JP2776203B2 - Manufacturing method of cold rolled steel sheet excellent in non-aging at normal temperature - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for efficiently producing a cold-rolled steel sheet for processing using a relatively inexpensive ordinary low-carbon Al-killed steel as a raw material, which hardly causes aging at room temperature.

[0002]

2. Description of the Related Art Cold-rolled steel sheets used in automobiles and the like are required to have excellent workability (formability). Therefore, in an annealing process for manufacturing such cold-rolled steel sheets, uniform and sufficient heat treatment is required. It was usual to employ the "box annealing method" to impart properties. However, recently, the improvement of productivity has been more strongly called out, and partly because of the progress of production technology, the “continuous annealing method” has been gradually applied to the production of the cold-rolled steel sheet. Was.

However, since the continuous annealing method is a process in which annealing and overaging are completed in a short time, it is difficult to sufficiently reduce the solid solution C in the steel sheet during the processing time, and therefore, the continuous annealing method has been obtained. There is a problem that cold-rolled steel sheets are susceptible to aging at room temperature. It is needless to say that such a cold-rolled steel sheet that is susceptible to aging at room temperature, when stored at room temperature for a long period of time, regenerates the yield elongation that has disappeared in the skin pass and is likely to wrinkle during press forming.

Therefore, in order to obtain a "cold rolled steel sheet which is unlikely to cause aging at room temperature" even by the continuous annealing method, it is necessary to reduce C as much as possible in the steel making stage and to add a carbonitride forming element such as Ti or Nb. "IF steel" has been developed to fix the solute C present as precipitates. And, by using this ultra-low carbon IF steel as a material, even when the continuous annealing method is applied to the annealing after cold rolling, the cold-rolled steel sheet having the non-aging property similar to the box annealing method can be overaged. However, in this case, it is necessary to perform vacuum degassing for a long time to adjust the composition in the process of melting the IF steel, so that there is a problem that economic disadvantage cannot be avoided. Was pointed out.

On the other hand, apart from this, although a process applying a continuous annealing method, as a method for producing a non-ageing cold-rolled steel sheet using a relatively inexpensive ordinary low carbon steel as a material, a method after cold rolling is used. Once the steel plate is soaked for annealing,
A method of rapidly cooling to a low temperature range of 0 ° C. and subsequently reheating the same has also been proposed (for example, see Japanese Patent Application Laid-Open No. Hei 3-2329). In this method, fine carbide is precipitated in ferrite grains by lowering the quenching end point temperature after performing soaking for annealing, and then further heated to grow it to reduce solid solution C. It was devised based on the idea of causing a cold rolled steel sheet having significantly improved non-ageing properties, and had considerable expectations. However, to implement this method, it is necessary to add a new reheating device to the equipment in the annealing step.
This became a major problem in equipment.

In view of the above, the object of the present invention is to use a relatively inexpensive ordinary low-carbon steel instead of an economically disadvantageous ultra-low-carbon steel as the material steel, and to use a continuous equipment with ordinary equipment. The purpose was to establish a means for stably producing cold-rolled steel sheets for processing excellent in non-aging at room temperature by applying annealing.

[0007]

Means for Solving the Problems The present inventors have conducted intensive studies in order to achieve the above object, and have obtained the following findings through them. That is, in order to improve the room-temperature non-aging property of the cold-rolled steel sheet, it is necessary to reduce solid solution C in the cold-rolled steel sheet. In order to reduce this solid solution C, cementite is finely precipitated in ferrite grains. It is known that it is effective to shorten the diffusion distance of C necessary for growing cementite.

For example, in the method disclosed in the above-mentioned Japanese Patent Application Laid-Open No. Hei 3-2329, the cold-rolled steel sheet which has been subjected to the soaking step of annealing is rapidly cooled to a low temperature range of 200 to 300 ° C. into ferrite grains. Fine cementite is precipitated, and in the next reheating step, the precipitated cementite is grown to reduce solid solution C. However, in this method, equipment is limited because a new reheating device is required as described above.

However, as a result of a number of experiments conducted by the present inventors, the C content was particularly reduced to 0.013 to 0.033% (hereinafter, referred to as “C” content).
% Represents the component ratio is higher than the conventional Al-killed steel sheet was adjusted to have a weight%) seven hundred to seven hundred eighty ° C. Oh - Sutenai
If quenched from a high temperature which bets are still present, 300-400
It was found that even when the quenching was stopped at a relatively high temperature of ℃, a sufficient amount of cementite was precipitated in the ferrite grains. Moreover, in this case, it was also confirmed that cementite was coarsened without reheating because the quenching stop temperature was high, and solute C was reduced.

