JP2000290748A - Hot rolled steel sheet for working excellent in notch fatigue resistance and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To allow a steel sheet to have superfine particles and to impart an excellent notch fatigue resistance and fatigue characteristics thereto by allowing it to have a structure in which the average particle size of ferrite is controlled to be below a specified value, the average particle size of 2nd phase particles is controlled to be a value equal to or below a specified value, the aspect ratio is controlled to a value equal to or below a specified value, and the ratio in which the interval between the most adjacent 2nd phase particles is made to be a value of the 2nd phase particle size or above is controlled to the ratio equal to or above the specified one and allowing the notch sensitivity coefficient and tensile strength to satisfy a specified relation. SOLUTION: This steel sheet is the one formed of ferrite as the main phase and a 2nd phase, the average particle size of ferrite is controlled to <4 μm, the average particle size of the 2nd phase particles is controlled to <=8 μm, the aspect ratio is controlled to <=2.0, and the ratio in which the interval between the most adjacent 2nd phase particles is made the one equal to or above the particle size of the 2nd phase particles is controlled to >=80%. The relation between the notch sensitivity coefficient (q) in the formula (α: the stress concentration coefficient in the notched part with a pierced hole shape, and β: the ratio of deterioration in fatigue strength caused by a notch = (the smooth fatigue strength of the steel sheet)/(the fatigue strength to a notch with a pierced hole shape in the steel sheet) and static tensile strength TS (MPa) lies in the relation in the inequality II (where TS>500 MPa).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet which is advantageous for use in automobiles, home appliances, mechanical structures, constructions, etc., and in particular, has ultra-fine grains as hot-rolled. The present invention relates to a hot-rolled steel sheet which has excellent ductility, toughness, strength-elongation balance, fatigue resistance, and particularly notch fatigue resistance, and has small anisotropy of these properties.

[0002]

2. Description of the Related Art Steel materials used for automobiles, home appliances, mechanical structures, and construction are required to have excellent mechanical properties such as strength, workability, and toughness. Since it is effective to refine the structure as a means for comprehensively improving these mechanical properties, many production methods for obtaining a fine structure have been proposed. Further, in the case of high-strength steel, in recent years, the target has been shifting to the development of a high-strength steel sheet that can achieve both low cost and high-performance characteristics. Further, in order to protect an occupant in the event of a collision, a steel sheet for automobiles is required to have not only high strength but also excellent impact resistance. For this reason, miniaturization of the structure in high-strength steels has become an important issue for the purpose of suppressing deterioration in ductility, toughness, durability ratio, and the like due to higher tensile strength.

[0003] As means for refining the structure, a large rolling reduction method, a controlled rolling method, a controlled cooling method and the like are known. As for the large rolling reduction method, there are proposals represented by, for example, JP-A-58-123823 and JP-B-5-65564. The key point of the structure refinement mechanism in these proposals is to apply a large pressure to the austenite grains to promote γ → α strain-induced transformation. However, these methods can achieve a certain degree of miniaturization, but in addition to the problem that it is difficult to realize with a general hot strip mill, such as making the rolling reduction per pass 40% or more, large rolling reduction As a result, the crystal grains are flattened, and thus there is a problem that mechanical properties become anisotropic, and the fracture absorption energy decreases due to separation.

On the other hand, as an example of applying the controlled rolling method and the controlled cooling method, there is a precipitation-strengthened steel sheet containing Nb or Ti.
These steel sheets are subjected to low-strength finish rolling using the precipitation strengthening action of Nb and Ti, and are subjected to low-temperature finish rolling using the recrystallization suppression action of austenite grains provided by Nb and Ti, to form unrecrystallized austenite. The ferrite crystal grains are refined by γ → α strain-induced transformation from the grains. However, these steel sheets have a problem that the anisotropy of mechanical properties is large. For example, in the case of automotive steel sheets subjected to press forming, the forming limit is determined by the characteristic level in the direction with the lowest ductility, so in steel sheets with large anisotropy, the effect of refining the structure may not appear as a characteristic at all. .
The same applies to the case where it is used for a structural material or the like. In some cases, the anisotropy such as toughness and fatigue strength, which are important for a structural material or the like, increases, and the effect of making the structure finer may not appear as a characteristic at all.

Japanese Patent Application Laid-Open No. 2-301540 discloses that a material steel is made into a microstructure in which at least a part thereof is made of ferrite, and is raised to a temperature range above a transformation point (Ac ₁ point) while performing plastic working. After the heating, the temperature is kept in a temperature range of at least _one point of Ac for a certain period of time, and a part or the whole of the structure is once transformed back into austenite, and then ultrafine austenite grains appear, and then cooled. Average grain size is 5μ
It describes that the structure is mainly composed of isotropic ferrite crystal grains of m or less. However, even this method has not completely eliminated anisotropy.

Recently, there has been proposed a method in which austenite grains before hot rolling are extremely refined and rolled, and dynamic recrystallization and further controlled cooling are used to refine the structure, for example, as disclosed in JP-A-9-87798. JP, JP-A-9-143570, JP-A-10
No. -8138. JP-A-9-87798 discloses that a slab containing Mn: 1.0 to 2.5 wt%, Ti: 0.05 to 0.30 wt%, or Ti: 0.05 to 0.30 wt% and Nb: 0.30 wt% or less is 950 to 1100 ° C. , And rolling at a rolling reduction of 20% or more per pass is performed at least twice, and hot rolling is performed at a finishing rolling temperature of the Ar ₃ transformation point or higher, and then at 20 ° C / s or higher. Cool at a cooling rate of 350-55
Wound at 0 ° C, 75% by volume or more of polygonal ferrite having an average crystal grain size of less than 10 µm, and retained austenite of 5 to 20%.
A method for producing a high-tensile hot-rolled steel sheet having a volume% structure is disclosed.

