JP4379618B2 - High-tensile hot-rolled steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-tensile hot-rolled steel sheet and a method for producing the same. In particular, the present invention relates to a hot-rolled steel sheet having fine crystal grains having excellent surface properties, high strength and excellent workability, and a method for producing the same, which are suitable as materials for members used in automobiles and various industrial machines.

  High-tensile hot-rolled steel sheets manufactured by continuous hot rolling are widely applied to various industrial machines such as automobiles as relatively inexpensive structural materials, and are processed into a predetermined shape by forming such as pressing. Often. For this reason, excellent workability is required for the high-tensile hot-rolled steel sheet.

Generally, when the strength of a steel sheet increases, the workability (for example, ductility) decreases. However, as a steel sheet having both excellent strength and ductility, so-called “residual austenite”, that is, transformation-induced plasticity of austenite remaining untransformed ( Hereinafter, a steel sheet using “TRIP”) is known. For example, Patent Document 1 discloses, in mass%, about 0.2% C, about 1.5% Si, and about 1.5%. The steel containing Mn before and after is hot-rolled, finish-rolled in the vicinity of Ar ₃ point, accelerated and cooled at a cooling rate of 40 ° C./s or more, and then wound up in the vicinity of 400 ° C. A method for manufacturing a steel sheet is disclosed.

  Further, Patent Document 2 discloses a steel sheet containing retained austenite which has a reduced content of Si, and instead contains a large amount of Al and has excellent ductility and hole expansibility, and a method for manufacturing the same.

  On the other hand, for strengthening steel, solid solution strengthening, precipitation strengthening, transformation strengthening and fine grain strengthening (strengthening by crystal grain refinement) are known. Of these, grain refinement is generally ductile. The strength can be increased without lowering. However, in order to obtain a sufficient strengthening effect by refining the crystal grains, it is necessary to refine the ferrite grain size to at least 3 μm or less. Techniques for obtaining such a microstructure, refinement of the structure and residual austenite Several methods for producing a steel having an excellent “strength-ductility balance” in which a high strength and an excellent ductility are made compatible by combining the TRIP phenomenon have been proposed.

For example, Patent Document 3 states that “in weight percent, a steel having a composition containing C: 0.05 to 0.3% and Mn: 0.5 to 3%, and the balance being substantially Fe, is Ac. Cooling at a cooling rate of 5 ° C./s or more and less than 100 ° C./s from a temperature of ₃ points or more to 650 ° C. or less, and a temperature at which a low temperature phase such as a ferrite phase, a bainite phase, or a martensite phase starts to precipitate Up to 60% or more of the cross-sectional area reduction rate at the end of processing with respect to the start of processing in the temperature range up to 30% or more per pass, and then at air cooling or higher cooling rate “A method for producing a steel having a fine-grained ferrite structure” comprising “cooling to a temperature of 400 ° C. or lower” is disclosed.

  Patent Document 4 discloses that “C: 0.05-0.30 wt%, Si: 0.30-2.0 wt%, Mn: 1.0-2.5 wt%, Al: 0.003-0. Steel structure containing 100 wt%, Nb: 0.05 to 0.50 wt%, the balance being the composition of Fe and inevitable impurities, residual austenite being 5 to 20 vol%, and the balance being mainly composed of polygonal ferrite Among the polygonal ferrite grains, the fine grains having a particle size of 8 μm or less occupy 85% or more of the total number ratio and the average particle diameter is 5 μm or less. A high-tensile hot-rolled steel sheet excellent in toughness, fatigue resistance and strength-elongation balance and a method for producing the same are disclosed.

JP 63-4017 A Japanese Patent Laid-Open No. 5-112846 JP 2001-98322 A Japanese Patent Laid-Open No. 11-1747

  The object of the present invention is suitable as a material for members used in automobiles and various industrial machines, has high strength and excellent workability, and also has excellent surface properties by suppressing the generation of so-called `` island scales ''. It is providing the hot-rolled steel plate which has a fine crystal grain, and its manufacturing method.

  Specifically, the formation of island scales is suppressed by reducing the Si content to less than 0.5%, the hole expandability is enhanced by containing Al, and the retained austenite is included in the structure. To improve the ductility, and further increase the strength of the hot rolled steel sheet by refining the crystal grains of polygonal ferrite to an appropriate size of 3.0 μm or less, and a steel sheet having the above-mentioned characteristics at a practical temperature of about 800 ° C. or more. And a method of manufacturing by hot rolling.

  According to the technique proposed in Patent Document 1 described above, an excellent “strength-ductility balance” of 24000 MPa ·% or more in terms of “TS × EL”, which is the product of tensile strength (TS) and total elongation (EL). However, since the hole expandability, which is one of the important indexes of workability, is low, it cannot be used as a material for parts that require stretch flangeability. Further, since 0.5% or more of Si is contained in order to secure retained austenite, a surface failure of the steel plate called “island scale” occurs.

  Al, like Si, has an effect of suppressing the precipitation of cementite, and austenite can remain in the structure. In addition, the effect of the Al is more remarkable than the case where Si is contained at the same mass ratio. Further, Al promotes uniform and fine formation of polygonal ferrite while improving the hole expandability. There is also an action which suppresses generation of coarse bainite which deteriorates.

