The present invention relates to a hot-rolled steel sheet and a method for manufacturing the same. More specifically, the present invention is a hot-rolled steel sheet used for structural members such as automobiles, and has a high tensile strength of 980 MPa or more, ductility, hole expandability and The present invention relates to a hot-rolled steel sheet having excellent punching property and a method for manufacturing the same.
Background technology
[0002]
In recent years, the automobile industry has been required to reduce the weight of the vehicle body from the viewpoint 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. In order to achieve both weight reduction of the vehicle body and collision safety, increasing the strength of the steel sheet used is one of the effective methods, and the development of high-strength steel sheet is being promoted from such a background.
[0003]
However, as the strength of the steel sheet increases, the formability of the steel sheet generally decreases, and there is a problem that the mechanical properties such as ductility and hole expandability (an index indicating the stretch flangeability of the steel sheet) decrease. be. Therefore, in the development of high-strength steel sheets, it is an important issue to increase the strength without deteriorating these mechanical properties.
[0004]
In Patent Document 1, the component composition is mass%, C: 0.4 to 0.8%, Si: 0.8 to 3.0%, Mn: 0.1 to 0.6%, and the balance is Composed of iron and unavoidable impurities, the steel structure contains 80% or more of pearlite and 5% or more of retained austenite in terms of area ratio to the total structure, and the average lamella spacing of the pearlite is 0.5 μm or less, and the orientation difference. High strength characterized in that the effective crystal grain size of ferrite surrounded by large grain boundaries of 15 ° or more is 20 μm or less, and the number of carbides having a circle equivalent diameter of 0.1 μm or more is 5 or less per 400 μm 2. Highly ductile steel sheets are described. Further, in Patent Document 1, according to the above-mentioned high-strength high-ductility steel plate, while pearlite is the main structure, the lamella spacing thereof is reduced to increase the yield strength (YS), and the effective ferrite grains are made finer. By increasing the ductility (λ) and further increasing the 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 or more. It is described that the tensile strength (TS) × elongation (EL) is 14000 MPa ·% or more, and the ductility (λ) can be 35% or more.
[0005]
In Patent Document 2, in terms of weight%, C: 0.60 to 1.20%, Si: 0.10 to 0.35%, Mn: 0.10 to 0.80%, and P: 0, which is larger than 0. 03% or less, and S: greater than 0 and containing 0.03% or less, Ni: 0.25% or less (including 0), Cr: 0.30% or less (including 0), and Cu: 0. Containing any one or more of 25% or less (including 0), consisting of the balance Fe and other unavoidable impurities, and the width of cementite being larger than 0 and 0.2 μm or less, the cementite and cementite. A high carbon hot-rolled steel sheet having a fine pearlite structure having an interval of more than 0 and 0.5 μm or less is described. Further, Patent Document 2 describes that the high carbon hot-rolled steel sheet has a fine pearlite structure, so that the final product can be made durable and strong.
[0006]
In Patent Document 3, the component composition is mass%, C: 0.3 to 0.85%, Si: 0.01 to 0.5%, Mn: 0.1 to 1.5%, P: 0. Contains 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 from Fe and unavoidable impurities. The structure is a rolled pearlite structure, and a high-strength steel plate characterized in that the ratio of the amount of solid-melted C calculated by a predetermined formula is 50% or more is described. Further, Patent Document 3 describes that the above-mentioned high-strength steel plate is excellent in bending workability and can realize high strength of 1500 MPa or more in tensile strength.
[0007]
In Patent Document 4, a step of rough rolling a continuously cast slab having a C content of 0.8 mass% or less to produce a rough bar, and a finishing temperature of the rough bar at (Ar 3 transformation point -20) ° C. or higher. A step of manufacturing a steel strip by finish rolling, a step of primary cooling the steel strip after finish rolling to a temperature of 500 to 800 ° C. at a cooling rate exceeding 120 ° C./sec, and a step of primary cooling the steel after primary cooling. The step of allowing the strip to cool for 1 to 30 sec, the step of secondary cooling the steel strip after cooling at a cooling rate of 20 ° C./sec or more, and the step of secondary cooling the steel strip after the secondary cooling are performed at 650. A method for manufacturing a thin steel sheet having a step of winding at a winding temperature of ° C. or lower is described. Further, Patent Document 4 describes that according to the above-mentioned manufacturing method, a thin steel sheet having excellent workability including stretch flangeability and having various strength levels having uniform mechanical properties can be obtained.
[0008]
In Patent Document 5, in terms of mass%, C: 0.70 to 0.95%, Si: 0.05 to 0.4%, Mn: 0.5 to 2.0%, P: 0.005 to 0. It contains 03%, S: 0.0001 to 0.006%, Al: 0.005 to 0.10%, and N: 0.001 to 0.01%, and the balance consists of Fe and unavoidable impurities. Moreover, a soft high carbon steel plate is described in which the structure has 100 or more voids per 1 mm 2 of the observed structure. Further, Patent Document 5 describes that by having the above structure, it is possible to provide a soft high carbon steel sheet having excellent punching property. In addition, in Patent Document 5, in order to obtain the above-mentioned soft high carbon steel sheet, a manufacturing method including cooling, winding, pickling, and then annealing a softened box is performed under predetermined conditions. It is taught.