[0007] JP-A-9-143570 discloses that Ti: 0.05-0.
A steel containing one or two of 3 wt% and Nb: 0.10 wt% or less is heated to a temperature of 950 to 1100 ° C., and rolling is performed at least twice so that a rolling reduction per pass is 20% or more. above is performed, the finish rolling temperature and hot rolled so that the Ar ₃ transformation point or higher, Ar ₃ transformation point to 750 ° C. and cooled at 20 ° C. / s or more cooling rate, the temperature range of 750 ° C. below to 600 ° C. For 5-20 seconds at 550 ° C again at a cooling rate of 20 ° C / s or more.
Cool to below temperature and wind at below 550 ° C,
A method for producing a high-tensile hot-rolled steel sheet having an ultrafine structure with a ferrite content of 80% by volume or more and an average ferrite grain size of less than 10 μm is disclosed.

[0008] JP-A-10-8138 discloses that Mn: 1.0 wt%
Hereinafter, Ti: 0.05 to 0.30 wt%, or all or 1 of Ti
Steel slab containing twice the amount of Nb
After heating to a temperature of 1100 ° C, rolling at a rolling reduction of 20% or more per pass is performed at least twice or more, and hot rolling is performed at a finishing rolling temperature of the Ar ₃ transformation point or higher. s
A method for producing a high-tensile hot-rolled steel sheet having an ultrafine grain structure composed of ferrite and retained austenite, which is cooled at the above-mentioned cooling rate and wound at 350 to 550 ° C., is disclosed.

[0009]

SUMMARY OF THE INVENTION
JP-A-9-87798, JP-A-9-143570, JP-A-10-813
Although the technology described in Japanese Patent Publication No. 8 focuses on the refinement of crystal grains, the grain size can be obtained up to about 3.6 μm. Although the ductility is improved, the anisotropy of the mechanical properties is hardly said to be acceptably small, particularly from the viewpoint of the workability of the steel sheet for automobiles, and it is necessary to further reduce the anisotropy. For these reasons, a hot-rolled steel sheet having an ultrafine structure, small anisotropy, and excellent workability has been demanded.

Further, among steel sheets for automobiles, steel sheets applied to automobile parts such as wheels and frames to which repeated stress is applied may have higher fatigue strength than static strength, particularly notch fatigue strength. Has been requested. For this reason, several techniques for improving the notch fatigue strength of a hot-rolled high-strength steel sheet have been proposed (for example, see Japanese Patent Application Laid-Open No. H05-205139).
-179346).

However, the prior art merely attempts to increase the notch fatigue strength itself, and does not include the viewpoint of essentially improving the reduction of the notch fatigue strength relative to the smooth fatigue strength. . For this reason, only a solution including an excessive increase in the smooth fatigue strength can be presented, and an increase in material cost and a decrease in workability due to excessive specifications cannot be avoided. In the case of a material whose fatigue strength changes greatly due to the difference in the shape of the notch hole, design changes and standardization of pressed products (details such as drilling may differ depending on the vehicle type)
, Which hinders rapid development of parts and reduction of manufacturing costs.

Although refinement of crystal grains is generally said to improve fatigue strength, the notch fatigue strength of conventional steel sheets still exceeds 2/3 of smooth fatigue strength and 1/2 of tensile strength. And the above problem has not been solved. The present invention advantageously solves the above-mentioned problems of the prior art, can be easily manufactured by a general hot strip mill, has ultra-fine grains, has small anisotropic mechanical properties, and has a cutting resistance. An object of the present invention is to propose a hot-rolled steel sheet for processing excellent in notch fatigue characteristics, particularly notch sensitivity, and a method for manufacturing the same.

[0013]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to achieve the above-mentioned objects. As a result, the conventional means for refining the structure considered only miniaturization of ferrite, which is the main phase. However, it was conceived that no consideration was given to the distribution form of the second phase. In a steel sheet manufactured by the conventional microstructure refining means, the second phase is distributed in a band shape or a cluster shape, and the present inventors have found that such a distribution of the second phase is, for example, anisotropic in ductility. It was thought that the workability such as pressability was degraded and the notch fatigue resistance was further degraded, and it was conceived that the second phase should be finely dispersed in an island shape. We have:
The miniaturization of the second phase reduces the particle size difference between the second phase and the main phase, improves local elongation, and improves the local deformability due to the miniaturization of the second phase, so that the shearing at the notch portion is achieved. It was presumed that the deformed surface became smooth, the generation of initial cracks in the notch was suppressed, and the notch fatigue resistance was improved.

The present inventors have further studied a method for dispersing the second phase finely and in an island shape in addition to miniaturizing the main phase. As a result, the present inventors have found that when heating during hot rolling, the austenite grain size of the rolled material is extremely reduced, and then hot rolling is performed. Recrystallization occurs, and the crystal grains after rolling are greatly refined.
It has been found that if the initial austenite grains are refined, dynamic recrystallization occurs even at a lower temperature, a higher strain rate range, and a lower strain range as in finish rolling.

