JPH03277717A - Manufacture of hot-rolled steel plate and method for heat-treating processed product thereof - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention has low strength and is easy to process until it is processed into high-strength parts for automobiles or industrial equipment, for example, and can be aged by appropriate heating after processing. The present invention relates to a hot-rolled steel sheet whose strength is increased by this phenomenon, and a heat treatment method for processed products of this hot-rolled steel sheet.

(Prior Art) Hot-rolled steel sheets are widely used as a relatively inexpensive structural material in various industrial appliances including the above-mentioned automobiles.

Many of these applications involve members that are formed by press working, and therefore hot-rolled steel sheets are often required to have excellent workability. On the other hand, structural members are also required to have high strength, but it is usually difficult to achieve both high strength and excellent workability.

Therefore, various materials have been developed that have low strength and good workability at the stage before processing (and can be made high in strength through appropriate heat treatment after processing). So-called bake-hardened high-strength steel plates, which are easy to process and whose yield strength increases by hardening during the baking process after processing, have already been put into practical use.Recently, baking-hardened hot-rolled steel plates have been used. Studies are underway, and a patent has been filed regarding this.

For example, Japanese Patent Publication No. Sho 62-188021 discloses a method for manufacturing high-strength hot-rolled steel sheets of which baking is hardening, in which steel with a specific chemical composition containing a large amount of N is cooled after hot rolling. has been done. In addition, Japanese Patent Application Laid-Open No. 5397925 discloses that Al-killed steel containing Nb is heated to 500°C after hot rolling.
A manufacturing method is disclosed that involves winding at the following temperatures: The invention described in Japanese Patent Publication No. 62-188021 utilizes the strain aging of solid solution N to obtain hardening properties, but according to the experimental results of the inventors of the present application, seizure is caused by subsequent yielding. Although the strength increased significantly, the increase in tensile strength was slight.

On the other hand, the invention described in JP-A-53-97925 suppresses the precipitation of Nb by low-temperature winding and uses strain aging of solid solution Nb after processing to increase strength. The precipitation rate of Ti, which will be described later, is extremely fast, and low-temperature winding alone cannot sufficiently suppress these precipitations, and the increase in strength after heating is small. For example, even if the hot-rolled steel sheet obtained by this method is heated at a temperature of 500 to 700°C, the increase in tensile strength is as small as less than 15 kgf/mm''.

If the increase in tensile strength is small, the improvement in fatigue properties is small.

It has been reported that there is a strong correlation between fatigue properties and tensile strength, and that the fatigue properties increase as the tensile strength increases. (Teyo et al.; Resident Metal vol. 1.33 (1981)
Nα4p, 121).

Therefore, if the increase in tensile strength is small, the effect of improving fatigue properties required for high-strength parts of automobiles and industrial equipment, which are the main uses of these steel sheets, will be small, and the steel sheets will have little practical value.

(Problem to be Solved by the Invention) The problem to be solved by the present invention is to create a hot-rolled steel sheet that has low strength and is easy to process until it is processed into high-strength parts for automobiles or industrial equipment, and that can be increased in strength by heat treatment after processing. The object of the present invention is to provide an optimal heat treatment method.

(Means for solving the problem) If the precipitation of Ti and Nb compounds is used as described above,
That is, if Ti and Illb are dissolved in solid solution at the manufacturing stage of the hot rolled steel sheet and precipitation is suppressed, the hot rolled steel sheet as it is will have low strength and excellent workability. After processing this hot rolled steel sheet, Ti,
If Nb is precipitated, the tensile strength will increase. but,
The conventional method of simply winding at a low temperature cannot sufficiently suppress the precipitation of Ti and Nb. However, the present inventors have found that by adjusting the C and Mn contents in the steel, or by adding an appropriate amount of B in addition to this, the precipitation of Ti and Nb can be reliably suppressed, and transformation strengthening can also be achieved. suppressed,
It was discovered that hot-rolled steel sheets as they are have low strength and excellent workability, and that heating after processing significantly increases the tensile strength.

The gist of the present invention based on the above knowledge is the following (1) to (3).

(1) In weight%, C: 0.02-0.07%, Si
: 1.5 or less, Mn: 0.6-2.0%, Ti
and 0.10 to 0.35% in total of one or two types of Nb
A steel billet containing 1] is subjected to hot rolling at a temperature of 00°C or higher, the hot rolling is finished at a temperature of Ars point or higher, and then,
A method for producing a hot-rolled steel sheet, which comprises cooling to a temperature of 500°C or less at a cooling rate of 20°C/second or more, and then winding it at a temperature of 300 to 500°C.

(2) In weight%, C: 0.02-0.07%, Si:
1.5% or less, Mn: 0.1-2.0%, B:
0.0003-0.0050%, 1 of T1 and Nb
A piece of rope containing 0.10 to 0.35% of the species or two species in total,
After hot rolling at a temperature of 1100°C or higher, finishing hot rolling at a temperature of Ars point or higher, and subsequently cooling to a temperature of 500°C or lower at a cooling rate of 20°C/sec or higher, 30°C
Hot rolling characterized by winding at a temperature of 0 to 500°C! 1
Method for manufacturing ii board.

