JP2004211199A - High strength hot rolled steel sheet having excellent fatigue resistance and excellent strength-ductility balance, and method of producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet which has an excellent strength-ductility balance, and excellent fatigue resistance. <P>SOLUTION: A steel stock having a composition comprising, by mass, 0.05 to 0.3% C, 0.01 to 2.5% Si and 0.5 to 3.0% Mn is heated, and is then successively subjected to: a hot rolling stage; a primary cooling stage where cooling is performed to a temperature range of 650 to 950°C at a cooling rate of 5 to 500°C/s; a strain impartation stage where repeated bending and rebending working by a leveller is performed and a strain of 0.05 to 2.0 is imparted to the outermost surface part of the hot rolled steel sheet; and a secondary cooling stage where cooling is performed to a temperature of ≤650°C at a cooling rate of 10 to 300°C. Thus, the high strength hot rolled steel sheet is obtained having a crystal grain size gradient structure in which the volume fraction of polygonal ferrite as the main phase is ≥70%, the volume fraction of a secondary phase other than that is ≥5%, and the average crystal grain size of the polygonal ferrite gradually decreases from the center of the sheet thickness toward the surface layer. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

  TECHNICAL FIELD The present invention relates to a high-strength hot-rolled steel sheet excellent in strength-ductility balance and suitable for applications such as automobile parts and mechanical structure parts, and more particularly to improvement of fatigue resistance characteristics. In the present invention, “high strength” refers to a hot-rolled steel sheet having a tensile strength of 440 MPa or more. Further, “excellent in strength-ductility balance” refers to a hot-rolled steel sheet having (tensile strength TS) × (elongation El) of 20,000 MPa% or more. Further, the hot-rolled steel sheet includes a hot-rolled steel strip.

  Steel sheets used for automobile parts, mechanical structural parts, and the like are required to have high strength, excellent workability, and excellent fatigue resistance. It is well known that it is effective to reduce the crystal grains of steel sheets in order to improve mechanical properties such as strength, workability, and fatigue resistance. Various methods have been studied. A typical example is a so-called controlled rolling method described in Patent Document 1.

  The feature of the controlled rolling method is that rolling starts on the high temperature side of the austenite recrystallization temperature range, and refines austenite (hereinafter simply referred to as γ) grains by using dynamic recrystallization or static recrystallization. And rolling again in the γ non-recrystallization temperature range, which is the lower temperature side of the γ range, to introduce lattice defects such as dislocations in the γ grains, and to promote the γ → α transformation starting therefrom. In view of the above, it is an object to realize finer ferrite (hereinafter simply referred to as α) grains. In other words, increasing the number of γ grain boundaries, which are α nucleation sites during the γ → α transformation, and introducing a larger amount of lattice defects such as dislocations, generate a large number of α grains and refine the crystal grains. It is to try to.

Further, Patent Document 2 discloses that a slab is water-cooled before rolling and / or during rolling after reheating to bring a surface layer into a (γ + α) two-phase region or a single-phase state of α, and a cumulative rolling reduction of 20% The above-mentioned rolling is performed, the rolling is finished after the temperature rises to the AC3 transformation point or more by recuperation, and the surface layer is subjected to a predetermined amount of strain or more by repeatedly bending at a temperature range of the Ar3 transformation point or more, thereby continuously forming the surface layer. There has been proposed a method for manufacturing a thick steel plate in which crystal grains are refined to improve brittle crack propagation stopping characteristics.
JP-A-63-223124 JP 7-76726 A

  In order to refine γ grains and introduce a large amount of lattice defects, it is necessary to impart as much strain as possible to the steel sheet. However, since the slab thickness, sheet bar thickness, and product thickness are determined, there is a limit to the amount of strain that can be applied in the rolling process. For this reason, it is said that the crystal grain obtained by the general controlled rolling method has a limit of miniaturization to an average crystal grain size of about 5 μm.

  Generally, the effect of improving mechanical properties such as strength associated with the refinement of crystal grains is inversely proportional to the square root of the crystal grain size. Therefore, the effect of improving the mechanical properties is small when the crystal grains are refined in the range of 50 to 5 μm as in a general controlled rolling method. In order to further improve the mechanical properties, further refinement of the crystal grains to have a size of 5 μm or less has been desired.

  Further, the technique described in Patent Literature 2 needs to cool a surface layer only to a two-phase (γ + α) or an α-single phase for a thick steel plate, and then reheat the steel sheet to a γ-single phase again. When this technique is applied to a hot-rolled steel sheet, which is a thin steel sheet, it is very difficult and practically impossible to transform α → γ by reheating from inside because the steel sheet is thin. Further, the technique described in Patent Document 2 aims at improving the brittle crack propagation arresting property, and does not consider the improvement of the fatigue resistance property at all.

