JPH0835013A - Production of cold rolled steel sheet excellent in secondary workability - Google Patents

Abstract

PURPOSE:To produce a cold rolled steel sheet excellent in secondary workability after soft-nitriding treatment as well as in press formability. CONSTITUTION:A steel as a stock, having a composition which contains <=0.0015% C, <=0.3% Mn, 0.01-0.04% P, <=(48/140.N% Ti, (11/14).[N-(14/48).Ti] to 0.0020% B, and 0.010-0.090& Al and in which the contents of Si, S, and N as impurities are regulated to <=0.3%, <=0.02%, and <=0.005%, respectively, is hot-rolled, coiled, cold-rolled, and annealed at a temp. not lower than the recrystallization temp. In this method, temper rolling can further be applied at 0.3-1.5% rolling rate after annealing. By this method, the cold rolled steel sheet, having superior press formability and secondary workability after soft- nitriding treatment, can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold rolled steel sheet which is excellent in press formability and is excellent in secondary workability after soft nitriding.

[0002]

2. Description of the Related Art In order to improve wear resistance and the like, soft nitriding of cold-rolled steel sheet is carried out after press forming, and sometimes light working is further carried out. Secondary workability becomes a big problem.

For such applications, a low carbon Al-killed steel by a box annealing method having both formability and secondary workability after soft nitriding has been used. However, in recent years, there has been a demand for a material having better formability, and as such a material, for example, there is a decarburization annealed material of low carbon Al killed steel. This material has good moldability and workability after soft nitriding treatment, but has the drawback that it has many manufacturing steps and its cost is extremely high.

A material having better formability than the above box annealed low carbon Al killed steel can be obtained by continuous annealing of an ultra low carbon steel. Steels of this type obtained by continuous annealing include, for example, cold-rolled steel sheets with excellent deep drawability disclosed in Japanese Patent Laid-Open No. 60-9830, in which a small amount of solid solution C, etc., is added, and a small amount of Ti-added steel is used. There is an IF steel in which solid solution C, N, etc. are eliminated by adding Ti, Nb, etc. to an ultra-low carbon steel, such as the cold-rolled steel sheet excellent in press formability disclosed in Japanese Patent Laid-Open No. 61-113724. .

When these steel types are actually tested, the formability is extremely good as compared with the box annealed low carbon Al killed steel, but the workability after the soft nitriding treatment is extremely poor. At present, a steel sheet having both press formability and secondary workability after soft nitriding has not been developed yet.

As a steel for soft nitriding treatment, JP-A-55-76046
There is also a steel in which Ti is added to an ultra-low carbon steel as shown in Japanese Patent Publication No. This steel is intended to easily increase the depth of the hardened layer after the soft nitriding treatment, and does not show any improvement in press formability or workability after the soft nitriding treatment.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method capable of producing a cold rolled steel sheet which is excellent in press formability and secondary workability after soft nitriding.

[0008]

The summary of the present invention is as follows.
Press moldability of (1) and (2) and 2 after soft nitriding
It is a method of manufacturing a cold rolled steel sheet having excellent secondary workability.

(1)% by weight, C: 0.0015% or less,
Mn: 0.3% or less, P: 0.01% or more and 0.04% or less, Ti: (48/14) / N% or less, B: [(11/14)-
{N- (14/48) Ti}]% or more and 0.0020% or less, Al:
It contains 0.010% or more and 0.090% or less, and the balance is Fe and inevitable impurities. Si in the impurities is 0.3% or less, and S is
Press forming and soft nitriding, characterized in that a material steel having 0.02% or less and N of 0.005% or less is hot-rolled, wound, cold-rolled, and annealed in a temperature range higher than the recrystallization temperature. A method for producing a cold-rolled steel sheet having excellent secondary workability after treatment (hereinafter referred to as a first method).

