JPH0747777B2 - Manufacturing method of hot-rolled steel sheet with excellent low temperature heat hardening - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention shows that although the strength is relatively low and the processing is easy, the strength is remarkably increased by low temperature heating such as paint baking after the processing. The present invention relates to a method for manufacturing a hot-rolled steel sheet for processing suitable as a high-strength member for automobiles or industrial machines.

<Prior art and its problems> Currently, so-called "hot rolled steel sheet" manufactured by continuous hot rolling
Is widely used in various industrial machines such as automobiles as a relatively inexpensive structural material, but it requires excellent workability because there are many members molded by press working for that purpose. It is often done. On the other hand, on the other hand, high strength is also an important required property of structural members, and it is necessary to simultaneously impart "high strength" and "excellent workability" to hot-rolled steel sheets, which are usually difficult to achieve. Was a big issue.

However, in the field of cold-rolled steel sheets, so-called "bake-hardening high-strength steel sheets", which are soft and easy to work before processing and harden during baking coating after processing to increase the yield strength, have already been put into practical use. In recent years, bake hardening type hot rolled steel sheets have been actively researched, and some proposals regarding these have come to be seen.

For example, Japanese Examined Patent Publication (Kokoku) No. 62-188021 discloses a method for producing a bake hardening type high strength hot rolled steel sheet which comprises hot rolling a steel containing a large amount of N and having a specific chemical composition and immediately quenching. This method utilizes the strain aging of solid solution N to obtain bake hardenability. According to the experiments conducted by the present inventors, the bake hardenable high-strength hot-rolled steel sheet obtained by the method has a significantly improved yield strength after baking, but a slight increase in tensile strength is obtained. It was judged that they do not have sufficiently satisfactory characteristics.

That is, as reported in “Sumitomo Metals” vol.33 (1981), No.4, page 121, the fatigue strength of hot-rolled steel sheet has a strong correlation with the tensile strength. There is a fact that the fatigue strength increases as the strength increases. Therefore, in the hot-rolled steel sheet according to Japanese Patent Publication No. Sho 62-188021 with a small increase in tensile strength, the degree of improvement in fatigue properties is also small, and it is required for high strength members for automobiles and industrial machines, which are the main applications of these steel sheets. It has to be said that it is of little practical value because it is not possible to ensure sufficient fatigue characteristics.

Therefore, the object of the present invention is to soften before working, and to increase the tensile strength effective for improving fatigue properties by simply reheating at a relatively low temperature after working. It was placed in the realization of a strong steel plate.

<Means for Solving the Problems> The inventors of the present invention have conducted a large number of experiments in order to achieve the above-mentioned object, and as a result, have repeatedly found that “a high N steel containing a specific amount of Cu is used. When hot rolling is performed to control Cu precipitation by controlling the cooling / winding conditions after rolling, it has good workability as rolled, and then at low temperature such as paint baking. When subjected to heat treatment, hot-rolled steel sheets exhibiting a marked increase in strength (tensile strength) due to the effect of accelerating strain aging due to the precipitation of Cu and the interaction between Cu and N, etc. can be stably obtained. " Was obtained.

The present invention has been made on the basis of the above findings and the like. “C: 0.18% or less (hereinafter,% representing a component ratio is% by weight), Si: 1.2% or less, Mn: 2.0% or less, sol .Al: 0.10% or less, N: 0.0050 to 0.0200% or less, Cu: 0.8 to 2.0%, Ni: 0.01 to 1.0%, or Nb: 0.01 to 0.10%, V: 0.01 to 0.10%, REM (Rare earth element): 0.002 to 0.10%, Ca: 0.002 to 0.01%, Zr: 0.01 to 0.10%, containing at least one kind, and the balance consisting essentially of Fe and unavoidable impurities. Finishing rolling is completed in the temperature range of Ar ₃ points or higher, followed by quenching at a cooling rate of 10 ° C / sec or higher and winding in the temperature range of 500 ° C or lower, which results in excellent heat resistance for processing at low temperatures. The feature is that it enables stable production of rolled steel sheets. "

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

<Action> A) Chemical composition C C is a preferable component that enhances the strength of the steel sheet, but 0.18%
If it is contained in excess of C, the weldability will be deteriorated.
The content was set to 0.18% or less.

