HOT-ROLLED STEEL SHEET AND METHOD FOR PRODUCING THE SAME

Abstract

A hot-rolled steel sheet is provided comprising a predetermined chemical composition and a metallic structure comprising, in area ratio, pearlite: 90 to 100%, pseudo-pearlite: 0 to 10% and pro-eutectoid ferrite: 0 to 1%, wherein the pearlite has an average lamellar spacing of 0.20 µm or less, and the pearlite has an average pearlite block size of 20.0 µm or less.A method is provided for producing a hot-rolled steel sheet comprising heating a plate to 1100°C or more, rolling it by hot rolling, wherein a finish roll exit side temperature is 820 to 920°C, cooling the steel sheet by primary cooling to a point Ae1 by an average cooling rate of 40 to 80°C / s, then cooling the steel sheet by secondary cooling from point Ae1 to a coiling temperature at an average cooling rate of less than 20°C / s, and coiling the steel sheet at a coiling temperature of 540 to 700°C.

Description

HOT-ROLLED STEEL SHEET AND METHOD FOR PRODUCING THE SAME c^nonn / Lznz / B / YiAi Field of invention [1] The present invention relates to a hot-rolled steel sheet and a method for producing the same, more particularly to a hot-rolled steel sheet used for a structural member of an automobile, etc., which is high strength with a tensile strength of 980 MPa or more, and which is excellent in ductility, hole expansion capability and stamping capability, and a method for producing the same. Background of the invention [2] In recent years, the automotive industry has sought to reduce the weight of car bodies to improve fuel efficiency. On the other hand, stricter regulations regarding collision safety have necessitated the addition of reinforcing components to body frames, leading to increased weight. To achieve lighter car bodies while maintaining collision safety, one effective method is to increase the strength of the steel sheets used. Given this context, efforts are underway to develop high-strength steel sheets. [3] However, there is a problem that as the strength of steel sheet increases, its formability generally decreases, and, for example, ductility, hole expansion capacity (an indicator of the steel sheet's stretch flanging), and other mechanical properties decline. Therefore, in the development of high-strength steel sheet, achieving higher strength without compromising these mechanical properties has become a major challenge. [4] PTL 1 describes a high-strength, high-ductility steel sheet comprising a composition of constituents containing, in mass %, C: 0.4 to 0.8%, Si: 0.8 to 3.0% and Mn: 0.1 to 0.6% and a remainder of iron and unavoidable impurities, and a steel microstructure including, in area ratio to the full microstructure, pearlite in 80% or more and residual austenite in 5% or more, an average pearlite lamellar spacing of 0.5 pm or less, an effective crystal grain size of ferrite surrounded by large-angle grain boundaries with orientation differences of 15° or more of 20 pm or less, and carbides having an equivalent circle diameter of 0.1 pm or more of 5 or less per 400 pm2.Furthermore, PTL 1 describes that according to the above high-strength, high-ductility steel sheet, it is possible to make pearlite the main structure while reducing its lamellar spacing to increase the yield strength (YS) and making the effective ferrite grains finer to increase the stretch flanging (λ) and, furthermore, making the residual austenite disperse to increase the elongation (EL) and thus ensure a tensile strength (TS) of 980 MPa or more, a yield ratio YR (= YS / TS) of 0.8 or more, a tensile strength (TS) x elongation (EL) of 14000 MPa-% or more, and a stretch flanging capacity (λ) of 35% or more. [5] PTL 2 describes a hot-rolled high-carbon steel sheet consisting of, by mass %, C: 0.60 to 1.20%, Si: 0.10 to 0.35%, Mn: 0.10 to 0.80%, P: greater than 0 and 0.03% or less, and S: greater than 0 and 0.03% or less, one or more of Ni: 0.25% or less (inclusive of 0), Cr: 0.30% or less (inclusive of 0) and Cu: 0.25% or less (inclusive of 0) and a remainder of Fe and other unavoidable impurities, and containing micropearlite structures having a cementite width greater than 0 and 0.2 pm or less and a cementite-to-cementite spacing greater than 0 and 0.5 pm or less. Furthermore, PTL 2 describes that, since hot-rolled high-carbon steel sheet has micro-pearlite structures, the final finished product can have durability and strength. [6] PTL 3 describes a high-strength steel sheet comprising a constituent composition consisting of, in mass %, C: 0.3 to 0.85%, Si: 0.01 to 0.5%, Mn: 0.1 to 1.5%, P: 0.035% or less, S: 0.02% or less, Al: 0.08% or less, N: 0.01% or less, Cr: 2.0 to 4.0% and a remainder of Fe and unavoidable impurities, and a microstructure containing rolled pearlite structures, wherein a ratio of the amount of dissolved C calculated by a predetermined formula is 50% or more. Furthermore, PTL 3 describes that, in accordance with the above high-strength steel sheet, excellent bending capacity and increased strength of a tensile strength of 1500 MPa or more can be achieved. [7] PTL 4 describes a method for producing thin-gauge steel sheet comprising the rough rolling of a continuous cast plate with a C content of 0.8% by mass or less to prepare a rough bar, and the finish rolling of the rough bar at a finishing temperature of (Ar 3 transformation point -20) °C or higher to prepare a steel strip, primary cooling of the steel strip after finish rolling to 500 to 800 °C at a cooling rate of more than 120 °C / sec, allowing the steel strip to cool for 1 to 30 seconds after primary cooling, secondary cooling of the steel strip after cooling at a cooling rate of 20 °C / sec or higher, and coiling of the steel strip after secondary cooling at a coiling temperature of 650 °C or lower. Furthermore, PTL 4 describes that, according to the above production method, a thin-gauge steel sheet with excellent workability, including draw flanging, is obtained, and that it has uniform mechanical properties of various strength levels. [8] PTL 5 describes a high-carbon mild steel sheet containing, by mass, C: 0.70 to 0.95%, Si: 0.05 to 0.4%, Mn: 0.5 to 2.0%, P: 0.005 to 0.03%, S: 0.0001 to 0.006%, Al: 0.005 to 0.10%, N: 0.001 to 0.01%, and a remainder of Fe and unavoidable impurities, and a microstructure having 100 or more voids per 1 mm² of the observed microstructure. Furthermore, PTL 5 describes that, with the above composition, it is possible to provide a high-carbon mild steel sheet with excellent stamping properties. Furthermore, to obtain the above high-carbon mild steel sheet, PTL 5 teaches a production method comprising cooling, rolling, and pickling a hot-rolled steel sheet under predetermined conditions, and then performing a softening box annealing. List of appointments Patent literature [9] PTL 1: Japanese Unexamined Patent Publication c^nonn / Lznz / E / YiAi No. 2016-098414 PTL 2: Publication of an unexamined Japanese patent No. 2011-530659 PTL 3: Publication of an unexamined Japanese patent No. 2011-099132 PTL 4: Publication of an unexamined Japanese patent No. 2001-164322 PTL 5: Publication of an unexamined Japanese patent No. 2011-012316 Summary of the invention Technical problem

[10] In PTL 1, a steel material containing no Cr or containing 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 this constituent composition and production method, the average lamellar spacing of the pearlite cannot necessarily be made small enough. Consequently, in the high-strength, high-ductility steel sheet described in PTL 1, there was still room for improvement in terms of enhancing the mechanical properties.

