JPWO2016132545A1 - Hot rolled steel sheet - Google Patents

Abstract

This hot-rolled steel sheet has a predetermined chemical composition, and the structure includes a total area ratio of 75 to 95% ferrite and bainite and 5 to 20% martensite; Is defined as a grain boundary, and a region surrounded by the grain boundary and having an equivalent circle diameter of 0.3 μm or more is defined as a crystal grain, the orientation difference within the grain is 5 to 14 °. The ratio of the certain crystal grains is 10 to 60% in terms of area ratio.

Description

  The present invention relates to a hot-rolled steel sheet excellent in workability, and particularly to a composite-structure hot-rolled steel sheet excellent in stretch flangeability.

  In recent years, in response to demands for weight reduction of various members for the purpose of improving fuel efficiency of automobiles, thinning by increasing the strength of steel plates such as iron alloys used for members, and application to various members of light metals such as Al alloys Is underway. However, when compared with heavy metals such as steel, light metals such as Al alloys have the advantage of high specific strength but have the disadvantage of being extremely expensive. For this reason, the application of light metals such as Al alloys is limited to special applications. Therefore, in order to apply the weight reduction of various members to a cheaper and wider range, it is required to reduce the thickness by increasing the strength of the steel sheet.

  When the strength of a steel plate is increased, generally material properties such as formability (workability) deteriorate. Therefore, in the development of high-strength steel sheets, it is an important issue to increase the strength without deteriorating the material properties. In particular, steel plates used as automobile members such as inner plate members, structural members, and suspension members have stretch flangeability, burring workability, ductility, fatigue durability, impact resistance, corrosion resistance, etc., depending on their applications. Therefore, it is important to make these material properties and strength compatible.

  For example, steel plates used for structural members and suspension members that account for approximately 20% of the body weight of automobile parts are subjected to blanking and punching by shearing or punching, and then stretch flange processing and burring processing. The press molding is mainly performed. Therefore, these steel plates are required to have good stretch flangeability.

  For example, Patent Document 1 discloses a hot-rolled steel sheet that defines the martensite fraction, size, number density, and average martensite spacing and is excellent in elongation and hole-expandability. Patent Document 2 discloses a hot-rolled steel sheet excellent in burring workability obtained by limiting the average particle diameter of ferrite and the second phase and the carbon concentration of the second phase. Patent Document 3 discloses a hot-rolled steel sheet having excellent workability, surface properties, and plate flatness, which is obtained by holding at a temperature range of 750 to 600 ° C. for 2 to 15 seconds and then winding at a low temperature.

  However, in Patent Document 1 described above, the primary cooling rate after the end of hot rolling must be secured at 50 ° C./s or more, which increases the load on the apparatus. In addition, when the primary cooling rate is set to 50 ° C./s or more, there is a problem that material variation due to variation in cooling rate occurs.

  In addition, as described above, in recent years, there has been an increasing demand for application of high-strength steel sheets to automobile members. When a high-strength steel sheet is cold-formed and formed, cracks are likely to occur from the edge of the part that becomes stretch flange forming during forming. This is thought to be due to the fact that work hardening proceeds only at the edge due to strain introduced into the punched end face during blanking. Conventionally, a hole expansion test has been used as a test evaluation method for stretch flangeability. However, in the hole-expansion test, fracture occurs with almost no circumferential strain distributed, but in actual part machining, strain distribution exists, so the strain around the fractured part and the effect of the stress gradient on the fracture limit. Exists. Therefore, in the case of a high-strength steel plate, cracks may occur due to strain distribution when cold pressing is performed even if the hole expansion test shows sufficient stretch flangeability.

  In the techniques disclosed in Patent Documents 1 to 3, it is disclosed that in any of the inventions, the hole expandability is improved by defining only the structure observed with an optical microscope. However, it is unclear whether sufficient stretch flangeability can be secured even when the strain distribution is taken into consideration.

Japanese Unexamined Patent Publication No. 2013-19048 Japanese Unexamined Patent Publication No. 2001-303186 Japanese Laid-Open Patent Publication No. 2005-213566

The present invention has been devised in view of the above-described problems.
An object of the present invention is to provide a high-strength hot-rolled steel sheet excellent in stretch flangeability, which can be applied to a member that requires high stretch flangeability while having high strength. In the present invention, stretch flangeability is an index of stretch flangeability in consideration of strain distribution, and the limit forming height H (mm) and tensile strength of the flange obtained as a result of testing by the vertical stretch flange test method. The value evaluated by the product of TS (MPa) is shown, and excellent in stretch flangeability means that the product of the limit molding height H (mm) and the tensile strength TS (MPa) is 19500 (mm · MPa) or more. Indicates that there is. Moreover, high strength indicates that the tensile strength is 590 MPa or more. The upper limit of the strength is not particularly defined, but it is difficult to ensure a strength of over 1470 MPa within the range of the structure defined in the present invention.

  According to the conventional knowledge, the improvement of stretch flangeability (hole expandability) is, as shown in Patent Documents 1 to 3, including inclusion control, tissue homogenization, single organization, and / or hardness difference between tissues. This was done by reducing the amount of In other words, conventionally, improvement of hole expandability, workability, and the like has been achieved by controlling the structure observed with an optical microscope.

