JP6136478B2 - High-strength hot-rolled steel sheet excellent in toughness and rigidity in the rolling direction and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength hot-rolled steel sheet mainly used for structural members of construction equipment, such as booms of large cranes, and in particular has high yield strength and excellent toughness and rigidity in the rolling direction. The present invention relates to a high-strength hot-rolled steel sheet and a method for producing the same. The high-strength hot-rolled steel sheet described in the present invention refers to a sheet having a thickness range of 4.0 to 10 mm.

  Crane booms for construction machines are becoming longer and larger with the recent increase 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. Until now, in order to increase the strength, solid solution strengthening elements such as Si and Mn and precipitation strengthening elements such as Ti and Nb have been added in a large amount to the steel components. For example, Patent Documents 1 to 5 all increase the amount of Ti added in order to utilize Ti precipitation strengthening, but Patent Documents 1 to 3 provide 0.12% or more in order to fully utilize precipitation strengthening. Ti addition and high-temperature heating at 1250 ° C. or higher are 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 the hot-rolled sheet is (Ms point + 50 ° C.) or less. And then tempering by winding after holding at a cooling stop temperature of ± 100 ° C.

  On the other hand, the rigidity of the member is reduced as the thickness is reduced. However, the reduction in rigidity cannot be suppressed even if the strength is increased. In addition, in order to reduce the thickness, it is necessary to increase the Young's modulus of the material together with increasing the strength. Generally, the Young's modulus of iron is said to be 206 GPa, but it is well known that the Young's modulus in one direction can be increased by aligning the crystal orientation of iron in a specific direction by rolling or the like. For example, in the case of a long member such as a boom of a large crane, since the rolling direction of hot rolling is the longitudinal direction of the member, the rigidity as a member can be improved by increasing the Young's modulus in the rolling direction. Examples of techniques for increasing the Young's modulus in the rolling direction include Patent Documents 8 and 9, for example. However, these techniques have a problem that the load on the rolling mill is high because a high shearing force is applied to the steel sheet surface during hot rolling in order to achieve a very high Young's modulus in the rolling direction.

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 JP 2007-46146 A JP 2008-274395 A

  The present invention has been made in view of the above problems, and is rolled up at 200 ° C. or less, is inexpensive without adding a large amount of expensive alloy elements, has high yield strength, and has high toughness and rolling. An object of the present invention is to provide a high-strength hot-rolled steel sheet having excellent directional rigidity and a method for producing the same.

  With regard to the above problems, the present inventors have optimized the component range and hot rolling conditions in a component system containing Nb and Ti, thereby achieving both toughness and rigidity in the rolling direction, and a high-strength hot-rolled steel sheet exceeding YP 880 MPa. It was found that can be obtained. That is, in order to increase the rigidity in the rolling direction, Nb and Ti are added, recrystallization in the austenite region is suppressed, and hot rolling is performed, so that αfiber orientation {100 } To {111} <011> is effective. In any of these orientations, since <011> of crystals are aligned in the rolling direction, a Young's modulus in the rolling direction of about 220 GPa can be obtained. On the other hand, the addition of excess Ti and Nb and the increase in the reduction ratio in the non-recrystallized region lead to the formation of Ti and Nb carbides at the grain boundaries and the formation of a microstructure extending in the rolling direction, resulting in the toughness of the steel sheet. Reduce. As a result of intensive studies, the present inventors have newly found the optimum component range of Nb, Ti or B, and hot rolling conditions in order to ensure rigidity in the rolling direction while ensuring toughness. Further, by cooling to 200 ° C. or lower at 20 ° C./s or higher, the strength is increased by martensitic transformation, and a high degree of integration without randomization from the austenite texture formed during hot-rolling processing. The present inventors have found that a transformation texture having the following can be obtained.

