JP2020164938A - Medium carbon steel sheet and method for producing the same - Google Patents

Abstract

To provide a medium carbon steel sheet that is soft and has low r-value in-plane anisotropy, and to provide a method for producing the same.SOLUTION: The medium carbon steel sheet contains C: 0.10 mass% or more and 0.70 mass% or less, has a yield stress of 400 MPa or less, an in-plane anisotropy index Δr of an r value of -0.2 or more and 0.2 or less, and a difference between rmax and rmin of 0.3 or less. Here, Δr=(r0-2r45+r90)/2, and rmax and rmin are the maximum value and the minimum value of r0, r45, and r90, respectively.SELECTED DRAWING: Figure 1

Description

The present invention relates to a medium carbon steel sheet preferably used as a material for deep drawing, for example, and a method for producing the same.

Medium- and high-carbon steel sheets used as materials for deep-drawing products are required to be soft because (i) the molding load during deep-drawing is desired to be reduced, and (ii) the vertical walls of the deep-drawing products. Since it is preferable that the height of the portion is as uniform as possible in the circumferential direction of the molded product and there is no variation, it is generally required that the in-plane anisotropy of the Rankford value (r value) is small. Therefore, so far, techniques such as Patent Documents 1 to 4 have been studied.

The technique of Patent Document 1 is, in order to suppress variations in the height of the vertical wall portion in the deep-drawn processed product, in terms of mass%, C: 0.15 to 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, the balance consists of Fe and unavoidable impurities, and the carbide spheroidization rate is 90. Provided is a carbon steel sheet in which carbides are dispersed in ferrite so as to have an average carbide particle size of 0.4 μm or more. This carbon steel sheet has an anisotropy Δr of −1.0 to 1.0.

The technique of Patent Document 2 is to provide a high carbon steel sheet molded into an automobile part or the like, particularly a high carbon steel sheet having good dimensional accuracy after molding and heat treatment of a cylindrical part, in terms of mass%, C: 0. .25 to 0.60%, Mn: 0.25 to 1.50%, Cr: 0.60% or less, Ti: 0.0l to 0.060%, B: 0.0003 to 0 if necessary. We provide high carbon steel sheets containing 0050%. In this high carbon steel sheet, (222) / (200) <5.5-5 × in relation to the X-ray integrated intensity ratio between the (222) plane and the (200) plane and the C amount of the high carbon steel sheet. By satisfying C (%), the roundness of the molded product and after quenching is good.

The technique of Patent Document 3 provides a high carbon steel sheet having low in-plane anisotropy and a method for manufacturing the same, which requires high dimensional accuracy in molding and is compatible with parts subjected to heat treatment such as quenching and tempering. C: 0.2% to 1.5%, Si: 0.10% to 0.35%, Mn: 0.1% to 0.9%, P: 0.03% or less, S: A high carbon steel sheet having a component system of 0.035% or less, Cu: 0.03% or less, Ni: 0.025% or less, Cr: 0.3% or less, and has an average carbide particle size of less than 0.5 μm. We provide high carbon steel sheets from. This high carbon steel sheet has an in-plane anisotropy index Δr of r value of more than −0.15 to less than 0.15.

The technique of Patent Document 4 is C: 0.25 to 0.75%, sol.Al: 0.01 to 0.10%, N: 0.0020 to 0.0100%, and 2 ≦ (sol.Al / N) A steel material having a steel composition satisfying ≦ 20 is hot-rolled at a winding temperature of 550 to 680 ° C., pickled, and then cold-rolled at a rolling reduction of 20 to 80%, and subsequently in the range of 650 ° C. to Ac1. Box annealing and temper rolling at temperature were performed, the average particle size of the carbides in the steel was 0.5 μm or more, the spheroidization rate ≥ 90% was satisfied, and the (222) plane and (200) planes and (200) planes were formed in the texture of the steel strip. ) A high-carbon cold-rolled steel strip in which the relationship between the X-ray integrated strength ratio with the surface and the C amount of the high-carbon steel sheet satisfies (222) / (200) ≥ 6-8.0 × C (%). The manufacturing method is provided. This high carbon steel strip has an average r value of ≧ 0.80 and an in-plane anisotropy index of Δr ± 0.020.

JP-A-2018-141184 Japanese Unexamined Patent Publication No. 2005-097656 Japanese Unexamined Patent Publication No. 2003-089846 Japanese Unexamined Patent Publication No. 2000-328172

However, in recent years, in deep-drawn molded products, not only the height of the vertical wall portion is as uniform as possible in the circumferential direction of the molded product (variation is small), but also the plate thickness of the vertical wall portion fluctuates in the circumferential direction of the molded product. It has come to be required to be small. Therefore, one aspect of the present invention provides a soft medium carbon steel sheet having a small r-value in-plane anisotropy and a method for producing the same as a material having material properties suitable for obtaining such a deep-drawn molded product. The purpose is to do.

As a result of diligent studies, the present inventors have determined that the medium carbon steel sheet containing C: 0.10% by mass or more and 0.70% by mass or less is softened (indicating a yield stress of 400 MPa or less) and an r value. The present invention was conceived after obtaining new knowledge about a means for achieving both reduction of in-plane anisotropy. More specifically, the present inventors appropriately soften the medium carbon steel sheet by annealing in a state where strain is accumulated in the structure, and the base iron ferrite having a random orientation in the structure is effective. The present invention was conceived by finding that the in-plane anisotropy of the r value can be effectively reduced.

That is, the medium carbon steel sheet according to one aspect of the present invention is a medium carbon steel sheet containing C: 0.10% by mass or more and 0.70% by mass or less, and has a yield stress of 400 MPa or less and an in-plane anisotropic r value. It is characterized in that the sex index Δr is −0.2 or more and 0.2 or less, and the difference between r _max and r _min is 0.3 or less. Here, Δr = (r ₀ -2r ₄₅ + r ₉₀ ) / 2, and r ₀ , r _45, and r ₉₀ rank in the 0 °, 45 °, and 90 ° directions with respect to the rolling direction, respectively. Ford value. Further, r _{max and} r _min are the maximum value and the minimum value of the above r ₀ , r ₄₅ , and r ₉₀ , respectively.

