JPS5825732B2 - Method for producing a low yield ratio, high strength composite structure steel sheet with excellent artificial age hardenability after processing - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a hot-rolled steel sheet with a low yield ratio and high strength, which has a composite structure as hot-rolled and has excellent artificial age hardenability after processing.

Here, a composite structure is one in which the main structure components are a ferrite phase, a martensitic phase, and a small amount of retained austenite phase, and a low yield ratio refers to the ratio of yield strength/tensile strength in the as-hot-rolled/rolled state. The ratio (yield ratio) is 0.6 or less, and high strength refers to the case where the tensile strength is approximately 40 K.
f/m'f/ refers to the above case.

Further, the term "artificial age hardenability after processing" refers to an increase in the yield strength of a steel sheet after applying processing strain, which is further increased by heating at a temperature of about 170 to 200C.

The reason why this is excellent is that the amount of increase varies from dog to dog and has little variation depending on the coil location.

In recent years, the automobile industry has been actively trying to reduce the weight of vehicles with the main purpose of reducing fuel consumption.

In order to ensure sufficient vehicle body strength even when the thickness of the material is reduced to reduce weight, it is absolutely necessary to use high-strength steel plates.

However, conventional high-strength steel plates generally have too high a yield ratio, which causes springback during press forming, and low work hardening ability, which tends to cause local strain to concentrate and crack during deformation. ,
Although the necessity was recognized, widespread dissemination was difficult.

Therefore, recently, steel plate users have a yield ratio of about 0.6 or less and a tensile strength of 40 Ky/mr7! There is a trend to demand high-strength steel sheets with a lower yield ratio (that is, higher work hardening ability).

In addition, steel sheets of the type (type of steel) have a high work hardening ability, so they exhibit a fairly high yield strength after forming. Further increasing the yield strength of W (is expected).

As a technology for economically producing high-strength hot-rolled steel sheets with a low yield ratio, one of the inventors of the present invention has developed a method of hot-rolling low-carbon steel in a ferrite-austenite two-phase coexistence region, followed by rapid cooling to a temperature of 350°C or less. (Japanese Unexamined Patent Publication No. 51-79628) and hot rolling finishing of Cr-added steel in the two-phase coexistence region 5
Method for rolling up at temperatures below 00℃ (Japanese Patent Application No. 53-39163)
invented.

In this way, it became possible to manufacture high-strength hot-rolled steel sheets with a low yield ratio.
Although it has become possible to obtain steel sheets with less springback and high work hardenability during forming, the following problems still remain.

That is, the steel plate according to the prior invention does not necessarily have sufficient artificial age hardenability after forming, and has a low level of variation with many variations depending on the steel plate coil portion.

For example, when artificial aging is performed at 180°C for 30 minutes after 3% tensile deformation, the increase in yield strength is about 3 to 4 Ky/mrft, excluding work hardening due to tensile deformation, and depending on the coil part, the increase is about 1 to 4 Ky/mrft. It shows a low value of 2 KP/7716 degrees.

It is extremely important industrially to improve the artificial age hardenability and to eliminate variations depending on coil parts, and one object of the present invention is to advantageously solve the above-mentioned problems.

Another objective is to greatly improve the ductility of the steel sheet, which can be achieved by containing S i f 1% or more and 2 or less as a steel component.

The characteristics of the present invention are C: 0.03 to 0.13%, Mn: 0.8 to 1.7%, Alo, 1% or less, S
i1.0 to 2.0φ (excluding 10%), containing 0.5% or less of Cr as needed, and the balance consisting of iron and unavoidable impurities, is hot-rolled to a temperature of 780°C or more and 890°C or less. Reach finishing temperature, 30℃/S or more 500'C/
It is rapidly cooled at an average cooling rate of less than 230°C and rolled up, and the upper and lower limits of the variation in the winding temperature are within a temperature range of 100 deg C, and the upper limit of the variation in the winding temperature is 230 deg C. The gist of this invention is a method for producing a low yield ratio, high-strength composite steel sheet with excellent post-working artificial age hardenability by adjusting the temperature so as not to exceed 'C.

The technical idea and reasons for limiting the constituent elements of the present invention are as follows.

Hot-rolling finishing temperature of steel plate (refers to finishing exit temperature).