[0010] Further, B co-exists with N in a specific range, thereby effectively acting to secure non-aging properties. Particularly, the quenching start temperature of the cold-rolled steel sheet after the soaking process is increased to 700 to 780 ° C. It was also found that the effect was very pronounced in some cases.

The present invention has been completed based on the above findings, etc., "C: 0.013 to 0.033%, Si: 0.1% or less, Mn: 0.
05 to 0.25%, P: 0.020% or less, S: 0.002 to 0.015%, sol.A
l: 0.01 to 0.10%, N: 0.0040% or less, or if necessary, 0.0005% N content
Under the condition adjusted to ~ 0.0040%, the steel slab which further contains B: 0.0001 ~ 0.0040% and the remainder is made of unavoidable impurities is A
r After hot rolling at ₃ transformation point or higher, winding at 600 to 700 ° C, then cold rolling, and then (1) heating the cold rolled steel sheet to a temperature of 740 to 930 ° C. (10) up to the temperature range of 700 to 780 ° C.
(3) Then, the austenite remains at 700 to 780 ° C.
Quenched with 90 ° C. / sec or more cooling rate at temperatures Ikima of 300 to 400 ° C. from the region, it was subjected to overaging treatment 90-240 seconds in the temperature range of (4) and 300 to 400 ° C., to room temperature A continuous annealing process consisting of a series of steps of cooling, thereby enabling stable and cost-effective production of cold-rolled steel sheets for processing with excellent non-aging at room temperature with good productivity. " Have.

[0012]

As described above, the present invention uses a relatively inexpensive "ordinary low carbon aluminum killed steel" as a material steel, and applies a continuous annealing method with high productivity to annealing to achieve "normal temperature aging". The method relates to a method of stably producing a “small cold-rolled steel sheet for processing”, and rapidly cools a steel sheet having a C content adjusted to 0.013 to 0.033% from a temperature range of 700 to 780 ° C.
The cause of precipitation and growth of a sufficient amount of cementite in ferrite grains when quenching is stopped at ~ 400 ° C is not necessarily clear. However, the following can be considered.

That is, the temperature of 700 to 780 ° C. is a region where austenite remains, and when quenching is performed from this temperature range, a rapid transformation from austenite to ferrite occurs, so that C is bound at the grain boundary. No time to spread to.
As a result, it is considered that cementite is easily precipitated in the ferrite grains, and the C is supersaturated to a large extent, so that it is likely to grow. If the cementite finely precipitated in the ferrite grains grows in this way, the solid solution C in the cold-rolled steel sheet is sufficiently reduced, which leads to a reduction in the aging at room temperature.

Next, the reason why the chemical composition of the raw steel and the processing conditions are limited as described above in the present invention will be described. A) Chemical composition of base steel C: As described above, the amount of solid solution C must be reduced as much as possible in order to ensure non-aging properties of cold-rolled steel sheets. It is necessary to dissolve a large amount of C in ferrite to increase the degree of supersaturation of C after quenching so that cementite is easily precipitated in ferrite grains. However, if the C content of the base steel is less than 0.013%, the degree of supersaturation of C in the ferrite is small, and it cannot be precipitated as cementite in the ferrite grains after quenching, so that the amount of solid solution C increases, and the non-aging property is reduced. to degrade. On the other hand, 0.033
If C is contained in excess of%, cementite will precipitate at the grain boundaries of the ferrite. Therefore, near the grain boundaries, the degree of supersaturation of C will be insufficient, so that cementite will not be precipitated, and the amount of solid solution C will also increase. Therefore, C content is 0.013-0.033%
However, the C content is preferably adjusted to 0.013 to 0.027%.

Si: Since Si is a solid solution strengthening element and increases the strength of a cold-rolled steel sheet to deteriorate workability, its content should be limited to a range of 0.1% or less where this adverse effect is acceptable. did.

Mn: Mn combines with S to form MnS which can be a precipitation nucleus of cementite, and has an effect of promoting the precipitation of cementite. In order to precipitate cementite finely so as to be advantageous in securing non-aging property, it is preferable to form MnS finely. From this point, it is better that the Mn content is small. However, when the Mn content is too low and the amount of solute S increases, low melting point FeS is formed,
It causes ear cracks during rolling. Therefore, the Mn content is determined to be in the range of 0.05 to 0.25%, which can prevent edge cracks during rolling and does not hinder fine precipitation of cementite.

P: Since P has an effect of deteriorating weldability, secondary workability, and surface treatment, the content thereof is limited to a range of 0.020% or less where these adverse effects are acceptable.

