阅读说明：本技术 碳含量降低的中锰冷轧带钢中间产品以及用于提供此种钢中间产品的方法 (Medium manganese cold rolled steel strip intermediate product with reduced carbon content and method for providing such steel intermediate product ) 是由 D·克里赞 K·斯坦纳 R·施奈德 于 2019-07-04 设计创作，主要内容包括：本发明涉及一种提供具有改进的fts值的中锰冷轧钢中间产品的方法,其合金包括：-碳组分(C)为0.003重量％<C<0.12重量％,-锰组分(Mn)为3.5重量％<Mn<12重量％,-作为合金组分的硅组分(Si)和/或铝组分(Al),其中Si重量％+Al重量％<1,-任选的另外的合金组分,-任选的微合金组分,特别是钛组分(Ti)和/或铌组分(Nb)和/或钒含量(V),和-其中,合金的其余部分包含铁(Fe)和熔体中不可避免的杂质,其中,所述方法包括在冷轧步骤之后进行的以下步骤：在最高退火温度为684℃-(517℃×以重量％计的碳组分)下进行亚温闭箱退火工艺。(The present invention relates to a method of providing a medium manganese cold rolled steel intermediate product having an improved fts value, the alloy comprising: -a carbon component (C) of 0.003 wt% < C <0.12 wt%, -a manganese component (Mn) of 3.5 wt% < Mn <12 wt%, -a silicon component (Si) and/or an aluminium component (Al) as alloying components, wherein Si wt% + Al wt% <1, -optionally further alloying components, -optionally microalloying components, in particular titanium components (Ti) and/or niobium components (Nb) and/or vanadium content (V), and-wherein the rest of the alloy contains iron (Fe) and unavoidable impurities in the melt, wherein the method comprises the following steps performed after the cold rolling step: a sub-temperature closed-box anneal process is performed at a maximum anneal temperature of 684 ℃ (-517℃ x carbon composition by weight%).)

1. A method of providing a medium manganese cold rolled steel strip intermediate product, the alloy comprising:

a carbon content (C) of 0.003 to 0.12% by weight C,

-a manganese content (Mn) of between 3.5% by weight and Mn 12% by weight,

-a silicon component (Si) and/or an aluminium component (Al) as alloying components, wherein Si wt% + Al wt% <1,

-optionally further alloy components,

-optional microalloying components, in particular titanium content (Ti) and/or niobium content (Nb) and/or vanadium content (V), and

-wherein the remainder of the alloy comprises iron (Fe) and unavoidable impurities in the melt,

wherein the method comprises the following steps performed after the cold rolling step:

-performing a sub-temperature box annealing (s.2.1, s.2.2) at a maximum annealing temperature (T2) of 684 ℃ - (517 ℃xcarbon content in wt%).

2. The method according to claim 1, characterized in that the sub-temperature box annealing process (s.2.1, s.2.2) comprises a heating step (E2), a holding phase (H2) with a holding time (Δ 2) and a cooling sequence (Ab2), wherein the holding time (Δ 2) lasts more than 1000 minutes and less than 6000 minutes, preferably less than 5000 minutes.

3. The method according to claim 1 or 2, characterized in that the cold-rolled steel strip intermediate product exhibits a fts value of at least 40% by selecting an annealing temperature (T2) which depends on the carbon content in weight% and is less than the maximum annealing temperature.

4. Method according to claim 1, 2 or 3, characterized in that the cold-rolled steel strip intermediate product exhibits a value of fts at a minimum uniform elongation (A) by selecting an annealing temperature (T2) which depends on the carbon content in weight% and which is less than the maximum annealing temperature_g) 10% and tensile strength (R)_m) 590MPa to 1350MPa is at least 104 xe^(-0.001*Rm)Wherein the fts value is determined on unnotched flat tensile samples of the cold rolled steel strip intermediate product.

