JPH0735537B2 - High ductility high strength hot rolled steel sheet manufacturing method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to the production of high-ductility and high-strength hot-rolled steel sheets, and in particular, a tensile strength of 70 kgf / mm ² suitable for press forming of automobile wheel discs, bumpers and the like. The present invention relates to the method for manufacturing a hot rolled steel sheet.

(Prior art and problems to be solved) In recent years, from the viewpoint of reducing fuel consumption of automobiles, thinning by increasing the strength of members has been studied, and for wheel discs, hot rolling with a tensile strength of 60 to 70 kgf / mm ² class is performed. Steel sheets are currently in practical use. However, since increasing the strength of such a member is accompanied by deterioration of press formability, considerable efforts have been made in terms of pressing technology as well, but recently, there has been an increasing demand for steel sheets with higher strength. Improvement of ductility of high-strength hot-rolled steel sheets has become a big problem.

Currently used 60-70 kgf / mm ² grade hot rolled steel sheets are strengthening mechanism by solid solution strengthening of Si, Mn, Cr, etc., precipitation strengthening utilizing fine precipitation of carbonitrides such as Nb, Ti, bainite. Strength is designed by the composite of strengthening by transformation structure such as. On the other hand, from the viewpoint of improving ductility or press formability, shape control of non-metallic inclusions by adding REM and Ca, C
Reduction of pearlite amount by making the amount 0.10% or less,
Measures such as improving the work hardening index of steel by adopting a ferrite / martensite structure have been adopted, but 70 kg
These measures alone are not always sufficient for improving the ductility of high-strength steel sheets exceeding f / mm ² .

That is, the strength equivalent to the current 70 kgf / mm ² grade hot rolled steel sheet −
In order to achieve a ductility balance [TS (kgf / mm ² ) x El (%) value in the range of 1700 to 2000 at a plate thickness of 2.6 mm) with higher strength steel sheets, the above ductility improvement measures are not sufficient. Therefore, basic review including strength design is required.

In particular, demand for high ductility thin hot-rolled steel sheets with a tensile strength of 80-100 kgf / mm ² is emerging not only for wheel parts but also for bumpers, and it is expected that demand will increase rapidly in the future.
At present, there is a strong demand for the emergence of steel sheets that satisfy such customer requirements at the mass production level.

The present invention has been made to meet the above-mentioned demand, and has a tensile strength of 70 kgf / mm ² or more and a strength-ductility balance of TS.
An object of the present invention is to provide a method for producing a high-ductility high-strength hot-rolled steel sheet satisfying × El ≧ 2000 (kgf / mm ² ×%).

(Means for Solving the Problems) In order to achieve the above-mentioned object, the present inventor has a tensile strength of 70 kg.
With a high strength of f / mm ² or more, the strength-ductility balance is the same as that of tensile strength of 70 kgf / mm ² class We have earnestly conducted research with the aim of establishing it. As a result, it was found that the intended purpose can be achieved by strengthening Cu precipitation and improving ductility by retained austenite. Therefore, on the basis of this knowledge, experiments and studies have been conducted in more detail on the adjustment of chemical components and production conditions, and the present invention has been made here.

That is, the present invention, C: 0.10 ~ 0.25%, Si: 1.0 ~ 2.5
%, Mn: 1.5 to 3.0%: Cu: 0.7 to 1.5% and solAl: 0.01 to 0.1
0%, Ni: 0.5% or less, Cr: 0 /
5% or less, P: 0.1% or less, V: 0.05% or less, Nb: 0.05% or less, Ti: 0.05% or less, and B: 0.002% or less, and one or more kinds are contained, and the balance is Fe and Steel consisting of unavoidable impurities is hot-rolled at a finishing temperature of 800 ° C or higher, cooled at an average cooling rate of 25 ° C / s or higher, and then coiled in the temperature range of 350 to 450 ° C to obtain a volume ratio. A gist of the present invention is to provide a method for producing a high ductility hot rolled steel sheet having a tensile strength of 70 kgf / mm ² or more, which is characterized by obtaining a structure containing 5% or more of retained austenite.