The same applies below), and immediately after finishing, the pro-eutectoid ferrite (ψ phase) is rapidly precipitated due to the deformation-induced transformation accompanying rolling, or the hot rolling finishing temperature itself is lowered to reduce the ferrite (C4 and austenite) phase. By bringing the temperature to a two-phase coexistence temperature range with (γ), a microstructure containing a mixture of fine pro-eutectoid α and untransformed γ is obtained, and by rapid cooling, the untransformed γ becomes martensite (αl) and some residual γ shall be.

The essential element for forming such a composite structure is C.
, Mn.

If the CO content is less than 0.03% and the Mn content is less than 0.8%, the desired composite structure cannot be obtained and the tensile strength will be insufficient.

On the other hand, when the C content exceeds 0.13% and the dynamic amount exceeds 1.7%, the Ar3 temperature decreases significantly, and as a result, the hot rolling finishing temperature at which a microstructure containing a sufficient amount of pro-eutectoid α should be obtained becomes significantly lower. As a result, the processed structure in the pro-eutectoid α remains without being sufficiently recovered, impairing ductility.

Therefore, the amount of C is 0.03% or more and 0.13φ or less,
The amount of Mn is 0.8% or more and 1.7% or less.

Si is an extremely effective element in actual manufacturing because it provides a suitable composite structure, expands the hot rolling finishing temperature range in which the yield ratio can be lowered, and eases the hot rolling working conditions.

A more important effect of Si is that it greatly improves the ductility of steel sheets.

Figure 1 shows the method of the present invention and the method of the previously applied invention (%
-1229, hereinafter referred to as Hikari Invention), shows the relationship between tensile strength and ductility of various composition steels shown in Table 1. % or more (including 1%), it can be seen that the level of ductility compared to tensile strength is significantly superior.

An increase in the amount of Si causes an increase in hot-rolling scale flaws and a deterioration in the paintability of the product to some extent, but it is important to note that the ductility of the steel sheet is extremely important for press-formed products and the requirements for the surface quality of the steel sheet are very strict. In cases where Si is not required, for example, for automobile wheel discs, suspension arms, axle bouncing, frame members, etc., steel in which the amount of Si is intentionally increased to a certain extent is extremely advantageously applied.

However, when the Si content reaches about 2% or more, the above-mentioned problems with the surface properties further increase, and the suitable finishing temperature for the steel sheet becomes extremely high, and rapid cooling after finishing makes it difficult to wind at extremely low temperatures as specified in the present invention. Since it is difficult to achieve this amount in actual operation, the upper limit of the amount of Si is set at 2%.

On the other hand, Cr is added in a small amount and expands the suitable temperature range for hot rolling finishing, but in relation to the amount of Mn, the amount of Mn + Cr
%≧1.7, the suitable range of finishing temperature tends to be narrowed, and Mn%+Cr%=1.3 to 1
．． The most desirable effect is exhibited at about 5%.

Therefore, in view of the limited range of Mn amount in the present invention,
It is sufficient that the Cr content is approximately 0.5% or less**.

FIG. 2 shows the influence of the Mn content, Cr content, and Si content on the tensile strength and yield ratio of the steel plate after final rolling in relation to the finishing temperature.

Figure 2 shows the preferred finishing temperature range (initial thickness 30#) for the invention steel and the comparative invention steel shown in Table 2.

Heating at 1150°C, reaching the finishing temperature in 4 passes, cooling at 50'C/s for 3 layers; winding temperature 100'C), and it is clear from the figure that the Si according to the present invention
In steel containing more than 1% and less than 2%, the range for obtaining a satisfactory low yield ratio is limited to 780°C or more and 890°C or less.

That is, compared to the preferred finishing temperature range of 750°C or more and 860°C or less for steel with a Si content of 1% or less in the previous invention,
In the steel according to the present invention with a Si content of more than 1φ and less than 2φ, the suitable finishing temperature range shifts to a slightly higher temperature side.

Furthermore, in the high-Si steel of the present invention, the addition of Cr is effective in sufficiently lowering the yield ratio that can be reached, rather than expanding the suitable finishing temperature range.

Note that Al is necessary as a deoxidizing element, but if it exceeds 0.1φ, alumina-based inclusions may increase and ductility may be inhibited, so Al should be kept at 10.1% or less.

After finishing the hot rolling, the steel plate is rapidly cooled, and the untransformed γ mixed in the pro-eutectoid α is treated as αI and residual γ.

