JP2015190015A - High strength hot rolled steel sheet and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet excellent in bendability and low temperature toughness and a manufacturing method therefor.SOLUTION: There is provided a high strength hot rolled steel sheet excellent in bendability and low temperature toughness by making a composition containing, by mass%, C:0.020% to 0.080%, Si:0.05% to 0.50%, Mn:1.20% to 2.20%, P:0.001% to 0.020%, S:0.0001% to 0.0050%, Al:0.005% to 0.050%, N:0.0010% to 0.0060%, Nb:0.040% to 0.080%, Mo:0.01% to 0.50%, Ti:0.005% to 0.050%, Cr:0.01% to 0.50%, Ca:0.0005% to 0.0050% and the balance Fe with inevitable impurities, and a structure having an area ratio of a tempered martensitic phase of 90% to 100% and an average crystal particle diameter of the tempered martensitic phase of 1.0 μm to 5.0 μm at a position of 1.5 mm from a surface of a steel sheet in a sheet thickness direction, a fluctuation amount of a Vickers hardness in a sheet width direction ΔHv of 50 or less, the area ratio of a bainitic phase of 90% to 98%, the area ratio of a martensitic phase of 1% to 5%, and the average crystal particle diameter of the martensitic phase of 0.5 μm to 5.0 μm and the area ratio of a retained austenitic phase of 1% to 5% at a sheet thickness center position of the steel sheet and a thickness of an iron oxide coating of the surface of the steel sheet of 0.1 μm to 10 μm.

Description

  The present invention relates to a high-strength hot-rolled steel sheet excellent in the balance of bending, low-temperature toughness and strength, and a method for producing the same, suitable as a material for steel pipes used in the fields of pipelines, oil well pipes, civil engineering and construction.

  In recent years, from the viewpoint of cost reduction, there is a growing need for ERW steel pipes and spiral steel pipes made of hot-rolled steel sheets instead of UOE steel pipes made of thick plate materials. In addition, with the increase in size of structures, there is an increasing need for increasing the strength, diameter, and thickness of steel pipe materials. Furthermore, low temperature toughness is required when the steel pipe is used in a cold region, and bendability is required when it is deformed during or after laying. Under such circumstances, many studies have been made on high-strength hot-rolled steel sheets for steel pipes having both low temperature toughness and bendability, and various techniques have been proposed.

  For example, Patent Document 1 includes mass%, C: 0.05 to 0.15%, Si: 0.05 to 0.5%, Mn: 1.0 to 2.0%, P: 0.02% or less, S: 0.01% or less, Al: 0.005 to 0.045. %, Nb: 0.005 to 0.08%, and the steel slab composed of the remaining iron and inevitable impurities is heated, hot-rolled and cooled under predetermined conditions, so that the entire structure at the 1/4 thickness position can be obtained. A technique for producing a high-strength steel sheet having a bainite area ratio of 80% or more has been proposed. Patent Document 1 describes that a high-strength steel sheet having an excellent balance between strength and low-temperature toughness can be obtained by forming a steel sheet structure having a bainite fraction of 80% or more.

  Further, Patent Document 2 relates to a high-strength hot-rolled steel sheet in terms of mass%: C: 0.08 to 0.25%, Si: 0.01 to 1.0%, Mn: 0.8 to 2.1%, P: 0.025% or less, S: 0.005% or less. , Al: 0.005 to 0.10%, a composition comprising the balance Fe and inevitable impurities, a bainite phase or a tempered martensite phase as a main phase, and a cross section in which the average grain size of prior austenite grains is parallel to the rolling direction And a structure having a structure of 20 μm or less and 15 μm or less in a cross section perpendicular to the rolling direction has been proposed. Patent Document 2 discloses a high-strength hot-rolled steel sheet having excellent bendability and low-temperature toughness by controlling the average grain size of prior austenite grains as described above, with the bainite phase or tempered martensite phase as the main phase. Is obtained.

  Further, in Patent Document 3, in mass%, C: 0.02 to 0.08%, Si: 0.01 to 0.50%, Mn: 0.5 to 1.8%, P: 0.025% or less, S: 0.005% or less, Al: 0.005 to 0.10 %, Nb: 0.01 to 0.10%, Ti: 0.001 to 0.05%, and C, Ti and Nb are contained so as to satisfy (Ti + (Nb / 2)) / C <4, the balance being Fe and inevitable After heating and hot rolling a steel material composed of impurities, the average cooling rate at a position 1 mm from the surface to the plate thickness direction is over 80 ° C / s, and the position from the surface to the plate thickness direction is 1 mm. At least twice the cooling process consisting of the first stage cooling that cools to the cooling stop temperature in the temperature range below the Ms point and the second stage cooling that performs air cooling for 30 s or less. Thickness of 11mm or more is obtained by sequentially applying the third stage of cooling at an average cooling rate of 1mm in the direction at a temperature exceeding 80 ° C / s to the specified cooling stop temperature and winding at the specified winding temperature. Made of hot rolled steel sheet A technique has been proposed.

  Further, in Patent Document 3, by producing a hot-rolled steel sheet according to the above method, the structure at a position of 1 mm in the thickness direction from the surface is a tempered martensite single phase structure or a mixed structure of bainite and tempered martensite. The structure at the center of the sheet thickness is bainite and / or bainitic ferrite as the main phase, and has a structure composed of a second phase of 2% or less by volume. The surface is 1 mm from the surface in the sheet thickness direction. It is described that a thick high-tensile hot-rolled steel sheet having a difference ΔHV between the Vickers hardness HV 1 mm at the position and the Vickers hardness HV1 / 2t at the center position of the sheet thickness of 50 points or less can be obtained. Furthermore, Patent Document 3 discloses a thick-walled high-tensile hot-rolled steel sheet having a thickness of 11 mm or more and excellent low-temperature toughness by making the structure of the hot-rolled steel sheet uniform in the thickness direction as described above. It is described that it is obtained.

