JPH04311518A - Production of cold rolled steel strip of ferritic stainless steel excellent in drilling workability - Google Patents

Abstract

PURPOSE:To remarkably improve the drilling workability of a cold rolled steel strip in which sufficient properties could not been obtd. by a single rolling method and a double rolling method approximately uniformly distributing its draft which have been used heretofore by specifying the first and second draft. CONSTITUTION:In a ferritic stainless steel contg., by weight, <=0.1% C, <=1% Si, <=1% Mn, 16 to 30% Cr, 0.1 to 1.0% Nb, <=0.1% N and the balance Fe with inevitable impurities, at the time of subjecting a hot rolled steel strip in which its hot rolling finishing temp. is regulated to >=850 deg.C to cold rolling, it is subjected to primary cold rolling at >=60% draft, is thereafter subjected to annealing and pickling, is successively subjected to secondary cold rolling at 20 to 40% draft and is thereafter subjected to annealing and pickling, by which the cold rolled steel strip of the ferritic stainless steel excellent in drilling workability is manufactured. Its drilling workability in which sufficient properties could not be obtd. by a single rolling method and a double rolling method approximately uniformly distributing its draft which have been used heretofore can be improved.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a cold-rolled ferritic stainless steel strip having excellent hole expandability.

0002

BACKGROUND OF THE INVENTION Conventionally, ferritic stainless steel, in which C and N are stabilized with Nb, has been used as a stainless steel with excellent workability, corrosion resistance, and surface quality. These steel types usually contain Cr: 16-30%, C: 0.1% or less,
N: 0.1% or less, Si: 1% or less, Mn: 1% or less,
Contains Nb: 0.1 to 1.0%, and in order to further improve corrosion resistance and workability, Cu: 1% or less, Mo: 4% or less, Ni: 1% or less, Ti: 0.2% or less. , Zr:0.
2% or less, V: 0.2% or less, and Ta: 0.2% or less. In order to improve the workability of these ferritic stainless steels, a manufacturing process in which the hot rolling end temperature is less than 850° C. is often used. This method uses low-temperature rolling to retain a large amount of rolling strain during hot rolling, promotes sufficient recrystallization during subsequent annealing, and prevents the formation of {001} <110> texture that impedes workability. It is something to do.

[0003] The low-temperature hot rolling technology for Nb-stabilized steel is also disclosed in the literature (Tetsu to Hagane Vol. 66 (1980) No. 11, S1167), and the present inventors have also fabricated cold-rolled sheets according to the disclosed technology. The workability was investigated, and although it was good in the conical cup test etc., it did not show good workability in the hole expansion test.

[0004] Furthermore, it contains Nb, which is the object of the present invention,
Moreover, steel types with a relatively high Cr content have high hot strength, so if hot rolled at a low temperature, scratches are likely to occur during rolling or coil winding. When scratches occur, surface grinding using a grinder or the like is required to remove the scratches from the hot rolled steel strip, which ultimately leads to a significant increase in product cost. Therefore, it is necessary to set the hot rolling end temperature to 850° C. or higher and to perform rolling with low hot strength.

[0005] When the hot rolling end temperature is 850°C, {
001} <110> Directional average value of Lankford value (r value), which is an index of workability because texture is likely to occur

[Equation 1] rL is the R value parallel to the rolling direction, rC is the r value perpendicular to the rolling direction, and rX is the r value at 45 degrees to the rolling direction). A double rolling method is used in the rolling process.

[0006] The double rolling method refers to the first cold rolling of a hot-rolled annealed steel strip, followed by annealing and pickling, followed by a second cold rolling to final plate thickness, final annealing, and acid washing. This is a method of washing and making products. By repeating rolling and annealing twice in this manner, the {001}<110> texture can be destroyed and workability can be improved.

