JP4523899B2 - High-strength hot-rolled steel sheet with excellent stretch flangeability and fatigue characteristics - Google Patents

Description

  The present invention relates to a high-strength hot-rolled steel sheet excellent in stretch flangeability and fatigue characteristics.

  High-strength hot-rolled steel sheets are widely used for automobile parts, especially frames and arms called suspension systems. These parts are required to have high fatigue characteristics from the viewpoint of durability against vibration during traveling. Several steel plates have been proposed for such a requirement.

  For example, (Patent Document 1) proposes a steel plate (generally called a DP steel plate) in which fine Cu precipitates or solid solutions are dispersed in a ferrite-structure and martensite-phase composite structure steel sheet.

  Such DP steel sheet is known to be inferior in stretch flangeability evaluated by a hole expansion test, although it is excellent in the balance between strength and ductility and fatigue characteristics. One of the reasons is that DP steel sheet is a composite of a soft ferrite phase and a hard martensite phase, so the boundary between both phases cannot follow deformation during hole expansion processing, and it tends to be the starting point of fracture. It is thought that it is from.

  In contrast, hot-rolled steel sheets having excellent stretch flangeability and fatigue characteristics have been proposed. One example is (Patent Document 2), in which the structure of the steel sheet is mainly composed of a bainite phase, the difference in hardness from other constituent phases is reduced, and the formation of coarse carbides is avoided.

Japanese Patent Laid-Open No. 11-199973 Japanese Patent Laid-Open No. 2001-200331

  Although the hot-rolled steel sheet having a steel sheet structure mainly composed of bainite phase and suppressing the formation of coarse carbides as disclosed in (Patent Document 2) certainly exhibits excellent stretch flangeability, DP containing Cu It cannot be said that the fatigue properties are necessarily superior to those of steel plates. Moreover, if only coarse carbides are suppressed, generation of cracks cannot be suppressed when severe hole expansion processing is performed.

  According to the studies by the present inventors, it has been found that these causes are the inclusions mainly composed of oxides in the steel sheet.

  When subjected to repeated deformation, internal defects are generated around the coarse cluster inclusions on or near the surface of the steel sheet, and the fatigue properties are deteriorated by propagating as cracks. This is because it is easy to become a starting point of crack generation during hole expansion processing. Therefore, it is desirable to make the inclusions in the steel as fine as possible.

  Generally, deoxidation of steel is performed using Al, but alumina inclusions generated by Al deoxidation are easily clustered and remain in the steel as coarse inclusions. This is considered to reduce the fatigue characteristics and stretch flangeability (hole expanding workability) as described above, but hot rolled steel sheets with excellent stretch flangeability and fatigue characteristics from the viewpoint of inclusion spheroidization control. The proposed example is not seen.

  In view of such a situation, the present inventors do not produce (a) alumina inclusions that are likely to be coarsened, so that Al is hardly deoxidized, and (b) inclusions are not clustered and coarsened. In addition, (c) deoxidation method that modifies into spherical inclusions that do not easily start cracking, and (d) elucidation of additive elements that do not degrade fatigue properties The present invention was completed by examining the components and the production method.

  The summary is as follows.

(1) C: 0.03-0.10 mass%, Si: 0.05-1.5 mass%, Mn: 1.0-3.0 mass%, P: 0.05 mass% or less, S: 0.01% by mass or less, N: 0.0005 to 0.01% by mass, acid-soluble Al: 0.005% by mass or less, Ti: 0.005% by mass or more, one or two of Nd or Pr total: 0.0002 to 0.04 wt% has free and further, satisfies the following expression,
−0.05 ≦ {Ti− (48/12) × C− (48/14) × N− (48/32) × S} ≦ 0.2
The balance is steel composed of iron and unavoidable impurities. In the steel, the total inclusion composition of one or two of Nd oxide or Pr oxide is 3 to 90% by mass, Ti oxide things 10-97 mass%, see containing inclusions ranging _Al 2 _{O 3} is less 50 mass%, and, in the number ratio of inclusions comprises at least spherical, fusiform inclusions than 50%, A high-strength hot-rolled steel sheet excellent in stretch flangeability and fatigue characteristics, characterized in that inclusions of 0.5 μm or more and 10 μm or less are present at 1000 / cm ² or more and 100,000 / cm ² or less .