The present invention will be described in more detail below.

First, an outline of a basic experiment that has led to the knowledge of the present invention will be described.

Steels having the chemical compositions shown in Table 1 were melted in a laboratory, then cut and cast to obtain a 30 mm-thick slab for hot rolling. These test materials are steels having a relatively large amount of C, Si and Mn, steel B is a C-Si-Mn system, and steel C is steel B to which Cu is added by about 1%.

Then, hot rolling, slab heating temperature 1150 ℃, finishing temperature
Finish rolling to 3.2mm thickness at 860 ℃, and after shower cooling (average cooling rate about 35 ℃ / s), coiling temperature is 650 each.
Simulations were carried out for coil winding at ℃, 500 ℃, and 400 ℃. Steel plate cooled to room temperature is 2m after front and back surface grinding
It was made m-thick and subjected to a tensile test (using JIS No. 5 test piece) and a micro survey. The strength-ductility balance of these steel sheets is shown in FIG. 1 for each winding temperature. In the same figure, an iso TS × El curve which is often used for evaluation of strength-ductility balance is shown.

For both steels, the TS-El balance was slightly reduced as the coiling temperature dropped from 650 ° C to 500 ° C.
S-El balance is improved. In particular, in the case of winding steel C at 400 ° C, a tensile strength close to 100 kgf / mm ² was obtained, and TS-El
It is noteworthy that the balance is greatly improved.

Table 2 shows the results of microstructure observation of these steel sheets. It can be seen from the table that when steel C is wound at 400 ° C., a relatively large amount of retained austenite is present in the bainite-ferrite structure.

As described above, the following investigation was carried out in connection with the fact that the steel C has a higher strength in the low temperature winding region and a larger amount of retained austenite than the steel B.

That is, a heat treatment sample is cut out from a hot rolled steel sheet, subjected to an austenitizing treatment in a salt bath for 1050 ° C x 10 minutes, then air-cooled to 860 ° C, and immersed in a 400 ° C salt bath to harden the steel. And the change of the retained austenite amount with the immersion time was investigated. The results are shown in FIG.

In the figure, in the case of Steel B, the hardness gradually decreases with the immersion time at 400 ° C., whereas in Steel C, it is hardened by immersion for up to 80 minutes. This hardening is considered to be caused by the precipitation of ε-Cu. On the other hand, the amount of retained austenite in Steel B increased to about 5% and then decreased to about 2% due to this increase in immersion time, but in Steel C, the amount of retained austenite increased to about 10%. Does not decrease.

The changes in hardness and retained austenite amount due to immersion at 400 ° C are considered to vary greatly depending on the chemical composition of the steel, heat treatment conditions, and pretreatment conditions of the test material. From the above results, steel C is treated under appropriate conditions. As a result, it became clear that the steel can be strengthened and a relatively large amount of austenite can be produced.

Therefore, in order to clarify the effect of the retained austenite amount on the improvement of the ductility of the steel, the above-mentioned two kinds of steels were hot-rolled under various conditions, and the TS-El balance was investigated. The results are shown in FIG. From this, it is understood that the TS-El balance is improved in the region where the amount of retained austenite is 5% or more.

These experimental results show a guideline for producing a hot-rolled steel sheet having higher strength and excellent ductility from a viewpoint completely different from that of the conventional hot-rolled high-strength hot-rolled steel sheet. That is, the material coiled after shower cooling achieves strengthening of the transformation structure during shower cooling by hardening of unprecipitated Cu, and produces a large amount of retained austenite as the transformation progresses after coiling. Depending on the time elapsed thereafter, ε
-Cu can be expected to precipitate finely and further increase the strength of the steel sheet.

The present invention has been completed based on the knowledge obtained in such basic experiments, and the reasons for limiting each condition will be described below.