  The present invention advantageously solves the above-mentioned problems of the prior art, the average crystal grain size is fine, and the average crystal grain size of the surface layer portion is finer than that of the central portion, high strength and excellent in strength-ductility balance, Another object of the present invention is to provide a high-strength hot-rolled steel sheet excellent in fatigue resistance and a method for producing the same. An object of the present invention is to provide a high-strength hot-rolled steel sheet having a strength one rank higher than a conventional strength level even if the composition of the steel sheet is the same.

  Means for Solving the Problems The present inventors have diligently studied measures for improving fatigue resistance as well as strength and workability in order to solve the above problems. As a result, the present inventors, in order to improve the fatigue resistance along with the strength and workability, the thickness average crystal grain size is fine, and the average crystal grain size of the surface layer is to be a steel sheet finer than the central part. Came to the point that it was important. Specifically, the average grain size of the steel sheet is reduced to 6 μm or less, the average grain size of the surface layer is made smaller than that of the central portion, and the average grain size of the surface layer is reduced to the average grain size of the entire sheet thickness. It has been found that it is important to make the fineness up to 90% to 30%.

  The present inventors have found that the fatigue resistance characteristics of a steel sheet depend on the crystal grain size near the surface layer rather than the average crystal grain size of the steel sheet, and the steel sheet after finish rolling is cooled to a predetermined temperature range, and then repeatedly. It has been found that by performing bending / unbending processing and cooling again, the crystal grains in the surface layer of the steel sheet become finer than in the center of the steel sheet, and the fatigue resistance characteristics are remarkably improved.

  When the steel sheet is cooled to a predetermined temperature range and subjected to repeated bending and bending back processing, bending and bending back strain can be accumulated in the surface layer of the steel sheet without changing the thickness of the steel sheet. In particular, the accumulated bending strain increases from the center of the plate thickness toward the surface layer. Therefore, at the time of the subsequent γ → α transformation, more α grains can be generated in the outermost layer, and after cooling, the thickness average crystal grain size can be reduced, and the grain refinement of the surface layer of the steel sheet is particularly effective. Can be planned. This has revealed that a hot-rolled steel sheet having high strength, excellent strength-ductility balance, and markedly improved fatigue resistance can be obtained.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) In mass%, C: 0.05-0.3%, Si: 0.01-2.5%, Mn: 0.5-3.0%, composition consisting of balance Fe and unavoidable impurities and polygonal ferrite as main phase by volume Grain size gradient containing 70% or more by volume fraction and 5% by volume or more of the second phase other than polygonal ferrite, and the average crystal grain size of the polygonal ferrite gradually decreases from the center of the plate thickness toward the surface layer. A high-strength hot-rolled steel sheet having an excellent fatigue resistance characteristic characterized by having a structure and an excellent balance between strength and ductility.
(2) The high-strength hot-rolled steel sheet according to (1), wherein the strength-ductility balance is 21000 MPa% or more.
(3) In (1) or (2), the average crystal grain size in the surface layer of the polygonal ferrite is 1 μm or less, and the average thickness of the polygonal ferrite is 1.8 μm or less in the entire thickness range. A high-strength hot-rolled steel sheet characterized by the following.
(4) In any one of (1) to (3), in addition to the above composition, one or two members selected from Ti: 0.005 to 0.25% and Nb: 0.005 to 0.2% by mass%. A high-strength hot-rolled steel sheet comprising:
(5) In any one of the constitutions (1) to (4), in addition to the above-mentioned composition, in mass%, Cu: 0.2% or less, Ni: 0.2% or less, Cr: 0.2% or less, Mo: 0.2% or less, A high-strength hot-rolled steel sheet characterized in that one or more selected from B: 0.2% or less and V: 0.2% or less are contained in a total of 1.0% or less.
(6) Mass%: C: 0.05-0.3%, Si: 0.01-2.5%, Mn: 0.5-3.0%, steel material with composition consisting of balance Fe and unavoidable impurities is heated, including finish rolling A hot rolling step of performing hot rolling, and a primary cooling step of cooling the hot-rolled steel sheet after the finish rolling to a temperature range of 650 ° C to 950 ° C at a cooling rate of 5 ° C / s to 500 ° C / s. The hot-rolled steel sheet cooled to the temperature range is subjected to a repeated bending / bending process by a leveler, a strain applying step of applying a strain of 0.05 or more and 2.0 or less to the outermost part of the hot-rolled steel sheet, and the strain applying step is performed. A secondary cooling step of cooling the hot-rolled steel sheet to a temperature of 650 ° C. or less at a cooling rate of 10 ° C./s or more and 300 ° C./s or less, characterized by excellent fatigue resistance and strength. Manufacturing method of high strength hot rolled steel sheet with excellent ductility balance.
(7) The method for producing a high-strength hot-rolled steel sheet according to (6), wherein the temperature range in which the cooling in the primary cooling step is stopped is 800 ° C. or less, and the strain in the strain applying step is more than 1.0.
(8) In (6) or (7), in addition to the above composition, one or two selected from Ti: 0.005 to 0.25% and Nb: 0.005 to 0.2% by mass% are further contained. A method for producing a high-strength hot-rolled steel sheet.
(9) In any one of (6) to (8), in addition to the above-mentioned composition, further, in mass%, Cu: 0.2% or less, Ni: 0.2% or less, Cr: 0.2% or less, Mo: 0.2% or less, A method for producing a high-strength hot-rolled steel sheet, comprising one or more selected from B: 0.2% or less and V: 0.2% or less in total of 1.0% or less.

