JP6354274B2 - Hot-rolled steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a hot-rolled steel sheet and a method for producing the same.

  In recent years, for the purpose of reducing the weight of various steel sheets, including improving the fuel efficiency of automobiles, increasing the strength of steel sheets such as iron alloys and applying light metals such as Al alloys have been promoted. However, although light metals such as Al alloys have the advantage of higher specific strength than heavy metals such as steel, they have the disadvantage of being extremely expensive, so their application is limited to special applications. Therefore, in order to promote the weight reduction of various steel plates at a lower cost and in a wider range, it is necessary to increase the strength of the steel plates.

  Increasing the strength of steel sheets generally involves deterioration of material properties such as formability (workability). For this reason, in the development of a high-strength steel sheet, it is important 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 are required to have stretch flange workability, burring workability, ductility, fatigue durability, corrosion resistance, and the like. It is important how to achieve the balance in a high dimension. For example, steel plates used for automobile members such as structural members and suspension members that account for approximately 20% of the weight of the vehicle body are subjected to blanking and drilling by shearing, punching, etc., followed by stretch flange processing and burring processing. Therefore, very severe hole expansibility (λ value) is required.

  In a steel sheet used for such a member, flaws and microcracks are generated on the end surface formed by shearing and punching, and cracks develop from these generated flaws and microcracks, leading to fatigue failure. There is concern. For this reason, in order to improve fatigue durability on the end surface of the said steel material, it is required not to produce a flaw, a microcrack, etc. Cracks are generated in parallel with the plate surfaces of the end faces as wrinkles and minute cracks generated on these end faces. This crack is called “peeling”. This “peeling” occurs about 80% particularly in a 540 MPa grade steel plate and almost 100% in a 780 MPa grade steel plate. Further, this “peeling” occurs without correlation with the hole expansion rate. For example, it occurs even when the hole expansion rate is 50% or 100%.

  For example, as a steel plate excellent in hole expansibility (λ value), a ferrite main phase steel plate precipitation strengthened by fine precipitates such as Ti and Nb and a method for producing the same have been reported.

  In Patent Document 1, in mass%, C: 0.01 to 0.10%, Si: 1.0% or less, Mn: 2.5% or less, P: 0.08% or less, S: 0.005% Hereinafter, mainly containing ferrite containing Al: 0.015 to 0.050%, Ti: 0.10% to 0.30%, and having an average grain size of 5 μm or less surrounded by an orientation difference of 15 ° or more. A high-strength hot-rolled steel sheet having excellent flangeability and a method for producing the same are disclosed. Patent Documents 2 and 3 disclose a technique of a high-tensile hot-rolled steel sheet that achieves excellent stretch flangeability while having high strength by adding Mo to refine the precipitates.

JP 2002-105595 A JP 2002-322540 A JP 2002-322541 A

  The method for producing a hot-rolled steel sheet described in Patent Document 1 has a problem that the winding process cannot be performed at temperatures higher than 500 ° C. to lower than 600 ° C. in order to avoid TiC coherent precipitation.

  In the hot-rolled steel sheet and its manufacturing method described in Patent Documents 2 and 3, there is a problem that the manufacturing cost is high because it is essential to add Mo, which is an expensive alloy element, in an amount of 0.07% or more.

  Furthermore, according to the additional test by the present inventors, in the steel having the chemical composition of the cited reference 2 or 3, “peeling” occurs after punching, and in the techniques described in these references, it is formed by shearing or punching. It is impossible to completely suppress wrinkles and microcracks at the end face.

The present invention has been made to solve the above-described problems, and has an object to provide a high-strength hot-rolled steel sheet having excellent workability while having high strength, that is, excellent in hole expansibility . To do. Another object of the present invention is to provide a production method that can stably produce a high-strength hot-rolled steel sheet excellent in hole expansibility at low cost.

“Excellent hole expansibility ” means that, in the hole expansion test method described in the Japan Iron and Steel Federation Standard JFS T 1001-1996, a hole expansion rate of 150% or more for a steel plate of 500 MPa class and 80% or more for a steel plate of 780 MPa or more. This means that the hole expansion rate can be achieved. "Excellent peel resistance" means that there is no fracture surface crack after the above-mentioned hole expansion test method.

  As a result of intensive studies to achieve the above object, the present inventors have obtained the following knowledge.

1) to suppress the precipitation of cementite, and by securing the solid solution C, it is possible to achieve both excellent hole expandability and excellent "peeling" property.

2) By reducing Si as much as possible, the transformation temperature decreases, and TiC precipitation in a high temperature range that causes fluctuations in steel sheet strength can be suppressed.

3) By using Cr as an essential element, it is possible to suppress precipitation of cementite having a large aspect ratio and a large aspect ratio that degrades hole expansibility , and it is possible to secure solid solution C.

4) Moreover, by containing Cr, Cr solid-dissolves in TiC, the precipitation amount of fine composite carbide increases, and precipitation strengthening can be achieved.

5) In the production of steel sheets, the temperature and rolling reduction of rough rolling and finish rolling are limited to a certain range, and the cooling speed range from the start of winding to 450 ° C is controlled, so that the size and number density of TiC are appropriate. reduction can, be in the range coiling starting temperature of 500 to 650 ° C., it is possible to increase the strength of the steel sheet, hole expandability and resistance to "peeling" property together.

  This invention is made | formed based on such knowledge, and makes a summary the following hot-rolled steel plate and its manufacturing method.

