JP6070642B2 - Hot-rolled steel sheet having high strength and excellent low-temperature toughness and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength hot-rolled steel sheet having high strength and excellent low-temperature toughness that is suitable as a material for spiral steel pipes, ERW steel pipes, and the like, and a method for producing the same.

  In recent years, with the increase in size of structures, there has been an increasing demand for high-strength and thick-walled steel pipe materials. There is a demand for a steel pipe material with an excellent balance of toughness.

  In response to the above, various technologies have been proposed for hot-rolling materials for steel pipes. For example, Patent Document 1 discloses a method for producing a high-strength steel sheet excellent in the balance between strength and low-temperature toughness, with a bainite phase area ratio of 80% or more with respect to the entire structure at a thickness of 1/4, and a control method thereof. ing.

  Patent Document 2 discloses a high strength hot rolling excellent in low temperature toughness that includes a bainitic ferrite phase with an average particle size of 10 μm or less as a main phase and a bulk martensite phase with an aspect ratio of less than 5.0 and an area ratio of 1.4 to 15%. A steel plate and a manufacturing method thereof are disclosed.

  In Patent Document 3, after the hot rolling is finished, the first stage cooling, the second stage cooling, the first stage cooling (repetition), the second stage cooling (repetition), and the third stage cooling are performed, so that the plate is removed from the surface. The structure at the position of 1 mm in the thickness direction is either a tempered martensite single phase structure or a mixed structure of bainite and tempered martensite phases, and the structure at the center of the thickness is bainite phase and / or bainitic ferrite. The phase is the main phase and has a structure consisting of 2% or less of the second phase by volume. Vickers hardness Hv1mm at the position of 1mm from the surface in the thickness direction and Vickers hardness Hv1 / 2t at the center position of the thickness. A thick high-tensile hot-rolled steel sheet excellent in low-temperature toughness having a difference ΔHv of 50 points or less and a method for producing the same are disclosed.

  Patent Document 4 discloses that the pro-eutectoid ferrite fraction is 3% or more and 20% or less, the other is a low-temperature transformation product, the number-average crystal grain size representing the grain size of pro-eutectoid ferrite, and the continuous cooling transformation structure. A high-strength hot-rolled steel sheet for spiral pipes, in which both of the area average particle diameters representing the particle diameters of the steel sheet are individually controlled, and a method for producing the same are described.

JP2013-7101A JP 2012-188731 A JP 2010-196164 A JP 2013-173998 A

  However, it is extremely difficult to obtain a high-strength hot-rolled steel sheet excellent in high strength and low-temperature toughness suitable as a steel pipe material in any of the above-described conventional techniques. The technique disclosed in Patent Document 1 is a structure mainly composed of a bainite phase that has no knowledge about the crystal grain size and shape, and there is concern about improvement in low temperature toughness and strength balance.

  The technique disclosed in Patent Document 2 has a slow cooling rate after the end of hot rolling, and contains Mo and Ni, which are expensive elements, in order to ensure hardenability and has a problem in cost. Moreover, only the average grain size of bainitic ferrite is controlled, and there is no knowledge about low temperature toughness when the shape of individual crystal grains is different even though the average grain size is the same, and there is a problem in improving the balance between strength and low temperature toughness. is there.

  In the technique described in Patent Document 3, the cooling process after hot rolling becomes complicated, and there is a problem in mass production stability. Moreover, there is a concern about variation in characteristics due to coil width and cooling unevenness in the longitudinal direction. In addition, the surface layer is mainly tempered martensite phase, and the center of the plate thickness is mainly bainite phase, and the structure changes in the plate thickness direction. Therefore, it is difficult to ensure an excellent balance between low temperature toughness and strength.

  The technique described in Patent Document 4 considers the average crystal grain size and standard deviation, but is composed of a soft pro-eutectoid ferrite phase and a hard low-temperature transformation phase, and is composed of a single phase. There are many interfaces between the soft phase and the hard phase as compared with the texture. For this reason, voids are easily generated and cracks are also easily propagated, so that it is difficult to ensure excellent toughness.

