JP5377832B2 - Hot-rolled steel sheet and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength hot-rolled steel plate having both of high strength and high toughness, which can realize a high tensile strength exceeding 580 MPa without rolling a plate with a high reduction rate at a low-temperature region, has toughnesses, particularly, large upper shelf energy among them, and is superior in fracture toughness as well. <P>SOLUTION: The hot-rolled steel plate contains ferrite as the main phase, which has an average grain size of less than 1.00 &mu;m in a position of 100 &mu;m deep from the surface of the steel plate, and contains dislocations of 7&times;10<SP>12</SP>/m<SP>2</SP>or less by mean density in the above position. Alternatively or furthermore, the hot-rolled steel plate contains the crystal grains of ferrite surrounded by high angle grain boundaries or clusters of the crystal grains, which have an average aspect ratio of 2.0 or less in the above position. <P>COPYRIGHT: (C)2009,JPO&amp;INPIT

Description

  The present invention relates to a hot-rolled steel sheet and a method for manufacturing the same, and more particularly to a hot-rolled steel sheet suitable as a material for a high-strength member used in automobiles, industrial machines, and the like, and a method for manufacturing the same.

  It has long been known as the Hall-Petch law that the strength is increased by making the crystal grains of the steel material finer.

  In order to increase the strength of a steel material without adding an alloying element, it is considered that the refinement of crystal grains is the most excellent means. This is because reduction or omission of alloy elements and improvement in recyclability can be expected.

  It should be noted that other strengthening means such as precipitation strengthening, dislocation strengthening and transformation strengthening inevitably involve a decrease in toughness, but in the case of crystal grain refinement strengthening, an effect of improving toughness as well as strengthening can be expected. Many high-tensile steels with fine crystal grains and methods for producing the same have been proposed.

  As conventional fine graining means, for example, a large reduction rolling method represented by Patent Documents 1 to 3 and the like can be mentioned.

That is, Patent Document 1 is characterized in that rolling is performed at a total rolling reduction of 80% or more in a temperature range composed of an austenite region near the Ar ₃ transformation point, and a fine ferrite structure is generated by inducing ferrite transformation by rolling. “A method for producing an ultrafine-grain hot-rolled steel sheet” is disclosed. According to the technique proposed in Patent Document 1, a fine-grained structure with an average particle diameter of less than 5 μm is realized with simple composition steel, which is a steel having a component based on ordinary carbon steel.

  Patent Document 2 discloses a technique that achieves an improvement in low-temperature toughness by reducing the average crystal grain size of ferrite grains to 3 μm or less for a dual-phase steel of “ferrite + martensite” or “ferrite + bainite”. ing. In the technique proposed in Patent Document 2, since the ferrite grains are refined by utilizing the recovery and recrystallization phenomenon of ferrite by rolling the ferrite region, the final rolling is large in a low temperature region of 750 to 600 ° C. A steel material having a 50% fracture surface transition temperature (vTrs) in the Charpy impact test of −110 to −147 ° C. is obtained by applying cumulative strain.

Further, in Patent Document 3, the final rolling is performed in the temperature range of 550 ° C. to Ac ₁ point and the total strain is in the range of 1 to 10, and by applying a large reduction in the low-temperature supercooled austenite region, Techniques that have achieved improvements are disclosed. According to the technique proposed in Patent Document 3, a fine grain structure composed of ferrite grains having a grain size of 1.4 μm or less is realized, and a steel material having a low temperature toughness in which the above vTrs is less than −196 ° C. is obtained.

  The refinement mechanism in the techniques proposed in Patent Documents 1 to 3 described above is a supercooled austenite grain (hereinafter, austenite is also referred to as “γ”) or ferrite grain (hereinafter, ferrite is also referred to as “α”) at a low temperature. The main point is to promote the γ → α strain-induced transformation or the recrystallization of α by applying a large pressure to.

  However, there is a manufacturing restriction that applying a large reduction in a low temperature range is difficult to achieve with a general rolling mill.

  Furthermore, in a fine-grained material produced by applying a large pressure in a low-temperature region, for example, as described in Non-Patent Document 1, although vTrs moves to the low-temperature side due to the refinement of crystal grains, The problem is that energy is reduced.

