JPH04198422A - Manufacture of hot rolled steel sheet for working - Google Patents

Abstract

PURPOSE:To provide a steel sheet with low strength and excellent workability and to heighten the strength by heat treatment after working by specifying the chemical componental compsn. of a steel as well as its hot rolling and cooling conditions. CONSTITUTION:The compsn. of a steel is formed of, by weight <=0.20%C, <=2.0%Si, 0.1 to 2.5%Mn, <=0.10%P, <=0.05% S, <=0.10% sol Al, 0.5 to 3.0% Cu, 0.1 to 1.5% and the balance Fe with inevitable impurities. This slab is reheated to >=1000 deg.C, and after that, its continuous hot rolling is finished at the finishing temp. of (the Ar3 point-100 deg.C) to less than the Ar3 point. Next, the slab is cooled to <=500 deg.C at >=10 deg.C/sec cooling rate abd is coiled. If required, the above steel compsn. is incorporated with one or more kinds among Ca, Zr and rate earth elements, one or more kinds among Nb, Ti and V and one or more kinds of B and Mo. This hot rolled steel sheet is suitable as the material for automobiles, industrial equipment or the like requiring workability and high strength.

Description

[Detailed Description of the Invention] <Industrial Field of Application> This invention is applicable to automotive applications, which have relatively low strength and are easy to work with, but which can be strengthened by an aging phenomenon if an appropriate heat treatment is applied after processing. The present invention relates to a method of manufacturing a hot rolled steel sheet suitable as a high strength member for industrial machinery.

<Prior art and its problems> Currently, so-called "hot-rolled steel sheets" are manufactured by continuous hot rolling.
It is widely used as a relatively inexpensive structural material in automobiles and various other industrial machinery, but many of its uses require excellent workability as many parts are formed by press working. Often. However, on the other hand, having high strength is also an important characteristic required for structural members, and for this reason, it is difficult to achieve both "high strength" and "excellent workability" with hot rolled steel sheets. At the same time, a major challenge was to provide the same characteristics as the

However, in the field of cold-rolled steel sheets, so-called "bake-hardened high-strength steel sheets" have already been put into practical use, which are soft and easy to work with before being processed, and harden and increase yield strength during baking painting after processing. As a result, research on bake-hardened hot-rolled steel sheets has recently become active, and several proposals related to these have been made.

For example, JP-A-61-188021 discloses a method for producing a bake-hardened high-strength hot-rolled steel sheet, which comprises hot rolling steel of a specific chemical composition containing a large amount of N, and immediately quenching the steel. . This method utilizes the strain aging of solid solution N to obtain bake hardenability, but according to experiments by the present inventors, the bake hardenable high-strength hot rolled steel sheet obtained by this method is hardenable by baking. The results showed that although the subsequent yield strength was significantly improved, the increase in tensile strength was only slight, and it was judged that the properties were not fully satisfactory. That is, [“
As reported in "Sumitomo Metals" Vol. 33 (1981), No. 4, p. 1211, there is a strong correlation between the fatigue strength and tensile strength of hot-rolled steel sheets, and Indeed, it is a fact that fatigue properties increase.Therefore, the increase in tensile strength is small in the above-mentioned JP-A-62-1880.
The degree of improvement in fatigue properties of the hot-rolled steel sheets according to Publication No. 21 is small, and the fatigue properties required for high-strength parts for automobiles and industrial machinery, which are the main uses of these steel plates, cannot be sufficiently secured, so they are not practical. I had to say that it was of little value.

Separately, by adding Cu to the material steel and rolling it at a low temperature after hot rolling, it is possible to
There is also a proposal to produce a hot-rolled steel sheet that can be reheated at ~700°C to precipitate Cu and increase its tensile strength (Japanese Unexamined Patent Publication No. 79717/1983).

However, it has been pointed out that this proposed method requires a long-time reheating treatment or a high-temperature reheating treatment to achieve the desired strength in areas where the degree of work is low during molding. It was done.