S: S combines with Mn to form MnS which can be a precipitation nucleus of cementite, and has an effect of promoting the precipitation of cementite in ferrite grains after quenching in the annealing step. However, if the content is 0.002%, the desired effect due to the above-mentioned effects cannot be obtained, while, on the other hand, 0.
Excessive addition of more than 015% leads to an excessive amount of nonmetallic inclusions of MnS, leading to deterioration of workability. Further, a large amount of S forms FeS having a low melting point, and causes ear cracks during rolling. Therefore, the S content was determined to be 0.002 to 0.015%.

Al: Since Al is added as a deoxidizing agent and has a function of fixing solid solution N and improving non-aging property, so as to secure a desired effect by these functions.
l. Al content must be 0.01% or more. However, excessive addition exceeding 0.10% forms non-metallic inclusions and deteriorates workability. Therefore, the Al content is 0.
It was determined as 01 to 0.10%.

N: When N exceeds 0.0040%,
The amount of solute N increases, and it becomes impossible to sufficiently suppress aging at room temperature. Therefore, the upper limit of the N content is set to 0.0040%. However, when B is added to steel, N
And precipitates as BN, which serves as a precipitation nucleus of cementite to promote the precipitation of cementite. Therefore, a favorable result for further improvement of non-aging property is obtained. And
Since this effect becomes remarkable when the N content is 0.0005% or more, the lower limit of the N content is set to 0.0005% in the B-added steel, but is preferably adjusted to 0.0008 to 0.0020%.

B: B is an element that is effective in suppressing normal-temperature aging of the cold-rolled steel sheet, and is therefore a component that is added as necessary to ensure better non-aging properties. That is, B
Is thought to have the effect of precipitating as BN to reduce solid solution N and promoting the precipitation of cementite with BN as a nucleus. In addition, segregated at the crystal grain boundaries as solid solution B,
This reduces the amount of solid solution C at the grain boundaries and suppresses the precipitation of cementite at the grain boundaries. Further, since the ferrite grains after annealing are coarsened, the workability is improved by softening based on the ferrite grains, and cementite is easily precipitated in the grains. That is, the quenching start temperature after soaking in annealing is 700
When the temperature is increased to about 780 ° C., the aging property is remarkably improved in the B-added steel. However, since the nucleation of ferrite is suppressed by the addition of B, it occurs after the ferrite growth is supercooled. This is probably because the grain boundary segregation of solid solution C is suppressed. However, if the content is less than 0.0001%, a sufficient effect cannot be obtained.
Although the effect is saturated even if it is contained in excess of 0040%, the B content is set to 0.0001 to 0.0040%.
It is good to adjust to 0.0003-0.0020%.

B) Processing Conditions Hot rolling must be completed at the Ar ₃ transformation point or higher.
This is because, when rolling is completed at a temperature lower than the Ar ₃ transformation point, the ferrite phase is processed and the texture changes, which causes a reduction in the r value (that is, a reduction in workability). The winding temperature after hot rolling is set to 600 to 700 ° C. That is, when the winding temperature is lower than 600 ° C., the agglomeration of carbides becomes insufficient and the r-value of the cold-rolled steel sheet is reduced. Problems such as coarsening of the crystal grain size occur.

The heating temperature of the steel slab (slab) at the time of hot rolling is not particularly limited.
It can be said that the temperature is preferably set to 50 to 1250C. Also,
The slab to be inserted into the heating furnace may be a slab that has been kept at a high temperature after casting or a slab that has been cooled to room temperature.

The coil after hot rolling is usually cold rolled after pickling. Although the rolling reduction in this cold rolling is not particularly defined, it is preferably 70 to 94% in order to secure a good r value.

The soaking in annealing after cold rolling is 740 to 9
This is performed by maintaining the temperature in a temperature range of 30 ° C. for 10 to 300 seconds. At this time, if the heating temperature is less than 740 ° C. or the holding time is less than 10 seconds, the recrystallization is insufficient, and the grain growth after the recrystallization is insufficient, resulting in poor workability. On the other hand, when the heating temperature exceeds 930 ° C., the texture is randomized, and the r value decreases. Further, even if the temperature is maintained at the soaking temperature for more than 300 seconds, the effect is saturated and the productivity is reduced.

After the above soaking, the steel sheet is cooled at a cooling rate of 10 ° C./sec or less up to 700 to 780 ° C., which is the quenching start temperature, in order to grow ferrite having an r value.

