JP4207527B2 - Manufacturing method of hot-rolled steel sheet - Google Patents

Description

【００８２】
【表２】

【００８３】
得られた鋼板から試験片を採取し、組織及び常温での引張特性を調査した。
【００８４】
組織は、光学顕微鏡又は電子顕微鏡を用いて相の判定をするとともに、フェライトの平均粒径と面積率（したがって、体積率）を求めた。ここで、フェライトの「平均粒径」は、前述のように、切断法によって得られた平均切片長さを１．１２８倍することによって求めた。
【００８５】
常温での引張特性はＪＩＳ５号試験片を用いて調査した。
【００８６】
表３に、前記の各調査結果をまとめて示す。
【００８７】
【表３】

【００８８】
表３から明らかなように、試験番号１〜１１の本発明の方法で製造された熱延鋼板は、いずれも主相であるフェライトの平均粒径が５μｍ以下という微細な組織を有し、しかも、引張強度（ＴＳ）と全伸び（Ｅｌ）の積である「ＴＳ×ＥＬ」の値が１４０００ＭＰａ・％以上という優れた「強度−延性バランス」を有する熱延鋼板となっている。
【００８９】
これに対して、製造条件が本発明で規定する条件から外れた試験番号１２及び１３の熱延鋼板は、目標とする「ＴＳ×ＥＬ」の値に未達で「強度−延性バランス」に劣っている。
【００９０】
すなわち、試験番号１２は、最終から１段前の圧延機での圧延後の平均冷却速度と冷却後の温度が本発明の規定条件から外れるため、フェライトの平均粒径が６．１μｍと大きく、このため所望の「ＴＳ×ＥＬ」が得られておらず、「強度−延性バランス」に劣っている。
【００９１】
試験番号１３は、最終圧延機での圧下量が本発明の規定条件を上回るので加工フェライトが生成し、このため所望の「ＴＳ×ＥＬ」が得られておらず、「強度−延性バランス」に劣っている。
【００９２】
【発明の効果】
本発明によれば、熱延のままで主相であるフェライトの平均粒径が５μｍ以下という微細な組織を有し、しかも、引張強度（ＴＳ）と全伸び（Ｅｌ）の積である「ＴＳ×ＥＬ」の値が１４０００ＭＰａ・％以上という優れた「強度−延性バランス」を有する微細粒熱延鋼板を容易に得ることができる。本発明の方法で製造された熱延鋼板は、自動車や各種の産業機械に用いられる高強度部材の素材として好適である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a hot-rolled steel sheet, and more specifically, a method for producing a hot-rolled steel sheet suitable as a material for a high-strength member used in automobiles and various industrial machines. The present invention relates to a method for producing a hot-rolled steel sheet for processing having excellent workability.
[0002]
[Prior art]
Steel materials used as materials for structural members of transportation machines including automobiles and various industrial machines are required to have excellent 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.
[0003]
As the means for refining the structure in the prior art, for example, Patent Documents 1 to 3 propose a technique relating to “large rolling rolling”, and Patent Documents 4 and 5 propose a technique relating to “controlled rolling / controlled cooling”. .
[0004]
That is, in Patent Document 1, in the subsequent stage of continuous hot rolling, the rolling reduction is 40% or more and the average strain rate is 60 seconds. ^-1 In addition, there is disclosed a technique for refining the structure by large rolling, in which a rolling reduction of 40% or more is continuously applied within 2 seconds. However, the technique proposed in the above-mentioned Patent Document 1 requires a reduction amount per pass of 40% or more, which is difficult to realize with a general hot strip mill. Furthermore, it is difficult to control the plate thickness.
[0005]
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. However, in the method proposed in Patent Document 2, the temperature measurement and the measurement of the plate thickness and the plate width, which are normally performed on the exit side of the finishing tandem rolling mill, are hindered, and thus the productivity is lowered.
[0006]
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. However, in a general hot strip mill, it is extremely difficult to stably control the rolling temperature in the dynamic recrystallization temperature range.