The method for producing a medium carbon steel sheet according to one aspect of the present invention is to cold-roll a hot-rolled steel sheet or an annealed steel sheet containing C: 0.10% by mass or more and 0.70% by mass or less at a rolling ratio of 50% or more. After a cold rolling step of applying to obtain a cold-rolled sheet and heating the cold-rolled sheet at a temperature rise rate of 30 ° C./h or more in a temperature range of 400 ° C. to 650 ° C., Ac1 transformation of 650 ° C. or more. It includes an annealing step of annealing the cold-rolled sheet by holding it at an annealing temperature below the point.

Further, the method for producing a medium carbon steel sheet according to one aspect of the present invention is to combine a hot-rolled steel sheet or an annealed steel sheet containing C: 0.10% by mass or more and 0.70% by mass or less with a cold rolling ratio of 50% or more. After a cold rolling step of rolling to obtain a cold-rolled sheet and heating the cold-rolled sheet so that the temperature rise rate is 30 ° C./h or more in a temperature range of 400 ° C. to 650 ° C., the Ac1 transformation point. The annealing step of annealing the cold-rolled sheet by holding it at the above annealing temperature is included, and the annealing temperature in the annealing step is equal to or higher than the Ac1 transformation point and equal to or lower than the Ac1 transformation point + 60 ° C. It is a feature.

According to one aspect of the present invention, it is possible to provide a medium carbon steel sheet which is soft and has a small in-plane anisotropy of r-value and a method for producing the same as a material having suitable material properties for obtaining a deep-drawn molded product. it can.

(A) is a diagram for explaining a method for manufacturing a medium carbon steel sheet according to the first embodiment of the present invention, (b) is a diagram for explaining a cold rolling process, and (c) is a cold-rolled coil. It is a figure for demonstrating the state of annealing of. It is a figure for demonstrating the manufacturing method of the medium carbon steel sheet in Embodiment 2 of this invention.

Hereinafter, embodiments of the present invention will be described. The following description is intended to better understand the gist of the invention, and does not limit the present invention unless otherwise specified. Further, in the present application, "A to B" indicates that it is A or more and B or less.

First, the outline of the findings found by the present inventors is as follows.

First, a means for softening the medium carbon steel sheet will be described. Spheroidization of cementite in the structure by spheroidizing the cold rolled sheet of medium carbon steel sheet at an annealing temperature of 650 ° C or higher and lower than Ac1 transformation point or Ac1 transformation point or higher and Ac1 transformation point + 60 ° C or lower. At the same time, recovery and recrystallization occur to increase the grain size of cementite ferrite. As a result, the medium carbon steel sheet can be appropriately softened.

Next, a basic means for reducing the in-plane anisotropy of the Rankford value (hereinafter, r value) of the medium carbon steel sheet will be described. In general, the r value in a certain direction of a steel sheet depends on the orientation state (degree of orientation in a specific direction) of the crystal orientation of each crystal grain in a large number of ground iron ferrite crystal grains existing in the structure of the steel sheet. To do. The crystal orientation of the crystal grains in the structure of the steel sheet is created by cold rolling and recrystallization annealing. At this time, when a large number of recrystallized grains having a specific crystal orientation are generated (having a texture), the in-plane anisotropy of the r value becomes large. On the other hand, when many recrystallized grains having random crystal orientations are generated, the in-plane anisotropy of the r value becomes small.

The medium carbon steel sheet has a metal structure in which cementite is dispersed in ferrite, and when cold rolling is performed, the strain generated by the cold rolling mainly accumulates at the ferrite grain boundaries and the ferrite / cementite interface. When the medium carbon steel plate after cold rolling is recrystallized below the Ac1 transformation point, strain-free recrystallized ferrite is generated from the ferrite grain boundaries where strain has accumulated and the ferrite / cementite interface, and the recrystallized ferrite takes time. It grows with the passage of time. At this time, recrystallized ferrite having an aggregate structure is generated from the ferrite grain boundaries, and recrystallized ferrite having a random orientation is generated from the ferrite / cementite interface.

Therefore, the present inventors have conceived that the in-plane anisotropy of the r value can be improved by increasing the ratio of recrystallized ferrite generated from the ferrite / cementite interface in the medium carbon steel sheet.

[Embodiment 1]
Hereinafter, an embodiment of the present invention will be described.

<Medium carbon steel sheet>
Before explaining in detail the method for producing a medium carbon steel sheet according to the embodiment of the present invention, which was conceived based on the above findings, the medium carbon steel sheet according to the embodiment of the present invention will be described.

(Steel composition)
The steel composition (component composition) of the medium carbon steel sheet in the present embodiment is shown below.

(C)
The present invention targets medium carbon steel (steel having a carbon content corresponding to so-called subeutectoid steel) having a C (carbon) content of 0.10% by mass or more and 0.70% by mass or less in the steel. .. C is the most basic alloying element in carbon steel, and the amount of cementite and the metallographic structure when heated to the Ac1 transformation point or higher vary greatly depending on the content thereof. In steels having a C content of less than 0.10% by mass, the amount of cementite is small, and there are few ferrite grains having a random orientation generated from the ferrite / cementite interface when recrystallization occurs by annealing after cold rolling. Therefore, it is difficult to improve the in-plane anisotropy of the r-value in the medium carbon steel sheet.

On the other hand, when the C content exceeds 0.70% by mass, the amount of cementite in the metal structure increases and the material becomes hard. Therefore, when cold rolling with a rolling ratio of 50% or more is performed in order to accumulate a large amount of strain at the ferrite / cementite interface in the structure, problems such as a significant increase in the number of rolling passes and cracks at the edges due to work hardening occur. May occur. As a result, manufacturability and handleability deteriorate.

