JP5222711B2 - Medium and high carbon steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a medium-high carbon steel plate subjected to spheroidizing annealing, a medium-high carbon steel plate before the annealing, and a method for producing the latter medium-high carbon steel plate.

  Cementite spheroidization in medium and high carbon steel sheets and workability and hardenability of spheroidized steel sheets are extremely important characteristics.

Non-Patent Documents 1 to 3 below report that spheroidizing annealing is promoted by making the structure before spheroidizing annealing of medium and high carbon steel into a fine ferrite / pearlite structure or bainite structure. As shown in Non-Patent Document 4, the controlled rolling technique for making the microstructure before spheroidizing annealing into a fine ferrite / pearlite structure is mainly a hot steel bar for the purpose of improving cold forgeability after spheroidizing annealing. Although it has been put into practical use in rolling, it has not been put into practical use in hot thin plate continuous rolling. Concerning hot thin plate continuous rolling, as in Non-Patent Document 5, a bainite structure (but not a fine one) is built by rapid cooling in the cooling bed after finish rolling to improve workability after spheroidizing annealing. There are reports.
Matsumoto Chieto, Katagiri Yukio, Shinoda Kenichi: Iron and Steel, 67 (1981) 204. Tadazo Sudo, Kenji Aihara, Susumu Kamihara: Iron and Steel, 69 (1983) 357. Toshiyuki Hoshino, Takuhisa Tabata, Kimio Mine: Iron and Steel, 70 (1984) 210. Hiroshi Kanezuki, Masaaki Katsumata, Yuji Sawada: Iron and Steel, 76 (1990) 73. Satoshi Fujita: Doctoral dissertation on “Improvement of workability by microstructure control of high carbon steel sheet”, Nagaoka University of Technology (2006) JP 59-229413 A1 JP, 2002-273501, A Claim 9 and Table 5

  Non-Patent Document 5 reports that, as described above, workability after spheroidizing annealing is improved by building a bainite structure in hot thin plate continuous rolling. However, since the technique of this document performs spheroidizing annealing based on a bainite structure which is not fine, there is a disadvantage that spheroidizing of cementite is slowed and the spheroidizing annealing time is extended. For this reason, in reality, even after the spheroidizing annealing, the steel sheet may not exhibit sufficient workability and hardenability.

Further, Patent Document 1 discloses a method for producing fine-grained ferritic steel. However, when the rolling reduction shown in this method is performed at a rolling reduction of 40% or more × 2 stages, the strain per stage is reduced. Since it is too large, a remarkable processing heat generation may occur, which may be a problem.
Furthermore, Patent Document 2 discloses a method for producing fine-grained steel that is rolled so that the cumulative strain is 0.9 or more. However, in Patent Document 2, a ferrite grain size of 3.5 μm is the minimum. Only things have been obtained.

  The present invention provides a spheroidized annealed medium-high carbon steel sheet imparted with high workability and hardenability by uniformly dispersing fine cementite, and also a suitable medium-high carbon steel sheet before spheroidizing annealing and a method for producing the same Is to provide.

(1) The medium-high carbon steel sheet of the present invention is a medium-high carbon steel sheet containing C = 0.14 to 0.85% and subjected to spheroidizing annealing, and has an average crystal grain size of 0.6 μm or less. A cementite having a particle size of 4.0 μm or less has an average center-to-center distance λ of (1.2−0.3 × C) μm or less (C on the left indicates a carbon content ratio), and the standard deviation σ of the center-to-center distance Is dispersed so as to be equal to or less than (0.6 × λ) μm.
Further, the chemical components of the medium and high carbon steel sheet of the present invention are C: 0.14-0.85 mass%, Si: 0.01-1.00 mass%, Mn: 0.10-2.00 mass%, P It is preferable that ≦ 0.04% by mass, S ≦ 0.03% by mass, Al: 0.002 to 0.08% by mass, with the balance being iron and inevitable impurities.

  A steel plate in which fine cementite is uniformly dispersed as described above in medium and high carbon steel sheets has the effect of improving the workability by slowing the generation of voids and void extension during processing (for example, Masahisa Nakamura and Masami Iida: Iron And steel, 61 (1975) 349, Satoshi Inoue and Shuji Kinoshita: Plasticity and processing, 14 (1973), 291), exhibiting excellent workability and hardenability. The inventors have recently developed a medium and high carbon steel sheet in which the particle size of cementite and its dispersion state are particularly defined as described above, and such a steel sheet can be produced commercially and has exceptional workability and quenching. It has been found that it exerts sex.