Therefore, the object of the present invention is to produce hot rolled sheets that are soft during forming and whose tensile strength, which is effective in improving fatigue properties, can be significantly increased by simply applying heat treatment at a low temperature for a short time after forming. The goal was to realize steel plates.

Means for Solving the Problems> Therefore, the present inventors conducted research while repeating numerous experiments in order to achieve the above object, and as a result, they came to the knowledge shown below.

In the hot-rolled steel sheet obtained according to the method of JP-A-53-79717, the reason why the post-processing heat treatment for the second part requires a long time or high temperature is that the ferrite produced Because there are almost no "dislocations that serve as nucleation sites for Cu precipitation during reheating treatment (post-processing heat treatment)," low-temperature, short-time heat treatment is sufficient to cause sufficient Cu precipitation to increase the desired strength. There is something for every problem.

(bl) However, by hot-rolling and winding Cu-added steel with a specific chemical composition under specific conditions, dislocations are created inside the ferrite that serve as the precipitation sites for Cu during reheating treatment, resulting in a low workability. In addition, a significant increase in strength can be obtained by heat treatment at a low temperature for a short time, and a hot-rolled steel sheet with very little over-aging softening can be obtained even if the heat treatment is held for a long time. Specific methods include: (a) A method of introducing dislocations into ferrite produced in the hot rolling process by setting the finishing temperature below the Ar+ point.

■) Austenite 1 by adding Mo and B to the material steel
By suppressing the transformation to ferrite and lowering the ferrite formation temperature during the cooling process after hot rolling, inite has a large amount of internal dislocations. This means not only henite but also acygular ferrite that does not contain carbides inside.
Hereinafter, this is a method for generating these structures (hereinafter collectively referred to as bainite).

is valid. In addition, "Hainite" has a large amount of dislocations inside and has the property of being less softened during heat treatment, but it is preferable that the metal structure has 65% or more of bainite to prevent Cu precipitation during heat treatment. 2. The degree of strength increase is also greater.

The present invention was completed based on the above-mentioned findings, etc. rc: 0.20% or less (hereinafter, the component ratio is not expressed and % is expressed as weight %).

Si: 2.0% or less, Mn: 0.
1-2.5%.

p: 0.10% or less, s: o, o 5% or less.

sol, lie: 0.10% or less, Cu
: 0.5-3.0%.

Contains Ni: 0.1 to 1.5%, or further contains B: 0.0003 to 0.0080%, Mo:
0.1-3.0%.

Ca: 0.0002~O, old%, Zr:
0.01~0.10%5 Rare earth elements: 0.002~0
．． 10%.

Nb: 0.005-0.20%, Ti:
0.005-0.20%.

v: o, oos ~ 0.20%, and the balance consists of Fe and unavoidable impurities, directly after casting, or after reheating to 1000°C or higher, B or Mo added steel is cast. In this case, [Ar3 points -1
Continuous hot finish rolling is finished at a finishing temperature of 00℃] or higher, or in other cases, CA, r3 point - 100℃] or higher and lower than Ar3 point, and cooling is performed at a rate of 10℃/sec or higher. The major feature is that hot-rolled steel sheets for processing with excellent low-temperature heat hardening properties can be stably produced by cooling the steel sheet at a speed of 500° C. or less and then winding it up.

Hereinafter, the reason why the chemical composition and processing conditions of the steel material are limited as described above in the present invention will be explained in detail together with the effects of the following.

<Effect> 8) Chemical composition C is a preferable component that increases the strength of steel sheets, but when its content increases, hot-rolled steel sheets that have been transformationally strengthened by C become significantly softened with the precipitation of carbides during post-processing heat treatment. However, even if the strength due to the precipitation of Cu could be secured, the increase in strength after heat treatment would be insufficient. This phenomenon becomes particularly noticeable when the C content exceeds 0.20%. Therefore, although the upper limit of the C content is set at 0.20%, it is preferable to reduce it as much as possible.

Needle Si is a preferable component that improves the strength and ductility of steel sheets through solid solution strengthening. However, since adding more than necessary causes deterioration of weldability, the Si content was set at 2.0% or less.