  That is, Non-Patent Document 1 shows that when the particle size is 3 μm or less, the upper shelf energy is reduced due to the development of separation, and the generation of flat particles, a specific texture, specifically, the plate It has been shown that {100} and {111} textures developed in the plane are factors that promote separation. In addition, since the material with low upper shelf energy does not ensure fracture toughness even in a ductile fracture region, the range of application as a structural material is limited.

  Patent Document 4 discloses that steel containing a precipitate-generating element such as Nb, Ti, V, and the like undergoes predetermined γ → α transformation and α → γ reverse transformation, so that separation does not occur in the Charpy test, and the upper shelf A method for producing a steel having a fine-grained structure with a particle size of 1.2 to 3.1 μm in which energy does not decrease is disclosed. However, in order to obtain a fine grain structure by the technique proposed in Patent Document 4, it is essential to add so-called “microalloy” such as Nb and Ti.

JP 58-123823 A Japanese Patent Laid-Open No. 10-168542 JP 2001-234242 A JP 2002-332521 A Shuji Kajihara: “Characteristics and applications of ultrafine grained steel”, 177 and 178th Nishiyama Memorial Technology Course Advancement of ultrafine grain technology-opening up new possibilities in steel Advanced Technology, (2002) , P. 161, Japan Steel Association

  As described above, the conventional crystal grain refining technique requires the precipitation of pinning particles such as Nb and Ti carbonitride by a process that is difficult to realize with a general rolling mill such as low temperature and large rolling reduction. Is. By adopting such a conventional grain refinement method, in any case, it is possible to increase the strength or lower the 50% fracture surface transition temperature (hereinafter referred to as “vTrs”) in the Charpy impact test. However, since it does not contribute to the improvement of the upper shelf energy, it has been impossible to provide a fine-grained material excellent in fracture toughness.

  Therefore, the object of the present invention is to refine the crystal grains without carrying out large rolling in a low temperature region, and to achieve high strength exceeding the tensile strength of 580 MPa, and toughness, in particular, the upper shelf energy is large and fracture toughness is large. The present invention also provides a high-strength hot-rolled steel sheet having both high strength and high toughness and a method for producing the same.

  Another object of the present invention is to use a so-called “C-Mn steel”, which has a simple composition not containing microalloys, to refine crystal grains without carrying out large rolling in a low temperature region, and to have a tensile strength of 580 MPa. It is possible to provide a high-strength hot-rolled steel sheet having both high strength and high toughness, and a method for producing the same, which can realize higher strength exceeding the above, and has high toughness, in particular, high upper shelf energy and excellent fracture toughness.

The gist of the present invention resides in the hot-rolled steel sheet shown in the following (1) to (3) and the method for producing the hot-rolled steel sheet shown in (4) .

(1) In mass%,
C: 0.1-0.2%
Mn: 0.5 to 1.0%
Si: 0.3% or less, and
sol. Al: 0.05% or less,
Is a hot-rolled steel sheet having a structure in which the balance is Fe and impurities, and the main phase is ferrite occupying a ratio of 70% or more, and the ferrite at a depth of 100 μm from the steel sheet surface. A hot-rolled steel sheet for high-strength members, having an average particle size of less than 1.00 μm and an average dislocation density in the ferrite at the position of 7 × 10 ¹² / m ² or less.

(2) In mass%,
C: 0.1-0.2%
Mn: 0.5 to 1.0%
Si: 0.3% or less, and
sol. Al: 0.05% or less,
Is a hot-rolled steel sheet having a structure in which the balance is Fe and impurities, and the main phase is ferrite occupying a ratio of 70% or more, and the ferrite at a depth of 100 μm from the steel sheet surface. High strength , characterized in that the average grain size is less than 1.00 μm, and the average aspect ratio of ferrite grains and aggregates of grains surrounded by a large-angle grain boundary at the above position is 2.0 or less Hot rolled steel sheet for parts .

