JP7368692B2 - Manufacturing method of medium carbon steel plate - Google Patents

Description

The present invention relates to a medium carbon steel plate suitable for use as a material for deep drawing, for example, and a method for manufacturing the same.

Medium- to high-carbon steel sheets used as materials for deep-drawn products are required to be (i) soft because it is desired to reduce the forming load during deep-drawing, and (ii) to reduce the strength of the vertical walls of deep-drawn products. Since it is preferable that the height of the molded part be as uniform as possible and without variation in the circumferential direction of the molded article, it is generally required that the in-plane anisotropy of the Lankford value (r value) be small. Therefore, techniques such as those disclosed in Patent Documents 1 to 4 have been studied so far.

The technology of Patent Document 1 uses C: 0.15 to 2.0%, Si: 0.40% or less, Mn in mass % to suppress variations in the height of the vertical wall portion in a deep drawn product. : 0.5% or less, P: 0.03% or less, S: 0.03% or less, Cr: 2.0% or less, with the remainder consisting of Fe and inevitable impurities, and the carbide spheroidization rate is 90. % or more and the average carbide grain size is 0.4 μm or more. This carbon steel plate has an anisotropy Δr of −1.0 to 1.0.

The technology of Patent Document 2 is designed to provide a high carbon steel sheet that is molded into automobile parts and the like, particularly a high carbon steel sheet with good dimensional accuracy after molding and heat treatment of cylindrical parts. .25 to 0.60%, Mn: 0.20 to 1.50%, Cr: 0.60% or less, additionally Ti: 0.0l0 to 0.060%, B: 0.0003 to 0 We provide high carbon steel sheets containing .0050%. This high carbon steel plate has the following relationship between the X-ray integrated intensity ratio of the (222) plane and the (200) plane and the C content of the high carbon steel plate: (222)/(200)<5.5-5× By satisfying C (%), the molded product and the roundness after quenching are good.

The technology of Patent Document 3 provides a high carbon steel plate with low in-plane anisotropy and a method for manufacturing the same, which is compatible with parts that require high dimensional accuracy in forming processing and undergoes heat treatment such as quenching and tempering. In order to do so, 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 composition system of 0.035% or less, Cu: 0.03% or less, Ni: 0.025% or less, Cr: 0.3% or less, and the average carbide grain size is less than 0.5 μm. We offer high carbon steel sheets. This high carbon steel plate has an in-plane anisotropy index Δr of r value of more than −0.15 to less than 0.15.

The technology of Patent Document 4 has C: 0.25 to 0.75%, sol.Al: 0.01 to 0.10%, N: 0.0020 to 0.0100%, and 2≦(sol.Al/ A steel material having a steel composition satisfying N)≦20 is hot rolled at a coiling temperature of 550 to 680°C, and after pickling, cold rolled at a rolling reduction of 20 to 80%, and subsequently rolled at a rolling temperature of 650°C to Ac1. Box annealing and temper rolling are performed at high temperatures, and the average grain size of carbides in the steel is 0.5 μm or more, the spheroidization rate is 90% or more, and the texture of the steel strip is such that the (222) plane and the (200 ) surface and the C content of the high carbon steel sheet satisfies (222)/(200)≧6-8.0×C (%). We provide the manufacturing method. This high carbon steel strip has an average r value≧0.80 and an in-plane anisotropy index Δr±0.020.

Japanese Patent Application Publication No. 2018-141184 Japanese Patent Application Publication No. 2005-097659 Japanese Patent Application Publication No. 2003-089846 Japanese Patent Application Publication No. 2000-328172

However, in recent years, in deep-drawn products, not only is the height of the vertical wall portion as uniform as possible (with little variation) in the circumferential direction of the molded product, but also changes in the thickness of the vertical wall portion in the circumferential direction of the molded product have been improved. Small things are now required. Therefore, one aspect of the present invention provides a medium carbon steel sheet that is soft and has small in-plane anisotropy of r value, and a method for manufacturing the same, as a material having material properties suitable for obtaining such a deep drawn product. The purpose is to

As a result of intensive studies, the present inventors have found that medium carbon steel sheets containing C: 0.10% by mass or more and 0.70% by mass or less are softened (yield stress shows 400 MPa or less) and r value. The present invention was conceived by obtaining new knowledge regarding a means for achieving both the reduction of the in-plane anisotropy and the reduction of the in-plane anisotropy. More specifically, the present inventors conducted annealing with strain accumulated in the microstructural structure to appropriately soften the medium carbon steel sheet and to improve the effect of the base ferrite having random orientation in the microstructural structure. The present invention was conceived based on the discovery that the in-plane anisotropy of the r value can be effectively reduced by incorporating

That is, the medium carbon steel sheet in 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, has a yield stress of 400 MPa or less, and has an in-plane anisotropy of r value. It is characterized in that the sexual 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 are the ranks in the 0° direction, 45° direction, and 90° direction, respectively, with respect to the rolling direction. Ford value. Furthermore, r _max and r _min are the maximum and minimum values of r ₀ , r ₄₅ , and r ₉₀ , respectively.