  The technique proposed in Patent Document 2 utilizes the above-mentioned effect of Al well, reduces the Si content to 1.0% or less, and instead replaces Al with sol. After finishing the hot rolling at 780 to 840 ° C., the steel containing 0.8% or more of Al is accelerated and cooled to 300 to 450 ° C. at a cooling rate of 10 to 50 ° C./s. After finishing the hot rolling at 780 to 940 ° C. by the winding method or after cooling the steel to 600 to 700 ° C. at a cooling rate of 10 ° C./s or more, and for 2 to 10 seconds. After air cooling, by the method of accelerated cooling to 300-450 ° C. at a cooling rate of 20 ° C./s or more and winding, “strength-ductility balance” with a value of “TS × EL” of 24000 MPa ·% or more and hole expansion A steel sheet having excellent workability of 90% or more is obtained. However, Al exhibits a remarkable effect in terms of improving workability, but has a lower solid solution strengthening ability than Si, so it is disadvantageous in terms of increasing strength, and the ferrite stabilizing element Al is sol. . Inclusion of 0.8% or more in the amount of Al leads to destabilization of austenite, so it may be necessary to excessively increase the hot rolling temperature, and there is room for improvement in terms of productivity and manufacturing cost. is there.

  According to the technique proposed in Patent Document 3, a fine ferrite grain structure of 3 μm or less can be obtained even with a low-carbon steel having a simple composition. However, it is necessary to perform so-called “low temperature and large pressure” rolling of 30% or more per pass in a low temperature region of 650 ° C. or lower, and it is difficult to apply to industrial scale production.

  In the technique proposed in Patent Document 4, NbC is precipitated by containing 0.05 to 0.50% of Nb, the austenite grains are refined by NbC, and the austenite grains are dynamically recrystallized during rolling. Through refinement, a fine ferrite grain structure of 2 to 4 μm can be easily obtained even in the case of hot rolling at a temperature of about 800 ° C. or higher, which is carried out industrially. In addition, since the retained austenite is included in the structure, a steel sheet having an excellent “strength-ductility balance” of 26000 MPa ·% or more in terms of “TS × EL” can be obtained. However, since it is a technique for precipitating a large amount of NbC, the amount of retained austenite inevitably obtained is lowered. Therefore, in order to ensure the amount of retained austenite stably and reliably, and to achieve high strength. As shown in Table 1 of Examples of Patent Document 4, 1% or more of Si is contained in the same manner as Patent Document 1 described above, and in this case, hole expansibility and surface properties are deteriorated. To do.

  In order to obtain a hot-rolled steel sheet having fine crystal grains, the present inventors have high strength and excellent workability, and further, the generation of so-called “island scales” is suppressed and the surface properties are also excellent. Researched earnestly. As a result, the following findings (a) to (d) were obtained.

(A) Hot rolling performed in multiple passes is completed at a temperature of Ar ₃ or higher (approximately 800 ° C. or higher), and then cooled to 720 ° C. within 0.4 seconds after the completion of rolling at the time of primary cooling performed thereafter. Then, extremely fine ferrite grains are obtained. In rolling in a temperature range of Ar ₃ or higher, strain is accumulated in austenite, and the strain is efficiently frozen in austenite by cooling to 720 ° C. within 0.4 seconds after completion of rolling. When the temperature is 720 ° C. or lower, the transformation from austenite to ferrite is activated, and a large number of ferrite grains are generated with the accumulated strain as a nucleus to form a fine ferrite structure.

(B) According to the above method (a), the generation of ferrite is remarkably promoted, so that a large amount of fine ferrite can be generated even with a steel composition having a very high hardenability. It is necessary to contain a certain amount of Al in order to leave austenite in steel with low Si to suppress generation of “island scale”, but in the case of steel containing Al, austenite becomes unstable. Ar ₃ point rises remarkably. Therefore, in order to hot-roll steel containing Al in the austenite region, it is necessary to contain C and Mn as austenite stabilizing elements to lower the Ar ₃ point. However, since the hardenability is remarkably increased in such a steel composition, the volume ratio of ferrite in the structure is reduced under normal hot rolling conditions, and a structure mainly composed of polygonal ferrite cannot be obtained. However, according to the treatment described in the above (a), a large amount of fine ferrite can be obtained even in a steel with high hardenability, and moreover, C is concentrated in untransformed austenite. To a structure containing residual austenite.

  (C) Normally, the remaining austenite is present in the form of lath at the interface of the bainitic ferrite, but the residual austenite obtained by the treatment (a) is fine and uniform in the ferrite grain boundaries and grains. scatter. Therefore, ductility and hole expansibility are further improved, and the anisotropy of mechanical properties is extremely reduced.

  (D) Strengthening can be achieved even if the content of Si having a high solid solution strengthening capacity is reduced by refining and strengthening with fine ferrite. In addition, the formation of island scale is suppressed by reducing the ferrite grain size and reducing the Si content, and the surface properties are extremely good.

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

  The gist of the present invention resides in the high-tensile hot-rolled steel sheets shown in the following (1) to (4) and the high-tensile hot-rolled steel sheets shown in (5) to (6).

  (1) By mass%, C: 0.05 to 0.30%, Si: less than 0.5%, Mn: 0.5 to 3.0% and Al: 0.1 to 2.0% In addition, the sum of the contents of Si and Al satisfies 0.5 to 2.0%, the balance is the chemical composition of Fe and impurities, and the volume ratio in the structure is 5% or more austenite and 60% or more polygonal ferrite. The average crystal grain size of the austenite is 2.0 μm or less, the average crystal grain size of the polygonal ferrite is more than 1.0 μm to 3.0 μm, and the C content in the austenite Is a high-tensile hot-rolled steel sheet characterized by being 0.7% to 2.0% by mass.

(2) The high-tensile hot-rolled steel sheet according to (1), wherein the sum of the contents of Si and Al satisfies the following formula (1).
0.5 ≦ Si + Al ≦ (13 × C + Mn + 10) / 10 (1),
However, the element symbol in the formula (1) represents the content in steel in mass% of the element.

