JP2018141184A - Carbon steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a carbon steel plate as a work piece that can reduce variations in the heights of vertical walls of a deep-drawing product.SOLUTION: A carbon steel plate contains, in mass%, C: 0.15-2.0%, Si: 0.40% or less, Mn: 0.5% or less, P: 0.03% or less, S: 0.03% or less, Cr: 2.0% or less with the balance being Fe and inevitable impurities, where, carbides are dispersed in ferrite so that the spheroidized carbide rate is 90% or more and the average carbide grain size is 0.4 μm or more, and the anisotropy Δr is -1.0 to 1.0.SELECTED DRAWING: None

Description

  The present invention relates to a carbon steel sheet characterized by the anisotropy of the Rankford value.

  Carbon steel sheets with a C content of approximately 0.1 to 2.0% by mass in steel can be hardened and have a certain degree of workability in the annealed state. Widely used as a material for bearing parts. In manufacturing parts, generally, punching and bending are performed, and relatively mild drawing and stretch flange molding may be performed. Further, when the part shape is complicated, it is often produced by welding two or three parts. These processed parts are finished into parts for various uses through heat treatment.

  However, in recent years, in order to reduce the manufacturing cost of parts, the integral molding of parts and the simplification of parts processing have been promoted. This means that it must withstand processing with a higher processing rate (= a large amount of plastic deformation) when viewed from the material side. In other words, with the advancement of processing technology, higher workability has been required for the carbon steel sheet itself. Particularly in recent years, there is an increasing need for steel plate materials that can withstand not only punching and bending, but also advanced deep drawing.

Japanese Examined Patent Publication No. 4-56088 discloses a method for producing a high carbon cold-rolled steel sheet having good drawability. In this manufacturing method, cold rolling and annealing treatment are performed on steel whose chemical components are regulated within a specific range to graphite cementite in the steel, and then cold rolling and recrystallization annealing are further performed. It is described that, by subjecting a steel sheet graphitized with cementite to cold rolling and annealing, a high carbon steel sheet having a high r value that has not been obtained in the past and having deep drawability comparable to that of a mild steel sheet can be obtained. However, this method requires the addition of a specific element for graphitization, and in addition, the manufacturing process is long, resulting in high costs.
Japanese Patent Laid-Open No. 11-61272 improves the r value of high carbon steel mainly composed of ferrite + cementite by annealing and cold rolling a high carbon hot rolled steel sheet having a bainite structure having a specific composition. Is disclosed. In this method, in addition to the addition of a specific element, in order to obtain a bainite structure in hot rolling, winding at a low temperature is necessary, and the productivity is poor. In any case, since specific additive elements are required, these techniques are not widely applicable to the production of general medium and high carbon steel types.

  Further, when deep drawing is performed, the difference (rank difference) in the Rankford value (r value) in the 45 ° direction with respect to the rolling direction of the steel sheet, the sheet width direction perpendicular to the rolling direction, and the rolling direction is different. If the directionality is large, the height of the workpiece vertical wall differs after drawing, and it is necessary to cut the vertical wall to make the height uniform. Therefore, there is a problem that the material cost and the process cost increase.

Japanese Patent Publication No. 4-56088 JP 11-61272 A

  The present invention has been devised to solve such problems, and aims to obtain excellent anisotropy that can be expressed even in general carbon steel sheets without adding special elements. To do.

Therefore, the present invention provides a carbon steel sheet having excellent anisotropy in terms of mass%, C: 0.15 to 2.0%, Si: 0.40% or less, Mn: 0.5% or less, P: 0. 0.03% or less, S: 0.03% or less, Cr: 2.0% or less, with the balance being Fe and inevitable impurities, and a carbide spheroidization rate defined by (a) below is 90% or more In addition, the carbides are dispersed in the ferrite so that the average carbide particle size defined in the following (b) is 0.4 μm or more, and the anisotropy Δr defined in the following (c) is -1. It is characterized by being 0-1.0.
(A) Carbide spheroidization ratio: In the observation of the metal structure of the cross section of the steel sheet, the ratio (p / q) of the maximum length p of carbide to the maximum length q in the direction perpendicular to the total number of carbides in the observation field is less than 3. The ratio (%) of the number of carbides. However, the observation visual field is a region where the total number of carbides is 300 or more.
(B) Average carbide particle size: A value obtained by averaging the equivalent circle diameters measured for individual carbides in the observation field in the observation of the metal structure of the cross section of the steel sheet for all the measured carbides. However, the observation visual field is a region where the total number of carbides is 300 or more.
(C) Regarding the Rankford value (r value), the rolling direction (r ₀ ) of the steel sheet, the sheet width direction (r ₉₀ ) perpendicular to the rolling direction, and the 45 ° direction (r ₄₅ ) with respect to the rolling direction Is calculated from the relational expression consisting of ((r ₀ + r ₉₀ ) / 2) −r ₄₅ .

