JP2012224915A - High strength hot rolled steel sheet excellent in moldability and fracture characteristic and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet excellent in moldability and fracture characteristics and a method for producing the same.SOLUTION: The steel sheet includes, in mass%, 0.01-0.3% of C, 0.001-2.0% of Si, 0.01-2.0% of Mn, 0.02% or less of P, 0.001-0.01% of S, 0.005-1.0% of Al; 0.02% or less of N, 0.0001-0.02% of REM, and 0.0001-0.01% of Ca, with the balance comprising Fe, The sum of the lengths in the rolling direction per 1 mmcross section of: an inclusion group having a length in the rolling direction of 30 μm or more and comprising an aggregate of the inclusions having a long diameter of 3 μm or more arranged with a space of 50 μm or less relative to another adjacent inclusion on the straight line in the rolling direction; and the inclusion having a length in the rolling direction of 30 μm or more arranged with a space of more than 50 μm relative to another adjacent inclusion on the straight line in the rolling direction is 0.38 mm or less, and a number density of the inclusions having an equivalent circle diameter of 1.0 μm or more is 200 pieces/mmor more.

Description

  The present invention relates to a high-strength hot-rolled steel sheet excellent in formability and fracture characteristics and a method for producing the same.

  In recent years, it has been desired to reduce the weight of automobiles in order to reduce automobile fuel consumption. Among automotive parts, strength parts such as arms can be reduced in weight by improving the formability such as elongation (uniform elongation) and hole expansibility of the steel plate as a raw material and enabling a complex cross-sectional structure. . This is because, by improving the formability, the section modulus necessary for obtaining the component strength can be obtained not by the plate thickness but by the sectional shape.

  In improving the hole expandability, the hole expansion rate, which is the evaluation index, has a relatively large variation. Therefore, in improving the hole expandability, not only the average value (λave) of the hole expandability but also the variation. It is important to reduce the standard deviation σλ of the hole expansion rate that is an index to be expressed.

  For steel plates used as undercarriage members of automobiles, etc., with respect to such characteristic values, steel plates having a strength level of 440 MPa or more, an average value λave of the hole expansion rate of 100% or more, and a standard deviation σ of the hole expansion rate of 15% or less. The proposal of was desired. The uniform elongation (U.El) is required to be 17%, preferably 19% or more.

In addition, when a strong impact load is applied to the undercarriage component such as when the automobile rides on the curbstone, ductile fracture may occur starting from the punched surface of the undercarriage component. In particular, the higher the strength of the steel plate, the higher the notch sensitivity, so there is a greater concern about the breakage from the punched end face, so it is important to prevent the breakage. For this reason, about the steel plate used as structural members, such as a suspension part, it is necessary to improve the fracture characteristic. As an index representing the fracture characteristics, there is a crack initiation resistance value Ji (J / m ² ) which is a characteristic value obtained by a notched three-point bending test as described later. The crack initiation resistance value Ji represents the resistance against the occurrence of cracks (start of fracture) from the steel sheet constituting the structural member when an impact load is applied. Improvement is important in order not to impair the safety of the structural member when an impact load is applied (Ji ≧ 870000 J / m ² ). Conventionally, there has not been proposed a technique that aims to improve such characteristic values by focusing on such characteristic values, in particular, crack initiation resistance value Ji, which is a characteristic value obtained by a three-point bending test with notches. It was.

  In addition, from the viewpoint of preventing fatigue failure due to repeated stress caused by impacts on the road surface and loads on automobile parts during acceleration / deceleration, automotive steel sheets are also required to have high fatigue strength in addition to the above. The fatigue limit ratio (fatigue strength / tensile strength) is required to be 0.5 or more.

  For the production of a high-strength steel sheet excellent in hole expansibility, a technique as shown in Patent Document 1 is known. This is a technology for obtaining good hole expansibility by finely dispersing pearlite or cementite with fine ferrite matrix for steel with a tensile strength of 440 MPa or more. In addition, high hole expansibility is obtained by controlling the structure, and because of this, restrictions are imposed on the components and production conditions, which leads to an increase in production cost. Also, no mention is made of fracture properties, fatigue properties and uniform elongation.

JP-A-6-299236

Therefore, the present invention has been devised in view of the above-mentioned problems, and the object of the present invention is excellent in uniform elongation and hole expansion properties, and also in excellent fracture characteristics and fatigue characteristics. Another object of the present invention is to provide a high-strength hot-rolled steel sheet excellent in formability and fracture characteristics and a method for producing the same.
In particular, it has been conventionally known that the uniform elongation and hole expansibility of thin steel sheets are in a contradictory relationship, and it is an important issue to provide a technique for improving them together.

  In order to solve the above-mentioned problems, the present inventor has invented a hot-rolled steel sheet having excellent formability and fracture characteristics described below and a method for producing the same after intensive studies.

  The gist of the present invention is as follows.

(1) Invention 1 is mass%,
C: 0.01 to 0.3%
Si: 0.001 to 2.0%,
Mn: 0.01 to 2.0%,
P: 0.02% or less,
S: 0.001 to 0.01%,
Al: 0.005 to 1.0%,
N: 0.02% or less,
REM: 0.0001 to 0.02%,
Ca: 0.0001 to 0.01%
And the balance is a steel plate made of Fe and inevitable impurities,
The following formulas (1), (2), (3) are satisfied,
The microstructure is composed of only the ferrite structure, or the ferrite structure, and a mixed structure of one or both of the bainite structure and the pearlite structure, and is spaced 50 μm or less from other adjacent inclusions on the straight line in the rolling direction. The inclusion group of inclusions having a major axis of 3 μm or more arranged in a rolling direction and an inclusion group having a rolling direction length of 30 μm or more and another inclusion adjacent on a straight line in the rolling direction with a spacing of more than 50 μm, The sum of the lengths in the rolling direction per 1 mm ² cross section with the inclusions having a rolling direction length of 30 μm or more is 0.38 mm or less, and the number density of inclusions having an equivalent circle diameter of 1.0 μm or more is 200 / an excellent hot-rolled steel sheet in formability and fracture properties characterized by mm ² or more and 2000 / mm ² or less.
{[S] / 32-([Ca] / 40 + [REM] / 140)} × 32 ≦ 0.003 Formula (1)
{([Ca] / 40 + [REM] / 140)} × 32 ≧ 0.001 Formula (2)
[S] ≧ 0.001 Formula (3)
[S], [Ca], [REM]: Content in mass% of each component