(1) Chemical composition is mass%, C: 0.01 to 0.1%, Si: 0.3% or less, Mn: 0.4 to 3%, P: 0.1% or less, S: 0 0.03% or less, Al: 0.001-1%, N: 0.01% or less, Cr: 0.05-1%, Nb: 0.003-0.05%, Ti: 0.003-0. 2%, Cu: 0 to 1.2%, Ni: 0 to 0.6%, Mo: 0 to 1%, V: 0 to 0.2%, Ca: 0 to 0.005%, REM: 0 to 0% 0.02%, B: 0 to 0.002%, balance: Fe and impurities, satisfying the relationship of the following formulas (1) and (2):
In the metal structure, has a cementite having an area ratio of 1% or less and an average particle diameter of 2 μm or less, and the Cr concentration in the cementite is 0.5 to 40% by mass on average, The area ratio of cementite having a particle diameter of 0.5 μm or less and an aspect ratio of 5 or less in the total cementite is 60% or more, the average particle diameter of the composite carbide of Ti and Cr is 10 nm or less, and the number density is 1 × 10 ¹³ pieces / mm ³ or more ,
A hot-rolled steel sheet having a tensile strength of 500 MPa or more and excellent hole expansibility and peeling resistance .
0.005 ≦ [Si] / [Cr] ≦ 2 (1)
0.5 ≦ [Mn] / [Cr] ≦ 20 (2)
However, [Si], [Cr] and [Mn] in the above formula mean the content (mass%) of each element.

  (2) The said chemical composition is the mass%, Cu: 0.2-1.2%, Ni: 0.1-0.6%, Mo: 0.05-1%, V: 0.02-0 The hot rolled steel sheet according to the above (1), which contains one or more selected from 2%.

  (3) The hot rolling of (1) or (2) above, wherein the chemical composition contains, by mass%, Ca: 0.0005 to 0.005% and / or REM: 0.0005 to 0.02% steel sheet.

  (4) The hot rolled steel sheet according to any one of (1) to (3), wherein the chemical composition is B% by mass and contains B: 0.0002 to 0.002%.

  (5) The hot-rolled steel sheet according to any one of (1) to (4) above, which is galvanized.

(6) The method for producing a hot-rolled steel sheet according to any one of (1) to (4), comprising the following steps (A) to (E).
(A) A step of heating the steel ingot or steel slab having the chemical composition of any one of (1) to (4) to 1150 to 1280 ° C,
(B) A step of rough rolling the heated steel ingot or steel slab in a temperature range of 1050 ° C. or higher and a cumulative reduction ratio of 40% or higher to obtain a rough bar;
(C) In the condition that the rough bar has a finish rolling start temperature of 1000 ° C. or higher, a cumulative rolling reduction ratio of 70% or higher, a final rolling reduction ratio of 3 to 25%, and a finish rolling finish temperature of 820 to 980 ° C. A step of performing finish rolling to obtain a steel plate,
(D) a step of cooling the obtained steel sheet to a temperature range of 500 to 650 ° C. at an average cooling rate exceeding 15 ° C./second;
(E) The process of winding the cooled steel plate on the conditions that the average cooling rate to 450 degreeC is 0.008-1.0 degree-C / sec.

ただし、上記式中の各記号の意味は次の通りである。
［Ｎｂ］：Ｎｂの含有量（質量％）
［Ｔｉ］：Ｔｉの含有量（質量％）
ｔ：最終圧延パスの１つ前の圧延完了から最終圧延パスの圧延開始までの時間（秒）
Ｔ：最終圧延パスの１つ前の圧延パスの圧延完了温度（℃） (7) The method for producing a hot-rolled steel sheet according to (6), wherein the step (C) is performed under a condition that satisfies the following formula.

However, the meaning of each symbol in the above formula is as follows.
[Nb]: Nb content (% by mass)
[Ti]: Ti content (% by mass)
t: Time (seconds) from the completion of rolling immediately before the final rolling pass to the start of rolling in the final rolling pass
T: Rolling completion temperature (° C.) of the rolling pass immediately before the final rolling pass

  (8) The method for producing a hot-rolled steel sheet according to (6) or (7), wherein the steel sheet after winding is pickled and then immersed in a galvanizing bath to galvanize the surface of the steel sheet.

  (9) The method for producing a hot-rolled steel sheet according to (8), wherein the galvanized steel sheet is alloyed.

ADVANTAGE OF THE INVENTION According to this invention, the high intensity | strength hot-rolled steel plate excellent in hole expansibility can be provided. This steel plate is suitable for use in members that require strict workability and hole expandability. In addition, according to the present invention, a high-strength hot-rolled steel sheet having a steel plate grade of 500 MPa class or higher, further 780 MPa class or higher, and excellent in hole expansibility and “peeling resistance” can be stably manufactured at low cost. For this reason, this invention has high industrial value.

Hereinafter, a high-strength hot-rolled steel sheet (hereinafter simply referred to as “hot-rolled steel sheet”) excellent in hole expansibility will be described in detail as an embodiment for carrying out the present invention. In the following description, “%” for the content of each element means “mass%”.

1. Chemical composition of hot-rolled steel sheet C: 0.01 to 0.1%
C is an element that combines with Nb, Ti or the like to form precipitates in the steel sheet and contributes to strength improvement by precipitation strengthening. The content of C is less than 0.01% can not be obtained the effect, and if it exceeds 0.1%, an increase in iron-based carbide serving as a starting point for cracking during hole expansion processing, the hole The spread value deteriorates. For this reason, the C content is set to 0.01 to 0.1%. The C content is preferably 0.08% or less, and more preferably 0.07% or less. The lower limit of the C content is preferably 0.03%.

Si: 0.3% or less Si has the effect of suppressing precipitation of iron-based carbides such as cementite in the material structure and contributing to the improvement of ductility and hole expansibility , but if its content is excessive, ferrite Transformation is likely to occur, and TiC is likely to precipitate at high temperatures. Precipitation at high temperatures tends to cause variations in the amount of precipitation, resulting in material variations such as strength and hole expansibility . Precipitation in a high temperature region decreases the amount of solid solution C at the grain boundaries and degrades the peel resistance. Therefore, the Si content is set to 0.3% or less. The Si content is desirably 0.1% or less. Thus, since Si content is reduced, in order to suppress precipitation of iron-based carbides, it is necessary to contain Nb and Ti and to limit the manufacturing process. These will be described later. The lower limit of the Si content is not particularly defined, but when the generation of scale defects such as scales and spindle scales is suppressed, the Si content is preferably 0.01% or more. A more preferable Si content is 0.03% or more.

