JPH0257634A - Manufacture of high-strength steel plate and heat treatment for worked product of same - Google Patents

Abstract

PURPOSE:To manufacture a high-strength steel plate for working having superior toughness by subjecting a slab with a specific composition to continuous hot rolling, cooling, and winding in succession under respectively specified conditions. CONSTITUTION:A slab having a composition which contains, by weight, 0.005-0.060% C, <=1.20% Si, <=1.5% Mo, and 0.02-0.20%, in total, of Ti and/or V and in which the content of P as an impurity is regulated to <=0.035% is heated up to >=1100 deg.C. Subsequently, the above slab is subjected to continuous hot rolling in which the final-path draft in finish rolling and finish-rolling temp. are regulated to >=30% and >=920 deg.C, respectively, and cooling is started within 3sec after the conclusion of rolling, and the rolled plate is cooled rapidly down to <=700 deg.C at >=20 deg.C/sec cooling rate and wound up at <=500 deg.C. By this method, the high-strength steel plate can be obtained, and this steel plate is formed into the prescribed shape and heated to 300-800 deg.C for >=1min so as to be formed into a worked product.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a steel plate suitable for high-strength members for automobiles or industrial equipment, specifically, a steel plate that is processed at a relatively low strength until it is subjected to forming processing. The present invention relates to a method for producing a high-strength steel plate for processing that is easy to process and has excellent toughness and can be increased in strength by an appropriate heat treatment after processing, and a method for heat treating the processed product.

(Prior art) So-called hot-rolled steel sheets manufactured by continuous hot rolling are
As a relatively inexpensive structural material, it is widely used in various industrial equipment including the above-mentioned automobiles. Many of these applications involve parts 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 in the raw material stage before processing, and that can be increased in strength by appropriate heat treatment after processing. In the case of cold-rolled steel sheets, high-strength steel sheets that are soft and easy to process before being processed, and which harden and increase their yield strength when baked and painted after processing, so-called bake-hardened high-strength steel sheets, have already been put into practical use. . Recently, studies have been progressing on baking-hardened hot-rolled steel sheets, and patents related to this have also been filed.

For example, Japanese Patent Publication No. Sho 62-180021 discloses a method of rapidly cooling steel with a specific chemical composition containing a large amount of N after hot rolling, as a method for manufacturing high-strength hot-rolled steel sheets of which baking is hardening. There is. This method uses the strain aging of solid solution N to obtain hardening properties, but according to the experimental results of the inventors of this application, the baking properties obtained by this method are similar to hardening type high-strength hot rolling. Although the yield strength of the steel plate significantly increased after baking, the increase in tensile strength was small. The reason for this is thought to be that the main strengthening mechanism is fixation of dislocations due to solid solution N, and the strengthening mechanism does not reach the level of strengthening due to precipitates.

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 (Hirakumi et al.: Resident Metals 1.33 (1981).
4 P, 121).

Therefore, if the increase in tensile strength is small, the effect of improving fatigue properties required for high-strength parts for 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 of the present invention is to significantly increase the tensile strength of a material that is soft before processing and is effective in improving fatigue properties by reheating it at a relatively low temperature after processing. It is an object of the present invention to provide a method for producing a high-strength steel plate for processing that has good toughness, and an optimal method for heat treating processed products made from the steel plate.

(Means for Solving the Problems) The present inventors have developed a continuous hot rolling condition when processing a steel piece having a specific chemical composition containing one or both of Ti and (2) into a steel plate by continuous hot rolling. We have found that the above problem can be achieved by adjusting the . That is, (1) If the hot rolling finishing temperature is set to a high temperature and the winding temperature is lowered to suppress the precipitation of Ti and (2), a steel plate containing a large amount of solid solute Ti and solid solute V can be obtained.

This steel plate is soft before being processed, and solid solution T+ and solid solution ① precipitate due to the relatively low temperature after processing, increasing its strength.

