WO2025171304A1

WO2025171304A1 - Improved spot weld strength in third generation advanced high-strength steel

Info

Publication number: WO2025171304A1
Application number: PCT/US2025/015074
Authority: WO
Inventors: Robert James COMSTOCK; Mohan Subramanian; Grant A. THOMAS
Original assignee: Cleveland Cliffs Steel Properties Inc
Current assignee: Cleveland Cliffs Steel Properties Inc
Priority date: 2024-02-08
Filing date: 2025-02-07
Publication date: 2025-08-14
Anticipated expiration: 2026-08-08
Also published as: TW202536210A

Abstract

The present disclosure relates to an improvement in third generation advanced high-strength steels, and specifically improvements related to alloying to improve spot welding toughness. By adding quantities of boron, titanium, calcium, or any combination thereof to established compositions of third-generation advanced high-strength steels, the weld toughness of the steel can be strengthened by reducing sulfur and phosphorus segregation.

Description

IMPROVED SPOT WELD STRENGTH IN THIRD GENERATION ADVANCED HIGH-STRENGTH STEEL

PRIORITY

[0001] This application claims priority to U.S. Provisional Patent Application Serial No. 63/551,135 filed February 8, 2024, entitled “Improved Spot Weld Strength in Third Generation Advanced High Strength Steel,” the disclosure of which is incorporated by reference herein.

BACKGROUND

[0002] The automotive industry seeks cost-effective steels that are lighter for more fuel efficient vehicles and stronger for enhanced crash -resistance, while still being formable. A type of steel offered to meet these needs is third generation advanced high strength steels (AHSS). The goal for these materials is to lower the cost compared to other advanced high strength steels by reducing the amount of expensive alloys in the compositions, while still improving both formability and strength. When manufacturing third generation AHSS for use in applications such as automotive applications, the steel material will need joined in certain instances, e.g., joining coiled sheets to allow for continuous processing. Welding is one such method of joining third generation AHSS. While a variety of third generation AHSS have been made and a variety of techniques for joining third generation AHSS have been used, it is believed that no one prior to the inventor(s) has made or used an invention as described herein.

SUMMARY

[0003] Sulfur and phosphorous segregation have shown to reduce the weld toughness of third generation AHSS spot welds. However, improvements are seen when using an alloying approach with third generation AHSS that includes additions, by weight percent, of up to 0.050% titanium, 0.001 to 0.010% boron, and the balance being iron and residuals. Separately or in combination, calcium may be included up to 0.01% by weight to also promote improved weld toughness. In one example, a third generation advanced high strength steel comprises (by weight percent) 0.10% to 0.30% carbon, 1.5% to 3.0% manganese, up to 0.050% titanium, 0.001% to 0.010% boron, and the balance being iron and residuals. In other examples up to 0.010% calcium may be included. The steels may be uncoated or coated with an aluminum-based or a zinc-based coating applied to at least one outer surface.

[0004] Exemplary steels described herein may have a yield strength of at least 850 MPa and an ultimate tensile strength of at least 1180 MPa. Additionally such exemplary steels may have a total elongation of at least 15%.

[0005] Exemplary steels described herein may have a multiphase microstructure including one or more phases of martensite, bainite, or austenite. In some instances, the steels can comprise 5% to 20% martensite and 10% to 15% retained austenite with the balance being tempered martensite and bainite.

[0006] Also, a method of improving the weld toughness of a third generation advanced high strength steel is described and at least in part includes preparing a steel melt having a composition in weight % comprising 0.10% to 0.30% carbon, 1.5% to 3.0% manganese, up to 0.05 % titanium, 0.001% to 0.01% boron, with the balance being iron and residuals. Thereafter the steel melt is cast into a slab and then rapidly cooled from a temperature of about 2500 F to below about 1700 F at a rate greater than about 20 F per second. Thereafter the slab thickness is reduced by rolling. Additionally one or more heat treatments may be used after forming the slab from the steel melt.