  (3) Instead of a part of Fe, in mass%, Nb: 0.005 to 0.10%, Ti: 0.005 to 0.20%, and V: 0.005 to 0.20% The high-tensile hot-rolled steel sheet according to (1) or (2), which contains one or more kinds.

  (4) Instead of a part of Fe, in mass%, Ca: 0.0002 to 0.01%, Zr: 0.002 to 0.10%, and REM (rare earth element): 0.002 to 0.10 The high-tensile hot-rolled steel sheet according to any one of (1) to (3) above, which contains one or more of%.

(5) A method for producing a high-tensile hot-rolled steel sheet having the chemical composition according to any one of (1) to (4) above, wherein a steel ingot or steel slab having the chemical composition is hot-rolled to form Ar After completion of hot rolling at a temperature of ₃ points or more, primary cooling is performed, cooling to 720 ° C. within 0.4 seconds, and further, the primary cooling is performed in a temperature range of 720 to 550 ° C. Stop at T ₁ ° C, then cool down from T ₁ ° C to 500 ° C by secondary cooling in ₁ to 30 seconds, and further perform the secondary cooling to a temperature T ₂ ° C in the temperature range of 500 to 300 ° C. After that, a method for producing a high-tensile hot-rolled steel sheet, which is wound up in a temperature range of T _{2 to} 300 ° C.

  (6) The method for producing a high-tensile hot-rolled steel sheet according to the above (5), wherein the primary cooling is performed to 720 ° C. within 0.2 seconds.

  Here, since the volume ratio of a certain phase is known to be equal to the area ratio, the volume ratio of the polygonal ferrite and austenite in the structure was determined by, for example, a normal two-dimensional evaluation method. What is necessary is just to determine from the ratio of polygonal ferrite and austenite.

  The “austenite” in the present invention refers to so-called “residual austenite” in which austenite remains without being transformed.

“Polygonal ferrite” refers to ferrite having an aspect ratio of 2 or less, and can be confirmed by image analysis processing of a structure observation image obtained by an optical microscope or a scanning electron microscope. The aspect ratio is “dRD / dND” or “dND / dRD” where dRD is the ferrite grain size in the rolling direction on the plane cut in parallel to the rolling direction and dND is the ferrite grain size in the direction perpendicular to the rolling direction. Is represented.
The “average crystal grain size” as used in the present invention refers to the ASTM nominal grain size obtained by multiplying the average grain section length obtained by the so-called “section method” by 1.12.

  “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 means one containing “slab” as defined in JIS G 0203 (1984).

  Hereinafter, the invention related to the high-tensile hot-rolled steel sheet of (1) to (4) and the invention related to the manufacturing method of the high-tensile hot-rolled steel sheet of (5) to (6) are respectively referred to as “the invention of (1)” to It is referred to as “(6) invention”. Also, it may be collectively referred to as “the present invention”.

  Since the high-tensile hot-rolled steel sheet of the present invention is excellent in “strength-ductility balance”, it can be used as a material for high-strength structural members used in automobiles and various industrial machines, and has high hole expansibility and excellent performance. In addition, since it also has a surface texture, its application range is very wide compared to conventional high strength and high ductility steel sheets. Moreover, the high-tensile hot-rolled steel sheet of the present invention can be manufactured relatively easily by practical hot rolling at about 800 ° C. or higher by the method of the present invention.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, "%" display of the content of each element means "mass%".

(A) Chemical composition of steel C: 0.05 to 0.30%
C has the effect of stabilizing austenite at high temperatures. For this reason, the transformation temperature from austenite to ferrite is lowered and the finishing temperature of hot rolling can be lowered, so that the refinement of ferrite crystal grains is promoted. C also concentrates in austenite as the ferrite transformation proceeds, and has the effect of stabilizing the austenite and strengthening the steel sheet.

  However, if the C content is less than 0.05%, a sufficient austenite stabilizing effect cannot be obtained, so that a desired retained austenite amount cannot be ensured. On the other hand, if the content exceeds 0.30%, the ferrite transformation after hot rolling is delayed, the volume fraction of ferrite is reduced, and the weldability is significantly deteriorated. Therefore, the content of C is set to 0.05 to 0.30%. In addition, the minimum with preferable content of C is 0.10%, and a preferable upper limit is 0.20%.

Si: Less than 0.5% Si stabilizes ferrite, promotes the formation of polygonal ferrite, promotes the concentration of C in untransformed austenite, and further acts to delay the precipitation of cementite. The amount of austenite remaining untransformed, that is, the amount of retained austenite is increased. Si also has the effect of increasing the strength of the steel sheet by solid solution strengthening of polygonal ferrite. In order to surely obtain such an effect, it is preferable that the Si content is 0.05% or more.

  However, when the Si content is 0.5% or more, the formation of island scale becomes remarkable and the surface properties of the steel sheet deteriorate. Therefore, the Si content is less than 0.5%. The Si content must be 0.5 to 2.0% in total with the Al content. This will be described later.

Mn: 0.5 to 3.0%
Mn has an effect of increasing strength. In addition, since the austenite at high temperature is stabilized and the transformation temperature from austenite to ferrite is lowered, the finishing temperature of hot rolling can be lowered, and therefore the refinement of ferrite crystal grains is promoted.

  However, if the Mn content is less than 0.5%, it is difficult to obtain the above effect. On the other hand, if the content exceeds 3.0%, it becomes difficult to generate a sufficient amount of polygonal ferrite in the cooling process after hot rolling. Therefore, the Mn content is set to 0.5 to 3.0%. In addition, it is preferable that content of Mn shall be 0.8 to 3.0%.

