WO2009078261A1

WO2009078261A1 - Steel sheets and process for manufacturing the same

Info

Publication number: WO2009078261A1
Application number: PCT/JP2008/071597
Authority: WO
Inventors: Nobusuke Kariya; Kazuhiro Seto
Original assignee: Jfe Steel Corporation
Priority date: 2007-12-19
Filing date: 2008-11-20
Publication date: 2009-06-25
Also published as: EP2246450A4; EP2246450A1; EP2246450B1; KR20130035276A; CN101903547B; CN101903547A; KR20100076073A

Abstract

Provided are steel sheets which are soft and have excellent workability and excellent hardenability and a process for manufacturing the same. A steel sheet characterized by having a composition which contains by mass C: 0.3 to 0.7%, Si: 0.1% or below, Mn: 0.20% or below, P: 0.01% or below, S: 0.01% or below, Al: 0.05% or below, and N: 0.0050% or below with the balance being Fe and unavoidable impurities and a structure which comprises ferrite, graphite and cementite and in which the total volume fraction of ferrite, graphite and cementite accounts for at least 95% of the whole structure and the ratio of the volume of graphite to the total volume of graphite and cementite (degree of graphitization) is 5% or above, wherein the mean particle diameter of graphite and cementite is 5μm or below; and a steel sheet characterized by having the above composition and the above structure, wherein the sum of the volume fractions of graphite and cementite present in ferrite grains accounts for 15% or below of the total of graphite and cementite.

Description

Technical field

The present invention relates to a steel plate suitable for applications such as automobile parts, and more particularly to a steel plate excellent in workability and hardenability and a method for producing the same. Background art

Steel plates used for tools or automobile parts (gears, missions) are often used after being processed into a desired shape and then subjected to heat treatment such as quenching and tempering. Such copper plates are required to have excellent workability because they are processed into various complicated shapes. Recently, there has been a strong demand for reduction in manufacturing costs for these parts, enabling processing technologies aimed at eliminating processing steps and changing processing methods, for example, increasing the thickness of automobile drive system parts using high-carbon steel sheets. As a result, double-acting machining technology has been developed and realized in part. As a result, the demands on workability for steel sheets used in the above-mentioned automobile parts and the like are becoming increasingly severe, and they are required to be softer and have higher ductility. For example, when processing by cold forging, lower yield stress is required. In addition, excellent stretch flangeability is desired when hole expansion (parling) is performed after punching. In order to meet these demands, techniques for improving the workability by graphing C in steel are being studied. For example, in Patent Document 1, in mass%, C: 0.40 to 0.80%, Si: 0.20 to 2.00%, Mn: 0.20 to 1.50%, Al: 0 · 001 to 0 · 150¾, Ρ: 0.018% or less , S: 0.010¾ or less, Ν: 0.0050% or less, with balance Fe and inevitable impurities, mainly composed of ferrite phase and graphite phase, TS≤60kgf / 讓² A method of manufacturing a steel plate suitable for a tiller claw component having good workability, toughness, and hardenability is disclosed. Further, Patent Document 2 includes, in mass%, C: 0.10 to 0.45%, Si: 0.05 to 1.00%, Μη: 0 · 05 to 0.50%, Nb: 0.005 to 0 · 1%, Α1: 0.01 to 10.00%, Ν: 0 002 to 0.010¾, B: 3 to 50ppm, Ca: 0.001 0.01% and Ni: 0 to 2.00%, the balance consists of Fe and impure impurities, in impurities P: 0.012% or less, S: a hot-rolled steel sheet is 0.008% or less, after 0. L～10hr maintained at a temperature in the range of a _Cl to Ac ₃ point, the cooling rate of 20 to 100 ° C / hr At room temperature, and then box anneal in the temperature range of 650-750 Thus, a method for producing a medium carbon steel sheet excellent in workability is disclosed, characterized by graphitizing 50 or more area areas of cementite in steel. Further, in Patent Document 3, in mass%, C: 0.20 to 1.00%, Si: more than 0.20%, 1. 20% or less,: η: 0 · 05 to 0.50%, Ν: 0 005 to 0.01 5%, Β: 0 · 2 ΧΝ% to 0.8 Χ Ν¾, and Al: less than 0.05%, and 1. OX (N-B)% to 5. OX (N_B) %, With the balance Fe and unavoidable impurities, P: 0.020% or less and S: 0.010% or less as impurities, ferrite, graphite and cementite A high carbon thin steel sheet having a work structure and good workability and a method for producing the same are disclosed.

