JP5101627B2 - High yield ratio cold rolled steel sheet with excellent weather resistance - Google Patents

Description

  The present invention relates to a high-strength weather-resistant cold-rolled steel sheet used for construction, railway vehicles, containers and the like and a method for producing the same, and more specifically, a high-strength weather-resistant cold-rolled steel sheet having a high yield ratio and the production thereof. It is about the method.

  Conventionally, materials such as stainless steel and aluminum have been used for the purpose of reducing the weight and extending the service life of containers and railway vehicles. Properties required for such products include bending workability, weldability, durability, and the like. Further, in the case of a structure for transportation, since it is often subjected to an impact when cargo is loaded and loaded, it is necessary to suppress deformation due to the impact. For this purpose, it is preferable to apply a material having a high yield ratio. .

  The yield ratio (Yield-ratio) is defined as the ratio of the tensile strength to the yield strength among the material values obtained from the tensile test. A high yield ratio at the same tensile strength level means that the yield strength of the material is high. It means that. That is, steel having a high yield ratio has a high yield strength, which is a material characteristic of the elastic region, so that the resistance to deformation increases even when subjected to an impact, and the ability to suppress deformation increases. is there. When used for applications such as containers, it is preferable to ensure a yield ratio of 80% or more. Especially, containers and the like have to withstand various marine or land climatic conditions according to transportation conditions, so they have excellent weather resistance. The use of steel was required.

As an example, the conventional a weatherproof rolled steel SPA-C (Industry Standard KS-D3542 and JIS-G3125) but have been used, these steels have a tensile strength as low as ^secondary 50 kg / mm, more When manufacturing a large product, the increase in transportation costs due to its own weight was a limiting factor. In addition, there are high strength cold-rolled steel materials with a tensile strength of 60 to 80 kg / mm ² for automobile structural members, but in the case of these materials as well, by producing with an emphasis on strength characteristics, the desired weather resistance can be achieved. There was a problem that it could not be demonstrated.

Recently, in order to cope with cost saving and environmental problems in the container industry, attempts have been made to greatly reduce the weight of the container itself to produce a larger container, thereby greatly increasing the efficiency of transportation. Yes. In particular, a request for a steel sheet having a weather resistance and a high strength of 80 kg / mm ² or more, and a technique relating to a method for manufacturing these materials have been proposed.

As an example, in Patent Document 1, C: 0.008% or less, Si: 0.5 to 2.5%, Mn: 0.1 to 3.5%, P: 0.03 to 0.20%, S : 0.01% or less, Cu: 0.05 to 2.0%, Al: 0.005 to 0.1%, and N: 0.008% or less, Cr: 0.05 to 6.0%, Steel containing Ni: 0.05-2.0%, Mo: 0.05-3.0%, B: 0.0003-0.002% is heated at 1100-1300 ° C., 800-950 A method for producing a hot-rolled steel strip was proposed, characterized in that rolling was completed at ℃ and winding was performed at 400 to 700 ℃. However, it only a small portion of Example Tensile Strength 60~70kg / mm ² class in the art, since the most exhibited a tensile strength of 50 kg / mm ² class, to ensure 80 kg / mm ² class tensile strength I can't. In addition, the addition of a large amount of a hardenability improving element such as Cr or Mo among the constituent components causes a problem that weldability deteriorates and manufacturing costs increase.

As another example, in Patent Document 2, C: 0.15% or less, Si: 0.7% or less, Mn: 0.2 to 1.5%, P: 0.03 to 0.15%, S: 0.02% or less, Cu: 0.4% or less, Al: 0.01-0.1%, Cr: 0.1% or less, Ni: 0.4-4.0%, and Mo: A method in which steel containing 0.1 to 1.5% is heated to 1050 to 1300 ° C., hot-rolled at 950 ° C. or more and 40% or more, and then rolled at 900 to 750 ° C. and air-cooled. Proposed. However, even in this case, the tensile strength is almost 50 kg / mm class ² and only a part of it has a tensile characteristic of 60 kg / mm class ^2. This technique is mainly applied to steel sheets with a tensile strength of 50 kg / mm class ^2. I can say that. In addition, P is added in a large amount of 0.03 to 0.15% to mention the effect of improving the corrosion resistance in the seawater atmosphere. However, the addition of a large amount of P induces center segregation of cold-rolled material. As a result, there is a problem that the workability of the steel sheet is drastically lowered.

