JP5376927B2 - Manufacturing method of high proportional limit steel plate with excellent bending workability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a sheet of plain steel, which has adequate bending workability and a high proportion limit that is effective for improving deflection-resistant strength, and can be used for components of household electrical appliances, electrical machinery and apparatuses, reinforcing members for buildings, automotive parts and the like. <P>SOLUTION: This steel sheet has a component composition comprising, by mass%, 0.001-0.20% C, 1.5% or less Si, 2.5% or less Mn, 0.10% or less P, 0.01% or less S, 0.001-0.10% acid soluble Al, 0.015% or less N, further predetermined amounts of Ti and Nb as needed, or further Cr, Mo and B, and the balance Fe with unavoidable impurities. The manufacturing method includes: hot-rolling a steel slab having the component composition; pickling the steel plate; cold-rolling the hot-rolled and pickled plate at a cold-rolling rate of 10-80%; and heating the cold-rolled sheet to a temperature range of 200-600&deg;C. In the obtained steel sheet, a ratio of the proportion limit to a tensile strength obtained with a tensile test is 0.65 or higher. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention is an object of home appliances, electronic equipment, architecture, automobile fields, etc., and is excellent in bending workability and plate thickness accuracy (including flatness) and is obtained by a tensile test in a structural member mainly subjected to bending. The present invention relates to a method for producing a high-strength steel sheet having a high proportional limit.

Structural members used for home appliance / electronic device parts, building reinforcement members, automobile parts, and the like are required to have high strength and excellent ductility and toughness.
For example, austenitic SUS301, SUS304 and ferrite-based SUH409 cold rolled materials have been put to practical use as materials for CPU socket frames or CPU fixing covers (Patent Documents 1 and 2). The former stainless steel plate can be increased in strength using work-induced martensitic transformation by cold rolling, and is generally used for leaf springs, frames and the like. The latter stainless steel secures the required rigidity by increasing the longitudinal elastic modulus in a certain direction.

On the other hand, high-strength steel plates such as frame materials and band materials for cathode-ray tubes have been developed as materials for electronic equipment, even for ordinary steel plates compared to stainless steel (Patent Documents 3, 4, 5, and 6).
JP 2004-68033 A Japanese Patent Application No. 2005-027269 JP 2001-59129 A JP-A-8-67945 JP 11-158548 A JP 2007-211131 A

Since stainless steel contains a large amount of Cr and Ni as component elements, it has a drawback of being expensive although it is excellent in corrosion resistance. In order to reduce the production cost of the material, it is advantageous to adopt plain steel instead of stainless steel.
On the other hand, if cheap ordinary steel is assumed, it is possible to achieve high strength by appropriately combining work strengthening, solid solution strengthening, transformation strengthening and precipitation strengthening, but any strengthening means can increase yield strength. Even if possible, the proportional limit considered to be the limit of substantial elastic deformation cannot be increased.
Therefore, even if the fracture strength as the structure can be ensured, the substantial plastic deformation capacity cannot be increased, and the initial deflection resistance as the structure cannot be increased.

For example, the CPU socket frame or CPU fixing cover shown in Patent Document 6 requires a high-strength material from the viewpoint of thinning and weight reduction, and if a 0.2% proof stress of a tensile test requires 600 N / mm ² or more. At the same time, since bending is performed, it is described that it is essential that bending be possible with a radius of the bending tip of 0.7 mm or less.
The stainless steel proposed in Patent Documents 1 and 2 satisfies this requirement, but the proportional limit is not necessarily large in the ordinary steel proposed in Patent Document 6. For this reason, the fixing pressure of the CPU socket frame or CPU fixing cover of Patent Document 6 is low. Therefore, it has been difficult to achieve sufficient thinning and weight reduction.

  The present invention has been devised to solve such problems, and is based on ordinary steel, which is cheaper than stainless steel plate, and can be used for home appliances / electric equipment parts, building reinforcement members, automobile parts, etc. An object of the present invention is to provide a method for producing a steel sheet having a high proportional limit effective for improving good bending workability and bending strength.