That is, the high-strength hot-rolled steel sheet excellent in the toughness and the rigidity in the rolling direction of the present invention and the production method thereof
[1] By mass%, C: 0.05% to 0.2%, Si: 0.01% to 0.6%, Mn: 0.5% to 2.5%, 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 0.010% or less, B: 0.0003% or more, 0.005% or less, Ti: 48N / 14 + 0.01% or more, 0.14% or less within the range satisfying the following formula (1), and the balance Has a steel composition consisting of iron and inevitable impurities,
Yield strength of not less than 880 MPa, the rolling direction Young's modulus Ri der least 210GPa, {100} ~ {111 } <011> maximum X-ray random intensity ratio of the orientation unit at the thickness direction 1/2 position 3 .5 above, {110} <001> high-strength hot-rolled steel sheet in which the X-ray random intensity ratio of orientation and excellent rigidity toughness and rolling direction, wherein 2.0 or less der Rukoto.
0.2 ≦ (1.3 × Nb (mass%) + Ti * (mass%)) × B * (ppm) ≦ 1.0 (1)
{However, in the above formula (1), Ti * = (Total Ti (mass%) − 48N / 14), and when Ti * <0, Ti * = 0. Further, when B ≦ 14 ppm, B * = (− 0.05 × B ² + 1.5B (ppm)), and when B> 14 ppm, B * = 11.2. }
[2] The high-strength hot-rolled steel sheet with excellent toughness and rigidity in the rolling direction according to [1], further comprising Nb: 0.005% to 0.09% by mass% .
[3] Furthermore, by mass, Mo: 0.02% or more, 0.5% or less, Cr: 0.1% or more, 2.0% or less, 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 A high-strength hot-rolled steel sheet having excellent toughness and rigidity in the rolling direction as described in [1] or [2].
[4] Furthermore, the composition contains one or more of Ca, Mg, Zr, and REM in a mass percentage of 0.0005% or more and 0.05% or less in total. 3] A high-strength hot-rolled steel sheet having excellent toughness and rigidity in the rolling direction .
[5] [1] to [4] A method of manufacturing a high strength hot rolled steel sheet according to any one of, [1] - 1150 ° C. The slab having a steel composition according to any one of [4] After heating to 1250 ° C. or less, hot rolling is performed under the conditions that the total rolling reduction at 1000 ° C. or less is 20% or more and 60% or less, and the finishing temperature is 850 ° C. or more and 950 ° C. or less. A method for producing a high-strength hot-rolled steel sheet excellent in toughness and rigidity in the rolling direction, wherein the steel sheet is cooled at a cooling rate of at least / sec and wound in a coil shape at a temperature of 200 ° C. or less.
[ 6 ] The steel sheet wound in the shape of a coil is cooled to room temperature, and then annealed at a maximum temperature of 150 to 500 ° C, and is excellent in toughness and rigidity in the rolling direction according to [ 5 ] A method for producing a high strength hot rolled steel sheet.
It has each composition of.

  According to the high-strength hot-rolled steel sheet excellent in toughness and rolling direction rigidity according to the present invention and the method for producing the same, the above structure provides high yield strength and high strength with high Young's modulus in the rolling direction while ensuring toughness. A hot-rolled steel sheet can be realized at low cost. Furthermore, by achieving both a high Young's modulus and toughness in the rolling direction, high shearing force is not applied to the surface of the steel sheet during hot rolling, and the load on the rolling mill can be reduced, thereby improving productivity. 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.

It is a figure explaining the high-strength hot-rolled steel sheet excellent in toughness and the rigidity of a rolling direction which is embodiment of this invention, and its manufacturing method, and showed the main direction of the austenite phase on ODF of (phi) 2 = 45 degree cross section It is a schematic diagram.

  Hereinafter, a high-strength hot-rolled steel sheet that is an embodiment of the present invention and a manufacturing method thereof will be described. In addition, since this embodiment is described in detail in order to better understand the purpose of the high-strength hot-rolled steel sheet having excellent toughness and rolling direction rigidity according to the present invention and its manufacturing method, there is no particular designation. As long as the present invention is not limited thereto.

