JP2009149924A - Steel sheet and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a soft steel sheet having superior stretch-flangeability and excellent workability and its manufacturing method. <P>SOLUTION: The steel sheet has a composition consisting of, by mass, 0.3 to 0.7% C, ≤0.1% Si, <0.15% Mn, ≤0.01% P, ≤0.01% S, ≤0.05% Al, ≤0.0050% N and the balance Fe with inevitable impurities and also has a structure containing ferrite, graphite and cementite. Moreover, the sum of the respective volume fractions of ferrite, graphite and cementite comprises ≥95% of the whole structure, and the volume fraction of graphite in the whole graphite and cementite is ≥5%, and further, the sum of the volume fractions of graphite and cementite present in ferrite grains comprises ≤15% of the whole graphite and cementite. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

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

  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 a steel sheet is required to have excellent workability because it is processed into various complicated shapes. Recently, there has been a strong demand for reducing the manufacturing cost of these parts, and it has become possible to increase the thickness of automobile drive system parts using high-carbon steel sheets, for example, processing technologies aimed at eliminating processing steps and changing processing methods. As a result, double-acting machining technology has been developed and realized in part. Accordingly, steel sheets used for the above-mentioned automobile parts and the like are becoming increasingly demanding for workability and are required to be softer and more ductile. For example, when working by cold forging, a lower yield stress is required. Furthermore, when performing hole expansion processing (burring) after punching, excellent stretch flangeability is desired.

In order to meet these demands, a technique for improving the workability by graphitizing C in steel has been 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 0.150%, P: 0.018% or less, S: 0.010% Below, N: 0.0050% or less, consisting of the balance Fe and inevitable impurities, having a structure mainly composed of ferrite phase and graphite phase, and workability, toughness, and quenching with a soft material of TS ≦ 60kgf / mm ² A steel plate suitable for a tiller claw component with good properties and a method for manufacturing the same are disclosed. Patent Document 2 includes mass%, C: 0.10 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.05 to 0.50%, Nb: 0.005 to 0.1%, Al: 0.01 to 1.00%, N: 0.002. ~ 0.010%, B: 3 ~ 50ppm, Ca: 0.001 ~ 0.01% and Ni: 0 ~ 2.00%, the balance consists of Fe and inevitable impurities, P in impurities: 0.012% or less, S: 0.008% or less The hot rolled steel sheet is held at a temperature range of Ac ₁ to Ac _{3 for} 0.1 to ₁₀ hours, then cooled to room temperature at a cooling rate of 20 to 100 ° C./hr, and then box annealed at a temperature range of 650 to 750 ° C. Thus, a method for producing a medium carbon steel sheet excellent in workability, characterized in that 50 area% or more of cementite in steel is graphitized is disclosed. Further, in Patent Document 3, in mass%, C: 0.20 to 1.00%, Si: more than 0.20% and 1.20% or less, Mn: 0.05 to 0.50%, N: 0.005 to 0.015%, B: 0.2 × N% to 0.8 × N%, and Al: less than 0.05%, and 1.0 × (NB)% to 5.0 × (NB)% in an amount satisfying, consisting of the balance Fe and inevitable impurities, P: 0.020% or less as impurities, A high-carbon thin steel sheet having a chemical composition of S: 0.010% or less, a workability having a structure composed of ferrite, graphite, and cementite, and a method for producing the same are disclosed.
Japanese Unexamined Patent Publication No. 1-025946 JP 7-258743 A JP-A-4-202744

  Conventionally, in order to improve the workability by graphitizing C in steel, it has been essential to add a large amount of Si, as described in Patent Documents 1 and 3. However, by adding Si, the ferrite itself becomes hard and it becomes difficult to obtain good workability. In addition, as in Patent Document 2, it is a component system to which B and Nb are added, and by performing annealing twice under predetermined conditions, graphitization and high ductility can be achieved without necessarily increasing the amount of Si added. Technology to be achieved has also been developed, but performing annealing twice leads to increased costs. Here, Patent Document 2 is a technique that attempts to graphitize 50% or more of cementite in steel, and as a component composition of steel disclosed in the examples, the amount of Si is large and exceeds 0.20%. ing. Moreover, although the steel plates 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 this invention is to provide the steel plate excellent in the workability which is soft and has the outstanding stretch flangeability, and its manufacturing method.

