JP4460343B2 - High-strength hot-rolled steel sheet excellent in punching workability and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength hot-rolled steel sheet having a tensile strength of 690 MPa or more excellent in burring workability, stretch flangeability and ductility represented by hole expansibility, and uses such excellent workability for automobile parts such as members. It can be used effectively as a material for foot parts such as arms and arms and chassis.

  In recent years, application of light metals such as Al alloys and high-strength steel sheets to automobile members has been promoted for the purpose of reducing the weight in order to improve the fuel efficiency of automobiles. However, although light metals such as Al alloys have the advantage of high specific strength, their application has been limited to special applications because they are significantly more expensive than steel. In order to promote weight reduction of automobiles in a wider range, the application of inexpensive high-strength steel sheets is strongly demanded.

  In general, the higher the strength of a material, the lower the ductility and the worse the workability (formability). Steel materials are no exception, and attempts have been made so far to achieve both high strength and high ductility. On the other hand, in addition to these characteristics, materials used for suspension parts such as automobile suspension arms are required to have high burring workability (hole expandability). However, as the strength increases, the hole expandability tends to decrease in the same way as the ductility. When applying high-strength steel sheets to undercarriage parts of automobiles with complex shapes, the hole expandability is reduced. This is an important consideration. Against this background, hot-rolled steel sheets with excellent ductility and hole expansibility have been developed, which are mainly composed of structures such as ferrite and bainitic ferrite, and have reduced so-called hard second phase and cementite phase such as pearlite and martensite structures. ing.

  Recently, along with the expansion of application of these high-strength hot-rolled steel sheets, there is a problem that defects such as peeling or cracking occur on the end face of the plate punched out from the material. These defects not only significantly impair the design of the end face of the product, but also have a risk of affecting the fatigue strength by becoming a stress concentration part such as a notch due to the high strength steel plate. Although many studies have not been made on the punchability of ordinary steel, for example, Japanese Patent Application Laid-Open No. 9-49053 (Patent Document 1) discloses control of precipitates in steel and Japanese Patent Application Laid-Open No. 2003-41342 ( Patent Document 2) discloses an invention that is improved by refining the structure of the steel sheet surface layer.

JP-A-9-49053 JP 2003-41342 A

However, the above prior art is basically a technique related to the punchability of a low-strength ultra-low carbon steel sheet, and it is difficult to say that it can be applied to a high-strength hot-rolled steel sheet having a tensile strength of 690 MPa or more, which is the subject of the present invention. That is, the invention described in Patent Document 1 relates to the generation of burrs at the time of punching, which is likely to occur because of a soft steel plate. Specifically, Ti ₄ C ₂ S ₂ is precipitated in the steel and thereby the inside of the punching end surface is formed. This is a technique for reducing burrs by generating voids in the shearing surface portion of the steel and advancing cracks. That is, in the invention described in Patent Document 1, there is a possibility that defects on the end face are promoted even though burrs can be reduced.

  The invention described in Patent Document 2 is also related to the soft cold-rolled steel sheet and the prevention of burr generation, and no consideration is given to the properties of the punched end face itself. Therefore, the present invention is hardly considered in the above prior art, and has a tensile strength of 690 MPa or more excellent in ductility and hole expansibility expected to be applied to high-strength hot-rolled steel, particularly automobile undercarriage parts. An object of the present invention is to provide a technique for preventing defects on a punched end face on the premise of a high-strength hot-rolled steel sheet.

  The inventors first performed punching with various clearances using a high-strength hot-rolled steel sheet having a tensile strength of 690 MPa or more and excellent in ductility and hole expansibility, and visually examined the end face properties. As a result, it was confirmed that cracks and crevices that could not be confirmed when punched with a clearance of around 12.5% normally recommended in the hole expansion test were clearly generated by increasing the clearance. Therefore, in the following, the investigation was advanced using a clearance of 17 to 23%.

  Next, in consideration of the chemical composition of the hot-rolled steel sheet and the manufacturing process of the hot-rolled steel sheet produced on an industrial scale by the continuous hot rolling equipment currently used normally, the production conditions affecting the defects on this punched end face As a result of diligent research on the effects of steel, the basic steel composition and manufacturing process are controlled by controlling the P content in the steel to a small level, so that defects can be generated without any change to that of conventional high-strength hot-rolled steel sheets. The present invention has been made by finding out that it can be prevented.

That is, the gist of the present invention is as follows.
(1) In mass%, C: 0.01 to 0.07%, Si: 0.77 to 2%, Mn: 0.05 to 3%, Al: 0.005 to 0.5%, N ≦ 0.005%, S ≦ 0.005%, Ti: 0.03-0.2%, P ≦ 0.01%, B: 0.0002-0.002% , the balance being Fe and inevitable impurities a steel consisting of ferrite or bainitic ferrite structure and area ratio up phase, is 3% or less hard second phase and cementite area ratio and a tensile strength of Ri der than 690 MPa, when punching A high-strength hot-rolled steel sheet excellent in punching workability, characterized by having a clearance of 17 to 23% and no defects in the end surface properties and cracks .