Furthermore, in Patent Document 3, C: 0.02 to 0.12%, Si: 0.5% or less, Mn: 0.1 to 2.0%, P: 0.07 to 0.15%, S: 0.02% or less, Cu: 0.25 to 0.55% or less, Al: 0.01~0.05%, Cr: 0.3~1.25%, N 2: 0.006% or less, Ti : Steel containing 0.06 to 0.20% is controlled in the range of 12.1X _{ti, eff} (%) / Mn (%)> 1.0, and reheated at 1180 ° C. or higher, at 880 to 950 ° C. After hot rolling, a technique of winding at 650 ° C. or lower is provided. In this technique, the Ti content is added as a precipitate control element in conjunction with the Mn addition amount. However, the tensile strength in the examples of this technology is 60 kg / mm ² class, lower than 80 kg / mm ² class as a target in the present invention.

Japanese Patent Application Laid-Open No. 07-207408 Japanese Patent Laid-Open No. 11-21622 Japanese Patent Laid-Open No. 06-104858

Therefore, in the present invention, in order to solve the above-described problems, it is intended to ensure weather resistance and high yield ratio characteristics having high strength characteristics of 80 kg / mm ² or more.

  In order to achieve the above object, the high yield ratio type weatherproof cold-rolled steel sheet of the present invention is C: 0.08-0.20%, Si: 0.1-0.5%, Mn: 0% by weight. .9-2.0%, P: 0.02% or less, S: 0.01% or less, Al: 0.02-0.07%, Nb: 0.03-0.06%, Ni: 0.0. 05-0.30%, Cu: 0.2-0.5%, Cr: 0.3-0.6%, B: 0.001-0.004%, Co: 0.02-0.08% And the remaining Fe and other inevitable impurities.

The cold-rolled steel sheet preferably has a tensile strength of 80 kg / mm ² or more.

Moreover, the cold-rolled steel plate manufacturing method of this invention is weight%, C: 0.08-0.20%, Si: 0.1-0.5%, Mn: 0.9-2.0%, P : 0.02% or less, S: 0.01% or less, Al: 0.02 to 0.07%, Nb: 0.03 to 0.06%, Ni: 0.05 to 0.30%, Cu: 0.2 to 0.5%, Cr: 0.3 to 0.6%, B: 0.001 to 0.004%, Co: 0.02 to 0.08%, the remaining Fe and others The steel consisting of inevitable impurities is reheated at 1150 to 1300 ° C., hot-rolled at a finish rolling temperature of 850 to 950 ° C., cooled at a cooling rate of 20 to 40 ° C. per second, and 500 to 650 ° C. after wound at a temperature, comprising to continuous annealing at 500 ° C. or higher to a ₁ or less transformation point and cold rolling.

  ADVANTAGE OF THE INVENTION According to this invention, while ensuring a weather resistance and a mechanical characteristic simultaneously, a high yield ratio can be obtained and the steel plate with high added value, such as a use as which impact resistance is requested | required, can be provided. In addition, since the continuous annealing operation is performed in a relatively low temperature region, energy saving and an improvement effect on the annealing workability can be obtained at the same time.

  Hereinafter, the present invention will be described in detail.

  The inventors of the present invention have completed the present invention by repeating research and experiments for satisfying weatherability as well as various processing characteristics and ensuring high yield characteristics and high yield ratio characteristics. The present invention secures a high yield ratio as well as high strength characteristics by controlling the amount of addition of Cr, B, Nb, etc. in a weathering component system of Cu-Co.

  C is preferably 0.08 to 0.20% by weight (hereinafter simply referred to as%).

  C is an element added to improve the strength of the steel sheet, and the tensile and yield strength increases as the addition amount increases, but if added excessively, the workability of the material decreases, so that The upper limit is preferably 0.20%. On the other hand, if the C content is less than 0.08%, there is a problem that the precipitation strengthening effect cannot be obtained sufficiently.

  Si is preferably 0.1 to 0.5%.

Si is an element that provides the effect of deoxidation and solid solution strengthening of molten steel, and also improves the corrosion resistance by forming a dense oxide of Fe ₂ SiO ₄ together with Fe on the surface layer of the steel at a high temperature, In order to obtain this effect, it is preferable to add at least 0.1% or more. Therefore, Si must be added to improve the weather resistance, but if added excessively, there is a problem that the weldability is lowered, and the plating property is lowered. preferable.

  Mn is preferably 0.9 to 2.0%.

