WO2010131303A1 - Hot rolled steel sheet having excellent punching workability and fatigue properties, hot dip galvanized steel sheet, and method for producing the same - Google Patents

Abstract

Provided is a hot rolled steel sheet comprising, by mass%, 0.025 to 0.15% of C, 0.01 to 1.0% or less of Si, 1.0 to 2.5% of Mn, 0.02% or less of P, 0.005% or less of S, 0.5% or less of Al, 0.04 to 0.10% of Ti, and 0.007% or less of N, as well as Fe and inevitable impurities as the balance, wherein the Mn/Ti ratio is 15 or greater; Nb is not added; the ferrite volume percentage is 30% or greater and the balance comprises one of or both pearlite and bainite; the average aspect ratio of the crystal grain corresponding ellipsoid is 5 or less; the average distribution density on the ferrite grain boundary surface of Ti carbide having a grain size of 20 nm or greater is 10 grains/µm or less; the brittle fracture rate of surface fractured by punching is less than 20%; and the maximum tensile strength is 590 MPa or greater.

Description

Hot-rolled steel sheet, hot-dip galvanized steel sheet excellent in punching workability and fatigue characteristics, and manufacturing method thereof

The present invention is a hot-rolled steel sheet having a tensile strength of 590 MPa or more, which can prevent damage accompanied by unevenness on the punched end face even when punched under severe processing conditions, and further has excellent fatigue properties of the base material and the punched portion. The present invention relates to a plated steel sheet and a method for producing them. This steel plate is suitable as a material for automobiles, truck frames, members, chassis, and the like.
This application claims priority on May 11, 2009 based on Japanese Patent Application No. 2009-114633 filed in Japan, the contents of which are incorporated herein by reference.

From the viewpoint of reducing CO ₂ emissions, efforts are being made to reduce the weight of automobiles and trucks, and the application of high-strength steel sheets to members is advancing rapidly. On the other hand, when the strength of the material is increased, defects with irregularities are generated on the punched fracture surface, and fatigue characteristics may be deteriorated starting from this portion. In particular, in the undercarriage members and frame members of automobiles and trucks, establishment of a technique for ensuring good fatigue characteristics in the punched portion is strongly demanded.

As a method for overcoming such a problem, Patent Documents 1 and 2 disclose a technique for improving fatigue characteristics by forming a DP structure containing ferrite and martensite. These techniques are characterized by adding a specific amount of a ferrite-forming element such as Si. However, if Si is high, the roughness of the base material / scale interface may increase and the fatigue characteristics may deteriorate. Moreover, when galvanizing the hot-rolled steel sheet, there is a problem that the adhesion of the plating is lowered due to the Si concentrated layer on the surface. Furthermore, these techniques are characterized in that ferrite is formed by providing an air cooling process in the middle of cooling after hot rolling. However, since it is difficult to make the air-cooling conditions uniform over the entire length of the hot-rolled coil, there has been a problem that the variation in structure and material is large along the length of the coil (Problem 1).

Patent Document 3 proposes a method of obtaining a hot-rolled steel sheet having a high notch fatigue strength by forming a composite structure of ferrite and bainite and suppressing banding of bainite. However, it is necessary to perform air cooling during the cooling after hot rolling, and there is a problem that the material cannot be made constant over the entire length of the coil (Problem 1).

Patent Documents 4 to 7 propose a method for improving cracking and fatigue characteristics of a punched portion in a steel sheet to which a carbide forming element such as Ti, Nb, or V is added. However, in the methods shown in Patent Documents 4 to 7, there is a case where cracks may occur in the punched end face in the clearance situation in the actual member (Problem 2), or because of containing high Si, In some cases, the roughness of the scale interface increased and the fatigue characteristics deteriorated (Problem 3). Furthermore, when galvanizing the hot-rolled steel sheet, there is a problem that the plating does not adhere due to the Si concentrated layer on the surface (Problem 4).

Patent Documents 8 to 10 propose a method of improving cracking and fatigue characteristics of a punched portion by utilizing B in a steel sheet to which carbide forming elements such as Ti, Nb, and V are added. However, in the methods of Patent Documents 8 to 10, although the tendency to suppress cracking is observed, the effect is not sufficient. For this reason, when punching with a worn punch or shear that can be used in an actual member forming processing line, there is a problem that cracks or large irregularities are generated on the end face and the fatigue characteristics of the punched portion deteriorate (problem 5). .

Patent Document 11 discloses that ferrite and bainite are main structures, and by controlling the grain size and fraction of precipitates in ferrite and the form of bainite, high strength excellent in elongation characteristics, stretch flange characteristics, and fatigue characteristics. A method for obtaining a hot-rolled steel sheet has been proposed. However, even with this method, when punched with a worn punch or shear, cracks and large irregularities may occur on the end face (Problem 5). Moreover, since it is necessary to provide air cooling holding in the middle of cooling after hot rolling, there was a problem in productivity.

Patent Document 12 proposes a method for improving surface defects and productivity in a continuous casting process in a steel sheet to which carbide forming elements such as Ti, Nb, and V are added. However, in this method, since the hot rolling conditions are not optimized, there is a problem that when punching with a worn punch or shear, cracks or large irregularities are generated on the end face, and the fatigue characteristics of the punched portion deteriorate. (Problem 5).

Japanese Patent Laid-Open No. 11-158547 JP 2007-321201 A JP 05-179346 A JP 2002-161340 A JP 2002-317246 A JP 2003-342684 A JP 2004-250749 A JP 2004-315857 A JP 2005-298924 A JP 2008-266726 A JP 2007-9322 A JP 2007-138238 A

The present invention has been made in view of the above problems 1 to 5, and prevents damage caused by unevenness on the punched end face even when punching is performed under severe processing conditions such as a tight clearance and a worn shear or punch. It is excellent in fatigue characteristics of the base material and punched parts, and can be manufactured with high productivity without variation in the material in the longitudinal direction of the coil without providing air cooling during cooling after hot rolling. It is an object of the present invention to provide a hot-rolled steel sheet, a hot-dip galvanized steel sheet having a tensile strength of 590 MPa or more, and a method for producing them.

