KR20040045592A - A method for manufacturing steel sheets for dummy with excellent weldability and repeated heat treatment property - Google Patents

Abstract

PURPOSE: A method for manufacturing steel sheet for dummy having superior weldability and repeated heat treatment characteristics is provided to secure stable operation temperature by properly controlling constituents and manufacturing process. CONSTITUTION: The method comprises a step of hot finish rolling to a temperature of 910 to 930 deg.C a steel slab comprising 0.002 to 0.005 wt.% of C, 0.1 to 0.5 wt.% of Mn, 0.04 wt.% or less of Si, 0.015 wt.% or less of S, 0.03 to 0.10 wt.% of Al, 0.003 wt.% or less of N, 0.03 to 0.15 wt.% of Mo, 0.04 to 0.10 wt.% of Nb, 0.0005 to 0.003 wt.% of B and a balance of Fe and other inevitable impurities, wherein atomic ratio of (Nb+Mo)/C is 4.0 to 5.9, and weight ratio of B/N is 0.6 to 1.1; a step of coiling the hot finish rolled steel sheet at a temperature of 580 to 680 deg.C; and a step of cold rolling the coiled steel sheet to a reduction ratio of 80 to 90%, wherein content of B is 0.001 to 0.002 wt.%, and wherein the cold rolling step is performed to a reduction ratio of 84 to 86%.

Description

A method for manufacturing steel sheets for dummy with excellent weldability and repeated heat treatment property}

The present invention relates to a cold-rolled steel sheet for a pile connected to the material in order to control the annealing temperature of the material in the continuous annealing process, more specifically carbon nitride in the aluminum-killed steel of the ultra-low carbon base (Base) Properly manage the addition amount of Mo, Nb, and B, the (Nb + Mo) / C atomic ratio, and B / N weight ratio as well as resistance weldability and repeatability by optimizing hot rolling condition, winding temperature and cold reduction rate It is related with the manufacturing method of this excellent dummy steel plate.

Black plate (BP), which is a steel material used for container materials such as cans, is mostly thin in thickness, so it is suitable for the temper grade based on the Rockwell surface hardness value HR30T. It is classified into material of T1 (HR30T 46-52)-T6 (HR30T 67-73). To make cans for storing contents by using tin-plated discs, tin (Tin, element symbol Sn), etc. is coated on the surface of the disc to give corrosion resistance, cut to a certain size, and then processed into round or square. The method of processing a container is a method of processing without welding, such as a two-piece can in which the container is composed of two parts of a lid and a body, and the configuration of the can is a body, an upper lid, The three-piece can consists of three parts of the bottom lid, and is divided into a method of fastening the body by welding or bonding.

Soft tin plated discs of T3 (HR30T 54 ~ 60) grade or less applied to applications requiring high workability are mainly manufactured by the annealing method, and T4 (HR30T 58 ~ 64) used for applications requiring pressure resistance characteristics. Hard tin-plated discs of) or higher grades have been manufactured by a continuous annealing process. However, the continuous annealing method in the annealing process that imparts the target workability and characteristics in cold rolled steel sheet has the advantages of lower product production cost and superior material uniformity, flatness and surface characteristics compared to the annealing method. Efforts have been made to produce a continuous annealing process. When the tin-plated disc is manufactured by continuous annealing process of low carbon steel, it is necessary to produce the fluting or stretcher strain, Cause the same machining defects. In addition, there is a problem in that it is difficult to produce a soft material of the T2 (HR30T 50 ~ 56) grade or less due to the grain refinement effect by solid solution elements and carbon in steel.

The prior art for solving the above problems is Japanese Patent Laid-Open No. 7-11333. The prior art adds carbon nitride forming elements such as Ti or Nb to ultra low carbon steel or lowers the amount of solid solution in the steel through decarburization annealing in order to manufacture soft tin plated discs by continuous annealing. It is about how to suppress. However, soft tin-plated discs have a higher operating temperature in the continuous annealing plant than 70 ~ 100 ℃ compared to hard materials made of medium and low carbon steel. Therefore, to produce these materials together in the same continuous annealing unit, Dummy material, which is an ultra-thin connecting coil, is injected. The ultra-thin connecting coil is for connecting the front and rear coils by resistance welding for continuous plate in the heat treatment furnace, and it is important to secure weldability. In addition, the connection coil not only passes through various temperature ranges due to the characteristics of use, but since these operations are repeatedly performed, the less material softening caused by high temperature annealing is advantageous. Therefore, the higher the recrystallization temperature of the connecting material, the better. However, the existing connection coils in which Ti is added to the low and low carbon steels have low recrystallization temperature of the material, thereby reducing the repetitive workability and annealing workability in the continuous annealing process. There is a problem that the variation in material caused.