Al: 0.1 to 2.0%
Al is an especially important element in the present invention. That is, Al, like Si, stabilizes ferrite, promotes the formation of polygonal ferrite and promotes the concentration of C in untransformed austenite, and further delays the precipitation of cementite. Increases the amount that remains untransformed, that is, the amount of retained austenite. And the effect | action is more remarkable than the case where Si is contained by the same mass ratio. Al also has the effect of promoting uniform and fine formation of polygonal ferrite and enhancing hole expansibility.

  However, when the Al content is less than 0.1%, it is difficult to obtain the above effect. On the other hand, excessive addition of Al destabilizes austenite at high temperatures, excessively raises the finishing temperature (completion temperature) of hot rolling, and makes stable continuous casting difficult. In particular, if the Al content exceeds 2.0%, the hot rolling completion temperature must be increased, and nozzle clogging occurs, resulting in a decrease in continuous castability. Therefore, the Al content is set to 0.1 to 2.0%. The preferable lower limit of the Al content is 0.3%, and the preferable upper limit is 1.5%. The Al content must be 0.5 to 2.0% in total with the Si content. This will be described later.

Sum of Si and Al content:
The content of Si and Al is less than 0.5% for the above-mentioned Si and 0.1 to 2.0% for Al, and the sum of the contents of both elements is 0.5 to It is necessary to satisfy 2.0%.

  When the sum of the contents of Si and Al (hereinafter also referred to as “Si + Al”) is less than 0.5%, the action of delaying the precipitation of cementite becomes insufficient, and part of the cementite is generated. For this reason, the amount of retained austenite decreases, and a desired retained austenite amount cannot be ensured. On the other hand, if the sum of Si and Al content exceeds 2.0%, even if the Al content is 2.0% or less, the austenite at high temperature becomes unstable and the finishing temperature of hot rolling (Completion temperature) rises excessively, and it becomes difficult to carry out continuous casting stably.

  For this reason, in the invention of (1), “Si + Al” is set to 0.5 to 2.0%. The preferable lower limit of “Si + Al” is 1.0%, and the preferable upper limit is 1.8%.

In order to increase the strain accumulation efficiency in austenite and refine the ferrite and further enhance the effect of strengthening the refinement, it is effective to set the Ar ₃ point to 950 ° C. or less in actual production. When expressed as a steel content in mass% of the element, the value of “Si + Al” is preferably “(13 × C + Mn + 10) / 10” or less.

  For this reason, in the invention of (2), it is defined that the sum of the contents of Si and Al satisfies the formula (1).

  Therefore, the chemical composition of the high-tensile hot-rolled steel sheet according to the invention of (1) above contains elements from C to Al in the above-described range, and “Si + Al” satisfies 0.5 to 2.0%, and the balance Is defined as consisting of Fe and impurities. The chemical composition of the high-tensile hot-rolled steel sheet according to the invention (2) is further defined so that “Si + Al” satisfies the formula (1).

  In addition to the above-described component elements, the high-tensile hot-rolled steel sheet according to the present invention contains, as necessary, one or more elements selected from at least one of the first group and the second group described later. An optional additive element may be added and contained.

  Hereinafter, the optional additive element will be described.

First group: Nb: 0.005-0.10%, Ti: 0.005-0.20% and V: 0.005-0.20%
Nb, Ti and V are all precipitated as carbonitrides on the ferrite ground, and have the effect of increasing the strength of the steel sheet. The above precipitate has an action of suppressing the coarsening of austenite and ferrite and promoting the refinement of crystal grains. In order to reliably obtain such an effect, it is preferable to contain at least one of 0.005% or more.

  However, even if Nb is contained in an amount exceeding 0.10% and Ti or V is contained in an amount exceeding 0.20%, the above effects are saturated and the cost is increased. Further, since a large amount of C is consumed for precipitation of carbonitride, the amount of retained austenite is reduced, and the desired amount of retained austenite cannot be ensured.

  Therefore, when Nb, Ti and V are added, the respective contents are 0.005 to 0.10% for Nb, 0.005 to 0.20% for Ti and 0.005 to 0.20% for V. It is good to do. The more preferable lower limit of each content in the case of adding is 0.008% for Nb, 0.008% for Ti, and 0.008% for V. Moreover, the upper limit with more preferable each content in the case of adding is 0.08% for Nb, 0.15% for Ti, and 0.15% for V. In addition, said Nb, Ti, and V can be added only with any 1 type or 2 or more types of composite.

Second group: Ca: 0.0002 to 0.01%, Zr: 0.002 to 0.10%, and REM (rare earth element): 0.002 to 0.10%
Ca, Zr, and REM all have an effect of improving the cold workability by adjusting the shape of inclusions. In order to reliably obtain this effect, it is preferable to contain at least one of 0.0002% or more of Ca, 0.002% or more of Zr, and 0.002% or more of REM.

  However, if Ca is contained in excess of 0.01%, and if Zr or REM is contained in excess of 0.10%, the inclusions in the steel increase so much that the workability decreases. .

  Therefore, when Ca, Zr or REM is added, the respective contents are 0.0002 to 0.01% for Ca, 0.002 to 0.10% for Zr, and 0.002 to 0.10 for REM. % Is good. The more preferable lower limit of each content in the case of adding is 0.0005% for Ca, 0.01% for Zr, and 0.01% for REM. Moreover, the upper limit with more preferable each content in the case of adding is 0.005% of Ca, 0.05% of Zr, and 0.05% of REM. In addition, said Ca, Zr, and REM can be added only with any 1 type or 2 or more types of composite.

  Therefore, the chemical composition of the high-tensile hot-rolled steel sheet according to the invention of (3) is that of the steel of the invention of (1) or (2) for the purpose of increasing the strength of the steel sheet and refining crystal grains. Instead of a part of Fe, one or more of Nb: 0.005 to 0.10%, Ti: 0.005 to 0.20%, and V: 0.005 to 0.20% were included.