JP2013-7101A JP 2013-1117068 A JP 2010-196164 A

However, any of the above prior arts is a high-strength hot-rolled steel sheet suitable as a material for steel pipes, that is, high strength and excellent low-temperature toughness, and further deformed due to molding conditions during pipe making or crustal deformation after laying. It is extremely difficult to obtain a high-strength hot-rolled steel sheet that has sufficient bendability to withstand.
In the technique proposed in Patent Document 1, although a high-strength hot-rolled steel sheet excellent in low-temperature toughness can be obtained, there is a problem in securing workability, particularly bendability. Specifically, at the time of bending, there is a concern that the outermost layer side of the steel plate is greatly deformed and cracks are generated.

  In the technique proposed in Patent Document 2, the C content of the hot-rolled steel sheet is large, and low temperature toughness and weldability may not be ensured. Moreover, the tensile strength TS of the hot-rolled steel sheet is excessively high, and sufficient bendability cannot be obtained. In the technique proposed in Patent Document 3, the manufacturing process of the hot-rolled steel sheet, particularly the cooling process after hot rolling becomes complicated, and there is a problem in mass production stability. In addition, the improvement in the bendability of the steel sheet contributes greatly to the uniformity of the surface layer characteristics rather than the uniformity of the characteristics in the thickness direction, but in the technique proposed in Patent Document 3, the coil width of the hot-rolled steel sheet, In the longitudinal direction, there is a concern about characteristic variation due to uneven cooling. Therefore, the technique proposed in Patent Document 3 has a problem in ensuring the bendability of the hot-rolled steel sheet.

  The present invention solves the above-mentioned problems of the prior art, and is a hot-rolled steel sheet suitable as a steel pipe material, that is, a high-strength hot-rolled steel sheet having high strength and excellent low-temperature toughness, and also excellent bendability, and its An object is to provide a manufacturing method.

The present inventors diligently studied means for significantly improving the bendability and the low temperature toughness while ensuring the strength of the hot rolled steel sheet.
As described above, to improve the bendability of the hot-rolled steel sheet, it is extremely important to make the surface layer characteristics uniform. Moreover, in order to improve the bendability of the hot-rolled steel sheet, it is necessary to soften the surface layer to some extent. However, when manufacturing a hot-rolled steel sheet, cooling unevenness occurs in the cooling step after hot rolling, and characteristic variations due to cooling unevenness occur in the sheet width direction and the longitudinal direction. Moreover, since the steel sheet surface layer is more easily cooled than the plate thickness center position, martensite is generated and the hardness is likely to increase.

  Therefore, the present inventors first studied a means for suppressing the above-mentioned cooling unevenness to reduce the characteristic variation of the steel sheet surface layer and to suppress the increase in hardness. As a result, by controlling the hot rolling and cooling conditions and keeping the thickness of the iron oxide film on the steel sheet surface to 10 μm or less and making the thickness uniform, the coil width and the longitudinal direction of the hot rolled steel sheet can be uniformly cooled. It became possible, and the knowledge that uniform hardness characteristics were obtained in the steel sheet surface layer was obtained. In addition, by controlling the hot rolling and cooling conditions and making the steel sheet surface layer fine tempered martensite, it was found that the increase in the hardness of the steel sheet surface layer is suppressed and the bendability is improved without accompanied by a decrease in toughness. . Furthermore, it has been clarified that by utilizing the retained austenite inside the steel plate, the ductility is improved and the bendability of the hot-rolled steel plate is further improved.

  On the other hand, in order to ensure the low temperature toughness of the hot-rolled steel sheet, it is effective to make the inside of the steel sheet have a structure mainly composed of bainite having an excellent strength-toughness balance. In order to increase the strength of a hot-rolled steel sheet, it is effective to use a martensite phase inside the steel sheet. By utilizing the martensite phase in this way, it is possible to increase the strength of the hot-rolled steel sheet with an alloy-saving component. In general, the retained austenite phase and martensite phase are not preferable for toughness, but it is possible to obtain excellent low-temperature toughness by optimizing the structure fraction and controlling the crystal grain size of the martensite phase. It was made clear.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] By mass%, C: 0.020% to 0.080%, Si: 0.05% to 0.50%, Mn: 1.20% to 2.20%, P: 0.001% to 0.020%, S: 0.0001% to 0.0050% Below, Al: 0.005% to 0.050%, N: 0.0010% to 0.0060%, Nb: 0.040% to 0.080%, Mo: 0.01% to 0.50%, Ti: 0.005% to 0.050%, Cr: 0.01 % And 0.50% or less, Ca: 0.0005% or more and 0.0050% or less, and the balance is composed of Fe and inevitable impurities, and the tempered martensite phase at a position 1.5mm from the steel sheet surface in the sheet thickness direction. It has a structure in which the area ratio is 90% or more and 100% or less and the average crystal grain size of the tempered martensite phase is 1.0 μm or more and 5.0 μm or less. Vickers hardness variation ΔHv of the steel sheet is 50 or less, and the area ratio of the bainite phase is 90% or less at the center of the plate thickness of the steel sheet. 98% or less, the area ratio of the martensite phase is 1% or more and 5% or less, the average crystal grain size of the martensite phase is 0.5 μm or more and 5.0 μm or less, and the area ratio of the retained austenite phase is 1% or more and 5% or less. A high-strength hot-rolled steel sheet having a structure, wherein the thickness of the iron oxide film on the steel sheet surface is 0.1 μm or more and 10 μm or less.

