JPH0639616B2 - Method for producing hot rolled steel sheet with excellent workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention provides a hot-rolled steel sheet which is excellent in drawability and other workability and which can be imparted with excellent strength by coating baking treatment after forming. The present invention relates to a manufacturing method.

<Prior art and its problems> In recent years, in the field of use of steel plates for processing such as steel plates for automobiles, it has been promoted to reduce the thickness and reduce the weight of products by adopting high-tensile steel plates. In general, the workability and strength of steel sheets are in a contradictory relationship. Therefore, in order to satisfy both of these characteristics, various measures must be taken in the quality design of the steel sheet. Various studies and proposals have been made.

However, for example, many automobile parts made of steel sheet are manufactured by press forming into a very complicated shape, and in the case where a steel sheet having better formability than the conventional mild steel sheet is required. However, it is almost impossible to apply a general high-strength steel sheet to such an application.

However, recently, the development of a bake hardenable steel sheet that shows better workability than conventional mild steel sheets during press forming and that has a higher yield point due to the paint bake treatment performed after this press forming step has increased the strength. It has been made. However, most of the hardened steel sheets developed so far are cold rolled steel sheets manufactured through a cold rolling process, which is extremely disadvantageous when applied to a strength base metal such as a steel sheet for automobiles. Met. That is, the cold-rolled steel sheet basically has a different application from the hot-rolled steel sheet, and it was very rarely used as a strength base material in terms of performance and price.

On the other hand, since hot-rolled steel sheet is originally used as a strength base metal, raising the "yield point" is not so important except for some applications, and as a hot-rolled steel sheet, Considering the application, it is necessary that the "tensile strength" be increased by the paint baking treatment after molding.

Therefore, from the above viewpoints, in order to realize a hot rolled steel sheet whose tensile strength is improved by coating baking treatment after forming, etc., a) low carbon steel containing Nb is hot-rolled to 500 ° C or less. By coiling at a low temperature, while suppressing the precipitation of Nb compound to ensure good workability, it is possible to sufficiently precipitate Nb compound in the subsequent aging treatment, the precipitation hardening of high tensile strength. A method for achieving this (Japanese Patent Laid-Open No. 53-9792)
No. 5), b) V-added low carbon steel is hot-rolled to 450-57.
By winding at a low temperature of 0 ° C., the hardening due to the precipitation of V compound and the hardening of the matrix due to the phase transformation are suppressed to ensure good workability, and the subsequent aging treatment provides sufficient V.
A method in which precipitation of a compound is carried out and high tensile strength is achieved by precipitation hardening (Japanese Patent Laid-Open No. 53-79718).
No.), c) Cu-added low carbon steel is hot-rolled to 350-45
By winding at a low temperature of 0 ° C., the action of Cu suppresses the precipitation of the strengthening element compound to ensure good workability, and the subsequent aging treatment causes sufficient precipitation of the strengthening element containing Cu. Thus, a method for achieving high tensile strength by the flexural hardening (Japanese Patent Laid-Open No. 53-79717), etc. was proposed, but in the methods a) and b) above, Nb or There is a problem that the precipitation hardening action of V cannot be completely suppressed, so that the strength of the material increases and the workability cannot be sufficiently improved. On the other hand, in the method of c), it is an element having a small solid solution strengthening ability.
Since the use of Cu does not deteriorate the workability of the hot-rolled sheet, it cannot suppress the precipitation of part of "added Cu" in the cooling process after winding, and heat treatment after pressing. At times, the amount of solid solution Cu that contributes to strengthening is reduced, so that, for example, "170
It was pointed out that the increase in tensile strength is not sufficient by heat treatment at a low temperature for a short time of "holding at 0 ° C for about 20 minutes".