(2) Invention 2 satisfies the following formulas (2) ′ and (3) ′, and the number density of inclusions having an equivalent circle diameter of 1.0 μm or more is 400 / mm ² or more and 2000 / mm ² or less. The hot-rolled steel sheet according to the first aspect of the invention.
{([Ca] / 40 + [REM] / 140)} × 32 ≧ 0.004 Expression (2) ′
[S] ≧ 0.004 Formula (3) ′
[S], [Ca], [REM]: Content in mass% of each component

(3) Invention 3 is further mass%,
B: 0.0005-0.003%,
Cu: 0.001 to 1.0%,
Cr: 0.001 to 1.0%,
Mo: 0.001 to 1.0%,
Ni: 0.001 to 1.0%,
V: 0.01-0.2%
Ti: 0.001 to 0.02%,
Nb: 0.001 to 0.05%,
The high strength hot-rolled steel sheet having excellent formability and fracture characteristics according to the first and second aspects of the invention, characterized by containing any one or more of the above.

  (4) Invention 4 has an average cooling rate of 3 ° C./second or more between 1350 ° C. and 1250 ° C. after casting a steel slab containing the component according to any one of Inventions 1 to 3 into a slab. Cooling to 1250 ° C or lower at a cooling rate, and subsequently, in the subsequent hot rolling step, the rolling reduction of 1150 ° C or higher in rough rolling is set to 70% or lower, and finish rolling is finished at an end temperature of Ar3 or higher and Ar3 + 200 ° C or lower. A high-strength hot-rolled steel sheet excellent in formability and fracture characteristics, characterized in that it is subsequently cooled at a cooling rate of 10 ° C./sec or higher and subsequently wound in a temperature range of 400 ° C. or higher and 700 ° C. or lower. It is a manufacturing method.

  According to the 1st invention-the 4th invention, it becomes possible to obtain the steel plate which is excellent in hole expansibility and uniform elongation, and also excellent in a fracture characteristic and a fatigue characteristic.

(A) is a figure for demonstrating a three-point bending test with a notch, (b) is side sectional drawing of the cross section which passes along the notch of the test piece with a notch of (a), (c) is three with a notch. It is a side view which shows the state of the side surface of the test piece with a notch obtained by carrying out a forced fracture after performing a point bending test. (A) is a load displacement curve obtained by a notched three-point bending test, and (b) is a crack propagation amount Δa obtained by performing a notched three-point bending test under variously changed strokes. It is a graph which shows the relationship of "J". It is a figure which shows a fatigue test piece. It is a schematic diagram for demonstrating the sum total M of the rolling direction length of an inclusion. It is a figure which shows the relationship between the sum total M of the rolling direction length of an inclusion, and average value (lambda) ave of a hole expansion rate. It is a figure which shows the relationship between the sum total M of the rolling direction length of an inclusion, and standard deviation (sigma) (lambda) of a hole expansion rate. It is a figure which shows the relationship between the value of Formula 1, and the sum total M of the rolling direction length of an inclusion. It is a figure which shows the average cooling rate between 1350 degreeC-1250 degreeC, and the extending | stretching inclusion length M of the slab after casting. It is a figure which shows the relationship between the sum total M of the rolling direction length of an inclusion, and the crack generation resistance value Ji. It is a figure which shows the relationship between the sum total M of the rolling direction length of an inclusion, and a fatigue limit ratio. It is a figure which shows the relationship between the number density of inclusions with an equivalent circle diameter of 1 μm or more and uniform elongation. It is a figure which shows the relationship between the value of Formula 2, and the number density of inclusions with a circle equivalent diameter of 1 μm or more.

Embodiments of the present invention will be described below.
First, as a form for carrying out the present invention, a high-strength hot-rolled steel sheet excellent in formability such as hole expandability, uniform elongation, and fracture characteristics and a method for producing the same will be described.

  First, the basic research results that led to the completion of the present invention will be described.

  The present inventor investigates the controlling factors for the uniform elongation, hole expansibility, and fracture characteristics of a steel sheet whose microstructure is a ferrite structure, a ferrite structure, and a mixed structure of one or both of a bainite structure and a pearlite structure. The following examination was conducted.

  The inventor performs hot rolling, cooling, winding, etc. under the conditions shown in Table 2 below for the test steels composed of steel components A to M as shown in Table 1 to be described later. A 9 mm hot rolled steel sheet was produced.

  The resulting hot-rolled steel sheet was examined for its microstructure, inclusions, as well as its hole strength, fracture properties, fatigue properties, such as its tensile strength, uniform elongation, average value λave of hole expansion rate and its standard deviation σλ, and so on. .

  Tensile strength and uniform elongation were obtained by manufacturing No. 5 test piece described in JIS Z 2201 so that the longitudinal direction of the test piece was parallel to the plate width direction from the 1/2 plate width part of the test steel. In accordance with the method described in JIS Z 2241, a tensile test was performed from the test piece.

  As for the hole expanding property, a test piece having a length in the rolling direction of 150 mm and a length in the width direction of 150 mm was produced from the 1/2 sheet width part of the test steel, and described in Japan Iron and Steel Federation Standard JFS T 1001-1996. According to the method, a hole expansion test was conducted and evaluated. In the evaluation, 20 test pieces were manufactured from one test steel, and the average of the hole expansion ratio obtained by arithmetically averaging the measured values obtained by performing the hole expansion test on each manufactured test piece. The value λave and its standard deviation σ are to be evaluated. In addition, the average value λave of the hole expansion rate referred to here is obtained by averaging the measured values obtained by performing the hole expansion test on 20 test pieces per one test steel, and the standard deviation σ is as follows: It calculated | required based on Numerical formula (4). In the following mathematical formula (4), λi is the hole expansion rate of each of the 20 test pieces.