(2) Further, in mass%, V: 0.01 to 0.2%, Nb: 0.01 to 0.2%, Mo: 0.01 to 0.2%, or one or more of them are contained. And Ti / 48 + V / 51 + Nb / 93 + Mo / 96-C / 12-N / 14-S / 32 ≧ 0%, and high strength heat excellent in punching workability as described in (1) above Rolled steel sheet.

( 3 ) Furthermore, the above-mentioned (1) or ( 1), further comprising one or two of Ca: 0.0005 to 0.02% and REM: 0.0005 to 0.2% in mass%. 2) A high-strength hot-rolled steel sheet having excellent punchability as described.
(4) the (1) to (3) A method of manufacturing a steel sheet according to any one of the above (1) to a steel slab having a component according to any one of (3) In hot rolling, the hot rolling heating temperature is set to 1200 ° C. or higher, and after finishing hot finish rolling at the Ar ₃ transformation point or higher, it is excellent in punching workability characterized by winding at 450 ° C. to 650 ° C. It exists in the manufacturing method of a strength hot-rolled steel sheet.

  The present invention provides a high-strength hot-rolled steel sheet excellent in hole expansibility and ductility with a tensile strength of 690 MPa or more, in which no defect occurs in the punched end face even when punched with a large clearance, and a method for producing the same. It is an invention with high industrial value that is effective as a material for automobile parts, for example, foot parts such as members and arms, and chassis, taking advantage of its excellent workability.

The present invention is described in further detail below.
First, the reasons for limiting the microstructure of the steel in the present invention will be described.
The microstructure of the present invention has a ferrite or bainitic ferrite structure as a phase with the largest area ratio, and the hard second phase that greatly reduces hole expansibility as the balance is 3% or less (including 0%) in area ratio. ). Here, the hard second phase is typically martensite or pearlite, but also includes retained austenite that may be transformed into martensite in the punching process that is performed prior to the hole expansion process. Furthermore, cementite is also hard and deteriorates the hole expandability.

  Therefore, in the present invention, the ferrite or bainitic ferrite structure is defined as the phase with the largest area ratio, and the hard second phase and cementite are defined as 3% or less in area ratio. The area ratio of cementite is the number of areas of the ferrite or bainitic ferrite structure and the hard second phase, and preferably the area ratio of cementite is 1% or less. The existence state of cementite here includes not only those present in a lump at the ferrite grain boundary or the like, but also those generated at the lath-like boundary in the bainite structure.

Next, the reasons for limiting the chemical components of the present invention will be described. The amount of chemical component is mass%. If the C content exceeds 0.07%, the workability deteriorates, so the content is made 0.07% or less. Moreover, since intensity | strength will fall if it is less than 0.01%, it shall be 0.01% or more.
Si is effective as a solid solution strengthening element for increasing the strength, but in order to obtain a desired strength, it is necessary to contain 0.01% or more. However, if it exceeds 2%, workability deteriorates. Therefore, the Si content is set to 0.01% or more and 2% or less.

Mn is effective for increasing the strength as a solid solution strengthening element, but 0.05% or more is necessary to obtain a desired strength. Further, if over 3% is added, slab cracking occurs, so the content is made 3% or less.
Al needs to be added in an amount of 0.005% or more for deoxidation of molten steel, but the cost is increased, so the upper limit is made 0.5%. Moreover, when adding too much, a nonmetallic inclusion will be increased and elongation will be degraded, Therefore Preferably it is 0.3% or less.

N is an element that forms coarse TiN in steel, and should be reduced as much as possible. However, mischievous reduction leads to an increase in steelmaking cost, and 0 in consideration of steelmaking ability. 0.005% or less.
If S is too large, the hole expandability deteriorates as inclusions such as MnS, and cracks during hot rolling should be caused. Therefore, it should be reduced as much as possible, but 0.005% or less is acceptable.

Ti contributes to an increase in the strength of the steel sheet by precipitation strengthening. However, if the content is less than 0.03%, this effect is insufficient. On the other hand, if the content exceeds 0.2%, generation of coarse TiN cannot be avoided. Therefore, the Ti content is set to 0.03% or more and 0.2% or less.
P is the most important element to be controlled in the present invention. By suppressing this element to 0.01% or less, defects on the punched end face can be prevented. The reason for this is not clear, but by reducing P, the amount of P segregated at the grain boundaries is reduced, grain boundary embrittlement is suppressed, and further, the amount of segregation of C increases because the grain boundary segregation of P is reduced. This is thought to increase the grain boundary strength. Preferably, P is 0.005% or less.