Further, the present inventors refined the initial austenite grains of the rolled material, and further reduced the dynamic recrystallization temperature in the austenite (γ) range at a low temperature range during hot rolling, and further reduced the conventional grain size. Compared to the technique, it has been found that by reducing the pressure relatively, the second phase particles in addition to the ferrite grains can be made finer, and the second phase particles can be dispersed and formed in an island shape. By repeatedly reducing the pressure in the dynamic recrystallization temperature range of the γ range, the recovery and recrystallization of the γ grains occur immediately after rolling, the γ grains are refined, and the ferrite formed by the γ → α transformation from the γ grains The average particle size is 4μm
And the second phase particles are also finely dispersed in an island shape. Thereby, the contradictory characteristics of strength and workability can be improved in a well-balanced manner.

Further, the present inventors set the rolling finishing temperature of the hot rolling at (Ar ₃ transformation point + 30 ° C.) or more and (Ar ₃ transformation point + 100 ° C.) or less, preferably (Ar ₃ transformation point + 60 ° C.) By limiting to the following, it is possible to disperse the crystal orientation of ferrite grains without aligning (orienting) the ferrite grains to a specific crystal orientation, suppress the propagation of fatigue cracks, and improve the notch fatigue resistance. Obtained.

The present invention has been completed by further study based on the above findings. That is, the present invention
A hot-rolled steel sheet comprising ferrite as a main phase, wherein the ferrite has an average grain size of less than 4 μm and has ultra-fine grains and excellent notch fatigue resistance. The present invention also provides a hot-rolled steel sheet having ferrite as a main phase and having a structure composed of a main phase and second phase particles, wherein the average particle size of the ferrite is less than 4 μm, Average particle size is less than 8μm, aspect ratio is
2.0 or less and the distance between the nearest second phase particles is
It has a structure in which the ratio of the phase particles to the particle size is 80% or more, and the following formula (1): q = (β-1) / (α-1) (1) : Notch sensitivity coefficient, α: Stress concentration coefficient of pierced hole-shaped notch, β: Reduction rate of fatigue strength due to notch = (Smooth fatigue strength of steel sheet) / (Pierce hole notch fatigue strength of steel sheet) The notch sensitivity coefficient q defined in ()) is related to the static tensile strength TS by the following equation (2): q ≦ 0.0012 × TS−0.3 where TS> 500 MPa (2) (where q: Notch sensitivity coefficient, TS: hot-rolled steel sheet for processing which has ultra-fine grains and excellent notch fatigue resistance characterized by satisfying the tensile strength (MPa) of the steel sheet. The second phase particles are one or more selected from pearlite, bainite, martensite, and retained austenite. Masui.

Further, in the present invention, the hot-rolled steel sheet contains, by weight%, C: 0.01 to 0.3%, Si: 2.0% or less, and Mn: 3.0%.
Below, P: 0.5% or less, Ti: 0.03-0.3%, Al: 0.2%
It is preferable to have a composition including the following and the balance being Fe and unavoidable impurities. Further, in the present invention, the hot-rolled steel sheet contains, by weight%, C: 0.01 to 0.3%, Si: 2.0% or less, M:
n: 3.0% or less, P: 0.5% or less, Ti: 0.03-0.3%,
Al: contains 0.2% or less, and further contains one or two selected from Group A: Nb: 0.3% or less, V: 0.3% or less, and has a composition consisting of the balance of Fe and inevitable impurities. preferable.

Further, in the present invention, the hot-rolled steel sheet is, by weight%, C: 0.01-0.3%, Si: 2.0% or less, Mn: 3.0%.
Below, P: 0.5% or less, Ti: 0.03-0.3%, Al: 0.2%
Group B: Cu: 1.0% or less, Mo: 1.0
% Or less, Ni: 1.0% or less, Cr: 1.0% or less, and preferably has a composition comprising the balance of Fe and unavoidable impurities.

Further, in the present invention, the hot-rolled steel sheet is, in terms of% by weight, C: 0.01 to 0.3%, Si: 2.0% or less, Mn: 3.0%.
Below, P: 0.5% or less, Ti: 0.03-0.3%, Al: 0.2%
It is preferable that the composition contains the following and further contains 0.005% or less in total of one or more of the group C: Ca, REM, and B, and has a composition comprising the balance of Fe and inevitable impurities.

Further, in the present invention, the hot-rolled steel sheet has a C content of 0.01 to 0.3%, a Si content of 2.0% or less, and a Mn content of 3.0% by weight.
Below, P: 0.5% or less, Ti: 0.03-0.3%, Al: 0.2%
Group A: Nb: 0.3% or less, V: 0.3%
% Or less, and Group B: Cu:
1.0% or less, Mo: 1.0% or less, Ni: 1.0% or less, Cr: 1.
It preferably contains one or more selected from 0% or less, and has a composition consisting of the balance of Fe and unavoidable impurities.

In the present invention, the hot-rolled steel sheet is, by weight%, 0.01% to 0.3% of C, 2.0% or less of Si, and 3.0% of Mn.
Below, P: 0.5% or less, Ti: 0.03-0.3%, Al: 0.2%
Group A: Nb: 0.3% or less, V: 0.3%
% Or less, and Group C: Ca,
0.005% in total of one or more of REM and B
It is preferable to have a composition comprising the following and the balance being Fe and unavoidable impurities.