[2] In the above [1], in addition to the above composition, by mass%, V: 0.001% to 0.100%, Cu: 0.01% to 0.50%, Ni: 0.01% to 0.50%, B: 0.0001 A high-strength hot-rolled steel sheet containing at least one selected from% to 0.0040%.

[3] The continuous cast slab having the composition described in [1] or [2] is reheated to a temperature range of 1050 ° C. or more and 1300 ° C. or less, and after rough rolling and before finish rolling, the impact pressure is 0.1 MPa or more. Descaling is performed with high-pressure water of 10.0 MPa or less, finish rolling is started within 0.1 s to 5.0 s after descaling, the reduction rate in the non-recrystallization temperature range is 40% to 90%, and the finish rolling finish temperature is Finish rolling in the temperature range of 700 ° C or more and 900 ° C or less is started, and cooling is started within 1 s or more and 10 seconds after the finish rolling is finished. On the steel sheet surface, the average cooling rate in the temperature range of 750 ° C or less and 650 ° C or more is 50 ℃ / s or more and 500 ℃ / s or less, and (average cooling rate in the temperature range of 750 ° C or less and 650 ° C or more) / (average cooling rate from the cooling start temperature to the cooling end temperature)> 1.10, 200 ° C or more and 400 ° C or less It is cooled to the cooling stop temperature of 1, and then wound in a temperature range of 400 ° C to 600 ° C Process for producing a high strength hot rolled steel sheet.

  According to the present invention, a high-strength hot-rolled steel sheet excellent in bending and low-temperature toughness suitable for steel pipe materials used in the fields of pipelines, oil well pipes, civil engineering and construction, etc. is obtained by conventional hot rolling equipment, It is extremely useful industrially.

Hereinafter, the present invention will be specifically described.
First, the reason for limiting the component composition of the high-strength hot-rolled steel sheet of the present invention will be described. In addition,% showing the following component composition shall mean the mass% (mass%) unless there is particular notice.

C: 0.020% or more and 0.080% or less
C contributes to improving the strength of the steel sheet. In order to secure the desired steel sheet strength of the present invention, for example, a tensile strength of 650 MPa or more, the C content needs to be 0.020% or more. On the other hand, when the C content exceeds 0.080%, a martensite phase or a retained austenite phase is excessively generated, which adversely affects the toughness or bendability of the steel sheet. Therefore, the C content is 0.020% or more and 0.080% or less. Preferably they are 0.035% or more and 0.065% or less.

Si: 0.05% or more and 0.50% or less
Si contributes to the strength improvement of the steel sheet by solid solution strengthening, and the strength improvement effect is recognized when the content is 0.05% or more. However, when the Si content exceeds 0.50%, the weldability of the steel sheet decreases. In addition, the iron oxide film becomes thick, it becomes difficult to cool the steel plate uniformly, and the bendability decreases. Therefore, the Si content is 0.05% or more and 0.50% or less. Preferably it is 0.10% or more and 0.40% or less.

Mn: 1.20% to 2.20%
Mn contributes to increasing the strength of the steel sheet through improving hardenability. In order to suppress the formation of pearlite and ferrite and to obtain a steel sheet having desired bainite or martensite described later, the Mn content needs to be 1.20% or more. On the other hand, when the Mn content exceeds 2.20%, the center segregation becomes remarkable, and the bendability of the steel sheet is greatly reduced. Moreover, when the Mn content exceeds 2.20%, the low temperature toughness of the steel sheet also decreases. Therefore, the Mn content is 1.20% or more and 2.20% or less. Preferably it is 1.40% or more and 2.00% or less.

P: 0.001% to 0.020%
P has an adverse effect on the toughness and weldability of the steel sheet, so the lower the content, the better. However, 0.020% or less is acceptable. On the other hand, excessive reduction of P hinders productivity, so the lower limit of P content is 0.001%.

S: 0.0001% or more and 0.0050% or less
S exists as MnS in the steel, adversely affects the ductility, and lowers the bendability of the steel sheet. The smaller the S content, the better. However, 0.0050% or less is acceptable. On the other hand, since excessive reduction of S inhibits productivity, the lower limit of the S content is set to 0.0001%.

Al: 0.005% to 0.050%
Al is added for the purpose of deoxidation, and when it is contained in an amount of 0.005% or more, it exhibits a deoxidation effect. On the other hand, if the Al content exceeds 0.050%, it is present in the steel as inclusions, which adversely affects the bendability and toughness of the steel sheet. Therefore, the Al content is 0.005% or more and 0.050% or less. Preferably they are 0.020% or more and 0.045% or less.

N: 0.0010% or more and 0.0060% or less
N exists as a nitride in steel, and if it exceeds 0.0060%, cracks occur during slab casting, and adversely affects the bendability of the steel sheet. The lower the N content, the better. On the other hand, excessive reduction reduces productivity. Therefore, the N content is 0.0010 or more and 0.0060% or less.

Nb: 0.040% to 0.080%
Nb contributes to improving the strength of the steel sheet by precipitation strengthening. Moreover, Nb contributes to the improvement of the strength and toughness of the steel sheet by reducing the crystal grain size. In order to express these effects, the Nb content is set to 0.040% or more. On the other hand, if the Nb content exceeds 0.080%, the amount of fine precipitates is excessively increased and the toughness of the steel sheet is deteriorated. Therefore, the Nb content is 0.040% or more and 0.080% or less. Preferably it is 0.050% or more and 0.070% or less.