V, Nb, and Mo are both added as precipitation strengthening elements that supplement Ti and for the purpose of fixing C that contributes to the formation of cementite by this precipitation. However, if each content is less than 0.01%, this effect is insufficient. On the other hand, if the content exceeds 0.2%, Ti is supplemented when TiN is formed, contributing to the formation of coarse precipitates. Resulting in. Accordingly, the contents of V, Nb, and Mo are both 0.01% or more and 0.2% or less, but these are allowed to contain not only one type but also two or more types in the above range. Furthermore, these elements are required to satisfy the following formula (element names mean mass% of chemical components of each element).
Ti / 48 + V / 51 + Nb / 93 + Mo / 96-C / 12-N / 14-S / 32 ≧ 0
This formula is a well-known formula that prescribes that precipitate constituent elements such as Ti, V, Nb, and Mo are added as necessary to sufficiently fix C, N, and S in steel. Yes, this makes it possible to reduce the amount of cementite.

B segregates at the grain boundary and plays a role of supplementing the grain boundary strengthening. However, if the content is less than 0.0002%, this effect is insufficient. On the other hand, if the content exceeds 0.002%, workability such as ductility is deteriorated. Therefore, the B content is set to 0.0002% or more and 0.002% or less.
Ca and REM are elements that are detoxified by changing the form of non-metallic inclusions that become the starting point of destruction or deteriorate the workability. However, if less than 0.0005% is added, there is no effect even if Ca is added, and if Ca exceeds 0.02% and if REM exceeds 0.2%, the effect is saturated. Addition of 0005 to 0.02% and REM: 0.0005 to 0.2%.

Next, the reasons for limiting the production method of the present invention will be described below.
In this invention, when hot-rolling the slab obtained by casting the molten steel which adjusted the component so that it might become the target component content, the heating temperature shall be 1200 degreeC or more. This is because when the temperature is lower than 1200 ° C., precipitation constituent elements such as V, Nb, and Mo including Ti in the steel are not sufficiently re-dissolved, and thus precipitation strengthening becomes insufficient. The upper limit of the heating temperature is not particularly provided, but if it is 1400 ° C. or higher, the amount of scale-off increases and the yield decreases, so the reheating temperature is preferably less than 1400 ° C. In the present invention, in addition to the case where the above slab is cooled and reheated in a heating furnace, the slab may be directly sent to a hot rolling mill as it is.

In the hot rolling process, finish rolling is performed after finishing the rough rolling, but the finishing temperature needs to be finished in a temperature range equal to or higher than the Ar ₃ transformation point. This is because if the rolling temperature falls below the Ar ₃ transformation point during hot rolling, strain remains and ductility decreases. The upper limit of the finishing temperature is not particularly required to obtain the effect of the present invention, but it is desirable that the upper limit of the finishing temperature is 1000 ° C. or less because scale soot may be generated in operation.

  After finishing rolling, the steel sheet is cooled to a specified winding temperature, but the cooling rate is not particularly required to obtain the effect of the present invention. However, if the cooling rate is too slow, the size of precipitates that precipitate during cooling after hot rolling becomes coarse and does not contribute to the increase in strength due to precipitation strengthening, but it is hard like cementite, which is harmful to hole expandability. Since a harmful tissue such as a phase or pearlite may be generated, the lower limit of the cooling rate is preferably 20 ° C./s or more. On the other hand, by providing a low cooling speed region (15 ° C./second or less) such as air cooling for a short time (1 to 20 seconds) in the cooling zone in the ferrite formation region (about 500 to 850 ° C.) during cooling. Further, the cooling conditions such as promoting the ferrite transformation may be adjusted.

  If the coiling temperature is less than 450 ° C., precipitation after winding becomes insufficient, precipitation strengthening cannot be obtained, and solid solution C may remain in the steel, reducing workability, and hole expandability. Hard martensite, which is harmful to water, may occur. On the contrary, if the cooling rate exceeds 650 ° C., the same phenomenon as that which is a concern when the cooling rate is too slow, that is, not only does the precipitate size coarsen and contributes to the increase in strength due to precipitation strengthening, There is a possibility that a hard phase such as cementite harmful to spreadability and harmful tissues such as pearlite may be generated. Therefore, the coiling temperature is set to 450 to 650 ° C. The steel of the present invention has the same effect even if the surface is subjected to surface treatment (for example, galvanizing), and does not depart from the present invention.

