Title of Invention: Hot Rolled Steel Plate
Technical field
[0001]
TECHNICAL FIELD The present invention relates to a hot-rolled steel sheet, and more specifically, a hot-rolled steel sheet used for structural members of automobiles and the like, which has high strength and excellent ductility, and can suppress the formation of voids during punching. It relates to inter-rolled steel sheets.
Background technology
[0002]
In recent years, the automobile industry has been demanding lighter vehicle bodies from the perspective of improving fuel efficiency. On the other hand, due to the tightening of regulations on collision safety, it is necessary to add reinforcing parts to the body frame, which leads to an increase in weight. Increasing the strength of the steel plate used is one of the effective ways to achieve both weight reduction and collision safety of the vehicle body.
[0003]
On the other hand, most automotive parts are made by press molding. In general, the formability of a steel sheet decreases as the strength increases, and for example, ductility indexes such as elongation and hole expansion ratio decrease. If these ductility indices are below a certain level, it is not possible to form the desired member shape. Therefore, in the development of high-strength steel sheets, it is important to increase the strength while maintaining these mechanical properties above a certain level. ing.
[0004]
For example, in the prior art, it has been proposed to include a predetermined amount of pearlite in the steel sheet structure in order to increase the strength while maintaining the ductility index of the steel sheet.
[0005]
In Patent Document 1, the component composition is, in mass%, C: 0.4 to 0.8%, Si: 0.8 to 3.0%, Mn: 0.1 to 0.6%, and the balance is It consists of iron and unavoidable impurities, and the steel structure contains 80% or more of pearlite and 5% or more of retained austenite in terms of area ratio to the entire structure, and the average lamellar spacing of the pearlite is 0.5 μm or less, and the misorientation A high strength characterized in that the effective grain size of ferrite surrounded by large-angle grain boundaries of 15° or more is 20 μm or less, and the number of carbides having an equivalent circle diameter of 0.1 μm or more is 5 or less per 400 μm 2 A highly ductile steel sheet is described. In addition, according to Patent Document 1, according to the above high-strength and high-ductility steel sheet, while pearlite is the main structure, the lamellar spacing is reduced to increase the yield strength (YS) and the effective ferrite grains are refined. By increasing stretch flangeability (λ) and further increasing elongation (EL) by dispersing retained austenite, the tensile strength (TS) is 980 MPa or more and the yield ratio YR (= YS / TS) is 0.8. As described above, it is described that a tensile strength (TS)×elongation (EL) of 14000 MPa·% or more and a stretch flangeability (λ) of 35% or more can be secured.
[0006]
In Patent Document 2, in weight %, C: 0.60 to 1.20%, Si: 0.10 to 0.35%, Mn: 0.10 to 0.80%, P: 0. 03% or less, and S: greater than 0 and 0.03% or less, Ni: 0.25% or less (including 0), Cr: 0.30% or less (including 0), and Cu: 0.03% or less. containing any one or more of 25% or less (including 0), the balance being Fe and other unavoidable impurities, and the width of cementite being greater than 0 and 0.2 μm or less, and the cementite and cementite A high-carbon hot-rolled steel sheet (hot-rolled steel sheet) characterized by having a fine pearlite structure in which the spacing is greater than 0 and 0.5 μm or less is described. In addition, Patent Document 2 describes that the fraction of the fine pearlite structure is 90% or more, and further, the above-mentioned high-carbon hot-rolled steel sheet has a fine pearlite structure, so that the final product has durability and strength. It is stated that it can be carried
[0007]
In Patent Document 3, the component composition in mass% is C: 0.3 to 0.85%, Si: 0.01 to 0.5%, Mn: 0.1 to 1.5%, P: 0.5%. 035% or less, S: 0.02% or less, Al: 0.08% or less, N: 0.01% or less, Cr: 2.0 to 4.0%, and the balance is Fe and unavoidable impurities It describes a high-strength steel sheet characterized in that the structure is a rolled pearlite structure, and the ratio of the amount of dissolved C calculated by a predetermined formula is 50% or more. Moreover, Patent Document 3 describes that the high-strength steel sheet described above has excellent bending workability and can achieve a high tensile strength of 1500 MPa or more.
[0008]
In Patent Document 4, it has a predetermined chemical composition, and the metal structure has an area ratio of pearlite: 90 to 100%, pseudo pearlite: 0 to 10%, and proeutectoid ferrite: 0 to 1%, and the pearlite is 0.20 μm or less, and the average pearlite block diameter of the pearlite is 20.0 μm or less. Further, Patent Document 4 describes that with the above configuration, a hot-rolled steel sheet having a high tensile strength of 980 MPa or more and excellent ductility, hole expansibility and punchability can be obtained. . US Pat. No. 6,200,000 was published after the priority date of the present application and represents related art rather than known prior art.
prior art documents
patent literature
[0009]
Patent document 1: JP 2016-098414 A
Patent Document 2: Japanese Patent Publication No. 2011-530659
Patent document 3: JP 2011-099132 A
Patent Document 4: International Publication No. 2020/179737
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0010]
In the case of a steel sheet containing a relatively large amount of pearlite, when the steel sheet is punched or sheared, microvoids are likely to occur starting from the carbide or the interface between the carbide and the matrix phase on the punched or sheared edge. These voids cause deterioration of the formability and/or fatigue resistance of the steel sheet after punching or shearing. Therefore, when a steel sheet containing a relatively large amount of pearlite is used, generation of voids in the steel sheet after punching or shearing becomes a problem. For example, Patent Literature 4 discusses improvement of punchability, but does not necessarily consider sufficiently from the viewpoint of suppressing the occurrence of such voids.
[0011]
Therefore, an object of the present invention is to provide a hot-rolled steel sheet that has high strength, excellent ductility, and excellent suppression of void formation during punching due to a novel structure.