[11] The hot-rolled high-carbon steel sheet described in PTL 2, like the high-ductility, high-strength steel sheet described in PTL 1, contains no Cr or only a relatively small amount. Furthermore, PTL 2 states that, due to its micro-pearlite structure, the finished product can be given durability and strength, as explained above, but it does not disclose the specific tensile strength. Additionally, PTL 2 does not sufficiently examine the improvement of other mechanical properties, such as ductility and hole expansion capacity.

[12] PTL 3 describes a high-strength steel sheet having a tensile strength of 1500 MPa or more, but it does not sufficiently study the improvement of hole expansion capacity and other mechanical properties. In reality, the high-strength steel sheet described in PTL 3 is produced by preparing a billet with pearlite structures as its main phases through a pearlite-forming treatment in an annealing furnace, followed by cold rolling at a rolling speed of 90% or more. However, in the case of this production method, due to the prior cold rolling, a microstructure is formed with the directions of the cementite stratified within the pearlite aligned with the rolling direction.However, since this microstructure reduces the hole expansion capacity, with the high-strength steel sheet described in PTL 3, it is difficult to achieve a hole expansion capacity suitable for use as an automotive steel sheet. c^nonn / Lznz / E / YiAi

[13] Furthermore, in the manufacture of automotive parts, etc., stamping processes using press machines are often involved, but there is a particular problem that, if a high-strength steel sheet is stamped, the increased strength of the steel sheet easily leads to cracks (stamping cracks) on the stamped end faces. On the other hand, PTL 1 to 4 also do not sufficiently study the improvement of the stamping capacity of a high-strength steel sheet.

[14] In this regard, PTL 5 describes how it is possible to produce a high-carbon mild steel sheet with excellent stamping capabilities, as explained above. However, in PTL 5, box softening annealing is used as the heat treatment to obtain the high-carbon mild steel sheet, and therefore the carbides become spherical, preventing the formation of fine lamellar structures. Thus, with the high-carbon mild steel sheet described in PTL 5, there was still room for improvement in terms of mechanical properties.

[15] Therefore, the present invention aims to provide a hot-rolled steel sheet that is high-strength with a tensile strength of 980 MPa or more and that is excellent in ductility, hole expansion capability and stamping capability, and a method for producing the same by a novel configuration. Solution to the problem

[16] The inventors studied the chemical composition and microstructure of a hot-rolled steel sheet to achieve the above goal. As a result, the inventors discovered that it is important to make the structure of the hot-rolled steel sheet primarily pearlite, which has a good balance of strength and ductility, and also to properly control the microstructure of the pearlite.More specifically, the inventors discovered that by including pearlite in the hot-rolled steel sheet in an area ratio of 90% or more, it is possible to ensure ductility; on the other hand, by not including residual austenite, it is possible to ensure stamping capacity; and furthermore, by making the pearlite blocks (corresponding to the regions where the ferrite that forms the pearlite aligns in the crystal orientation) thinner, it is possible to suppress the appearance of cracks at the time of local deformation and ensure hole expansion capacity; and furthermore, by making the lamellar spacing of the pearlite finer while maintaining the pearlite fraction of 90% or more, it is possible to increase the strength of the hot-rolled steel sheet without diminishing ductility and hole expansion capacity, thus completing the present invention.Since increasing the strength of hot-rolled steel sheet by making the pearlite lamellar spacing finer is not related to improving ductility and hole expansion capacity, by controlling the structure in the above way, it is possible to obtain excellent ductility and hole expansion capacity even with greater strength.

[17] The present invention was completed based on the foregoing findings. Specifically, it is as follows: (1) A hot-rolled steel sheet comprising a chemical composition comprising, in % by mass, c^nonn / Lznz / E / YiAi C: 0.50 to 1.00%, 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: 0.50 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 to 0.0100%, Ca: 0 to 0.0050%, REM: 0 to 0.0050% and the remainder: Fe and impurities, and a metallic structure comprising, in area ratio, pearlite: 90 to 100%, pseudo-pearlite: 0 to 10%, and pro-eutectoid ferrite: 0 to 1%, wherein the pearlite has an average lamellar spacing of 0.20 μη or less, and the pearlite has an average pearlite block size of 20.0 pm or less. (2) Hot-rolled steel sheet in accordance with (1) above, wherein the chemical composition comprises, in % by mass, one or more of Cu: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Mo: 0.01 to 0.50%, Nb: 0.01 to 0.10%, V: 0.01 to 1.00%, and Ti: 0.01 to 1.00%. (3) Hot-rolled steel sheet in accordance with (1) or (2) above, wherein the chemical composition comprises, in % by mass, B: 0.0005 to 0.0100%. c^nonn / Lznz / E / YiA (4) Hot-rolled steel sheet in accordance with any of (1) to (3) above, wherein the chemical composition comprises, in % by mass, one or both of Ca: 0.0005 to 0.0050% and REM: 0.0005 to 0.0050%. (5) Hot-rolled steel sheet conforming to any of (1) to (4) above, wherein the hot-rolled steel sheet has a tensile strength of 980 MPa or more. (6) A method for producing a hot-rolled steel sheet comprising heating a plate having the chemical composition of any of the above (1) to (4) to 1100°C or more, hot rolling, including finish rolling of the heated plate, wherein the exit-side temperature of the finish rolling is 820 to 920°C, primary cooling the resulting steel sheet to a point Ael at an average cooling rate of 40 to 80°C / s, then secondary cooling the steel sheet from point Ael to a coiling temperature at an average cooling rate of less than 20°C / s, and coiling the steel sheet at a coiling temperature of 540 to 700°C. Advantageous effects of the invention

[18] In accordance with the present invention, it is possible to obtain a hot-rolled steel sheet that is high strength with a tensile strength of 980 MPa or more and that is excellent in ductility, hole expansion capability, and stamping capability. Brief description of the drawings

[19] Figures 1(a), 1(b) and 1(c) are reference views showing pearlite, pseudopearlite and proeutectoid ferrite. c^nonn / ίζηζ / Β / γίΛΐ Description of modalities

[20] Hot-rolled steel sheet The hot-rolled steel sheet according to one embodiment of the present invention comprises a chemical composition comprising, in % by mass, C: 0.50 to 1.00%, 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: 0.50 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 to 0.0100%, Ca: 0 to 0.0050%, REM: 0 to 0.0050% and the remainder: Fe and impurities, and a metallic structure comprising, in area ratio, pearlite: 90 to 100%, pseudo-pearlite: 0 to 10%, and pro-eutectoid ferrite: 0 to 1%, wherein the pearlite has an average lamellar spacing of 0.20 μη or less, and the pearlite has an average pearlite block size of 20.0 pm or less.