  Furthermore, it is also characterized by having a component composition containing Ni: 0.3% or less, Cu: 0.3% or less, and Mo: 1.0% or less in mass%.

  According to the present invention, there is an effect that variation in height of the vertical wall portion of the deep-drawn product can be suppressed.

  The present inventors have studied in detail the means for improving the anisotropy of a steel sheet in a general carbon steel type. As a result, it was found that (1) it was not possible to improve the anisotropy by simply spheroidizing the carbide, and (2) the anisotropy was influenced by the dispersion form of the carbide in the steel sheet. . Specifically, it has been found that improvement can be achieved by further spheroidizing carbides and dispersing carbides having a certain average particle size or more in ferrite.

  Hereinafter, matters for specifying the present invention will be described.

  In this invention, C: Carbon steel containing 0.15-2.0 mass% is made into object. C is an alloy element which is the most basic in carbon steel, and the quenching hardness and the amount of carbide vary greatly depending on its content. Steel with a C content of less than 0.15% by mass cannot provide sufficient quenching hardness when applied to various machine structural parts. On the other hand, if the C content exceeds 2.0% by mass, the toughness after hot rolling is lowered and the manufacturability and handleability of the steel strip are deteriorated, and sufficient ductility cannot be obtained even after annealing. This makes it difficult to apply to parts with a high degree of processing. Therefore, in the present invention, steel with a C content in the range of 0.15 to 2.0% by mass is targeted from the viewpoint of providing a raw steel plate having appropriate quenching hardness and workability.

When Si is added excessively, the ferrite is hardened by the solid solution strengthening action, which causes cracks during molding. Further, when the Si content is increased, scale flaws tend to be generated on the surface of the steel sheet during the production process, leading to a reduction in surface quality. Therefore, when adding Si, the content is made 0.4% by mass or less.
Mn is an additive element that enhances the hardenability of the steel sheet and is effective for toughening, and it is preferable to have a content of 0.10% by mass or more. In order to prevent sufficient hardenability and deterioration of workability due to ferrite hardening, the content is 0.5% by mass or less.

Since P deteriorates ductility and toughness, the content is set to 0.03% by mass or less.
S is an element that forms MnS inclusions. Since deep drawability deteriorates when the amount of inclusions increases, it is desirable to reduce the S content in the steel as much as possible. If the carbide dispersion form prescribed | regulated by this invention is implement | achieved, the improvement effect of deep drawability will be acquired also with respect to the general commercial steel which has not reduced especially S content. However, even when the C content increases to nearly 0.40 mass%, the S content is set to 0.03 mass% or less in order to ensure high deep drawability.

  Cr is an element that improves hardenability and increases temper softening resistance. However, if a large amount of Cr exceeding 2.0% by mass is contained, it is difficult to soften even if three-stage annealing is performed, and press formability and workability before quenching deteriorate. Therefore, when adding Cr, it is made into the range of 2.0 mass% or less.