  An object of the present invention is to solve such problems and to provide a hot-rolled steel sheet having high strength and excellent low-temperature toughness and a method for producing the hot-rolled steel sheet.

  In order to achieve the above object, the present inventors have achieved both hot strength and excellent low temperature toughness, that is, hot rolling with improved low temperature toughness while maintaining a high strength of tensile strength: 650 MPa or more. In order to obtain a steel plate, it was studied earnestly. As a result, the hot rolling conditions and the cooling conditions after the hot rolling are controlled, the average crystal grain size of the bainitic ferrite phase is refined, the crystal grain shape (maximum length a in the rolling direction a, aspect ratio a / By optimizing b) and controlling the sum of the area ratios of the martensite phase, bainite phase, and residual austenite phase, it is possible to obtain a hot-rolled steel sheet having excellent low-temperature toughness while maintaining high strength The knowledge that it was possible was obtained.

This invention is made | formed based on the above knowledge, and makes the following a summary.
[1] By mass%, C: 0.02% to 0.12%, Si: 0.05% to 0.45%, Mn: 1.0% to 2.0%, P: 0.001% to 0.020%, S: 0.0001% to 0.0050% In the following, Al: 0.005% to 0.050%, N: 0.0010% to 0.0060%, Nb: 0.020% to 0.080%, Ti: 0.005% to 0.050%, Ca: 0.0005% to 0.0050%, The balance is composed of Fe and inevitable impurities. The bainitic ferrite phase has an area ratio of 90 to 98% and martensite phase, bainite phase, and retained austenite phase. A hot-rolled steel sheet having a structure containing 2 to 10% in total area ratio and having high strength and excellent low-temperature toughness.
The bainitic ferrite phase has an average crystal grain size of 0.5 to 5.0 μm, a maximum length a parallel to the rolling direction of the crystal grains of 100 μm or less, and a maximum length a parallel to the rolling direction of the crystal grains and the plate The aspect ratio a / b of the crystal grains, which is the ratio of the maximum length b parallel to the thickness direction, is 3 or more and 10 or less.
[2] In addition to the above component composition, further, in mass%, V: 0.001% to 0.10%, Cu: 0.01% to 0.50%, Ni: 0.01% to 0.50%, Cr: 0.01% to 0.50% The high strength and low temperature described in [1] above, which contains one or more selected from Mo: 0.01% to 0.50%, B: 0.0001% to 0.0040% Hot rolled steel sheet with excellent toughness.
[3] A method for producing a hot-rolled steel sheet according to [1] or [2] above, wherein the steel material is heated at a temperature of 1050 ° C. to 1250 ° C., heating temperature × heating time: 1000 ° C. · hour or more. Reheat at 10000 ° C / hour or less, perform rough rolling, then finish rolling start temperature: 900 ° C or less, difference between finish rolling start temperature and finish rolling end temperature: 100 ° C or less, finish rolling reduction: 50% or more Finish rolling is performed at 70% or less, cooling is started within 1 second to 10 seconds after finish rolling, and the average cooling rate in the temperature range of steel sheet surface temperature: 750 ° C or lower and 650 ° C or higher is 50 ° C / s or higher and 500 ° C A method for producing a hot-rolled steel sheet having high strength and excellent low-temperature toughness, wherein the steel sheet is cooled at / s or less and wound in a temperature range of a steel sheet surface temperature of 400 ° C to 600 ° C.
In addition, in this specification, all% which shows the component of steel is the mass%.
In the present invention, the high-strength hot-rolled steel sheet is a hot-rolled steel sheet having a tensile strength (TS) of 650 MPa or more.