Further, in the present invention, the hot-rolled steel sheet is, by weight%, C: 0.01-0.3%, Si: 2.0% or less, Mn: 3.0%.
Below, P: 0.5% or less, Ti: 0.03-0.3%, Al: 0.2%
Group B: Cu: 1.0% or less, Mo: 1.0
% Or less, Ni: 1.0% or less, Cr: 1.0% or less, and group C: containing at least one or more of Ca, REM and B in an amount of 0.005% or less in total. Rest
It preferably has a composition consisting of Fe and unavoidable impurities.

Further, in the present invention, the hot-rolled steel sheet is, in terms of% by weight, C: 0.01 to 0.3%, Si: 2.0% or less, Mn: 3.0%.
Below, P: 0.5% or less, Ti: 0.03-0.3%, Al: 0.2%
Group A: Nb: 0.3% or less, V: 0.3%
% Or less, one or two selected from the following: Group B: Cu: 1.0%
In the following, one or two or more kinds selected from Mo: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, and Group C: Ca,
0.005% of REM, one or more of B in total
It is preferable to have a composition comprising the following and the balance being Fe and unavoidable impurities.

In the present invention, C: 0.01 to 0.2% by weight.
Rolled material containing 3%, Si: 2.0% or less, Mn: 3.0% or less, P: 0.5% or less, Ti: 0.03-0.3%, Al: 0.2% or less is reheated to 1150 ° C or less, or 1150 ° C or less. ℃ or less, and then hot-rolled to form a hot-rolled steel sheet,
The hot rolling is performed by reducing the number of passes by 5 or more in a low dynamic recrystallization temperature range, and the final reduction in the low dynamic recrystallization temperature range is 15 to 30%, and the reduction other than the final reduction is 4 to 4%. 20%, the rolling finish temperature FDT is (Ar ₃ transformation point + 30 ° C) or more, and (A
r ₃ Transformation point + 100 ° C) or less, cooling is started within 2 seconds after the completion of the hot rolling, cooled at a cooling rate of 30 ° C / sec or more, and wound into a coil. This is a method for producing a hot-rolled steel sheet for processing having grains and excellent notch fatigue resistance, and in the present invention, the temperature at which the coil is wound around the coil is preferably 350 to 600 ° C. Also,
In the present invention, in the hot rolling, it is preferable that the steel sheet is heated by the heating means and / or the rolling roll is heated by the heating means.

[0026]

BEST MODE FOR CARRYING OUT THE INVENTION A hot-rolled steel sheet for processing according to the present invention comprises:
It can be applied as a steel sheet for a wide range of fields and applications, such as a mild steel sheet, a steel sheet for an automobile structure, a high-strength steel sheet for an automobile for processing, a steel sheet for a household appliance, and a steel sheet for a structure. The hot-rolled steel sheet of the present invention is a steel sheet having a main phase of ferrite and a structure including a main phase and second phase particles other than ferrite. The ferrite, which is the main phase, has a volume fraction of at least 50% or more, preferably 70% or more.

The main phase, ferrite, has an average grain size of less than 4 μm. If the ferrite grains are refined, the target strength can be secured with a smaller amount of alloying elements compared to conventional high-tensile steel, and the deterioration of properties other than strength is small, and the subsequent plating property is also good. . On the other hand, when the average grain size of the ferrite is 4 μm or more, the workability is generally remarkably reduced, and the increase in strength due to the refinement of the crystal grains is small. For this reason,
The average grain size of the ferrite was limited to less than 4 μm. Further, the ferrite phase preferably disperses the crystal orientation of ferrite grains without orienting a specific crystal orientation. As the dispersibility of the crystal orientation of the ferrite, it is preferable that the number of crystal grains in which the average of the inclination angles θ of the crystal grain boundaries between adjacent crystal grains is 15 ° or more is 80% or more in number ratio. In addition, it is more preferably at least 90%. Thereby, the propagation of fatigue cracks is suppressed, and the notch fatigue resistance is improved. Note that the inclination angle θ of the crystal grain boundary between adjacent crystal grains is determined by Electron Back
The crystal orientation was determined for each crystal of the cross section ferrite by the Scattaring Diffraction Pattern.

The second phase particles are particles having an average particle size of 8 μm or less and an aspect ratio of 2.0 or less. When the average particle size of the second phase particles exceeds 8 μm, the improvement in toughness and ductility is reduced, so that the average particle size of the second phase particles is 8 μm.
m. When the particle size of the second phase particles is larger than the particle size of the ferrite particles as the main phase, uniform deformation does not occur during processing, necking and wrinkles are generated, and the surface properties are poor. The particle size ratio between the second phase and the main phase (= second phase particle size / main phase particle size) is preferably 2.2 or less. By setting it to 2.2 or less, the structure becomes uniform and the local deformability improves,
It is possible to smooth the shear deformation surface at the notch and suppress the occurrence of cracks in the early stage of fatigue.

Further, when the aspect ratio of the second phase particles exceeds 2.0, the anisotropy of the mechanical properties increases. In particular, the effect on the characteristics in the 45 ° and 90 ° directions of the rolling direction is great. Therefore, the aspect ratio of the second phase particles is 2.0
Limited to the following. In the present invention, the ferrite, the second
The average particle size of the phase particles is defined as the average particle size in a cross section in the rolling direction according to a conventional method. The aspect ratio of the second phase particles refers to the ratio between the major axis and the minor axis of the second phase particles.