(3) In mass%,
C: 0.1-0.2%
Mn: 0.5 to 1.0%
Si: 0.3% or less, and
sol. Al: 0.05% or less,
Is a hot-rolled steel sheet having a structure in which the balance is Fe and impurities, and the main phase is ferrite occupying a ratio of 70% or more, and the ferrite at a depth of 100 μm from the steel sheet surface. Ferrite crystal grains and crystals having an average grain size of less than 1.00 μm, an average dislocation density in the ferrite at the position of 7 × 10 ¹² / m ² or less, and surrounded by a large tilt grain boundary at the position A hot-rolled steel sheet for high-strength members, wherein the average aspect ratio of the aggregate of grains is 2.0 or less.

(4) In mass%,
C: 0.1-0.2%
Mn: 0.5 to 1.0%
Si: 0.3% or less, and sol. Al: 0.05% or less,
The steel ingot or steel slab having a chemical composition consisting of Fe and impurities in the balance is subjected to hot rolling to complete rolling at a temperature range of Ae3 transformation point ± 50 ° C. The cooling is started within 3 seconds, is cooled to a temperature range of 700 to 550 ° C. at an average cooling rate of 600 ° C./s or more, and held in the temperature range to transform the ferrite (1) The manufacturing method of the hot-rolled steel sheet for high strength members in any one of-(3) .

  The main phase refers to a phase that accounts for 70% or more of the structure. The proportion of the main phase in the organization may be 100%.

  The grain size of ferrite is the grain size in a cross section parallel to the rolling direction, and the large tilt grain boundary is defined as a grain boundary having an orientation difference of 15 ° or more between grain boundaries.

  The aggregate of crystal grains refers to a so-called “colony”, and the aspect ratio of the crystal grains and the aggregate of crystal grains is the ratio of the major axis to the minor axis in a section parallel to the rolling direction, that is, “major axis / short axis”. "Diameter". The average aspect ratio of crystal grains and aggregates of crystal grains is a value obtained by dividing the sum of the aspect ratios obtained for crystal grains and aggregates of crystal grains by the total number of crystal grains and aggregates of crystal grains. It is defined as

  Moreover, temperature and a cooling rate point out the temperature and cooling rate in the surface of the hot-rolled steel plate used as object.

  Hereinafter, the invention relating to the hot-rolled steel sheet shown in the above (1) to (4) and the invention relating to the method for producing a hot-rolled steel sheet shown in (5) are referred to as “present invention (1)” to “present invention (5), respectively. ) ". Also, it may be collectively referred to as “the present invention”.

  The hot-rolled steel sheet of the present invention has high strength exceeding tensile strength of 580 MPa, and has high toughness, especially high shelf energy and excellent fracture toughness, so it is applicable to high-strength members used in automobiles and industrial machinery. Thus, it is extremely useful industrially because it is possible to further increase its safety. Furthermore, since it is possible to achieve both high strength and high toughness while reducing the alloying elements of steel as much as possible, it has a great merit from the viewpoint of economy and recyclability. This hot-rolled steel sheet can be manufactured by the method of the present invention.

  Hereinafter, the reason why the structure, chemical composition, and production conditions, which are the requirements of the present invention, are limited as described above will be described in detail together with the operation thereof.

(A) Organization:
(A-1) Ferrite particle size:
The hot rolled steel sheet according to the present invention has ferrite as a main phase. For this reason, the strength and toughness are governed by the grain size of the ferrite. And if the average particle diameter of the ferrite in the depth position of 100 micrometers from the steel plate surface becomes 1.00 micrometers or more, desired high intensity | strength of 580 MPa or more will no longer be obtained. Moreover, the vTrs moves to the high temperature side and the toughness also decreases.

  From the above, in the present invention (1) to the present invention (3), the average grain size of ferrite at a depth position of 100 μm from the steel sheet surface is defined as less than 1.00 μm.

  The lower limit of the average particle diameter of the ferrite at the position is preferably 0.50 μm from the viewpoint of ensuring ductility.

  As described above, the main phase refers to a phase whose proportion in the structure is 70% or more. Moreover, the particle diameter of a ferrite points out the particle diameter in the cross section parallel to a rolling direction.

(A-2) Dislocation density in ferrite:
If the ferrite contains many dislocations, the toughness will decrease.