  According to the present invention, a high-strength hot-rolled steel sheet excellent in strength-ductility balance and excellent in fatigue resistance can be easily and inexpensively provided, and an industrially remarkable effect is achieved. The high-strength hot-rolled steel sheet of the present invention is a steel sheet suitable for various uses such as for automobile parts and mechanical structural parts.

  The hot-rolled steel sheet of the present invention contains, by mass%, C: 0.05 to 0.3%, Si: 0.01 to 2.5%, Mn: 0.5 to 3.0%, preferably S: 0.02% or less, P: 0.03% or less, and Al: 0.03% or less. : N: 0.02% or less, N: 0.02% or less, with the balance being Fe and unavoidable impurities.

  First, the reasons for limiting the composition of the hot-rolled steel sheet of the present invention will be described. Hereinafter, mass% in the composition is simply described as%.

C: 0.05-0.3%
C is an element effective for solid solution strengthening and structure strengthening of steel, and in order to obtain such an effect, the present invention needs to contain 0.05% or more. If it is less than 0.05%, sufficient strength cannot be obtained. On the other hand, if it exceeds 0.3%, the ductility decreases. Therefore, in the present invention, C is limited to 0.05 to 0.3%.

Si: 0.01-2.5%
Si is an element effectively acting on solid solution strengthening of steel, and in the present invention, the content of 0.01% or more is required. On the other hand, if the content exceeds 2.5%, the releasability of the scale decreases and the surface quality deteriorates. For this reason, Si was limited to 0.01 to 2.5%. Incidentally, the content is preferably 0.1 to 1.5%.

Mn: 0.5 to 3.0%
Mn is an element having an effect of lowering the transformation point at the time of cooling the steel sheet. In the present invention, Mn needs to be contained at 0.5% or more. On the other hand, if the content exceeds 3.0%, the ductility decreases. For this reason, Mn is limited to the range of 0.5 to 3.0%. From the viewpoint of increasing the strength-ductility balance, the content is preferably 0.7 to 2.5%.

S: 0.02% or less S exists as a sulfide, and lowers the cleanliness of the steel and decreases the ductility and fatigue resistance of the steel. Therefore, S is desirably limited to 0.02% or less. In addition, it is preferably 0.01% or less.

P: 0.03% or less P is an element which increases the strength of steel by forming a solid solution, but is easily segregated at the crystal grain boundary, and a large amount of P causes deterioration of the toughness of the steel. For this reason, P is preferably limited to 0.03% or less. Note that the content is more preferably 0.02% or less.

Al: 0.2% or less
Al is an element which has a strong bonding force with oxygen and is usually used as a deoxidizing agent. However, if it exceeds 0.2%, the ductility of the steel sheet decreases. Therefore, it is desirable to limit Al to 0.2% or less. In addition, it is preferably 0.05% or less.

N: 0.02% or less N is an element that increases the strength of steel by forming a solid solution. However, when N is contained in a large amount, the formability of the steel sheet is reduced. For this reason, it is preferable to limit N to 0.02% or less.

  In the present invention, in addition to the above-mentioned basic composition, one or two selected from 0.005 to 0.25% of Ti and 0.005 to 0.2% of Nb can be further contained.

  Both Ti and Nb form carbides, nitrides, and carbonitrides, suppress grain growth and effectively act on grain refinement, and contribute to precipitation strengthening, and contribute to the strengthening of steel sheets. It is an element that effectively works for improvement, and can be selected and contained as needed. In order to obtain such effects, it is desirable to contain Ti: 0.005% or more and Nb: 0.005% or more, respectively. On the other hand, if the content exceeds 0.25% of Ti and 0.2% of Nb, the effect is saturated, and the effect corresponding to the content cannot be expected. For this reason, it is preferable to limit the content to 0.005 to 0.25% for Ti and 0.005 to 0.2% for Nb.

  In the present invention, in addition to the above-described compositions, Cu: 0.2% or less, Ni: 0.2% or less, Cr: 0.2% or less, Mo: 0.2% or less, B: 0.2% or less, V: 0.2% or less One or more selected from the above may be contained in a total of 1.0% or less.

  Cu, Ni, Cr, Mo, B, and V are all elements that increase the strength of steel, and one or two or more of them can be selected and contained as necessary. If the content exceeds Cu: 0.2%, Ni: 0.2%, Cr: 0.2%, Mo: 0.2%, B: 0.2%, and V: 0.2%, and if the total content exceeds 1.0%, the workability will increase. Decreases. Therefore, one or two or more selected from Cu: 0.2% or less, Ni: 0.2% or less, Cr: 0.2% or less, Mo: 0.2% or less, B: 0.2% or less, V: 0.2% or less Is preferably limited to 1.0% or less in total.