In the present invention, the quenching start temperature is set to 700
It is important to set the temperature as high as 780 ° C. (more preferably 720-760 ° C.). As already mentioned, 70
0 to 780 ° C. is a region where austenite remains. However, when quenching is performed from this temperature range, austenite is rapidly transformed into ferrite, so that there is no time for C to diffuse to grain boundaries, and as a result, It is considered that cementite easily precipitates and grows (coarsened) in ferrite grains, which leads to improvement of non-aging property.

Here, if the quenching start temperature is lower than 700 ° C., austenite disappears and the treatment effect is not recognized. On the other hand, if the temperature exceeds 780 ° C., there is a problem in sheet passing such as a heat buckle. At the same time, the ratio of residual ferrite becomes too low, ferrite newly forms nuclei and the texture is randomized, and the amount of solid solution C in the remaining ferrite becomes too low, causing precipitation of cementite in the grains. Less likely to happen.

It is necessary to carry out the rapid cooling at a cooling rate of 90 ° C./sec or more, and to stop the rapid cooling at 300 to 400 ° C. Because, when the cooling rate is less than 90 ° C./sec, the amount of cementite precipitated at the grain boundary becomes too large and the degree of supersaturation of solid solution C in the ferrite grains is insufficient, so that cementite does not precipitate in the grains, and as a result, This is because the amount of solid solution C increases and the non-aging property at room temperature deteriorates.

In the present invention, the quenching stop temperature is set to 300 to 400.
° C, and the overaging treatment is performed in a temperature range of 300 to 400 ° C. This temperature range is a temperature at which cementite precipitates and grows in ferrite grains, and by performing the overaging treatment in this temperature range. The solid solution C can be sufficiently reduced and the desired non-aging property can be secured. When quenching is stopped in a temperature range higher than 400 ° C., the amount of cementite precipitated at the grain boundary becomes too large, resulting in insufficient supersaturation of solid solution C in ferrite grains, and therefore, cementite does not precipitate in the grains. . On the other hand, if quenching is continued to a temperature range lower than 300 ° C., cementite will not sufficiently precipitate in the ferrite grains, and further, there will be no opportunity for the precipitated cementite to grow. The quenching stop temperature is preferably set to 300 to 350 ° C.

The overaging treatment time is 90 to 240 seconds. If the time is less than 90 seconds, the growth of cementite is insufficient, so that the solute C is not sufficiently reduced. Even if the aging treatment is performed, the effect is saturated, which causes a deterioration in productivity. In the overaging process,
There is no problem even if "gradient overaging" in which the temperature is gradually lowered in the overaging zone in continuous annealing is adopted.

The cold-rolled steel sheet obtained by the method of the present invention is usually subjected to temper rolling of 0.4 to 2.0% in order to eliminate the yield elongation after annealing. There is no problem even if the cold-rolled steel sheet obtained by the method of the present invention is further subjected to surface treatment such as electroplating and used.

Next, the present invention will be described with reference to examples.

EXAMPLES After smelting 50 kg of steel having the chemical composition shown in Table 1 in a vacuum melting furnace, a slab having a thickness of 60 mm was manufactured by hot forging. Hot rolling was started immediately after heating this slab to 1170 ° C., and a 4.0 mm-thick hot-rolled steel sheet was produced at a finishing temperature of 900 ° C., and then cooled at an average cooling rate of 20 ° C./sec to 640 ° C. Rolled up.

[0034]

[Table 1]

The hot-rolled steel sheet was pickled, then cold-rolled to a thickness of 0.8 mm, and then continuously annealed under the conditions shown in Table 2. FIG. 1 shows a schematic diagram of a temperature change in the annealing step.

[0036]

[Table 2]

Next, the steel sheet after the continuous annealing is further
A temper rolling of 1.2% was performed, and a JIS No. 5 tensile test piece was sampled from the obtained cold-rolled steel sheet in the rolling direction and subjected to a tensile test. In addition, "yield elongation" and "deterioration of elongation" under accelerated aging at 100 ° C. for 60 minutes were examined using JIS No. 5 tensile test specimens taken from the rolling direction, and the room temperature aging was evaluated. Furthermore, 2
After heat treatment at 170 ° C. for 20 minutes after the% tensile prestrain, a tensile test was performed again, and the value obtained by subtracting the deformation stress before heat treatment from the yield strength after heat treatment was evaluated as “BH amount”. In addition,
Needless to say, the lower the BH amount, the better the non-aging property at normal temperature.

The results are shown in Table 2. The following can be understood from the results shown in Table 2. That is, in each of the cold-rolled steel sheets (Test Nos. 1 to 5, Test Nos. 10 to 12) manufactured according to the conditions specified in the present invention, the occurrence of yield elongation due to accelerated aging is 0.2% or less, and the reduction in elongation is not more than 0.2%. 2
% Or less, and the BH amount is 30 MPa or less, indicating good room temperature non-aging properties.