Mn: 0.4 to 3%
Mn is an element that contributes to strength improvement by solid solution strengthening and quenching strengthening. Mn content is not possible to obtain this effect is less than 0.4%, the Mn content exceeds 3%, not only the effect is saturated, excessive excellent is increased hole expandability hardenability continuous Formation of a cooling transformation structure becomes difficult. For this reason, Mn content was 0.4 to 3%. In order to improve the hardenability and facilitate the formation of a continuously cooled transformed structure having excellent hole expansibility , Mn is preferably contained in an amount of 0.5% or more, more preferably 0.6% or more. A preferable upper limit of Mn is 2.4%.

P: 0.1% or less P is an impurity that is inevitably mixed during the refining of steel, and is an element that segregates at the grain boundary and decreases toughness as the content increases. For this reason, the P content is preferably as low as possible. If it exceeds 0.1%, the workability and weldability are adversely affected. In particular, in order to improve hole expandability and weldability, the P content is preferably 0.02% or less, and more preferably 0.015% or less. The lower limit of P is not particularly defined, but excessive reduction deteriorates the manufacturing cost, so 0.005% or more is preferable.

S: 0.03% or less S is an impurity that is inevitably mixed during refining of steel. If the content is too large, not only will cracking occur during hot rolling, but the A-type will deteriorate hole expansibility. Inclusions are generated. For this reason, the S content should be reduced as much as possible, but is acceptable up to 0.03%. In order to improve hole expansibility, it is preferable to set it as 0.01% or less, and it is more preferable to set it as 0.005% or less. Although the lower limit of S is not particularly defined, excessive reduction deteriorates the manufacturing cost, so 0.001% or more is preferable.

Al: 0.001 to 1%
Al is an effective element for molten steel deoxidation in the steelmaking process of the steel sheet, and is contained in an amount of 0.001% or more. When the content is excessive, the cost is increased, so the upper limit is set to 1%. Al increases nonmetallic inclusions and may deteriorate ductility and toughness. Therefore, the Al content is preferably 0.10% or less, and more preferably 0.05% or less. The lower limit of the Al content is preferably 0.01% or more.

N: 0.01% or less N is an impurity that is inevitably mixed during the refining of steel, and is an element that forms a nitride by combining with Ti, Nb, and the like. This nitride is likely to precipitate and coarsen at a relatively high temperature, and may become a starting point of cracking during hole expansion processing. Further, as described later, it is preferable that the amount of this nitride is small in order to effectively use Nb and Ti. Therefore, the N content is set to 0.01% or less. When the hot-rolled steel sheet of the present invention is used as a member that causes aging deterioration, the N content is preferably 0.006% or less. This is because the content exceeding this causes aging deterioration remarkably. In addition, when the hot-rolled steel sheet of the present invention is used as a member on the premise that the hot-rolled steel sheet of the present invention is left at room temperature for 2 weeks or more and then subjected to processing, the N content is preferably 0.005% or less. It is more preferable to set it to 0.004% or less. This is for aging deterioration countermeasures. Furthermore, when using the hot-rolled steel sheet of the present invention as a member that is allowed to stand in a high temperature environment (for example, a member that is left in a high temperature environment in summer, a member that exceeds the equator when exported by a ship, etc.) The N content is preferably less than 0.003%. Although the lower limit of N is not particularly defined, excessive reduction deteriorates the manufacturing cost, so 0.001% or more is preferable.

Cr: 0.05-1.0%
Cr is one of the most important elements in the present invention. Cr suppresses pearlite transformation and improves the hole expansibility by controlling the size and shape of cementite by solid solution in cementite, and increases the number density of precipitates by dissolving in TiC precipitates. The amount of reinforcement can be increased. For this reason, Cr content is made to contain 0.05% or more. On the other hand, even if the content exceeds 1.0%, this effect is saturated and the cost is increased, and the chemical conversion treatment performance is remarkably lowered. Therefore, the Cr content is set to 0.05 to 1.0%. In order to obtain the above effect more reliably, the Cr content is desirably 0.2% or more, and more desirably 0.4% or more.

Nb: 0.003 to 0.05%
Nb is one of the most important elements in the present invention. Nb precipitates finely as carbide during cooling after rolling or after winding, and improves strength by precipitation strengthening. Furthermore, Nb fixes C as a carbide and suppresses the formation of cementite, which is harmful to hole expansibility . In order to obtain these effects, it is necessary to contain 0.003% or more of Nb. On the other hand, even if the Nb content exceeds 0.05%, these effects are saturated. Therefore, the Nb content is set to 0.003 to 0.05%. The Nb content is preferably more than 0.005%.

Ti: 0.003-0.2%
Ti is one of the most important elements in the present invention. Like Nb, it precipitates finely as carbide during cooling after rolling and after winding, and the strength is improved by precipitation strengthening. Furthermore, Ti fixes C as a carbide and suppresses the formation of cementite, which is harmful to hole expansibility . In order to acquire these effects, it is necessary to contain 0.003% or more of Ti. On the other hand, even if the content exceeds 0.2%, these effects are saturated. For this reason, the content of Ti is set to 0.003 to 0.2%. The Ti content is preferably 0.005% or more.