DESCRIPTION OF DRAWINGS

[0007] While the specification concludes with claims which particularly point out and distinctly claim the invention, it is believed the present invention will be better understood from the following description of certain examples taken in conjunction with the accompanying examples and figures.

[0008] FIG. l is a plot of measured weld strength versus predicted weld strength.

[0009] FIG. 2 shows a micrograph of a third generation AHSS of the steel control example of Table 1, taken at a magnification of 2000X.

[0010] FIG. 3 shows the micrograph of FIG. 2 with the retained austinite highlighted in green. [0011] FIG. 4 shows the micrograph of FIG. 2 taken at a magnification of 5000X.

[0012] FIG. 5 shows the micrograph of FIG. 4 with the retained austinite highlighted in green.

[0013] The figures are not intended to be limiting in any way, and it is contemplated that various embodiments of the invention may be carried out in a variety of other ways. The accompanying figures incorporated in and forming a part of the specification show aspects of the present invention, and together with the description serve to explain the principles of the invention; it being understood, however, that this invention is not limited to what is shown in the figures.

DETAILED DESCRIPTION

[0014] The following description of certain examples should not be used to limit the scope of the present invention. Other examples, features, aspects, embodiments, and advantages will become apparent to those skilled in the art from the following description. As will be realized, the invention is capable of other different and obvious aspects, all without departing from the invention. Accordingly, the examples, figures, and descriptions should be regarded as illustrative in nature and not restrictive.

[0015] The following description details a composition and/or alloying approach and method to improve the weld strength in third generation AHSS. Third generation AHSS rely on retained austenite, in the range of 5% to 25% by weight percent, in a bainite or martensite matrix and potentially some amount of ferrite or precipitates to develop the desired enhanced properties. When the retained austenite is subjected to plastic deformation, it transforms to martensite and increases the overall strength of the steel through work hardening. In some instances, third generation AHSS provide improved formability at higher tensile strength levels. Some such example steels have yield strengths of approximately 700 MPa to 1300 MPa, ultimate tensile strengths of approximately 900 MPa to 1500 MPa, and tensile elongations of approximately 7% to 25%.

[0016] When welding third generation AHSS, applicants have discovered that the presence of sulfur and phosphorous segregation have shown to reduce the toughness of third generation AHSS spot welds as measured with quasi static cross tension tests. The improvements described herein provide a composition and/or alloying approach and method to improve the toughness of spot welding with third generation AHSS. In one example, a third generation AHSS is made with reduced sulfur and phosphorus to improve strength and ductility of spot welds. To achieve this, microalloying elements such as titanium and boron are added. In some cases, calcium is added to the steel to improve the strength, ductility, and weld toughness of the steel. In addition to iron and other impurities incidental to steelmaking, and the microalloying elements mentioned above, third generation AHSS often include carbon, manganese, and boron in concentrations sufficient to obtain one or more of the above benefits. In some examples third generation AHSS may also include silicon and/or aluminum.

[0017] Carbon acts to stabilize the austenite phase and inhibit the deformation-induced martensitic transformation. However, carbon also increases the probability of forming carbides, which reduces toughness. Carbides also reduce the amount of carbon available to stabilize austenite. Accordingly, third generation AHSS described herein include carbon in the range of 0.10% to 0.30% by weight.

[0018] Manganese stabilizes the austenite phase. The amount of manganese will help determine the material’s strength. Accordingly, third generation AHSS described herein include manganese in the range of 1.5% to 3.0% by weight.

[0019] Boron acts to increase the hardenability, strength, and ductility of the steel and improve the hot workability and surface quality of the steel. Additions of boron displace phosphorous at grain boundaries, and the presence of phosphorous has been shown to reduce the weld toughness of AHSS spot welds. However, too much boron degrades corrosion resistance and workability. Accordingly, third generation AHSS described herein include boron in the range of 0.001-0.01% by weight.