In order to achieve the object, the method for producing a high proportionality limit steel sheet having excellent bending workability according to the present invention includes C: 0.001 to 0.20 mass%, Si: 1.5 mass% or less, Mn: 2.5 mass% or less, P: 0.10. mass% or less, S: 0.01 wt% or less, acid-soluble Al: 0.001 to 0.10 mass%, N: 0.015 mass% include the following, among the steel sheet to have a component composition and the balance being Fe and unavoidable impurities hot rolling The hot-rolled pickled sheet that has been pickled and then cold-rolled at a cold rolling rate of 10 to 80% is obtained by a tensile test by heating the cold-rolled sheet to a temperature in the range of 200 to 400 ° C. It is characterized in that a high proportional limit steel sheet having a ratio of proportional limit to tensile strength of 0.65 or more is obtained .

As the above component composition, 0.005 to 0.15 mass% and Ti satisfying the following formula (1), 0.005 to 0.15 mass% and Nb satisfying the following formula (2), or 0.005 to 0.15 mass% and the following (3 It is preferable that two types of Ti and Nb satisfying the formula are included.
Ti <(48/14 × N + 48/12 × C + 48/32 × S) × 2 (1)
Nb <(93/14 × N + 93/12 × C) × 2 (2)
Ti−48 / 32 × S + 48/93 × Nb <(48/14 × N + 48/12 × C) × 2 (3)
Furthermore, you may contain 1 type, or 2 or more types, Cr: 0.2-1.5%, Mo: 0.1-0.6%, B: 0.0005-0.005%.

According to the present invention, a steel plate with a high proportional limit having good bending workability and high deflection strength can be obtained. For this reason, in applications such as home appliance / electric equipment parts, building reinforcement members, and automobile parts, it is possible to reduce the thickness and weight when ensuring the same performance as before.
The steel plate provided by the present invention can be used as a material suitable for a CPU socket frame or CPU fixing cover for attaching a CPU such as a computer to a wiring board, as well as other home appliance / electronic device parts, building reinforcement members, automobile parts. It can be applied to any structural member that is mainly subjected to bending, and the effect of reducing the thickness and weight is very large.

In general, methods for increasing the strength of steel include work strengthening, solid solution strengthening, transformation strengthening, and precipitation strengthening. However, in the process strengthening and transformation strengthening, the initial dislocation density is high, and there are innumerable movable dislocations contributing to the initial plastic deformation, so that the proportional limit is low and the elastic range is exceeded with low strength. Also, in solid solution strengthening and precipitation strengthening, the dislocation density as a whole is low even when the cold-rolled material is recrystallized and annealed, but the initial movable dislocations are sufficient, so the proportional limit must be increased. It is difficult.
On the other hand, if steel sheets with a high dislocation density in ferrite by processing strengthening are used as materials, annealing at a relatively low temperature range below the recrystallization temperature causes precipitation of solute C and solute N simultaneously with rearrangement of dislocations. Since the movable dislocation is fixed, the proportional limit is high and the elastic range can be maintained up to a high strength level.

The inventors diligently studied a means for increasing the proportional limit while maintaining good bending workability from the above viewpoint. As a result, cold rolling was performed on a steel sheet having a specific composition range, and annealing was performed at a specific temperature range below the recrystallization temperature, thereby causing rearrangement of dislocations while suppressing excessive recovery, thereby causing a proportional limit. It has been found that it is possible to improve the bending workability simultaneously with the improvement of the above.
Hereinafter, the action and effect of the chemical components of the base steel in the present invention, the reason for limiting the content, and the production method will be described individually.

First, the action / effect of each alloy component and the reason for limiting the content will be described.
C: 0.001 to 0.20 mass%
In addition to being effective for increasing the strength of steel strip, C is an element effective for fixing movable dislocations and improving the proportional limit during low-temperature annealing after work strengthening by making it exist as solid solution C. In order to obtain the effect, it is necessary to contain at least 0.001% by mass or more. On the other hand, if it is excessively contained, bending workability and weldability deteriorate, so the allowable upper limit is 0.20% by mass.