As a factor that increases the dislocation density in ferrite,
[1] Rolling in a ferrite region or a two-phase region of ferrite and austenite,
[2] Occurrence of γ → α strain-induced transformation due to rolling applied in the low-temperature supercooled austenite region,
There are two points.

  On the other hand, according to the detailed study by the present inventors, the dislocation density in the ferrite grains formed by static transformation at the time of cooling after rolling in a high temperature austenite region is extremely low, and such dislocation density It has been revealed for the first time that a structure composed of extremely low ferrite contributes to the improvement of toughness, especially the upper shelf energy by the Charpy impact test.

That is, even when the average grain size of ferrite at a depth position of 100 μm from the steel sheet surface is less than 1.00 μm, the upper shelf energy when the average dislocation density in the ferrite exceeds 7 × 10 ¹² / m ² is For example, the ferrite having the same chemical composition has an average grain diameter of the same or lower than that of coarse-grained steel having a diameter of 5 μm or more, and the merit of the fine-grained structure cannot be obtained.

However, when the average dislocation density in the ferrite having an average grain size of less than 1.00 μm at the above position is 7 × 10 ¹² / m ² or less, the upper shelf energy is increased and the toughness, especially the fracture toughness is improved. .

Therefore, in the present invention (1) and the present invention (3), the average dislocation density in the ferrite having an average particle size of less than 1.00 μm at the position is defined as 7 × 10 ¹² / m ² or less.

The lower limit of the average dislocation density in the ferrite having an average particle size of less than 1.00 μm is preferably as low as possible, so it is not necessary to specify the lower limit, but the dislocation density of the steel material after annealing is usually 1 × 10 ¹⁰ / m ^2. Since it is around, it is desirable to set it as 1 × 10 ¹⁰ / m ² .

(A-3) Ferrite morphology and grain boundary properties:
In the structure made of isotropic ferrite grains, the upper shelf energy shows a high value, whereas in the structure containing flat ferrite grains, the upper shelf energy is lowered.

  Therefore, the present inventors conducted a detailed study on the factors that govern the upper shelf energy.

  As a result, the unit of the structure that dominates the upper shelf energy is a large tilt grain boundary, that is, a ferrite crystal grain and an aggregate of crystal grains surrounded by a grain boundary having an orientation difference between grain boundaries of 15 ° or more (hereinafter referred to as a grain boundary). Simply called “colony”), that is, ferrite grains and colonies surrounded by large-angle grain boundaries become the smallest fracture surface unit by impact test, while the orientation difference between grain boundaries is It became clear for the first time that ferrite surrounded by small-angle grain boundaries of less than 15 ° would not be an obstacle to impact cracking.

  Even if the average grain size of ferrite at a depth of 100 μm from the steel sheet surface is less than 1.00 μm, the average aspect ratio of ferrite crystal grains and colonies surrounded by the large-angle grain boundaries at the above positions is 2. When the upper shelf energy exceeds 0, the upper shelf energy is of the same chemical composition, for example, the average grain size of ferrite at the above position is about the same as or less than that of coarse-grained steel of 5 μm or more. Cannot be obtained.

  However, when the average grain size of ferrite at the position is less than 1.00 μm and the average aspect ratio of the ferrite crystal grains and colonies surrounded by the large tilt grain boundary at the position is 2.0 or less In this case, the upper shelf energy is increased and the toughness, especially the fracture toughness is improved.

  Therefore, in the present invention (2) and the present invention (3), the average aspect ratio of the crystal grains and colonies surrounded by the large-angle grain boundaries at the above positions is defined as 2.0 or less.

  The lower limit of the average aspect ratio of the ferrite crystal grains and colonies surrounded by the high-inclination grain boundary at the above position does not need to be specified, and is assumed to be 1 which is a perfect equiaxed shape.

(B) Chemical composition:
Conventionally, fine graining from the viewpoint of chemical composition has been achieved by adding so-called “microalloys” such as Ti and Nb to precipitate carbonitrides such as Ti and Nb and utilizing these pinning effects. It has been. However, fine precipitates typified by Ti and Nb carbonitrides deteriorate toughness.

  For this reason, the hot-rolled steel sheet according to the present invention is made of so-called “C-Mn steel” having a simple composition.