  Mn is an effective element for strengthening by solid solution, and is an important element for improving the hot workability by increasing the strength of steel. On the other hand, the softness and workability of the material due to the formation of MnS. It is also an element that damages. A low Mn content is advantageous for workability, but there is a problem with strength, and 0.9% or more is preferably added in order to ensure the target strength. On the other hand, when Mn is added excessively, there is a problem that economic efficiency is deteriorated due to a large amount of expensive alloy elements added and weldability is impaired. Therefore, the upper limit is preferably 2.0%.

  P is preferably 0.02% or less.

  Since P plays a role of improving the corrosion resistance of steel, it is preferable to be added in a large amount in terms of corrosion resistance. However, P is the element that causes the largest segregation at the time of casting. It becomes a factor to make. Therefore, the content is preferably limited to 0.02% or less.

  S is preferably 0.01% or less.

  S is known as an element effective for improving the corrosion resistance, but forms non-metallic inclusions that act as a corrosion initiation point by combining with Mn in steel, so it is preferable to reduce its content as much as possible. . Accordingly, the S content is limited to 0.01% or less, and more preferably 0.005% or less.

  Al is preferably 0.02 to 0.07%.

  Al is generally an effective element for improving the deoxidation and corrosion resistance of molten steel, but when added in excess, there is a problem that the amount of inclusions in the steel is increased and workability is lowered. The content is preferably set to 0.02 to 0.07%.

  Nb is preferably 0.03 to 0.06%.

Nb is not only an effect of delaying the recrystallization of ferrite, but also an element that has an effect of increasing the strength of the steel sheet by binding and precipitation with C, N _2, etc. in the steel. In order to ensure, it is preferable to add 0.03% or more. On the other hand, if the amount of Nb added exceeds 0.06%, it may cause an increase in manufacturing cost and a decrease in hot rolling workability.

  Ni is preferably 0.05 to 0.3%.

  Ni is an element that not only plays a role in preventing casting cracks that generally occur during casting in Cu-added steel, but also improves corrosion resistance, and in order to exert such an effect, 0.05% or more is added. It is preferable to do. However, if the Ni content exceeds 0.3%, the corrosion resistance is worsened, and the cost increases due to excessive use of expensive alloy elements.

  Cu is preferably 0.2 to 0.5%.

  Cu is an element that improves the corrosion resistance by forming a stable rust layer in a corrosive atmosphere, and it is preferable to add 0.2% or more in order to ensure the target corrosion resistance. However, if the added amount of Cu exceeds 0.5%, it may cause grain boundary cracking during continuous casting and may roughen the surface state of the hot-rolled steel sheet.

  It is preferable that Cr is 0.3 to 0.6%.

  Cr is an element that plays a role of forming a stable rust layer like Cu, and 0.3% or more is preferably added in order to secure corrosion resistance and obtain strength. On the other hand, if the added amount of Cr exceeds 0.6%, it acts as a factor inducing the corrosiveness of the pores, and the manufacturing cost is rapidly increased.

  B is preferably 0.001 to 0.004%.

  B is an element that not only improves the hardenability of the steel but also delays recrystallization of the ferrite phase, and 0.001% or more may be added to obtain a target strength level in a low temperature region. preferable. On the other hand, if the B content exceeds 0.004%, the hardenability is increased and the formation of a hard phase such as bainite is promoted in the hot rolling process, so that the cold rollability cannot be ensured.

  Co is preferably 0.02 to 0.08%.

  Co is an element that promotes the formation of a surface layer corrosion inhibiting product by reacting with Cu and Cr added to ensure corrosion resistance in the steel. It is preferable to add 02% or more. However, if the added amount of Co exceeds 0.08%, it will act as a factor that increases the manufacturing cost rather than the effect of improving the corrosion resistance.

  The steel of the present invention is composed of inevitable impurities and the remaining Fe while containing the above components. Alloy elements that improve the properties may be added to the weather resistant steel as needed, and even if alloy elements not described in the examples of the present invention are added, they are not excluded from the scope of the present invention. .

The cold-rolled steel sheet of the present invention has high strength and high yield ratio characteristics. According to an embodiment of the present invention, the tensile strength may be at 80 kg / mm ² or more, for example those having a substantially 80 to 110 kg / mm ² approximately tensile strength. The yield ratio may be 80% or more, for example, approximately 85 to 94%. The softness has a characteristic of 10% or more.

  Such characteristics are obtained by low-temperature annealing of the present invention described later while satisfying the above component system. By low temperature annealing, all of the rolling deformation grains are not recovered and a part of them is present in the microstructure. The microstructure of the present invention may be ferrite or a mixed grain structure of ferrite and pearlite, and is not limited thereto.