(2) C: 0.03-0.10 mass%, Si: 0.05-1.5 mass%, Mn: 1.0-3.0 mass%, P: 0.05 mass% or less, S: 0.01% by mass or less, N: 0.0005 to 0.01% by mass, acid-soluble Al: 0.005% by mass or less, Ti: 0.005% by mass or more, one or two of Nd or Pr Total: 0.0002 to 0.04 % by mass and Nb, further satisfying the following formula,
−0.05 ≦ {Ti + (48/93) × Nb− (48/12) × C− (48/14) × N− (48/32) × S} ≦ 0.2
The balance is steel composed of iron and unavoidable impurities. In the steel, the total inclusion composition of one or two of Nd oxide or Pr oxide is 3 to 90% by mass, Ti oxide things 10-97 mass%, see containing inclusions ranging _Al 2 _{O 3} is less 50 mass%, and, in the number ratio of inclusions comprises at least spherical, fusiform inclusions than 50%, A high-strength hot-rolled steel sheet excellent in stretch flangeability and fatigue characteristics, characterized in that inclusions of 0.5 μm or more and 10 μm or less are present at 1000 / cm ² or more and 100,000 / cm ² or less .

( 3 ) Stretch flangeability and fatigue as described in (1) or (2) above, containing Cu: 0.2-2.0 mass%, Ni: 0.1-1.0 mass% High-strength hot-rolled steel sheet with excellent characteristics.

( 4 ) The area ratio of the bainitic ferrite phase in the steel sheet is 80 to 100%, which is excellent in stretch flangeability and fatigue characteristics according to any one of the above (1) to (3) High strength hot rolled steel sheet.

  According to the method of the present invention, a high-strength hot-rolled steel sheet excellent in stretch flangeability and fatigue characteristics can be obtained.

  First, the experiment that led to the completion of the present invention will be described.

  The present inventors contain C: 0.05% by mass, Si: 0.05% by mass, Mn: 1.5% by mass, P: 0.02% by mass or less, S: 0.001% by mass, and the balance The molten steel in which Fe is Fe was deoxidized using various elements, and a steel ingot was manufactured through addition of Ti and Nb.

  The obtained steel ingot was hot-rolled to obtain a 4 mm hot-rolled steel sheet. These steel plates were subjected to a hole expansion test and a fatigue test, and the inclusion particle size distribution, morphology and average composition in the steel plates were investigated.

  As a result, the steel sheet deoxidized by adding at least Nd and Pr with almost no deoxidation with Al is most excellent in stretch flangeability and fatigue characteristics, because the spherical inclusions produced by the deoxidation are It was found that the inclusions are difficult to become the starting point of crack generation and the path of crack propagation during repetitive deformation and hole expansion because of fine dispersion in the steel sheet.

  Based on this, the hot rolling conditions were studied and the present invention was completed.

  The reasons for limiting the present invention will be described below. First, the reasons for limiting chemical components will be described.

  C is an essential element for securing the strength of the steel sheet, and at least 0.03 mass% is necessary to obtain a high-strength steel sheet. However, if excessively contained, the formation of cementite phase, which is undesirable for stretch flange characteristics, is avoided even if carbides are formed by Ti or Nb for the purpose of improving workability or cooling conditions are used as described later. Therefore, the content is 0.10% by mass or less.

  Si is an element effective for securing the strength without deteriorating the stretch flangeability. At least 0.05% by mass is necessary, but if it is excessively contained, it is not preferable for the stretch ferrite property. Since it becomes easy to produce | generate a phase, the upper limit shall be 1.5 mass%.

  Mn is an element effective for increasing the strength of a steel sheet together with C and Si, and it is necessary to contain 1.0% by mass or more, but if it exceeds 3.0% by mass, ductility deteriorates. The upper limit is 3.0% by mass.

  P is effective as a solid solution strengthening element, but since there is a concern about deterioration of workability due to segregation, it is necessary to make it 0.05% by mass or less. The lower limit value of P includes 0% by mass.

  In addition to forming inclusions such as MnS and deteriorating stretch flangeability, S has a harmful effect such as combining with Ti contained for the purpose of making C a carbide and reducing its yield. Therefore, it should be suppressed as much as possible, but is acceptable if it is 0.01% by mass or less. The lower limit value of S includes 0% by mass.

  N combines with Ti to be contained for the purpose of converting C into carbide and lowers the yield. Therefore, it should be suppressed as much as possible, but is acceptable if it is 0.01% by mass or less. On the other hand, since it costs to make it less than 0.0005 mass%, 0.0005 mass% is made the lower limit.