[0007] When a double rolling method is used, the rolling reduction ratio during the first rolling and the rolling reduction during the second rolling are generally set to be approximately equal. For example, if the thickness of the hot-rolled annealed steel strip is 4 mm and the finished plate thickness is 1 mm, by setting the intermediate plate thickness to 2 mm, the first rolling reduction is 50%,
The second rolling reduction rate is also 50%. If the rolling reduction rate is distributed evenly in this way, it will be more effective than if the reduction ratio was distributed more intensively to either the first or second process.

[Math 2] will improve.

[0008]

[Problem to be solved by the invention] Generally

[Equation 3] has a good correlation with workability, and the higher the value, the better the workability. However, hole enlarging is required for burring, etc., which is performed to join plates to pipes, and in this case, it is not always possible to

[Equation 4] and workability did not match. 4mm → 2 as in the previous example
mm → 1 mm double rolling method (rolling ratio distribution 50%: 50%
), the hole expandability did not improve.

[0009] The present invention has a hot rolling end temperature of 850°C or higher, Cr: 16-30%, C: 0.1% or less, N:
0.1% or less, Si: 1% or less, Mn: 1% or less, and Nb: 0.1 to 1.0%, and optionally Cu.
: 1% or less, Mo: 4% or less, Ni: 1% or less, Ti:
0.2% or less, Zr: 0.2% or less, V: 0.2% or less, and Ta: 0.2% or less. The object of the present invention is to provide a method for manufacturing a cold-rolled steel strip.

0010

[Means for Solving the Problems] The present inventors have conducted day and night research to improve the hole-expanding process, and have found that in order to improve the hole-expanding process,

rmin value (usually rx) than [Equation 5]
I learned that it is necessary to increase the In the case of steel strips whose hot rolling end temperature is 850°C or higher, {0
01}〈110〉Since it is necessary to destroy the texture,
In cold rolling, the rmin value does not improve even if a single rolling method is used, and a double rolling method is used, and the first rolling reduction is 60% or more, and the second rolling reduction is 20 to 40%. It was found that the rmin value was improved the most when the hole expansion method was used.

[0011] If the rolling reduction rate is not distributed primarily to the first rolling, but the first rolling reduction rate and the second rolling rate are equally distributed,

[Number 6
] is improved, but the rmin value is not sufficiently improved, and the hole expandability is inferior to that obtained by the method of the present invention. Moreover, contrary to the present invention, the second rolling ratio is set to 60% or more, and the first rolling ratio is set to 2.
If it is 0 to 40%, the {001}<110> texture will not be sufficiently destroyed in steel strips whose hot rolling end temperature is 850° C. or higher, and the rmin value will not be sufficiently improved.

That is, in weight percent, C: 0.1% or less, Si
: 1% or less, Mn: 1% or less, Cr: 16-30%, N
b: 0.1 to 1.0%, N: 0.1% or less, and optionally further contains Cu: 1% or less, Mo: 4% or less, Ni
: 1% or less, Ti 0.2% or less, Zr: 0.2% or less,
In ferritic stainless steel, it may contain one or more selected from the group consisting of V: 0.2% or less and Ta: 0.2% or less, with the remainder consisting of unavoidable impurities and Fe. When cold rolling a hot-rolled steel strip with a rolling finish temperature of 850°C or higher, the first cold rolling is performed at a rolling reduction of 60% or higher, followed by annealing, pickling, and subsequent cold rolling at a rolling reduction of 20 to 20%. 40% second cold rolling, then annealing,
By pickling, a ferritic stainless steel strip with excellent hole expandability is produced.

The present invention will be explained in more detail below.

Nb-stabilized ferritic stainless steel is used as a stainless steel with excellent workability, corrosion resistance, and surface quality. These steel types usually have C: 0.1% or less,
Si: 1% or less, Mn: 1% or less, Cr: 16-30%
, Nb: 0.1 to 1.0%, and N: 0.1% or less. Since C and N are stabilized by Nb, the workability and surface quality are good, and since the Cr content is relatively high, the corrosion resistance is also good. In order to further improve the corrosion resistance and workability of these steel types, Cu: 1% or less, Mo: 4% or less, Ni: 1% or less, Ti: 0.2% or less, Zr: 0.2
% or less, V: 0.2% or less, and Ta: 0.2% or less.