<Means for Solving the Problems> From the above, the present inventor has a sufficiently good workability, and the tensile strength is remarkably increased by a simple heat treatment such as coating baking after the molding step. As a result of repeated studies from various viewpoints in order to find a more stable manufacturing method for rising hot rolled steel sheets, the following findings were obtained. That is, (a) Certainly, in hot-rolled steel sheets, Cu is usually reported as one of the strength-imparting elements by precipitation hardening.
The effect of increasing the strength when added is not so remarkable. However, the elongation that greatly contributes to the workability is strongly influenced by the presence or absence of Cu addition and the coiling temperature, and the hot rolled Cu
When the coiling temperature of the added steel exceeds 300 ° C. in particular, the phenomenon that the elongation of the obtained hot-rolled sheet significantly deteriorates appears. Therefore, a detailed study of these phenomena revealed that Cu-added steel had a small amount of Cu precipitation in the cooling process after winding, which did not significantly affect the strength, but had a large adverse effect on elongation. Was determined to have caused.

Therefore, conversely, when the behavior of Cu when the Cu-added hot-rolled steel sheet was wound at 300 ° C or lower was examined, in this case, most of the added Cu was kept in a solid solution state and the elongation was increased. It is clear that most of the Cu retained in this solid solution precipitates at one time during the paint baking process after press molding and contributes significantly to the subsequent increase in tensile strength. Became.

(b) However, when the hot rolled steel sheet is wound at 300 ° C. or lower as described above, if the hot rolled steel sheet has a C level that is about the level of ordinary low carbon steel, aging deterioration due to solid solution C occurs and the degree of elongation deterioration also occurs. Since it shows a large value, sufficient workability required as a steel sheet for working cannot be obtained.

However, if the C content of the raw material steel is reduced to the range of the extremely low carbon steel, the concern of the characteristic deterioration due to the solid solution C is eliminated.

(c) Furthermore, when strain is introduced into the Cu-added ultra-low carbon steel hot-rolled steel sheet as described above, the work-induced precipitation can be utilized during the subsequent hardening heat treatment, and the heat treatment for a shorter time than in the paint baking treatment is performed. This is more preferable because the tensile strength can be sufficiently increased. Then, the introduction of strain into the hot-rolled steel sheet can be easily carried out by temper rolling or the like having a higher elongation than in the usual case.

The present invention has been made on the basis of the above findings and the like. "C: 0.0050% or less (hereinafter,% representing a component ratio is% by weight), Mn: 0.30% or less, S: 0.010% or less, al: 0.010~0.080%, Cu: 0.65~1.20 % or as a constituent component, or more Ni: also contains 0.20 to 0.40%, the balance of the component composition of Fe and unavoidable impurities steel, Ar ₃ Hot rolling at a finishing temperature above the point, 3
Winding below 00 ° C, or elongation after winding: 1
By imparting a strain of up to 5%, it has excellent formability that is sufficiently satisfactory for automobile applications, etc., and sufficient tensile strength can be obtained only by short-time heat treatment (for example, paint baking) after press molding. It is possible to stably manufacture hot-rolled steel sheets for processing that can secure a large rise. "

Hereinafter, in the present invention, the reason why the component composition of the raw steel, the hot rolling / winding conditions, and the applied strain amount are limited as described above will be described in detail together with its action.

A) Component ratio of raw steel C Since the hot-rolled steel sheet obtained in the present invention is wound up at 300 ° C or lower, when the C content exceeds 0.0050%, solid solution C increases and strength increases. The decrease in elongation due to aging deterioration becomes remarkable. Therefore, the C content is determined to be 0.0050% or less.
The lower the amount of C, the more preferable.

Mn When the Mn content exceeds 0.30%, the solid solution strengthening effect of Mn becomes remarkable, and it becomes impossible to secure good elongation in the hot-rolled steel sheet, so the Mn content was limited to 0.30% or less.

S S is an impurity element that produces sulfide-based inclusions and deteriorates the workability of the hot-rolled steel sheet, but if the content is up to 0.010, the degree of workability deterioration falls within the acceptable range.
The content was set to 0.010% or less.

Al Al is a component added as a deoxidizing agent for steel, and its content must be 0.010% or more to stabilize the deoxidizing effect. On the other hand, when Al is contained in 0.080 or more, the steel sheet is hardened, and the amount of alumina-based inclusions is increased to deteriorate the workability, so the Al content is 0.010.
It was set at ~ 0.080%.