[0020] Titanium acts to increase the hardenability, strength, and ductility of the steel and improve the hot workability and surface quality of the steel. Additions of titanium combine with nitrogen in the alloy to form titanium-nitride. This prevents the nitrogen from combining with boron, which subsequently allows boron to displace phosphorous at grain boundaries. However, too much titanium degrades corrosion resistance and workability. Accordingly, third generation AHSS described herein include titanium in the range of 0.005-0.05% by weight.

[0021] Calcium acts to increase the hardenability, strength, and ductility of the steel and improve the hot workability and surface quality of the steel. Additions of calcium combine with sulfur to form calcium-sulfide. This prevents the sulfur from combining with manganese to form manganese-sulfide. The presence of manganese-sulfide has been shown to form elongated inclusions that reduce toughness, while the presence of calcium-sulfide has not. However, too much calcium can degrade workability. Accordingly, third generation AHSS described herein include calcium in the range of 0.00-0.01% by weight.

[0022] Sulfur is an impurity element inevitably contained in steel, and the content of sulfur may preferably be limited. Accordingly, it can be important in some cases to manage the upper limit of the content of sulfur. When the content of sulfur exceeds 0.015% by weight, ductility and weldability of a steel sheet may be more likely to deteriorate. In the present disclosure, sulfur is managed with addition of other elements to minimize the formation of manganese sulfide and the inclusions that can occur with that and thus deteriorate weldability.

[0023] Phosphorous is an element capable of obtaining a solid solution strengthening effect, but levels of phosphorous that are greater than 0.005% by weight have the potential to degrade weldability. By way of example, when the content exceeds 0.05% by weight, weldability is degraded and the risk of incurring brittleness of a steel is increased. In the present disclosure it is desirable to displace phosphorous at the critical grain boundaries to improve the spot welding strength.

[0024] Silicon is a solid solution strengthener, but can decrease toughness. Silicon inhibits the formation of carbides, which is necessary for the processing of third generation AHSS. Accordingly, third generation AHSS described herein include silicon in the range of 0.2% to 2.0% by weight. Aluminum also inhibits the formation of carbides and can replace some, or all, of the silicon in third generation AHSS. [0025] The AHSS can be processed using conventional steel making, roughing, and finishing processes. By way of example only, and not limitation, the steel can be continuously cast to produce slabs which are rapidly cooled and then rolled to reduce slab thickness. One or more intermediate annealing and reduction steps may be required before reaching the desired thickness. The steels of the present application can remain uncoated but can also be coated with an aluminum-based coating or a zinc-based coating after cold rolling and before hot stamping. Such coating can be applied to the steel sheet using processes known in the art, including hot dip coating or electrolytic coating.

[0026] EXAMPLES

[0027] Table 1 shows exemplary steels and their compositions for a third generation AHSS that has been tested for improved spot welding. FIGS. 2-5 show micrographs of an exemplary third generation AHSS. The third generation AHSS was prepared using standard steel making lab processes except for the control and F, which was made in a commercial process or plant heat. The concentrations shown in Table 1 are in weight percentage. Table 2 shows a corresponding table with the concentrations shown in atomic percentage. The balance of the steels in Tables 1 and 2 are substantially iron and residuals.

[0028] TABLE 1 : Weight % [0029] TABLE 2: Atomic %

[0030] When considering the mechanism of improved spot welding described herein, using the composition information in atomic percentage provides clarity. Therefore, the following paragraphs will refer frequently to the atomic percentages. The conversion from weight percentages to atomic percentages is within the capabilities of those of ordinary skill in the art and requires no further explanation here.

[0031] When considering the differences among the alloying approaches with the steel examples shown in Table 2, steel A represents a low phosphorous and low sulfur example. Steel B represents the low phosphorous but without the low sulfur. Steel C represents the low sulfur but without the low phosphorous. Steel D represents the low sulfur without the low phosphorous and with higher boron. Steels F, G and H are examples of higher titanium, mid-level phosphorous, low sulfur, added calcium, and higher boron.