5. The method according to any of claims 1 to 4, characterized in that a single-step annealing process (GR1) is applied, wherein only the closed-box annealing process (S.2.1) with a sub-temperature annealing temperature (T2) is performed, the sub-temperature annealing temperature (T2) being higher than A_c1Temperature and below the maximum annealing temperature defined by equation 684 ℃ - (352 ℃xcarbon content in wt.%).

6. The method according to any of the claims 1 to 4, characterized in that a two-step annealing process (GR2) is applied, wherein a full austenite annealing (S.1) is applied before the sub-temperature box-closed annealing (S.2.2).

7. The method according to claim 6, characterized in that the complete austenite annealing process (S.1) is at a temperature higher than A_c3At an annealing temperature (T1), wherein the annealing temperature (T1) is preferably maintained during a holding time (Δ 1), the holding time (Δ 1) being at least 10 seconds and preferably between 10 seconds and 6000 minutes.

8. The method according to claim 6 or 7, wherein a two-step annealing process (GR2) is applied, wherein above A is performed first_c3Complete austenite annealing at temperature (S.1), then at a temperature higher than A_c1Temperature below maximum annealingThe sub-temperature closed-box annealing method (S.2.2) is carried out at a sub-temperature annealing temperature of the firing temperature.

9. The method of any one of claims 1 to 8, wherein the carbon content (C) is 0.003 wt.% to 0.08 wt.%.

10. The process according to any one of claims 1 to 9, characterized in that the manganese content (Mn) is 4% by weight or more and 10% by weight or less, in particular 5% by weight or more and 8% by weight or less.

11. The method of any of claims 1 to 10, wherein the alloy comprises a silicon content (Si) of 0 wt.% Si ≦ 1 wt.%, particularly 0.2 wt.% Si ≦ 0.9 wt.%.

12. The method of any one of claims 1 to 11, wherein the alloy comprises an aluminum content (Al) of 0 wt.% Al ≦ 1 wt.%, particularly 0.01 wt.% Al ≦ 0.7 wt.%.

13. The method of any one of claims 1 to 12, wherein the alloy comprises a chromium content (Cr) of 0 wt.% to Cr <1 wt.%.

14. The method according to any one of claims 1 to 13, wherein the alloy comprises a sulfur content (S) of less than 60 ppm.

15. The method of any one of claims 1 to 14, wherein the alloy comprises one or more of the following microalloy constituents:

-a titanium content (Ti),

-a niobium content (Nb),

-vanadium content (V).

16. The method of claim 15, wherein the microalloyed components, taken together, are in a maximum proportion of 0.15 percent by weight.

17. A steel intermediate product provided according to the method of any one of claims 1 to 5, characterized in that it has a microstructure in the following proportions:

the residual austenite content is ≥ 10% and ≤ 60%,%, preferably ≥ 10% and ≤ 40%,

the alpha ferrite content is not less than 20% and not more than 90%, and preferably not less than 50% and not more than 80%, and

-the content of cementite is not less than 0% and not more than 5%.

18. A steel intermediate product provided according to the method of any one of claims 6 to 8, characterized in that it has a microstructure in the following proportions:

the martensite content is ≥ 0% and ≤ 20%, and preferably ≥ 0% and ≤ 10%,

a residual austenite content of 10% or more and 60% or less, preferably 10% or more and 40% or less,

the alpha ferrite content is not less than 20% and not more than 90%, and preferably not less than 50% and not more than 80%, and

-the content of cementite is not less than 0% and not more than 5%.

Drawings

Exemplary embodiments of the present invention are described in more detail below with reference to the accompanying drawings.