[0007]
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. However, in the case of the technique proposed in Patent Document 4, the particle size inside the steel sheet is as large as 10 μm or more, and the contribution to the strengthening of the entire steel material is very small just by making the surface layer finer. .
[0008]
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. However, in the case of steel whose Ti content is lower than the specified value, the dynamic recrystallization in the first stage is insufficient and it is difficult to refine the crystal grains. In the case of steel without addition of Ti, Even if the above rolling technique is applied, the particle size is only 11 μm or more.
[0009]
[Patent Document 1]
Japanese Patent Publication No. 5-65564
[Patent Document 2]
Japanese Patent Publication No.4-111608
[Patent Document 3]
Japanese Patent Laid-Open No. 11-152544
[Patent Document 4]
JP-A-9-137248
[Patent Document 5]
Japanese Patent Laid-Open No. 11-92859
[0010]
[Problems to be solved by the invention]
This invention is made | formed in view of the said present condition, The objective is to provide the manufacturing method of a fine grain hot rolled sheet steel suitable as a raw material of the high strength member used for a motor vehicle or various industrial machines. Specifically, it is a method for efficiently producing a hot-rolled steel sheet having a fine structure in which the average particle diameter of ferrite as a main phase is 5 μm or less while being hot-rolled, and further, the tensile strength (TS) and the total It is to provide a method for producing a fine-grain hot-rolled steel sheet excellent in workability having an excellent “strength-ductility balance” in which the value of “TS × EL”, which is a product of elongation (El), is 14000 MPa ·% or more. .
[0011]
[Means for Solving the Problems]
The gist of the present invention resides in the method for producing a hot-rolled steel sheet shown in the following (1) to (5).
[0012]
(1) A method of manufacturing a hot-rolled steel sheet that performs finish rolling using a tandem rolling mill train after rough rolling, and Ar is used in a rolling mill one stage before the end of the tandem rolling mill train. ₃ After rolling at a temperature above the point, “Ar” with an average cooling rate of 50 ° C./second or more ₃ A method for producing a hot-rolled steel sheet, which is cooled to a temperature range of “point −50 ° C.” or less and then rolled at a rolling reduction of 20% or less in the final rolling mill of the tandem rolling mill row.
[0013]
(2) A method of manufacturing a hot-rolled steel sheet that performs finish rolling using a tandem rolling mill train after rough rolling, and Ar is used in a rolling mill one stage before the end of the tandem rolling mill train. ₃ After rolling at a temperature above the point, “Ar” is used at an average cooling rate of 100 ° C./second or more. ₃ A method for producing a hot-rolled steel sheet, which is cooled to a temperature range of “point −100 ° C.” or less and then rolled at a rolling reduction of 20% or less in the final rolling mill of the tandem rolling mill row.
[0014]
(3) A method of manufacturing a hot-rolled steel sheet that performs finish rolling using a tandem rolling mill train after rough rolling, wherein Ar is used in a rolling mill one stage before the end of the tandem rolling mill train. ₃ After rolling at a temperature above the point, “Ar” is used at an average cooling rate of 200 ° C./second or more. ₃ A method for producing a hot-rolled steel sheet, which is cooled to a temperature range of “point −100 ° C.” or less and then rolled at a rolling reduction of 20% or less in the final rolling mill of the tandem rolling mill row.
[0015]
(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 being the method for producing a hot-rolled steel sheet according to any one of (1) to (3) above for steel having a chemical composition composed of Fe and impurities.
[0016]
(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 hot-rolled steel sheet according to any one of (1) to (3), wherein the steel has a chemical composition comprising:
[0017]
(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%.
[0018]
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.
[0019]
The rolling reduction (%) is the thickness of the material to be rolled before rolling in the n-th pass. _ni The thickness of the rolled material after rolling is t _no {(T _ni -T _no ) / T _ni } It means what is required by x100.
[0020]
“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.
