JP2017057472A - Hot rolled steel sheet and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot rolled steel sheet having high strength and yet excellent in stretch-flangeability and low temperature toughness, and a production method allowing for stable production of the steel sheet.SOLUTION: The hot rolled steel sheet is provided that contains C:0.03% to 0.25%, Si:0.001% to 2.0%, Mn:0.5% to 4.0%, P:0.10% or less, S:0.01% or less, sol.Al:0.001% to 1.0%, B:0.0001% to 0.005%, and N:0.01% or less. In the hot rolled steel sheet, total area ratio of a martensite phase and lower bainite structure is 85% or ore, average grain diameter of crystal grains is 20 μm or less, crystal grain with its aspect ratio of 0.30 or less is 50% or less by area ratio and average of X ray random intensity ratios of {100}<011> to {211}<011> orientation groups is 6.0 or less and maximum thereof is 8.0 or less.SELECTED DRAWING: None

Description

  The present invention relates to a hot-rolled steel sheet and a manufacturing method thereof. More specifically, the present invention relates to a high-strength hot-rolled steel sheet excellent in workability and low-temperature toughness that is suitable as a material for use in automobiles, home appliances, machine structures, buildings, and the like, and a method for producing the same.

  Steel sheets used as materials for structural members of automobiles and other transportation machines and various industrial machines have strength, workability such as elongation and hole expansibility, low temperature toughness, and uniformity of these properties, etc. Various characteristics are required.

  In particular, steel plates used for automobile inner plate members, structural members, suspension members, etc. are required to have stretch flangeability, burring workability, ductility, fatigue durability, impact resistance, corrosion resistance, etc., depending on the application. Therefore, it is important how to exhibit these material characteristics and high strength in a high-dimensional and well-balanced manner. In addition, the steel plate used for such a member is not easily destroyed even if it is subjected to impacts such as a collision after being mounted on a car as a part after molding. In order to ensure impact resistance particularly in cold regions, There was a need to improve toughness as well. This low temperature toughness is defined by vTrs (Charpy fracture surface transition temperature) and the like. For this reason, it is also necessary to consider the impact resistance itself of the steel material. That is, in addition to excellent workability, low temperature toughness is required as a very important characteristic for thin steel sheets for parts including the above parts.

  As for the method for improving the low temperature toughness of the high-strength steel sheet, for example, Patent Documents 1 and 2 disclose the production method thereof, a method using a martensite phase with an adjusted aspect ratio as a main phase (Patent Document 1), an average Low temperature toughness is improved by a method (Patent Document 2) in which carbides are finely precipitated in ferrite having a particle size of 5 to 10 μm. However, the steel sheet manufactured by applying the techniques disclosed in Patent Documents 1 and 2 is not mentioned at all about the stretch flangeability focused on in this study, and is applied to a member that performs burring. There is a concern that molding defects may occur. In addition, although there is knowledge of improving low temperature toughness in the steel pipe field and the thick plate field, there is a similar concern because formability as thin as that of a thin sheet is not required.

  For the method of improving stretch flangeability in high-strength steel sheets, a metal structure control method for steel sheets that improves local ductility is also disclosed, including inclusion control, single structure, and inter-structure hardness. Non-Patent Document 1 discloses that if the difference is reduced, it is effective for bendability and stretch flangeability. In addition, by controlling the finishing temperature of hot rolling, the reduction rate and temperature range of finishing rolling, promoting the recrystallization of austenite, suppressing the development of rolling texture, and randomizing the crystal orientation, the strength and ductility Non-patent document 2 discloses a technique for improving stretch flangeability. From Non-Patent Documents 1 and 2, it is considered that stretch flangeability can be improved by making the metal structure and rolling texture uniform, but no consideration is given to both low temperature toughness and stretch flangeability.

  The coexistence of stretch flangeability and low temperature toughness is mentioned in Patent Document 3, and a technique is disclosed in which an appropriate amount of retained austenite and bainite are dispersed in a ferrite phase whose hardness and particle size are controlled. The technique of the present invention is a structure having a martensite phase as a main phase, which is different from the above-described invention.

JP 2011-52321 A JP 2011-17044 A Japanese Patent Laid-Open No. 7-252592

K. Sugimoto et al, `` ISIJ International '' (2000) Vol.40, p.920 Kishida, “Nippon Steel Technical Review” (1999) No. 371, p. 13 Hirofumi Inoue, Naoji Inabe “Determination of crystal orientation distribution function from incomplete pole figure by iterative series expansion method” Journal of the Japan Institute of Metals, The Japan Institute of Metals, August 1994, Vol. 58, No. 8, p. 892-898

  The present invention has been devised in view of the above-mentioned problems, and its object is to stably produce a hot-rolled steel sheet having high strength and excellent stretch flangeability and low-temperature toughness, and the steel sheet. It aims at providing the manufacturing method which can be performed.