Therefore, in the present invention, steel having a C content in the range of 0.10% by mass or more and 0.70% by mass or less is targeted. For applications that require higher workability, the C content is preferably 0.50% by mass or less.

(Si)
Si (silicon) is an alloying element that acts as an antacid. If the Si content is less than 0.02% by mass, the effect cannot be sufficiently obtained. On the other hand, Si is one of the elements having a great influence on the workability of the annealed steel sheet. If Si is added in excess, the ferrite is hardened by the solid solution strengthening action, which causes cracks during the molding process. Further, when the Si content increases, scale defects tend to occur on the surface of the steel sheet in the manufacturing process, which causes deterioration of the surface quality. Therefore, when Si is added, the content is adjusted to 0.50% by mass or less. Therefore, the Si content is preferably 0.02% by mass or more and 0.50% by mass or less, and more preferably 0.10% by mass or more and 0.40% by mass or less.

(Mn)
Mn (manganese) is an alloying element that improves hardenability and is added as needed. If the Mn content exceeds 2.0% by mass, the steel sheet becomes hard and the workability is lowered. The Mn content is preferably 2.0% by mass or less, and more preferably 0.1% by mass or more and 1.0% by mass or less.

(Cr)
Cr (chromium) is an element that improves hardenability and increases temper softening resistance, and is added as necessary. However, if a large amount of Cr exceeding 1.6% by mass is contained, it becomes difficult to soften even if annealing is performed, and the workability before quenching deteriorates. Therefore, when Cr is added, it is desirable to contain it in the range of 1.6% by mass or less. The Cr content is preferably 0.1% by mass or more and 1.2% by mass or less.

(P, S)
P (phosphorus) and S (sulfur) are alloying elements that reduce toughness. Therefore, in order to improve toughness, it is preferable to reduce it as much as possible. When ensuring the toughness of medium carbon steel parts used as various mechanical parts, the P content and the S content are allowed up to 0.03% by mass, respectively. The P content and the S content are each preferably 0.025% by mass or less, more preferably 0.02% by mass or less.

The present invention can also be applied to steels to which the following elements have been added for the purpose of improving properties such as hardenability and toughness. As a range that does not impair moldability, Mo is 0.5% by mass or less, Cu is 0.3% by mass or less, Ni is 2.0% by mass or less, Al is 0.1% by mass or less, and Ti is 0.3% by mass. % Or less, V is 0.3% by mass or less, Nb is 0.5% by mass or less, and B is 0.01% by mass or less.

The rest other than the above components are Fe and unavoidable impurities. Here, the unavoidable impurities mean components such as O and N that are difficult to remove. These components are inevitably mixed in at the stage of melting the steel piece (slab).

(Characteristic)
The medium carbon steel sheet in the present embodiment has a yield stress of 400 MPa or less at room temperature, an in-plane anisotropy index Δr of Rankford value of −0.2 or more and 0.2 or less, and r _max and r _min of each other. The difference is 0.3 or less. Such mechanical properties are realized by the medium carbon steel sheet in the present embodiment having a metal structure (structure structure) having a specific annealed structure by being manufactured by a method (condition) described later.

(I) Yield stress In the medium carbon steel sheet of the present embodiment, the cementite particles are relatively spherical and coarse in the metal structure, and the distance between the cementite particles is relatively wide. The wider the distance between the cementite particles (the smaller the number of cementite particles per unit volume), the wider the portion where the soft ferrite is continuously present, and the easier it is to be deformed when processed. As a result, the medium carbon steel sheet in the present embodiment has a yield stress of 400 MPa or less at room temperature (for example, 20 ° C. to 25 ° C.). The yield stress may be measured by the test method of JIS Z2241.

(Ii) In-plane anisotropy index of Rankford value The Rankford value (r value) is an index used to evaluate the deformation anisotropy in the plate width direction and the plate thickness direction during processing of a metal material. It is also called a plastic working strain ratio. Specifically, when a tensile test is performed using a plate-shaped test piece, the r value of the plate-shaped test piece is determined based on the plate width and the plate thickness before and after the tensile test. However, since it is difficult to accurately grasp the change in plate thickness with a thin plate such as a steel plate (for example, the plate thickness is about 1 mm), the r value is set as follows based on the assumption that the volume is constant before and after plastic working. Ask.

r = ln (W / W ₀ ) / ln (L ₀・ W ₀ / L ・ W)
Here, W ₀ and L ₀ are the plate width and the distance between the gauge points in the parallel portion of the plate-shaped test piece before the tensile test, respectively. Further, W and L are the plate width and the distance between the gauge points in the parallel portion of the plate-shaped test piece after the tensile test, respectively.

Usually, a tensile test is performed so that the elongation strain is 10 to 20%, and the r value obtained at that time is called a Rankford value. Also in the medium carbon steel sheet of the present embodiment, the Rankford value is obtained based on the result of conducting a tensile test so that the elongation strain is 10 to 20%. In the following description herein, the r-value means a Rankford value.

Then, the in-plane anisotropy index Δr is calculated by the following formula.

Δr = (r ₀ -2r ₄₅ + r ₉₀ ) / 2
Here, the medium carbon steel sheet in the present embodiment is manufactured by undergoing various rolling treatments and annealing treatments. Relative to the rolling direction (direction steel sheet rolling rolls rotating is pushed) in the rolling process, in a plate surface, and r ₀ the r value of the 0 ° direction relative to the rolling direction. Similarly, r ₄₅ and r ₉₀ are an r-value in the 45 ° direction and an r-value in the 90 ° direction with respect to the rolling direction in the plate surface, respectively.

In the medium carbon steel sheet of the present embodiment, the base iron ferrite exists so as to have a random crystal orientation in the metal structure, and the in-plane anisotropy index Δr of the Rankford value is −0.2 or more. It is 2 or less. The closer the value of Δr is to 0, the smaller the in-plane anisotropy.

