JPH01188630A - Manufacture of cold rolled steel sheet having superior press formability - Google Patents

Abstract

PURPOSE:To manufacture a cold rolled steel sheet having superior deep drawability by hot rolling a dead soft Ti-contg. steel having a specified compsn., specifying temp. conditions after finish rolling, and subjecting the resulting steel sheet to coiling, cold rolling and continuous annealing. CONSTITUTION:When a steel sheet consisting of, by weight, <=0.005% C, <=1.0% Si, <=2.0% Mn, <=0.15% P, <=0.05% S, <=0.1% sol. Al, <=0.008% N, 0.004-0.2% Ti and the balance Fe or further contg. 0.004-0.05% Nb optionally in combination with 0.0005-0.0030% B is hot rolled, finish rolling is finished at the Ar3 point or above. The resulting steel sheet is cooled from the Ar3 point or above to (Ar3 point-10 deg.C)-(Ar3 point-100 deg.C) at 50-400 deg.C/sec cooling rate, held for 2-20 sec and coiled at <=650 deg.C. The steel sheet is then cold rolled and continuously annealed as usual.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a method of manufacturing a cold rolled steel plate for press forming.

[Prior Art] A method of manufacturing a cold-rolled steel sheet with excellent press formability by adding 1<1> or Nb to ultra-low carbon steel is well known. JP-A No. 6], -276930 discloses hot rolling conditions, cooling conditions, coiling conditions, cold rolling conditions, and continuous annealing conditions of extremely low C-Ti-Nb steel with adjusted composition within a specific range. control to
This is a method for manufacturing cold-rolled thin sheets. A method is disclosed in which cooling is started within 0.5 seconds after finish rolling in hot rolling to refine the hot rolling crystal structure and improve deep drawability. However, in this method, cooling is started immediately after hot rolling, and a large amount of water vapor is generated immediately after the finishing mill. This method makes H1 measurement of thickness and temperature difficult, and therefore hot rolling control becomes difficult with a normal rolling mill. In this invention, the rolled material is cooled at an average cooling rate of 10° C./s or more, but there is no specific description of the limitation on the cooling rate, so 10° C./s or more refers to, for example, 30° C./s in the example. .

On the other hand, cold-rolled steel sheets containing Ti or Ti and Nb are
When carbides and Nb carbonitrides are sufficiently precipitated in the hot rolled sheet, high deep drawability, that is, high press formability can be obtained.

In the box annealing method, the annealing time is long, so even if the material is wound at a low temperature, T'' carbide and Nb carbonitride precipitate during annealing, resulting in good deep drawability, that is, good press formability. However, since the continuous annealing method has a short annealing time, 11 carbides and Nb carbonitrides are difficult to precipitate during annealing. Therefore, in the continuous annealing method, hot-rolled steel sheets are wound up at high temperatures, and then Ti
In many cases, carbides and Nb carbonitrides are precipitated. However, when coiled at high temperatures, the surface of the steel plate is greatly oxidized, reducing pickling properties.Also, even when coiled at high temperatures, the inner and outer peripheries of the coil cool at a high rate, which impairs the uniformity of the coil. leading to.

[Problems to be Solved by the Invention] The present invention involves producing a hot-rolled steel sheet with precipitated Ti carbides and Nb carbonitrides by low-temperature winding, and using this to continuously produce a steel sheet with good press formability, that is, deep drawability. The objective is to disclose a manufacturing method using an annealing method.

[Means for Solving the Problems] The present invention provides (]) in weight%, C: 0.005 or less, S
l: 1.0 or less, Mn: 2.0 or less, P: 0.15 or less, S: 0.05 or less, 5olAfl: 0.1 or less, N: 0.008 or less, Ti: 0.004-0
．． 2 and B as necessary: 0.0005 to 0.003
When hot rolling a steel containing 0 and the remainder consisting of Fe and unavoidable impurities, finish rolling at a temperature of Ar3 or higher, and then cooling start temperature at Ar3 or higher, The cooling end temperature is (Ar3-1.0°C)-(Ar
3-1oo'c) temperature range at 50℃/s -400
A cold-rolled steel sheet with excellent press formability, characterized by being cooled at a cooling rate of ℃/s, then held for 2s to 20s, coiled at 650℃ or less, and then cold-rolled and continuously annealed according to a conventional method. (2) In weight%, C: 0.005 or less, Si: 1.0
below.