FIG. 1 shows a graph plotting the elongation after break A of various steels (prior art)₈₀(percent) relative to tensile Strength R_m(MPa) high schematic view of the relationship;

FIG. 2 shows a highly schematic drawing plotting thickness-at-break strain (FTS) (percent) versus Uniform Elongation (UE) (percent) for DP and CP steels (prior art);

FIG. 3 shows a highly schematic drawing plotting thickness strain at break (fts) (percent) versus temperature used during annealing for three medium manganese alloys of the invention having different carbon contents;

FIG. 4 shows a highly schematic drawing plotting thickness strain at break (fts) (percent) versus temperature for a medium manganese steel alloy of the present invention subjected to a single anneal, annealing pass 1 (GR1), and a double anneal, annealing pass 2 (GR2), at fts values;

FIG. 5A shows a highly schematic graph plotting thickness strain at break (fts) (percent) versus Uniform Elongation (UE) (percent) for DP steel, CP steel, and the medium manganese steel alloy of the present invention subjected to annealing pass 1 (GR 1);

FIG. 5B shows a highly schematic graph plotting thickness strain at break (fts) (percent) versus Uniform Elongation (UE) (percent) for DP steel, CP steel, and the medium manganese steel alloy of the present invention subjected to annealing pass 2 (GR 2);

FIG. 6 shows a highly schematic drawing plotting the annealing temperature versus carbon content for various manganese steel alloys of the invention, in particular illustrating an experimentally determined annealing temperature T at which the maximum retained austenite amount is reached_RAmaxAs a function of carbon content; furthermore, in the figure, the maximum annealing temperature T for single and double annealing can be found_ANmaxTo achieve an increased fts value;

figure 7 shows a highly schematic drawing plotting the thickness strain at break (fts) (percent) against different strength ratings rm (mpa);

FIG. 8 is a schematic of an exemplary temperature-time diagram of a single step temperature treatment (GR1) of a steel (intermediate) product of the present invention;

fig. 9 shows a schematic of an exemplary temperature-time diagram of a two-step temperature treatment (GR2) of a steel (intermediate) product according to the invention.

Detailed Description

The cold-rolled steel strip intermediate product of the invention is produced by reducing the carbon content of the starting alloy. It has been shown that the fts value can be increased by significantly reducing the carbon content. By reducing the carbon content, the hardness differential in the structure is reduced. This relationship has been confirmed and quantified on the basis of studies which have shown that there are limitations to the carbon content. In the context of the present invention, only alloys with a carbon content of less than 0.12 wt.% are used.

The fts value was determined on flat tensile test pieces of unnotched steel. It is necessary to determine the initial thickness d of the intermediate steel product₀And thickness d at fracture surface₁. The fts values were calculated as follows: (d)₀-d₁)/d₀×100％。

Figure 3 shows a graph of fts values versus annealing temperature for various steel alloys of the present invention. Specifically, examining several samples herein includes:

-a carbon content (C) of 0 to 0.12% by weight, and

-a manganese content (Mn) of 6% by weight,

wherein the alloy contains silicon (Si) and aluminum (Al) according to the following formula Si wt% + Al wt% <1, and

the remainder of the alloy contains iron (Fe) and unavoidable impurities of the respective melt.

Different correlations can be derived from fig. 3, as described below. For example, if alloy 1 (abbreviated as leg.1) having the following composition is annealed at different temperatures, the fts value decreases significantly with increasing annealing temperature:

-a carbon content (C) of 0.12% by weight,

-a manganese content (Mn) of 6% by weight,

-a silicon content (Si) and/or an aluminum content (Al) as alloy components, wherein Si wt% + Al wt% <1, and

iron (Fe) and inevitable impurities in the remainder of the alloy.

Similar observations can also be made for alloys 2 and 3 (abbreviated to leg.2 and leg.3).

Furthermore, it was shown that the fts value increased significantly with decreasing carbon content. Leg.2 has the following composition:

-a carbon content (C) of 0.056% by weight,

-a manganese content (Mn) of 6% by weight,

-a silicon content (Si) and/or an aluminum content (Al) as alloy components, wherein Si wt% + Al wt% <1, and

the balance of the alloy being iron (Fe) and unavoidable impurities.