Means to solve problems
[0012]
In order to achieve the above objectives, the present inventors investigated the chemical composition and structure of hot-rolled steel sheets. As a result, the present inventors have found that it is important to make the structure of the hot-rolled steel sheet mainly pearlite having an excellent strength-ductility balance, and in addition to appropriately control the microstructure of the pearlite. rice field. More specifically, the present inventors have found that the microstructure does not contain bainite and martensite, which cause a decrease in ductility, and instead contains pearlite at an area ratio of 90% or more in the hot-rolled steel sheet. The ductility is secured, and in addition, by refining the lamellar spacing of the pearlite while maintaining the pearlite fraction of 90% or more, the strength of the hot-rolled steel sheet is increased without impairing the ductility. The inventors have found that voids during punching can be suppressed by reducing the proportion of spherical cementite, and have completed the present invention.
[0013]
The present invention has been completed based on the above findings, and is specifically as follows.
(1) The chemical composition is mass%,
C: 0.30-0.80%,
Si: 0.01 to 0.50%,
Mn: 0.50 to 2.00%,
P: 0.100% or less,
S: 0.0100% or less,
Al: 0.100% or less,
N: 0.0100% or less,
Cr: 0.30 to 1.00%,
Ti: 0 to 1.00%,
Nb: 0 to 0.10%,
　V: 0 to 1.00%,
Cu: 0 to 1.00%,
Ni: 0 to 2.00%,
Mo: 0 to 0.40%,
B: 0 to 0.0100%,
Ca: 0 to 0.0050%,
REM: 0 to 0.005%, and
The balance: Fe and impurities,
The microstructure is the area ratio,
　Perlite: 90-100%, and
Proeutectoid ferrite: 0 to 10%,
the average lamellar spacing of the perlite is 0.08 to 0.30 μm,
A hot-rolled steel sheet characterized in that, among the cementite constituting the pearlite, the proportion of cementite having a major axis length of more than 0.3 μm and an aspect ratio of less than 3.0 is less than 15%.
(2) The chemical composition is mass%,
Ti: 0.01 to 1.00%,
Nb: 0.01 to 0.10%,
　V: 0.01 to 1.00%,
Cu: 0.01 to 1.00%,
Ni: 0.10 to 2.00%,
Mo: 0.01 to 0.40%,
B: 0.0005 to 0.0100%,
Ca: 0.0005 to 0.0050%, and
REM: 0.0005-0.005%
The hot-rolled steel sheet according to (1) above, comprising one or more selected from the group consisting of:
(3) The hot-rolled steel sheet according to (1) or (2) above, which has a tensile strength of 780 MPa or more.
Effect of the invention
[0014]
According to the present invention, it is possible to obtain a hot-rolled steel sheet with a high tensile strength of 780 MPa or more, excellent ductility, and capable of suppressing the formation of voids during punching.
Brief description of the drawing
[0015]
[Fig. 1] (a) is a diagram showing a typical microstructure of a hot-rolled steel sheet corresponding to an example, and (b) is a diagram showing void generation after punching in a hot-rolled steel sheet corresponding to a comparative example. It is a diagram.
MODE FOR CARRYING OUT THE INVENTION
[0016]

　The hot-rolled steel sheet according to the embodiment of the present invention has, in mass%,
C: 0.30-0.80%,
Si: 0.01 to 0.50%,
Mn: 0.50 to 2.00%,
P: 0.100% or less,
S: 0.0100% or less,
Al: 0.100% or less,
N: 0.0100% or less,
Cr: 0.30 to 1.00%,
Cr: 0.30 to 1.00%,
Ti: 0 to 1.00%,
Nb: 0 to 0.10%,
　V: 0 to 1.00%,
Cu: 0 to 1.00%,
Ni: 0 to 2.00%,
Mo: 0 to 0.40%,
B: 0 to 0.0100%,
Ca: 0 to 0.0050%,
REM: 0 to 0.005%, and
The balance: Fe and impurities,
The microstructure is the area ratio,
　Perlite: 90-100%, and
Proeutectoid ferrite: 0 to 10%,
the average lamellar spacing of the perlite is 0.08 to 0.30 μm,
It is characterized in that the proportion of cementite having a major axis length of more than 0.3 μm and an aspect ratio of less than 3.0 in the cementite constituting the pearlite is less than 15%.
[0017]
First, the chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention and the slab used for its manufacture will be described. In the following description, "%", which is the unit of content of each element contained in the hot-rolled steel sheet and slab, means "% by mass" unless otherwise specified.
[0018]
[C: 0.30 to 0.80%]
　C is an essential element for ensuring the strength of hot-rolled steel sheets. In order to sufficiently obtain such effects, the C content is made 0.30% or more. The C content may be 0.35% or more, 0.36% or more, 0.37% or more, 0.40% or more, 0.45% or more, or 0.50% or more. On the other hand, when C is contained excessively, cementite precipitates, and a sufficient pearlite fraction may not be obtained, or ductility and weldability may deteriorate. Therefore, the C content should be 0.80% or less. The C content may be 0.77% or less, 0.75% or less, 0.70% or less, or 0.65% or less.
[0019]
[Si: 0.01 to 0.50%]
　Si is an element used for deoxidizing steel. However, if the Si content is excessive, the chemical convertibility deteriorates, and the punchability of the steel sheet deteriorates due to the residual austenite in the microstructure of the steel sheet. Therefore, the Si content should be 0.01 to 0.50%. Si content may be 0.05% or more, 0.10% or more or 0.15% or more and/or 0.45% Below, it may be 0.40% or less or 0.35% or less.