  Further, the chemical composition of the high-tensile hot-rolled steel sheet according to the invention of (4) is for the purpose of improving cold workability, and the Fe of the steel of any one of the inventions of (1) to (3). In place of a part, Ca: 0.0002 to 0.01%, Zr: 0.002 to 0.10%, and REM (rare earth element): 0.002 to 0.10% are included. It was supposed to be.

  Note that “REM (rare earth element)” is a generic name for a total of 17 elements of Sc, Y and lanthanoid, and the content of REM indicates the total content of the above-described elements as already described.

  Examples of impurities mixed in the steel include P, S, Cu, Ni, Cr, and Mo. For example, the content of P and S is preferably regulated as follows.

P: 0.05% or less Since P adversely affects toughness and ductility, its content is preferably suppressed to 0.05% or less. In order to disperse the polygonal ferrite more uniformly, the P content is more preferably 0.010% or less.

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

  Steel having the above-described chemical composition is produced by, for example, a converter, an electric furnace, a flat furnace, or the like. For the production of the steel ingot, any means of “ingot-making method” or “continuous casting method” injected into the mold may be used.

(B) Structure of high-tensile hot-rolled steel sheet The high-tensile hot-rolled steel sheet of the present invention contains 5% or more austenite and 60% or more polygonal ferrite in volume ratio, and further, the average of the austenite. It is necessary that the crystal grain size is 2.0 μm or less, and the average crystal grain size of the polygonal ferrite exceeds 1.0 μm and reaches 3.0 μm.

  First, regarding the type of phase, it is assumed that 5% or more austenite and 60% or more polygonal ferrite are contained in a volume ratio. When the austenite in the structure is less than 5% in volume ratio, sufficient ductility is achieved. It is because it cannot be obtained. Furthermore, even when the polygonal ferrite occupying the structure is less than 60% by volume, sufficient ductility and hole expansibility cannot be ensured.

  Since the ductility improves as the amount of austenite in the structure increases, all the structures other than the main phase polygonal ferrite occupying 60% or more of the structure in volume ratio may be austenite.

  Further, in order to ensure further excellent ductility and hole expansibility, the volume ratio of polygonal ferrite in the structure is preferably 70% or more, and 95% (that is, the structure other than polygonal ferrite is only austenite). Or a volume ratio of the austenite is 5%).

  The high-tensile hot-rolled steel sheet of the present invention may include a third phase other than the polygonal ferrite and austenite. Here, examples of the phase other than polygonal ferrite and austenite include bainite.

  Next, regarding the size of the phase, the average crystal grain size of austenite is 2.0 μm or less, and the average crystal grain size of polygonal ferrite is more than 1.0 μm to 3.0 μm. This is because when the particle diameter exceeds 2.0 μm, hard and coarse martensite is generated by the “TRIP” phenomenon, and the local ductility deteriorates. On the other hand, when the average crystal grain size of polygonal ferrite is 1.0 μm or less, the strength is increased, but the ductility is remarkably lowered, and the “strength-ductility balance” is sharply reduced. This is because when the diameter exceeds 3.0 μm, the effect of strengthening refinement cannot be obtained sufficiently.

  The smaller the austenite size, the more uniformly and more widely dispersed, so that the “TRIP” phenomenon occurs efficiently and ductility is improved. Therefore, the lower limit value of the average crystal grain size of austenite in the high-tensile hot-rolled steel sheet of the present invention may not be specified. Note that “residual austenite” that is usually observed has an average crystal grain size of 0.05 μm or more. Further, when the aspect ratio of austenite exceeds 2, the anisotropy of the mechanical properties of the steel sheet may increase, so the austenite aspect ratio is preferably 2 or less.

  For the reasons described above, the high-strength hot-rolled steel sheet of the present invention contains 5% or more austenite and 60% or more polygonal ferrite in volume ratio, and the average grain size of the austenite is 2 The average crystal grain size of the polygonal ferrite exceeds 1.0 μm and reaches 3.0 μm.

  As described above, since the volume ratio of a certain phase is known to be equal to the area ratio, the volume ratio of the austenite and polygonal ferrite in the structure is, for example, a normal two-dimensional evaluation method. May be determined from the ratio of austenite and polygonal ferrite obtained by the above. The same applies to the other phases.

  The “austenite” in the present invention refers to so-called “residual austenite” in which austenite remains without transformation.

  “Polygonal ferrite” refers to a ferrite having an aspect ratio of 2 or less, and can be confirmed by image analysis processing of a structure observation image obtained by an optical microscope or a scanning electron microscope, as described above. The aspect ratio of ferrite is “dRD / dND” or “dND” where dRD is the ferrite grain size in the rolling direction on the plane cut in parallel to the rolling direction and dND is the ferrite grain size in the direction perpendicular to the rolling direction. / DRD ".

  Similarly, the aspect ratio of the austenite is “dRD / dND” or “a” when the austenite grain size in the rolling direction on the plane cut in parallel with the rolling direction is dRD and the austenite grain size in the direction perpendicular to the rolling direction is dND. dND / dRD ”, which can be confirmed by performing image analysis processing on a tissue observation image obtained by an optical microscope or a scanning electron microscope.

  As described above, the “average crystal grain size” in the present invention refers to the ASTM nominal grain size obtained by multiplying the average grain section length obtained by the so-called “intersection method” by 1.12.

(C) C content in austenite of high-tensile hot-rolled steel sheet The high-tensile hot-rolled steel sheet of the present invention has a C content in the austenite in the structure described in the above section (B) of 0.7 to 2.0%. Must be the ones.