Cu Cu is a solid solution in the state of hot-rolled sheet, and it is an element that precipitates during short-time heat treatment (paint baking treatment, etc.) after the press forming process, and the precipitation strengthening action increases the tensile strength of the steel sheet. , If the Cu content is less than 0.65%, the increase in tensile strength is not sufficient, while excessive addition exceeding 1.20% is Cu.
The Cu content is limited to 0.65 to 1.20% because it causes hot brittleness due to S and leads to the occurrence of surface cracks in the hot rolling process.

Ni Ni is a component added as needed to prevent hot embrittlement of steel due to addition of Cu, but if the content is 0.20% or less, a sufficient effect of preventing hot embrittlement cannot be obtained, while 0.40
%, The effect is saturated and the cost of steel is increased. Therefore, when Ni is added, the content is set to 0.20 to 0.40%.

B) Hot rolling / winding conditions Hot rolling finishing temperature When the finishing temperature of hot rolling is lower than the temperature of Ar ₃ point, the workability of the obtained hot rolled steel sheet deteriorates significantly. Therefore, the finishing temperature for hot rolling was set to the range of Ar ₃ points or higher.

Winding temperature When the winding temperature is higher than 300 ℃, Cu added in the steel
Precipitates in the cooling process after winding, which lowers the elongation of the hot-rolled steel sheet and deteriorates the workability. Therefore, the winding temperature is set to 300 ° C. or lower. The lower limit of the winding temperature is not particularly limited.

Strain imparting amount After forming, in order to sufficiently increase the tensile strength of the hot-rolled steel sheet by heat treatment for a shorter time than the paint baking treatment, it is preferable to introduce a strain that promotes work-induced precipitation in the hot-rolled steel sheet, Therefore, it is desirable to perform temper rolling or the like. In this case, in order to cause the work-induced precipitation, it is necessary to give a strain of 1% or more in terms of elongation. On the other hand, the application of strain causes an increase in the yield point and a decrease in elongation of the base material, but if the strain is 5% or less in terms of elongation, the performance equivalent to that of a normal low carbon steel sheet can be secured. Therefore, although the amount of strain to be applied is determined to be 1 to 5% in terms of elongation, it is preferably adjusted to the range of 2 to 3%.

There are no particular restrictions on the method of introducing strain, and it is practical to carry out the method using an ordinary temper rolling mill or leveler.

Also, there are no particular restrictions on the heating conditions during hot rolling, and in recent years, the "hot charging method" has been developed for the purpose of energy saving, in which a continuous hot slab is inserted into a heating furnace at a high temperature, or the hot slab is directly used. It goes without saying that the effects of the present invention are not impaired even if the "direct roll method" of rolling is adopted.

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

<Example> Example 1 A slab having the chemical composition shown in Table 1 was obtained by continuous casting, heated to 1200 ° C, hot-rolled at a finishing temperature of 920 ° C, and wound at various temperatures. A hot rolled steel sheet with a thickness of mm was manufactured.

Next, a JIS No. 5 test piece was sampled from each of the obtained steel plates and the tensile properties were investigated.

The results obtained in this way are sorted by the winding temperature,
It is shown in FIG.

As is clear from the results shown in FIG. 1, the Cu-added steel sheet produced under the conditions specified in the present invention shows a sufficient elongation value, which shows excellent workability, It can be seen that in the comparative example in which the taking temperature is higher than 300 ° C., the elongation is largely deteriorated and the workability is not sufficient.

Example 2 Similarly, a slab having a chemical composition as shown in Table 1 was used for 1200
After heating to 0 ° C and hot rolling at a finishing temperature of 920 ° C, it was wound at various temperatures to obtain a hot rolled steel sheet having a thickness of 3.0 mm, and further temper-rolled with an elongation of 2%.

Then, from each hot-rolled steel sheet manufactured in this way, JI
A No. S5 test piece was sampled and the tensile properties were investigated.

The obtained results are summarized by winding temperature and shown in FIG.

As is clear from the results shown in FIG. 2, the Cu-added steel sheet produced by hot rolling and winding according to the conditions specified in the present invention and further temper rolling also shows a sufficient elongation value, While exhibiting excellent workability, the winding temperature is 300 ° C.
It can be seen that the comparative example, which was higher than the above, had a large deterioration in elongation and did not have sufficient workability.