Hereinafter, the reasons for limiting the chemical components of the steel sheet of the present invention will be described.
C is an element necessary for increasing the strength of the steel sheet and for ensuring the strength after quenching. For this reason, the lower limit of the C content is set to 0.14%. On the other hand, since the workability decreases as the content increases, the upper limit of the C content is set to 0.85% by mass. A more preferable lower limit of the C amount is 0.20%, and a more preferable upper limit of the C amount is 0.70%.
Si is an element that increases the strength of the steel sheet. However, if its content is high, the workability tends to decrease, the carbide tends to be graphitized and the hardenability is impaired, and it is also the cause of the surface quality defects of the steel sheet. Therefore, the upper limit of the amount of Si was set to 1.00% by mass. On the other hand, from the viewpoint of cost required for reduction, the lower limit of the Si amount is set to 0.01%. A more preferable lower limit of the Si amount is 0.10%, and a more preferable upper limit of the Si amount is 0.25%.

Mn is an element that increases the strength of the steel sheet and increases the hardenability. Moreover, it has the effect | action which fixes S which exists as an impurity in steel as MnS, and prevents hot brittleness. Therefore, the lower limit was made 0.10% by mass. On the other hand, if the content is high, the workability is lowered, so the upper limit of the Mn amount is 2.00% by mass. A more preferable lower limit of the amount of Mn is 0.60%, and a more preferable upper limit of the amount of Mn is 0.80%.
P is an element that enhances the strength of the steel sheet, but if contained, the workability, impact property, and weldability may be deteriorated, and the upper limit of the P content is set to 0.04% by mass. Moreover, the minimum with preferable P amount is 0.010%, and the more preferable upper limit of P amount is 0.025%.
S is an element that causes hot brittleness and at the same time becomes an inclusion in the steel as MnS and TiS, and lowers the workability of the steel sheet. Therefore, it is desirable that S be as low as possible, and the upper limit of the amount of S is 0.03% by mass. It was. A preferable upper limit of the amount of S is 0.010%, and a lower limit of the amount of S is, for example, 0.001%.

  Al is an element used as a deoxidizing material, and also has a role of fixing N in steel as AlN, so the lower limit of the amount of Al was set to 0.002% by mass. On the other hand, if the content is increased, it can cause surface quality defects of the steel sheet, so the upper limit of the Al content was set to 0.08 mass%. A more preferable lower limit of the Al amount is 0.010%, and a more preferable upper limit of the Al amount is 0.035%.

(2) Moreover, in addition to the steel as described in (1), the medium-high carbon steel sheet of the present invention includes at least one of Cr: 0.05 to 2.00% by mass or Mo: 0.05 to 0.50% by mass. May be included.

Cr is an element that increases the strength of the steel sheet and increases the hardenability. For this reason, the lower limit of the Cr amount was set to 0.05% by mass. On the other hand, if the content is high, the workability deteriorates, so the upper limit of the Cr content was 2.00% by mass. A more preferable lower limit of the Cr content is 0.04%, and a more preferable upper limit of the Cr content is 1.00%.
Mo, like Cr, is an element that increases the strength of the product and increases the hardenability. For this reason, the lower limit of the Mo amount was set to 0.05% by mass. On the other hand, when the content is increased, the workability is lowered, so the upper limit of the Mo amount is set to 0.50% by mass. A more preferable upper limit of the Mo amount is 0.20%.

(3) Further, the medium and high carbon steel sheet of the present invention may contain Ti: 0.005 to 0.07 mass% and B: 3 to 60 ppm in addition to the steel steel described in (1) and (2). is there.

B is an element that can improve the hardenability. For this reason, the lower limit of the amount of B is set to 3 ppm. On the other hand, when the content is increased, the toughness after the heat treatment is lowered, so the upper limit of the B amount was set to 60 ppm. A more preferable lower limit of the B amount is 1 ppm, and a more preferable upper limit of the B amount is 20 ppm.
In the steel containing B, Ti is added in order to make this B act effectively on the hardenability. Therefore, the lower limit of Ti is set to 0.005% by mass. On the other hand, since the workability decreases when the Ti content increases, the upper limit is set to 0.07% by mass.

  In addition, it is desirable to reduce the amount of elements inevitably mixed such as O, N, Cu, Ni, Nb, V, Ca, Zr, and Mg in order to improve workability and impact characteristics.