In the punching hole expansion test performed here, a punching punch having a diameter of 10 mm is used, and the punching clearance obtained by dividing the gap between the punching punch and the die hole by the plate thickness of the test piece is 12.5%. A punch hole having an initial hole diameter (D0) of 10 mm is provided in the test piece, and then a conical punch having a vertex angle of 60 ° is pushed into the punch hole from the same direction as the punch, and cracks generated on the punch end face are in the thickness direction. The hole inner diameter Df at the time of penetration was measured, and the hole expansion rate λ (%) was determined from the following equation (5). Here, the plate thickness of the crack was visually observed.
λ (%) = {(Df−D0) / D0} × 100 (5)

The fracture characteristics were evaluated based on the crack initiation resistance value Ji (J / m ² ) obtained by the following notched three-point bending test.

In the three-point bending test with notches, five or more notched test pieces 1 as shown in FIG. 1 (a) and FIG. 1 (b) are formed from one test steel so that the longitudinal direction is parallel to the plate width direction. Produced. As shown in FIG. 1B, a test piece having a plate thickness B of 2.6 mm, a notch depth a of 2.6 mm, and a ligament b of 2.6 mm was used. For the obtained notched specimen, as shown in FIG. 1 (a), both ends in the longitudinal direction are the support points 2 and the center is the load point 3, and the displacement (stroke) of the load point. A three-point bending test with a notch was performed under the condition that the forced displacement 4 was varied. A notched specimen that has been subjected to a notched three-point bending test under conditions of a predetermined stroke is generated by a notched three-point bending test by performing a heat treatment that is held in air at 250 ° C. for 30 minutes and then air-cooled. The fracture surface was oxidized and colored. Thereafter, the notched test piece was cooled to liquid nitrogen temperature with liquid nitrogen, and then the notched test piece was forcibly broken so that a crack extended from the notch to the notch depth direction at that temperature. After the forced fracture, as shown in FIG. 1 (c), the fracture surface caused by the notched three-point bending test becomes clear from the fracture surface caused by the forced fracture due to oxidation coloring and the fracture surface caused by the bending test. Therefore, the crack propagation amount Δa (m) was obtained based on the following formula (6).
Δa = (D1 + D2 + D3) / 3 Formula (6)
Here, D1: Fracture surface length generated by a bending test at a position where the plate thickness is ¼, D2: Fracture surface length generated by a bending test at a position where the plate thickness is 1/2, D3: Plate thickness 3/4 It means the fracture surface length generated by the bending test at the position.

FIG. 2A is a load displacement curve obtained by a three-point bending test with a notch performed under conditions of a predetermined stroke. From this load displacement curve, the processing energy A (J) corresponding to the energy applied to the test piece was obtained by the test, and from this, the thickness B (m) of the test piece and the ligament b (m), 2 X The value of processing energy A / {plate thickness B x ligament b} was determined, and this was defined as "J (J / m ² )". Here, the ligament b means the length in the notch depth direction of the portion other than the notch of the cross section including the notch in the notched test piece. Further, as shown in FIG. 3B, the crack propagation amount Δa () of each notched test piece obtained from the notched test piece obtained after the notched three-point bending test under variously changed strokes. “J (J / m ² )” for each m) was plotted. Then, as shown in FIG. 2 (b), a vertical axis value J that is the intersection of the linear regression line with respect to Δa and J plotted as shown in FIG. 2B and the slope through the origin = 3 × (YP + TS) / 2 is obtained, and this is used. The crack initiation resistance value Ji (J / m ² ), which is a value representing the crack initiation resistance of the test steel, was used. The crack generation resistance value Ji is a value corresponding to the processing energy per unit area necessary for generating a crack, and the occurrence of cracks (breakage of fracture) from the steel sheet constituting the structural member when an impact load is applied. Represents the resistance to (onset).

  As for fatigue characteristics, fatigue test piece 5 with surface hot rolling as shown in FIG. 3 is processed, repeated bending stress is applied to the center part of the test piece, and fatigue is the number of repetitions until the test piece is subjected to fatigue failure. The life was measured and evaluated by this fatigue life. At this time, the condition of the repetitive stress applied to the test piece is complete swing, that is, when the stress amplitude = σ0, the time change of the stress is the maximum stress = σ0, the minimum stress = −σ0, the average value of the stress = The stress is applied so that a sine wave of 0 is applied. The stress amplitude is gradually changed to measure the number of repetitions until the specimen breaks down due to fatigue. The test piece does not break due to fatigue breakage even when the number of repetitions is 10,000,000. The maximum stress amplitude was determined and used as fatigue strength. A value obtained by dividing the fatigue strength by the tensile strength was determined and used as the fatigue limit ratio. Other test conditions were based on JIS Z 2275.

The microstructure was examined by cutting out and polishing the steel plate so that a cross section having the normal direction in the width direction of the steel plate (hereinafter referred to as the L cross section) was exposed from the 1/2 plate width position of the steel plate, and corroded by the Nital reagent. The ¼ plate thickness position of the steel plate was observed by using an optical microscope at a magnification of 200 to 500 times. In the investigation of inclusions, a sample having the same cross section was mirror-polished and observed at a magnification of × 500.
When investigating inclusions, measure the total length M (mm / mm ² ) of the inclusions in the rolling direction from the viewpoint of suppressing ductile fracture to improve hole expansion, and improve uniform elongation. Therefore, in order to investigate the relationship between fine inclusions and uniform elongation, inclusions with an equivalent circle diameter of 1 μm or more were also evaluated and quantified.

  First, the length M of the stretched inclusions is measured, and the result of considering the guideline for improving the hole expansibility will be described.