As a result of studying to solve the above problems, the present inventors incorporated an appropriate amount of Ti without containing Nb, Mn / Ti ratio, structure fraction, crystal grain form, Ti-based carbide distribution, By optimizing the size and texture, it is possible to produce hot rolled steel sheets and galvanized steel sheets with excellent fatigue characteristics even under severe punching conditions using clearance and worn shears and punches with high productivity. I found it. This invention is made | formed based on the said knowledge, The summary is as follows.

The hot-rolled steel sheet having excellent punchability and fatigue characteristics according to the present invention is C: 0.025 to 0.15%, Si: 1.0% or less, and Mn: 1.0 to 2.5% by mass. P: 0.02% or less, S: 0.005% or less, Al: 0.5% or less, Ti: 0.04 to 0.10%, and N: 0.007% or less, with the balance being It has a component composition consisting of Fe and inevitable impurities, Mn / Ti ratio: 15 or more, Nb is not added, the ferrite volume fraction is 30% or more, and the balance is one of pearlite and bainite Or it is composed of two types, the average aspect ratio of the equivalent ellipse of the crystal grains is 5 or less, the average distribution density of Ti carbides of 20 nm or more on the grain interface is 10 pieces / μm or less, and the brittle fracture of the punched fracture surface The area ratio is less than 20%, and the maximum tensile strength is 590 MPa or more. .
The hot-rolled steel sheet having excellent punchability and fatigue characteristics of the present invention may have a thickness of 5 to 10 mm.
Furthermore, 0.0003 to 0.005 mass% of B may be contained.
Further, one or both of Zr and V may be contained in a total amount of 0.002 to 0.08 mass%.
Further, one or more selected from Cr, Cu, Ni, Mo and W may be contained in a total amount of 0.02 to 2.0% by mass.
Further, one or more selected from Ca, Mg, La and Ce may be contained in a total amount of 0.0003 to 0.01% by mass.

The hot-dip galvanized steel sheet excellent in punching workability and fatigue characteristics of the present invention has the above-described hot-rolled steel sheet of the present invention and a plating layer or alloyed plating layer provided on the surface of the hot-rolled steel sheet. *

The method for producing a hot-rolled steel sheet excellent in punching workability and fatigue characteristics according to the present invention comprises heating a steel slab comprising the above-described composition of the hot-rolled steel sheet according to the present invention to 1100-1300 ° C. and then a temperature of 1000 ° C. or higher. The rough bar is finished under the conditions that the rough rolling is carried out under the conditions ending in step 1 and the cumulative rolling reduction of the final three-stage rolling is 25% or more and the final rolling temperature Tf satisfies the formula (1). A step of rolling into a rolled material, and after the finish rolling, the rolled material is air-cooled for 1 to 5 seconds, and subsequently cooled to a temperature of 700 ° C. or lower at a minimum cooling rate of 8 ° C./s or higher. A step of forming a rolled steel sheet, and a step of winding the hot-rolled steel sheet within a range of 500 to 650 ° C.
Tf> 840 + 800 × [% Ti] (1)
In the method for producing a hot-rolled steel sheet having excellent punchability and fatigue characteristics according to the present invention, the time from the end of the rough rolling to the start of the finish rolling may be 45 seconds or more.
You may further have the process of annealing at Ac3 temperature or less with respect to the said rolling material or the said hot-rolled steel plate after the said finish rolling.

The method for producing a hot-dip galvanized steel sheet having excellent punchability and fatigue characteristics according to the present invention comprises the steps of producing a hot-rolled steel sheet by the above-described method for producing a hot-rolled steel sheet, and pickling the hot-rolled steel sheet. Then, it has the process of heating below Ac3 temperature, and the process of immersing in a galvanization bath and then galvanizing the steel plate surface.
In the manufacturing method of the hot dip galvanized steel sheet excellent in the punching workability and fatigue characteristics of the present invention, the method may further include a step of performing galvanizing alloying treatment on the hot dip galvanized steel sheet.

According to the present invention, it is possible to provide a hot-rolled steel sheet and a galvanized steel sheet having a tensile strength of 590 MPa or more, little end face damage in the punched portion, and excellent fatigue characteristics of the base material and the punched portion at a low cost. Therefore, the industrial contribution is extremely remarkable. In particular, even when the punching hole is drilled under severe conditions such as punching with a worn punch or shear with strict clearance, the end face damage of the punched portion is small and excellent fatigue characteristics can be obtained. Furthermore, since the steel plate of the present invention has excellent paint corrosion resistance, the thickness of the undercarriage members and frame members of automobiles and trucks can be reduced, which can greatly contribute to the weight reduction of the vehicle body. In addition, since air cooling is not maintained during cooling after hot rolling, the occurrence of variations in structure and material in the longitudinal direction of the coil can be suppressed, and high-quality steel sheets can be manufactured stably.
Thus, the present invention has a very remarkable effect.

It is a figure which shows the relationship (in the case of C amount: 0.08%, coiling temperature: 600 degreeC) with the brittle fracture surface ratio of the punching fracture surface of a hot-rolled steel plate, a crystal grain average aspect-ratio, and precipitation Ti amount. It is a figure which shows the relationship (in the case of C amount: 0.08%, coiling temperature: 600 degreeC) with the brittle fracture surface ratio of the punching fracture surface of a hot-rolled steel plate, Mn content, and Ti content.