Patent Document 1: Japanese Patent Laid-Open No. 1-025946

Patent Document 2: JP-A-7-258743

Patent Document 3: Japanese Patent Laid-Open No. 4-202744 Disclosure of Invention

Conventionally, in order to improve the workability by graphitizing C in copper, it has been essential to add a large amount of Si, as described in Patent Documents 1 and 3. However, the addition of Si hardens the ferrite itself and makes it difficult to obtain good processability. In addition, as shown in Patent Document 2, by using a component system containing B and Nb and performing annealing twice under predetermined conditions, graph addition and high ductility can be achieved without necessarily adding a large amount of Si. Technology to achieve this has been developed, but performing annealing twice will increase costs. Here, Patent Document 2 is a technique for graphitizing 50% or more of the cementite in copper, and the component composition of copper disclosed in the Example of Patent Document 2 has a large amount of Si, 0. The amount exceeds 20%. In addition, the copper plates described in Patent Documents 1 to 3 are soft and have excellent bending workability and elongation properties in tensile tests, but depending on the heating conditions during the quenching of the steel sheet, the graph items may be cemented. May not be sufficiently dissolved, resulting in poor quenching. Moreover, although the steel sheets described in Patent Documents 1 to 3 are soft, they have a problem that they are not necessarily excellent in stretch flangeability, which is an index for hole expansion workability after punching.

An object of the present invention is to provide a steel plate that is soft, has excellent workability, has excellent hardenability, has excellent stretch flangeability, and has excellent workability, and a method for producing the same. As a result of repeated investigations on the above-mentioned problems of the prior art, the present inventors have found that even in a high carbon steel, even when the Si content is very low, specifically 0.1% or less, the graph items and It was found that by controlling the distribution of cementite, good workability could be obtained without necessarily increasing the graphing rate, and good hardenability and stretch flangeability could be secured. That is, as a result of diligent research on the influence of the structure on the strength, hardenability and stretch flangeability of steel sheets containing 0.3 to 0.7 mass ¾, the following was found. .

(1) For softening, the organization should include ferrites, graph items, and cementite.The total volume ratio of ferrite, graph items, and cementite in the entire organization should be 95% or more. It is effective to make the volume ratio of the graph items in the entire image 5% or more.

(2) In order to improve hardenability, the average particle size of graphite and cementite must be 5 / zm or less.

(3) Cooling conditions after hot rolling are extremely important for controlling the grain size of graphite and cementite.

(4) In order to improve stretch flangeability, the total volume ratio of graphite and cementite in the ferrite particles that occupy the entire graphite and cementite must be 15% or less. is there.

(5) Cooling conditions after hot rolling are extremely important for controlling the volume ratio of graphite and cementite present in ferrite grains.

The present invention has been made based on such knowledge,

% By mass: C: 0.3 to 0.7¾, Si: 0.1% or less, Mn: 0.20% or less, P: 0.01% or less, S: 0.01% or less, A1: 0 .05% or less, N: 0.0050% or less, with the balance consisting of Fe and unavoidable impurities, having a structure including flylite, graphite, and cementite, and occupying the entire structure The total volume ratio of ferrite, graph item, and cementite is 95% or more, and the volume ratio of graph items (graph rate) in the entire graph item and cementite is 5% or more. Graphite and cementite The copper plate is characterized in that the average particle size of the copper is 5 μπι or less.

The copper plate of the present invention further contains at least one selected from Ni: 3.0% or less, B: 0.005% or less, and Cu: 0.1% or less by mass%. Is preferred. The steel sheet of the present invention is a hot-rolled sheet obtained by hot rolling a steel having the above composition at a finishing temperature of 800 to 950 ° C, and the hot-rolled sheet after the hot rolling has an average of 50/3 or more. After cooling to a cooling stop temperature of 500 ° C or lower at a cooling rate, cutting is performed at a cutting temperature of 450 ° C or lower, and the hot-rolled sheet after the cutting is manufactured by a method of annealing at an annealing temperature of 720 or lower. it can.

The present invention

In mass%, C: 0.3 to 0.7¾, Si: 0.1% or less, Mn: less than 0.15%, P: 0.01% or less, S: 0.01% or less, A 1: 0 .05% or less, N: 0.005% or less, a composition comprising the balance Fe and inevitable impurities, having a structure including ferrite, graphite, and cementite, and accounting for the entire structure The total volume ratio of graph and graphite and cementite is 95% or more, the volume ratio of graphitems (graphite ratio) in the entire graphitem and cementite is 5¾ or more, Provided is a copper plate characterized in that the total volume ratio of graphite and cementite existing in ferrite grains occupying the whole is 15¾ or less.

The steel sheet of the present invention may further contain at least one selected from the following in terms of mass%: Ni: 3.0¾ or less, B: 0.005% or less, Cu: 0.1% or less. preferable.