  Inclusions form voids in the steel during deformation of the steel sheet, promote ductile fracture, and become a factor of deteriorating hole expandability. The influence of inclusions that deteriorates the hole expansion property is that the stress concentration near the inclusions increases as the shape of the inclusions extends longer in the rolling direction, and increases accordingly. Conventionally, it has been known that as the length in the rolling direction of a single inclusion is larger, the hole expandability is greatly deteriorated.

  Here, the inventor also includes an inclusion group composed of a collection of inclusions in which stretched inclusions and spherical inclusions are distributed within a predetermined interval in the rolling direction, which is a crack propagation direction. It was found that, as with a single stretched inclusion, it affects the hole expandability deterioration. This is presumably because a large stress concentration is generated in the vicinity of the inclusion group due to a synergistic effect of strain introduced in the vicinity of each inclusion constituting the inclusion group when the steel plate is deformed. Quantitatively, an inclusion group consisting of a collection of inclusions arranged at an interval of 50 μm or less with respect to other inclusions adjacent on a straight line in the rolling direction is the length in the rolling direction of the inclusion group. It has been found that it affects the hole expandability to the same extent as a single inclusion stretched to the same length.

  Therefore, in evaluating the hole expansibility, inclusions having shapes and positions as described below are to be measured.

  First, as shown in FIG. 4A, a group of inclusions arranged with an interval of 50 μm or less with respect to other inclusions adjacent on a straight line in the rolling direction is regarded as one inclusion group. The length L1 in the rolling direction was measured, and only those having a length in the rolling direction of 30 μm or more were evaluated. Further, as shown in FIG. 4 (b), the length L2 in the rolling direction is measured even for an inclusion having an interval of more than 50 μm with respect to another inclusion adjacent on the straight line in the rolling direction. Of these, only those having a length in the rolling direction of 30 μm or more were evaluated. Here, the reason why the length in the rolling direction is limited to 30 μm or more as a measurement target is because inclusions having a length in the rolling direction less than this are considered to have little influence on the deterioration of the hole expanding property. . In addition, the straight line of the rolling direction here means the virtual straight line extended in the rolling direction.

  The inclusions to be measured are limited to those having a major axis of 3.0 μm or more. This is because inclusions whose major axis is less than this have little effect on the deterioration of hole expansibility. The major axis here means the longest diameter in the cross-sectional shape of the observed inclusions, and is often the diameter in the rolling direction.

  Moreover, as shown in FIG.4 (c), even if it is an inclusion whose rolling direction length is 30 micrometers or more, the space | interval of 50 micrometers or less is vacant with respect to the other inclusion adjacent on the straight line of a rolling direction. Inclusions were measured as being part of the inclusion group. Hereinafter, inclusions that are not included in the inclusion group and have a rolling direction length of 30 μm or more are referred to as “stretched inclusions”.

The rolling direction length L1 of the inclusion group to be evaluated and the rolling direction length L2 of the elongated inclusion are the same as the rolling direction length L1 of all the inclusion groups observed in one field of view. The length L2 in the rolling direction for all the stretched inclusions observed in the film is measured, and these are summed to obtain L (mm). Based on the obtained L, the following formula (7) is obtained. The numerical value M (mm / mm ² ) is obtained, and the obtained M is defined as the sum M of the length in the rolling direction of inclusions per unit area (1 mm ² ), and the hole expansion property is evaluated by this sum M. It was. In addition, S in Formula (7) is the area (mm < ² >) of the observed visual field.
M = L / S (7)
Here, the sum M per unit area is determined from the total length L of the inclusions in the rolling direction instead of the average value for the following reason.

  At the time of deformation of the steel sheet, if the number of inclusion groups and stretched inclusions is small, the voids generated around these inclusion groups etc. break and the crack propagates, whereas the number of these inclusion groups etc. When the number is large, surrounding voids such as inclusions are connected without interruption, and long continuous voids are formed to promote ductile fracture. Such an influence of the number of inclusion groups etc. cannot be expressed by the average value of the rolling direction lengths of the inclusion groups etc., but can be expressed by the sum M of the rolling direction lengths of the inclusion groups etc. Therefore, the sum M per unit area of the length in the rolling direction of the inclusions was determined.

  Further, when the steel plate is deformed, cracks are generated and propagated at the stress concentration portion due to the deformation, with the inclusion group and the stretched inclusion as the starting point. When the sum M of the lengths of inclusions in the rolling direction is large, this tendency becomes strong, and the crack initiation resistance value Ji decreases.

  From the above viewpoint, the sum M of the lengths of inclusions in the rolling direction was measured, and based on this, the average value λave of the hole expansion ratio and the crack initiation resistance value Ji were evaluated.

  The steel sheet obtained under the hot rolling conditions as described above has a tensile strength distributed in the range of 450 to 480 MPa, and the microstructure is one of ferrite structure, ferrite structure and bainite structure and pearlite structure or The tissue consisted of both mixed tissues.

  FIG. 5 is a diagram showing the relationship between the total length M (inclusion length) of the inclusions in the rolling direction and the average value λave of the hole expansion ratio, and FIG. 6 is the total length in the rolling direction of the inclusions. It is a figure which shows the relationship between M and standard deviation (sigma) (lambda) of a hole expansion rate.

  As shown in FIG. 5, the average value λave of the hole expansion ratio of the steel sheet is found to be better as the total sum M in the rolling direction length of the inclusion is smaller. Further, as shown in FIG. 6, the standard deviation σλ of the hole expansion rate is also better as the total sum M of the inclusions in the rolling direction is smaller.

From these FIG. 5 and FIG. 6, when the total length M of inclusions in the rolling direction is 0.38 mm / mm ² or less, the average value λave of the hole expansion rate is 100% or more and the standard deviation σ is 15% or less. You can see that you can. Therefore, in the present invention, the total sum M of the lengths of the inclusions in the rolling direction is set to 0.38 mm / mm ² or less.