The inventors first investigated the cause of damage formation on the punched fracture surface that occurs when punched or punched. As a result, it was found that the punched fracture surface has two forms, a ductile fracture surface and a brittle fracture surface, and when a brittle fracture surface appears, large irregularities or microcracks are generated on the fracture surface. It was also found that the greater the unevenness, the easier the fatigue fracture starting from the punched portion. *

Next, the inventors observed the brittle fracture surface in detail and investigated the structural factors that cause brittle fracture. As a result, as individual factors that affect the brittle fracture surface ratio of the punched fracture surface, (a) the form of crystal grains, (b) the amount and size of alloy carbides such as TiC on the grain boundaries, and (c) the steel sheet It was found that there is a texture and that all these factors are within the proper range, so that no brittle fracture surface appears on the fracture surface even under severe processing conditions.
For example, as shown in FIG. 1, the degree of elongation of crystal grains in the rolling direction and the amount of precipitated TiC have a strong correlation with the degree of occurrence of brittle fracture surfaces. Although the cause of this is not clear, the more the crystal grains are elongated, the more the strain concentration during punching is localized at the grain boundaries. Further, when a large amount and large alloy carbide are precipitated on the grain boundary, generation of voids during processing is promoted. From the above, it is presumed that it is easily peeled and broken along the grain boundary to become a brittle fracture surface.

Next, the inventors have a tensile strength of 590 MPa or more, and can be manufactured under manufacturing conditions in which air cooling is not performed during the cooling after hot rolling, and the brittle fracture of the punched fracture surface at a Si content that can be galvanized. A search was made for the component conditions that would result in a steel plate that is less likely to occur.
As a result, the inventors have found that desired characteristics can be obtained by containing an appropriate amount of Ti without containing Nb and further optimizing the Mn / Ti ratio as shown in FIG.

Hereinafter, the present invention will be described in detail.
First, the reasons for limiting the components will be described. The component content is% by mass.
C is necessary as an element for precipitating carbides and controlling the microstructure. If it is less than 0.025%, it is difficult to obtain a tensile strength of 590 MPa or more. If it exceeds 0.15%, the unevenness of the punched fracture surface becomes large and the fatigue characteristics are deteriorated. Therefore, the C content is 0.025 to 0.15%. From the viewpoint of weldability, a more preferable upper limit is 0.12%.

Si has a function of improving the fatigue characteristics of the material. However, if the content exceeds 1.0%, the unevenness of the base material / scale interface increases and the fatigue characteristics deteriorate, and in the case of black skin steel plate (with scale), the coating corrosion resistance decreases due to the red scale. It is done. Therefore, the Si content is 1.0% or less.
On the other hand, when performing galvanization, if it exceeds 0.6%, plating adhesion may be lowered, so 0.6% or less is a preferred range. From the viewpoint of fatigue and plating adhesion, a more preferable range is 0.1% or less. When the content of Si is less than 0.01%, the shape of the welded portion is deteriorated, and the fatigue characteristics of the welded portion are deteriorated. For this reason, it is preferable that the minimum of Si amount is 0.01%.

Mn has a function of controlling the fraction of the microstructure and the precipitation of the alloy carbide that occurs with the transformation by controlling the transformation. If it is less than 1.0%, sufficient punching workability cannot be secured. On the other hand, if it exceeds 2.5%, macro segregation of Mn becomes prominent, and the punched portion cracks due to segregation. Therefore, the Mn content is 1.0 to 2.5%. A more preferable upper limit of the Mn content is 2.0%.

P acts as a solid solution strengthening element and can be used to adjust the strength of the steel sheet. However, since it segregates at the grain boundary and causes grain boundary cracking during punching, the upper limit was made 0.02%. Although the minimum of P amount is not limited, Usually, 0.001% or more is contained. *

When S is precipitated as sulfides such as MnS, it induces brittle fracture of the punched fracture surface, so it is desirable to reduce it as much as possible. However, up to 0.005% is acceptable, so the upper limit was made 0.005%. Although the minimum of S amount is not limited, Usually, 0.0003% or more is contained.

Al is added as an element that promotes the formation of ferrite. However, if it exceeds 0.5%, the γ → α transformation temperature increases, the size of the alloy carbide such as TiC formed along with the γ → α transformation increases, and the brittle fracture of the punched fracture surface is promoted. For this reason, the upper limit was limited to 0.5%. 0.1% or less is a more preferable range.
In addition, Al is an element effective for improving the cleanliness of steel as a deoxidizing element. In order to acquire this effect, it is desirable to make it contain 0.003% or more.

Ti is an important element in the present invention, and mainly adjusts the strength of the steel sheet by being dispersed in the steel as TiC. However, when the Ti content is less than 0.04%, it is difficult to obtain a tensile strength of 590 MPa or more. On the other hand, if it exceeds 0.10%, the brittle fracture surface ratio of the punched fracture surface increases, and fatigue failure starting from the punched portion tends to occur. Therefore, the Ti content is 0.04 to 0.10%. A more preferable upper limit is 0.08% or less. In this case, good fatigue characteristics can be obtained even under more severe punching conditions. *

N combines with Ti to form TiN. If it exceeds 0.007%, the punching workability is lowered, and the amount of fine alloy carbides such as TiC that contribute to strengthening of steel is reduced. Therefore, the N content is 0.007% or less. A more desirable upper limit is 0.004% or less. Although a minimum is not specifically limited, Usually, 0.001% or more is contained.

In the present invention, Nb is not added. Nb is known as an element that suppresses recrystallization of γ during finish rolling, but when Nb is contained within the content range of other elements in the present invention, the crystal grains of the steel sheet are flattened. Furthermore, the size and amount of alloy carbides precipitated on the grain boundaries are increased. For this reason, the brittle fracture surface ratio of the punched fracture surface is remarkably increased. Therefore, it is desirable not to contain Nb, but the upper limit is 0.003% which can be contained as an inevitable impurity. *

The Mn / Ti ratio is an important component parameter in the present invention. In the present invention, as will be described later, in order to reduce the brittle fracture surface ratio of the punched fracture surface, TiC existing at the ferrite grain boundary is adjusted to be less than 20 nm. Usually, when ferrite is transformed, TiC starts to precipitate, so when ferrite transformation occurs at a high temperature, coarse TiC is precipitated. For this reason, in order to suppress the growth of TiC so that the grain size of TiC existing at the ferrite grain boundary is less than 20 nm, the TiC to the ferrite grain boundary is suppressed at a temperature as low as possible to suppress the grain growth. It is necessary to promote precipitation.