According to the present invention, a hot-rolled steel sheet having high strength and excellent low-temperature toughness can be obtained. And such a hot-rolled steel plate can be stably manufactured by the existing hot-rolling equipment.
Moreover, the hot-rolled steel sheet of the present invention is suitable as a material for spiral steel pipes, ERW steel pipes and the like.
As described above, the present invention is an industrially extremely useful invention.

Hereinafter, the present invention will be specifically described.
First, the reasons 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% unless there is particular notice.

C: 0.02% to 0.12%
C contributes to strength improvement. When C is less than 0.02%, it is difficult to ensure strength. On the other hand, if the content exceeds 0.12%, the material toughness or welded portion toughness is deteriorated. Therefore, C is 0.02% or more and 0.12% or less.

Si: 0.05% or more and 0.45% or less
Si is related to strength. Strength improvement is recognized when 0.05% or more of Si is contained. On the other hand, if it exceeds 0.45%, Si oxide remains in the weld and the weld zone toughness decreases. Therefore, Si is made 0.05% to 0.45%.

Mn: 1.0% to 2.0%
Mn contributes to strength improvement through the effect of improving hardenability. In order to obtain a desired strength, it is necessary to contain 1.0% or more of Mn. On the other hand, if the content exceeds 2.0%, a hard martensite phase is excessively generated, and the toughness is greatly reduced. Therefore, Mn is 1.0% or more and 2.0% or less.

P: 0.001% to 0.020%
Since P adversely affects toughness and welded portion toughness, the lower the value, the better. However, 0.020% or less is acceptable. On the other hand, reducing P more than necessary inhibits productivity, such as lengthening the de-P process, so the lower limit of P is 0.001%. Therefore, P is 0.001% or more and 0.020% or less.

S: 0.0001% or more and 0.0050% or less,
In hot-rolled steel sheets, S forms coarse MnS and expands during hot rolling, which adversely affects toughness. The smaller the S, the better. However, 0.0050% or less is acceptable. On the other hand, reducing S more than necessary hinders productivity, such as lengthening the de-S step, so the lower limit of S is set to 0.0001%. Therefore, S is 0.0001% or more and 0.0050% or less.

Al: 0.005% to 0.050%
Al is used as a deoxidizing agent, and a deoxidizing effect can be obtained with a content of 0.005% or more. On the other hand, if it exceeds 0.050%, it is present in the steel as inclusions, which adversely affects the toughness of the material and the welded part when used by welding. Therefore, Al is made 0.005% or more and 0.050% or less.

N: 0.0010% or more and 0.0060% or less
N exists as a precipitate of Al, Ti and Nb in the steel and is effective for improving the toughness by refining crystal grains, and it is necessary to contain 0.0010% or more. On the other hand, if the content exceeds 0.0060%, the toughness deteriorates. Therefore, N is 0.0010% or more and 0.0060% or less.

Nb: 0.020% to 0.080%
Nb contributes to improvement of strength and toughness by reducing the crystal grain size. The said effect is acquired by containing 0.020% or more of Nb. On the other hand, if the content exceeds 0.080%, the deformation resistance during hot rolling becomes too high, and hot rolling becomes difficult. Therefore, Nb is made 0.020% or more and 0.080% or less.

Ti: 0.005% to 0.050%
Ti contributes to strength improvement by precipitation strengthening. In order to obtain the effect, it is necessary to contain 0.005% or more of Ti. On the other hand, if the Ti content exceeds 0.050% and the Ti content is excessive, the toughness of the material or welded portion deteriorates. Therefore, Ti is made 0.005% or more and 0.050% or less.

Ca: 0.0005% or more and 0.0050% or less
Ca changes the precipitation form of MnS, which deteriorates toughness, and contributes to the improvement of toughness. The said effect is acquired by containing 0.0005% or more of Ca. On the other hand, if the content exceeds 0.0050%, Ca-based inclusions are excessively present in the steel, and the cleanliness of the steel sheet is lowered. Therefore, Ca is 0.0005% or more and 0.0050% or less.