Next, the medium-high carbon steel sheet before spheroidizing annealing is composed of a ferrite / pearlite structure having a ferrite grain size of 3 μm or less and a pearlite lamellar spacing of 0.1 μm or less, or a bainite packet size of 3 μm or less. What consists of a certain bainite structure is good. If the steel sheet having such a structure, subjected to annealing temperature _{(Ac 1 -150) ℃ ~Ac 1} ℃, ( annealing × 20 hours, for example, 690 ℃ ~710 ℃) spheroidizing annealing at conditions such annealing time 2~100hr As a result, the steel sheet is obtained by uniformly dispersing fine cementite as described above.

The medium and high carbon steel sheet before spheroidizing annealing is
i) C = 0.14 to 0.85%, Si = 0.01 to 1.00%, Mn = 0.10 to 2.00%, P ≦ 0.04%, S ≦ 0.03%, Al = 0.002 to 0.08% included, the balance being steel and inevitable impurities (such as slabs that are intermediate products manufactured in the steelmaking stage)
ii) Hot-strip continuous rolling from a surface temperature of 1100 ° C or higher,
iii) During the rolling, in each of the final three stages of finish rolling (rolling machine in the latter three stages), the rolling is performed so that the strain rate is 60 / s or more and the reduction rate is 30% / stage or more, Immediately after each stand, water cooling is performed so that the temperature drop of the rolled material becomes 30 ° C./step or more, and the final finish rolling temperature is 800 ° C. or less (Ar ₃ +30) ° C. or more (Ar ₃ is a transformation from austenite to ferrite). By referring to the temperature), low temperature large rolling is performed to accumulate the strain,
iv) The cooling rate between the finishing temperature after finishing rolling (finishing temperature−50) ° C. is 50 ° C./second or more, and the winding temperature is 400 to 650 ° C.,
It is good to manufacture by such a method. As a result, a medium-high carbon steel sheet having a fine ferrite / pearlite structure or a fine bainite structure as described above can be produced smoothly. By spheroidizing annealing, a medium-high carbon steel sheet in which the fine cementite is uniformly dispersed is obtained. A cold rolling process may be further included before and after the spheroidizing annealing.

The spheroidized annealed medium-high carbon steel sheet of the present invention has a uniform dispersion of fine cementite, so spheroidizing annealing is reliably performed in a relatively short time, and void generation / extension in the processed / deformed part is achieved. , Exhibiting excellent workability and fatigue characteristics, and an excellent fracture surface shape can be obtained during punching.
Furthermore, the finely dispersed cementite dissolves relatively easily in austenite and improves hardenability, and is particularly suitable for induction hardening that must be performed in a short time.
Moreover, according to the steel plate before spheroidizing annealing and the manufacturing method thereof according to the present invention, the preferred spheroidizing annealed medium-high carbon steel plate as described above can be easily manufactured.

  Embodiments of the invention will be introduced. The manufacturing process of the steel sheet according to the invention is as follows. The steps in parentheses () are optional and may be included, but are not performed in the examples shown below.

  Heating furnace → rough rolling → finish rolling → water cooling zone → winding → (cold rolling) → spheroidizing annealing → (cold rolling / softening annealing)

The results of the components are shown in Table 1. Steel type A is S45C of JIS standard, Steel B is SAE1070 of JIS standard, Steel C is S35C of JIS standard, Steel D is a component of JIS S35C with lower S, Steel type E is S22CB of JIS standard, steel type F has components corresponding to SCM420 defined by JIS. For each steel type, there is no addition of special elements other than those shown in Table 1, and the balance is iron and inevitable impurities. The steel types A to F were each made into 200 mm thick slabs with a thickness of 40 mm by 7 passes in rough rolling, and then subjected to 6 passes of finish rolling under conditions 1 to 3 in Table 2 to obtain 3 mm thick hot rolled steel sheets. Table 2 shows the production history of Condition 1 (Comparative Example), Condition 2 (Example (Ferrite / Pearlite)), and Condition 3 (Example (Bainite)). Incidentally Ar ₃ temperature of the steel type A of component 705 ° C., Ar ₃ temperature of the steel type B is 623 ° C., Ar ₃ temperature of the steel type C is 717 ° C., Ar ₃ temperature of the steel type D is 718 ° C., Ar ₃ temperature of the steel type E Is 673 ° C.

Here, with regard to F4, F5, and F6, which are the final three stages of finish rolling in hot sheet continuous rolling, the strain rate is set to 60 / s or more, the rolling reduction is set to 30% or more, and immediately after each stand, the temperature is 30 ° C. or more. Describe the reason for water cooling.
In order to build a fine ferrite / pearlite or fine bainite structure in thin sheet hot rolling, accumulation of strain is required. However, in F1, F2, and F3, which are three stages before finish rolling, strain is released and does not accumulate due to recovery and recrystallization between stands, and strain is accumulated only in F4, F5, and F6, which are the last three stages of finish rolling. (For example, Keiji Morimoto, Ichiro Takeshi, Takaro Kurahashi, Jun Yanagimoto: Iron and Steel 88 (2002), 747.).

