JP6191268B2 - High yield ratio high strength hot-rolled steel sheet with less variation in strength in the coil width direction and excellent toughness, and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength hot-rolled steel sheet that is mainly used as a structural member for construction equipment, such as a boom of a large crane, and particularly has high yield strength and small variation in the sheet width direction. The present invention relates to a high yield ratio high strength hot rolled steel sheet excellent in toughness characterized by the following and a method for producing the same. In addition, although the size of the steel plate of this invention is not limited, it mainly targets the steel plate with a plate thickness of 10 mm or less and a plate width of 1200 mm or more.

  The boom of cranes for construction machinery is becoming longer and larger with the recent rise in construction objects. Therefore, in order to reduce the weight of the boom itself and increase the lifting and carrying capacity, the steel plate that is the material tends to be thinned, and higher yield strength is required. In addition, as cranes become larger, there is a growing need for steel plates that are wider than before. So far, in order to increase the strength, a large amount of solid solution strengthening elements such as Si and Mn and precipitation strengthening elements such as Ti and Nb have been added to steel components. For example, Patent Documents 1 to 5 all increase the Ti addition amount in order to utilize Ti precipitation strengthening, but Patent Documents 1 to 3 include 0.12% or more of Ti addition in order to sufficiently utilize precipitation strengthening. Heating at a high temperature of 1250 ° C. or higher is essential.

  Patent Documents 6 and 7 are inventions related to high-strength steel sheets that ensure strength and toughness by using a martensite phase or a tempered mansite phase as the main phase, and cool the hot-rolled sheet to (Ms point + 50 ° C.) or less. Then, after holding at a cooling stop temperature of ± 100 ° C., tempering is performed by winding. However, it is very difficult to control the coiling temperature of the hot-rolled sheet in the range of 200 to 500 ° C., which causes a material variation in the coil.

  On the other hand, the hot-rolling material for booms is often used by cutting in the rolling direction. Therefore, the material variation in the width direction directly leads to the strength variation as a member, which causes a shape defect. On the other hand, the widening of the steel sheet increases the material variation in the width direction, so the reduction in the strength variation in the width direction is a very important issue.

JP-A-7-138638 JP-A-5-230529 JP-A-5-271865 JP 2002-97545 A JP 2004-250744 A JP 2011-52320 A JP 2011-52321 A

  The present invention has been made in view of the above problems, and by subjecting a steel sheet wound at 200 ° C. or lower to batch annealing, there is little strength variation in the width direction of the sheet, and a high yield ratio high strength steel sheet excellent in toughness and The object is to provide a manufacturing method thereof.

  With regard to the above problems, the present inventors have further optimized the component range in the component system in which Mo and B are added in combination, thereby achieving both the strength level and the degree of strength variation in the steel sheet, and after hot rolling. It was found that high-strength hot-rolled steel sheets with high toughness and high yield ratio can be obtained with the strength variation reduced by batch annealing. That is, by adding Mo and B in combination, the cold speed dependence of hardenability is minimized, strength variation is reduced, and the balance with other hardenability improving components is optimized to achieve desired strength. The level is to be obtained. In addition, after hot rolling, by cooling to room temperature once, the occurrence of strength variation in the coil is suppressed as much as possible, and further, by performing batch annealing in the temperature range of 150 to 350 ° C, the strength variation in the coil is reduced. It is another object of the present invention to provide a high-strength hot-rolled steel sheet having excellent toughness and high yield ratio, and a method for producing the same.