  If the C content in the austenite is less than 0.7%, the “TRIP” phenomenon occurs at the initial stage of deformation when the steel sheet is processed, and the ductility is not improved. On the other hand, when it exceeds 2.0%, since austenite becomes too stable, “TRIP” does not occur and ductility is lowered.

  Therefore, in the high-tensile hot-rolled steel sheet of the present invention, the C content in the austenite in the structure described in the above section (B) is set to 0.7 to 2.0%.

  In addition, the minimum with preferable C content in the said austenite is 0.9%, and a preferable upper limit is 1.6%.

(D) Manufacturing method of high-tensile hot-rolled steel sheet The chemical composition described in the item (A), the structure described in the item (B), and the C content in the austenite described in the item (C). The high-strength hot-rolled steel sheet according to the inventions of the invention to (4) has, for example, the chemical composition described in the item (A), “hot ingot or steel slab and hot at a temperature of Ar ₃ point or higher. After completing the rolling in between, primary cooling is performed to cool to 720 ° C. within 0.4 seconds, and further, the primary cooling is stopped at a temperature T ₁ ° C. of 720 to 550 ° C., then, wound cooled to a temperature range of T ₂ to 300 ° C. until the 30 seconds at a temperature T ₂ ° C. in a temperature range of 500 to 300 ° C. from T ₁ ° C. by the secondary cooling "that characterized According to the invention of (5), it can be manufactured relatively easily.

(D-1) Hot rolling:
The hot rolling is preferably performed at an Ar ₃ point or higher in order to transform from austenite to ferrite after rolling. Since the effect of accumulating work strain introduced into austenite by rolling is increased and the refinement of crystal grains is promoted, the completion temperature of the hot rolling is preferably closer to the Ar ₃ point. In addition, when “Si + Al” of a steel ingot or steel slab that is a material to be rolled satisfies the above formula (1), the Ar ₃ point becomes 950 ° C. or less, and it becomes easy to increase the accumulation effect of processing strain. Fine polygonal ferrite grains having an average crystal grain size exceeding 1.0 μm and up to 2 μm can be easily obtained by primary cooling treatment after completion of hot rolling described later.

Therefore, in the invention of (5), the hot rolling is completed at a temperature not lower than the Ar ₃ point.

If the steel ingots or steel slabs to be used for hot rolling are those described in the following (A) to (C), the hot rolling completion temperature not lower than Ar ₃ can be easily ensured.

(B) after the ingot or casting of the left cast that is not temperature drops to Ar ₃ point or less of the temperature region directly to the hot working, the steel strip which is not a temperature drop to a temperature range of below ₃ points Ar,
(B) to a temperature range of Ac ₃ point or more cold piece after ingot or hot working was reheated to a temperature range of Ac ₃ point or more to Hiyakatamari after casting reheated slab,
(C) A steel piece obtained by reheating a cold-worked cold piece to a temperature range of Ac ₃ or higher.

In addition, the upper limit of the heating temperature in the case of reheating to the temperature range of ₃ points or more of (b) and (c) is not particularly limited. However, from the viewpoint of ensuring high productivity and uniform mechanical properties at low cost, it is preferable that the temperature is about 900 to 1350 ° C., and precipitates such as TiC and NbC are sufficiently dissolved in austenite. In the case of a steel type that is not necessary, in order to refine the initial austenite crystal grains and to facilitate refinement of the ferrite grains after hot rolling, it is preferable that the temperature is relatively low 900 to 1100 ° C. .

  In the hot rolling, it is preferable to use a reverse mill or a tandem mill. In particular, from the viewpoint of industrial productivity, at least the last several stages are preferably rolled using a tandem mill.

The reduction in hot rolling is preferably such that the sheet thickness reduction rate in the temperature range of Ar ₃ point to “Ar ₃ point + 100 ° C.” is 40% or more, Ar ₃ point to “Ar ₃ point + 80 ° C.” More preferably, the plate thickness reduction rate in the temperature range is 60% or more. The above rolling does not have to be performed in one pass, and may be a continuous multiple-pass rolling.

  The amount of reduction per pass in the multipass rolling is preferably 15 to 60% in terms of sheet thickness reduction rate. From the viewpoint of easily reducing the size of ferrite produced by transformation from austenite in which strain is accumulated, it is preferable to increase the amount of reduction per pass, but by adjusting the cooling conditions after completion of rolling, 1 Fine polygonal ferrite ferrite grains having an average crystal grain size of 3.0 μm or less can be obtained by rolling a plurality of passes with a reduction amount per pass of 40% or less.

(D-2) Cooling and winding after hot rolling:
After the hot rolling is completed, in order to transform the austenite to ferrite as a driving force without releasing the processing strain introduced in the austenite, first, in order to generate a fine ferrite grain structure, After the rolling is completed, primary cooling is performed to cool to 720 ° C. within 0.4 seconds, and the primary cooling is preferably stopped at a temperature T ₁ ° C. in a temperature range of 720 to 550 ° C.

  When the cooling time is stopped or slowed down at a temperature exceeding 720 ° C. and the time from hot rolling to 720 ° C. after completion of hot rolling exceeds 0.4 seconds, it is introduced by processing before fine ferrite is formed. In some cases, the strain is released, or the existence form of the strain is changed so that it is not effective for nucleation of ferrite, and the ferrite grains are remarkably coarsened.

  When the temperature of the steel sheet reaches 720 ° C. due to primary cooling, the steel enters a temperature range in which ferrite transformation is activated. In addition, since the temperature range of the ferrite transformation is a temperature range of 720 to 550 ° C., when the primary cooling is performed to a temperature range lower than 550 ° C., the volume ratio of ferrite (polygonal ferrite) is drastically reduced and low temperatures such as bainite and martensite. A transformation phase may form.