Leg.3 has the following composition:

-a carbon content (C) of 0.0% by weight,

-a manganese content (Mn) of 6% by weight,

-a silicon content (Si) and/or an aluminum content (Al) as alloy components, wherein Si wt% + Al wt% <1, and

the balance of the alloy being iron (Fe) and unavoidable impurities.

In other words, if a high fts value is desired, such a medium manganese alloy should not be annealed too high, and it should preferably have a low carbon content. The directional arrow pointing to-C (upward in fig. 3) indicates that decreasing carbon content results in an increased fts value.

The reduction in annealing temperature results in a higher chemical enrichment of the austenite, resulting in a smaller grain size and more stable retained austenite. It has been found that, in the case of the alloy according to the invention, the proportion of retained austenite is advantageously ≥ 10% and ≤ 60%. These effects result in an increase in the value of fts.

The effect of various annealing methods on the resulting fts values was also examined. Herein, the 1 st annealing route (hereinafter GR1) with a sub-warm box annealing process (method s.2.1 in fig. 8) and the 2 nd annealing route (hereinafter GR2) with a complete austenite annealing step (performed in a closed box or continuous annealing line) followed by a sub-warm box annealing process (method s.1+ s.2.2 in fig. 9) were examined.

Fig. 4 shows a graph plotting fts values versus annealing temperature for the steel alloy of the present invention, wherein the effect of the 1 st annealing path is compared to the effect of the 2 nd annealing path. Specifically, a steel alloy sample of the present invention was examined herein, comprising:

-a carbon content (C) of 0.1% by weight,

-a manganese content (Mn) of 6% by weight,

-a silicon content (Si) and/or an aluminum content (Al) as alloy components, wherein Si wt% + Al wt% <1,

whereby the remainder of the alloy comprises iron (Fe) and unavoidable impurities in the respective melt.

Those alloy samples that were subjected to annealing pass GR1 (method s.2.1 in fig. 8) of only one sub-temperature closed-box anneal are indicated by black squares in fig. 4. Here, as already discussed in connection with fig. 3, if the alloy sample has a carbon content of less than 0.12 wt.%, the reduction in annealing temperature results in an increase in the fts value. In fig. 4, this effect is illustrated by the black square arrow.

The diamonds filled with white in FIG. 4 show alloy samples that passed through annealing pass 2 GR2 with full austenite annealing followed by a sub-warm box annealing process (method S.1+ S.2.2 in FIG. 9). For example, if a first alloy sample is subjected to annealing pass number 1 GR1 and a second, identical second alloy sample is subjected to annealing pass number 2 GR2, the second alloy sample exhibits a fts value that is higher than the fts value of the first alloy sample. In fig. 4, this effect is illustrated by the white square arrow.

This leads to an optimization of the microstructure if a double annealing GR2 is carried out with a complete austenite annealing step (method s.1 in fig. 9) followed by a sub-temperature closed-box annealing method (method s.2.2 in fig. 9). Specifically, it has been shown that ferrite strength increases and the stability of retained austenite increases.

Further studies of these alloy samples showed that comparing a first alloy sample passing through the 1 st annealing route GR1 with an identical second alloy sample passing through the 2 nd annealing route GR2 found that the 2 nd annealing route GR2 also resulted in an increase in uniform elongation UE, i.e. the choice of annealing route and the choice of parameters for each annealing route (holding temperature H1 or H2, holding time Δ 1 or Δ 2, etc.) had an effect not only on the fts value, but also on the UE value.

FIG. 5A shows a graph of fts values versus Uniform Elongation (UE) for various steel alloys of the present invention. This relates to the steel alloy of the invention subjected to annealing route 1 GR 1. Similar to the graph of fig. 2, a steel alloy belonging to CP or DP steel is also shown here. In this graph, the steel alloy of the present invention is located in the cross-hatched area. Based on this highly schematic representation, it can be seen that the alloy steels of the invention achieve significantly higher UE values compared to CP steels. However, they achieved a significantly higher fts value compared to DP steel.

