or less.   In mass%, C: 0.02-0.2%, Mn: 0.05-3.0%, Si: 0.001-3.0%, P: 0.001-0.2% and Al: The fine-grain hot-rolled steel sheet containing 0.001 to 3%, the balance being a steel having a chemical composition consisting of Fe and impurities, and having an average grain size of ferrite of 5 µm or less according to any one of claims 1 to 3 Manufacturing method. In mass%, C: 0.02-0.2%, Mn: 0.05-3.0%, Si: 0.001-3.0%, P: 0.001-0.2% and Al: A chemical composition containing 0.001 to 3%, further including at least one component selected from one or more of the following groups (a) to (c), the balance being substantially composed of Fe and impurities: The manufacturing method of the fine grain hot rolled sheet steel whose average particle diameter of the ferrite in any one of Claim 1 to 3 which makes object the steel of a composition is 5 micrometers or less.
(A) Ca: 0.0002 to 0.010%, Zr: 0.01 to 0.10%, and REM (rare element): 0.002 to 0.10%
(B) Nb: 0.005-0.10%, Ti: 0.005-0.20% and V: 0.005-1.0%
(C) Cr: 0.05-1.0% and Mo: 0.05-1.0%