It is desirable to suppress acid-soluble Al as much as possible because the oxide is likely to cluster and become coarse. However, it is allowed to use up to 0.005% by mass as a preliminary deoxidizer. This is because when the acid-soluble Al concentration exceeds 0.005% by mass, the content of Al ₂ O _{3 in the} inclusions exceeds 50% by mass and inclusions are clustered.

From the viewpoint of preventing clustering, the acid-soluble Al concentration should be low, and the lower limit value includes 0% by mass. The acid-soluble Al concentration is an analytical method that measures the amount of Al dissolved in an acid, and utilizes the fact that dissolved Al dissolves in an acid and Al ₂ O ₃ does not dissolve in an acid.

  Since Ti is hardly deoxidized with Al in the present invention, 0.005% by mass or more is necessary as a deoxidizing material. This is because when the Ti concentration is less than 0.005 mass%, the Ti oxide content in the inclusions is less than 10 mass%, and the inclusion composition, form, and particle size that are good in fatigue characteristics and stretch flangeability cannot be controlled.

  Ti and Nb function to improve the workability of the steel sheet by fixing C, S and N as precipitates. On the other hand, when Ti or Nb is added more than necessary, they are present in the steel as free Ti or free Nb, which raises the recrystallization temperature and facilitates the presence of a hot-worked structure. Damage.

  And the range of the optimal addition amount of Ti and Nb can be appropriately expressed by using the following middle side described using the chemical equivalent of each element as an index.

  Here, the following middle side means the total of the free Ti amount and Nb amount that are not combined with C, N or S to form carbides, nitrides or sulfides. Is represented by the equivalent Ti amount (Nb amount is converted to Ti amount because Nb has the same properties as Ti).

That is, if the value of the middle side is less than −0.05, the ductility and stretch flangeability are inferior, and if it exceeds 0.2, the ductility deteriorates. For the above reasons,
−0.05 ≦ {Ti + (48/93) × Nb− (48/12) × C− (48/14) × N− (48/32) × S} ≦ 0.2
It is limited to satisfy the relationship.

When Nb is not added, the Nb content is limited to 0 so that the following relationship is satisfied.
−0.05 ≦ {Ti− (48/12) × C− (48/14) × N− (48/32) × S} ≦ 0.2

Nd and Pr modify cluster-like Ti oxides (eg, Ti ₂ O ₃ , Ti ₃ O ₅ ) produced by Ti deoxidation, and are fine spherical and have good fatigue characteristics and hole expansion properties. _{(e.g., Nd 2 O 3, NdO 2} ) -Pr oxide _{(e.g., Pr 2 O 3, PrO 2} ) -Ti oxide inclusions, Nd oxide -Ti oxide inclusions, or, Pr oxide -It has the effect of making a Ti oxide inclusion (some of the inclusions may contain a part of Al ₂ O ₃ due to the effect of Al preliminary deoxidation or refractory melting).

  In order to obtain such an inclusion modification effect, the total concentration of one or two of Nd and Pr needs to be 0.0002 mass% or more and 0.04 mass% or less. If the total concentration of one or two of Nd or Pr is less than 0.0002% by mass, the Ti oxide cannot be modified, and if it exceeds 0.04% by mass, the Ti oxide is reduced. It becomes Pr oxide and is difficult to become a composite inclusion to be controlled.

  Moreover, Cu and Ni are mentioned as a selective element.

  Since Cu can be used as a solid solution strengthening element or a precipitation strengthening element to increase the strength of the steel sheet, it is preferable because the fatigue strength can be further improved by further adding to the above elements.

  However, if 0.2% by mass or more is not added, the effect is small and only the cost is increased. Therefore, in the case of adding 0.2% by mass, the lower limit is preferably set. On the other hand, if the content exceeds 2.0% by mass, the surface property of the steel sheet after hot rolling is deteriorated, so it is preferable that the upper limit is 2.0% by mass.

  Ni has an effect of relieving the deterioration of hot rolled surface properties caused by Cu, and it is desirable to add about half of Cu as a guide. Therefore, the lower limit is 0.1% by mass. On the other hand, even if added over 1.0% by mass, the effect is saturated and leads to an increase in cost. Therefore, it is preferable to set the upper limit to 1.0% by mass.

  Next, conditions for inclusions in the steel sheet will be described.

  In order to obtain a steel sheet excellent in stretch flangeability and fatigue characteristics, it is important that inclusions in the steel sheet are spherically and finely dispersed so that cracks are unlikely to become a starting point of crack generation or a path of crack propagation. The average composition, morphology, and particle size distribution of inclusions in the steel sheet of the present invention were investigated.