As a result of repeated investigations on the above-mentioned problems of the prior art, the present inventors have found that the distribution of graphite and cementite is high even when the Si content is very low, specifically 0.1% or less. It has been found that by controlling, good workability can be obtained without necessarily increasing the graphitization rate, and further good stretch flangeability can be secured. That is, as a result of diligent research on the influence of the structure on the strength and stretch flangeability of the steel sheet containing C: 0.3 to 0.7 mass%, the following was found.
I) For softening, a structure containing ferrite, graphite, and cementite is used. The total volume ratio of ferrite, graphite, and cementite in the entire structure is 95% or more. Is effective.
II) In order to improve stretch flangeability, the total volume ratio of graphite and cementite in the ferrite grains occupying the entire graphite and cementite needs to be 15% or less.
III) 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, in mass%, C: 0.3-0.7%, Si: 0.1% or less, Mn: less than 0.15%, P: 0.01% or less, S: 0.01% Hereinafter, Al: 0.05% or less, N: 0.0050% or less, having a composition comprising the balance Fe and inevitable impurities, having a structure containing ferrite, graphite, and cementite, and ferrite and graphite occupying the entire structure The total volume ratio of cementite is 95% or more, the volume ratio of graphite (graphite ratio) is 5% or more in the entire graphite and cementite, and the total volume ratio of graphite and cementite in the ferrite grains in the entire graphite and cementite A steel sheet is provided, characterized in that is 15% or less.

  The steel sheet of the present invention preferably 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%.

  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 ° C./s or more. After cooling to a cooling stop temperature of 600 ° C. or lower at a cooling rate, winding can be performed at a winding temperature of 550 ° C. or lower, and the hot-rolled sheet after winding can be manufactured by annealing at an annealing temperature of 720 ° C. or lower.

  According to the present invention, a steel plate that is soft and has excellent stretch flangeability and excellent workability can be produced. In particular, the steel sheet of the present invention can be manufactured at low cost and easily 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 such as stretch flangeability, so it is suitable for thickening processing of automobile drive system parts. Even when applied to parts with complex shapes, multiple parts This eliminates the need for processing and welding, thereby improving the productivity of automobile parts and reducing costs.

  Below, the steel plate excellent in workability which is this invention, and its manufacturing method are demonstrated in detail. Note that “%” representing the amount of a component means “% by mass” unless otherwise specified.

1) Composition
C: 0.3-0.7%
C is an element that forms graphite. If the amount of C is less than 0.3%, the hardness after quenching cannot be secured. On the other hand, if it exceeds 0.7%, the steel sheet becomes hard even if graphitized, and the workability is lowered. For this reason, the C amount is set to 0.3 to 0.7%.

Si: 0.1% or less
If the Si content exceeds 0.1%, the ferrite becomes hard and the workability decreases. Therefore, the Si content is 0.1% or less, preferably 0.05% or less.

Mn: less than 0.15%
If the amount of Mn is 0.15% or more, graphite formation is inhibited, so Mn is less than 0.15%, preferably 0.10% or less.

P: 0.01% or less
P is preferably segregated at grain boundaries to reduce workability and stabilize cementite to inhibit graphite formation. Therefore, P is desirably reduced 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 to reduce workability, and also stabilizes cementite to inhibit graphite formation. Therefore, the S content is 0.01% or less, preferably 0.007% or less.

Al: 0.05% or less
Al is an element that binds to solute N to form AlN, detoxifies the adverse effect of solute N having an action of inhibiting graphite formation, and promotes graphite formation with AlN as a nucleus. For this reason, the Al content is preferably 0.003% or more, but if it exceeds 0.05%, the cleanliness of the steel is lowered, so the Al content is 0.05% or less, preferably 0.04% or less.

N: 0.0050% or less
When the amount of N exceeds 0.0050%, the action of stabilizing the solid solution N cementite becomes remarkable, and the formation of graphite is inhibited. Therefore, the N 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, and Cu: 0.1% or less is contained for the following reasons.