[0021]
Hereinafter, the inventions related to the methods for producing hot-rolled steel sheets (1) to (5) above are referred to as inventions (1) to (5), respectively.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
The present inventors have repeated research and development in order to achieve the above-described object. As a result, in the so-called “tandem hot rolling” in which finish rolling is performed using a tandem rolling mill train after rough rolling, Ar in a rolling mill one stage before the last of the tandem rolling mill train ₃ Rolled at a temperature above the point, and then “Ar” at an average cooling rate of 50 ° C./second or more. ₃ It was found that a structure having fine ferrite crystal grains can be obtained by cooling to a temperature range of “point −50 ° C.” or less and then rolling with a final rolling mill at a rolling reduction of 20% or less.
[0023]
In addition, if quenching is performed between the rolling mill one stage before the final stage of the tandem rolling mill row and the final rolling mill, the “immediate quenching method” that is performed on the exit side of the final rolling mill known in the past can be measured. It is possible to measure the temperature, thickness, and width at the delivery side of the final rolling mill, which is difficult to achieve, and it is possible to control the temperature with high accuracy and shape control such as thickness, width, etc. It turns out that production can be realized.
[0024]
Furthermore, it has been clarified that if the rolling reduction in the final rolling mill, which is the most important factor in controlling the shape of the steel sheet, is reduced, rolling with high productivity and excellent shape of the steel sheet is possible. In addition, if the rolling mill from the last to the first stage performs lubricated rolling for the purpose of reducing rolling load and increasing the reduction ratio for promoting fine graining, and performing the non-lubricating rolling in the final rolling mill, lubricated rolling It also became clear that rolling with high productivity and excellent sheeting performance was possible.
[0025]
The reason why the structure having fine ferrite crystal grains can be obtained by the above-described finish rolling in “tandem hot rolling” is not necessarily clear, but by rapid cooling before rolling by the final rolling mill, by the rolling mill one stage before the final The strain applied to the austenite during rolling is maintained in a state in which the rolling is performed in the final rolling mill while the strain is accumulated, and between the rolling mill one stage before the final rolling mill and the final rolling mill. It is presumed that ferrite nucleation is promoted by consuming the latent time for ferrite nucleation and receiving rolling in the final rolling mill.
[0026]
The present inventions (1) to (5) have been completed based on the above findings.
[0027]
Hereinafter, each requirement of the present invention will be described in detail.
(A) Manufacturing conditions for hot-rolled steel sheet
The method for producing a hot-rolled steel sheet according to the present invention defines rolling conditions in finish rolling by a tandem rolling mill as essential requirements as follows.
[0028]
That is, “A rolling mill one stage before the end of the tandem rolling mill row is Ar ₃ After rolling at a temperature higher than the point, "Ar" ₃ It is cooled to a temperature range of “point −50 ° C.” or less, and then rolled at a rolling reduction of 20% or less with a final rolling mill.
[0029]
By the above treatment, nucleation of ferrite is promoted, and a fine ferrite having a desired average particle diameter of 5 μm or less is obtained.
[0030]
The rolling temperature at the rolling machine one stage before the end of the tandem rolling mill row is Ar ₃ If it is less than the point, not only does it cause the formation of processed ferrite, but the strain accumulation in the soft ferrite becomes insufficient due to insufficient strain accumulation in the untransformed austenite, so the ferrite can be refined to the desired size. Can not do it. 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. The cooling temperature after rolling in the rolling machine one stage before the end of the tandem rolling mill row is “Ar. ₃ Even when the temperature exceeds "point-50 ° 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. The rolling reduction in the final rolling mill is preferably 15% or less, and more preferably 10% or less.
[0031]
Therefore, in the invention of (1), Ar is used in the rolling mill one stage before the end of the tandem rolling mill row. ₃ After rolling at a temperature higher than the point, "Ar" ₃ It was decided to cool to a temperature range of “point −50 ° C.” or less and then to roll at a rolling reduction of 20% or less with a final rolling mill.
[0032]
Here, from the viewpoint of accumulating rolling distortion, the rolling temperature in the rolling machine one stage before the final stage of the tandem rolling mill row is Ar. ₃ Above the point, "Ar ₃ It is preferable to set it to "point +100 degreeC" or less. More preferably, Ar ₃ Above the point, "Ar ₃ Point + 60 ° C. ”or less.