The steel sheet of the present invention is a hot-rolled sheet obtained by hot-rolling steel having the above composition at a finishing temperature of 800 to 950 ° C, and the hot-rolled sheet after the hot-rolling is performed at 50 / s or more. After cooling to a cooling stop temperature of 600 ° C or less at an average cooling rate, cutting at a cutting temperature of 550 ° C or less, and annealing the hot-rolled sheet after the cutting at an annealing temperature of 720 ° C or less Can be manufactured.

According to the present invention, it has become possible to produce a copper plate that is soft and has excellent workability and also has excellent hardenability. In particular, the steel sheet of the present invention can be easily manufactured at low cost because it only needs to control the components and the cooling conditions after hot rolling. In addition, the steel sheet of the present invention is soft and excellent in workability, so it is suitable for thickening processing of automobile drive system parts. Even if it is applied to parts with complicated shapes, it can process and weld multiple parts. This eliminates the need to improve the productivity of automobile parts and reduce costs. Furthermore, the steel sheet of the present invention does not suffer from quenching failure due to unmelted graph eye and cementite during heating at high frequency or the like. According to the present invention, it has become possible to produce a steel sheet that is soft and has excellent stretch flangeability and excellent workability. In particular, the steel sheet of the present invention can be easily produced at low cost because it is only necessary to control the components and the cooling conditions after hot rolling. In addition, the steel sheet of the present invention is soft and excellent in workability such as stretch flangeability, so it is suitable for thickening processing of automobile drive system parts. Even if it is applied to parts with complex shapes, it is not necessary to process or weld multiple parts, improving the productivity of automobile parts and reducing costs. Brief Description of Drawings

Fig. 1 is a graph showing the relationship between cementite 卜, the average particle diameter d of graph items, and ΔΗν. FIG. 2 is a graph showing the relationship between the cementite present in the ferrite grains, the volume ratio S of the graph items, and the average. BEST MODE FOR CARRYING OUT THE INVENTION

Below, the manufacturing method of the steel plate excellent in workability which is this invention is demonstrated in detail. “%” Representing the amount of a component means “mass ¾” unless otherwise specified.

1) Composition

C: 0.3 ~ 7

C is an element that forms a graph item. If the amount of C is less than 0.3%, the hardness after quenching cannot be secured, and if it exceeds 0.7%, the copper plate becomes hard even if it is graphitized, and the workability decreases. Therefore, the C content is 0.3-0.7%.

Si: 0.1% or less

When the Si content exceeds 0.1%, the ferrite becomes hard and the workability deteriorates. Therefore, the Si content is 0.1% or less, preferably 0.05% or less.

Mn: 0.20% or less

If the amount of Mn exceeds 0.20%, graphite formation is inhibited, so Mn is 0.20% or less, preferably 0.10% or less.

P: 0.01% or less

P is segregated at grain boundaries and lowers workability, and has the effect of stabilizing cementite and inhibiting graphite formation, so it is desirable to reduce it as much as possible. Therefore, the P content is 0.01% or less, preferably 0.008% or less.

S: 0.01% or less

It is desirable to reduce S as much as possible because S forms sulfides such as MnS and lowers workability, and stabilizes cementite and inhibits the formation of graphite. Therefore, the S content is 0.01% or less, preferably 0.007% or less. A1: Less than 0.05¾

Al is an element that binds to solute N to form A1N, detoxifies the adverse effects of solute N, which has the effect of inhibiting graphite formation, and promotes graphite formation using A1N as a nucleus.

For this reason, the amount of A1 is preferably 0.003% or more, but if it exceeds 0.05%, the cleanliness of the steel decreases and the workability deteriorates, so the amount of A1 is 0.05% or less, preferably 0.04% or less. To do.

Ν: 0 0050% or less

When the amount of soot exceeds 0.0050%, the action of stabilizing the cementite of solid solution soot becomes remarkable and the formation of graphite is inhibited. Therefore, the soot content is 0.0050%, preferably 0.0040% or less.

The balance is Fe and inevitable impurities, but it is preferable that at least one selected from Ni: 3.0% or less, B: 0.005% or less, Cu: 0.1% or less is contained for the following reasons. .

Ni: 3.0¾ or less

Ni is an element that promotes the formation of graphite and is also an element effective for improving hardenability. To obtain these effects, Ni is preferably contained in an amount of 0.1% or more, but the Ni content exceeds 3.0%. And the effect is saturated. For this reason, the Ni content is 3.0% or less, preferably 0 · 1 to 3 · 0¾, more preferably 0 · 3 to 1 · 0%.