[0020]
[Mn: 0.50 to 2.00%]
Mn is an element that delays the phase transformation of steel and is effective in preventing phase transformation from occurring during cooling. However, when the Mn content is excessive, micro-segregation or macro-segregation tends to occur, degrading the hole expansibility. Therefore, the Mn content should be 0.50 to 2.00%. The Mn content may be 0.60% or more, 0.70% or more, or 0.90% or more, and/or 1.90% or less, 1.70% or less, or 1.50% or less, good too.
[0021]
[P: 0.100% or less]
The lower the P content is, the better. If it is excessive, the formability and weldability are adversely affected, and the fatigue properties are also deteriorated, so the P content is made 0.100% or less. It is preferably 0.050% or less, more preferably 0.040% or less or 0.030% or less. The P content may be 0%, but excessive reduction leads to an increase in cost, so it is preferably 0.0001% or more.
[0022]
[S: 0.0100% or less]
S forms MnS and acts as a starting point of fracture, which significantly reduces the hole expansibility of the steel plate. Therefore, the S content should be 0.0100% or less. The S content is preferably 0.0090% or less, more preferably 0.0070% or less or 0.0060% or less. The S content may be 0%, but excessive reduction leads to an increase in cost, so it is preferably 0.0001% or more.
[0023]
[Al: 0.100% or less]
Al is an element used for deoxidizing steel. However, if the Al content is excessive, inclusions increase, degrading the workability of the steel sheet. Therefore, the Al content is set to 0.100% or less. Although the Al content may be 0%, it is preferably 0.001% or more or 0.003% or more. On the other hand, the Al content may be 0.070% or less, 0.050% or less, or 0.040% or less.
[0024]
[N: 0.0100% or less]
N combines with Al in the steel to form AlN, and the pinning effect prevents the pearlite block diameter from increasing, thereby improving the toughness of the steel. However, when the N content becomes excessive, the effect saturates, rather causing a decrease in toughness. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0090% or less or 0.0070% or less. From this point of view, there is no need to set a lower limit for the N content, and it may be 0%, but reducing the N content to less than 0.0010% increases the steelmaking cost. Therefore, the N content is preferably 0.0010% or more.
[0025]
[Cr: 0.30 to 1.00%]
Cr has the effect of refining the lamellar spacing of pearlite, which can ensure the strength of the steel sheet. In addition, Cr has the effect of suppressing the spheroidization of cementite, and can suppress the spheroidization of cementite in the coiled steel sheet. Therefore, in order to reduce the proportion of coarse spherical cementite in the pearlite and suppress the generation of voids during punching, it is necessary to contain a certain amount of Cr or more. For this reason, the lower limit of the Cr content is made 0.30%, preferably 0.40%, more preferably 0.45% or 0.50%. Furthermore, since Cr stabilizes cementite, the inclusion of Cr can expand the pearlite formation region to the low carbon side. Therefore, by containing Cr in an appropriate amount, ie, an amount of 0.30% or more, it is possible to achieve a pearlite fraction of 90% or more even with a relatively low C content. On the other hand, excessive addition of Cr delays the pearlite transformation and causes hard structures such as bainite and martensite, which may make it difficult to achieve a pearlite fraction of 90% or more. Alternatively, too much Cr may result in too small an average lamellar spacing of pearlite, resulting in decreased ductility with increased tensile strength. Therefore, the upper limit of Cr content is set to 1.00%, preferably 0.90%, more preferably 0.85% or 0.80%.
[0026]
The hot-rolled steel sheet according to the embodiment of the present invention and the basic chemical composition of the slab used for its production are as described above. Furthermore, the hot-rolled steel sheets and slabs may contain the following optional elements, if necessary. In addition, the lower limit of the content when the arbitrary element is not included is 0%.
[0027]
[Ti: 0 to 1.00%]
[Nb: 0 to 0.10%]
[V: 0 to 1.00%]
Ti, Nb and V are elements that contribute to the improvement of steel sheet strength through carbide precipitation. The Ti, Nb and V contents may be 0%, but in order to obtain the above effect, one selected from these may be contained alone or two or more may be combined as needed. good. However, if any element is contained excessively, a large amount of carbides are formed and the toughness of the steel sheet is lowered. Therefore, the Ti content is 1.00% or less or 0.60% or less, the Nb content is 0.10% or less or 0.08% or less, and the V content is 1.00% or less or 0.60% or less. It is preferable to have On the other hand, in order to sufficiently obtain the above effect, the lower limits of the contents of Ti, Nb and V are preferably 0.01% or 0.05% for each element.
[0028]
[Cu: 0 to 1.00%]
Cu is an element that dissolves in steel and can increase strength without impairing toughness. Although the content of Cu may be 0%, it may be contained as necessary in order to obtain the above effect. However, if the content is excessive, an increase in precipitates may cause minute cracks on the surface during hot working. Therefore, the Cu content is preferably 1.00% or less or 0.60% or less. In order to sufficiently obtain the above effects, the Cu content is preferably 0.01% or more, more preferably 0.05% or more.
[0029]
[Ni: 0 to 2.00%]
Ni is an element that dissolves in steel and can increase strength without impairing toughness. Although the Ni content may be 0%, it may be contained as necessary in order to obtain the above effects. However, Ni is an expensive element, and excessive addition causes an increase in cost. Therefore, the Ni content is preferably 2.00% or less or 1.00% or less. In order to sufficiently obtain the above effects, the Ni content is preferably 0.10% or more, more preferably 0.20% or more.
[0030]
[Mo: 0 to 0.40%]
Mo is an element that increases the strength of steel. Although the Mo content may be 0%, it may be contained as necessary in order to obtain the above effect. However, when the content is excessive, the toughness is significantly lowered due to the increase in strength. Therefore, the Mo content is preferably 0.40% or less or 0.20% or less. In order to sufficiently obtain the above effect, the Mo content is preferably 0.01% or more, more preferably 0.05% or more.