Mo: 0.01% or more and 0.50% or less
Mo contributes to improving the strength of the steel sheet through improving hardenability. In order to suppress the formation of pearlite and ferrite and to obtain a steel sheet having desired martensite and retained austenite described later, the Mo content needs to be 0.01% or more. On the other hand, when the Mo content exceeds 0.50%, a martensite phase is excessively generated, and the bendability and toughness of the steel sheet are greatly reduced. Therefore, the Mo content is 0.01% or more and 0.50% or less. Preferably they are 0.05% or more and 0.25% or less.

Ti: 0.005% to 0.050%
Ti contributes to improving the strength of the steel sheet by precipitation strengthening. In order to realize the effect, the Ti content needs to be 0.005% or more. On the other hand, if the Ti content exceeds 0.050% and becomes excessive, the toughness and weldability of the steel sheet deteriorate. Therefore, Ti content shall be 0.005% or more and 0.050% or less. Preferably it is 0.010% or more and 0.030% or less.

Cr: 0.01% to 0.50%
Cr contributes to increasing the strength of the steel sheet through improving hardenability. In order to suppress the formation of pearlite and ferrite and obtain a steel sheet having desired martensite or retained austenite described later, the Cr content needs to be 0.01% or more. On the other hand, when the Cr content exceeds 0.50%, martensite is excessively generated, and the toughness and bendability of the steel sheet are greatly reduced. Therefore, the Cr content is 0.01% or more and 0.50% or less. Preferably it is 0.10% or more and 0.35% or less.

Ca: 0.0005% or more and 0.0050% or less
Ca granulates the form of the plate-like S inclusions and contributes to improvement of the bendability and toughness of the steel plate. In order to achieve this effect, the Ca content needs to be 0.0005% or more. On the other hand, if the Ca content exceeds 0.0050%, excessive Ca-based inclusions are present in the steel, which adversely affects the bendability of the steel sheet. Therefore, Ca is 0.0005% or more and 0.0050% or less. Preferably it is 0.0010% or more and 0.0030% or less.

  Although the above components are basic components, the high-strength hot-rolled steel sheet according to the present invention may further include V: 0.001% or more and 0.100% or less, Cu: 0.01% or more, as necessary, in addition to the above components. 0.50% or less, Ni: 0.01% or more and 0.50% or less, B: 0.0001% or more and 0.0040% or less may be included.

V: 0.001% or more and 0.100% or less
V contributes to improving the strength of the steel sheet by precipitation strengthening, and the V content is preferably 0.001% or more in order to achieve the effect. On the other hand, if the V content exceeds 0.100% and becomes excessive, the toughness and weldability of the steel sheet may be deteriorated. Therefore, when V is contained, the content is preferably 0.001% or more and 0.100% or less. More preferably, it is 0.020% or more and 0.070% or less.

Cu: 0.01% to 0.50%
Cu contributes to improving the strength of the steel sheet, and in order to exhibit this effect, the Cu content is preferably 0.01% or more. On the other hand, when Cu content exceeds 0.50%, it becomes a factor of hot brittleness. Therefore, when it contains Cu, it is preferable to make the content into 0.01% or more and 0.50% or less. More preferably, it is 0.10% or more and 0.30% or less.

Ni: 0.01% or more and 0.50% or less
Ni contributes to improving the strength and toughness of the steel sheet. In order to exhibit such an effect, the Ni content is preferably 0.01% or more. On the other hand, the Ni content may exceed 0.50%, but if it exceeds 0.50%, the effect tends to be saturated. Therefore, when it contains Ni, it is preferable that the content shall be 0.01% or more and 0.50% or less. More preferably, it is 0.10% or more and 0.30% or less.

B: 0.0001% or more and 0.0040% or less
B contributes to increasing the strength of the steel sheet through improving hardenability. In order to exhibit such an effect, the B content is preferably 0.0001% or more. On the other hand, if the B content exceeds 0.0040%, there is a risk of adversely affecting the toughness of the welded portion when welding the steel sheet. Therefore, when it contains B, it is preferable to make the content into 0.0001% or more and 0.0040% or less. More preferably, it is 0.0005% or more and 0.0020% or less.

  In the high-strength hot-rolled steel sheet of the present invention, components other than the above are Fe and inevitable impurities. Inevitable impurities include, for example, Co, W, Pb, Sn and the like, and the content of these elements can be allowed if it is 0.0050% or less.

Next, the reasons for limiting the structure, surface layer hardness characteristics, and iron oxide film thickness of the high-strength hot-rolled steel sheet of the present invention will be described.
In the high-strength hot-rolled steel sheet of the present invention, the area ratio of the tempered martensite phase is 90% or more and 100% or less and the average grain size of the tempered martensite phase is 1.5 mm from the steel sheet surface. It has a tissue that is 1.0 μm or more and 5.0 μm or less. In the high-strength hot-rolled steel sheet of the present invention, the fluctuation amount ΔHv of the Vickers hardness in the sheet width direction is 50 or less at a position 1.5 mm from the sheet surface in the sheet thickness direction.

Area ratio of tempered martensite phase: 90% or more and 100% or less Average crystal grain size of tempered martensite phase: 1.0 μm or more and 5.0 μm or less In the present invention, the steel sheet surface layer is substantially fine tempered martensite. By adopting a single-phase structure and a uniform structure in which the hardness is suppressed to some extent, the bendability of the steel sheet is improved. When the area ratio of the tempered martensite phase is less than 90% at a position 1.5 mm from the steel sheet surface, other phases different in hardness from the tempered martensite phase are mixed in the steel sheet surface layer, and the hardness is It becomes non-uniform and the bendability of the steel sheet deteriorates. Therefore, the area ratio of the tempered martensite phase is 90% or more and 100% or less at a position 1.5 mm away from the steel sheet surface. Preferably they are 95% or more and 100% or less.