(Iii) If the maximum, minimum r ₀ , r ₄₅ , and r ₉₀ values of the Rankford values increase in this order, for example, r ₀ = 0.8, r ₄₅ = 1, and r ₉₀ = 1. If it is 2, the value of Δr is 0 (within the range of −0.2 or more and 0.2 or less). _{However, r 0,} r _45, and the maximum value _{and, r 0,} r _45, and the minimum value with each other is the difference between 0.4 next to one of the _{r 90,} actually plane anisotropy of the _{r 90} It can be said that the sex is great. Therefore, medium carbon steel in this _embodiment, the maximum value of r _{0, r 45,} and _{r 90,} the absolute value of the mutual difference between the minimum value of r _{0, r 45,} and _{r 90} is It is stipulated that it is 0.3 or less.

<Manufacturing method of medium carbon steel sheet>
The method for producing a medium carbon steel sheet which is soft and has a small in-plane anisotropy in the present embodiment will be described below with reference to FIG. FIG. 1A is a diagram for explaining a method for manufacturing a medium carbon steel sheet in the present embodiment, and shows an example of an annealing cycle. In FIG. 1A, the horizontal axis represents time t and the vertical axis represents temperature TE. The annealing cycle shown in FIG. 1 (a) is an example, and the specific annealing conditions (temperature control) may be appropriately changed as long as the conditions described later are satisfied. In FIG. 1, the portions (1), (2-1), and (2-2) surrounded by the dotted lines are used as reference numbers for explaining the state at that time.

As shown in FIG. 1A, the method for producing a medium carbon steel sheet in the present embodiment includes a cold rolling step (S1) in which a hot-rolled steel sheet or an annealed steel sheet to be annealed is cold-rolled. , The temperature raising step (S2) in which the cold rolled sheet is heated in a heating furnace to raise the temperature to the vicinity of the Ac1 transformation point, and the annealing temperature is heated to an annealing temperature of 650 ° C. or higher and lower than the Ac1 transformation point following the raising step. A first temperature holding step (S3) for keeping the temperature uniform, and a cooling step (S4) for lowering the temperature of the annealed sheet annealed through the above S2 and S3 to room temperature are included. Each of these steps will be described below. In this embodiment, the above S2 to S4 are collectively referred to as an annealing step.

(Cold rolling process)
First, a hot-rolled steel sheet from which scale has been removed by hot rolling and then pickling, or an annealed steel sheet obtained by subjecting the hot-rolled steel sheet to primary annealing is prepared. The hot-rolled steel sheet or annealed steel sheet may be manufactured by a general method. Usually, hot-rolled or annealed steel sheets are manufactured as coils. The primary annealing may be, for example, a process of spheroidizing cementite by maintaining the temperature below the Ac1 transformation point or above the Ac1 transformation point. The hot-rolled steel sheet and the annealed steel sheet have the steel composition of the medium carbon steel sheet of the present embodiment described above.

The hot-rolled steel sheet has a structure mainly composed of layered pearlite and proeutectoid ferrite. Further, the annealed steel sheet has a structure mainly composed of base iron ferrite and spheroidized cementite. Hot-rolled steel sheets and annealed steel sheets have less strain accumulation in the structure.

Here, in the following description, the terms are defined as follows. The ferrite in the structure of the hot-rolled steel sheet or annealed steel sheet after being cold-rolled is referred to as processed ferrite. Then, the crystal grains newly generated at the ferrite grain boundary and the ferrite / cementite interface by annealing the hot-rolled steel sheet or the annealed steel sheet are referred to as recrystallized ferrite.

The recrystallized ferrites are (i) ferrite (hereinafter, irregular ferrite) formed so that the crystal orientations of the crystal grains have a random relationship with each other, and (ii) an aggregate structure in which each crystal grain has a specific crystal orientation. (Hereinafter referred to as oriented ferrite), which is formed so as to form the above.

In the present embodiment, the base iron ferrite in the medium carbon steel sheet after annealing is a processed ferrite (at least a phase in which the crystal orientation of the processed ferrite is maintained) and a recrystallized ferrite when the processed ferrite remains after annealing. including. Alternatively, the base iron ferrite in the medium carbon steel sheet after annealing may consist only of recrystallized ferrite when all the processed ferrite has disappeared due to annealing.

In the method for producing a medium carbon steel sheet in the present embodiment, a hot-rolled steel sheet or an annealed steel sheet is cold-rolled (finish-rolled). FIG. 1B is a diagram for explaining the cold rolling step S1.

As shown in FIG. 1 (b), the coil 1 of the hot-rolled steel sheet or the annealed steel sheet is cold-rolled using a cold rolling mill 2 to have a rolling ratio (rolling ratio) of 50% or more, and then cooled. A cold-rolled coil 3 made of a rolled plate is manufactured. The cold rolling mill 2 may be generally used for finish rolling, and is, for example, a Zendimia cold rolling mill or a tandem rolling mill.

In the cold rolling step S1, when the cold rolling ratio is low, the strain is not sufficiently accumulated at the ferrite / cementite interface, and the strain is mainly accumulated at the ferrite grain boundaries. On the other hand, as the rolling ratio increases, the amount of strain accumulated at the ferrite / cementite interface increases. In order to suitably generate and grow recrystallized grains from the ferrite / cementite interface by annealing after cold rolling, it is necessary to perform cold rolling with a rolling ratio of 50% or more. By performing cold rolling with a rolling ratio of 50% or more, sufficient strain can be accumulated at the ferrite / cementite interface, and therefore, when recrystallization occurs in the structure structure in the cold rolled plate, from the ferrite grain boundaries. The proportion of recrystallized ferrite (irregular ferrite) generated from the ferrite / cementite interface can also be increased.

It is not necessary to set an upper limit of the rolling ratio in the cold rolling step S1, but if it exceeds 90%, work hardening becomes remarkable, which causes an increase in cost due to an increase in the number of cold rolling passes and in some cases. May cause problems such as cracking at the edge of the steel plate.