  The balance other than the above components is Fe and unavoidable impurities. As unavoidable impurities, Ca: 0.01% or less, O: 0.01% or less can be tolerated.

  Next, the metallographic structure of the hot-rolled steel sheet of the present invention will be described.

  The hot-rolled steel sheet of the present invention has the above-described composition, has polygonal ferrite as a main phase, and has a second phase other than polygonal ferrite, and the average crystal grain size of the polygonal ferrite gradually increases from the center of the sheet thickness toward the surface layer. It has a grain size gradient structure that becomes smaller.

  Polygonal ferrite, which is the main phase, has a volume fraction of 70% or more. If the polygonal ferrite content is less than 70%, the ductility (elongation) is greatly reduced, and the strength-ductility balance is reduced. The polygonal ferrite is preferably set to 95% or less in volume fraction from the viewpoint of securing strength. The hot-rolled steel sheet according to the present invention has a crystal grain size gradient structure in which the polygonal ferrite as the main phase gradually decreases in size from the center of the sheet thickness toward the surface layer. When the polygonal ferrite in the surface portion has a finer crystal grain size than the central portion, fatigue resistance is significantly improved. In the hot-rolled steel sheet of the present invention, the average crystal grain size in the entire thickness direction is 6 μm or less, and the average crystal grain size in the surface layer portion is 90 to 30% of the average crystal grain size in the sheet thickness. Is preferred.

  When the average crystal grain size of the surface layer portion exceeds 90% as compared with the average thickness grain size, the improvement in fatigue resistance characteristics is not remarkable. On the other hand, when the average thickness of the sheet exceeds 6 μm, the effect of improving the strength due to the refinement of the crystal grains decreases. The surface layer portion in the present invention refers to a region from the outermost layer to 200 μm in the thickness direction (depth direction). More preferably, the average crystal grain size in the plate thickness is 1.8 μm or less, and the average crystal grain size in the surface layer is 1 μm or less.

  In the present invention, the second phase means a structure other than the main phase, polygonal ferrite. The second phase has a total volume fraction of 5% or more. If the volume fraction of the second phase is less than 5%, the strength of the steel sheet is greatly reduced, and the strength-ductility balance is reduced. In addition, it is preferable to set the volume of the second phase to 30% or less from the viewpoint of ensuring ductility. As the second phase, one or more of pearlite, bainite, martensite, and retained austenite are preferably used. In particular, it is preferable to increase the amount of martensite when improving the strength, and it is preferable to increase the amount of retained austenite when improving the ductility.

  Next, a preferred method for producing the hot-rolled steel sheet of the present invention will be described.

  The molten steel having the composition described above is smelted by a known smelting method such as a converter or an electric furnace, and is made into a steel material such as a slab by a known casting method such as a continuous casting method. In the present invention, the method for producing the steel material is not particularly limited. In general, any known method is suitable.

  Next, the steel material is preferably heated to a temperature of 1000 ° C. or more and 1200 ° C. or less, and is subjected to a hot rolling step to be a hot-rolled steel sheet. The heating in the hot rolling may be a direct-feed rolling without heating or slightly heating when the steel material is at a rolling temperature or higher.

  The heated steel material is converted into a sheet bar by rough rolling, and then subjected to finish rolling to obtain a hot-rolled steel sheet having a predetermined thickness. When a sheet bar or a thin slab is used as a material, it goes without saying that rough rolling may be omitted.

  In the production method of the present invention, the hot rolling conditions are not particularly limited. However, in order to reduce the thickness average crystal grain size of the steel sheet to 6 μm or less, rough rolling is performed at a cumulative rolling reduction of 70% in a temperature range of 900 to 1150 ° C. Preferably, the finish rolling is performed at a cumulative rolling reduction of 70 to 98% in a temperature range of 800 to 1050 ° C., and the exit temperature of the finishing mill is set to 800 to 1000 ° C.

  After the finish rolling, the hot-rolled steel sheet subjected to the finish rolling is immediately subjected to a primary cooling step of cooling to a predetermined temperature range. In the primary cooling step, the hot-rolled steel sheet is cooled to a temperature range of 650 ° C to 950 ° C at a cooling rate of 5 ° C / s to 500 ° C / s.

  If the cooling rate in the primary cooling step is less than 5 ° C./s, the γ grains are coarsened during cooling, and the crystal grains cannot be sufficiently refined. On the other hand, since a cooling rate exceeding 500 ° C./s is difficult to achieve with the current cooling facility capacity, it is preferable to limit the cooling rate in the primary cooling step to 5 to 500 ° C./s. Note that, as the cooling rate increases, the grain growth of γ grains after finish rolling can be suppressed, so that the rate is preferably 30 ° C./s or more.