  The inventors focused on the area ratio, grain size and texture of the martensite phase and the lower bainite structure with respect to the low temperature toughness and hole expandability of the high strength hot rolled steel sheet mainly composed of the low temperature transformation phase. The impact was investigated. As a result, the martensite phase and the lower bainite structure are the main phase, the particle size is made fine, and the texture is optimized to combine excellent hole expansibility and low temperature toughness with high strength. It has been found that hot-rolled steel sheets can be produced.

This invention is made | formed based on the said knowledge, The place made into the summary is as follows.
(1) By mass%, C: 0.03% or more and 0.25% or less, Si: 0.001% or more and 2.0% or less, Mn: 0.5% or more and 4.0% or less, P: 0.00. 10% or less, S: 0.010% or less, sol. Al: 0.001% or more and 1.0% or less, B: 0.0001% or more and 0.005% or less, N: 0.01% or less, with the balance being Fe and inevitable impurities. Have
The total area ratio of the martensite phase and the lower bainite structure is 85% or more at the 1/4 thickness position, and the average grain size of the crystal grains surrounded by the boundary with a crystal orientation difference of 15 ° or more is 20 μm or less. The crystal grains having an aspect ratio of 0.30 or less are 50% or less in area ratio,
The average value of the X-ray random intensity ratio of the {100} <011> to {211} <011> orientation groups at the plate thickness center position is 6.0 or less, and the maximum value is 8.0 or less. Hot rolled steel sheet.
(2) The chemical composition is mass% in place of part of the Fe, Ti: 0.2% or less, Nb: 0.1% or less, V: 0.5% or less, and Mo: 0.5 The hot-rolled steel sheet according to (1) above, which contains one or more selected from the group consisting of% or less.
(3) The chemical composition is selected from the group consisting of Cu: 1.0% or less, Ni: 1.0% or less, and Cr: 2.0% or less in mass% instead of part of the Fe. The hot-rolled steel sheet according to (1) or (2) above, wherein the hot-rolled steel sheet contains at least one kind.
(4) The chemical composition is selected from the group consisting of Ca: 0.01% or less, Mg: 0.01% or less, and REM: 0.01% or less in mass% instead of part of the Fe. The hot-rolled steel sheet according to any one of (1) to (3) above, wherein the hot-rolled steel sheet contains at least one kind.
(5) In producing the hot-rolled steel sheet according to any one of (1) to (4) above, a slab having the chemical composition according to any one of (1) to (4) above. Or a method for producing a hot-rolled steel sheet, which is subjected to multi-pass hot rolling on a steel slab to form a hot-rolled steel sheet,
In the multi-pass hot rolling, the rolling reduction of the rolling pass immediately before the final rolling pass is 20% or more, the rolling reduction of the final rolling pass is 10% or more,
The rolling temperature and rolling finishing temperature of the rolling pass immediately before the final rolling pass are Ar ₃ or higher and 1100 ° C. or lower and T0 (° C.) or higher determined by the formula (1),
The time t1 (s) between passes from the completion of the rolling pass immediately before the final rolling pass to the start of rolling of the final rolling pass, and the time t2 (s) from the completion of the final rolling pass (finish rolling) to the start of cooling. Respectively satisfying formula (2) and formula (3),
From the start of cooling to the temperature range of Ms (° C.) or less determined by equation (5), cooling is performed at an average cooling rate of CR (° C./s) or more calculated by equation (4) and 20 ° C./s or more. A method for producing a hot-rolled steel sheet.
T0 (° C.) = 850 + 350 {(% Ti) +2 (% Nb) +0.3 (% V)} (1)
0.002 / exp (−6080 / (T1 + 273)) ≦ t1 ≦ 2.0 (2)
0.002 / exp (-6080 / (T2 + 273)) ≦ t2 ≦ 4.0 (3)
CR (° C./s)=50 {5.6-4.8 (% C) -0.5 (% Si) -1.1 (% Mn) -1180 (% B) -0.9 (% Cr) -2.1 (% Mo)} (4)
Ms (° C.) = 561-474 (% C) −33 (% Mn) −17 (% Ni) −21 (% Mo) (5)
Where the meaning of each symbol is as follows:
t1: Time (seconds) between passes from the completion of rolling of the rolling pass immediately before the final rolling pass to the start of rolling of the final rolling pass.
T1: Rolling temperature (° C.) of the rolling pass immediately before the final rolling pass.
t2: Time (seconds) from the completion of finish rolling until cooling is started.
T2: Rolling finish temperature (° C)

  According to the present invention, it is possible to stably produce a high-strength hot-rolled steel sheet having high strength and excellent hole expandability and low-temperature toughness. If this steel plate is used, it is easy to process and can withstand use in extremely cold regions, and thus the industrial contribution is extremely remarkable.

  The hot-rolled steel sheet and the manufacturing method thereof according to the present invention will be described in detail below. In the following description, all percentages relating to the chemical composition of steel are mass%.