  In the present invention, the balance other than the above is Fe and inevitable impurities. Inevitable impurities include Sb, Sn, Co, W, etc. These contents are acceptable if Sb: 0.01% or less, Sn: 0.1% or less, Co: 0.01% or less, W: 0.01% or less it can.

  The above is the basic component of the high-strength hot-rolled steel sheet of the present invention. The hot-rolled steel sheet of the present invention is, for example, for the purpose of improving toughness and increasing strength, V: 0.001% or more and 0.10% or less, Cu : 0.01% to 0.50%, Ni: 0.01% to 0.50%, Cr: 0.01% to 0.50%, Mo: 0.01% to 0.50%, B: 0.0001% to 0.0040% Or 2 or more types can be contained.

V: 0.001% to 0.10%
V contributes to improvement in strength. In order to acquire the effect, it is necessary to contain V 0.001% or more. On the other hand, if it exceeds 0.10% and excessively contains V, the toughness and weldability may deteriorate. Therefore, when it contains V, it is 0.001% or more and 0.10% or less.

Cu: 0.01% to 0.50%
Cu contributes to strength improvement. In order to acquire such an effect, it is necessary to contain 0.01% or more of Cu. On the other hand, if it exceeds 0.50%, it may cause hot brittleness and surface defects. Therefore, when it contains Cu, it is 0.01% or more and 0.50% or less.

Ni: 0.01% or more and 0.50% or less
Ni contributes to improvement of strength and toughness. In order to obtain such an effect, it is necessary to contain 0.01% or more of Ni. On the other hand, the content may exceed 0.50%, but the effect tends to be saturated. Therefore, when it contains Ni, it is 0.01% or more and 0.50% or less.

Cr: 0.01% to 0.50%
Cr is related to strength by improving hardenability. In order to suppress the formation of a pearlite phase and a ferrite phase and obtain a desired martensite phase or residual austenite phase, it is necessary to contain 0.01% or more of Cr. On the other hand, if the content exceeds 0.50%, weldability may be greatly reduced. Therefore, when it contains Cr, it is 0.01% or more and 0.50% or less.

Mo: 0.01% or more and 0.50% or less
Mo contributes to strength improvement. In order to improve hardenability, suppress the formation of a pearlite phase and a ferrite phase, and obtain a desired martensite phase and a retained austenite phase, it is necessary to contain 0.01% or more of Mo. On the other hand, when the content exceeds 0.50%, a martensite phase is excessively generated, and the toughness may be greatly reduced. Therefore, when it contains Mo, it is 0.01% or more and 0.50% or less.

B: 0.0001% or more and 0.0040% or less
B improves the hardenability and contributes to the improvement of strength. In order to acquire such an effect, it is necessary to contain B 0.0001% or more. On the other hand, if the content exceeds 0.0040%, the weld joint toughness may be adversely affected. Therefore, when it contains B, it is 0.0001% or more and 0.0040% or less.

Next, the reason for limiting the structure of the high-strength hot-rolled steel sheet of the present invention will be described.
The high-strength hot-rolled steel sheet of the present invention has an average crystal grain size of 0.5 to 5.0 μm, a maximum length a parallel to the rolling direction of the crystal grains is 100 μm or less, and a maximum length a parallel to the rolling direction of the crystal grains Bainitic ferrite phase with an area ratio of 90 to 98% in which the aspect ratio a / b of the crystal grains, which is the ratio of the maximum length b parallel to the plate thickness direction, is 3 to 10, and the martensite phase, bainite phase, It has a structure containing 2 to 10% of the total area ratio of any one or more of the retained austenite phases.