Si: 1.5% by mass or less
Si has a strength improving effect. When it is added for the purpose of improving strength, it is desirable to add 0.05% by mass or more. However, if it is added in excess of 1.5% by mass, the strength increases, but the workability deteriorates, and plate cracking occurs during cold rolling. This produces a spinel-type Si-based oxide that is inferior in peelability during hot rolling, so that the surface properties of the product are significantly deteriorated. Therefore, the upper limit of the addition amount is 1.5% by mass.

Mn: 2.5% by mass or less
Mn is an alloy element that has a strength improving action. The effect of addition is seen when the content is 0.10% by mass or more, but when it is added in excess of 2.5% by mass, the strength increases with the addition amount, but the pearlite band structure of hot-rolled sheet due to Mn segregation is remarkable. The bending workability is extremely deteriorated. For this reason, the upper limit of the Mn content is set to 2.5% by mass.

P: 0.10% by mass or less
P is an effective component for increasing strength. For the purpose of improving the strength, it is desirable to contain 0.01% by mass or more. However, if the content exceeds 0.10% by mass, the strength increases with the content. The band structure of pearlite becomes remarkable and the bending workability is extremely deteriorated. Also, the low temperature toughness is significantly degraded. For this reason, the upper limit of the P content is 0.10% by mass.

S: If contained in a large amount of 0.01% by mass or less , it causes high temperature embrittlement. Further, when the S content is increased, MnS of string-like inclusions is generated, and the bending workability is remarkably deteriorated. Accordingly, it is preferable that the amount be as small as possible, but up to 0.01% by mass is acceptable, so the upper limit was set to 0.01% by mass.

Acid-soluble Al: 0.001 to 0.10% by mass
It is an alloy component added as a deoxidizing agent, and in order to obtain a sufficient deoxidizing effect, it is necessary to add 0.001% by mass or more as acid-soluble Al. However, even if added in excess of 0.10% by mass, the effect of deoxidation is saturated, leading to an increase in production cost. In addition, Al fixes solid solution N as AlN, so that the effect of improving the bending workability is increased. On the other hand, the effect of improving the proportional limit by solid solution N cannot be expected. Therefore, the addition amount can be appropriately controlled according to the required characteristics. In other words, when the proportional limit improvement effect due to solute N is required, the Al content is preferably less than 27/14 × N, and conversely, when very good bending workability is required. It is desirable to add 27/14 × N or more.

N: 0.015 mass% or less
Like C, it is an element effective as a solid solution N to improve the proportionality limit due to the fixation of movable dislocations, but at the same time deteriorates workability. In order to obtain the effect of improving the proportional limit as solute N, the content is preferably 0.001% by mass or more. However, if it is contained in an amount of 0.015% by mass or more, bubbles due to N ₂ gas that could not be dissolved in the steel ingot at the time of melting and solidification are generated and become internal defects. In addition, workability is significantly deteriorated. Therefore, the content is 0.015% by mass or less. In addition, when emphasizing workability, it is desirable to make it 0.005 mass% or less.

Ti: 0.005-0.15 mass%
It is an element that forms precipitates with N, C, and S and is effective in increasing the strength by precipitation strengthening. It is added as necessary. In order to obtain the effect, addition of at least 0.005% by mass or more is necessary. However, even if added in excess of 0.15% by mass, the effect of increasing the strength is saturated and the manufacturing cost is increased.
On the other hand, when adding Ti alone, adding more than the following formula (1), the solid solution C and N necessary for fixing the mobile dislocations are completely fixed as precipitates, and the effect of improving the proportional limit is achieved. Disappear. Therefore, the amount of Ti added is limited to 0.005 to 0.15 mass% and an amount satisfying the following expression (1).
Ti <(48/14 × N + 48/12 × C + 48/32 × S) × 2 (1)