HaMn = S, which exists as an impurity in steel, is MnS.
14- It has the effect of suppressing cracks that occur during hot rolling by fixing, but if the content is less than 0.1%, the desired effect due to the above effect cannot be obtained; If the content exceeds 2.5%, a large amount of martensite will be mixed into the manufactured hot rolled steel sheet, which will increase the strength of the steel sheet too much and deteriorate workability.This martensite will soften during heat treatment. is large, resulting in a small increase in strength during post-processing heat treatment. Therefore, the Mn content was determined to be 0.1 to 2.5%.

P is an impurity element that inevitably mixes into steel, and since it does not adversely affect weldability, the lower the content, the more preferable it is. However, since the desired weldability can be ensured by suppressing the S content to 0.10% or less, the upper limit of the S content can be set to 0.
．． It was set at 10%.

S is also an impurity element that inevitably mixes into steel, and segregates at austenite grain boundaries during hot rolling to form FeS with a low melting point, causing cracks during hot rolling. It is also an impurity element that forms MnS-based inclusions and reduces workability. Therefore, the lower the S content is, the more preferable it is, but the upper limit of the S content is set at 0.05% because desired hot workability and formability can be ensured if the S content is suppressed to 0.05% or less.

S01. 8β Aβ is a component added as a deoxidizing agent for steel, but from the viewpoint of ensuring the cleanliness of the steel, the static content was determined to be 0.10% or less in soZ and Aβ content.

Normal Cu has the effect of precipitating independently into the matrix during heat treatment after forming and strengthening the steel sheet, but if its content is less than 0.5%, the desired effect due to the above effect cannot be obtained; Even if the content exceeds 3.0%, the effect will be saturated and it will be economically disadvantageous. Therefore, the Cu content was determined to be 0.5 to 3.0%.

Ni has the effect of preventing cracks that occur during hot rolling of Cu-added steel, but if its content is less than 0.1%, the desired effect of the above effect cannot be obtained; .5%
Even if the content exceeds 15%, the effect becomes saturated and it becomes economically disadvantageous, so the previous content of H was set at 0.1 to 1.5%.

These components have the effect of forming haynite containing a large amount of dislocations inside during the hot rolling process and promoting strength increase due to Cu precipitation during post-forming heat treatment. If necessary, one or two types may be added, but the details of the reason for limiting the amount added are as follows.

a) B B is a preferable element that has a strong tendency to segregate at grain boundaries and does not cause solid solution strengthening, and therefore does not deteriorate the workability of hot rolled steel sheets. The B segregated at the grain boundaries suppresses the transformation of austenite to ferrite, lowers the ferrite formation temperature, and generates bainite containing a large amount of dislocations inside. However, the above-mentioned effect of B cannot be obtained at a content of less than 0.0003%, and on the other hand, even if the content exceeds 0.0080%, the effect is saturated. For this reason, the S content was determined to be 0.0003 to 0.00%, ooso%.

b) M.

Like B, Mo is ferrite I from austenite.
It suppresses transformation, lowers the temperature at which ferrite is formed, and produces haynite that contains a large amount of dislocations inside. Furthermore, when Mo is added, recovery of dislocations in heinite during heat treatment is suppressed, MO□C is precipitated, and softening of the matrix is suppressed, resulting in a large increase in strength. However, these effects due to Mo occur when the content is 0.1
If the content is less than 3.0%, it is not sufficient, while if the content exceeds 3.0%, the effect is saturated and it is economically disadvantageous.

Therefore, the Mo content was determined to be 0.1 to 3.0%.