(2) Furthermore, if the final pass of finish rolling is made with a large reduction ratio and the steel plate is rapidly cooled immediately after rolling, a fine ferrite structure is formed and the toughness is improved.

The present invention is based on such knowledge, and its gist is in (i) and (ii) below.

(i) CF 0.005-0.060%, S in weight%
t: 1.20% or less, Mn: 1.5% or less, T
The total amount of one or both of i and V is 0.02 to 0.20
%, and P as an impurity is 0.035% or less, is heated to a temperature of 1100°C or higher, and then the final pass reduction rate of finish rolling is set to 30% or more, and the finish rolling temperature is set to 920°C.
℃ or higher, cooling is started within 3 seconds after the end of rolling, and the cooling rate is 700℃ or higher at a cooling rate of 20"C/sec or higher.
A method for manufacturing a high-strength steel sheet, which comprises rapidly cooling the steel sheet to a temperature of 0.degree. C. or less, and then winding it at a temperature of soo'c or less.

(ii) After processing and forming the high-strength steel plate manufactured by the method described in claim 1 into a predetermined shape,
A method for heat treating processed products, characterized by heating at a temperature of 00°C or less for 1 minute or more.

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

First, the reason why the chemical composition of the steel slab and the manufacturing conditions in the method for manufacturing a high-strength steel plate of the first invention of the present application are limited as described above will be explained together with the effects.

C: C combines with Ti and ■ to form TiC during reheating after processing.
Or/and is an important element that precipitates as VC and improves tensile strength. If the C content is less than 0°005%, the desired precipitation hardening cannot be expected;
If it exceeds 0%, the structure before processing and reheating after processing will be hardened, resulting in poor workability and, at the time of reheating, this hardened portion will rather soften considerably. For this reason, even if TiC and/or VC are precipitated, the increase in tensile strength as a whole is small. For this reason, the C content is set to o, oos.
~0.060%. Preferably 0.015-0.0
It is 40%.

Si: Si is a preferred element that improves strength and ductility through solid solution hardening. However, if added more than necessary, weldability deteriorates, so the content should be 1.20% or less.

Mn: Like Si, Mn is also a preferable element that improves strength and ductility through solid solution hardening. However, if the C content is more than necessary, it will promote the formation of low-temperature transformed structures such as hard martensite and bainite, which are mainly non-diffusion-transformed, and the same problem as when the C content is too high will occur, so it should be less than 1.5%. The content of Preferably it is 1.0% or less.

Ti and ■: Ti and ■ are the most important elements in the present invention, and their content is 0.02 to 0.2 in one or two types &@ amount.
Set to 0%.

? + and V combine with C during reheating after processing, forming TiC
Precipitation hardening occurs as 5V (C, N), but if the total content is less than 0.02%, the amount of Ti and ■ dissolved in solid solution is also small, so the desired precipitation hardening cannot be obtained. If the content is more than 0.20%, the copper plate will be hardened by low-temperature transformation strengthening, and the same problem as when the content of C and Mn is too large will occur.

The steel piece used as the raw material in the first invention of the present application may contain at least the elements in the above range, and if necessary, Nb of 0.10% or less, Ca of 0.0100% or less, 0 Nb, which may be a steel piece containing one or more types of B at 0.0030% or less, has the effect of promoting precipitation hardening as an auxiliary, and Ca has the effect of improving workability. , B have the effect of improving warp cracking resistance.

Note that the amount of P as an impurity is preferably as low as possible. In particular, since the steel sheet manufactured by the method of the present invention is likely to suffer from temper brittleness during reheating, it is best to suppress it to 0.035% or less. More preferably, it is 0.010% or less.

The first invention of the present application provides a steel piece having such a chemical composition,
Hot rolling is carried out under the conditions described in (a) to (C) below,
It is characterized by rapidly cooling the rolled steel plate and then winding it up.

(a) Continuous hot rolling: The hot rolling of the method of the present invention obtains a hot-rolled II plate with a predetermined thickness, and at the same time dissolves as much Ti or The purpose is to suppress precipitation of V as carbonitrides. Furthermore, the purpose is to prevent deterioration of toughness by high-temperature finishing.