A method for manufacturing a medium carbon steel sheet in one aspect of the present invention includes cold rolling a hot rolled steel sheet or annealed steel sheet containing C: 0.10% by mass or more and 0.70% by mass or less at a rolling reduction of 50% or more. A cold rolling process to obtain a cold-rolled sheet by heating the cold-rolled sheet at a temperature increase rate of 30°C/h or more in the temperature range from 400°C to 650°C, followed by Ac1 transformation at 650°C or more. and an annealing step of annealing the cold rolled sheet by holding the cold rolled sheet at an annealing temperature below a point.

Further, in the method for producing a medium carbon steel sheet in one aspect of the present invention, a hot rolled steel sheet or annealed steel sheet containing C: 0.10% by mass or more and 0.70% by mass or less is subjected to cold rolling at a rolling ratio of 50% or more. A cold rolling process to obtain a cold-rolled sheet by rolling, and after heating the cold-rolled sheet at a temperature increase rate of 30°C/h or more in the temperature range from 400°C to 650°C, the Ac1 transformation point is an annealing step of annealing the cold-rolled sheet by holding it at a higher annealing temperature; Features.

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

(a) is a diagram for explaining a method for manufacturing a medium carbon steel sheet in Embodiment 1 of the present invention, (b) is a diagram for explaining a cold rolling process, and (c) is a diagram for explaining a cold rolling coil. It is a figure for explaining the state of annealing. It is a figure for demonstrating the manufacturing method of the medium carbon steel plate in Embodiment 2 of this invention.

Embodiments of the present invention will be described below. It should be noted that the following description is provided for better understanding of the gist of the invention, and is not intended to limit the invention unless otherwise specified. Furthermore, in this application, "A to B" indicates that the number is greater than or equal to A and less than or equal to B.

First, the outline of the findings discovered by the present inventors will be explained as follows.

First, a method for softening a medium carbon steel sheet will be explained. By performing spheroidizing annealing on a cold-rolled medium carbon steel plate at an annealing temperature of 650°C or higher and lower than the Ac1 transformation point or higher than the Ac1 transformation point and lower than the Ac1 transformation point + 60°C, cementite in the microstructure is spheroidized. At the same time, the crystal grain size of the base ferrite is increased by causing recovery and recrystallization. This allows the medium carbon steel plate to be appropriately softened.

Next, basic means for reducing the in-plane anisotropy of the Lankford value (hereinafter referred to as r value) of a medium carbon steel sheet will be explained. In general, the r value in a certain direction in a steel sheet depends on the orientation state (degree of orientation in a specific direction) of the crystal orientation of each crystal grain in the crystal grains of base ferrite that exist in large numbers in the microstructure of the steel sheet. do. The crystal orientation of the crystal grains in the microstructure of the steel sheet is created by cold rolling and recrystallization annealing. At this time, if many recrystallized grains with 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 with random crystal orientation are generated, the in-plane anisotropy of the r value becomes small.

Medium carbon steel sheets have a metal structure in which cementite is dispersed in ferrite, and when cold rolling is performed, the strain caused by cold rolling is mainly accumulated at ferrite grain boundaries and ferrite/cementite interfaces. When a cold-rolled medium carbon steel sheet is subjected to recrystallization annealing below the Ac1 transformation point, unstrained recrystallized ferrite is generated from the ferrite grain boundaries where strain has accumulated and the ferrite/cementite interface, and the recrystallized ferrite grows over time. It grows with the passage of time. At this time, recrystallized ferrite having a texture is generated from the ferrite grain boundaries, and recrystallized ferrite with random orientation is generated from the ferrite/cementite interface.

Therefore, the present inventors came up with the idea that the in-plane anisotropy of the r value could be improved by increasing the proportion of recrystallized ferrite generated from the ferrite/cementite interface in a medium carbon steel sheet.

[Embodiment 1]
An embodiment of the present invention will be described below.

<Medium carbon steel plate>
Before explaining in detail the method for producing a medium carbon steel plate according to an embodiment of the present invention, which was conceived based on the above-mentioned knowledge, a medium carbon steel plate according to an embodiment of the present invention will be explained.

(steel composition)
Below, the steel composition (component composition) of the medium carbon steel sheet in this embodiment will be shown.