As the Mn content increases, the ferrite transformation temperature decreases. Therefore, the particle size of TiC precipitated at the ferrite grain boundary can be reduced. Therefore, it is necessary to increase the Mn content so as to suppress the ferrite transformation. Furthermore, in order to suppress the production | generation of TiC, ensuring intensity | strength, it is necessary to adjust the density | concentration of Ti. Therefore, it is necessary to set a lower limit for the Mn / Ti ratio.
If the Mn / Ti ratio is less than 15, ferrite starts to form at a high temperature during cooling after hot rolling, so that TiC that precipitates simultaneously with ferrite formation becomes coarse and forms at grain boundaries. For this reason, the brittle fracture surface ratio of the punched portion increases, and the fatigue characteristics of the punched portion decrease. Therefore, the Mn / Ti ratio is 15 or more.
In order to make the brittle fracture surface ratio of the punched end face 5% or less, the Mn / Ti ratio is preferably 15 or more. Furthermore, in order to make the brittle fracture surface ratio of the punched end face 3% or less, the Mn / Ti ratio is more preferably 18 or more.

Furthermore, as a selective component, one or more of B, Zr, V, Cr, Cu, Ni, Mo, W, Ca, Mg, La, and Ce may be included as necessary.

B can be used to suppress the γ → α transformation and adjust the metal structure. If it is less than 0.0003%, the effect may not be sufficiently obtained. Moreover, when it exceeds 0.005%, workability will deteriorate. Therefore, the B content is set to 0.0003 to 0.005%. *

Zr and V can form alloy carbide together with Ti and can be used for adjusting the strength of the steel sheet. If the total amount of one or both is less than 0.002%, the effect may not be sufficiently obtained. On the other hand, if it exceeds 0.08%, the fatigue characteristics of the punched portion will deteriorate. Therefore, the content of one of Zr and V or the total amount of both is set to 0.002 to 0.08%, and the amount is preferably 0.03% or less.

Cr, Cu, Ni, Mo, and W are solid solution strengthening elements useful for adjusting the strength of the steel sheet, and contain 0.02% or more in total of one or more of Cr, Cu, Ni, Mo, and W. Can be made. On the other hand, if the content exceeds 2.0% in total, the surface quality is lowered and the fatigue characteristics may be lowered. *

Ca, Mg, La, and Ce are elements useful for controlling the form and distribution of inclusions, and one or more of these elements can be contained in a total of 0.0003% or more. On the other hand, if the total of one or more of Ca, Mg, La, and Ce exceeds 0.01%, the surface quality may deteriorate, so the upper limit is preferably made 0.01% or less. *

Next, the metal structure of the hot rolled steel sheet according to the present invention will be described.
The hot-rolled steel sheet of the present invention has ferrite as a main phase, and the balance consists of either one or both of pearlite and bainite.
In the present invention, the observation of the metal structure of the steel sheet may be performed with an optical microscope in accordance with JIS G 0551. Since the degree of elongation of crystal grains in the rolling direction correlates with the occurrence of unevenness in the punched portion, a sample for observing the structure is taken with a plate thickness section (referred to as an L section) parallel to the rolling direction as the observation surface. The observation surface may be etched with a nital etchant after polishing.
The area ratio of ferrite, bainite, and pearlite can be measured by a point count method or image analysis using a structural photograph taken with an optical microscope.
Further, the ferrite particle size may be measured by a cutting method or a comparison method in accordance with JIS G 0551, and a structure photograph taken with an optical microscope can be obtained by image analysis.

In the present invention, the ferrite is a mixed structure of polygonal ferrite (PF) and pseudo-polygonal ferrite (Quasi-Polygonal Ferrite, hereinafter referred to as αq). When the total of the polygonal ferrite and the pseudo-polygonal ferrite is less than 30%, the fatigue characteristics of the base material are deteriorated. Therefore, the volume ratio of ferrite is 30% or more. The volume ratio of this ferrite is more preferably 50% or more. The upper limit is not particularly limited, but is substantially 98% or less.

The pseudo-polygonal ferrite does not show an internal structure by etching like the polygonal ferrite (PF), but the shape is ash and is clearly distinguished from the polygonal ferrite. Here, assuming that the perimeter of the target crystal grain is lq and the equivalent circle diameter is dq, the crystal grain whose ratio (lq / dq) satisfies lq / dq ≧ 3.5 is a pseudopolygonal. Ferrite.

The structure other than ferrite consists of one or two of pearlite and bainite. When pearlite and bainite are compared, bainite has better fatigue characteristics at the punched portion. In the component range of the chemical composition of the present invention, the pearlite volume fraction is preferably 0 to 15%, and in this case, a better punched end face can be obtained.

In the hot-rolled steel sheet of the present invention, martensite and residual γ are not particularly included, but each may contain up to 2% in volume fraction as a lower limit that can be observed with an optical microscope. *

The average aspect ratio of the equivalent ellipse of the crystal grains is related to the cracking and unevenness generation behavior of the punched end face. When the average aspect ratio exceeds 5, the crack becomes prominent and fatigue cracks starting from the punched part It tends to occur. Therefore, the average aspect ratio of the equivalent ellipse of crystal grains is set to 5 or less. The average aspect ratio is preferably 3.5 or less, so that cracking does not occur even in a more severe punching process. The lower limit is not particularly limited, but 1 corresponding to a circle is a substantial lower limit.
Here, the average aspect ratio is a value obtained by observing the structure of the L cross-section, measuring (ellipse major axis length) / (elliptical minor axis length), and averaging about 50 or more crystal grains. In addition, the crystal grain here refers to a grain surrounded by a large tilt grain boundary having a grain boundary tilt angle of 10 ° or more.