  However, in view of the fact that the stretch flangeability in the presence of strain distribution cannot be improved even if only the structure observed with an optical microscope is controlled, the inventors of the present invention have no difference in orientation within each grain. Focused on, and proceeded with intensive studies. As a result, it has been found that stretch flangeability can be greatly improved by controlling the ratio of crystal grains having a misorientation in the crystal grains of 5 to 14 ° to all crystal grains within a certain range.

  This invention is comprised based on said knowledge, The summary is as follows.

(1) The hot-rolled steel sheet according to one embodiment of the present invention has a chemical component of mass%, C: 0.04 to 0.18%, Si: 0.10 to 1.70%, Mn: 0.50. -3.00%, Al: 0.010-1.00%, B: 0-0.005%, Cr: 0-1.0%, Mo: 0-1.0%, Cu: 0-2. 0%, Ni: 0 to 2.0%, Mg: 0 to 0.05%, REM: 0 to 0.05%, Ca: 0 to 0.05%, Zr: 0 to 0.05% , P: 0.050% or less, S: 0.010% or less, N: 0.0060% or less, the balance is composed of Fe and impurities, and the structure is an area ratio, 75 to 95% in total. Including the ferrite and bainite of 5 to 20% martensite, and in the structure, a boundary having an orientation difference of 15 ° or more is a grain boundary, and is surrounded by the grain boundary, and If the equivalent diameter is the region is 0.3μm or more was defined as the crystal grains, the proportion of the grain misorientation is 5 to 14 ° in the grains, the area ratio, 10 to 60%.
(2) In the hot-rolled steel sheet described in (1) above, the tensile strength is 590 MPa or more, and the product of the tensile strength and the limit forming height in the vertical stretch flange test is 19500 mm · MPa or more. Also good.
(3) In the hot-rolled steel sheet according to the above (1) or (2), the chemical component is B: 0.0001 to 0.005% in mass%, Cr: 0.01 to 1.0%, Mo. : 0.01 to 1.0%, Cu: 0.01 to 2.0%, Ni: 0.01 to 2.0%, or one or more of them may be contained.
(4) In the hot-rolled steel sheet according to any one of (1) to (3), the chemical component is Mg: 0.0001 to 0.05% in mass%, REM: 0.0001 to 0. 0.05%, Ca: 0.0001 to 0.05%, or Zr: 0.0001 to 0.05% may be contained.

  According to the above aspect of the present invention, it is possible to provide a high-strength hot-rolled steel sheet that is excellent in stretch flangeability and can be applied to a member that requires high stretch flangeability while having high strength.

It is the analysis result by EBSD in the 1 / 4t part (position of 1/4 thickness of the plate thickness from the surface in the plate thickness direction) of the hot rolled steel plate according to the present embodiment. It is a figure which shows the shape of a vertical shape molded article used for a vertical stretch flange test method.

Hereinafter, a hot-rolled steel sheet according to an embodiment of the present invention (hereinafter may be referred to as a hot-rolled steel sheet according to the present embodiment) will be described in detail.
The hot-rolled steel sheet according to this embodiment has a chemical composition of mass%, C: 0.04 to 0.18%, Si: 0.10 to 1.70%, Mn: 0.50 to 3.00%. Al: 0.010 to 1.00%, and if necessary, B: 0.005% or less, Cr: 1.0% or less, Mo: 1.0% or less, Cu: 2. 0% or less, Ni: 2.0% or less, Mg: 0.05% or less, REM: 0.05% or less, Ca: 0.05% or less, Zr: 0.05% or less , P: 0.050% or less, S: 0.010% or less, N: 0.0060% or less, and the balance is Fe and impurities.
Moreover, the structure of the hot-rolled steel sheet according to the present embodiment includes 75 to 95% of ferrite and bainite and 5 to 20% martensite in total, and the orientation difference is 15 °. When the above boundary is defined as a grain boundary, and a region surrounded by the grain boundary and having a circle-equivalent diameter of 0.3 μm or more is defined as a crystal grain, the crystal having an orientation difference within the grain of 5 to 14 ° The ratio of the grains is 10 to 60% in terms of area ratio.

  First, the reasons for limiting the chemical components of the hot-rolled steel sheet according to this embodiment will be described. % Of content of each component is mass%.

C: 0.04 to 0.18%
C is an element that contributes to improving the strength of steel. In order to obtain this effect, the lower limit of the C content is 0.04%. On the other hand, when the C content is less than 0.04%, the proportion of crystal grains having an in-grain orientation difference of 5 to 14 ° is lowered. Also from this point, the lower limit of the C content is 0.04%. A preferable lower limit of the C content is 0.045%, and a more preferable lower limit of the C content is 0.05%. On the other hand, when the C content exceeds 0.18%, stretch flangeability and weldability deteriorate. In addition, the hardenability becomes excessive, the number of crystal grains having an in-grain misorientation of more than 14 ° increases, and the ratio of crystal grains having an in-grain misorientation of 5 to 14 ° decreases. Therefore, the upper limit of C content is 0.18%. The upper limit of the preferable C content is 0.17%, and the more preferable upper limit of the C content is 0.16%.