Β: 0.005% or less

Β is a useful element that combines with Ν to form ΒΝ and acts as a core for graphite formation, and also an effective element for improving hardenability. To obtain these effects, 0.0005% It is preferable to contain the above, but if the amount of soot exceeds 0.005%, the effect is saturated. For this reason, the soot amount is 0.005% or less, preferably 0.0005 to 0.005%, more preferably 0.0010 to 0.0040%.

Cu: 0.1% or less

Cu is an element that promotes the formation of graphite, and is also an element effective for improving hardenability. To obtain these effects, it is preferably contained in an amount of 0.01% or more, more preferably 0.02% or more. However, when the amount of Cu exceeds 0.1%, the effect is saturated. Therefore, the Cu content is 0.1% or less, and more preferably 0.07% or less.

2) Organization In order to soften the steel sheet and improve the bending strength and elongation properties in tensile tests, the structure should include ferrite, graphite, and cementite. The sum of the volume fractions of cementite must be 95% or more, and the graph item ratio in the graph items and the entire cementite must be 5% or more. At this time, the present invention also includes the case where the graph item rate is 100%, that is, when all cementites are converted into graph items, the same effect can be obtained. If the sum of the volume fractions of fillite, graphite, and cementite is less than 95%, that is, if the volume fraction of other phases exceeds 5%, the workability deteriorates. In addition, when the graph item rate is less than 5%, the workability deteriorates.

Here, the volume ratio of ferrite, graphite, and cementite was obtained as follows. In other words, after polishing the 1/4 position of the thickness cross section in the rolling direction of the copper plate, it corroded with nital, and with an optical microscope, observed 5 spots per field at a magnification of 400 times, 10 fields (50 places in total), These images are analyzed with Media Cybernetics image analysis software “Image Pro Plus ver. 4.0” to determine the area of ferrite, graphite, and cementite, and the percentage of the total observation area ( The area ratio was defined as the volume fraction of each of ferrite, graphite, and cementite. In addition, the ratio (area ratio) of the area (Sgr) of the graph item to the sum of the area (Sgr) of the graph item and the area (Scm) of the cementite (volume ratio of the graph item) ) That is, the graph item rate (¾) can be expressed by the following equation.

Graph item rate = {Sgr / (Sgr + Scm)} X 100

Only by controlling the sum of the volume ratios of ferrite, graphite, and cementite, and the ratio of graphite, it is not always possible to obtain excellent hardenability, especially when induction hardening is performed. That is, in the present invention, in order to ensure excellent hardenability, the average particle size of cementite and graphite needs to be 5 m or less. More preferably, it is as follows. The present inventors have made various studies in order to obtain excellent hardenability. An example of examination is shown below. C: 0.55%, Si: 0.01%, Mn: 0.10%, P: 0.003%, S: 0.0006%, A1: 0.005%, N: 0.0019% , Ni: 0.50%, Β: 0-0013%, steel slab composed of the balance Fe and inevitable impurities is heated to 1150 ° C, then rough-rolled for 5 passes, and finished for 7 passes. A hot rolled sheet having a thickness of 4.0 mm was obtained at ° C, picked at a cutting temperature of 430 ° C, pickled, and batch-annealed at 720 ° C for 40 hours. At this time, in order to change the grain size of cementite and graphite, the temperature range from finish rolling to the take-off temperature was changed within the range of average cooling rate from air cooling (5 ° C) to 200 ° C / s. Cooled down. The structure and hardenability were investigated as follows. In the same way as above, the thickness 1/4 position of the cross section in the rolling direction is polished. ・ After nital corrosion, 5 sections, 10 magnifications at 1500 magnifications in each area (50 fields in total), a scanning electron microscope Using the image analysis software described above, two points on the circumference of the cementite or graph item and the equivalent ellipse of the cementite or graph item (the same area as the cementite and graph item, and primary and secondary) The diameter passing through the center of gravity of the ellipse with the same second moment) was measured in increments of 2 degrees and averaged to obtain each particle size. The average particle size of cementite and graphite was obtained by averaging the particle sizes of cementite and graphite obtained by observing 50 fields of view.

Hardenability: A disk specimen having a diameter of 100 mm was collected, and the outer peripheral edge of the disk specimen was heated to 1000 ° C at a frequency of 100 kHz using a high-frequency heat treatment apparatus, and then immediately cooled with water. And at 8 positions along the circumferential direction of the disk specimen after the heat treatment, measure the Vickers hardness Hv [load: 49N (= 5kgf)] of the front and back surfaces 1.5mm inside from the outer edge, The difference Δ Ην between the maximum Hv and the minimum Ην was obtained. If this ΔΗν is 8 or less, it can be said that it is excellent in burn-in.