If this cooling rate is less than 30° C./S, the untransformed γ is transformed into pearlite, and the tendency to become α7g and residual γ is significantly reduced.

On the other hand, if the cooling rate is 500°C/S or more, there is no room for the solid solution carbon in the pro-eutectoid α to diffuse into untransformed γ, and the processed structure in the pro-eutectoid α due to finish rolling (favorable In the case of relatively low temperatures in the finishing temperature range), elongation deteriorates significantly because there is not enough room for recovery.

Therefore, the cooling rate is limited to 30°C/S or more and less than 500°C/S.

The reason why the winding temperature is limited to 230°C or less is that if the winding is carried out at a temperature exceeding 230°C, the fraction of untransformed γ that has already been transformed into bainite will increase, and the tendency to become σ and residual γ will be reduced. This is because, in the end, it becomes difficult to obtain a sufficiently low yield ratio.

The above is basically a technique for producing a low yield ratio, high strength composite structure steel, but in order to significantly improve the post-processing artificial age hardenability of the resulting steel plate, the following requirements must be met.

In other words, the upper and lower limits of the variation in winding temperature are 1010
The temperature should be adjusted so that the temperature is within 0 de and the upper limit of the variation in winding temperature does not exceed 230° C.**.

This limiting requirement will be explained in detail with reference to the embodiments as follows.

Example 1 Limited composition steel according to the invention (0,085% C, 1,10%
Si, 1.15%Mn, 0.014%P,
0.003%S, 0.023φAl) was hot-rolled and finished on a production hot-rolling line (after rough rolling, the finishing temperature was 800 in 7 passes).
~840℃j2.5fi thickness), then rapidly cooled (average 40℃/S
), then a series of coils were wound at varying winding temperatures, the coils were allowed to cool to room temperature, and tensile test pieces were taken from each part of the coils to determine the yield ratio and the artificial age hardening properties after processing (3). After applying tensile strain and further heating at 180° C. for 30 minutes, the deformation strength was measured at room temperature, and the increment compared with the tensile stress was determined.

A few examples of the results are shown in Table 3. In the case of coil number 1 listed in Table 3, the coiling temperature includes a portion exceeding 230°C, which is the limited range of the present invention, and the yield ratio of the obtained coil is high, and the artificial age hardenability after processing is at a low level. It is.

Coil No. 2 is obtained when the winding temperature is 230° C. or lower, but the yield ratio and post-processing artificial age hardenability still vary widely and are insufficient characteristics.

Here, there is a tendency that is completely contrary to the common sense of those skilled in the art.

That is, the yield ratio and post-processing artificial age hardenability of the portion where the winding temperature is low are rather deteriorated compared to the portion where the winding temperature is high.

According to the common sense of those skilled in the art, the lower the winding temperature is, the more σ formation will be achieved and a suitable composite structure will be formed, so the yield ratio should be lower. Since it is easy to secure a sufficient amount of solid solution of carbon during precipitation hardening, the artificial age hardenability after processing should increase.

Coil numbers 1 and 2 listed in Table 2 show results contrary to this.

The technical requirement of adjusting the range of variation in winding temperature in the present invention is based on consideration of such a phenomenon that is difficult to understand, and this consideration will be described in detail later.

Coil number 3 in Table 2 is a case where the winding temperature level is around 200° C. and the variation in winding temperature is within a temperature range of 10100 de, and coil number 4 is a case where the winding temperature level is even lower.

Both low yield ratio and post-processing artificial age hardening properties show stable and good results in these cases.

Example 2 The difficult-to-understand phenomenon shown in Example 1 can be explained by the following experiments and considerations.

When a steel plate with a winding temperature distribution as shown in Fig. 3a is rolled up, the low-temperature region The temperature history is as shown in Figure 3.

That is, the low temperature region X is recuperated to a considerable extent by heat conduction from the high temperature region Y.

After undergoing these thermal histories, yield ratio and artificial age hardening after processing (after 3 tensile cycles, heating at 180°C for 30 minutes, yield strength is measured at room temperature, and the difference from 3% tensile stress is determined)
The influence of
1,69 polySi, 1.28%Mn, 0.010'%
P, 0.005%S, 0.12%Cr, 0.025%
AA heating: 1100℃ heating, 3 passes finishing temperature 850'
Fig. 4 shows the results of an investigation into the following conditions: C, 2.5M thickness, average cooling of 50'C/s, and charging into a furnace set at the winding temperature during cooling.