  Further, when the cooling stop temperature in the primary cooling step is lower than 650 ° C., the γ → α transformation is largely completed, and strain cannot be accumulated in the γ grains by subsequent processing (repeated bending / bending back processing). , The transformation nuclei during the γ → α transformation cannot be increased, and the crystal grains cannot be sufficiently refined. On the other hand, if the cooling stop temperature in the primary cooling step is higher than 950 ° C., the strain applied by the subsequent processing (repeated bending / bending-back processing) is remarkably recovered, and the strain can be sufficiently accumulated. Therefore, the crystal grains cannot be sufficiently refined. Note that the cooling stop temperature is preferably set to 700 to 800 ° C., which is a temperature range in which recovery of the strain applied thereafter hardly occurs.

  The hot-rolled steel sheet cooled to the predetermined temperature range by the primary cooling step is then subjected to repeated bending and bending back processing by a leveler, and subjected to a strain imparting step of imparting a strain of 0.05 or more and 2.0 or less to the outermost part of the steel sheet. .

  The strain ε imparted to the outermost part of the steel sheet by the repeated bending and bending back processing by the leveler is approximately expressed by the equation (1).

ε = (N−2) × (2t) δ / L ² (1)
Here, ε: strain, t: plate thickness, δ: leveler tightening amount, 2L: leveler work roll (hereinafter simply referred to as WR) center axis interval, N: number of leveler WRs. It is defined as the distance from the state where the steel plate is sandwiched by the leveler WR and the leveler WR is tightened. FIG. 2 shows a bending state of the steel sheet 1 in one bending operation by a leveler. Here, δ is the leveler tightening amount, r is the leveler work roll radius, and 2L is the leveler work roll center axis interval.

  If the strain ε at the outermost portion of the steel sheet provided by the bending process with the leveler is less than 0.05, the amount of accumulated strain is small, and it is not possible to refine the crystal grains. On the other hand, it is practically difficult to apply a strain exceeding 2.0. For example, from equation (1), if N is increased to 109 or more, the strain ε can be increased beyond 2.0 (in addition, δ = 19 mm, 2L = 180 mm, t = 4 mm), but the equipment length exceeds 9.9 m. Becomes too long to be realistic. Further, as is apparent from FIG. 2, the leveler tightening amount is limited due to the arrangement of the work rolls, and it is difficult to increase the strain ε beyond 2.0. In addition, by reducing the leveler WR center axis interval 2L to less than 5.5 mm from equation (1) (where δ = 7 mm, N = 29 and t = 4 mm), the strain ε can be increased beyond 2.0. Since the WR diameter must be extremely small, the deflection of the WR increases and the shape of the steel sheet deteriorates.

  In addition, in the repeated bending and bending back processing using a leveler, even at the position of 1/4 of the plate thickness, a strain of half of the outermost layer of the plate thickness can be given, and at the position of 1/4 of the plate thickness and other parts, Crystal grains can be refined according to the amount of strain applied. That is, by the strain imparting by the repeated bending and bending back processing by the leveler, it is possible to obtain a crystal grain size gradient structure in which the grain size of polygonal ferrite gradually decreases from the sheet thickness center toward the sheet thickness outermost part. . The strain ε applied to the outermost part of the steel sheet is more preferably 0.2 or more from the viewpoint of grain refinement and 1.0 or less from the equipment condition.

  In order to reduce the average thickness of the hot-rolled steel sheet to 1.8 μm or less and the average crystal grain size of the surface layer to 1 μm or less, the cooling stop temperature in the primary cooling step is 800 ° C. or less, It is preferable to set the distortion to be more than 1.0. If the cooling stop temperature in the primary cooling step exceeds 800 ° C. or the strain applied in the strain applying step is 1.0 or less, the above-described fine structure cannot be obtained. Further, a finer structure can be obtained by using a composition containing Ti: 0.05 to 0.15% and Nb: 0.05 to 0.15%.

  Next, the strained hot-rolled steel sheet is subjected to a secondary cooling step of cooling to a temperature of 650 ° C. or lower at a cooling rate of 10 to 300 ° C./s.

  If the cooling rate in the secondary cooling step is less than 10 ° C./s, the α grains after transformation will grow, and the crystal grain size will increase. Further, depending on the chemical composition and the hot rolling conditions, the γ → α transformation greatly proceeds during cooling, and the amount of the second phase formed decreases. In either case, the strength decreases. On the other hand, when the cooling rate in the secondary cooling step exceeds 300 ° C./s, the amount of polygonal ferrite, which is the main phase, decreases, and the ductility decreases. For this reason, it is preferable to limit the cooling rate in the secondary cooling step to a range of 10 to 300 ° C./s. Note that the temperature is more preferably 30 ° C./s or more.