Bainitic ferrite phase When the average crystal grain size of the bainitic ferrite phase is large, the propagation resistance of cracks in the impact test becomes small, the crack progresses easily, and it becomes difficult to ensure good toughness. Therefore, it is important to control the structure finely and uniformly. When the average crystal grain size exceeds 5.0 μm, the toughness is adversely affected. The average grain size may be small, but the lower limit is 0.5 μm from the viewpoint of productivity and cost. Therefore, the average crystal grain size of the bainitic ferrite phase is 0.5 to 5.0 μm. Even in the case of the same average crystal grain size, localized coarse grains are the propagation points of cracks in the case of a structure in which various sizes and crystal grain sizes are mixed, compared to a structure having a uniform size and crystal grain size. Low temperature toughness may be poor. From this point, in addition to the average crystal grain size, shape control of individual crystal grains is also important.

  When the maximum length a parallel to the rolling direction exceeds 100 μm, the crack propagation resistance is reduced in the impact test, and the crack progresses easily, making it difficult to ensure good toughness. In addition, when the aspect ratio a / b of the crystal grain, which is the ratio of the maximum length a parallel to the rolling direction and the maximum length b parallel to the plate thickness direction, is smaller than 3, elliptical crystal grains that are not flat are In the impact test, the crack propagation resistance becomes small, the crack progresses easily, and it becomes difficult to ensure good toughness. On the other hand, after rough rolling, multi-pass rolling in the finish rolling process, rolling at a large rolling reduction, etc., and spreading in the rolling direction, and sufficiently flattened crystal grains with an aspect ratio a / b greater than 10, the progress of cracks Is not easy, but its effect tends to saturate. Therefore, the maximum length a parallel to the rolling direction in the crystal grain is 100 μm or less, and the aspect ratio of the crystal grain is the ratio of the maximum length a parallel to the rolling direction and the maximum length b parallel to the plate thickness direction in the crystal grain. a / b is 3 or more and 10 or less.

  In an impact test, in the case of a mixed structure in which a hard phase such as a bainitic ferrite phase (matrix phase) and a martensite phase exists, the crack propagates from the interface between the soft matrix phase and the hard phase, or the hard phase itself. The toughness is reduced. Therefore, control of the area ratio of each structure is important for improving toughness. By setting the area ratio of the bainitic ferrite phase to 90% or more and 98% or less at the 1/4 position of the plate thickness, a uniform structure can be obtained inside the plate thickness and excellent toughness can be secured. When the area ratio of the bainitic ferrite phase is less than 90%, other phases such as a martensite phase having higher strength than the bainitic ferrite phase are excessively mixed and the toughness is deteriorated. On the other hand, when the area ratio of bainitic ferrite phase exceeds 98%, YR = YS / TS (YR: Yield ratio, YS: Yield strength, TS: Tensile strength) becomes excessively high, and work hardening ability is increased. It is small and inferior in strain dispersibility at the time of deformation, and there is a risk of local buckling at the time of steel pipe laying, which is not preferable. Therefore, the area ratio of the bainitic ferrite phase is 90 to 98%.

Total area ratio of one or more of the martensite phase, bainite phase, and retained austenite phase: 2 to 10%
The balance other than the bainitic ferrite phase is composed of at least one of a martensite phase, a bainite phase, and a retained austenite phase. The bainite phase, the martensite phase, and the retained austenite phase that are transformed from austenite are hard and contribute to improving the strength of the hot-rolled steel sheet. In order to exert such an effect, the total area ratio of the bainite phase, the martensite phase, and the retained austenite phase needs to be 2% or more. On the other hand, when the total area ratio of the bainite phase, the martensite phase, and the retained austenite phase exceeds 10%, these phases function as a propagation path of cracks and the toughness deteriorates. Therefore, the total area ratio of the bainite phase, the martensite phase, and the retained austenite phase is 2% or more and 10% or less.

  The average crystal grain size, the maximum length a of the crystal grains parallel to the rolling direction, the maximum length b of the crystal grains parallel to the plate thickness direction, and the area ratio of each phase are measured by the methods of the examples described later. can do.

  Next, a method for producing a hot-rolled steel sheet having high strength and excellent low-temperature toughness according to the present invention will be described.