[0033]
The average cooling rate after rolling in the rolling machine one stage before the end of the tandem rolling mill row is preferably 100 ° C./second or more, and more preferably 200 ° 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.
[0034]
In addition, after rolling in the rolling machine one stage before the last of the tandem rolling mill row, “Ar” ₃ It is preferable to cool to a temperature range of “point −100 ° C.” or less, and “Ar ₃ It is more preferable to cool to a temperature range of “point −200 ° C.” or lower. This cooling may be performed up to a temperature at which processed ferrite is not generated by rolling at a rolling reduction of 20% or less performed in the next final rolling mill, for example, a temperature in the vicinity of 600 ° C.
[0035]
In order to reduce the average grain size of ferrite to 3 μm or less, the average cooling rate after rolling in the rolling machine one stage before the end of the tandem rolling mill row is set to 200 ° C./second or more, and “Ar ₃ It is preferable to cool to a temperature range of “point −100 ° C.” or lower.
[0036]
For the reason described above, in the invention of (2), Ar is used in the rolling mill one stage before the last in the tandem rolling mill row. ₃ The average cooling rate after rolling at a temperature equal to or higher than the point is 100 ° C./second or higher, and the cooling temperature range is “Ar. ₃ It was set as "point-100 degreeC" or less. In the invention of (3), Ar is used in the rolling machine one stage before the end of the tandem rolling mill row. ₃ The average cooling rate after rolling at a temperature equal to or higher than the point is 200 ° C./second or higher, and the cooling temperature range is “Ar. ₃ It was set as "point-100 degreeC" or less.
[0037]
In addition, the rolling in the final rolling mill of the tandem rolling mill row is a draining function for preventing the cooling water after rolling in the rolling mill one stage before from the final rolling mill from flowing out to the outlet side of the final rolling mill. It has a function of preventing tension of the material to be rolled by applying tension to the material to be rolled with the rolling mill before the stage, and also a cooling effect by heat removal from the roll. Therefore, in the final rolling mill of the tandem rolling mill row, the material to be rolled and the roll are simply brought into contact with each other, and the rolling reduction ratio may be 0%. 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.
[0038]
The temperature in the present invention refers to the temperature on the surface of the steel sheet, the “average cooling rate” refers to the temperature difference before and after cooling divided by the cooling time, and the rolling reduction (%) is the coverage before the n-th rolling. Rolled material thickness t _ni The thickness of the rolled material after rolling is t _no {(T _ni -T _no ) / T _ni } It is as already described that it points out what is calculated | required by * 100.
[0039]
From the viewpoint of suppressing the growth of crystal grains during finish rolling, it is preferable that the start temperature of finish rolling in the tandem rolling mill is lowered. However, if the temperature before the rolling side tip of the material to be rolled enters the tandem rolling mill row is lowered, the temperature drop at the rear edge or edge increases, so the rear edge or edge of the material to be rolled In order to prevent the temperature drop at the end of the tandem rolling mill, the auxiliary heating device is used to maintain the temperature of the material to be rolled before finishing rolling in the tandem rolling mill, especially the rear end and edge of the material to be rolled. Also good. In this case, by setting the heating temperature by the auxiliary heating device to 1100 ° C. or lower, a microstructure having ferrite of a desired size can be obtained stably and reliably over the entire area of the material to be rolled.
[0040]
Therefore, it is preferable to heat the material to be rolled to a temperature of 1100 ° C. or less by an auxiliary heating device before the finish rolling in the tandem rolling mill row. In addition, said heating can ensure the rolling in an austenite area | region as finishing rolling in a tandem rolling mill row | line | column. ₃ Heating to a temperature above the point is sufficient, but heating to a temperature of 950 ° C. or higher is more preferable.