The high-strength hot-rolled steel sheet (hereinafter, simply referred to as “high-strength hot-rolled steel sheet”) having excellent toughness and rigidity in the rolling direction according to the present invention is mass%, and 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 %: 0.05% or less, Al: 0.01% or more, 0.2% or less, N: 0.0001% or more, 0.010% or less, B: 0.0003% or more, 0.005% or less Ti: 48N / 14 + 0.01% or more and 0.14% or less in a range satisfying the following formula (1), with the balance being a steel composition consisting of iron and inevitable impurities, with a yield strength of 880 MPa. The Young's modulus in the rolling direction is 210 GPa or more , and {100} to { The maximum X-ray random intensity ratio of the 111} <011> orientation group is 3.5 or more, and the X-ray random intensity ratio of the {110} <001> orientation is 2.0 or less .
0.2 ≦ (1.3 × Nb (mass%) + Ti * (mass%)) × B * (ppm) ≦ 1.0 (1)
However, in the above formula (1), Ti * = (Total Ti (mass%) − 48N / 14), and when Ti * <0, Ti * = 0. Further, when B ≦ 14 ppm, B * = (− 0.05 × B ² + 1.5B (ppm)), and when B> 14 ppm, B * = 11.2.
Below, the reason for limitation of the steel characteristic and manufacturing conditions in this invention is demonstrated in detail.

[Steel composition]
Regarding the component composition in the high-strength hot-rolled steel sheet of the present invention, the reasons for limiting each element will be described in detail below. In the following description, “%” represents mass% unless otherwise specified.

(C: Carbon) 0.05% or more and 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 specified in the present invention cannot be satisfied. On the other hand, if 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.

(Si: silicon) 0.01% or more and 0.6% or less Si is added in an amount of 0.01% or more in order 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 scale are generated on the surface and the surface quality is remarkably lowered. Therefore, the upper limit is made 0.6%. From this viewpoint, the amount of Si added is more preferably 0.3% or less, and further preferably 0.15% or less.

(Mn: Manganese) 0.5% or more and 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 point of view, the amount of Mn added is preferably 2.2% or less, and more preferably 2.0% or less.

(P: Phosphorus) 0.001% or more and 0.1% or less P is added as an element for increasing the strength of the steel sheet according to the required strength level. However, if the amount of P added is large, it segregates to the grain boundaries, thereby deteriorating local ductility, weldability, and toughness. Therefore, the upper limit value of P is 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 due to P can be ignored, and if it is less than this, the cost increases, so 0.001% is made the lower limit.

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

(Al: aluminum) 0.01% or more, 0.2% or less Al needs to be added 0.01% or more as a deoxidizer. 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 and 0.010% or less N is an element inevitably contained in steel, but forms BN and reduces solid solution B to deteriorate hardenability. Therefore, the content is made 0.010% or less. From this point of view, 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 0.001% or more.

(B: Boron) 0.0003% or more and 0.005% or less B is an important element in the present invention and an inexpensive hardenability improving element. In addition, B is added in combination with Nb and Ti to expand the non-recrystallization temperature range in the austenite phase, promote the development of a working texture that improves the Young's modulus in the rolling direction, and can be used to select variants during quenching. And has an effect of strengthening the {100} to {111} <011> orientation group, particularly the {211} <011> orientation. Therefore, in the present invention, 0.0003% or more of B is added, and 0.0006% or more is desirable from the above viewpoint. On the other hand, even if B is added in excess of 0.005%, not only a special effect is not obtained but also the toughness is deteriorated, 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 Since Ti inhibits precipitation of BN by forming TiN at a high temperature, the following formula {48N / 14 + 0.01}% or more Added. Further, solute Ti is positively added because it delays recrystallization and promotes the development of the working texture during hot rolling. On the other hand, even if Ti is added in an amount of more than 0.14%, not only the recrystallization delay effect is not obtained, but also the toughness and weldability are reduced, so this value is made the upper limit.

(Nb: niobium) 0.005% or more and 0.09% or less In the present invention, it is more desirable to add Nb in a predetermined range in addition to the above essential elements. Here, Nb, like Ti, is an element that suppresses recrystallization and promotes the formation of a processed texture, so it is desirable to add 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.

  Furthermore, in the present invention, it is more desirable to add one or more of Mo, Cr, W, Cu, Ni, and V as the element for improving the steel characteristics as necessary.