First, the reasons for limiting the chemical components in the present invention will be explained.

C: C needs to be 0.10% or more to secure a tensile strength of 70 kgf / mm ² or more and generate 5% or more of retained austenite. However, if it exceeds 0.25%, the weldability deteriorates, which is not preferable. Therefore, the amount of C is set in the range of 0.10 to 0.25%.

Si: Si is a very important component along with C and Mn for the formation of retained austenite. However, if it is less than 1.0%, a sufficient amount of retained austenite cannot be obtained. On the other hand, if it exceeds 2.5%, the cost rises significantly and the problem of cold cracking of the slab occurs. Therefore, the Si amount is set to the range of 1.0 to 2.5%.

Mn: Mn is an effective component for strengthening steel by a transformation structure such as bainite. However, if it is less than 1.5%, the tensile strength (TS) becomes insufficient and the generation of retained austenite decreases. On the other hand, if it exceeds 3.0%, the cost will increase significantly and the strengthening effect of steel will be saturated. Therefore, the amount of Mn is
The range is 1.5 to 3.0%.

Cu: The addition of Cu is one of the main requirements constituting the present invention,
It is an essential component for strengthening steel. However, if it is less than 0.7%, ε-Cu precipitation does not sufficiently occur during cooling after coil winding, and steel cannot be strengthened.If it exceeds 1.5%, this effect is saturated and surface defects occur. It is not preferable. Therefore, the Cu content should be in the range of 0.7 to 1.5%.

SolAl: The steel of the present invention is an Al-killed steel, and 0.01% or more of SolAl is necessary to sufficiently deoxidize the steel. However, if it exceeds 0.10%, not only is the cost increased, but also the ductility of the steel is likely to deteriorate due to inclusion of non-metallic inclusions, which is not preferable. Therefore, the content of SolAl is 0.01 to 0.10%.

Although the main elements of the steel of the present invention are as described above, solid solution strengthening, strength adjustment of the steel by transformation structure strengthening, suppression of slab surface flaws, corrosion resistance improvement, Ni, Cr, P,
One or more of V, Nb, Ti and B can be added in a predetermined amount. When added, the amount of each element is Ni: 0.5
%, Cr is 0.5% or less, P is 0.1% or less, V, Nb and Ti
Is 0.05% or less, and B is 0.002% or less. If these upper limits are exceeded, stable formation of retained austenite cannot be achieved.

Next, the manufacturing conditions of the present invention will be described.

The steel is melted, cast and hot rolled by conventional methods.

However, hot rolling must be performed at a finishing temperature of 800 ° C or higher. If the finishing temperature is rolled at less than 800 ° C, the degree of austenite / ferrite rolling is increased, the transformation structure of the steel cannot be sufficiently strengthened, and the material variation in the coil plate width direction becomes large.

Further, it is necessary to cool the steel sheet after finish rolling at an average cooling rate of 25 ° C./s or more and then wind the coil in a temperature range of 350 to 450 ° C. In the slow cooling of less than 25 ° C / s, the austenite → ferrite reaction becomes active and the strength of 70 kgf / mm ² or more cannot be obtained, and the generation of residual austenite is small,
Highly ductile steel sheet cannot be obtained. Although the upper limit of the cooling rate is not specified from a metallurgical point of view, 100 ° C./s or less is desirable in the sense that the flatness of the thin steel sheet is kept good. The upper limit (450 ° C) and the lower limit (350 ° C) of the coiling temperature are mainly determined from the precipitation behavior of ε-Cu and the formation behavior of retained austenite. If the temperature is lower than 350 ° C, the precipitation of ε-Cu is suppressed and sufficient strengthening cannot be achieved, and the amount of retained austenite formed is also small, and the ductility cannot be improved. On the other hand, if the temperature exceeds 450 ° C, the steel will be overaged and strengthened and then softened, and a sufficient amount of retained austenite cannot be secured.