[0041]
After the material to be rolled passes through the final rolling mill of the tandem rolling mill row, the temperature, sheet thickness and width of the rolled material are measured, and the results are used to determine the rolling conditions in the tandem rolling mill row and the tandem rolling mill row. By reflecting in the auxiliary heating conditions before rolling at, a steel sheet having a fine ferrite structure of a desired size over the entire area of the material to be rolled and more excellent in dimensional accuracy can be obtained more stably and reliably. In addition, what is necessary is just to cool a to-be-rolled material after measuring said temperature, board thickness, and board width.
[0042]
Therefore, after rolling by the final rolling mill in the tandem rolling mill row, it is preferable to measure the temperature, plate thickness, and plate width of the rolled material, and then cool the rolled material.
[0043]
The cooling of the material to be rolled after finish rolling in the tandem rolling mill row, the winding temperature and the cooling conditions after winding are not particularly limited, and may be appropriately determined according to the structure of the hot rolled steel sheet to be manufactured. .
[0044]
For example, when the structure other than ferrite, which is the main phase, is collectively referred to as the second phase, when it is desired to form a structure containing pearlite or cementite as the second phase, the formation of low-temperature transformation phases such as bainite and martensite is avoided. Cooling and winding may be performed under conditions. In addition, when a composite structure such as so-called “DP steel (duplex steel)” or “TRIP steel” is obtained 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.
[0045]
From the viewpoint of making the ferrite grain size very fine, cooling should be started within a very short time after finish rolling in the tandem rolling mill, for example, within 0.5 seconds after the finish rolling. Is preferred. However, such cooling immediately after finishing finish rolling hinders the above-mentioned temperature, plate thickness and width measurement and leads to a decrease in productivity. A rise is inevitable.
[0046]
Here, the rolling conditions other than the finish rolling by the tandem rolling mill described so far do not need to be specified in particular, and it is only necessary to perform rolling in a normal austenite region.
[0047]
The inventions of the above (1) to (3) are not particularly defined with respect to a method for producing a steel ingot or steel slab to be rolled. For example, any of rimmed steel, capped steel, semi-killed steel or killed steel melted by a converter, an electric furnace, a flat furnace or the like may be used. Further, the target steel may be a steel ingot using any means of “ingot forming method” or “continuous casting method” injected into a mold.
[0048]
Steel ingots and steel slabs used for rough rolling in a roughing mill are once cooled and then ₃ Reheated to a temperature above the point or Ar after casting ₃ Steel ingot that has not dropped to a temperature range below the point or Ar after hot working ₃ It may be any steel slab that has not fallen to a temperature range below the point. In addition, from the viewpoint of fine graining, after cooling once, Ac ₃ What was reheated to the temperature more than a point is more preferable. 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.
[0049]
In addition, after once cooling the steel ingot and steel slab, Ac ₃ The heating temperature in the case of reheating to a temperature above the point is preferably 1200 ° C. or less which does not coarsen the austenite crystal grains. Moreover, in order to ensure rolling temperature and reduce the load of a rolling mill, it is preferable to set it as 1000 degreeC or more. More preferably, it is 1100 degreeC or more.
[0050]
Ar after casting ₃ Steel ingot that has not dropped to a temperature range below the point or Ar after hot working ₃ For any steel slab that has not fallen to a temperature range below the temperature point, after casting or hot working, the steel ingot or steel slab can be cooled to a temperature range of 1200 ° C. or less, and then rough rolled. Desirable for suppressing 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.
[0051]
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 of the second stage before the temperature rise of the material to be rolled due to reduction. Lubricating rolling is preferably performed from the last to the first rolling mill from the viewpoint of sheet feeding.
(B) Chemical composition of the target steel
The inventions (1) to (3) are not particularly specified with respect to the chemical composition of the steel to be produced. However, even with hot rolling, the average grain size of ferrite as the main phase has a fine structure of 5 μm or less, and “TS × EL” is the product of tensile strength (TS) and total elongation (El). In order to obtain a fine-grained hot-rolled steel sheet having excellent workability having an excellent “strength-ductility balance” of 14000 MPa ·% or more, the chemical composition of the steel to be manufactured is set to (4) and (5). It should be specified in the invention. In the following description, “%” of the content of each element means “mass%”.