Prior art literature
Patent documents
[0009]
Patent Document 1: Japanese Unexamined Patent Publication No. 2016-098414
Patent Document 2: Japanese Patent Application Laid-Open No. 2011-530569
Patent Document 3: Japanese Unexamined Patent Publication No. 2011-099132
Patent Document 4: Japanese Unexamined Patent Publication No. 2001-164322
Patent Document 5: Japanese Unexamined Patent Publication No. 2011-012316
Outline of the invention
Problems to be solved by the invention
[0010]
In Patent Document 1, a steel sheet that does not contain Cr or contains Cr in a relatively small amount is hot-rolled, then cold-rolled, and then subjected to a predetermined heat treatment to produce a steel sheet. However, with such a component composition and manufacturing method, the average lamella spacing of pearlite cannot always be sufficiently reduced, and therefore, the high-strength high-ductility steel sheet described in Patent Document 1 still has an improvement in mechanical properties. There was room for improvement.
[0011]
The high carbon hot-rolled steel sheet described in Patent Document 2 does not contain Cr or contains Cr in a relatively small amount, as in the case of the high-strength high-ductility steel sheet described in Patent Document 1. Further, in Patent Document 2, as described above, it is described that the final product can have durability and strength because it has a fine pearlite structure, but the specific tensile strength is not disclosed. In addition, in Patent Document 2, no sufficient study has been made from the viewpoint of improving other mechanical properties, for example, mechanical properties such as ductility and hole widening property.
[0012]
Although Patent Document 3 discloses a high-strength steel plate having a tensile strength of 1500 MPa or more, no sufficient study has been made from the viewpoint of improving mechanical properties such as hole expandability. In fact, the high-strength steel sheet described in Patent Document 3 is prepared by preparing a steel piece having a pearlite structure as a main phase by a pearlite treatment using an annealing furnace, and then cold-rolling the steel with a rolling ratio of 90% or more. Although it is manufactured, in the case of such a manufacturing method, the above-mentioned cold rolling forms a microstructure in which the directions of the layered cementites in the pearlite are aligned in the rolling direction. However, since such a microstructure reduces the hole expandability, it is difficult to achieve the hole expandability suitable for use in an automobile steel sheet with the high-strength steel sheet described in Patent Document 3.
[0013]
Further, in the processing of automobile parts and the like, a punching process by a press machine is often included, but in particular, in the case of punching a high-strength steel sheet, cracking (punching) is performed at the punched end face due to the high strength of the steel sheet. There is a problem that cracking) is likely to occur. On the other hand, in Patent Documents 1 to 4, no sufficient study has been made from the viewpoint of improving the punching property of the high-strength steel sheet.
[0014]
In relation to this, Patent Document 5 describes that it is possible to provide a soft high carbon steel sheet having excellent punching property as described above, but Patent Document 5 describes that the soft high carbon steel sheet can be obtained. Since the softened box is annealed as a heat treatment, the carbides are spheroidized and a fine lamella structure cannot be obtained. Therefore, in the soft high carbon steel sheet described in Patent Document 5, there is still room for improvement in terms of improvement of mechanical properties.
[0015]
Therefore, an object of the present invention is to provide a hot-rolled steel sheet having a tensile strength of 980 MPa or more and excellent ductility, drilling property and punching property, and a method for manufacturing the same, by a novel structure.
Means to solve problems
[0016]
The present inventors examined the chemical composition and structure of the hot-rolled steel sheet in order to achieve the above object. As a result, the present inventors have found that it is important that the structure of the hot-rolled steel sheet is mainly composed of pearlite having an excellent strength-ductility balance, and in addition, it is important to appropriately control the microstructure of the pearlite. rice field. More specifically, the present inventors ensure ductility by containing pearlite in a hot-rolled steel sheet in an area ratio of 90% or more, while ensuring punching property by not including retained austenite. In addition, by refining the pearlite block (corresponding to the region where the crystal orientations of the ferrites that make up pearlite are aligned), it is possible to suppress the occurrence of cracks during local deformation and ensure hole malleability. Furthermore, it was found that the strength of hot-rolled steel sheets can be increased without impairing ductility and hole-expanding properties by making the pearlite spacing finer while maintaining a pearlite fraction of 90% or more. Completed the invention. Since increasing the strength of hot-rolled steel sheets by miniaturizing the pearlite lamella spacing does not compete with the improvement of ductility and hole expandability, controlling the structure as described above provides excellent ductility even at higher strengths. And it becomes possible to achieve hole malleability.
[0017]
The present invention has been completed based on the above findings, and specifically, it is as follows.
(1) The chemical composition is mass%,
C: 0.50 to 1.00%,
Si: 0.01-0.50%,
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: 0.50 to 2.00%,
Cu: 0 to 1.00%,
Ni: 0 to 1.00%,
Mo: 0 to 0.50%,
Nb: 0 to 0.10%,
V: 0 to 1.00%,
Ti: 0 to 1.00%,
B: 0-0.0100%,
Ca: 0 to 0.0050%,
REM: 0 to 0.0050%, and
Remaining: Fe and impurities,
The metal structure is the area ratio,
Pearlite: 90-100%,
Pseudo pearlite: 0-10%, and
Initialized ferrite: 0 to 1%,
The average lamella spacing of the pearlite is 0.20 μm or less,
A hot-rolled steel sheet characterized in that the average pearlite block diameter of the pearlite is 20.0 μm or less.
(2) The chemical composition is mass%.