[0031]
[B: 0 to 0.0100%]
B segregates at grain boundaries and has the effect of increasing grain boundary strength. The B content may be 0%, but may be contained as necessary in order to obtain the above effects. However, if the content is excessive, the effect will be saturated and the raw material cost will increase. Therefore, the B content is preferably 0.0100% or less. More preferably, the B content is 0.0080% or less or 0.0060% or less. In order to sufficiently obtain the above effects, the B content is preferably 0.0005% or more, more preferably 0.0010% or more.
[0032]
[Ca: 0 to 0.0050%]
Ca is an element that controls the morphology of non-metallic inclusions that act as fracture starting points and cause deterioration in workability, thereby improving workability. Although the Ca content may be 0%, it may be contained as necessary in order to obtain the above effect. However, if the content is excessive, the effect will be saturated and the raw material cost will increase. Therefore, the Ca content is preferably 0.0050% or less. More preferably, the Ca content is 0.0045% or less or 0.0040% or less. In order to sufficiently obtain the above effects, the Ca content is preferably 0.0005% or more, more preferably 0.0010% or more.
[0033]
[REM: 0 to 0.005%]
REM is an element that improves the toughness of welds by adding a small amount. Although the REM content may be 0%, it may be contained as necessary in order to obtain the above effect. However, if it is added excessively, the weldability deteriorates. Therefore, the REM content is preferably 0.005% or less or 0.004% or less. In order to sufficiently obtain the above effect, the REM content is preferably 0.0005% or more, more preferably 0.001% or more. REM is a general term for a total of 17 elements including Sc, Y and lanthanoids, and the content of REM means the total amount of the above elements.
[0034]
　In the hot-rolled steel sheet according to the embodiment of the present invention, the balance other than the above components consists of Fe and impurities. Impurities are those that are mixed in from raw materials such as ores, scraps, or the manufacturing environment, and are allowed within a range that does not adversely affect the hot-rolled steel sheet according to the embodiment of the present invention.
[0035]
Next, the reason for limiting the structure of the hot-rolled steel sheet according to the embodiment of the present invention will be explained.
[0036]
[Perlite: 90-100%]
By making the microstructure of the steel sheet mainly composed of pearlite, it is possible to obtain a steel sheet with excellent ductility while maintaining high strength. If the area ratio of pearlite is less than 90%, the strength or ductility cannot be ensured and/or the ferrite-pearlite boundary that can become the starting point of void generation during punching increases due to non-uniformity of the structure. Therefore, the area ratio of pearlite in the microstructure of the hot-rolled steel sheet according to the embodiment of the present invention is set to 90% or more. Perlite is preferably 95% or more, 96% or more, 97% or more, 98% or more or 99% or more, and may be 100%.
[0037]
[Proeutectoid ferrite: 0 to 10%]
The residual structure other than pearlite may have an area ratio of 0%, but if there is a residual structure, it is limited to proeutectoid ferrite. Therefore, the area ratio of pro-eutectoid ferrite is set to 0 to 10%. By using pro-eutectoid ferrite as the residual structure, it is possible to ensure good ductility and punchability. In the present invention, "pro-eutectoid ferrite" does not substantially contain cementite precipitated as primary crystals in the cooling stage after hot rolling, that is, the cementite fraction in the crystal grains is less than 1% in terms of area ratio. It is called ferrite. The pro-eutectoid ferrite may have an area ratio of 5% or less, 4% or less, 3% or less, 2% or less, or 1% or less, for example. In the hot rolled steel sheet according to the embodiment of the present invention, retained austenite, proeutectoid cementite, bainite and martensite are absent or substantially absent in the microstructure. "Substantially absent" means that the total area percentage of these tissues is less than 0.5%. It is difficult to accurately measure the total amount of such microstructures, and the effect thereof can be ignored. It is possible to judge Hot-rolled steel sheets according to embodiments of the present invention encompass the range of hypereutectoid steels with a C content greater than 0.77%. In general, hypereutectoid steels may form proeutectoid cementite within a certain temperature range during cooling, depending on the composition and cooling rate. However, as long as the C content does not exceed 0.8% as in the hot-rolled steel sheet according to the embodiment of the present invention, the temperature range in which the pro-eutectoid cementite is formed is sufficiently narrow, and the pro-eutectoid cementite Since the formation is slow, the temperature of the steel sheet falls below the formation temperature range of pro-eutectoid cementite before the formation of pro-eutectoid cementite during cooling, and little pro-eutectoid cementite is formed. For example, thermalIn the method for producing a cold-rolled steel sheet, the cooling process is carried out at a relatively high cooling rate, which results in a short holding time within the formation temperature range of the pro-eutectoid cementite. Even with a hypereutectoid range of 80%, little or no proeutectoid cementite is formed. Therefore, the area ratio of proeutectoid cementite is less than 1%.
[0038]
[Average lamellar spacing of perlite: 0.08 to 0.30 μm]
The average lamellar spacing of pearlite has a strong correlation with the strength of the steel sheet, and the smaller the average lamellar spacing, the higher the strength. If the average lamellar spacing is too large, a tensile strength of 780 MPa or more cannot be obtained. 25 μm or less or 0.20 μm or less. On the other hand, if the average lamellar spacing is too small, the ductility may decrease as the tensile strength increases. Therefore, the lower limit of the average lamellar spacing of pearlite is set to 0.08 μm. The lower limit of the average lamellar spacing of pearlite is preferably 0.09 μm, more preferably 0.10 μm.