Due to the above-mentioned production suitability, a structure containing residual austenite with a volume ratio of 5% or more is obtained, and a steel ductile steel strength heat with a tensile strength of 70 kgf / mm ² or more and TS × El ≧ 2000 (kgf / mm ² ×%) A rolled steel sheet is obtained. If the residual austenite content is less than 5% by volume, a sufficient strength-postponement balance cannot be obtained as shown in FIG.

The present invention was completed on the basis of a hot rolling experiment using ordinary slab heating, but even if HCR (heating piece charging into a heating furnace) or HDR (direct rolling) technology is applied, the gist thereof is impaired. Needless to say. Further, it is also possible to carry out a plating treatment using the present steel plate as an original plate.

The development of hot-rolled high-strength retained austenitic steel sheet has been reported (see “Iron and Steel” Vol 72, No. 13, s138).
4) In the present invention, the specific behavior of Cu is utilized to establish appropriate manufacturing conditions from the viewpoint of strengthening the steel and promoting the formation of retained austenite. ing.

Next, examples of the present invention will be described.

(Example) Steel having the chemical composition shown in Table 3 was melted, cast, skinned, cast, and then heated to 1200 ° C to obtain a finishing temperature of 870 ° C and 3.2.
After hot rolling to a thickness of mm, the coil was wound at various temperatures at an average cooling rate of about 35 ° C / s.

With respect to the obtained hot rolled steel sheet, the mechanical properties of the steel sheet having a thickness of 2.0 mm by front and back surface grinding were investigated. The results are also shown in Table 3.

From the table, a steel plate satisfying the conditions of the present invention (invention steel)
Despite its high strength, TS × El ≧ 2000 (kgf / mm ²
X%) and has an excellent TS-El balance.

(Effect of the Invention) As described in detail above, according to the present invention, the tensile strength is 70 kgf /
High strength of mm ² or more and TS × El ≧ 2000 (kgf / mm ² ×
%) High ductility and high strength hot rolled steel sheet can be obtained,
It is possible to sufficiently meet the demand for a thin hot-rolled steel sheet having higher strength and ductility.

[Brief description of drawings]

FIG. 1 is a diagram showing strength-elongation balance of hot rolled steel sheet,
The figure shows the time-dependent changes in the retained austenite amount and hardness in the hot-rolled steel sheet, and FIG. 3 is a diagram showing the relationship between the retained austenite amount in the hot-rolled steel sheet and the TS × El value.

Claims (2)

[Claims] 1. In weight% (hereinafter the same), C: 0.10 to 0.25
%, Si: 1.0 to 2.5%, Mn: 1.5 to 3.0%: Cu: 0.7 to 1.5% and
solAl: steel containing 0.01 to 0.10%, the balance consisting of Fe and unavoidable impurities, hot-rolled at a finishing temperature of 800 ° C or higher, and cooled at an average cooling rate of 25 ° C / s or higher, and then 350 to 450
By coiling in the temperature range of ℃, volume ratio is 5
% Of retained austenite is obtained, and a method for producing a high ductility hot rolled steel sheet having a tensile strength of 70 kgf / mm ² or more. 2. C: 0.10 to 0.25%, Si: 1.0 to 2.5%, Mn: 1.5
-3.0%, Cu: 0.7-1.5%, solAl: 0.01-0.10%, Ni: 0.5% or less, Cr: 0.5% or less, P: 0.1% or less, V: 0.05% or less, Nb: 0.05% Below, Ti: 0.05% or less and
B: Steel containing one or more of 0.002% or less, the balance of which is Fe and inevitable impurities, is hot-rolled at a finishing temperature of 800 ° C or more, and cooled at an average cooling rate of 25 ° C / s or more. Then, by coiling in a temperature range of 350 to 450 ° C., a structure containing residual austenite with a volume ratio of 5% or more is obtained, and a high ductility hot rolling with a tensile strength of 70 kgf / mm ² or more. Steel plate manufacturing method.