  The average composition of inclusions can be obtained by analyzing the composition of a plurality of randomly selected inclusions (for example, about 20) and calculating the average concentration.

As a result, with the average composition of inclusions, the total of one or two of Nd oxide or Pr oxide is 3 to 90% by mass, Ti oxide is 10 to 97% by mass, and Al ₂ O ₃ is 50% by mass. It has been found that the stretch flangeability and fatigue characteristics are improved in the steel sheet whose composition is controlled to be in the range of% or less.

  When the total of one or two of Nd oxide or Pr oxide is less than 3% by mass with an average composition, the inclusion modification effect due to the addition of Nd or Pr is small. If the total of one or two types exceeds 90% by mass, over-reformation occurs, and in any case, inclusions do not become fine spheroidized, so that one or two types of Nd oxide or Pr oxide are used in the average composition. The total lower limit was 3% by mass, and the upper limit was 90% by mass.

  In addition, when the Ti oxide is more than 97% by mass in average composition, the modification with Nd oxide or Pr oxide is insufficient, and conversely, even if the Ti oxide is less than 10% by mass, the inclusions are refined. Since spheroidization becomes difficult, the upper limit value of the Ti oxide was 97% by mass and the lower limit value was 10% by mass in the average composition.

Furthermore, it is preferable not to contain Al ₂ O ₃ in the inclusions from the viewpoint of fine spheroidization, but the Al ₂ O ₃ content in the inclusions is increased due to the influence of Al preliminary deoxidation and refractory melting. Sometimes. In this case, the inclusion of Al ₂ O ₃ is allowed in the inclusions with an average composition limited to 50% by mass or less.

The lower limit value includes 0% by mass. This is because when the Al ₂ O ₃ content in the inclusions exceeds 50% by mass, the modification effect by Nd and Pr is impaired, and the inclusions are clustered.

  In the present invention, inevitable impurity oxides mixed from slag, refractory, etc. are allowed in addition to the oxides having the above composition.

  Regarding the form of inclusions, a plurality of inclusions selected at random can be observed with an optical microscope. For example, 100 randomly selected inclusions are observed at 100 times and 1000 times of an optical microscope, and are classified into spherical, spindle, cluster and others, and the number ratio of spherical and spindle inclusions is obtained. Is recommended.

  In addition, the spherical shape means that the major axis and minor axis of the inclusions are observed as being substantially circular, and the spindle shape means that the major axis / minor axis of the inclusion is 3 or less and is observed in an elliptical shape. Is defined as one in which two or more inclusion particles are densely packed, and the other as, for example, an angular single piece.

  As a result, it was found that stretch flangeability and fatigue characteristics are improved in a steel sheet in which the number ratio of spherical and spindle inclusions is 50% or more. When the number ratio of spherical and spindle-shaped inclusions is less than 50%, cluster-like inclusions are relatively increased, and the stretch flangeability and fatigue characteristics are deteriorated. Therefore, the lower limit is set to 50%.

  The particle size distribution of the inclusion can be obtained by observing the inclusion with an optical microscope (for example, 100 times and 1000 times) and measuring the particle size. Here, the particle size means the equivalent circle diameter.

  Regarding the particle size of inclusions, when the number of large inclusions exceeding 10 μm harmful to stretch flangeability decreases, the number of fine inclusions of 0.5 μm to 10 μm increases, and the number of inclusions of this size increases the stretch flangeability. In order to correspond well with the above, attention was focused on 0.5 μm or more and 10 μm or less.

As a result, it has been found that stretch flangeability and fatigue characteristics are improved in a steel sheet in which inclusions of 0.5 μm or more and 10 μm or less are present at 1000 / cm ² or more and 100,000 / cm ² or less.

If the number of inclusions of 0.5 μm or more and 10 μm or less is less than 1000 / cm ² , large inclusions that are harmful to stretch flangeability exceeding 10 μm are observed in the steel sheet and become the starting point of cracking. The value was 1000 pieces / cm ² . Further, when the 10μm following inclusions than 0.5μm is 100,000 / cm ² ultra present, the flange formability and fatigue properties extends the number is too large inclusions decreases, the upper limit value is 100,000 / It was cm ^2.

  Finally, the structure of the steel plate will be described.

  In order to obtain excellent stretch flangeability, it is preferable to use a structure having bainitic ferrite as the main phase.