(C)
The present invention targets medium carbon steel (steel having a carbon content corresponding to so-called hypoeutectoid steel) in which the C (carbon) content in the steel is 0.10% by mass or more and 0.70% by mass or less. . C is the most basic alloying element in carbon steel, and its content greatly changes the amount of cementite and the metal structure when heated to the Ac1 transformation point or higher. Steel with a C content of less than 0.10% by mass has a small amount of cementite, and there are few ferrite grains with 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 medium carbon steel sheets.

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 is performed at a rolling reduction of 50% or more to accumulate a large amount of strain at the ferrite/cementite interface in the microstructure, problems such as a significant increase in the number of rolling passes and cracking of edges due to work hardening may occur. may occur. As a result, manufacturability and handling properties deteriorate.

Therefore, the present invention targets steel in which the C content is in the range of 0.10% by mass or more and 0.70% by mass or less. In applications requiring higher workability, the C content is preferably 0.50% by mass or less.

(Si)
Si (silicon) is an alloying element that acts as a deoxidizing agent. 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 that has a large influence on the workability of annealed steel sheets. If Si is added in excess, the ferrite will harden due to solid solution strengthening, which will cause cracks to occur during molding. Furthermore, when the Si content increases, scale flaws tend to occur on the surface of the steel sheet during the manufacturing process, leading to a decrease in surface quality. Therefore, when adding Si, the content should be 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 necessary. If the Mn content exceeds 2.0% by mass, the steel plate will become hard and workability will decrease. The Mn content is preferably 2.0% by mass or less, 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 when annealing is performed, and workability before quenching deteriorates. Therefore, when adding Cr, it is desirable to add it within a 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, P content and S content of up to 0.03% by mass are permissible. The P content and the S content are each preferably at most 0.025% by mass, more preferably at most 0.02% by mass.

The present invention can also be applied to steel to which the following elements are added for the purpose of improving properties such as hardenability and toughness. As a range that does not inhibit 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 can be added up to 0.3% by mass, Nb can be added up to 0.5% by mass, and B can be added up to 0.01% by mass.

The remainder other than the above components is Fe and unavoidable impurities. Here, unavoidable impurities refer to components such as O and N that are difficult to remove. These components are unavoidably mixed into the steel slab at the stage of melting it.

(Characteristic)
The medium carbon steel sheet in this embodiment has a yield stress of 400 MPa or less at room temperature, an in-plane anisotropy index Δr of the Lankford value of −0.2 or more and 0.2 or less, and r _max and r _min mutually The difference is 0.3 or less. Such mechanical properties are realized because the medium carbon steel sheet in this embodiment has a metal structure (tissue structure) consisting of a specific annealing structure by being manufactured by the method (conditions) described below.

(i) Yield Stress In the medium carbon steel sheet of this embodiment, the cementite particles are relatively spherical and coarse in the metal structure, and the intervals between the cementite particles are relatively wide. The wider the distance between cementite particles (the smaller the number of cementite particles per unit volume), the larger the area where soft ferrite exists continuously, and the easier it is to deform during processing. As a result, the medium carbon steel plate in this 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 Lankford value Lankford value (r value) is an index used to evaluate deformation anisotropy in the width direction and thickness direction during processing of metal materials. It is also called the plastic working strain ratio. Specifically, when performing a tensile test using a plate-shaped test piece, the r value of the plate-shaped test piece is determined based on the plate width and thickness before and after the tensile test. However, for thin plates like steel plates (for example, about 1 mm thick), it is difficult to accurately measure changes in plate thickness, so based on the assumption that the volume is constant before and after plastic working, the r value is calculated as follows. demand.

r=ln(W/W ₀ )/ln(L ₀・W ₀ /L・W)
Here, W ₀ and L ₀ are the plate width and gage distance in the parallel part of the plate-shaped test piece before the tensile test, respectively. Moreover, W and L are the plate width and gauge distance in the parallel part 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 the Lankford value. Also for the medium carbon steel plate of this embodiment, the Lankford value is determined based on the results of a tensile test conducted so that the elongation strain is 10 to 20%. In the following description of this specification, the r value means the Lankford value.

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

Δr=(r ₀ -2r ₄₅ +r ₉₀ )/2
Here, the medium carbon steel plate in this embodiment is manufactured by being subjected to various rolling treatments and annealing treatments. Based on the rolling direction in this rolling process (the direction in which the steel plate is extruded from the rotating rolling rolls), the r value in the 0° direction with respect to the rolling direction within the plate surface is defined as r ₀ . Similarly, r ₄₅ and r ₉₀ are the r value in the 45° direction and the r value in the 90° direction with respect to the rolling direction, respectively, within the plate plane.