  Further, when the average crystal grain size of the tempered martensite phase exceeds 5.0 μm at a position 1.5 mm from the steel sheet surface in the thickness direction, the toughness of the steel sheet is adversely affected. Furthermore, coarse grains are mixed, which inhibits the uniformity of the steel sheet surface layer and adversely affects the bendability of the steel sheet. Accordingly, the average crystal grain size of the tempered martensite phase is 5.0 μm or less at a position 1.5 mm from the steel plate surface in the plate thickness direction. On the other hand, the average crystal grain size may be fine, but the lower limit is 1.0 μm from the viewpoint of productivity and cost.

  Note that the structure at a position 1.5 mm from the surface of the steel sheet may contain a ferrite phase, bainitic ferrite phase, bainite phase, residual austenite phase, and the like in addition to the fine tempered martensite phase. The total area ratio of these phases is preferably 5% or less, more preferably 3% or less, and even more preferably 0% (that is, tempered martensite single phase).

Vickers hardness fluctuation amount in the plate width direction ΔHv: 50 or less In the present invention, the structure of the steel sheet surface layer is a substantially fine tempered martensite single phase as described above, and the hardness distribution of the steel sheet surface layer is made uniform. By doing so, the bendability is further improved. By making the hardness distribution of the steel sheet surface layer uniform, uniform deformation is possible in the steel sheet surface layer during bending deformation, and excellent bendability can be ensured. When the fluctuation amount ΔHv of the Vickers hardness in the plate width direction exceeds 50 at a position 1.5 mm from the steel plate surface, the hardness unevenness in the steel plate surface layer becomes large, and there is a region where the ductility is locally different. The surface layer has a non-uniform structure, and the bendability of the steel sheet deteriorates. Therefore, the ΔHv is set to 50 or less. Preferably it is 40 or less.

  The high-strength hot-rolled steel sheet of the present invention has the above structure and hardness characteristics in the steel sheet surface layer, and the area ratio of the bainite phase is 90% or more and 98% or less at the center of the sheet thickness of the steel sheet. It has a structure in which the area ratio of the phase is 1% to 5%, the average crystal grain size of the martensite phase is 0.5 μm to 5.0 μm, and the area ratio of the residual austenite phase is 1% to 5%.

Area ratio of bainite phase: 90% or more and 98% or less In the present invention, the toughness of the steel sheet is made by making the structure inside the steel sheet a structure mainly composed of the bainite phase having an excellent strength-toughness balance and making the inside of the steel sheet a uniform structure. Secure. When the area ratio of the bainite phase is less than 90% at the plate thickness center position of the steel plate, other phases, for example, a martensite phase having higher strength than the bainite phase are mixed in a large amount, and the toughness of the steel plate is deteriorated. On the other hand, if the area ratio of the bainite phase exceeds 98% at the plate thickness center position of the steel plate, the desired martensite phase and residual austenite phase described later cannot be secured, and the strength and bendability of the steel plate are reduced. . Therefore, the area ratio of the bainite phase is 90% or more and 98% or less at the center position of the thickness of the steel sheet. Preferably they are 95% or more and 97% or less.

Martensite phase area ratio: 1% or more and 5% or less The martensite phase is hard and contributes to improving the strength of the steel sheet. In order to exhibit this effect, the area ratio of the martensite phase needs to be 1% or more at the center of the plate thickness of the steel plate. On the other hand, when the area ratio of the martensite phase exceeds 5% at the plate thickness center position of the steel plate, the toughness of the steel plate deteriorates. Therefore, the area ratio of the martensite phase is 1% or more and 5% or less at the center position of the steel sheet thickness. Preferably they are 1.5% or more and 3.0% or less.

Average grain size of martensite phase: 0.5 μm or more and 5.0 μm or less If the average grain size of the martensite phase exceeds 5.0 μm at the center of the plate thickness of the steel sheet, the coarse martensite phase will be distributed roughly. As a result, the bendability and toughness of the steel sheet deteriorate. Therefore, the average crystal grain size of the martensite phase is 5.0 μm or less at the center of the plate thickness of the steel plate. Preferably it is 2.5 μm or less. The average crystal grain size is preferably as small as possible, but the lower limit is set to 0.5 μm from the viewpoint of productivity and cost.

Area ratio of retained austenite phase: 1% or more and 5% or less The retained austenite phase contributes to improving workability such as bendability of the steel sheet by improving ductility. In order to exert this effect, the area ratio of the retained austenite phase needs to be 1% or more at the center position of the steel sheet thickness. On the other hand, if the area ratio of the retained austenite phase exceeds 5% at the center of the thickness of the steel sheet, it acts as a propagation path for cracks and the toughness of the steel sheet deteriorates. Therefore, the area ratio of the retained austenite phase is 1% or more and 5% or less at the center of the plate thickness of the steel plate. Preferably they are 1.5% or more and 2.5% or less.

  In addition, the structure in the center position of the plate thickness of the steel sheet may include pearlite, cementite, and the like in addition to the bainite phase, the retained austenite phase, and the martensite phase. The total area ratio of pearlite, cementite, etc. is preferably 3% or less.

Thickness of iron oxide film on steel sheet surface: 0.1 μm or more and 10 μm or less Uniform thickness of steel sheet iron oxide film achieves a uniform structure in the sheet width direction by uniformly cooling the steel sheet in hot rolling. And contributes to excellent bendability. If there is a region where the thickness of the iron oxide film exceeds 10 μm, it is difficult to cool the steel plate uniformly, the strength in the plate width direction becomes non-uniform, and the bendability decreases. On the other hand, the thinner the iron oxide film, the better, but the lower limit is set to 0.1 μm from the viewpoint of productivity and cost.