The fracture surface form of the punched fracture surface correlates with the unevenness of the punched fracture surface and the occurrence of microcracking, and affects the fatigue characteristics of the member having the punched portion. When the brittle fracture surface ratio in the fracture surface is 20% or more, irregularities on the fracture surface become large, and minute cracks may occur. This promotes the generation of fatigue cracks in the punched portion, so the appropriate range is limited to less than 20%. The brittle fracture surface ratio in the fracture surface is preferably 10% or less.
The brittle fracture surface ratio in the fracture surface is a value measured by punching a sample steel plate with a shear or a punch under a clearance condition of 10 to 15% of the plate thickness and observing the fracture surface formed.

The texture of the steel sheet affects the fatigue characteristics of the punched part through the occurrence of cracks in the fractured surface of the punched part and the residual stress distribution. If the X-ray random intensity ratio of the {112} <110> orientation and {332} <113> orientation of the plate surface at the center portion of the plate thickness exceeds 5, respectively, cracking of the punched portion fracture surface may occur. Therefore, the X-ray random intensity ratio in the above orientation is preferably 5 or less. More preferably, it is 4 or less. In this case, cracks do not occur even when punched with a worn punch used in mass production. 1 which is completely random is a practical lower limit.

If fine Ti-based carbides are present on the grain boundaries and the crystal grains are flat, the brittle fracture surface ratio of the punched fracture surface increases and the fatigue characteristics deteriorate. According to the observations by the inventors, it is considered that Ti-based carbides having a particle size of 20 nm or more tend to induce void generation at the time of strain concentration and cause grain boundary destruction. When the average distribution density of Ti carbides of 20 nm or more exists on the grain interface exceeding 10 per grain boundary length of 1 μm, the brittle fracture surface ratio increases and the fatigue characteristics of the member are lowered. For this reason, the upper limit is 10 / μm. 6 / μm or less is a more preferable range. About a minimum, it is so preferable that it is low from a viewpoint of brittle fracture surface suppression. However, if it is 0.1 piece / μm or less, the effect is saturated and a brittle fracture surface hardly occurs.
The precipitate distribution on the grain interface is measured by observing the L-section cut sample with an SEM.

If the thickness of the hot-rolled steel sheet of the present invention is less than 5 mm, the elongation of crystal grains tends to be slightly advanced, and the brittle fracture surface ratio of the punched fracture surface may increase. On the other hand, if the plate thickness exceeds 10 mm, the cooling rate after finish rolling to 700 ° C. or less is decreased, and the brittle fracture surface ratio of the punched fracture surface may increase. Therefore, the thickness of the hot rolled steel sheet is preferably 5 to 10 mm.

Then, the reason for limitation of the manufacturing method of the hot rolled steel plate and plated steel plate concerning this invention is demonstrated.
Prior to hot rolling, the steel slab needs to be heated to 1100 ° C or higher. If this temperature is less than 1100 ° C., it is difficult to obtain sufficient strength. This is considered to be because when the temperature is lower than 1100 ° C., the Ti-based carbide is not sufficiently dissolved, and as a result, the precipitate becomes coarse. As for the heating temperature of a steel piece, 1140 degreeC or more is more preferable. If it exceeds 1300 ° C., the unevenness of the scale / base metal interface becomes large and the fatigue characteristics of the base material deteriorate, so the upper limit is 1300 ° C.

粗 The heated steel slab is roughly rolled into a rough bar. This rough rolling needs to be completed at 1000 ° C. or higher. This is because if the finish temperature is less than 1000 ° C., the crystal grains after finish hot rolling are flattened and cracks in the punched portion fracture surface occur.

Next, the rough bar is finish-rolled to obtain a rolled material.
You may heat-process after rough rolling until completion of finish rolling. As a result, the temperature in the width direction and the longitudinal direction of the plate becomes uniform, and the material variation in the coil of the product is also reduced. The heating method is not particularly specified. What is necessary is just to perform by methods, such as a furnace heating, induction heating, electrical heating, and high frequency heating.
Similarly, descaling may be performed. This may reduce the surface roughness and improve the fatigue characteristics. The descaling method is not particularly specified, but the most common method is a high-pressure water stream.

The time from rough rolling to finish rolling affects the fracture surface morphology of the punched fracture surface through the recrystallization behavior of the γ phase during rolling. If the time from the end of rough rolling to the start of finish rolling is less than 45 seconds, the brittle fracture surface ratio of the punched end surface may increase. For this reason, it is preferable to set the time from the end of rough rolling to the start of finish rolling to 45 seconds or more, thereby further promoting recrystallization of austenite and making the crystal grains more spherical. *

In hot finish rolling, the final three-stage cumulative reduction ratio and final rolling temperature are important conditions in the present invention because they affect the flattening of crystal grains after transformation.
If the final three-stage cumulative rolling reduction is less than 25%, the recrystallization of γ does not proceed sufficiently, and the crystal grains become flat after transformation. For this reason, the range is limited to 25% or more to promote recrystallization of austenite so that the crystal grains become spherical.
The final three-stage cumulative reduction ratio is calculated by the following equation, where N is the total number of rolling mills in finish rolling.
100 × [(Outside plate thickness after rolling in the N-3th rolling mill counted from the rolling entry side) − (Outside plate thickness after rolling in the Nth rolling mill counted from the rolling entry side)] / (Outside plate thickness after rolling on the N-3rd rolling mill counted from the rolling entry side)

The final rolling temperature Tf is changed according to the Ti content (% Ti) because it affects the aspect ratio of the crystal grains and the distribution state of the alloy carbide, which affect the fracture surface form of the punched end face. When the final rolling temperature is 840 + 800 × [% Ti] or less, the punched end face is cracked. Therefore, the final rolling temperature Tf is set so as to satisfy the following formula (1).
Tf> 840 + 800 × [% Ti] (1)
It was confirmed that the more preferable range of the final rolling temperature Tf is 840 + 1000 × [% Ti] or more. Although there is no particular limitation on the upper limit, the hot rolling roll is likely to be worn out at high temperatures, and is usually performed at 1000 ° C. or lower.