Cu: 0 to 1.2%
Ni: 0 to 0.6%
Mo: 0 to 1%
V: 0 to 0.2%
Since Cu, Ni, Mo, and V are elements that have the effect of improving the strength of the hot-rolled steel sheet by precipitation strengthening or solid solution strengthening, one or more of these elements may be contained. Even if the content of these elements exceeds a certain amount, the effect is saturated and the economy is deteriorated. Therefore, when each element is contained, the upper limit of Cu is 1.2% and the upper limit of Ni is 0. 0.6%, the upper limit of Mo is 1%, and the upper limit of V is 0.2%. In order to sufficiently obtain the above effects, it is preferable that Cu is 0.2% or more, Ni is 0.1% or more, Mo is 0.05% or more, and V is 0.02% or more.

Ca: 0 to 0.005%
REM: 0 to 0.02%
Ca and REM (rare earth elements) are elements that improve the workability by controlling the form of non-metallic inclusions that are the starting point of destruction and cause deterioration of workability, and therefore contain one or more of these elements. You may let them. Even if the content of these elements exceeds a certain amount, the effect is saturated and the economy is deteriorated. Therefore, when each element is contained, the upper limit of Ca is 0.005% and the upper limit of REM is 0. 0.02%. In order to sufficiently obtain the above effects, it is preferable that Ca is contained in an amount of 0.0005% or more and REM is contained in an amount of 0.0005% or more, respectively.

  Note that REM is a generic name for a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM means the total amount of the above elements.

B: 0 to 0.002%
When B segregates at the grain boundary and exists together with the solid solution C, B has an effect of increasing the grain boundary strength, and may be contained. However, when the content exceeds 0.002%, slab cracking occurs. Therefore, when B is contained, the content is made 0.002% or less. B has the effect of improving the hardenability and facilitating the formation of a continuous cooling transformation structure that is a favorable microstructure for hole expansibility . Therefore, B is preferably contained in an amount of 0.0005% or more, and 0.001% or more. It is more preferable to make it contain.

In the hot-rolled steel sheet of the present invention, as described above, it is not enough to set the content of each element within a certain range, and it is extremely important to adjust the balance of the content of each element. It is necessary to satisfy the relationship between the expressions (1) and (2).
0.005 ≦ [Si] / [Cr] ≦ 2 (1)
0.5 ≦ [Mn] / [Cr] ≦ 20 (2)
However, [Si], [Cr] and [Mn] in the above formula mean the content (mass%) of each element.

In the present invention, it is extremely important to control the size and amount of TiC and the size and morphology of cementite. The precipitation behavior of TiC and cementite varies depending on the balance of the content of Si and Cr, and the content ratio ([Si] / [Cr]) needs to be in the range of 0.005 to 2. When [Si] / [Cr] is less than 0.005, the hardenability is excessively increased and TiC is hardly precipitated in a low temperature range. On the other hand, when [Si] / [Cr] exceeds 2, TiC precipitates in a high temperature range, resulting in material fluctuations and a decrease in the amount of solute C, which deteriorates the “peeling resistance”. Furthermore, coarse cementite precipitates and the hole expansibility deteriorates. A preferred range is 0.01-1.

  Both Mn and Cr are elements that enhance the hardenability, and suppress the precipitation of TiC by suppressing the ferrite transformation at high temperature, thereby contributing to the stabilization of the material. However, since Mn and Cr have different effects on controlling the precipitation of cementite and enhancing the hardenability, the content ratio ([Mn] / [Cr]) needs to be in the range of 0.5-20. When [Mn] / [Cr] is less than 0.5, the hardenability is excessively increased and TiC is hardly precipitated in a low temperature region. On the other hand, when [Mn] / [Cr] exceeds 20, it is difficult to control the size and form of the desired cementite. A preferred range is 1-10.

  The chemical composition of the hot-rolled steel sheet according to the present invention includes the above-mentioned elements within the specified ranges, and the balance is Fe and impurities. An impurity means the component mixed by raw materials and other factors, such as an ore and a scrap, when manufacturing steel materials industrially.

2. Metallurgical factors such as microstructure of hot-rolled steel sheet (1) Cementite Area ratio: 1% or less Average particle size: 2μm or less Stretch flange workability and burring workability represented by hole expansion values are determined during punching or shearing. It is affected by voids that are the starting point of cracks that sometimes occur. Voids are likely to occur in places where the hardness difference in the metal structure is large. In particular, when cementite is included, voids are generated due to excessive stress concentration at the interface between the cementite and the matrix. Therefore, the area ratio of the cementite precipitated in the steel sheet is limited to 1% or less, and the average particle size of the cementite is limited to 2 μm or less. If or when the average particle size area ratio of cementite exceeds 1% is more than 2μm, the hole expandability is deteriorated for the reasons described above. The lower the area ratio, the smaller the average particle diameter, and the worse the hole expandability is eliminated. Therefore, the lower limit is not specified. However, in the measurement of cementite described later, the area ratio is 0.01% and the average particle diameter is about 0.02 μm. Is the measurement limit.

Cr concentration in cementite: 0.5-40% by mass on average
By dissolving 0.5% by mass or more of Cr in cementite, it is easy to form cementite that is relatively small with respect to the grain size of the parent phase grains. Therefore, the hole expandability is improved. However, if the Cr concentration exceeds 40 wt%, it changed from cementite to Cr carbides, which may degrade the hole expandability and resistance to "peeling resistance".

Cementite having a particle size of 0.5 μm or less and an aspect ratio of 5 or less: 60% or more For cementite, in addition to the area ratio and average particle size, the particle size is 0.5 μm or less and the aspect ratio is 5 or less. An area ratio of a certain cementite to the total cementite is set to 60% or more. As a result, the cementite grains are relatively small with respect to the parent phase grains and the anisotropy to deformation is small, so that stress concentration is not mechanically concentrated, and voids are less likely to be generated, thereby improving hole expandability.