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

(iii) Maximum value, minimum value of Lankford value When the values of r ₀ , r ₄₅ , and r ₉₀ 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 a range of −0.2 or more and 0.2 or less). However, the difference between the maximum value of r ₀ , r ₄₅ , and r ₉₀ and the minimum value of r ₀ , r ₄₅ , and r ₉₀ is 0.4, and in reality, the in-plane anisotropy It can be said that it has a lot of sex. Therefore, in the medium carbon steel plate in this embodiment, the absolute value of the difference between the maximum value of r ₀ , r ₄₅ , and r ₉₀ and the minimum value of r ₀ , r ₄₅ , and r ₉₀ is It stipulates that it is 0.3 or less.

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

As shown in FIG. 1(a), the method for manufacturing a medium carbon steel sheet in this embodiment includes a cold rolling step (S1) in which a hot rolled steel sheet or annealed steel sheet to be annealed is cold rolled. , a temperature raising step (S2) of heating the cold rolled sheet in a heating furnace to raise the temperature to around the Ac1 transformation point; A first temperature holding step (S3) in which the plate is soaked and held, and a cooling step (S4) in which the temperature of the annealed plate annealed through the above S2 and S3 is lowered to room temperature. Each of these steps will be explained below. In this embodiment, the above steps S2 to S4 are collectively referred to as an annealing step.

(cold rolling process)
First, a hot-rolled steel sheet that has been hot-rolled and then pickled to remove scale, or an annealed steel sheet that has been subjected to primary annealing is prepared. This hot rolled steel plate or annealed steel plate may be manufactured by a general method. Typically, hot rolled or annealed steel sheets are manufactured as coils. The above-mentioned primary annealing may be a process in which the cementite is spheroidized, for example, by holding the cementite at a temperature lower than the Ac1 transformation point or at a temperature higher than 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 this embodiment described above.

Hot-rolled steel sheets have a microstructure mainly composed of layered pearlite and pro-eutectoid ferrite. In addition, an annealed steel sheet has a microstructure mainly composed of base iron ferrite and spheroidized cementite. Hot-rolled steel sheets and annealed steel sheets have little strain accumulation in their microstructures.

Here, in the following explanation, terms are defined as follows. The ferrite in the microstructure of the hot rolled steel sheet or annealed steel sheet after cold rolling is referred to as processed ferrite. By annealing the hot-rolled steel sheet or annealed steel sheet, crystal grains newly generated at ferrite grain boundaries and ferrite/cementite interfaces are referred to as recrystallized ferrite.

Recrystallized ferrite consists of (i) ferrite that is generated so that the crystal orientations of each crystal grain have a random relationship with each other (hereinafter referred to as irregular ferrite), and (ii) a texture in which each crystal grain has a specific crystal orientation. ferrite (hereinafter referred to as oriented ferrite) that is generated to form a ferrite.

In this embodiment, the base ferrite in the medium carbon steel sheet after annealing is composed of processed ferrite (at least a phase in which the crystal orientation of processed ferrite is maintained) and recrystallized ferrite when 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 processed ferrite has disappeared due to annealing.

In the method for manufacturing a medium carbon steel sheet in this embodiment, a hot rolled steel sheet or an annealed steel sheet is subjected to cold rolling (finish rolling). FIG. 1(b) 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 annealed steel sheet is subjected to cold rolling using a cold rolling mill 2 at a rolling ratio (reduction ratio) of 50% or more. A cold-rolled coil 3 made of a rolled sheet is manufactured. The cold rolling mill 2 may be one commonly used for finish rolling, such as a Sendzimir cold rolling mill or a tandem rolling mill.

In the cold rolling step S1, when the cold rolling rate is low, 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 rate increases, the amount of strain accumulated at the ferrite/cementite interface increases. In order to appropriately generate and grow recrystallized grains from the ferrite/cementite interface by annealing after cold rolling, it is necessary to perform cold rolling at a rolling ratio of 50% or more. By performing cold rolling at a rolling reduction of 50% or more, sufficient strain can be accumulated at the ferrite/cementite interface. Therefore, when recrystallization occurs in the microstructure in the cold-rolled sheet, the ferrite grain boundaries It is also possible to increase the proportion of recrystallized ferrite (irregular ferrite) generated from the ferrite/cementite interface.

There is no particular need to set an upper limit for the rolling ratio in the cold rolling process S1, but when it exceeds 90%, work hardening becomes significant, leading to an increase in cost due to an increase in the number of cold rolling passes, and in some cases In this case, problems such as cracking of the edge of the steel plate may occur.