Cu: 0.01-1.00%,
Ni: 0.01-1.00%, and
Mo: 0.01-0.50%
Nb: 0.01-0.10%,
V: 0.01-1.00%, and
Ti: 0.01-1.00%
The hot-rolled steel sheet according to (1) above, which comprises one or more of the above.
(3) The hot-rolled steel sheet according to (1) or (2) above, wherein the chemical composition contains B: 0.0005 to 0.0100% in mass%.
(4) The chemical composition is mass%.
Ca: 0.0005 to 0.0050%, and
REM: 0.0005 to 0.0050%
Includes one or two of The hot-rolled steel sheet according to any one of (1) to (3) above, which is characterized by being squeezed.
(5) The hot-rolled steel sheet according to any one of (1) to (4) above, which has a tensile strength of 980 MPa or more.
(6) A step of heating a slab having the chemical composition according to any one of (1) to (4) above to 1100 ° C. or higher.
A hot rolling process including finish rolling of a heated slab, wherein the output side temperature of the finish rolling is 820 to 920 ° C.
The process of primary cooling the obtained steel sheet to Ae 1 point at an average cooling rate of 40 to 80 ° C / sec, and then secondary cooling from Ae 1 point to the winding temperature at an average cooling rate of less than 20 ° C / sec, and
The process of winding the steel sheet at a winding temperature of 540 to 700 ° C.
A method for manufacturing a hot rolled steel sheet, which comprises.
The invention's effect
[0018]
According to the present invention, it is possible to obtain a hot-rolled steel sheet having a high tensile strength of 980 MPa or more and excellent ductility, drilling property and punching property.
A brief description of the drawing
[0019]
FIG. 1 is a reference diagram showing pearlite, pseudo-pearlite and proeutectoid ferrite.
Embodiment for carrying out the invention
[0020]

The hot-rolled steel sheet according to the embodiment of the present invention has a chemical composition of% by mass.
C: 0.50 to 1.00%,
Si: 0.01-0.50%,
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: 0.50 to 2.00%,
Cu: 0 to 1.00%,
Ni: 0 to 1.00%,
Mo: 0 to 0.50%,
Nb: 0 to 0.10%,
V: 0 to 1.00%,
Ti: 0 to 1.00%,
B: 0-0.0100%,
Ca: 0 to 0.0050%,
REM: 0 to 0.0050%, and
Remaining: Fe and impurities,
The metal structure is the area ratio,
Pearlite: 90-100%,
Pseudo pearlite: 0-10%, and
Initialized ferrite: 0 to 1%,
The average lamella spacing of the pearlite is 0.20 μm or less,
The average pearlite block diameter of the pearlite is 20.0 μm or less.
[0021]
First, the chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention and the slab used for manufacturing the same will be described. In the following description, "%", which is a unit of the content of each element contained in the hot-rolled steel sheet and the slab, means "mass%" unless otherwise specified.
[0022]
[C: 0.50 to 1.00%]
C is an essential element for ensuring the strength of hot rolled steel sheets. In order to sufficiently obtain such an effect, the C content is set to 0.50% or more. The C content may be 0.53% or more, 0.55% or more, 0.60% or more, or 0.65% or more. On the other hand, if C is excessively contained, cementite may be precipitated, and a sufficient pearlite fraction may not be obtained, or ductility or weldability may be deteriorated. Therefore, the C content is set to 1.00% or less. The C content may be 0.95% or less, 0.90% or less, 0.85% or less, 0.80% or less, or 0.75% or less. Further, in the hot rolled steel sheet according to the embodiment of the present invention, the ratio of the solid melt C amount (the amount obtained by subtracting the C amount precipitated as cementite from the C content) to the total C content (C content) in the steel is Generally less than 50%. More specifically, in the embodiment of the present invention in which such cold rolling is not performed, although the amount of solid melt C may increase when strong processing is performed at a high rolling reduction in cold rolling. In such hot rolled steel sheets, the proportion of the amount of solid melt C is generally much lower than 50%, for example, 30% or less, 20% or less, or 10% or less.
[0023]
[Si: 0.01-0.50%]
Si is an element used for deoxidizing steel. However, if the Si content is excessive, the chemical conversion treatment property is deteriorated, and austenite remains in the microstructure of the steel sheet, so that the punching property of the steel sheet is deteriorated. Therefore, the Si content is set to 0.01 to 0.50%. The Si content may be 0.05% or higher, 0.10% or higher or 0.15% or higher, and / or 0.45% or lower, 0.40% or lower or 0.30% or lower. May be good.
[0024]
[Mn: 0.50 to 2.00%]
Mn is an effective element for delaying the phase transformation of steel and preventing the phase transformation from occurring during cooling. However, when the Mn content becomes excessive, microsegregation or macrosegregation is likely to occur, and the hole expandability is deteriorated. Therefore, the Mn content is set to 0.50 to 2.00%. The Mn content may be 0.60% or higher, 0.70% or higher or 0.90% or higher, and / or 1.90% or lower, 1.70% or lower, 1.50% or lower or 1. It may be 30% or less.
[0025]
[P: 0.100% or less]
The lower the P content is, the more preferable it is, and if it is excessive, it adversely affects the moldability and weldability and also deteriorates the fatigue characteristics, so it should be 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 it is preferably 0.0001% or more because excessive reduction causes an increase in cost.