Next, the manufacturing method of the high intensity | strength hot-rolled steel plate of this invention is demonstrated.
The high-strength hot-rolled steel sheet of the present invention is subjected to rough rolling and finish rolling after reheating a slab (slab) having the above composition obtained by continuous casting, and then cooled under predetermined conditions, It can be manufactured by winding it around a coil at temperature.

Reheating temperature of continuous cast slab: 1050 ° C or higher and 1300 ° C or lower If the reheating temperature of continuous cast slab exceeds 1300 ° C, the austenite grains during heating become coarse, resulting in the final crystal grain size after hot rolling It becomes coarse and the bendability and toughness of the steel sheet deteriorate. On the other hand, when the reheating temperature of the continuous cast slab is less than 1050 ° C., precipitation strengthening elements such as Ti and Nb are not sufficiently dissolved, and it becomes difficult to secure a desired steel plate strength. In addition, it is difficult to ensure a predetermined finish rolling end temperature. Therefore, the reheating temperature is set to 1050 ° C. or higher and 1300 ° C. or lower. Preferably they are 1100 degreeC or more and 1250 degrees C or less.

  Hot rolling is usually composed of rough rolling and finish rolling, but the conditions for rough rolling are not particularly limited in the present invention. After rough rolling, after descaling with high-pressure water under the following conditions, finish rolling is performed under the following conditions.

Descaling after rough rolling and before finish rolling: high pressure water with impact pressure of 0.1MPa or more and 10.0MPa or less If the impact pressure of high pressure water exceeds 10.0MPa, the surface roughness of the steel sheet increases, surface properties deteriorate, and bendability Decreases. On the other hand, when the collision pressure of the high-pressure water is less than 0.1 MPa, the thickness of the iron oxide film on the surface of the steel sheet becomes thick, a uniform structure cannot be obtained in the sheet width direction, and bendability decreases. Therefore, the collision pressure of the high pressure water is set to 0.1 MPa or more and 10.0 MPa or less. Preferably, it is 1.0 MPa or more and 8.0 MPa or less.

Time to start finish rolling after descaling: 0.1 s or more and within 5.0 s Even if appropriate descaling is performed before finish rolling, a new iron oxide film is formed over time. If the time until descaling starts after descaling exceeds 5.0 seconds, the thickness of the iron oxide film on the surface of the steel sheet increases, a uniform structure cannot be obtained in the sheet width direction, and the bendability decreases. Therefore, the time from descaling to the start of finish rolling should be within 5.0 seconds. Note that the shorter the time from descaling to the start of finish rolling, the better. However, the lower limit is set to 0.1 seconds from the viewpoint of productivity and cost.

Reduction ratio in non-recrystallization temperature range: 40% or more and 90% or less In finish rolling, if the reduction ratio in non-recrystallization temperature range is less than 40%, grain refinement by rolling recrystallization becomes insufficient, Crystal grains become coarse and the bendability and toughness of the steel sheet deteriorate. In order to obtain fine grains, it is preferable that the rolling reduction in the non-recrystallization temperature range is high. However, if the rolling reduction exceeds 90%, the deformation resistance becomes high and rolling becomes difficult. Therefore, in the finish rolling, the rolling reduction in the non-recrystallization temperature range is 40% or more and 90% or less. Preferably they are 60% or more and 85% or less.

Finish rolling finish temperature: 700 ° C or more and 900 ° C or less If the finish rolling finish temperature is less than 700 ° C, it becomes a mixed grain structure of expanded, coarse and fine grains, and the structure becomes non-uniform, or the ferrite phase in the surface layer It becomes difficult to obtain a tempered martensite phase having a desired area ratio, and the bendability and toughness of the steel sheet deteriorate. On the other hand, when the finish rolling finish temperature exceeds 900 ° C., the crystal grains become coarse, and the bendability and toughness of the steel sheet deteriorate. Accordingly, the finish rolling finish temperature is set to 700 ° C. or more and 900 ° C. or less. Preferably they are 740 degreeC or more and 840 degrees C or less. These temperatures are temperatures on the steel sheet surface.
After finishing rolling, accelerated cooling is performed under the following conditions.

Time after finishing rolling until cooling starts: 1 s or more and within 10 s If the time until finishing accelerated cooling after finishing rolling exceeds 10 seconds, the thickness of the iron oxide film on the surface of the steel sheet increases, and the plate A uniform structure cannot be obtained in the width direction, and the bendability decreases. Therefore, the time from the end of finish rolling to the start of accelerated cooling is within 10 seconds. The earlier the cooling start time after finish rolling is, the better. However, the lower limit is set to 1 second from the viewpoint of productivity and cost.

Average cooling rate in the temperature range of 750 ° C or lower and 650 ° C or higher: 50 ° C / s or higher and 500 ° C / s or lower Suppresses the formation of polygonal ferrite phase and pearlite phase with low dislocation density, and tempered martensite phase and plate on the surface layer In order to secure a desired amount of bainite phase at the thickness center and to achieve both bendability and toughness, the average cooling rate in the temperature range of 750 ° C. or lower and 650 ° C. or higher needs to be 50 ° C./s or higher. Preferably, it is 100 ° C./s or more. On the other hand, the cooling rate in the temperature range may be high, but the above effect tends to be saturated when the average cooling rate exceeds 500 ° C./s.
In addition, the said temperature range and average cooling rate are the values in the steel plate surface.