Example 3 A slab having the chemical composition shown in Table 2 was obtained by continuous casting, heated to 1200 ° C., and finishing temperature: 920 ° C.
Hot-rolled steel sheet was rolled at 250 ° C. and a hot-rolled steel sheet having a thickness of 3.0 mm was manufactured.

Then, JIS No. 5 test pieces were sampled from each of the obtained steel plates and subjected to an aging test at 30 ° C. for 30 days.

Table 3 shows the changes in the properties due to the natural aging.

As is clear from the results shown in Table 3, in the case of ordinary low carbon steel, the deterioration of properties (characteristics related to workability) due to aging is so great that sufficient workability cannot be expected. It can be confirmed that good workability is maintained even after natural aging when the component system of is an ultra low carbon steel system according to the present invention.

Example 4 A slab having the same chemical composition as shown in Table 2 was used for 1200
2 after heating to ℃ and hot rolling at finishing temperature: 920 ℃
It was wound at 50 ° C. to obtain a hot rolled steel sheet having a thickness of 3.0 mm, and further temper-rolled with an elongation ratio of 2%. Then, JIS No. 5 test pieces were taken from each of the steel plates thus obtained, and the temperature was 30 ° C.
A 30-day aging test was conducted.

Table 4 shows the change in properties due to the natural aging.

From the results shown in Table 4, when the raw material steel is made of the ultra-low carbon steel according to the present invention, good workability is maintained after natural aging even in the strain-added hot-rolled steel sheet. I understand that.

Example 5 A slab having the chemical composition shown in Table 5 was obtained by continuous casting, hot rolled under the conditions shown in Table 6 and wound to 2.8 m.
A hot rolled steel sheet with a thickness of m was manufactured.

Next, JIS No. 5 tensile test pieces were taken from each of the obtained hot-rolled steel sheets to investigate the mechanical properties of the hot-rolled sheet and to examine the change in tensile strength due to the paint baking treatment after press forming. Mechanical properties were also investigated for those subjected to heat treatment at 170 ° C. for 20 minutes after% strain, and changes in tensile strength were investigated.

The results are also shown in Table 6.

As is clear from the results shown in Table 6, the hot-rolled steel sheet manufactured according to the conditions specified in the present invention is soft and has a high elongation, and the paint baking treatment is performed. It can be confirmed that the tensile strength of 15 kgf / mm ² or more can be increased by the heat treatment corresponding to.

On the other hand, in Test Nos. 24 to 26, the increase in the tensile strength after heat treatment of the hot-rolled steel sheet obtained because the Cu content of the material steel is insufficient is low, and in Test Nos. 27 and 28,
It is understood that the workability (especially elongation) of the hot-rolled steel sheet obtained is poor because the winding temperature is high, and the degree of increase in tensile strength after heat treatment is low.

Example 6 Similarly, a slab having a chemical composition as shown in Table 5 was hot rolled under the conditions shown in Table 7 and wound to obtain a hot rolled steel sheet having a thickness of 2.8 mm. Quality rolling was performed.

Then, JIS No. 5 test pieces were taken from each of the obtained hot-rolled steel sheets to investigate the mechanical properties, and in order to grasp the change in the tensile strength due to the coating oblique baking treatment after the press forming step, the test pieces were made into 500 pieces. Mechanical properties were also investigated for those that were heat-treated at 30 ° C. for 30 seconds, and changes in tensile strength were investigated.

The results are also shown in Table 7.

As is clear from the results shown in Table 7, the hot-rolled steel sheet manufactured according to the conditions specified in the present invention shows high elongation in softness even after temper rolling, and moreover, in the paint baking treatment. It can be seen that the tensile strength of 15 kgf / mm ² or more can be increased by the corresponding short-time heat treatment.

On the other hand, Test Nos. 34 to 36 have a low increase in tensile strength after heat treatment of the hot-rolled steel sheet obtained because the Cu content of the material steel is insufficient, and in Test Nos. 37 and 38, It can be seen that the hot rolling steel sheet obtained is inferior in heat treatment (especially elongation) due to the high taking temperature, and the degree of increase in tensile strength after heat treatment is low.