That is, the high yield ratio high strength hot-rolled steel sheet and the method for producing the same according to the present invention with less variation in strength in the coil width direction and excellent toughness are as follows.
(1) In mass%,
C: 0.05% or more, 0.2% or less,
Si: 0.01% or more, 0.6% or less,
Mn: 0.5% or more, 2.5% or less,
P: 0.001% or more, 0.1% or less,
S: 0.0005% or more, 0.05% or less,
Al: 0.01% or more, 0.2% or less,
N: 0.0001% or more, 0.010% or less,
Mo: 0.05% or more, 0.5% or less,
Ti: 48N / 14 + 0.01% or more, 0.14% or less,
B: 0.0003% or more and 0.005% or less in a range satisfying the following formula (1), the balance having a steel composition consisting of iron and inevitable impurities, yield strength of 960 MPa or more, yield ratio There is 0.83 or more and variation in the yield strength of the plate width direction Ri der within 50 MPa, the strength variation in the coil width direction impact absorbed energy at -40 ℃ characterized -32J ultra der Rukoto High yield ratio high strength hot-rolled steel sheet with little toughness.
70 ≦ 300 × C (mass%) + 33 × Mn (mass%) + 22 × Cr (mass%) + 11 × Mo (mass%) + 11 × Si (mass%) + 17 × Ni (mass%) ≦ 100 (1)
(2) Furthermore, in mass%,
Nb: 0.005% or more and 0.09% or less. The high yield ratio high strength hot-rolled steel sheet with less strength variation in the coil width direction and excellent toughness as described in (1).
(3) Furthermore, in mass% ,
W: 0.01% or more, 2.0% or less,
Cu: 0.04% or more, 2.0% or less,
Ni: 0.02% or more, 1.0% or less,
V: 0.001% or more, 0.10% or less,
The high yield ratio high strength hot-rolled steel sheet according to any one of (1) and (2), wherein the strength variation in the coil width direction is small and the toughness is excellent. .
(4) Furthermore, in mass% ,
One or more of Mg and Zr are contained in a total of 0.0005% to 0.05% in the coil width direction according to any one of (1) to (3), High yield ratio high strength hot-rolled steel sheet with low strength variation and excellent toughness.
(5) A method for producing the high-strength hot-rolled steel sheet according to any one of (1) to (4), wherein a slab having the steel component according to any one of (1) to (4) is 1100 ° C. After heating to 1250 ° C. or lower, hot rolling is performed under conditions where the finishing temperature is 850 ° C. or higher and 950 ° C. or lower. The coil according to any one of (1) to (4), wherein the coil is cooled to room temperature and then annealed in a batch annealing furnace (BAF annealing) in a temperature range of 150 to 350 ° C. A method of producing a high-strength hot-rolled steel sheet having high yield ratio and high toughness with little variation in strength in the width direction.

  According to the high yield ratio high strength hot-rolled steel sheet and its manufacturing method with less strength variation in the coil width direction and excellent toughness according to the present invention, with the above configuration, the strength variation in the sheet width direction is small, and the yield strength is 960 MPa or more. A high-yield-ratio high-strength steel sheet excellent in toughness with a yield ratio of 0.83 or more can be obtained. Therefore, for example, by applying the present invention to a structural member of a construction machine such as a boom of a large crane, the boom itself can be reduced in weight and the lifting and carrying capacity can be increased, and the work efficiency can be improved. Its social contribution is immeasurable because it can fully enjoy the benefits of significant improvement.

  Hereinafter, the high-strength steel plate which is an embodiment of the present invention and the manufacturing method thereof will be described. This embodiment will be described in detail in order to better understand the purpose of the high yield ratio high strength hot-rolled steel sheet and its manufacturing method with less variation in strength in the coil width direction and excellent toughness according to the present invention. Therefore, the present invention is not limited unless otherwise specified.

The high yield ratio high strength hot-rolled steel sheet (hereinafter sometimes simply referred to as “high-strength hot-rolled steel sheet”) having little strength variation in the coil width direction and excellent toughness according to the present invention is expressed by mass%, and C: 0.05 %: 0.2% or less, Si: 0.01% or more, 0.6% or less, Mn: 0.5% or more, 2.5% or less, P: 0.001% or more, 0.1% or less S: 0.0005% or more, 0.05% or less, Al: 0.01% or more, 0.2% or less, N: 0.0001% or more, 0.010% or less, Mo: 0.05% or more 0.5% or less, Ti: 48N / 14 + 0.01% or more, 0.14% or less, B: 0.0003% or more and 0.005% or less, so as to satisfy the following formula (1) The balance has a steel composition consisting of iron and inevitable impurities, the yield strength is 960 MPa or more, the yield ratio is 0.83 or more, and The variation in the yield strength in the plate width direction is set to be within 50 MPa, which is roughly configured.
70 ≦ 300 × C (mass%) + 33 × Mn (mass%) + 22 × Cr (mass%) + 11 × Mo (mass%) + 11 × Si (mass%) + 17 × Ni (mass%) ≦ 100 1)
Below, the reason for limitation of the steel characteristic and manufacturing conditions in this invention is demonstrated in detail.