(2) The material steel having the composition of (1) above is hot-rolled, wound, cold-rolled, annealed in a temperature range not lower than the recrystallization temperature, and then rolled at a rolling ratio of 0.3 to 1.5%. A method for producing a cold-rolled steel sheet having excellent press formability and secondary workability after soft nitriding, which is characterized by performing temper rolling (hereinafter, referred to as the second
Method).

In all of the above steel materials, the lower limit of Mn is 0.01
It is desirable to set it as%.

[0012]

The present inventor has conducted various investigations with the aim of improving the secondary workability of the ultra-low carbon steel after the soft nitriding treatment. As a result, the reason why the secondary workability is poor is that it is a solid solution in the steel sheet. It was found that when Ti is present, it is combined with C and N that enter from the atmosphere during the soft nitriding treatment and finely precipitates as a carbonitride of Ti, which deteriorates the toughness.

That is, by setting the Ti content to be substantially equal to or less than N equivalent, it is possible to prevent the formation of Ti-based carbonitrides during the soft nitriding treatment and to avoid the deterioration of the toughness. Found that can improve.

Further, a small amount of B is added in addition to Ti, and TiN
Other than N is precipitated as BN, and C
The present invention has been made based on the finding that by reducing the content as much as possible, good press formability can be obtained at the same time.

The reason why the chemical composition of the raw material steel which is the object of the method of the present invention is limited as described above will be explained below. % Of alloy components means% by weight.

C: 0.0015% or less By reducing the C content in the steel sheet as much as possible, both excellent press formability and secondary workability after soft nitriding can be achieved. Therefore, the lower the C content in the material steel, the more desirable. If the C content exceeds 0.0015%, the solid solution C content becomes too much, so that the yield stress (YS) increases and the r
Value and elongation are low, and press formability is poor. Therefore, C
The upper limit of the content is 0.0015%.

Mn: 0.3% or less Mn is to prevent hot embrittlement due to S during hot rolling,
The Mn content must be 8 times or more the S content. Usually S
Exists inevitably around 0.01%, so the Mn content is 0.
10% or more is desirable. On the other hand, if it exceeds 0.3%, it coexists with C and develops an unfavorable texture for improving the r value. Therefore, the upper limit of Mn content is
0.3%.

P: 0.01-0.04% In a component system in which the precipitation amount of carbonitride is small, the crystal grain size of the hot-rolled sheet tends to be large in ordinary hot rolling. Therefore, after cold rolling and recrystallization, R-value becomes low. Therefore, in order to make the hot rolled sheet finer, P having this effect is appropriately contained. To obtain this effect, it is necessary to contain 0.01% or more. However, if the P content exceeds 0.04%, the ductility deteriorates.

Ti: (48/14) .N% or less Ti is one of the most important elements in the present invention, and is contained in order to reduce the amount of solute N. The reason for limitation is as follows. .

In order to eliminate solid solution N, the Ti content should be equal to or less than the N equivalent.

When the Ti content exceeds N equivalent, other than TiN
Most of Ti is precipitated as TiS, TiC, Ti ₄ C ₂ S ₂ , but
Part of it remains as solid solution Ti in the steel sheet. If this residual solid solution Ti is present, it is finely precipitated as Ti (CN) during the soft nitriding treatment, so that the secondary workability after the soft nitriding treatment is deteriorated.
Therefore, the Ti content is set to (48/14) N% or less.

However, if the Ti content is too small with respect to the N equivalent, the B content, which will be described later, increases, and a texture unfavorable for drawability is formed. A preferable lower limit of the Ti content for preventing this is 0.6 × (48/14) · N%.

B: [(11/14). {N- (14/48) .Ti}]
% Or more and 0.0020% or less B is the most important element in the present invention together with Ti,
When the Ti content is N equivalent or less as described above, solid solution N
However, due to the effect of this solid solution N, it is not possible to obtain a texture that is preferable for drawability after recrystallization annealing. Therefore, B must be added in order to precipitate the solid solution N as BN, and B must be contained together with Ti so as to be N equivalent or more. Therefore, the B content is [(1
1/14) ・ {N- (14/48) ・ Ti}]% or more.