(Average cooling rate in the temperature range of 750 ° C or lower and 650 ° C or higher) / (Average cooling rate from the cooling start temperature to the cooling end temperature) ≧ 1.10
In the present invention, the average cooling rate CR1 in the temperature range of 750 ° C. or lower and 650 ° C. or higher is increased with respect to the average cooling rate CR2 from the accelerated cooling start temperature to the accelerated cooling end temperature.
By increasing the cooling rate (CR1) in the high temperature range in the initial stage of cooling, coarsening of the crystal grains is suppressed, and the tempered martensite phase of the steel sheet surface layer and the martensite phase at the center of the sheet thickness are set to the desired average crystal grain size. And bendability and toughness can be obtained. In order to exhibit such an effect, it is necessary to control the average cooling rates CR1 and CR2 so that (CR1 / CR2) ≧ 1.10. Preferably, (CR1 / CR2) ≧ 1.12. In addition, these average cooling rates are the cooling rates in the steel plate surface.

Cooling stop temperature: 200 ° C or more and 400 ° C or less If the cooling stop temperature exceeds 400 ° C, a desired amount of tempered martensite phase cannot be obtained on the surface layer, and a desired amount of bainite phase is obtained even at the center position of the plate thickness. I can't. On the other hand, when the cooling stop temperature is lower than 200 ° C., a desired tempered martensite phase is obtained in the surface layer, but a desired bainite phase or residual austenite phase cannot be obtained at the plate thickness center position. Therefore, the cooling stop temperature is set to 200 ° C. or more and 400 ° C. or less. Preferably they are 220 degreeC or more and 350 degrees C or less. In addition, these cooling stop temperature is the temperature in the steel plate surface.

Winding temperature: 400 ° C or more and 600 ° C or less If the winding temperature exceeds 600 ° C, polygonal ferrite phase and pearlite phase are generated, and the steel sheet cannot be formed into the desired structure, and excellent bendability and toughness are achieved. I can't get it. On the other hand, when the coiling temperature is less than 400 ° C., the bainite transformation is insufficient, the residual austenite phase does not remain, and it becomes difficult to secure the desired bainite phase and the residual austenite phase, resulting in excellent bendability and toughness. Cannot be obtained. Therefore, the winding temperature is set to 400 ° C. or more and 600 ° C. or less. Preferably they are 460 degreeC or more and 560 degrees C or less. In addition, these winding temperature is the temperature in the steel plate surface. In addition, the steel plate after the cooling stop is allowed to stand for a predetermined time, so that the surface of the steel plate is raised from the cooling stop temperature (200 ° C. to 400 ° C.) to the coiling temperature (400 ° C. to 600 ° C.) by reheating.

A slab having the composition shown in Table 1 (continuous cast slab, wall thickness: 220 mm) is reheated to the temperature shown in Table 2, rough-rolled, descaled under the conditions shown in Table 2, and finish-rolled as shown in Table 2. Hot rolling is performed under the conditions, and after the hot rolling is finished, the steel is cooled under the cooling conditions shown in Table 2, wound up in a coil having a predetermined dimension (width: 1500 mm) at the winding temperature shown in Table 2, and shown in Table 2. A hot-rolled steel sheet (steel strip) having a thickness was used.
Test specimens were collected from the obtained hot-rolled steel sheet, and the following structure observation, thickness measurement of the iron oxide film on the steel sheet surface, hardness test, tensile test, impact test, and bending test were performed.

(1) Microstructure observation The microstructure at the position 1.5mm below the surface of the hot-rolled steel sheet and the central position of the plate thickness is imaged by observing three or more fields of view at each plate thickness position using a scanning electron microscope SEM (magnification: 2000 times). The area ratios of tempered martensite (1.5 mm position below the steel sheet surface) and bainite (plate thickness center position) as the main phase, martensite as the second phase at the sheet thickness center position, and retained austenite were measured.
The area ratio of retained austenite at the center of the plate thickness was determined by X-ray diffraction. Specifically, a specimen for X-ray diffraction is taken from a hot-rolled steel sheet parallel to the plate surface, ground and polished (chemical polishing), and the surface of the polished specimen is the thickness of the steel sheet (1/2) t position. Then, for the specimen after polishing, the (200), (211), (220) face of bcc iron and (200), (220), (311) of fcc iron using Mo Kα ray with an X-ray diffractometer. ) The integrated intensity of the surface was measured, the intensity ratio of the integrated reflection intensity from each surface of fcc iron to the integrated reflection intensity from each surface of bcc iron was determined, and this was defined as the area ratio of residual austenite.
In addition, the martensite area ratio at the center position of the plate thickness is the sum of martensite and retained austenite in a massive and smooth surface area on the scanning electron microscope image, and the total area ratio of martensite and retained austenite is obtained. The total area ratio was determined by subtracting the area ratio of retained austenite determined by X-ray diffraction.
The average crystal grain size of tempered martensite (1.5mm below the surface of the steel sheet) was measured by measuring the area of tempered martensite using an SEM photograph taken and counting the tempered martensite grains. The average grain area a was calculated from the area and the number of grains, and was obtained by a quadrature method with a grain size d = √a. The average crystal grain size of martensite (plate thickness center position) was also determined by the same method as described above.

(2) Measurement of the thickness of the iron oxide film on the surface of the steel sheet Scanning electron microscope SEM shows the structure of the surface of the hot-rolled steel sheet where the iron oxide film is deficient, peeled, or where no voids are observed at the interface between the film and the ground iron. (Magnification: 2000 times) using 10 field observations at each plate thickness position, imaged, measured the maximum thickness of the iron oxide film in each image, and determined as the 10-point simple average of the measured maximum thickness This was the thickness of the iron oxide film on the surface of the hot rolled steel sheet.