[Steel composition]
[(C: carbon) 0.05% or more, 0.2% or less]
C is an element that can ensure strength at low cost, and is an essential element of the present invention. In order to satisfy the strength, if C is less than 0.05%, the strength defined in the present invention cannot be satisfied. On the other hand, when C exceeds 0.2%, the strength is excessively increased, ductility is lowered, and weldability is also deteriorated.
For this reason, in this invention, content of C was prescribed | regulated to 0.05% or more and 0.2% or less. A more preferable range of the C content is 0.10% or more and 0.15% or less.

[(Si: silicon) 0.01% or more, 0.6% or less]
Si is added in an amount of 0.01% or more to ensure strength. Further, from the viewpoint of weldability, it is desirable to add 0.1% or more of Si. However, if Si is added in excess of 0.6%, defects called Si scales are generated on the surface and the surface quality is remarkably lowered, so 0.6% is made the upper limit. From this viewpoint, the amount of Si added is more preferably 0.3% or less, and still more preferably 0.15% or less.

[(Mn: Manganese) 0.5% or more, 2.5% or less]
Mn is added in an amount of 0.5% or more from the viewpoint of securing strength. From this point of view, Mn is preferably added in an amount of 1.0% or more, and more preferably 1.3% or more. Further, if the amount of Mn added exceeds 2.5%, the weld crack sensitivity deteriorates, so the upper limit is made 2.5% or less. From this viewpoint, the amount of Mn added is desirably 2.2% or less, and more desirably 2.0% or less.

[(P: phosphorus) 0.001% or more, 0.1% or less]
P is added according to the strength level required as an element for increasing the strength of the steel sheet. However, if the amount of P added is large, it segregates to the grain boundaries, thereby degrading local ductility, weldability and toughness. Therefore, the P upper limit is set to 0.1%. From this viewpoint, P is preferably 0.05% or less. On the other hand, if it is less than 0.001%, the deterioration effect of P is negligible. If it is less than 0.001%, the cost is increased, so 0.001% is made the lower limit.

[(S: sulfur) 0.0005% or more, 0.05% or less]
S is an element that degrades local ductility, weldability, and toughness by generating MnS, and is preferably an element that does not exist in steel, so the upper limit is made 0.05%. From this viewpoint, S is desirably 0.01% or less. On the other hand, if S is less than 0.0005%, the cost is increased, so this is set as the lower limit.

[(Al: aluminum) 0.01% or more, 0.2% or less]
Al needs to be added in an amount of 0.01% or more as a deoxidizing material. On the other hand, even if Al is added excessively, the steel is embrittled and weldability is lowered, so 0.2% is made the upper limit.

[(N: nitrogen) 0.0001% or more, 0.010% or less]
N is an element inevitably contained in the steel, but forms BN, reduces the solid solution B, and deteriorates the hardenability, so the content is made 0.010% or less. From this viewpoint, N is preferably added in an amount of 0.006% or less. On the other hand, unnecessarily reducing N increases the cost in the steelmaking process, so the content is controlled to 0.0001% or more.

[(Mo: molybdenum) 0.05% or more, 0.5%]
Mo is an important element in the present invention. Mo is an element for improving hardenability, but by adding it in combination with B, there is an effect of reducing the cooling rate dependency of the hardenability. For this reason, it is possible to suppress intensity variation in the coil. Moreover, since there is little deterioration of toughness compared to Mn, 0.05% or more is added. From this viewpoint, it is desirable to add Mo by 0.1% or more. On the other hand, even if Mo is added in an amount of 0.5% or more, a special effect is not obtained, and the alloy cost is only increased, so 0.5% is made the upper limit. From this viewpoint, the addition amount of Mo is desirably 0.3% or less.

[(B: boron) 0.0003% or more, 0.005% or less]
B is also an important element in the present invention. B is an inexpensive hardenability-improving element, and by adding it together with Mo, there is an effect of reducing the strength-dependent cooling rate dependency and reducing the strength variation in the coil. Therefore, in this invention, B is added 0.0003% or more. From this viewpoint, B is preferably added in an amount of 0.0006% or more. On the other hand, adding more than 0.005% of B not only gives a special effect but also causes toughness deterioration, so 0.005% is made the upper limit. From this point of view, B is preferably added in an amount of 0.003% or less.