  The area ratio of the bainitic ferrite phase in the steel sheet is preferably 80% or more, more preferably 90% or more, and even more preferably 100%. Further, the balance may contain 20% or less of bainite phase or polygonal ferrite phase, and it is desirable to avoid the martensite phase as much as possible. In addition to the steel sheet structure control, the inclusions are made into fine spheroids and dispersed, whereby excellent fatigue characteristics can be obtained at the same time.

  About the manufacturing method of the steel plate of this invention, it is as follows.

  First, for the melting of molten steel, an alloy is added to the molten steel blown in the converter by a conventional method, or the molten steel blown in the converter and subsequently vacuum degassed, so that it falls within the component range of the present invention. This can be done by adjusting.

  The order of addition of the alloy is not particularly defined, but it is preferable to add one or two of Nd or Pr after Ti addition. This is because the modification controllability is better when the Ti oxide is once generated by adding Ti and then the N oxide or Pr is added to modify the Ti oxide. The slab is obtained by continuously casting the molten steel produced in this manner by a conventional method.

  Next, hot rolling conditions for producing a high strength hot rolled steel sheet will be described.

  The heating temperature of the slab before hot rolling is preferably 1150 ° C. or higher in order to dissolve TiC, NbC, etc. in the steel. By solid-dissolving these, formation of polygonal ferrite is suppressed in the cooling process after rolling, and a structure mainly composed of bainitic ferrite phase which is preferable for stretch flangeability can be obtained.

  On the other hand, when the heating temperature before hot rolling exceeds 1250 ° C., oxidation of the slab surface becomes significant, and in particular, wedge-shaped surface defects resulting from selective oxidation of grain boundaries remain after descaling, Since the surface quality after rolling is impaired, the upper limit is preferably set to 1250 ° C.

Rolling is performed after heating to the above temperature range, and the finish rolling completion temperature is important in the structure control of the steel sheet in the process. When the finish rolling completion temperature is less than Ar ₃ point + 50 ° C., the crystal grain size of the surface layer portion is coarse, which is not preferable in terms of fatigue characteristics. On the other hand, when Ar ₃ point + 150 ° C. is exceeded, a polygonal ferrite phase which is not preferable for stretch flangeability is likely to be generated. Therefore, the upper limit is preferably Ar ₃ point + 150 ° C.

  Moreover, the average cooling rate of the steel sheet after finish rolling is set to 40 ° C./second or more, and cooling to the range of 300 to 500 ° C. suppresses the formation of polygonal ferrite phase, and mainly bainitic ferrite phase. It is important to obtain an organization.

  When the average cooling rate is less than 40 ° C./second, a polygonal ferrite phase is easily generated, which is not preferable. On the other hand, there is no need to set an upper limit on the cooling rate in terms of structure control, but too high a cooling rate may cause uneven cooling of the steel sheet, and it may be necessary to manufacture equipment that enables such cooling. Would require a large amount of money, which would lead to an increase in the price of the steel sheet. From such a viewpoint, the upper limit of the cooling rate is preferably set to 100 ° C./second.

  On the other hand, when the cooling stop temperature is lower than 300 ° C., a martensite phase that is not preferable for stretch flangeability is generated, so the lower limit is set to 300 ° C. Therefore, the winding temperature of the hot-rolled coil is preferably set to 300 ° C. or higher in order to suppress generation of a martensite phase that extremely deteriorates stretch flangeability.

  On the other hand, when the cooling stop temperature exceeds 500 ° C., the formation of polygonal ferrite phase cannot be suppressed, and in steels containing Cu, Cu is locally precipitated in the ferrite phase to improve fatigue characteristics. Since there exists a possibility that an effect may be reduced, it is preferable to set it as 500 degrees C or less. Therefore, by winding the hot-rolled coil at 500 ° C. or lower, TiC and NbC are precipitated in the subsequent cooling process, thereby greatly reducing the amount of solute C in the ferrite phase and improving stretch flangeability.

  Examples of the present invention will be described below together with comparative examples.

  A steel slab having chemical components shown in Table 1 was hot-rolled under the conditions shown in Table 2 to obtain a hot-rolled sheet having a thickness of 3.2 mm. The steel sheet thus obtained was examined for strength, ductility, stretch flangeability, cross-sectional structure, fatigue limit ratio, and inclusion particle size distribution, morphology, and average composition.