Mo contributes to the improvement of hardenability and temper softening resistance in the same manner as Cr when added in a small amount. However, if a large amount of Mo exceeding 1.0% by mass is contained, it is difficult to soften even if three-stage annealing is performed, and press formability and workability before quenching deteriorate. Therefore, when adding Mo, it is set as 1.0 mass% or less.
Cu improves the surface properties of the steel sheet because it improves the peelability of the oxide scale produced during hot rolling. However, if the content exceeds 0.3% by mass, fine cracks are likely to occur on the surface of the steel sheet due to the embrittlement of the molten metal. Therefore, when Cu is contained, the range is 0.3% by mass or less.
Ni is an alloy component that improves hardenability and prevents low temperature brittleness. In addition, since Ni has an action to counteract the adverse effect of molten metal embrittlement which is a problem due to the addition of Cu, when adding about 0.2 mass% or more of Cu, the same amount of Ni as the amount of added Cu should be added. Is extremely effective. However, if a large amount of Ni exceeding 0.3% by mass is contained, it is difficult to soften even if three-stage annealing is performed, and press formability and workability before quenching deteriorate. Therefore, when adding Ni, it is set as the range of 0.3 mass% or less.

  Next, the matter for specifying the metal structure of the steel sheet of the present invention will be described.

[Carbide spheroidization rate]
The carbide spheroidization rate is as defined above, which represents how much of all carbides are considered to be “spheroidized carbides”. Here, as a condition for a certain carbide to be regarded as “spheroidized carbide”, the ratio of the maximum length p of the carbide to the maximum length q in the direction perpendicular thereto in the metallographic observation plane of the cross section of the steel sheet (p / Q) was less than 3. For example, most of the carbides in recycled perlite have the above ratio (p / q) of 3 or more. On the other hand, in the carbide grown starting from the undissolved carbide remaining after heating at the A _C1 point or higher, the ratio (p / q) is less than 3.

  It is difficult to accurately determine and define the shape of the carbide three-dimensionally, and it is complicated to determine the suitability of the product steel plate. On the other hand, it is easy to observe the planar metal structure of the cross section of the steel plate. When the present inventors grasped the degree of spheroidization using the ratio of p and q (p / q) as described above for the carbide shape observed in the metal structure of the steel sheet cross section, the influence of the carbide shape. It was confirmed that can be evaluated appropriately. As a result of various experiments, the number of “spheroidized carbides” having the above ratio (p / q) of less than 3 accounts for 90% or more of the total number of carbides, and an average carbide described later. It was found that the steel sheet exhibits high anisotropy when the distance falls within a specific range. In order to increase the reliability of the numerical value, the observation field of view is a region where the total number of carbides is 300 or more.

[Average carbide particle size]
The average carbide particle size is a value obtained by averaging the equivalent circle diameters measured for individual carbides within the observation field for all the measured carbides in the observation of the metal structure of the cross section of the steel sheet. Specifically, the area of each carbide is measured, and the equivalent circle diameter is calculated from the area. The area can be measured using an image processing apparatus. And the sum total of the circle equivalent diameter of all the measured carbide | carbonized_materials is calculated | required, and the value which remove | divided the sum total with the total number of measurement carbide | carbonized_materials is made into an average carbide particle diameter. In order to increase the reliability of the numerical value, the observation visual field is an area where the total number of measured carbides is 300 or more.

  As a result of experiments by the present inventors, it was found that, from the viewpoint of anisotropy, the above-mentioned carbide spheroidization ratio should be 90% or more, and the average carbide particle size should be 0.4 μm or more. . Thereby, anisotropy ((DELTA) r) can be made into the range of -1.0-1.0.

A steel sheet having the above metal structure can be obtained by devising an annealing method. For example, it can be realized by annealing appropriately combining heating in a specific temperature range immediately below and immediately above the A ₁ transformation point of the steel sheet. Specifically, for example, with respect to hot-rolled steel sheet or cold-rolled steel sheet, after the heat of the first stage which holds more than 0.5 hours at a temperature range of less than _{A C1 -50 ℃ ~A C1, A} C1 ~ The second stage heating is maintained for 0.5 to 20 hours in the temperature range of A _C1 + 100 ° C., and the third stage heating is maintained for 2 to 60 hours in the temperature range of A _r1 −80 ° C. to A _r1. And the steel plate which has the appropriate metal structure prescribed | regulated by this invention by giving the 3 stage annealing which makes the cooling rate from the 2nd stage holding temperature to the 3rd stage holding temperature 5-30 degrees C / hour. It can manufacture suitably.