[(Ti: Titanium) 48N / 14 + 0.01% or more, 0.14% or less]
Ti forms TiN at a high temperature to inhibit the precipitation of BN and contributes to an increase in strength due to carbide formation and solid solution. Therefore, it is added in the following formula {48N / 14 + 0.01}% or more. N in the formula is the content (% by mass) of nitrogen (N). On the other hand, even if Ti is added in excess of 0.14%, not only a further increase in strength is not obtained, but also the toughness and weldability are lowered, so this value is made the upper limit.

[(Nb: niobium) 0.005% or more, 0.09% or less]
In the present invention, it is desirable to add Nb in a predetermined range in addition to the above essential elements. Here, Nb, like Ti, contributes to the increase in strength through the suppression of recrystallization, the refinement of the structure, and the precipitation of carbides, and particularly the improvement of the yield strength. Therefore, Nb is preferably added in an amount of 0.005% or more. . On the other hand, addition of Nb exceeding 0.09% significantly deteriorates the toughness and ductility of toughness, so this value is made the upper limit. A more preferable range of Nb is 0.015% or more and 0.05% or less, and a further preferable range is 0.015% or more and 0.03% or less.

[(Cr: chrome) 0.1% or more, 2.0% or less]
[(W: tungsten) 0.01% to 2.0%]
Cr and W are both elements that have the effect of improving hardenability and forming carbides to increase strength. Therefore, it is desirable to add 0.1% (Cr) or more and 0.01% (W) or more, respectively. On the other hand, addition of more than 2.0% (Cr) and more than 2.0% (W) respectively reduces ductility and weldability. From the above viewpoint, Cr is desirably added as necessary within a range of 0.1% to 2.0% and W within a range of 0.01% to 2.0%.

[(Cu: copper) 0.04% or more, 2.0% or less]
Cu is an element that increases the steel sheet strength and improves the corrosion resistance and the peelability of the scale, so it is desirable to add 0.04% or more. On the other hand, since addition of Cu exceeding 2.0% causes surface defects, it is desirable to add as necessary within a range of 0.04% to 2.0%.

[(Ni: nickel) 0.02% to 1.0%]
Ni is an element that increases the strength of the steel sheet and improves the toughness, so it is desirable to add 0.02% or more. On the other hand, since addition of Ni exceeding 1.0% causes ductile deterioration, it is desirable to add as necessary within a range of 0.02% to 1.0%.

[(V: Vanadium) 0.001% or more, 0.10% or less]
V is an element effective in improving the strength. However, the effect cannot be obtained with addition of V less than 0.001%, and the addition of more than 0.10% conversely causes a decrease in toughness, so the range is made 0.001 to 0.10%. .
Furthermore, in the present invention, one or more of Ca, Mg, Zr, and REM (rare earth elements) are used alone or in total as 0.0005% or more, 0.05 as elements for improving the steel characteristics. % Or less.
Ca, Mg, Zr, and REM improve toughness by controlling the shape of sulfides and oxides. For this purpose, it is necessary to add one or more of these elements alone or in total of 0.0005% or more. However, excessive addition of these elements deteriorates workability, so the upper limit was made 0.05%.
In addition to the above elements, the steel of the present invention contains elements inevitably mixed such as Sn and As, with the balance being iron and unavoidable impurities.

[Relationship between each element]
Next, the following formula (1) which is a relational expression of each element will be described.
In the high-strength hot-rolled steel sheet of the present invention, in order to obtain the effects of the present invention, the steel composition needs to satisfy the relationship of the following formula (1).

70 ≦ 300 × C (mass%) + 33 × Mn (mass%) + 22 × Cr (mass%) + 11 × Mo (mass%) + 11 × Si (mass%) + 17 × Ni (mass%) ≦ 100 1)

If the value of the formula (1) is less than 70, sufficient strength cannot be obtained. From this viewpoint, it is more desirable to set the value of the above expression (1) to 70 or more. On the other hand, if the value of the above formula (1) exceeds 100, the strength of the hot-rolled sheet becomes too high and the load on the winding device becomes too high. From this point of view, it is desirable to set the value of the above expression (1) to 100 or less. A more preferable range of the formula (1) is 80 or more and 95 or less, and a more preferable range is 85 or more and 90 or less.