<Chemical composition of steel>
C: 0.03% or more and 0.25% or less C has an effect of increasing the strength of the martensite phase. If the C content is less than 0.03%, it is difficult to obtain the desired strength. Therefore, the C content is 0.03% or more. Preferably it is 0.05% or more. On the other hand, if the C content exceeds 0.25%, the hole expandability and the base metal low temperature toughness are lowered, and the weldability is also lowered. Therefore, the C content is 0.25% or less. Preferably it is 0.2% or less, More preferably, it is 0.15% or less.

Si: 0.001% or more and 2.0% or less Si has an effect of increasing the strength of the steel sheet through solid solution strengthening and improvement of hardenability, and also has a deoxidizing action. When the Si content is less than 0.001%, it is difficult to obtain the effect by the above action. Therefore, the Si content is 0.001% or more. Preferably it is 0.1% or more. On the other hand, when added excessively, it becomes difficult to obtain a desired structure by promoting the α transformation. Therefore, the content upper limit is set to 2.0%. Preferably it is 1.50% or less, More preferably, it is 1.25% or less.

Mn: 0.5% or more and 4.0% or less Mn has the effect of increasing the strength of the steel sheet through solid solution strengthening and improving hardenability. When the Mn content is less than 0.5%, it is difficult to obtain the effect by the above action. Therefore, the Mn content is 0.5% or more. Preferably it is 1.0% or more, More preferably, it is 1.5% or more. On the other hand, even if added over 4%, this effect is saturated. Therefore, the Mn content is 4% or less.

P: 0.10% or less P is an element contained as an impurity and has the effect of reducing the low temperature toughness and workability of the steel sheet. Therefore, the P content is 0.10% or less. Preferably, it is 0.06% or less, more preferably 0.03% or less, still more preferably 0.015% or less.

S: 0.010% or less S is an element contained as an impurity, and has the effect of reducing the low temperature toughness and workability of the steel sheet. For this reason, S content shall be 0.010% or less. Preferably it is 0.005% or less, More preferably, it is 0.003% or less, More preferably, it is 0.001% or less.

sol. Al: 0.001% or more and 1.0% or less Al has an action of making steel healthy by deoxidation. sol. If the Al content is less than 0.001%, it is difficult to obtain the effect by the above action. Therefore, sol. The Al content is 0.001% or more. Preferably it is 0.01% or more, More preferably, it is 0.02% or more. On the other hand, sol. Even if the Al content exceeds 1.0%, the effect of the above action is saturated, and the desired structure is difficult to obtain due to the effect of promoting ferrite transformation. Therefore, sol. The Al content is 1.0% or less, preferably 0.6% or less, more preferably 0.4% or less, and still more preferably 0.2% or less.

N: 0.01% or less N is an element contained as an impurity and has the effect of reducing the workability of the steel sheet. For this reason, N content shall be 0.01% or less. Preferably it is 0.006% or less, More preferably, it is 0.005% or less.

B: 0.0001% or more and 0.005% or less B segregates at the γ grain boundary and has the effect of significantly improving the hardenability by adding a small amount and increasing the strength of the steel. Further, it is contained because it effectively works to prevent fracture cracks by increasing the grain boundary strength. If it is contained excessively, recrystallization of austenite during hot rolling is suppressed, the rolling load is increased, and a desired texture is difficult to obtain. Therefore, the B content is 0.005% or less. Preferably, it is 0.003% or less, more preferably 0.002% or less. On the other hand, it is desirable to satisfy 0.0001% or more in order to reliably obtain the above action. Preferably it is 0.0006% or more, More preferably, it is 0.001% or more.

  The following elements are optional elements that can be included in the hot-rolled steel sheet of the present invention as needed.

[Ti: 0.2% or less, Nb: 0.1% or less, V: 0.5% or less, and Mo: one or more selected from the group consisting of 0.5% or less]
Ti, Nb, V and Mo are precipitated as carbides or nitrides and have an action of increasing the strength of the steel sheet. Moreover, these precipitates have the effect | action which suppresses the coarsening of austenite and accelerates | stimulates refinement | miniaturization of a structure | tissue. Therefore, you may contain 1 type, or 2 or more types of these elements. However, if Ti is contained in excess of 0.2%, Nb is contained in excess of 0.1%, V is contained in excess of 0.5%, and Mo is 0.5%. If the content exceeds 50%, a large amount of coarse carbides or nitrides precipitate in the steel before being subjected to hot rolling, resulting in deterioration of workability of the hot-rolled steel sheet. In addition, hole expandability and low temperature toughness are reduced by the precipitation of a large amount of carbides and nitrides. Therefore, the content of each element is Ti: 0.2% or less, Nb: 0.1% or less, V: 0.5% or less, and Mo: 0.5% or less. Ti is preferably 0.1% or less, and more preferably 0.05% or less. Nb is preferably 0.05% or less, and more preferably 0.03% or less. V is preferably 0.3% or less. About Mo, it is preferable to set it as 0.3% or less. Furthermore, from the viewpoint of facilitating the formation of ferrite, the total content of Ti and Nb is preferably 0.1% or less, more preferably 0.03% or less, and 0.01% or less. It is particularly preferable to do this. In order to obtain the effect of the above operation more reliably, any of Ti: 0.001% or more, Nb: 0.001% or more, V: 0.01% or more, and Mo: 0.001% or more is satisfied. It is preferable.