Alloy samples having the following compositions were prepared and subjected to annealing route 1, GR1 (see table 1). For these alloys, a tensile strength R of 663MPa to 873MPa can be achieved_m. The fts value for this alloy sample ranged from about 48% to 74%, while the UE value ranged from about 14% to 32%.

FIG. 5B shows another graph of fts values versus Uniform Elongation (UE) for various steel alloys of the present invention. However, this is the steel alloy of the invention subjected to annealing pass 2 GR 2. In this graph, the steel alloy of the present invention is located in the cross-hatched area. It can also be seen here that the steel alloy of the invention achieves significantly higher UE values compared to CP steel. However, they achieved a significantly higher fts value compared to DP steel.

Alloy samples having the following compositions were prepared here and subjected to annealing pass 2 GR2 (see table 2). For these alloys, a tensile strength R of 597MPa to 996MPa can be achieved_m. The fts values for these alloy samples ranged from about 51% to 75%, and the UE values ranged from about 10% to 36%.

Table 3 provides the mechanical property values as a result of the different temperature treatments. For each temperature treatment, a tensile strength of 820 to 875MPa and a uniform elongation of 27 to 31% were achieved. The fts value obtained proved to be advantageous. According to FIG. 9, a complete austenite annealing S.1 as part of a two-stage annealing procedure GR2 is preferred, wherein a relatively long holding time of 1000 min. ltoreq.H 1. ltoreq.6000 min is set. After this complete austenite annealing, a sub-temperature annealing s.2.2 is performed, as shown in fig. 9.

In summary, the following can be assumed for the tested alloy compositions of the invention:

the following characteristic values can be achieved with the alloy composition of the invention if annealing is carried out according to the process requirements of the invention;

intermediate products of medium manganese cold-rolled steel strip with fts values greater than 40% can be produced;

in particular, medium manganese cold rolled steel strip intermediate products can be produced by single annealing GR1 (see fig. 8) having the following fts values: 48 percent to fts percent to 74 percent (see figure 5A);

in particular, a medium manganese cold rolled steel strip intermediate product may be produced by double annealing GR2 (see fig. 9) having the following fts values: 51 percent to fts percent to 75 percent (see figure 5B);

intermediate manganese cold-rolled steel strip intermediate products with UE values greater than 10% can be produced;

in particular, a medium manganese cold rolled steel strip intermediate product can be produced with the following UE values: UE is more than or equal to 14% and less than or equal to 32% (see figure 5A);

in particular, a medium manganese cold rolled steel strip intermediate product can be produced with the following UE values: UE is more than or equal to 10% and less than or equal to 36% (see FIG. 5A). Here, UE is defined as a minimum requirement of 10%.

In summary, for the alloy compositions of the invention studied, the following can be presumed:

by reducing the carbon content of the medium manganese alloy, the fts value can be increased;

by lowering the sub-temperature annealing temperature T2 for annealing s.2.1 or s.2.2 of such medium manganese alloys, the fts value can be increased;

the fts value can be increased by selecting the annealing route (annealing route GR1 or GR 2);

the steel intermediate product can be further optimized by appropriate reduction of the silicon and aluminium alloy composition;

the steel intermediate product can be further optimized by optionally reducing the sulfur content.

These assumptions, summarized above in simplified and purely schematic form, give the developer a lot of freedom in the definition of existing alloys. This will be illustrated by the following examples.

When using double annealing (GR2), an alloy may be used whose carbon content itself is slightly higher than single annealing GR1, since double annealing (GR2) achieves a higher value of fts than single annealing (GR 1).