Si: 0.10 to 1.70%
Si is an element that contributes to improving the strength of steel. Si is an element having a role as a deoxidizer for molten steel. In order to obtain these effects, the lower limit of the Si content is 0.10%. A preferable lower limit of the Si content is 0.12%, and a more preferable lower limit of the Si content is 0.15%. On the other hand, when the Si content exceeds 1.70%, the Ar3 transformation temperature becomes too high, so that hot rolling in the γ region becomes difficult, and processed ferrite is generated, and the orientation difference within the grains is 5 to 14 °. The ratio of the crystal grains of the material decreases, and the stretch flangeability deteriorates. In addition, surface flaws may occur. Therefore, the upper limit of Si content is set to 1.70%. The upper limit of the preferable Si content is 1.60%, and the more preferable upper limit of the Si content is 1.50%.

Mn: 0.50 to 3.00%
Mn is an element that contributes to improving the strength of steel by solid solution strengthening and / or improving the hardenability of steel. In order to obtain this effect, the lower limit of the Mn content is 0.50%. The lower limit of the preferable Mn content is 0.65%, and the lower limit of the more preferable Mn content is 0.70%. On the other hand, if the Mn content exceeds 3.00%, the proportion of crystal grains having an orientation difference in the grains of 5 to 14 ° is lowered, and the stretch flangeability is deteriorated. Therefore, the upper limit of the Mn content is 3.00%. The upper limit of the preferable Mn content is 2.60%, and the upper limit of the more preferable Mn content is 2.30%.

Al: 0.010 to 1.00%
Al is an element effective as a deoxidizer for molten steel. In order to obtain this effect, the lower limit of the Al content is 0.010%. A preferable lower limit of the Al content is 0.015%, and a more preferable lower limit of the Al content is 0.020%. On the other hand, when the Al content exceeds 1.00%, weldability, toughness and the like deteriorate. Therefore, the upper limit of the Al content is set to 1.00%. The upper limit of the preferable Al content is 0.90%, and the more preferable upper limit of the Al content is 0.80%.

P: 0.050% or less P is an impurity. Since P deteriorates toughness, workability, weldability, etc., its content is preferably as low as possible. However, when the P content exceeds 0.050%, the stretch flangeability is significantly deteriorated. Therefore, the P content may be limited to 0.050% or less. More preferably, it is 0.040% or less. The lower limit of the P content is not particularly required, but excessive reduction is not desirable from the viewpoint of manufacturing cost, so the P content may be 0.005% or more.

S: 0.010% or less S is an element that not only causes cracking during hot rolling, but also forms A-based inclusions that degrade stretch flangeability. Therefore, the lower the S content, the better. However, when the S content exceeds 0.010%, the stretch flangeability is significantly deteriorated. Therefore, the upper limit of the S content may be limited to 0.010%. More preferably, it is 0.005% or less. The lower limit of the S content is not particularly defined, but excessive reduction is not desirable from the viewpoint of manufacturing cost, so the S content may be 0.001% or more.

N: 0.0060% or less N is an element that forms AlN during cooling after hot rolling and lowers the formability of the steel sheet. In particular, when the N content exceeds 0.0060%, the stretch flangeability is significantly deteriorated. Therefore, the upper limit of N content is limited to 0.0060%. A more preferable upper limit of the N content is 0.0040%. The lower limit of the N content is not particularly defined, but excessive reduction is not desirable from the viewpoint of manufacturing cost, so the N content may be 0.0010% or more.

The above chemical elements are basic components contained in the hot-rolled steel sheet according to this embodiment, and the chemical composition including these basic elements, the balance being Fe and impurities, is the same as that of the hot-rolled steel sheet according to this embodiment. Basic composition. Impurities are components mixed into steel from various raw materials such as ores and scraps when manufacturing alloys such as As and Sn industrially. It means that it is allowed as long as it does not adversely affect the properties of the hot-rolled steel sheet.
However, for the purpose of further improving the strength and toughness, one or more of B, Cr, Mo, Cu, Ni, Mg, REM, Ca, and Zr may be included in the ranges described below as necessary. . Since these elements are not necessarily contained, the lower limit of the content is 0%. Among the elements other than the above, Nb and Ti suppress recrystallization and deteriorate workability. Therefore, it is desirable that Nb is less than 0.005% and Ti is less than 0.015%.

B: 0.0001 to 0.0050%
B is an element that enhances the hardenability and contributes to increasing the strength of steel. In order to obtain this effect, the B content is preferably 0.0001% or more. On the other hand, when the B content exceeds 0.0050%, workability deteriorates. In addition, bainite having a large orientation dispersion is easily formed during quenching, and the proportion of crystal grains having an in-grain orientation difference of 5 to 14 ° is reduced. Therefore, even when B is contained, the upper limit of the B content is preferably 0.0050%.

Cr: 0.01 to 1.0%
Cr is an element that contributes to improving the strength of steel. Cr is an element having a cementite suppressing effect. When obtaining these effects, it is desirable that the Cr content be 0.01% or more. On the other hand, if the Cr content exceeds 1.0%, the ductility decreases. Therefore, even when Cr is contained, it is desirable that the upper limit of Cr content be 1.0%.

Mo: 0.01 to 1.0%
Mo is an element that has the effect of improving hardenability and forming carbides to increase strength. When obtaining these effects, it is desirable that the Mo content be 0.01% or more. On the other hand, if the Mo content exceeds 1.0%, ductility and weldability may be reduced. Therefore, even when Mo is contained, the upper limit of the Mo content is preferably 1.0%.