[0052]
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%.
[0053]
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. . 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%.
[0054]
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%.
[0055]
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%, and in order to further improve the workability, the upper limit is 0. .05% is extremely preferable.
[0056]
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, it is good to make the upper limit of Al content into 0.10% from a viewpoint of economical efficiency.
[0057]
The chemical composition of the steel according to the invention of (4), which is the production object of the inventions of (1) to (3), is composed of the elements from C to Al, with the balance being Fe and impurities. .
[0058]
The chemical composition of the steel described in the invention of (5), which is the production object of the invention of (1) to (3), is the following, instead of a part of Fe of the steel described in the invention of (4). It comprises at least one component selected from one or more of groups (a) to (c).
[0059]
(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%.
[0060]
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.
[0061]
REM refers to a total of 17 elements of Sc, Y, and lanthanoid as described above, and may be added in the form of misch metal. As described above, the REM content in the present invention indicates the total content of the above elements.
[0062]
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.
[0063]
In addition, since both Cr and Mo described in the group (c) have an effect of increasing the strength by transformation strengthening, Cr and Mo may be contained alone or in combination within the range described below. And may be contained.
[0064]
(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. .
[0065]
(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. .
[0066]
(C) Group (Cr and Mo):
Cr and Mo are both elements that have the effect of increasing the strength 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%.
[0067]
About the element of said (a) group to (c) group, you may contain the element chosen from a some group in combination.
[0068]
Examples of impurities mixed in the steel include S and N. For example, the content of S and N is preferably regulated as follows.
[0069]
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.
[0070]
N:
Since N deteriorates workability, the content is desirably suppressed to 0.01% or less. The N content is preferably 0.006% or less.
[0071]
Further, Cu and Ni may be contained in an amount of 0.05 to 1.0% from the viewpoint of transformation strengthening or corrosion resistance improvement.
[0072]
In addition, the hot-rolled steel sheet manufactured by the inventions (1) to (5) has a fine structure in which the average particle diameter of ferrite as the main phase is 5 μm or less. The proportion of the ferrite as the main phase in the structure is at least 50% or more by volume. The proportion of ferrite in the structure is preferably 60% or more by volume. When the structure other than ferrite, which is the main phase, is collectively referred to as the second phase, the second phase is composed of one or more of pearlite, cementite, bainite, martensite and untransformed austenite (so-called “residual austenite”). It is an organization.
[0073]
When the average grain size of ferrite, which is the main phase, is 5 μm or less, the target strength can be secured with a small alloy content compared to conventional steel sheets, and there is little deterioration in properties other than strength. Will also be good.
[0074]
When the average particle diameter of ferrite exceeds 5 μm, the degree of strength increase due to the refinement of the structure is remarkably reduced, and it is necessary to increase the content of the alloy element, resulting in an increase in cost. In order to save the alloy elements and ensure the strength stably and reliably, the average particle diameter of the ferrite is preferably 4 μm or less, and more preferably 3 μm or less. On the other hand, in order to secure a value of 14000 MPa ·% or more stably and surely in “TS × EL”, which is the product of tensile strength (TS) and total elongation (El), to improve the workability, The average particle diameter of the ferrite is preferably 1 μm or more, and more preferably 2 μm or more.
[0075]
According to the inventions of (1) to (5) above, according to the inventions of (4) and (5), the average particle diameter of ferrite is 2 to 3 μm and occupies the structure stably and reliably. The volume ratio of ferrite can be 60% or more.
[0076]
Here, the “average particle diameter” of ferrite refers to a value obtained by multiplying the average intercept length obtained by a cutting method by 1.128.
[0077]
In addition, it is known that the volume ratio of a certain phase is equal to the area ratio. Therefore, the volume ratio of the ferrite in the structure is determined, for example, from the ratio of ferrite obtained by a normal two-dimensional evaluation method. do it.
[0078]
In addition, 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, or electroplating, it is possible to obtain a surface-treated steel sheet that also has excellent corrosion resistance. it can.