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

(Temperature raising process)
FIG. 1C is a diagram for explaining how the cold rolled coil 3 (ie, cold rolled sheet) is annealed. As shown in FIG. 1C, the cold-rolled coil 3 is housed in a 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 processes from the temperature raising step S2 to the cooling step S4 are performed in the heating furnace 4. Hereinafter, the cold rolled coil 3 (ie, cold rolled sheet) to be annealed will be referred to as an annealing target material.

The relationship between the conditions defined in the temperature raising step S2 in this embodiment and the state (2-1) of the microstructure of the material to be annealed will be described below. In the temperature raising step S2, heating is performed in a temperature range from 400° C. to 650° C. at a temperature increasing rate of 30° C./h or more. If the temperature increase rate in the temperature range from 400°C to 650°C is slow, only strain recovery will proceed by the time the recrystallization temperature is reached, and recrystallized grains with random orientation from the ferrite/cementite interface (irregular ferrite ) production is inhibited. By increasing the temperature to 650° C. at a temperature increasing rate of 30° C./h or higher, more recrystallized grains from the ferrite/cementite interface are generated than recrystallized grains (oriented ferrite) at the ferrite grain boundaries. The recrystallization temperature of medium carbon steel sheets is affected by the degree of processing strain and alloying elements, but it is generally completed when heated to 650°C.

Even if slow heating or soaking is performed between the temperature raising step S2 and the first temperature holding step S3, there is no effect on the anisotropy improvement effect. Therefore, the temperature increasing step S2 may include a process of increasing the temperature to 650° C. at a temperature increasing rate of 30° C./h or more, and then slowly heating it, holding it uniformly heated, and the like.

(First temperature holding step)
The relationship between the conditions defined in the first temperature holding step S3 of this embodiment and the state (2-2) of the structure of the annealing target material will be described below. In the first temperature holding step S3, cementite is spheroidized by soaking at an annealing temperature of 650°C or higher and lower than the Ac1 transformation point, and recrystallized grains (irregular ferrite) generated with random orientation grow. do. As irregular ferrite grows, processed ferrite is incorporated into irregular ferrite, and the amount of processed ferrite decreases. Further, since the amount of oriented ferrite produced in the temperature raising step S2 is small, the oriented ferrite is difficult 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 this embodiment, cooling is performed from an annealing temperature of 650° C. or higher and lower than the Ac1 transformation point. The microstructure of the medium carbon steel sheet after cooling is the same as the microstructure 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 metallographic structure (tissue structure) mainly composed of irregular ferrite having random orientation.

In the cooling step S4 of this embodiment, the temperature decreasing 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 forcedly cooled by blowing cooling gas.

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

(Advantage of invention)
In the method for manufacturing a medium carbon steel sheet in this embodiment, during annealing after performing cold rolling at a rolling reduction of 50% or more, heating is performed in a temperature range from 400°C to 650°C at a temperature increase 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 medium carbon steel sheets. Specifically, a medium carbon steel plate having a yield stress of 400 MPa or less and a small in-plane anisotropy of r value can be obtained. In the medium carbon steel sheet in this embodiment, 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. By using the medium carbon steel plate of the present invention for deep drawing, a molded product with small variations in thickness and diameter can be obtained.

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

In the method for producing a medium carbon steel plate in the first embodiment, the material to be annealed is annealed by soaking and holding 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 this embodiment, soaking is carried out at an annealing temperature equal to or higher than the Ac1 transformation point.

A method for manufacturing a medium carbon steel plate in this embodiment will be explained using FIG. 2. FIG. 2 is a diagram for explaining the method for manufacturing a medium carbon steel plate in this embodiment. In FIG. 2, the horizontal axis represents time t, and the vertical axis represents temperature TE. Note that the annealing cycle shown in FIG. 2 is an example, and the specific annealing conditions (temperature control) may be changed as appropriate within a range that satisfies the conditions described below. In FIG. 2, parts (1) to (5) surrounded by dotted lines are used as reference numbers to explain the state at that point.

Note that the chemical 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 manufacturing a medium carbon steel sheet according to the present embodiment includes a cold rolling step (S11) in which a hot rolled steel sheet or annealed steel sheet to be annealed is subjected to cold rolling; A first temperature raising step (S12) in which the plate is heated in a heating furnace to raise the temperature to around the Ac1 transformation point, and following the first temperature raising step, the temperature of the cold rolled sheet is raised to an annealing temperature equal to or higher than the Ac1 transformation point. A second temperature raising step (S13) is included. The method for manufacturing a medium carbon steel sheet according to the present embodiment further includes, following S13, a second temperature holding step (S14) of heating the cold rolled sheet to an annealing temperature and maintaining the temperature; It includes a slow cooling step (S15) of lowering the temperature of the annealed plate annealed through S14. In this embodiment, the above steps S12 to S15 are collectively referred to as an annealing process. After S15, the annealed plate is cooled to room temperature to obtain the medium carbon steel plate of this embodiment. According to the method for producing a medium carbon steel sheet in this embodiment, it is possible to promote spheroidization and coarsening of cementite (increase in the cementite particle spacing due to coarsening of cementite particles), and to produce an even softer medium carbon steel sheet. Obtainable. Each of these steps will be explained below.