[One or more selected from the group consisting of Cu: 1.0% or less, Ni: 1.0% or less, and Cr: 2.0% or less]
Cu, Ni and Cr have the effect of further improving the strength of the steel sheet by precipitation strengthening and solid solution strengthening. Therefore, you may contain 1 type, or 2 or more types of these elements. However, when Cu and Ni exceed 1.0% and Cr exceeds 2.0%, the workability deteriorates remarkably. Moreover, when adding Cu, in order to prevent the grain boundary embrittlement of a slab, it is desirable to add Ni more than 1/2 of the addition amount of Cu. In order to more reliably obtain the effect of the above action, it is preferable to satisfy any of Cu: 0.02% or more, Ni: 0.02% or more, and Cr: 0.02% or more.

[One or more selected from the group consisting of Ca: 0.01% or less, Mg: 0.01% or less, and REM: 0.01% or less]
Ca, Mg, and REM (rare earth elements) have the effect of refining oxides and nitrides that precipitate during solidification and improving the soundness of the steel ingot or slab. Therefore, you may contain 1 type, or 2 or more types of these elements. However, in the case of Ca, the content exceeds 0.01%. In the case of Mg, the content exceeds 0.01%. In the case of REM, the content exceeds 0.01%. However, the effects of the above actions are saturated, and the cost increases. Therefore, the respective contents are set to Ca: 0.01% or less, Mg: 0.01% or less, and REM: 0.01% or less. In order to more reliably obtain the effect of the above action, it is preferable to satisfy any of Ca: 0.0002% or more, Mg: 0.0002% or more, and REM: 0.0002% or more. Here, REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the content of REM refers to the total content of these elements. In the case of a lanthanoid, it is industrially added in the form of misch metal.

<Steel structure>
[Total area ratio of martensite phase and lower bainite structure: 85% or more]
The martensite phase and the lower bainite structure are hard, homogeneous and fine structures and are suitable for combining high strength, excellent hole expansibility and low temperature toughness. If the total area ratio of the martensite phase and the lower bainite structure is less than 85%, it is difficult to ensure the desired strength and combine excellent stretch flangeability and low temperature toughness. Therefore, the total area ratio of the martensite phase and the lower bainite structure is 85% or more. Preferably it is 90% or more, More preferably, it is 95% or more. There is no particular upper limit, and it may be 100%. The martensite phase includes an autotempered tempered martensite phase.

[Other organizations: 15% or less]
Other structures that are optional structures include ferrite, upper bainite, retained austenite, pearlite, and grain boundary cementite. If these structures exceed 15% in area ratio, the void starting point increases with the increase in the heterogeneous interface, leading to deterioration of hole expansibility. Therefore, the upper limit is made 15% or less. Preferably it is 8% or less, More preferably, it is 5% or less. Since the smaller the better, the lower limit is not particularly limited. The lower limit may be 0%. By satisfying the total area ratio of the martensite phase and the lower bainite structure: 85% or more, the other structures: 15% or less are automatically satisfied.

[Average grain size of crystal grains surrounded by a boundary with a crystal orientation difference of 15 ° or more: 20 μm or less]
When the average grain size of the martensite and bainite blocks and ferrite grains such as ferrite grains, which are steel structures of the present invention, is coarse, the fracture surface unit at the time of fracture increases, and the low-temperature toughness decreases. Therefore, the average grain size of the crystal grains is 20 μm or less. Preferably it is 15 micrometers or less, More preferably, it is 13 micrometers or less. Since the average particle size is preferably as small as possible, the lower limit is not particularly limited.

As the average grain size of the crystal grains of the present invention, a value obtained by EBSD analysis is adopted. Specifically, with the boundary having a crystal orientation difference of 15 ° or more as a grain boundary, the value calculated by the formula shown in the following [Equation 1] is taken as the average grain size. In the formula, N is the number of crystal grains included in the evaluation area of the average grain size, Ai is the area of the i-th (i = 1, 2,..., N) grain, and di is equivalent to the circle of the i-th crystal grain. Indicates diameter. These data are easily obtained by EBSD analysis. Specifically, it is obtained by distinguishing phases using the crystal structure definition of iron face-centered cubic lattice (FCC) and body-centered cubic lattice (BCC), and analyzing only the phase recognized as BCC. It is done. At the same time, since the major axis and minor axis of each crystal grain are also obtained, the area ratio of crystal grains having an aspect ratio of 0.30 or less can be easily obtained.