Further, in the present invention, the distance between the nearest second phase particles is set to 80% or more of the ratio of the second phase particles to the diameter of the second phase particles or more. This means that the second phase particles are distributed not in a band shape or a cluster shape but in an island shape. If the ratio of the distance between the nearest second phase particles being equal to or larger than the particle size of the second phase particles is less than 80%, the anisotropy of the mechanical properties increases, so that uniform deformation does not occur during processing and necking. Wrinkles occur and surface properties are poor.

In the present invention, the second phase particles are pearlite and bainite. In the present invention, the second phase particles are one or more selected from pearlite, bainite, martensite and retained austenite. It is preferred that It is preferable that the volume fraction of the second phase particles is 40% or less, preferably 20 to 30%. When the volume fraction of the second phase particles increases, the required strength is easily achieved,
If it exceeds 30%, mechanical properties, particularly ductility, deteriorate.

With the above structure, the hot-rolled steel sheet of the present invention has the following formula (1): q = (β-1) / (α-1) (1) Notch sensitivity coefficient, α: Stress concentration coefficient of pierced notch, β: Reduction rate of notched fatigue strength due to notch = (Smooth fatigue strength of steel sheet) / (Pierced notch fatigue strength of steel sheet)) The notch sensitivity coefficient q defined is related to the static tensile strength TS by the following equation (2): q ≦ 0.0012 × TS−0.3 where TS> 500 MPa (2) (where q: off Notch sensitivity coefficient, TS: Satisfies the static tensile strength (MPa) of the steel sheet. q is defined by the equation (1) and indicates notch sensitivity in consideration of the shape of the notch hole. As q increases, the notch fatigue resistance decreases.

In general, the higher the strength (TS) of the steel sheet, the larger q becomes, and it becomes difficult to make q small. In the conventional technology, the notch sensitivity coefficient q is 0.0012 × TS
Normally, the value exceeds -0.3, and there is no technology that stably satisfies the expression (2). However, according to the present invention, the notch resistance can be improved to an unprecedented level. .

Next, a preferable chemical composition of the hot-rolled steel sheet of the present invention will be described. C: 0.01 to 0.3% C is an inexpensive reinforcing component, and contains a necessary amount according to the desired steel sheet strength. If the C content is less than 0.01%, the crystal grains become coarse, and it is not possible to achieve the average ferrite grain size of less than 4 μm intended in the present invention. In addition, C content is 0.3
%, The workability deteriorates and the weldability also deteriorates. For this reason, C is preferably in the range of 0.01 to 0.3%. More preferably, it is in the range of 0.05 to 0.2%.

Si: 2.0% or less Si effectively contributes to an increase in strength while improving strength-elongation balance as a solid solution strengthening component. In addition, it effectively acts to suppress the formation of ferrite and obtain a structure having a desired second phase volume fraction, but excessive addition deteriorates ductility and surface properties. Therefore, the content of Si is desirably 2.0% or less. In addition, preferably 0.01 to 1.0%, more preferably
0.05 to 1.0%.

Mn: 3.0% or less Mn contributes to the refinement of crystal grains through the action of lowering the Ar ₃ transformation point, and has the strength—through the action of promoting the formation of martensite and retained austenite in the second phase. It has the effect of improving the ductility balance and strength-fatigue strength balance. Further, Mn has a function of detoxifying harmful dissolved S as MnS. However, a large amount of addition hardens the steel,
Rather, it degrades the strength-ductility balance. For these reasons, it is desirable that Mn be 3.0% or less. The content is more preferably 1.0 to 2.0%.

P: 0.5% or less P is useful as a strengthening component and can be added according to the desired steel sheet strength. However, excessive addition segregates at grain boundaries and causes embrittlement. Therefore, it is desirable that P be 0.5% or less. The content is preferably 0.005 to 0.2%. Ti: 0.03-0.3% Ti exists as TiC and effectively acts to refine the initial austenite grains during the hot rolling heating stage and induce dynamic recrystallization in the subsequent hot rolling process. I do. In order to exert such an effect, at least 0.03%
The above content is necessary, but if the content exceeds 0.3%, the effect is saturated and an effect corresponding to the content cannot be expected.
For this reason, Ti is desirably in the range of 0.03 to 0.3%. In addition, more preferably, it is 0.08 to 0.20%.

In the present invention, if necessary, the following groups A and B
Group, one or more of group C can be contained. Group A: 1 selected from Nb: 0.3% or less, V: 0.3% or less
Species or two types Nb and V both form carbonitrides and have the effect of refining the initial austenite grains in the hot rolling and heating stage, and are contained in an overlapping manner with Ti as necessary. By doing
Further, it effectively acts on the occurrence of dynamic recrystallization. But 0.
Even if it is contained in a large amount exceeding 3%, the effect is saturated and an effect commensurate with the content cannot be expected. Therefore, both Nb and V are 0.3%
It is desirable to do the following. It is desirable that both Nb and V be added at 0.001% or more.

Group B: Cu: 1.0% or less, Mo: 1.0% or less,
One or more selected from Ni: 1.0% or less and Cr: 1.0% or less. Cu, Mo, Ni, and Cr can be contained as a strengthening component, if necessary. On the contrary, the strength-ductility balance is deteriorated. Therefore, Cu, Mo, N
It is desirable that both i and Cr be 1.0% or less. In order to sufficiently exhibit the above-mentioned effects, it is preferable to contain at least 0.01% or more.