Therefore, in the invention of (5), after the hot rolling is completed, the primary cooling is performed to cool to 720 ° C. within 0.4 seconds, and the primary cooling is performed at a temperature of 720 to 550 ° C. It was decided to stop at the temperature T ₁ ° C of the zone.

  The time for cooling to 720 ° C. after completing the hot rolling is more preferably within 0.2 seconds. This is because freezing of working strain into austenite becomes easier, and the transformation from austenite to ferrite using the working strain as a driving force makes it possible to obtain a more stable and reliable fine polygonal ferrite grain structure. .

  Therefore, in the invention of (6), it is decided to cool to 720 ° C. within 0.2 seconds by the primary cooling after completing the hot rolling.

  More preferably, water cooling is used for the primary cooling, and the cooling rate is 400 ° C./s or more.

  In addition, after completion of hot rolling, primary cooling is performed and cooling to 700 ° C. within 0.4 seconds makes it easier to freeze the working strain into austenite, thereby driving the working strain to the driving force. By the transformation from austenite to ferrite, a fine polygonal ferrite grain structure can be obtained more stably and reliably. For this reason, after completing the hot rolling, it is more preferable to perform primary cooling and cool to 700 ° C. within 0.4 seconds.

  Furthermore, in order to suppress the generation of low-temperature transformation phases such as bainite and martensite as much as possible and obtain a sufficient amount of polygonal ferrite, the primary cooling stop temperature is further set to a temperature range of 700 to 600 ° C. preferable.

After the above primary cooling, the secondary cooling is performed from ₁ to 30 seconds from T ₁ ° C to 500 ° C, and the secondary cooling is further performed to a temperature T ₂ ° C in the temperature range of 500 to 300 ° C. good to take up at a temperature range of T ₂ ~300 ℃ after.

When secondary cooling from T ₁ ° C to 500 ° C is performed in less than 1 second, the amount of polygonal ferrite produced is small, so the concentration of C in the untransformed austenite becomes insufficient and the volume ratio is 5%. In some cases, the desired amount of austenite cannot be retained. On the other hand, if it takes more than 30 seconds to cool from T ₁ ° C to 500 ° C, pearlite is generated during this secondary cooling, so the amount of retained austenite is reduced and a sufficient “TRIP” effect is obtained. It may not be possible.

  When the winding temperature exceeds 500 ° C., pearlite is generated and the amount of retained austenite decreases, so that a sufficient “TRIP” effect may not be obtained, and winding is performed in a temperature range below 300 ° C. In some cases, the formation of martensite is promoted and the ductility is lowered.

Therefore, in the invention of the above (5), the primary cooling is stopped at T ₁ ° C., and then, from T ₁ ° C. to 500 ° C. is cooled by secondary cooling in ₁ to 30 seconds, and the secondary cooling is further performed. was and be wound in a temperature range of T ₂ ~300 ℃ after up to a temperature T ₂ ℃ temperature range of 500~300 ℃.

  In addition, when surface treatment such as hot dip galvanizing, alloying hot dip galvanizing, and electroplating is performed on the high-tensile hot-rolled steel sheet according to the present invention, it has excellent corrosion resistance in addition to excellent ductility and hole expansibility. The provided surface-treated steel sheet can be obtained.

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

  Steel having the chemical composition shown in Table 1 was melted in a 150 kg high-frequency vacuum melting furnace, each steel ingot was hot forged by a normal method, and a steel piece (steel plate) having a width of 150 mm and a thickness of 30 mm; did.

  Next, each steel slab was heated to a temperature of 1000 to 1250 ° C. and hot-rolled for 5 passes to finish a steel sheet having a thickness of 1.5 mm. In addition, the concrete temperature which heated the steel slab is steel A, steel C-E, steel G, steel I, steel K, steel M, and steel O 1250 degreeC, steel B, steel F, steel H, and steel J Is 1150 ° C, steel L and steel N are 1000 ° C.

After the hot rolling was completed, cooling and winding processes were performed under the conditions shown in Table 2. The total rolling reduction (sheet thickness reduction rate) in the 5-pass hot rolling is 95%, and the rolling completion temperature is approximately “Ar ₃ point + 20 ° C.”.

  For each steel plate thus obtained, the structure, mechanical properties, surface properties, and C content in austenite were investigated.

  The texture was investigated for phase identification, area ratio (and hence volume ratio) of each of austenite and ferrite, average grain size and aspect ratio. The phase was identified by cross-sectional observation in the plate thickness direction using a scanning electron microscope. For the average crystal grain size of austenite and ferrite, use scanning electron micrographs taken at three positions at a depth of 1/8, 1/4 and 1/2 of the plate thickness from either surface of the steel plate. Then, the average grain slice length at each position was measured by the intercept method, and the arithmetic average value was multiplied by 1.12. In addition, about the austenite whose aspect ratio exceeds 2, since the film-like form is exhibited, since it is difficult to measure the average crystal grain size, the film width was measured as a reference value. The volume ratio of the ferrite was determined from the arithmetic average value obtained by measuring the same position as the ferrite particle diameter measurement position described above by the so-called “mesh method”. The volume ratio of austenite was determined by X-ray diffraction measurement.

  Regarding the mechanical properties, tensile properties and stretch flangeability were investigated by the following methods.

  Tensile properties were collected from each steel sheet in the RD direction (that is, the rolling direction) and TD direction (that is, the rolling width direction) from No. 5 tensile test pieces described in JIS Z 2201 (1998), and subjected to a tensile test at room temperature. Tensile strength (TS) and total elongation (EL) were measured.

  For stretch flangeability, 100 mm square specimens are taken from each steel sheet, a punched hole with a diameter of 10 mm is punched in the center, and this hole is expanded with a conical punch with a tip angle of 60 °. The critical hole expansion rate (HER) calculated from the hole diameter when a crack penetrated the edge of the film was evaluated.