[0026]
[S: 0.0100% or less]
S forms MnS and acts as a starting point of fracture, which significantly reduces the hole expandability of the steel sheet. Therefore, the S content is 0.0100% or less. The S content is preferably 0.0090% or less, more preferably 0.0060% or less, or 0.0010% or less. The S content may be 0%, but it is preferably 0.0001% or more because excessive reduction causes an increase in cost.
[0027]
[Al: 0.100% or less]
Al is an element used for deoxidizing steel. However, if the Al content is excessive, inclusions increase and the workability of the steel sheet is deteriorated. Therefore, the Al content is set to 0.100% or less. The Al content may be 0%, but is preferably 0.005% or more or 0.010% or more. On the other hand, the Al content may be 0.080% or less, 0.050% or less, or 0.040% or less.
[0028]
[N: 0.0100% or less]
N combines with Al in steel to form AlN, which inhibits the increase in pearlite block diameter due to the pinning effect. However, when the N content becomes excessive, the effect is saturated and rather causes a decrease in toughness. Therefore, the N content is 0.0100% or less. The N content is preferably 0.0090% or less, 0.0080% or less, or 0.0050% or less. From this point of view, it is not necessary to set the lower limit of the N content and it may be 0%, but the steelmaking cost increases in order to reduce the N content to less than 0.0010%. Therefore, the N content is preferably 0.0010% or more.
[0029]
[Cr: 0.50 to 2.00%]
Cr has the effect of reducing the lamella spacing of pearlite, which can ensure the strength of the steel sheet. In order to sufficiently obtain such an effect, the lower limit of the Cr content is set to 0.50%, preferably 0.60%. On the other hand, if Cr is excessively added, structures such as pseudo-pearlite and bainite are likely to appear, and it becomes difficult to make the pearlite fraction 90% or more. Therefore, the upper limit of the Cr content is 2.00%, 1.50%, 1.25%, preferably 1.15%.
[0030]
The basic composition of the hot-rolled steel sheet according to the embodiment of the present invention and the slab used for manufacturing the same is as described above. Further, the hot-rolled steel sheet and the slab may contain the following optional elements, if necessary. The content of these elements is not essential and the lower limit of the content of these elements is 0%.
[0031]
[Cu: 0 to 1.00%]
Cu is an element that can be dissolved in steel to increase its strength without impairing its toughness. The Cu content may be 0%, but may be contained as necessary in order to obtain the above effects. However, if the content is excessive, the increase in precipitates may cause minute cracks on the surface during hot processing. Therefore, the Cu content is preferably 1.00% or less or 0.60% or less, and more preferably 0.40% or less or 0.25% 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.
[0032]
[Ni: 0 to 1.00%]
Ni is an element that can be dissolved in steel to increase its strength without impairing its toughness. The Ni content may be 0%, but 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 1.00% or less or 0.80% or less, and more preferably 0.60% or less or 0.30% or less. In order to sufficiently obtain the above effects, the Ni content is preferably 0.10% or more, and more preferably 0.20% or more.
[0033]
[Mo: 0 to 0.50%]
Mo is an element that increases the strength of steel. The Mo content may be 0%, but may be contained as necessary in order to obtain the above effects. However, if the content is excessive, the decrease in toughness with the increase in strength becomes remarkable. Therefore, the Mo content is preferably 0.50% or less or 0.40% or less, and more preferably 0.20% or less or 0.10% or less. In order to sufficiently obtain the above effects, the Mo content is preferably 0.01% or more, and more preferably 0.05% or more.
[0034]
[Nb: 0 to 0.10%]
[V: 0 to 1.00%]
[Ti: 0 to 1.00%]
Since Nb, V and Ti contribute to the improvement of the strength of the steel sheet by the precipitation of carbides, one selected from these may be contained alone or in combination of two or more, if necessary. However, if any of the elements is excessively contained, a large amount of carbides are generated, which reduces the toughness of the steel sheet. Therefore, the Nb content is preferably 0.10% or less or 0.08% or less, more preferably 0.05% or less, and the V content is preferably 1.00% or less or 0.80% or less, 0.50. % Or less or 0.20% or less is more preferable, the Ti content is preferably 1.00% or less or 0.50% or less, and 0.20% or less or 0.04% or less is more preferable. On the other hand, the lower limit of the Nb, V and Ti contents may be 0.01% or 0.03% for any element.
[0035]
[B: 0 to 0.0100%]
Since B has the effect of segregating at the grain boundaries and reddening the grain boundary strength, it may be contained if necessary. However, if the content is excessive, the effect is saturated and the raw material cost increases. Therefore, the B content is 0.0100% or less. The B content is preferably 0.0080% or less, 0.0060% or less, or 0.0020% or less. In order to sufficiently obtain the above effects, the B content is preferably 0.0005% or more, and more preferably 0.0010% or more.
[0036]
[Ca: 0 to 0.0050%]
Ca is an element that controls the morphology of non-metal inclusions that are the starting point of fracture and cause deterioration of workability, and may be contained as necessary. However, if the content is excessive, the effect is saturated and the raw material cost increases. Therefore, the Ca content is set to 0.0050% or less. The Ca content is preferably 0.0040% or less or 0.0030% or less. In order to obtain the above effect sufficiently, the Ca content is preferably 0.0005% or more.