[0039]
[Proportion of cementite having a major axis length of more than 0.3 μm and an aspect ratio of less than 3.0 among cementites constituting pearlite: less than 15%]
The aspect ratio of cementite is the value obtained by dividing the length of the major axis of cementite that appears on the observation surface by the length of the minor axis. Cementite having a major axis length of more than 0.3 μm and an aspect ratio of less than 3.0 is defined here as coarse spherical cementite. Such coarse spheroidal cementite becomes a starting point for void generation during steel plate punching, and by keeping the ratio of coarse spheroidal cementite to the total cementite below a certain level, it is possible to obtain the effect of suppressing void generation during steel plate punching. It was found by the studies of the present inventors. In order to obtain such an effect, coarse spherical cementite should be less than 15%, preferably 14% or less, more preferably 12% or less or 10% or less, relative to the total cementite in the pearlite. The lower limit of this ratio is 0%, but it may be 1% or 3%. Although the details will be described later, the aspect ratio is the ratio between the length of the major axis and the length of the minor axis of the ellipsoid obtained by applying ellipsoidal approximation processing to each cementite by image processing.
[0040]
[Certification method and measurement method for perlite and residual tissue]
The perlite and residual tissue fractions are obtained as follows. First, a sample is taken from a position of 1/4 or 3/4 of the plate thickness from the surface of the steel plate so that the cross section parallel to the rolling direction and thickness direction of the steel plate becomes the observation surface. Subsequently, the observation surface is mirror-polished, corroded with a picral corrosive solution, and then subjected to structural observation using a scanning electron microscope (SEM). The measurement area is an area of ​​80 μm×150 μm, that is, an area of ​​12,000 μm 2 . For example, the perlite area ratio is calculated from a tissue photograph at a magnification of about 5000 times using the point counting method. Here, pearlite is a region surrounded by grain boundaries where the ferrite crystal orientation difference is 15° or more, where the ferrite phase and the cementite phase are mixed, and the cementite has a lamellar and/or spherical form. and certify. Therefore, for example, pearlite has a layered (lamellar) dispersed structure of ferrite phase and cementite, as well as a structure mainly composed of cementite dispersed in clusters. It also includes a structure containing more than 50% in terms of area ratio with respect to the total amount of cementite. The latter pearlite mainly composed of cementite dispersed in lumps is small, and may be 10% or less of the total pearlite. Cementite in pearlite is about 210 nm at the largest (about 100 nm on average), and does not exceed 300 nm. Further, it is an aggregate of lath-shaped crystal grains, and has a plurality of iron-based carbides having a major axis of 20 nm or more inside the laths, and these carbides are a single variant, that is, an iron-based carbide elongated in the same direction. Those belonging to the group are identified as bainite. In addition, a region that is a massive or film-like iron-based carbide and has an equivalent circle diameter of 300 nm or more is identified as proeutectoid cementite. Inclusions observed in the pearlite structure are basically cementite, and using a scanning electron microscope with an energy dispersive X-ray spectrometer (SEM-EDS), etc., individual inclusions are identified as cementite or iron-based carbides. need not be identified. Only when there is doubt that the inclusion is cementite or iron-based carbide, the inclusion may be analyzed using SEM-EDS or the like separately from SEM observation, if necessary. Both proeutectoid ferrite and retained austenite have an area fraction of cementite inside of less than 1%, and if such a structure exists, after observing the structure by SEM, electron beam backscatter diffraction (EBSD) , the bcc structure structure is determined to be proeutectoid ferrite, and the fcc structure structure is determined to be retained austenite.
[0041]
[Method for measuring average lamellar spacing]
　The average lamellar spacing is obtained as follows. First, a sample is taken from a position of 1/4 or 3/4 of the plate thickness from the surface of the steel plate so that the cross section parallel to the rolling direction and thickness direction of the steel plate becomes the observation surface. Subsequently, the observation surface is mirror-polished, corroded with a picral corrosive solution, and then subjected to structural observation using a scanning electron microscope (SEM). The measurement area is 80 μm×150 μm, that is, 12,000 μm 2 (magnification is, for example, 5000 times), and 10 or more points are selected where the cementite layer crosses the plane of the microstructure photograph perpendicularly. By corroding with a picral etchant and measuring, we can obtain information in the depth direction, so we can see where it crosses the cementite layer vertically. By selecting and measuring 10 or more such locations, the lamellar spacing S is determined at each location, and the average lamellar spacing is obtained by averaging them. The method for measuring the lamellar spacing at each location is as follows. First, draw a straight line perpendicular to the cementite layer so as to cross 10 to 30 cementite layers, and let the length of the straight line be L. Let N be the number of cementite layers that the straight line crosses. At this time, the lamellar spacing S at the location is obtained by S=L/N. In the measurement of the average lamellar spacing, pearlite in which ferrite phase and cementite are dispersed in layers (lamellar form) is to be measured, and the average lamellar spacing is not to be measured for structures mainly composed of cementite dispersed in lumps.
[0042]
[Method for measuring ratio R of cementite having major axis length of more than 0.3 μm and aspect ratio of less than 3.0 among cementites constituting pearlite]
The value of R above is obtained as follows. First, a sample is taken from a position of 1/4 or 3/4 of the plate thickness from the surface of the steel plate so that the cross section parallel to the rolling direction and thickness direction of the steel plate becomes the observation surface. Subsequently, the observation surface is mirror-polished, corroded with a picral corrosive solution, and then subjected to structural observation using a scanning electron microscope (SEM). The measurement area is 80 μm×150 μm, that is, 12,000 μm 2 (magnification is, for example, 5000 times), and the obtained image is subjected to binarization processing so that dark areas are ferrite and bright areas are cementite. Of these, for each cementite, ellipsoid approximation is performed by image processing, and the length of the major axis and the length of the minor axis of the ellipsoid are calculated as the length of the major axis and the length of the minor axis of each cementite. and the aspect ratio of each cementite is defined by the following equation.
[aspect ratio] = [long axis length]/[short axis length]
In one field of view of 80 μm × 150 μm, the total area of ​​cementite having a long axis length of more than 0.3 μm and an aspect ratio of less than 3.0 as defined by the above method is calculated by image processing, The value obtained by dividing this by the total area of ​​all cementite and expressed as a percentage is the value of R defined in the present invention.