On the other hand, when the B content exceeds 0.0020% and becomes excessive, the r value is lowered. Therefore, the upper limit is 0.0020%
And

If all of N is precipitated with B alone, the B content increases and the r value decreases, so it is essential to add Ti and B in combination.

Al: 0.010 to 0.090% Al is added for deoxidation. 0.010% for deoxidizing effect
It is necessary to contain the above. On the other hand, the Al content is 0.090%
If it exceeds, the deoxidizing effect will be saturated and only increase the cost, so the upper limit was made 0.090%.

Si: 0.3% or less As the Si content increases, the ductility deteriorates. For this reason, it is treated as an unavoidable impurity. If the Si content exceeds 0.3%, it will exceed the allowable limit of ductility deterioration.
The upper limit was set to 0.3%.

S: 0.02% or less S is an unavoidable impurity, and if its content is high, the Mn content for preventing hot brittleness also increases, resulting in a poor r value as described above. Therefore, it is desirable to make it as low as possible. The allowable upper limit is 0.02%.

N: 0.005% or less N is an unavoidable impurity. Since the drawability deteriorates when solid solution N is present, N must be precipitated as TiN and BN as described above. If the N content exceeds 0.005%, not only the Ti and B contents for precipitating these nitrides increase, but also the ductility is impaired. Therefore, the upper limit of the N content is set to 0.005%.

Next, the reasons for limiting the manufacturing process of the method of the present invention and a part of the conditions will be described. The first method of the present invention is to hot-roll the above-mentioned raw material steel, then wind it, cold-roll it, and then anneal it in a temperature range not lower than the recrystallization temperature.

Hot rolling: The rolling is carried out according to the usual method and conditions, but the completion (finishing) temperature is preferably set to ₃ or more points of Ar in order to obtain a good deep drawability in a cold rolled product.

Winding: There are no particular restrictions on the winding temperature after hot rolling. This means that Ti and B are Ti by the finish of rolling.
This is because the coiling temperature does not affect the formation of these nitrides because they are substantially precipitated as N and BN.

Cold rolling: The rolling is carried out according to the usual method and conditions. The higher the cold rolling rate, the better the r value.
The cold rolling rate is preferably 70% or more.

Annealing conditions: As the annealing method, either box annealing or continuous annealing may be used. If the temperature is less than the recrystallization temperature, the rolling structure remains and the pressability of the steel sheet deteriorates.

In the second method of the present invention, after the above annealing, temper rolling with a rolling rate of 0.3 to 1.5% is further performed.
This temper rolling is carried out as needed when it is desired to make the steel sheet have a low YS. To obtain this effect, the rolling rate is 0.3%
Temper rolling is required. On the other hand, when temper rolling with a rolling ratio of more than 1.5% is performed, YS increases, and ductility deteriorates.

[0036]

【Example】

(Test 1) A material steel slab having the chemical composition shown in Table 1 was manufactured and hot-rolled, the finishing temperature was 890 to 920 ° C, and the winding temperature was 5
The hot-rolled sheet thickness was set to 80 mm to 630 ° C and 3.8 mm. This was pickled and cold-rolled to a plate thickness of 0.8 mm.

[0037]

[Table 1]

Next, the cold rolled steel sheet thus obtained was annealed in a continuous annealing furnace under the conditions of an annealing temperature of 820 ° C. and a holding time of about 60 seconds. Then, 0.6% temper rolling was performed, and test pieces were cut out from each of the obtained steel plates, and the tensile properties were investigated. The results are shown in Table 2.

[0039]

[Table 2]

The r _m value in Table 2 represents the three-direction average r value,
It was calculated using the formula of (r0 ° + 2r45 ° + r90 °) / 4. Other characteristic values are test values in the 0 ° direction (rolling direction).