  The grains having a crystal orientation difference of 15 ° or more are mainly ferrite grains, martensite and bainite blocks. Here, the block is an assembly of laths having the same orientation. A region surrounded by a boundary having a crystal orientation difference of 15 ° or more is defined as a block. In the ferrite grain size measurement method according to JIS G0552, grain size may be calculated even for grains having a crystal orientation difference of less than 15 °, and martensite and bainite blocks are not calculated. On the other hand, in the EBSD analysis, the crystal structure and the crystal orientation can be evaluated, not the distinction between the structures of ferrite, martensite, and bainite. Therefore, in the present invention, the average grain size of the crystal grains surrounded by the boundary having a crystal orientation difference of 15 ° or more and the area ratio of the crystal grains having the following aspect ratio of 0.30 or less adopt values obtained by EBSD analysis. To do. Here, the number of crystal grains needs to be at least 1000 or more.

[Area ratio of crystal grains having an aspect ratio of 0.30 or less: 50% or less]
The aspect ratio is a value obtained by dividing the minor axis of the crystal grain by the major axis, and takes a value from 0 to 1. The smaller the aspect ratio, the flatter the crystal grains, and the closer to 1, the more equiaxed grains. As the number of flat crystal grains having an aspect ratio of 0.30 or less increases, the characteristic anisotropy increases and the hole expansibility decreases, and at the same time, the low-temperature toughness decreases. Therefore, in the present invention, the area ratio of crystal grains having an aspect ratio of 0.30 or less is set to 50% or less. Since the tissue is preferably equiaxed, the smaller the area ratio, the better. Preferably it is 45% or less, More preferably, it is 40% or less, More preferably, it is 35% or less, Most preferably, it is 30% or less.

  In addition, the above-mentioned steel structure (crystal structure area ratio, crystal grain size, aspect ratio) is a structure at a 1/4 depth position of the plate thickness from the steel plate surface. This depth position is an intermediate point between the surface of the steel plate and the center of the plate thickness, and shows the average steel structure of the hot-rolled steel plate.

<Group organization>
[Average value of X-ray random intensity ratio of {100} <011> to {211} <011> orientation group at the thickness center position is 6.0 or less and the maximum value is 8.0 or less]
When the texture of {211} <011> to {100} <011> orientation develops at the plate thickness center position, the hole expansibility decreases. For this reason, hole expansibility can be improved by reducing these azimuth | directions. Therefore, the average value of the X-ray random intensity ratios in these directions is 6.0 or less, and the maximum value is 8.0 or less. Preferably they are 5.0 or less, 7.0 or less, More preferably, it is 4.0 or less, 6.0 or less, More preferably, it is 3.0 or less, 5.0 or less. The lower the better.

  Here, {hkl} represents a crystal plane parallel to the rolling surface, and <uvw> represents a crystal direction parallel to the rolling direction. That is, {hkl} <uvw> indicates a crystal in which {hkl} is oriented in the normal direction of the plate surface and <uvw> is oriented in the rolling direction.

  The X-ray random intensity ratio is the ratio of the X-ray diffraction intensity to the random sample, and the {211} <011> to {100} <011> orientation group X-ray random intensity ratio is an iterative series expansion. ODF analysis is performed by the method, and the X-ray random intensity ratio of φ1 = 0 °, φ2 = 45 °, and φ = 0-35 ° in the Bunge method. The average value of these X-ray random intensity ratios should just be 6.0 or less, and the maximum value should be 8.0 or less. Although the measurement method is not particularly limited, a method of obtaining an incomplete pole figure of three or more surfaces such as (110), (200), (211), etc. by X-ray diffraction, and then obtaining by the method described in Non-Patent Document 3 above. Alternatively, there is a method of measuring using EBSD. However, in the latter case, the number of crystal grains must be at least 3000. The incomplete pole figure is a pole figure obtained by the reflection method. A random sample is a sample having an irregular distribution without having a crystal orientation.

<Mechanical properties>
The hot rolled steel sheet obtained by the present invention has high strength and excellent low temperature toughness and hole expansibility by controlling the steel structure and texture. However, when the tensile strength of the steel sheet is small, effects such as weight reduction and rigidity improvement are small. Therefore, it is preferable that the tensile strength (TS) of a steel plate is 960 MPa or more. More preferably, it is 1050 MPa or more, More preferably, it is 1100 MPa or more, Most preferably, it is 1150 MPa or more.

  As for the low temperature toughness of the manufactured hot-rolled steel sheet, the fracture surface transition temperature in the Charpy test is preferably −40 ° C. or lower. Hole expandability is evaluated by TS × HER (HER: hole expansion rate specified in Japan Iron and Steel Federation Standard JFS-T1001-1996), which is an index of balance with strength, and TS × HER is 50000 MPa ·% or more. Is more preferably 52500 MPa ·% or more, and particularly preferably 55000 MPa ·% or more.

<Manufacturing method>
Although it does not specifically limit about the manufacturing method of the hot-rolled steel plate of this invention, It can obtain easily with the following manufacturing methods.