Cu: 0.01 to 2.0%
Cu is an element that increases the strength of the steel sheet and improves the corrosion resistance and the peelability of the scale. When obtaining these effects, it is desirable that the Cu content be 0.01% or more. More desirably, it is 0.04% or more. On the other hand, if the Cu content exceeds 2.0%, there is a concern that surface defects may occur. Therefore, even when Cr is contained, the upper limit of the Cr content is desirably 2.0%, and more desirably 1.0%.

Ni: 0.01% to 2.0%
Ni is an element that increases the steel sheet strength and improves the toughness. When obtaining these effects, the Ni content is preferably 0.01% or more. On the other hand, when the Ni content exceeds 2.0%, the ductility decreases. Therefore, even when Ni is contained, the upper limit of the Ni content is desirably 2.0%.

Ca: 0.0001 to 0.05%
Mg: 0.0001 to 0.05%
Zr: 0.0001 to 0.05%
REM: 0.0001 to 0.05%
Ca, Mg, Zr, and REM are all elements that improve toughness by controlling the shape of sulfides and oxides. Therefore, for this purpose, it is desirable to contain one or more of these elements in an amount of 0.0001% or more. More preferably, it is 0.0005%. However, when the content of these elements is excessive, stretch flangeability deteriorates. Therefore, even when these elements are contained, the upper limit of the content is preferably 0.05%.

Next, the structure (metal structure) of the hot-rolled steel sheet according to this embodiment will be described.
The hot-rolled steel sheet according to the present embodiment needs to contain 75 to 95% in total and 5 to 20% martensite in terms of area ratio in the structure observed with an optical microscope, including ferrite and bainite. By setting it as such a composite structure, strength and stretch flangeability can be improved with good balance. If the total area ratio of ferrite and bainite is less than 75%, stretch flangeability is deteriorated. Further, if the total area ratio of ferrite and bainite is more than 95%, the strength is lowered and the ductility is lowered, so that it is difficult to ensure the characteristics generally required for automobile members and the like. The fraction (area ratio) of ferrite and bainite need not be limited, but if the ferrite fraction exceeds 90%, sufficient strength may not be obtained, so the ferrite fraction should be 90% or less. It is desirable. More desirably, it is 70% or less. On the other hand, if the bainite fraction exceeds 60%, there is a concern that the ductility will decrease, so it is desirable that the bainite fraction be less than 60%. More desirably, it is less than 50%.
In the hot-rolled steel sheet according to the present embodiment, the remaining structure other than ferrite, bainite, and martensite is not particularly limited, and may be, for example, retained austenite or pearlite. However, when a structure other than ferrite, bainite, and martensite is included in excess of 5% in total, stretch flangeability and ductility are deteriorated. For this reason, the ratio of the remaining structure is preferably 5% or less in terms of area ratio. More preferably, it is 3% or less, and further preferably 0%.

The tissue fraction (area ratio) can be obtained by the following method. First, a sample taken from a hot rolled steel sheet is etched with nital. After the etching, image analysis is performed on the structure photograph obtained with a field of view of 300 μm × 300 μm at a position of ¼ depth of the plate thickness using an optical microscope, so that the area ratio of ferrite and pearlite, and bainite and martensite are obtained. Get the total area ratio with the site. Next, using a sample that has undergone repeller corrosion and performing an image analysis on a structural photograph obtained with a field of view of 300 μm × 300 μm at a position of ¼ depth of the plate thickness using an optical microscope, residual austenite and martensite are obtained. Calculate the total area ratio with the site.
Furthermore, the volume fraction of retained austenite is obtained by X-ray diffraction measurement using a sample that has been chamfered from the normal direction of the rolling surface to ¼ depth of the plate thickness. Since the volume ratio of retained austenite is equivalent to the area ratio, this is defined as the area ratio of retained austenite.
By this method, the area ratios of ferrite, bainite, martensite, retained austenite, and pearlite can be obtained.

The hot-rolled steel sheet according to the present embodiment uses an EBSD method (electron beam backscatter diffraction pattern analysis method) often used for crystal orientation analysis after controlling the structure observed with an optical microscope to the above range. It is necessary to control the proportion of crystal grains having an in-grain orientation difference of 5 to 14 °. Specifically, when a boundary having an orientation difference of 15 ° or more is defined as a grain boundary, and a region surrounded by the grain boundary and having an equivalent circle diameter of 0.3 μm or more is defined as a crystal grain, all crystal grains Of these, the proportion of crystal grains having an orientation difference within the grains of 5 to 14 ° needs to be 10 to 60% in terms of area ratio.
Since the crystal grains having such an in-granular orientation difference are effective for obtaining a steel sheet having an excellent balance between strength and workability, by controlling the ratio, the stretch flange is maintained while maintaining the desired steel sheet strength. Can be greatly improved. When the ratio of crystal grains having an orientation difference within the grains of 5 to 14 ° is less than 10% in terms of area ratio, stretch flangeability is deteriorated. Moreover, ductility will fall that the ratio of the crystal grain whose orientation difference within a grain | grain is 5 to 14 degree is more than 60% in an area ratio.
Here, it is considered that the difference in crystal orientation within the grain has a correlation with the dislocation density contained in the crystal grain. In general, an increase in the dislocation density in the grains brings about an improvement in strength while lowering workability. However, in the crystal grains in which the orientation difference within the grains is controlled to 5 to 14 °, the strength can be improved without reducing the workability. Therefore, in the hot-rolled steel sheet according to this embodiment, the proportion of crystal grains having an orientation difference in the grains of 5 to 14 ° is controlled to 10 to 60%. A crystal grain having an orientation difference of less than 5 ° is excellent in workability, but it is difficult to increase the strength. A crystal grain having an orientation difference of more than 14 ° in the grain has different deformability within the crystal grain. Does not contribute to improvement of stretch flangeability.