(Cold rolling process/first temperature raising process)
The cold rolling step S11 and the first temperature raising step S12 may be performed in the same manner as the cold rolling step S1 and the temperature raising step S2 in the first embodiment described above, respectively. Therefore, the explanation will be omitted.

(Second temperature increase step)
The state (3) of the structure of the annealing target material in the second temperature raising step S13 of this embodiment will be described below. In the second temperature raising step S13, heating is performed to a temperature higher than the Ac1 transformation point. Generally, when medium carbon steel is heated to the Ac1 transformation point or higher, austenite is generated by dissolving cementite. In the second temperature raising step S13 in this embodiment, during recrystallization during temperature raising, irregular ferrite with random orientation generated from the ferrite/cementite interface is preferentially transformed into austenite.

Here, it is known that during the transformation from ferrite to austenite, the generated austenite has a specific crystal orientation relationship with the original ferrite. Therefore, austenite generated in the second temperature raising step S13 of this embodiment has the same orientation relationship as irregular ferrite having random orientation (inherits the crystal orientation of 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 this embodiment and the state (4) of the structure of the annealing target material will be described below. In the second temperature holding step S14, the irregular austenite produced in the second temperature raising step S13 grows as cementite melts by soaking and holding at a temperature equal to or higher than the Ac1 transformation point. This results in 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 of the cementite does not dissolve.

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

In the second temperature holding step S14, the temperature equal to or higher than the Ac1 transformation point for soaking and holding is referred to as an annealing temperature. The annealing temperature in the second temperature holding step S14 of this embodiment is equal to or higher than the Ac1 transformation point and equal to or lower than the Ac1 transformation point +60°C. The state of existence (density) of undissolved cementite after the second temperature holding step S14 determines the size of cementite particles in the medium carbon steel sheet of this 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, once the size of the cementite particles is determined, the spacing between the cementite particles (the number of cementite particles per unit volume) is determined. The wider the interval between cementite particles, the larger the continuous portion of soft ferrite, which facilitates deformation during processing. That is, the larger the interval between cementite particles in a medium carbon steel sheet after annealing, the softer it becomes.

In the second temperature holding step S14, when the annealing temperature becomes high, undissolved cementite in the microstructure disappears and the amount of ferrite present decreases. When there is no undissolved cementite, cementite needs to be newly nucleated and precipitated during the phase transformation (ferrite + cementite is generated from austenite) that occurs during the slow cooling in the slow cooling step S15. Therefore, compared to the case where there is undissolved cementite, the transformation in the slow cooling step S15 is started in a supercooled state. In this case, the driving force for transformation becomes large and a pearlite structure, which is advantageous in terms of transformation speed, is generated. As a result, the metal structure after annealing becomes a ferrite+pearlite structure. Therefore, when the heating temperature is higher than the Ac1 transformation point, a spherical cementite structure is not obtained after annealing, and the formability of the steel sheet is poor (a hard steel sheet is obtained).

Therefore, from the viewpoint of suppressing cementite spheroidization and pearlite formation, 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 defined in the slow cooling step S15 of this embodiment and the state (5) of the structure of the annealing target material will be described below.

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

(1) High temperature region above the Ac1 transformation point: As the temperature decreases, austenite decreases (C concentration in austenite increases) and recrystallized ferrite grows. As a result, ferrite having the crystal orientation of the remaining ferrite increases in the microstructure.

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

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

(2-ii) When ferrite newly nucleates at the austenite/undissolved cementite interface, ferrite nucleates and grows at the austenite/undissolved cementite interface. As a result, the transformation progresses and irregular ferrite is generated in the microstructure.

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

(3) Lower temperature range than the Ac1 transformation point: residual ferrite grows (ferrite interface decreases, that is, ferrite grain size increases).

Further, in the steel sheet as a whole after annealing, the amount of ferrite generated due to the growth of residual ferrite increases.