Therefore, the rolling ratio in the cold rolling step S1 is preferably 50% or more and 90% or less.

(Heating process)
FIG. 1C is a diagram for explaining the state of annealing of the cold-rolled coil 3 (that is, the cold-rolled plate). As shown in FIG. 1 (c), the cold-rolled coil 3 is housed in the heating furnace 4 and the inside of the furnace is heated to perform box annealing (batch-type annealing) of the cold-rolled coil 3. That is, the processing of the temperature raising step S2 to the cooling step S4 is performed in the heating furnace 4. Hereinafter, the cold-rolled coil 3 (that is, the cold-rolled plate) to be annealed is referred to as an annealing target material.

The relationship between the conditions defined in the heating step S2 in the present embodiment and the state of the structural structure (2-1) of the material to be annealed will be described below. In the temperature raising step S2, the temperature range from 400 ° C. to 650 ° C. is heated at a temperature rising rate of 30 ° C./h or more. When the temperature rise rate in the temperature range from 400 ° C to 650 ° C is slow, only strain recovery proceeds by the time the recrystallization temperature is reached, and recrystallized grains having a random orientation from the ferrite / cementite interface (irregular ferrite). ) Is inhibited. By raising the temperature to 650 ° C. at a heating rate of 30 ° C./h or higher, more recrystallized grains from the ferrite / cementite interface are generated than the recrystallized grains (aligned ferrite) at the ferrite grain boundaries. The recrystallization temperature of the medium carbon steel sheet is affected by the degree of processing strain and alloying elements, but it is almost completed when heated to 650 ° C.

Even if slow heat or soaking heat is applied between the temperature raising step S2 and the first temperature holding step S3, there is no effect on the anisotropy improving effect. Therefore, in the temperature raising step S2, after raising the temperature to 650 ° C. at a heating rate of 30 ° C./h or more, the heat may be slowed down, the heat may be kept uniform, and the like.

(First temperature holding step)
The relationship between the conditions defined in the first temperature holding step S3 of the present embodiment and the state of the structural structure (2-2) of the material to be annealed will be described below. In the first temperature holding step S3, cementite is spheroidized and recrystallized grains (irregular ferrite) generated having random orientations grow by soaking heat at an annealing temperature of 650 ° C. or higher and lower than the Ac1 transformation point. To do. As the irregular ferrite grows, the processed ferrite is incorporated into the irregular ferrite, and the abundance of the processed ferrite decreases. Further, since the amount of oriented ferrite produced in the temperature raising step S2 is small, it is difficult for the oriented ferrite to grow in the first temperature holding step S3. Then, as the irregular ferrite grows, the oriented ferrite can be incorporated into the irregular ferrite.

(Cooling process)
In the cooling step S4 of the present embodiment, cooling is performed from an annealing temperature of 650 ° C. or higher and lower than the Ac1 transformation point. The structure of the medium carbon steel sheet after cooling is the same as the structure finally formed in the first temperature holding step S3. The medium carbon steel sheet cooled to room temperature in the cooling step S4 has a metal structure (structure structure) mainly composed of irregular ferrite having a random orientation.

In the cooling step S4 of the present embodiment, the temperature lowering rate is not particularly limited. In the cooling step S4, the heating furnace 4 may be air-cooled or allowed to cool, or may be forcibly cooled by blowing a cooling gas.

As described above, in the cold rolling steps S1 to the first temperature holding step S3, by appropriately controlling the rolling rate, the rate of temperature rise, and the annealing temperature, the softness and the in-plane anisotropy of the r value are improved. A medium carbon steel sheet can be obtained.

(Advantage of invention)
In the method for producing a medium carbon steel sheet in the present embodiment, in annealing after cold rolling with a rolling ratio of 50% or more, a temperature range from 400 ° C. to 650 ° C. is heated at a heating rate of 30 ° C./h or more. After that, annealing is performed at an annealing temperature of 650 ° C. or higher and lower than the Ac1 transformation point. This made it possible to improve the in-plane anisotropy of the r-value in the medium carbon steel sheet. Specifically, a medium carbon steel sheet having a yield stress of 400 MPa or less and a small r-value in-plane anisotropy can be obtained. The medium carbon steel sheet in the present embodiment has an in-plane anisotropy index Δr of r value of −0.2 or more and 0.2 or less, and the difference between r _max and r _min is 0.3 or less. By using the medium carbon steel sheet of the present invention for deep drawing, a molded product having a small variation in thickness and diameter can be obtained.

[Embodiment 2]
Other embodiments of the present invention will be described below. The configuration other than that described in the present embodiment is the same as that in the first embodiment.

In the method for producing a medium carbon steel sheet according to the first embodiment, the material to be annealed was annealed by maintaining the soaking temperature at an annealing temperature of 650 ° C. or higher and lower than the Ac1 transformation point. On the other hand, in the method for producing a medium carbon steel sheet in the present embodiment, the soaking temperature is maintained at an annealing temperature equal to or higher than the Ac1 transformation point.

The method for producing a medium carbon steel sheet in the present embodiment will be described with reference to FIG. FIG. 2 is a diagram for explaining a method for manufacturing a medium carbon steel sheet in the present embodiment. In FIG. 2, the horizontal axis represents time t and the vertical axis represents temperature TE. The annealing cycle shown in FIG. 2 is an example, and specific annealing conditions (temperature control) may be appropriately changed as long as the conditions described later are satisfied. In FIG. 2, the portions (1) to (5) surrounded by the dotted lines are used as reference numbers for explaining the state at that time.

The composition of the medium carbon steel sheet in this embodiment is the same as that in the first embodiment described above.