  If the cooling stop temperature in the secondary cooling step is set to a high temperature exceeding 650 ° C., α grains are likely to be coarsened, and fine crystal grains having a thickness average grain size of 6 μm or less cannot be obtained. For this reason, the cooling stop temperature in the secondary cooling step was set to 650 ° C. or less. However, depending on the chemical composition and hot rolling conditions, if the cooling stop temperature in the secondary cooling step is set to a high temperature exceeding 600 ° C, the γ → α transformation proceeds even after the cooling stop, and the amount of the second phase generated And the strength may be reduced. On the other hand, when the temperature is lower than 300 ° C., a problem occurs that the hot-rolled steel sheet becomes extremely hard and becomes difficult to be wound into a coil. For this reason, the cooling stop temperature in the secondary cooling stop step is more preferably set to 300 ° C. or more and 600 ° C. or less. The cooling stop temperature in the secondary cooling step is important in controlling the type and amount of the second phase, and depends on the chemical composition, hot rolling conditions, and cooling conditions, but a better balance between strength and ductility is obtained. In order to achieve this, it is preferable to cool to a temperature range in which pearlite, bainite and the like are remarkably generated. From this viewpoint, the cooling stop temperature in the secondary cooling step is more preferably set to 450 ° C. or more and 600 ° C. or less.

  The hot-rolled steel sheet that has undergone the secondary cooling step is then wound into a coil to obtain a product.

  The hot-rolled steel sheet of the present invention is preferably manufactured using the hot rolling line shown in FIG. The steel material S is heated in a heating furnace (not shown), or directly fed in a hot state from an upstream process, turned into a sheet bar SB by a row of rough rolling mills 2, and then finish-rolled by a row of finishing mills 3. A hot-rolled steel sheet 1 having dimensions is set. Reference numeral 3a in the finishing mill train 3 denotes a work roll, and 3b denotes a backup roll. On the exit side of the finishing mill train 3, a first cooling device 4, a downstream leveler 5, and a second cooling device 6 are arranged in that order, and a winding device 7 is further provided. Hot-rolled steel sheet 1 is wound in a coil shape. In addition, a number of table rollers (not shown) are provided between the above-described main facilities, and transport the steel material S being rolled.

  The leveler 5 includes three or more work rolls 5a arranged in a staggered manner, or a backup roll 5b that further backs up the work rolls 5a. The level of the work roll of the leveler is preferably 300 mm or less, because the strain that can be imparted by the repeated bending / bending process by the leveler can be increased.

  Next, the present invention will be described in more detail based on examples.

(Example 1)
A steel material (slab thickness: 260 mm) having the composition shown in Table 1 was used. These slabs are heated to 1100 ° C, and the heated slabs are subjected to rough rolling at a cumulative rolling reduction of 85% in 7 passes in a temperature range of 1000 to 1100 ° C to form sheet bars, and then to 950 to 1050 ° C. A hot-rolled steel sheet having a thickness of 4 mm was obtained by performing a finish rolling process in which the rolling was performed at a cumulative draft of 90% in seven passes in a temperature range of 70 ° C. and the exit temperature of the finishing mill at 950 ° C. In addition, what rolled the hot-rolled steel plate after finish rolling in the shape of a coil is hereafter called a coil.

  Immediately after the finish rolling in the hot rolling step, the first half (the first half of the coil) of the hot-rolled steel sheet was cooled to 450 ° C under the condition of a cooling rate of 30 ° C / sec, and was taken up as a comparative example. On the other hand, the second half of the hot-rolled steel sheet (first half of the coil) is subjected to a primary cooling step of cooling to a cooling stop temperature of 850 ° C. immediately after the finish rolling in the hot rolling step, at a cooling rate of 30 ° C./sec. Then, at that temperature, a strain applying step of repeatedly bending and bending back with a leveler was performed so that the strain at the outermost portion of the steel sheet was 0.5. The hot-rolled steel sheet after the strain imparting step was subjected to a secondary cooling step of cooling again to 450 ° C. under the condition of a cooling rate of 30 ° C./sec, and then wound up to obtain an example of the present invention. The levelers used in the strain applying step had a leveler WR diameter of 2r: 170 mm, a center axis interval of the leveler WR of 2 L: 180 mm, the number of leveler WRs N: 29, and a leveler tightening amount δ: 19 mm.