  Hereinafter, the reason for limiting the chemical composition will be described. In addition, “%” of the content of the chemical component means “mass%”.

C: 0.1-0.2%
C is an important element that effectively increases the strength of the steel and controls the austenite / ferrite transformation. Further, the Ae ₃ transformation point is lowered, contributing to the refinement of crystal grains. However, if the content is less than 0.1%, the intended ferrite particle size and strength cannot be obtained. On the other hand, when the C content exceeds 0.2%, excessive cementite and pearlite are formed, resulting in a decrease in toughness. Therefore, the content of C is set to 0.1 to 0.2%. In addition, the desirable range of C content is 0.13 to 0.17%.

Mn: 0.5 to 1.0%
Mn lowers the Ae ₃ transformation point and contributes to refinement of crystal grains. Further, Mn dissolved in ferrite has the effect of increasing the strength of the steel. However, if the Mn content is less than 0.5%, the desired ferrite particle size and strength cannot be obtained. On the other hand, if the Mn content exceeds 1.0%, it becomes difficult to obtain a structure mainly composed of ferrite, and the toughness is lowered. Therefore, the Mn content is set to 0.5 to 1.0%.

Si: 0.3% or less Si is an element that may be contained as an impurity in steel, and if contained excessively, it not only adversely affects toughness but also raises the Ae ₃ transformation point, leading to coarsening. In particular, when the content exceeds 0.3%, coarsening accompanying a decrease in toughness and an increase in the Ae ₃ transformation point becomes significant. Therefore, the Si content is set to 0.3% or less. Si has a deoxidizing effect and an effect of increasing the strength as a solid solution strengthening element, so that it can be added as needed for the purpose of improving the cleanliness and strength of the steel. In order to obtain the above effect, the Si content is preferably 0.1 to 0.3%.

sol. Al: 0.05% or less Al is an element that may be contained as an impurity in the steel, and if contained excessively, inclusions such as oxides are formed, resulting in a decrease in toughness. The amount is sol. If it exceeds 0.05% with Al (acid-soluble Al), the toughness will be significantly reduced. Therefore, Al sol. The content of Al was set to 0.05% or less. In addition, since Al has a deoxidation action, it can be added as necessary. When it is desired to reliably obtain the above-mentioned effect, the sol. The content of Al is desirably 0.005 to 0.05%.

  For the above reasons, the chemical composition of the hot rolled steel sheet according to the present invention (4) is C, Mn, Si and sol. Al is contained in the above-described range, and the balance is made of Fe and impurities.

  In the method for producing a hot-rolled steel sheet according to the present invention (5), the above amounts of C, Mn, Si and sol. A steel ingot or steel slab comprising Al and Fe and impurities as the balance was used.

(C) Manufacturing method:
The hot-rolled steel sheet according to the present invention (4) is a hot-rolling that completes rolling in a temperature range of Ae ₃ transformation point ± 50 ° C. on a steel ingot or steel slab having the chemical composition described in the above-mentioned item (B). Then, cooling is started within 0.3 seconds after the completion of the rolling, cooling is performed to a temperature range of 700 to 550 ° C. at an average cooling rate of 600 ° C./s or more, and ferrite is transformed in the temperature range. It can be produced by the present invention (5) characterized by the above.

First, in hot rolling, when the rolling completion temperature exceeds “Ae ₃ transformation point + 50 ° C.”, a fine grain structure cannot be obtained. On the other hand, when the rolling completion temperature is lower than “Ae ₃ transformation point−50 ° C.”, the grains transformed by ferrite during rolling are further rolled to form a non-uniform structure. Therefore, it was defined that the hot rolling is completed in the temperature range of Ae ₃ transformation point ± 50 ° C.

  Next, in order to obtain a ferrite having an average grain size of less than 1.00 μm at least at a depth position of 100 μm from the steel sheet surface, it is necessary to transform the ferrite from the processed austenite. If the time until the start exceeds 0.3 seconds, the austenite starts to recover and recrystallize.

  And the ferrite which uses the recovered and recrystallized austenite as a matrix does not become a fine one having an average particle diameter of less than 1.00 μm. Therefore, it was defined that the cooling was started within 0.3 seconds after the completion of the rolling.