  In addition, the inventor found that the inclusion group and the elongated inclusions that cause the deterioration of the hole expanding property and the like by increasing the total length M of the inclusions in the rolling direction are MnS elongated by rolling. I found it. And, in order to improve the hole expandability, etc., as a result of examining the manufacturing method for suppressing these inclusions, it was found that it is important to simultaneously control the following two points of the composition and the slab cooling condition: did.

  First, in the composition, in order to suppress MnS, it is important to reduce S in steel. From this viewpoint, the upper limit of the amount of S in the present invention is set to 0.01%.

Moreover, in REM and Ca addition steel, the amount of S couple | bonded with Mn can be reduced by making the sulfide of REM and Ca precipitate, and extending | stretched MnS can be reduced. From this viewpoint, when the relationship between the numerical value on the left side of the following mathematical formula (1) and the sum M of the lengths in the rolling direction of inclusions was investigated, the numerical value on the left side of the following mathematical formula (1) was 0 as shown in FIG. It was found that the total M of 0.38 mm / mm ² or less, which is the object of the present invention, can be obtained if it is 0.003 or less.
{[S] / 32-([Ca] / 40 + [REM] / 140)} × 32 ≦ 0.003 Formula (1)
[S], [Ca], [REM]: Content in mass% of each component

  Further, the relationship between the cooling condition of the slab and the length M of the stretched inclusions is investigated, and even in the composition satisfying the above conditions, stretched inclusions are generated unless the cooling rate at the surface temperature of the slab after casting is set to a predetermined value or more. There was found. Specifically, as shown in FIG. 8, the cooling rate in the temperature range of 1350 ° C. to 1250 ° C. needs to be 3 ° C./second or more. This is because MnS precipitates coarsely when the cooling rate is low. Since coarse MnS tends to be stretched by rolling, it is considered that the stretch inclusion length M increases depending on the conditions for coarse precipitation.

  Further, the relationship between the rough rolling conditions and the stretched inclusion length M was investigated, and although not shown, it was found that the stretched inclusion length M increases when the rolling reduction at a relatively high temperature is high. This is presumably because MnS is easily stretched by rolling at a high temperature. From this point of view, it is necessary that the rolling reduction rate at 1150 ° C. or higher in rough rolling be 70% or less.

  As shown in FIGS. 9 and 10, it was also found that reducing the stretch inclusion length M also improves the fatigue limit ratio and crack initiation resistance value Ji value. The reason why they are improved by the reduction of the stretched inclusions is considered to be that the stretched inclusions become stress concentration points when stress is applied, and that is the starting point of fracture.

  Next, the present inventors investigated the relationship between fine inclusions and uniform elongation. As a result, the following was found.

  As shown in FIG. 11, it was found that the uniform elongation increases as the number density of relatively fine inclusions (equivalent circle diameter of 1 μm or more) increases. The reason is considered as follows. That is, at the time of deformation in a tensile test or the like, dislocations generated by the deformation are fixed around those fine inclusions, and the dislocations that are fixed prevent movement of other dislocations, thereby increasing the stress required for deformation. Become. For this reason, it is considered that work hardening of steel is promoted and uniform elongation is increased.

  As a result of intensive investigations on the number density of inclusions having an equivalent circle diameter of 1 μm or more, as shown in FIG. 12, the larger the left piece (([Ca] / 40 + [REM] / 140)} × 32) of equation 2, Turned out to be. At that time, it was found that the number of fine inclusions is small when the amount of S is low. This is because fine inclusions are obtained by controlling the form of sulfide by adding both REM and Ca, and the number of fine inclusions increases as the amount of S to be granulated increases. Conceivable.

The following can be seen from FIGS. That is, in order to obtain uniform elongation ≧ 17%, the number density of inclusions having an equivalent circle diameter of 1 μm or more needs to be 200 pieces / mm ² or more, preferably 200 to 600 pieces / mm ^2. It is necessary to satisfy the following expressions (2) and (3).
{([Ca] / 40 + [REM] / 140)} × 32 ≧ 0.001 Formula (2)
[S] ≧ 0.001 Formula (3)
[S], [Ca], [REM]: Content of each component in mass% On the other hand, in order to obtain uniform elongation ≧ 19%, the number density of inclusions having an equivalent circle diameter of 1 μm or more is 400 / mm. ^For this purpose, it is necessary to satisfy the following equations (2) ′ and (3) ′.
{([Ca] / 40 + [REM] / 140)} × 32 ≧ 0.004 Expression (2) ′
[S] ≧ 0.004 Formula (3) ′
[S], [Ca], [REM]: Content in% by mass of each component In the present invention, if the number density of inclusions having an equivalent circle diameter of 1 μm or more is too large, it is fine (1 μm or more). ), There is a possibility of deteriorating the hole expanding property. From this viewpoint, the upper limit of the number density of inclusions having an equivalent circle diameter of 1 μm or more is set to 2000 / mm ² .

  Then, the reason for limitation of the composition of the steel plate in this invention is demonstrated. Hereinafter, mass% in the composition is simply referred to as%.

C: 0.01 to 0.3%
C contributes to strengthening of the steel by increasing the hard second phase. If it is too low from this viewpoint, the strength deteriorates, so the lower limit is made 0.01%. Further, when the content of C is too large, a phase that becomes a starting point of ductile fracture such as cementite is generated, and the average value λave of the hole expansion rate and the crack initiation resistance value Ji are deteriorated. For this reason, content of C shall be 0.3% or less.

Si: 0.001 to 2.0%
Si is an element contributing to the improvement of tensile strength as a solid solution strengthening element, and it is preferable to add from this viewpoint. However, if Si is added excessively, the effect is saturated while increasing the cost. For this reason, content of Si shall be 0.001% or more and 2.0% or less.

Mn: 0.01 to 2.0%
Mn is an element that contributes to improving the tensile strength of the steel sheet as a solid solution strengthening element. Mn must be contained in an amount of 0.01% or more in order to obtain the intended tensile strength of the present invention. Further, if the content of Mn is more than 2.0%, slab cracking during hot rolling tends to occur. For this reason, content of Mn shall be 0.01-2.0%.