The proportion of crystal grains having an orientation difference within the grains of 5 to 14 ° can be measured by the following method.
First, with respect to the vertical cross section in the rolling direction at the 1/4 depth position (1/4 t portion) of the thickness t from the steel sheet surface, an area of 200 μm in the rolling direction and 100 μm in the normal direction of the rolling surface is measured at a measurement interval of 0.2 μm. Crystal orientation information is obtained by EBSD analysis. Here, the EBSD analysis is performed at an analysis speed of 200 to 300 points / second using an apparatus constituted by a thermal field emission scanning electron microscope (JSMOL JSM-7001F) and an EBSD detector (TSL HIKARI detector). To do. Next, with respect to the obtained crystal orientation information, a region having an orientation difference of 15 ° or more and an equivalent circle diameter of 0.3 μm or more is defined as a crystal grain, and an average orientation difference in the crystal grain is calculated. The ratio of crystal grains having an orientation difference of 5 to 14 ° is obtained. The crystal grains and the average orientation difference within the grains defined above can be calculated using software “OIM Analysis (registered trademark)” attached to the EBSD analyzer.
The “intragranular orientation difference” in the present invention represents “Grain Orientation Spread (GOS)”, which is the orientation dispersion in crystal grains, and the value is an error in plastic deformation of stainless steel by the EBSD method and the X-ray diffraction method. Analysis of Orientation ”, Hidehiko Kimura et al., Transactions of the Japan Society of Mechanical Engineers (A), 71, 712, 2005, p. As described in 1722-1728, it is obtained as an average value of misorientation between a reference crystal orientation and all measurement points in the same crystal grain. In this embodiment, the reference crystal orientation is an orientation obtained by averaging all measurement points in the same crystal grain, and the value of GOS is the software “OIM Analysis (registered trademark) Version 7.0” attached to the EBSD analyzer. .1 ".

  FIG. 1 is an example of an EBSD analysis result of a 100 μm × 100 μm region of a vertical cross section in the rolling direction at a 1/4 t portion of the hot-rolled steel sheet according to the present embodiment. In FIG. 1, a boundary having an orientation difference of 15 ° or more is displayed as a grain boundary, and a region having 5 to 14 ° is displayed in gray. Martensite is displayed in black in the figure.

  In this embodiment, stretch flangeability is evaluated by a saddle stretch flange test method using a saddle molded product. Specifically, as shown in FIG. 2, a saddle-shaped molded product simulating an elongated flange shape composed of a straight portion and an arc portion is pressed, and stretch flangeability is obtained using the limit molding height at that time. evaluate. In the vertical stretch flange test of the present embodiment, when a corner molded product with a corner radius of curvature R of 50 to 60 mm and an opening angle θ of 120 ° is used, and the clearance when punching the corner is 11% The limit molding height H (mm) is measured. Here, the clearance indicates the ratio of the gap between the punching die and the punch and the thickness of the test piece. Since the clearance is actually determined by the combination of the punching tool and the plate thickness, 11% means that the range of 10.5 to 11.5% is satisfied. The determination of the limit forming height was made by visually observing the presence or absence of cracks having a length of 1/3 or more of the plate thickness after forming, and determining the limit forming height at which no cracks exist.

  The hole-expansion test that has been used as a test method for stretch flange forming has hitherto been fractured with almost no distribution in the circumferential direction. The gradient is different. In addition, the hole expansion test is not an evaluation reflecting the original stretch flange molding, such as an evaluation at the time when a through-thickness breakage occurs. On the other hand, in the vertical stretch flange test used in the present embodiment, the stretch flangeability considering the strain distribution can be evaluated, so that the evaluation reflecting the original stretch flange molding is possible.

  In the hot-rolled steel sheet according to the present embodiment, the area ratio of each structure observed in an optical microscope structure such as ferrite and bainite is directly related to the ratio of crystal grains whose orientation difference in the grains is 5 to 14 °. It is not a thing. In other words, for example, even if there are hot-rolled steel sheets having the same ferrite area ratio and bainite area ratio, the ratio of crystal grains having an in-grain orientation difference of 5 to 14 ° is not always the same. Therefore, the characteristics corresponding to the hot-rolled steel sheet according to this embodiment cannot be obtained only by controlling the ferrite area ratio, bainite area ratio, and martensite area ratio. This is as shown in the examples described later.

  The hot-rolled steel sheet according to this embodiment can be obtained, for example, by a manufacturing method including the following hot rolling process and cooling process.

<Hot rolling process>
In a hot rolling process, the slab which has the chemical component mentioned above is heated, hot-rolled, and a hot-rolled steel plate is obtained. The slab heating temperature is preferably 1050 ° C. or more and 1260 ° C. or less. If the slab heating temperature is lower than 1050 ° C., it is difficult to ensure the hot rolling end temperature, which is not preferable. On the other hand, when the slab heating temperature exceeds 1260 ° C., the yield decreases due to scale-off, and therefore the heating temperature is preferably 1260 ° C. or less.