Group C: one or two of Ca, REM and B
0.005% or less in total of more than one species Ca, REM, and B all have the effect of improving the workability by controlling the shape of the sulfide and increasing the grain boundary strength, and can be contained as necessary. However, excessive content may adversely affect cleanliness and recrystallization, so that the total content is desirably 0.005% or less. The content is more preferably 0.004% or less.

Al: 0.2% or less Al is an element acting as a deoxidizing agent, and contains 0.2% or less. If the content of Al exceeds 0.2%, the amount of inclusions increases and the surface properties deteriorate. For this reason, Al is preferably set to 0.2% or less. Note that the content is more preferably 0.05% or less. In the hot-rolled steel sheet of the present invention, except for the composition described above,
The balance consists of Fe and inevitable impurities.

Next, a method for producing a hot-rolled steel sheet according to the present invention will be described. The molten steel adjusted to the above component composition range is made into a rolled material by continuous casting or ingot-bulking rolling, and the rolled material is subjected to hot rolling to obtain a hot-rolled steel sheet. The hot rolling may be reheating rolling in which the rolled material is cooled and then reheated, or may be direct rolling or hot charge rolling. Also, like the thin slab continuous casting method,
The continuously cast slab may be directly hot-rolled. When reheating, it is desirable to heat to 1150 ° C. or lower in order to make the initial austenite grains fine. Also, in the case of direct rolling, it is preferable to start rolling after cooling to 1150 ° C. or lower in order to promote dynamic recrystallization. In order to set the finish rolling temperature in the austenite range, the reheating temperature or the direct rolling start temperature is preferably set to 800 ° C. or higher.

In the present invention, when hot rolling is performed on a rolled material having the above-described temperature, it is preferable that the rolling is repeatedly performed by at least 5 passes or more in a low dynamic recrystallization temperature range. The austenite grains are refined by repeatedly reducing the temperature in the low temperature range of the dynamic recrystallization temperature. Since the finer austenite grains progress as the number of times of the dynamic recrystallization increases, it is preferable to reduce the pressure in at least 5 passes and more than 5 consecutive passes. With less than 5 passes, the degree of refinement of the austenite grains is small, and it is difficult to achieve fine grains having an average ferrite grain size of less than 4 μm. If the number of passes is excessively increased, the grain refinement proceeds excessively, the ferrite grain size becomes too small, and a specific crystal orientation may be easily oriented. Path.

The rolling reduction in the low dynamic recrystallization temperature range is 1 except for the final rolling pass in the low dynamic recrystallization temperature range.
4% or more and 20% or less per pass. If the rolling reduction per pass is less than 4%, dynamic recrystallization does not occur. Meanwhile, 1
If the rolling reduction per pass exceeds 20%, the anisotropy of the mechanical properties tends to increase, and the rolling reduction per pass is:
4 to 20%.

In the final rolling pass in the low temperature region of the dynamic recrystallization temperature, the rolling reduction is set to 15 to 30% in order to make the second phase finer and to form an island-like distribution. If the rolling reduction is less than 15%, the fineness and island distribution are insufficient, while if it exceeds 30%, the second
As the aspect ratio of the phase increases and the load on the rolling mill increases, the anisotropy of the mechanical properties increases. In addition, it is preferably 22 to 30%.

The dynamic recrystallization temperature referred to in the present invention is a strain obtained by simulating rolling conditions by a measuring device capable of controlling temperature and strain independently (for example, “Processing Four Master” manufactured by Fuji Denki Koki Co., Ltd.). -A value measured in advance from the relationship of stress shall be used. The dynamic recrystallization temperature varies depending on the steel composition, heating temperature, reduction ratio, reduction distribution, etc., but is said to exist within a temperature range of 850 to 1100 ° C, usually in a range of 250 to 100 ° C. The temperature width of the dynamic recrystallization temperature range increases as the rolling reduction per pass is higher or as the heating temperature is lower. Since rolling in the dynamic recrystallization region contributes more or less to refinement of crystal grains, it does not regulate rolling in the high temperature region of the dynamic recrystallization temperature. However, from the viewpoint of refining the structure, rolling in a low temperature range of the dynamic recrystallization temperature range is advantageous because the number of transformation sites of the γ → α transformation is remarkably increased.

Therefore, in the present invention, when rolling in the dynamic recrystallization temperature range, the rolling conditions particularly in the low dynamic recrystallization temperature range are specified as described above. That is, in order to promote the refinement of austenite grains, the temperature from (lower limit temperature of dynamic recrystallization) + 80 ° C., preferably (lower limit temperature of dynamic recrystallization) + 60 ° C. to the lower limit temperature of dynamic recrystallization Apply a reduction of 5 passes or more in the range.

In order to secure the number of times of rolling in the low temperature region of the dynamic recrystallization temperature, a heating means is provided between the rolling stands or between the rolling stands to suppress the temperature of the material being rolled during rolling. Is preferably heated. In particular, it is effective to install the heating means at a position (stand) where the temperature drops significantly. FIG. 1 shows an example of the heating means.
Shown in

The heating means shown in FIG. 1 (a) is a high-frequency heating device, which generates an induced current by applying an alternating magnetic field to the material to be rolled to heat the material to be rolled.
Further, instead of the high-frequency heating device, as shown in FIG. 1 (b), an electric heater may be used to heat the roll, or it may be heated by direct electric heating. In addition, at the time of hot rolling, it goes without saying that the reduction may be performed while applying lubrication.