圧 延 The rolled material is air-cooled immediately after the final rolling. This air cooling time affects the flattening of the crystal grains after transformation in connection with the recrystallization of γ. If the air cooling time immediately after the final rolling is less than 1 second, the brittle fracture surface ratio of the punched end face increases. Therefore, this air cooling time is 1 second or more. More preferably, it is 2 seconds or more. If the air cooling time exceeds 5 seconds, coarse TiC will precipitate and it will be difficult to ensure the strength, and the properties of the punched end face will deteriorate, so this is the upper limit.

Following the air cooling immediately after the final rolling, the rolled material is cooled to form a hot-rolled steel sheet. This cooling is an important process affecting the properties of the punched end face and the strength fluctuation in the coil longitudinal direction. Cooling to 700 ° C. or lower at a minimum cooling rate of 8 ° C./s or higher.
When the cooling stop temperature exceeds 700 ° C., alloy carbide tends to precipitate coarsely on the grain boundaries, and the brittle fracture surface ratio of the punched end face increases. On the other hand, even when the minimum cooling rate up to 700 ° C. is less than 8 ° C./s, alloy carbides are likely to be coarsely precipitated on the grain boundaries, and it is easy to induce cracking of the punched end face, and the coil length of the final product The intensity fluctuation in the direction becomes large.
Here, the minimum cooling rate of 8 ° C./s or more means that the cooling rate between the temperatures from the air cooling end temperature to 700 ° C. is not always lower than 8 ° C./s. For this reason, for example, it means that air cooling is not performed within this temperature section. Thus, in the present invention, air cooling is not performed in the middle of the cooling process by water cooling as in the prior art.
The cooling stop temperature is more preferably 680 ° C. or lower, and the minimum cooling rate is more preferably 15 ° C./s or higher. The upper limit of the minimum cooling rate is not particularly defined, but if it exceeds 80 ° C./s, it becomes difficult to cool uniformly in the hot-rolled coil, and the strength fluctuation in the coil becomes large. For this reason, it is preferable that it is 80 degrees C / s or less.

Next, the cooled hot-rolled steel sheet is wound up. The winding temperature is 540 to 650 ° C. When the coiling temperature is less than 540 ° C., the ferrite fraction decreases and the fatigue characteristics of the base material deteriorate. Moreover, when it exceeds 650 degreeC, TiC coarsens and precipitates in large quantities. As a result, it becomes difficult to ensure a tensile strength of 590 MPa or more, and fatigue cracks starting from the punched portion are likely to occur.

The hot-rolled steel sheet thus obtained may be reheated (annealed). In this case, when the reheating temperature exceeds the Ac3 temperature, TiC precipitates at the grain boundaries, and the tensile strength of the steel sheet and the fatigue strength of the punched portion are reduced. For this reason, the suitable range of reheating temperature is restrict | limited to Ac3 temperature or less. The heating method is not particularly specified, and may be performed by methods such as furnace heating, induction heating, current heating, and high frequency heating.
The heating time is not particularly defined, but when the heating and holding time of 550 ° C. or higher exceeds 30 minutes, the maximum heating temperature is desirably 700 ° C. or lower in order to obtain a tensile strength of 590 MPa or higher.
Note that the reheating (annealing) may be performed after the hot-rolled steel sheet is wound up and before the temperature reaches room temperature.

Since skin pass rolling or leveler rolling is effective in improving shape correction, aging, and fatigue properties, it may be performed after pickling or before pickling. When performing skin pass rolling, it is desirable that the upper limit of the rolling reduction be 3%. This is because if it exceeds 3%, the formability of the steel sheet is impaired. Moreover, you may perform pickling according to the objective.

Next, the hot dip galvanized steel sheet and the manufacturing method thereof according to the present invention will be described.
The hot dip galvanized steel sheet of the present invention is a steel sheet in which a plated layer or an alloyed plated layer is provided on the surface of the above-described hot rolled steel sheet of the present invention.
After pickling the hot-rolled steel sheet obtained by the above-described method, the steel sheet is heated using a continuous galvanizing facility or a continuous annealing galvanizing facility, hot-plated, and a plated layer is formed on the surface of the hot-rolled steel plate. .
When the heating temperature of the steel plate exceeds the Ac3 temperature, the tensile strength and fatigue limit of the steel plate are lowered, so the appropriate range of the heating temperature is limited to the Ac3 temperature or lower. The heating temperature is more preferably in the range of Ac3-30 ° C. or lower from the viewpoint of the fatigue characteristics of the punched part.
Further, after galvanizing, galvanizing alloying treatment may be performed to form an alloyed galvanized layer.

The plating type is not limited to galvanizing, and other plating types may be used as long as the upper limit of the heating temperature is Ac3 temperature.
Moreover, you may abbreviate | omit the process of heating the hot-rolled steel plate after pickling.

In the present invention, the production method preceding hot rolling is not particularly limited. That is, following the smelting by a blast furnace, a converter, an electric furnace or the like, the components are adjusted so that the desired component content is obtained by various secondary scouring. Then, it may be cast by a method such as thin continuous slab casting in addition to normal continuous casting and casting by ingot method. Scrap may be used as a raw material. In the case of a slab obtained by continuous casting, it may be sent directly to a hot rolling mill as it is at a high temperature slab, or may be hot rolled after being cooled to room temperature and then reheated in a heating furnace.