In winding condition 4, which assumes a case where a portion that has become excessively low temperature recuperates, a low yield ratio is not achieved and the artificial age hardenability after working is poor.

Under winding conditions 2, 3, 5, and 6, all exhibited equally good characteristics.

Among these, for example, winding condition 6 provides a temperature history that is the limit at which low-temperature regions can recuperate when the variation in winding temperature depending on the steel plate region is 30'C to 1308C (in reality, high-temperature regions also occur). Since the temperature gradually decreases after winding, the limit of heat recovery is lower than 130° C., which is advantageous for the thermal history of the actual coil.

), it is clear that variations of this degree, that is, within a temperature range of 10100 de, do not adversely affect the characteristics.

The same can be said about winding condition 3.

On the other hand, winding condition 4 has a variation in winding temperature of 500C to 2
This reproduces the recuperation limit of the low temperature region at 20° C., and even though the average winding temperature is lower than that under winding condition 3, the characteristics are significantly deteriorated.

This means that the variation in winding temperature is equivalent to a temperature difference of 170 d.
This shows that if it reaches eg C, it will cause significant deterioration.

Further, winding condition 1 has a high yield ratio and poor artificial age hardening properties after working, so 270°C is shown to be too high as a winding temperature.

From the above embodiments, the requirements of the present invention regarding the winding process are limited as described above.

The metallurgical phenomena related to the winding process are generally considered as follows.

If the winding temperature (CT) is too high, it will become impossible to transform during the subsequent slow cooling, and the γ phase will undergo bainite transformation, making it difficult to achieve a low yield ratio through the formation of a composite structure (for example, Winding conditions 1) in FIG. 4 are considered as follows for a CT range below a certain level where the γ phase undergoes martensitic transformation rather than bainitic transformation.

After being slowly cooled and reaching room temperature (RT) after being rolled up, a small amount of residual γ is always observed in the composite structure steel sheet along with α and α'.

Therefore, as shown in Fig. 5, when the steel plate finishes hot-rolling at the finishing temperature FT and is cooled to reach the winding temperature CT, the structure of the steel plate consists of an α phase, a residual γ phase, and perhaps a slightly extended phase. It is thought to consist of

That is, the martensitic transformation start temperature Ms point and the martensitic transformation end temperature Mf point of the γ phase, which exists on the way to the winding temperature CT after being cooled after hot rolling finishing, are estimated to have the following temperature order. .

Ms > CT (230℃) > RT > M fo Now,
When the γ phase is rapidly cooled to a temperature T such that Ms > T > Mf, the γ phase undergoes a perfect transformation by a specific rate f(1) determined by T.

This f(T) increases as T decreases within this range (e.g. W, Hume-Rotheryy The
Structure of A11oys of fr
on; An Elementary Introduct
ionjl 966.

Pergamon Pressy England)Q
In other words, f(T) varies from almost o% to almost 1 depending on the temperature T.
It can vary up to 00%.

On the other hand, the reason why the yield ratio of composite structure steel is low is that α of the γ phase
As a result of the volume expansion associated with the l-transformation, the surrounding α phase is elastically strained, and mobile dislocations occur frequently within the α phase near the boundary with the α phase and the α phase (Moriyo, Furukawa, Sahara, Endo; Tetsu to Hagane, v□l.

64 (1978) All, 8.740).

By the way, a composite structure steel with a fairly low CT and therefore with a fairly large proportion of f(T) elongation,
When reheated to a sufficiently high temperature by coil recuperation (
For example, under winding conditions 4) in Figure 4, the mobile dislocations in the α phase described above are fixed by solid solution carbon atoms, and the α phase also tends to be slightly tempered and decomposed into the α phase and carbides, resulting in the above elasticity. As a result of the strain being relaxed, the yield ratio increases and the characteristic of composite structure steel, which is a low yield ratio, is lost. At the same time, the solute carbon atoms, which should be effective by restraining dislocations during artificial aging after processing, At this point of recuperation, the heat is already consumed due to the fixation of mobile dislocations.

Therefore, in such a case, the artificial age hardenability after processing is poor.

If αl is formed at a rate where CT is not very low and therefore f (T) is small, the amount of solute carbon atoms and the amount of d will be small, which will be ineffective for the reasons mentioned above, even if some reheating is performed. Therefore, the above-mentioned adverse effects are naturally reduced (for example, winding condition 3 in FIG. 4).