[0037]
[REM: 0 to 0.0050%]
REM is an element that improves the toughness of welds by adding a small amount. The REM content may be 0%, but may be contained as necessary in order to obtain the above effects. However, if it is added in an excessive amount, the weldability is deteriorated. Therefore, the REM content is preferably 0.0050% or less or 0.0040% or less. It's good. In order to sufficiently obtain the above effects, the REM content is preferably 0.0005% or more, and more preferably 0.0010% or more. In addition, REM is a general term for a total of 17 elements of Sc, Y and lanthanoid, and the content of REM means the total amount of the above elements.
[0038]
In the hot-rolled steel sheet according to the embodiment of the present invention, the balance other than the above-mentioned components consists of Fe and impurities. Impurities are components that are mixed in by various factors in the manufacturing process, including raw materials such as ore and scrap, when hot-rolled steel sheets are industrially manufactured.
[0039]
Next, the reason for limiting the structure of the hot-rolled steel sheet according to the embodiment of the present invention will be described.
[0040]
[Pearlite: 90-100%]
By making the metal structure of the steel sheet mainly pearlite, it is possible to make a steel sheet with excellent ductility and hole expansion while maintaining high strength. If the area ratio of pearlite is less than 90%, ductility cannot be ensured and / or perforation widening cannot be ensured due to the non-uniformity of the structure. Therefore, the pearlite content in the metal structure of the hot-rolled steel sheet according to the embodiment of the present invention is 90% or more in area ratio, preferably 95% or more, 96% or more, 97% or more, 98% or more or 99. % Or more, and may be 100%.
[0041]
[Pseudo pearlite: 0-10%]
[Initialized ferrite: 0 to 1%]
The residual structure other than pearlite may be 0%, but if a residual structure is present, it consists of at least one of pseudo-pearlite and proeutectoid ferrite. It is possible to ensure good punching performance by constructing the residual structure from at least one of pseudo-pearlite and proeutectoid ferrite, that is, by not including retained austenite in the residual structure. In the present invention, the "pseudo-pearlite" is a structure mainly composed of cementite dispersed in a lump, as opposed to pearlite in which a ferrite phase and cementite are dispersed in a layered state (lamellar), and more specifically, such a lump-like structure. It refers to a structure containing cementite in an area ratio of more than 50% with respect to the total amount of cementite in the structure, and may contain lamellar cementite in a part thereof. Further, in the present invention, the "initialized 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 1% in area ratio. It refers to less than a ferrite (see, for example, the reference diagram of FIG. 1 (c)). The area ratio of the pseudo pearlite is 0 to 10%, and the area ratio may be, for example, 8% or less, 6% or less, 4% or less, 3% or less, 2% or less, or 1% or less. The area ratio of the proeutectoid ferrite is 0 to 1%, and the area ratio may be, for example, 0.8% or less or 0.6% or less. 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 metal structure. By "substantially nonexistent" is meant that the total area ratio of these tissues is less than 0.5%. Since it is difficult to accurately measure the total amount of such minute tissues and the effect is negligible, if the total amount of these tissues is less than 0.5%, it does not exist. It is possible to judge.
[0042]
[Average lamella spacing of pearlite: 0.20 μm or less]
The average lamellar spacing of pearlite (excluding the above-mentioned pseudo pearlite) has a strong correlation with the strength of the steel sheet, and the smaller the average lamellar spacing, the higher the strength. Further, if the components are the same, the smaller the average lamella spacing, the better the hole expanding property of the steel sheet. If the average lamella spacing exceeds 0.20 μm, a tensile strength of 980 MPa or more cannot be obtained and / or the hole expandability is reduced. Therefore, the average of pearlite in the metal structure of the hot-rolled steel sheet according to the embodiment of the present invention. The lamella spacing is 0.20 μm or less, preferably 0.15 μm or less or 0.10 μm or less. The lower limit of the average lamella spacing of pearlite is not particularly limited, but may be, for example, 0.05 μm or 0.07 μm.
[0043]
[Average pearlite block diameter: 20.0 μm or less]
The pearlite block corresponds to a region in which the crystal orientations of the ferrites constituting pearlite (excluding the above-mentioned pseudo pearlite) are aligned. Here, the average pearlite block diameter of pearlite has a correlation with the local ductility and toughness of the steel sheet, and the smaller the average pearlite block diameter, the better the hole expandability. If the average pearlite block diameter exceeds 20.0 μm, the hole expandability deteriorates. Therefore, the average pearlite block diameter in the metal structure of the hot-rolled steel sheet according to the embodiment of the present invention is preferably 20.0 μm or less, preferably 18. It is 9.0 μm or less, more preferably 16.0 μm or less. The lower limit of the average pearlite block diameter of pearlite is not particularly limited, but may be, for example, 3.0 μm, 5.0 μm, or 7.0 μm.
[0044]
[Certification method and measurement method for pearlite and residual tissue]
The fraction of pearlite and the remaining tissue is calculated 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 the thickness direction of the steel plate becomes the observation surface. Subsequently, the observation surface is mirror-polished, corroded with a Piclar corrosive solution, and then the structure is observed using a scanning electron microscope (SEM). The magnification was set to 5000 times (measurement area: 80 μm × 150 μm), and the area where cementite was layered was recognized as pearlite from the obtained tissue photograph using a point calculation method (for example, the reference diagram in FIG. 1 (a) is used. See), calculate the fraction. On the other hand, in the case of a structure in which the ferrite phase and cementite are not dispersed in layers but mainly in cementite dispersed in a mass, it is recognized as pseudo-pearlite (for example, see the reference diagram in FIG. 1 (b)). Calculate the fraction. 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 lath, and these carbides are a single variant, that is, an iron-based carbide extending in the same direction. Those belonging to the group are recognized as bainite. Further, a region of lumpy or film-like iron-based carbide having a circle-equivalent diameter of 300 nm or more is recognized as proeutectoid cementite. In the case of a structure as shown in FIGS. 1 (a) or 1 (b), the observed inclusions are basically cementite, using a scanning electron microscope (SEM-EDS) with an energy dispersive X-ray spectroscope or the like. It is not necessary to identify individual inclusions as cementite or iron-based carbides. Only when there is doubt that it is cementite or iron-based carbide, inclusions 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 less than 1% inside, and if there is such a structure, after microstructure observation by SEM, electron backscatter diffraction (EBSD). The structure of the bcc structure is determined to be proeutectoid ferrite, and the structure of the fcc structure is determined to be retained austenite.