In fig. 6, various effects observed on the basis of the alloy composition of the present invention are shown in the figure. The graph has an ordinate representing the annealing temperature and an abscissa representing the carbon content of the alloy composition. Inputting an experimentally determined maximum annealing temperature T as a function of carbon content when achieving an elevated fts value_ANmax。

The dotted line connecting the white diamonds represents the experimentally determined annealing temperature T of the alloy subjected to the double annealing method (GR2)_ANMax. The short dashed line connecting the black squares represents the experimentally determined annealing temperature T of the alloy subjected to a single annealing process (GR1)_ANmax. The solid line connecting the white circles represents the experimentally determined annealing temperature T as a function of the carbon content when the maximum amount of retained austenite is reached_RAmax。

Alloy compositions containing 6 wt.% manganese (Mn) have been investigated here. As shown on the abscissa, the carbon content varies from 0 wt% to 0.12 wt%.

The dotted line in fig. 6 may be represented by the following equation (1), where T_ANmaxIs the maximum annealing temperature. Equation (1) defines the maximum annealing temperature T2 for the sub-temperature anneal s.2.2 of fig. 9.

T_ANmax＝684℃-(517℃×C％) (1)

The short-dashed line in fig. 6 can be represented by the following equation (2). Equation (2) defines the maximum annealing temperature T2 for the sub-temperature anneal s.2.1 of fig. 8.

T_ANmax＝684℃-(352℃×C％) (2)

The results of which are summarized in the study of fig. 6, demonstrate that it is possible to act at relatively high annealing temperatures with low carbon content to obtain an increased fts value. At higher carbon contents, the annealing temperature T2 must be lowered to obtain an increased value of fts.

It can also be concluded from the results summarized in fig. 6 that it is so effective to reduce the carbon content to a level close to 0 wt.%, that even exceeding the temperature T during annealing of such an alloy composition can be used_RAmaxWithout thereby decreasing the fts value of the annealing temperature T2. That is, in the alloys of the present invention, a reduction in carbon content is a particularly effective measure.

It can also be concluded from the results summarized in fig. 6 that at higher carbon contents, e.g. 0.05 to 0.12 wt.%, higher fts values can be obtained by lowering the annealing temperature T2. The higher the carbon content of the alloy of the invention, the greater the reduction in the annealing temperature T2 must be.

If annealed twice as shown in FIG. 9, the annealing temperature T2 only needs to be relative to T when the carbon content exceeds 0.056 wt.%_RAmaxAnd decreases.

In fig. 7, further aspects of the invention are shown in this figure. The abscissa is the strength rating R in MPa_mThe ordinate is fts values in percent. The minimum fts value is shown by the slanted dashed line, where, as a boundary condition, the UE value is assumed to be at least 10%, i.e., UE ≧ 10%. The dotted line can be mathematically described by equation (3).

fts_min＝104×e^(-0.001*Rm) (3)

In fig. 7, the range defined by the rectangle denoted by reference numeral 4 is shown, which includes the alloy of the present invention. For alloys in the range 4, it is ensured that they have a good local deformability on the one hand and a good overall deformability on the other hand. UE values were always above 10% and fts values were always above 40%.

Some characteristic properties of the alloys of the invention are summarized in table 4.

Table 5 summarizes some alloy compositions and their characteristic properties. These alloy compositions are shown in table 5 in combination with the annealing temperatures selected according to the invention, since they are outside the claimed range 4.

Sample No.3.1 only reached a UE value of 8.1%. 8.1% is less than 10% of the minimum UE value. One of the reasons for not reaching the minimum UE value is the carbon content, which is 0.18 wt%, above the upper limit of 0.12 wt% set forth herein. Furthermore, the minimum requirement of fts value of 40% of equation 3 is not met.

Although sample No.3.2 achieved a sufficiently high UE value, the fts value of 29% was significantly lower than fts_min40%. From equation (2) the annealing temperature T2 is calculated, which according to the invention should be a maximum of 612.80 ℃ for this particular alloy. However, sample No.3.2 was annealed at a relatively high temperature of 680 ℃, which resulted in a value of fts that was too low.

Although sample No.3.3 obtained a sufficiently high UE value, the fts value of 47% was much lower than the required fts value of 57% according to equation 3. One of the reasons for the failure to reach the minimum fts value is that the manganese content is 1.83 wt%, which is below the lower limit of 3.5 wt% set forth herein.