The present invention is to solve the above problems, and the amount of carbon nitride forming elements, the addition amount of Mo, Nb, B alone, (Nb + Mo) / C atomic ratio, B / N weight ratio, In addition, the present invention provides a method of manufacturing a dummy steel sheet having improved cyclic heat treatment properties by improving resistance weldability and delaying recrystallization of materials by optimizing hot rolling conditions, winding temperature and cold rolling rate. have.

The present invention for achieving the above object is by weight, C: 0.002-0.005%, Mn: 0.1-0.5%, Si: 0.04% or less, S: 0.015% or less, Al: 0.03-0.10%, N: 0.003 % Or less, Mo: 0.03 to 0.15%, Nb: 0.04 to 0.10%, B: 0.0005 to 0.003%, remaining Fe and other inevitable impurities, (Nb + Mo) / C atomic ratio: 4.0 to 5.9 and B / N weight ratio: after the hot-rolled steel to satisfy the 0.6 ~ 1.1 at 910 ~ 930 ℃ hot-rolled at 580 ~ 680 ℃, and cold rolling at a reduction ratio of 80 ~ 90%.

Hereinafter, the present invention will be described in more detail.

The present invention relates to an ultra-thin cold rolled steel sheet used as a connecting dummy coil, and to improve the resistance weldability and repeated heat treatment required for the connecting dummy coil.

First, the reason for component limitation of this invention is demonstrated.

C: 0.002-0.005 wt%

The C is an element that causes hardening of the material by aging deterioration and grain refinement. If the C content is less than 0.002% by weight, abnormal grain growth occurs during the hot rolling step, which causes material variation due to thickness variation during cold rolling. In addition, precipitation of fine carbide-based precipitates is suppressed, making it difficult to secure high-temperature annealing workability, and when it exceeds 0.005% by weight, it not only causes hardening of the material by aging deterioration and grain refinement, but also increases additional manufacturing cost due to decarburization during the steelmaking process. It is preferable to limit the content to 0.002 to 0.005% by weight.

Mn: 0.1-0.5 wt%

The Mn is an effective ingredient for suppressing red brittleness by sulfur, and in order to obtain the effect, it should be added at least 0.1 wt%. However, when the amount of Mn added exceeds 0.5% by weight, the hardenability is increased to form a hard two-phase structure such as bainite, which causes material variation, and also causes micro-segregation to cause cold rolling and formability. Since it deteriorates, it is preferable to limit the content to 0.1 to 0.5% by weight.

Si: 0.04 wt% or less

The Si bonds with oxygen to form an oxide layer on the surface of the steel sheet, causing defects such as poor coloration, thereby deteriorating plating workability, and when added in excess of 0.04% by weight, the material becomes brittle and causes fracture during cold rolling. It is preferable to limit the content to 0.04% by weight or less since it deteriorates the sex.

S: 0.015% by weight or less

The S is an element that forms manganese-sulfide-based precipitates by combining with Mn in the steel. When the S content exceeds 0.015% by weight, these precipitates are coarsened and the material softens rapidly and the recrystallization temperature is lowered. Since it becomes difficult to ensure the property, it is preferable to limit the content to 0.015% by weight or less.

Al: 0.03-0.10 weight

The Al is an element added to prevent material deterioration due to deoxidizer and aging in aluminum-kilted steel, and in order to obtain the above effect, Al should be added at least 0.03% by weight. In addition, since surface inclusions such as aluminum oxide (Al ₂ O ₃ ) increase rapidly to deteriorate the surface properties of the performance and hot rolled material, it is preferable to limit the content to 0.03 to 0.10% by weight.

N: 0.003 wt% or less

The N is a representative intrusion type element that exhibits reinforcing characteristics by invading steel, and when added in excess of 0.003% by weight, it is not only able to manufacture a soft material, but also causes deformation aging and acts as a factor of processing defects. It is desirable to limit it to 0.003% by weight or less.

Mo: 0.03-0.15 wt%

The Mo is a key element exhibiting the effect of delaying the recrystallization of the steel by reducing the size of the precipitate precipitated by reacting with carbon, etc., in order to secure the recrystallization target of the present invention should be added at least 0.03% by weight, 0.15% by weight When added in excess of%, the recrystallization temperature synergistic effect is not only to increase the production cost due to excessive addition of saturated and expensive Mo, but also to increase the hardenability to promote the formation of a hard phase such as martensite phase, which is cold rollable. Since it worsens, it is preferable to limit the content to 0.03 to 0.15 weight%.