The reasons for limiting the steel composition are as follows.

[0016]C is an element that greatly affects workability and corrosion resistance, and if the amount of C is large, it becomes hard, and also forms Cr carbide, causing intergranular corrosion, so the content must be kept at 0.1% or less.

[0017]Si is an element necessary as a deoxidizing agent,
Adding too much will harm processability, so the upper limit is set at 1.0%.
And so.

[0018] Mn has deoxidizing and desulfurizing effects,
If added in large amounts, corrosion resistance will be impaired, so the upper limit is set at 1.0%.
And so.

[0019] If Cr is less than 16%, sufficient corrosion resistance cannot be maintained, and if it exceeds 30%, workability deteriorates, so the content is limited to a range of 16 to 30%.

[0020] Nb combines with C and N to form Nb carbonitride, and is essential for ensuring corrosion resistance and workability because it suppresses precipitation of Cr carbonitride. If it is less than 0.1%, the fixation of C and N is insufficient, and if it exceeds 1.0%, there is excessive N.
0.1 to 1.0 because the workability decreases due to b.
% range.

[0021] Like C, N is an element that affects workability and corrosion resistance, and if the amount of N is large, it becomes hard and also forms Cr nitrides, causing intergranular corrosion, so it should be kept at 0.1% or less. There is a need.

[0022]Cu is an element that improves corrosion resistance such as sulfuric acid resistance, but if added in excess, it deteriorates pitting corrosion resistance, so the upper limit was set at 1.0%.

[0023] Mo is an extremely effective element for corrosion resistance, but when added in a large amount, it reduces workability, so the upper limit is set at 4.
It was set to 0%.

[0024] Ni is an element that improves sulfuric acid resistance, but
The upper limit was set at 1.0% because the larger the amount added, the harder it becomes.

[0025]Ti, Zr, V, and Ta are C, like Nb.
It combines with N to form carbonitrides to improve corrosion resistance and workability, but if the amount added is too large, it becomes an excessive solid solution element and reduces workability, so the upper limit was set at 0.2%.

The steel type targeted by the present invention has the disadvantage of high hot strength because it contains Nb and has a large amount of Cr. In order to prevent scratches that occur during hot rolling and coil winding, it is necessary to The hot rolling end temperature must be 850°C or higher.

[0027] When cold rolling a hot rolled steel strip whose hot rolling end temperature is 850°C or higher, it is necessary to use a double rolling method in order to improve workability. In the single rolling method, the {001}<110> texture generated in the hot rolled steel strip cannot be destroyed, so the hole expandability is not improved.

[0028] rmin, which is a problem in hole expandability
Since the value is significantly improved when the cold rolling reduction is 60% or more, the reduction is prioritized in one of the two rolling steps. At that time, if the rolling reduction is prioritized in the second rolling, the rolling reduction in the first rolling will inevitably become smaller, which makes it difficult to sufficiently destroy the {001} <110> texture that has occurred in the hot-rolled steel strip. become unable. Comparing the case where the rolling reduction ratio is mainly allocated to the first rolling and the case where the rolling reduction ratio is mainly allocated to the second rolling, it is found that the rmin is better when the rolling reduction is mainly allocated to the first rolling.
The value becomes larger.

However, if the reduction ratio is extremely concentrated in the first rolling and the reduction ratio in the second rolling becomes less than 20%, coarsening of crystal grains will occur in the subsequent annealing.
On the contrary, the min value decreases. Furthermore, even if the rolling reduction ratio in the second rolling exceeds 40%, there is not much effect, and the rolling reduction ratio in the first rolling decreases accordingly, so that the rmin value deteriorates.

[0030] As for annealing after rolling, 900 to 1050
It is desirable to carry out under the conditions of 10 to 100 seconds at ℃.