[0043]
[Mechanical properties]
A hot-rolled steel sheet having the chemical composition and structure described above can achieve a high tensile strength, specifically a tensile strength of 780 MPa or more. The reason why the tensile strength is set to 780 MPa or more is to satisfy the demand for weight reduction of automobile bodies. The tensile strength is preferably 880 MPa or higher, more preferably 980 MPa or higher. Although it is not necessary to specify the upper limit, for example, the tensile strength may be 1500 MPa or less or 1400 MPa or less. Similarly, the hot-rolled steel sheet having the above chemical composition and structure can achieve high ductility, more specifically 15% or more, preferably 17% or more, more preferably 20% or more. Full elongation can be achieved. Although the upper limit does not have to be specified, for example, the total elongation may be 40% or less or 30% or less. Tensile strength and total elongation were determined by taking a No. 5 tensile test piece of JIS Z2241 (2011) from the direction perpendicular to the rolling direction of the hot-rolled steel plate and performing a tensile test in accordance with JIS Z2241 (2011). measured.
[0044]
[Thickness]
A hot rolled steel sheet according to an embodiment of the present invention generally has a thickness of 1.0 to 6.0 mm. Although not particularly limited, the plate thickness may be 1.2 mm or more or 2.0 mm or more and/or may be 5.0 mm or less or 4.0 mm or less.
[0045]

Next, a preferred method for manufacturing the hot-rolled steel sheet according to the embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing a hot-rolled steel sheet according to embodiments of the present invention, wherein the hot-rolled steel sheet is manufactured by a manufacturing method as described below. It is not intended to be limited to what is manufactured.
[0046]
A preferred method for manufacturing a hot-rolled steel sheet according to an embodiment of the present invention includes a step of heating a slab having the chemical composition described above to 1150°C or higher,
A hot rolling process including finish rolling of a heated slab, wherein the final pass reduction ratio of the finish rolling is 20% or more and the delivery side temperature FT is 750 to 850 ° C.,
The obtained steel sheet is cooled from the finish rolling delivery side temperature to the primary cooling end temperature shown below at an average cooling rate of 40 to 200 ° C./sec (primary cooling), then allowed to cool for 2 to 20 seconds, and cooled for 10 to A cooling step including cooling (secondary cooling) to a temperature of 560 ° C. or less at an average cooling rate of 200 ° C./sec, wherein the temperature Tc calculated by the following formula 1 or the outlet temperature FT-70 ° C. A cooling step in which the primary cooling end temperature is in the range of Ts to Ts + 20 ° C., where Ts is the lower temperature, and
A process of winding the steel sheet at a winding temperature of 400 to 550°C
is characterized by including
　Tc (°C) = 412.7 + 411.9 x [C] + 21.0 x [Si] + 2.7 x [Mn] + 114.4 x [Cr] ... Formula 1
Here, [C], [Si], [Mn] and [Cr] are the content [mass %] of each element. Each step will be described in detail below.
[0047]
[Slab heating process]
First, a slab having the chemical composition described above is heated before hot rolling. The heating temperature of the slab is set to 1150° C. or higher so that Ti carbonitrides and the like are fully dissolved again. Although the upper limit is not particularly specified, it may be 1250° C., for example. Also, the heating time is not particularly limited, but may be, for example, 30 minutes or more and/or 120 minutes or less. The slab to be used is preferably cast by a continuous casting method from the viewpoint of productivity, but may be produced by an ingot casting method or a thin slab casting method.
[0048]
[Hot rolling process]
(rough rolling)
In this method, for example, the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness. Conditions for the rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.
[0049]
(finish rolling)
a heated slab or additionally required The slab that has been rough-rolled according to is then subjected to finish rolling, and the final pass reduction in the finish rolling is controlled to 20% or more, and the delivery side temperature FT is controlled to 750 to 850°C. When the final pass reduction ratio of finish rolling is less than 20% and/or the delivery side temperature FT is more than 850°C, the accumulation of working strain in austenite during cooling is insufficient, pearlite transformation is delayed, and winding It becomes difficult to complete the pearlite transformation before removal, and a pearlite fraction of 90% or more cannot be achieved. Therefore, the final pass reduction in finish rolling is set to 20% or more, preferably 25% or more, and more preferably 30% or more. Although the upper limit of the final pass rolling reduction is not particularly required, it may be, for example, 50% or less. Similarly, in order to achieve a pearlite fraction of 90% or more, the upper limit of the outlet temperature FT of the finishing temperature is set to 850°C, preferably 830°C, more preferably 820°C. From this point of view, it is not necessary to set a lower limit to the delivery side temperature FT of the finish rolling if the Ar is 3 points or more, but the lower the temperature, the greater the deformation resistance of the steel sheet, which places a great burden on the rolling mill and increases the equipment. It can cause trouble. Therefore, the lower limit of the delivery side temperature FT of finish rolling is set to 750°C.
[0050]
[Cooling process]
After finish rolling, the steel plate is cooled. The cooling process is further subdivided into primary cooling, standing cooling (air cooling), and secondary cooling.
[0051]
(Average cooling rate of primary cooling: 40 to 200° C./sec)
In the cooling process, the steel is cooled from the delivery side temperature FT of the finish rolling to the primary cooling end temperature at an average cooling rate of 40°C/second or more. If the average cooling rate to the primary cooling end temperature is less than 40° C./sec, a large amount of pro-eutectoid ferrite and/or pro-eutectoid cementite precipitate, and the target pearlite fraction (90% or more) cannot be achieved. may disappear. The average cooling rate may be 42° C./s or higher, or 45° C./s or higher. The average cooling rate is preferably 200° C./second or less in order to obtain a desired structure, and may be 100° C./second or less. Note that the primary cooling end temperature can be appropriately selected within the range of Ts to Ts+20° C. described below.