[21] First, the chemical composition of a hot-rolled steel sheet according to an embodiment of the present invention and a plate used for its production will be explained. In the following explanation, the % content units of the elements contained in the hot-rolled steel sheet and plate mean % by mass unless otherwise specified.

[22] C: 0.50% to 1.00% Carbon (C) is an essential element for ensuring the strength of hot-rolled steel sheet. To achieve sufficient strength, the C content is 0.50% or more. The C content can also be 0.53% or more, 0.55% or more, 0.60% or more, or 0.65% or more. On the other hand, if the C content is excessive, cementite precipitates, sometimes resulting in insufficient pearlite formation or decreased ductility or weldability. For this reason, the C content is 1.00% or less. The C content can also be 0.95% or less, 0.90% or less, 0.85% or less, 0.80% or less, or 0.75% or less. Furthermore, in hot-rolled steel sheet according to the embodiment of the present invention, the ratio, with respect to the total amount of C in the steel (C content), of the amount of dissolved C (C content less the amount of C that precipitates as cementite) is generally less than 50%.More specifically, if heavy work is done under a high rolling reduction in cold rolling, the amount of dissolved C sometimes increases, but in hot-rolled steel sheet according to the embodiment of the present invention where such cold rolling is not performed, the ratio of the amount of dissolved C is generally considerably less than 50%, for example, it is 30% or less. 20% or less, or 10% or less.

[23] Yes: 0.01 to 0.50% Silicon (Si) is an element used to deoxidize steel. However, if the Si content is excessive, chemical convertibility decreases and austenite remains in the microstructure of the steel sheet, thus impairing the steel sheet's stamping capacity. For this reason, the Si content is 0.01 to 0.50%. The Si content may also be 0.05% or more, 0.10% or more, or 0.15% or more, and / or it may be 0.45% or less, 0.40% or less, or 0.30% or less.

[24] Mn: 0.50% to 2.00% Manganese (Mn) is an effective element for delaying the phase transformation of steel and preventing it from occurring during mid-cooling. However, if the Mn content becomes excessive, micro- or macro-segregation can easily occur, impairing hole expansion capacity. For this reason, the Mn content is typically 0.50 to 2.00%. The Mn content can also be 0.60% or more, 0.70% or more, or 0.90% or more, and / or it can be 1.90% or less, 1.70% or less, 1.50% or less, or 1.30% or less.

[25] P: 0.100% or less The lower the phosphorus content, the better, but if it is excessive, it has a detrimental effect on formability and weldability and also causes a drop in fatigue properties; therefore, the content is 0.100% or less. Preferably, it is 0.050% or less, very preferably 0.040% or less, or 0.030% or less. The phosphorus content can also be 0%, but an excessive reduction invites an increase in costs, so the content is preferably 0.0001% or more.

[26] S: 0.0100% or less Sulfur (S) forms manganese sulfide (MnS), which acts as a starting point for fracture and causes a significant drop in the hole expansion capacity of the steel sheet. For this reason, the sulfur content is 0.0100% or less. The sulfur content is preferably 0.0090% or less, and very preferably 0.0060% or less or 0.0010% or less. The sulfur content can also be 0%, but excessive reduction leads to increased costs, so the content is preferably 0.0001% or more.

[27] Al: 0.100% or less Aluminum (Al) is used to deoxidize steel. However, if the Al content is excessive, inclusions increase and impair the workability of the steel sheet. For this reason, the Al content is 0.100% or less. The Al content can also be 0%, but it is preferably 0.005% or more, or 0.010% or more. Alternatively, the Al content can be 0.080% or less, 0.050% or less, or 0.040% or less.

[28] Ν: 0.0100% or less Nitrogen (N) in steel combines with aluminum (Al) to form AlN, which inhibits the growth of pearlite blocks due to a locking effect. However, if the N content becomes excessive, this effect saturates, and a decrease in toughness actually occurs. For this reason, the N content is 0.0100% or less. Preferably, the N content is 0.0090% or less, 0.0080% or less, or 0.0050% or less. From this perspective, it is not necessary to establish a lower limit for the N content. The content can also be 0%. However, reducing the N content to less than 0.0010% will increase steelmaking costs. Therefore, the N content is preferably 0.0010% or more.

[29] Cr: 0.50% to 2.00% Chromium (Cr) has the effect of making the pearlite lamellar spacing finer, thus ensuring the strength of the steel sheet. To achieve this effect, the lower limit for Cr content is 0.50%, preferably 0.60%. On the other hand, excessive Cr addition leads to the easy formation of structures such as pseudo-pearlite and bainite, making it difficult to obtain a pearlite fraction of 90% or more. For this reason, the upper limit for Cr content is 2.00%, 1.50%, or 1.25%, preferably 1.15%.

[30] The basic composition of the constituents of the hot-rolled steel sheet according to one embodiment of the present invention and the plate used for its production is as explained above. In addition, the hot-rolled steel sheet and plate may contain, if necessary, any of the following optional elements. The inclusion of these elements is not essential. The lower limits for the content of these elements are 0%.

[31] Cu: 0 to 1.00% Copper (Cu) is an element capable of dissolving in steel and improving its strength without compromising its toughness. The Cu content can be 0%, but additional Cu can be added as needed to achieve the desired effect. However, if the content is excessive, due to increased precipitation, microcracks may sometimes form on the surface during hot working. Therefore, the Cu content is preferably 1.00% or less, or 0.60% or less, very preferably 0.40% or less, or 0.25% or less. To achieve a sufficiently strong effect, the Cu content is preferably 0.01% or more, very preferably 0.05% or more.