Nb: 0.04-0.10 wt%

The Nb is a key element added to improve the high temperature annealing properties of the connecting steel sheet by forming a precipitate such as niobium carbide by combining with carbon, which is a solid solution element in the steel, and in order to obtain the effect, 0.04 wt% It should be added above, but if it is added in excess of 0.10% by weight, the strength of the hot-rolled sheet material is not significantly improved, but the strength of the hot-rolled sheet material is rapidly increased, so it is difficult to secure the cold rolling property of the ultra-thin connection steel sheet which requires a high cold rolling rate. It is preferable to limit the content to 0.04 to 0.10% by weight.

B: 0.0005 to 0.003 wt%

B is a key element effective in delaying recrystallization of steel by combining with nitrogen in the steel to form a precipitate, and is effective in softening the material. When the content of B is less than 0.0005% by weight, B, which is present in solid solution, strengthens grain boundaries in the resistance welding process, thereby suppressing grain growth at the weld heat affected zone, thereby suppressing plate breakage at the coil connection site. If it is added in excess of 0.003%, the recrystallization delay effect of the steel is saturated, and the effect of improving the workability of the high temperature is not only saturated, but also the workability of the product is remarkably lowered. Therefore, the content is limited to 0.0005 to 0.003% by weight. It is preferable. When the content of B is controlled to 0.001 to 0.002% by weight, the recrystallization retardation effect of the steel according to grain boundary reinforcement is further improved, and therefore, more preferable.

In addition to the above composition, the remainder is composed of Fe and other unavoidable impurities.

Meanwhile, in the present invention, in order to effectively control solid solution elements such as carbon and nitrogen present in the steel to ensure resistance weldability and high temperature workability in the annealing process, the relationship between these elements and the elements forming the compound is considered. It is also necessary to manage the addition ratio.

(Nb + Mo) / C atomic ratio: 4.0 to 5.9

In the case of Nb, Mo, and C, in order to secure the target characteristics, the amount of addition of each element is satisfied and the atomic ratio of C to Nb and Mo, that is, carbide forming elements, (Nb + Mo) / C is managed. As a result, precipitation of carbide-based precipitates, which are advantageous for suppressing recrystallization behavior, can be promoted. When the (Nb + Mo) / C atomic ratio is less than 4.0, solid carbons in the steel are not sufficiently precipitated in the form of carbide, which causes a material deterioration due to aging. In addition, since the formation of fine precipitates is not promoted, the recrystallization delay effect is reduced, making it difficult to secure high temperature workability in the continuous annealing process. On the other hand, when the (Nb + Mo) / C atomic ratio exceeds 5.9, the recrystallization delay effect of the steel due to the fine precipitate is saturated, and the excess additive elements promote coarsening of the precipitate, resulting in poor high temperature repeatability. do. In addition, the production cost is increased by increasing the amount of expensive Nb and Mo added, the hot rolled material is cured to deteriorate cold rolling. Therefore, it is preferable to limit the (Nb + Mo) / C atomic ratio to 4.0 to 5.9.

B / N weight ratio: 0.6 to 1.1

On the other hand, in the relationship between B and N for forming nitride-based precipitates, high temperature workability and resistance weldability can be secured by managing the weight ratio of N to B. When the B / N weight ratio is less than 0.6, all the B in the steel in the hot rolling step is combined with N to suppress abnormal grain growth of the weld heat affected zone during the resistance welding process, and the recrystallization delay effect is not so great to be used as a connection material. Inadequate On the other hand, when the B / N weight ratio exceeds 1.1, the size of the nitride grows and the recrystallization delay effect is reduced, thereby deteriorating high temperature repeatability. Therefore, the B / N weight ratio is preferably limited to 0.6 to 1.1.

The steel formed as described above is homogenized in an austenitic single-phase zone, and then hot-rolled at 910-930 ° C. The homogenization treatment (slave reheating) is usually carried out in an austenite single phase region where the initial austenite structure can be coarsened. In addition, if the finishing hot rolling temperature is less than 910 ℃, the rolling temperature during the hot rolling is lowered to the ferrite region, the coexistence of coarse grains and fine grains coexist, causing material variation and also act as a factor that worsens cold rolling properties In addition, when the temperature exceeds 930 ° C, the precipitate size is coarsened, the material is softened, and the recrystallization delay effect is decreased. Thus, there is a problem in securing high temperature workability in the annealing process. Therefore, the finishing hot rolling temperature is limited to 910-930 ° C. desirable.