The following examples further illustrate the present invention.
A to R steels having the chemical components shown in Table 1 were produced by the following method. First, a steel slab was produced by casting, and then the steel slab was reheated and rough-rolled under the conditions shown in Tables 2 to 4 to form a rough bar. Next, the rough bar was finish-rolled to form a rolled material having a thickness of 5 to 10 mm, and then cooled and wound up as a hot-rolled steel plate.
Steels F-1 and G-2 were produced by further reheating the hot-rolled steel sheet.

The indication about the chemical composition in the table is mass%. In Table 1, Ac3 is a value calculated by the following equation.
Ac3 = 910−203√ (% C) + 45 ×% Si−30 ×% Mn−11 ×% Cr + 700 ×% P + 400 ×% Al + 400 ×% Ti
In the formula,% C,% Si,% Mn,% Cr,% P,% Al, and% Ti indicate the contents of C, Si, Mn, Cr, P, Al, and Ti, respectively.

The chemical composition of the steel in the table is the steel no. Steel No. 1 in Table 1 with the same alphabet. It corresponds to the chemical composition of steel.
“SRT” in the table indicates the slab heating temperature. “RFT” indicates the rough rolling end temperature. “T1” indicates the time from the end of rough rolling to the start of finish rolling. “Red3” indicates the cumulative reduction ratio of the final three stages in finish rolling. “Tf” indicates the final finish rolling temperature. “T2” indicates an air cooling time immediately after the final finish rolling. “CRmin” indicates the minimum cooling rate between SCTs after air cooling. “SCT” indicates the water cooling stop temperature. “CT” indicates a winding temperature.

Steels A-5, B-5, C-5, G-1, H-2, I-1, J-2, and R-1 are hot dip galvanized steel sheets, and after pickling hot rolled steel sheets In the continuous annealing galvanization line, it annealed at the annealing temperature shown in Table 5, and then performed galvanization and manufactured.
The galvanizing immersion temperature was 450 ° C. and the plating alloying temperature was 500 ° C.

First, the metal structure, texture, and grain boundary of the produced steel plate were observed.
The observation of the metal structure of the steel sheet was performed with an optical microscope in accordance with JIS G 0551 as described above. In addition, as described above, the area ratio of each tissue was measured by a point count method or image analysis using a tissue photograph.
As described above, the average aspect ratio of the crystal grains is obtained by observing the structure of the L cross-section, measuring (ellipse major axis length) / (elliptical minor axis length) and averaging for 50 or more crystal grains. Asked.
The number of Ti-based carbides existing on the grain boundaries was measured by SEM observation.
The above measurement results are shown in Tables 6-8. Note that “Ngb” in the table indicates the distribution density of Ti-based carbides having a particle size of 20 nm or more on the grain interface.

Next, the strength characteristics, brittle fracture surface ratio, fatigue characteristics, and paint corrosion resistance of the steel sheets were evaluated.
The strength characteristics of the steel sheet were evaluated by the following method. First, the specimen was processed into a No. 5 test piece described in JIS Z 2201. And the tensile test was done with respect to this No. 5 test piece according to the method of JISZ2241, and the maximum tensile strength (TS), yield strength (YS), and elongation (EI) were calculated | required.
The fatigue properties of the steel sheet were evaluated according to the fatigue limit (σwp) by conducting a plane bending fatigue test in accordance with JIS Z2275 under the condition of stress ratio = −1.
The fatigue characteristics of the punched portion were evaluated by the following method. A No. 2 fatigue test piece was prepared by punching a φ10 punched hole in the center of the steel plate with a clearance of 20% of the plate thickness. And this No. 2 fatigue test piece was tested by the said method, and it evaluated by the fatigue limit ((sigma) wpp). The punching was used 500 times or more, and a punch with a worn end corner was used. In this way, punching holes were formed under severe conditions where the clearance was severe and the punch was worn, and the fatigue characteristics of the punched portion were evaluated.
Prior to the fatigue test, the punched fracture surface was observed with a microscope to evaluate the brittle fracture surface ratio in the fracture surface. Here, all the fracture surfaces other than the dimple-like ductile fracture surface were evaluated as brittle fracture surfaces.
The coating corrosion resistance of black-skinned hot-rolled steel sheets and plated steel sheets is determined by applying a cut wrinkle to the sample surface that has been subjected to chemical conversion treatment and electrodeposition coating, performing a 240 hr SST (salt spray) test, and having a peel width of 3 mm or less. Good (Good) was evaluated and those exceeding 3 mm were evaluated as bad (Bad).
The above measurement results are shown in Tables 9-11. As described above, “σwp” in the table indicates the bending fatigue limit of the original plate, and “σwpp” indicates the bending fatigue limit of the punched hole material.

Steel A-2 to 4, Steel B-3 to 4, Steel C-2 to 4, Steel D-2, Steel E-2, Steel H-2, Steel K-2, Steel L-2 This is an example in which a brittle fracture surface is generated on the punched end face because the conditions and the cooling condition after finish rolling are outside the proper range.
Steel A-6 is an example in which the air-cooling time after finish rolling is outside the proper range, so that the tensile strength is less than 590 MPa and the brittle fracture surface ratio of the punched end face is high.
Steel B-4 and Steel C-4 are examples in which the coiling temperature of the hot-rolled steel sheet is out of the range, the tensile strength is less than 590 MPa, and a brittle fracture surface is generated on the punched fracture surface.
Steel I-2 is an example in which the tensile strength is less than 590 MPa because the winding temperature is low.
Steel B-2 and Steel K-2 are examples in which a brittle fracture surface was generated on the punched fracture surface because the rough rolling end temperature was out of the range.
Steel F-2 is an example in which a brittle fracture surface was generated on the punched fracture surface because the slab heating temperature was outside the range.
Steel G-2 is an example in which the post-heat treatment (annealing) temperature is higher than the Ac3 temperature, and a brittle fracture surface is generated on the punch fracture surface.
Steels N to S are examples in which a brittle fracture surface is generated on the punched fracture surface because the amount of Ti, Nb, or Mn / Ti ratio is outside the proper range.