(Mo: molybdenum) 0.02% or more, 0.5% or less (Cr: chromium) 0.1% or more, 2.0% or less (W: tungsten) 0.01% or more, 2.0% or less Mo, Cr and W are both elements that have the effect of improving the hardenability and forming carbides to increase the strength. Therefore, it is desirable to add at 0.02% or more (Mo), 0.1% or more (Cr), and 0.01% or more (W), respectively. On the other hand, addition of more than 0.5% (Mo), more than 2.0% (Cr), and more than 2.0% (W) respectively reduces ductility and weldability. From the above viewpoint, Mo is 0.02% to 0.5%, Cr is 0.1% to 2.0%, W is 0.01% to 2.0%, It is desirable to add as needed.

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

(Ni: nickel) 0.02% or more and 1.0% or less 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 and 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 0.001 to 0.10% or less. To do.

  Furthermore, in this invention, as an element for improving a steel characteristic, 1 type (s) or 2 or more types of Ca, Mg, Zr, and REM (rare earth elements) are total, or it is 0.0005% or more, 0.00. It can be contained at 05% 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 in total, or 0.0005% or more alone. 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.

(Relational formula for 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).
0.2 ≦ (1.3 × Nb (mass%) + Ti * (mass%)) × B * (ppm) ≦ 1.0 (1)
However, in the above formula (1), Ti * = (Total Ti (mass%) − 48N / 14), and when Ti * <0, Ti * = 0. Further, when B ≦ 14 ppm, B * = (− 0.05 × B ² + 1.5B (ppm)), and when B> 14 ppm, B * = 11.2.

  If the value of the above formula (1) is less than 0.2, a sufficient hot-rolled texture does not develop, so that a Young's modulus in the rolling direction of 210 GPa or more cannot be obtained. From this viewpoint, it is more desirable to set the value of the above expression (1) to 0.3 or more. On the other hand, if the value of the above formula (1) exceeds 1.0, it becomes a structure extended in the rolling direction by non-recrystallization zone rolling, and Nb and Ti carbides are generated at the grain boundaries, so that the toughness deteriorates. This value is the upper limit. From this point of view, it is more desirable that the value of the above equation (1) is 0.8 or less.

[Young's modulus in the rolling direction]
In the high-strength hot-rolled steel sheet of the present invention, the Young's modulus in the rolling direction is regulated to 210 GPa or more, and high rigidity is ensured.

Since the Young's modulus varies depending on the crystal orientation, the crystal orientation in the hot-rolled sheet needs to satisfy the following regulations. That is, in the steel sheet containing Nb, Ti, B, etc., the non-recrystallized rolling / transformation texture develops at the position where the thickness is 1/2, and the {100} to {111} <011> orientation group is strong. Become. In order to set the Young's modulus in the rolling direction of the steel sheet to 210 GPa or more, the maximum X-ray random intensity ratio of this orientation group needs to be 3.5 or more. From this viewpoint, it is desirable that the maximum X-ray random intensity ratio of the {100} to {111} <011> orientation groups be 4 or more. Moreover, the upper limit of the maximum X-ray random intensity ratio of the {100} to {111} <011> orientation groups is not particularly defined, but a special effect is not obtained even if the accumulation is 8 or more.
On the other hand, the {110} <001> orientation relatively increases when the rolling reduction of the hot rolling is low and the working texture does not develop. Since this orientation is an orientation that lowers the Young's modulus in the rolling direction, the X-ray random intensity ratio is 2.0 or less. Moreover, although the upper limit of the X-ray random intensity ratio in the {110} <001> orientation is not particularly defined, 0 is the lower limit by definition.

  Here, the maximum X-ray random intensity ratio of {100} to {111} <011> orientation group and the X-ray random intensity ratio of {110} <001> orientation are measured by X-ray diffraction {110}. , {100}, {211}, {310} pole diagrams, a crystal orientation distribution function (Orientation Distribution Function, ODF) representing a three-dimensional texture calculated by a series expansion method based on a plurality of pole figures. Find it from The X-ray random intensity ratio referred to in the present invention is obtained by measuring the X-ray intensity of a standard sample having no accumulation in a specific orientation and a test material under the same conditions by an X-ray diffraction method or the like. It is a numerical value obtained by dividing the X-ray intensity of the sample by the X-ray intensity of the standard sample.