When the ratio of crystal grains having an orientation difference within the grains of 5 to 14 ° is set to 10 to 60% in terms of area ratio, in the hot rolling performed on the heated slab, three stages after the final rolling (final) It is important to perform the cooling described later after setting the cumulative strain of (3 passes) to more than 0.6 to 0.7. This is because crystal grains having an orientation difference in the grains of 5 to 14 ° are formed by transformation in a para-equilibrium state at a relatively low temperature, so that the dislocation density of austenite before transformation is limited to a certain range and thereafter This is because by limiting the cooling rate to a certain range, it is possible to control the generation of crystal grains having an in-grain orientation difference of 5 to 14 °. That is, by controlling the cumulative strain in the subsequent three stages of finish rolling and the subsequent cooling, it is possible to control the nucleation frequency and the subsequent growth rate of the crystal grains whose orientation difference within the grains is 5 to 14 °, The resulting area ratio can also be controlled. More specifically, the dislocation density of austenite introduced by finish rolling is mainly related to the nucleation frequency, and the cooling rate after rolling is mainly related to the growth rate.
If the cumulative strain in the last three stages of finish rolling is 0.6 or less, the proportion of crystal grains having an in-grain misorientation of 5 to 14 ° is less than 10%, which is not preferable. Further, if the cumulative strain of the last three stages of finish rolling is more than 0.7, recrystallization of austenite during hot rolling occurs, the accumulated dislocation density at the time of transformation is lowered, and the orientation difference in the grains is 5 to 5. Since the ratio of the crystal grain which is 14 degrees will be less than 10%, it is not preferable.
The cumulative strain (εeff.) Of the last three stages of finish rolling referred to in the present embodiment can be obtained by the following equation (1).
εeff. = Σεi (t, T) (1)
here,
εi (t, T) = εi0 / exp {(t / τR) ^2/3 },
τR = τ0 × exp (Q / RT),
τ0 = 8.46 × 10 ⁻⁶ ,
Q = 183200J,
R = 8.314 J / K · mol,
εi0 represents the logarithmic strain at the time of rolling, t represents the accumulated time until immediately before cooling in the pass, and T represents the rolling temperature in the pass.

The rolling end temperature of the hot rolling is preferably Ar3 ° C to Ar3 + 60 ° C. If the rolling end temperature exceeds Ar3 + 60 ° C., the crystal grain size of the hot-rolled sheet is increased, the workability is reduced, and the proportion of crystal grains having an in-grain orientation difference of 5-14 ° is decreased, which is not preferable. . Further, when the rolling end temperature is less than Ar3, it becomes hot rolling in a two-phase region, the ferrite phase is processed, the ductility and hole expandability of the hot-rolled steel sheet are reduced, and the orientation difference in the grains is 5-14. This is not preferable because the proportion of crystal grains at a temperature is lowered.
Hot rolling includes rough rolling and finish rolling. Finish rolling is performed using a tandem rolling mill in which a plurality of rolling mills are linearly arranged and continuously rolled in one direction to obtain a predetermined thickness. Is preferred. Moreover, when performing finish rolling using a tandem rolling mill, cooling is performed between the rolling mill and the rolling mill (inter-stand cooling), and the maximum temperature of the steel sheet during finish rolling is in the range of Ar3 + 60 ° C. or higher and Ar3 + 150 ° C. or lower. It is preferable to control so that. If the maximum temperature of the steel sheet during finish rolling exceeds Ar3 + 150 ° C, the grain size becomes too large and the toughness deteriorates, and there is a concern that the proportion of crystal grains having an in-grain orientation difference of 5 to 14 ° will decrease. Is done. On the other hand, if the maximum temperature of the steel sheet during finish rolling is less than Ar 3 + 60 ° C., there is a concern that the finish temperature of finish rolling cannot be secured.
If hot rolling is performed under the conditions as described above, the range of dislocation density of austenite before transformation can be limited, and as a result, crystal grains having an orientation difference in the grains of 5 to 14 ° can be obtained at a desired ratio. Can be obtained.

Ar3 is calculated by the following formula (2) in consideration of the influence on the transformation point due to the reduction.
Ar3 = 970-325 × [C] + 33 × [Si] + 287 × [P] + 40 × [Al] −92 × ([Mn] + [Mo] + [Cu]) − 46 × ([Cr] + [Ni ]) ... (2)
Here, [C], [Si], [P], [Al], [Mn], [Mo], [Cu], [Cr], and [Ni] are C, Si, P, Al, The content in mass% of Mn, Mo, Cu, Cr and Ni is shown. The element not contained is calculated as 0%.