Ni: 3.0% or less
Ni is an element that promotes the formation of graphite and is an element that is also effective in improving hardenability.To obtain such an effect, it is preferably contained in an amount of 0.1% or more, but when the amount of Ni exceeds 3.0%, The effect is saturated. Therefore, the Ni content is 3.0% or less, preferably 0.1 to 3.0%, more preferably 0.3 to 1.0%.

B: 0.005% or less
B is a useful element that combines with N to form BN and acts as a nucleus for graphite formation, and also works effectively to improve hardenability. To obtain these effects, 0.0005% or more Although it is preferable to contain, if the amount of B exceeds 0.005%, the effect is saturated. Therefore, the B content 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 such an effect, it is preferably contained in an amount of 0.01% or more, more preferably 0.02% or more. If the amount of Cu exceeds 0.1%, the effect is saturated. For this reason, the amount of Cu is made 0.1% or less, more preferably 0.07% or less.

2) Structure In order to soften the steel sheet and to improve the bending workability and elongation characteristics in tensile tests, the structure should contain ferrite, graphite and cementite, and the total volume fraction of ferrite, graphite and cementite in the entire structure should be It is necessary to set it to 95% or more and to make the graphite ratio in the entire graphite and cementite 5% or more. At this time, the present invention includes the case where the graphite ratio is 100%, that is, when all cementite is graphitized, since the same effect can be obtained. If the total volume ratio of ferrite, graphite, and cementite is less than 95%, that is, if the volume ratio of phases other than these exceeds 5%, workability deteriorates. On the other hand, if the graphite ratio is less than 5%, the workability deteriorates.

Here, the volume ratios of ferrite, graphite, and cementite were determined as follows. That is, after polishing the thickness 1/4 position of the thickness cross section in the rolling direction of the steel sheet, it corrodes with Nital, and with an optical microscope, observe 5 places per field of view at a magnification of 400 times, 10 fields (50 places in total), These images are subjected to image analysis processing by the image analysis software “Image Pro Plus ver.4.0” manufactured by Media Cybernetics, to determine the areas of ferrite, graphite, and cementite, and the ratio (area ratio) of the total observation area to ferrite, The volume ratios of graphite and cementite were used. The ratio (area ratio) of the graphite area (Sgr) to the sum of the graphite area (Sgr) and the cementite area (Scm) was defined as the volume ratio of graphite (graphite ratio). That is, the graphite ratio (%) can be expressed by the following formula.
Graphite rate = {Sgr / (Sgr + Scm)} x 100
However, excellent stretch flangeability cannot always be obtained simply by controlling the sum of the volume fractions of ferrite, graphite and cementite, and the graphite ratio. That is, in the present invention, in order to ensure excellent stretch flangeability, the total volume ratio of cementite and graphite present in the ferrite grains needs to be 15% or less. More preferably, it is 10% or less.

  The present inventors have made various studies in order to obtain excellent stretch flangeability. An example of study is shown below. C: 0.55%, Si: 0.01%, Mn: 0.10%, P: 0.003%, S: 0.0006%, Al: 0.005%, N: 0.0018%, Ni: 0.50%, B: 0.0013%, balance Fe and inevitable After heating the steel slab made of impurities to 1150 ° C, rough rolling is performed for 5 passes, 7-pass finish rolling is performed at a finishing temperature of 870 ° C to form a hot rolled sheet with a thickness of 4.0 mm, and winding at a winding temperature of 520 ° C. After taking, it was pickled and subjected to batch annealing at 720 ° C. for 40 hours. At this time, in order to change the amount and distribution state of cementite and graphite, the temperature range from the finish rolling to the coiling temperature is changed by changing the average cooling rate from air cooling (5 ° C / s) to 200 ° C / s. did. Then, the structure and stretch flangeability were investigated as follows.