Nb: 0.005-0.15 mass%
Like Ti, it combines with C and N to form precipitates, and is an element effective for increasing strength by precipitation strengthening, and refines the metal structure to improve strength. It is added as necessary. In order to obtain the effect, it is preferable to add at least 0.005 mass% or more. However, even if added in excess of 0.15% by mass, the effect of increasing the strength is saturated and the manufacturing cost is increased.
On the other hand, when adding Nb alone, adding an amount of Nb that exceeds the following formula (2), the solid solution C and N necessary for fixing the mobile dislocations are completely fixed as precipitates, and the effect of improving the proportionality limit is achieved. Disappear. Therefore, the Nb addition amount is limited to 0.005 to 0.15 mass% and an amount satisfying the following expression (2).
Nb <(93/14 × N + 93/12 × C) × 2 (2)

Ti + Nb: 0.005-0.15 mass%
When adding Ti and Nb in combination, addition of Ti and Nb in excess of the following formula (3) will completely fix the solid solution C and N necessary for fixing the mobile dislocation as a precipitate, The improvement effect is lost. Therefore, in the case of combined addition of Ti and Nb, the total amount added is limited to 0.005 to 0.15 mass% and an amount satisfying the following expression (3).
Ti−48 / 32 × S + 48/93 × Nb <(48/14 × N + 48/12 × C) × 2 (3)

Cr: 0.2 to 1.5 mass%
Mo: 0.1-0.6% by mass
B: 0.0005 to 0.005 mass%
Cr, Mo, and B are elements that improve the hardenability of the steel material and are effective in increasing the strength through the formation of the hard second phase, and can be added as necessary. In order to obtain the effect, addition of Cr: 0.2% by mass or more, Mo: 0.1% by mass or more, and B: 0.0005% by mass or more is necessary. However, even if Cr exceeds 1.5% by mass, Mo: 0.6% by mass, and B: 0.005% by mass, the effect is saturated and the production cost is increased. Therefore, these addition amounts are limited to the ranges of Cr: 0.2 to 1.5 mass%, Mo: 0.1 to 0.6 mass%, and B: 0.0005 to 0.005 mass%, respectively.

Next, details of the operation of the manufacturing conditions will be described.
Hot rolling conditions:
Although there is no particular limitation, if the finishing temperature is less than the Ar3 transformation point, the hot strength fluctuation accompanying the transformation is large and the thickness accuracy is deteriorated. . In addition, the higher the coiling temperature, the more the ductility tends to be improved. However, when the coiling is performed at a temperature exceeding 700 ° C, the oxide scale grows thick and the pickling property deteriorates. Is desirable.
In the present invention, the same effect can be obtained by using either a hot-rolled steel sheet or a cold-rolled steel sheet for the final cold rolling, but cold rolling is required when strict sheet thickness accuracy is required. It is desirable to use a steel plate.

Cold rolling:
The number of dislocation densities required to raise the proportional limit due to the fixation of movable dislocations due to low-temperature annealing can be sufficiently secured even if the cold rolling rate is less than 10%, but high strength without adding a large amount of expensive additive elements In order to achieve this, a cold rolling rate of less than 10% is not sufficient. On the other hand, even if cold rolling exceeding 80% is performed, a great increase in strength due to work strengthening cannot be expected, which in turn leads to an increase in manufacturing cost. Therefore, the cold rolling rate is specified in the range of 10 to 80%.

Heat treatment temperature:
Due to the heat treatment, the solid solutions C and N fix the movable dislocations, and the proportional limit is improved. In order to obtain the effect, it is necessary to heat to a temperature of 200 ° C. or higher. On the other hand, when heated to a temperature exceeding 600 ° C., recovery and recrystallization occur and the dislocations move easily, and the movable dislocations once fixed by the solute C and N are released from the fixing, and the effect of improving the proportional limit is obtained. Disappear. Therefore, the heating temperature is limited to a range of 200 to 600 ° C.
Note that the heating time is not particularly limited, and even if the temperature is raised to a predetermined temperature and then cooled without soaking, an effect of improving the proportional limit is recognized. On the other hand, if the soaking time is too long, a part of the fixed dislocations fixed is released particularly in a relatively high temperature range of 500 to 600 ° C., so that the soaking time is preferably within 20 hours.