  Here, the particle size and aspect ratio of cementite and the area ratio of cementite having a predetermined shape were measured as follows. A transmission electron microscope sample was taken from a 1/4 thickness of the sample cut from the 1/4 W or 3/4 W position of the steel plate width of the test steel, and observed with a transmission electron microscope at an acceleration voltage of 200 kV. The observed precipitate was confirmed to be cementite by analyzing the diffraction pattern. Furthermore, the Cr concentration in cementite was measured using an energy dispersive X-ray spectrometer attached to a transmission electron microscope. The cementite particle size, aspect ratio, and the area ratio occupied by cementite having a particle size of 1 μm or less were calculated using commercially available image analysis software “Image-Pro” for 10 fields of view arbitrarily observed at a magnification of 5000 times.

(2) Microstructure of parent phase The microstructure of the parent phase in the hot-rolled steel sheet of the present invention is not particularly limited, but a continuous cooling transformation structure (Zw) is desirable in order to obtain better hole expansibility . In addition, the microstructure of the matrix of the hot rolled steel sheet to which the present invention is applied has a volume fraction of polygonal ferrite (PF) of 20% or less in order to achieve both workability and ductility represented by uniform elongation. May be included. Incidentally, the volume fraction of the microstructure refers to the area fraction in the measurement visual field. In the case of a continuously cooled transformation structure, the solid solution C in the crystal grains is transformed while remaining in the grains. Therefore, the probability that solute C exists at the grain boundary is low.

  Here, the continuous cooling transformation structure (Zw) in the present invention is the Japan Iron and Steel Institute Basic Research Group, Bainite Research Group / Edition; Recent Research on Bainite Structure and Transformation Behavior of Low Carbon Steels-Final Report of Bainite Research Group -As described in (1994 Japan Iron and Steel Institute), the transformation in the intermediate stage between the microstructure containing polygonal ferrite or pearlite produced by the diffusion mechanism and the martensite produced by the non-diffusion shearing mechanism A microstructure defined as an organization. That is, the continuous cooling transformation structure (Zw) is mainly a basic ferrite (α ° B) (α ° B in the photo book) and a Granular as described in the above-mentioned reference pages 125 to 127 as an optical microscope observation structure. It is defined as a microstructure composed of bainite ferrite (αB) and quasi-polygonal ferrite (αq), and further containing a small amount of retained austenite (γr) and martensite-austentite (MA). Note that αq does not reveal an internal structure by etching like polygonal ferrite (PF), but the shape is ash and is clearly distinguished from PF. Here, αq is a grain whose ratio (lq / dq) satisfies lq / dq ≧ 3.5 when the perimeter length lq of the target crystal grain is dq and the equivalent circle diameter is dq. The continuous cooling transformation structure (Zw) in the present invention is defined as a microstructure containing one or more of α ° B, αB, αq, γr, and MA. Note that a small amount of γr and MA is 3% or less in total.

This continuous cooling transformation structure (Zw) is difficult to distinguish by optical microscope observation in etching using a nital reagent. Therefore, the determination is made using EBSP-OIM ^™ . In the EBSP-OIM ^™ (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy) method, an electron beam is formed by irradiating a backscattered pond with a high-tilt sample in a scanning electron microscope (Scanning Electron Microscope). The crystal orientation at the irradiation point is measured in a short time by taking a picture with a high sensitivity camera and processing the computer image.

The EBSP method can quantitatively analyze the microstructure and crystal orientation of the bulk sample surface, and the analysis area can be analyzed up to a minimum resolution of 20 nm as long as it is within the region that can be observed with the SEM, depending on the resolution of the SEM. The analysis by the EBSP-OIM ^TM method is performed by mapping several tens of thousands of points to be analyzed in a grid pattern at equal intervals over several hours. For polycrystalline materials, the crystal orientation distribution and crystal grain size in the sample can be seen. In the present invention, an image that can be discriminated from an image mapped with the azimuth difference of each packet as 15 ° may be conveniently defined as a continuous cooling transformation structure (Zw).

(3) Composite carbide of Ti and Cr Average particle size: 10 nm or less Since the coarse composite carbide of Ti and Cr hardly contributes to precipitation strengthening, the average particle size is set to 10 nm or less. The average particle size of the composite carbide is preferably 7 nm or less. The lower limit of the average particle size of the composite carbide is not particularly defined, but the precipitation strengthening mechanism is changed from the Orowan mechanism to the Cutting mechanism, and a desired precipitation strengthening amount may not be obtained. preferable.

Number density: 1 × 10 ¹³ pieces / mm ³ or more If the number density of the composite carbide is less than 1 × 10 ¹³ pieces / mm ³ , sufficient precipitation strengthening action cannot be obtained, and ductility, hole expansibility and peeling resistance are ensured. However, the desired tensile strength (TS) cannot be obtained. The number density of the composite carbide is preferably 5 × 10 ¹³ pieces / mm ³ or more.

Cr is dissolved in TiC and has the effect of controlling the form of the composite carbide and increasing the number density. In order to obtain this effect, the amount of Cr solid solution in TiC is preferably 2 to 30% by mass. When the amount of Cr solid solution in TiC is less than 2% by mass, precipitation strengthening due to the addition of Cr may be insufficient, and when it exceeds 30% by mass, the composite carbide is likely to become Cr carbide. This is because precipitation strengthening may not be obtained.

  Here, the size of the composite carbide and the Cr concentration in the composite carbide were measured by the three-dimensional atom probe measurement method as follows.

  First, a needle-like sample is prepared from a sample to be measured by cutting and electrolytic polishing using a focused ion beam processing method in combination with an electrolytic polishing method as necessary. In the three-dimensional atom probe measurement, the accumulated data can be reconstructed and obtained as an actual distribution image of atoms in real space. The number density of the composite carbide can be obtained from the volume of the three-dimensional distribution image of the composite carbide and the number of the composite carbide. The size of the composite carbide is defined as the diameter calculated from the observed number of constituent atoms of the composite carbide and its lattice constant, assuming that the composite carbide is spherical. Here, an effective composite carbide having a particle size of 0.5 nm or more is used. The diameter of 30 or more complex carbides is arbitrarily measured, and the average value is obtained. The Cr content was arbitrarily calculated by measuring the number of Ti and Cr atoms in 30 or more composite carbides, and calculating the ratio between them.