In addition, after grinding and nital corrosion, the cross section in the rolling direction parallel section in the same manner as described above, the cross section was observed at an optical microscope over 10 fields (total of 50 fields) at a magnification of 400 at each location, Using the image analysis software described above, cementite and graphite existing on the ferrite grain boundary and cementite and graphite existing on the ferrite grain boundary are identified, the occupied area S _on of the cementite and graphite existing on the ferrite grain boundary, and ferrite Measure the occupied area S _in of cementite and graphite present in the grain, and calculate the area ratio of cementite and graphite present in the ferrite grain from the following equation, and calculate the area ratio of the cementite and graphite in the ferrite grain occupying the entire cementite and graphite. And the volume ratio S (%) of graphite. That is, S (%) can be expressed by the following formula.
S = {S _in / (S _on + S _in )} × 100
Here, the cementite grains or graphite grains having at least a portion existing on the ferrite grain boundary are the same as the cementite grains or graphite grains existing on the ferrite grain boundary. The area of cementite or graphite grains not having a portion present on the ferrite grain boundary was measured as the area occupied by cementite grains or graphite grains present in the ferrite grains.

Stretch flangeability: A test piece (100 x 100 mm) for hole expansion test was taken and punched using a punching tool with a punch diameter of 10 mm and a die diameter of 11.6 mm (clearance: plate thickness 20%) at the center of the test piece. . After that, the punched hole is pushed up by a cylindrical flat bottom punch (diameter 50mmΦ, shoulder R8mm) to expand the hole, and the hole diameter d (mm) is measured when a plate thickness penetration crack occurs at the hole edge. The hole expansion rate λ (%) was calculated from the equation, and the same test was performed 6 times to obtain the average λ (%).
λ = 100 × (d-10) / 10
FIG. 1 shows the relationship between the volume fraction S and average λ of cementite and graphite present in the ferrite grains. 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 λ of 60% or more is obtained, and excellent stretch flangeability is obtained.

  As a result of various studies based on the above studies, the inventors have determined that the total volume ratio of cementite and graphite present in ferrite grains is 15% or less in order to ensure excellent stretch flangeability. It has been found that it is necessary to make it 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 a large amount of cementite or graphite is present in the ferrite grains, fine cracks are likely to occur at the interface between cementite or graphite and ferrite during the punching process, and they propagate and bond from the initial stage during the hole expansion test. It tends to lead to cracks in the penetration. On the other hand, since the diffusion rate of carbon is fast at the ferrite grain boundary, the coarsening of the cohesion is promoted more than in the ferrite grain, and cementite and graphite on the ferrite grain boundary are more likely to be coarser than those in the ferrite grain. The interval between grains and graphite grains tends to be wide. For this reason, cementite or graphite on the ferrite grain boundary slows crack propagation compared to cementite or graphite in the ferrite grain.

3) Production conditions The following are preferred production conditions for the steel sheet of the present invention. 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 ° C
When the finish temperature during hot rolling is less than 800 ° C, the increase in rolling load becomes significant. When the finish temperature exceeds 950 ° C, the scale to be produced becomes thick and the pickling property decreases, and a decarburized layer is formed on the steel sheet surface layer. Therefore, the temperature is set to 800 to 950 ° C.

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. As a result, ferrite and cementite precipitate finely. For this reason, C is likely to diffuse into the ferrite grain boundaries during annealing after winding, which promotes the aggregation, coarsening, and graphitization of cementite on the ferrite grain boundaries, and reduces cementite and graphite in the ferrite grains. Stretch flangeability is improved. Further, 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, the formation of graphite with dislocations as the core during annealing is facilitated, the softening proceeds, and the workability is improved. From 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 limited, 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 in cooling after hot rolling: 600 ° C or less The minimum temperature that needs to be cooled at the cooling rate as described above, that is, when the cooling stop temperature exceeds 600 ° C, it is first precipitated during cooling until winding While ferrite is generated, pearlite is generated, and cementite and graphite present in the ferrite grains are increased during annealing after winding, resulting in a decrease in stretch flangeability, so that it is 600 ° C. or less, preferably 550 ° C. or less. To do. The lower limit of the cooling stop temperature is not particularly required, but is preferably 200 ° C. or higher in order to ensure the shape of the steel sheet.