P: 0.02% or less P is an impurity that is inevitably mixed, and is an element that causes an increase in the amount of segregation at the grain boundary as the content increases, leading to deterioration of the average value λave of the hole expansion rate. For this reason, the P content is preferably as low as possible. From this viewpoint, the P content is 0.02% or less.

S: 0.001 to 0.01%
S is an inevitably mixed impurity, and if the content is too large, a large amount of MnS is generated in the steel when the steel slab is heated, and this is stretched by hot rolling to sum the length in the rolling direction of the inclusions. M is increased, and the average value λave of the hole expansion rate, crack initiation resistance value Ji, and crack propagation resistance value T, which are the object of the present invention, cannot be obtained. For this reason, the content of S is 0.01% or less.
On the other hand, when S becomes a fine sulfide by Ca and REM, it contributes to an increase in uniform elongation. From this viewpoint, the lower limit of S is 0.001%.

Al: 0.005 to 1.0%
Al is an element necessary for deoxidation of molten steel. It is also effective for strengthening steel. It is necessary to add 0.005% or more from the viewpoint of deoxidation of molten steel. Even if added over 1.0%, the effect of deoxidation of molten steel and the effect of strengthening of steel are saturated, but it is not economical. From the above viewpoint, the Al content is set to 0.005% or more and 1.0% or less.

N: 0.02% or less N is an element contained in steel as an inevitable impurity. If this amount is excessive, the steel material is likely to be aged, and the surface cleanliness is likely to deteriorate due to hip breakage or the like, so 0.02% is made the upper limit.

REM: 0.0001 to 0.02%
REM (rare earth element) reduces the stretched MnS by forming granulated material, reduces the total length M of inclusions in the rolling direction, and improves the average value λave of the hole expansion rate and the crack initiation resistance value Ji Element. From this point, REM needs to satisfy the above-described formula (1). If the REM content is less than 0.0001%, the effect of spheroidizing a sulfide such as MnS cannot be obtained sufficiently, so the lower limit is made 0.0001%. By adding REM, more fine inclusions containing REM are generated, and MnS is deposited around the fine inclusions later. Since MnS dispersed and precipitated around hard and fine inclusions is not easily stretched by rolling, stretched MnS is less likely to occur even in steels to which a predetermined amount of REM is added even if S is relatively high. In this respect, addition of 25 ppm or more is preferable. When the content of REM is more than 0.02%, such an effect is saturated and economic efficiency is lowered. For this reason, content of REM shall be 0.02% or less. As REM, La, Ce or the like can be used, but it is easy to use misch metal.

Ca: 0.0001 to 0.010%
Ca is an element that fixes S in steel as spherical CaS, suppresses the generation of MnS, and reduces the total length M in the rolling direction. From this point, it is necessary to set the content so as to satisfy the above-described formula (1). If the Ca content is less than 0.0001%, the effect of spheroidizing a sulfide such as MnS cannot be obtained sufficiently, so the lower limit is made 0.0001%. Further, if the Ca content is more than 0.010%, a large amount of calcium aluminate that tends to become inclusions in a stretched shape is generated, which may increase the total length M of inclusions in the rolling direction. is there. For this reason, the upper limit of the Ca content is 0.010% or less.

Further, S, REM, and Ca need to be contained so as to satisfy the above formula (1) in order to reduce as much as possible MnS that causes hole expandability deterioration. By satisfy | filling this Numerical formula (1), the amount of MnS in steel reduces and the effect of reducing the total M of the rolling direction length of an inclusion is acquired. Thereby, the effect of improving the average value λave of the hole expansion rate, the crack initiation resistance value Ji, and the crack propagation resistance value T is obtained. When the left side of the mathematical formula (1) is more than 0.003, the effect intended by the present invention cannot be obtained for these characteristic values.
{[S] / 32-([Ca] / 40 + [REM] / 140)} × 32 ≦ 0.003 Formula (1)
[Ti], [S], [Ca], [REM]: Content in mass% of each component

As described above, when a large amount of fine particles of REM and Ca are contained in the steel, the uniform elongation is improved (uniform elongation ≧ 17%). From this viewpoint, the equations (2) and ( 3) needs to be satisfied.
{([Ca] / 40 + [REM] / 140)} × 32 ≧ 0.001 Formula (2)
[S] ≧ 0.001 Formula (3)

Further, in order to further improve the uniform elongation (uniform elongation ≧ 19%), it is necessary to satisfy the expressions (2) ′ and (3) ′ from this viewpoint.
{([Ca] / 40 + [REM] / 140)} × 32 ≧ 0.004 Expression (2) ′
[S] ≧ 0.004 Formula (3) ′
[S], [Ca], [REM]: Content in mass% of each component

The above are the basic elements related to the steel of the present invention. In the present invention, in addition to the above, the following elements may be appropriately added to increase the strength of the steel. The lower limit of the addition amount of these elements is set in order to obtain the effect of increasing the strength, while the upper limit is set in order to prevent the economy from being impaired by the addition of the element.
B: 0.0005-0.003%,
Cu: 0.001 to 1.0%,
Cr: 0.001 to 1.0%,
Mo: 0.001 to 1.0%,
Ni: 0.001 to 1.0%,
V: 0.01-0.2%
Ti: 0.001 to 0.02%,
Nb: 0.001 to 0.05%

In the steel sheet according to the present invention, the remaining balance of the basic component is composed of Fe and inevitable impurities. Inevitable impurities include O, Zn, Pb, As, Sb, etc. Even if these are included in the following ranges, the effects of the present invention are not lost.
O ≦ 0.005%,
Zn ≦ 0.05%,
Pb ≦ 0.05%,
As ≦ 0.05%,
Sb ≦ 0.05%,
In the present invention, Zr, Sn, Co, W, and Mg may be contained in total of 1% or less as necessary.