It winds up at 580-680 degreeC after the said finish hot rolling. The hot rolled winding process is a step having a close relationship with AlN precipitation and material softening. When the coiling temperature is less than 580 ° C, the precipitation of aluminum nitride is suppressed, thereby not exerting a great effect on suppression of recrystallization, and a large material deviation occurs in the hot rolled sheet width direction, which acts as a factor of deteriorating cold rolling property. If the temperature exceeds 680 ° C, hot rolled crystal grains and precipitates increase in size, thereby lowering the recrystallization temperature, thereby causing a problem in the repeated heat treatment workability in the annealing process. Therefore, the winding temperature is preferably limited to 580 to 680 ° C.

After the winding, cold rolling is performed at a reduction ratio of 80 to 90%. The cold reduction rate is a factor that is closely related to hot rolling workability and recrystallization behavior of steel. As the cold rolling reduction rate decreases, the strain energy acting as the driving force of the recrystallization phenomenon decreases, thereby retarding the recrystallization, but the thickness of the hot rolled sheet produced for the purpose of connecting thin materials should be thinner. A problem occurs. In order to obtain a thin hot rolled sheet, the temperature drop (Drop) is severely generated, it is difficult to control the winding shape, not only increase the load of the hot rolling process, but also change the cooling pattern to increase the material variation, thereby reducing the cold rolling. On the other hand, if the cold reduction rate is increased, the strain energy accumulated in the material is increased to promote recrystallization, so there is a problem that it is difficult to secure high-temperature repeatability during heat treatment. If the cold reduction rate is less than 80%, ultra-thin hot rolled material should be produced. In this case, not only the load is applied to the hot rolling process but also the cold rolling property is bad. Since it worsens, it is preferable to limit the said cold rolling reduction to 80 to 90%. In addition, managing the cold reduction rate at 84 to 86% is more effective in managing hot rolling workability and recrystallization behavior.

Hereinafter, the present invention will be described in more detail with reference to Examples.

EXAMPLE

The slab continuously cast the steel composition as shown in Table 1 was reheated in the austenitic single-phase zone, and was finished hot rolled under the conditions shown in Table 2 below. Thereafter, it was cooled to the winding step at a cooling rate of 20 ℃ per second. After winding up to the winding temperature shown in Table 2, it was cold rolled at the reduction ratio of the following Table 2. The resistance weldability, recrystallization temperature, cold rolling property, high temperature heat treatment property, and annealing repeatability of the inventive material and the comparative material manufactured according to the conditions of Table 2 using the inventive steel and the comparative steel are shown in Table 2 below.

division Steel grade Chemical composition (% by weight) C Mn Si S Al N Nb Mo B (Nb + Mo) / C atomic ratio B / N weight ratio Invention steel One 0.0027 0.35 0.010 0.008 0.054 0.0025 0.043 0.044 0.0016 4.099 0.640 2 0.0035 0.39 0.011 0.010 0.039 0.0024 0.052 0.069 0.0019 4.388 0.792 3 0.0036 0.31 0.012 0.011 0.041 0.0027 0.064 0.091 0.0024 5.462 0.889 Comparative steel One 0.0067 0.57 0.014 0.013 0.047 0.0054 0.027 - 0.0005 0.521 0.093 2 0.0410 0.31 0.011 0.009 0.045 0.0046 - 0.055 0.0014 0.168 0.304 3 0.0035 0.94 0.610 0.010 0.106 0.0031 0.045 0.059 0.0024 3.772 0.774 4 0.0025 0.28 0.010 0.009 0.033 0.0028 0.037 0.241 - 13.981 0

As can be seen in Table 2, in the case of the invention materials (1 to 4) manufactured under the manufacturing conditions of the present invention using steel satisfying the component range of the present invention, resistance weldability, recrystallization temperature, cold rolling property, It can be seen that both the high temperature heat treatment property and the annealing repeatability in the continuous annealing process are excellent.

In addition, in the case of the invention material 3 whose cold reduction rate is 84%, it turns out that recrystallization temperature appears high compared with the invention material 4 whose cold reduction rate is 88%.

On the other hand, the comparative materials (1 to 10) that do not fall outside the component range of the present invention or manufactured under the manufacturing conditions do not satisfy the characteristics aimed at in the present invention, or cause problems such as breakage of the plate during repeated heat treatment. It was difficult to apply to a steel sheet.

Comparison in which the coiling temperature was lower than the range of the present invention at 450 ° C. using a steel having a composition of C, Mo, Nb, etc., (Nb + Mo) / C atomic ratio, and B / N weight ratio satisfying the component range of the present invention. In the case of ash (1), it was difficult to secure high temperature annealing workability due to the fact that the distribution and size of precipitates were not effective in suppressing recrystallization. A problem occurred.