On the other hand, the cold rolled steel sheet according to Test No. 6 in which the annealing temperature is lower than the specified value of the present invention cannot obtain a good texture and has a low r-value. Further, the cold-rolled steel sheet according to Test No. 7 in which the quenching start temperature is higher than the specified value of the present invention, the cold-rolled steel sheet according to Test No. 8 in which the quenching start temperature is lower than the specified value in the present invention, and the overaging temperature according to the present invention. In each of the cold-rolled steel sheets according to Test No. 9 higher than the specified value, sufficient cementite does not precipitate in the ferrite grains, and the deterioration of elongation due to solid solution C and the occurrence of yield elongation due to accelerated aging are large. Furthermore, the cold-rolled steel sheets according to Test Nos. 13 and 14, in which the C content is out of the range specified in the present invention, have large degradation of elongation and high yield elongation due to solid solution C due to accelerated aging similarly to the above.

FIG. 2 shows that the steel slabs of steel a (B-added steel) and steel b (B-free steel) in Table 1 were hot-rolled under the above-mentioned conditions (the conditions of the above-described embodiment). A cold-rolled steel sheet produced by cold rolling and subjected to a continuous annealing treatment in which only the quenching start temperature was changed under the conditions of Test No. 1 in Table 2 above had an elongation of 1.2%.
"100 ° C x 6"
FIG. 2 is a graph obtained by examining and arranging the “change in the amount of yield elongation after the 0-minute acceleration aging”.
By setting the quenching start temperature to 700 to 780 ° C., it can be confirmed that the occurrence of yield elongation is suppressed to 0.2% or less, which is not a problem.

[0041]

[Summary of Effects] As described above, according to the present invention,
Industrially extremely useful effects are obtained, for example, a cold rolled steel sheet which is excellent in non-aging property at room temperature and suitable as a steel sheet for interior or exterior of automobiles can be manufactured at low cost and with high productivity.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a change in temperature and time in an annealing step employed in an example.

FIG. 2 is a graph showing the relationship between the quenching start temperature and the amount of yield elongation after acceleration aging at 100 ° C. for 60 minutes.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) C21D 9/46-9/48 C21D 8/02-8/04

Claims (2)

(57) [Claims] C. 0.013 to 0.033%, Si: 0.1% or less, Mn: 0.05 by weight ratio
Up to 0.25%, P: 0.020% or less, S: 0.002 to 0.015%, sol.A
l: 0.01~0.10%, N: the billet balance consisting of unavoidable impurities with containing 0.0040% or less, Ar ₃
Hot rolling at the transformation point or higher, winding at 600 to 700 ° C., followed by cold rolling, followed by continuous annealing comprising a series of steps described below, at room temperature. Manufacturing method of cold rolled steel sheet for processing with excellent non-aging properties. (1) The cold-rolled steel sheet is heated and maintained at a temperature range of 740 to 930 ° C for 10 to 300 seconds. (2) Subsequently, a temperature range of 700 to 780 ° C is maintained at 10 ° C.
(3) Then, the austenite remains at 700 to 780 ° C.
Quenched with 90 ° C. / sec or more cooling rate at temperatures Ikima of 300 to 400 ° C. from the region, it was subjected to overaging treatment 90-240 seconds in the temperature range of (4) and 300 to 400 ° C., to room temperature Cooling. 2. C: 0.013 to 0.033%, Si: 0.1% or less, Mn: 0.05 by weight ratio
Up to 0.25%, P: 0.020% or less, S: 0.002 to 0.015%, sol.A
l: A steel slab containing 0.01 to 0.10%, N: 0.0005 to 0.0040%, and B: 0.0001 to 0.0040%, with the balance being unavoidable impurities.
After hot rolling at r ₃ transformation point or higher, winding at 600 to 700 ° C., and then performing cold rolling, then successively performing a continuous annealing process comprising a series of steps shown below. , A method for producing cold-rolled steel sheets for processing excellent in non-aging at room temperature. (1) The cold-rolled steel sheet is heated and maintained at a temperature range of 740 to 930 ° C for 10 to 300 seconds. (2) Subsequently, a temperature range of 700 to 780 ° C is maintained at 10 ° C.
(3) Then, the austenite remains at 700 to 780 ° C.
Quenched with 90 ° C. / sec or more cooling rate at temperatures Ikima of 300 to 400 ° C. from the region, it was subjected to overaging treatment 90-240 seconds in the temperature range of (4) and 300 to 400 ° C., to room temperature Cooling.