As shown in FIG. 2, the method for producing a medium carbon steel sheet in the present embodiment includes a cold rolling step (S11) in which a hot-rolled steel sheet or an annealed steel sheet to be annealed is cold-rolled, and the cold-rolling step (S11). Following the first heating step (S12) in which the plate is heated in a heating furnace to raise the temperature to the vicinity of the Ac1 transformation point, and the annealing temperature above the Ac1 transformation point, the temperature of the cold-rolled plate is raised. The second heating step (S13) is included. Then, the method for producing the medium carbon steel sheet in the present embodiment further includes, following S13, a second temperature holding step (S14) in which the cold rolled sheet is heated to an annealing temperature to maintain the temperature, and S12 to the above. The slow cooling step (S15) of lowering the temperature of the annealed sheet annealed through S14 is included. In the present embodiment, the above S12 to S15 are collectively referred to as an annealing step. After the above S15, the annealed sheet is cooled to room temperature to obtain the medium carbon steel sheet of the present embodiment. According to the method for producing a medium carbon steel sheet in the present embodiment, spheroidization and coarsening of cementite (increase in cementite particle spacing due to coarsening of cementite particles) can be promoted, and a softer medium carbon steel sheet can be obtained. Obtainable. Each of these steps will be described below.

(Cold rolling process / first heating process)
The cold rolling step S11 and the first heating step S12 may be subjected to the same processing as in the cold rolling step S1 and the heating step S2 in the above-described first embodiment, respectively. Therefore, the description thereof will be omitted.

(Second heating step)
The state (3) of the structural structure of the material to be annealed in the second heating step S13 of the present embodiment will be described below. In the second temperature raising step S13, heating is performed above the Ac1 transformation point. Generally, when medium carbon steel is heated above the Ac1 transformation point, austenite is produced by melting cementite. In the second temperature raising step S13 in the present embodiment, during recrystallization during temperature rising, irregular ferrite having a random orientation generated from the ferrite / cementite interface is preferentially transformed into austenite.

Here, it is known that the austenite produced during the transformation from ferrite to austenite has a specific crystal orientation relationship with the original ferrite. Therefore, the austenite produced in the second heating step S13 of the present embodiment has the same orientation relationship as the irregular ferrite having a random orientation (it inherits the crystal orientation of the ferrite before transformation). This austenite is called irregular austenite.

(Second temperature holding step)
The relationship between the conditions defined in the second temperature holding step S14 of the present embodiment and the state (4) of the structural structure of the material to be annealed will be described below. In the second temperature holding step S14, the irregular austenite produced in the second temperature raising step S13 grows with the dissolution of cementite by keeping the heat equalized above the Ac1 transformation point. Then, it becomes a metal structure in which undissolved cementite is dispersed in ferrite + austenite. Here, in the second temperature holding step S14, the soaking time is set so that all cementite is not dissolved.

Here, even if a large amount of oriented ferrite having an texture is generated from the ferrite grain boundaries during recrystallization in the first temperature raising step S12, it is difficult to generate austenite having a crystal orientation relationship with the oriented ferrite. This is because the growth of the irregular austenite causes the oriented ferrite having an texture to be absorbed (transformed so as to be incorporated) into the irregular austenite.

In the second temperature holding step S14, the temperature above the Ac1 transformation point at which the heat is kept uniform is referred to as an annealing temperature. The annealing temperature in the second temperature holding step S14 of the present embodiment is at least the Ac1 transformation point and at least the Ac1 transformation point + 60 ° C. The presence state (density) of undissolved cementite after the second temperature holding step S14 determines the size of cementite particles in the medium carbon steel sheet of the present embodiment. This is because in the slow cooling step S15, undissolved cementite grows spherically and coarsely, and austenite transforms into ferrite.

Since the amount of cementite after annealing is determined by the C content, if the size of the cementite particles is determined, the spacing of the cementite particles (the number of cementite particles per unit volume) is determined. The wider the spacing between the cementite particles, the larger the continuous portion of the soft ferrite, and the easier it is to deform when processed. That is, the larger the distance between the cementite particles in the annealed medium carbon steel sheet, the softer it becomes.

In the second temperature holding step S14, when the annealing temperature becomes high, the undissolved cementite in the structure structure disappears and the abundance of ferrite decreases. When the undissolved cementite disappears, it is necessary to newly nucleate and precipitate cementite in the phase transformation (ferrite + cementite is generated from austenite) that occurs during the slow cooling in the slow cooling step S15. Therefore, the transformation in the slow cooling step S15 is started in a supercooled state as compared with the case where there is undissolved cementite. In this case, the driving force of transformation becomes large, and a pearlite structure which is advantageous in terms of transformation speed is generated. As a result, the metallographic structure after annealing becomes a ferrite + pearlite structure. Therefore, when the heating temperature above the Ac1 transformation point is high, a spherical cementite structure cannot be obtained after annealing, and the formability of the steel sheet is inferior (a hard steel sheet can be obtained).

Therefore, from the viewpoint of suppressing the spheroidization of cementite and the formation of pearlite, the annealing temperature in the second temperature holding step S14 needs to be Ac1 + 60 ° C. or lower.

(Slow cooling process)
In the slow cooling step S15, slow cooling is performed from a heating temperature equal to or higher than the Ac1 transformation point. The relationship between the conditions specified in the slow cooling step S15 of the present embodiment and the state (5) of the structural structure of the material to be annealed will be described below.

In medium carbon steel, the crystal orientation of the residual ferrite has a great influence on the crystal orientation distribution after annealing. This will be described below based on the state (estimated state) of the tissue structure in each temperature range (step) in the slow cooling step S15. Here, for convenience of explanation, the ferrite existing in the structure structure in the above state (4) is referred to as residual ferrite. The residual ferrite is mainly recrystallized ferrite.

(1) High temperature region from the Ac1 transformation point: Austenite decreases (C concentration in austenite increases) as the temperature decreases, and recrystallized ferrite grows. As a result, ferrite having a crystal orientation of residual ferrite increases in the structure.

(2) During A1 transformation (austenite → ferrite + cementite): The following two patterns (i) and (ii) can be considered for the formation of ferrite.