In the present invention, the rolling conditions other than the rolling in the low temperature range of the dynamic recrystallization temperature are not particularly limited, but the rolling finishing temperature FDT is not less than (Ar ₃ transformation point + 30 ° C.) and (Ar ₃ transformation point + 100 ° C.) The following is assumed. If the rolling finish temperature is less than (Ar ₃ transformation point + 30 ° C), the ductility and toughness of the steel sheet deteriorate, the anisotropy of mechanical properties increases, the orientation of ferrite grains becomes uniform, and the notch fatigue resistance deteriorates. . By setting the rolling finishing temperature FDT to (Ar ₃ transformation point + 30 ° C.) or more,
The strain introduced during rolling can be recovered to some extent, and without orienting a specific crystal orientation,
It is possible to reduce the size of the region in which the crystal orientation is uniform. This suppresses the propagation of fatigue cracks and improves the notch fatigue resistance.

On the other hand, if the rolling finishing temperature FDT exceeds (Ar ₃ transformation point + 100 ° C.), the recovery of strain after rolling progresses remarkably, recrystallization is promoted, and the ferrite grain size and the second
The phase grain size increases, the strength and workability decrease, and the improvement in ductility and toughness decreases. The rolling finish temperature FDT
Is, from the viewpoint of miniaturization of ferrite grains and second phase grains,
It is preferable to set it in the range of (Ar ₃ transformation point + 30 ° C.) to (Ar ₃ transformation point + 60 ° C.).

In the hot-rolled steel sheet which has been hot-rolled under the above-described conditions, the austenite grains at this point are almost equiaxed crystal grains. , Γ → α transformation, the growth of ferrite grains is suppressed, and the structure is refined. For this reason, cooling is started within 2 seconds, preferably within 1 second after the end of rolling. If the start of cooling exceeds 2 seconds after the end of rolling, grain growth becomes remarkable.

The cooling rate is 30 ° C./sec or more.
If the cooling rate is less than 30 ° C./sec, grain growth of ferrite grains occurs, so that fineness cannot be achieved, and it is difficult to distribute the second phase finely and in an island shape. The hot-rolled steel sheet cooled at a cooling rate of 30 ° C./sec or more, preferably to a temperature range of 350 to 600 ° C., is immediately wound around a coil. The cooling rate after winding is not particularly limited. It is determined appropriately according to the steel sheet to be manufactured. However, when the winding temperature is high, the second phase has a structure mainly composed of pearlite, and the ferrite grains tend to grow. On the other hand, if the winding temperature is too low, the second phase has a structure mainly composed of martensite. Therefore, the winding temperature is 350
It is desirable that the temperature be within the range of -600 ° C.

[0054]

EXAMPLE Molten steel having the composition shown in Table 1 was smelted in a converter and made into a slab (rolled material) by a continuous casting method. These slabs are heated, hot-rolled, and cooled after rolling under the conditions shown in Table 2 and wound on a coil to form a hot-rolled steel sheet (sheet thickness 1.8 to 4.0).
mm). The rolling reduction in each rolling pass of hot rolling is
Except for the final rolling pass, the content was 5 to 18%. In addition, steel plate No. 2,
For No. 16, lubricated rolling was performed. In addition, steel plate No. 13
This is a conventional example of a method for refining the structure using reverse transformation, in which the steel is once cooled to 600 ° C. during the rolling, then heated again to 850 ° C., and then rolled.

Next, the structure and mechanical properties of these steel sheets were examined, and the results are shown in Table 3. The microstructure was obtained by using an optical microscope or an electron microscope for the cross section in the rolling direction of the steel sheet, using the volume fraction of ferrite, the particle size, the particle size of the second phase particles, the aspect ratio of the second phase particles, and the distribution of the second phase particles. The condition was measured. In addition, the interval between the nearest second phase particles was measured, and the ratio of the interval being equal to or larger than the particle size of the second phase particles was determined, and the distribution was defined as the second phase.

The tensile properties (yield point YS, tensile strength TS, elongation El) of the steel sheet in the rolling direction, the direction perpendicular to the rolling direction, and the 45 ° direction were measured using JIS No. 5 test pieces. From the measured elongation, ΔEl = 1/2 · (El ₀ +
El ₉₀ )-Anisotropy of elongation ΔEl of each steel sheet defined by El ₄₅
Was calculated. Here, El ₀ represents an elongation value in the rolling direction, El ₉₀ represents an elongation value in a direction perpendicular to the rolling direction, and El ₄₅ represents an elongation value in a 45 ° direction in the rolling direction.

The ductility-brittle transition temperature vTrs (° C.) was examined using a 2 mm V notch Charpy test piece of the original thickness. Further, with respect to the rolling direction of the steel sheet, the fatigue properties were measured using a smooth plane bending test piece shown in FIG. 2 and a notched flat bending test piece shown in FIG. The notched flat bending test piece was punched with 10%
(A stress concentration coefficient α = 2.216) is formed in the center of the test piece. Fatigue test, a complete plane of Reversed bending fatigue test of Schenk, 10 ⁷ times repeated stress that does not break after load fatigue strength sigma _W (smooth fatigue strength), sigma
_P (notch fatigue strength). The notch coefficient β (= (σ _P / σ _w ) and the notch sensitivity coefficient q defined by the equation (1) were calculated from σ _P and σ _W.

Table 3 shows the results.