On the other hand, test number 39 is an example of the case where the hot rolling finishing temperature was lower than the Ar ₃ point, but in this case the elongation value of the hot-rolled steel sheet obtained is low and may not show sufficient workability. it is obvious.

Example 7 Cu addition shown in Table 1 as "Steel subject to the present invention" The steel was heated to 1200 ° C., hot-rolled at a finishing temperature of 920 ° C., and then rolled at 250 ° C. to produce a hot-rolled steel sheet having a thickness of 2.8 mm.

Next, each of the obtained hot-rolled steel sheets was temper-rolled at various elongation rates, and then JIS No. 5 tensile test pieces were sampled to measure the mechanical properties of the hot-rolled steel sheets.

Further, the mechanical properties of each of the hot-rolled steel sheets after the above temper rolling subjected to heat treatment at 500 ° C. for 30 seconds were also investigated,
The relationship between the tensile strength increased by the heat treatment and the "elongation ratio during temper rolling" given to the elongation of the base material was shown.

These results are shown in FIG.

As is clear from the results shown in FIG. 3, the Cu-added hot-rolled steel sheet to which strain is applied within the range of the elongation rate specified in the present invention has a low carbon steel sheet-like elongation and ΔTS (heat pressure / heat treatment). Change in tensile strength as rolled and tensile strength after heat treatment) is 15 kg
It can be seen that the tensile strength of f / mm ² or more can be improved.

<Summary of Effects> As described above, according to the present invention, the tensile strength is remarkably improved by the simple heat treatment (paint baking treatment etc.) after the molding process as well as the excellent molding processability. However, it is possible to produce a hot-rolled steel sheet which is suitable as a steel sheet for automobiles in a very stable manner.

[Brief description of drawings]

FIG. 1 and FIG. 2 are graphs showing the relationship between the tensile properties of hot-rolled steel sheets and the winding temperature. FIG. 3 is a graph showing the relationship between the “elongation ratio during temper rolling” and mechanical properties of hot rolled steel sheets.

Claims (4)

[Claims] 1. The content ratio of the components in% by weight is C: 0.0050% or less, Mn: 0.30% or less, S: 0.010% or less, Al: 0.010 to 0.080%, Cu: 0.65 to 1.20%, and the balance Fe and A method for producing a hot-rolled steel sheet having excellent workability, which comprises hot rolling a steel having an ingredient composition consisting of inevitable impurities at a finishing temperature of Ar ₃ or higher and winding it at 300 ° C or lower. 2. The content ratio of components in% by weight C: 0.0050% or less, Mn: 0.30% or less, S: 0.010% or less, Al: 0.010 to 0.080%, Cu: 0.65 to 1.20%, Ni: 0.20 to Steel with a composition of 0.40% and the balance being Fe and unavoidable impurities is hot-rolled at a finishing temperature of Ar ₃ or higher and wound at 300 ° C or lower. Steel plate manufacturing method. 3. The content ratio of the components in% by weight is C: 0.0050% or less, Mn: 0.30% or less, S: 0.010% or less, Al: 0.010 to 0.080%, Cu: 0.65 to 1.20 with the balance being Fe and unavoidable. Steel having a chemical composition consisting of specific impurities is hot-rolled at a finishing temperature of Ar ₃ or higher and wound at 300 ° C. or lower, and then subjected to strain of elongation 1 to 5%. A method for producing a hot rolled steel sheet having excellent workability. 4. The content ratio of components in% by weight C: 0.0050% or less, Mn: 0.30% or less, S: 0.010% or less, Al: 0.010 to 0.080%, Cu: 0.65 to 1.20%, Ni: 0.20 to Steel with a composition of 0.40% and the balance Fe and unavoidable impurities is hot-rolled at a finishing temperature of Ar ₃ or higher and wound at 300 ° C or lower, and then strained at an elongation of 1 to 5%. A method for producing a hot-rolled steel sheet having excellent workability, which is characterized by being added.