[0032] With the examples described herein, and the objective to improve spot welding with third generation AHSS, one consideration pertains to the nitrogen and boron interaction. With free or excess nitrogen, boron will form with nitrogen to make boron nitride (BN). The formation of boron nitride is undesirable from the perspective that it then prevents the boron from being available to displace the phosphorous in critical boundary regions, and displacing the phosphorous improves the welding strength. When considering the steel examples in Table 2, where titanium is present, the titanium will form with nitrogen to form titanium nitride (TiN) and this occupies the excess nitrogen so that formation of boron nitride is avoided, which in turn keeps boron available to displace phosphorous at the critical boundary regions. Accordingly, welding strength can be improved by intentionally forming a non-boron containing compound with free or excess nitrogen such as, for example, a nitrogen and titanium containing compound. For clarity of terms, the term “free” or “excess” when describing a composition element is understood as follows. For excess titanium and excess nitrogen, an atom of titanium will combine with an atom of nitrogen. If titanium atoms are left over when all nitrogen atoms are consumed to form TiN, the leftover titanium is referred to as excess titanium. If nitrogen atoms are left over when all titanium atoms are consumed to form TiN, the leftover nitrogen is referred to as excess nitrogen. For excess boron, an excess nitrogen atom will combine with a boron atom to form BN. Boron that is remaining after all BN is formed is referred to as excess boron. For excess calcium and excess sulfur, an atom of calcium will combine a sulfur atom to form CaS. If calcium atoms are left over when all sulfur atoms are consumed to form CaS, the leftover calcium is referred to as excess calcium. If sulfur atoms are left over when all calcium atoms are consumed to form CaS, the leftover sulfur is referred to as excess sulfur.

[0033] Generally, Table 3 below shows an updated version of Table 2 with columns added to show atomic percentages after the formation of titanium nitride.

[0034] TABLE 3 : Atomic % After TiN Formation

[0035] As mentioned above, in the example steels being considered, if excess nitrogen remains after the formation of the titanium nitride, that excess nitrogen will form with the boron to form boron nitride, which is undesirable for the reasons stated above. As shown above in Table 3, steels F, G and H shows compositions where there is no excess nitrogen, and the other examples, while having some excess nitrogen, all have less excess nitrogen compared to the control steel example. Additionally, in example steels F, G, and H the amount of excess nitrogen was less than or equal to the amount of titanium nitride. Table 4 below shows an updated version of Table 3 with columns added to show atomic percentages after the formation of titanium nitride and any excess nitrogen that then forms with boron to make boron nitride. Note that the titanium nitride column has been removed for space reasons but would be present in the same amounts shown in Table 3 above.

[0036] TABLE 4: Atomic % After TiN and BN Formation

[0037] Based on the data shown in Table 4, the amount of boron remaining available to displace the phosphorous in the critical boundary regions can be seen. For instance, with steel examples A, B, C, and the control, there is no excess boron present after the formation of boron nitride. However, with steel examples D, F, G and H, excess boron is present, and this excess boron is then available to displace the phosphorous in the critical boundary regions.

[0038] With the examples described herein, and the objective to improve spot welding with third generation AHSS, another consideration pertains to the sulfur and manganese interaction. With free or excess sulfur, manganese will form with sulfur to make manganese sulfide (MnS). The formation of manganese sulfide is undesirable from the perspective that it causes stringer inclusions to form, and these inclusions degrade toughness. Additions of calcium will cause the sulfur to bond with the calcium to form calcium sulfide (CaS), which then assists in preventing manganese sulfide formation and the associated inclusions from the manganese sulfide. This mechanism can also contribute to improved spot welding strength with third generation AHSS. Table 5 below shows an updated version of Table 4 with columns added to show atomic percentages after the formation of calcium sulfide. Note that the titanium nitride and boron nitride columns have been removed for space reasons but would be present in the same amounts shown in respective Table 3 and Table 4 above.