FIG. 1 shows an ODF of a φ2 = 45 ° cross section in which the crystal orientation of the present invention is displayed.
As indicated by the point on the axis of Φ = 0 ° in FIG. 1, the {100} to {111} <011> orientation groups are strictly Φ = 0 ° and Φ1 = 0 to 54.74 °. It refers to a range. However, since a measurement error due to test piece processing or sample setting may occur, the maximum value of the X-ray random intensity ratio of the {100} to {111} <011> orientation groups is the hatched portion in FIG. The maximum X-ray random intensity ratio in the range of φ = 0 to 5 ° and φ1 = 0 to 55 ° shown in FIG.

For the same reason as described above, in the cross section of φ2 = 45 ° of the three-dimensional texture, the {110} <001> orientation is φ = 85 to 90 ° and φ1 = 85 with the position indicated by the point in FIG. The maximum value in a range of ˜90 ° is represented as the intensity ratio of each direction.
Here, as for the crystal orientation, the orientation perpendicular to the plate surface is usually represented by [hkl] or {hkl}, and the orientation parallel to the rolling direction is represented by (uvw) or <uvw>. {Hkl} and <uvw> are generic names of equivalent planes, and [hkl] and (uvw) indicate individual crystal planes. That is, since the present invention is intended for the bcc structure, for example, (111), (−111), (1-11), (11-1), (−1-11), (−11-1) , (1-1-1), (-1-1-1) planes are equivalent and cannot be distinguished. In such a case, these orientations are collectively referred to as {111}.

  The ODF is also used to display the orientation of a crystal structure with low symmetry, and is generally expressed as φ1 = 0 to 360 °, Φ = 0 to 180 °, and φ2 = 0 to 360 °. The direction is displayed in [hkl] (uvw). However, in the present invention, since a highly symmetrical bcc crystal structure is targeted, Φ and φ2 are expressed in the range of 0 to 90 °. In addition, the range of φ1 varies depending on whether or not symmetry due to deformation is taken into account when performing calculation. In the present invention, φ1 = 0 to 90 ° in consideration of symmetry. That is, a method of selecting an average value in the same direction at φ1 = 0 to 360 ° on an ODF of 0 to 90 ° is selected. In this case, [hkl] (uvw) and {hkl} <uvw> are synonymous. Therefore, for example, the X-ray random intensity ratio of (100) [0-11] of the ODF in the φ2 = 45 ° cross section shown in FIG. 1 is the X-ray random intensity ratio in the {100} <011> orientation.

The Young's modulus is measured by a transverse resonance method at room temperature in accordance with JIS Z 2280. That is, the Young's modulus is calculated from the following equation (2) by applying vibration without fixing the sample, measuring the primary resonance frequency by gradually changing the vibration frequency of the oscillator.
E = 0.946 × (l / h) 3 × m / w × f2 (2)
In the above formula (2), E: dynamic Young's modulus (N / m ² ), l: length of test piece (m), h: thickness of test piece (m), m: mass (kg ), W: width (m) of the test piece, f: primary resonance frequency (s-1) of the transverse resonance method.

  Note that if the scale remains on the surface of the test piece or there are irregularities, the measurement accuracy decreases. Therefore, the surface of the test piece is measured after making it a smooth metal surface by mechanical grinding or the like.

The X-ray diffraction sample is produced as follows.
First, the steel plate is polished to a predetermined position in the plate thickness direction by mechanical polishing, chemical polishing, or the like, and if necessary, distortion is removed by electrolytic polishing or chemical polishing. Adjust so that In addition, since it is difficult to accurately set the measurement surface to a 1/2 plate thickness portion, a sample is prepared so that the measurement surface is within a range of 3% of the plate thickness with the target position as the center. do it. If an abnormality such as segregation is observed at the center of the plate thickness, the sample may be prepared by avoiding the segregated portion within the range of 7/16 to 9/16 of the plate thickness. When measurement by X-ray diffraction is difficult, a statistically sufficient number of measurements may be performed by an EBSP (Electron Back Scattering Pattern) method or an ECP (Electron Channeling Pattern) method.