[Yield strength (YP)]
In the high-strength hot-rolled steel sheet of the present invention, the yield strength (YP) is specified to be 960 MPa or more.
In the present invention, a high-strength hot-rolled steel sheet having a yield strength of 960 MPa or more can be realized by controlling the steel composition within the above-described range and further setting each manufacturing condition to the conditions described later. In this way, by increasing the yield strength to 960 MPa or more, for example, even when the steel plate thickness is reduced to about 4.0 to 10 mm, a sufficiently high strength can be secured as a member, and the weight can be reduced. Can contribute. When measuring the yield strength of high-strength hot-rolled steel sheets, the distance between 1/20 to 1/5 of the total sheet width and the central part of the sheet thickness from both edges in the sheet width direction of the steel sheet. It is preferable that the yield strengths of the tensile test pieces prepared from the above are 960 MPa or more, respectively.

[Yield Strength Ratio (YR)]
On the other hand, an excessive increase in the maximum strength (TS) lends an excessive load to the apparatus during the molding of the member, leading to a defective shape of the member or a damage to the molding apparatus. Therefore, the yield strength ratio YR (= YP / TS), which is the ratio of YP and TS, is 0.83 or more.

[Variation in yield strength in the width direction]
If the variation in yield strength in the coil width direction (plate width direction) exceeds 50 MPa, the variation is set to 50 MPa or less because it causes warping after cutting the steel plate or shape failure during pressing. The variation in this case is the maximum / minimum value of the yield strength of the tensile test specimens produced from the two edges, at a distance of 1/20 to 1/5 of the total plate width and at three locations in the center of the plate thickness. Point to the difference.

[Production method]
A method for producing a high yield ratio high strength hot-rolled steel sheet having less variation in strength in the coil width direction and excellent toughness according to the present invention will be described below.
In the method for producing a high-strength hot-rolled steel sheet of the present invention, a slab having the above steel components is heated to 1100 ° C. or more and 1250 ° C. or less, and then hot-rolled under a condition that the finishing temperature is 850 ° C. or more and 950 ° C. or less. Then, after cooling at a cooling rate of 10 ° C./second or more, winding in a coil shape at a temperature of 200 ° C. or less and once cooling to room temperature, annealing by a batch annealing furnace (BAF annealing) in a temperature range of 150 to 350 ° C. It is a method of applying.

  First, steel is melted and cast by a conventional method to obtain a steel slab (slab) to be subjected to hot rolling. The steel slab may be a forged or rolled steel ingot, but from the viewpoint of productivity, the steel slab is preferably produced by continuous casting, or may be produced by a thin slab caster or the like. Alternatively, a process such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after the molten steel is cast may be employed.

  Usually, the steel slab is cooled after casting, and then heated again for hot rolling. In this case, the heating temperature of the steel slab when hot rolling is 1100 ° C. or higher. In order to perform heating uniformly at a temperature of less than 1100 ° C., it is necessary to hold for a long time and the productivity is lowered, so this temperature is set as the lower limit. On the other hand, if the steel slab is heated to over 1250 ° C., the crystal grain size of the steel sheet becomes coarse and the workability may be impaired, so this value is made the upper limit.

  In the production method of the present invention, the finishing temperature is 850 ° C. or higher. When the hot rolling is finished at less than 850 ° C., the hot rolling load becomes too high, the hardenability is lowered, and the strength is lowered. On the other hand, if the finishing temperature exceeds 950 ° C., the γ grain size becomes coarse and the toughness decreases, so this temperature is set as the upper limit. After hot rolling, cooling is performed at a cooling rate of 10 ° C./s or higher in any temperature range up to 200 ° C. or lower. If the cooling rate is less than 10 ° C./s, sufficient strength cannot be obtained, so this value is set as the lower limit. From this viewpoint, a cooling rate of 25 ° C./s or higher is desirable. Although the upper limit of the cooling rate is not particularly defined, in order to cool at a rate of 100 ° C./s or more, excessive capital investment is required, but since a special effect cannot be obtained, it is 100 ° C./s or less. It is realistic to do.

  In the manufacturing method of the present invention, the coiling temperature is 200 ° C. or less. When the coiling temperature exceeds 200 ° C., the martensite transformation does not occur sufficiently, the strength is lowered, and it is difficult to control to a uniform coiling temperature. The value is the upper limit. Although the lower limit of the coiling temperature is not particularly defined, cooling to room temperature or lower requires excessive facilities, but on the other hand, no special effect is obtained.