[0039] TABLE 5: Atomic % After CaS Formation

[0040] From Table 5, example steels control, F, G, and H, included calcium addition as evidenced by the presence of calcium sulfide. With these example steels control, F, G and H, the included calcium amount was not high enough to avoid the presence of any excess sulfur after formation of calcium sulfide. Consequently, the excess sulfur, after calcium sulfide formation, is available to bond with the manganese to form manganese sulfide. As stated above, formation of manganese sulfide is undesirable due to the formation of inclusions that decrease toughness. Despite the presence of some excess sulfur with these example steels, with the example steels F, G, and H the excess sulfur is relatively low compared with the control example steel. Specifically, in example steels F, G, and H the amount of excess sulfur was less than or equal to the amount of calcium sulfide. Thus, steel examples F, G, and H would have fewer inclusions from manganese sulfide formations compared to the control steel example, and thus improved spot welding performance.

[0041] As discussed above, the excess boron will displace the phosphorous at the grain boundaries. This benefit can be approximated by subtracting the excess boron from the phosphorus. With the example steels, it is desirable to not use more boron than necessary to displace the phosphorous and so compositions would typically use boron in amounts where the excess boron will not exceed the phosphorous on an atomic percentage basis. Table 6 below shows an updated version of Table 5 with columns added to show the excess boron subtracted from the phosphorus. In some embodiments, the atomic % of P within the steel minus the excess B (i.e., the amount of boron free to displace phosphorous at grain boundaries) can be greater than -0.004 such as, for example, greater than or equal to -0.002 and less than or equal to 0.044 in some embodiments. Note that the calcium sulfide, carbon, and silicon columns have been removed for space reasons, but would be present in the same amounts shown in Table 5 above.

[0042] TABLE 6: Atomic % of P-(Excess B)

[0043] Table 7 below shows an updated version of Table 6 with the measured weld strengths for the control and steel examples A through H. Note that the manganese has been removed for space reasons but would be present in the same amounts shown in Table 6 above. Strengths are compared to the excess sulfur and the phosphorous minus the excess boron. For the tested weld strengths, the weld schedule used two constant- current weld pulses to fabricate the weld coupons for the cross tension tests. Weld current yielding a weld size of 6 +/- 0.3 mm was employed across all the materials. Also, in a traditional automotive manufacturing process, different automotive structural components are joined together to form a Body-In-White (BIW) structural assembly. This structural assembly further goes through the body paint treatment. After painting, these assemblies undergo baking in ovens at temperatures typically in the range of 160 to 190C for 10-30 minutes to cure the paint. As shown in Table 7, weld strength tests were performed with steel examples that had both undergone this baking process (with BH condition in Table 7) and not undergone this baking process (no BH condition in Table 7). In certain instances, a given example steel may have only been tested in one of these baking conditions: no BH or with BH. As such a blank result for a weld strength in Table 7 means the example steel of that row was not tested for the weld strength condition indicated in the column.

[0044] TABLE 7: Atomic % and Measured Weld Strengths

[0045] From Table 7 in conjunction with Table 2, the impact of sulfur can be seen comparing steel example A with steel example B. Most composition parameters were consistent between these two examples except for the sulfur. Steel example A had the low sulfur while steel example B had the higher sulfur. As seen from the weld strength data, there was a decrease in weld strength of 225 kN per atomic percent of sulfur. This is seen where steel example A has a non-BH weld strength of 6.7 kN and it has 0.0012 atomic % excess S. Steel example B has 5.1 kN strength and 0.0083 atomic % excess S. Both have the same amount of P. Accordingly, the change in weld strength per excess S difference between steel example B and steel example A is calculated as (5.1 - 6.7 kN)/(0.0083 - 0.0012% excess S) = -225 kN/at% excess S. In both these examples the calcium level was 0.0000% on an atomic and weight percentage basis.