Figure 1 shows the relationship between the average particle diameter d of cementite and graphite and Δ Ην. It can be seen that when the average particle size d of cementite and graphite is less than ΔΔν is less than 8, and excellent hardenability can be obtained.

As a result of various studies based on the above-mentioned studies, the inventors need to make the average particle size of cementite slag and graphite below to ensure excellent hardenability. It has been found that it is necessary to make it preferably 3 μππι or less. The reason why excellent hardenability can be obtained by specifying yarn and weave in this way is considered as follows. In other words, when the cementite or graphite average particle size is below, the cementite and graphite are almost completely dissolved during high-frequency heating, and the hardness after quenching can be made uniform.

3) Manufacturing conditions

Below, the preferable manufacturing conditions of the steel plate of this invention are shown. In addition, the manufacturing method of the steel plate of this invention is not limited to the following.

Finishing temperature during hot rolling: 800 ~ 950

If the finishing temperature during hot rolling is less than 800 ° C, the rolling load will increase significantly, and if it exceeds 950 ° C, the resulting scale will become thicker and the pickling property will decrease, and the surface of the steel plate will be decarburized. Since this may occur, the temperature should be 800 to 950 ° C. Average cooling rate after hot rolling: 50 ° C / s or more

The steel sheet after hot rolling is immediately cooled at an average cooling rate of 50 / s or more to the cooling stop temperature described later. When the average cooling rate is less than 50 ° C / s, ferrite grains tend to grow during cooling, and large ferrite grains are formed. During subsequent annealing, graphite cementite is thought to be formed with ferrite grain boundaries and inclusions as nuclei, so if ferrite grains are large, graphite formed with grain boundaries as nuclei. And cementite becomes coarse and hardenability decreases. In addition, if the average cooling rate is slow, coarse perlites are generated, and graphite and cementite are formed through the splitting, agglomeration, and coarsening of the parrites, so the graphite and cementite become coarser. , Hardenability decreases. When the average cooling rate is set to 50 ° C / s or more, the rolling strain introduced into the austenite by hot rolling tends to remain in the structure after the transformation, resulting in an increase in the dislocation density. There is also the merit of promoting the formation of graphite using the dislocations as the core. Based on the above, the average cooling rate is 50 ° C / s or higher, preferably 80 ° C / s or higher. The upper limit of the average cooling rate is not particularly required, but is preferably 200 ° C./s or less in order to suppress the deterioration of the shape of the steel sheet and ensure the shape of the steel sheet.

Cooling stop temperature for cooling after hot rolling: 500 ° C or less

When the minimum temperature that needs to be cooled at the cooling rate as described above, that is, the cooling stop temperature, exceeds 500 ° C, proeutectoid ferrite is generated during cooling until scraping, and coarse parlite is generated. When it is generated and annealed after scraping, the cementite becomes coarse and the hardenability deteriorates, so the temperature is set to 500 ° C or lower, preferably 470 ° C or lower. The lower limit of the cooling stop temperature does not need to be specified, but is preferably 200 ° C. or higher in order to secure the shape of the steel sheet.

Cutting temperature: 450 ° C or less

The hot-rolled sheet is cooled immediately after cooling, but at that time, if the milling temperature exceeds 450 ° C, coarse pearlite is generated, and the cementite and graphite become coarse during annealing, resulting in hardenability. Decreases. Therefore, the coiling temperature should be 450 ° C or less. In order to sufficiently obtain the cooling effect after! ^-Rolling as described above, it is preferable that the cutting temperature is lower than the cooling stop temperature. Also, since the shape of the hot-rolled sheet is likely to deteriorate, the cutting temperature is preferably 200 ° C or higher.

Annealing temperature: 720 ° C or less After removing scales by pickling, etc., the hot rolled sheet of cocoon removal is annealed in order to promote spheroidization and graphiteization of cementite and to make it softer. At that time, if the annealing temperature exceeds 720, coarse pearlite is generated during cooling, resulting in a decrease in hardenability. Further, if the annealing temperature is less than 600 ° C, the annealing time becomes extremely long, so the annealing temperature is preferably 600 ° C or higher.

The annealing time is not particularly limited. However, the annealing time should be 8 hours or longer in order to form graphite, and the ferrite grains may be excessively coarsened, resulting in reduced ductility. It is preferable to set it to lOOhr or less.