If it is gradually cooled without being reheated, f (T) will increase sequentially as T decreases as mentioned above, and mobile dislocations will occur in α accordingly, but the main amount of α' will be formed. At T, the temperature is too low for solid solution carbon to rapidly precipitate at the mobile dislocations, and eventually the conditions will be such that the mobile dislocations will remain as they are (for example, 4) Winding condition 2), so there is no problem with the results.

In addition, CT is quite low, and f (T) is at a small rate so that d is formed and is not reheated (for example, winding condition 5 in Figure 4), or even if it is reheated, the temperature reached level is low. Needless to say, there is no problem at all when the fixation of mobile dislocations due to low solid solution carbon is difficult to occur quickly (for example, winding condition 6 in FIG. 4).

From the above considerations, it was concluded that in order to improve the artificial age hardenability after processing, it is extremely important to suppress fluctuations in the winding temperature, and the examples described above demonstrate this.

Some additional notes regarding the technical requirements of the present invention are as follows.

In the present invention, since the finishing temperature is lower than in the case of normal hot rolling, the processed structure due to finish rolling tends to remain in the pro-eutectoid α phase, but it is necessary to wait 1 to 2 seconds before cooling starts. As a result, the processed structure is sufficiently recovered, and there is no concern that it will have an adverse effect on ductility.

Since this requirement is almost automatically achieved in conventional hot strip equipment, no particular limitation is considered necessary.

Furthermore, although limiting the winding temperature is the gist of the present invention, in actual operation, the work may be facilitated by slightly deviating from the limited range to a higher temperature at the tip or tail of the coil.

Since the tip and tail portions are rapidly cooled after winding the coil, as long as within 5φ of the total length of the coil, respectively, deviate from the limited range, no major inconvenience will occur as a practical result.

The steel of the present invention contains rare earth elements (RE
M) or Ca or the like can be added.

The amount added depends on the content of impurity S, and it is recommended that REM/S < 5 and Ca/S < 3 in terms of weight ratio.

In addition, in order to suppress softening near the weld that may occur when the steel of the present invention is subjected to spot welding, flash butt welding, arc welding, etc., Nb5LTt of 0.2% or less each is added.
tW, and M of 0.5φ or less.

Any one or two or more of these may be added to the steel of the present invention.

[Brief explanation of the drawing]

Figure 1 is a chart showing the relationship between tensile strength and elongation of various component steel sheets, Figure 2 is a chart showing the hot rolling finishing temperature range that provides a suitable low yield ratio for various component steel sheets, and Figure 3 is the winding temperature distribution. Fig. 4 is a chart showing the winding simulation experimental conditions, yield ratio, and post-processing artificial age hardenability; It is a chart explaining consideration of changes in steel structure accompanying finishing, cooling, rolling, and slow cooling processes.

Claims (1)

[Claims] I C0.03-0.13%, Mn0.8-1.7%
, A10.1% or less, Si1. O~2.0% (However, 1.
(excluding 0%) steel consisting of balance iron and unavoidable impurities is hot-rolled to a finishing temperature of 780°C or more and 890°C or less, and an average cooling rate of 30°C/s or more and less than 500°C/s. It is rapidly cooled to a temperature of 230℃ or less and then rolled up, and the upper and lower limits of the variation in the winding temperature are 100°C.
A low yield ratio high strength composite structure steel sheet aiming at artificial age hardenability after working, characterized by adjusting the temperature so that it falls within the temperature range of deg C and the upper limit of variation in the winding temperature does not exceed 230°C. manufacturing method. 2 CO, 03-0.13%, Mn 0.8-1.
7%, A[0.1% or less, Si1. A steel consisting of O ~ 2-0 rice (excluding 1.0%), Cr 0.5% or less, the balance iron and unavoidable impurities is hot rolled to a finishing temperature of 780°C or higher and 890°C or lower. , 30℃/S or more5
It is rapidly cooled at an average cooling rate of less than 00°C/S to a temperature of 230°C or less and then rolled up, and the upper and lower limits of the variation in the winding temperature are within a temperature range of 10100 de, and the upper limit of the variation in the winding temperature is A method for producing a low yield ratio, high-strength composite steel sheet with excellent post-working artificial age hardenability, the method comprising adjusting the temperature so as not to exceed 230°C.