(3) A method for heat treatment of a processed product, which comprises heating a processed product of a hot-rolled steel sheet manufactured by the method described in (1) or (2) above at a temperature of 500 to 750°C for 1 minute or more. .

(Function) Hereinafter, the constituent elements of the present invention and their functions will be explained in detail.

[Contents of steel billets]

C: C combines with T], Nb during heat treatment after processing to form T1, N
It precipitates as a carbide (b) and increases the strength of the steel plate.

If the content is less than 0.02%, the above effects cannot be expected, and 0.
．． If the content exceeds 0.07%, the amount of martensite and bainite mixed in the hot rolled steel sheet will increase, and the workability of the steel sheet will deteriorate. Therefore, the C content is set to 0.02 to 0.07%. The preferred content is 0.03 to 0.06%.

Si: Si is a preferred element that improves strength and ductility through solid solution strengthening. However, if more than necessary, weldability deteriorates, so the content was set to 1.5% or less.

Mn: Like B, which will be described later, Mn has the effect of lowering the Ara point, that is, the temperature of austenite-ferrite transformation, and suppressing the precipitation of Ti and Nb carbides associated with a decrease in solid solubility in ferrite. However, in the case of a steel billet that does not contain B, if the Mn content is less than 0.6%, the decrease in the Ars point is small;
In the case of a steel slab containing B, the effect of B is added, but if the Mn content is less than 0.1%, the decrease in the Ar2 point is small, so the precipitation of T1 and Nb carbides cannot be sufficiently suppressed. . On the other hand, if Mn is contained at 2.0% regardless of the presence or absence of B, the amount of martensite and bainite mixed in the hot rolled steel sheet will increase, and the workability of the steel sheet will deteriorate.

B: B can be added as necessary. B segregates at austenite grain boundaries, lowers the temperature of austenite-ferrite transformation, and decreases the solid solubility of Ti in ferrite.
This has the effect of suppressing the precipitation of XNb carbides. 0.000
If the content is less than 3%, the effect is small and 0.0050
Even if the content exceeds %, the effect will be saturated. Therefore, when adding B, the content should be 0.0003 to 0.005
It is preferable to set it to 0%. Since B itself has no effect of solid solution strengthening, the strength increase in as-hot-rolled steel due to B addition is smaller than that of Mn.

Ti and Nb: According to the method of the present invention, Ti and Nb exist in a solid solution state in a hot rolled steel sheet, and are precipitated as carbides by heat treatment at 500 to 750°C for 1 minute or more after processing.
Significantly hardens the steel plate. Its content is T1 and Nb
If the total amount of one or both of the above is less than 0.10%, the amount of solid solution is small, so precipitation hardening due to heat treatment is small. on the other hand,
Even if the total content of one or both of them exceeds 0.35%, no further effect can be obtained. Therefore, the total amount of Ti and Nb was 0.10 to 0.35% either alone or in combination. The preferred content is 0.15-0.35%.

The object of the present invention can be achieved as long as the raw steel slab contains at least the above components, but in addition to these components, it may also contain the following components.

Sol, A j! : ^l is added as a deoxidizing agent and has the effect of increasing the cleanliness of steel, so Sol, AI! It may be contained in a range of 0.10% or less.

Cu: Cu precipitates independently without combining with C and has the effect of strengthening the steel sheet, so it may be contained in an amount of 0.5% or more, but it may not be contained in an amount exceeding 3.0%. However, the effect is saturated. In the case of sprocketing with Cu, cracking during hot rolling can be prevented by containing 1.5% or less of Ni.

Ca, Zr, and rare earth elements: If these components are contained in excess, there will be too many inclusions in the steel and cold workability will deteriorate, but if they are contained in appropriate amounts, they can adjust the shape of the inclusions. and improves cold workability,
Ca: 0.002~0.01%, Zr: 0.01~
0.10%, rare earth element: One or more kinds may be contained in the range of 0.002 to 0.10%.

Note that impurity P has a negative effect on weldability, and S has a negative effect on weldability.
Since S-based inclusions are formed and workability is degraded, it is desirable to suppress each to 0.05% or less.

[Hot rolling]

For hot rolling, a steel billet having the above composition is supplied at a temperature of 1100° C. or higher. This is to create a state in which T1 and Nb are dissolved in solid solution. Therefore, in the case of a steel billet that has been cooled to a lower temperature after casting, it should be heated to 1100°C or higher to remove Ti and N.
It is necessary to solidify b again, but it is sent directly after casting and 11
If the steel piece has a temperature of 00°C or higher, Ti, N
Since b is in solid solution, hot rolling may be performed as it is.