A test piece was sampled from the obtained hot-rolled steel sheet and subjected to a structure examination, a tensile test, and a fatigue test.
(1) Structural investigation A test specimen was sampled from the center of the obtained hot-rolled steel sheet in the width direction, and the cross section in the C direction was measured using a scanning electron microscope. Centering on each position up to the 3.9 mm position at intervals, 100 fields in the plate width direction (50 μm each in the center distribution), 75 μm in the thickness direction (37.5 μm each in the center distribution), a total of 20 fields of view at a magnification of 1000 times. Observe, using an image analyzer for each, measure the volume fraction of polygonal ferrite, the volume fraction of the second phase, the average crystal grain size of the polygonal ferrite, to determine the average value in each field of view, The values at each center position in the plate thickness direction are taken, and they are further averaged in the plate thickness direction to obtain a volume fraction Vα (%) of polygonal ferrite, a volume fraction V2 (%) of the second phase, and a value of polygonal ferrite. The thickness average crystal grain size was defined as D2 (μm). In addition, the area ratio was measured in each visual field and converted into a volume fraction. In addition, the average crystal grain size of polygonal ferrite is determined in accordance with the provisions of JIS G 0552, by calculating the average cross-sectional area of the crystal grains, assuming that it is circular, and converting it to a crystal grain size. did. The average crystal grain size D1 for the surface layer portion is centered at a position of 0.1 mm in the thickness direction from the outermost portion and is 100 μm in the width direction (50 μm each for center distribution) and 75 μm in the thickness direction (center vibration). Observe the field of view of 37.5 μm) at a magnification of 1000 times and determine the average cross-sectional area of the crystal grains within the field of view in the same manner as described above in accordance with the provisions of JIS G 0552. Was calculated.
(2) Tensile test From the obtained hot-rolled steel sheet, a JIS No. 5 test piece having a test piece longitudinal direction in the L direction (rolling longitudinal direction) in accordance with the provisions of JIS Z 2201 was sampled, and JIS Z 2241 A tensile test was performed in accordance with the regulations, and a tensile strength TS and an elongation El were measured.
(3) Fatigue test From the obtained hot-rolled steel sheet, in accordance with the provisions of JIS Z 2275, take the No. 1 test piece whose longitudinal direction is the L direction of the steel sheet and comply with the provisions of JIS Z 2275. Then, a swing plane bending fatigue test was performed to determine a fatigue limit FL.

  Table 2 shows the obtained results.

  FIG. 3 shows the grain size distribution of steel No. A in the plate thickness direction. In the comparative example (the first half of the coil) in which the repetitive bending / bending process with the leveler was not performed, the particle diameter was substantially uniform in the thickness direction. On the other hand, in the example of the present invention (the latter half of the coil) subjected to repeated bending / bending back processing with a leveler, the average crystal grain size D1 for the surface layer portion is larger than that for the entire region in the thickness direction. The grain size is smaller than the grain size D2, and the grain size is gradually reduced from the center of the sheet thickness toward the surface layer of the sheet thickness.

  In each of the examples of the present invention, the main phase is polygonal ferrite, and the crystal grain size of the surface layer is as fine as 75% or less as compared with the average crystal grain size, and the crystal grain size gradient structure has a tensile strength TS of 440 MPa or more. High strength hot-rolled steel sheet with excellent strength-ductility balance, high fatigue limit FL, excellent strength-ductility balance, and excellent fatigue resistance characteristics, with a strength-ductility balance TS × El of 20,000 MPa% or more. ing. On the other hand, the comparative examples out of the range of the present invention have a low strength-ductility balance, a low fatigue limit FL and insufficient fatigue resistance, or a tensile strength of less than 440 MPa and high strength is not obtained. As a result, a high-strength hot-rolled steel sheet having excellent strength-ductility balance and excellent fatigue resistance has not been obtained.

With respect to the results shown in Table 2, the relationship between the tensile strength and the elongation is shown in FIG. 4, and the relationship between the fatigue limit and the tensile strength is shown in FIG. From FIG. 4, it can be seen that the present invention example has a better strength-ductility balance than the comparative example. Also, from FIG. 5, it can be seen that, in comparison with the same tensile strength, the example of the present invention shows a high fatigue limit and is excellent in fatigue resistance characteristics. The crystal grain is refined by the repeated bending and bending back processing by the leveler which is an example of the present invention, and a remarkable increase in fatigue limit is recognized when compared at the same tensile strength.
(Example 2)
The steel materials (slab thickness: 260 mm) of steel Nos. A to D shown in Table 1 were used. These slabs are heated to 1100 ° C, and the heated slabs are subjected to rough rolling at a temperature range of 1000 to 1100 ° C in seven passes with a cumulative rolling reduction of 85% to form sheet bars, and then 950 to 1050 ° C. In the temperature range of 7 mm, a hot rolling process was performed in which finishing rolling was performed in 7 passes with a cumulative rolling reduction of 90% and the exit temperature of the finishing mill at 950 ° C. to obtain a hot-rolled steel sheet having a thickness of 4 mm. Immediately after the finish rolling in the hot rolling step, a primary cooling step, a strain imparting step, and a secondary cooling step under the conditions shown in Table 3 were performed and wound. The leveler used in the strain imparting process has a leveler WR diameter of 2r: 100 mm φ, a center axis interval of the leveler WR 2 L: 120 mm, and the number of levelers WR N: 49. By changing the tightening amount δ of the leveler, The amount of distortion was adjusted. Further, the change of the cooling rate in the primary cooling step and the secondary cooling step was performed by changing the cooling water amount density and the length of the zone for injecting the cooling water in the conveying direction of the steel material S in the hot rolling line. .

  About the obtained hot-rolled steel sheet, the structure | tissue investigation, the tensile test, and the fatigue test were implemented like Example 1. FIG. The results obtained are shown in Table 3.