  Next, the reasons for limiting the microstructure and inclusions of the hot-rolled steel sheet according to the present invention will be described.

  The microstructure needs to be a ferrite structure or a mixed structure of one or both of a ferrite structure and a bainite structure and a pearlite structure. This is because, in these structures, the hardness of the entire microstructure becomes relatively uniform, ductile fracture is suppressed, and the average value λave of the hole expansion rate and crack initiation resistance value Ji that are the object of the present invention are obtained. This is because it becomes possible. Further, in the microstructure, a structure called island martensite (MA) which is a mixture of martensite and retained austenite may remain slightly. Since this promotes ductile fracture and degrades the average value λave of the hole expansion rate, it is preferable that it does not remain, but an area fraction of 3% or less is acceptable.

The inclusions must have a total length M in the rolling direction of 0.38 mm / mm ² or less. This is because when the sum M exceeds 0.38 mm / mm ² , ductile fracture is significantly promoted during deformation of the steel sheet, and the average value λave of the hole expansion rate and crack initiation resistance value Ji targeted by the present invention are It can no longer be obtained. The sum M may be zero.

  The inclusion here refers to, for example, MnS in steel.

  Moreover, the measurement method of these microstructures and inclusions, and the definition of the total sum M of the inclusions in the rolling direction are as described above.

  Next, the manufacturing method for manufacturing the hot rolled steel sheet according to the present invention will be described.

  In the steelmaking process, for example, after obtaining hot metal in a blast furnace, etc., decarburization treatment and alloy addition are performed in a converter, and then the desulfurization treatment, deoxidation treatment, etc. are performed on the molten steel that has been produced using various secondary refining equipment. By carrying out the above, molten steel having the target component content is produced.

  In the secondary refining process, it is necessary not to perform desulfurization using a desulfurization material, but to add Ca and REM in an amount satisfying the formula (1) and suppress the stretched MnS in terms of cost. In order to obtain a high uniform elongation, it is necessary to satisfy the expressions (2) and (3). In order to obtain higher elongation, it is preferable to satisfy the expressions (2) ′ and (3) ′.

After refining under the above conditions, a slab is obtained by continuous casting. The cooling rate of the slab needs to be 3 ° C./second or more in the temperature range of 1350 ° C. to 1250 in order to reduce the stretch inclusion length M. If the above conditions are satisfied, it may be sent directly to a hot rolling mill as it is a high-temperature slab, or alternatively, it may be hot-rolled after being reheated by a heating furnace after being cooled to room temperature. . Also, as an alternative to obtaining hot metal with a blast furnace, iron scrap may be used as a raw material, and after melting this in an electric furnace, various secondary refining may be performed to obtain molten steel with a desired component content. .
If the cooling rate of the slab in the temperature range of 1350 ° C. to 1250 ° C. is too large, the slab after cooling may be cracked. From this viewpoint, the cooling rate of the slab in the temperature range of 1350 ° C. to 1250 ° C. is preferably 50 ° C./second or less.

  Next, manufacturing conditions for hot rolling a steel piece obtained by continuous casting or the like will be described.

  First, when the slab obtained by continuous casting or the like is once cooled to a low temperature, it is appropriately heated again in a heating furnace so that predetermined hot rolling can be performed. The heating temperature at this time needs to be sufficiently high in order to perform rough rolling and finish rolling in the austenite region at the Ar3 temperature or higher. From this viewpoint, it is desirable to heat to 1150 ° C or higher.

  Subsequently, rough rolling is performed on the steel piece extracted from the heating furnace, and then finish rolling is performed.

In rough rolling, it is preferable to reduce the rolling reduction at a high temperature from the viewpoint of further reducing the length M of the stretched inclusions. From this point of view, it is necessary that the rolling reduction of 1150 ° C. or higher is 70% or lower.
If the rolling reduction of 1150 ° C. or higher is too small, a large rolling at a low temperature (1150 ° C. or lower) is necessary to obtain a predetermined plate thickness. In this case, the rolling load becomes excessive, which is not preferable in operation. From this point of view, the rolling reduction of 1150 ° C. or higher is preferably 40% or higher.

  In the subsequent finish rolling step, the end temperature is made Ar3 ° C. or higher and Ar3 + 200 ° C. or lower. The reason why the end temperature is set to Ar3 ° C. or higher is that the surface layer of the steel sheet obtained when the end temperature is equal to or lower than the Ar3 temperature is mixed and the formability deteriorates. The reason why the upper limit of the finish rolling temperature is Ar3 + 200 ° C. is that if the finish rolling temperature is too high, the particle size becomes excessive and the tensile strength deteriorates.

Ar3 is obtained from the following mathematical formula (8). [C], [Si], etc. in the following mathematical formula (8) mean the content of each component in mass% in the steel.
Ar3 = 868-396 * [C] + 25 * [Si] -68 * [Mn] -36 * [Ni] -21 * [Cu] -25 * [Cr] + 30 * [Mo] (8)
Subsequently, the steel plate obtained by the finish rolling process is cooled by a run-out table or the like. In this cooling step, the cooling rate needs to be 10 ° C./sec or more. This is because if the cooling rate is less than 10 ° C./sec, the crystal grain size becomes coarse and the strength decreases.
If the cooling rate is too high, phase transformation (ferrite transformation) during cooling is suppressed, and the strength of the resulting steel sheet becomes excessive. From this viewpoint, the cooling rate is preferably 60 ° C./second or less.

  Subsequently, the cooled steel plate is wound up by a winding device or the like. In this winding process, it is necessary to wind the steel sheet in a temperature range of 700 ° C. or lower. This is because if the steel sheet is wound up in a temperature range exceeding 700 ° C., the crystal grain size becomes coarse and the strength is reduced. The winding temperature needs to be 400 ° C. or higher. This is because, when the coiling temperature is less than 400 ° C., the solid solution C and solid solution N of the steel sheet increase, the aging property is deteriorated, the waist breaks easily, and the surface properties of the steel sheet may be impaired. is there.