When hot rolling continues until the temperature is lower than the Ar3 point,
It is important to finish the process at a temperature equal to or higher than the Arz point, since the processed structure will be mixed into the ferrite grains produced by the transformation and the processability will deteriorate. The hot rolling is desirably completed at a temperature as high as possible above the Ar3 point, for example at a temperature of 900°C or above, more preferably at a temperature of 1000°C or above.

Since Ti and Nb precipitate even during hot rolling, precipitation is suppressed by finishing rolling at a high temperature.

After hot rolling, the hot rolled steel sheet is cooled to a temperature of 500°C or less at a cooling rate of 20°C/sec or more, and wound into a coil at a temperature of 300 to 500"C, preferably 300 to 400"C. When the cooling rate is slower than 20℃/sec, when the cooling end temperature exceeds 500℃, and when the winding temperature is 500℃
If it exceeds this, carbides of Ti and Nb will precipitate, and the steel sheet will become hardened and have poor workability. If the winding temperature is less than 300°C, the mixing ratio of haynite and martensite will be high, and the steel plate will be significantly hardened and its workability will deteriorate.

to degrade.

The hot-rolled steel sheet manufactured under the above conditions has a large amount of Ti and Nb in solid solution, so it has low strength and excellent workability. If the next heat treatment is performed after processing, Ti and Nb will precipitate as carbides, resulting in a significant increase in tensile strength.

[Heat treatment after processing]

The heat treatment is performed after forming the hot rolled steel plate into a predetermined shape.
The heating is performed at a temperature of 0 to 750°C for 1 minute or more. 5
For heat treatment below 00°C or for less than 1 minute, T1 and N
Strengthening due to precipitation of carbides in b cannot be obtained, and thermal distortion and scale occur when heat treated at a temperature exceeding 750°C. In this heat treatment, the entire steel plate after processing may be heated, or only the necessary portions may be heated.

(Example) 50 kg of steel with the chemical composition shown in Table 1 is melted in a vacuum melting furnace, hot forged or molded into a 601-thick slab, and then heated to a 211-11 thick slab under the conditions shown in Table 2. It was rolled into a steel plate and wound up.

JIS No. 5 tensile test pieces were cut out from these hot-rolled steel sheets, and the tensile strength of the hot-rolled steel sheets was measured. Also, as a result of measuring 3 Ar points using Formaster, all Ar points were 9.
The temperature was less than 10°C.

Next, these hot rolled steel plates after rolling were heat treated under the conditions shown in Table 2, and tensile test pieces of the same shape were cut out and tensile tested to measure the amount of increase in strength after the heat treatment. These results are also shown in Table 2.

(Hereinafter, blank space) The hot-rolled steel sheets manufactured by the method of the present invention have low strength and excellent workability before heat treatment, and after heat treatment,
A remarkable increase in strength of 5 kg/nv" or more was obtained.

On the other hand, Nα7 has a slow cooling rate after hot rolling, Nα8 has a high cooling end temperature and high coiling temperature, and Nα17 has a low Mn and B content.
Since Nb carbide is precipitated, the increase in strength is small after heat treatment. Seed 16 with a low C content also had a small increase in strength due to insufficient precipitation of Ti and Nb carbides during heat treatment. Nα6, whose finishing temperature is below the Ar3 point, is subjected to ferrite strain during hot rolling, while Nα9, which has a low cooling end temperature and low coiling temperature, and Nα15 and Nα18, which have high C and Mn contents, are due to transformation strengthening. It has high strength as hot-rolled, but during post-processing heat treatment it becomes softer or almost no increase in strength is observed.

(Effect of the invention)

Claims (3)

[Claims] (1) In weight%, C: 0.02-0.07%, Si: 1
．． 5% or less, Mn: 0.6-2.0%, Ti and Nb
A steel billet containing a total of 0.10 to 0.35% of one or two types of is subjected to hot rolling at a temperature of 1100°C or higher,
Hot rolling is completed at a temperature of 20℃/
A method for producing a hot-rolled steel sheet, which comprises cooling to a temperature of 500°C or less at a cooling rate of seconds or more, and then winding at a temperature of 300 to 500°C. (2) In weight%, C: 0.02 to 0.07%, Si: 1
．． 5% or less, Mn: 0.1-2.0%, B: 0.000
A steel billet containing 3 to 0.0050% and 0.10 to 0.35% in total of one or both of Ti and Nb is subjected to hot rolling at a temperature of 1100°C or higher, and at a temperature of Ar_3 or higher. After finishing hot rolling and subsequently cooling to a temperature of 500°C or less at a cooling rate of 20°C/sec or more, 300 to 500
A method for producing a hot-rolled steel sheet, characterized by winding at a temperature of °C. (3) A method for heat treating a processed product, which comprises heating a processed product of a hot-rolled steel sheet manufactured by the method according to claim (1) or (2) at a temperature of 500 to 750°C for 1 minute or more.