[0046] The impact on the weld strength of the boron displacing the phosphorus can be seen comparing steel example A with steel example C from Table 7 in conjunction with Table 2. Steel example A was the lower phosphorous level while steel example C was the higher phosphorous level. The sulfur amount was consistent between steel examples A and C. As shown from the weld strength data of Table 7, the weld strength decreased by 79 kN per atomic percent of the difference in phosphorous and excess boron. This is seen where steel example C has a non-BH weld strength of 4.1 kN and it has 0.044 atomic % P. Steel example A has 6.7 kN with 0.011 atomic % P. Both samples have approximately the same amount of excess S. Accordingly, the change in weld strength per P difference between steel example C and steel example A is calculated as (4.1 - 6.7 kN)/(0.044 - 0.011% P) = -78.8 kN/at% P.

[0047] Based on the above measured impacts of sulfur and the difference in phosphorus and excess boron, the spot weld strength can be predicted. For instance, based on steel example A from Table 7, the force in a spot weld test would equal 6.7 kN when the excess sulfur is 0.0012 atomic % and the difference in phosphorous and excess boron is 0.011 atomic %. Additionally, the strength would decrease by 225 kN per atomic percent of sulfur above 0.0012 atomic % and decrease by 79 kN per atomic percent of the difference in phosphorous and excess boron above 0.011 atomic %. Based on this an equation, Equation 1 below, can be used to predict the spot weld strength.

[0048] EQUATION 1 : Predicted Weld Strength

F = 6.7 + ((Excess S - 0.0012) x (-225)) + ((P - Excess B - 0.011) x (-79)) where Excess S, P, and Excess B are in atomic percentage where F is predicted weld strength is in kN [0049] Table 8 below shows the example steels and how the measured weld strengths compare to the calculated or predicted weld strength by using Equation 1. Furthermore, FIG. 1 illustrates a plot of measured weld strength versus predicted weld strength using Equation 1 for the predicted weld strength. Table 8 demonstrates that weld strength can be improved by controlling excess S, P minus excess B, or both. For example, when excess S is less than 0.0029, P minus excess B is less than 0.026, or both and the predicted weld strength is greater than or equal to 4.1, the example embodiments all achieved a higher weld strength than the control in both the non-baked condition and the baked condition. Additionally, when the predicted weld strength is greater than or equal to 5.5, the example embodiments all achieved an improvement in weld strength of greater than 25% relative to the control in the non-baked condition. When the predicted weld strength is greater than or equal to 6.8, the example embodiments all achieved an improvement in weld strength of greater than 44% relative to the control in the baked condition. Referring to the ideal fit of FIG. 1, it is believed that predicted weld strength is greater than or equal to 5.2 generally corresponds to an improvement in weld strength relative to the control.

[0050] TABLE 8: Comparison of Predicted to Measured Weld Strengths

[0051] Referring to FIG. 1, the data from Table 8 is plotted. The scatter plot shows circular markers representing steel examples that were not subjected to a baking process similar to the paint curing process described above. Triangular markers represent steel examples that did undergo a baking process prior to weld testing. For those steel examples that show data for both the baked and not baked condition, it is clear from the data that the effect of the bake process increases the weld strength results. Accordingly, the embodiments provided herein may have additional utility with applications that include baking after welding such as, for example, automotive painting processes. The dashed line of FIG. 1 represents the fit line and supports the correlation seen with Equation 1 to the data set.

[0052] As demonstrated by the above examples, an improvement in the spot weld toughness of third generation AHSS is achieved through a novel alloying approach. For example, improvements were seen with additions of boron, titanium, or calcium to compositions of third-generation AHSS. Such approaches were successful in improving the weld toughness of the steel based on reducing the sulfur and phosphorus segregation.

[0053] It should be understood that any one or more of the teachings, embodiments, examples, etc. described herein may be combined with any one or more of the other teachings, embodiments, examples, etc. that are described herein. The following-described teachings, embodiments, examples, etc. should therefore not be viewed in isolation relative to each other. Various suitable ways in which the teachings herein may be combined will be readily apparent to those of ordinary skill in the art in view of the teachings herein. Such modifications and variations are intended to be included within the scope of the claims.