According to the invention, the alloy therefore consists of the following composition:

a carbon content (C) of 0.003 to 0.12% by weight C,

-a manganese content (Mn) of between 3.5% by weight and Mn 12% by weight,

-a silicon content (Si) and/or an aluminum content (Al) as alloy components, wherein Si wt% + Al wt% <1, optionally further comprising the following alloy components:

-optional microalloying components, in particular titanium content (Ti) and/or niobium content (Nb) and/or vanadium content (V), and

the remainder of the alloy comprises iron (Fe) and unavoidable impurities in the melt.

In at least some embodiments, the carbon content (C) is 0.003 wt.% C.ltoreq.0.08 wt.% and/or the manganese content (Mn) is 4 wt.% Mn.ltoreq.10 wt.%, in particular 6 wt.% Mn.ltoreq.10 wt.%, since in this case a particularly high fts value can be achieved.

In at least some embodiments, the silicon content (Si) is 0 wt.% Si ≦ 1 wt.%. In particular, the silicon content (Si) is 0.2% by weight or more and 0.9% by weight or less of Si.

In at least some embodiments, the aluminum content (Al) is 0 wt.% Al ≦ 1 wt.%. In particular, the aluminum content (Al) is 0.01 to 0.7 wt.% Al.

In at least some embodiments, the alloy includes a sulfur content (S) of less than 60ppm, in weight%.

In at least some embodiments, the alloy includes a chromium content (Cr) of 0 wt.% Cr 1 wt.%.

In at least some embodiments, the alloy comprises one or more of the following microalloy components:

-a titanium content (Ti),

-a niobium content (Nb),

-vanadium content (V).

In at least some embodiments, the titanium content (Ti), if present, is 0 wt% < Ti ≦ 0.12 wt%.

In at least some embodiments, the microalloyed components together comprise a maximum proportion of 0.15 percent by weight of the alloy.

The information about the composition of the alloy is to be understood here as a percentage by weight. The remainder of the alloy comprises iron (Fe) and impurities inevitable in such melts. The data in weight percent always amount to 100 weight%.

As mentioned above, the method of the invention comprises a special annealing step performed after the cold rolling step:

a sub-temperature closed-box anneal S.2.1 or S.2.2 is performed, with a maximum anneal temperature T2 of 684 ℃ - (517 ℃. times.carbon content in wt.%). The carbon content in weight% is also referred to herein as C%. If the sub-temperature box anneal process is part of a one-step anneal process, the maximum anneal temperature T2 may even be lower than these values as represented by the equation 684 ℃ - (352 ℃. times.carbon content in weight%).

Exemplary details of the one-step annealing process GR1 are shown in fig. 8. In the sub-temperature box annealing process s.2.1, the alloy is heated to a holding temperature T2. In fig. 8, heating is indicated by E2. The alloy is then held at a holding temperature T2 for a holding time Δ 2. Followed by cooling. In fig. 8, cooling is indicated by Ab 2. In table 6 below, exemplary parameters for the one-step annealing process GR1 of the present invention are given:

the sub-temperature closed-box annealing, also referred to as sub-temperature annealing for short, is carried out at a holding temperature T2 in the α + γ two-phase region. A. the_c3And A_c1The region in between (see fig. 8 and 9) is referred to as the α + γ biphase region.

Complete Austenitic annealing method S.1 (see FIG. 9) A in the region where the holding temperature T1 is higher than the monophase γ_c3Temperature, i.e. T1>A_c3The process is carried out as follows.