[32] Ni: 0 to 1.00% Ni is an element that can be dissolved in steel to increase strength without reducing toughness. The Ni content can also be 0%, but Ni can be added as needed to achieve this effect. However, Ni is an expensive element. Excessive addition leads to increased costs. Therefore, the Ni content is preferably 1.00% or less, or 0.80% or less, very preferably 0.60% or less, or 0.30% or less. To sufficiently achieve this effect, the Ni content is preferably 0.10% or more, very preferably 0.20% or more. [ 33] Mo: 0 to 0.50% Molybdenum (Mo) is an element that increases the strength of steel. The Mo content can also be 0%, but Mo can be included as needed to achieve this effect. However, if the content is excessive, the decrease in toughness that accompanies the increase in strength becomes noticeable. Therefore, the Mo content is preferably 0.50% or less, or 0.40% or less, very preferably 0.20% or less, or 0.10% or less. To sufficiently achieve this effect, the Mo content is preferably 0.01% or more, very preferably 0.05% or more.

[34] Nb: 0 to 0.10% V: 0 to 1.00% Ti: 0 to 1.00% Nitrogen (Nb), vanadium (V), and titanium (Ti) contribute to improving the strength of steel sheets by precipitating carbides. Therefore, one of these elements may be included alone as needed, or two or more may be included in combination. However, if any of these elements is included in excess, a large amount of carbides will form, reducing the toughness of the steel sheet. For this reason, the Nb content is preferably 0.10% or less, or 0.08% or less, very preferably 0.05% or less; the V content is preferably 1.00% or less, or 0.80% or less, very preferably 0.50% or less, or 0.20% or less; and the Ti content is preferably 1.00% or less, or 0.50% or less, very preferably 0.20% or less, or 0.04% or less. On the other hand, the lower limit values ​​for the contents of Nb, V and Ti can be, for all elements, 0.01% or 0.03%.

[35] B: 0 to 0.0100% Boron (B) has the effect of segregating grain boundaries and increasing intergranular strength, so it can be included as needed. However, if the content is excessive, the effect becomes saturated and raw material costs increase. For this reason, 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. To sufficiently achieve the aforementioned effect, the B content is preferably 0.0005% or more, and very preferably 0.0010% or more.

[36] Ca: 0 to 0.0050% Calcium (Ca) is an element that controls the shape of non-metallic inclusions, which act as fracture starting points and cause deterioration in workability. It also improves workability, so it can be included as needed. However, if the content is excessive, the effect becomes saturated and raw material costs increase. For this reason, the Ca content is 0.0050% or less. The Ca content is preferably 0.0040% or less, or 0.0030% or less. To sufficiently achieve the aforementioned effect, the Ca content is preferably 0.0005% or more.

[37] REM: 0 to 0.0050% REM is an element that improves the toughness of the weld zone when added in small quantities. The REM content can also be 0%, but this can be included as needed to achieve the desired effect. However, if added in excess, weldability will deteriorate. For this reason, the REM content is preferably 0.0050% or less, or 0.0040% or less. To achieve a sufficiently strong effect, the REM content is preferably 0.0005% or more, and very preferably 0.0010% or more. It should be noted that REM is the general term for a total of 17 elements, including Se, Y, and the lanthanides. The REM content refers to the total amount of these elements.

[38] In hot-rolled steel sheet according to one embodiment of the present invention, the remainder, apart from the constituents explained above, is composed of Fe and impurities. The impurities are constituents, etc., that enter due to various factors in the production process, such as ore, scrap, and other similar raw materials when hot-rolled steel sheet is industrially produced.

[39] The reasons for the limitation of the structure of the hot-rolled steel sheet in accordance with an embodiment of the present invention will now be explained.

[40] Perlite: 90 to 100% By making the metallic structure of the steel sheet composed primarily of pearlite, it is possible to obtain a steel sheet that maintains high strength while also possessing excellent ductility and hole expansion capacity. If the pearlite is present in an area ratio of less than 90%, ductility and / or hole expansion capacity cannot be guaranteed due to irregularities in the structure. For this reason, the pearlite content in the metallic structure of hot-rolled steel sheet according to one embodiment of the present invention is an area ratio of 90% or more, preferably 95% or more, 96% or more, 97% or more, 98% or more, or 99% or more. It can also be 100%.

[41] Pseudoperlite: 0 to 10% Pro-eutectoid ferrite: 0 to 1% The remaining structure other than pearlite may be 0%, but if such a remaining structure is present, it is composed of at least one of either pseudopearlite or proeutectoid ferrite. By configuring the remaining structure from at least one of either pseudopearlite or proeutectoid ferrite, i.e., by not including residual austenite in the remaining structure, good stamping capability can be ensured. In the present invention, pseudopearlite means, unlike pearlite in which the ferrite and cementite phases are dispersed in a layered (lamellar) state, structures composed mainly of lumpy cementite, more specifically structures containing such cementite in the form of lumps in an area ratio of more than 50% with respect to the total amount of cementite in the structures, and may contain some lamellar cementite.Furthermore, in the present invention, pro-eutectoid ferrite means ferrite that precipitates as primary crystals in the cooling step after hot rolling and that substantially does not contain cementite, i.e., has a cementite fraction in the crystal grains with an area ratio of less than 1% (e.g., see the reference view in Figure 1(c)). It should be noted that pseudopearlite may be present in an area ratio of 0 to 10%, e.g., an area ratio of 8% or less, 6% or less, 4% or less, 3% or less, 2% or less, or 1% or less. Pro-eutectoid ferrite may be present in an area ratio of 0 to 1%, e.g., an area ratio of 0.8% or less or 0.6% or less.In a hot-rolled steel sheet according to an embodiment of the present invention, residual austenite, pro-eutectoid cementite, bainite, and martensite are either absent from or substantially absent from the metallic structure. Substantially absent means that the area ratios of these structures are, even in total, less than 0.5%. It is difficult to accurately measure the total amount of such fine structures. Moreover, their effects can be disregarded. Therefore, when the total amount of these structures is less than 0.5%, they can be considered absent. c^nonn / Lznz / E / YiAi

[42] Average lamellar spacing of perlite: 0.20 pm or less The average lamellar spacing of the pearlite (excluding the aforementioned pseudo-pearlite) is strongly correlated with the strength of the steel sheet. The smaller the average lamellar spacing, the greater the strength obtained. Furthermore, if the constituents are the same, the smaller the average lamellar spacing, the better the hole expansion capacity of the steel sheet. With an average lamellar spacing greater than 0.20 pm, a tensile strength of 980 MPa or more is not obtained and / or the hole expansion capacity decreases. Therefore, the average lamellar spacing of the pearlite in the hot-rolled steel sheet metal structure according to one embodiment of the present invention is 0.20 pm or less, preferably 0.15 pm or less, or 0.10 pm or less.It should be noted that the lower limit value of the average lamellar spacing of perlite is not particularly limited, but, for example, can be 0.05 pm or 0.07 pm.