[0059]

[Table 1]

[0060]

[Table 2]

[0061]

[Table 3]

[0062]

[Table 4]

[0063]

[Table 5]

In each of the steel sheets of the present invention, the average particle diameter of the ferrite is less than 4 μm, the average particle diameter of the second phase particles is 8 μm or less, the aspect ratio is 2.0 or less, and
The ratio at which the phase particle interval is equal to or greater than the average particle size of the second phase particles is 80
%, And has an elongation value of 25% or more and a tensile strength TS of 500 MPa or more, and the strength-elongation TS × El is also 20,000 MPa ·%.
As described above, the anisotropy ΔE1 of elongation is also small. further,
The notch sensitivity coefficient q satisfies the expression (2), and is a hot-rolled steel sheet having excellent workability and excellent notch fatigue resistance.

On the other hand, the average grain size of ferrite,
Comparative examples in which any one of the particle size of the phase particles, the aspect ratio, the interval between the second phase particles, and the inclination angle of the adjacent crystal grain boundary of the ferrite grains is 15 ° or more are out of the range of the present invention. , Elongation, ΔEl, or strength-elongation balance TS × El, the notch sensitivity coefficient q does not satisfy the expression (2), and the notch fatigue resistance decreases.

[0066]

According to the present invention, it has ultra-fine grains, good mechanical properties, small mechanical property anisotropy, excellent workability, and excellent notch fatigue resistance. The hot rolled steel sheet can be easily manufactured by ordinary rolling equipment, and has a remarkable industrial effect.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an example of a heating means suitable for carrying out the present invention.

FIG. 2 is an explanatory view showing the shape and dimensions of a smooth plane bending test piece.

FIG. 3 is an explanatory view showing the shape and dimensions of a notched flat bending test piece.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Roll stand 2 Work roll 3 Backup roll 4 Rolled material 5 High frequency induction heating device 6 Heater heating device

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Eiko Yasuhara 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Technical Research Institute of Kawasaki Steel Corporation (72) Inventor Akio Tosaka 1-Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture (72) Inventor Osamu Furukun 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba F-term (reference) 4K037 EA01 EA02 EA05 EA06 EA09 EA11 EA13 EA15 EA16 EA17 EA19 EA20 EA23 EA25 EA27 EA28 EA31 EA32 EA36 EB08 EB09 EB11 FA02 FB07 FC03 FC04 FC07 FD04 FE01 FE02 FE06 JA06

Claims (4)

[Claims] 1. A hot-rolled steel sheet having ferrite as a main phase and having a structure comprising a main phase and second phase particles, wherein the ferrite has an average particle size of less than 4 μm, and Particle size is 8μm or less, aspect ratio is 2.0 or less,
In addition, a notch sensitivity coefficient q defined by the following formula (1) has a structure in which the distance between the nearest second phase particles is equal to or larger than the particle size of the second phase particles is 80% or more. Satisfies the following expression (2) in relation to the static tensile strength TS. Note q = (β-1) / (α-1) (1) q ≦ 0.0012 × TS-0.3 where TS> 500 MPa (2) where q: notch sensitivity coefficient, α: Stress concentration coefficient of pierced notch, β: reduction rate of fatigue strength due to notch = (smooth fatigue strength of steel sheet) / (pierced notch fatigue strength of steel sheet), TS: static tension of steel sheet Strength (MPa) 2. The hot-rolled steel sheet is, by weight, C: 0.01 to 0.3%, Si: 2.0% or less, Mn: 3.0% or less, P: 0.5% or less, Ti: 0.03 to 0.3%, Al: 0.2. The hot-rolled steel sheet for processing having excellent notch fatigue resistance according to claim 1, wherein the hot-rolled steel sheet has a composition comprising the balance of Fe and inevitable impurities. 3. The hot-rolled steel sheet is, by weight, C: 0.01 to 0.3%, Si: 2.0% or less, Mn: 3.0% or less, P: 0.5% or less, Ti: 0.03 to 0.3%, Al: 0.2. The notch fatigue resistance according to claim 1, wherein the composition further contains one or more of the following groups A to C, and has a composition consisting of a balance of Fe and unavoidable impurities. Hot rolled steel sheet for processing with excellent properties. Note A: 1 selected from Nb: 0.3% or less, V: 0.3% or less
Species or two or more, Group B: one or more selected from Cu: 1.0% or less, Mo: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, Group C: Ca, REM, B 0.005% or less in total of one or more of the above 4. A rolled material containing, by weight, C: 0.01 to 0.3%, Si: 2.0% or less, Mn: 3.0% or less, P: 0.5% or less, Ti: 0.03 to 0.3%, and Al: 0.2% or less. Is re-heated to 1150 ° C or lower, or subjected to hot rolling after the temperature becomes 1150 ° C or lower, and in making a hot-rolled steel sheet, the hot rolling is performed at a dynamic recrystallization temperature in a low temperature range of 5 or more passes. The final reduction in the low temperature range of the dynamic recrystallization temperature is 15 to 30%, and the reduction other than the final reduction is 4%.
And the rolling finish temperature FDT is (Ar ₃ transformation point +30).
℃) or more and (Ar ₃ transformation point + 100 ℃) or less, and cooling is started within 2 seconds after the completion of the hot rolling.
A method for producing a hot-rolled steel sheet for processing having excellent notch fatigue resistance, wherein the steel sheet is cooled at a cooling rate of at least ° C / sec and wound around a coil.