  When the average cooling rate is less than 600 ° C./s, austenite recovery occurs during cooling, and fine ferrite having an average particle size of less than 1.00 μm cannot be obtained. Therefore, the average cooling rate when cooling is started within 0.3 seconds after completion of the rolling is set to 600 ° C./s or more.

  The upper limit of the average cooling rate is 2000 ° C./s due to the limitation of cooling equipment.

  Furthermore, in order to transform the ferrite, the cooling at the average cooling rate of 600 ° C./s or higher is stopped in the temperature range of 700 to 550 ° C. That is, when the cooling stop temperature is lower than 550 ° C., a structure having ferrite as a main phase cannot be obtained. On the other hand, when the cooling stop temperature exceeds 700 ° C., ferrite grains grow and a fine grain structure having an average grain size of less than 1.00 μm cannot be obtained. Therefore, it is defined that cooling is performed to a temperature range of 700 to 550 ° C. at an average cooling rate of 600 ° C./s or more, and ferrite transformation is performed in the temperature range. In addition, it is desirable to cool to a temperature range of 650 to 600 ° C. at an average cooling rate of 600 ° C./s or more, and to transform the ferrite in the temperature range.

  The cooling after causing the ferrite transformation described above is not particularly specified, and may be determined by appropriately selecting from, for example, air cooling, forced air cooling, mist cooling or water cooling.

  In addition, as already stated, temperature and a cooling rate point out the temperature and cooling rate in the surface of the hot-rolled steel plate used as object.

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

After holding a slab having a thickness of 35 mm, a width of 100 to 200 mm, and a length of 50 to 100 mm made of steel A and steel B having the chemical composition shown in Table 1 for 1 hour in a heating furnace having a furnace atmosphere temperature of 1000 ° C After 3 passes of rough rolling, 3 passes of finish rolling were performed to produce a steel sheet having a thickness of 1.2 to 2.1 mm. Table 1 also shows the Ae ₃ transformation points of the two steels.

  Steel A in Table 1 is a steel whose chemical composition is within the range defined by the present invention (4) and the present invention (5). On the other hand, steel B is a precipitation strengthened steel containing Nb and Ti, and is a steel whose chemical composition deviates from the conditions defined in the present invention (4) and the present invention (5).

  Table 2 shows the details of the hot rolling conditions for manufacturing the steel sheet and the cooling conditions after completion of the hot rolling. In addition, after cooling stop shown in Table 2, it was set as the air cooling in air | atmosphere.

  The structural characteristics of each steel plate obtained as described above were investigated as follows.

  That is, with respect to a cross section parallel to the rolling direction of each steel plate, a scanning electron microscope (hereinafter referred to as “SEM”) was used to measure each position of a depth position of 100 μm from the surface, a depth position of ¼ thickness from the surface, and a central portion of the thickness. And the magnification was set to 3000 times, and the ferrite grain size was calculated from the obtained SEM image by the so-called “slice method”, and the average grain size was determined from this.

  Further, with respect to a cross section parallel to the rolling direction of the steel sheet, a thin film sample was taken from a depth position of 100 μm from the surface, and a bright field was observed using a transmission electron microscope (hereinafter referred to as “TEM”) at a magnification of 40000 times. Observe and photograph, measure the number of intersections (N) between an arbitrary straight line and dislocation lines from the obtained TEM image, calculate the dislocation density (ρ) in ferrite by the following formula, and calculate the average The dislocation density was determined.

ρ = 2N / Lt.
Where L: length of an arbitrary straight line,
t: film thickness.

When no dislocation contrast was observed in the ferrite, the average dislocation density was determined to be less than 1 × 10 ¹² / m ² .

  Further, using a high-resolution field emission scanning microscope (FE-SEM), a crystal orientation analysis is performed using an EBSP (Electron Back Scattering Pattern) method at a depth position of 100 μm from the surface of a cross section parallel to the rolling direction of the steel sheet. Then, the ratio of the large tilt grain boundary was obtained. As already described, the large-angle grain boundary refers to a grain boundary having an orientation difference between grain boundaries of 15 ° or more.