[43] Average perlite block size: 20.0 pm or less A pearlite block corresponds to a region where the ferrite that forms the pearlite (excluding the pseudo-pearlite mentioned above) is aligned in a crystalline orientation. Here, the average pearlite block size correlates with the local ductility and toughness of the steel sheet. The smaller the average pearlite block size, the greater the hole expansion capability. With an average pearlite block size greater than 20.0 pm, the hole expansion capability deteriorates. Therefore, the average pearlite block size of the metal structure of the hot-rolled steel sheet according to one embodiment of the present invention is 20.0 pm or less, preferably 18.0 pm or less, and most preferably 16.0 pm or less. It should be noted that the lower limit of the average pearlite block size is not particularly restricted, but, for example, it could be 3.0 pm or 5.0 pm.0 pm or 7.0 pm.

[44] Method of judgment and method of measurement of perlite and remaining structure The pearlite fractions and remaining structure are determined as follows. First, samples are taken from positions 1 / 4 or 3 / 4 of the steel sheet surface thickness so that cross-sections parallel to the rolling direction and the steel sheet thickness become the observed surfaces. These observed surfaces are then polished to a mirror finish, etched with a picral etchant, and subsequently examined for structure using a scanning electron microscope (SEM). The magnification is 5000X (measurement region: 80 pm x 150 pm). From the resulting structural image, using the point calculation method, regions where cementite forms layers are judged to be pearlite (e.g., see reference view in Figure 1(a)), and the pearlite fraction is calculated.On the other hand, structures in which the ferrite and cementite phases are not layered but are composed mainly of clumpy cementite are judged to be pseudopearlite (e.g., see the reference figure in Figure 1(b)), and a fraction thereof is calculated. Furthermore, lattice-like crystal grain assemblies containing a plurality of iron-based carbides with principal axes of 20 nm or more within the lattice, and where these carbides belong to groups of iron-based carbides of simple variants (i.e., drawn in the same directions), are judged to be bainite. Additionally, clumped or film-like regions of iron-based carbides with equivalent circle diameters of 300 nm or more are considered pro-eutectoid cementite. In the case of structures such as in Figure 1(a) or 1(b), the observed inclusions are basically cementite.It is not necessary to use a scanning electron microscope (SEM-EDS) equipped with an energy-dispersive X-ray spectroscope, etc., to identify individual inclusions as cementite or iron-based carbides. SEM-EDS, etc., can be used to analyze inclusions independently of SEM examination, as it is only required when there is doubt as to whether they are cementite or iron-based carbides. Both pro-eutectoid ferrite and residual austenite have less than 1% cementite area fractions within them. If these structures, after examination by c^nonn / Lznz / E / YiAi, SEM, electron backscatter diffraction (EBSD) is used for analysis and bcc structures are judged to be pro-eutectoid ferrite and fcc structures are judged to be residual austenite.

[45] Method for measuring average laminar spacing The average lamellar spacing is determined as follows: First, samples are taken from positions 1 / 4 or 3 / 4 of the surface thickness of the steel sheet so that the cross-sections are parallel to the rolling direction, and the direction of the steel sheet thickness becomes the observed surfaces. Next, the observed surfaces are polished to a mirror finish, etched with a picral etcher, and then examined for structure using a scanning electron microscope (SEM). The magnification is 5000X (measurement region: 80 pm x 150 pm). Ten or more locations are selected where the cementite layer vertically penetrates the surface of the structural photograph paper. Information on the depth direction is obtained by measuring etching with a picral etcher, thus identifying the locations where the cementite layer penetrates vertically.By selecting measurements from 10 or more of these locations, the lamellar spacings S are found at the respective locations. The average of these measurements is then taken to obtain the average lamellar spacing. The method for measuring lamellar spacing at individual locations is as follows: First, a vertical line is drawn through the cementite layers to cut through 10 to 30 cementite layers. The lengths of the lines are given by L. The number of cementite layers that cross this line is defined as N. At this point, the lamellar spacing S at that location is found by S = L / N.

[46] Method for measuring the average size of the perlite block The average size of the pearlite block is measured using EBSD. First, samples are taken from positions 1 / 4 or 3 / 4 of the steel sheet surface thickness so that cross-sections parallel to the rolling direction and the steel sheet thickness direction become the observed surfaces. Next, the observed surfaces are polished to a mirror finish, EBSD is used to measure the iron crystal orientation, and the crystal grain boundaries are located. A crystal grain boundary is defined as a boundary where the crystal orientation changes by 15°. The measurement region is 100 pm x 200 pm, and the distance between measurement points is 0.2 pm per step. Finally, the equivalent diameter of the circle is found within the area of ​​the region enclosed by the crystal grain boundaries.The average value of the equivalent circle diameters calculated for all crystal grains in the measurement region by the area fraction method is defined as the average size of the pearlite block. c^nonn / Lznz / E / YiAi

[47] Mechanical properties According to the hot-rolled steel sheet with the above chemical composition and structure, high tensile strength can be achieved, specifically a tensile strength of 980 MPa or more. A tensile strength of 980 MPa or more is required to meet the demand for lighter car bodies. A tensile strength of 1050 MPa or more is preferred, and 1100 MPa or more is highly preferred. The upper limit does not need to be specifically prescribed, but, for example, the tensile strength could be 1500 MPa or less, 1400 MPa or less, or 1300 MPa or less. Similarly, according to the hot-rolled steel sheet with the above chemical composition and structure, high ductility can be achieved, more specifically an overall elongation of 13% or more, preferably 15% or more, and highly preferred 17% or more.The upper limit value does not have to be specifically prescribed, but, for example, the total elongation can be 30% or less, or 25% or less. Furthermore, depending on the hot-rolled steel sheet having the above chemical composition and structure, excellent hole expansion capacity can be achieved; more specifically, a hole expansion capacity of 45% or more, preferably 50% or more, and very preferably 55% or more, can be achieved. The upper limit value does not have to be specifically prescribed, but, for example, the hole expansion capacity can be 80% or less, or 70% or less. Tensile strength and total elongation are measured by taking a JIS No. 5 tensile test specimen from a direction perpendicular to the rolling direction of the hot-rolled steel sheet and subjecting it to a tensile test based on JIS Z2241 (2011).On the other hand, hole expansion capacity is measured by means of a hole expansion test modality based on JIS Z2256 (2010). c^nonn / ίζηζ / Β / γίΛΐ

[48] ​​Thickness Hot-rolled steel sheet according to one embodiment of the present invention generally has a thickness of 1.0 to 6.0 mm. Although not particularly limited, the thickness may be 1.2 mm or more, or 2.0 mm or more, and / or 5.0 mm or less, or 4.0 mm or less.