Si and Mn Si and Mn are preferable components that improve the strength and ductility of the steel sheet through solid solution strengthening. However, if added more than necessary, weldability deteriorates, so the Si content is 1.2% or less,
The Mn content was set to 2.0% or less.

sol.Al Al is added as a deoxidizer for steel, but as sol.Al 0.10
If it is contained in excess of%, the amount of alumina-based inclusions increases and the workability deteriorates. Therefore, the addition amount of Al was set to 0.10% or less in terms of sol.Al content.

Cu, and N Cu and N are very important components in the present invention. That is, Cu does not impede the workability by existing in a solid solution state in the as-hot-rolled steel sheet, and is precipitated and tensile strength when subjected to a low temperature heat treatment such as a paint baking treatment after forming. It has the effect of improving the height. Also, N remarkably promotes strain aging due to interaction with Cu when it is subjected to the low temperature heat treatment such as the above-mentioned paint baking treatment, resulting in a remarkable increase in tensile strength not seen in the absence of Cu. Bring

However, the Cu content is less than 0.8% or the N content is 0.050
If it is less than 0.1%, the desired high strength cannot be expected even after heat treatment. On the other hand, if more than 2.0% of Cu is contained, cracking tends to occur during hot rolling, and if more than 0.0200% of N is contained. Remarkable aging deterioration occurs, and nest defects due to gas generation easily occur. Therefore, the Cu content is
The content of N is set to 0.8 to 2.0% and the content of N is set to 0.0050 to 0.0200%.

Ni Ni has the effect of preventing cracking during hot rolling due to the addition of Cu, but if its content is less than 0.01% the desired effect due to the above action cannot be obtained, while on the other hand it exceeds 1.0%. Even if it is contained, the effect is saturated and it is economically disadvantageous, so the Ni content was set to 0.01 to 1.0%.

Nb, V, REM (rare earth elements), Ca and Zr These components have the effect of improving the strength or cold workability of the steel sheet, so one or more of them may be contained as necessary. The reason for limiting the content of the component is as follows.

That is, Nb and V have the action of strengthening the steel sheet by combining with C and N and precipitating as carbonitrides, but if the contents of Nb and V are each less than 0.01%, the desired effect due to the above action is obtained. On the other hand, on the other hand, even if the content of each exceeds 0.10%, the strength increasing effect is saturated, which is economically disadvantageous. Therefore, the content of each of these components is set to 0.01 to 0.10%.

Further, REM, Ca and Zr all have the effect of adjusting the shape of inclusions to improve the cold workability of the steel sheet, but their contents are less than 0.002% for Ca, less than 0.01% for Zr and REM. In the case of less than 0.002%, the desired effect due to the above action cannot be obtained, while 0.01% for Ca, 0.10% for Zr and 0 for REM.
On the contrary, when the content of each element exceeds 10%, the amount of inclusions in the steel becomes too large, which causes deterioration of cold workability. Therefore, Ca content is 0.002-0.01%, Zr content is 0.01-0.10
%, And the REM content was 0.002 to 0.10%.

Note that P, which is known as an element that is inevitably mixed into steel
Has the effect of improving the strength and ductility of the steel sheet through solid solution strengthening, but on the other hand, it adversely affects the weldability. Therefore, in order to secure the desired weldability, the P content is 0.07%.
% Or less is preferable. Also, S, which is also known as an element that is unavoidably mixed into steel, forms MnS-based inclusions and reduces workability, so the S content is 0.05.
% Or less is desirable.

B) Manufacturing conditions for hot-rolled steel sheet During hot rolling, it does not matter whether a direct-feed slab is used or a slab is reheated, but it is transformed when the finish rolling temperature is less than Ar ₃ points. The work structure is mixed in the ferrite grains and the workability deteriorates.
Therefore, the finish rolling temperature was set at ₃ Ar or higher.

Further, after the hot rolling is completed, unless the material is rapidly cooled to a region of 500 ° C. or less at a cooling rate of 10 ° C./sec or more, strength is increased due to precipitation of Cu, which also leads to deterioration of workability. Therefore, it was decided that the hot-rolled steel sheet after hot rolling should be cooled at a cooling rate of 10 ° C / sec or higher and wound in a temperature range of 500 ° C or lower.

The hot-rolled steel sheet according to the present invention manufactured under the above conditions,
For example, in the case of baking coating applied after forming into a predetermined shape, it is necessary to remarkably enhance the strength by "strain aging due to interaction of solid solution Cu and N" and "promotion of Cu precipitation due to strain". As stated, but instead of baking coating, 800 ° C
It has been confirmed that, if the reheating treatment is performed at a low temperature below, a significant increase in strength is caused by the coprecipitation hardening action of Cu and nitride, as in the case of applying baking coating. The term "" broadly refers to heat treatment at 800 ° C or lower.