[0045]
[Measurement method of average lamella interval]
The average lamella interval is calculated 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 the thickness direction of the steel plate becomes the observation surface. Subsequently, the observation surface is mirror-polished, corroded with a Piclar corrosive solution, and then the structure is observed using a scanning electron microscope (SEM). The magnification is 5000 times (measurement area: 80 μm × 150 μm), and 10 or more points where the cementite layer crosses perpendicularly to the paper surface of the tissue photograph are selected. By corroding with Piclar corrosive liquid and measuring, information in the depth direction can be obtained, so that the part vertically crossing the cementite layer can be known. By selecting and measuring 10 or more such points, the lamella interval S is obtained at each point, and the average of them is taken to obtain the average lamella interval. The method of measuring the lamella interval at each location is as follows. First, a straight line is drawn perpendicular to the cementite layer so as to cross 10 to 30 cementite layers, and the length of the straight line is L. The number of cementite layers crossed by the straight line is N. At this time, the lamella interval S at the relevant location is obtained by S = L / N.
[0046]
[Measuring method of average pearlite block diameter]
The average pearlite block diameter is measured using EBSD. 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 the thickness direction of the steel plate becomes the observation surface. Subsequently, the observation surface is mirror-polished, the crystal orientation of iron is measured using EBSD, and the grain boundaries are obtained. Grain boundaries are defined as boundaries where the crystal orientation changes by 15 °. The measurement area is 100 μm × 200 μm, and the measurement point interval is 0.2 μm pitch. Finally, the circle-equivalent diameter is obtained from the area of ​​the region surrounded by the crystal grain boundaries, and the average value of the circle-equivalent diameters calculated for all the crystal grains in the measurement region by the Area Fraction method is defined as the average pearlite block diameter. ..
[0047]
[Mechanical characteristics]
According to the hot-rolled steel sheet having the above-mentioned chemical composition and structure, high tensile strength, specifically, tensile strength of 980 MPa or more can be achieved. The reason why the tensile strength is 980 MPa or more is to satisfy the demand for weight reduction of the vehicle body in the automobile. The tensile strength is preferably 1050 MPa or more, more preferably 1100 MPa or more. The upper limit value does not need to be specified in particular, but for example, the tensile strength may be 1500 MPa or less, 1400 MPa or less, or 1300 MPa or less. Similarly, according to the hot rolled steel sheet having the above chemical composition and structure, high ductility can be achieved, more specifically 13% or more, preferably 15% or more, more preferably 17% or more. Full growth can be achieved. The upper limit does not need to be specified, but for example, the total elongation may be 30% or less or 25% or less. Further, according to the hot-rolled steel sheet having the above-mentioned chemical composition and structure, excellent drilling property can be achieved, more specifically 45% or more, preferably 50% or more, more preferably 55%. The above hole expansion rate can be achieved. The upper limit value does not need to be specified in particular, but for example, the hole expansion rate may be 80% or less or 70% or less. Tensile strength and total elongation are measured by collecting a JIS No. 5 tensile test piece from a direction perpendicular to the rolling direction of the hot-rolled steel sheet and performing a tensile test in accordance with JIS Z2241 (2011). On the other hand, the hole expansion rate is measured by performing a hole expansion test in accordance with JIS Z2256 (2010).
[0048]
[Plate thickness]
The hot hot-rolled steel sheet according to the embodiment of the present invention generally has a plate 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.
[0049]

The method for manufacturing a hot-rolled steel sheet according to an embodiment of the present invention is a step of heating a slab having the chemical composition described above to 1100 ° C. or higher.
A hot rolling process including finish rolling of a heated slab, wherein the output side temperature of the finish rolling is 820 to 920 ° C.
The process of primary cooling the obtained steel sheet to Ae 1 point at an average cooling rate of 40 to 80 ° C / sec, and then secondary cooling from Ae 1 point to the winding temperature at an average cooling rate of less than 20 ° C / sec, and
The process of winding the steel sheet at a winding temperature of 540 to 700 ° C.
It is characterized by including. Hereinafter, each step will be described in detail.
[0050]
[Slab heating process]
First, the slab having the chemical composition described above is heated before hot rolling. The heating temperature of the slab is set to 1100 ° C. or higher in order to sufficiently re-dissolve Ti carbonitride and the like. The upper limit is v Although not particularly specified, it may be, for example, 1250 ° C. 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 manufactured by an ingot forming method or a thin slab casting method.
[0051]
[Hot rolling process]
(Rough rolling)
In this method, for example, a heated slab may be roughly rolled before finish rolling in order to adjust the plate thickness or the like. The rough rolling is not particularly limited as long as the desired seat bar size can be secured.