[0079]
Hereinafter, the present invention will be described in more detail with reference to examples.
[0080]
【Example】
Steel having the chemical composition shown in Table 1 was heated, roughly rolled, finish-rolled, cooled and wound using the experimental rolling mill under the conditions shown in Table 2 to obtain a steel plate having a plate thickness of 2.3 mm. .
[0081]
[Table 1]

[0082]
[Table 2]

[0083]
Test pieces were collected from the obtained steel plates, and the microstructure and tensile properties at room temperature were investigated.
[0084]
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.
[0085]
Tensile properties at room temperature were investigated using JIS No. 5 test pieces.
[0086]
Table 3 summarizes the results of the above investigations.
[0087]
[Table 3]

[0088]
As apparent from Table 3, the hot-rolled steel sheets produced by the methods of the present invention having test numbers 1 to 11 all have a fine structure in which the average particle diameter of ferrite as the main phase is 5 μm or less, and The steel sheet is a hot-rolled steel sheet having an excellent “strength-ductility balance” in which the value of “TS × EL”, which is the product of tensile strength (TS) and total elongation (El), is 14000 MPa ·% or more.
[0089]
On the other hand, the hot-rolled steel sheets with test numbers 12 and 13 whose manufacturing conditions deviated from the conditions specified in the present invention did not reach the target “TS × EL” value and were inferior in “strength-ductility balance”. ing.
[0090]
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, For this reason, the desired “TS × EL” is not obtained, and the “strength-ductility balance” is inferior.
[0091]
In Test No. 13, since the reduction amount in the final rolling mill exceeds the specified condition of the present invention, processed ferrite is generated, and thus the desired “TS × EL” is not obtained, and the “strength-ductility balance” is achieved. Inferior.
[0092]
【The invention's effect】
According to the present invention, the average grain size of ferrite as a main phase remains hot rolled and has a fine structure of 5 μm or less, and “TS” is the product of tensile strength (TS) and total elongation (El). It is possible to easily obtain a fine-grain hot-rolled steel sheet having an excellent “strength-ductility balance” having a value of “× EL” of 14000 MPa ·% or more. The hot-rolled steel sheet produced by the method of the present invention is suitable as a material for high-strength members used in automobiles and various industrial machines.

Claims (5)

A method of producing a hot-rolled steel sheet that performs finish rolling using a tandem rolling mill train after rough rolling, after rolling at a temperature of Ar ₃ or higher in a rolling mill one stage before the final stage of the tandem rolling mill train, It is cooled to a temperature range of “Ar ₃ points−50 ° C.” or less at an average cooling rate of 50 ° C./second or more, and then rolled at a rolling reduction of 20% or less in the final rolling mill of the tandem rolling mill row. A method for producing a hot-rolled steel sheet. A method of producing a hot-rolled steel sheet that performs finish rolling using a tandem rolling mill train after rough rolling, after rolling at a temperature of Ar ₃ or higher in a rolling mill one stage before the final stage of the tandem rolling mill train, It is cooled to a temperature range of “Ar ₃ points-100 ° C.” or less at an average cooling rate of 100 ° C./second or more, and then rolled at a rolling reduction of 20% or less in the final rolling mill of the tandem rolling mill row. A method for producing a hot-rolled steel sheet. A method of producing a hot-rolled steel sheet that performs finish rolling using a tandem rolling mill train after rough rolling, after rolling at a temperature of Ar ₃ or higher in a rolling mill one stage before the end of the tandem rolling mill train, It is cooled to a temperature range of “Ar ₃ points−100 ° C.” or less at an average cooling rate of 200 ° C./second or more, and then rolled at a rolling reduction of 20% or less in the final rolling mill of the tandem rolling mill row. A method for producing a hot-rolled steel sheet. 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 method for producing a hot-rolled steel sheet according to any one of claims 1 to 3, wherein 0.001 to 3% is included and the balance is made of steel having a chemical composition composed of Fe and impurities. 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 method for producing a hot-rolled steel sheet according to any one of claims 1 to 3, wherein the composition steel is a target.
(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%