Ca+ Zr and earth? (REM) Since each of these components has the effect of adjusting the shape of inclusions and improving cold workability, one or more of these components may be added as necessary. However, the Ca content is 0.
If the content of Zr is less than 0.002%, the content of Zr is less than 0.01%, and the content of rare earth elements is less than 0.002%, the desired effect cannot be obtained. In the case of Zr, O, 1.0%,
In the case of rare earth elements, if each content exceeds 0.10%, inclusions in the steel will increase too much and cold workability will deteriorate, so the content of each element should be Ca 0.0002-0.01%, Zr 0.0002-0.01%.
Old ~0.10%, rare earth elements 0.002~0.10%
It was determined that

Nb, Ti, and V All of these components strengthen the steel sheet by precipitating as carbonitrides, and also improve MA+Fx by reducing the amount of solid solute C.
Since it has the effect of improving processability, one or more types may be added as necessary. However, if the content is less than 0.005% of each, the desired effect due to the above-mentioned action cannot be obtained, and on the other hand, if the content exceeds 0.20% of each, the effect is saturated.

Therefore, the content of Nb, Ti or ■ is each 0.00
It was set at 5% to 0.20%.

B) Hot rolling conditions In hot rolling, it does not matter whether a direct slab is used or the slab is reheated.

However, when heating the slab from the viewpoint of ensuring finishing temperature, it is difficult to achieve the objective unless the heating temperature is 1000° C. or higher.

In addition, if the finishing temperature of hot rolling is lower than the temperature of [Ar5 point - 100°C], the amount of processing on the ferrite grains generated by transformation becomes too large, and dislocations are recovered during reheating treatment and softened. As a result, the amount of strength increase decreases, and the workability of the hot-rolled steel sheet deteriorates. Therefore,
The finishing temperature of hot rolling needs to be higher than [Ar3 point - 100°C]. On the other hand, there is no need to set an upper limit for the hot rolling finishing temperature because when B or Mo is added, the transformation from austenite to ferrite is suppressed, resulting in a bainite structure containing many dislocations. However, if B or Mo is not added, it is necessary to generate ferrite during hot rolling and actively introduce dislocations into the ferrite by hot rolling, so the upper limit of the finishing temperature should be less than 3 points Ar. There is a need. By setting the finishing temperature to such a relatively low temperature, strain in the hot rolling is sufficiently introduced, and the amount of increase in strength during reheating treatment is significantly increased. In addition,
Even in the case of adding B or Mo, the hot rolling finishing temperature should be lower than [Ar: + point -100°C] for the same reason.
Preferably, the temperature is below the point r.

Furthermore, in the present invention, cooling and winding conditions after rolling are also very important. In other words, after hot rolling, 10°C
Only by cooling to the 5M range of 500°C or less at a cooling rate of /sec or more and coiling, the precipitation of Cu in the cooling process after rolling is suppressed and Cu is present in a solid solution state in the hot rolled steel sheet. This means that it can be done. And this solid solution C
Since u precipitates with dislocations as nuclei by heat treatment at 400 to 700° C. after processing, a remarkable increase in tensile strength can be ensured by heat treatment at low temperature and for a short time.

Next, the effects of the present invention will be explained in more detail with reference to Examples.

<Example> 50 kg of steel having the chemical composition shown in Table 1 is melted in a vacuum melting furnace, and cast into a mold and directly or hot forged with a δ trowel 6
After forming slabs with a thickness of 0 μm, each of the slabs was hot-rolled under the conditions shown in Table 2 to obtain a hot-rolled steel plate with a thickness of 211 m. Note that air cooling was performed from the stop of cooling after hot rolling to the time of winding. Separately, three Ar points were measured by thermal expansion.

Next, a test piece (JISS No.) was taken from the obtained hot-rolled steel sheet and the tensile strength (TS) and elongation (EL) of the hot-rolled steel sheet were measured. X 15 minutes 1. f550°c x 30 minutes” and [55°”CX 12
The increase in tensile strength (ΔTS) due to the heat treatment was also measured by applying a heat treatment of 1:0, and the results are also shown in Table 2.

As is clear from the results shown in Table 2, the hot-rolled steel sheet manufactured under the conditions specified in the present invention not only has excellent workability (low strength, high elongation), but also has excellent workability (low strength, high elongation). ×15
20kgf/- or more with heat treatment of 1350℃ x 30 minutes
It can be seen that the increase in strength is rapid and the amount of increase in strength is large, with a heat treatment of 25 kgf/mf or more after J minutes of heat treatment.