The following examples further illustrate the present invention. Steels A to N having chemical components shown in Table 1 were melted in a converter and continuously cast to obtain a slab for hot rolling. However, the display about the chemical composition in a table | surface is the mass%. In Table 1, the value of the left side of the following formula, which is a requirement of the present invention, is also shown.
Ti / 48 + V / 51 + Nb / 93 + Mo / 96-C / 12-N / 14-S / 32 ≧ 0% These were reheated at the heating temperature (SRT) shown in Table 2, and after rough rolling, the finishing temperature (FT) was 1 After rolling to a plate thickness of 0.2 to 6.0 mm, each was wound at a winding temperature (CT). Cooled at 20 to 60 ° C./second from finish rolling to winding (No. D1 was cooled at 30 ° C./second to 720 ° C. after finishing rolling and air cooled at 8 ° C./second to 720 to 670 ° C. And then cooled to the coiling temperature at 30 ° C./second).

  The sample cut out from the 1/4 W position of the steel plate width of the hot-rolled sheet thus obtained was polished into a cross section in the rolling direction, etched with a Nital reagent, and observed at a magnification of 200 to 500 times using an optical microscope. If there is a structure name of the phase with the largest area ratio and a hard second phase, the structure name and each area ratio are displayed from the microstructure, and an image from the structure observed by etching the sample with the Picral reagent. The area ratio of cementite calculated by analysis is also shown in Table 2. The tensile properties were evaluated according to a test method described in JIS Z 2241 after processing the test material into a No. 5 test piece described in JIS Z 2201. The hole expansion test was evaluated according to the test method described in Japan Iron and Steel Federation Standard JFS T 1001-1996.

In addition, as described above, the occurrence of defects on the punched end face was determined by punching a 10 mm diameter hole in the same manner as in the hole expansion test with the clearance at the time of punching being 17 to 23% and visually observing the end face state. Table 2 shows the test results. A1 to J1 in accordance with the present invention and the reference examples have a tensile strength TS of 690 MPa or more, an elongation EL of 20% or more, and a hole expansion value λ of 90% or more. It is a rolled steel sheet, and there is no defect in the punched end face.

Other than the above is out of the scope of the present invention for the following reasons. That is, K1 is steel K, except for the P content, and other components and manufacturing conditions satisfy the requirements of the present invention, so that the tensile properties and hole expansibility are inferior to those of the present invention. All the punched edge defects occurred. On the other hand, L1 has a B addition amount of 38 ppm in the steel L , which exceeds the upper limit of the present invention, and as a result of the ferrite transformation being delayed, cementite is precipitated and the hole expandability is inferior.

M 1 is inferior in tensile strength to 690 MPa or less because the amount of C of steel M is small and outside the present invention . N1 does not satisfy the conditions of the component formula defined in the present invention in steel N. Particularly, N is small in Ti amount itself and is outside the scope of the present invention, but these all have a structure containing more than 3% cementite. In particular, the hole expandability is deteriorated .

In addition, as a steel component, C2 is one in which the characteristics are insufficient because the manufacturing conditions are out of the scope of the present invention. That is , in C2 , since the heating temperature SRT is low and outside the present invention, precipitate constituent elements such as Ti cannot be sufficiently dissolved, precipitation strengthening is insufficient, and A1 of the reference example condition and the inventive example C1 In comparison with the tensile strength TS, the amount of cementite is large and the hole expansibility is lowered .

Claims (4)

In mass%, C: 0.01 to 0.07%, Si: 0.77 to 2%, Mn: 0.05 to 3%, Al: 0.005 to 0.5%, N ≦ 0.005 %, S ≦ 0.005%, Ti: 0.03 to 0.2%, P ≦ 0.01%, B: 0.0002 to 0.002% , the balance being Fe and inevitable impurities a is a ferrite or bainitic ferrite structure and area ratio largest phase, 3% or less in the hard second phase and cementite area ratio, tensile strength der least 690MPa is, the clearance at the time of punching A high-strength hot-rolled steel sheet excellent in stamping workability, characterized in that it has 17 to 23% and has no defects in end face properties and cracks . Furthermore, it contains one or more of V: 0.01 to 0.2%, Nb: 0.01 to 0.2%, Mo: 0.01 to 0.2% in mass%, and The high-strength hot-rolled steel sheet having excellent punchability according to claim 1, wherein Ti / 48 + V / 51 + Nb / 93 + Mo / 96-C / 12-N / 14-S / 32 ≧ 0% is satisfied. Furthermore, it contains the 1 type or 2 types of Ca: 0.0005-0.02% and REM: 0.0005-0.2% in the mass%, The punching of Claim 1 or 2 characterized by the above-mentioned. High-strength hot-rolled steel sheet with excellent workability. A method of manufacturing a steel sheet according to any one of claims 1 to 3 upon hot rolling of the steel strip having components according to any one of claims 1 to 3 hot rolling heating temperature Is a high strength hot-rolled steel sheet excellent in punching processability, characterized in that the hot finish rolling is finished at an Ar ₃ transformation point or higher, and then rolled up at 450 to 650 ° C.