(3) Hardness measurement About the rolling direction cross section of a hot-rolled steel sheet, hardness Hv was measured using the Vickers hardness meter (test force 9.8N, load 1kgf). The measurement position was 1.5 mm in the thickness direction from the surface of the hot-rolled steel sheet. Hardness measurement at each position was performed at N = 5, and the obtained measurement results were simply averaged to obtain hardness. Sheet width direction measurement position is 25mm from one edge, 1/16, 1/8, 1/4, 1/2, 3/4, 7/8, 15/16 position in the sheet width direction, and opposite edge The position was 25 mm. The fluctuation amount ΔHv of the Vickers hardness in the sheet width direction was obtained by subtracting the minimum value from the maximum value of the measured hardness.

(4) Tensile test Flat full thickness tensile test piece (plate thickness: full thickness, parallel part length) so that the direction perpendicular to the rolling direction (C direction) is the longitudinal direction from the plate thickness center position of the hot rolled steel plate (60 mm, distance between gauges: 50 mm, gauge width: 38 mm), and a tensile test was carried out at room temperature in accordance with the provisions of ASTM E8M-04 to determine the tensile strength TS. The case where the tensile strength of the hot-rolled steel sheet was 650 MPa or more was evaluated as “high-strength hot-rolled steel sheet”.

(5) Charpy impact test V-notch test piece (length 55mm x height 10mm x width 10mm) so that the direction perpendicular to the rolling direction (C direction) is the longitudinal direction from the center of the thickness of the hot-rolled steel sheet The sample was collected and subjected to a Charpy impact test in accordance with the provisions of JIS Z 2242 to obtain a ductile-brittle fracture surface transition temperature (° C.). In addition, the test piece was set to three, the arithmetic average of the obtained ductility-brittle fracture surface transition temperature was calculated | required, and it was set as the ductility-brittle fracture surface transition temperature vTrs of the steel plate. The case where vTrs was −80 ° C. or less was evaluated as “good toughness”.

(6) Bending test JIS Z 2248 No. 1 bending test piece was taken from the 1/4 position of the width of the hot-rolled steel sheet so that the direction perpendicular to the rolling direction (C direction) was the longitudinal direction, and the bending radius / plate A 180 ° U bending test with a thickness of 2.0 was performed. The outer surface of the bend was visually observed, and the case where no occurrence of cracks (cracks, hair cracks) was observed was evaluated as “bendability was good”.
The results obtained as described above are shown in Table 3.

  As shown in Table 3, the hot-rolled steel sheet of the invention example was good in both bendability and toughness (low temperature toughness). On the other hand, the hot rolled steel sheet of the comparative example could not obtain sufficient characteristics in either one or both of bendability and toughness (low temperature toughness).

Claims (3)

% By mass
C: 0.020% to 0.080%, Si: 0.05% to 0.50%,
Mn: 1.20% or more and 2.20% or less, P: 0.001% or more and 0.020% or less,
S: 0.0001% to 0.0050%, Al: 0.005% to 0.050%,
N: 0.0010% to 0.0060%, Nb: 0.040% to 0.080%,
Mo: 0.01% to 0.50%, Ti: 0.005% to 0.050%,
Cr: 0.01% or more and 0.50% or less, Ca: 0.0005% or more and 0.0050% or less, with the balance being composed of Fe and inevitable impurities,
A structure in which the area ratio of the tempered martensite phase is 90% or more and 100% or less and the average crystal grain size of the tempered martensite phase is 1.0 μm or more and 5.0 μm or less at a position 1.5 mm from the steel sheet surface. Have
At a position 1.5 mm from the steel sheet surface in the plate thickness direction, the fluctuation amount ΔHv of the Vickers hardness in the plate width direction is 50 or less,
At the center of the thickness of the steel sheet, the area ratio of the bainite phase is 90% to 98%, the area ratio of the martensite phase is 1% to 5%, and the average crystal grain size of the martensite phase is 0.5 μm to 5.0 μm. Hereinafter, it has a structure in which the area ratio of the retained austenite phase is 1% or more and 5% or less,
A high-strength hot-rolled steel sheet, wherein the thickness of the iron oxide film on the steel sheet surface is 0.1 μm or more and 10 μm or less.   In addition to the above composition, V: 0.001% or more and 0.100% or less, Cu: 0.01% or more and 0.50% or less, Ni: 0.01% or more and 0.50% or less, B: 0.0001% or more and 0.0040% or less The high-strength hot-rolled steel sheet according to claim 1, wherein the high-strength hot-rolled steel sheet according to claim 1 is contained.   The continuous cast slab comprising the composition according to claim 1 or 2 is reheated to a temperature range of 1050 ° C to 1300 ° C, and after high-pressure water having a collision pressure of 0.1 MPa to 10.0 MPa after rough rolling and before finish rolling. After descaling, finish rolling is started within 0.1s to 5.0s, the rolling reduction in the non-recrystallization temperature range is 40% to 90%, and the finish rolling finish temperature is 700 ° C to 900 ° C After finishing the finish rolling, cooling is started within 1 s to 10 s, and the average cooling rate in the temperature range of 750 ° C. to 650 ° C. is 50 ° C./s to 500 ° C. / s or less, and (average cooling rate in the temperature range of 750 ° C or lower and 650 ° C or higher) / (average cooling rate from the cooling start temperature to the cooling end temperature) ≥ 1.10, cooling to a cooling stop temperature of 200 ° C or higher and 400 ° C or lower And then winding in a temperature range of 400 ° C to 600 ° C. A method for producing rolled steel sheets.