[0054] Having shown and described the various examples above, further adaptations of the methods and systems described herein may be accomplished by appropriate modifications by one of ordinary skill in the art without departing from the scope of the present invention. Accordingly, the scope of the present invention should be considered in terms of the following claims and is understood not to be limited to the details of structure and operation shown and described in the specification and drawings.

Claims

I/We claim:

1. A third generation advanced high strength steel comprising, by weight percent, 0.10% to 0.30% carbon, 1.5% to 3.0% manganese, up to 0.050% titanium, 0.001% to 0.010% boron, and the balance being iron and residuals.

2. The steel of claim 1 further comprising up to 0.010% calcium.

3. The steel of claim 1 or claim 2, wherein the steel has two outer surfaces, and further comprises an aluminum-based or a zinc-based coating applied to at least one outer surface.

4. The steel of claim 1 or claim 2, wherein the steel has a yield strength of at least 700 MPa and an ultimate tensile strength of at least 900 MPa.

5. The steel of claim 4, wherein the steel has a total elongation of at least 7%.

6. The steel of claim 1 or claim 2, wherein the steel has a multiphase microstructure including one or more phases of martensite, bainite, or austenite.

7. The steel of claim 6, comprising 5% to 20% martensite and 10% to 15% retained austenite with the balance being tempered martensite and bainite.

8. The steel of claim 1 or claim 2, wherein the steel includes a weld having a spot weld strength of 4.1 kN or more in a cross tension test, when the weld has a weld size of 5.7 mm to 6.3 mm and the weld was made using a weld schedule of two constant-current weld pulses.

9. The steel of claim 8, wherein the steel is baked prior to cross tension testing, wherein the baking temperature range is 160 degrees C to 190 degrees C and the baking duration is 10 minutes to 30 minutes, and wherein the spot weld strength is 5.1 kN or more.

10. The steel of claim 8, wherein the spot weld strength is 4.9 kN or more.

11. The steel of claim 1, comprising phosphorous, and wherein at least some of the boron is present in grain boundaries of the steel.

12. The steel of claim 11, wherein the boron comprises excess boron, and wherein on an atomic percentage basis the excess boron in the steel minus the phosphorus is greater than or equal to -0.004%.

13. The steel of claim 12, comprising boron nitride.

14. The steel of claim 1, comprising titanium nitride.

15. The steel of claim 14, comprising excess nitrogen, wherein the steel comprises more titanium nitride than the excess nitrogen on an atomic percentage basis.

16. The steel of claim 1, comprising excess sulfur, phosphorous, and excess boron, wherein: a predicted weld strength of the steel is greater than or equal to 4.1; the steel has less than 0.0029 excess sulfur on an atomic percentage basis, less than 0.026 more of the excess boron than the phosphorous on an atomic basis, or both; and where F is the predicted weld strength, S_excess is the excess sulfur in atomic percentage, P is the phosphorous in atomic percentage, and B_excess is the excess boron in atomic percentage.

17. A method of improving the weld toughness of a third generation advanced high strength steel comprising the steps of:

(a) preparing a steel melt having a composition in weight % comprising 0.10% to 0.30% carbon, 1.5% to 3.0% manganese, up to 0.05 % titanium, 0.001% to 0.01% boron, and the balance being iron and residuals;

(b) casting a slab from the steel melt;

(c) rapidly cooling the slab from a temperature of about 2500 F to below about 1700 F at a rate greater than about 20 F per second; and (d) rolling the slab to reduce thickness of the slab.