Steel slabs having the components shown in Table 1 were heated to 1200 ° C. and subjected to rough rolling and finish rolling to produce hot-rolled steel strips having a thickness of 2.0 mm. The hot rolling finishing temperature and the hot rolling coiling temperature were as shown in Table 2. The hot rolling finishing temperature is at least the Ar ₃ transformation point.
The obtained hot-rolled sheet was pickled and then cold-rolled to a thickness of 1.0 mm. At this time, the cold rolling reduction was as shown in Table 3.
The obtained cold-rolled steel strip was subjected to a soaking treatment at 200 ° C., 400 ° C., and 680 ° C. for 8 hours in an ammonia decomposition gas atmosphere using a batch annealing furnace and cooled in the furnace. A test piece was taken from the obtained steel strip and subjected to a tensile test and a bending test at room temperature.

In addition, the JIS Z2201 No. 5 test piece collected in the C direction (perpendicular to the rolling direction) was used for the tensile test. Moreover, the V block method (90 degree V bending test according to JIS Z2248) was used for evaluation of bending workability. Change the punch tip R, bend the specimen V 90 degrees with a bending axis parallel to the rolling direction (C-direction bending), and the tip bend R is 0.5mm and no cracks were observed. ) And those in which cracks were observed were evaluated as x (defect).
The results obtained are shown in Table 4.

No.1 to No.20 having the chemical composition and cold rolling rate within the scope of the present invention have a high proportional limit of PL / TS ≧ 0.65 by performing heat treatment at 200 ° C. × 8 h and 400 ° C. × 8 h. In addition, it can be seen that it also has excellent bending workability.
On the other hand, the comparative examples of No. 31 to No. 39 having chemical compositions outside the scope of the present invention are inferior in either or both of the bending workability and the PL / TS value regardless of the heat treatment conditions.
Even if the chemical composition is within the range of the present invention, PL / TS ≧ 0.65 cannot be satisfied in the cold-rolled state without heat treatment, and in many samples, the bending workability is inferior and the tip bend R0.5mm is 90 °. Bending will cause cracks.
On the other hand, when the heat treatment at 680 ° C. × 8 h is performed, the metal structure is recrystallized, so that bending workability is greatly improved, but PL / TS ≧ 0.65 cannot be satisfied.

Claims (3)

C: 0.001 to 0.20 mass%, Si: 1.5 mass% or less, Mn: 2.5 mass% or less, P: 0.10 mass% or less, S: 0.01 mass% or less, acid-soluble Al: 0.001 to 0.10 mass%, N: 0.015 includes the following mass%, the steel sheet balance to have a component composition consisting of Fe and unavoidable impurities hot-rolled pickling plate pickled after hot rolling, cold rolling ratio: cold-rolled at 10% to 80% After the application, the cold-rolled sheet is heated to a temperature in the range of 200 to 400 ° C. to obtain a high proportional limit steel sheet having a ratio between the proportional limit obtained by a tensile test and the tensile strength of 0.65 or more. A method for producing highly proportional steel sheets with excellent workability . The steel plate further comprises 0.005 to 0.15 mass% and Ti that satisfies the following formula (1), 0.005 to 0.15 mass% and Nb that satisfies the following formula (2), or 0.005 to 0.15 mass% and the following formula (3): high proportional limit steel manufacturing method having excellent bending workability as claimed in claim 1 in which contains two kinds of Ti and Nb satisfy.
Ti <(48/14 × N + 48/12 × C + 48/32 × S) × 2 (1)
Nb <(93/14 × N + 93/12 × C) × 2 (2)
Ti−48 / 32 × S + 48/93 × Nb <(48/14 × N + 48/12 × C) × 2 (3) Steel plate, further, Cr: 0.2~1.5%, Mo: 0.1~0.6%, B: a 0.0005 to 0.005 percent of one or bending workability according to claim 1 or 2 are those containing two or more An excellent method for producing high proportional steel sheets.