[0052]
(Finish rolling)
The heated slab or, if necessary, the rough-rolled slab is then subjected to finish-rolling, and the exit temperature in the finish-rolling is controlled to 820 to 920 ° C. If the output temperature of the finish rolling is more than 920 ° C., the austenite becomes coarse and does not satisfy the condition of the average pearlite block diameter of the final product (that is, 20.0 μm or less). Therefore, the upper limit of the exit temperature of the finishing temperature is 920 ° C, preferably 900 ° C, and more preferably 880 ° C. From this point of view, it is not necessary to set a lower limit for the output side temperature of finish rolling if the Ar is 3 points or more. Can be the cause of. Therefore, the lower limit of the output side temperature of finish rolling is set to 820 ° C.
[0053]
[Cooling process]
After finishing rolling, cool the steel sheet. The cooling process is further subdivided into primary cooling and secondary cooling.
[0054]
(Primary cooling at an average cooling rate of 40 to 80 ° C / sec to Ae1 point)
In the primary cooling, the temperature is cooled from the output side temperature of the above finish rolling to Ae 1 point at an average cooling rate of 40 to 80 ° C./sec. If the average cooling rate to the above temperature is less than 40 ° C./sec, proeutectoid ferrite and / or proeutectoid cementite may precipitate, making it impossible to achieve the target value (90% or more) of the pearlite fraction. The average cooling rate of the primary cooling may be 43 ° C./sec or higher or 45 ° C./sec or higher. On the other hand, if the average cooling rate becomes too high, the steel sheet cannot be cooled uniformly, and there is a possibility that the material may vary. Therefore, the average cooling rate of the primary cooling is 80 ° C./sec or less, and may be, for example, 70 ° C./sec or less. In addition, Ae1 (° C.) can be obtained by using the following formula.
Ae1 (° C) = 723-10.7 x [Mn] + 29.1 x [Si]
However, the [element symbol] in the formula indicates the content of each element in% by mass.
[0055]
(Secondary cooling at an average cooling rate of less than 20 ° C / sec from Ae1 point to winding temperature)
Subsequently, in the secondary cooling, cooling is performed from the Ae point 1 to the winding temperature (that is, the temperature range of 540 to 700 ° C.) at an average cooling rate of less than 20 ° C./sec. By slowing the cooling rate as compared with the primary cooling in this way, it is possible to generate a pearlite structure in which the direction of the lamellar is more random, and it is possible to make the lamellar spacing finer and improve the hole expanding property. On the other hand, if the average cooling rate up to the above temperature range is high, the lamella spacing becomes non-uniform in the steel sheet, which may deteriorate the hole widening property, or a large amount of pseudo pearlite is generated and the pearlite fraction is increased. There is a risk that the target value (90% or more) cannot be achieved. Therefore, the average cooling rate of the secondary cooling is set to less than 20 ° C./sec, preferably 15 ° C./sec or less, more preferably 10 ° C./sec or less, and most preferably 10 ° C./sec or less. The secondary cooling is preferably performed immediately after the completion of the primary cooling in order to surely suppress the formation of ferrite.
[0056]
[Winding process]
After the cooling process, wind up the steel sheet. The temperature of the steel sheet at the time of winding is 540 to 700 ° C. By controlling the take-up temperature to 540-700 ° C, the structure is appropriately transformed during take-up and the average lamella spacing of pearlite is made finer, so that hot rolling is performed without impairing ductility and hole expandability. It is possible to increase the strength of the steel sheet. On the other hand, when the winding temperature is less than 540 ° C., other structures such as pseudo pearlite and bainite appear, and it becomes difficult to achieve the above pearlite fraction of 90% or more. Therefore, the winding temperature may be 540 ° C. or higher, and may be 550 ° C. or higher or 600 ° C. or higher. Further, when the winding temperature exceeds 700 ° C., the average lamella spacing of pearlite becomes large, and sufficient strength and / or hole expandability cannot be ensured. Therefore, the winding temperature is 700 ° C. or lower, and may be 680 ° C. or lower or 650 ° C. or lower. The conditions after the winding process are not particularly limited.
[0057]
Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited to these examples.
Example
[0058]
In the following examples, the hot-rolled steel sheet according to the embodiment of the present invention was manufactured under various conditions, and the mechanical properties of the obtained hot-rolled steel sheet were investigated.
[0059]
First, a slab having the chemical composition shown in Table 1 was produced by a continuous casting method. Next, a hot-rolled steel sheet having a plate thickness of 3 mm was manufactured from these slabs under the heating, hot rolling, cooling, and winding conditions shown in Table 2. The secondary cooling in the cooling step was performed immediately after the completion of the primary cooling. The balance other than the components shown in Table 1 is Fe and impurities. The chemical composition of the sample collected from the manufactured hot-rolled steel sheet was equivalent to the chemical composition of the slab shown in Table 1. In addition, in the hot-rolled steel sheets of all the examples, the ratio of the solid solution C amount was 10% or less.