Mn:2. O or less, P: 0.15 or less, S: 0.05 or less, S.

IAQ: 0.1 or less, N: 0.008 or less, T
i: 0.004~0゜2, Nb: 0.004
~0.05 as necessary, B: 0.0005~0.
When hot rolling a steel containing 0030 and the balance consisting of Fe and unavoidable impurities, finish rolling is completed at a temperature of Ar3 or higher, and then the cooling start temperature is set to Ar3.
The cooling end temperature is (Ar3-100C)-(Ar
3-100℃), 50℃/s -400℃
Cold rolled with excellent press formability, characterized by cooling at a cooling rate of ℃/s, then holding for 25 to 20 seconds, coiling at 650℃ or less, and then cold rolling and continuous annealing according to conventional methods. This is a method for manufacturing steel plates.

[Action Co. The present invention will be specifically explained below.

C shall be 0.005% or less. In order to improve press workability, it is better to have less C. Furthermore, if C exceeds o or oos%, the amount of Ti or Nb added increases to fix it, resulting in an increase in cost.

Sl is 1.0% or less. Si is an effective element for increasing strength and can be added depending on the required tensile strength, but if it exceeds 1.0%, hot-dip plating properties and chemical conversion properties are impaired.

Like Mr+t+Si, it can be added depending on the required tensile strength, but if the Mn content is 2.0% or more in ultra-low coal, the refining cost will be high, and if the Mn content is too high, the workability will be impaired. .

Since S forms sulfide-based inclusions and deteriorates press formability, it is better to have a smaller amount, and it is set to 0.05% or less. P is an element effective in increasing strength, and is actively added when high tensile strength is desired. However, if it exceeds 0.15%, weld brittleness tends to occur.

5olA fl is contained in order to deoxidize the molten steel and improve the yield of Ti and Nb. However, if added in excess, the press formability of the steel plate will be impaired, so the upper limit is set at 0.1%.

N is o, oog% or less. If the N content is too high, the amount of li, T1, and Nb added will increase, resulting in an increase in cost.
Moreover, press formability is impaired.

In the present invention, Ti and Nb are added to precipitate and fix C and N in the steel and to obtain good press formability.

When Ti is less than 0.004%, C and N are not sufficiently precipitated and fixed, resulting in poor aging properties of the cold rolled steel sheet. Ti content is 0
．． 2% is sufficient, and excessive addition is not preferable from the economic point of view. Note that Ti nitride precipitates in the slab because its solubility product in steel is small. Ti carbide has a large solubility product, and most of it is precipitated during the coiling stage of hot rolling or during sintering, but as described later, in the present invention, it is precipitated during low-temperature coiling.

Nb is also contained in an amount of 0.004 to 0.05% for the same reason. Although Nb carbonitride is also precipitated during the winding stage of hot rolling and annealing, in the present invention, it is precipitated during low-temperature winding as will be described later.

In the present invention, B is added when suppressing secondary processing brittleness. When added in an amount of o, oos% or more, secondary work brittleness is significantly improved. However, even if 0.003% or more is added, the effect is saturated.

The finish rolling temperature of the hot rolling of the present invention is Ar3 point or higher. If the Ar point is 3 or less, coarse grains are generated in the hot rolled sheet or a processed structure remains, reducing the deep drawability after cold rolling and annealing.