[Yield strength (YP)]
In the high-strength hot-rolled steel sheet of the present invention, the yield strength (YP) is specified to be 880 MPa or more.
In the present invention, a high-strength hot-rolled steel sheet having a yield strength of 880 MPa or more can be realized by controlling the steel composition to the above-described range and further setting each manufacturing condition to the conditions described later. Thus, by increasing the yield strength to 880 MPa or more, for example, even when the plate thickness of the steel sheet is reduced to about 4.0 to 10 mm, a sufficiently high strength can be ensured.

[Production method]
A method for producing a high-strength hot-rolled steel sheet excellent in toughness and rigidity in the rolling direction according to the present invention will be described below.
The method for producing a high-strength hot-rolled steel sheet according to the present invention comprises heating the slab having the above steel components to 1150 ° C. or more and 1250 ° C. or less, then the total rolling reduction at 1000 ° C. or less is 20% or more and 60% or less, and the finishing temperature Is a method in which hot rolling is performed at a temperature of 850 ° C. or higher and 950 ° C. or lower, followed by cooling at a cooling rate of 20 ° C./second or higher and winding at a temperature of 200 ° C. or lower.

  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.

  Typically, the steel slab is cooled after casting and reheated for hot rolling. In this case, the heating temperature of the steel slab when hot rolling is set to 1150 ° C. or higher. If this temperature is lower than 1150 ° C., Ti and Nb are not sufficiently re-dissolved, and the recrystallization suppressing effect is not exhibited, so this temperature is set as the lower limit. On the other hand, when 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 temperature is made the upper limit.

  In the method for producing a high-strength hot-rolled steel sheet according to the present invention, the finish rolling temperature and the rolling reduction are extremely important. In the present invention, the temperature in hot rolling is 1000 ° C. or lower, and the total rolling reduction (total rolling reduction) up to the final pass is 20% or more. Even after hot rolling at over 1000 ° C., the texture after processing recrystallizes, increasing the X-ray random intensity ratio of {110} <111> to {110} <112> orientation groups in the 1/2 plate thickness part. This is because the effect cannot be obtained. Further, if the total rolling reduction at 1000 ° C. or less is less than 20%, the texture cannot be developed because sufficient processing in the non-recrystallized region cannot be obtained. From this point of view, it is desirable that the total rolling reduction in hot rolling is 30% or more. On the other hand, if the total rolling reduction exceeds 60%, the steel sheet structure becomes an elongated structure extending in the rolling direction after hot rolling, and the toughness decreases. From this point of view, the total rolling reduction in hot rolling is preferably 50% or less.

  In the production method of the present invention, the hot rolling finishing temperature is 850 ° C. or higher. When hot rolling is finished at less than 850 ° C., the hot rolling load becomes too high, so this temperature is set as the lower limit. On the other hand, if the finishing temperature exceeds 950 ° C., sufficient rolling cannot be performed in the non-recrystallized region, and the Young's modulus does not improve, so this temperature is set as the upper limit.

  Next, after the hot rolling, cooling is performed at a cooling rate of 20 ° C./s or higher in any temperature range up to 200 ° C. or lower. When the cooling rate is less than 20 ° C./s, variant selection at the time of transformation does not occur, so that the texture is randomized, the Young's modulus is lowered, and YP is lowered as the ferrite transformation proceeds. Therefore, 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. If the coiling temperature exceeds 200 ° C., the martensite transformation does not occur sufficiently and the strength decreases, so this value is made the upper limit. The lower limit of the coiling temperature is not particularly defined, but cooling to room temperature or lower requires excessive facilities and does not provide a special effect.