The pickling after annealing does not need to be particularly specified, but it is preferable to perform it under the following conditions: salt treatment, sulfuric acid pickling, and nitric acid pickling.

[0032]

[Examples] The present invention will be specifically explained below based on Examples.

(Example 1) Table 1 shows the composition of the sample materials used in the test. Slabs having these compositions were hot rolled under the following conditions. Hot rolling end temperature: 860°C Dimensions after hot rolling: thickness 4mm x width 1020mm

0034
] This hot-rolled steel strip was annealed at 1020° C. for 1 minute, and further descaled by shot blasting and pickling. The hot-rolled steel strip after annealing and pickling was made into a cold-rolled annealed steel strip with a thickness of 1 mm through the following steps.

[0035] A JIS No. 13B test piece was prepared from the obtained cold rolled steel strip, and the r value was measured by a tensile test. In addition, a hole expansion test was conducted under the following conditions. Blank outer diameter: 80mmφ Initial hole diameter: 10mmφ 30° conical punch Punch diameter: 50mmφ

The hole expandability was evaluated by expanding the initial hole using a conical punch and determining the hole expansion limit (λB) from the hole diameter when the hole edge broke. The larger the λB value, the better the hole expandability. Hole expansion limit (λB) = (hole diameter at break - initial hole diameter)
/Initial hole diameter

Table 2 shows the cold rolling conditions and workability. Also, when changing the distribution of the first rolling reduction rate and the second rolling reduction rate,

FIG. 1 shows the changes in the rmin value. When the rolling reduction ratio of the first rolling and the second rolling is equally distributed as 50%:50%,

[Equation 8] is high, but the rmin value is low, and therefore the hole expandability is also poor. In addition, if the rolling reduction is prioritized in the second rolling, rmi
Although the n value improves slightly, it is not sufficient. On the other hand, the method of the present invention in which the rolling reduction is concentrated in the first rolling and the rolling reduction in the second rolling is kept within an appropriate range has a significantly high rmin value and excellent hole expansion workability.

[0038]

[Table 1]

[0039]

[Table 2]

FIG. 1 shows the results of investigating the influence of reduction distribution on workability for steel type A. From this figure, it is clear that the hole expandability (λB) is improved when rolling is performed at the rolling reduction ratio distributed within the range of the present invention.

[0041]

[Effects of the Invention] The present invention makes it possible to significantly improve the hole expandability, which has not been able to obtain sufficient properties with the conventionally used single-rolling method and double-rolling method in which the rolling reduction is distributed almost evenly. It has become possible.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the influence of reduction distribution on workability for steel type A.

Claims (2)

[Claims] [Claim 1] In weight %, C: 0.1% or less, Si:
1% or less, Mn: 1% or less, Cr: 16-30%, Nb
: 0.1 to 1.0% and N: 0.1% or less, with the remainder consisting of unavoidable impurities and Fe, hot rolled with a hot rolling end temperature of 850°C or higher. When cold rolling a steel strip, after first cold rolling with a rolling reduction of 60% or more, annealing and pickling are performed, followed by a second cold rolling with a rolling reduction of 20 to 40%. A method for producing a cold-rolled steel strip of ferritic stainless steel having excellent hole expandability, the method comprising: followed by annealing and pickling. 2. The ferritic stainless steel is made of C
: 0.1% or less, Si: 1% or less, Mn: 1% or less, C
r: 16-30%, Nb: 0.1-1.0% and N:
Contains 0.1 or less, further Cu: 1% or less, Mo: 4%
Below, Ni: 1% or less, Ti: 0.2% or less, Zr: 0
．． 2% or less, V: 0.2% or less, and Ta: 0.2%
The method for manufacturing a cold rolled steel strip of ferritic stainless steel with excellent hole expandability according to claim 1, which contains one or more selected from the group consisting of the following, and the remainder is unavoidable impurities and Fe. .