  The high-strength hot-rolled steel sheet of the present invention is obtained by reheating a steel material having the above composition at a heating temperature of 1050 ° C. to 1250 ° C., a heating temperature × heating time of 1000 ° C. · hour to 10000 ° C. · hour, Next, finish rolling is performed at a finish rolling start temperature of 900 ° C. or less, a difference between the finish rolling start temperature and the finish rolling end temperature: 100 ° C. or less, and a finish rolling reduction ratio of 50% to 70%. Cooling is started within 1 second to 10 seconds after completion, and the steel sheet surface temperature is 750 ° C or lower and the average cooling rate in the temperature range of 650 ° C or higher is 50 ° C / s or higher and 500 ° C / s or lower, and the steel plate surface temperature is 400 ° C. It can be manufactured by winding in a temperature range of 600 ° C. or lower.

  Details will be described below.

  The manufacturing method of the steel material is not particularly limited, and any conventional method in which the molten steel having the above composition is melted in a converter or the like and is made into a steel material such as a slab by a casting method such as continuous casting. Is also applicable. Note that the ingot-making / bundling method may be used.

Reheating: heating temperature 1050 ° C. or more and 1250 ° C. or less, heating temperature × heating time: 1000 ° C. · hour or more and 10000 ° C. · hour or less The heating conditions are important for controlling the average crystal grain size and the shape of individual crystal grains. If the heating temperature is 1250 ° C, or if the heating temperature x heating time exceeds 10000 ° C / hour, the austenite grains during heating become coarse, the average crystal grain size finally obtained after hot rolling becomes coarse, and the toughness deteriorates. . On the other hand, when the temperature is lower than 1050 ° C., or when the heating temperature × heating time is less than 1000 ° C. · hour, solid solution of precipitation strengthening elements such as Ti and Nb becomes insufficient, and it becomes difficult to secure desired strength. Therefore, the heating temperature is from 1050 ° C. to 1250 ° C., and the heating temperature × heating time is from 1000 ° C. · hour to 10000 ° C. · hour.

Finish rolling: The finish rolling start temperature is 900 ° C or less, the difference between the finish rolling start temperature and the finish rolling finish temperature is 100 ° C or less, and the finish rolling reduction ratio is 50% or more and 70% or less. This is important from the viewpoint of increasing the number of nucleation sites and finally obtaining fine crystal grains. It is also important in terms of flattening the crystal grains, increasing the propagation resistance of cracks in the impact test, preventing the cracks from progressing easily, and ensuring excellent toughness. When the finish rolling start temperature exceeds 900 ° C., the crystal grains become coarse and the toughness deteriorates. If the difference between the finish rolling start temperature and finish temperature exceeds 100 ° C, the time between passes during finish rolling becomes longer, and strain recovery between passes occurs, so that sufficient strain does not accumulate during rolling and processing. The bainitic ferrite transformed from the austenite formed is coarsened. In addition, the maximum length a parallel to the rolling direction in each crystal grain becomes longer, and the toughness is deteriorated. Therefore, the finish rolling start temperature is 900 ° C. or less, and the difference between the finish rolling temperature and the finish rolling end temperature is 100 ° C. or less.
When the finish rolling reduction is less than 50%, bainitic ferrite that is transformed from the processed austenite is coarsened. In addition, the maximum length a parallel to the rolling direction in each crystal grain becomes longer, and the toughness is deteriorated. In order to obtain fine grains, a higher rolling reduction is preferable. However, if the rolling reduction exceeds 70%, deformation resistance increases and rolling becomes difficult. Therefore, the finish rolling reduction is set to 50% to 70%.