  The above is the manufacturing conditions of the hot rolling process according to the present invention. After the hot rolling process, skin pass rolling may be performed for the purpose of correcting the shape of the steel sheet. Moreover, you may pickle after completion | finish of a hot rolling process.

  Moreover, you may make it improve the corrosion resistance of a steel plate by carrying out a plating process by the hot dipping method after completion | finish of a hot rolling process. Further, in addition to hot dipping, alloying treatment may be performed.

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

  First, steel having a composition of steel components A to M as shown in Table 1 was used as a slab by continuous casting, and hot rolling was performed under the conditions shown in Table 2-1, thereby obtaining a steel sheet having a thickness of 2.9 mm. Table 2-2 also shows the microstructure, inclusions, and mechanical properties of the obtained steel sheet. The measurement method of the microstructure and inclusions and the measurement method of the mechanical properties are as described above. The underline in Tables 1 and 2 means outside the scope of the present invention or outside the preferred range.

  Steel Nos. 1 to 10 satisfy the requirements of the present invention in terms of composition and production conditions. Good hole expansion ratio λ (%), variation in hole expansion ratio σλ (%), uniform elongation U.V. El, crack initiation resistance value (Ji value), and fatigue limit ratio are shown.

  Steel Nos. 5 and 6 have particularly good uniform elongation because the compositions also satisfy the formulas (2) ′ and (3) ′.

  In steel No. 11, S is higher than a predetermined value, so that a large amount of stretch inclusions are generated and the formula (1) is not satisfied. Therefore, the hole expansion ratio λ (%) and the hole expansion ratio variation σλ (%) The crack initiation resistance value Ji and the fatigue limit ratio are inferior.

  Steel Nos. 12 and 13 have compositions that do not satisfy the value of Formula 1. For this reason, a large amount of stretch inclusions are generated, and therefore, the hole expansion ratio λ (%), the hole expansion ratio variation σλ (%), Ji, and the fatigue limit ratio are inferior.

  Steel numbers 14, 15, and 16 do not satisfy Formula (2) and Formula (3). For this reason, uniform elongation is small.

  Steel Nos. 17 and 18 are slow in cooling the slab. For this reason, the stretch inclusion length M is large, and therefore the hole expansion ratio λ (%), the hole expansion ratio variation σλ (%), Ji, and the fatigue limit ratio are inferior.

  Steel No. 19 has a rolling reduction of 1150 ° C. or higher in rough rolling larger than a predetermined value. Therefore, the stretch inclusion length M is small, and the hole expansion ratio λ (%), the hole expansion ratio variation σλ (%), Ji, and the fatigue limit ratio are particularly inferior.

1 notched test piece 2 support point 3 load point 4 forced displacement 5 fatigue test piece

Claims (4)

% By mass
C: 0.01 to 0.3%
Si: 0.001 to 2.0%,
Mn: 0.01 to 2.0%,
P: 0.02% or less,
S: 0.001 to 0.01%,
Al: 0.005 to 1.0%,
N: 0.02% or less,
REM: 0.0001 to 0.02%,
Ca: 0.0001 to 0.01%
And the balance is a steel plate made of Fe and inevitable impurities,
The following formulas (1), (2), (3) are satisfied,
The microstructure is composed of only a ferrite structure, or a ferrite structure, and a mixed structure of one or both of a bainite structure and a pearlite structure,
A group of inclusions having a rolling direction length of 30 μm or more, comprising a collection of inclusions having a major axis of 3 μm or more arranged at intervals of 50 μm or less with respect to other inclusions adjacent to each other on a straight line in the rolling direction; The total sum of the lengths in the rolling direction per 1 mm ² in cross section with the inclusions having a rolling direction length of 30 μm or more with an interval of more than 50 μm with respect to other inclusions adjacent on the straight line in the direction is 0.38 mm or less. And a number density of inclusions having an equivalent circle diameter of 1.0 μm or more is 200 / mm ² or more and 2000 / mm ² or less, a hot-rolled steel sheet excellent in formability and fracture characteristics.
{[S] / 32-([Ca] / 40 + [REM] / 140)} × 32 ≦ 0.003 Formula (1)
{([Ca] / 40 + [REM] / 140)} × 32 ≧ 0.001 Formula (2)
[S] ≧ 0.001 Formula (3)
[S], [Ca], [REM]: Content in mass% of each component The following expressions (2) ′ and (3) ′ are satisfied,
^2. The hot rolled steel sheet according to claim 1, wherein the number density of inclusions having an equivalent circle diameter of 1.0 μm or more is 400 / mm ² or more and 2000 / mm ² or less.
{([Ca] / 40 + [REM] / 140)} × 32 ≧ 0.004 Expression (2) ′
[S] ≧ 0.004 Formula (3) ′
[S], [Ca], [REM]: Content in mass% of each component Furthermore, in mass%,
B: 0.0005 to 0.003%
Cu: 0.001 to 1.0%,
Cr: 0.001 to 1.0%,
Mo: 0.001 to 1.0%,
Ni: 0.001 to 1.0%
V: 0.01 to 0.2%
Ti: 0.001 to 0.02%
Nb: 0.001 to 0.05%
The high-strength hot-rolled steel sheet having excellent formability and fracture characteristics according to claim 1, comprising at least one of the above.   After casting the steel slab containing the component according to any one of claims 1 to 3 to form a slab, a temperature between 1350 ° C and 1250 ° C is 1250 ° C or less at an average cooling rate of 3 ° C / second or more. In the subsequent hot rolling step, the rolling reduction of 1150 ° C. or higher in rough rolling is set to 70% or lower, and finish rolling is performed at an end temperature of Ar3 or higher and Ar3 + 200 ° C. or lower, followed by a cooling rate. Is produced at a temperature of 10 ° C./sec or higher, followed by winding in a temperature range of 400 ° C. or higher and 700 ° C. or lower. A method for producing a high-strength hot-rolled steel sheet having excellent formability and fracture characteristics.