  Further, for the cross section parallel to the rolling direction of each steel sheet, the major axis and minor axis of ferrite grains and aggregates of crystal grains surrounded by a large-angle grain boundary at a depth of 100 μm from the surface were measured, and the aspect ratio was measured. The ratio, that is, “major axis / minor axis” was calculated, and the average aspect ratio was determined therefrom. As already mentioned, the average aspect ratio of crystal grains and crystal grain aggregates is the sum of the aspect ratios obtained for crystal grains and crystal grain aggregates, together with the crystal grains and crystal grain aggregates. The value divided by the total number.

  Table 3 summarizes the structural characteristics of the steel sheets investigated as described above. In addition, in Table 3, the depth position of the plate thickness ¼ from the surface is expressed as “1/4 t depth position”. Moreover, what was shown by "-" in Table 3 means not having investigated.

  The mechanical properties of each steel plate were investigated as follows.

  That is, the No. 5 test piece described in JIS Z 2201 (1998) was collected from each steel plate obtained as described above in parallel with the rolling direction, and was pulled at a tensile rate of 0.17 mm / sec in the room temperature atmosphere. Tests were performed and the yield strength (LYS), tensile strength (TS), uniform elongation (UEL) and total elongation (EL) were measured.

In addition, each steel plate is subjected to 2 mm V notch processing perpendicular to the rolling direction, and then 5 to 9 sheets are set as a set according to the plate thickness so that the total thickness of the stack, that is, the width of the test piece is 10 mm. One steel plate was ground and adjusted, and this was fixed as a set with a bolt to prepare a specimen for Charpy impact test, which was subjected to the Charpy impact test. Then, 50% fracture surface transition temperature (vTrs), absorption energy at ₋₁₀₀ ° C. (vE ₋₁₀₀ ), absorption energy at 0 ° C. (vE ₀ ), and absorption energy at 20 ° C. (vE ₂₀ ) were measured.

  Table 4 summarizes the above test results.

From Table 4, in the case of the steel plates of test numbers 1 to 4 of the present invention example, as shown in Table 3, the average particle diameter of ferrite at a depth of 100 μm from the surface is 0.84 to 0.95 μm, both extremely Fine, and the average dislocation density in the ferrite at the position is not more than 7 × 10 ¹² / m ² or / and the ferrite grains and aggregates of grains surrounded by the high-angle grain boundaries at the positions Since the average aspect ratio satisfies the rule of 2.0 or less, even in a so-called “C-Mn steel” having a simple composition not containing microalloys, LYS exceeds 588 MPa and TS also exceeds 598 MPa. It is clear that high strength has been obtained and the goal of increasing strength is achieved, in which TS exceeds 580 MPa.

  Moreover, vTrs by the Charpy impact test of the steel plates of the test numbers 1 to 4 are all below -196 ° C, and it is clear that the steel has extremely good low temperature toughness.

  Further, as described above, the steel plates having the test numbers 1 to 4 have a low average dislocation density in the ferrite, or more than 90% of the ferrite grain boundaries are large tilt grain boundaries, and have a large tilt angle. Since the average aspect ratio of the crystal grains and colonies surrounded by the grain boundaries is 2.0 or less, the test number 6 and the test number in which the ferrite grain size using the same steel A is coarse are used. The upper shelf energy is larger than that of the steel plate No. 7, and the fracture toughness at a high temperature such as room temperature is improved.

On the other hand, in the case of the steel plate of test number 5 of the comparative example, the average particle diameter of ferrite at a depth position of 100 μm from the surface is very fine as 0.75 μm. Therefore, the tensile properties and vTrs according to the Charpy impact test are good. However, the average dislocation density in the ferrite at the position is 7 × 10 ¹² / m ² or less, and the average aspect ratio of the ferrite grains and the aggregate of the grains surrounded by the large-angle grain boundary at the position is 2 It does not satisfy any of the regulations of 0.0 or less. For this reason, the upper shelf energy in the Charpy impact test is lower than that of the steel plates of Test No. 6 and Test No. 7 having a coarse ferrite grain size using the same steel A, and is inferior in terms of fracture toughness. I understand that.