Next, the present invention will be explained in detail. In the present invention, the cooling start temperature is Ar3 point or higher and the cooling end temperature is (Ar3-10℃) ~
(Ar3-100°C) temperature range is 50°C/5-40
Cool at a cooling rate of 0° C./s and then hold for 2 to 20 seconds. This rapid cooling and holding is accompanied by the following effects that were previously unknown. That is, conventionally, when winding is performed at a low temperature of 650° C. or lower, Ti carbide and Nb carbonitride are difficult to generate nuclei due to the low winding temperature, and the precipitation rate is slow, so the precipitates do not complete precipitation. The cold-rolled steel sheet obtained by cold rolling and continuous annealing is a product with low press formability. However, when the rapid cooling of the present invention is performed, Ti carbides and Nb carbonitrides precipitate uniformly within the crystal grains during holding, and even during the slow cooling process after winding at a temperature of 650°C or less, the already precipitated Ti carbides and Nb carbonitride grows. As a result, a cold-rolled steel sheet with good press formability can be obtained even by continuous annealing. The reason for this is that the cooling start temperature is Ar3 point or higher and the cooling end temperature is (Ar3-10℃).
By rapidly cooling in the temperature range of ~(Ar3-100℃), dislocations are introduced into the generated α grains, forming Ti carbides and Nb
This is thought to be to provide a site for carbonitride precipitation. For this purpose, a cooling rate of 50° C./s or higher is required. The cooling rate may be 400°C/s or more, but the cooling rate is 400°C/s or more.
s is an easily achievable range.

The cooling end temperature is (Ar3-10℃)-(Ar3 100
℃). If the cooling end temperature is too high, the volume fraction of the α phase will be small and precipitation will occur only in some crystal grains. On the other hand, if it is too low, the precipitation rate will be slow, and the introduced dislocations will not be effectively precipitated, and the dislocations will disappear before they are precipitated. Therefore, the rapid cooling end temperature is (Ar3-10℃) ~ (Ar3-100℃)
It is. Preferably the quenching end temperature (Ar3-10℃)
~(Ar3-80°C), most preferably (Ar3-1
0°C) to (Ar3-50°C), even higher deep drawability can be obtained.

Next, in the present invention, the temperature is maintained for 2 to 20 seconds in order to precipitate Ti carbide and Nb carbonitride. In the present invention, holding means cooling at a cooling rate of 20°C/s or less, that is, cooling with a passing roll on a runout table, weak water cooling with cooling water, partial cooling by draining water such as temperature measurement, etc. This refers to maintaining a hot rolled sheet at a cooling rate of air cooling or lower.

In order to reduce the temperature drop during holding, it is preferable to use a heater using electricity, gas, etc., a heat insulating cover, etc. to prevent the temperature drop during holding and increase the amount of precipitation. If the holding time is 28 or more, a large number of Ti carbides and Nb carbonitrides can be precipitated on the dislocation lines, but this holding time is more preferably 4S or more, most preferably 68 or more. A longer holding time is preferable because the amount of precipitation increases, but if the holding time is 20 seconds or more, the length of the line from the finishing mill to the winding machine becomes significantly long and the equipment becomes large-scale.

In addition, in order to make the holding time as long as possible, the cooling equipment that performs rapid cooling before holding is installed in the finishing rolling mill to the extent that it does not interfere with the operation of the plate thickness gauge and thermometer that are normally placed at the exit side of the finishing rolling mill. It is preferable to place it as close to the machine as possible.

When the temperature at which holding ends is lowered, the rate of precipitation decreases and the holding effect becomes smaller. Preferably (Ar3
-100°C) or higher is preferable.

In the present invention, winding is performed at a temperature of 650°C or higher. At temperatures below 650°C, there is little oxidation on the surface of the steel plate and the pickling properties are good.
Moreover, a hot-rolled sheet can be obtained in which both the inner and outer circumferential parts of the coil are homogeneous over the entire length.

In addition, during hot rolling, the slab heating temperature is 1.000 to 1.
If the temperature is 300°C, good deep drawability can be obtained. Preferably, when the temperature is [000~]]50°C, even better deep drawability can be obtained. Similar effects can also be obtained when direct rolling is performed after continuous casting.

The hot-rolled steel sheet produced according to the present invention is cold rolled and continuously annealed by conventional methods. The conditions for cold rolling and continuous annealing are not particularly limited, but the cold rolling rate is 40 to 95%, preferably 7.
When it is 0 to 90%, a cold rolled steel sheet with very high press formability can be obtained. Further, in continuous annealing, too high annealing temperature is not preferred, but under normal continuous annealing conditions, a cold rolled steel sheet with excellent press formability can be obtained.