[0060]
[table 1]

[0061]
[Table 2]

[0062]
JIS No. 5 tensile test pieces were collected from the hot-rolled steel sheet thus obtained from a direction perpendicular to the rolling direction, and a tensile test was conducted in accordance with JIS Z2241 (2011) to obtain tensile strength (TS) and total elongation. (El) was measured. In addition, a hole expansion test was conducted in accordance with JIS Z2256 (2010), and the hole expansion rate (λ) was measured. For punching performance, a hole with a diameter of 10 mm is punched with a punching clearance of 12.5%, and the properties of the end face are visually observed. "Pass (x)" was given, and if not accepted, "pass (○)" was given. Hot-rolled steel sheet with high strength and excellent ductility, drilling and punching properties when TS is 980 MPa or more, El is 13% or more, λ is 45% or more, and the evaluation of punching property is passed. Evaluated as. The results are shown in Table 3 below.
[0063]
[Table 3]

[0064]
As is clear from Table 3, in Examples 1, 2, 8 to 11 and 19 to 25, the tensile strength is 980 MPa or more, El is 13% or more, λ is 45% or more, and the evaluation of punching property is performed. As a result, it was possible to obtain a hot-rolled steel sheet having high strength and excellent ductility, drilling property and punching property.
[0065]
On the other hand, in Comparative Example 3, since the winding temperature was over 700 ° C., the average lamella spacing of pearlite was coarsened to over 0.20 μm. Therefore, TS980 MPa or more and λ45% or more could not be achieved. In Comparative Example 4, since the average cooling rate of the primary cooling in the cooling step was less than 40 ° C./sec, a large amount of proeutectoid ferrite was generated, and the pearlite fraction was less than 90%. Therefore, λ45% or more could not be achieved. In Comparative Example 5, since the average cooling rate of the secondary cooling was high, the pseudo pearlite increased and the pearlite fraction became less than 90%. Therefore, λ45% or more could not be achieved. In Comparative Example 6, since the winding temperature in the winding step was lower than 540 ° C., the pseudo pearlite increased and the pearlite fraction became less than 90%. Therefore, El 13% or more and λ 45% or more could not be achieved. In Comparative Example 7, the pearlite block became coarse because the temperature on the exit side of the finish rolling in the hot rolling process exceeded 920 ° C., and the average pearlite block diameter became more than 20.0 μm. Therefore, λ45% or more could not be achieved.
[0066]
In Comparative Example 12, since the Cr content was high, pseudo-pearlite increased and bainite was mixed, and the pearlite fraction was less than 90%. Therefore, El 13% or more and λ 45% or more could not be achieved. In Comparative Example 13, TS980 MPa or more could not be achieved because the C content was low. In Comparative Example 14, TS980 MPa or more could not be achieved because the Cr content was low. Further, in Comparative Example 14, since the finish rolling outside temperature in the hot rolling process exceeded 920 ° C., the average pearlite block diameter exceeded 20.0 μm, and λ45% or more could not be achieved. In Comparative Examples 15 and 16, since the Si content was excessive, residual austenite was mixed in the residual structure, and the punching property was rejected. In Comparative Example 17, since the C content was high, the pearlite fraction was less than 90% due to the mixture of proeutectoid cementite in the residual structure. Therefore, El 13% or more and λ 45% or more could not be achieved. In Comparative Example 18, λ45% or more could not be achieved because the Mn content was high.
The scope of the claims
[Claim 1]
The chemical composition is mass%,
C: 0.50 to 1.00%,
Si: 0.01-0.50%,
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: 0.50 to 2.00%,
Cu: 0 to 1.00%,
Ni: 0 to 1.00%,
Mo: 0 to 0.50%,
Nb: 0 to 0.10%,
V: 0 to 1.00%,
Ti: 0 to 1.00%,
B: 0-0.0100%,
Ca: 0 to 0.0050%,
REM: 0 to 0.0050%, and
Remaining: Fe and impurities,
The metal structure is the area ratio,
Pearlite: 90-100%,
Pseudo pearlite: 0-10%, and
Initialized ferrite: 0 to 1%,
The average lamella spacing of the pearlite is 0.20 μm or less,
A hot-rolled steel sheet characterized in that the average pearlite block diameter of the pearlite is 20.0 μm or less.
[Claim 2]
The chemical composition is by mass%
Cu: 0.01-1.00%,
Ni: 0.01-1.00%, and
Mo: 0.01-0.50%
Nb: 0.01-0.10%,
V: 0.01-1.00%, and
Ti: 0.01-1.00%
The hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet contains one or more of the above.
[Claim 3]
The hot-rolled steel sheet according to claim 1 or 2, wherein the chemical composition contains B: 0.0005 to 0.0100% in% by mass.
[Claim 4]
The chemical composition is by mass%
Ca: 0.0005 to 0.0050%, and
REM: 0.0005 to 0.0050%
The hot-rolled steel sheet according to any one of claims 1 to 3, wherein the hot-rolled steel sheet contains one or two of the above.
[Claim 5]
The hot-rolled steel sheet according to any one of claims 1 to 4, which has a tensile strength of 980 MPa or more.
[Claim 6]
A step of heating a slab having the chemical composition according to any one of claims 1 to 4 to 1100 ° C. or higher.
A hot rolling process including finish rolling of a heated slab, wherein the output side temperature of the finish rolling is 820 to 920 ° C.
The process of primary cooling the obtained steel sheet to Ae 1 point at an average cooling rate of 40 to 80 ° C / sec, and then secondary cooling from Ae 1 point to the winding temperature at an average cooling rate of less than 20 ° C / sec, and
The process of winding the steel sheet at a winding temperature of 540 to 700 ° C.
A method for manufacturing a hot rolled steel sheet, which comprises.