Cooling start time after hot rolling (finish rolling): 1 second or more and within 10 seconds When finishing rolling finishes and the time to start cooling exceeds 10 seconds, the crystal grains become coarse during cooling after the end of rolling. , A fine structure cannot be obtained, and toughness decreases. The earlier the cooling start time after finishing rolling is, the better. However, the lower limit is set to 1 second from the viewpoint of productivity and cost. Accordingly, the cooling starts from 1 second to 10 seconds after finishing rolling.

Average cooling rate in the temperature range of steel sheet surface temperature 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 bainitic ferrite phase In order to secure the desired amount and obtain excellent low temperature toughness, the average cooling rate in the temperature range of 750 ° C. or lower and 650 ° C. or higher is required to be 50 ° C./s or higher. The cooling rate may be high, but the effect tends to saturate when the cooling rate exceeds 500 ° C./s. The temperature range of 750 ° C or lower and 650 ° C or higher is the temperature range where polygonal ferrite and pearlite are transformed from austenite during continuous cooling when the cooling rate is slow, and it is desired by controlling the average cooling rate in that temperature range. An amount of bainitic ferrite phase can be secured. Therefore, the average cooling rate in the temperature range of 750 ° C. or lower and 650 ° C. or higher on the steel sheet surface is 50 ° C./s or higher and 500 ° C./s or lower.

Steel sheet surface temperature during winding: Temperature range of steel sheet surface temperature of 400 ° C or higher and 600 ° C or lower If the coiling temperature exceeds 600 ° C, a polygonal ferrite phase and a pearlite phase are generated, and the desired structure cannot be obtained. High toughness cannot be obtained. On the other hand, when the temperature is lower than 400 ° C., it becomes difficult to secure a desired bainitic ferrite phase, and excellent toughness cannot be obtained. Therefore, the steel sheet surface temperature is taken up in a temperature range of 400 ° C to 600 ° C.

  After winding, temper rolling may be performed according to a conventional method, or the scale formed on the surface may be removed by pickling. Alternatively, a plating treatment such as hot dip galvanization or electrogalvanization or chemical conversion treatment may be performed.

Using the slab (continuous cast slab) (slab thickness: 220 mm) having the composition shown in Table 1, hot rolling is performed under the hot rolling conditions shown in Table 2, and after the hot rolling is finished, the cooling conditions shown in Table 2 are given. Then, it was wound around a coil having a predetermined size at a winding temperature shown in Table 2 to obtain a hot rolled steel sheet (steel strip) having a thickness of 10 to 25 mm.
Test pieces were collected from the hot-rolled steel sheet obtained as described above, and subjected to structure observation, tensile test, and impact test. The tissue observation method and various test methods are as follows.

(1) Structure observation The area ratio of each structure of the hot rolled steel sheet at the 1/4 thickness position of the structure is observed and imaged using a scanning electron microscope SEM (magnification: 1000 to 5000 times) over 3 fields of view. The area ratio of the bainitic ferrite phase at the 1/4 thickness position and the total area ratio of the bainite phase, martensite phase, and retained austenite phase were determined.
Further, the structure of the hot-rolled steel sheet at the 1/4 thickness position was imaged by observing 3 or more fields of view using SEM (magnification: 2000 times), and the average crystal grain size of the bainitic ferrite phase was obtained.

  The total area ratio of the bainite phase, martensite phase, and retained austenite phase is defined as the sum of the martensite phase and retained austenite phase on the SEM photograph taken and the area where fine carbides are observed. The phases were determined, and the areas of these phases were summed and determined as a ratio to the observed area. Moreover, the area ratio was calculated | required by making remainder the bainitic ferrite phase.

The average crystal grain size of the bainitic ferrite phase is calculated by counting the grains of the bainitic ferrite phase using an imaged SEM photograph and calculating the average grain area A using the area ratio of the bainitic ferrite phase. Obtained by the quadrature method with d = √A.
The maximum length a parallel to the rolling direction and the maximum length b parallel to the plate thickness direction for each crystal grain are 3 fields at the 1/4 position of the plate thickness using an optical microscope (magnification: 400 to 1000 times). Using the optical micrographs observed and imaged above, a and b were measured for each of a plurality of coarse crystal grains, and the longest value among the measured values was determined as the maximum length.