[0052]
(Primary cooling end temperature: Ts to Ts + 20°C)
The cooling is completed within the temperature range of Ts to Ts+20°C, where Ts is the lower temperature of the temperature Tc or the delivery side temperature FT-70°C of the finish rolling. Here, Tc is the precipitation temperature of cementite, and is represented by the following formula 1.
　Tc (°C) = 412.7 + 411.9 x [C] + 21.0 x [Si] + 2.7 x [Mn] + 114.4 x [Cr] ... Formula 1
Here, [C], [Si], [Mn] and [Cr] are the content [mass %] of each element. If the primary cooling end temperature is lower than Ts, the pearlite transformation is delayed and no pearlite transformation occurs during the subsequent standing cooling. As a result, a pearlite fraction of 90% or more cannot be achieved, or pearlite transformation occurs after winding. For example, when pearlite transformation progresses after winding at a temperature of 550° C. or lower, the amount of pearlite generated at such a low temperature increases, and the average lamellar spacing of pearlite may become smaller than 0.08 μm. Also, if the primary cooling end temperature is higher than Ts+20° C., ferrite transformation occurs before pearlite transformation and a relatively large amount of proeutectoid ferrite is generated. Therefore, the primary cooling end temperature is specified as described above.
[0053]
(Cooling time: 2 to 20 seconds)
After the completion of the primary cooling, it is left to cool for 2 to 20 seconds to form pearlite with few coarse spherical carbides. If the cooling time is less than 2 seconds or 0 seconds, phase transformation (pearlite transformation) does not proceed sufficiently in the cooling process, and a pearlite fraction of 90% or more cannot be achieved, or pearlite transformation occurs after winding. It will happen. For example, when pearlite transformation progresses after winding at a temperature of 550° C. or lower, the amount of pearlite generated at such a low temperature increases, and the average lamellar spacing of pearlite may become smaller than 0.08 μm. Therefore, in order to complete the phase transformation with a pearlite fraction of 90% or more in the cooling step, the cooling time is set to 2 seconds or longer, preferably 3 seconds or longer, and more preferably 5 seconds or longer. Although there is no particular need to set an upper limit for the cooling time, the upper limit for the cooling time is set to 20 seconds from the viewpoint of productivity. The upper limit of the cooling time may be 15 seconds.
[0054]
(Secondary cooling)
　Secondary cooling is performed between the above cooling and the following winding process. As described above, by standing to cool for 2 seconds or more after the end of the primary cooling, the phase transformation with a pearlite fraction of 90% or more can be completed, and furthermore, the coiling temperature is set to 550 ° C. Spheroidization of cementite can be suppressed by the following. For this reason, there are no particular restrictions on the cooling for 2 to 20 seconds in the cooling step and the cooling during the winding step, other than cooling at an average cooling rate of 10 to 200° C./second. Although the average cooling rate of the secondary cooling does not significantly affect the microstructure of the steel sheet, the higher the average cooling rate, the more likely the temperature of the steel sheet becomes uneven. Therefore, the average cooling rate of secondary cooling is set to 200° C./second or less, and may be 100° C./second or less. From the viewpoint of productivity, the average cooling rate of secondary cooling is set to 10° C./second or more, and may be 20° C./second or more. Also, the end temperature of the secondary cooling need not be the same as the coiling temperature, and may be 560° C. or lower from the viewpoint of controlling the coiling temperature. Although the lower limit of the secondary cooling end temperature is not particularly limited, the secondary cooling end temperature may be 400° C. or higher, for example. Winding may be performed immediately after the end of secondary cooling, or cooling (air cooling) may be performed after the end of secondary cooling until winding.
[0055]
[Winding process]
After the cooling process, the steel sheet that has undergone a certain amount of phase transformation during cooling is wound up. The temperature of the steel sheet during winding is 400 to 550°C. If the coiling temperature exceeds 550 ° C, the time in the temperature range where the subsequent cementite spheroidization and coarsening occurs becomes longer, so the layered cementite in the pearlite generated during cooling becomes spheroidized, resulting in spheroidization during punching. A large number of coarse spheroidal cementites that can be void origins are generated. As a result, a structure that does not satisfy the characteristics of less than 15% of cementite having a long axis length of more than 0.3 μm and an aspect ratio of less than 3.0 among the cementite constituting pearlite is formed. There is Therefore, the coiling temperature is 550° C. or lower, and may be 540° C. or lower or 530° C. or lower. Moreover, when the coiling temperature is less than 400° C., a hard structure such as bainite or martensite is generated, so that the elongation of the steel sheet is lowered. Therefore, the winding temperature is set to 400° C. or higher, and may be 420° C. or higher or 440° C. or higher. In this manufacturing method, as described above, the pearlite transformation is completed in the cooling step by allowing the coil to cool for 2 to 20 seconds after the end of primary cooling, so the winding temperature does not particularly affect the average lamellar spacing of pearlite.
[0056]
The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.
Example
[0057]
In the following examples, hot-rolled steel sheets according to embodiments of the present invention were produced under various conditions, and the mechanical properties of the obtained hot-rolled steel sheets were investigated.
[0058]
First, a slab having the chemical composition shown in Table 1 was manufactured by continuous casting. Then, from these slabs, hot-rolled steel sheets with a thickness of 2.5 mm were produced under the heating, hot rolling, cooling, and coiling conditions shown in Table 2. The balance other than the components shown in Table 1 is Fe and impurities. Further, the chemical composition obtained by analyzing the sample taken from the manufactured hot-rolled steel plate was the same as the chemical composition of the slab shown in Table 1.