Fig. 9 shows exemplary details of the two-step annealing process GR 2. In the complete austenite annealing process S.1, the alloy is heated to a holding temperature T1. In fig. 9, this heating is denoted by E1. The alloy is then held at a holding temperature T1 for a holding time Δ 1. And then cooled. In fig. 9, the cooling is indicated by Ab 1. In the subsequent sub-temperature box annealing process s.2.2, the alloy is heated to a holding temperature T2. In fig. 9, this heating is denoted by E2. The alloy is then held at a holding temperature T2 for a holding time Δ 2. And then cooled. In fig. 9, the cooling is indicated by Ab 2. In the following table 7, exemplary parameters of the two-stage annealing process GR2 of the present invention are given:

from the various figures and the description of these figures, it is important that the annealing temperature T2 of the sub-temperature closed box annealing process is not too high for reaching high fts values above 40%. For annealing in sub-ambient temperature closed boxesThe maximum annealing temperature T2 of the process is always lower than A_c3The upper limit is defined by equation (1) or (2).

The properties of the cold-rolled steel strip intermediate product according to the invention are influenced in particular by the selection of the annealing temperatures T1 and/or T2, wherein in particular the temperature T2 depends on the carbon content in% by weight and is always below the maximum annealing temperature A_c3。

The fts value of the cold rolled steel strip intermediate product of the present invention is at the minimum uniform elongation (A) according to equation (3)_g) 10% and tensile strength (R)_m) At least 104 × e when 590MPa to 1350MPa^(-0.001*Rm). These fts values were measured on unnotched flat tensile specimens of the cold-rolled steel strip intermediate product.

The cold-rolled steel strip intermediate product of the invention is particularly characterized in that it has a microstructure in the following proportions if the single-step annealing process GR1 of fig. 8 is used:

the residual austenite content is not less than 10% and not more than 60%,

the content of-alpha-ferrite is not less than 20% and not more than 90%, and

content of cementite ((Fe, Mn)₃C) Is more than or equal to 0 percent and less than or equal to 5 percent.

The cold-rolled steel strip intermediate product of the invention is particularly characterized in that it has a microstructure in the following proportions if the two-step annealing process GR2 of fig. 9 is used:

the martensite content is more than or equal to 0% and less than or equal to 20%,

the residual austenite content is not less than 10% and not more than 60%,

the content of-alpha-ferrite is not less than 20% and not more than 90%, and

content of cementite ((Fe, Mn)₃C) Is more than or equal to 0 percent and less than or equal to 5 percent.

This microstructure, which has a martensite content, a residual austenite content, an alpha-ferrite content and a cementite content, provides the special properties of the cold-rolled steel strip intermediate product of the invention.

Drawings and formula notation:

medium manganese steel zone	1
		TRIP steel zone	2
TBF and Q&P steel area	3
		Region(s)	4
Austenite phase	γ
		Two phase region	α+γ
Elongation after fracture%	A
		Cooling during austenite annealing	Ab1
Cooling in sub-temperature annealing process	Ab2
		Temperature at the onset of austenite formation/austenite start temperature, in deg.C	Ac1
Temperature at the end of austenite form/austenite finish temperature, in deg.C	Ac3
		Uniform elongation%	Ag
The elongation after fracture in% was measured at a length of 80mm	A80
		Carbon content, wt.%	C％
Heating during austenitic annealing	E1
		Heating in sub-temperature annealing process	E2
Initial thickness of intermediate steel product	d0
		Thickness of fracture surface of intermediate steel product	d1
Retention time in a complete austenite annealing process	Δ1
		Hold time in sub-temperature annealing process	Δ2
Thickness strain at break, Unit%	fts
		Minimum value of fracture thickness strain,% unit	ftsmin
Ferrite phase	α
		Annealing path	GR
Retention during full austenite annealing	H1
		Maintenance during sub-temperature annealing	H2
Tensile strength in MPa	Rm
		Complete austenite annealing	S.1
Annealing at sub-temperature	S.2.1,S.2.2
		Time	t
Holding temperature during complete austenite annealing	T1
		Holding temperature during sub-temperature annealing	T2
Maximum annealing temperature in deg.C	TANmax
		The annealing temperature, in deg.C, at which the maximum amount of retained austenite is reached	TRAmax
Uniform elongation%	UE