3. Next, the reason for limitation of the manufacturing method of the hot-rolled steel sheet to which the present invention is applied will be described in detail.
In this invention, the manufacturing method of the steel slab which has said chemical composition performed prior to a hot rolling process is not specifically limited. That is, as a method for producing a steel slab having the above chemical composition, the components are adjusted so that the desired component content is obtained in various secondary scouring steps following the smelting step using a blast furnace, converter, electric furnace or the like. Then, the casting process may be performed by a method such as thin slab casting in addition to normal continuous casting or ingot casting. In addition, you may use a scrap for a raw material. When a slab is obtained by continuous casting, it may be sent directly to a hot rolling mill with a high-temperature slab, or may be hot-rolled after being reheated in a heating furnace after being cooled to room temperature.

(1) Heating process The steel ingot or steel piece which has said chemical composition is heated at 1150-1280 degreeC before a hot rolling process. When the heating temperature is less than 1150 ° C., Nb and Ti carbonitrides are not sufficiently dissolved in the base material. In this case, the effect of improving the strength using precipitation strengthening cannot be obtained by fine precipitation of Nb and Ti as carbides during cooling after rolling or after winding. Further, the presence of coarse Nb and Ti carbonitrides degrades hole expansibility . On the other hand, when the heating temperature exceeds 1280 ° C., the scale loss increases. In addition, the heating time in this heating step is not particularly defined, but in order to sufficiently dissolve the Nb carbonitride, it is desirable to hold it for 30 minutes or more after reaching the above heating temperature. However, this is not the case when the cast slab is directly fed and rolled at a high temperature.

(2) Rough rolling process After the slab heating process, the rough rolling is performed under a condition that the cumulative rolling reduction is 40% or more and the rough rolling finish temperature is 1050 ° C. or more with respect to the slab extracted from the heating furnace without waiting. Go to get a coarse bar.

  In this rough rolling step, when the rough rolling end temperature is less than 1050 ° C., Ti and Nb are coarsely precipitated in the austenite as carbides, and the workability of the steel sheet is deteriorated. In addition, the hot deformation resistance in rough rolling is increased, and there is a risk that the rough rolling operation may be hindered. The upper limit of the rough rolling end temperature is not particularly defined, but is preferably 1150 ° C. If the temperature is too high, the secondary scale produced during rough rolling will grow too much, making it difficult to remove the scale by descaling or finish rolling to be performed later. Moreover, if the cumulative rolling reduction in this temperature range is less than 40%, the solidification structure at the time of casting is fully destroyed, the crystal structure cannot be equiaxed, and the workability of the steel sheet is hindered. Therefore, the rough rolling process is performed under conditions where the end temperature is 1050 ° C. or higher and the cumulative rolling reduction is 40% or higher.

  In addition, about the obtained rough bar, you may make it perform endless rolling which joins each rough bar between a rough rolling process and a finish rolling process, and performs a finish rolling process continuously. At that time, the coarse bar may be wound once in a coil shape, stored in a cover having a heat retaining function as necessary, and rewound again before joining.

In addition, during the hot rolling process, it may be required to control the temperature variation in the rolling direction, the plate width direction, and the plate thickness direction of the rough bar to be small. In this case, if necessary, between the rough rolling mill in the rough rolling process and the finish rolling mill in the finish rolling process, or between each stand in the finish rolling process, the rolling direction of the rough bar, the sheet width direction, and You may heat a rough bar using the heating apparatus which can control the variation in the temperature in a plate | board thickness direction. Various heating means such as gas heating, energization heating, induction heating, etc. can be considered as the heating device method, but if the variation in temperature in the rolling direction, plate width direction and plate thickness direction of the coarse bar can be controlled to be small Any known means may be used.

(3) Finish rolling step The obtained rough bar is subjected to finish rolling under the conditions of cumulative reduction ratio: 70% or more, final pass reduction ratio: 3 to 25%, and finish rolling temperature: 820 to 980 ° C. Get.

  The finish rolling start temperature is not particularly limited as long as the finish rolling finish temperature described later can be secured. However, if the finish rolling temperature is too low, coarse TiC and NbC are precipitated during finish rolling, resulting in a precipitation strengthening amount. descend. Therefore, the finish rolling start temperature is preferably 1000 ° C. or higher, and preferably 1100 ° C. or lower from the viewpoint of refining recrystallized austenite grains.

  Here, refinement of recrystallized austenite before transformation is effective for improving the workability of the steel sheet, but if the cumulative rolling reduction is less than 70%, it cannot be sufficiently refined, and recrystallization is sufficient. Without proceeding, the proportion of unrecrystallized austenite increases and the workability deteriorates. Further, in the finish rolling process, when the rolling reduction of the final pass is less than 3%, the sheet passing shape deteriorates, and there is a concern that the coil winding shape and the product plate thickness accuracy during hot coil formation may be adversely affected. On the other hand, when the rolling reduction of the final pass exceeds 25%, the dislocation density inside the hot-rolled steel sheet increases more than necessary due to the introduction of excessive strain. After the finish rolling process, the region having a high dislocation density has a high strain energy, and thus is easily transformed into a ferrite structure. Since the ferrite formed by such transformation precipitates without dissolving so much carbon, the carbon contained in the mother layer tends to concentrate at the interface between austenite and ferrite, and coarse Nb, Ti carbide is easily precipitated. Thus, when solid solution N and Ti reduce in a finish rolling process, the strength improvement of a steel plate cannot be expected for the reason mentioned above. Therefore, the rolling reduction of the final pass in the finish rolling process is limited to 3 to 25%.