[49] Method for producing hot-rolled steel sheet The method for producing a hot-rolled steel sheet according to one embodiment of the present invention comprises heating a plate having the chemical composition explained above to 1100°C or more, hot rolling, including finish rolling of the heated plate, wherein the temperature of the exit side of the finish rolling is 820 to 920°C, Cool the resulting steel sheet by primary quenching to a point Ael at an average cooling rate of 40 to 80°C / s, then cool the steel sheet by secondary quenching from point Ael to a rolling temperature at an average cooling rate of less than 20°C / s, and roll the steel sheet at a rolling temperature of 540 to 700°C. Each step will be explained in detail below.

[50] Plate heating First, a plate with the chemical composition described above is heated before rolling in a hot furnace. The plate is heated to 1100°C or higher to sufficiently redissolve the Ti carbonitrides, etc. The upper limit is not specifically prescribed, but could be, for example, 1250°C. Similarly, the heating time is not strictly limited, but could be, for example, 30 minutes or more, or 120 minutes or less. Note that, from a productivity standpoint, the cast iron is preferably produced by continuous casting, but can also be produced by ingot casting or thin-plate casting.

[51] Hot rolled Rough laminated In the present method, for example, the heated plate can be rough-rolled before the final rolling to adjust the thickness, etc. Rough rolling is not particularly limited in terms of conditions as long as the desired dimensions of the sheet bar are ensured.

[52] Finish laminate The heated plate, or the plate additionally subjected to finish rolling as required, is then subjected to finish rolling. The exit-side temperature of the finish rolling is controlled to 820 to 920°C. If the exit-side temperature of the finish rolling exceeds 920°C, the austenite becomes coarser, and the average pearlite block size requirement of the final finished product (i.e., 20.0 pm or less) is no longer met. For this reason, the upper limit for the exit-side temperature of the finish rolling is 920°C, preferably 900°C, and most preferably 880°C. From this perspective, it is not necessary to provide a lower limit for the exit-side temperature of the finish rolling as long as it is at or above the Ar3 point, but the lower the temperature, the greater the resistance to deformation of the steel sheet.A heavy load is applied to the laminating machine and can lead to equipment problems. For this reason, the lower limit for the output temperature of the finished laminate is 820°C.

[53] Cooling Once the final rolling process is complete, the steel sheet is cooled. This cooling process is further subdivided into primary cooling and secondary cooling.

[54] Primary cooling to point Ael with an average cooling rate of 40 to 80°C / s In primary cooling, the steel sheet is cooled from the previous exit-side temperature of the finish roll at an average cooling rate of 40 to 80°C / s to the Ael point. If the average cooling rate to the previous temperature is less than 40°C / s, pro-eutectoid ferrite and / or pro-eutectoid cementite precipitates, and the previous target pearlite fraction (90% or more) is likely to be impossible to achieve. The average cooling rate of primary cooling can be 43°C / s or 45°C / s. On the other hand, if the average cooling rate is too high, the steel sheet can no longer cool uniformly, and variations in quality are likely to occur. Therefore, the average cooling rate of primary cooling can be 80°C / s or less. For example, it is 70°C / s or less. It should be noted that Ael (°C) can be found using the following formula: Ael (°C) = 723 to 10.7 x [Mn] + 29.1 x [Si] where, in the formula, the symbols of the elements in parentheses respectively show the content of the elements in % by mass.

[55] Secondary cooling from point Ael to the winding temperature by means of an average cooling rate of less than 20°C / s Next, in secondary cooling, the steel sheet is cooled from point Ael to the rolling temperature (i.e., the 540 to 700°C temperature range) at an average cooling rate of less than 20°C / s. By making the cooling rate slower than the primary cooling in this way, it is possible to form more random pearlite structures in the lamellar direction and to achieve finer lamellar spacing, thus improving hole expansion capacity. On the other hand, if the average cooling rate to this temperature range is high, the lamellar spacing becomes uneven within the steel sheet, and the hole expansion capacity is likely to deteriorate, or a large amount of pseudo-pearlite will form, making it unlikely that the target pearlite fraction (90% or more) can be achieved.Therefore, the average cooling rate of the preceding secondary cooling is less than 20°C / s and is preferably 15°C / s or less, very preferably 10°C / s or less. Secondary cooling is preferably performed immediately after the end of primary cooling to reliably suppress ferrite formation. c^nonn / Lznz / E / YiAi

[56] Winding After cooling, the steel sheet is rolled up The temperature of the steel sheet during rolling is between 540 and 700°C. By controlling the rolling temperature to between 540 and 700°C, it is possible to ensure the structure transforms appropriately during rolling, resulting in a finer average pearlite lamellar spacing and, consequently, higher strength in the hot-rolled steel sheet without compromising ductility and hole expansion capacity. On the other hand, if the rolling temperature is below 540°C, other structures such as pseudo-pearlite and bainite appear, making it difficult to guarantee a pearlite fraction of 90% or higher. Therefore, the rolling temperature is 540°C or higher, and can also be 550°C or higher, or even 600°C or higher. Furthermore, if the winding temperature is over 700°C, the average lamellar separation of the pearlite becomes the largest and sufficient force and / or the hole expansion capacity can no longer be assured.Therefore, the winding temperature can be 700°C or less, 680°C or less, or 650°C or less. Post-winding conditions are not particularly restricted.

[57] The following examples will be used to explain the present invention in more detail, but the present invention is not limited by these examples in any way. c^nonn / Lznz / E / YiAi Examples

[58] In the following examples, hot-rolled steel sheets were produced in accordance with an embodiment of the present invention under various conditions, and the mechanical properties of the resulting hot-rolled steel sheets were investigated.

[59] First, the continuous casting method was used to produce plates with the chemical compositions shown in Table 1. Then, the heating, hot rolling, cooling, and coiling conditions shown in Table 2 were used to produce 3 mm thick hot-rolled steel sheets from these plates. Secondary cooling in the quenching step was performed immediately after the end of primary cooling. It should be noted that, in addition to the components shown in Table 1, the remainder consists of Fe and impurities. Furthermore, the chemical compositions obtained by analyzing samples taken from the produced hot-rolled steel sheets were the same as the chemical compositions of the plates shown in Table 1. Additionally, in the hot-rolled steel sheets of all examples, the proportions of dissolved C were 10% or less.