[0059]
[table 1]

[0060]
[Table 2]

[0061]
A No. 5 tensile test piece of JIS Z2241 (2011) was taken from the hot-rolled steel sheet thus obtained from the direction perpendicular to the rolling direction, and a tensile test was performed according to JIS Z2241 (2011). Tension (TS) and total elongation (El) were measured. The presence or absence of void generation during punching was evaluated by the following method. First, a hole with a diameter of 10 mm is punched with a punching clearance of 12.5%, and the steel sheet is cut along a cross section parallel to the rolling direction passing through the center point of the hole. Next, after mirror-polishing the cross section, the microstructure is revealed with Picral, and an area within 50 μm from the end face is observed with a SEM at a magnification of 5000. If voids with an equivalent circle diameter of 0.2 μm or more are observed, "Void presence" was determined, and if not observed, "void absence" was determined. A hot-rolled steel sheet with TS of 780 MPa or more, El of 15% or more, and no void generation during punching was evaluated as a hot-rolled steel sheet with high strength, ductility, and excellent void generation suppression. The results are shown in Table 3 below. The residual structure in Table 3 indicates structures other than pearlite, and thus means that structures other than the structures indicated as residual structures except for pearlite are not included.
[0062]
[Table 3]

[0063]
As is clear from Table 3, in Examples 1 to 5, 14, 17 to 19, and 24 to 27 within the scope of the present invention, the tensile strength is 780 MPa or more, and El is 15% or more, And since no voids are generated during punching, a hot-rolled steel sheet having high strength and excellent ductility and suppression of void generation could be obtained. As shown in FIG. 1( a ), in the hot-rolled steel sheets corresponding to these examples, the microstructure is mainly pearlite, the lamellar spacing of the pearlite is refined, and the coarse spherical cementite in the pearlite It can be seen that the ratio of
[0064]
On the other hand, in Comparative Example 6, the reduction in the final pass of the finish rolling was low, so the phase transformation was not promoted, the pearlite fraction decreased, and sufficient tensile strength was not obtained. In Comparative Example 7, since the delivery side temperature of finish rolling was high, the phase transformation was not promoted, the pearlite fraction decreased, and sufficient tensile strength was not obtained. In Comparative Example 8, since the average cooling rate was low, ferrite transformation occurred during cooling, the pearlite fraction decreased, and sufficient tensile strength could not be obtained. In Comparative Examples 9 and 15, since the primary cooling end temperature was low, bainite was formed and sufficient ductility was not obtained. In Comparative Examples 10 and 16, since the end temperature of the primary cooling was high, a relatively large amount of proeutectoid ferrite was formed during standing cooling, and sufficient tensile strength was not obtained. In Comparative Example 11, since the cooling time was short, phase transformation was not completed during cooling, and bainite was generated in the winding process, resulting in insufficient ductility. In Comparative Example 12, since the coiling temperature was low, bainite was similarly formed and sufficient ductility was not obtained. In Comparative Example 13, since the coiling temperature was high, the cementite was spheroidized after coiling, and the proportion of coarse spherical cementite increased, resulting in the generation of voids during punching. In Comparative Example 20, since the C content was low, the pro-eutectoid ferrite fraction increased and sufficient tensile strength was not obtained. In Comparative Example 21, since the C content was excessive, proeutectoid cementite was generated and sufficient ductility was not obtained. In Comparative Example 22, since the Cr content was low, the effect of suppressing cementite spheroidization by Cr was not sufficiently exhibited, and the ratio of coarse spherical cementite increased and voids were generated during punching. In Comparative Example 23, since the Cr content was excessive, the average lamellar spacing of pearlite was excessively refined, and the ductility decreased as the tensile strength improved. In Comparative Example 28, since the cooling time was 0 seconds, pearlite transformation occurred after winding, and the winding temperature was high, so that the cementite was spheroidized after winding, and the proportion of coarse spherical cementite increased. However, voids were generated during punching. In Comparative Example 29, since the cooling time was 0 seconds, pearlite transformation progressed after winding at a low temperature, the average lamellar spacing of pearlite was excessively refined, and ductility decreased as tensile strength improved. low got down. FIG. 1(b) shows a hot-rolled steel sheet corresponding to a comparative example in which voids were observed during punching. Referring to this, it is recognized that many minute voids are generated.
The scope of the claims
[Claim 1]
The chemical composition, in mass%,
C: 0.30-0.80%,
Si: 0.01 to 0.50%,
Mn: 0.50 to 2.00%,
P: 0.100% or less,
S: 0.0100% or less,
Al: 0.100% or less,
N: 0.0100% or less,
Cr: 0.30 to 1.00%,
Ti: 0 to 1.00%,
Nb: 0 to 0.10%,
　V: 0 to 1.00%,
Cu: 0 to 1.00%,
Ni: 0 to 2.00%,
Mo: 0 to 0.40%,
B: 0 to 0.0100%,
Ca: 0 to 0.0050%,
REM: 0 to 0.005%, and
The balance: Fe and impurities,
The microstructure is the area ratio,
　Perlite: 90-100%, and
Proeutectoid ferrite: 0 to 10%,
the average lamellar spacing of the perlite is 0.08 to 0.30 μm,
A hot-rolled steel sheet characterized in that, among the cementite constituting the pearlite, the proportion of cementite having a major axis length of more than 0.3 μm and an aspect ratio of less than 3.0 is less than 15%.
[Claim 2]
The chemical composition, in % by mass,
Ti: 0.01 to 1.00%,
Nb: 0.01 to 0.10%,
　V: 0.01 to 1.00%,
Cu: 0.01 to 1.00%,
Ni: 0.10 to 2.00%,
Mo: 0.01 to 0.40%,
B: 0.0005 to 0.0100%,
Ca: 0.0005 to 0.0050%, and
REM: 0.0005-0.005%
The hot-rolled steel sheet according to claim 1, comprising one or more selected from the group consisting of
[Claim 3]
The hot-rolled steel sheet according to claim 1 or 2, characterized by having a tensile strength of 780 MPa or more.