To melt the steel of the present invention, either a converter or an electric furnace can be used. The steel melted in this way is made into slabs by ingot-bundling or continuous forging. Stebs are usually hot-rolled (reheated) and then hot-rolled. In the case of a slab produced by continuous forging, direct feed rolling in which heat is maintained for the purpose of suppressing the temperature drop may be applied as it is. When the slab is reheated and hot rolled, the slab heating temperature is preferably 1280 ° C or lower in order to avoid deterioration of the surface state due to scale. Hot rolling can be performed only by finish rolling, omitting rough rolling. In order to ensure the finishing temperature, the material to be rolled may be heated by a heating means such as a sheet heater during hot rolling. The thickness of the hot-rolled sheet is not particularly limited as long as the production conditions of the present invention can be maintained, but 1.0 to 10.0 mm is preferable. The annealed steel sheet can be temper-rolled as necessary. Examples are given in Example 1.

Only by controlling the sum of the volume ratios of the ferrite, graphite, and cementite and the graphite ratio, excellent stretch flangeability cannot always be obtained. That is, in the present invention, in order to ensure excellent stretch flangeability, the total volume ratio of cementite and glassite present in the ferrite particles must be 15% or less. More preferably, it is 10% or less. The present inventors conducted various studies in order to obtain excellent stretch flangeability. An example of the study is shown below. C: 0.55%, Si: 0.01%, Mn: 0.13, P: 0.003%, S: 0.0006%, A1: 0.005%, Ν: 00018%, Ni: 0. 50%, B: 0.001%, steel slab composed of the balance Fe and inevitable impurities is heated to 115 0 ° C, followed by rough rolling for 5 passes and finish rolling for 7 passes at a finishing temperature of 870 ° C. In this way, a hot-rolled sheet having a thickness of 4.0 mm was obtained, picked at a cutting temperature of 520 ° C, pickled, and subjected to patch annealing at 720 ° C for 40 hours. At this time, in order to change the amount and distribution state of cementite and graph items, The temperature range from finish rolling to the take-off temperature was changed by changing the average cooling rate in the range of air cooling (5 ° C / s;) to 200 ° C / s. Then, the structure and stretch flangeability were investigated as follows. In the same way as above, the thickness 1/4 position of the cross section in the rolling direction is polished. After the nital corrosion, the cross-section is observed at 5 locations, with 10 magnifications at a magnification of 400 at each location (total 50 views) with an optical microscope. Using the above-mentioned image analysis software, the cementite 卜 and graphite existing on the ferrite grain boundary and the cementite and graph item existing in the ferrite grain are identified, and the cementite existing on the ferrite grain boundary is identified. And the area occupied by the graph item. _n , and the occupied area S _in of cementite and graph items existing in ferrite grains are measured, and the area ratio of cementite and graph items existing in ferrite grains is calculated from The volume fraction S (¾) of cementite and graph items existing in the ferrite grains occupying the entire graph and graph items. That is, S (%) can be expressed by the following formula.

S = {S _in / (S _on + S _in )} X 100

Here, for cementite grains or graphite grains that have at least a portion existing on the ferrite grain boundary, the area of the single cementite grain or the entire graphite grain is determined by Measured as the area occupied by cementite grains or graphite grains existing on the grain boundaries, and the area of cementite grains or graphite grains that do not have a portion present on the ferrite grain boundaries The area occupied by cementite grains or graphite grains present in the grains was measured.

Stretch flangeability: Specimen for hole expansion test (100 X 100 was sampled and punched using a punching tool with a punch diameter of 10 mm and a die diameter of 11.6 (clearance: 20% thickness) at the center of the specimen. After that, the punched hole was pushed up with a cylindrical flat bottom punch (diameter 50 1) and shoulder R 8mm) to expand the hole, and the hole diameter d was measured when a through-thickness crack occurred at the hole edge. Then, the hole expansion rate 1 (%) was calculated from the following formula, and the same test was performed 6 times to obtain the average (%).

λ = 100 Χ (d-10) / 10

Figure 2 shows the relationship between the cementite and graphite volume fraction S present in the ferrite grains and the average λ. It can be seen that when the volume fraction S of cementite and graphite present in the ferrite grains is 15% or less, an average of 60¾ or more is obtained, and excellent stretch flangeability is obtained.

As a result of various studies based on the above studies, the inventors have found that the total body of cementite and graphite present in the ferrite grains is necessary to secure excellent stretch flangeability. It has been found that the volume fraction needs to be 15% or less, more preferably 10% or less. The reason why good stretch flangeability can be obtained by defining the structure in this way is considered as follows. In other words, if there is a large amount of cementite or graphite in the ferrite grains, fine cracks are likely to occur at the interface between the cementite and graphite during the punching process. From Propagation 'Combination, easy to lead to cracks through the plate thickness. On the other hand, since the diffusion rate of carbon is fast at the ferrite grain boundary, coarsening of the cohesion is promoted more than in the ferrite grain, and the cementite graphite on the ferrite grain boundary is those in the ferrite grain. It is easier to coarsen, and the interval between each cementite grain and graphite grain tends to be wide. For this reason, cementite on the ferrite grain boundary slows crack propagation compared to cementite and graphite in the ferrite grain.