  For the surface properties, the surface of each steel plate was photographed with an optical microscope at an observation magnification of 10 times or 50 times, and the area ratio of the island scale was calculated by image analysis. In addition, the case where the area ratio of the island scale was less than 5% was evaluated as “◎”, the case where it was 5% or more and less than 20% was evaluated as “◯”, and the case where it was 20% or more was evaluated as “x”.

  The C content in austenite was calculated from the lattice constants of ferrite and austenite obtained by X-ray diffraction measurement.

  Table 3 shows the results of investigation of the C content in the structure and austenite. In addition, except for the steel plate of test number 18, all the ferrites in the steel plates of other test numbers were polygonal ferrites having an aspect ratio of 2 or less. For this reason, in Table 3, only the aspect ratio related to austenite is displayed in a magnitude relationship with 2.

  Table 4 summarizes the survey results of mechanical properties and surface properties.

  As is apparent from Tables 3 and 4, the hot rolled steel sheets of Test Nos. 1 to 12 having the chemical composition and structure defined in the present invention have high strength and good elongation characteristics. Anisotropy is extremely small. Furthermore, the area ratio of the island scale is “で” of less than 5%, and the surface property is also excellent.

  On the other hand, even if it has a chemical composition defined in the present invention, the mechanical properties of the hot-rolled steel sheets having test numbers 13 to 18 whose structure deviates from the provisions defined in the present invention are those of the above test numbers 1 to 12. It is clear that it is inferior to hot-rolled steel sheets.

  That is, the hot rolled steel sheet of test number 13 has a low strength because the average crystal grain size of polygonal ferrite exceeds 3.0 μm.

  The hot-rolled steel sheet of test number 14 has a bainite-based structure in which the average crystal grain size of polygonal ferrite exceeds 3.0 μm, and the generated amount is 21.2% by volume and greatly lower than 60%. Therefore, the ductility is extremely low.

  The hot-rolled steel sheets of Test No. 15 and Test No. 17 are each a mixed structure of polygonal ferrite and pearlite, a mixed structure of polygonal ferrite and martensite, and does not contain austenite. The content of C in the austenite is also outside the definition of the present invention. For this reason, the mechanical properties are greatly inferior.

  The hot rolled steel sheet of Test No. 16 has an average crystal grain size of polygonal ferrite exceeding 3.0 μm and does not contain austenite. Therefore, the content of C in austenite is also outside the scope of the present invention. ing. For this reason, the mechanical properties are greatly inferior.

  Further, as described above, the hot rolled steel sheet of test number 18 has a processed structure in which the ferrite has an aspect ratio exceeding 2 as described above, and the average crystal grain size of the processed ferrite exceeds 3.0 μm, so the ductility is extremely low. .

  In addition, the hot-rolled steel sheets having test numbers 19 to 22 that deviate from the chemical composition defined in the present invention are also out of the structure defined in the present invention, and at least one of strength, ductility, and surface properties is inferior. Yes.

Since the high-tensile hot-rolled steel sheet of the present invention is excellent in “strength-ductility balance”, it can be used as a material for high-strength structural members used in automobiles and various industrial machines. This high-tensile hot-rolled steel sheet can be produced relatively easily by practical hot rolling at about 800 ° C. or higher by the method of the present invention.

Claims (6)

  In a mass%, C: 0.05 to 0.30%, Si: less than 0.5%, Mn: 0.5 to 3.0% and Al: 0.1 to 2.0% and Si and The sum of the Al contents satisfies 0.5 to 2.0%, the balance is the chemical composition of Fe and impurities, and the structure contains 5% or more austenite and 60% or more polygonal ferrite by volume. Furthermore, the average crystal grain size of the austenite is 2.0 μm or less, the average crystal grain size of the polygonal ferrite is more than 1.0 μm to 3.0 μm, and the C content in the austenite is mass%. And a high-tensile hot-rolled steel sheet characterized by being 0.7 to 2.0%. The high-tensile hot-rolled steel sheet according to claim 1, wherein the sum of the contents of Si and Al satisfies the following formula (1).
0.5 ≦ Si + Al ≦ (13 × C + Mn + 10) / 10 (1)
However, the element symbol in the formula (1) represents the content in steel in mass% of the element.   Instead of a part of Fe, in mass%, Nb: 0.005 to 0.10%, Ti: 0.005 to 0.20% and V: 0.005 to 0.20% The high-tensile hot-rolled steel sheet according to claim 1 or 2, comprising:   Instead of a part of Fe, by mass%, Ca: 0.0002 to 0.01%, Zr: 0.002 to 0.10%, and REM (rare earth element): 0.002 to 0.10% The high-tensile hot-rolled steel sheet according to any one of claims 1 to 3, comprising one or more of the following. A method of manufacturing a high-tensile hot-rolled steel sheet having a chemical composition according to any one of claims 1 to 4, the steel ingot or steel slab of the chemical composition in the hot rolling to Ar ₃ point or more temperature After the hot rolling is completed, the primary cooling is performed to cool to 720 ° C. within 0.4 seconds, and further, the primary cooling is stopped at a temperature T ₁ ° C. in the temperature range of 720 to 550 ° C. Then, from T ₁ to 500 ° C. is cooled by secondary cooling in ₁ to 30 seconds, and further, the secondary cooling is performed to a temperature T ₂ ° C. in a temperature range of 500 to 300 ° C., and then T _{2 to} 300 A method for producing a high-tensile hot-rolled steel sheet, which is wound in a temperature range of ° C. The method for producing a high-tensile hot-rolled steel sheet according to claim 5, wherein the primary cooling is performed within 0.2 seconds to 720 ° C.