  When the finish rolling finish temperature is less than 820 ° C., coarse TiC and NbC are likely to precipitate in austenite during finish rolling. On the other hand, the rolling load may become excessively high and stable rolling may be difficult. On the other hand, if the finish rolling finish temperature exceeds 980 ° C., the γ grains grow and become coarse before the start of cooling after the finish of rolling. In addition, the region in which ferrite for obtaining ductility can be reduced decreases, and as a result, the ductility may be deteriorated. Therefore, the finish rolling end temperature in the finish rolling process is set to a temperature range of 820 to 980 ° C.

ただし、上記式中の各記号の意味は次の通りである。
［Ｎｂ］：Ｎｂの含有量（質量％）
［Ｔｉ］：Ｔｉの含有量（質量％）
ｔ：最終圧延パスの１つ前の圧延完了から最終圧延パスの圧延開始までの時間（秒）
Ｔ：最終圧延パスの１つ前の圧延パスの圧延完了温度（℃） The finish rolling step is preferably performed under conditions that satisfy the following formula.

However, the meaning of each symbol in the above formula is as follows.
[Nb]: Nb content (% by mass)
[Ti]: Ti content (% by mass)
t: Time (seconds) from the completion of rolling immediately before the final rolling pass to the start of rolling in the final rolling pass
T: Rolling completion temperature (° C.) of the rolling pass immediately before the final rolling pass

When the above equation is satisfied, austenite recrystallization is promoted and grain growth of austenite is suppressed 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. Therefore, the recrystallized austenite grains are reduced in size during rolling, which makes it easier to obtain a steel structure suitable for ductility and hole expansibility .

(4) Cooling step After the finish rolling step, the obtained steel sheet is cooled to a temperature range of 500 to 650 ° C at an average cooling rate of more than 15 ° C / second.

  During cooling from the end of the finish rolling process to the winding process, competition occurs between the formation of cementite and precipitation nuclei such as TiC and NbC. For this reason, when the average cooling rate during this period is 15 ° C./second or less, the formation of cementite precipitation nuclei is prioritized, and in the subsequent winding process, it grows to a cementite of more than 2 μm at the grain boundary, and the hole expandability. Will deteriorate. Therefore, the lower limit of the cooling rate is set to more than 15 ° C./second. Although the upper limit of the cooling rate in the cooling step is not particularly limited, the effect of the present invention can be obtained, but it is desirable to set the upper limit at 300 ° C./second in consideration of plate warpage due to thermal strain. When the cooling stop temperature is less than 500 ° C., sufficient precipitation does not occur during the subsequent winding, and the desired steel plate strength cannot be obtained. On the other hand, when the temperature exceeds 650 ° C., cementite is easily generated, and a desired microstructure cannot be obtained.

In the cooling process, it is desirable that the microstructure is a continuous cooling transformation structure (Zw) in order to obtain better hole expansibility , but the cooling rate for obtaining this microstructure is over 15 ° C./second. If there is enough.

(5) Winding step The cooled steel sheet is wound under the condition that the average cooling rate up to 450 ° C is 0.008 to 1.0 ° C / second.

  When winding a steel sheet cooled to a temperature of 500 to 650 ° C., precipitation and growth of cementite, TiC, and NbC occur in the temperature range from the cooling end temperature to 450 ° C. Therefore, the cooling rate in this temperature range is controlled. It is necessary to. If the average cooling rate during that time is less than 0.008 ° C./sec, growth of TiC and NbC precipitates will occur. On the other hand, if it exceeds 1.0 ° C./sec, precipitation will be insufficient, and it will be difficult to obtain the desired steel sheet strength. It becomes. Therefore, the cooling rate in this temperature range is set to 0.008 to 1.0 ° C./second.

(6) Other processes After the completion of all processes, it is desirable to perform skin pass rolling with a rolling reduction of 0.1 to 2% for the purpose of improving the ductility by correcting the shape of the steel sheet and introducing movable dislocations. Moreover, after completion | finish of all the processes, you may pickle with respect to the hot-rolled steel plate obtained as needed for the purpose of the removal of the scale adhering to the surface of the obtained hot-rolled steel plate. Furthermore, after pickling, the obtained hot-rolled steel sheet may be subjected to in-line or off-line skin pass with a reduction rate of 10% or less or cold rolling to a reduction rate of about 40%.

  The hot-rolled steel sheet to which the present invention is applied may be further subjected to heat treatment in a hot dipping line in any case after casting, after hot rolling, and after cooling. You may make it perform a surface treatment separately. By applying the plating in the hot dipping line, the corrosion resistance of the hot rolled steel sheet is improved.

  In addition, when galvanizing the hot-rolled steel plate after pickling, the obtained steel plate may be immersed in a galvanizing bath and may be alloyed as necessary. By performing the alloying treatment, the hot-rolled steel sheet is improved in resistance to various types of welding such as spot welding in addition to the improvement in corrosion resistance.

  However, when assuming a hot-rolled steel sheet manufactured in a hot-rolling process having a winding process instead of a thick-sheet manufacturing process, the upper limit of the thickness of the hot-rolled steel sheet of the present invention is 12 mm.

  A 300 kg steel ingot having the chemical composition shown in Table 1 was melted in a high-frequency vacuum melting furnace and made into a 70 mm thick steel slab using a test rolling mill.

  Using this steel slab, hot rolling was carried out in a small test tandem mill to finish a steel sheet having a thickness of 2.0 to 3.6 mm. After completion of rolling, the steel sheet was cooled to a predetermined coiling temperature, and then charged into a furnace set at the coiling temperature, and cooled to 450 ° C. at a predetermined cooling rate. Then, the furnace was cooled to obtain a hot rolled steel sheet. These conditions are shown in Table 2.