  In any of the examples of the present invention, the main phase is polygonal ferrite, and the average grain size D1 in the surface layer portion is 75% or less as compared with the average thickness grain size D2 in the entire thickness direction. This is a high-strength hot-rolled steel sheet having an inclined structure, a tensile strength of 440 MPa or more, a high strength-ductility balance exceeding 20,000 MPa%, and a high fatigue limit. Comparative examples outside the scope of the present invention cannot satisfy any of the above-mentioned structures and mechanical properties.

  In the present invention example, even if not containing Ti, Nb, the average crystal grain size of the plate thickness is less than 2.0μm, the average crystal grain size of the surface layer portion is 1.2μm or less and polygonal ferrite is fine, according to the present invention Thus, a hot-rolled steel sheet can be obtained which does not contain expensive alloy elements and retains excellent mechanical properties equal to or more than those of the case where expensive alloy elements are contained. Further, when Ti and Nb are contained, the average crystal grain size of the surface layer portion can be 1.0 μm or less, and the average crystal grain size of the plate thickness can be 1.8 μm or less. It can be said that the high-strength hot-rolled steel sheet of the present invention is a material excellent in recyclability.

It is explanatory drawing which shows typically an example of the hot rolling line suitable for implementation of this invention. It is explanatory drawing which shows typically the outline of distortion imparting to the steel plate using a leveler. 4 is a graph showing a particle size distribution in a plate thickness direction in Examples. It is a graph which shows the relationship between tensile strength and elongation in an example. It is a graph which shows the relationship between tensile strength and a fatigue limit in an example.

Explanation of reference numerals

DESCRIPTION OF SYMBOLS 1 Steel plate 2 Rough rolling mill row 3 Finish rolling mill row 3a Work roll 3b Backup roll 4 First cooling device 5 Leveler 6 Second cooling device 7 Winding device

Claims (9)

mass%
C: 0.05-0.3%, Si: 0.01-2.5%,
Mn: 0.5 to 3.0%
And a composition comprising the balance of Fe and unavoidable impurities, and 70% or more by volume fraction of polygonal ferrite as a main phase, and 5% or more by volume fraction of a second phase other than polygonal ferrite. A high-strength hot-rolled steel sheet with excellent fatigue resistance and an excellent balance of strength-ductility, characterized by having a crystal grain size gradient structure in which the average crystal grain size of polygonal ferrite gradually decreases from the center of the sheet thickness toward the surface layer. .   The high-strength hot-rolled steel sheet according to claim 1, wherein the strength-ductility balance is 21000 MPa% or more.   The average crystal grain size in the surface layer of the polygonal ferrite is 1 μm or less, and the plate thickness average crystal grain size covering the entire thickness of the polygonal ferrite is 1.8 μm or less. The high-strength hot-rolled steel sheet as described.   4. The method according to claim 1, further comprising one or two selected from Ti: 0.005 to 0.25% and Nb: 0.005 to 0.2% in mass% in addition to the composition. A high-strength hot-rolled steel sheet according to crab.   In addition to the above composition, in mass%, Cu: 0.2% or less, Ni: 0.2% or less, Cr: 0.2% or less, Mo: 0.2% or less, B: 0.2% or less, V: 0.2% or less The high-strength hot-rolled steel sheet according to any one of claims 1 to 4, further comprising 1.0% or less of the selected one or more kinds. mass%
C: 0.05-0.3%, Si: 0.01-2.5%,
Mn: 0.5 to 3.0%
A hot rolling step of heating a steel material having a composition comprising the balance of Fe and unavoidable impurities and performing hot rolling including finish rolling, and applying a hot rolled steel sheet having the finish rolling of 5 ° C./s to 500 ° C. / S at a cooling rate of not more than 650 ° C and not more than 950 ° C, and a hot rolled steel sheet cooled to the above temperature range is subjected to repeated bending and unbending by a leveler. A strain imparting step of imparting a strain of 0.05 to 2.0 to the surface part, and cooling the hot-rolled steel sheet having passed through the strain imparting step to a temperature of 650 ° C. or less at a cooling rate of 10 ° C./s to 300 ° C./s. A method for producing a high-strength hot-rolled steel sheet having excellent fatigue resistance and an excellent balance of strength and ductility, characterized by sequentially performing the following cooling step.   The method for producing a high-strength hot-rolled steel sheet according to claim 6, wherein the temperature range in which cooling in the primary cooling step is stopped is set to 800 ° C or less, and the strain in the strain applying step is set to more than 1.0.   8. The composition according to claim 6, further comprising one or two selected from Ti: 0.005 to 0.25% and Nb: 0.005 to 0.2% in mass% in addition to the composition. Production method of high-strength hot-rolled steel sheet.   In addition to the above composition, in mass%, Cu: 0.2% or less, Ni: 0.2% or less, Cr: 0.2% or less, Mo: 0.2% or less, B: 0.2% or less, V: 0.2% or less The method for producing a high-strength hot-rolled steel sheet according to any one of claims 6 to 8, wherein one or more of the selected ones are contained in a total of 1.0% or less.

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