3) Manufacturing conditions

Finishing temperature during hot rolling: 800-950 ° C

If the finishing temperature during hot rolling is less than 800, the rolling load increases remarkably, and if it exceeds 950 ° C, the scale to be produced becomes thick and the pickling property decreases, and a decarburized layer is formed on the surface of the copper plate. Since it may occur, it should be 800-950.

Average cooling rate after hot rolling: 50 ° C / s or more

If the steel sheet after hot rolling is immediately cooled to the cooling stop temperature described later at an average cooling rate of 50 ° C / s or more, the formation of proeutectoid ferrite is suppressed and ferrite and cementite precipitate finely. . For this reason, C is likely to diffuse into the ferrite grain boundaries during annealing performed after milling, and the cementite on the ferrite grain boundaries is agglomerated, coarsened, and promotes graphiteization. If cementite is reduced, the graph item is reduced and stretch flangeability is improved. In addition, the rolling strain introduced into the austenite by hot rolling tends to remain in the structure after transformation, resulting in an increase in dislocation density. As a result, graphite formation with dislocations as the core during annealing is facilitated, softening proceeds, and workability is improved. Therefore, the average cooling rate should be 50 ° C / s or higher, preferably 80 ^ or higher. The upper limit of the average cooling rate need not be specified, but is preferably set to 200 ° C./s or less in order to suppress the deterioration of the shape of the steel plate and ensure the shape of the steel plate. Cooling stop temperature for cooling after hot rolling: 600 ° C or less

When the minimum temperature that needs to be cooled by the cooling rate as described above, that is, the cooling stop temperature, exceeds 600 ° C, proeutectoid light is generated during cooling up to the removal, and pallet light is generated. However, the cementite / dalaite present in the ferrite grains increases during annealing after milling, leading to a decrease in stretch flangeability. Therefore, the temperature should be 600 ° C or lower, preferably 550 ° C or lower. The lower limit of the cooling stop temperature does not need to be specified, but is preferably 200 or more in order to ensure the shape of the steel sheet.

Cutting temperature: 550 ° C or less

The hot-rolled sheet is cooled immediately after cooling, but if the coiling temperature exceeds 550 ° C, no. One line is formed, and the cementite present in the ferrite grains increases during annealing, and the stretch flangeability decreases. Therefore, the trapping temperature is 550 and below. In order to sufficiently obtain the cooling effect after the hot rolling described above, the winding temperature is preferably lower than the cooling stop temperature. Further, since the shape of the hot-rolled sheet is likely to deteriorate, the securing temperature is preferably 200 ° C. or higher, and more preferably 450 ° C. or higher, in securing the shape of the steel plate.

Annealing temperature: 720 ° C or less

The hot-rolled sheet after scraping is subjected to annealing to remove the scale by pickling, etc., and to promote spheroidization and graphiteization of cementite and softening. At that time, if the annealing temperature exceeds 720 ° C, pearlite is generated during cooling and the stretch flangeability is deteriorated. Also, if the annealing temperature is less than 600 ° C, the cementite and graphite present in the ferrite grains increase and the stretch flangeability tends to deteriorate, so the annealing temperature should be 600 ° C or higher. It is preferable.

The annealing time does not need to be particularly limited, but it can be 8 hours or longer in order to form graphite and reduce cementite in the ferrite grains to reduce graphite. Since the grains may become excessively coarse and the ductility may be lowered, it is preferable to set it to lOOhr or less.

To melt the steel of the present invention, either a converter or an electric furnace can be used. The steel melted in this way is made into slabs by ingot-bundling or continuous forging. Slabs are usually heated (reheated) and then hot rolled. In the case of a slab manufactured by continuous forging, even if direct feed rolling is used, the rolling is carried out as it is or for the purpose of suppressing the temperature drop. Good. When the slab is reheated and hot rolled, the slab heating temperature is preferably 1280 ° C or lower in order to avoid deterioration of the surface state due to scale. Hot rolling can be performed only by finish rolling, omitting rough rolling. In order to ensure the finishing temperature, the material to be rolled may be heated by a heating means such as a sheet bar heater during hot rolling. The thickness of the hot-rolled sheet is not particularly limited as long as the production conditions of the present invention can be maintained, but 1.0 to 10.0 mm is preferable. Hot-rolled sheet is pickled

Or after removing the surface scale by shot blasting, etc., it is annealed by hot rolling. The annealed steel sheet can be temper-rolled as necessary. Examples are given in Example 2. Example

Example 1

Slabs of copper Nos. A to S having the composition shown in Table 1 were heated to 1250 ° C, hot-rolled under the conditions shown in Table 2, and pickled, and then annealed under the conditions shown in Table 2, Plate thickness 4. (½! 11 steel plates 1½. 1 to 22 were prepared. Then, the above method was used to evaluate the graphite rate, the average grain size of cementite and graphite, and the hardenability. Δ Ην In addition, JIS No. 5 tensile test specimens were collected along the rolling direction and subjected to a tensile test to determine yield stress ΥΡ, tensile strength TS, and elongation El.