  A hot rolled steel sheet is manufactured by subjecting the slab having the above chemical composition to multi-pass hot rolling. The slab to be subjected to hot rolling may be obtained by continuous casting or casting / bundling rolling, but may be obtained by adding hot working or cold working to them. Moreover, the reheated slab may be used for hot rolling, or a slab in a high temperature state after continuous casting or after partial rolling may be used as it is. If the hot rolling finishing temperature mentioned later can be ensured, there will be no restriction | limiting in particular. The temperature of the slab used for hot rolling is generally 900 to 1350 ° C. Multi-pass hot rolling can be performed using a lever mill or a tandem mill. From the viewpoint of industrial productivity, it is preferable to use a tandem mill for at least the last several stages.

[Rolling ratio of rolling pass one step before final rolling pass: 20% or more]
[The rolling reduction of the final rolling pass is 10% or more]
By increasing the rolling reduction of the rolling pass immediately before the final rolling pass as described above, the recrystallized austenite grains are mainly refined, and further, the rolling reduction of the final rolling pass is increased as described above. The recrystallization of austenite is promoted and refined, and in combination with the cooling conditions after hot rolling described later, a hot rolled steel sheet having a steel structure and a texture suitable for low temperature toughness and hole expansibility is manufactured. can do. Therefore, the rolling reduction of the rolling pass immediately before the final rolling pass is set to 20% or more. Preferably it is 22% or more. More preferably, it is 25% or more. The rolling reduction of the final rolling pass is 10% or more. It is preferably 14% or more, more preferably 18% or more, and particularly preferably 22% or more.

  On the other hand, from the viewpoint of ensuring good flatness of the steel sheet, the rolling reduction of the rolling pass immediately before the final rolling pass is preferably 60% or less. More preferably, it is 50% or less. Further, the rolling reduction of the final rolling pass is preferably 50% or less. More preferably, it is 45% or less.

  In order to obtain the desired fine structure and texture more reliably, it is preferable to increase the total rolling reduction after the finish rolling, and for that purpose, the steel sheet temperature is reduced from the rolling finishing temperature + 100 ° C to the rolling finishing temperature. It is preferable to set it to 50% or more. More preferably, it is 60% or more, and most preferably 65% or more.

[Rolling temperature and rolling finishing temperature of the rolling pass immediately before the final rolling pass: Ar ₃ or higher and 1100 ° C. or lower and T0 (° C. or higher) determined by the formula (1)]
T0 = 850 + 350 {(% Ti) +2 (% Nb) +0.3 (% V)} (1)
In the present invention, by repeating the processing and recrystallization of the austenite phase in the final stage of finish rolling, the structure is refined and the development of the texture is suppressed. Therefore, the rolling temperature and rolling finishing temperature of the rolling pass immediately before the final rolling pass are set to Ar ₃ or higher and 1100 ° C. or lower and T0 (° C.) or higher determined by Equation (1). Here, the rolling temperature of the rolling pass one step before the final rolling pass refers to the steel plate surface temperature after the rolling pass one step before the final rolling pass, and the rolling finish temperature refers to the steel plate surface temperature after the final rolling pass. Shall be pointed to. By setting both of these temperatures to Ar ₃ or higher, ferrite transformation between the final rolling passes and during rolling can be prevented. By setting it to T0 or more, recrystallization of austenite is moderately promoted between rolling passes, and the recrystallized austenite grains are refined. After hot rolling, the cooling conditions after hot rolling described later are compatible. Thus, a hot-rolled steel sheet having a steel structure and a texture suitable for low temperature toughness and hole expansibility is obtained. If it is less than T0, austenite before cooling after hot rolling becomes extremely flat, and in the hot rolled steel sheet as the final product, it will exhibit a microstructure that is elongated in the rolling direction, and the plastic anisotropy becomes large, and the hole expandability and Low temperature toughness decreases. Preferably it is T0 + 20 degreeC or more, More preferably, it is T0 + 40 degreeC or more. On the other hand, when these temperatures exceed 1100 ° C., the structure becomes coarse and the low-temperature toughness decreases. Therefore, the rolling temperature and rolling finishing temperature of the rolling pass immediately before the final rolling pass are set to 1100 ° C. or lower. Preferably it is 1070 degrees C or less, More preferably, it is 1040 degrees C or less. In addition, these temperatures are the surface temperature of steel materials, and can be measured with a radiation thermometer or the like.