In addition, in the case of the comparative material 2 having a high coiling temperature of 760 ° C, the cold rolling property and the like were good, but the recrystallization temperature was lowered to 750 ° C or lower by coarsening of precipitates, thereby ensuring high temperature annealing workability in the continuous annealing process. There was no.

In addition, in the case of the comparative material (3) having a low cold reduction rate of 75%, as the thickness of the hot rolled material became too thin, the wound shape of the hot rolled sheet worsened, which significantly decreased the workability, and it was difficult to secure cold rollability. .

On the other hand, in the case of the comparative material 4 having a high cold reduction rate of 95%, the deformation energy due to cold rolling is increased to promote recrystallization, so that recrystallization is completed at a temperature of 750 ° C. or lower, and it is difficult to secure annealing repeatability and the like. It was.

On the other hand, compared with the chemical composition of the present invention, the content of C, Mn, N is high, the content of Mo, Nb and B is low, and the weight ratio of (Nb + Mo) / C atomic ratio and B / N satisfies the scope of the present invention. The comparative material (5) manufactured outside the winding and cold rolling condition manufacturing range of the present invention method using the comparative steel 1 that cannot be compared and the comparative material (6) satisfying the manufacturing range are not prepared due to the precipitation of fine precipitates. The recrystallization was completed at 750 ° C. or lower, so that high temperature heat treatment and annealing repetitive workability could not be secured, and thus, suitable characteristics for the target ultrathin material connection material could not be obtained, and welding properties could not be obtained.

In addition, a comparative material in which the addition amount of C, N and Nb was out of the component range of the inventive method and the coiling temperature was lowered to 570 ° C using Comparative Steel 2 having a (Nb + Mo) / C atomic ratio lower than that of the present invention (7 ) Was able to secure the resistance weldability, but the aging phenomenon caused the material variation of the material, and recrystallization was completed at 700 ℃ or below, which is a relatively low temperature range. Therefore, it was difficult to apply to a connection material that requires high temperature annealing characteristics.

In addition, although the comparative material 8 manufactured in the manufacturing range of this invention using the comparative steel 3 with the addition amount of Mn, Si, Al, and N higher than the composition of the invention steel, recrystallization property etc. were securable, Si, Mn As a result, segregation of the material is severely generated and defects such as scale are generated, which acts as a factor to greatly reduce the cold rolling property. In addition, a surface thickening such as Si occurs during the high temperature heat treatment, thereby causing a problem of significantly lowering the annealing workability.

In addition, although the other components satisfy the component range of the present invention, the preparation of the present invention using Comparative Steel 4 in which the addition amount of Mo and B and the weight ratio of (Nb + Mo) / C atomic ratio and B / N are outside the scope of the invention. Comparative materials 9 to 10 manufactured in the range of the weld heat affected zone is significantly coarse during resistance welding, which acts as a factor of fracture of the weld plate, which greatly reduces resistance weldability. In addition, the hot rolling of the plate thickness (hunting, width direction deviation) was not able to secure the cold rolling characteristics, the production cost increases as the expensive Mo is added too much.

As described above, the present invention improves resistance weldability and high temperature annealing characteristics and the like through the control of appropriate components and manufacturing processes, and also ensures a stable working temperature as it ensures repeated annealing workability as a connecting material. Therefore, it is possible to reduce the occurrence of material deviation of the product, thereby preventing reprocessing due to material deviation, and reducing manufacturing cost.

Claims (3)

By weight%, C: 0.002-0.005%, Mn: 0.1-0.5%, Si: 0.04% or less, S: 0.015% or less, Al: 0.03-0.10%, N: 0.003% or less, Mo: 0.03-0.15%, Nb: 0.04 ~ 0.10%, B: 0.0005 ~ 0.003%, steel with remaining Fe and other unavoidable impurities, satisfying (Nb + Mo) / C atomic ratio: 4.0 ~ 5.9 and B / N weight ratio: 0.6 ~ 1.1 After finishing hot rolling to 910 ~ 930 ℃ and wound at 580 ~ 680 ℃, cold rolling at a reduction ratio of 80 ~ 90%, the manufacturing method of a pile steel sheet excellent in resistance weldability and repeated heat treatment. The method of claim 1, wherein the content of the B is 0.001 to 0.002% by weight, the method for producing a pile steel sheet excellent in resistance weldability and repeated heat treatment. The method for manufacturing a dummy steel sheet having excellent resistance weldability and repeated heat treatment property according to claim 1, wherein the cold rolling rate is 84 to 86%.