For this purpose, it is necessary to subject a steel billet having the above chemical composition heated to a temperature of 1100° C. or higher to rolling. - Once cooled to a temperature below 1100°C, the billet must be reheated above this temperature.

However, 11
If the steel billet is maintained at a temperature of 00° C. or higher, it can be directly rolled.

If the heating temperature is lower than 1100° C., the amount of solid solution of Ti and (2) will be small (and the amount of increase in tensile strength during reheating after processing will be small).

For hot rolling, the final pass reduction rate of finish rolling is 30% or more,
It is necessary to perform continuous hot rolling with a finish rolling temperature of 920° C. or higher.

If the finishing temperature is lower than 920℃, solid solution Ti or solid solution V
precipitates as carbonitrides, and as a result, the amount of increase in tensile strength during reheating after processing becomes small. A preferable finish rolling temperature is 950°C or higher.

On the other hand, if the final pass reduction ratio of finish rolling is less than 30%, the effect of refining austenite grains is insufficient and the toughness deteriorates. A preferable final pass reduction rate is 40% or more.

(b) Cooling after rolling: Cooling conditions are set to suppress the growth of austenite grains after rolling to prevent them from becoming coarse, and to refine the ferrite grains produced after transformation to improve toughness. .

To do this, the rolled steel plate must be heated at least 3 times after rolling.
Start cooling within seconds, and at a cooling rate of 20℃/second or more
It is necessary to rapidly cool down to below 00°C.

If the cooling start time exceeds 3 seconds after the end of rolling, the growth of austenite grains cannot be suppressed, and
If the cooling rate is slower than 20° C./sec or if the cooling end temperature is higher than 700° C., the ferrite grains formed after transformation become coarse and the toughness decreases. A preferable cooling start time is within 2 seconds from the end of rolling.

Incidentally, since it is preferable that the cooling rate be as high as possible, there is no need to particularly limit the cooling rate. The upper limit rate of cooling that can be carried out industrially is about 100° C./sec.

(C) Winding: Winding conditions are determined to ensure sufficient solid solution Ti content and solid solution amount necessary to increase bowing strength during reheating after molding.

For this purpose, it is necessary to wind up the steel plate after cooling at a temperature of 500° C. or lower. At a winding temperature exceeding 500°C, solid solution Ti and solid solution (2) precipitate, resulting in little increase in tensile strength during reheating after processing.

As mentioned above, the steel sheets manufactured under the conditions (a) to (C) are soft and easy to work because they contain a large amount of Ti and V in solid solution when subjected to processing, and when reheated after processing
Since i and solid solution V precipitate and harden as TiC or/and VC, the tensile strength increases greatly.

In order to greatly increase the tensile strength, the reheating is preferably performed under the heat treatment conditions of the second invention of the present application.

That is, after processing and forming a high-strength steel plate manufactured by the above method into a predetermined shape, it is heated at a temperature of 300°C or higher and 800°C or lower.
It is heated for at least 5 minutes, preferably for at least 5 minutes.

If the heating temperature is less than 300°C, precipitation of solid solution Ti and solid solution (■) will not occur, and if the heating time is less than 1 minute, sufficient precipitation of these will not occur, and as a result, the bow strength will not increase at all. Or the rise is small. Moreover, at heating temperatures exceeding 800° C., thermal distortion occurs in the processed product.

This reheating can be done locally or globally depending on the use of the processed product. Further, the heating means is not particularly limited. For example, high frequency induction heating can be used to heat the entire area, and a gas burner can be used to locally heat the area.

According to the findings of the present inventors, it is preferable that the processed product is processed by 5% or more.The processing introduces precipitation sites and increases the tensile strength more effectively in a short time.

Next, the present invention will be further explained by examples.