Steels having chemical compositions shown in Table 1 were melted. The quenching hardness in the table indicates the hardness when the test material is kept as it is at 900 ° C. for 5 minutes and then water-quenched.
In Table 1, steel type A had a low C content of 0.07% by mass, so the hardness after quenching was low, and the hardness required for machine parts could not be obtained. About the steel plate except steel grade A, hot rolling which changed various hot rolling coil winding temperature was performed, and the hot rolling structure was changed. The obtained hot-rolled steel sheet was subjected to cold rolling and annealing under various conditions after pickling, thereby changing the ferrite grain boundary presence rate and texture of the carbide of the steel sheet. Then, it used for the tension test and measured (DELTA) r value.

The carbide spheroidization rate is determined by observing the inside of a certain area of the cross section of the steel sheet with a scanning electron microscope, and the ratio (p / q) between the maximum length p of carbide and the maximum length q in the perpendicular direction is less than 3. Counting as “spheroidized carbide”, the ratio of the number of “spheroidized carbides” in the total number of measured carbides was calculated. At that time, the total number of measured carbides was in the range of 300 to 1000.
Further, an average carbide particle size D was determined for the region where the carbide spheroidization ratio was measured using an image processing apparatus (manufactured by Nireco Corporation, LUZEX III U).

The tensile test was performed by creating three JIS No. 5 tensile test pieces of L (rolling direction), D (45 degrees with respect to the rolling direction), and T (90 degrees with respect to the rolling direction) between the parallel part marks. The distance was 50 mm and the plate thickness was 1.0 mm. In the tensile test, a tensile elongation of 15% was given, the plate width between the gauge points at that time was measured, and the r value was calculated by the following formula.
r = ln (Wo / Wx) / ln (LxWx / LoWo)
Here, Wo and Lo are the plate width and the distance between the gauge points before the test, and Wx and Lx indicate the sheet width and the distance between the gauge points after giving 15% tensile elongation.

  These test results are shown in Table 2.

  In Table 2, since the steel plate of the comparative example had an average carbide of less than 0.4 μm, the anisotropy (Δr) of the steel plate was increased. In contrast, the average carbide particle size in the examples was 0.4 μm or more, and the anisotropy (Δr) was also in the range of −1 to 1.

In this invention, the carbon steel plate excellent in anisotropy was implement | achieved by controlling a carbide | carbonized_material spheroidization rate, an average carbide | carbonized_material particle size, and a dispersion state to carbide | carbonized_material. This steel sheet can be suitably used as a material for various machine parts such as automobile parts having a complicated part shape.

Claims (2)

In mass%, C: 0.15-2.0%, Si: 0.40% or less, Mn: 0.5% or less, P: 0.03% or less, S: 0.03% or less, Carbide so that the balance is Fe and inevitable impurities, the carbide spheroidization rate defined by (a) below is 90% or more, and the average carbide particle size defined by (b) is 0.4 μm or more Is dispersed in ferrite, and the anisotropic Δr defined by (c) below is −1.0 to 1.0.
(A) Carbide spheroidization ratio: In the observation of the metal structure of the cross section of the steel sheet, the ratio (p / q) of the maximum length p of carbide to the maximum length q in the direction perpendicular to the total number of carbides in the observation field is less than 3. The ratio (%) of the number of carbides. However, the observation visual field is a region where the total number of carbides is 300 or more.
(B) Average carbide particle size: A value obtained by averaging the equivalent circle diameters measured for individual carbides in the observation field in the observation of the metal structure of the cross section of the steel sheet for all the measured carbides. However, the observation visual field is a region where the total number of carbides is 300 or more.
(C) Regarding the Rankford value (r value), the rolling direction (r ₀ ) of the steel sheet, the sheet width direction (r ₉₀ ) perpendicular to the rolling direction, and the 45 ° direction (r ₄₅ ) with respect to the rolling direction Is calculated from the relational expression consisting of ((r ₀ + r ₉₀ ) / 2) −r ₄₅ . Furthermore, in mass%, a component composition containing one or more selected from Cr: 2.0% or less, Mo: 1.0% or less, Ni: 0.3% or less, and Cu: 0.3% or less The carbon steel plate according to claim 1.