Winding temperature: 550 ° C or less The hot-rolled sheet after cooling is immediately wound, but at that time, if the winding temperature exceeds 550 ° C, pearlite is generated, and cementite and graphite existing in the ferrite grains are annealed during annealing. It increases and stretch flangeability falls. Therefore, the coiling temperature is 550 ° C or lower. 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 coiling temperature is preferably 200 ° C. or higher and more preferably 450 ° C. or higher when ensuring the shape of the steel plate.

Annealing temperature: 720 ° C. or less The hot-rolled sheet after winding is subjected to annealing to remove the scale by pickling or the like and then to promote the spheroidization and graphitization of cementite and to soften it. At that time, if the annealing temperature exceeds 720 ° C., pearlite is generated during cooling, and the stretch flangeability is deteriorated. Further, 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. Therefore, the annealing temperature is preferably 600 ° C. or more.

  Note that the annealing time is not particularly limited, but to form graphite and reduce cementite and graphite in the ferrite grains to be 8 hours or more, and the ferrite grains are excessively coarsened, Since there is a risk of reducing ductility, it is preferably 100 hours or less.

  To melt the steel of the present invention, both a converter and an electric furnace can be used. The steel thus melted is made into a slab by ingot-bundling rolling or continuous casting. The slab is usually heated (reheated) and then hot rolled. In addition, in the case of the slab manufactured by continuous casting, you may apply direct feed rolling which rolls as it is or in order to suppress a temperature fall. 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 is preferably 1.0 to 10.0 mm. The hot-rolled sheet is subjected to hot-rolled sheet annealing after removing the surface scale by pickling or shot blasting. The annealed steel sheet can be temper-rolled as necessary.

  Slabs of steel 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, pickled, and then annealed under the conditions shown in Table 2, Steel plates No. 1 to 22 having a thickness of 4.0 mm were produced. Then, the graphite ratio, the volume ratio S of cementite and graphite existing in the ferrite grains occupying the entire cementite and graphite, and the average λ, which is an index of stretch flangeability, were obtained by the above method. In addition, JIS No. 5 tensile test specimens were collected along the rolling direction and subjected to a tensile test to determine yield stress YP, tensile strength TS, and elongation El. A similar test was performed twice for each sample to obtain an average value, and this average value was used as the characteristic value of the steel sheet.

  The results are shown in Table 3. It can be seen that all the steel plates of the present invention have low YP, low TS, high El, and high λ, are soft, and have excellent workability including stretch flangeability. As shown in Table 3, the structure of the steel sheet of the present invention was substantially composed of ferrite, cementite, and graphite, and it was confirmed that the total volume ratio of these was 95% or more.

It is a figure which shows the relationship between the volume ratio S and average (lambda) of the cementite and graphite which exist in a ferrite grain.

Claims (3)

  In mass%, C: 0.3-0.7%, Si: 0.1% or less, Mn: less than 0.15%, P: 0.01% or less, S: 0.01% or less, Al: 0.05% or less, N: 0.0050% or less, the balance It has a composition composed of Fe and inevitable impurities, has a structure containing ferrite, graphite, and cementite, and the total volume ratio of ferrite, graphite, and cementite in the entire structure is 95% or more, and accounts for the entire graphite and cementite. A steel plate characterized by having a volume ratio of graphite (graphite ratio) of 5% or more and a total volume ratio of graphite and cementite present in ferrite grains occupying the entire graphite and cementite is 15% or less.   2. The steel sheet according to claim 1, wherein the steel sheet has a composition containing at least one selected from Ni: 3.0% or less, B: 0.005% or less, and Cu: 0.1% or less in mass%. .   The steel having the composition according to claim 1 or 2 is hot-rolled by hot rolling at a finishing temperature of 800 to 950 ° C, and the hot-rolled plate after the hot rolling has an average of 50 ° C / s or more. After cooling to a cooling stop temperature of 600 ° C. or less at a cooling rate, winding at a coiling temperature of 550 ° C. or less, and annealing the hot rolled sheet after the winding at an annealing temperature of 720 ° C. or less Manufacturing method.

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