(2-i) When ferrite is not newly nucleated at the austenite / undissolved cementite interface, A1 transformation proceeds due to the growth of residual ferrite. In this case, ferrite having a crystal orientation of residual ferrite increases in the structure.

(2-ii) When ferrite is newly nucleated at the austenite / undissolved cementite interface, ferrite is nucleated and grown at the austenite / undissolved cementite interface. As a result, transformation proceeds and irregular ferrite is formed in the structure.

Along with the formation of ferrite in (2-i) and (2-ii) above, cementite is formed in the form in which undissolved cementite grows.

(3) Low temperature region from the Ac1 transformation point: Residual ferrite grows (the ferrite interface decreases, that is, the grain size of ferrite increases).

Then, in the entire steel sheet after annealing, the amount of ferrite generated by the growth of residual ferrite increases.

Therefore, in the method for producing a medium carbon steel sheet in the present embodiment, it is preferable to sufficiently generate irregular ferrite in the above-mentioned first heating step S12. Therefore, in the method for producing a medium carbon steel sheet in the present embodiment, cold rolling of 50% or more is performed in the cold rolling step S11, and the temperature is raised by 30 ° C./h to a temperature of 650 ° C. in the first heating step S12. It stipulates the temperature rate.

The base iron ferrite in the final annealed structure after the slow cooling step S15 contains a large amount of ferrite having a random orientation (irregular ferrite) formed during recrystallization or formed or grown by the phase transformation of irregular austenite.

By slowly cooling until the above transformation is completed, a structure in which spherical and coarse cementite is dispersed in the base iron ferrite is obtained, and the medium carbon steel sheet is softened. In order to sufficiently obtain spheroidization of cementite, it is preferable that the cooling from the heating above the Ac1 transformation point is slowly cooled at 5 to 30 ° C./h until the phase transformation is completed. If the cooling rate is less than 5 ° C./h, annealing takes a very long time, which hinders productivity. If the cooling rate is faster than 30 ° C./h, even if sufficient undissolved cementite remains, the diffusion of elements may not catch up and pearlite may be generated. From the viewpoint of productivity and workability of the steel sheet, the cooling rate in the slow cooling step S15 is preferably 5 to 30 ° C./h.

As described above, in annealing using heating to the Ac1 transformation point or higher, the in-plane anisotropy of the rolling rate and r value is appropriately controlled by appropriately controlling the rolling rate, heating rate, annealing temperature, and cooling rate. A medium carbon steel sheet with improved is obtained.

(Advantage of invention)
In the method for producing a medium carbon steel sheet in the present embodiment, in annealing after cold rolling with a rolling ratio of 50% or more, a temperature range from 400 ° C. to 650 ° C. is heated at a heating rate of 30 ° C./h or more. After that, annealing is performed at an annealing temperature equal to or higher than the Ac1 transformation point and equal to or lower than the Ac1 transformation point + 60 ° C. Then, after annealing, the mixture is slowly cooled at a cooling rate of 5 to 30 ° C./h. This makes it possible to improve the in-plane anisotropy of the r-value and to make the steel sheet of medium carbon steel softer. Specifically, a medium carbon steel sheet having a yield stress of 400 MPa or less and a small r-value in-plane anisotropy can be obtained. The medium carbon steel sheet in the present embodiment has an in-plane anisotropy index Δr of r value of −0.2 or more and 0.2 or less, and the difference between r _max and r _min is 0.3 or less. By using the medium carbon steel sheet of the present invention for deep drawing, a molded product having a small variation in thickness and diameter can be obtained.

[Appendix]
The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the claims, and the embodiments obtained by appropriately combining the technical means disclosed in the different embodiments. Is also included in the technical scope of the present invention.

Table 1 shows the chemical composition and Ac1 transformation point of the molten test steel.

The steels having the components shown in Table 1 were hot-rolled, and the obtained steel sheet was pickled to remove scale. The obtained hot-rolled steel sheet was first annealed under the following conditions. Part of it was subjected to the next process without primary annealing.
Condition (a): [Ac1 transformation point-100 ° C. to Ac1 transformation point] × 10 to 60 hours Holding condition (b): [Ac1 transformation point to Ac1 transformation point + 50 ° C.] × 4 to 20 hours, and then to Ar1 point or less The condition (b) is slowly cooled at a cooling rate of 30 ° C./h or less, and includes the case where the temperature is maintained at the Ac1 transformation point or lower before and after the heating and holding at the Ac1 transformation point or higher.

Further, the hot-rolled steel sheet and the annealed steel sheet annealed according to the conditions (a) and (b) were subjected to finish cold rolling at various rolling ratios, and then finish annealing was performed under various annealing conditions. The rolling ratio in the finish cold rolling and the annealing cycle of the finish annealing are shown in Table 2 below. Then, the yield stress of the obtained annealed sheet and the in-plane anisotropy of the r value were measured.

For the tensile test, JIS No. 5 tensile test pieces in three directions of L (rolling direction), D (45 ° with respect to the rolling direction) and T (90 ° with respect to the rolling direction) were prepared, and between the gauge points of the parallel portions. The plate thickness was 1.0 mm with the distance being 50 mm. In the tensile test, a tensile elongation of 15% was given, the plate width within the gauge points at that time was measured, and the r value was calculated by the following formula.

r = ln (W _X / W ₀ ) / ln (L ₀ · W ₀ / L _X · W _X )
Here, W ₀ and L ₀ are the plate width and the distance between the gauge points before the test, and W _X and L _x are the plate width and the distance between the gauge points after applying 15% tensile elongation.

As an index of the in-plane anisotropy of the r value, the Δr value of each test material was calculated by the following equation.
Δr value = (r ₀ -2r ₄₅ + r ₉₀ ) / 2
Since the closer the Δr value is to 0, the smaller the anisotropy is, the in-plane anisotropy of the r value was evaluated by the absolute value | Δr value |. In addition, _x of r _x indicates the cutting direction of the test piece with respect to the rolling direction. For example, r ₄₅ is the r value measured by the test pieces taken in the direction of 45 ° to the rolling direction.