[60] Table Ae1 p'C] £ Ll¿ £ 726 725 719 721 751 752 715 706 or EN 716 716 717 EN (X > > 1 i 4 < i < < < • > 0.0030 ><D O > i 0.0035 i 4 < < gs * > > > ) CÜ > > 1 i 08000 ii < 1 0.0087 0.0084 > > > I > < 0.90 > i •1 1 4 < zoo < 017 < < > i 0.19 > 4 • > 1 < > >060 Nb < t 800 i • ij 0 02 > 1 0.03 < l < > 4 O. .§ Qj Mo ( » i J i 4 > l 1 • • < o • 4 OW • i 4 i 4 < > 0.79 > > ) i (3⁄4 in mass, the Cü • i 8 • • - 1 > í. 1.58 0.92 0.55 2.03 0.70 gold 05 Oj 1.21 1.50 0.80 180 08'0 0.79 09'0 ion chemistry οε0οοο 0.0089 0.000 0.00 0 0 0.0031 0.0030 ιεοοο 0.0030 goo 0.0086 00031 0.0032 0.0031 0.0028 0.0027 □isaduio < 010ο0 0.0087 s 0.010 0.030 1 0.030 0.030 o «> 0.086 0.030 0.031 0.030 0.029 0.031 ω σι oooo 0.0020 0 0087 <o o o © Z1000 00006 0.0008 01000 0.0020 0.0020 § 0.0014 ozooo 0.0021 0,0020 0.0029 οεοοο Q. 010Ό 0.070 0.092 6100 0.011 6000 0.010 0.010 010 0 0.010 0.018 880Ό 0.009 O o £100 010Ό 0100 Mn 960 00 EZT 990 101 0.97 8C1 0.70 o! <Tj 035 04 IO o G3i 1.30 ιει 0¿0 0.70 i¿0 A 0.20 0.27 0.34 0.45 0.21 0.20 0.20 πτ m 11Ό 610 εοο 0.06 zoo 0.03 0.07 o 0.71 0.76 0.64 0 54 0.94 0.70 3 0.70 F<T0 © .1,0.5 0.72 69'0 0.70 0.70 0.71 0.52 Tipo o S g «3 < ω O O LU uj Tj -4 ξ: z: O o. <3. £ ω and Ό w <0 Home £ S c^nonn / Lznz / E / YiAi

[61] Table <N Winding | Winding temperature [°C] 26 | | 079 710 | 1009 | 099 | 079 | 029 640 | | 089 | 079 | 80 | 1079 | 079 | 099 400 | 540 | | 029 540 | | 029 620 | 099 640 | 00 640 | 1099 | Cooling | Secondary cooling | Average cooling rate p°C / s 00 00 03 <200 ...<n DO 5 -40 o> ro sr £ 5 5 5 s 04 in 04 Lfb 04 U1 | Hot Rolling j Finishing Roll Outlet Side Temperature ['C] 1 098 ¡ | £28 | ¡ 852 ¡ 860 | 016 j | 840 j | mi 880 | 848 I 905 198 ¡ 848 j [ 978 j 951 | 902 ¡ ¿48 ¡ i 875 ¡ I 706 088 883 188 880 288 879 ¡ 188 ¡ | Heating | Heating Time [min] O •oo O «Jó soo § O <a 5 O O s© a O Ό O •-G 5 'O 0 Ό O o a o O *0 5 Temperatura de calentamiento ÍC] 1200 | 1200 | | 1200 | 1200 | | oozi | 1200 | | oozi 1250 ¡ 1250 | 1 0971 1 1250 | | 1200 I | oozi 1200 | 1 oozi 1 oozi I 1200 I | oozi |1250 | | 1250 | | 0971 j | 1250 II 0921 1250 | 1250 ¡ Type of steel < < < c < < £Ω OO 113 w tpi 33 -t “>l X} -1 szo Class <3 Test No. - 04 ob LT? O CO co o - 04 S y LÓ £ «3 O o w £ c^nonn / Lznz / E / YiAi

[62] A JIS No. 5 tensile test piece was taken from each of the hot-rolled steel sheets thus obtained in a direction perpendicular to the rolling direction and subjected to a tensile test based on JIS Z2241 (2011) to measure tensile strength (TS) and total elongation (El). In addition, it was subjected to a hole expansion test based on JIS Z2256 (2010) to measure hole expansion capacity (λ). Stamping capacity was evaluated by punching a 10 mm diameter hole with a punching clearance of 12.5%, visually examining the end face properties, judging the case where a crack of 0.5 mm or more in size was observed on the end face as a failure (Poor), and judging the case where no crack was observed as a pass (Good).In cases where TS is 980 MPa or higher, El is 13% or higher, λ is 45% or higher, and the stamping capacity is rated as passing, the product is evaluated as a hot-rolled steel sheet with high strength, excellent ductility, hole expansion capacity, and stamping capacity. The results are shown in Table 3 below.