  In addition, after cooling the wound coil to room temperature, you may further heat-process with the highest ultimate temperature of 150-500 degreeC as needed. With such heat treatment, YP rises due to carbide precipitation or tempering of martensite. The annealing method may be any method such as continuous annealing or BAF annealing. Moreover, since a sufficient effect cannot be obtained by annealing at less than 150 ° C., the lower limit for annealing after winding is 150 ° C. Also, annealing at over 500 ° C. causes toughness deterioration due to precipitation of Nb or Ti carbides and a significant decrease in strength due to coarsening of carbides, so 500 ° C. is the upper limit.

As described above, according to the high strength hot-rolled steel sheet having excellent toughness and rigidity in the rolling direction and the manufacturing method thereof according to the present invention, the above configuration provides high yield strength, and also ensures the toughness and the rolling direction. A high-strength hot-rolled steel sheet having a high Young's modulus can be realized at low cost. Also, by making the Young's modulus and toughness in a high rolling direction compatible, a high shearing force is not applied to the steel sheet surface during hot rolling, the load on the rolling mill can be reduced, and productivity and maintainability are improved.
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-strength hot-rolled steel sheet having excellent toughness and rolling direction rigidity according to the present invention and a method for producing the same will be described, and the present invention will be described more specifically. However, the present invention is originally limited to the following examples. However, the present invention can be carried out with appropriate modifications within a range that can meet the gist of the preceding and following descriptions, all of which are included in the technical scope of the present invention.

  In this example, first, steel having the composition shown in Table 1 below was melted and hot rolled under the conditions shown in Tables 2 and 3 below. Tables 4 and 5 below show the results of investigating the properties of the obtained hot-rolled steel sheet. Moreover, about some hot-rolled steel plates, after winding, it annealed with the coil in the BAF furnace. The maximum temperature reached at this time is shown in Tables 2 and 3 below.

Young's modulus was measured by the transverse resonance method described above.
The tensile properties (YP) were evaluated by collecting JIS No. 5 tensile test pieces from a direction perpendicular to the rolling direction.
The toughness was evaluated by a Charpy impact test. At this time, a JIS Z2202 test piece was produced 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.

Further, the maximum value of the {100} to {111} <011> orientation group and the X-ray random intensity ratio of the {110} <001> orientation at the position where the plate thickness is 1/2 thickness are as follows. It was measured. First, the steel plate was mechanically polished and buffed, then further electropolished to remove strain, and X-ray diffraction was performed using a sample adjusted so that the 1/2 plate thickness portion became the measurement surface. Note that X-ray diffraction of a standard sample having no accumulation in a specific orientation was performed under the same conditions.
Next, ODF was obtained by the series expansion method based on {110}, {100}, {211}, {310} pole figures obtained by X-ray diffraction. And from this ODF, the maximum value of {100} to {111} <011> orientation group and the X-ray random intensity ratio of {110} <001> orientation were obtained.

Details of the results of this example are described below.
As is clear from the results shown in Tables 4 and 5, when steel having the chemical components specified in the present invention is hot-rolled under appropriate conditions, YP is 880 MPa or more and Young's modulus in the rolling direction is 210 GPa or more. Thus, a hot-rolled steel sheet having high impact energy could be obtained (examples of the present invention in the remarks column of Tables 1 to 6).

  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 Y (see Tables 1 and 2). Production No. In Nos. 40 and 42, the addition amounts of C and Mn are below the appropriate range, and the strength is insufficient. In addition, production No. 43 and 45 are cases where the toughness is deteriorated due to the addition amount of Ti or Nb being too high. In addition, production No. 41, 44, and 46 are cases where the Young's modulus did not become 210 GPa or more because the amount of Ti or B added was too low or the above formula (1) was not satisfied.

  Production No. Although 3, 8, 13, 27, and 35 all satisfied the chemical composition of the present invention, the Young's modulus was less than 210 GPa. Production No. In No. 3, since the heating temperature was low, Ti and Nb were not sufficiently re-dissolved, so that recrystallization was not suppressed and the texture was randomized. In addition, production No. In No. 8, the texture was not sufficiently developed because the rolling reduction at 1000 ° C. or lower was too low. In addition, production No. No. 13 was insufficiently processed in the non-recrystallized region because the finishing temperature was too high. In addition, production No. In No. 27, since the cold speed was slow, variant selection at the time of transformation did not occur, and the texture was randomized. In addition, production No. No. 35 has a reduced strength because the heat treatment temperature after hot rolling was too high.