18. The method of claim 17, wherein the residuals comprise sulfur and nitrogen.

19. The method of claim 18, wherein the composition of the steel melt comprises up to 0.010 % calcium.

20. The method of claim 19, comprising forming calcium sulfide with at least a portion of the calcium and the sulfur.

21. The method of claim 18, wherein the composition of the steel melt comprises up to 0.050% titanium.

22. The method of claim 19, comprising forming titanium nitride with at least a portion of the titanium and nitrogen.

23. The method of claim 18, comprising forming boron nitride with at least a portion of the boron and nitrogen.

24. The steel of any of claims 1 to 7, comprising phosphorous at up to 0.05% by weight percent.

25. The steel of any of claims 1 to 7 and 24, wherein at least some of the boron is present in grain boundaries of the steel.

26. The steel of any of claims 1 to 7 and 24 to 25, wherein at least some of the boron is excess boron.

27. The steel of claim 26, wherein on an atomic percentage basis the excess boron in the steel minus the phosphorus is greater than or equal to -0.004%.

28. The steel of claim 26, wherein on an atomic percentage basis the excess boron in the steel minus the phosphorus is greater than or equal to -0.002% and less than or equal to 0.044%.

29. The steel of any of claims 1 to 7 and 24 to 28, comprising boron nitride.

30. The steel of any of claims 1 to 7 and 24 to 29, comprising titanium nitride.

31 . The steel of claim 30, comprising excess nitrogen, wherein the steel comprises more titanium nitride than the excess nitrogen on an atomic percentage basis.

32. The steel of any of claims 1 to 7 and 24 to 31, comprising excess sulfur, phosphorous, and excess boron, wherein: a predicted weld strength of the steel is greater than or equal to 4.1; and where F is the predicted weld strength, S_excess is the excess sulfur in atomic percentage, P is the phosphorous in atomic percentage, and B_excess is the excess boron in atomic percentage.

33. The steel of claim 32, wherein the steel has less than 0.0029 excess sulfur on an atomic percentage basis, less than 0.026 more of the excess boron than the phosphorous on an atomic basis, or both.

34. The steel of any of claims 32 to 33, wherein the predicted weld strength is greater than or equal to 5.2.

35. The steel of claim 34, wherein the predicted weld strength is greater than or equal to 5.5.

36. The steel of any one of claims 1 to 7 and claims 24 to 35, wherein the steel includes a weld having a spot weld strength of 4.1 kN or more in a cross tension test, when the weld has a weld size of 5.7 mm to 6.3 mm and the weld was made using a weld schedule of two constant-current weld pulses.

37. The steel of claim 36, wherein the steel is baked prior to cross tension testing, wherein the baking temperature range is 160 degrees C to 190 degrees C and the baking duration is 10 minutes to 30 minutes, and wherein the spot weld strength is 5.1 kN or more.

38. The steel of claim 37, wherein the spot weld strength is 4.9 kN or more.

39. The steel of claim 38, wherein the spot weld strength is 5.1 kN or more.

40. The steel of claim 39, wherein the spot weld strength is 6.2 kN or more.

41. The steel of claim 40, wherein the spot weld strength is 6.6 kN or more.

42. The steel of claim 41, wherein the spot weld strength is 6.7 kN or more.

43. The steel of claim 42, wherein the spot weld strength is 7.0 kN or more.

44. The steel of claim 43, wherein the spot weld strength is 8.0 kN or more.

45. The steel or method of any one of claims 1 to 44, wherein the carbon is 0.18% to 0.22%.

46. The steel or method of any one of claims 1 to 45, wherein the manganese is 2.27% to 2.6%.

47. The steel or method of any one of claims 1 to 46, wherein the titanium is up to 0.33%.

48. The steel of any one of claims 1 to 47, wherein the boron is 0.0005% to 0.0045%.

49. The steel of any one of claims 1 to 48, further comprising silicon, aluminum, or a combination of silicon with aluminum of 0.8% to 1.61%.

50. The steel of any one of claims 1 to 49, comprising calcium at up to 0.0005% by weight percent.

51. The method of any of claims 17 to 23, comprising phosphorous at up to 0.05% by weight percent.