(Example) A steel having the chemical composition shown in Table 1 was melted in an experimental 50 kg vacuum melting furnace, hot forged or a slab with a thickness of 60+ m was produced in a mold, and hot-forged under the conditions shown in Table 2. Roll and cool;
Then, it was wound up to produce a hot-rolled steel sheet tentatively yJ-211Il.

The slabs manufactured by hot forging were reheated to the temperatures shown in Table 2 after forging, while the slabs cast from the molds were kept at 1200°C and directly rolled as hot pieces. .

A test piece was taken from each hot-rolled steel sheet obtained, and the tensile strength (TS) and elongation ([if) as hot-rolled and after giving 8% pre-strain to the test piece were shown in the same table. Increase in tensile strength after heat treatment at reheating temperature and reheating time (ΔT
S) and Charpy characteristics (νTrs) were measured. These results are shown in Table 2 (hereinafter referred to as blank space).As is clear from Table 3, Nα obtained by the method of the present invention
Although the steel plates of floors 1 to 9 have relatively low tensile strength and high elongation when hot-rolled, that is, they are soft and have good workability, they are heated to 7.
An increase in tensile strength (ATS) of ~17 kgf/+m'' was obtained. Also, a vTrs of -40°C or less was obtained.

On the other hand, those obtained under rolling, cooling, winding and reheating conditions outside the range specified in the present invention,
Sufficient solid solution T such as Toru 10, N 11ll and Ni 16
The increase in tensile strength is small because i and/or solid solution (2) are not obtained, or the ferrite grains become coarse as in N[L12) 13 and Nα14, resulting in poor toughness. Also, N
Like α15, sufficient solid solution Ti and/or solid solution V cannot be obtained, and the ferrite grains also become coarser, resulting in a small increase in tensile strength and poor toughness, N111? and 1
As in k18, sufficient precipitation of solid solution T+ and/or solid solution V does not occur, so the increase in tensile strength is small or some of the properties are poor.

In addition, even if the manufacturing conditions satisfy the conditions of the present invention, steel slabs with chemical compositions outside the range specified by the present invention may have solid solute Ti and/or solid solute V as in Nα19 and Nα24. Either the increase in tensile strength is small because no precipitation occurs, or the increase in tensile strength is large due to transformation strengthening when hot-rolled as in Magneto 20, Na22, and Ni25.
During reheating after working, the reduction in transformation strengthening and the contribution of precipitation strengthening from solid solution Ti and/or solid solution 2 cancel each other out, resulting in a small increase in tensile strength.
Although the amount of increase in
No. 3 has poor toughness due to high P content.

(Effects of the Invention) As explained above, according to the method of the present invention, a steel plate with excellent toughness, which is soft before processing and has excellent workability, and whose tensile strength is greatly increased by heating at a relatively low temperature after processing, can be produced. be able to.

In addition, it is sufficient to heat the material after processing at a lower temperature than the quenching temperature used in conventional quench-strengthened high-strength steel sheets, and there is no need to speed up the subsequent cooling.
Almost no distortion occurs in the processed product.Therefore, the above-mentioned steel sheet obtained by the method of the present invention is ideal for high-strength members for automobiles and various industrial equipment.

Claims (2)

[Claims] (1) In weight%, C: 0.005-0.060%, Si
: 1.20% or less, Mn: 1.5% or less, Ti and V
Containing one or both of 0.02 to 0.20% in total amount,
A steel billet containing 0.035% or less of P as an impurity is
After heating to a temperature of 100 ° C. or higher, continuous hot rolling is performed with a final pass reduction rate of 30% or higher and a finish rolling temperature of 920 ° C. or higher, and cooling is started within 3 seconds after the end of rolling, A method for producing a high-strength steel sheet, which comprises rapidly cooling the steel sheet to a temperature of 700°C or less at a cooling rate of 20°C/second or more, and then winding it at a temperature of 500°C or less. (2) After processing and forming the high-strength steel plate manufactured by the method described in claim 1 into a predetermined shape,
A method for heat treating processed products, characterized by heating at a temperature of 0°C or less for 1 minute or more.