After cold rolling and annealing to form a cold rolled steel sheet, various platings such as galvanizing, tin plating, chrome plating, etc. may be performed in subsequent steps depending on the purpose. Also, after annealing,
Subsequently, hot-dip galvanizing or the like may be performed depending on the application. In this case, it is preferable to use steel to which Nb and Ti are added because a galvanized steel sheet with better adhesion of the plating layer can be obtained.

Furthermore, after annealing, skin pass rolling, rust prevention treatment, application of lubricant, etc. may be performed as necessary.

[Example 1] A slab of 245 mm thick was made by continuous casting of steel melted according to the usual process. The chemical composition of the steel is shown in Table 1.

After that]] 1. at 50°C. ．． After soaking for 5 hours, rough rolling,
Finish rolling was performed and coiled at a predetermined temperature to form a hot coil. Thereafter, after pickling, 80% cold rolling was performed, continuous annealing was performed at 760° C. for 40 seconds, and 0.6% pre-chamber rolling was performed to produce a cold rolled steel sheet.

Table 2 shows the hot rolling conditions and the r value of the cold rolled steel sheets. As shown in Table 2, steel with chemical composition within the range of the present invention is used, and the steel is cooled and held at the rolling end temperature, cooling start temperature, cooling end temperature, and cooling rate within the range of the present invention. It can be seen that this makes it possible to produce a cold-rolled steel sheet with excellent deep drawability.

The Lankford value (r value) was used as an index of deep drawability. The r value used was the average value of the values in the rolling direction, a direction tilted by ±45° from the rolling direction, and a direction perpendicular to the rolling direction.

[Effects of the Invention] The present invention has excellent pickling properties and
2 Table *Holding: Air cooling (cooling rate: 5℃/5ee) *Holding:
Heat insulation cover (cooling rate = 0.2°C/5ee) The material of the inner and outer periphery of the coil is stable.

Furthermore, although the present invention involves low-temperature winding, Ti carbide and Nb
Because carbonitrides are precipitated, cold rolling using continuous annealing is a method that can produce cold rolled steel sheets with excellent deep drawability, and has great industrial effects.

Patent applicant: Nippon Steel Corporation

Claims (2)

[Claims] (1) In weight%, C: 0.005 or less, Si: 1.0 or less, Mn: 2.0 or less, P: 0.15 or less, S: 0.05 or less, SolAl: 0.1 or less, N: 0.
008 or less, Ti: 0.004 to 0.2, B: 0.0005 to 0.0030 as required, and the balance is Fe.
During hot rolling of steel with components consisting of and unavoidable impurities, finish rolling is completed at a temperature of Ar3 or higher, and then the cooling start temperature is Ar3 or higher and the cooling end temperature is (Ar
3-10℃) to (Ar3-100℃), 50℃
Cool at a cooling rate of ℃/s to 400℃/s, then 2s
A method for producing a cold-rolled steel sheet with excellent press formability, characterized by holding the steel sheet for ~20 seconds, winding it at 650°C or less, and then cold rolling and continuous annealing according to a conventional method. (2) In weight%, C: 0.005 or less, Si: 1.0 or less, Mn: 2.0 or less, P: 0.15 or less, S: 0.05 or less, SolAl: 0.1 or less, N: 0.
008 or less, Ti: 0.004 to 0.2, Nb: 0.0
04 to 0.05, if necessary, B: 0.0005 to 0.0030, and the balance consists of Fe and unavoidable impurities, finish rolling at a temperature of Ar3 point or higher during hot rolling. After that, the temperature range where the cooling start temperature is Ar3 point or higher and the cooling end temperature is (Ar3-10℃) to (Ar3-100℃) is cooled at a cooling rate of 50℃/s to 400℃/s. , then held for 2s to 20s and wound at 650°C or less,
A method for producing a cold-rolled steel sheet with excellent press formability, which is then subjected to cold rolling and continuous annealing according to a conventional method.