  Moreover, in the case of the steel plates of test number 6 and test number 7, the average particle diameters of ferrite at a depth position of 100 μm from the surface are as coarse as 4.8 μm and 5.2 μm, respectively. For this reason, TS is 481.6MPa and 464.8MPa, respectively, and the target of high strength that TS exceeds 580MPa is not achieved. In addition, the Charpy impact characteristics are only comparable to those of conventional steel plates proposed in Patent Document 1 and the like.

In the case of the steel plate of test number 8, in addition to steel B being a precipitation strengthened steel containing Nb and Ti, the average grain size of ferrite at a depth of 100 μm from the surface is 0.82 μm. High strength is obtained by the synergistic effect of the pulverization and refinement. However, the average dislocation density in ferrite at a depth of 100 μm from the surface is 7 × 10 ¹² / m ² or less, and the ferrite grains and aggregates of grains surrounded by the large-angle grain boundaries at the positions It does not satisfy any of the regulations that the average aspect ratio is 2.0 or less. For this reason, the upper shelf energy in the Charpy impact test is as low as 48 J, which is inferior in terms of fracture toughness.

  The hot-rolled steel sheet of the present invention has high strength exceeding tensile strength of 580 MPa, and has high toughness, especially high shelf energy and excellent fracture toughness, so it is applicable to high-strength members used in automobiles and industrial machinery. Thus, it is extremely useful industrially because it is possible to further increase its safety. Furthermore, since it is possible to achieve both high strength and high toughness while reducing the alloying elements of steel as much as possible, it has a great merit from the viewpoint of economy and recyclability. This hot-rolled steel sheet can be manufactured by the method of the present invention.

Claims (4)

% By mass
C: 0.1-0.2%
Mn: 0.5 to 1.0%
Si: 0.3% or less, and
sol. Al: 0.05% or less,
Is a hot-rolled steel sheet having a structure in which the balance is Fe and impurities, and the main phase is ferrite occupying a ratio of 70% or more, and the ferrite at a depth of 100 μm from the steel sheet surface. A hot-rolled steel sheet for high-strength members, having an average particle size of less than 1.00 μm and an average dislocation density in the ferrite at the position of 7 × 10 ¹² / m ² or less. % By mass
C: 0.1-0.2%
Mn: 0.5 to 1.0%
Si: 0.3% or less, and
sol. Al: 0.05% or less,
Is a hot-rolled steel sheet having a structure in which the balance is Fe and impurities, and the main phase is ferrite occupying a ratio of 70% or more, and the ferrite at a depth of 100 μm from the steel sheet surface. High strength , characterized in that the average grain size is less than 1.00 μm, and the average aspect ratio of ferrite grains and aggregates of grains surrounded by a large-angle grain boundary at the above position is 2.0 or less Hot rolled steel sheet for parts . % By mass
C: 0.1-0.2%
Mn: 0.5 to 1.0%
Si: 0.3% or less, and
sol. Al: 0.05% or less,
Is a hot-rolled steel sheet having a structure in which the balance is Fe and impurities, and the main phase is ferrite occupying a ratio of 70% or more, and the ferrite at a depth of 100 μm from the steel sheet surface. Ferrite crystal grains and crystals having an average grain size of less than 1.00 μm, an average dislocation density in the ferrite at the position of 7 × 10 ¹² / m ² or less, and surrounded by a large tilt grain boundary at the position A hot-rolled steel sheet for high-strength members, wherein the average aspect ratio of the aggregate of grains is 2.0 or less. % By mass
C: 0.1-0.2%
Mn: 0.5 to 1.0%
Si: 0.3% or less, and sol. Al: 0.05% or less,
A steel ingot or steel slab having a chemical composition comprising Fe and impurities in the balance, and hot rolling to complete rolling at a temperature range of Ae ₃ transformation point ± 50 ° C. starting cooling within .3 seconds at 600 ° C. / s or more average cooling rate to a temperature range of seven hundred to five hundred and fifty ° C. performs cooling, claim 1 and held at said temperature range, characterized in that to ferrite transformation To 3. The method for producing a hot-rolled steel sheet for high-strength members according to any one of items 1 to 3 .