  The wound coil is once cooled to room temperature, and then subjected to heat treatment in a batch annealing furnace (BAF furnace) so that the coil remains in the temperature range of 150 to 350 ° C. and has a holding time of 0.5 to 8 hours (hereinafter referred to as “coil”). In addition, annealing in a batch annealing furnace may be referred to as BAF annealing). Since annealing at a temperature lower than 150 ° C. results in insufficient carbide precipitation and tempering of martensite, YP cannot be increased, so this temperature is set as the lower limit. On the other hand, holding above 350 ° C. causes a significant decrease in TS and causes a variation in strength within the coil, so this temperature is made the upper limit.

  Further, if the holding time is less than 0.5 hours, YP cannot be increased due to insufficient carbide precipitation and martensite tempering, so this holding time is made the lower limit. On the other hand, if the holding time exceeds 8 hours, the TS is remarkably lowered and causes a variation in strength within the coil, so this holding time is made the upper limit.

  As described above, according to the present invention, according to the high yield hot rolled steel sheet having high yield ratio and high strength with little toughness variation in the coil width direction according to the present invention and the manufacturing method thereof, the above structure has high yield strength and yield ratio. Further, it is possible to realize a high-strength hot-rolled steel sheet with small strength variation in the coil width direction while ensuring toughness.

  Therefore, for example, by applying the present invention to a structural member of a construction machine such as a boom of a large crane, the boom itself can be reduced in weight and the lifting and carrying capacity can be increased, and the work efficiency can be improved. Its social contribution is immeasurable because it can fully enjoy the benefits of significant improvement.

  Hereinafter, examples of the high yield ratio high strength hot-rolled steel sheet and its manufacturing method with less strength variation in the coil width direction and excellent toughness will be given and the present invention will be described more specifically. The present invention is not limited to the examples, and can be carried out with appropriate modifications within a range that can be adapted to the purpose described above and below, all of which are included in the technical scope of the present invention. .

[Example 1]
In this example, first, steel having the composition shown in Table 1 below was melted, and hot rolling and annealing in a BAF furnace were performed under the conditions shown in Tables 2 and 3 below. Tables 2 and 3 below also show the results of investigating the properties of the obtained hot-rolled sheet.

  As for the tensile properties, JIS No. 5 tensile test specimens were sampled from the direction perpendicular to the rolling direction, and the tensile properties were evaluated. The plate width of the hot-rolled plate is 1500 mm to 2200 mm, and the tensile test piece is placed so that the position (-L, -R) 150 mm from the both edge portions of the plate and the center position (-C) of the plate width are the center of the parallel portion. Produced.

  The toughness was evaluated by a Charpy impact test. At this time, JIS Z 2202 test pieces were prepared from the center position of the plate width with the direction perpendicular to the rolling direction as the longitudinal direction, and the absorbed energy at a test temperature of −40 ° C. was measured. In addition, about the steel plate less than 10 mm in thickness, it converted into the value of a 10 mm full size test piece.

Details of the results of this example are described below.
As is apparent from the results shown in Tables 2 and 3, when the steel having the chemical components specified in the present invention is hot-rolled under appropriate conditions, the yield strength is 960 MPa or more and the yield strength ratio is 0.83 or more. It was possible to obtain a hot rolled steel sheet having excellent toughness with a variation in yield strength in the sheet width direction within 50 MPa and an impact absorption energy at −40 ° C. exceeding 32 J. However, no. Reference numerals 11, 12, 16, 17, 23, and 24 are reference examples.

  On the other hand, production No. Nos. 40 to 46 are steel Nos. Whose chemical components are outside the scope of the present invention. This is a comparative example using R to X. Production No. 40 and No. 43 to 45 all have too many additional elements and exceed the upper limit of Formula 1, and the toughness is deteriorated. On the other hand, production No. In 41 and 46, the value of Equation 1 is below the lower limit, and the yield strength is reduced. In addition, production No. Since the amount of Ti added is low because TiN is not formed and becomes BN, the solid solution B that contributes to improving the hardenability is lowered and the strength is lowered. Production No. In No. 46, Mo is not added, so that the hardenability is not sufficiently secured, and the variation of the yield strength in the coil width direction is large.