Therefore, in the method for manufacturing a medium carbon steel sheet in this embodiment, it is preferable to sufficiently generate irregular ferrite in the first temperature raising step S12. Therefore, in the method for producing a medium carbon steel sheet in this embodiment, cold rolling is performed by 50% or more in the cold rolling step S11, and the temperature is increased by 30° C./h to 650° C. in the first temperature increasing step S12. It stipulates that it should be the temperature rate.

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

By slow cooling until the above transformation is completed, a structure in which coarse spherical cementite is dispersed in the base ferrite is obtained, and the medium carbon steel sheet becomes soft. In order to obtain sufficient spheroidization of cementite, cooling from heating above the Ac1 transformation point is preferably carried out gradually at a rate of 5 to 30° C./h until the phase transformation is completed. When the cooling rate is less than 5° C./h, annealing takes a very long time, which impairs productivity. If the cooling rate is faster than 30° C./h, even if a sufficient amount of undissolved cementite remains, the diffusion of elements may not catch up and pearlite may be produced. From the viewpoint of productivity and workability of the steel plate, 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, by appropriately controlling the rolling rate, heating rate, annealing temperature, and cooling rate, the in-plane anisotropy of the rolling rate and r value can be improved. A medium carbon steel sheet with improved properties is obtained.

(Advantage of invention)
In the method for manufacturing a medium carbon steel sheet in this embodiment, during annealing after performing cold rolling at a rolling reduction of 50% or more, heating is performed in a temperature range from 400°C to 650°C at a temperature increase rate of 30°C/h or more. After that, annealing is performed at an annealing temperature of not less than the Ac1 transformation point and not more than the Ac1 transformation point +60°C. After annealing, it is slowly cooled at a cooling rate of 5 to 30°C/h. This has made it possible to improve the in-plane anisotropy of the r value and to make the medium carbon steel sheet softer. Specifically, a medium carbon steel plate having a yield stress of 400 MPa or less and a small in-plane anisotropy of r value can be obtained. In the medium carbon steel sheet in this embodiment, 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. By using the medium carbon steel plate of the present invention for deep drawing, a molded product with small variations in thickness and diameter can be obtained.

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

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

Steel having the components shown in Table 1 was hot rolled, and the obtained steel plate was pickled to remove scale. The obtained hot rolled steel sheet was subjected to primary annealing under the following conditions. Some of the samples were subjected to the next step without being subjected to primary annealing.
Condition (a): [Ac1 transformation point - 100°C to Ac1 transformation point] × 10 to 60 h holding condition (b): [Ac1 transformation point to Ac1 transformation point + 50°C] × 4 to 20 h hold, then down to Ar1 point or less Slow cooling at a cooling rate of 30° C./h or less Note that condition (b) also includes the case where the temperature is maintained at a temperature below the Ac1 transformation point before and after the heating is maintained at a temperature above the Ac1 transformation point.

Further, the hot-rolled steel sheets and the annealed steel sheets annealed under 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 finish cold rolling and the annealing cycle in finish annealing are shown in Table 2 below. Then, the yield stress and in-plane anisotropy of the r value of the obtained annealed plate were measured.

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

r=ln( _Wx / _W0 )/ln( _L0・_W0 / _LX・_WX )
Here, W ₀ and L ₀ are the sheet width and gauge distance before the test, and W _X and L _x are the sheet width and gauge distance after 15% tensile elongation is applied.

As an index of the in-plane anisotropy of the r value, the Δr value of each sample material was calculated using the following formula.
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 based on its absolute value |Δr value|. Note that _x in rx indicates the cutting direction of the test piece with respect to the rolling direction. For example, _r45 is an r value measured using a test piece taken at 45° to the rolling direction.

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

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

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

The steels of Comparative Example 1 (Nos. 2, 4, 7, 10, 16, 19, 21, 26, 29, 31) whose cold rolling reduction is lower than 50% have |Δr values| of 0. 2 or more, and r _max - r _min is 0.3 or more, and the in-plane anisotropy of the r value is large compared to others. In addition, Comparative Examples 2 (Nos. 6, 13, 14, 17, and 24), in which the heating temperature of finish annealing is lower than the range of the present invention even when the rolling reduction of finish cold rolling is 50% or more, are of the same steel type and are within the range of the present invention. Harder than those subjected to specified finish annealing.

On the other hand, the present invention example (No. 3, 5, 8, 9, 11, 12, 15, 18, 20, 22, 23, 25, 27, 28, 30, 32, 33), the in-plane anisotropy of the r value was is smaller and softer than Comparative Example 2 of the same steel type. Thus, it can be seen that by applying the present invention, a medium carbon steel sheet that is soft and has small in-plane anisotropy of r value can be obtained.