  The results are shown in Table 3 for each combination of steel and conditions. The strength and ductility were obtained by a tensile test of a JIS No. 5 specimen taken in parallel with the rolling direction.

  Stretch flangeability is measured by measuring the hole diameter D (mm) at the time when a through-thickness crack is generated by expanding a punched hole with a diameter of 10 mm with a 60 ° conical punch in the center of a 150 mm × 150 mm steel plate. The hole expansion value λ = (D−10) / 10.

Further, the fatigue limit ratio used as an index representing fatigue characteristics is a value (σW) obtained by dividing 2 × 10 ⁶ times strength (σW) obtained by a method based on JIS Z 2275 by the strength (σB) of the steel sheet. / ΣB).

  Note that the test piece is a No. 1 test piece defined in the same standard, with a parallel part of 25 mm, a radius of curvature R of 100 mm, and a thickness of 3.0 mm obtained by equally grinding both surfaces of the original plate (hot rolled plate). .

  Furthermore, the inclusions were observed 100 times and 1000 times with an optical microscope, and the particle size and morphology of 100 randomly selected inclusions were measured. Furthermore, using a quantitative analysis function of a scanning electron microscope, composition analysis was performed on 20 inclusions randomly selected.

  As is apparent from Table 3, in the steel sheet of the present invention, spherical and spindle inclusions are finely dispersed in the steel sheet, and as a result, a steel sheet having excellent strength, ductility, stretch flangeability and fatigue characteristics can be obtained. However, in the comparative example, since the inclusion composition is not the composition defined in the present invention, the coarse inclusions are increased by the reduced amount of fine inclusions in the steel sheet, so that the strength, ductility, stretch flangeability and fatigue characteristics are increased. It is falling.

Claims (4)

C: 0.03-0.10 mass%, Si: 0.05-1.5 mass%, Mn: 1.0-3.0 mass%, P: 0.05 mass% or less, S: 0.01 % By mass or less, N: 0.0005 to 0.01% by mass, acid-soluble Al: 0.005% by mass or less, Ti: 0.005% by mass or more, one or both of Nd or Pr: 0 It has .0002～0.04 mass% containing, further, satisfies the following expression,
−0.05 ≦ {Ti− (48/12) × C− (48/14) × N− (48/32) × S} ≦ 0.2
The balance is steel composed of iron and unavoidable impurities. In the steel, the total inclusion composition of one or two of Nd oxide or Pr oxide is 3 to 90% by mass, Ti oxide things 10-97 mass%, see containing inclusions ranging _Al 2 _{O 3} is less 50 mass%, and, in the number ratio of inclusions comprises at least spherical, fusiform inclusions than 50%, A high-strength hot-rolled steel sheet excellent in stretch flangeability and fatigue characteristics, characterized in that inclusions of 0.5 μm or more and 10 μm or less are present at 1000 / cm ² or more and 100,000 / cm ² or less . C: 0.03-0.10 mass%, Si: 0.05-1.5 mass%, Mn: 1.0-3.0 mass%, P: 0.05 mass% or less, S: 0.01 % By mass or less, N: 0.0005 to 0.01% by mass, acid-soluble Al: 0.005% by mass or less, Ti: 0.005% by mass or more, one or both of Nd or Pr: 0 .0002 to 0.04 % by mass and Nb, further satisfying the following formula,
−0.05 ≦ {Ti + (48/93) × Nb− (48/12) × C− (48/14) × N− (48/32) × S} ≦ 0.2
The balance is steel composed of iron and unavoidable impurities. In the steel, the total inclusion composition of one or two of Nd oxide or Pr oxide is 3 to 90% by mass, Ti oxide things 10-97 mass%, see containing inclusions ranging _Al 2 _{O 3} is less 50 mass%, and, in the number ratio of inclusions comprises at least spherical, fusiform inclusions than 50%, A high-strength hot-rolled steel sheet excellent in stretch flangeability and fatigue characteristics, characterized in that inclusions of 0.5 μm or more and 10 μm or less are present at 1000 / cm ² or more and 100,000 / cm ² or less . Cu: 0.2-2.0 mass%, Ni: 0.1-1.0 mass% is contained, The high intensity | strength excellent in the stretch flangeability and fatigue characteristic of Claim 1 or 2 characterized by the above-mentioned Hot rolled steel sheet. The high-strength hot-rolled steel sheet excellent in stretch flangeability and fatigue characteristics according to any one of claims 1 to 3 , wherein the area ratio of the bainitic ferrite phase in the steel sheet is 80 to 100%.