<Cooling process>
Cooling is performed on the hot-rolled steel sheet subjected to the hot rolling controlled as described above. In the cooling step, the hot-rolled steel sheet that has been hot-rolled is cooled to a temperature range of 650 to 750 ° C. at a cooling rate of 10 ° C./s or more (first cooling). Hold for 10 seconds, and then cool to 100 ° C. or lower at a cooling rate of 30 ° C./s or higher (second cooling).
If the cooling rate of the first cooling is less than 10 ° C./s, the proportion of crystal grains having an orientation difference in the grains of 5 to 14 ° is less than 10%, which is not preferable. Further, if the cooling stop temperature of the first cooling is less than 650 ° C., the proportion of crystal grains having an orientation difference in the grains of 5 to 14 ° is less than 10%, which is not preferable. On the other hand, when the cooling stop temperature of the first cooling is higher than 750 ° C., the martensite fraction becomes too low and the strength decreases, and the proportion of crystal grains having an in-grain misorientation of 5 to 14 ° Since it exceeds 60%, it is not preferable. When the holding time at 650 to 750 ° C. is less than 3 seconds, the martensite fraction becomes too high and the ductility is lowered, and the ratio of crystal grains having an in-grain orientation difference of 5 to 14 ° is 10%. Since it becomes less than, it is not preferable. When the holding time at 650 to 750 ° C. exceeds 10 seconds, the martensite fraction decreases, the strength decreases, and the proportion of crystal grains having an in-grain misorientation of 5 to 14 ° is less than 10%. This is not preferable. Further, when the cooling rate of the second cooling is less than 30 ° C./s, the fraction of martensite is lowered and the strength is lowered, and the proportion of crystal grains having an in-grain orientation difference of 5 to 14 ° Since it exceeds 60%, it is not preferable. If the cooling stop temperature of the second cooling is higher than 100 ° C., the proportion of crystal grains having an orientation difference in the grains of 5 to 14 ° exceeds 60%, which is not preferable.
The upper limit of the cooling rate in the first cooling and the second cooling is not particularly limited, but may be 200 ° C./s or less in consideration of the facility capacity of the cooling facility.

According to the manufacturing method described above, in terms of area ratio, 75 to 95% of ferrite and bainite and 5 to 20% martensite are included, and a boundary having an orientation difference of 15 ° or more is defined as a grain boundary. When a region surrounded by grain boundaries and having a circle-equivalent diameter of 0.3 μm or more is defined as a crystal grain, the ratio of the crystal grains having an orientation difference within the grain of 5 to 14 ° is an area ratio of 10 A tissue that is ~ 60% can be obtained.
In the above-described manufacturing method, it is important to introduce the work dislocations into the austenite by controlling the hot rolling conditions and to leave the work dislocations introduced by controlling the cooling conditions appropriately. That is, since the hot rolling conditions and the cooling conditions influence each other, it is important to control these conditions simultaneously. Regarding conditions other than those described above, known methods may be used, and there is no need to specifically limit them.

  Examples of the hot-rolled steel sheet according to the present invention will be given below to describe the present invention more specifically. However, the present invention is not limited to the following examples, and can be implemented with appropriate modifications within a range that can be adapted to the purpose described above and below. Included in the scope.

First, a steel slab was manufactured by melting steel having chemical components shown in Table 1 below and performing continuous casting. And this steel slab was heated to the temperature shown in Table 2, and rough rolling was performed. After rough rolling, finish rolling was performed under the conditions shown in Table 2 to obtain a hot-rolled steel sheet having a sheet thickness of 2.2 to 3.4 mm. Ar3 (° C.) described in Table 2 was obtained from the chemical components shown in Table 1 using the following formula (2).
Ar3 = 970-325 × [C] + 33 × [Si] + 287 × [P] + 40 × [Al] −92 × ([Mn] + [Mo] + [Cu]) − 46 × ([Cr] + [Ni ]) ... (2)
Here, [C], [Si], [P], [Al], [Mn], [Mo], [Cu], [Cr], and [Ni] are C, Si, P, Al, It is the content in terms of mass% of Mn, Mo, Cu, Cr, Ni, and is 0 when not contained.
Further, in Table 2, the cumulative strain in the third stage after the finish rolling is a value obtained from the following equation (1).
εeff. = Σεi (t, T) (1)
here,
εi (t, T) = εi0 / exp {(t / τR) ^2/3 },
τR = τ0 × exp (Q / RT),
τ0 = 8.46 × 10 ⁻⁶ ,
Q = 183200J,
R = 8.314 J / K · mol,
εi0 represents the logarithmic strain at the time of rolling, t represents the accumulated time until immediately before cooling in the pass, and T represents the rolling temperature in the pass.
The blank in Table 1 means that the analysis value was less than the detection limit.