Steel L-1 was an example of the present invention, but had an Al content of 0.3% and Ngb of 8 pieces / μm. On the other hand, in other examples of the present invention, the Al content was 0.1% or less and the Ngb was 0.7 to 5 / μm. Thus, the Al content is preferably 0.1% or less. This suppresses the increase in the size of alloy carbides such as TiC formed with the γ → α transformation, and keeps the average distribution density (Ngb) of Ti-based carbides having a particle size of 20 nm or more on the ferrite grain interface low. Can do.

The hot-rolled steel sheet of the present invention is excellent in fatigue characteristics of the base material and the punched portion. Moreover, the manufacturing method of the hot-rolled steel sheet of the present invention does not require air cooling during the cooling after hot rolling, and can manufacture the hot-rolled steel sheet of the present invention with high productivity. For this reason, the present invention can be suitably applied to a member that requires good fatigue characteristics in a punched portion, such as an undercarriage member or a frame member of an automobile or a truck, and a manufacturing process thereof.

Claims (13)

1. % By mass
   C: 0.025 to 0.15%,
   Si: 0.01 to 1.0% or less,
   Mn: 1.0 to 2.5%
   P: 0.02% or less,
   S: 0.005% or less,
   Al: 0.5% or less,
   Ti: 0.04 to 0.10%,
   And N: 0.007% or less,
   The balance has a component composition consisting of Fe and inevitable impurities,
   Mn / Ti ratio: 15 or more,
   Nb is not added,
   The volume fraction of ferrite is 30% or more, and the balance consists of one or two of pearlite and bainite,
   The average aspect ratio of the equivalent ellipse of the crystal grains is 5 or less,
   The average distribution density of Ti-based carbide having a particle size of 20 nm or more on the ferrite grain interface is 10 pieces / μm or less,
   The brittle fracture surface ratio of the punched fracture surface is less than 20%,
   A hot-rolled steel sheet excellent in punching workability and fatigue characteristics, characterized by having a maximum tensile strength of 590 MPa or more.
2. The hot-rolled steel sheet having excellent punching workability and fatigue properties according to claim 1, wherein the plate thickness is 5 to 10 mm.
3. The hot-rolled steel sheet having excellent punching workability and fatigue properties according to claim 1, further comprising B in an amount of 0.0003 to 0.005 mass%.
4. The hot rolled steel sheet having excellent punchability and fatigue properties according to claim 1, further comprising one or both of Zr and V in a total amount of 0.002 to 0.08 mass%.
5. The punching workability and fatigue according to claim 1, further comprising 0.02 to 2.0 mass% in total of one or more selected from Cr, Cu, Ni, Mo and W Hot-rolled steel sheet with excellent characteristics.
6. The punching workability and fatigue characteristics according to claim 1, further comprising 0.0003 to 0.01 mass% in total of one or more selected from Ca, Mg, La, and Ce. Excellent hot-rolled steel sheet.
7. A hot-dip galvanized steel sheet excellent in punching workability and fatigue characteristics, comprising the hot-rolled steel sheet according to claim 1 and a plating layer or an alloyed plating layer provided on the surface of the hot-rolled steel sheet.
8. A step of heating the steel slab comprising the component composition according to claim 1 to 1100 to 1300 ° C. and then roughly rolling the steel slab at a temperature of 1000 ° C. or higher to form a rough bar;
   A step in which the rolling reduction of the final three-stage rolling is 25% or more, and the final rolling temperature Tf finish-rolls the rough bar under a condition satisfying the formula (1) to form a rolled material;
   A step of air-cooling the rolled material for 1 to 5 seconds after completion of the finish rolling, and subsequently cooling to a temperature of 700 ° C. or less at a minimum cooling rate of 8 ° C./s to obtain a hot-rolled steel sheet;
   A method for producing a hot-rolled steel sheet excellent in punching workability and fatigue characteristics, comprising a step of winding the hot-rolled steel sheet within a range of 540 to 650 ° C.
   Tf> 840 + 800 × [% Ti] (1)
9. The method from the end of the rough rolling to the start of the finish rolling is 45 seconds or more, and the method for producing a hot-rolled steel sheet excellent in punching workability and fatigue characteristics according to claim 8.
10. The hot-rolled steel sheet having excellent punching workability and fatigue characteristics according to claim 8, further comprising a step of annealing the rolled material or the hot-rolled steel sheet at an Ac3 temperature or lower after the finish rolling. Manufacturing method.
11. A process for producing a hot-rolled steel sheet by the method for producing a hot-rolled steel sheet according to claim 8;
    Pickling the hot-rolled steel sheet; and
    Then, the manufacturing method of the hot dip galvanized steel plate excellent in the punching workability and fatigue characteristics which has the process of immersing in a galvanizing bath and galvanizing the said steel plate surface.
12. A process for producing a hot-rolled steel sheet by the method for producing a hot-rolled steel sheet according to claim 8;
    Heating the hot-rolled steel sheet at an Ac3 temperature or lower after pickling;
    Next, a method for producing a hot dip galvanized steel sheet excellent in punching workability and fatigue characteristics, comprising a step of galvanizing the steel sheet surface by dipping in a galvanizing bath.
13. The method for producing a hot-dip galvanized steel sheet having excellent punchability and fatigue characteristics according to claim 11 or 12, further comprising a step of subjecting the hot-dip galvanized steel sheet to a galvanizing alloying process.
    

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