(2) Tensile test A flat full-thickness tensile test piece (plate) so that the direction perpendicular to the rolling direction (C direction) is the longitudinal direction from the position of 1/4 of the plate width of the obtained hot-rolled steel sheet. Thickness: total thickness, parallel part length: 60 mm, distance between gauges: 50 mm, gauge part width: 38 mm), and a tensile test at room temperature in accordance with ASTM E8M-04 regulations. Asked. 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”.

(3) Charpy impact test V notch so that the direction perpendicular to the rolling direction (C direction) is the longitudinal direction from the position of 1/4 of the sheet width of the hot-rolled steel sheet obtained and the position 1 mm below the sheet thickness surface layer. A test piece (length 55 mm × height 10 mm × width 10 mm) 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.). The number of test pieces was three, and the arithmetic average of the obtained ductile-brittle fracture surface transition temperature was determined to obtain the ductile-brittle fracture surface transition temperature (vTrs) of the steel sheet. The case where vTrs was −120 ° C. or lower was evaluated as “good toughness”.

  The results obtained as described above are shown in Table 3.

  As shown in Table 3, in the inventive examples, the tensile strength was 650 MPa or more, and the toughness (low temperature toughness) was also good. That is, a high-strength hot-rolled steel sheet that has both high strength and excellent low-temperature toughness has been obtained. On the other hand, in the comparative example, sufficient toughness (low temperature toughness) was not obtained. In some comparative examples, the tensile strength was less than 650 MPa.

Claims (3)

In mass%, C: 0.02% to 0.12%, Si: 0.05% to 0.45%, Mn: 1.0% to 2.0%, 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: 0.020% to 0.080%, Ti: 0.005% to 0.050%, Ca: 0.0005% to 0.0050%, with the balance being Fe And having a component composition consisting of inevitable impurities,
Have a structure that satisfies the following and contains a bainitic ferrite phase with an area ratio of 90 to 98% and one or more of a martensite phase, a bainite phase, and a retained austenite phase in a total area ratio of 2 to 10%. Hot-rolled steel sheet with high strength and excellent low-temperature toughness.
The bainitic ferrite phase has an average crystal grain size of 0.5 to 5.0 μm, a maximum length a parallel to the rolling direction of the crystal grains of 100 μm or less, and a maximum length a parallel to the rolling direction of the crystal grains and the plate The aspect ratio a / b of the crystal grains, which is the ratio of the maximum length b parallel to the thickness direction, is 3 or more and 10 or less.   In addition to the above component composition, in addition, by mass, V: 0.001% to 0.10%, Cu: 0.01% to 0.50%, Ni: 0.01% to 0.50%, Cr: 0.01% to 0.50%, Mo The high-strength and low-temperature toughness according to claim 1, characterized by containing one or more selected from: 0.01% or more and 0.50% or less, B: 0.0001% or more and 0.0040% or less Hot rolled steel sheet. A method for producing a hot-rolled steel sheet according to claim 1 or 2,
The steel material is reheated at a heating temperature of 1050 ° C to 1250 ° C, heating temperature x heating time: 1000 ° C · hour to 10000 ° C · hour, and subjected to rough rolling.
Next, finish rolling start temperature: 900 ° C. or less, difference between finish rolling start temperature and finish rolling end temperature: 100 ° C. or less, finish rolling reduction: 50% to 70%,
Cooling starts within 1 second to 10 seconds after finishing rolling, and the steel sheet surface temperature is cooled at an average cooling rate of 50 ° C / s to 500 ° C / s in the temperature range of 750 ° C or lower to 650 ° C or higher. A method for producing a hot-rolled steel sheet having high strength and excellent low-temperature toughness, characterized by winding in a temperature range of 400 ° C to 600 ° C.