  Production No. Although 5, 13, 22, 27, 31, and 34 all satisfy the present invention as chemical components, the yield strength or yield strength ratio does not satisfy the lower limit of the present invention. No. In No. 5, the heating temperature and the finishing temperature were high, and the γ grain size was coarsened, so the toughness decreased. Production No. 13 and 31 were reduced in strength because the coiling temperature CT was too high. Production No. No. 22 had a slow cooling speed, so that the martensite transformation did not occur sufficiently and the strength was lowered. Production No. In No. 27, since the BAF annealing temperature was too high, both YP and TS were lowered, and the toughness was also deteriorated. On the other hand, production No. Since 34 did not perform BAF annealing after hot rolling, YP and YR depart from the scope of the present invention. Production No. As for 5, 10, 31 all, toughness has fallen. Production No. In the case of No. 5, the FT was too high and the γ particle size was too large. Regarding 10 and 31, toughness deteriorated because some precipitates were formed during winding. No. Regarding 10 and 31, due to the large variation in the temperature history from cooling to winding, the material variation in the width direction is also extremely large.

[Example 2]
Table 4 shows the steel No. of Table 1. The change of the mechanical property at the time of changing BAF annealing temperature using J slab is shown.

As is apparent from the results shown in Table 4, when the heat treatment is performed within the range of the BAF annealing temperature specified in the present invention, the yield strength is 960 MPa or more, the yield strength ratio is 0.83 or more, and the yield strength in the plate width direction is Although it is possible to obtain a hot-rolled steel sheet having excellent toughness with variations within 50 MPa and impact absorption energy at −40 ° C. exceeding 32 J, YP is below the lower limit when the BAF annealing temperature is too low. On the other hand, when the annealing temperature in the BAF furnace is too high, both YP and TS are lowered. However, no. Reference numerals 48 to 52 are reference examples.

  The hot-rolled steel sheet shown in the present invention can be used in construction equipment such as a crane boom, for example, and the boom itself can be reduced in weight and lifted and transported capacity can be increased. Its social contribution is immeasurable because it can be enjoyed.

Claims (5)

% By mass
C: 0.05 or more and 0.2% or less,
Si: 0.01% or more, 0.6% or less,
Mn: 0.5% or more, 2.5% or less,
P: 0.001% or more, 0.1% or less,
S: 0.0005% or more, 0.05% or less,
Al: 0.01% or more, 0.2% or less,
N: 0.0001% or more, 0.010% or less,
Mo: 0.05% or more, 0.5% or less,
Ti: 48N / 14 + 0.01% or more, 0.14% or less,
B: 0.0003% or more and 0.005% or less in a range satisfying the following formula (1), the balance having a steel composition composed of iron and inevitable impurities, yield strength of 960 MPa or more, and and the yield ratio is 0.83 or more and variation in the yield strength of the plate width direction Ri der within 50 MPa, the coil width direction impact absorbed energy at -40 ℃ characterized -32J ultra der Rukoto High yield ratio high strength hot-rolled steel sheet with low strength variation and excellent toughness.
70 ≦ 300 × C (mass%) + 33 × Mn (mass%) + 22 × Cr (mass%) + 11 × Mo (mass%) + 11 × Si (mass%) + 17 × Ni (mass%) ≦ 100 (1) In addition,
Nb: 0.005% or more and 0.09% or less, The high yield ratio high strength hot-rolled steel sheet having less toughness variation in the coil width direction and excellent toughness according to claim 1. Furthermore, in mass% ,
W: 0.01% or more, 2.0% or less,
Cu: 0.04% or more, 2.0% or less,
Ni: 0.02% or more, 1.0% or less,
V: 0.001% or more, 0.10% or less,
The high yield ratio high strength hot-rolled steel sheet with less variation in strength in the coil width direction and excellent toughness according to claim 1 or 2, characterized by containing at least one of the following. Furthermore, in mass% ,
The intensity variation in the coil width direction according to any one of claims 1 to 3, characterized by containing one or more of Mg and Zr in a total of 0.0005% to 0.05%. High yield ratio high strength hot-rolled steel sheet with little toughness. After heating the slab which has the steel component in any one of Claims 1-4 to 1100 degreeC or more and 1250 degrees C or less, a finishing temperature is hot-rolled on conditions with 850 degreeC or more and 950 degrees C or less, Then, 10 degreeC The steel sheet is cooled at a cooling rate of at least / sec, wound in a coil shape at a temperature of 200 ° C. or lower, once cooled to room temperature, and then annealed in a batch annealing furnace in a temperature range of 150 to 350 ° C. The manufacturing method of the high yield ratio high intensity | strength hot-rolled steel plate with few intensity dispersion | variation in the coil width direction as described in any one of 1-4, and excellent in toughness.