[Time t1 from the completion of the rolling pass immediately before the final rolling pass to the start of the final rolling pass satisfies the equation (2), and the time t2 from the end of the rolling finish to the start of cooling satisfies the equation (3)]
0.002 / exp (−6080 / (T1 + 273)) ≦ t1 ≦ 2.0 (2)
0.002 / exp (-6080 / (T2 + 273)) ≦ t2 ≦ 4.0 (3)
Where the meaning of each symbol is as follows:
t1: Time (seconds) between passes from the completion of rolling of the rolling pass immediately before the final rolling pass to the start of rolling of the final rolling pass.
T1: Rolling temperature (° C.) of the rolling pass immediately before the final rolling pass.
t2: Time (seconds) from the completion of finish rolling until cooling is started.
T2: Rolling finish temperature (° C)
In the present invention, it is important to refine the texture and suppress the development of the texture. When the accumulated rolling reduction before transformation from austenite to martensite is increased, the structure becomes finer, but the texture develops and the hole expandability deteriorates. On the other hand, if the accumulated rolling reduction is reduced, the development of the texture can be suppressed, but the structure becomes coarse and it is difficult to obtain high low temperature toughness. Therefore, the time between the passes in the latter half of the finish rolling and the time from the completion of the finish rolling to the start of cooling are appropriately controlled, and the refining of austenite and the development of the texture are suppressed in each case. For that purpose, it is important to satisfy the above formulas (2) and (3). The lower limit is determined as the time required for recrystallization of austenite, and the upper limit is determined to prevent coarsening of the structure due to the growth of recrystallized austenite grains. Note that “start cooling” means the start of water cooling in the run-out table after finish rolling.

[Cooling at an average cooling rate of 20 ° C./s or more and CR or more obtained by Equation (4) from the start of cooling to a temperature range of Ms or less obtained by Equation (5)]
CR (° C./s)=50 {5.6-4.8 (% C) -0.5 (% Si) -1.1 (% Mn) -1180 (% B) -0.9 (% Cr) -2.1 (% Mo)} (4)
Ms (° C.) = 561-474 (% C) −33 (% Mn) −17 (% Ni) −21 (% Mo) (5)
After the rolling finish, the desired structure is obtained by cooling from austenite to a martensitic temperature range by water cooling. When the cooling rate is slow, ferrite and bainite are formed during the cooling process, and the strength may be lowered and the hole expansibility may be lowered. Further, if the cooling stop temperature is not sufficiently low, the desired martensite area ratio cannot be obtained sufficiently, and the strength and the low temperature toughness may be lowered. For this reason, the average cooling rate from the start of cooling to the stop of cooling is set to be equal to or higher than the CR obtained by Expression (4). Preferably it is CR + 10 ° C./s or more, more preferably CR + 20 ° C./s or more, further preferably CR + 30 ° C./s or more. Further, the cooling stop temperature is set to be equal to or lower than Ms obtained by Expression (5). Preferably it is Ms-50 degrees C or less, More preferably, it is Ms-100 degrees C or less, Most preferably, it is Ms-200 degrees C or less.

  Generally, after cooling to a temperature range of Ms or less, winding is performed.

  Thus, the hot-rolled steel sheet manufactured by the method according to the present invention may be appropriately subjected to known temper rolling for the purpose of, for example, shape correction. Moreover, it is good also as a plated steel plate by plating. The plating may be either electroplating or hot dip plating, and the type of plating is not particularly limited, but is generally zinc-based plating including zinc plating and zinc alloy plating. Examples of the plated steel sheet include an electrogalvanized steel sheet, an electrogalvanized nickel-plated steel sheet, a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet, and a hot-dip zinc-aluminum alloy plated steel sheet. The plating adhesion amount may be a general amount.

  Although it does not specifically limit about the plate | board thickness of the steel plate of this invention, when a plate | board thickness is too thick, since a structural difference differs remarkably by a plate | board surface layer and an inside, 6 mm or less is preferable. On the other hand, if the plate thickness is too thin, it is difficult to pass the plate at the time of hot rolling. Preferably, it is 1.5 mm or more.

  After melting and casting steel having the chemical composition shown in Table 1, a steel piece having a thickness of 30 mm was formed by hot forging. The obtained steel slab was heated to 1250 ° C. and subjected to hot rolling under the conditions shown in Table 2 using a small test tandem mill, and finished to a plate thickness of 3 mm. Table 2 shows the manufacturing conditions. The total rolling reduction from the steel sheet temperature from the rolling finishing temperature + 100 ° C. to the rolling finishing temperature was 48% for trial No. 14, 63% for trial No. 21, and 65% or more for others.

  About the obtained hot-rolled steel sheet, the cross section of the steel sheet in the direction parallel to the rolling direction was observed using a scanning electron microscope, and the structure at the 1/4 depth position of the sheet thickness from the steel sheet surface was investigated. The rate was measured. Moreover, it measured by 0.5 micrometer space | interval using EBSD, and calculated | required the area ratio of the crystal grain whose average particle diameter and aspect-ratio are 0.30 or less.

  A method for measuring the average value and the maximum value of the X-ray random intensity ratio of the {100} <011> to {211} <011> orientation groups will be described. Α-fiber obtained by obtaining an incomplete pole figure of (200), (110), (211) at the center position of the plate thickness by an X-ray diffraction test and performing ODF analysis by the iterative series expansion method described above From the values (φ1 = 0 °, φ2 = 45 °, Φ = 0-90 ° in the Bunge method), the average value and the maximum value of the X-ray random intensity ratio in the range of Φ = 0-35 ° were determined.