From the steel sheet after continuous annealing, the thickness was 0.8 mm and the diameter was
We made a sample of 50mmφ cylindrical drawing product (drawing ratio 1.8),
It was subjected to a soft nitriding treatment. The soft nitriding process is a gas soft nitriding method,
The sample was kept at 580 ° C for 60 minutes in an atmosphere consisting mainly of RX + NH ₃ gas.

The workability after the soft nitriding treatment was evaluated by the method shown in FIG. 1 after the cylindrical drawn product 1 after the gas soft nitriding treatment was put on the truncated cone 2 and then compressed from above by a jig 3 having a horizontal plane. Then, the movement distance of the jig 3 up to the destruction was used. The results are also shown in Table 2.

[0043] Table As shown in 2, the steel plates produced according to the present invention a method (No.1~5), both elongation and r _m value is high, the yield stress (YS) is low. Further, the secondary workability after the soft nitriding treatment is also good.

[0044] As for comparative example, because it exceeds the upper limit C content in Nanba6,11, YS is high, a low elongation and r _m values. In No. 7, since the P content exceeds the upper limit, YS is high and elongation is low. Since the Mn content in the No.8 exceeds the upper limit, r _m value is low. In Nos. 9 and 10, since the Ti content exceeds the upper limit, the mechanical properties are good, but the secondary workability after soft nitriding is poor. In No. 12 and 13, Ti
Since the contents of B and B are N equivalents or less, the r _m value is low.

(Test 2) Material steel slabs having the compositions shown in Table 3 were manufactured and subjected to continuous annealing under the same conditions as in Test 1. After that, temper rolling was performed by changing the rolling ratio (elongation) in the range of 0 to 2.0%.

A test piece was cut out from each of the obtained steel plates and the tensile properties were investigated. Table 4 shows the results. Characteristic values are test values in the 0 ° direction.

[0047]

[Table 3]

[0048]

[Table 4]

As shown in Table 4, when the temper rolling ratio is No. 2 to 5 within the range of the present invention, YS is low and elongation is high. No. 1 has good elongation because of no temper rolling, but has high YS.

In No. 6, since the temper rolling rate is too high, YS is high and the elongation is low.

[0051]

According to the method of the present invention, it is possible to produce a cold-rolled steel sheet having both good formability and secondary workability after soft nitriding.

[Brief description of drawings]

FIG. 1 is a schematic diagram illustrating a method of evaluating and processing workability after soft nitriding.

[Explanation of symbols]

 1: Cylindrical drawn product, 2: Frustum, 3: Jig

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C22C 38/14

Claims (2)

[Claims] 1. By weight%, C: 0.0015% or less,
Mn: 0.3% or less, P: 0.01% or more and 0.04% or less, Ti: (48/14) N%
Below, B: [(11/14)-{N- (14/48) -Ti}]% or more and 0.00
20% or less, Al: 0.010% or more and 0.090% or less, the balance consists of Fe and unavoidable impurities, and Si in the impurities is 0.3% or less,
The press formability is characterized in that a material steel having S of 0.02% or less and N of 0.005% or less is hot-rolled, wound, cold-rolled, and annealed in a temperature range higher than the recrystallization temperature. A method for producing a cold rolled steel sheet having excellent secondary workability after soft nitriding. 2. In% by weight, C: 0.0015% or less,
Mn: 0.3% or less, P: 0.01% or more and 0.04% or less, Ti: (48/14) N%
Below, B: [(11/14)-{N- (14/48) -Ti}]% or more and 0.00
20% or less, Al: 0.010% or more and 0.090% or less, the balance consists of Fe and unavoidable impurities, and Si in the impurities is 0.3% or less,
A material steel containing S of 0.02% or less and N of 0.005% or less is hot-rolled, wound, cold-rolled, annealed in a temperature range of a recrystallization temperature or higher, and then rolled at a rolling ratio of 0.3 to 1.5%. A method for producing a cold-rolled steel sheet having excellent press formability and secondary workability after soft nitriding, which is characterized by performing temper rolling.