Further, the difference r _max −r _min between the maximum value r _max and the minimum value r _min in each direction was also calculated, and the in-plane anisotropy of the r value was evaluated. In addition, the yield stress was measured as an index of softening.

Table 2 shows the rolling ratio of the finish cold rolling, the finish annealing conditions, the in-plane anisotropy of the r value of the annealed material, and the yield strength. In this example, the heating rate from 400 to 650 ° C. was 80 ° C./h.

As shown in Table 2, No. 1 using steel type A having a C amount lower than the range of the present invention. In No. 1, even after cold rolling and annealing within the range of the present invention, Δr was 0.35 and r _max −r _min was 0.45, and the in-plane anisotropy of the r value was large.

The steel of Comparative Example 1 (No. 2, 4, 7, 10, 16, 19, 21, 26, 29, 31) in which the rolling ratio of cold rolled rolling is lower than 50% has a | Δr value | of 0. 2 or more, r _max −r _min is 0.3 or more, and the in-plane anisotropy of r value is larger than other ones. Further, Comparative Example 2 (No. 6, 13, 14, 17, 24) in which the heating temperature of finish annealing is lower than the range of the present invention even when the rolling ratio of finish cold rolling is 50% or more is the same steel type in the present invention. It is harder than the one that has been annealed for the specified finish.

On the other hand, in the steel in which the amount of C is in the range of 0.10 to 0.70% specified in the present invention, the rolling ratio of the finish cold rolling and the finish annealing specified in the present invention are applied to the example of the present invention (No. In 3,5,8,9,11,12,15,18,20,22,23,25,27,28,30,32,33), the in-plane anisotropic of r value is compared with Comparative Example 1. Is small and is softer than Comparative Example 2 of the same steel type. As described above, it can be seen that by applying the present invention, a medium carbon steel sheet that is soft and has a small r-value in-plane anisotropy can be obtained.

An example of investigating the influence of the temperature rising rate from 400 to 650 ° C. at the time of finish annealing on the in-plane anisotropy of the r value using the B to I steels in Table 1 is shown. A hot-rolled plate made of B to I steel is annealed at 700 ° C. for 20 hours, cold-rolled at a rolling ratio of 65%, and then the temperature rise rate from 400 to 650 ° C. is changed to a temperature of 650 ° C. or higher. The finish was annealed with.

The results of these tests are shown in Table 3 together with the finishing annealing conditions.

The steel of the comparative example (No. 41, 44, 46, 48, 50, 54, 56, 58, 60, 62, 66) in which the heating rate from 400 to 650 ° C. is lower than 30 ° C./h in the finish annealing , | Δr value | is 0.2 or more, r _max −r _min is 0.3 or more, and the in-plane anisotropy of the r value is larger than that of other substances. On the other hand, in the finish annealing, the heating rate from 400 to 650 ° C. is within the specified range of the present invention (No. 42, 43, 45, 47, 49, 51, 52, 53, 55, It can be seen that in the steels of 57, 59, 61, 63, 64, 65, 67), the in-plane anisotropy of the r value is smaller than that of the comparative example.

1 Coil 2 Cold rolling mill 3 Cold rolled coil 4 Heating furnace

Claims (5)

C: A medium carbon steel sheet containing 0.10% by mass or more and 0.70% by mass or less.
The yield stress is 400 MPa or less, the in-plane anisotropy index Δr of the r value is −0.2 or more and 0.2 or less, and the difference between r _max and r _min is 0.3 or less. Medium carbon steel plate.
(here,
Δr = (r ₀ -2r ₄₅ + r ₉₀ ) / 2
r ₀ : Rankford value in the 0 ° direction with respect to the rolling direction r ₄₅ : Rankford value in the 45 ° direction with respect to the rolling direction r ₉₀ : Rankford value in the 90 ° direction with respect to the rolling direction r _max : The above r Maximum value among ₀ , r ₄₅ , and r ₉₀ r _min : Minimum value among the above r ₀ , r ₄₅ , and r ₉₀ ) Further, in mass%, Si: 0.02% or more and 0.50% or less, Mn: 2.0% or less, P: 0.03% or less, S: 0.03% or less, and Cr: 1.6%. The medium carbon steel sheet according to claim 1, wherein the medium carbon steel sheet contains the following. C: A cold rolling step of obtaining a cold-rolled sheet by cold-rolling a hot-rolled steel sheet or an annealed steel sheet containing 0.10% by mass or more and 0.70% by mass or less with a rolling ratio of 50% or more.
After heating the cold-rolled sheet so that the temperature rise rate is 30 ° C./h or more in the temperature range from 400 ° C. to 650 ° C., the cold-rolled sheet is held at an annealing temperature of 650 ° C. or higher and lower than the Ac1 transformation point. A method for producing a medium carbon steel sheet, which comprises an annealing step of annealing a plate. C: A cold rolling step of obtaining a cold-rolled sheet by cold-rolling a hot-rolled steel sheet or an annealed steel sheet containing 0.10% by mass or more and 0.70% by mass or less with a rolling ratio of 50% or more.
After heating the cold-rolled plate so that the temperature rise rate is 30 ° C./h or more in the temperature range from 400 ° C. to 650 ° C., the cold-rolled plate is annealed by holding it at an annealing temperature equal to or higher than the Ac1 transformation point. Including the annealing process and
A method for producing a medium carbon steel sheet, wherein the annealing temperature in the annealing step is equal to or higher than the Ac1 transformation point and equal to or lower than the Ac1 transformation point + 60 ° C. The hot-rolled steel sheet or annealed steel sheet has Si: 0.02% or more and 0.50% or less, Mn: 2.0% or less, P: 0.03% or less, S: 0.03% or less, in mass%. And Cr: The method for producing a medium carbon steel sheet according to claim 3 or 4, further containing 1.6% or less.

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