  In order to evaluate the mechanical properties of the hot-rolled steel sheet, the tensile test was conducted according to JIS Z 2241 and the hole expansion test was conducted according to JIS Z 2256. The product of the tensile strength TS and the hole expansion rate HER (TS × HER) is preferably 50000 MPa ·% or more. In the Charpy test, a steel sheet was processed into a 2.5 mm sub-size test piece in accordance with JIS Z 2242, and the fracture surface transition temperature vTrs (° C.) was evaluated.

  Table 3 shows the survey results of steel structure, texture and mechanical properties.

  As shown in Table 3, in the inventive examples according to the present invention, a high-strength hot-rolled steel sheet having excellent low-temperature toughness and hole expansibility and excellent in low-temperature toughness and hole expansibility is obtained.

  On the other hand, the comparative example whose chemical composition or manufacturing conditions are outside the scope of the present invention is inferior to the fracture surface transition temperature and / or the product of TS and hole expansion rate (TS × HER).

Claims (5)

% By mass
C: 0.03% to 0.25%,
Si: 0.001% to 2.0%,
Mn: 0.5% to 4.0%,
P: 0.10% or less,
S: 0.010% or less,
sol. Al: 0.001% to 1.0%,
B: 0.0001% to 0.005%,
N: 0.01% or less,
And the balance has a chemical composition consisting of Fe and inevitable impurities,
The total area ratio of the martensite phase and the lower bainite structure is 85% or more at the plate thickness 1/4 plate thickness position,
The average grain size of the crystal grains surrounded by the boundary having a crystal orientation difference of 15 ° or more is 20 μm or less,
The crystal grain whose aspect ratio is 0.30 or less is 50% or less in area ratio,
The average value of the X-ray random intensity ratio of the {100} <011> to {211} <011> orientation groups at the plate thickness center position is 6.0 or less, and the maximum value is 8.0 or less. Hot rolled steel sheet.   Instead of a part of the Fe, the chemical composition is, in mass%, Ti: 0.2% or less, Nb: 0.1% or less, V: 0.5% or less, and Mo: 0.5% or less. The hot-rolled steel sheet according to claim 1, comprising one or more selected from the group consisting of:   The chemical composition is selected from the group consisting of Cu: 1.0% or less, Ni: 1.0% or less, and Cr: 2.0% or less in mass%, instead of a part of the Fe. Or the hot-rolled steel sheet according to claim 1 or 2 containing two or more sorts.   The chemical composition is one type selected from the group consisting of Ca: 0.01% or less, Mg: 0.01% or less, and REM: 0.01% or less in mass%, instead of a part of the Fe. Or the hot-rolled steel sheet according to any one of claims 1 to 3, comprising two or more kinds. In producing the hot-rolled steel sheet according to any one of claims 1 to 4, a multi-pass heat is applied to a slab or steel slab having the chemical composition according to any one of claims 1 to 4. A method for producing a hot-rolled steel sheet that is subjected to hot rolling to obtain a hot-rolled steel sheet, wherein the rolling reduction of the rolling pass immediately before the final rolling pass in the multi-pass hot rolling is 20% or more,
The rolling reduction of the final rolling pass is 10% or more,
The rolling temperature and rolling finishing temperature of the rolling pass immediately before the final rolling pass are Ar ₃ or higher and 1100 ° C. or lower and T0 (° C.) or higher determined by the formula (1),
The time t1 (s) between passes from the completion of the rolling pass immediately before the final rolling pass to the start of rolling of the final rolling pass, and the time t2 (s) from the completion of the final rolling pass (finish rolling) to the start of cooling. Respectively satisfying formula (2) and formula (3),
From the start of cooling to the temperature range of Ms (° C.) or less determined by equation (5), cooling is performed at an average cooling rate of CR (° C./s) or more calculated by equation (4) and 20 ° C./s or more. A method for producing a hot-rolled steel sheet.
T0 (° C.) = 850 + 350 {(% Ti) +2 (% Nb) +0.3 (% V)} (1)
0.002 / exp (−6080 / (T1 + 273)) ≦ t1 ≦ 2.0 (2)
0.002 / exp (-6080 / (T2 + 273)) ≦ t2 ≦ 4.0 (3)
CR (° C./s)=50 {5.6-4.8 (% C) -0.5 (% Si) -1.1 (% Mn) -1180 (% B) -0.9 (% Cr) -2.1 (% Mo)} (4)
Ms (° C.) = 561-474 (% C) −33 (% Mn) −17 (% Ni) −21 (% Mo) (5)
Where the meaning of each symbol is as follows:
t1: Time (seconds) between passes from the completion of rolling of the rolling pass immediately before the final rolling pass to the start of rolling of the final rolling pass.
T1: Rolling temperature (° C.) of the rolling pass immediately before the final rolling pass.
t2: Time (seconds) from the completion of finish rolling until cooling is started.
T2: Rolling finish temperature (° C)