However, when the strain rate is slow, the strain rate is set to 60 / s or more because the strain is released between the stands of the final three stages of finish rolling as well as the three stages before finish rolling. According to a conventional report (Japanese Patent Laid-Open No. 59-229413, Hiroshi Yada et al.), When the strain rate is high, a significant processing heat is generated and the fine particles are not formed, so that the strain rate is 60 / s or less. Is different. In this production method, water cooling at 30 ° C. or higher is performed immediately after each of the final three stages of finish rolling to absorb the processing heat and accumulate strain. The rolling reduction is 30% or more. However, if the rolling reduction is less than 30%, sufficient strain does not accumulate and the structure after hot rolling does not become finer. The reason for setting the finishing temperature to Ar ₃ + 30 ° C. to 800 ° C. is that at a temperature of 800 ° C. or higher, strain does not accumulate due to recovery and recrystallization, and a fine structure is not formed. Further, when the finishing temperature is set to Ar ₃ transformation point + 30 ° C. or lower, a mixed grain structure is formed and workability is impaired.

  Table 3 shows the results of measuring the grain size, etc., of ¼ part in the thickness direction of the cross section in the rolling direction of the steel sheet by JIS G0551 steel-crystal grain size microscopic test method and Electron Back Scattering Pattern. FIG. 1 shows a comparative example, and FIGS. 2 and 3 show structural photographs of the example. Table 3 and FIGS. 1 to 3 relate to steel plates after finishing rolling (before spheroidizing annealing). Here, the ferrite grain size was measured by a microscope test method of JIS G0551 steel-crystal grain size after nital corrosion. The bainite grain size was measured with a structure analysis software “OEM ver4.0” manufactured by TSL, with large angle grain boundaries identified by Electron Back Scattering Pattern.

  Next, spheroidizing annealing was performed for each steel type. FIG. 4 shows a structure photograph after spheroidizing annealing in each of the comparative example and FIG. 5 and FIG. 6, and Table 5 shows the measurement results of the standard deviation of the cementite particle size, the cementite center distance, and the cementite center distance. Show. The spheroidizing annealing conditions for steel types A, C and D were a comparative example and an example, and an annealing temperature of 690 ° C. and an annealing time of 20 hours, and for steel type B, an annealing temperature of 700 ° C. and an annealing time of 20 hours for both comparative examples and examples. For steel types E and F, the annealing temperature was 710 ° C. and the annealing time was 20 hours in both the comparative example and the example. The distance between the centers of cementite was converted to Voronoi polygons using the image analysis software “Image Pro Plus ver.9.0” made by Media Cybernetics, and each Voronoi polygon was converted to a circle of the same area. The diameter was measured. A Voronoi polygon is a polygon formed by connecting a line passing through the middle of two adjacent points (Voronoi boundary line), and is also referred to as a “Dirichlet region”, “Wigner Deutsches cell”, or “Tiessen diagram”.

Table 5 shows the spheroidizing annealing conditions, the hole expansion ratio after spheroidizing annealing, and the laser quenching depth. The hole expansion test was performed by punching with a punching tool having a punch diameter of d ₀ 10 mm and a die diameter of 12 mm and then performing a hole expansion test. The hole diameter d ₁ (mm) at the time when the thick through crack occurred was measured to obtain the hole expansion ratio λ (%). However, the hole expansion ratio λ (%) is
λ = 100 × (d ₁ −d ₀ ) / d ₀
It is what I asked for. Laser hardenability is the result of quenching using a direct semiconductor laser under conditions of an output of 600 W and 500 mm / min.
As shown in Table 5, in the medium and high carbon steel plates of the examples, both the hole expansibility and the hardenability were greatly improved.

  Fig. 7 and Fig. 8 show the relationship between the carbon concentration and the average distance between cementite centers and the relationship between the average distance between cementite centers and the standard deviation of the distance between cementite centers. Each is shown. Unlike the comparative example, the medium-high carbon steel plate as an example has an average center-to-center distance λ of (1.2−0.3 × C) μm or less, and a standard deviation σ of the center-to-center distance is (0. 6 × λ) μm or less.