An example will be shown in which the influence of the temperature increase rate from 400 to 650°C during final annealing on the in-plane anisotropy of r value was investigated using B to I steels in Table 1. A hot-rolled plate made of B to I steel is annealed at 700°C for 20 hours, cold rolled at a rolling reduction of 65%, and then heated to a temperature of 650°C or higher by varying the heating rate from 400 to 650°C. Finish annealing was performed.

These test results are shown in Table 3 together with the final annealing conditions.

Steels of comparative examples (Nos. 41, 44, 46, 48, 50, 54, 56, 58, 60, 62, 66) whose temperature increase rate from 400 to 650 °C in final annealing are lower than 30 °C/h are , |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 large compared to others. On the other hand, examples of the present invention (No. 42, 43, 45, 47, 49, 51, 52, 53, 55, It can be seen that in the steels Nos. 57, 59, 61, 63, 64, 65, and 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 (3)

A cold rolling step of cold rolling a hot rolled steel plate or annealed steel plate at a rolling reduction of 50% or more to obtain a cold rolled plate;
After heating the cold-rolled sheet at a temperature increase rate of 30°C/h or more in the temperature range from 400°C to 650°C, the cold-rolled sheet is maintained at an annealing temperature of 650°C or more and less than the Ac1 transformation point. an annealing step of annealing the plate,
The hot rolled steel sheet or annealed steel sheet has, in mass %, C: 0.10% or more and 0.70% or less, Si: 0.02% or more and 0.50% or less, Mn: 2.0% or less, and P: 0. .03% or less, S: 0.03% or less, Al: 0.1% or less, and Cr: 1.6% or less,
A medium carbon steel plate obtained by cooling the annealed plate annealed in the annealing process from the annealing temperature to room temperature has a yield stress of 400 MPa or less and 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,
The hot rolled steel plate or annealed steel plate is
In mass%, Mo: 0.5% by mass or less, Cu: 0.3% by mass or less, Ni: 2.0% by mass or less, Ti: 0.3% by mass or less, V: 0.3% by mass or less, Nb Optionally contains one or more selected from the group consisting of: 0.5% by mass or less, and B: 0.01% by mass or less,
A method for producing a medium carbon steel sheet, characterized in that the remainder consists of Fe and inevitable impurities .
(here,
Δr=(r ₀ -2r ₄₅ +r ₉₀ )/2
r ₀ : Lankford value at 0° to the rolling direction r ₄₅ : Lankford value at 45° to the rolling direction r ₉₀ : Lankford value at 90° to the rolling direction r _max : The above r ₀ , r ₄₅ , and r ₉₀ ; r _min : the minimum value of r ₀ , r ₄₅ , and r ₉₀ ) A cold rolling step of cold rolling a hot rolled steel plate or annealed steel plate at a rolling reduction of 50% or more to obtain a cold rolled plate;
After heating the cold-rolled sheet at a temperature increase rate of 30°C/h or more in the temperature range from 400°C to 650°C, the cold-rolled sheet is annealed by holding at an annealing temperature of Ac1 transformation point or higher. an annealing step of applying,
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, in mass %, C: 0.10% or more and 0.70% or less, Si: 0.02% or more and 0.50% or less, Mn: 2.0% or less, and P: 0. .03% or less, S: 0.03% or less, Al: 0.1% or less, and Cr: 1.6% or less,
A medium carbon steel plate obtained by cooling the annealed plate annealed in the annealing process from the annealing temperature to room temperature has a yield stress of 400 MPa or less and 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,
The hot rolled steel plate or annealed steel plate is
In mass%, Mo: 0.5% by mass or less, Cu: 0.3% by mass or less, Ni: 2.0% by mass or less, Ti: 0.3% by mass or less, V: 0.3% by mass or less, Nb Optionally contains one or more selected from the group consisting of: 0.5% by mass or less, and B: 0.01% by mass or less,
A method for producing a medium carbon steel sheet, characterized in that the remainder consists of Fe and inevitable impurities .
(here,
Δr=(r ₀ -2r ₄₅ +r ₉₀ )/2
r ₀ : Lankford value at 0° to the rolling direction r ₄₅ : Lankford value at 45° to the rolling direction r ₉₀ : Lankford value at 90° to the rolling direction r _max : The above r ₀ , r ₄₅ , and r ₉₀ ; r _min : the minimum value of r ₀ , r ₄₅ , and r ₉₀ ) The annealing step is a slow cooling step in which the annealed plate is gradually cooled from the annealing temperature to a temperature lower than the Ac1 transformation point at which austenite phase transformation is completed at a cooling rate of 5 to 30° C./h. including;
The method for manufacturing a medium carbon steel plate according to claim 2 , wherein the medium carbon steel plate is obtained by being cooled to room temperature after the slow cooling step.

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