[63] Table Observations | Example | | ojúuiélg j Comp. Example comp. Example comp. 1 [ Example as. I | Comp. Example 1 | Example 1 1 Example | or E If w | Example 1 Comp. Example comp. | Comp. Example | | Comp. Example | | Comp. Example | Comp. Example | Comp. Example| | o|d«jal3 | | Example | QE If Lu | Example | Example Example Example m ro c 'Λ5 E m tu T3 re ó Stamping capacity | Good | | Good | Good Good I Good | I Good | | euang | I Good 1 1 Good I r5 C ¢1 ce I Good i Good Good | euang | | Hala | JS: X; Good | eueng | | Good | | Good | | Good | | Good | Good Good ra c Yes 3 m E co $ £7 a K ca <0 OÍ o -tf $ CT A 10 <íj «yes 40 o 10 o L0 10 CG The l%] in oo 03 $ a 10 03 03 £ O 00 £ a 01 3 £ Ό 10 in 10 TS [MPa] ¡ 1112 j [ 1148 ¡ 1080 I 1209 I | Z9EI ] 1 1040 IL J2?6 J 03 Oo ! 986 [ 1223 ! 1353 TOS l 233 ! 1184 1043 § | 1280 I [ via ¡ [ 1197 ¡ | 1076 I [ 1095 j ÍXU 11132 1281 Έ ε Φ Ό «3 gw □ w ω Average pearlite block size [im] I tu I | 154 u.® 18.2 I 19.5 ! I tai II 2M| 1 12 6 ! I______________10.6 | 1 m 1 TÍ in >> 18.6 I 30.6 I 15.2 ! 5Ή I ¡ 6-41 1 1 12.6 I | 13 8 ! I is.i ! | 14.8 S 11.6 SOI Average laminar spacing b <m] I oro oto I a UO | 400 í í 007 I | £10 ! 0.06 1 i 0.08 I i 0.12 | ί 0.11 I 0.07 | 0.25 3 | 81'0 0.20 I Ζ1Ό 0.08 | | Ó00 | gold \ 0.14 I 0.14 | SIO no 600 Remaining structure [54 de áreel Pseudoperlita: 9 I Pseudoperüta: 4 I Oí 10 Te g A <y TS ‘5 ·§ 3 P cu £0 o! *S 2S §2 1 s «í g ,S rí hf tu ce αχ w Cl; o Λ ώ § Ό <υ ÚL ¡ Pseudopertita; 61 10 O CL O O 3 O ιΛ Cl hj fe 0 0 3 es UJ c_ Pseudoperlfta: 5 I ΙΛ rg fe O 3 O! Vj Cl Pseudoperlha y feainita:_taia_L48. Pseudepetifta y ferriu.pro-.eutecLoi^^ ί Pseudoperlita: 9 | c5 xx ‘íw i ΙΛ A us í en ita. r es i d ya l;„6 | c > S Έ 1 s Sr 3 3 Λ 2X C & u üS e Ot «τ •Ξ S cu PssudQB^rlíiaiJl 1 ί Pseudoperlite; 2 | Pseudoperlite: 3 I QJ 0 35 tu •J1 CL s fe o Ό 3 ω w CL Pseudoperlite: 6 Pro-eutectoid ferrite: 1 Pseudoperlite. 7 Pro-eutectoid ferrite: 1 Pseudo-perlite: 8 Pro-eutectoid ferrite: 1 Perlite fraction [% of area] *C Yes roí 7X 9 10 o <1 cr* ΙΛ 10 o tí 80 coi coi 03 & 5 sG OO θ' Γ7 5: Espesor M 40! yes? 1 2.5 2.5 [......?:5.......¡ 1 2.5 ¡ 40 CSÍ 10 c< L 25 J 10 σί ! sz | 2.5 2.5 I 2.5 ¡ 1 2·5 1 40 e7 2.5 1 2.5 ¡ I 2.5 ¡ 1 2.5 ¡ I 2.5 ¡ 10 CM 2.5 CM 8S Steel type < «í < < < < < en o O LiJ LU eí 33 -i: u 2 O o- O C5 a Test No. - -I 40 o o& o O - CM en 10 £ £ OQ £ CM CN Γ7 OM 10. c^nonn / Lznz / E / γΐΛΐ

[64] As will be clear from Table 3, in each of Examples 1, 2, 8 to 11 and 19 to 25, the tensile strength was 980 MPa or more and El was 13% or more, λ was 45% or more, and the stamping capability was evaluated as passed, so it was possible to obtain a hot-rolled steel sheet of high strength and excellent ductility, hole expansion capability and stamping capability.

[65] In contrast to these, in Comparative Example 3, the winding temperature was over 700°C, so the average pearlite lamellar spacing increased to more than 0.20 pm. For this reason, a TS of 980 MPa or more and a λ of 45% or more could not be achieved. In Comparative Example 4, the average cooling rate of the primary cooling step was less than 40°C / s, pro-eutectoid ferrite formed in large quantities, and the pearlite fraction became less than 90%. For this reason, a λ of 45% or more could not be achieved. In Comparative Example 5, the average cooling rate of the secondary cooling was high, so pseudo-pearlite increased, and the pearlite fraction became less than 90%. For this reason, a λ of 45% or more could not be achieved.In Comparative Example 6, the winding temperature in the winding pass was less than 540°C, so the pseudo-pearlite increased and the pearlite fraction became less than 90%. For this reason, an El of 13% or more and a λ of 45% or more could not be achieved. In Comparative Example 7, the exit-side temperature of the finishing roll in the hot rolling pass was more than 920°C, so the pearlite blocks became thicker and the average pearlite block size reached more than 20.0 pm. For this reason, a λ of 45% or more could not be achieved.

[66] In Comparative Example 12, the Cr content was high, so pseudopearlite increased, bainite entered, and the pearlite fraction fell below 90%. For this reason, an El of 13% or more and a λ of 45% or more could not be achieved. In Comparative Example 13, the C content was low, so a TS of 980 MPa or more could not be achieved. In Comparative Example 14, the Cr content was low, so a TS of 980 MPa or more could not be achieved. Furthermore, in Comparative Example 14, the exit temperature of the finish roll in the hot rolling pass was over 920°C, so the average pearlite block size ended up being over 20.0 pm, and a λ of 45% or more could not be achieved. In comparative examples 15 and 16, the Si content was excessive, so residual austenite entered the remaining structure and the stamping ability became defective.In Comparative Example 17, the C content was high, so proeutectoid cementite entered the remaining structure and the pearlite fraction fell below 90%. For this reason, an El of 13% or more and a λ of 45% or more could not be achieved. In Comparative Example 18, the Mn content was high, so a λ of 45% or more could not be achieved.

Claims

1. A rolled steel sheet comprises a chemical composition comprising: C: 0.50 to 1.00%, 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: 0.50 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 to 0.0100%, Ca: 0 to 0.0050%, REM: 0 to 0.0050% and the remainder: Fe and impurities, and a metallic structure comprising of area, pearlite: 90 to 100%, hot that in % by mass, in relation pseudo-pearlite: 0 to 10%, and pro-eutectoid ferrite: 0 to 1%, wherein the pearlite has an average lamellar spacing of 0.20 pm or less, and the pearlite has an average pearlite block size of 20.0 pm or less.

2. The hot-rolled steel sheet according to claim 1, wherein the chemical composition comprises, in % by mass, one or more of Cu: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Mo: 0.01 to 0.50%, Nb: 0.01 to 0.10%, V: 0.01 to 1.00%, Ti: 0.01 to 1.00%, B: 0.0005 to 0.0100%, Ca: 0.0005 to 0.0050% and REM: 0.0005 to 0.0050%.

3. The hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet has a tensile strength of 980 MPa or more.

4. The hot-rolled steel sheet according to claim 2, wherein the hot-rolled steel sheet has a tensile strength of 980 MPa or more.

5. A method for producing a hot-rolled steel sheet comprising heating a plate having the chemical composition 5 of claim 1 or 2 to 1100°C or more, hot rolling, including finishing rolling the heated plate, wherein the exit temperature of the finishing roll is 820 to 920°C, cooling the resulting steel sheet by primary cooling to a point Ael at an average cooling rate of 40 to 80°C / s, then cooling the steel sheet by secondary cooling from point Ael to a coiling temperature at an average cooling rate of less than 20°C / s, and coiling the steel sheet at a coiling temperature of 540 to 700°C.