  Hereinafter, the manufacturing method of the cold-rolled steel sheet of this invention is demonstrated.

The method for producing the steel having the above composition will be specifically described with reference to an example. The steel having the above chemical composition is reheated at 1150 to 1300 ° C., finish hot rolled at 850 to 950 ° C., cooled at a cooling rate of 20 to 40 ° C./second, and wound at 500 to 650 ° C. and then, it relates to a method of producing a superior tensile strength 80 kg / mm ² or more high yield ratio cold rolled steel sheet in weather resistance by continuous annealing at a temperature 500～A ₁ transformation point during cold rolling and heat treatment is there.

  If the reheating temperature is less than 1150 ° C, the solidified structure formed at the time of casting is not sufficiently broken and the center segregation is well developed, so that the final formed crystal grains are generated, and the workability and impact toughness are increased. Is significantly reduced. In addition, when the reheating temperature exceeds 1300 ° C., the scale formation due to oxidation is promoted, the amount of decrease in the thickness of the slab is large, and the impact toughness is reduced due to the coarsening of crystal grains during the reheating, Since the economic loss due to the increase in the basic unit is large, the management range is limited to 1150 to 1300 ° C.

  When the finish hot rolling temperature is higher than 950 ° C., uniform hot rolling cannot be performed over the entire thickness, and the refinement of the crystal grains becomes insufficient, thereby reducing the impact toughness due to the coarsening of the crystal grains. appear. On the other hand, when the finish hot rolling temperature is less than 850 ° C., the hot rolling is finished in a low temperature region, and the mixing of crystal grains proceeds rapidly, resulting in a decrease in corrosion resistance and workability. It is preferable to limit the rolling temperature to 850 to 950 ° C.

  In the cooling step after hot rolling as described above, the cooling temperature is preferably 20 to 40 ° C. That is, if the cooling rate on the run-out table (ROT, Run-out-table) is less than 20 ° C. per second after finishing hot rolling, relatively coarse crystal grains are formed due to the promotion of crystal grain growth, resulting in a decrease in strength. Therefore, the lower limit is preferably 20 ° C. per second. On the other hand, if the cooling rate exceeds 40 ° C. per second, a hard second phase such as bainite is formed and the cold rolling property is remarkably lowered, so the cooling rate should be set to 20 to 40 ° C. per second. Is preferred.

In the winding process after cooling as described above, the winding temperature is preferably 500 to 650 ° C. When the hot rolling coiling temperature exceeds 650 ° C., a sufficient precipitation effect cannot be obtained, so that it is difficult to stably secure the target strength of 80 kg / mm ² by reducing the material strength. On the other hand, at a coiling temperature of less than 500 ° C., a hard phase is generated during cooling and maintenance, and rollability of the rolling mill is increased in the cold rolling process to ensure rollability. Therefore, it is preferable to limit the control range of the coiling temperature to 500 to 650 ° C.

The material that has been hot-rolled is rolled under normal cold rolling conditions and undergoes a continuous annealing process. At this time, it is necessary to appropriately manage the annealing temperature in order to ensure the target material characteristics. In the continuous annealing step, if the annealing temperature is lower than 500 ° C., there is a problem that deformed grains during cold rolling remain as they are, and the softness is drastically lowered, so that workability is lowered. In contrast, in the A ₁ transformation point or more annealing temperature, the yield strength martensite phase is formed by transformation during the cooling after annealing the yield ratio is lowered to 60% or less reduced resistance to deformation to ensure the upper limit line for that could not be annealing temperature is preferably set to a ₁ transformation point. In the present invention, some of the rolling deformation grains can remain on the cold-rolled steel sheet by low-temperature annealing.

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

After evaluating the standardized corrosion resistance index (CI) value and the weather resistance test on the steel having the composition as shown in Table 1, the evaluation results are shown in [Table 2]. The weather resistance test is a result of conducting a salt spray test (SST, Salt Spray Test) for 480 hours under conditions of 5% salt water (NaCl solution) at 30 ° C. Here, the corrosion resistance index (CI) is a weather resistance related evaluation index defined in ASTM G101, and it is known that the higher this value is, the better the weather resistance of the steel is. The index calculated based on is defined as follows.