With respect to the obtained hot-rolled steel sheet, the ratio of crystal grains having a structure fraction (area ratio) of each structure and an orientation difference within the grains of 5 to 14 ° was determined.
The tissue fraction (area ratio) was determined by the following method. First, a sample taken from a hot rolled steel sheet was etched with nital. After the etching, image analysis is performed on the structure photograph obtained with a field of view of 300 μm × 300 μm at a position of ¼ depth of the plate thickness using an optical microscope, so that the area ratio of ferrite and pearlite, and bainite and martensite are obtained. The total area ratio with the site was obtained. Next, using a sample that has undergone repeller corrosion and performing an image analysis on a structural photograph obtained with a field of view of 300 μm × 300 μm at a position of ¼ depth of the plate thickness using an optical microscope, residual austenite and martensite are obtained. The total area ratio with the site was calculated.
Furthermore, the volume fraction of retained austenite was determined by X-ray diffraction measurement using a sample which was chamfered from the normal direction of the rolling surface to ¼ depth of the plate thickness. Since the volume ratio of retained austenite is equivalent to the area ratio, this was defined as the area ratio of retained austenite.
By this method, area ratios of ferrite, bainite, martensite, retained austenite, and pearlite were obtained.
Moreover, the ratio of the crystal grain whose orientation difference within a grain is 5-14 degrees was measured with the following method. First, with respect to the vertical cross section in the rolling direction at the 1/4 depth position (1/4 t portion) of the thickness t from the steel sheet surface, an area of 200 μm in the rolling direction and 100 μm in the normal direction of the rolling surface is measured at a measurement interval of 0.2 μm. Crystal orientation information was obtained by EBSD analysis. Here, the EBSD analysis is performed at an analysis speed of 200 to 300 points / second using an apparatus constituted by a thermal field emission scanning electron microscope (JSMOL JSM-7001F) and an EBSD detector (TSL HIKARI detector). did. Next, with respect to the obtained crystal orientation information, a region having an orientation difference of 15 ° or more and an equivalent circle diameter of 0.3 μm or more is defined as a crystal grain, and an average orientation difference in the crystal grain is calculated. The ratio of crystal grains having an orientation difference of 5 to 14 ° was determined. The crystal grains and the average orientation difference within the grains defined above were calculated using software “OIM Analysis (registered trademark)” attached to the EBSD analyzer.

Next, in the tensile test, the yield strength and the tensile strength were determined, and the limit forming height was determined by the vertical stretch flange test. Further, the product of the tensile strength (MPa) and the limit molding height (mm) was evaluated as an index of stretch flangeability, and when the product was 19500 mm · MPa or more, it was determined that the stretch flangeability was excellent.
In the tensile test, a JIS No. 5 tensile test piece was taken from a direction perpendicular to the rolling direction, and the test was performed according to JISZ2241.
In addition, the vertical stretch flange test was performed using a vertical molded product having a corner radius of curvature of R60 mm and an opening angle θ of 120 °, with a clearance when punching the corner of 11%. Further, the limit forming height was determined as the limit forming height at which no cracks exist by visually observing the presence or absence of cracks having a length of 1/3 or more of the plate thickness after forming.
The results are shown in Table 3.

As is clear from the results shown in Table 3, when the chemical components defined in the present invention are hot-rolled under favorable conditions (test Nos. 1 to 17), the strength is 590 MPa or more and the index of stretch flangeability A high-strength hot-rolled steel sheet having a thickness of 19500 mm · MPa or more was obtained.
On the other hand, production No. 18 to 23, since the chemical component was outside the scope of the present invention, either or both of the structure observed with an optical microscope and the proportion of crystal grains having an orientation difference within the grain of 5 to 14 ° are present. The scope of the invention was not satisfied. As a result, the stretch flangeability did not satisfy the target value. In some cases, the tensile strength was also low.
No. 24-36 is a result of the manufacturing conditions deviating from the desired range. As a result, either or both of the structure observed with an optical microscope and the ratio of crystal grains having an orientation difference in the grains of 5 to 14 ° fall within the scope of the present invention. This is an example that was not satisfied. In these examples, the stretch flangeability did not satisfy the target value. In some cases, the tensile strength was also low.

  According to the present invention, it is possible to provide a high-strength hot-rolled steel sheet excellent in stretch flangeability that can be applied to a member that requires high stretch flangeability while being high in strength. Since these steel plates contribute to improving the fuel efficiency of automobiles, they have high industrial applicability.

Claims (4)

Chemical composition is mass%,
C: 0.04 to 0.18%,
Si: 0.10 to 1.70%,
Mn: 0.50 to 3.00%,
Al: 0.010 to 1.00%,
B: 0 to 0.005%,
Cr: 0 to 1.0%,
Mo: 0 to 1.0%,
Cu: 0 to 2.0%,
Ni: 0 to 2.0%,
Mg: 0 to 0.05%,
REM: 0 to 0.05%,
Ca: 0 to 0.05%,
Zr: 0 to 0.05%
Containing
P: 0.050% or less,
S: 0.010% or less,
N: 0.0060% or less,
Limited to
The balance consists of Fe and impurities;
The structure comprises, in area ratio, a total of 75-95% ferrite and bainite and 5-20% martensite;
In the structure, when a boundary having an orientation difference of 15 ° or more is defined as a grain boundary, and a region surrounded by the grain boundary and having an equivalent circle diameter of 0.3 μm or more is defined as a crystal grain, the orientation difference within the grain is defined. The ratio of the crystal grains in which is 5 to 14 ° is 10 to 60% in terms of area ratio;
A hot-rolled steel sheet characterized by that.   The hot-rolled steel sheet according to claim 1, wherein the tensile strength is 590 MPa or more, and the product of the tensile strength and the limit forming height in the vertical stretch flange test is 19500 mm · MPa or more.   The chemical component is B: 0.0001 to 0.005% in mass%, Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, Cu: 0.01 to 2.0. %, Ni: The hot-rolled steel sheet according to claim 1 or 2 containing one or more of 0.01 to 2.0%.   The chemical component is Mg: 0.0001-0.05%, REM: 0.0001-0.05%, Ca: 0.0001-0.05%, Zr: 0.0001-0.05% by mass. The hot-rolled steel sheet according to any one of claims 1 to 3, comprising one or more of%.