  Production No. Nos. 18, 27 and 35 have low YP and do not satisfy the specified range of the present invention. Production No. Since CT was too high, no. In No. 27, the martensite transformation did not occur sufficiently due to the slow cooling speed. No. No. 35 is an example in which the strength decreased due to the progress of carbide coarsening, martensite tempering, recovery and recrystallization because the heat treatment temperature was too high. In addition, production No. No. 30 is an example in which the toughness was lowered because the microstructure was flattened and the in-plane anisotropy was increased because the rolling reduction at 1000 ° C. or lower was too high.

  From the results of the examples described above, it is apparent that the present invention makes it possible to realize a high-strength hot-rolled steel sheet having high yield strength and excellent toughness and rigidity in the rolling direction.

  The high-strength hot-rolled steel sheet shown in the present invention reduces the weight of the boom itself and expands the lifting and carrying capacity, for example, by applying it to structural members of construction equipment including booms of large cranes. The social contribution is immeasurable because it is possible to fully enjoy the merit of significantly improving work efficiency.

Claims (6)

% By 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,
B: 0.0003% or more, 0.005% or less,
Ti: 48N / 14 + 0.01% or more, 0.14% or less is contained in a range satisfying the following formula (1), the balance has a steel composition consisting of iron and inevitable impurities,
Yield strength of not less than 880 MPa, the rolling direction Young's modulus Ri der than 210 GPa,
The maximum X-ray random intensity ratio of the {100} to {111} <011> azimuth group at the position of 1/2 in the plate thickness direction is 3.5 or more, and the X-ray random intensity ratio of the {110} <001> azimuth is 2. high strength hot rolled steel sheet having excellent rigidity toughness and rolling direction, wherein 0 or less der Rukoto.
0.2 ≦ (1.3 × Nb (mass%) + Ti * (mass%)) × B * (ppm) ≦ 1.0 (1)
{However, in the above formula (1), Ti * = (Total Ti (mass%) − 48N / 14), and when Ti * <0, Ti * = 0. Further, when B ≦ 14 ppm, B * = (− 0.05 × B ² + 1.5B (ppm)), and when B> 14 ppm, B * = 11.2. } Furthermore, in mass%,
Nb: 0.005% or more and 0.09% or less, The high-strength hot-rolled steel sheet having excellent toughness and rigidity in the rolling direction according to claim 1. Furthermore, in mass%,
Mo: 0.02% or more, 0.5% or less,
Cr: 0.1% or more, 2.0% or less,
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,
The high strength hot-rolled steel sheet having excellent toughness and rigidity in the rolling direction according to claim 1 or 2, wherein V: 0.001% or more and 0.10% or less is contained. . Furthermore, it contains at least 0.0005% or more and 0.05% or less of Ca, Mg, Zr, or REM in mass%. A high-strength hot-rolled steel sheet excellent in toughness and rigidity in the rolling direction according to item 1 . A method for producing the high-strength hot-rolled steel sheet according to any one of claims 1 to 4 ,
After the slab having the steel component according to any one of claims 1 to 4 is heated to 1150 ° C or higher and 1250 ° C or lower, the total rolling reduction at 1000 ° C or lower is 20% or higher and 60% or lower, and the finishing temperature is 850 ° C. As described above, hot rolling is performed under a condition of 950 ° C. or lower, then cooled at a cooling rate of 20 ° C./second or higher, and wound in a coil shape at a temperature of 200 ° C. or lower. A method for producing high-strength hot-rolled steel sheets with excellent rigidity. The steel sheet wound in the shape of a coil is cooled to room temperature, and further annealed at a maximum temperature of 150 to 500 ° C. The high strength excellent in toughness and rigidity in the rolling direction according to claim 5 A method for producing a hot-rolled steel sheet.

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