  A part of the obtained hot-rolled steel sheet was subjected to an alloying treatment after the pickling treatment, the immersion in the plating bath and the immersion in the plating bath. The plating bath immersion was performed at a Zn bath temperature of 430 to 460 ° C. The alloying treatment was performed at an alloying temperature of 500 to 600 ° C. Table 3 shows the material of the steel plate thus obtained.

The evaluation method of the obtained steel plate is the same as that described above. Here, the “microstructure” indicates a microstructure at ¼ t of the steel plate thickness. Further, the “tensile test” result shows the result of the C direction JIS No. 5 test piece. In Table 3, “TS” indicates tensile strength and “EI” indicates elongation. The “hole expansion” result indicates a result obtained by the hole expansion test method described in JFS T 1001-1996. The “fracture surface crack” result indicates a result of visually confirming the presence / absence of the fracture. The case where there is no fracture surface crack is indicated as “ none ”, and the case where there is a fracture surface crack is indicated as “ present ”.

As shown in Table 3, test numbers 1-5, 8-11, 15, 16, and 22-25 within the range defined by the present invention have high tensile strength (TS) and excellent strength-ductility. It has a balance (TS × El) and excellent strength-hole expansion balance (TS × λ), and has excellent “peeling resistance”. On the other hand, test numbers 6, 7, 12-14, 17-21, and 26-28 outside the range defined in the present invention are inferior in TS × El, TS × λ, and “peeling resistance”. .

ADVANTAGE OF THE INVENTION According to this invention, the high intensity | strength hot-rolled steel plate excellent in hole expansibility can be provided. This hot-rolled steel sheet is required for high strength and hole expandability, including automobile parts such as inner plate members, structural members, suspension members, shipbuilding, construction, bridges, offshore structures, pressure vessels, Ideal for line pipes and machine parts.

Claims (9)

Chemical composition is mass%,
C: 0.01 to 0.1%
Si: 0.3% or less,
Mn: 0.4 to 3%
P: 0.1% or less,
S: 0.03% or less,
Al: 0.001 to 1%,
N: 0.01% or less,
Cr: 0.05 to 1%,
Nb: 0.003 to 0.05%,
Ti: 0.003 to 0.2%,
Cu: 0 to 1.2%,
Ni: 0 to 0.6%,
Mo: 0 to 1%,
V: 0 to 0.2%,
Ca: 0 to 0.005%,
REM: 0 to 0.02%,
B: 0 to 0.002%,
Balance: Fe and impurities,
Satisfying the relationship of the following formulas (1) and (2):
In the metal structure, has a cementite having an area ratio of 1% or less and an average particle diameter of 2 μm or less, and the Cr concentration in the cementite is 0.5 to 40% by mass on average, The area ratio of cementite having a particle diameter of 0.5 μm or less and an aspect ratio of 5 or less in the total cementite is 60% or more, the average particle diameter of the composite carbide of Ti and Cr is 10 nm or less, and the number density is 1 × 10 ¹³ pieces / mm ³ or more ,
A hot-rolled steel sheet having a tensile strength of 500 MPa or more and excellent hole expansibility and peeling resistance .
0.005 ≦ [Si] / [Cr] ≦ 2 (1)
0.5 ≦ [Mn] / [Cr] ≦ 20 (2)
However, [Si], [Cr] and [Mn] in the above formula mean the content (mass%) of each element. The chemical composition is mass%,
Cu: 0.2 to 1.2%,
Ni: 0.1 to 0.6%,
Mo: 0.05 to 1%
V: 0.02-0.2%
Containing one or more selected from
The hot-rolled steel sheet according to claim 1. The chemical composition is mass%,
Ca: 0.0005 to 0.005% and / or REM: 0.0005 to 0.02%
Containing
The hot rolled steel sheet according to claim 1 or 2. The chemical composition is mass%,
B: 0.0002 to 0.002%
Containing
The hot-rolled steel sheet according to any one of claims 1 to 3.   The hot-rolled steel sheet according to any one of claims 1 to 4, wherein galvanizing is performed. Comprising the following steps (A) to (E),
The manufacturing method of the hot-rolled steel plate in any one of Claim 1 to 4.
(A) a step of heating the steel ingot or steel slab having the chemical composition according to any one of claims 1 to 4 to 1150 to 1280 ° C,
(B) A step of rough rolling the heated steel ingot or steel slab in a temperature range of 1050 ° C. or higher and a cumulative reduction ratio of 40% or higher to obtain a rough bar;
(C) In the condition that the rough bar has a finish rolling start temperature of 1000 ° C. or higher, a cumulative rolling reduction ratio of 70% or higher, a final rolling reduction ratio of 3 to 25%, and a finish rolling finish temperature of 820 to 980 ° C. A step of performing finish rolling to obtain a steel plate,
(D) a step of cooling the obtained steel sheet to a temperature range of 500 to 650 ° C. at an average cooling rate exceeding 15 ° C./second;
(E) The process of winding the cooled steel plate on the conditions that the average cooling rate to 450 degreeC is 0.008-1.0 degree-C / sec. The method for producing a hot-rolled steel sheet according to claim 6, wherein the step (C) is performed under a condition that satisfies the following formula.

However, the meaning of each symbol in the above formula is as follows.
[Nb]: Nb content (% by mass)
[Ti]: Ti content (% by mass)
t: Time (seconds) from the completion of rolling immediately before the final rolling pass to the start of rolling in the final rolling pass
T: Rolling completion temperature (° C.) of the rolling pass immediately before the final rolling pass   The method for producing a hot-rolled steel sheet according to claim 6 or 7, wherein the steel sheet after winding is pickled and then immersed in a galvanizing bath to galvanize the surface of the steel sheet.   The method for producing a hot-rolled steel sheet according to claim 8, wherein the steel sheet after galvanization is alloyed.