That is, corrosion resistance index (CI) = 26.11 (% Cu) +3.88 (% Ni) +1.2 (% Cr) +1.49 (% Si) +17.28 (% P) −7.29 ( % Cu) (% Ni) -9.10 (% Ni) (% P) -33.39 (% Cu) ²

As shown in Table 2, the comparative steel 2 is difficult to apply in terms of weather resistance because the corrosion resistance index is low and the corrosion weight loss is large, and the comparative steel 3 and the comparative steel 4 have slightly higher CI values. However, the corrosion weight loss by the salt spray test was 0.030 g / cm ² or more, and the weather resistance was low. In contrast, invention steels 1 and 2 and comparative steel 1 showed excellent weather resistance in terms of corrosion weight loss and corrosion resistance index.

  In Table 1 of Example 1, the inventive steels 1 and 2 and comparative steels 1 to 4 were used to manufacture cold-rolled steel sheets under the conditions shown in Table 3, and then mechanical properties and processing for each material. The results of evaluating the characteristics are shown in Table 4.

As shown in Table 4, in the case of invention materials 1 to 4 whose chemical components and production conditions satisfy the scope of the method of the present invention, a tensile strength of 80 kgf / mm ² or more, a yield ratio of 80% or more, and a softness of 10% or more are ensured. It was possible to produce high-yield-ratio-type weathering steel having high strength without causing cracks during banding.

  On the other hand, although the chemical composition range of invention steel was satisfy | filled, the comparative materials 1-5 which remove | deviated from the range of invention with manufacturing conditions were not able to obtain the target characteristic. That is, in the case of the comparative material 2 and the comparative material 5 whose annealing temperature is higher than that of the present invention, the tensile strength criterion is satisfied, but the yield is caused by the second phase generated by the transformation in the cooling stage due to the high annealing temperature. The strength was lowered, and the yield ratio was 70% or less. That is, there is a problem that the target yield ratio of 80% or more cannot be obtained and the deformation resistance is lowered.

  In the case of the comparative material 4 having an annealing temperature lower than the range of the invention, almost all of the deformed grains generated during the cold rolling remained, and the softness and banding workability could not be ensured. Also, in the case of the comparative material 1 whose hot rolling finishing temperature and winding temperature are out of the scope of the present invention and the comparative material 3 whose cooling rate is higher than the scope of the present invention, the softness is less than 5% and appropriate workability is ensured. I could not.

  Although Mn and Cr are not included in the composition of the present invention, when the production conditions are within the scope of the present invention for the comparative steel 1 having relatively excellent weather resistance (Comparative material 7), the flexibility and workability should be ensured. It was difficult. Furthermore, when the annealing temperature was raised to ensure softness and workability, a composite structure was generated, yield strength was lowered, and a yield ratio of 80% or more could not be obtained.

  It is clear from Table 4 when the production conditions change test was performed on the comparative steels 2 to 3 whose chemical composition was out of the composition range of the steel of the present invention and could not be secured even in the weather resistance (comparative materials 8 to 11). It was found that the appropriate range of workability and material could not be set.

Claims (3)

  % By weight, C: 0.08 to 0.20%, Si: 0.1 to 0.5%, Mn: 0.9 to 2.0%, P: 0.02% or less, S: 0.01 %: Al: 0.02 to 0.07%, Nb: 0.03 to 0.06%, Ni: 0.05 to 0.30%, Cu: 0.2 to 0.5%, Cr: 0 .3 to 0.6%, B: 0.001 to 0.004%, Co: 0.02 to 0.08%, consisting of the remaining Fe and other unavoidable impurities Rolled steel sheet. The tensile strength of the cold-rolled steel sheet is 80 kg / mm ² or more, The high yield ratio type weather-resistant cold-rolled steel sheet according to claim 1. % By weight, C: 0.08 to 0.20%, Si: 0.1 to 0.5%, Mn: 0.9 to 2.0%, P: 0.02% or less, S: 0.01 %: Al: 0.02 to 0.07%, Nb: 0.03 to 0.06%, Ni: 0.05 to 0.30%, Cu: 0.2 to 0.5%, Cr: 0 3 to 0.6%, B: 0.001 to 0.004%, Co: 0.02 to 0.08%, and the steel composed of the remaining Fe and other inevitable impurities at 1150 to 1300 ° C Reheat and hot-roll at a finish rolling temperature of 850 to 950 ° C, cool at a cooling rate of 20 to 40 ° C per second, wind up at a temperature of 500 to 650 ° C, and then cold-roll. high yield ratio-type production method of a cold-rolled steel sheet to continuous annealing at 500 ° C. or higher to a ₁ transformation point Te.