Next, the effects of the present invention will be described more specifically by way of examples.

<Example> Steel having the chemical composition shown in Table 1 was melted in a 50 kg vacuum melting furnace and cast into a mold to form a 60 mm thick slab directly or by hot forging. Hot rolling was performed under the conditions shown in Table 2 to obtain a 3.5 mm thick hot rolled steel sheet.

Next, a test piece is taken from the obtained hot-rolled steel sheet, the tensile strength (TS) and elongation of the hot-rolled steel sheet are measured, and a pre-strain of 8% is applied to the test piece before heating at a typical low temperature. The increase in tensile strength (ΔTS) after the baking treatment (170 ° C. × 20 minutes), which is a treatment, was also measured, 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 has excellent workability (low strength and high elongation), and is heated by baking. It can be seen that it shows a remarkable increase in strength (tensile strength) of 12 kgf / mm ² or more.

On the other hand, in the test number 6 in which the finishing temperature during hot rolling deviates from the lower limit defined by the present invention, the hot-rolled material exhibits high strength and the tensile strength after baking coating is high. The rise in is small.

Further, in the test number 7 in which the cooling rate after rolling is slower than the value specified in the present invention and in the test number 8 in which the winding temperature is higher than the value specified in the present invention, Cu is already precipitated in the hot-rolled sheet. In addition, the strength of the hot rolled sheet is high and the workability is not good, and the increase in strength after baking coating is small.

Furthermore, in the test numbers 20, 21, and 22 in which the C, Si, Mn contents in the steel sheets exceeded the upper limit values specified in the present invention, it was confirmed that the hot-rolled steel sheets obtained had arc welding defects. In addition, test number 27 in which the N content in the steel sheet exceeds the specified value of the present invention
In the steel ingot, nest-like defects are generated and the yield is poor, and the aging deterioration of the obtained hot-rolled steel sheet is large, which means that the N content is less than the specified value of the present invention, the test number 23 and the Cu content are In the test number 25 less than the specified value of the invention, the strength increase after baking coating of the obtained hot rolled steel sheet is small, and the Cu content is more than the specified value of the present invention, the test number 24 and the Ni content are It was confirmed that the test number 26, which is less than the specified value of the present invention, causes cracks during hot rolling.

<Summary of Effects> As described above, according to the present invention, it is relatively soft and has good workability, and the strength (tensile strength) is remarkably increased only by heating at a relatively low temperature after working. As described above, it is possible to stably mass-produce a hot-rolled steel sheet suitable as a high-strength member for automobiles or industrial machines, and the like, which is extremely useful in industry.

Claims (2)

[Claims] 1. By weight ratio, C: 0.18% or less, Si: 1.2% or less, Mn: 2.0% or less, sol.Al: 0.10% or less, N: 0.0050 to 0.0200% or less, Cu: 0.8 to 2.0%, Ni: A steel slab containing 0.01 to 1.0% and the balance consisting essentially of Fe and unavoidable impurities is hot-rolled to finish the finish rolling in a temperature range of Ar ₃ points or higher, and then the cooling rate: 10 ° C / A method for producing a hot-rolled steel sheet for processing, which is excellent in low-temperature heat-hardening property, characterized by being rapidly cooled for sec or more and wound in a temperature range of 500 ° C or less. 2. C: 0.18% or less, Si: 1.2% or less, Mn: 2.0% or less, sol.Al: 0.10% or less, N: 0.0050 to 0.0200% or less, Cu: 0.8 to 2.0%, by weight ratio. One or more of Ni: 0.01 to 1.0%, Nb: 0.01 to 0.10%, V: 0.01 to 0.10%, REM: 0.002 to 0.10%, Ca: 0.002 to 0.01%, Zr: 0.01 to 0.10% , The balance of which consists essentially of Fe and unavoidable impurities, is hot-rolled to finish the finish rolling in a temperature range of Ar ₃ points or higher, and then rapidly cooled at a cooling rate of 10 ° C / sec or higher. A method for producing a hot-rolled steel sheet for working, which is excellent in low-temperature heat-curing properties, characterized by winding in a temperature range of 500 ° C or lower.