FIG. 1 is a photomicrograph of a medium and high carbon steel sheet (Comparative Example) of steel type A after finishing rolling (before spheroidizing annealing) finish-rolled under condition 1. FIG. 2 is a micrograph of a medium and high carbon steel plate (Example) of steel type A after finish rolling (before spheroidizing annealing) finish-rolled under condition 2. FIG. 3 is a microstructural photograph of a medium-high carbon steel sheet (another example) of steel type A after finishing rolling (before spheroidizing annealing) finish-rolled under condition 3. FIG. 4 is a micrograph of a medium and high carbon steel sheet (Comparative Example) of steel type A that has been finish-rolled under condition 1 and spheroidized annealing. FIG. 5 is a micrograph of a medium and high carbon steel sheet (Example) of steel type A that was finish-rolled under condition 2 and spheroidized and annealed. FIG. 6 is a micrograph of a medium- and high-carbon steel plate (another example) of steel type A that has been finish-rolled under condition 3 and spheroidized and annealed. FIG. 7 is a graph showing the relationship between the carbon concentration and the average distance between cementite centers for medium and high carbon steel sheets of steel type A after spheroidizing annealing. FIG. 8 is a graph showing the relationship between the average distance between cementite centers and the standard deviation of the distance between cementite centers for medium and high carbon steel sheets of steel type A after spheroidizing annealing.

Claims (10)

  C = 0.14 to 0.85%, Si = 0.01 to 1.00%, Mn = 0.10 to 2.00%, P ≦ 0.04%, S ≦ 0.03%, Al = 0 0.002 to 0.08%, the balance being iron and inevitable impurities, and a spheroidizing annealed medium-high carbon steel sheet having an average grain size of 0.6 μm or less and a maximum grain size of 4. The cementite of 0 μm or less is dispersed so that the average center distance λ is (1.2−0.3 × C) μm or less and the standard deviation σ of the center-to-center distance is (0.6 × λ) μm or less. A medium-high carbon steel sheet characterized by   Furthermore, the medium-high carbon steel plate of Claim 1 containing 1 type or 2 types of Cr = 0.05-2.00% or Mo = 0.05-0.50%.   Furthermore, the medium-high carbon steel plate of Claim 1 or Claim 2 containing Ti = 0.005-0.07% and B = 3-60 ppm.   C = 0.14 to 0.85%, Si = 0.01 to 1.00%, Mn = 0.10 to 2.00%, P ≦ 0.04%, S ≦ 0.03%, Al = 0 0.002% to 0.08%, the balance being iron and inevitable impurities, and a ferrite / pearlite structure having a ferrite grain size of 3 μm or less and a pearlite lamellar spacing of 0.1 μm or less, or bainite An unannealed medium-high carbon steel sheet for spheroidizing annealing, comprising a bainite structure having a packet size of 3 μm or less.   Furthermore, the non-annealed medium-high carbon steel plate for spheroidizing annealing of Claim 4 containing 1 type or 2 types of Cr = 0.05-2.00% or Mo = 0.05-0.50%.   The unannealed medium-high carbon steel sheet for spheroidizing annealing according to claim 4 or 5, further comprising Ti = 0.005 to 0.07% and B = 3 to 60 ppm. A method for producing an unannealed medium-high carbon steel sheet for spheroidizing annealing according to claim 4,
C = 0.14 to 0.85%, Si = 0.01 to 1.00%, Mn = 0.10 to 2.00%, P ≦ 0.04%, S ≦ 0.03%, Al = 0 .002% to 0.08%, the balance being steel and unavoidable impurities, and continuously rolling a thin steel sheet from a surface temperature of 1100 ° C. or higher,
At the time of the rolling, in each of the final three stages of the finish rolling, rolling is performed so that the strain rate is 60 / s or more and the rolling reduction is 30% / stage or more, and the temperature drop of the rolled material is immediately after each of the stands. Water cooling is performed at 30 ° C./stage or more, the final finish rolling temperature is 800 ° C. or less (Ar ₃ +30) ° C. or more, and the cooling rate between the finish temperature after finish rolling (finishing temperature−50) ° C. is 50 ° C. The manufacturing method of the non-annealed medium-high carbon steel plate for spheroidizing annealing characterized by the above-mentioned.   Furthermore, manufacture of the non-annealed medium-high carbon steel plate for spheroidizing annealing of Claim 7 containing 1 type or 2 types of Cr = 0.05-2.00% or Mo = 0.05-0.50%. Method.   Furthermore, the manufacturing method of the unannealed medium-high carbon steel plate for spheroidizing annealing of Claim 7 or Claim 8 containing Ti = 0.005-0.07% and B = 3-60 ppm. Claims 4 to the medium and high carbon steel unannealed of claim 6 temperature _{(Ac 1 -150) ℃ ~Ac 1} ℃, method for producing medium and high carbon steel sheet annealing spheroidizing at time 2～100Hr.