On the other hand, in test numbers 9 and 43, which did not contain Mo or )3 and had a high hot rolling finishing temperature, the amount of strength increase was reduced by heat treatment at [550°C x 15 minutes] because Cu precipitation was slow. Small, less than 15 kgf/m4, [550”CX
The softening during heat treatment for 120 minutes is also large.

In addition, in test number 10.27 where the hot rolling finishing temperature is lower than the range specified in the present invention, in addition to 'strengthening due to Cu precipitation', 'softening due to dislocation recovery' occurs during heat treatment [550°C x Even after heat treatment for 30 minutes, the increase in strength is small, and since the ferrite in the hot-rolled steel sheet has already been subjected to severe processing, the strength-elongation balance (strength-elongation balance) of the hot-rolled steel sheet is
TS x El,) J is also bad.

In test number LL 28, which had a slow cooling rate after hot rolling, and test number 12.29, which had a high cooling end temperature, carbon was already present in the hot rolled sheet.
A large amount of u is precipitated, and the amount of increase in strength due to Cu precipitation during reheating treatment is small.

Summary of Effects> As explained above, according to the present invention, it is possible to stably mass-produce hot-rolled steel sheets for processing that have low strength and excellent workability, and whose strength increases significantly through proper heat treatment after processing. It is extremely useful in industry, as it can be applied to materials for automobiles, industrial equipment, etc. that require both workability and high strength, and further improve their performance. effect is brought about.

Claims (9)