The results are shown in Table 3. It can be seen that the steel sheets of the examples of the present invention all have low YP, low TS, high El, and low ΔΗν, are soft, excellent in workability, and excellent in hardenability. In addition, as shown in Table 3, the weave of the steel sheet of the example of the present invention was almost composed of ferrite, cementite, and graph items, and it was confirmed that the total volume ratio of these was 95% or more. .

table 1

(mass%)

Table 2

Table 3

*: F ferrite, G graph eye, C cementite Example 2

Table 4} 3¾ ^ Destructed Steel No. M—AS slab was squeezed into Table 5 and hot rolled in a “^” cow and pickled, then in Table 5 also in a “^ 1” ^ ί cow. «to ί Lou the 赚 No. 101~122 of row-\ ¾i? 4 Qmi the and, by the above ^, graphite rate, ft¾ rate of cementite and graphite to ferrite grains to the total cementite and graphite S, Find the average length of stretch flangeability ί | ¾ Also, along the pressure »direction, enter JIS No. 5 ¾g ^ and pull? S ^! US, calculate force YP, pull TS, elongation El. Also, decimate the same difficulty twice for each w to obtain a flat female, and use this average value as its special 14 value; ^

The results are shown in Table 6. All of the examples of the present invention are low YP, low TS, high El, and high; it can be seen that they are soft and have excellent [] properties including stretch flangeability. In addition, as shown in Table 6, the yarn job of the example of the present invention is almost composed of ferrite, cementite, and graphite, and these (^ is that the ratio of the total is 9¾¾.

Table 4

Table 5

Table 6

*: F: Ferrite, G: Graphite, C: Cementite

Claims

The scope of the claims

1.Quality *%, C: 0.3-0.71 Si: 0. 1% KTf, Mh: 0.20% ¾ lower, P: 0.01¾¾ lower, S: 0.01% ¾ lower, A1: 0.0¾¾ lower, N: 0.005 (including ¾¾ lower, ¾¾ Fe and §J¾ consisting of impure impurities, ferrite, dalafite, and cementite ^, a strong body of ferri cocoon, grafite, and cementite occupying the entire weave The total precision is 99¾¾, and the ratio of graphite and cementite in the entire graphite and cementite (graphite ratio) is 5¾ (¾, the average of graphite and cementite ^ is 5; zm or less. ! Plfc

2. In addition, quality. Therefore, it is difficult to have at least one kind selected from Ni: 3.0½¾, B: 0.00S% ¾, Cu: 0. Frozen «fc

3. Steel having the composition according to claim 1 or 2 is hot rolled at itJL at 800 to 950 ° C to obtain heat and the heat stress after the rolling is an average of 50 ° C / s or more Cool down to i ^ Pff at 500 ° C or less with ί ^ ¾, remove with 450 ° C or less, and take tflfe with «less than 720 ° C.

4. At S%, C: 0.3 ~ O.7 Si: 0. 1 lower, Mn: 0. 15 ^, P: 0.01 lower, S: 0.01 lower, Α1: 0.09¾¾ lower, Ν: 0 · 0050¾¾ including, ¾¾ Fe and yarns composed of inevitable impurities, ferrite, graphite and cementite, and ferrite. The total volume fraction of menthite is 9E% ¾, the fl¾ ratio of graphite occupying the entire graphite and cementite (graphite ratio) is 5% ¾, and the graphite in the ferrite grains occupying the entire graphite and cementite And the ratio of cementite (7 g + is under 15½¾ «^ fe

5. Furthermore, it is difficult to have at least one kind selected from the following: S:%, Ni: 3.0¾¾, B: 0.005½¾, Cu: 0. Claim 4

6. The steel having the thread castle of claim 4 or 5 is rolled at tt ± of 800 to 950 ° C, and the heat after rolling is ≥ 50 ° C / s. The average value is chilled to 600 ° C or below, 550 is taken with the following take-up, and after taking tiJ | B, it is supposed to be taken down with 720 ° C or less.