[Claims] (1) C: 0.20% or less, Si: 2.0% or less, Mn: 0.
1 to 2.5%, P: 0.10% or less, S: 0.05% or less, sol. Al: 0.10% or less, Cu: 0.5-3
．． After reheating a steel piece containing 0%, Ni: 0.1 to 1.5%, and the balance consisting of Fe and unavoidable impurities to 1000°C or higher, at a finishing temperature of [Ar_3 point - 100°C] or more but less than Ar_3 point. A method for producing a hot-rolled steel sheet, which comprises completing continuous hot finish rolling, cooling to 500° C. or lower at a cooling rate of 10° C./sec or higher, and then winding the steel sheet. (2) C: 0.20% or less, Si: 2.0% or less, Mn: 0.
1 to 2.5%, P: 0.10% or less, S: 0.05% or less, sol. Al: 0.10% or less, Cu: 0.5-3
．． 0%, Ni: 0.1-1.5%, Ca: 0.0002-0.01%, Zr: 0.01-0
．． 10%, rare earth elements: 0.002 to 0.10%, and the balance is Fe and unavoidable impurities. After reheating to 1000°C or higher [Ar_3 points =
Continuous hot finish rolling is finished at a finishing temperature of 100℃] or higher and lower than Ar_3 points, and then the continuous hot finish rolling is finished at a cooling rate of 10℃/sec or higher for 5
It is characterized by being wound up after being cooled to 00°C or less,
A method for producing hot rolled steel sheets. (3) C: 0.20% or less, Si: 2.0% or less, Mn: 0.
1 to 2.5%, P: 0.10% or less, S: 0.05% or less, sol. Al: 0.10% or less, Cu: 0.5-3
．． 0%, Ni: 0.1-1.5%, Nb: 0.005-0.20%, Ti: 0.005-0
．． 20%, V: 0.005 to 0.20%, and the balance is Fe and unavoidable impurities. After reheating to 1000°C or higher [Ar_3 points
Continuous hot finish rolling is finished at a finishing temperature of 100℃] or higher and lower than Ar_3 points, and then the continuous hot finish rolling is finished at a cooling rate of 10℃/sec or higher for 5
It is characterized by being wound up after being cooled to 00°C or less,
A method for producing hot rolled steel sheets. (4) C: 0.20% or less, Si: 2.0% or less, Mn: 0.
1 to 2.5%, P: 0.10% or less, S: 0.05% or less, sol. Al: 0.10% or less, Cu: 0.5-3
．． 0%, Ni: 0.1-1.5%, Ca: 0.0002-0.01%, Zr: 0.01-0
．． 10%, one or more of rare earth elements: 0.002-0.10%, Nb: 0.005-0.20%, Ti: 0.005-0
．． 20%, V: 0.005 to 0.20%, and the balance is Fe and unavoidable impurities. After reheating to 1000°C or higher [Ar_3 points
Continuous hot finish rolling is finished at a finishing temperature of 100℃] or higher and lower than Ar_3 points, and then the continuous hot finish rolling is finished at a cooling rate of 10℃/sec or higher for 5
It is characterized by being wound up after being cooled to 00°C or less,
A method for producing hot rolled steel sheets. (5) C: 0.20% or less, Si: 2.0% or less, Mn: 0.
1 to 2.5%, P: 0.10% or less, S: 0.05% or less, sol. Al: 0.10% or less, Cu: 0.5-3
．． 0%, Ni: 0.1 to 1.5%, and also contains one or more of B: 0.0003 to 0.0080%, Mo: 0.1 to 3.0%, and the balance is Fe. After reheating the steel billet containing unavoidable impurities to 1000℃ or higher [Ar_3 points-
100° C.] or higher, and after cooling to 500° C. or lower at a cooling rate of 10° C./sec or higher, the hot rolled steel sheet is coiled. (6) C: 0.20% or less, Si: 2.0% or less, Mn: 0.
1 to 2.5%, P: 0.10% or less, S: 0.05% or less, sol. Al: 0.10% or less, Cu: 0.5-3
．． 0%, Ni: 0.1 to 1.5%, and one or more of B: 0.0003 to 0.0080%, Mo: 0.1 to 3.0%, and Ca: 0.0002. ~0.01%, Zr: 0.01~0
．． 10%, rare earth elements: 0.002 to 0.10%, and the balance is Fe and unavoidable impurities. After reheating to 1000°C or higher [Ar_3 points-
100° C.] or higher, and after cooling to 500° C. or lower at a cooling rate of 10° C./sec or higher, the hot rolled steel sheet is coiled. (7) C: 0.20% or less, Si: 2.0% or less, Mn: 0.
1 to 2.5%, P: 0.10% or less, S: 0.05% or less, sol. Al: 0.10% or less, Cu: 0.5-3
．． 0%, Ni: 0.1 to 1.5%, and one or more of B: 0.0003 to 0.0080%, Mo: 0.1 to 3.0%, and Nb: 0.005 ~0.20%, Ti:0.005~0
．． 20%, V: 0.005 to 0.20%, and the balance is Fe and unavoidable impurities. After reheating to 1000°C or higher [Ar_3 points
100° C.] or higher, and after cooling to 500° C. or lower at a cooling rate of 10° C./sec or higher, the hot rolled steel sheet is coiled. (8) C: 0.20% or less, Si: 2.0% or less, Mn: 0.
1 to 2.5%, P: 0.10% or less, S: 0.05% or less, sol. Al: 0.10% or less, Cu: 0.5-3
．． 0%, Ni: 0.1 to 1.5%, and one or more of B: 0.0003 to 0.0080%, Mo: 0.1 to 3.0%, and Ca: 0.0002. ~0.01%, Zr: 0.01~0
．． 10%, one or more of rare earth elements: 0.002-0.10%, Nb: 0.005-0.20%, Ti: 0.005-0
．． 20%, V: 0.005 to 0.20%, and the balance is Fe and unavoidable impurities. After reheating to 1000°C or higher [Ar_3 points
100° C.] or higher, and after cooling to 500° C. or lower at a cooling rate of 10° C./sec or higher, the hot rolled steel sheet is coiled. (9) After casting the steel billet, 100% without reheating treatment.
The method for producing a hot rolled steel sheet according to any one of claims 1 to 8, wherein hot rolling is started from 0°C or higher.